The physiological variability of channel density in hippocampal CA1 pyramidal cells and interneurons explored using a unified data-driven modeling workflow

Every neuron is part of a network, exerting its function by transforming multiple spatiotemporal synaptic input patterns into a single spiking output. This function is specified by the particular shape and passive electrical properties of the neuronal membrane, and the composition and spatial distribution of ion channels across its processes. For a variety of physiological or pathological reasons, the intrinsic input/output function may change during a neuron's lifetime. This process results in high variability in the peak specific conductance of ion channels in individual neurons. The mechanisms responsible for this variability are not well understood, although there are clear indications from experiments and modeling that degeneracy and correlation among multiple channels may be involved. Here, we studied this issue in biophysical models of hippocampal CA1 pyramidal neurons and interneurons. Using a unified data-driven simulation workflow and starting from a set of experimental recordings and morphological reconstructions obtained from rats, we built and analyzed several ensembles of morphologically and biophysically accurate single cell models with intrinsic electrophysiological properties consistent with experimental findings. The results suggest that the set of conductances expressed in any given hippocampal neuron may be considered as belonging to two groups: one subset is responsible for the major characteristics of the firing behavior in each population and the other is responsible for a robust degeneracy. Analysis of the model neurons suggests several experimentally testable predictions related to the combination and relative proportion of the different conductances that should be expressed on the membrane of different types of neurons for them to fulfill their role in the hippocampus circuitry.

Complementary synaptic distribution of enzymes responsible for synthesis and inactivation of the endocannabinoid 2-arachidonoylglycerol in the human hippocampus

Clinical and experimental evidence demonstrates that endocannabinoids play either beneficial or adverse roles in many neurological and psychiatric disorders. Their medical significance may be best explained by the emerging concept that endocannabinoids are essential modulators of synaptic transmission throughout the central nervous system. However, the precise molecular architecture of the endocannabinoid signaling machinery in the human brain remains elusive. To address this issue, we investigated the synaptic distribution of metabolic enzymes for the most abundant endocannabinoid molecule, 2-arachidonoylglycerol (2-AG), in the postmortem human hippocampus. Immunostaining for diacylglycerol lipase-α (DGL-α), the main synthesizing enzyme of 2-AG, resulted in a laminar pattern corresponding to the termination zones of glutamatergic pathways. The highest density of DGL-α-immunostaining was observed in strata radiatum and oriens of the cornu ammonis and in the inner third of stratum moleculare of the dentate gyrus. At higher magnification, DGL-α-immunopositive puncta were distributed throughout the neuropil outlining the immunonegative main dendrites of pyramidal and granule cells. Electron microscopic analysis revealed that this pattern was due to the accumulation of DGL-α in dendritic spine heads. Similar DGL-α-immunostaining pattern was also found in hippocampi of wild-type, but not of DGL-α knockout mice. Using two independent antibodies developed against monoacylglycerol lipase (MGL), the predominant enzyme inactivating 2-AG, immunostaining also revealed a laminar and punctate staining pattern. However, as observed previously in rodent hippocampus, MGL was enriched in axon terminals instead of postsynaptic structures at the ultrastructural level. Taken together, these findings demonstrate the post- and presynaptic segregation of primary enzymes responsible for synthesis and elimination of 2-AG, respectively, in the human hippocampus. Thus, molecular architecture of the endocannabinoid signaling machinery supports retrograde regulation of synaptic activity, and its similar blueprint in rodents and humans further indicates that 2-AG's physiological role as a negative feed-back signal is an evolutionarily conserved feature of excitatory synapses.

Petilla terminology: nomenclature of features of GABAergic interneurons of the cerebral cortex

Neuroscience produces a vast amount of data from an enormous diversity of neurons. A neuronal classification system is essential to organize such data and the knowledge that is derived from them. Classification depends on the unequivocal identification of the features that distinguish one type of neuron from another. The problems inherent in this are particularly acute when studying cortical interneurons. To tackle this, we convened a representative group of researchers to agree on a set of terms to describe the anatomical, physiological and molecular features of GABAergic interneurons of the cerebral cortex. The resulting terminology might provide a stepping stone towards a future classification of these complex and heterogeneous cells. Consistent adoption will be important for the success of such an initiative, and we also encourage the active involvement of the broader scientific community in the dynamic evolution of this project.

Reciprocal inhibition of G-protein signaling is induced by CB(1) cannabinoid and GABA(B) receptor interactions in rat hippocampal membranes

Cannabinoid CB(1) and the metabotropic GABA(B) receptors have been shown to display similar pharmacological effects and co-localization in certain brain regions. Previous studies have reported a functional link between the two systems. As a first step to investigate the underlying molecular mechanism, here we show cross-inhibition of G-protein signaling between GABA(B) and CB(1) receptors in rat hippocampal membranes. The CB(1) agonist R-Win55,212-2 displayed high potency and efficacy in stimulating guanosine-5'-O-(3-[(35)S]thio)triphosphate, [(35)S]GTPgammaS binding. Its effect was completely blocked by the specific CB(1) antagonist AM251 suggesting that the signaling was via CB(1) receptors. The GABA(B) agonists baclofen and SKF97541 also elevated [(35)S]GTPgammaS binding by about 60%, with potency values in the micromolar range. Phaclofen behaved as a low potency antagonist with an ED(50) approximately 1mM. However, phaclofen at low doses (1 and 10nM) slightly but significantly attenuated maximal stimulation of [(35)S]GTPgammaS binding by the CB(1) agonist R-Win55,212-2. The observation that higher concentrations of phaclofen had no such effect rule out the possibility of its direct action on CB(1) receptors. The pharmacologically inactive stereoisomer S-Win55,212-3 had no effect either alone or in combination with phaclofen establishing that the interaction is stereospecific in hippocampus. The specific CB(1) antagonist AM251 at a low dose (1 nM) also inhibited the efficacy of G-protein signaling of the GABA(B) receptor agonist SKF97541. Cross-talk of the two receptor systems was not detected in either spinal cord or cerebral cortex membranes. It is speculated that the interaction might occur via an allosteric interaction between a subset of GABA(B) and CB(1) receptors in rat hippocampal membranes. Although the exact molecular mechanism of the reciprocal inhibition between CB(1) and GABA(B) receptors will have to be explored by future studies it is intriguing that the cross-talk might be involved in balance tuning the endocannabinoid and GABAergic signaling in hippocampus.

Quantitative ultrastructural differences between local and medial septal GABAergic axon terminals in the rat hippocampus

Functionally distinct subsets of hippocampal inhibitory neurons exhibit large differences in the frequency, pattern and short-term plasticity of GABA release from their terminals. Heterogeneity is also evident in the ultrastructural features of GABAergic axon terminals examined in the electron microscope, but it is not known if or how this corresponds to interneuron subtypes. We investigated the feasibility of separating morphologically distinct clusters of terminal types, using the approach of measuring several ultrastructural parameters of GABAergic terminals in the CA1 area of the rat hippocampus. Septo-hippocampal axon terminals were anterogradely labeled by biotinylated dextran amine and visualized by pre-embedding immunogold staining to delineate one homogeneous terminal population. Long series (100-150) of ultrathin sections were cut from stratum oriens and stratum radiatum of the CA1 area, and GABAergic terminals were identified by post-embedding immunogold staining. Stereologically unbiased samples of the total GABAergic axon terminal population and a random sample of the septal axon terminals were reconstructed in 3D, and several of their parameters were measured (e.g. bouton volume, synapse surface, volume occupied by vesicles, mitochondria volume). Septal terminals demonstrated significantly larger mean values for most parameters than the total population of local GABAergic terminals. There was no significant difference between terminals reconstructed in the basal and apical dendritic regions of pyramidal cells, neither for the septal nor for the local population. Importantly, almost all parameters were highly correlated, precluding the possibility of clustering the local terminals into non-overlapping subsets. Factor and cluster analysis confirmed these findings. Our results suggest that similarly to excitatory terminals, inhibitory terminals follow an "ultrastructural size principle," and that the terminals of different interneuron subtypes cannot be distinguished by ultrastructure alone.

Correlated species differences in the effects of cannabinoid ligands on anxiety and on GABAergic and glutamatergic synaptic transmission

Cannabinoid ligands show therapeutic potential in a variety of disorders including anxiety. However, the anxiety-related effects of cannabinoids remain controversial as agonists show opposite effects in mice and rats. Here we compared the effects of the cannabinoid agonist WIN-55,212 and the CB1 antagonist AM-251 in CD1 mice and Wistar rats. Special attention was paid to antagonist–agonist interactions, which had not yet been studied in rats. In mice, WIN-55,212 decreased whereas AM-251 increased anxiety. The antagonist abolished the effects of the agonist. In contrast, WIN-55,212 increased anxiety in rats. Surprisingly, the antagonist potentiated this effect. Cannabinoids affect both GABAergic and glutamatergic functions, which play opposite roles in anxiety. We hypothesized that discrepant findings resulted from species differences in the relative responsiveness of the two transmitter systems to cannabinoids. We investigated this hypothesis by studying the effects of WIN-55,212 on evoked hippocampal inhibitory and excitatory postsynaptic currents (IPSCs and EPSCs). IPSCs were one order of magnitude more sensitive to WIN-55,212 in mice than in rats. In mice, IPSCs were more sensitive than EPSCs to WIN-55,212. This is the first study showing that the relative cannabinoid sensitivity of GABA and glutamate neurotransmission is species-dependent. Based on behavioural and electrophysiological findings, we hypothesize that WIN-55,212 reduced anxiety in mice by affecting GABA neurotransmission whereas it increased anxiety in rats via glutamatergic mechanisms. In rats, AM-251 potentiated this anxiogenic effect by inhibiting the anxiolytic GABAergic mechanism. We suggest that the anxiety-related effects of cannabinoids depend on the relative cannabinoid responsiveness of GABAergic and glutamatergic neurotransmission.

CB1 cannabinoid receptors are enriched in the perisynaptic annulus and on preterminal segments of hippocampal GABAergic axons

Cannabinoids have been shown to modulate the inhibitory effect of cholecystokinin-containing GABAergic interneurons in the hippocampus via type 1 cannabinoid receptors (CB1 receptor). Although immunohistochemical studies, using pre-embedding techniques, have demonstrated that these receptors are abundant on GABAergic axon terminals, little is known about their exact location relative to the synapse. Here we used two recently developed antibodies against the CB1 receptor to study this question with the postembedding immunogold method, which allows the quantitative examination of receptor distribution along the axonal membrane, even within the synaptic active zone. CB1 receptor positive terminals target both the dendritic and somatic surface of neurons in the CA1 area of the rat hippocampus. We found no difference between these two populations of terminals either in their CB1 receptor density or in the distribution of receptors on their membrane. Recent studies suggest that endocannabinoids play a role in retrograde signaling at these synapses, i.e. signaling molecules diffuse from the postsynaptic membrane to nearby presynaptic terminals. Therefore, we examined the distribution of CB1 receptors on the terminal membranes. We found that they are rare in the synaptic active zone, but are enriched in the perisynaptic annulus, where they can directly influence synaptic calcium channels. Perisynaptic CB1 receptors represent about one tenth of all CB1 receptors in a terminal. In contrast, CB1 receptors have a lower density on the extrasynaptic membrane of terminals far from the postsynaptic cell. We estimated that these terminals contain exceptionally large numbers of CB1 receptors, i.e. a single axon terminal was usually labeled with more than 450 particles. An unexpected finding was that the density of CB1 receptors was significantly higher on preterminal axons than on synaptic terminals. These observations suggest that endocannabinoid signaling may subserve roles other than simply reducing transmitter release from axon terminals.

Cannabinoid CB1 receptor dependent effects of the NMDA antagonist phencyclidine in the social withdrawal model of schizophrenia

Clinical and laboratory findings suggest that cannabinoid signalling is implicated in schizophrenia. However, the interaction remains poorly understood, as data are often contradictory. Here we investigated wild-type (WT) and cannabinoid CB1 receptor-knockout (CB1-KO) mice in the phencyclidine-induced social withdrawal model of schizophrenia. N-methyl-D-aspartate (NMDA) antagonists (including phencyclidine) induce psychotic symptoms in humans, and are used to model schizophrenia in a variety of experimental conditions. In WTs, 5 mg/kg phencyclidine increased locomotion and stereotyped behaviours, and decreased social interactions. These changes are consistent with a schizophrenia-like effect. In CB1-KOs, phencyclidine decreased locomotion, enhanced ataxia and stereotypy more markedly than in WTs, but did not affect social interactions. Locomotion showed a significant negative correlation with both ataxia and stereotypy, suggesting that in CB1-KOs, the locomotor suppressive effect of phencyclidine was secondary to changes in these variables. Our findings demonstrate that CB1 gene disruption dramatically alters the behavioural effects of the NMDA antagonist phencyclidine, suggesting that the CB1 receptor is involved in schizophrenia. As social disruption and stereotypy respectively are believed to model negative and positive symptoms of schizophrenia, our findings tentatively suggest that cannabinoids are differentially involved in these two symptom categories. These findings require verification by experiments involving CB1 receptor blockers, as the genetic and pharmacological blockade of receptors may not always provide similar results.

Subcellular localization of type 1 cannabinoid receptors in the rat basal ganglia

Endocannabinoids, acting via type 1 cannabinoid receptors (CB1), are known to be involved in short-term synaptic plasticity via retrograde signaling. Strong depolarization of the postsynaptic neurons is followed by the endocannabinoid-mediated activation of presynaptic CB1 receptors, which suppresses GABA and/or glutamate release. This phenomenon is termed depolarization-induced suppression of inhibition (DSI) or excitation (DSE), respectively. Although both phenomena have been reported to be present in the basal ganglia, the anatomical substrate for these actions has not been clearly identified. Here we investigate the high-resolution subcellular localization of CB1 receptors in the nucleus accumbens, striatum, globus pallidus and substantia nigra, as well as in the internal capsule, where the striato-nigral and pallido-nigral pathways are located. In all examined nuclei of the basal ganglia, we found that CB1 receptors were located on the membrane of axon terminals and preterminal axons. Electron microscopic examination revealed that the majority of these axon terminals were GABAergic, giving rise to mostly symmetrical synapses. Interestingly, preterminal axons showed far more intense staining for CB1, especially in the globus pallidus and substantia nigra, whereas their terminals were only faintly stained. Non-varicose, thin unmyelinated fibers in the internal capsule also showed strong CB1-labeling, and were embedded in bundles of myelinated CB1-negative axons. The majority of CB1 receptors labeled by immunogold particles were located in the axonal plasma membrane (92.3%), apparently capable of signaling cannabinoid actions. CB1 receptors in this location cannot directly modulate transmitter release, because the release sites are several hundred micrometers away. Interestingly, both the CB1 agonist, WIN55,212-2, as well as its antagonist, AM251, were able to block action potential generation, but via a CB1 independent mechanism, since the effects remained intact in CB1 knockout animals. Thus, our electrophysiological data suggest that these receptors are unable to influence action potential propagation, thus they may not be functional at these sites, but are likely being transported to the terminal fields. The present data are consistent with a role of endocannabinoids in the control of GABA, but not glutamate, release in the basal ganglia via presynaptic CB1 receptors, but also call the attention to possible non-CB1-mediated effects of widely used cannabinoid ligands on action potential generation.

Altering cannabinoid signaling during development disrupts neuronal activity

In adult cortical tissue, recruitment of GABAergic inhibition prevents the progression of synchronous population discharges to epileptic activity. However, at early developmental stages, GABA is excitatory and thus unable to fulfill this role. Here, we report that retrograde signaling involving endocannabinoids is responsible for the homeostatic control of synaptic transmission and the resulting network patterns in the immature hippocampus. Blockade of cannabinoid type 1 (CB1) receptor led to epileptic discharges, whereas overactivation of CB1 reduced network activity in vivo. Endocannabinoid signaling thus is able to keep population discharge patterns within a narrow physiological time window, balancing between epilepsy on one side and sparse activity on the other, which may result in impaired developmental plasticity. Disturbing this delicate balance during pregnancy in either direction, e.g., with marijuana as a CB1 agonist or with an antagonist marketed as an antiobesity drug, can have profound consequences for brain maturation even in human embryos.

Spike timing of distinct types of GABAergic interneuron during hippocampal gamma oscillations in vitro

Gamma frequency (30-100 Hz) network oscillations occur in the intact hippocampus during awake, attentive behavior. Here, we explored the underlying cellular mechanisms in an in vitro model of persistent gamma-frequency oscillations, induced by bath application of 20 microm carbachol in submerged hippocampal slices at 30 +/- 1 degrees C. Current-source density analysis of the field oscillation revealed a prominent alternating sink-source pair in the perisomatic and apical dendritic regions of CA3. To elucidate the active events generating these extracellular dipoles, we examined the firing properties of distinct neuron types. Visually guided unit recordings were obtained from individual CA3 neurons followed by intracellular labeling for anatomical identification. Pyramidal cells fired at 2.82 +/- 0.7 Hz, close to the negative peak of the oscillation (0.03 +/- 0.65 msec), and often in conjunction with a negative spike-like component of the field potential. In contrast, all phase-coupled interneurons fired after this negative peak. Perisomatic inhibitory interneurons fired at high frequency (18.1 +/- 2.7 Hz), shortly after the negative peak (1.97 +/- 0.95 msec) and were strongly phase-coupled. Dendritic inhibitory interneurons fired at lower frequency (8.4 +/- 2.4 Hz) and with less fidelity and a longer delay after the negative peak (4.3 +/- 1.1 msec), whereas interneurons with cell body in the stratum radiatum often showed no phase relationship with the field oscillation. The phase and spike time data of individual neurons, together with the current-source density analysis, support a synaptic feedback model of gamma oscillations primarily involving pyramidal cells and inhibitory cells targeting their perisomatic region.

Distinct properties of carbachol- and DHPG-induced network oscillations in hippocampal slices

The aim of this study was to compare and contrast the properties of gamma oscillations induced by activation of muscarinic acetylcholine or metabotropic glutamate receptors in the CA3 region of rat hippocampal slices. Both carbachol and the group I metabotropic glutamate receptor agonist, (RS)-3,5-dihydroxyphenylglycine (DHPG), induced network oscillations in the gamma-frequency range (30-100 Hz). The M1 muscarinic receptor antagonist, pirenzepine, blocked carbachol-, but enhanced DHPG-induced oscillations, whereas LY 341495, an antagonist at metabotropic glutamate receptors, abolished DHPG-, but left carbachol-induced oscillations unchanged. There were significant differences in the peak frequency, maximal power, and spectral width of the two oscillations. Pharmacological experiments showed that both types of oscillation depend on fast excitatory and inhibitory synaptic transmission. Interestingly, activation of neurokinin-1 receptors by substance P fragment or enhancement of inhibitory synaptic currents by the benzodiazepine ligand, zolpidem, boosted DHPG-, but reduced the power of carbachol-induced oscillations. These results suggest that, although carbachol and DHPG might activate similar conductances in individual pyramidal cells, the oscillations they induce in slices involve different network mechanisms, most likely by recruiting distinct types of GABAergic interneuron.

Surviving CA1 pyramidal cells receive intact perisomatic inhibitory input in the human epileptic hippocampus

Temporal lobe epilepsy (TLE) is known to be linked to an impaired balance of excitation and inhibition. Whether inhibition is decreased or preserved in the human epileptic hippocampus, beside the excess excitation, is still a debated question. In the present study, quantitative light and electron microscopy has been performed to analyse the distribution, morphology and input-output connections of parvalbumin (PV)-immunopositive interneurons, together with the entire perisomatic input of pyramidal cells, in the human control and epileptic CA1 region. Based on the degree of cell loss, the patients with therapy-resistant TLE formed four pathological groups. In the non-sclerotic CA1 region of TLE patients, where large numbers of pyramidal cells are preserved, the number of PV-immunopositive cell bodies decreased, whereas axon terminal staining, and the distribution of their postsynaptic targets was not altered. The synaptic coverage of CA1 pyramidal cell axon initial segments (AISs) remained unchanged in the epileptic tissue. The somatic inhibitory input is also preserved; it has been decreased only in the cases with patchy pyramidal cell loss in the CA1 region (control, 0.637; epileptic with mild cell loss, 0.642; epileptic with patchy cell loss, 0.424 microm synaptic length/100 microm soma perimeter). The strongly sclerotic epileptic CA1 region, where pyramidal cells can hardly be seen, contains a very small number of PV-immunopositive elements. Our results suggest that perisomatic inhibitory input is preserved in the epileptic CA1 region as long as pyramidal cells are present. Basket and axo-axonic cells survive in epilepsy if their original targets are present, although many of them lose their PV content or PV immunoreactivity. An efficient perisomatic inhibition is likely to take part in the generation of abnormal synchrony in the non-sclerotic epileptic CA1 region, and thus participate in the maintenance of epileptic seizures driven, for example, by hyperactive afferent input.

Segregation of two endocannabinoid-hydrolyzing enzymes into pre- and postsynaptic compartments in the rat hippocampus, cerebellum and amygdala

Fatty acid amide hydrolase (FAAH) and monoglyceride lipase (MGL) catalyse the hydrolysis of the endocannabinoids anandamide and 2-arachidonoyl glycerol. We investigated their ultrastructural distribution in brain areas where the localization and effects of cannabinoid receptor activation are known. In the hippocampus, FAAH was present in somata and dendrites of principal cells, but not in interneurons. It was located mostly on the membrane surface of intracellular organelles known to store Ca(2+) (e.g. mitochondria, smooth endoplasmic reticulum), less frequently on the somatic or dendritic plasma membrane. MGL immunoreactivity was found in axon terminals of granule cells, CA3 pyramidal cells and some interneurons. In the cerebellum, Purkinje cells and their dendrites are intensively immunoreactive for FAAH, together with a sparse axon plexus at the border of the Purkinje cell/granule cell layers. Immunostaining for MGL was complementary, the axons in the molecular layer were intensively labelled leaving the Purkinje cell dendrites blank. FAAH distribution in the amygdala was similar to that of the CB(1) cannabinoid receptor: evident signal in neuronal somata and proximal dendrites in the basolateral nucleus, and hardly any labelling in the central nucleus. MGL staining was restricted to axons in the neuropil, with similar relative signal intensities seen for FAAH in different nuclei. Thus, FAAH is primarily a postsynaptic enzyme, whereas MGL is presynaptic. FAAH is associated with membranes of cytoplasmic organelles. The differential compartmentalization of the two enzymes suggests that anandamide and 2-AG signalling may subserve functional roles that are spatially segregated at least at the stage of metabolism.

Differential distribution of the KCl cotransporter KCC2 in thalamic relay and reticular nuclei

In the thalamus of the rat the reversal potential of GABA-induced anion currents is more negative in relay cells than in neurones of the reticular nucleus (nRt) due to different chloride extrusion mechanisms operating in these cells. The distribution of KCl cotransporter type 2 (KCC2), the major neuronal chloride transporter that may underlie this effect, is unknown in the thalamus. In this study the precise regional and ultrastructural localization of KCC2 was examined in the thalamus using immunocytochemical methods. The neuropil of all relay nuclei was found to display intense KCC2 immunostaining to varying degrees. In sharp contrast, the majority of the nRt was negative for KCC2. In the anterior and dorsal part of the nRt, however, KCC2 immunostaining was similar to relay nuclei and parvalbumin and calretinin were found to colocalize with KCC2. At the ultrastructural level, KCC2 immunoreactivity was mainly located in the extrasynaptic membranes of thick and thin dendrites and the somata of relay cells but was also found in close association with asymmetrical synapses formed by cortical afferents. Quantitative evaluation of KCC2 distribution at the electron microscopic level demonstrated that the density of KCC2 did not correlate with dendritic diameter or synaptic coverage but is 1.7 times higher on perisynaptic membrane surfaces than on extrasynaptic membranes. Our data demonstrate that the regional distribution of KCC2 is compatible with the difference in GABA-A reversal potential between relay and reticular nuclei. At the ultrastructural level, abundant extrasynaptic KCC2 expression will probably play a role in the regulation of extrasynaptic GABA-A receptor-mediated inhibition.

Uridine release during aminopyridine-induced epilepsy

Uridine, like adenosine, is released under sustained depolarization and it can inhibit hippocampal neuronal activity, suggesting that uridine may be released during seizures and can be involved in epileptic mechanisms. In an in vivo microdialysis study, we measured the extracellular changes of nucleoside and amino acid levels and recorded cortical EEG during 3-aminopyridine-induced epilepsy. Applying silver impregnation and immunohistochemistry, we examined the degree of hippocampal cell loss. We found that extracellular concentration of uridine, adenosine, inosine, and glutamate increased significantly, while glutamine level decreased during seizures. The release of uridine correlated with seizure activity. Systemic and local uridine application was ineffective. The number of parvalbumin- and calretinin-containing interneurons of dorsal hippocampi decreased. We conclude that uridine is released during epileptic activity, and suggest that as a neuromodulator, uridine may contribute to epilepsy-related neuronal activity changes, but uridine analogues having slower turnover would be needed for further investigation of physiological role of uridine.

CB1 cannabinoid receptors mediate anxiolytic effects: convergent genetic and pharmacological evidence with CB1-specific agents

Cannabinoids are known to modulate GABAergic and glutamatergic transmission in cortical areas, the former via CB1 and the latter via a novel receptor. Pharmacological data demonstrate that several widely used cannabinoid ligands bind to both receptors, which may explain the inconsistencies in their behavioural effects. Earlier we showed that the cannabinoid antagonist SR-141716A affected behaviour in both CB1 knockout and wild-type animals, and its effect (anxiolysis) was different from that of CB1 gene disruption (anxiogenesis). In the present experiments, we studied the effects of the CB1 antagonist AM-251, and the cannabinoid agonist WIN-55,212-2 in wild-type as well as in CB1 knockout mice. CB1 knockout mice showed higher scores of anxiety-like behaviour than the wild-type animals in the elevated plus-maze. Selective blockade of CB1 receptors by AM-251 (0.3, 1 and 3 mg/kg) increased anxiety-like behaviour dose-dependently in the wild-type mice but had no effect in the knockouts. In wild types, the cannabinoid agonist WIN-55,212-2 (1 and 3 mg/kg) caused a decrease in anxiety-like behaviour, which was abolished by the CB1-selective antagonist AM-251 (3 mg/kg). The same agonist did not change plus-maze behaviour in CB1 knockout animals. These data demonstrate at the behavioural level that AM-251 and, at low concentrations, WIN-55,212-2, are selective ligands of the CB1 cannabinoid receptor in mice. Our studies on the behavioural effects of the cannabinoid antagonist SR-141716A and the CB1 antagonist AM-251 show that the CB1 and the novel cannabinoid receptor mediate anxiolytic and anxiogenic effects, respectively. This suggests that agonists of the former, or antagonists of the latter, are promising new compounds in the pharmacotherapy of anxiety.

The spatial and temporal pattern of fatty acid amide hydrolase expression in rat hippocampus during postnatal development

GABAergic synaptic transmission is efficiently controlled by endogenous cannabinoids in cortical structures. Fatty acid amide hydrolase (FAAH) is one of the metabolizing enzymes of endocannabinoids in the brain. In this study we investigated the cellular and subcellular distribution of FAAH at various timepoints during the first postnatal weeks, when GABA is still depolarizing, and plays a crucial role in network events. FAAH immunoreactivity is strong in the CA3 region already at postnatal day 0 (P0), but in CA1 only after P8. During this period, FAAH levels in hilar mossy cells decrease and in granule cells slowly increase. Pyramidal cells express FAAH first in the soma and proximal dendrites, and gradually in more distal segments, reaching adult levels in the most distal dendrites only at P22. Transient expression of FAAH was found in a small number of stratum radiatum cells that may be interneurons and in ependymal cells at the border of the alveus and corpus callosum between P2 and P8. At the ultrastructural level, FAAH distribution at all ages was very similar to the adult pattern, i.e. it was largely associated with the membrane of cytoplasmic vesicles, mitochondria and endoplasmic reticulum. During postnatal development of the hippocampus, the spatio-temporal expression of FAAH correlates well with the general pattern of neuronal maturation, but not with the arrival of afferent pathways, which suggests that FAAH - and its major endocannabinoid substrate, anandamide - is unlikely to be involved in the presynaptic control of neurotransmission. Instead, FAAH may subserve general roles as the inactivating enzyme for many fatty acid amides, in addition to anandamide.

Context-dependent effects of CB1 cannabinoid gene disruption on anxiety-like and social behaviour in mice

Contrasting data were reported regarding the effects of cannabinoids on anxiety and social behaviour in both animals and humans. The cognitive effects of cannabinoids and their interactions with the HPA-axis raise the possibility that cannabinoid effects are context but not behaviour specific. To assess this hypothesis, we submitted CB1 receptor knock-out (CB1-KO) and wild-type (WT) mice to tests, which involved similar behaviours, but the behavioural context was different. The elevated plus-maze test was performed under less and more anxiogenic conditions, i.e. under low and high light, respectively. We also compared the social behaviour of the two genotypes in the resident/intruder and social interaction tests. Both tests represent a social challenge and induce similar behaviours, but involve different contexts. The behaviour of CB1-KO and WT mice was similar under low light, but CB1 gene disruption increased anxiety-like behaviour under the high light condition. CB1 gene disruption promoted aggressive behaviour in the home-cage, whereas it inhibited social behaviour in the unfamiliar cage. Thus, the anxiogenic-like effect was restricted to the more stressful unfamiliar environment. These data suggest that the effects of CB1 gene disruption were context and not behaviour specific. Novelty stress resulted in higher ACTH levels in CB1-KOs than in WTs, which suggests that context dependency occurred in conjunction with an altered HPA axis function. The present data at least partly explain contrasting effects of cannabinoids in different contexts as well as in different species and strains that show differential stress responses and coping strategies.

Postnatal development and migration of cholecystokinin-immunoreactive interneurons in rat hippocampus

The development of cholecystokinin-immunoreactive (CCK-IR) interneurons in the rat hippocampus was studied using immunocytochemical methods at the light and electron microscopic levels from early (P0-P8) to later postnatal (P12-P20) periods. The laminar distribution of CCK-IR cell bodies changed considerably during the studied period, which is suggested to be due to migration. CCK-IR cells appear to move from the molecular layer of the dentate gyrus to their final destination at the stratum granulosum/hilus border, and tend to concentrate in the distal third of stratum radiatum in CA1-3. The density of CCK-IR cells is rapidly decreasing during the first 4 postnatal days without any apparent reduction in their total number, therefore it is due to the pronounced growth of hippocampal volume in this period. Axons of CCK-IR interneurons formed symmetrical synapses already at P0, and by far the predominant targets were dendrites of presumed principal cells in all subfields of the hippocampus. These axon arbors began to concentrate around pyramidal cell bodies only at P8, at earlier ages CCK-IR axons crossed stratum pyramidale at right angles, and gave rise to varicose collaterals only outside this layer. The dendrites and somata of CCK-IR cells received synapses already at P0, but those were mostly symmetrical, apart from a few immature asymmetrical synapses. At P4, mature asymmetrical synapses with considerable amounts of synaptic vesicles were already commonly encountered. Thus, the innervation of CCK-IR interneurons apparently develops later than their output synapses, suggesting that they may be able to release transmitter before receiving any considerable excitatory drive. We conclude that CCK-IR cells represent one, if not the major, interneuron type that assists in the maturation of glutamatergic synapses (activation of N-methyl-D-aspartate receptors) via GABAergic depolarization of principal cell dendrites, and may contribute to the generation of giant depolarizing potentials. CCK-IR cells will change their function to perisomatic hyperpolarizing inhibition, as glutamatergic transmission in the network becomes operational.

Large variability in synaptic N-methyl-D-aspartate receptor density on interneurons and a comparison with pyramidal-cell spines in the rat hippocampus

Pyramidal cells receive input from several types of GABA-releasing interneurons and innervate them reciprocally. Glutamatergic activation of interneurons involves both alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and N-methyl-D-aspartate (NMDA) type glutamate receptors expressed in type I synapses, mostly on their dendritic shafts. On average, the synaptic AMPA receptor content is several times higher on interneurons than in the spines of pyramidal cells. To compare the NMDA receptor content of synapses, we used a quantitative postembedding immunogold technique on serial electron microscopic sections, and analysed the synapses on interneuron dendrites and pyramidal cell spines in the CA1 area. Because all NMDA receptors contain the obligatory NR1 subunit, receptor localisation was carried out using antibodies recognising all splice variants of the NR1 subunit. Four populations of synapse were examined: i). on spines of pyramidal cells in stratum (str.) radiatum and str. oriens; ii). on parvalbumin-positive interneuronal dendritic shafts in str. radiatum; iii). on randomly found dendritic shafts in str. oriens and iv). on somatostatin-positive interneuronal dendritic shafts and somata in str. oriens. On average, the size of the synapses on spines was about half of those on interneurons. The four populations of synapse significantly differed in labelling for the NR1 subunit. The median density of NR1 subunit labelling was highest on pyramidal cell spines. It was lowest in the synapses on parvalbumin-positive dendrites in str. radiatum, where more than half of these synapses were immunonegative. In str. oriens, synapses on interneurons had a high variability of receptor content; some dendrites were similar to those in str. radiatum, including the proximal synapses of somatostatin-positive cells, whereas others had immunoreactivity for the NR1 subunit similar to or higher than synapses on pyramidal cell spines. These results show that synaptic NMDA receptor density differs between pyramidal cells and interneurons. Some interneurons may have a high NMDA receptor content, whereas others, like some parvalbumin-expressing cells, a particularly low synaptic NMDA receptor content. Consequently, fast glutamatergic activation of interneurons is expected to show cell type-specific time course and state-dependent dynamics.

Role of endogenous cannabinoids in synaptic signaling

Research of cannabinoid actions was boosted in the 1990s by remarkable discoveries including identification of endogenous compounds with cannabimimetic activity (endocannabinoids) and the cloning of their molecular targets, the CB1 and CB2 receptors. Although the existence of an endogenous cannabinoid signaling system has been established for a decade, its physiological roles have just begun to unfold. In addition, the behavioral effects of exogenous cannabinoids such as delta-9-tetrahydrocannabinol, the major active compound of hashish and marijuana, await explanation at the cellular and network levels. Recent physiological, pharmacological, and high-resolution anatomical studies provided evidence that the major physiological effect of cannabinoids is the regulation of neurotransmitter release via activation of presynaptic CB1 receptors located on distinct types of axon terminals throughout the brain. Subsequent discoveries shed light on the functional consequences of this localization by demonstrating the involvement of endocannabinoids in retrograde signaling at GABAergic and glutamatergic synapses. In this review, we aim to synthesize recent progress in our understanding of the physiological roles of endocannabinoids in the brain. First, the synthetic pathways of endocannabinoids are discussed, along with the putative mechanisms of their release, uptake, and degradation. The fine-grain anatomical distribution of the neuronal cannabinoid receptor CB1 is described in most brain areas, emphasizing its general presynaptic localization and role in controlling neurotransmitter release. Finally, the possible functions of endocannabinoids as retrograde synaptic signal molecules are discussed in relation to synaptic plasticity and network activity patterns.

Comparison of single NMDA receptor channels recorded on hippocampal principal cells and oriens/alveus interneurons projecting to stratum lacunosum-moleculare (O-LM cells)

NMDA receptors participate in the glutamatergic excitation of both principal cells and GABAergic interneurons. The features of NMDA channels on specific interneurons, however, are not known. Therefore, we obtained direct measurements of single NMDA receptor channels on anatomically identified oriens/alveus interneurons projecting to stratum lacunosum-moleculare (O-LM cells) and compared them to those found on hippocampal principal cells using cell-attached recordings in in vitro slice preparations. The recorded channels could be blocked by ketamine, a membrane-permeable NMDA channel inhibitor. In the absence of Mg(2+), all O-LM cells had NMDA channels with a comparable slope conductance (approximately 60pS) to those measured on CA1 pyramidal cells or dentate granule cells. In addition, NMDA channels with smaller conductance (43-45 pS) were also found on two O-LM cells but not on principal cells. These results suggest that at least two types of NMDA channels are expressed on O-LM cells likely reflecting distinct subunit composition.

Loss of interneurons innervating pyramidal cell dendrites and axon initial segments in the CA1 region of the hippocampus following pilocarpine-induced seizures

In the pilocarpine model of chronic limbic seizures, vulnerability of GABAergic interneurons to excitotoxic damage has been reported in the hippocampal CA1 region. However, little is known about the specific types of interneurons that degenerate in this region. In order to characterize these interneurons, we performed quantitative analyses of the different populations of GABAergic neurons labeled for their peptide or calcium-binding protein content. Our data demonstrate that the decrease in the number of GAD mRNA-containing neurons in the stratum oriens of CA1 in pilocarpine-treated rats involved two subpopulations of GABAergic interneurons: interneurons labeled for somatostatin only (O-LM and bistratified cells) and interneurons labeled for parvalbumin only (basket and axo-axonic cells). Stratum oriens interneurons labeled for somatostatin/calbindin or somatostatin/parvalbumin were preserved. The decrease in number of somatostatin- and parvalbumin-containing neurons was observed as early as 72 hours after the sustained seizures induced by pilocarpine injection. Many degenerating cell bodies in the stratum oriens and degenerating axon terminals in the stratum lacunosum-moleculare were observed at 1 and 2 weeks after injection. In addition, the synaptic coverage of the axon initial segment of CA1 pyramidal cells was significantly decreased in pilocarpine-treated animals. These results indicate that the loss of somatostatin-containing neurons corresponds preferentially to the degeneration of interneurons with an axon projecting to stratum lacunosum-moleculare (O-LM cells) and suggest that the death of these neurons is mainly responsible for the deficit of dendritic inhibition reported in this region. We demonstrate that the loss of parvalbumin-containing neurons corresponds to the death of axo-axonic cells, suggesting that perisomatic inhibition and mechanisms controlling action potential generation are also impaired in this model.

Interneurons are the local targets of hippocampal inhibitory cells which project to the medial septum

A subset of GABAergic neurons projecting to the medial septum has long been described in the hippocampus. However, the lack of information about their local connectivity pattern or their correspondence with any of the well-established hippocampal interneuron types has hampered the understanding of their functional role. Retrograde tracing combined with immunostaining for neurochemical markers in the adult rat hippocampus showed that nearly all hippocampo-septal (HS) neurons express somatostatin (>95%) and, in the hilus and CA3 stratum lucidum, many contain calretinin (>45%). In contrast, in stratum oriens of the CA1 and CA3 subfields, the majority of HS neurons contain somatostatin (>86%) and calbindin (>73%), but not calretinin. Because somatostatin-positive hippocampal interneurons have been most extensively characterized in the stratum oriens of CA1, we focused our further analysis on HS cells found in this region. In 18-20-day-old rats, intracellularly filled CA1-HS cells had extensive local axon collaterals crossing subfield boundaries and innervating the CA3 region and the dentate gyrus. Electron microscopic analysis provided evidence that the axon terminals of CA1-HS cells form symmetrical synapses selectively on GABAergic interneurons, both locally and in the CA3 region. In addition, double retrograde labelling experiments revealed that many CA1-HS neurons of the dorsal hippocampus also have collateral projections to the ventral hippocampus. Thus, CA1-HS cells innervate inhibitory interneurons locally and in remote hippocampal regions, in addition to targeting mostly GABAergic neurons in the medial septum. This dual projection with striking target selectivity for GABAergic neurons may be ideally suited to synchronize neuronal activity along the septo-hippocampal axis.

Synaptic reorganization of calbindin-positive neurons in the human hippocampal CA1 region in temporal lobe epilepsy

The distribution, morphology, synaptic coverage and postsynaptic targets of calbindin-containing interneurons and afferent pathways have been analyzed in the control and epileptic CA1 region of the human hippocampus. Numerous calbindin-positive interneurons are preserved even in the strongly sclerotic CA1 region. The morphology of individual cells is altered: the cell body and dendrites become spiny, the radially oriented dendrites disappear, and are replaced by a large number of curved, distorted dendrites. Even in the non-sclerotic epileptic samples, where pyramidal cells are present and calbindin-immunoreactive interneurons seem to be unchanged, some modifications could be observed at the electron microscopic level: they received more inhibitory synaptic input, and the calbindin-positive excitatory afferents - presumably derived from the CA1, the CA2 and/or the dentate gyrus - are sprouted. In the strongly sclerotic tissue, with the death of pyramidal cells, calbindin-positive terminals (belonging to interneurons and the remaining excitatory afferents) change their targets. Our data suggest that an intense synaptic reorganization takes place in the epileptic CA1 region, even in the non-sclerotic tissue, before the death of considerable numbers of pyramidal cells. Calbindin-positive interneurons participate in this reorganization: they show plastic changes in response to epilepsy. The enhanced inhibition of inhibitory interneurons may result in the disinhibition of pyramidal cells or in an abnormal synchrony in the output region of the hippocampus.

Calretinin-containing interneurons innervate both principal cells and interneurons in the CA1 region of the human hippocampus

Hippocampal interneurons consist of functionally diverse cell types, most of them target the dendrites or perisomatic region of pyramidal cells with a few exceptions, like the calretinin-containing cells in the rat: they selectively innervate other interneurons. However, no electron microscopic data are available about the synaptic connections of calretinin-immunoreactive neurons in the human hippocampus. We aimed to provide these data to establish whether interneuron-selective interneurons indeed represent an essential feature of hippocampal circuits across distant species. Two types of calretinin-immunostained terminals were found in the CA1 region: one of them presumably derived from the thalamic reuniens nucleus, and established asymmetric synapses on dendrites and spines. The other type originating from local interneurons formed symmetric synapses on both pyramidal and interneuron dendrites. Distribution of postsynaptic targets showed that 26.8% of the targets were CR-positive interneuron dendrites, and 25.2% proved to be proximal pyramidal dendrites. CR-negative interneuron dendrites were also contacted (12.4%). Small caliber postsynaptic dendrites were not classified (28%). Somata were rarely contacted (7.6%). The present data suggest that calretinin-positive boutons do show a preference for other interneurons, but a considerable proportion of the targets are pyramidal cells. We propose that interneuron-selective inhibitory cells exist in the human Ammon's horn, and boutons innervating pyramidal cells derive from another cell type that might not exist in rodents.

The effects of genetic and pharmacological blockade of the CB1 cannabinoid receptor on anxiety

The aim of this study was to compare the effects of the genetic and pharmacological disruption of CB1 cannabinoid receptors on the elevated plus-maze test of anxiety. In the first experiment, the behaviour of CB1-knockout mice and wild-type mice was compared. In the second experiment, the cannabinoid antagonist SR141716A (0, 1, and 3 mg/kg) was administered to both CB1-knockout and wild type mice. Untreated CB1-knockout mice showed a reduced exploration of the open arms of the plus-maze apparatus, thus appearing more anxious than the wild-type animals, however no changes in locomotion were noticed. The vehicle-injected CB1-knockout mice from the second experiment also showed increased anxiety as compared with wild types. Surprisingly, the cannabinoid antagonist SR141716A reduced anxiety in both wild type and CB1 knockout mice. Locomotor behaviour was only marginally affected. Recent evidence suggests the existence of a novel cannabinoid receptor in the brain. It has also been shown that SR141716A binds to both the CB1 and the putative novel receptor. The data presented here supports these findings, as the cannabinoid receptor antagonist affected anxiety in both wild type and CB1-knockout mice. Tentatively, it may be suggested that the discrepancy between the effects of the genetic and pharmacological blockade of the CB1 receptor suggests that the novel receptor plays a role in anxiety.

Selective GABAergic innervation of thalamic nuclei from zona incerta

Thalamocortical circuits that govern cortical rhythms and ultimately effect sensory transmission consist of three major interconnected elements: excitatory thalamocortical and corticothalamic neurons and GABAergic cells in the reticular thalamic nucleus. Based on the present results, a fourth component has to be added to this scheme. GABAergic fibres from an extrareticular diencephalic source were found to selectively innervate relay cells located mainly in higher-order thalamic nuclei. The origin of this pathway was localized to zona incerta (ZI), known to receive collaterals from corticothalamic fibres. First-order nuclei were innervated only in zones showing a high density of calbindin-positive neurons. The large GABA-immunoreactive incertal terminals established multiple contacts preferentially on the proximal dendrites of relay cells via symmetrical synapses with multiple release sites. The distribution, ultrastructural characteristics and postsynaptic target selection of extrareticular terminals were similar to type II muscarinic acetylcholine receptor-positive boutons, which constituted up to 49% of all GABAergic terminals in the posterior nucleus. This suggests that a significant proportion of the GABAergic input into certain thalamic territories involved in higher-order functions may have extrareticular origin. Unlike the reticular nucleus, ZI receives peripheral and layer V cortical input but no thalamic feedback; it projects to brainstem centres and has extensive intranuclear recurrent collaterals. This indicates that ZI exerts a conceptually new type of inhibitory control over the thalamus. The proximally situated, multiple active zones of ZI terminals indicate a powerful influence on the firing properties of thalamic neurons, which is conveyed to multiple cortical areas via relay cells which have widespread projections to neocortex.

Pharmacological separation of cannabinoid sensitive receptors on hippocampal excitatory and inhibitory fibers

Our earlier studies demonstrated that in the hippocampus, cannabinoids suppress inhibitory synaptic transmission via CB(1) cannabinoid receptors, whereas a novel cannabinoid-sensitive receptor modulates excitatory synapses (Katona, I. et al., Journal of Neuroscience 19 (1999) 4544; Hájos, N. et al., European Journal of Neuroscience 12 (2000) 3239; Hájos, N. et al., Neuroscience 106 (2001) 1). The novel receptor does not correspond to CB(2), since this receptor type is not expressed in the brain (Munro, S. et al., Nature 365 (1993) 61). Recent binding experiments revealed that the synthetic cannabinoid WIN 55,212-2 binds with lower affinity to brain membranes of CB(1) receptor-knockout mice indicating that pharmacological differences exist between these two types of cannabinoid receptors in the hippocampus (Breivogel et al., Molecular Pharmacology 60 (2001) 155). To analyze this difference in detail, we first determined the EC(50) values of WIN 55,212-2 for excitatory and inhibitory transmission in rat hippocampal slices using whole-cell patch-clamp recordings. The estimated EC(50) value for inhibitory postsynaptic currents (IPSC) evoked by electrical stimulation in CA1 pyramidal cells was 0.24 microM, whereas for excitatory postsynaptic currents (EPSC) it was 2.01 microM, respectively. The cannabinoid antagonist, AM251, blocked the WIN 55,212-2-induced inhibition of evoked IPSCs, but not of EPSCs, providing evidence for its selectivity for CB(1). We then tested the hypothesis of whether the cannabinoid effect on hippocampal excitatory neurotransmission is mediated via receptors with an affinity for vanilloid ligands. Co-application of the vanilloid receptor antagonist capsazepine (10 microM) with cannabinoids (WIN55,212-2 or CP55,940) prevented the reduction of EPSCs, but not of IPSCs. The amplitude of evoked EPSCs was also suppressed by superfusion of the vanilloid receptor agonist capsaicin (10 microM), an effect which could also be antagonized by capsazepine. In contrast, capsaicin did not change the amplitude of evoked IPSCs. These results demonstrate that WIN 55,212-2 is an order of magnitude more potent in reducing GABAergic currents via CB(1) than in inhibiting glutamatergic transmission via the new CB receptor. The sensitivity of the new CB receptor (and EPSCs) to vanilloid ligands, but not to the cannabinoid antagonist AM251, represents another pharmacological tool to distinguish the two receptors, since CB(1) (and its effect on IPSCs) is not modulated by vanilloids, but is antagonized by AM251.

Brain monoglyceride lipase participating in endocannabinoid inactivation

The endogenous cannabinoids (endocannabinoids) are lipid molecules that may mediate retrograde signaling at central synapses and other forms of short-range neuronal communication. The monoglyceride 2-arachidonoylglycerol (2-AG) meets several criteria of an endocannabinoid substance: (i) it activates cannabinoid receptors; (ii) it is produced by neurons in an activity-dependent manner; and (iii) it is rapidly eliminated. 2-AG inactivation is only partially understood, but it may occur by transport into cells and enzymatic hydrolysis. Here we tested the hypothesis that monoglyceride lipase (MGL), a serine hydrolase that converts monoglycerides to fatty acid and glycerol, participates in 2-AG inactivation. We cloned MGL by homology from a rat brain cDNA library. Its cDNA sequence encoded for a 303-aa protein with a calculated molecular weight of 33,367 daltons. Northern blot and in situ hybridization analyses revealed that MGL mRNA is heterogeneously expressed in the rat brain, with highest levels in regions where CB(1) cannabinoid receptors are also present (hippocampus, cortex, anterior thalamus, and cerebellum). Immunohistochemical studies in the hippocampus showed that MGL distribution has striking laminar specificity, suggesting a presynaptic localization of the enzyme. Adenovirus-mediated transfer of MGL cDNA into rat cortical neurons increased MGL expression and attenuated N-methyl-D-aspartate/carbachol-induced 2-AG accumulation in these cells. No such effect was observed on the accumulation of anandamide, another endocannabinoid lipid. The results suggest that hydrolysis by means of MGL is a primary mechanism for 2-AG inactivation in intact neurons.

GABA(B) receptors in the median raphe nucleus: distribution and role in the serotonergic control of hippocampal activity

Previous studies have shown that serotonergic neurons of the median raphe nucleus have a suppressive effect on theta synchronization in the hippocampus. Median raphe lesion, suppression of 5-HT neuronal activity by administration of GABA(A) receptor antagonist or by glutamate blockade or depletion produced long-lasting non-interrupted hippocampal theta in freely behaving rats independent of behavior and in rats anesthetized with urethane. Serotonergic neurons show a characteristic sleep-wake pattern of activity and there is evidence that GABAergic mechanisms play an important role in their regulation. In this study we analyzed the distribution and subcellular localization of GABA(B) receptors in the midbrain raphe complex using combined 5-HT/GABA(B) receptor immunohistochemistry at the light and electron microscopic levels and studied the effects of their pharmacological manipulation on hippocampal electroencephalographic activity in urethane-anesthetized rats. We found that sustained infusion of the GABA(B) receptor agonist baclofen into the median raphe nucleus, using the microdialysis technique, elicited lasting theta activity in the hippocampus. The effect was antagonized by selective GABA(B) receptor antagonists. The predominant localization of GABA(B) receptors in the median, as well as in dorsal raphe was found on serotonergic neurons which strongly indicates that the increase in theta occurrence after baclofen injection resulted from suppression of the serotonergic output originating from the median raphe. On the electron microscopic level, we found GABA(B) receptors located extrasynaptically indicating that these receptors are preferentially activated by strong inputs, i.e. when GABA released from the synaptic terminals is sufficient to spill over from the synaptic cleft. Such conditions might be satisfied during rapid eye movement sleep when GABAergic neurons in the raphe are firing at their highest rate and in rhythmic synchronized bursts. Our data indicate that midbrain raphe GABA(B) mechanisms play an important role in behavioral state control and in hippocampal activity, in particular.

Simultaneous activation of gamma and theta network oscillations in rat hippocampal slice cultures

Hippocampal activity in vivo is characterized by concurrent oscillations at theta (4-15 Hz) and gamma (20-80 Hz) frequencies. Here we show that cholinergic receptor activation (methacholine 10-20 nm) in hippocampal slice cultures induces an oscillatory mode of activity, in which the intrinsic network oscillator (located in the CA3 area) expresses simultaneous theta and gamma network oscillations. Pyramidal cells display synaptic theta oscillations, characterized by cycles consisting of population EPSP-IPSP sequences that are dominated by population IPSPs. These rhythmic IPSPs most probably result from theta-modulated spiking activity of several interneurons. At the same time, the majority of interneurons consistently display synaptic gamma oscillations. These oscillatory cycles consist of fast depolarizing rhythmic events that are likely to reflect excitatory input from CA3 pyramidal cells. Interneurons comprising this functional group were identified morphologically. They include four known types of interneurons (basket, O-LM, bistratified and str. lucidum-specific cells) and one new type of CA3 interneuron (multi-subfield cell). The oscillatory activity of these interneurons is only weakly correlated between neighbouring cells, and in about half of these (44 %) is modulated by depolarizing theta rhythmicity. The overall characteristics of acetylcholine-induced oscillations in slice cultures closely resemble the rhythmicity observed in hippocampal field and single cell recordings in vivo. Both rhythmicities depend on intrinsic synaptic interactions, and are expressed by different cell types. The fact that these oscillations persist in a network lacking extra-hippocampal connections emphasizes the importance of intrinsic mechanisms in determining this form of hippocampal activity.

Preservation of perisomatic inhibitory input of granule cells in the epileptic human dentate gyrus

Temporal lobe epilepsy is known to be associated with hyperactivity that is likely to be generated or amplified in the hippocampal formation. The majority of granule cells, the principal cells of the dentate gyrus, are found to be resistant to damage in epilepsy, and may serve as generators of seizures if their inhibition is impaired. Therefore, the parvalbumin-containing subset of interneurons, known to provide the most powerful inhibitory input to granule cell somata and axon initial segments, were examined in human control and epileptic dentate gyrus. A strong reduction in the number of parvalbumin-containing cells was found in the epileptic samples especially in the hilar region, although in some patches of the granule cell layer parvalbumin-positive terminals that form vertical clusters characteristic of axo-axonic cells were more numerous than in controls. Analysis of the postsynaptic target elements of parvalbumin-positive axon terminals showed that they form symmetric synapses with somata, dendrites, axon initial segments and spines as in the control, but the ratio of axon initial segment synapses was increased in the epileptic tissue (control: 15.9%, epileptic: 31.3%). Furthermore, the synaptic coverage of granule cell axon initial segments increased more than three times (control: 0.52, epileptic: 2.10 microm synaptic length/100 microm axon initial segment membrane) in the epileptic samples, whereas the amount of somatic symmetric synapses did not change significantly. Although the number of parvalbumin-positive interneurons is decreased, the perisomatic inhibitory input of dentate granule cells is preserved in temporal lobe epilepsy. Basket and axo-axonic cell terminals - whether positive or negative for parvalbumin - are present, moreover, the axon collaterals targeting axon initial segments sprout in the epileptic dentate gyrus. We suggest that perisomatic inhibitory interneurons survive in epilepsy, but their somadendritic compartment and partly the axon loses parvalbumin or immunoreactivity for parvalbumin. The hyperinnervation of axon initial segments might be a compensatory change in the inhibitory network, but at the same time may lead to a more effective synchronization of granule cell firing that could contribute to the generation or amplification of epileptic seizures.

Novel cannabinoid-sensitive receptor mediates inhibition of glutamatergic synaptic transmission in the hippocampus

Psychoactive effects of cannabinoids are thought to be mediated, at least in part, by suppression of both glutamate and GABA release via CB1 cannabinoid receptor. Two types of cannabinoid receptor (CB1 and CB2) have been cloned so far. The CB1 receptors are abundantly expressed in the nervous system, whereas CB2 receptors are limited to lymphoid organs (Matsuda et al., 1990; Munro et al., 1993). Immunocytochemical and electrophysiological studies revealed that in the hippocampus CB1 receptors are expressed on axon terminals of GABAergic inhibitory interneurons (Tsou et al., 1999; Katona et al., 1999) and activation of these receptors decreases GABA release (Hájos et al., 2000). Other physiological studies pointed out the involvement of CB1 receptors in the modulation of hippocampal glutamatergic synaptic transmission and long-term potentiation (Stella et al., 1997; Misner and Sullivan, 1999), but anatomical studies could not confirm the existence of CB1 receptors on glutamatergic terminals. Here we examined cannabinoid actions on both glutamatergic and GABAergic synaptic transmission in the hippocampus of wild type (CB1+/+) and CB1 receptor knockout mice (CB1-/-). The synthetic cannabinoid agonist WIN55,212-2 reduced the amplitudes of excitatory postsynaptic currents in both wild type and CB1-/- mice, while inhibitory postsynaptic currents were decreased only in wild type mice, but not in CB1-/- animals. Our findings are consistent with a CB1 cannabinoid receptor-dependent modulation of GABAergic postsynaptic currents, but a novel cannabinoid-sensitive receptor must be responsible for the inhibition of glutamatergic neurotransmission.

Distribution of CB1 cannabinoid receptors in the amygdala and their role in the control of GABAergic transmission

Cannabinoids are the most popular illicit drugs used for recreational purposes worldwide. However, the neurobiological substrate of their mood-altering capacity has not been elucidated so far. Here we report that CB1 cannabinoid receptors are expressed at high levels in certain amygdala nuclei, especially in the lateral and basal nuclei, but are absent in other nuclei (e.g., in the central nucleus and in the medial nucleus). Expression of the CB1 protein was restricted to a distinct subpopulation of GABAergic interneurons corresponding to large cholecystokinin-positive cells. Detailed electron microscopic investigation revealed that CB1 receptors are located presynaptically on cholecystokinin-positive axon terminals, which establish symmetrical GABAergic synapses with their postsynaptic targets. The physiological consequence of this particular anatomical localization was investigated by whole-cell patch-clamp recordings in principal cells of the lateral and basal nuclei. CB1 receptor agonists WIN 55,212-2 and CP 55,940 reduced the amplitude of GABA(A) receptor-mediated evoked and spontaneous IPSCs, whereas the action potential-independent miniature IPSCs were not significantly affected. In contrast, CB1 receptor agonists were ineffective in changing the amplitude of IPSCs in the rat central nucleus and in the basal nucleus of CB1 knock-out mice. These results suggest that cannabinoids target specific elements in neuronal networks of given amygdala nuclei, where they presynaptically modulate GABAergic synaptic transmission. We propose that these anatomical and physiological features, characteristic of CB1 receptors in several forebrain regions, represent the neuronal substrate for endocannabinoids involved in retrograde synaptic signaling and may explain some of the emotionally relevant behavioral effects of cannabinoid exposure.

Evidence for presynaptic cannabinoid CB(1) receptor-mediated inhibition of noradrenaline release in the guinea pig lung

Using neurochemical method, evidence was obtained that cannabinoid CB(1) receptors are localized on noradrenergic terminals and their stimulation by WIN-55,212-2 reduces the release of [3H]noradrenaline evoked by axonal activity in a frequency-dependent manner. At stimulation rates of 1 and 3 Hz, there was significant inhibition of noradrenaline release, with IC(50) of WIN-55,212-2 41.5+/-2.6 and 320.5+/-28.2 nM, for 1 and 3 Hz, respectively. Cannabinoid CB(1) receptor antagonist SR 141716A completely prevented WIN-55,212-2 from reducing the release. The release of noradrenaline is negatively modulated by presynaptic alpha(2)-adrenoceptors. Because BRL-44408, an alpha(2B)-adrenoceptor, and prazosin, an alpha(1)- and alpha(2B)-adrenoceptor antagonist, both increased the release of [(3)H]noradrenaline, it seems likely that the alpha(2B) subtype is responsible for the negative feedback modulation of noradrenaline release. In the presence of alpha(2)-adrenoceptor antagonism, cannabinoid CB(1) receptor activation by WIN-55,212-2 was much more effective in inhibiting the release of [(3)H]noradrenaline. Using a specific antibody against the C-terminus of the rat cannabinoid CB(1) receptor and also against neuropeptide Y, ultrastructural evidence was obtained that cannabinoid CB(1) receptors are exclusively localized on neuropeptide Y-positive noradrenergic varicosities. Since the sympathetic innervation of the human airway smooth muscle is sparse, and mainly the circulating adrenaline relaxes the airways via activation of beta(2)-adrenoceptor localized on the smooth muscle, it is suggested that inhibition of noradrenaline release by cannabinoids, and the subsequent bronchospasm, may be limited to those cases when noradrenaline released from sympathetic varicosities is involved in airway relaxation.

Electrotonic profile and passive propagation of synaptic potentials in three subpopulations of hippocampal CA1 interneurons

To elucidate the role of dendritic morphology in signal transfer, the passive propagation of somatic and dendritic potentials was compared in multi-compartment models of three interneuron subpopulations in the CA1 region. Nine calbindin-, 15 calretinin- and 10 parvalbumin-containing cells were modelled incorporating the detailed geometry, the currents of the action potentials in the soma, and the AMPA, N-methyl-D-aspartate and GABA-B receptor-mediated postsynaptic currents in the dendrites. The cable properties show characteristic differences among the subpopulations. The morphotonic length of calbindin and calretinin cell dendrites is larger than of parvalbumin cells. Thus parvalbumin cells are more compact than calbindin or calretinin cells unless the ratio of their axial and membrane resistivities exceeds the ratios of the other two cell types by more than 33%. In calbindin cells, the distal parts of the extremely long dendrites that invade the alveus are virtually isolated from the soma for passively propagating signals. The synaptic potentials evoked at a given morphotonic distance from the soma show larger differences locally on the dendrites than on the soma in all subpopulations. Both the somatic and dendritic amplitude ratios are the smallest in PV cells. In calbindin cells the somatic amplitude of synaptic potentials evoked at the same morphotonic distance from the soma is similar regardless of the number of branchpoints along their path. In calretinin and parvalbumin cells, from dendrites with long primary segments synaptic potentials reach the soma with larger amplitude than from dendrites that are branching close to the soma. The dendrites with the larger impact on somatic membrane potential are usually the dendrites that enter the stratum lacunosum-moleculare. These results indicate that dendritic morphology plays a role in changing the effectiveness of synaptic potentials evoked at different dendritic locations, and in this way is likely to be an important factor in determining the integrative properties of the different neuron populations.

The KCl cotransporter, KCC2, is highly expressed in the vicinity of excitatory synapses in the rat hippocampus

Immunocytochemical visualization of the neuron-specific K+/Cl- cotransporter, KCC2, at the cellular and subcellular level revealed an area- and layer-specific diffuse labelling, and a discrete staining outlining the somata and dendrites of some interneurons in all areas of the rat hippocampus. KCC2 was highly expressed in parvalbumin-containing interneurons, as well as in subsets of calbindin, calretinin and metabotropic glutamate receptor 1a-immunoreactive interneurons. During the first 2 postnatal weeks, an increase of KCC2 staining was observed in the molecular layer of the dentate gyrus, correlating temporally with the arrival of entorhinal cortical inputs. Subcellular localization demonstrated KCC2 in the plasma membranes. Immunoreactivity in principal cells was responsible for the diffuse staining found in the neuropil. In these cells, KCC2 was detected primarily in dendritic spine heads, at the origin of spines and, at a much lower level on the somata and dendritic shafts. KCC2 expression was considerably higher in the somata and dendrites of interneurons, most notably of parvalbumin-containing cells, as well as in the thorny excrescences of CA3 pyramidal cells and in the spines of spiny hilar and stratum lucidum interneurons. The data indicate that KCC2 is highly expressed in the vicinity of excitatory inputs in the hippocampus, perhaps in close association with extrasynaptic GABAA receptors. A high level of excitation is known to lead to a simultaneous net influx of Na+ and Cl-, as evidenced by dendritic swelling. KCC2 located in the same microenvironment may provide a Cl- extrusion mechanism to deal with both ion and water homeostasis in addition to its role in setting the driving force of Cl- currents involved in fast postsynaptic inhibition.

Total number and distribution of inhibitory and excitatory synapses on hippocampal CA1 pyramidal cells

The integrative properties of neurons depend strongly on the number, proportions and distribution of excitatory and inhibitory synaptic inputs they receive. In this study the three-dimensional geometry of dendritic trees and the density of symmetrical and asymmetrical synapses on different cellular compartments of rat hippocampal CA1 area pyramidal cells was measured to calculate the total number and distribution of excitatory and inhibitory inputs on a single cell.A single pyramidal cell has approximately 12,000 microm dendrites and receives around 30,000 excitatory and 1700 inhibitory inputs, of which 40 % are concentrated in the perisomatic region and 20 % on dendrites in the stratum lacunosum-moleculare. The pre- and post-synaptic features suggest that CA1 pyramidal cell dendrites are heterogeneous. Strata radiatum and oriens dendrites are similar and differ from stratum lacunosum-moleculare dendrites. Proximal apical and basal strata radiatum and oriens dendrites are spine-free or sparsely spiny. Distal strata radiatum and oriens dendrites (forming 68.5 % of the pyramidal cells' dendritic tree) are densely spiny; their excitatory inputs terminate exclusively on dendritic spines, while inhibitory inputs target only dendritic shafts. The proportion of inhibitory inputs on distal spiny strata radiatum and oriens dendrites is low ( approximately 3 %). In contrast, proximal dendritic segments receive mostly (70-100 %) inhibitory inputs. Only inhibitory inputs innervate the somata (77-103 per cell) and axon initial segments. Dendrites in the stratum lacunosum-moleculare possess moderate to small amounts of spines. Excitatory synapses on stratum lacunosum-moleculare dendrites are larger than the synapses in other layers, are frequently perforated ( approximately 40 %) and can be located on dendritic shafts. Inhibitory inputs, whose percentage is relatively high ( approximately 14-17 %), also terminate on dendritic spines. Our results indicate that: (i) the highly convergent excitation arriving onto the distal dendrites of pyramidal cells is primarily controlled by proximally located inhibition; (ii) the organization of excitatory and inhibitory inputs in layers receiving Schaffer collateral input (radiatum/oriens) versus perforant path input (lacunosum-moleculare) is significantly different.

Distribution of chloride channel-2-immunoreactive neuronal and astrocytic processes in the hippocampus

The chloride homeostasis of neurons and non-neuronal cells is maintained in part by a voltage-sensitive inwardly rectifying chloride conductance through the chloride channel-2. This channel is activated by hyperpolarization and extracellular hypotonicity. In the present study, hippocampal sections were immunostained for chloride channel-2, and somata and dendrites of both pyramidal and non-pyramidal cells were found to be immunoreactive. In addition, glial processes in the vicinity of small blood vessels were also immunostained, whereas the neuropil of strata pyramidale and lacunosum-moleculare contained chloride channel-2-positive punctate structures. Electron microscopy and double immunostaining using antibodies against chloride channel-2 and glial fibrillary acidic protein confirmed that the dense network of chloride channel-2-positive processes corresponds to the end feet of astrocytes. The distribution of chloride channel-2-immunoreactive astrocytes was inhomogeneous throughout the hippocampus: strata oriens, pyramidale and lacunosum-moleculare of CA1-CA3 and the outer molecular layer of the dentate gyrus contained the majority of immunoreactive end feet, whereas the other layers showed sparse labeling. Subcellular studies demonstrated that, in addition to astrocytes, chloride channel-2 was localized in the membrane of dendrites, dendritic spines, cell bodies and axon initial segments of neurons, frequently close to, or within active zones of, symmetrical synapses.Thus, chloride channel-2 appears to be involved in transmembrane chloride movements associated with GABAergic synaptic transmission. The specific laminar distribution of chloride channel-2-positive astroglial processes coinciding with that of GABAergic axon terminals suggests that the network of astrocytes may be able to siphon and deliver Cl(-) ions to layers with intense GABAergic transmission, thereby increasing the efficacy of GABA(A) receptor-mediated inhibition.

GABAergic interneurons are the targets of cannabinoid actions in the human hippocampus

Cannabinoids have been shown to disrupt memory processes in mammals including humans. Although the CB1 neuronal cannabinoid receptor was identified several years ago, neuronal network mechanisms mediating cannabinoid effects are still controversial in animals, and even more obscure in humans. In the present study, the localization of CB1 receptors was investigated at the cellular and subcellular levels in the human hippocampus, using control post mortem and epileptic lobectomy tissue. The latter tissue was also used for [3H]GABA release experiments, testing the predictions of the anatomical data. Detectable expression of CB1 was confined to interneurons, most of which were found to be cholecystokinin-containing basket cells. CB1-positive cell bodies showed immunostaining in their perinuclear cytoplasm, but not in their somadendritic plasmamembrane. CB1-immunoreactive axon terminals densely covered the entire hippocampus, forming symmetrical synapses characteristic of GABAergic boutons. Human temporal lobectomy samples were used in the release experiments, as they were similar to the controls regarding cellular and subcellular distribution of CB1 receptors. We found that the CB1 receptor agonist, WIN 55,212-2, strongly reduced [3H]GABA release, and this effect was fully prevented by the specific CB1 receptor antagonist SR 141716A. This unique expression pattern and the presynaptic modulation of GABA release suggests a conserved role for CB1 receptors in controlling inhibitory networks of the hippocampus that are responsible for the generation and maintenance of fast and slow oscillatory patterns. Therefore, a likely mechanism by which cannabinoids may impair memory and associational processes is an alteration of the fine-tuning of synchronized, rhythmic population events.

Input-dependent synaptic targeting of alpha(2)-subunit-containing GABA(A) receptors in synapses of hippocampal pyramidal cells of the rat

Pyramidal cells, expressing at least 14 subunits of the heteropentameric GABA(A) receptor, receive GABAergic input on their soma and proximal dendrites from basket cells, activating GABA(A) receptors and containing either parvalbumin or cholecystokinin and vasoactive intestinal polypeptide. The properties of GABA(A) receptors are determined by the subunit composition, and synaptic receptor content governs the effect of the presynaptic neuron. Using a quantitative electron microscopic immunogold technique, we tested whether the synapses formed by the two types of basket cell show a difference in the subunit composition of GABA(A) receptors. Terminals of one of the basket cells were identified by antibodies to parvalbumin. Synapses made by parvalbumin-negative terminals showed five times more immunoreactivity for the alpha(2) subunit than synapses made by parvalbumin-positive basket cells, whose synapses were frequently immunonegative. This difference is likely to be due to specific GABA(A) receptor alpha subunit composition, because neither synaptic size nor immunoreactivity for the beta(2/3) subunits, indicating total receptor content, was different in these two synapse populations. Synapses established by axo-axonic cells on axon initial segments showed an intermediate number of immunoparticles for the alpha(2) subunit compared to those made by basket cells but, due to their smaller size, the density of the alpha(2) subunit immunoreactivity was higher in synapses on the axon. Because the two basket cell types innervate the same domain of the pyramidal cell, the results indicate that pyramidal cells have mechanisms to target GABA(A) receptors, under presynaptic influence, preferentially to distinct synapses. The two basket cell types act via partially distinct GABA(A) receptor populations.

Bidirectional control of airway responsiveness by endogenous cannabinoids

Smoking marijuana or administration of its main active constituent, delta9-tetrahydrocannabinol (delta9-THC), may exert potent dilating effects on human airways. But the physiological significance of this observation and its potential therapeutic value are obscured by the fact that some asthmatic patients respond to these compounds with a paradoxical bronchospasm. The mechanisms underlying these contrasting responses remain unresolved. Here we show that the endogenous cannabinoid anandamide exerts dual effects on bronchial responsiveness in rodents: it strongly inhibits bronchospasm and cough evoked by the chemical irritant, capsaicin, but causes bronchospasm when the constricting tone exerted by the vagus nerve is removed. Both effects are mediated through peripheral CB1 cannabinoid receptors found on axon terminals of airway nerves. Biochemical analyses indicate that anandamide is synthesized in lung tissue on calcium-ion stimulation, suggesting that locally generated anandamide participates in the intrinsic control of airway responsiveness. In support of this conclusion, the CB1 antagonist SR141716A enhances capsaicin-evoked bronchospasm and cough. Our results may account for the contrasting bronchial actions of cannabis-like drugs in humans, and provide a framework for the development of more selective cannabinoid-based agents for the treatment of respiratory pathologies.

Recurrent mossy fibers preferentially innervate parvalbumin-immunoreactive interneurons in the granule cell layer of the rat dentate gyrus

Detection of vesicular zinc and immunohistochemistry against markers for different interneuron subsets were combined to study the postsynaptic target selection of zinc-containing recurrent mossy fiber collaterals in the dentate gyrus. Mossy fiber collaterals in the granule cell layer selectively innervated parvalbumin-containing cells, with numerous contacts per cell, whereas the granule cells were avoided. Under the electron microscope, those boutons made asymmetrical contacts on dendrites and somata. These findings suggest that, in addition to the hilar perforant path-associated (HIPP) interneurons, the basket and chandelier cells also receive a powerful feed-back drive from the granule cells, and thereby are able to control population synchrony in the dentate gyrus. On the other hand, the amount of monosynaptic excitatory feed-back among granule cells is shown to be negligible.

Unusual target selectivity of perisomatic inhibitory cells in the hilar region of the rat hippocampus

Perisomatic inhibitory innervation of all neuron types profoundly affects their firing characteristics and vulnerability. In this study we examined the postsynaptic targets of perisomatic inhibitory cells in the hilar region of the dentate gyrus where the proportion of potential target cells (excitatory mossy cells and inhibitory interneurons) is approximately equal. Both cholecystokinin (CCK)- and parvalbumin-immunoreactive basket cells formed multiple contacts on the somata and proximal dendrites of mossy cells. Unexpectedly, however, perisomatic inhibitory terminals arriving from these cell types largely ignored hilar GABAergic cell populations. Eighty-ninety percent of various GABAergic neurons including other CCK-containing basket cells received no input from CCK-positive terminals. Parvalbumin-containing cells sometimes innervated each other but avoided 75% of other GABAergic cells. Overall, a single mossy cell received 40 times more CCK-immunoreactive terminals and 15 times more parvalbumin-positive terminals onto its soma than the cell body of an average hilar GABAergic cell. In contrast to the pronounced target selectivity in the hilar region, CCK- and parvalbumin-positive neurons innervated each other via collaterals in stratum granulosum and moleculare. Our observations indicate that the inhibitory control in the hilar region is qualitatively different from other cortical areas at both the network level and the level of single neurons. The paucity of perisomatic innervation of hilar interneurons should have profound consequences on their action potential generation and on their ensemble behavior. These findings may help explain the unique physiological patterns observed in the hilus and the selective vulnerability of the hilar cell population in various pathophysiological conditions.

Cannabinoids inhibit hippocampal GABAergic transmission and network oscillations

Using a new antibody developed against the C-terminus of the cannabinoid receptor (CB1), the immunostaining in the hippocampus revealed additional axon terminals relative to the pattern reported previously with an N-terminus antibody. Due to a greater sensitivity of this antibody, a large proportion of boutons in the dendritic layers displaying symmetrical (GABAergic) synapses were also strongly immunoreactive for CB1 receptors, as were axon terminals of perisomatic inhibitory cells containing cholecystokinin. Asymmetrical (glutamatergic) synapses, however, were always negative for CB1. To investigate the effect of presynaptic CB1 receptor activation on hippocampal inhibition, we recorded inhibitory postsynaptic currents (IPSCs) from principal cells. Bath application of CB1 receptor agonists (WIN55,212-2 and CP55,940) suppressed IPSCs evoked by local electrical stimulation, which could be prevented or reversed by the CB1 receptor antagonist SR141716A. Action potential-driven IPSCs, evoked by pharmacological stimulation of a subset of interneurons, were also decreased by CB1 receptor activation. We also examined the effects of CB1 receptor agonists on Ca2+-independent miniature IPSCs (mIPSC). Both agonists were without significant effect on the frequency or amplitude of mIPSCs. Synchronous gamma oscillations induced by kainic acid in the CA3 region of hippocampal slices were reversibly reduced in amplitude by the CB1 receptor agonist CP 55,940, which is consistent with an action on IPSCs. We used CB1-/- knock-out mice to confirm the specificity of the antibody and of the agonist (WIN55,212-2) action. We conclude that activation of presynaptic CB1 receptors decreases Ca2+-dependent GABA release, and thereby reduces the power of hippocampal network oscillations.

Nerve growth factor but not neurotrophin-3 is synthesized by hippocampal GABAergic neurons that project to the medial septum

Conventional uptake of neurotrophins takes place at axon terminals via specific receptors, and is followed by retrograde transport. Recent studies demonstrated that, with the exception of nerve growth factor, other neurotrophins may be delivered anterogradely to the region containing the receptor expressing neurons. In this study we used a triple labeling method that combines retrograde tract tracing, in situ hybridization and immunocytochemistry to examine whether non-principal cells projecting from the hippocampus to the septum synthesize nerve growth factor. Our results show that, on average, 59% of the horseradish peroxidase-labeled hippocamposeptal nonpyramidal neurons also display nerve growth factor messenger RNA hybridization signal. The ratio was slightly higher in the CA1 stratum oriens and the hilus of the dentate gyrus (64 and 62%, respectively) compared to stratum oriens of the CA3 region (58%). In addition, we demonstrated that many nerve growth factor-positive septally projecting neurons also contain the calcium-binding protein calbindin D-28K, whereas nerve growth factor-negative projecting cells mostly lack this neurochemical marker. In contrast to nerve growth factor, neurotrophin-3 has never been found in hippocamposeptal cells. Hippocamposeptal GABAergic cells are reciprocally connected with the medial septum, thus they are in a key position to regulate nerve growth factor release as a function of the activity level in the septohippocampal system. Furthermore, our results raise the intriguing possibility that nerve growth factor may be transported also in an anterograde manner. Regardless of the direction of transport, the presence of nerve growth factor in hippocamposeptal cells suggests that long distance fast synaptic mechanisms and slow neurotrophin action are coupled in these neurons.

Neurons immunoreactive for vasoactive intestinal polypeptide in the rat primary somatosensory cortex: morphology and spatial relationship to barrel-related columns

Vasoactive intestinal polypeptide (VIP) in neocortex affects neuronal excitability as well as cortical blood flow and metabolism. Interneurons immunoreactive for VIP (VIP-IR neurons) are characterized by their predominantly bipolar appearance and the radial orientation of their main dendrites. In order to determine whether the morphology of VIP-IR neurons is related to the functional organization of the cortex into vertical columns, we combined both immunostaining of neurons containing VIP and cytochrome oxidase histochemistry for visualizing barrels, morphological layer IV correlates of functional columns, in the primary somatosensory (barrel) cortex of rats. VIP-IR neurons were localized in supragranular (48%), granular (16%), and infragranular layers (36%) as well as in the white matter. In the granular layer, a clear trend that more neurons were located in interbarrel septa rather than in barrels could be observed, resulting in a neuronal density which was about one-third higher in the septal area. VIP-IR neurons from the different cortical layers were three-dimensionally reconstructed from serial sections by using a computer microscope system. The neurons were mostly bipolar. Striking morphological differences in both axonal and dendritic trees were found between neurons whose cell bodies were located in supragranular, granular, and the upper part of infragranular layers, and those whose cell bodies were located in the area below. The former had dendrites which often reached layer I, where they bifurcated several times, and axonal trees which were particularly oriented vertically, with a tangential extent smaller than the width of barrels. Therefore, these neurons were mostly confined to either a barrel- or septum-related column. By contrast, the dendrites of neurons of the latter group did not reach the granular layer. Furthermore, these neurons had axons with sometimes very long horizontal collaterals, which often spanned two, in one case three, barrel columns. It is proposed that the differential morphology of neurons with different locations as stated above parallels to some extent the divergence of input streaming into the corresponding layer-defined areas. As a possible consequence of this, VIP-IR neurons may be capable of adapting the excitability and metabolism of cortical compartments either in a spatially limited or more extensive way.