Differential Excitability of PV and SST Neurons Results in Distinct Functional Roles in Inhibition Stabilization of Up States

Up states are the best studied example of an emergent neural dynamic regime. Computational models based on a single class of inhibitory neurons indicate that Up states reflect bistable dynamic systems in which positive feedback is stabilized by strong inhibition and predict a paradoxical effect in which increased drive to inhibitory neurons results in decreased inhibitory activity. To date, however, computational models have not incorporated empirically defined properties of parvalbumin (PV) and somatostatin (SST) neurons. Here we first experimentally characterized the frequency-current (F-I) curves of pyramidal (Pyr), PV, and SST neurons from mice of either sex, and confirmed a sharp difference between the threshold and slopes of PV and SST neurons. The empirically defined F-I curves were incorporated into a three-population computational model that simulated the empirically derived firing rates of pyramidal, PV, and SST neurons. Simulations revealed that the intrinsic properties were sufficient to predict that PV neurons are primarily responsible for generating the nontrivial fixed points representing Up states. Simulations and analytical methods demonstrated that while the paradoxical effect is not obligatory in a model with two classes of inhibitory neurons, it is present in most regimes. Finally, experimental tests validated predictions of the model that the Pyr ↔ PV inhibitory loop is stronger than the Pyr ↔ SST loop.SIGNIFICANCE STATEMENT Many cortical computations, such as working memory, rely on the local recurrent excitatory connections that define cortical circuit motifs. Up states are among the best studied examples of neural dynamic regimes that rely on recurrent excitatory excitation. However, this positive feedback must be held in check by inhibition. To address the relative contribution of PV and SST neurons, we characterized the intrinsic input-output differences between these classes of inhibitory neurons and, using experimental and theoretical methods, show that the higher threshold and gain of PV leads to a dominant role in network stabilization.

Activity labeling in vivo using CaMPARI2 reveals intrinsic and synaptic differences between neurons with high and low firing rate set points

Neocortical pyramidal neurons regulate firing around a stable mean firing rate (FR) that can differ by orders of magnitude between neurons, but the factors that determine where individual neurons sit within this broad FR distribution are not understood. To access low- and high-FR neurons for ex vivo analysis, we used Ca2+- and UV-dependent photoconversion of CaMPARI2 in vivo to permanently label neurons according to mean FR. CaMPARI2 photoconversion was correlated with immediate early gene expression and higher FRs ex vivo and tracked the drop and rebound in ensemble mean FR induced by prolonged monocular deprivation. High-activity L4 pyramidal neurons had greater intrinsic excitability and recurrent excitatory synaptic strength, while E/I ratio, local output strength, and local connection probability were not different. Thus, in L4 pyramidal neurons (considered a single transcriptional cell type), a broad mean FR distribution is achieved through graded differences in both intrinsic and synaptic properties.

Gamma activity accelerates during prefrontal development

Gamma oscillations are a prominent activity pattern in the cerebral cortex. While gamma rhythms have been extensively studied in the adult prefrontal cortex in the context of cognitive (dys)functions, little is known about their development. We addressed this issue by using extracellular recordings and optogenetic stimulations in mice across postnatal development. We show that fast rhythmic activity in the prefrontal cortex becomes prominent during the second postnatal week. While initially at about 15 Hz, fast oscillatory activity progressively accelerates with age and stabilizes within gamma frequency range (30-80 Hz) during the fourth postnatal week. Activation of layer 2/3 pyramidal neurons drives fast oscillations throughout development, yet the acceleration of their frequency follows similar temporal dynamics as the maturation of fast-spiking interneurons. These findings uncover the development of prefrontal gamma activity and provide a framework to examine the origin of abnormal gamma activity in neurodevelopmental disorders.

Brainstem nucleus incertus controls contextual memory formation

Hippocampal pyramidal cells encode memory engrams, which guide adaptive behavior. Selection of engram-forming cells is regulated by somatostatin-positive dendrite-targeting interneurons, which inhibit pyramidal cells that are not required for memory formation. Here, we found that γ-aminobutyric acid (GABA)-releasing neurons of the mouse nucleus incertus (NI) selectively inhibit somatostatin-positive interneurons in the hippocampus, both monosynaptically and indirectly through the inhibition of their subcortical excitatory inputs. We demonstrated that NI GABAergic neurons receive monosynaptic inputs from brain areas processing important environmental information, and their hippocampal projections are strongly activated by salient environmental inputs in vivo. Optogenetic manipulations of NI GABAergic neurons can shift hippocampal network state and bidirectionally modify the strength of contextual fear memory formation. Our results indicate that brainstem NI GABAergic cells are essential for controlling contextual memories.

IKCa channels are a critical determinant of the slow AHP in CA1 pyramidal neurons

Control over the frequency and pattern of neuronal spike discharge depends on Ca2+-gated K+ channels that reduce cell excitability by hyperpolarizing the membrane potential. The Ca2+-dependent slow afterhyperpolarization (sAHP) is one of the most prominent inhibitory responses in the brain, with sAHP amplitude linked to a host of circuit and behavioral functions, yet the channel that underlies the sAHP has defied identification for decades. Here, we show that intermediate-conductance Ca2+-dependent K+ (IKCa) channels underlie the sAHP generated by trains of synaptic input or postsynaptic stimuli in CA1 hippocampal pyramidal cells. These findings are significant in providing a molecular identity for the sAHP of central neurons that will identify pharmacological tools capable of potentially modifying the several behavioral or disease states associated with the sAHP.

Fluorescence imaging of dendritic spines of Golgi-Cox-stained neurons using brightening background

We report a novel fluorescence imaging approach to imaging nonfluorescence-labeled biological tissue samples. The method was demonstrated by imaging neurons in Golgi-Cox-stained and epoxy-resin-embedded samples through the excitation of the background fluorescence of the specimens. The dark neurons stood out clearly against background fluorescence in the images, enabling the tracing of a single dendritic spine using both confocal and wide-field fluorescence microscopy. The results suggest that the reported fluorescence imaging method would provide an effective alternative solution to image nonfluorescence-labeled samples, and it allows tracing the dendritic spine structure of neurons.

Measuring CaMKII concentration in dendritic spines

Here, we present a method for measuring the concentration of endogenous protein in cellular compartments. Importantly, the method is applicable to compartments such as dendritic spines with dimensions often close to the resolution limit of optical microscopy. To our knowledge, a method with such capabilities has not yet been described. The method utilizes overexpression of the protein of interest, which is tagged with fluorescent protein. This is followed by immunostaining of both overexpressed and endogenous proteins. Expression of a volume marker is also required. We applied this method to measure the concentration of Ca/calmodulin kinase II (CaMKII) in different cellular compartments of hippocampal pyramidal neurons. It was found that the concentrations of CaMKIIα subunits in cell bodies, proximal dendrites, and spines on these dendrites are 71, 46, and 103 μM, respectively. Considering the 3:1 ratio of α to β CaMKII subunits in the hippocampus, the concentrations of total (α+β) CaMKII subunits in these compartments are 94, 61, and 138 μM, respectively.

Transcranial optogenetic stimulation for functional mapping of the motor cortex

We developed a method that uses Channelrhodopsin-2 (ChR2) for transcranial optogenetic stimulation. This method is based on scanning a light beam over the brain, thereby photostimulating ChR2-expressing neurons in intact mice. As a proof of principle, we applied this technique to the motor cortex of transgenic mice expressing ChR2 in cortical pyramidal cells. Photostimulation induced limb movements that were time-locked with millisecond precision and could be induced at frequencies up to 20 Hz. By scanning this light beam, we could map the distribution of neurons associated with limb movement. With this approach we could simultaneously define motor maps controlling two limbs and could reproducibly generate such cortical motor maps over periods of weeks. This method allows non-invasive mapping of brain circuitry in living animals and could help define the connection between behavior and brain circuitry.

Subfield-specific loss of hippocampal N-acetyl aspartate in temporal lobe epilepsy

PURPOSE

In patients with mesial temporal lobe epilepsy (MTLE) it remains an unresolved issue whether the interictal decrease in N-acetyl aspartate (NAA) detected by proton magnetic resonance spectroscopy ((1)H-MRS) reflects the epilepsy-associated loss of hippocampal pyramidal neurons or metabolic dysfunction.

METHODS

To address this problem, we applied high-resolution (1)H-MRS at 14.1 Tesla to measure metabolite concentrations in ex vivo tissue slices from three hippocampal subfields (CA1, CA3, dentate gyrus) as well as from the parahippocampal region of 12 patients with MTLE.

RESULTS

In contrast to four patients with lesion-caused MTLE, we found a large variance of NAA concentrations in the individual hippocampal regions of patients with Ammon's horn sclerosis (AHS). Specifically, in subfield CA3 of AHS patients despite of a moderate preservation of neuronal cell densities the concentration of NAA was significantly lowered, while the concentrations of lactate, glucose, and succinate were elevated. We suggest that these subfield-specific alterations of metabolite concentrations in AHS are very likely caused by impairment of mitochondrial function and not related to neuronal cell loss.

CONCLUSIONS

A subfield-specific impairment of energy metabolism is the probable cause for lowered NAA concentrations in sclerotic hippocampi of MTLE patients.

Inherited cortical HCN1 channel loss amplifies dendritic calcium electrogenesis and burst firing in a rat absence epilepsy model

While idiopathic generalized epilepsies are thought to evolve from temporal highly synchronized oscillations between thalamic and cortical networks, their cellular basis remains poorly understood. Here we show in a genetic rat model of absence epilepsy (WAG/Rij) that a rapid decline in expression of hyperpolarization-activated cyclic-nucleotide gated (HCN1) channels (I(h)) precedes the onset of seizures, suggesting that the loss of HCN1 channel expression is inherited rather than acquired. Loss of HCN1 occurs primarily in the apical dendrites of layer 5 pyramidal neurons in the cortex, leading to a spatially uniform 2-fold reduction in dendritic HCN current throughout the entire somato-dendritic axis. Dual whole-cell recordings from the soma and apical dendrites demonstrate that loss of HCN1 increases somato-dendritic coupling and significantly reduces the frequency threshold for generation of dendritic Ca2+ spikes by backpropagating action potentials. As a result of increased dendritic Ca2+ electrogenesis a large population of WAG/Rij layer 5 neurons showed intrinsic high-frequency burst firing. Using morphologically realistic models of layer 5 pyramidal neurons from control Wistar and WAG/Rij animals we show that the experimentally observed loss of dendritic I(h) recruits dendritic Ca2+ channels to amplify action potential-triggered dendritic Ca2+ spikes and increase burst firing. Thus, loss of function of dendritic HCN1 channels in layer 5 pyramidal neurons provides a somato-dendritic mechanism for increasing the synchronization of cortical output, and is therefore likely to play an important role in the generation of absence seizures.

Immunohistochemical study of the serotonergic neuronal system in an animal model of the mood disorder

The monoamine theory is one of the major hypotheses about the biological etiology of major depressive disorders. Recent pharmacological and postmortem investigations suggest that depressed patients have alterations in function of serotonergic neuronal system. However, the exact sites of alterations and the association between these alterations and the etiology of the disorder are still unclear. To elucidate these issues, we immunohistochemically examined vesicle monoamine transporter 2 (VMAT2), serotonin receptor type 1a (5HT1a), and serotonin transporter (5HTT) in the hippocampal region of reserpine-treated rats, an animal model of depression. The results showed more VMAT2-immunoreactive varicose fibers in the pyramidal cell layer of hippocampus and parahippocampal cortexes, and more intense 5HTT-immunoreactivity in the pyramidal cell layer and the area CA4 of hippocampus in the animal models compared to those of the controls. On the other hand, lower density of 5HT1a-immunoreactive deposits in the pyramidal cell layer of hippocampus and the parahippocampal cortex was observed in the animal models compared to those of the controls. These results suggest that a deficit of monoamines induces the alterations in the expression of the storage protein, the receptor and the transporter that are involved in the serotonergic neurotransmission in the hippocampal region. These alterations may underlie the changes of serotonergic system observed in the brains of patients with the depressive disorder.

GABAergic protection of hippocampal pyramidal neurons against glutamate insult: deficit in young animals compared to adults

Hypoxia-ischemia (HI) injury in neonatal animals leads to selective regional loss of neurons including CA1 and CA3 pyramidal neurons of the hippocampus as well as nonlethal pathologies. Glutamate-receptor over-activation and Ca2+ influx are involved in these neonatal changes. We examined glutamate-evoked Ca2+ responses in neonatal (PN 7-13) and young adult (PN 21-27) CA1 pyramidal neurons in acute slices from rats. In neonates, transient exposure to glutamate produced large Ca2+ increases throughout neurons, including distal dendrites (primary Ca2+ responses). Repeated exposures resulted in sustained Ca2+ increases in apical dendrites (secondary Ca2+ responses) that were independent of continued glutamate exposure. These responses propagated and invaded the soma, resulting in irrecoverably high Ca2+ elevations. In neurons from adults, identical glutamate exposure evoked primary Ca2+ responses only in somata and proximal apical dendrites. Repeated glutamate exposures in the adult neurons also led to secondary Ca2+ responses, but they arose in the peri-somatic region and then spread outward to distal apical dendrites, again without recovery. More stimuli were required to initiate secondary responses in neurons from adult versus neonates. Block of GABAA receptors in adults caused the primary and secondary responses to revert to the spatial pattern seen in the neonates and greatly increased their vulnerability to glutamate. These findings suggest that neurodegenerative secondary Ca2+ events may be important determinants of susceptibility to HI injury in the developing CNS and that immature CA1 neurons may be more susceptible to excitotoxic injury due at least in part to insufficient development of GABAergic inputs to their dendrites.

Compartment-dependent colocalization of Kir3.2-containing K+ channels and GABAB receptors in hippocampal pyramidal cells

G-protein-coupled inwardly rectifying K+ channels (Kir3 channels) coupled to metabotropic GABAB receptors are essential for the control of neuronal excitation. To determine the distribution of Kir3 channels and their spatial relationship to GABAB receptors on hippocampal pyramidal cells, we used a high-resolution immunocytochemical approach. Immunoreactivity for the Kir3.2 subunit was most abundant postsynaptically and localized to the extrasynaptic plasma membrane of dendritic shafts and spines of principal cells. Quantitative analysis of immunogold particles for Kir3.2 revealed an enrichment of the protein around putative glutamatergic synapses on dendritic spines, similar to that of GABA(B1). Consistent with this observation, a high degree of coclustering of Kir3.2 and GABA(B1) was revealed around excitatory synapses by the highly sensitive SDS-digested freeze-fracture replica immunolabeling. In contrast, in dendritic shafts receptors and channels were found to be mainly segregated. These results suggest that Kir3.2-containing K+ channels on dendritic spines preferentially mediate the effect of GABA, whereas channels on dendritic shafts are likely to be activated by other neurotransmitters as well. Thus, Kir3 channels, localized to different subcellular compartments of hippocampal principal cells, appear to be differentially involved in synaptic integration in pyramidal cell dendrites.

Voltage-gated ion channels in the axon initial segment of human cortical pyramidal cells and their relationship with chandelier cells

The axon initial segment (AIS) of pyramidal cells is a critical region for the generation of action potentials and for the control of pyramidal cell activity. Here we show that Na+ and K+ voltage-gated channels, together with other molecules involved in the localization of ion channels, are distributed asymmetrically in the AIS of pyramidal cells situated in the human temporal neocortex. There is a high density of Na+ channels distributed along the length of the AIS together with the associated proteins spectrin betaIV and ankyrin G. In contrast, Kv1.2 channels are associated with the adhesion molecule Caspr2, and they are mostly localized to the distal region of the AIS. In general, the distal region of the AIS is targeted by the GABAergic axon terminals of chandelier cells, whereas the proximal region is innervated, mostly by other types of GABAergic interneurons. We suggest that this molecular segregation and the consequent regional specialization of the GABAergic input to the AIS of pyramidal cells may have important functional implications for the control of pyramidal cell activity.

Imaging membrane potential in dendritic spines

Dendritic spines mediate most excitatory inputs in the brain. Although it is clear that spines compartmentalize calcium, it is still unknown what role, if any, they play in integrating synaptic inputs. To investigate the electrical function of spines directly, we used second harmonic generation (SHG) imaging of membrane potential in pyramidal neurons from hippocampal cultures and neocortical brain slices. With FM 4-64 as an intracellular SHG chromophore, we imaged membrane potential in the soma, dendritic branches, and spines. The SHG response to voltage was linear and seemed based on an electro-optic mechanism. The SHG sensitivity of the chromophore in spines was similar to that of the parent dendritic shaft and the soma. Backpropagation of somatic action potentials generated SHG signals at spines with similar amplitude and kinetics to somatic ones. Our optical measurements of membrane potential from spines demonstrate directly that backpropagating action potentials invade the spines.

Single channel properties of hyperpolarization-activated cation currents in acutely dissociated rat hippocampal neurones

The hyperpolarization-activated cation current (I(h)), mediated by HCN channels, contributes to intrinsic neuronal properties, synaptic integration and network rhythmicity. Recent studies have implicated HCN channels in neuropathological conditions including epilepsy. While native HCN channels have been studied at the macroscopic level, the biophysical characteristics of individual neuronal HCN channels have not been described. We characterize, for the first time, single HCN currents of excised inside-out patches from somata of acutely dissociated rat hippocampal CA1 pyramidal cells. Hyperpolarization steps elicited non-inactivating channel openings with an apparent conductance of 9.7 pS, consistent with recent reports of native and recombinant HCN channels. The voltage-dependent P(o) had a V(1/2) of -81 +/- 1.8 mV and slope -13.3 +/- 1.9 mV. Blockers of macroscopic I(h), ZD7288 (50 microM) and CsCl (1 mM), reduced the channel conductance to 8 pS and 8.4 pS, respectively. ZD7288 was slightly more effective in reducing the P(o) at depolarized potentials, whereas CsCl was more efficacious at hyperpolarized potentials. The unitary neuronal HCN channels had voltage-dependent latencies to first channel opening and two open states. As expected, ZD7288 and CsCl increased latencies and decreased the properties of both open states. The major endogenous positive modulator of macroscopic I(h) is cAMP. Application of 8Br-cAMP (10 microM) did not affect conductance (9.4 pS), but did increase P(o) and short and long open times. Thus, sensitivity to I(h) modulators supports the single h-channel identity of these unitary currents. Detailed biophysical analysis of unitary I(h) conductances is likely to help distinguish between homomeric and heteromeric expression of these channels - findings that may be relevant toward the pathophysiology of diseases such as epilepsy.

Comparative proteomic analysis using samples obtained with laser microdissection and saturation dye labelling

Comparative proteomic methods are rapidly being applied to many different biological systems including complex tissues. One pitfall of these methods is that in some cases, such as oncology and neuroscience, tissue complexity requires isolation of specific cell types and sample is limited. Laser microdissection (LMD) is commonly used for obtaining such samples for proteomic studies. We have combined LMD with sensitive thiol-reactive saturation dye labelling of protein samples and 2-D DIGE to identify protein changes in a test system, the isolated CA1 pyramidal neurone layer of a transgenic (Tg) rat carrying a human amyloid precursor protein transgene. Saturation dye labelling proved to be extremely sensitive with a spot map of over 5,000 proteins being readily produced from 5 mug total protein, with over 100 proteins being significantly altered at p < 0.0005. Of the proteins identified, all showed coherent changes associated with transgene expression. It was, however, difficult to identify significantly different proteins using PMF and MALDI-TOF on gels containing less than 500 mug total protein. The use of saturation dye labelling of limiting samples will therefore require the use of highly sensitive MS techniques to identify the significantly altered proteins isolated using methods such as LMD.

Presynaptic D1 dopamine receptors in primate prefrontal cortex: target-specific expression in the glutamatergic synapse

Dopaminergic modulation of glutamate neurotransmission in prefrontal cortex (PFC) microcircuits is commonly perceived as a basis for cognitive operations. Yet it appears that although the control of recurrent excitation between deep-layer prefrontal pyramids may involve presynaptic and postsynaptic D1 receptor (D1R) mechanisms, pyramid-to-interneuron communication will engage a postsynaptic D1R component. The substrate underlying such target-specific neuromodulatory patterns was investigated in the infragranular PFC with immunoelectron microscopy for D1R and parvalbumin, a marker for fast-spiking interneurons. In addition to their proverbial postsynaptic expression, gold-labeled D1Rs were distinctly distributed on perisynaptic/extrasynaptic membranes and the axoplasm of 13% of excitatory-like, presumably glutamatergic varicosities. Most importantly, presynaptic D1Rs were highly specific with regard to the cellular compartment and neurochemical identity of the postsynaptic neuron, being present in spine-targeting varicosities but distinctly absent from those synapsing with parvalbumin profiles often coexpressing D1Rs. We define therein an axonal D1 heteroreceptor component, apparently mediating volume neurotransmission, yet strategically positioned to convey target cell-specific modulation of the glutamatergic drive. We also indicate that presynaptic D1R mechanisms may indeed be associated with recurrent excitation in prefrontal microcircuits, consistent with physiological evidence for a role of these receptors in modulating the persistent activity-profile of neurons essential for working memory.

High intrinsic oxidative stress may underlie selective vulnerability of the hippocampal CA1 region

Oxidative stress (OS) causes extensive cell death in the CA1 but not the CA3 region of the hippocampus. We found that the CA1 region of hippocampus explants, cultured under normal conditions, had significantly higher superoxide levels and expressed both anti-oxidant genes and genes related to the generation of reactive oxygen species at significantly higher levels than the CA3. These observations were indicative of high intrinsic OS in CA1.

Localization of brain-derived neurotrophic factor to distinct terminals of mossy fiber axons implies regulation of both excitation and feedforward inhibition of CA3 pyramidal cells

Hippocampal dentate granule cells directly excite and indirectly inhibit CA3 pyramidal cells via distinct presynaptic terminal specializations of their mossy fiber axons. This mossy fiber pathway contains the highest concentration of brain-derived neurotrophic factor (BDNF) in the CNS, yet whether BDNF is positioned to regulate the excitatory and/or inhibitory pathways is unknown. To localize BDNF, confocal microscopy of green fluorescent protein transgenic mice was combined with BDNF immunohistochemistry. Approximately half of presynaptic granule cell-CA3 pyramidal cell contacts were found to contain BDNF. Moreover, enhanced neuronal activity virtually doubled the percentage of BDNF-immunoreactive terminals contacting CA3 pyramidal cells. To our surprise, BDNF was also found in mossy fiber terminals contacting inhibitory neurons. These studies demonstrate that mossy fiber BDNF is poised to regulate both direct excitatory and indirect feedforward inhibitory inputs to CA3 pyramdal cells and reveal that seizure activity increases the pool of BDNF-expressing granule cell presynaptic terminals contacting CA3 pyramidal cells.

Quantitative analysis of axon collaterals of single superficial pyramidal cells in layer IIb of the piriform cortex of the guinea pig

To understand the functional organization of the piriform cortex (PC), the axon collaterals of three pyramidal cells in layer IIb of the anterior PC and one pyramidal cell in layer IIb of the posterior PC were labeled and quantitatively analyzed by intracellular biocytin injection in the guinea pig. Single pyramidal cells in the anterior and posterior PCs have widely distributed axon collaterals, which exhibit little tendency for patchy concentrations inside as well as outside the PC. The total lengths of the axon collaterals of the three fully analyzed pyramidal cells ranged from 68 to 156 mm, more than 50% of which were distributed in the PC. The total number of boutons of the three cells ranged from 6000 to 14,000, 5000-7000 of which were distributed in the PC. It was estimated that individual pyramidal cells in layer IIb form synaptic contacts with 2200 to 3000 other pyramidal cells in the PC, indicating that single pyramidal cells in layer IIb receive input from a large number of other pyramidal cells. This high connectivity of the network of pyramidal cells in the PC can be regarded as the neural network operating parallel distributed processing, which may play an important role in experience-induced enhancement in odorant discrimination in the PC.

A primary culture system for biochemical analyses of neuronal proteins

Low-density cultures of embryonic rat hippocampal neurons have been widely used to investigate localization and function of neuronal proteins using immunocytochemistry and electrophysiology. These cultures provide a relatively homogeneous population of hippocampal pyramidal neurons and interneurons compared to post-natal mixed neuron/glial cultures from hippocampus, cerebral cortex, and cerebellum. However, the limited quantity of neurons and the difficulty in harvesting adequate amounts makes biochemical analyses of endogenous neuronal proteins in these low-density cultured neurons difficult. Here, we provide detailed methods to prepare cultures of embryonic rat hippocampal neurons suitable for biochemical analyses of both endogenously and exogenously expressed proteins. The procedures described here are also suitable for comprehensive studies of expression, localization, post-translational modification, and function of neuronal proteins in the same neuronal culture system.

Nectin-dependent localization of synaptic scaffolding molecule (S-SCAM) at the puncta adherentia junctions formed between the mossy fibre terminals and the dendrites of pyramidal cells in the CA3 area of the mouse hippocampus

BACKGROUND

Two types of intercellular junctions, synaptic junctions (SJs) and puncta adherentia junctions (PAs), are observed at the synapses between the mossy fibre terminals and the dendrites of pyramidal cells in the CA3 area of the hippocampus. SJs are associated with active zones and postsynaptic densities (PSDs) where neurotransmission occurs, whereas PAs are not associated with either of them. We have found that the nectin-afadin unit as well as the N-cadherin-catenin unit localizes at the PAs and that both the units cooperatively organize the PAs. Nectins are Ca2+-independent Ig-like cell-cell adhesion molecules and afadin is a nectin- and actin filament-binding protein that connects nectins to the actin cytoskeleton. Synaptic scaffolding molecule (S-SCAM) is a neural scaffolding protein which interacts with many proteins including neuroligin, NMDA receptors, neural plakophilin-related armadillo-repeat protein/delta-catenin, a GDP/GTP exchange protein for Rap1 small G protein (PDZ-Rap-GEP), and beta-catenin. S-SCAM has been suggested to be a component of PSDs, but its precise localization at the synapses remains unknown.

RESULTS

S-SCAM was not concentrated at the PSDs but highly concentrated and co-localized with nectins at both the sides of the PAs formed between the mossy fibre terminals and the dendrites of pyramidal cells in the CA3 area of the adult mouse hippocampus. S-SCAM co-localized with nectin-1 at the primitive synapses where the SJs and the PAs were not morphologically differentiated, and they co-localized during the maturation of the SJs and the PAs. Nectin-1 had a potency to recruit S-SCAM to the nectin-1-based cell-cell adhesion sites formed in cadherin-deficient L cells as a model system. This recruitment was dependent on the C-terminal PDZ domain-binding motif of nectin-1 which is necessary for the binding of afadin, suggesting that nectins recruit S-SCAM through afadin. Consistently, S-SCAM was co-immunoprecipitated with afadin by the anti-S-SCAM antibody from the mouse brain, but S-SCAM did not directly bind afadin.

CONCLUSION

These results indicate that S-SCAM localizes at the PAs in the CA3 area of the hippocampus in a nectin-dependent manner and suggest that S-SCAM serves as a scaffolding molecule at the PAs after maturation of the synapses and at the SJs during the maturation.

Expression and cellular distribution of multidrug resistance-related proteins in the hippocampus of patients with mesial temporal lobe epilepsy

PURPOSE

This study investigated the cellular distribution of different multidrug resistance (MDR)-related proteins such as P-glycoprotein (P-gp), the multidrug resistance-associated proteins (MRP) 1 and 2, and the major vault protein (MVP) in normal and sclerotic hippocampus of patients with medically refractory mesial temporal lobe epilepsy (MTLE).

METHODS

Single- and double-label immunocytochemistry was used on brain sections of control hippocampus and of hippocampus of refractory MTLE patients.

RESULTS

In TLE cases with hippocampal sclerosis (HS), all four MDR proteins examined that had low or no expression in control tissue were upregulated, albeit with different cellular distribution patterns. P-gp immunoreactivity (IR) was observed in astrocytes in regions with diffuse reactive gliosis. In 75% of HS cases, strong P-gp IR was detected in blood vessels, with prominent endothelial labeling. Reactive astrocytes displayed low MRP1 IR. However, glial MRP1 expression was noted in glial endfoot processes around blood vessels. Neuronal MRP1 expression was observed in hypertrophic hilar neurons and in a few residual neurons of the CA1 region. Hippocampal MRP2 expression was observed in the large majority of HS cases in blood vessels. Hypertrophic hilar neurons and blood vessels within the sclerotic hippocampus expressed major vault protein (MVP).

CONCLUSIONS

These findings indicate that MDR proteins are upregulated in concert in the hippocampus of patients with refractory MTLE, supporting their role in the mechanisms underlying drug resistance. The specific cell-distribution patterns within the sclerotic hippocampus suggest different cellular functions, not necessarily linked only to clinical drug resistance.

Selective neurofilament (SMI-32, FNP-7 and N200) expression in subpopulations of layer V pyramidal neurons in vivo and in vitro

There are two main types of layer V pyramidal neurons in rat cortex. Type I neurons have tufted apical dendrites extending into layer I, produce bursts of action potentials and project to subcortical targets (spinal cord, superior colliculus and pontine nuclei). Type II neurons have apical dendrites, which arborize in layers II-IV, do not produce bursts of action potentials and project to ipsilateral and contralateral cortex. The specific expression of different genes and proteins in these two distinct layer V neurons is unknown. To distinguish between distinct subpopulations, fluorescent microspheres were injected into subcortical targets (labeling type I neurons) or primary somatosensory cortex (labeling type II neurons) of adult rats. After transport, cortical sections were processed for immunohistochemistry using various antibodies. This study demonstrated that antigens recognized by SMI-32, N200 and FNP-7 antibodies were only expressed in subcortical (type I)--but not in contralateral (type II)--projecting neurons. NR1, NR2a/b, PLCbeta1, BDNF, NGF and TrkB antigens were highly expressed in all neuronal subpopulations examined. Organotypic culture experiments demonstrated that the development of neurofilament expression and laminar specificity does not depend on the presence of the subcortical targets. This study suggests specific markers for the subcortical projecting layer V neuron subpopulations.

Expression of serotonin1A and serotonin2A receptors in pyramidal and GABAergic neurons of the rat prefrontal cortex

Serotonergic 5-HT1A and 5-HT2A receptors are abundantly expressed in prefrontal cortex (PFC) and are targets of atypical antipsychotic drugs. They mediate, respectively, inhibitory and excitatory actions of 5-HT. The transcripts for both receptors are largely (approximately 80%) colocalized in rat and mouse PFC, yet their quantitative distribution in pyramidal and GABAergic interneurons is unknown. We used double in situ hybridization histochemistry to estimate the proportion of pyramidal and GABAergic neurons expressing these receptor transcripts in rat PFC. The number of GABAergic interneurons (expressing GAD mRNA) was a 22% of glutamatergic neurons (expressing vGluT1 mRNA, considered as putative pyramidal neurons). 5-HT2A receptor mRNA was present in a large percentage of pyramidal neurons (from 55% in prelimbic cortex to 88% in tenia tecta), except in layer VI, where it was localized only in 30% of those neurons. 5-HT2A receptor mRNA was present in approximately 25% of GAD-containing cells except in layer VI (10%). Likewise, approximately 60% of glutamatergic cells contained the 5-HT1A receptor transcript. We also found that approximately 25% of GAD-expressing cells contained the 5-HT1A receptor mRNA. These data help to clarify the role of 5-HT in prefrontal circuits and shed new light to the cellular elements involved in the action of atypical antipsychotics.

Anatomical evidence for transsynaptic influences of estrogen on brain-derived neurotrophic factor expression

Several studies have demonstrated that estrogen modulates brain-derived neurotrophic factor (BDNF) mRNA and protein within the adult hippocampus and cortex. However, mechanisms underlying this regulation are unknown. Although an estrogen response element (ERE)-like sequence has been identified within the BDNF gene, such a classical mechanism of estrogen-induced transcriptional activation requires the colocalized expression of estrogen receptors within cells that produce BDNF. Developmental studies have demonstrated such a relationship, but to date no studies have examined colocalization of estrogen receptors and BDNF within the adult brain. By utilizing double-label immunohistochemistry for BDNF, estrogen receptor-alpha (ER-alpha), and estrogen receptor-beta (ER-beta), we found only sparse colocalization between ER-alpha and BDNF in the hypothalamus, amygdala, prelimbic cortex, and ventral hippocampus. Furthermore, ER-beta and BDNF do not colocalize in any brain region. Given the recent finding that cortical ER-beta is almost exclusively localized to parvalbumin-immunoreactive GABAergic neurons, we performed BDNF/parvalbumin double labeling and discovered that axons from cortical ER-beta-expressing inhibitory neurons terminate on BDNF-immunoreactive pyramidal cells. Collectively, these findings support a potential transsynaptic relationship between estrogen state and cortical BDNF: By directly modulating GABAergic interneurons, estrogen may indirectly influence the activity and expression of BDNF-producing cortical neurons.

Radial organisation of neurons and dendrites in human cortical areas 25, 32, and 32'

Nissl cytoarchitectural and MAP-2 immunocytochemical evidence is presented for the radial organisation of neurons and neural processes in the human medial prefrontal cortex (mPFC). In Brodmann areas 25, 32, and 32', neuronal cell bodies are organised into short vertical stacks of 15-19 somata with pyramidal cells apical dendrites being arranged into distinct vertically oriented units spaced 52-59 microm apart. Such architecture may underlie specific functional aspects of information processing in the human mPFC.

The effect of chronic aluminum(III) administration on the nervous system of aged rats: Clues to understand its suggested role in Alzheimer's disease

The effect of chronic aluminum intake has been investigated in the brain of aged male Wistar rats to assess the potential role of the accumulation of this metal ion on the development of neurodegenerative features observed in Alzheimer's disease. AlCl3 x 6 H2O (2g/L) was administered to experimental animals for 6 months in the drinking water. The total content of Al (microg/g fresh tissue) was measured by inductively coupled plasma atomic emission spectrometry (ICP-AES), while the content of Cu, Zn and Mn was determined by flame AAS in the prosencephalon + mesencephalon, pons-medulla and cerebellum of control and Al(III)-treated animals. The area occupied by mossy fibres in the CA3 field of the hippocampus was estimated by a computer-assisted morphometric method following Timm's preferential staining. In Al(III)-treated rats the concentration of Cu, Zn and Mn did not increase significantly (p < 0.5) in prosencephalon + mesencephalon, nor in pons-medulla (p < 0.5) except for Cu (p < 0.05) in pons-medulla. In the cerebellum the only significant increase was seen for Zn (p < 0.01) while no change was observed for Cu and Mn. The area occupied by the mossy fibres in the hippocampal CA3 field was significantly increased (+32%) in aged Al(III)-treated rats. Since Cu, Zn and Mn are essential components of the cytosolic and mitochondrial superoxide dismutases, it is possible that the increased content of these ions in aged Al(III)-treated rats represents an increased amount of genetic expression of these antioxidant enzymes. Considering that the positivity to Timm's reaction is based on the presence of free or loosely bound Zn2+ ions within synaptic terminals and that Zn2+ ions are reported to be accumulated by hippocampal neurons when tissue injury occurs, the increased area of the mossy fibres in CA3 field of Al(III)-treated rats could indicate increased hippocampal damage in these animals. Taken together, the present findings indicate that the aging CNS is particularly susceptible to Al(III) toxic effects which may increase the cell load of oxidative stress and may contribute, as an aggravating factor, to the development of neurodegenerative events as observed in Alzheimer's disease.

Distribution and morphological characterization of phosphate-activated glutaminase-immunoreactive neurons in cat visual cortex

Phosphate-activated glutaminase (PAG) is the major enzyme involved in the synthesis of the excitatory neurotransmitter glutamate in cortical neurons of the mammalian cerebral cortex. In this study, the distribution and morphology of glutamatergic neurons in cat visual cortex was monitored through immunocytochemistry for PAG. We first determined the specificity of the anti-rat brain PAG polyclonal antibody for cat brain PAG. We then examined the laminar expression profile and the phenotype of PAG-immunopositive neurons in area 17 and 18 of cat visual cortex. Neuronal cell bodies with moderate to intense PAG immunoreactivity were distributed throughout cortical layers II-VI and near the border with the white matter of both visual areas. The largest and most intensely labeled cells were mainly restricted to cortical layers III and V. Careful examination of the typology of PAG-immunoreactive cells based on the size and shape of the cell body together with the dendritic pattern indicated that the vast majority of these cells were pyramidal neurons. However, PAG immunoreactivity was also observed in a paucity of non-pyramidal neurons in cortical layers IV and VI of both visual areas. To further characterize the PAG-immunopositive neuronal population we performed double-stainings between PAG and three calcium-binding proteins, parvalbumin, calbindin and calretinin, to determine whether GABAergic non-pyramidal cells can express PAG, and neurofilament protein, a marker for a subset of pyramidal neurons in mammalian neocortex. We here present PAG as a neurochemical marker to map excitatory cortical neurons that use the amino acid glutamate as their neurotransmitter in cat visual cortex.

Immunocytochemical localization of neurons containing the AMPA GluR2/3 subunit in the hamster visual cortex

AMPA glutamate receptors play a crucial role in brain functions such as synaptic plasticity and development. We have studied the chemo-architecture of the AMPA glutamate receptor subtype GluR2/3 in the hamster visual cortex by immunocytochemistry and compared it with the distribution of the calcium-binding proteins, calbindin D28K and calretinin. Anti-GluR2/3-immunoreactive (IR) neurons were predominantly located in layers II/III, V, and VI, and the majority of the labeled neurons were round or oval. However, many pyramidal cells in layer V were also labeled. Two-color immunofluorescence revealed that none of the GluR2/3-IR neurons contained calbindin D28 K or calretinin. Thus specific layers of neurons express the GluR2/3 subunit and these do not correlate with expression of calbindin D28K and calretinin.

Quantitative analysis of axon collaterals of single cells in layer III of the piriform cortex of the guinea pig

Recent physiological and morphological studies suggest that the piriform cortex (PC) functions like the association areas of the neocortex rather than the typical primary sensory area as was previously assumed. The axon connection patterns of single cells are important for understanding the functional organization of the PC. The axon collaterals of three single pyramidal cells and one spiny multipolar cell in layer III of the PC were labeled and quantitatively analyzed by intracellular injections of biocytin in guinea pigs. The individual pyramidal and spiny multipolar cells have highly distributed axon collaterals, which display little tendency for patchy concentrations, within the PC and multiple higher order behavior/reward/contextual-related areas, such as the prefrontal cortex, amygdaloid nuclei, and entorhinal cortex. For the pyramidal cells, the average length of axonal collaterals is 143 mm; the average number of boutons is 12,930. For the spiny multipolar cell, the length of the axonal collaterals is 88 mm; the number of boutons is 7,052. The pyramidal cells in the anterior subdivision of the PC (APC) have both rostrally and caudally directed intrinsic association fibers, whereas the pyramidal and spiny multipolar cells in the posterior subdivision (PPC) have predominantly caudally directed intrinsic association fibers in the PC. Our results reveal that the connection patterns of single cells in layer III resemble those of pyramidal cells in layer II, suggesting that the PC performs correlative functions analogous to those in the association area of other sensory systems. The rostrally-to-caudally directed connections in the APC provide a substrate for the recurrent process, whereas largely caudally directed connections in the PPC suggest the dominance of the feed-forward process.

Postnatal development of pre- and postsynaptic GABA markers at chandelier cell connections with pyramidal neurons in monkey prefrontal cortex

The protracted postnatal maturation of the primate prefrontal cortex (PFC) is associated with substantial changes in the number of excitatory synapses on pyramidal neurons, whereas the total number of inhibitory synapses appears to remain constant. In this study, we sought to determine whether the developmental changes in excitatory input to pyramidal cells are paralleled by changes in functional markers of inhibitory inputs to pyramidal neurons. The chandelier subclass of gamma-aminobutyric acid (GABA) neurons provides potent inhibitory control over pyramidal neurons by virtue of their axon terminals, which form distinct vertical structures (termed cartridges) that synapse at the axon initial segment (AIS) of pyramidal neurons. Thus, we examined the relative densities, laminar distributions, and lengths of presynaptic chandelier axon cartridges immunoreactive for the GABA membrane transporter 1 (GAT1) or the calcium-binding protein parvalbumin (PV) and of postsynaptic pyramidal neuron AIS immunoreactive for the GABA(A) receptor alpha(2) subunit (GABA(A) alpha(2)) in PFC area 46 of 38 rhesus monkeys (Macaca mulatta). From birth through 2 years of age, the relative densities and laminar distributions of these three markers exhibited different trajectories, suggesting developmental shifts in the weighting of at least some factors that determine inhibition at the AIS. In contrast, from 2 to 4 years of age, all three markers exhibited similar declines in density and length that paralleled the periadolescent pruning of excitatory synapses to pyramidal neurons. Across development, the predominant laminar location of PV-labeled cartridges and GABA(A) alpha(2)-immunoreactive AIS shifted from the middle to superficial layers, whereas the laminar distribution of GAT1-positive cartridges did not change. Together, these findings suggest that the maturation of inhibitory inputs to the AIS of PFC pyramidal neurons is a complex process that may differentially affect the firing patterns of subpopulations of pyramidal neurons at specific postnatal time points.

ATP modulation of excitatory synapses onto interneurons

Inhibitory interneurons play important roles in neuronal circuits, but the synaptic mechanisms that regulate excitatory input onto interneurons remain to be fully understood. We show that ATP-gated presynaptic P2X2 channels facilitate excitatory transmission onto stratum radiatum interneurons but not onto CA1 pyramidal neurons. ATP released endogenously during carbachol-induced oscillations facilitates excitatory synapses onto interneurons. Overall, these data provide evidence for the molecular identity, synaptic function, and interneuron synapse specificity of a presynaptic neurotransmitter-gated cation channel. The findings highlight a novel form of presynaptic facilitation for hippocampal interneurons and suggest a role for extracellular ATP in neuronal networks.

Increase in prefrontal cortex serotonin 2A receptors following estrogen treatment in postmenopausal women

OBJECTIVE

This study investigated the effect of estrogen on brain serotonin 2A (5-HT(2A)) receptors in postmenopausal women and whether there was any correlation of receptor changes with cognition and mood.

METHOD

Ten postmenopausal subjects underwent positron emission tomography measurements of 5-HT(2A) receptor binding with [(18)F]deuteroaltanserin before and after estrogen replacement therapy.

RESULTS

5-HT(2A) receptor binding was significantly increased after estrogen replacement therapy in the right prefrontal cortex (right precentral gyrus [Brodmann's area 9], inferior frontal gyrus [Brodmann's area 47], medial frontal gyrus [Brodmann's area 6, 10] and the anterior cingulate cortex [Brodmann's area 32]). In the inferior frontal gyrus [Brodmann's area 44]), receptor up-regulation was correlated with change in plasma estradiol. Verbal fluency and Trail Making Test performance, but not mood, were significantly improved by estrogen without correlation with receptor changes.

CONCLUSIONS

Estrogen increases 5-HT(2A) receptor binding in human prefrontal regions.

Insulin-like growth factor 1 is essential for normal dendritic growth

This study evaluated somatic and dendritic growth of neurons in the frontoparietal cortex of Igf1-/- brains. Pyramidal neuron density was increased by approximately 25% (P =.005) and soma size reduced by approximately 10% (P <.001). Golgi staining revealed that cortical layer II-III neurons exhibited a significant reduction in dendritic length and complexity in Igf1 null mice. Dendritic spine density and presumably synaptic contacts were reduced by 16% (P =.002). Similar findings were obtained for cortical layer V and piriform cortex pyramids. Supporting a reduction in synapses, synaptotagmin levels were reduced by 30% (P <.02) in the Igf1 null brain. Investigation of factors critically involved in dendritic growth and synaptogenesis showed an approximately 50% reduction in cortical CDC42 protein expression (P <.001) and an approximately 10% reduction in brain cholesterol levels (P <.01) in Igf1 null mice. Evidence is presented that Igf1 deletion causes disruptions in lipid and microtubule metabolism, leading to impaired neuronal somatic and dendritic growth. Published 2003 Wiley-Liss, Inc.

Ultrastructural localization of serotonin2A receptors in the middle layers of the rat prelimbic prefrontal cortex

Cortical serotonin(2A) receptors are hypothesized to be involved in the pathology and treatment of schizophrenia. Light microscopic studies in the rat prefrontal cortex have localized serotonin(2A) receptors to the dendritic shafts of pyramidal and local circuit neurons. Electrophysiological studies have predicted that these receptors are also located on glutamate terminals, whereas neurochemical studies have hypothesized that they are located on dopamine terminals in this area. The present study sought to determine the ultrastructural localization of immunoperoxidase labeling for serotonin(2A) receptors in the middle layers of the prelimbic portion of the rat prefrontal cortex. Serotonin(2A) receptor immunoreactivity was observed in 325 identifiable structures. Of these, 73% were postsynaptic profiles that were composed of either dendritic shafts (58%) or dendritic spine heads and necks (42%). Twenty-four percent of the labeled profiles were presynaptic axons and varicosities; most of these had morphological features that were characteristic of monoamine axons: thin diameter, lack of myelination, occasional content of dense-cored vesicles, and infrequent formation of synapses in single sections. The remainder of the labeled profiles (4%) were glial processes. These findings suggest that serotonin(2A) receptor-mediated effects within the rat prelimbic prefrontal cortex are primarily postsynaptic in nature, affecting both the spines of pyramidal cells and the dendrites of pyramidal and local circuit neurons in this area. The results further suggest that serotonin acts presynaptically via this receptor subtype, most likely at receptors on monoamine fibers, and only rarely directly on glutamate axons.

Nectin-dependent localization of ZO-1 at puncta adhaerentia junctions between the mossy fiber terminals and the dendrites of the pyramidal cells in the CA3 area of adult mouse hippocampus

Nectin and afadin constitute a novel intercellular adhesion system that organizes adherens junctions in cooperation with the cadherin-catenin system in epithelial cells. Nectin is a Ca(2+)-independent immunoglobulin-like adhesion molecule and afadin is an actin filament (F-actin)-binding protein that connects nectin to the actin cytoskeleton. At the puncta adhaerentia junctions (PAs) between the mossy fiber terminals and the dendrites of the pyramidal cells in the CA3 area of the adult mouse hippocampus, the nectin-afadin system also colocalizes with the cadherin-catenin system and has a role in the formation of synapses. ZO-1 is another F-actin-binding protein that localizes at tight junctions (TJs) and connects claudin to the actin cytoskeleton in epithelial cells. The nectin-afadin system is able to recruit ZO-1 to the nectin-based cell-cell adhesion sites in nonepithelial cells that have no TJs. In the present study, we investigated the localization of ZO-1 in the mouse hippocampus. Immunofluorescence and immunoelectron microscopy revealed that ZO-1 also localized at the PAs between the mossy fiber terminals and the dendrites of the pyramidal cells in the CA3 area of the adult mouse hippocampus, as described for afadin. ZO-1 colocalized with afadin during the development of synaptic junctions and PAs. Microbeads coated with the extracellular fragment of nectin, which interacts with cellular nectin, recruited both afadin and ZO-1 to the bead-cell contact sites in cultured rat hippocampal neurons. These results indicate that ZO-1 colocalizes with nectin and afadin at the PAs and that the nectin-afadin system is involved in the localization of ZO-1.

The alpha-latrotoxin mutant LTXN4C enhances spontaneous and evoked transmitter release in CA3 pyramidal neurons

Alpha-latrotoxin (LTX) stimulates vesicular exocytosis by at least two mechanisms that include (1) receptor binding-stimulation and (2) membrane pore formation. Here, we use the toxin mutant LTX(N4C) to selectively study the receptor-mediated actions of LTX. LTX(N4C) binds to both LTX receptors (latrophilin and neurexin) and greatly enhances the frequency of spontaneous and miniature EPSCs recorded from CA3 pyramidal neurons in hippocampal slice cultures. The effect of LTX(N4C) is reversible and is not attenuated by La3+ that is known to block LTX pores. On the other hand, LTX(N4C) action, which requires extracellular Ca2+, is inhibited by thapsigargin, a drug depleting intracellular Ca2+ stores, by 2-aminoethoxydiphenyl borate, a blocker of inositol(1,4,5)-trisphosphate-induced Ca2+ release, and by U73122, a phospholipase C inhibitor. Furthermore, measurements using a fluorescent Ca2+ indicator directly demonstrate that LTX(N4C) increases presynaptic, but not dendritic, free Ca2+ concentration; this Ca2+ rise is blocked by thapsigargin, suggesting, together with electrophysiological data, that the receptor-mediated action of LTX(N4C) involves mobilization of Ca2+ from intracellular stores. Finally, in contrast to wild-type LTX, which inhibits evoked synaptic transmission probably attributable to pore formation, LTX(N4C) actually potentiates synaptic currents elicited by electrical stimulation of afferent fibers. We suggest that the mutant LTX(N4C), lacking the ionophore-like activity of wild-type LTX, activates a presynaptic receptor and stimulates Ca2+ release from intracellular stores, leading to the enhancement of synaptic vesicle exocytosis.

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.

Pyramidal neuron local axon terminals in monkey prefrontal cortex: differential targeting of subclasses of GABA neurons

In monkey prefrontal cortex (PFC), approximately 50% of the local axon terminals of pyramidal neurons form synapses with the dendritic shafts of GABA neurons. Subclasses of GABA neurons can be distinguished by the presence of different calcium-binding proteins. For example, in monkey PFC, parvalbumin (PV)-containing cells comprise approximately 25% of GABA neurons and are predominantly located in layers 3b-4, whereas calretinin (CR)-containing cells, which are present in greatest density in layers 2-3a, constitute 50% of GABA neurons. Consequently, in order to determine the cell class and laminar specificity of the dendritic targets of pyramidal neuron local axon collaterals in monkey PFC area 9, we conducted ultrastructural analyses of local axon terminals labeled with the anterograde tracer, biotinylated dextran amine, and dendrites immunoreactive (IR) for PV or CR. In layer 3b, the majority of the local axon terminals targeted PV-IR dendritic shafts, whereas CR-IR dendritic shafts were targeted infrequently. This differential targeting was also present in layers 2-3a, although it was less pronounced. In addition, PV-IR dendrites had a significantly greater density of excitatory inputs than did CR-IR dendrites. These findings indicate that PV-containing interneurons, which have a potent inhibitory effect on pyramidal neurons, are selectively targeted by the excitatory local axon terminals of supragranular pyramidal neurons in monkey PFC. These connections may provide the anatomical substrate for the coordinated activity of pyramidal neurons and fast-spiking GABA neurons during working memory.

Role of protein kinase A in GABAA receptor dysfunction in CA1 pyramidal cells following chronic benzodiazepine treatment

One-week treatment with the benzodiazepine (BZ) flurazepam (FZP), results in anticonvulsant tolerance, associated with reduced GABAA receptor (GABAR) subunit protein and miniature inhibitory post-synaptic current (mIPSC) amplitude in CA1 neurons of rat hippocampus. Because protein kinase A (PKA) has been shown to modulate GABAR function in CA1 pyramidal cells, the present study assessed whether GABAR dysfunction is associated with changes in PKA activity. Two days after 1-week FZP treatment, there were significant decreases in basal (- 30%) and total (- 25%) PKA activity, and a 40% reduction in PKA RIIbeta protein in the insoluble fraction of CA1 hippocampus. The soluble component of CA1 showed a significant increase in basal (100%) but not total PKA activity. Whole-cell recording in vitro showed a 50% reduction in mIPSC amplitude in CA1 pyramidal cells, with altered sensitivity to PKA modulators. Neurons from FZP-treated rats responded to 8-bromo-cAMP with a significant increase (31%) in mIPSC amplitude. Likewise, vasoactive intestinal polypeptide (VIP), an endogenous PKA activator, caused a significant 36% increase in mIPSC amplitude in FZP-treated cells. Neither agent had a significant effect on mIPSC amplitude in control cells. This study supports a role for PKA in GABAR dysfunction after chronic FZP treatment.

Single-cell microarray analysis in hippocampus CA1: demonstration and validation of cellular heterogeneity

Laser capture microdissection in combination with microarrays allows for the expression analysis of thousands of genes in selected cells. Here we describe single-cell gene expression profiling of CA1 neurons in the rat hippocampus using a combination of laser capture, T7 RNA amplification, and cDNA microarray analysis. Subsequent cluster analysis of the microarray data identified two different cell types: pyramidal neurons and an interneuron. Cluster analysis also revealed differences among the pyramidal neurons, indicating that even a single cell type in vivo is not a homogeneous population of cells at the gene expression level. Microarray data were confirmed by quantitative RT-PCR and in situ hybridization. We also report on the reproducibility and sensitivity of this combination of methods. Single-cell gene expression profiling offers a powerful tool to tackle the complexity of the mammalian brain.

Role of the medial septum diagonal band of Broca cholinergic neurons in oestrogen-induced spine synapse formation on hippocampal CA1 pyramidal cells of female rats

Oestrogen is known to influence pyramidal cell spine synapse plasticity in the CA1 subfield of the hippocampus. Apart from direct oestrogen action on the hippocampus, oestrogen effects mediated by subcortical structures are known to be important. The purpose of this study was to investigate whether the medial septum diagonal band of Broca (MSDB) takes part in mediating oestrogen effects to the hippocampus. Special attention was given to the role of cholinergic MSDB neurons that project to the hippocampus, as a rather large population of them contains oestrogen receptors and, consequently, may be sensitive to oestrogen signals. Adult female rats were ovariectomized. Oestradiol- and cholesterol-filled cannulae (control) were implanted into the MSDB. To selectively eliminate the cholinergic population of MSDB neurons of oestrogen-treated animals, a group of rats was injected with 192 IgG-saporin (SAP) into the lateral ventricle 1 week before the cannula implant. Immunostaining with anti-choline acetyltransferase and parvalbumin (PA) showed that cholinergic but not PA-containing GABAergic neurons were substantially reduced in the MSDB of SAP rats. Comparative electron microscopic unbiased stereological analysis on the spine synapse density of CA1 area pyramidal cells was performed between all animal groups. Rats that received oestradiol-filled cannulae showed a higher (30%) spine synapse density than control animals. Oestrogen-treated rats that had received SAP treatment showed no significant difference to controls. Thus, this observation indicates that septo-hippocampal cholinergic neurons are involved in mediating oestrogen effects to the hippocampus. The relevance of this observation to mnemonic functions and Alzheimer's disease is discussed.

Distribution of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate-type glutamate receptor subunits (GluR2/3) along the ventral visual pathway in the monkey

By using immunohistochemical methods, we examined the distribution of cells expressing subunits of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)-selective glutamate receptors (GluR2/3) in the cortical areas of the occipitotemporal pathway in monkeys. GluR2/3-immunoreactive (-ir) cells were primarily pyramidal cells; this category, however, also included large stellate cells in layer IVB of the striate cortex (V1) and fusiform cells in layer VI of all the areas examined. GluR2/3 immunoreactivity differed among the areas in laminar distribution and intensity. In V1, GluR2/3-ir cells were identified mainly in layers II, III, IVB, and VI. The prestriate areas V2 and V4 and the inferior temporal areas TEO and TE contained GluR2/3-ir cells in layers II, III, and VI. In the TE, GluR2/3-ir cells were also abundant in layer V. In area 36 of the perirhinal cortex, neurons in layers II, III, V, and VI were labeled in a similar manner to the TE labeling, but with greater staining intensity and numbers, especially in layer V. Thus, GluR2/3 immunoreactivity increased rostrally along the pathway. Within V1 and V2, cells strongly stained for GluR2/3 formed clusters that colocalized with cytochrome oxidase (CO)-rich regions. These distinct laminar and regional distribution patterns of GluR2/3 expression may contribute to the specific physiological properties of neurons within various visual areas and compartments.

Differential nicotinic acetylcholine receptor subunit expression in the human hippocampus

Neuronal nicotinic acetylcholine receptors (nAChRs) are ligand-gated ion channels composed of alpha and beta subunits with specific structural, functional and pharmacological properties. In this study the distribution of alpha3, alpha4, alpha7, beta2 and beta4 nAChR subunits in the human hippocampus was investigated using immunohistochemistry. Most pyramidal neurons, pre-alpha cells of the entorhinal cortex and dentate granule cells were immunoreactive for all subunits. Small islands of alpha7 immunoreactive cells were present in the outer presubiculum. alpha4 and beta2, and alpha3, alpha4 and beta2 immunoreactive fibre tracts were present in the stratum radiatum and subiculum, respectively, suggesting nAChRs may play a role in modulating inputs to the hippocampus via Schaffer collaterals and along the perforant pathway. Some astrocytes were immunoreactive for alpha3, alpha7 and beta4 subunits. Immunoreactivity to all subunits was noted in association with blood vessels. These results indicate the involvement of multiple nAChR subtypes in the modulation of both neuronal and non-neuronal functions in the human hippocampus.

Corticothalamic cells in layers 5 and 6 of primary and secondary sensory cortex express GAP-43 mRNA in the adult rat

The expression of a presynaptic phosphoprotein, growth-associated protein (GAP)-43, is associated with synaptogenesis during development and synaptic remodeling in the adult. This study examined GAP-43 mRNA expression and distribution in primary and secondary areas of visual, auditory, and somatosensory cortex of the adult rat, by in situ hybridization with a digoxigenin-coupled mRNA probe, focusing particularly on the corticothalamic cells in layers 5 and 6. In the six cortical areas studied, GAP-43 mRNA was expressed predominantly in layers 5 and 6 and was greater in secondary than primary areas. There were densely labeled cells in layers 5 and 6 of all areas, which showed a restricted sublaminar distribution in primary areas and more even distribution in secondary areas. Combining retrograde transport of rhodamine beads with in situ hybridization in visual and auditory cortex showed that corticothalamic cells in layers 5 and 6 express GAP-43 mRNA. There are more of these GAP-43 mRNA positive corticothalamic cells in layer 5 of secondary areas than in primary areas. The evidence suggests that in the adult rat, plasticity related to GAP-43 is present in primary and secondary sensory cortex and more so in secondary areas.

Lack of thyroid hormone receptor alpha1 is associated with selective alterations in behavior and hippocampal circuits

Brain development and function are dependent on thyroid hormone (T3), which acts through nuclear hormone receptors. T3 receptors (TRs) are transcription factors that activate or suppress target gene expression in a hormone-dependent or -independent fashion. Two distinct genes, TRalpha and TRbeta, encode several receptor isoforms with specific functions defined in many tissues but not in the brain. Mutations in the TRbeta gene cause the syndrome of peripheral resistance to thyroid hormone; however, no alterations of the TRalpha gene have been described in humans. Here we demonstrate that mice lacking the TRalpha1 isoform display behavioral abnormalities of hippocampal origin, as shown by the open field and fear conditioning tests. In the open field test mutant mice revealed less exploratory behavior than wild-type mice. In the contextual fear conditioning test mutant mice showed a significantly higher freezing response than wild-type controls when tested 1 week after training. These findings correlated with fewer GABAergic terminals on the CA1 pyramidal neurons in the mutant mice. Our results indicate that TRalpha1 is involved in the regulation of hippocampal structure and function, and raise the possibility that deletions or mutations of this receptor isoform may lead to behavioral changes or even psychiatric syndromes in humans.