A new monoclonal antibody against the GABA-protein conjugate shows immunoreactivity in sensory neurons of the rat

GABA-mediated modulation of ATP-induced intracellular calcium responses in nodose ganglion neurons of the rat

We examined ATP-induced intracellular Ca(2+) ([Ca(2+)]i) responses in the neurons and satellite cells from one of the viscerosensory ganglia, the nodose ganglion (NG), as well as the GABA-mediated modulation of ATP-induced neuronal [Ca(2+)]i responses using intracellular calcium imaging. In neurons with satellite cells, ATP induced [Ca(2+)]i increases in both the neurons and satellite cells. The P2X receptor agonist, α,β-meATP, induced [Ca(2+)]i increases in neurons and this response was inhibited by the P2X receptor antagonist, PPADS. On the other hand, the P2Y receptor agonist, ADP, induced [Ca(2+)]i increases in satellite cells, and this response was inhibited by the P2Y receptor antagonist, MRS2179. RT-PCR detected the expression of P2X2, P2X3, P2Y1, and P2Y2 receptor mRNAs in NG extracts. Immunohistochemistry revealed that NG neurons and satellite cells were immunoreactive to P2X2 and P2X3, and P2Y1 and P2Y2 receptors, respectively. In isolated neurons, the ATP-evoked [Ca(2+)]i increase was inhibited by GABA. However, in neurons with satellite cells, the GABAA receptor antagonist, bicuculline, enhanced the ATP-induced [Ca(2+)]i increase in neurons. These results suggest that viscerosensory neuronal excitability may be modulated by GABA from satellite cells in NG.

Thyroid hormone-dependent development of early cortical networks: temporal specificity and the contribution of trkB and mTOR pathways

Early in neocortical network development, triiodothyronine (T3) promotes GABAergic neurons' population increase, their somatic growth and the formation of GABAergic synapses. In the presence of T3, GABAergic interneurons form longer axons and conspicuous axonal arborizations, with an increased number of putative synaptic boutons. Here we show that the increased GABAergic axonal growth is positively correlated with the proximity to non-GABAergic neurons (non-GABA). A differential innervation emerges from a T3-dependent decrease of axonal length in fields with low density of neuronal cell bodies, combined with an increased bouton formation in fields with high density of neuronal somata. T3 addition to deprived networks after the first 2 weeks of development did not rescue deficits in the GABAergic synaptic bouton distribution, or in the frequency and duration of spontaneous bursts. During the critical 2-week-period, GABAergic signaling is depolarizing as revealed by calcium imaging experiments. Interestingly, T3 enhanced the expression of the potassium-chloride cotransporter 2 (KCC2), and accelerated the developmental shift from depolarizing to hyperpolarizing GABAergic signaling in non-GABA. The T3-related increase of spontaneous network activity was remarkably reduced after blockade of either tropomyosin-receptor kinase B (trkB) or mammalian target of rapamycin (mTOR) pathways. T3-dependent increase in GABAergic neurons' soma size was mediated mainly by mTOR signaling. Conversely, the T3-dependent selective increase of GABAergic boutons near non-GABAergic cell bodies is mediated by trkB signaling only. Both trkB and mTOR signaling mediate T3-dependent reduction of the GABAergic axon extension. The circuitry context is relevant for the interaction between T3 and trkB signaling, but not for the interactions between T3 and mTOR signaling.

Cell-specific expression of calcineurin immunoreactivity within the rat basolateral amygdala complex and colocalization with the neuropeptide Y Y1 receptor

Neuropeptide Y (NPY) produces potent anxiolytic effects via activation of NPY Y1 receptors (Y1r) within the basolateral amygdaloid complex (BLA). The role of NPY in the BLA was recently expanded to include the ability to produce stress resilience and long-lasting reductions in anxiety-like behavior. These persistent behavioral effects are dependent upon activity of the protein phosphatase, calcineurin (CaN), which has long been associated with shaping long-term synaptic signaling. Furthermore, NPY-induced reductions in anxiety-like behavior persist months after intra-BLA delivery, which together indicate a form of neuronal plasticity had likely occurred. To define a site of action for NPY-induced CaN signaling within the BLA, we employed multi-label immunohistochemistry to determine which cell types express CaN and if CaN colocalizes with the Y1r. We have previously reported that both major neuronal cell populations in the BLA, pyramidal projection neurons and GABAergic interneurons, express the Y1r. Therefore, this current study evaluated CaN immunoreactivity in these cell types, along with Y1r immunoreactivity. Antibodies against calcium-calmodulin kinase II (CaMKII) and GABA were used to identify pyramidal neurons and GABAergic interneurons, respectively. A large population of CaN immunoreactive cells displayed Y1r immunoreactivity (90%). Nearly all (98%) pyramidal neurons displayed CaN immunoreactivity, while only a small percentage of interneurons (10%) contained CaN immunoreactivity. Overall, these anatomical findings provide a model whereby NPY could directly regulate CaN activity in the BLA via activation of the Y1r on CaN-expressing, pyramidal neurons. Importantly, they support BLA pyramidal neurons as prime targets for neuronal plasticity associated with the long-term reductions in anxiety-like behavior produced by NPY injections into the BLA.

Reduced GABA(A) receptor α6 expression in the trigeminal ganglion alters inflammatory TMJ hypersensitivity

Trigeminal ganglia neurons express the GABA(A) receptor subunit alpha 6 (Gabrα6) but the role of this particular subunit in orofacial hypersensitivity is unknown. In this report the function of Gabrα6 was tested by reducing its expression in the trigeminal ganglia and measuring the effect of this reduction on inflammatory temporomandibular joint (TMJ) hypersensitivity. Gabrα6 expression was reduced by infusing the trigeminal ganglia of male Sprague Dawley rats with small interfering RNA (siRNA) having homology to either the Gabrα6 gene (Gabrα6 siRNA) or no known gene (control siRNA). Sixty hours after siRNA infusion the rats received a bilateral TMJ injection of complete Freund's adjuvant to induce an inflammatory response. Hypersensitivity was then quantitated by measuring meal duration, which lengthens when hypersensitivity increases. Neuronal activity in the trigeminal ganglia was also measured by quantitating the amount of phosphorylated ERK. Rats in a different group that did not have TMJ inflammation had an electrode placed in the spinal cord at the level of C1 sixty hours after siRNA infusion to record extracellular electrical activity of neurons that responded to TMJ stimulation. Our results show that Gabrα6 was expressed in both neurons and satellite glia of the trigeminal ganglia and that Gabrα6 positive neurons within the trigeminal ganglia have afferents in the TMJ. Gabrα6 siRNA infusion reduced Gabrα6 gene expression by 30% and significantly lengthened meal duration in rats with TMJ inflammation. Gabrα6 siRNA infusion also significantly increased p-ERK expression in the trigeminal ganglia of rats with TMJ inflammation and increased electrical activity in the spinal cord of rats without TMJ inflammation. These results suggest that maintaining Gabrα6 expression was necessary to inhibit primary sensory afferents in the trigeminal pathway and reduce inflammatory orofacial nociception.

Inhibitory plasticity underlies visual deprivation-induced loss of receptive field refinement in the adult superior colliculus

Increasing evidence shows that sensory experience is not necessary for initial patterning of neural circuitry but is essential for maintenance and plasticity. We have investigated the role of visual experience in development and plasticity of inhibitory synapses in the retinocollicular pathway of an altricial rodent, the Syrian hamster. We reported previously that visual receptive field (RF) refinement in superior colliculus (SC) occurs with the same time course in long-term dark-reared (LTDR) as in normally-reared hamsters, but RFs in LTDR animals become unrefined in adulthood. Here we provide support for the hypothesis that this failure to maintain refined RFs into adulthood results from inhibitory plasticity at both pre- and postsynaptic levels. Iontophoretic application of gabazine, a GABA(A) receptor antagonist, or muscimol, a GABA(A) receptor agonist, had less of an effect on RF size and excitability of adult LTDR animals than in short-term DR animals or normal animals. Consistent with these physiological observations, the percentage of GABA-immunoreactive neurons was significantly decreased in the SC of LTDR animals compared to normal animals and to animals exposed to a normal light cycle early in development, before LTDR. Thus GABAergic inhibition in the SC of LTDR animals is reduced, weakening the inhibitory surround and contributing significantly to the visual deprivation-induced enlargement of RFs seen. Our results argue that early visually-driven activity is necessary to maintain the inhibitory circuitry intrinsic to the adult SC and to protect against the consequences of visual deprivation.

Cell-specific expression of neuropeptide Y Y1 receptor immunoreactivity in the rat basolateral amygdala

Activation of neuropeptide Y (NPY) Y1 receptors (Y1r) in the rat basolateral nuclear complex of the amygdala (BLA) produces anxiolysis and interferes with the generation of conditioned fear. NPY is important in regulating the output of the BLA, yet the cell types involved in mediating this response are currently unknown. The current studies employed multiple label immunocytochemistry to determine the distribution of Y1r-immunoreactivity (-ir) in glutamatergic pyramidal and GABAergic cell populations in the BLA using scanning laser confocal stereology. Pyramidal neurons were identified by expression of calcium-calmodulin dependent kinase II (CaMKII-ir) and functionally distinct interneuron subpopulations were distinguished by peptide (cholecystokinin, somatostatin) or calcium-binding protein (parvalbumin, calretinin) content. Throughout the BLA, Y1r-ir was predominately on soma with negligible fiber staining. The high degree of coexpression of Y1r-ir (99.9%) in CaMKII-ir cells suggests that these receptors colocalize on pyramidal cells and that NPY could influence BLA output by directly regulating the activity of these projection neurons. Additionally, Y1r-ir was also colocalized with the interneuronal markers studied. Parvalbumin-ir interneurons, which participate in feedforward inhibition of BLA pyramidal cells, represented the largest number of Y1r expressing interneurons in the BLA ( approximately 4% of the total neuronal population). The anatomical localization of NPY receptors on different cell populations within the BLA provides a testable circuit whereby NPY could modulate the activity of the BLA via actions on both projection cells and interneuronal cell populations.

Adenovector GAD65 gene delivery into the rat trigeminal ganglion produces orofacial analgesia

Background

Our goal is to use gene therapy to alleviate pain by targeting glial cells. In an animal model of facial pain we tested the effect of transfecting the glutamic acid decarboxylase (GAD) gene into satellite glial cells (SGCs) of the trigeminal ganglion by using a serotype 5 adenovector with high tropisms for glial cells. We postulated that GABA produced from the expression of GAD would reduce pain behavior by acting on GABA receptors on neurons within the ganglion.

Results

Injection of adenoviral vectors (AdGAD65) directly into the trigeminal ganglion leads to sustained expression of the GAD65 isoform over the 4 weeks observation period. Immunohistochemical analysis showed that adenovirus-mediated GAD65 expression and GABA synthesis were mainly in SGCs. GABAA and GABAB receptors were both seen in sensory neurons, yet only GABAA receptors decorated the neuronal surface. GABA receptors were not found on SGCs. Six days after injection of AdGAD65 into the trigeminal ganglion, there was a statistically significant decrease of pain behavior in the orofacial formalin test, a model of inflammatory pain. Rats injected with control virus (AdGFP or AdLacZ) had no reduction in their pain behavior. AdGAD65-dependent analgesia was blocked by bicuculline, a selective GABAA receptor antagonist, but not by CGP46381, a selective GABAB receptor antagonist.

Conclusion

Transfection of glial cells in the trigeminal ganglion with the GAD gene blocks pain behavior by acting on GABAA receptors on neuronal perikarya.

Neuronal subtype identity in the rat auditory brainstem as defined by molecular profile and axonal projection

The nuclei of the auditory brainstem harbor a diversity of neuronal cell types and are interconnected by excitatory as well as inhibitory ascending, descending, and commissural pathways. Classically, neurons have been characterized by size and shape of their cell body and by the geometry of their dendrites. Our study is based on the use of axonal tracers in combination with immunocytochemistry to identify and distinguish neuronal subtypes by their molecular signature in dorsal and ventral cochlear nucleus, lateral superior olive, medial superior olive, medial nucleus of the trapezoid body, and inferior colliculus of the adult rat. The presumed neurotransmitters glutamate, glycine, and GABA were used alongside the calcium-binding proteins parvalbumin, calretinin, and calbindin-D28k as molecular markers. Our data provide distinct extensions to previous characterizations of neuronal subtypes and reveal regularities and differences across auditory brainstem nuclei that are discussed for their functional implications.

Unique clustering of A-type potassium channels on different cell types of the main olfactory bulb

Theoretical and functional studies predicted a highly non-uniform distribution of voltage-gated ion channels on the neuronal surface. This was confirmed by recent immunolocalization experiments for Na+, Ca2+, hyperpolarization activated mixed cation and K+ channels. These experiments also indicated that some K+ channels were clustered in synaptic or non-synaptic membrane specializations. Here we analysed the subcellular distribution of Kv4.2 and Kv4.3 subunits in the rat main olfactory bulb at high resolution to address whether clustering characterizes their distribution, and whether they are concentrated in synaptic or non-synaptic junctions. The cell surface distribution of the Kv4.2 and Kv4.3 subunits is highly non-uniform. Strong Kv4.2 subunit-immunopositive clusters were detected in intercellular junctions made by mitral, external tufted and granule cells (GCs). We also found Kv4.3 subunit-immunopositive clusters in periglomerular (PGC), deep short-axon and GCs. In the juxtaglomerular region some calretinin-immunopositive glial cells enwrap neighboring PGC somata in a cap-like manner. Kv4.3 subunit clusters are present in the cap membrane that directly contacts the PGC, but not the one that faces the neuropil. In membrane specializations established by members of the same cell type, K+ channels are enriched in both membranes, whereas specializations between different cell types contain a high density of channels asymmetrically. None of the K+ channel-rich junctions showed any of the ultrastructural features of known chemical synapses. Our study provides evidence for highly non-uniform subcellular distributions of A-type K+ channels and predicts their involvements in novel forms of intercellular communication in the olfactory pathway.

Immunocytochemical demonstration of polyamines in nucleoli and nuclei

Although biochemical studies have shown that polyamines (PAs) occur in the nucleus, only few studies have examined the intranuclear distribution of these organic cations. By immunocytochemistry, we have previously demonstrated that PAs are located in ribosomes. We now show that PAs also are present in both nucleoli and nuclei of a variety of cell types. Detection of nucleolar and nuclear PAs required novel pretreatment procedures involving protease and/or DNase digestion of specimens prior to immunoreaction. Double fluorescence staining confirmed the localizations. This suggests that PAs may be important to the formation of ribosomes in nucleoli, as well as adds support to biochemical studies suggesting that PAs are involved in many biological events in the nucleus. Further biochemical studies will be needed to substantiate this hypothesis.

Immediate early gene expression invoked by electrical intracochlear stimulation in some but not all types of neurons in the rat auditory brainstem

Specific patterns of sensory activity may induce plastic remodeling of neurons and the communication network they form in the adult mammalian brain. Among the indicators for the initiation of neuronal remodeling is the expression of immediate early genes (IEGs). The IEGs c-fos and egr-1 encode transcription factors. Following spectrally and temporally precisely defined unilateral electrical intracochlear stimulation (EIS) that corresponded in strength to physiological acoustic stimuli and lasted for 2 h under anesthesia, we characterized those neuronal cell types in ventral (VCN) and dorsal cochlear nucleus (DCN), lateral superior olive (LSO) and central nucleus of the inferior colliculus (CIC) of the rat brain that expressed IEGs. We found that EIS affected only specific types of neurons. Whereas sub-populations of glutamatergic and glycinergic cells responded in all four regions, GABAergic neurons failed to do so except in DCN. Combining immunocytochemistry with axonal tracing, neurons participating in major ascending pathways, commissural cells of VCN and certain types of neurons of the descending auditory system were seen to respond to EIS with IEG expression. By contrast, principal LSO cells projecting to the contralateral CIC as well as collicular efferents of the DCN did not. In total, less than 50% of the identified neurons turned up expression of the IEGs studied. The pattern of IEG expression caused by unilateral EIS led us to suggest that dominant sensory activity may quickly initiate a facilitation of central pathways serving the active ear at the expense of those serving the unstimulated ear.

Gamma-aminobutyric acid-containing sympathetic preganglionic neurons in rat thoracic spinal cord send their axons to the superior cervical ganglion

Gamma-aminobutyric acid (GABA)-containing fibers have been observed in the rat superior cervical ganglion (SCG) and, to a lesser extent, in the stellate ganglion (STG). The aim of present study is to clarify the source of these fibers. No cell body showed mRNAs for glutamic acid decarboxylases (GADs) or immunoreactivity for GAD of 67 kDa (GAD67) in the cervical sympathetic chain. Thus, GABA-containing fibers in the ganglia are suggested to be of extraganglionic origin. GAD67-immunoreactive fibers were found not in the dorsal roots or ganglia, but in the ventral roots, so GABA-containing fibers in the sympathetic ganglia were considered to originate from the spinal cord. Furthermore, almost all GAD67-immunoreactive fibers in the sympathetic ganglia showed immunoreactivity for vesicular acetylcholine transporter, suggesting that GABA was utilized by some cholinergic preganglionic neurons. This was confirmed by the following results. 1) After injection of Sindbis/palGFP virus into the intermediolateral nucleus, some anterogradely labeled fibers in the SCG were immunopositive for GAD67. 2) After injection of fluorogold into the SCG, some retrogradely labeled neurons in the thoracic spinal cord were positive for GAD67 mRNA. 3) When the ventral roots of the eighth cervical to the fourth thoracic segments were cut, almost all GAD67- and GABA-immunoreactive fibers disappeared from the ipsilateral SCG and STG, suggesting that the vast majority of GABA-containing fibers in those ganglia were of spinal origin. Thus, the present findings strongly indicate that some sympathetic preganglionic neurons are not only cholinergic but also GABAegic.

A novel subpopulation of 5-HT type 3A receptor subunit immunoreactive interneurons in the rat basolateral amygdala

The amygdalar basolateral nuclear complex (BLC) has very high levels of the 5-HT type 3 receptor (5-HT(3)R). Previous studies have reported that 5-HT(3)R protein in the BLC is expressed in interneurons and that 5-HT(3)R mRNA is coexpressed with GABA and certain neuropeptides or calcium-binding proteins in these cells. However, there have been no detailed descriptions of the distribution of 5-HT(3)R+ neurons in the rat amygdala, and no quantitative studies of overlap of neurons expressing 5-HT(3)R protein with distinct interneuronal subpopulations in the BLC. The present investigation employed dual-labeling immunohistochemistry using antibodies to the 5-HT-3A receptor subunit (5-HT(3A)R) and specific interneuronal markers to address these questions. These studies revealed that there was a moderate density of nonpyramidal 5-HT(3A)R+ neurons in the BLC at all levels of the amygdala. In addition, immunostained cells were also seen in anterior portions of the cortical and medial nuclei. Although virtually all 5-HT(3A)R+ neurons in the BLC were GABA+, very few expressed neuropeptide or calcium-binding protein markers for individual subpopulations. The main interneuronal marker expressed by 5-HT(3A)R+ neurons was cholecystokinin (CCK), but only 8-16% of 5-HT(3)R+ neurons in the BLC, depending on the nucleus, were CCK+. Most of these CCK+/5-HT(3A)R+ double-labeled neurons appeared to belong to the subpopulation of large type L CCK+ interneurons. Very few 5-HT(3A)R+ neurons expressed calretinin, vasoactive intestinal peptide, or parvalbumin, and none expressed somatostatin or calbindin. Thus, the great majority of neurons expressing 5-HT(3A)R protein appear to constitute a previously unrecognized subpopulation of GABAergic interneurons in the BLC.

A local GABAergic system within rat trigeminal ganglion cells

We investigated the GABAergic system within the Sprague-Dawley rat (2-3-weeks old) trigeminal ganglion (TG). Reverse transcription-polymerase chain reaction (RT-PCR) analysis revealed expression of glutamate decarboxylase (GAD) 65 and GAD67 mRNAs and mRNAs encoding GABA(A) receptor subunits alpha1-6, beta1-3, gamma1-3, and delta. In situ hybridization revealed that GAD65 and GAD67 mRNAs were expressed in neuronal cell bodies but not satellite cells. Immunohistochemical analysis showed that only GAD65 was expressed in all neuronal cell bodies, and approximately 70% of all neurons exhibited GABA immunoreactivity. Satellite cells were strongly immunopositive for GABA. GABA(A) receptor alpha1, alpha5, beta2/3 and gamma1/2/3 subunit immunoreactivities were observed in the majority of neurons, but no immunoreactivity for alpha2 was observed. Two types of cells were identified in TG based on cell size and morphology, type A and B. The percentage of cells expressing alpha3, alpha4, alpha6, and delta subunits appeared to be dependent on cell size, as delta and alpha6 expression were only observed in small (B-type) neurons. In whole-cell patch clamp experiments, GABA application induced inward Cl- currents in all neurons examined. The EC50 for GABA varied from 5.3 to 240 microm, and the Hill Coefficient (nH) varied between 0.98 and 2.6 at -60 mV. We found that GABA was released from TG cells by increasing extracellular K+ concentration to 100 mm. We speculate that GABA acts as a nonsynaptically released diffusible neurotransmitter, which may modulate somatic inhibition of neurons within the TG.

Feedforward inhibitory control of sensory information in higher-order thalamic nuclei

Sensory stimuli evoke strong responses in thalamic relay cells, which ensure a faithful relay of information to the neocortex. However, relay cells of the posterior thalamic nuclear group in rodents, despite receiving significant trigeminal input, respond poorly to vibrissa deflection. Here we show that sensory transmission in this nucleus is impeded by fast feedforward inhibition mediated by GABAergic neurons of the zona incerta. Intracellular recordings of posterior group neurons revealed that the first synaptic event after whisker deflection is a prominent inhibition. Whisker-evoked EPSPs with fast rise time and longer onset latency are unveiled only after lesioning the zona incerta. Excitation survives barrel cortex lesion, demonstrating its peripheral origin. Electron microscopic data confirm that trigeminal axons make large synaptic terminals on the proximal dendrites of posterior group cells and on the somata of incertal neurons. Thus, the connectivity of the system allows an unusual situation in which inhibition precedes ascending excitation resulting in efficient shunting of the responses. The dominance of inhibition over excitation strongly suggests that the paralemniscal pathway is not designed to relay inputs triggered by passive whisker deflection. Instead, we propose that this pathway operates through disinhibition, and that the posterior group forwards to the cerebral cortex sensory information that is contingent on motor instructions.

Mechanisms of orofacial pain control in the central nervous system

Recent advances in the study of pain have revealed somatotopic- and modality-dependent processing and the integration of nociceptive signals in the brain and spinal cord. This review summarizes the uniqueness of the trigeminal sensory nucleus (TSN) in structure and function as it relates to orofacial pain control. The oral nociceptive signal is primarily processed in the rostral TSN above the obex, the nucleus principalis (Vp), and the subnuclei oralis (SpVo) and interpolaris (SpVi), while secondarily processed in the subnucleus caudalis (SpVc). In contrast, the facial nociceptive signal is primarily processed in the SpVc. The neurons projecting to the thalamus are localized mostly in the Vp, moderately in the SpVi, and modestly in the ventrolateral SpVo and the SpVc. Orofacial sensory inputs are modulated in many different ways: by interneurons in the TSN proper, through reciprocal connection between the TSN and rostral ventromedial medulla, and by the cerebral cortex. A wide variety of neuroactive substances, including substance P, gamma-aminobutyric acid, serotonin and nitric oxide (NO) could be involved in the modulatory functions of these curcuits. The earliest expression of NO synthase (NOS) in the developing rat brain is observed in a discrete neuronal population in the SpVo at embryonic day 15. NOS expression in the SpVc is late at postnatal day 10. The neurons receiving intraoral signals are intimately related with the sensorimotor reflexive function through the SpVo. In summary, a better understanding of the trigeminal sensory system--which differs from the spinal system--will help to find potential therapeutic targets and lend to developing new analgesics for orofacial-specific pain with high efficacy and fewer side effects.

Gamma-aminobutiric acid immunostaining in trigeminal, nodose and spinal ganglia of the cat

Gamma-aminobutyric acid (GABA) is a principal inhibitory neurotransmitter in the vertebrate nervous system. It is found mainly in local circuit neurons, but it has also been described in sensory organs and dorsal root ganglia (DRG). The present study describes the presence of GABA in primary afferent neurons of feline sensory ganglia: trigeminal ganglia (TrG), nodose ganglia (NG), and DRG. Quantitative analysis revealed that approximately 20% of the cells in the TrG, NG and DRG are GABAergic. GABA-expressing neurons varied in size. GABA-containing neuronal fibres were also observed in the neuropil. Some of these were in close apposition to both GABA-positive and GABA-negative ganglionic neuronal perikarya. The localization of GABA in small primary afferent neurons, which are considered to be nociceptors, suggests that the amino acid may function as a pain transmitter or modulator, whereas processing of other sensory modalities, such as somatosensory and proprioceptive, may also be affected by GABA.

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.

Parvalbumin-containing interneurons in the basolateral amygdala express high levels of the alpha1 subunit of the GABAA receptor

The basolateral amygdala (ABL) is essential for the amnestic effects of benzodiazepines in aversive learning tasks. Because the alpha1 subunit of the gamma-aminobutyric acid (GABA)(A) receptor is critical for these amnestic actions, knowledge of the neuronal localization of this subunit in the ABL should contribute to an understanding of the candidate neuronal mechanisms involved. To examine this question, we used dual-labeling immunohistochemical techniques to study the localization of the alpha1 subunit in the ABL. Our results suggest that the alpha1 subunit of the GABA(A) receptor is localized primarily in GABAergic interneurons in the ABL at the somal level, although the intense neuropil staining in the lateral nucleus suggests that distal dendrites of pyramidal projection neurons in this nucleus may also contain high levels of the alpha1 subunit. The great majority of alpha1-immunoreactive interneurons also exhibit immunoreactivity for the beta2/3 subunits of the GABA(A) receptor. Parvalbumin-positive (PV+) interneurons are the main interneuronal subpopulation exhibiting alpha1 immunoreactivity, but some calretinin-positive interneurons also express this subunit. These data suggest that certain subpopulations of GABAergic interneurons in the ABL, especially PV+ cells, receive a robust GABAergic innervation. Because the most likely source of this innervation is intrinsic, these results suggest that PV+ interneurons could constitute an important component of interneuronal networks in the ABL. These networks may be critical for the generation of synchronized rhythmic oscillations involved in consolidation of emotional memories. The activation of alpha1-containing GABA(A) receptors in the ABL by benzodiazepines may disrupt rhythmic oscillations critical for memory consolidation.

Age-related modifications of the morphological organization of pituicytes are associated with alteration of the GABAergic and dopaminergic innervation afferent to the neurohypophysial lobe

Ageing is known to induce a marked activation of astrocytes within various regions of the central nervous system. To date, the age-related factors responsible for these modifications are unknown. The neural lobe of the hypophysis (NL) is a particular brain region which does not contain neurons but does contain specialized astrocytes, called pituicytes, and numerous terminals of afferent axons, including (i) peptidergic neurohypophysial axons which terminate on the NL blood vessels, and (ii) axons containing both gamma amino-butyric acid (GABA) and dopamine (DA) which form contacts with pituicytes. Because evidence has recently been provided that GABA signalling mediates the morphological organization of astrocytes, the present study was designed to determine whether modifications of pituicytes during ageing were associated with modifications of the GABAergic axons innervating the NL. We show here that, in adult rats, GABA/DA axons form preferential synaptic-like contacts with pituicytes which express both GABAA and D2 dopamine receptors. We then show that, during ageing, pituicytes undergo dramatic modifications of their morphology, correlatively with marked modifications of the GABA/DA fibres innervating the NL. Lastly, in vitro experiments indicate that modifications of the morphology of pituicytes similar to those observed during ageing were obtained by incubating isolated NL of adult rats with a GABAA receptor agonist and/or a D2 dopamine receptor antagonist, whereas inverse modifications were observed when NL of aged rats were incubated with a GABAA receptor antagonist and a D2 dopamine receptor agonist. Taken together, these data suggest that the age-related morphological changes of pituicytes result from the alteration of the GABA/DAergic innervation of the NL.

Preliminary studies on GABA-immunoreactive neurons in the rat trigeminal ganglion

GABA-immunoreactive (ir) primary sensory neurons have been reported in many studies. However, the role of GABA in the primary sensory neurons and their targets is quite open to question. The present study aimed to examine the GABA-ir neurons in the rat trigeminal ganglion (TG), for the first step of functional study on them. Some neurons in the TG showed GABA-ir, which were ranged from large to small size. The total number of examined TG neurons from 6 ganglia was 2,531, of which 462 neurons (18.3%) showed GABA-ir the large subpopulation of GABA-ir TG neurons is likely to involve in nerve-muscle functions, whereas medium and small subpopulations might participate in cutaneous nociceptive sensory function. The present findings demonstrated a considerable number of sensory neurons containing GABA in the rat TG. Functional studies on GABA-ir neurons in the TG would be an interesting and important issues in future studies. The next aim of our study is to examine the size distribution of GABAergic neurons and the coexistence with other neurotransmitters in the rat TG.

GABA and GABA receptors in the central nervous system and other organs

Gamma-aminobutyrate (GABA) is a major inhibitory neurotransmitter in the adult mammalian brain. GABA is also considered to be a multifunctional molecule that has different situational functions in the central nervous system, the peripheral nervous system, and in some nonneuronal tissues. GABA is synthesized primarily from glutamate by glutamate decarboxylase (GAD), but alternative pathways may be important under certain situations. Two types of GAD appear to have significant physiological roles. GABA functions appear to be triggered by binding of GABA to its ionotropic receptors, GABA(A) and GABA(C), which are ligand-gated chloride channels, and its metabotropic receptor, GABA(B). The physiological, pharmacological, and molecular characteristics of GABA(A) receptors are well documented, and diversity in the pharmacologic properties of the receptor subtypes is important clinically. In addition to its role in neural development, GABA appears to be involved in a wide variety of physiological functions in tissues and organs outside the brain.

Comparative analysis of the chemical neuroanatomy of the mammalian trigeminal ganglion and mesencephalic trigeminal nucleus

A characteristic peculiarity of the trigeminal sensory system is the presence of two distinct populations of primary afferent neurons. Most of their cell bodies are located in the trigeminal ganglion (TG) but part of them lie in the mesencephalic trigeminal nucleus (MTN). This review compares the neurochemical content of central versus peripheral trigeminal primary afferent neurons. In the TG, two subpopulations of primary sensory neurons, containing immunoreactive (IR) material, are identified: a number of glutamate (Glu)-, substance P (SP)-, neurokinin A (NKA)-, calcitonin gene-related peptide (CGRP)-, cholecystokinin (CCK)-, somatostatin (SOM)-, vasoactive intestinal polypeptide (VIP)- and galanin (GAL)-IR ganglion cells with small and medium-sized somata, and relatively less numerous larger-sized neuropeptide Y (NPY)- and peptide 19 (PEP 19)-IR trigeminal neurons. In addition, many nitric oxide synthase (NOS)- and parvalbumin (PV)-IR cells of all sizes as well as fewer, mostly large, calbindin D-28k (CB)-containing neurons are seen. The majority of the large ganglion cells are surrounded by SP-, CGRP-, SOM-, CCK-, VIP-, NOS- and serotonin (SER)-IR perisomatic networks. In the MTN, the main subpopulation of large-sized neurons display Glu-immunoreactivity. Additionally, numerous large MTN neurons exhibit PV- and CB-immunostaining. On the other hand, certain small MTN neurons, most likely interneurons, are found to be GABAergic. Furthermore, NOS-containing neurons can be detected in the caudal and the mesencephalic-pontine junction portions of the nucleus. Conversely, no immunoreactivity to any of the examined neuropeptides is observed in the cell bodies of MTN neurons but these are encircled by peptidergic, catecholaminergic, serotonergic and nitrergic perineuronal arborizations in a basket-like manner. Such a discrepancy in the neurochemical features suggests that the differently fated embryonic migration, synaptogenesis, and peripheral and central target field innervation can possibly affect the individual neurochemical phenotypes of trigeminal primary afferent neurons.

Localization and mechanisms of action of cannabinoid receptors at the glutamatergic synapses of the mouse nucleus accumbens

Despite the role of excitatory transmission to the nucleus accumbens (NAc) in the actions of most drugs of abuse, the presence and functions of cannabinoid receptors (CB1) on the glutamatergic cortical afferents to the NAc have never been explored. Here, immunohistochemistry has been used to show the localization of CB1 receptors on axonal terminals making contacts with the NAc GABAergic neurons. Electrophysiological techniques in the NAc slice preparation revealed that cannabimimetics [WIN 55,212,2 (WIN-2) and CP55940] strongly inhibit stimulus-evoked glutamate-mediated transmission. The inhibitory actions of WIN-2 were dose-dependent (EC(50) of 293 +/- 13 nm) and reversed by the selective CB1 antagonist SR 141716A. In agreement with a presynaptic localization of CB1 receptors, WIN-2 increased paired-pulse facilitation, decreased miniature EPSC (mEPSC) frequency, and had no effect on the mEPSCs amplitude. Perfusion with the adenylate cyclase activator forskolin enhanced glutamatergic transmission but did not alter presynaptic CB1 actions, suggesting that cannabinoids inhibit glutamate release independently from the cAMP-PKA cascade. CB1 did not reduce evoked transmitter release by inhibiting presynaptic voltage-dependent Ca(2+) currents through N-, L-, or P/Q-type Ca(2+) channels, because CB1 inhibition persisted in the presence of omega-Conotoxin-GVIA, nimodipine, or omega-Agatoxin-IVA. The K(+) channel blockers 4-aminopyridine (100 micrometer) and BaCl(2) (300 micrometer) each reduced by 40-50% the inhibitory actions of WIN-2, and their effects were additive. These data suggest that CB1 receptors are located on the cortical afferents to the nucleus and can reduce glutamate synaptic transmission within the NAc by modulating K(+) channels activity.

Origins of spontaneous and noxious stimuli-evoked miniature EPSCs in substantia gelatinosa

The origins of spontaneous and noxious stimuli-evoked glutamatergic miniature excitatory postsynaptic currents (mEPSCs) in substantia gelatinosa (SG) neurons were investigated by using whole-cell voltage-clamp technique on adult rat spinal cord slice. The properties of mEPSCs of SG neurons from rats either neonatally capsaicin-treated or sciatic nerve ligated showed no difference from those of intact SG neurons, indicating independence of spontaneous mEPSCs on primary afferent fibers. In the presence of tetrodotoxin (TTX), capsaicin, which noxiously stimulated fine primary afferent fibers, caused increase of the mEPSCs frequency, but did not affect the amplitude profiles or mean amplitudes. TTX affected neither the spontaneous mEPSCs nor capsaicin-induced mEPSCs frequency increase. The results suggest that spontaneous mEPSCs in SG are mediated by presynaptic spontaneous glutamate release predominantly originating from interneuron terminals rather than from primary afferent terminals; under noxious stimulation, however, mEPSCs frequency increase is mediated by primary afferent excitation.

Neuroanatomical tracing at high resolution

Most techniques used for the study of the fiber connectivity in the central nervous system produce results which are visualized in the conventional light microscope or fluorescence microscope. Although in some cases this may be sufficient, often proof is necessary that fibers which enter a particular brain area indeed terminate here. Alternatively, it may be necessary to determine whether the axon terminals of traced fibers form synapses with specific processes of specific neurons. With the latter neurons all cellular elements are meant which can be labeled in some way. Evidence of synaptic connectivity necessitates visualization at a higher level of resolution, that is at the electron-microscopic level. In this contribution to the Special Issue we discuss several methods currently available to visualize individual tracers, and methods developed to visualize two different markers, that is one marker attached to a fiber or an axon terminal, and the second marker attached to a presumed pre- or postsynaptic neuronal element.

Distribution of parvalbumin, calretinin, and calbindin-D(28k) immunoreactivity in the rat amygdaloid complex and colocalization with gamma-aminobutyric acid

To understand the organization of inhibitory circuitries in the rat amygdala, the distribution of parvalbumin, calretinin, and calbindin immunoreactivity was investigated in the rat amygdaloid complex. Colocalization of various calcium-binding proteins with the inhibitory transmitter gamma-aminobutyric acid (GABA) was studied by using the mirror technique. Parvalbumin-immunoreactive (-ir) elements were located mostly in the deep amygdaloid nuclei, whereas the calretinin-ir and calbindin-ir staining were most intense in the cortical nuclei as well as in the central nucleus and the amygdalohippocampal area. Second, the distribution of immunopositive neurons largely parallelled the distribution of terminal and neuropil labeling. Third, immunostained neurons could be divided into four major morphologic types (types 1-4) based on the characteristics of the somata and the dendritic trees. The fourth lightly stained neuronal type that had a pyramidal GABA-negative soma was observed only in calretinin and calbindin preparations. Fourth, parvalbumin-ir terminals formed basket-like plexus and cartridges, which suggests that parvalbumin labels GABAergic inhibitory basket cells and axo-axonic chandelier cells, respectively. Colocalization studies indicated that 521 of 553 (94%) of parvalbumin-ir, 419 of 557 (75%) of calbindin-ir, and 158 of 657 (24%) of calretinin-ir neurons were GABA-positive in the deep amygdaloid nuclei. A high density of large GABA-negative calbindin-ir neurons was observed caudally in the medial division of the lateral nucleus and GABA-negative calretinin-ir neurons were observed in the magnocellular division of the accessory basal nucleus as well as in the intermediate and parvicellular divisions of the basal nucleus. These data suggest that in various amygdaloid areas, neuronal excitability is controlled by GABAergic neurons that contain different calcium-binding proteins. The appearance of basket-like plexus and cartridges in the parvalbumin preparations, but not in calretinin preparations, suggests that like in the hippocampus, the distribution of inhibitory terminals in the dendritic and perisomatic regions of postsynaptic neurons in the rat amygdala is organized in a topographic manner.

Calretinin in the entorhinal cortex of the rat: distribution, morphology, ultrastructure of neurons, and co-localization with gamma-aminobutyric acid and parvalbumin

Calretinin is a marker that differentially labels neurons in the central nervous system. We used this marker to distinguish subtypes of neurons within the general population of neurons in the entorhinal cortex of the rat. The distribution, morphology, and ultrastructure of calretinin-immunopositive neurons in this cortical area were documented. We further analyzed the co-localization of the marker with gamma-aminobutyric acid (GABA) and studied whether calretinin-positive neurons project to the hippocampal formation. Methods used included single-label immunocytochemistry at the light and electron microscopic level, retrograde tracing combined with immunocytochemistry, and double-label confocal laser scanning microscopy (CLSM). The entorhinal cortex contained calretinin-positive cells in a scattered fashion, in all layers except layer IV (lamina dissecans). Bipolar and multipolar dendritic configurations were present, displaying smooth dendrites. Bipolar cells had a uniform morphology whereas the multipolar calretinin cell population consisted of large neurons, cells with long ascending dendrites, horizontally oriented neurons, and small spherical cells. Retrograde tracing combined with immunocytochemistry showed that calretinin is not present in cells projecting to the hippocampus. Few synapic contacts between calretinin-positive axon terminals and immunopositive cell bodies and dendrites were seen. Most axon terminals of calretinin fibers formed asymmetrical synapses, and immunopositive axons were always unmyelinated. Results obtained in the CLSM indicate that calretinin co-exists in only 18-20% of the GABAergic cell population (mostly small spherical and bipolar cells). Thus, the entorhinal cortex contains two classes of calretinin interneurons: GABA positive and GABA negative. The first class is presumably a classical, GABAergic inhibitory interneuron. The finding of calretinin-immunoreactive axon terminals with asymmetrical synapses suggests that the second class of calretinin neuron is a novel type of a (presumably excitatory) interneuron.

Testing the disinhibition hypothesis of epileptogenesis in vivo and during spontaneous seizures

The "disinhibition" hypothesis contends that (1) seizures begin when granule cells in the dentate gyrus of the dorsal hippocampus are disinhibited and (2) disinhibition occurs because GABAergic interneurons are excessively inhibited by other GABAergic interneurons. We tested the disinhibition hypothesis using the experimental model that inspired it-naturally epileptic Mongolian gerbils. To determine whether there is an excess of GABAergic interneurons in the dentate gyrus of epileptic gerbils, as had been reported previously, GABA immunocytochemistry, in situ hybridization of GAD67 mRNA, and the optical fractionator method were used. There were no significant differences in the numbers of GABAergic interneurons. To determine whether granule cells in epileptic gerbils were disinhibited during the interictal period, IPSPs were recorded in vivo with hippocampal circuits intact in urethane-anesthetized gerbils. The reversal potentials and conductances of IPSPs in granule cells in epileptic versus control gerbils were similar. To determine whether the level of inhibitory control in the dentate gyrus transiently decreases before seizure onset, field potential responses to paired-pulse perforant path stimulation were obtained from the dorsal hippocampus while epileptic gerbils experienced spontaneous seizures. Evidence of reduced inhibition was found after, but not before, seizure onset, indicating that seizures are not triggered by disinhibition in this region. However, seizure-induced depression of inhibition may amplify and promote the spread of seizure activity to other brain regions. These findings do not support the disinhibition hypothesis and suggest that in this model of epilepsy seizures initiate by another mechanism or at a different site.

Human NT2 neurons express a large variety of neurotransmission phenotypes in vitro

The NT2 cell line, which was derived from a human teratocarcinoma, exhibits properties that are characteristic of a committed neuronal precursor at an early stage of development. NT2 cells can be induced by retinoic acid to differentiate in vitro into postmitotic central nervous system (CNS) neurons (NT2-N cells). The commitment of NT2-N cells to a stable neuronal phenotype is irreversible. Because it may be possible to transplant these human neurons to compensate for neuronal loss after traumatic injuries or neurodegenerative diseases of the CNS, knowledge of their phenotype is essential. This study aimed to characterize in detail the neurotransmission phenotype of NT2-N cells by using immunocytochemical methods. Single peroxidase immunostaining demonstrated that NT2-N cells expressed the gamma-aminobutyric acidergic (GABAergic), catecholaminergic, and cholinergic phenotypes to a large extent and expressed the serotonergic phenotype to a minor extent. NT2-N cells also expressed different neuropeptides, such as neuropeptide Y, oxytocin, vasopressin, calcitonin gene-related peptide, and Met- and Leu-enkephalin. Double fluorescence immunostaining further indicated that a large number of NT2-N cells could express GABA and another neurotransmitter or neuropeptide at the same time. Finally, electron microscopy demonstrated that these NT2 neurons elaborate classical synaptic contacts. The multipotentiality of these neurons, combined with their apparent functionality, suggests that they may represent useful material for a variety of therapeutic approaches aimed at replacing dead neurons after neurodegenerative diseases or lesions of the CNS.

Sparse colocalization of somatostatin- and GABA-immunoreactivity in the entorhinal cortex of the rat

We studied the regional and laminar distribution of neurons expressing immunoreactivity with antibodies against the neuropeptide somatostatin (SOM) in the entorhinal cortex of colchicine-treated rats. We further determined whether these neurons also express immunoreactivity with antibodies against the neurotransmitter gamma-aminobutyric acid (GABA). Frontally and horizontally cut brain sections were subjected to double immunofluorescence histochemistry and investigated in a two-laser confocal laser scanning fluorescence microscope. The exact position of each single- or double-labeled cell was obtained via the preparation of large-scale digital fluorescence images superimposed on a brightfield digital image obtained postscanning after decoverslipping and staining with cresyl violet. Three types of SOM-positive cells were found: big multipolar cells (10-15% of the SOM-positive cells), oval cells (15-20%), and small spherical cells (majority of SOM-positive cells). Most cells were seen in layer III. In addition, we found immunoreactive cells in the other layers, with the fewest cells in layers I and IV (lamina dissecans). Of the SOM-positive cells, 18% also expressed GABA immunoreactivity; of the GABA-positive cells, 8% were also immunoreactive for SOM. Double-labeled cells were mostly small spherical cells and, infrequently, multipolar. These data indicate that in the entorhinal cortex, a large proportion of the cells belonging to the SOM population do not express GABA. We speculate that there may be several subpopulations of SOM cells, of which the largest may consist of non-GABAergic, excitatory interneurons.

Brainstem viscerotopic organization of afferents and efferents involved in the control of swallowing

Cholera toxin horseradish peroxidase (CT-HRP), a sensitive antegrade and retrograde tracer, is effective at labeling swallowing motoneurons and their dendritic fields within the nucleus ambiguus (NA), nucleus of the solitary tract (NTS), dorsal motor nucleus of the vagus nerve, and hypoglossal nucleus. Using this tracer to label motoneurons within the NTS demonstrates that palatal, pharyngeal, and laryngeal afferents overlap considerably within the interstitial and intermediate subnuclei. These afferents have a pattern of distribution within the NTS similar to the labeling observed after application of the same tracer to the superior laryngeal nerve. Esophageal afferents, however, terminate entirely within the central (NTScen) subnucleus and do not overlap their distribution with palatal, pharyngeal, or laryngeal afferents. Within the nodose ganglion (NG), sensory neurons projecting to the soft palate and pharynx are located superiorly, and those projecting to the esophagus and stomach are located inferiorly, an organization that indicates rostrocaudal positioning along the alimentary tract. Sensory neurons within the NG and NTS contain, among others, the major excitatory and inhibitory amino acid neurotransmitters glutamate (Glu) and gamma-aminobutyric-acid (GABA). Both Glu and GABA help to coordinate esophageal peristalsis. Using pseudorabies virus as a transsynaptic tracer demonstrates the role of GABA and Glu as mediators of synaptic transmission within the swallowing central pattern generator, a fact further supported by the presence of specific receptors for each neurotransmitter within the NTScen. Anatomic studies using CT-HRP have been effective in revealing the total extent of extranuclear dendritic projections and the organization of dendrites within the confines of a nucleus; further studies have produced the following data. Motoneurons innervating the soft palate, pharynx, larynx, and cervical esophagus have extensive dendrites that extend into the adjacent reticular formation with a distinct pattern for each muscle group. Motoneurons of the musculature active during the buccopharyngeal phase of swallowing (soft palate, pharynx, cricothyroid, and cervical esophagus) have extensive dendritic arborizations that terminate within the adjacent reticular formation of the NA. Swallowing premotor neurons located in the reticular formation surrounding the NA are active during the buccopharyngeal phase of swallowing. These data provide an anatomic basis for interaction of swallowing motoneurons with premotor neurons located in this area. Motoneurons innervating all levels of the esophagus are confined to the compact formation (NAc), whereas those motoneurons projecting to the pharynx and cricothyroid muscle are located in the semicompact formation (NAsc). The intrinsic laryngeal muscles were represented within the loose formation (NAI) and the heart within the external formation. In contrast, the dendrites of motoneurons projecting to the thoracic and subdiaphragmatic esophagus are confined to the NAc. Both the NAsc and NAc have extensive longitudinal bundling of dendrites within the confines of the nucleus, resulting in the formation of a rostrocaudal dendritic plexus where dendrites crisscross between bundles. Intranuclear bundling of dendrites is evident in the soft palate, pharynx, and esophagus and is lacking only for the cricothyroid muscle. Moreover, ventrolateral- and dorsomedial-oriented dendritic bundles are present within the NAsc. In contrast to the longitudinal dendritic bundles, the ventrolateral- and dorsomedial-oriented dendritic bundles exit the NAsc and penetrate the adjacent reticular formation. The extensive bundling of motoneuronal dendrites within the NA supports the hypothesis that these structures serve as networks for the generation of complex motor activities, such as swallowing.

Distribution of parvalbumin immunoreactivity in the brain of the tench (Tinca tinca L., 1758)

The distribution of parvalbumin (PV) immunoreactivity in the tench brain was examined by using the avidin-biotin-peroxidase immunocytochemical method. This protein was detected in neuronal populations throughout all main divisions of the tench brain. In the telencephalic hemispheres, PV-immunopositive neurons were distributed in both the dorsal and ventral areas, being more abundant in the area ventralis telencephali, nucleus ventralis. In the diencephalon, the scarce distribution of PV-containing cells followed a rostrocaudal gradient, and the most evident staining was observed in the nucleus periventricularis tuberculi posterioris and in a few nuclei of the area praetectalis. In the mesencephalon, abundant PV-immunoreactive elements were found in the tectum opticum, torus semicircularis, and tegmentum. In the tectum opticum, PV-immunoreactivity presented a laminar distribution. Three PV-containing neuronal populations were described in the torus semicircularis, whereas in the tegmentum, the PV staining was mainly located in the nucleus tegmentalis rostralis and in the nucleus nervi oculomotorii. In the metencephalon, Purkinje cells were PV-immunopositive in the valvula cerebelli, lobus caudalis cerebelli, and in the corpus cerebelli. In the myelencephalon, PV immunoreactivity was abundant in the nucleus lateralis valvulae, in the nucleus nervi trochlearis, nucleus nervi trigemini, nucleus nervi abducentis, nucleus nervi glossopharyngei, and in the formatio reticularis. Mauthner cells were also PV immunostained. By contrast to other vertebrate groups, only a restricted population of PV-containing neurons was GABA-immunoreactive in the tench, demonstrating that this calcium-binding protein cannot be considered a marker for GABAergic elements in the teleost brain. This study demonstrates a low phylogenetic conservation of the distribution of PV comparing teleosts and tetrapods.

Adrenergic alpha2C-receptors reside in rat striatal GABAergic projection neurons: comparison of radioligand binding and immunohistochemistry

We have investigated the distribution of alpha2c-adrenergic receptors in the rat striatum and characterized the striatal neuron types expressing these receptors. Sequential double-labelled immunocytochemistry was performed with a polyclonal antibody against rat alpha2c-adrenoceptors and antibodies against GABA, Calbindin-D28k, parvalbumin and calretinin. The subregional distribution of alpha2c-adrenoceptor binding sites in the striatum was also quantitatively investigated using selective radioligands. Almost all lightly stained striatal GABAergic neurons, with the morphological characteristics of medium-sized spiny projection neurons (94% of GABAergic cells counted), contained alpha2c-adrenoceptor-immunoreactive structures. Intensely labelled GABAergic inteneurons (6%) were devoid of alpha2c-adrenoceptor immunoreactivity. The co-localization of calbindin- and alpha2c-adrenoceptor immunoreactivity in the majority of the cells confirmed the presence of alpha2c-adrenoceptors in the population of medium-sized spiny neurons. Furthermore, the alpha2c-adrenoceptor/calbindin double-labelling disclosed the existence of three neuronal subsets in the matrix compartment of the striatum: a large proportion (83%) of double-labelled neurons, a population of neurons (8%) that exhibited only alpha2c-adrenoceptor immunoreactivity without calbindin immunoreactivity, and a population of neurons (9%) immunoreactive for calbindin, but lacking alpha2c-adrenoceptors. In addition, alpha2c-adrenoceptor immunolabelled neurons were observed in calbindin-free striatal patches. Parvalbumin- and calretinin-positive neurons never displayed alpha2c-adrenoceptor immunoreactivity, confirming that striatal GABAergic interneurons are devoid of alpha2c-adrenoceptors. The present findings indicate that alpha2c-adrenoceptors are localized in GABAergic medium-sized spiny projection neurons but not in interneurons of the rat striatum, and that they may modulate both the direct and indirect pathways of the basal ganglia, as well as participate in the regulation of mesencephalic dopaminergic neurons.

Subpopulations of GABAergic and non-GABAergic rat dorsal horn neurons express Ca2+-permeable AMPA receptors

Subpopulations of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptors that are either permeable or impermeable to Ca2+ are expressed on dorsal horn neurons in culture. While both mediate synaptic transmission, the Ca2+ -permeable AMPA receptors provide a Ca2+ signal that may result in a transient change in synaptic strength [Gu, J.G., Albuquerque, C., Lee, C.J. & MacDermott, A.B. (1996) Nature, 381, 793]. To appreciate the relevance of these receptors to dorsal horn physiology, we have investigated whether they show selective expression in identified subpopulations of dorsal horn neurons. Expression of Ca2+-permeable AMPA receptors was assayed using the kainate-induced cobalt loading technique first developed by Pruss et al. [Pruss, R.M., Akeson, R.L., Racke, M.M. & Wilburn, J.L. (1991) Neuron, 7, 509]. Subpopulations of dorsal horn neurons were identified using immunocytochemistry for gamma-aminobutyric acid (GABA), glycine, substance P receptor (NK1 receptor) and the Ca2+-binding proteins, calretinin and calbindin D28K. We demonstrate that, in dorsal horn neurons in culture, kainate-induced cobalt uptake is selectively mediated by Ca2+-permeable AMPA receptors, and that a majority of GABA and NK1 receptor-expressing neurons express Ca2+-permeable AMPA receptors. GABAergic dorsal horn neurons are important in local inhibition as well as in the regulation of transmitter release from primary afferent terminals. NK1 receptor-expressing dorsal horn neurons include many of the projection neurons in the nociceptive spino-thalamic pathway. Thus, we have identified two populations of dorsal horn neurons representing important components of dorsal horn function that express Ca2+-permeable AMPA receptors. Furthermore, we show that several subpopulations of putative excitatory interneurons defined by calretinin and calbindin expression do not express Ca2+-permeable AMPA receptors.

Development of inhibitory circuitry in visual and auditory cortex of postnatal ferrets: immunocytochemical localization of GABAergic neurons

The goal of this study was to describe the development of gamma-aminobutyric acid (GABA)-containing neurons in visual and auditory cortex of ferrets. The laminar and tangential distribution of neurons containing excitatory, inhibitory, and neuromodulatory substances constrain the potential circuits which can form during development. Ferrets are born at an early stage of brain development, allowing examination of inhibitory circuit formation in cerebral cortex prior to thalamocortical ingrowth and cortical plate differentiation. Immunocytochemically labelled nonpyramidal GABA neurons were present from postnatal day 1 throughout development, in all cortical layers, and generally followed the inside-out pattern of neuronal migration into the cortical plate. Prior to postnatal day 14, pyramidal neurons with transient GABA immunoreactivity were also observed. The density of Nissl-stained and GABA-immunoreactive neurons was high early in development, declined markedly by postnatal day 20, then remained relatively constant until adulthood. However, examination of the proportion of GABA neurons revealed an unexpected late peak at postnatal day 60, then a decrease in adulthood. Visual and auditory cortex were similar in most respects, but the peak at postnatal day 60 and the final proportion of GABA neurons was higher in auditory cortex. The late peak suggests that inhibitory circuitry is stabilized relatively late in sensory cortical development, and thus that GABA neurons could provide an important substrate for experience-dependent plasticity at late stages of development.

Activation of peripheral GABAA receptors inhibits temporomandibular joint-evoked jaw muscle activity

We have previously shown that injection of mustard oil or glutamate into rat temporomandibular joint (TMJ) tissues, an experimental model of acute TMJ injury, can reflexly induce a prolonged increase in the activity of both digastric (jaw-opener) and masseter (jaw-closer) muscles. In this study, GABA was applied to the TMJ region by itself or in combination with glutamate, and the magnitude of evoked jaw muscle electromyographic (EMG) activity was measured. Application of GABA alone to the TMJ region did not evoke significant jaw muscle EMG activity when compared with normal saline controls. In contrast, co-application of GABA and glutamate into the TMJ region decreased the magnitude of glutamate-evoked EMG activity. This GABA-mediated inhibition of glutamate-evoked EMG activity followed an inverse dose-response relationship with an estimated median inhibitory dose (ID50) of 0.17 +/- 0.05 (SE) micromol and 0.031 +/- 0.006 micromol for the digastric and masseter muscles, respectively. Co-administration of the GABAA receptor antagonist bicuculline (0.05 micromol) but not the GABAB receptor antagonist phaclofen (0.05 or 0. 15 micromol) reversed the suppressive actions of GABA, indicating that this action of GABA may be mediated by peripheral GABAA receptors located within the TMJ region. Our results suggest that activation of peripheral GABAA receptors located within the TMJ region could act to decrease the transmission of nociceptive information.

The ontogenic appearance of tyrosine hydroxylase-, serotonin-, gamma-aminobutyric acid-, calcitonin gene-related peptide-, substance P-, and synaptophysin-immunoreactivity in rat pituitary gland

The initial appearance of tyrosine hydroxylase (TH)-, serotonin (5-HT)-, gamma-aminobutyric acid (GABA)-, calcitonin gene-related peptide- (CGRP), substance P-, and synaptophysin-immunoreactivity in the rat pituitary gland, and in the related brain regions was investigated. Several groups of TH-immunoreactive neurons were first detected in the brain stem on day E17, and in the hypothalamus on day E18, followed by TH-immunoreactivity in the median eminence and infundibulum on E19-E20. TH-positive fibers appeared in the posterior lobe on day E20 and in the intermediate lobe on day P0. 5-HT-immunoreactivity was first detected on day E17 in neurons and nerve fibers in the brain stem and in the median eminence, respectively. On day E18, a few 5-HT-immunoreactive fibers were detected in the posterior lobe of the pituitary, although they were consistently seen in the infundibulum from day E19. In newborn rats, some 5-HT-immunoreactive fibers, but no neurons, were seen in the hypothalamus. GABA immunoreactivity appeared on day E17 in several nerve fibers of the infundibulum and the posterior lobe. Some neurons in the cortex and ventral hypothalamus transiently expressed GABA-immunoreactivity on day E17. In newborn rats, a plexus of GABA-immunoreactive fibers was detected for the first time in the intermediate lobe. No CGRP-immunoreactive fibers could be detected in the prenatal pituitary. On day P10, CGRP-immunoreactive fibers were first observed in the anterior lobe. Later their number considerably increased, while only sporadic fibers could be found in the intermediate or posterior lobes. No substance P-immunoreactivity could be detected in any of the lobes in the embryonic or developing postnatal rat pituitary, instead the adult anterior lobe occasionally showed some substance P-immunoreactive fibers. Synaptophysin-immunoreactivity was first detected in the posterior lobe on day E20, followed shortly by its expression in the intermediate lobe in newborn rats. The time course of GABA and 5-HT expression revealed in the present study suggests that these transmitters, which are initially expressed in the developing pituitary clearly before synaptic maturation, may act as trophic molecules during the prenatal period.

Monoclonal antibody monospecific to glycine for brain immunocytochemistry

We have developed mouse monoclonal antibodies (AGLY-1-8, all IgG1 subisotype mAbs) against glycine (Gly) conjugated to bovine serum albumin using glutaraldehyde (GA)-NaBH4. Among these, AGLY-4 mAb was found to be the most useful for Gly immunocytochemistry (ICC) in functions of specificity and sensitivity without non-specific immunobinding. AGLY-4 was demonstrated to be monospecific to Gly by an enzyme-linked immunosorbent assay (ELISA) binding test, and not reactive to any of the other amino acids and peptides tested. Using this antibody, indirect immunoperoxidase staining was observed in different regions of the rat brain fixed with GA in combination with borohydride reduction. In contrast, immunoreactivity was quite low in tissues fixed only with GA. Absorption controls indicated that the immunostaining could be completely inhibited by 5 microg/ml of Gly-human serum albumin (HSA) conjugate prepared using GA and NaBH4, which was consistent with the results of an ELISA inhibition test. No cross-reaction occurred with other GA-conjugated amino acids. Dense ICC staining was observed in the rat neurons related to the auditory and vestibular centers, and modest immunostaining was seen in all the structures of the cerebellar cortex except for the Golgi cells which were strongly stained. These results were in complete agreement with the previous methods using polyclonal anti-Gly serum. Also, a new finding was that staining was noticed in certain cells widely distributed in the different brain regions. These results strongly suggest that the monoclonal antibody has a potential for elucidating the precise distribution of Gly-containing cells.

Neurochemical features and synaptic connections of large physiologically-identified GABAergic cells in the rat frontal cortex

Physiological and morphological properties of large non-pyramidal cells immunoreactive for cholecystokinin, parvalbumin or somatostatin were investigated in vitro in the frontal cortex of 18-22-day-old rats. These three peptides were expressed in separate populations including large cells. Cholecystokinin cells and parvalbumin cells made boutons apposed to other cell bodies, but differed in their firing patterns in response to depolarizing current pulses. Parvalbumin cells belonged to fast-spiking cells. Parvalbumin fast-spiking cells also included chandelier cells. In contrast, cholecystokinin cells were found to be regular-spiking non-pyramidal cells or burst-spiking non-pyramidal cells with bursting activity from hyperpolarized potentials (two or more spikes on slow depolarizing humps). Large somatostatin cells belonged to the regular-spiking non-pyramidal category and featured wide or ascending axonal arbors (wide arbor cells and Martinotti cells) which did not seem to be apposed to the somata so frequently as large cholecystokinin and parvalbumin cells. For electron microscopic observations, another population of eight immunohistochemically-uncharacterized non-pyramidal cells were selected: (i) five fast spiking cells including one chandelier cell which are supposed to contain parvalbumin, and (ii) three large regular-spiking non-pyramidal cells with terminals apposed to somata, which are not considered to include somatostatin cells, but some of which may belong to cholecystokinin cells. The fast-spiking cells other than a chandelier cell and the large regular-spiking non-pyramidal cells made GABA-positive synapses on somata (4% and 12% of the synapses in two small to medium fast-spiking cells, 22% and 35% of the synapses in two large fast-spiking cells, and 10%, 18% and 37% of the synapses in three large regular-spiking non-pyramidal cells). A few terminals of the fast-spiking and regular-spiking non-pyramidal cells innervated GABAergic cells. About 30% of the fast-spiking cell terminals innervated spines, but few of the regular-spiking non-pyramidal cell terminals did. A fast-spiking chandelier cell made GABA-positive synapses on GABA-negative axon initial segments. These results suggest that large GABAergic cells are heterogeneous in neuroactive substances, firing patterns and synaptic connections, and that cortical cells receive heterogeneous GABAergic somatic inputs.

Enkephalin-containing interneurons are specialized to innervate other interneurons in the hippocampal CA1 region of the rat and guinea-pig

Enkephalins are known to have a profound effect on hippocampal inhibition, but the possible endogenous source of these neuropeptides, and their relationship to inhibitory interneurons is still to be identified. In the present study we analysed the morphological characteristics of met-enkephalin-immunoreactive cells in the CA1 region of the rat and guinea-pig hippocampus, their coexistence with other neuronal markers and their target selectivity at the light and electron microscopic levels. Several interneurons in all subfields of the hippocampus were found to be immunoreactive for met-enkephalin. In the guinea-pig, fibres arising from immunoreactive interneurons were seen to form a plexus in the stratum oriens/alveus border zone, and basket-like arrays of boutons on both enkephalin-immunoreactive and immunonegative cell bodies in all strata. Immunoreactive boutons always established symmetric synaptic contacts on somata and dendritic shafts. Enkephalin-immunoreactive cells co-localized GABA, vasoactive intestinal polypeptide and calretinin. Postembedding immunogold staining for GABA showed that all the analysed enkephalin-immunoreactive boutons contacted GABAergic postsynaptic structures. In double-immunostained sections, enkephalin-positive axons were seen to innervate calbindin D28k-, somatostatin-, calretinin- and vasoactive intestinal polypeptideimmunoreactive cells with multiple contacts. Based on these characteristics, enkephalin-containing cells in the hippocampus are classified as interneurons specialized to innervate other interneurons, and represent a subset of vasoactive intestinal polypeptide- and calretinin-containing cells. The striking match of ligand and receptor distribution in the case of enkephalin-mediated interneuronal communication suggests that this neuropeptide may play an important role in the synchronization and timing of inhibition involved in rhythmic network activities of the hippocampus.

The absence of a major Ca2+ signaling pathway in GABAergic neurons of the hippocampus

The Ca2+/calmodulin-dependent protein phosphatase 2B or calcineurin (CN) participates in several Ca2+-dependent signal transduction cascades and, thus, contributes to the short and long term regulation of neuronal excitability. By using a specific antibody to CN, we demonstrate its absence from hippocampal interneurons and illustrate a physiological consequence of such CN deficiency. Consistent with the lack of CN in interneurons as detected by immunocytochemistry, the CN inhibitors FK-506 or okadaic acid significantly prolonged N-methyl-D-aspartate channel openings recorded in the cell-attached mode in hippocampal principal cells but not those recorded in interneurons. Interneurons were also devoid of Ca2+/calmodulin-dependent protein kinase IIalpha, yet many of their nuclei contained the cyclic AMP-responsive element binding protein. On the basis of the CN and Ca2+/calmodulin-dependent protein kinase IIalpha deficiency of interneurons, entirely different biochemical mechanisms are expected to govern Ca2+-dependent neuronal plasticity in interneurons versus principal cells.

Distinct interneuron types express m2 muscarinic receptor immunoreactivity on their dendrites or axon terminals in the hippocampus

In previous studies m2 muscarinic acetylcholine receptor-immunoreactive interneurons and various types of m2-positive axon terminals have been described in the hippocampal formation. The aim of the present study was to identify the types of interneurons expressing m2 receptor and to examine whether the somadendritic and axonal m2 immunostaining labels the same or distinct cell populations. In the CA1 subfield, neurons immunoreactive for m2 have horizontal dendrites, they are located at the stratum oriens/alveus border and have an axon that project to the dendritic region of pyramidal cells. In the CA3 subfield and the hilus, m2-positive neurons are multipolar and are scattered in all layers except stratum lacunosum-moleculare. In stratum pyramidale of the CA1 and CA3 regions, striking axon terminal staining for m2 was observed, surrounding the somata and axon initial segments of pyramidal cells in a basket-like manner. The co-localization of m2 with neurochemical markers and GABA was studied using the "mirror" technique and fluorescent double-immunostaining at the light microscopic level and with double-labelling using colloidal gold-conjugated antisera and immunoperoxidase reaction (diaminobenzidine) at the electron microscopic level. GABA was shown to be present in the somata of most m2-immunoreactive interneurons, as well as in the majority of m2-positive terminals in all layers. The calcium-binding protein parvalbumin was absent from practically all m2-immunoreactive cell bodies and dendrites. In contrast, many of the terminals synapsing on pyramidal cell somata and axon initial segments co-localized parvalbumin and m2, suggesting a differential distribution of m2 receptor immunoreactivity on the axonal and somadendritic membrane of parvalbumin-containing basket and axo-axonic cells. The co-existence of m2 receptors with the calcium-binding protein calbindin and the neuropeptides cholecystokinin and vasoactive intestinal polypeptide was rare throughout the hippocampal formation. Only calretinin and somatostatin showed an appreciable degree of co-localization with m2 (20% and 15%, respectively). Using retrograde tracing, some of the m2-positive cells in stratum oriens were shown to project to the medial septum, accouting for 38% of all projection neurons. The present results demonstrate that there is a differential distribution of m2 receptor immunoreactivity on the axonal vs the somadendritic membranes of distinct interneuron types and suggest that acetylcholine via m2 receptors may reduce GABA release presynaptically from the terminals of perisomatic inhibitory cells, while it may act to increase the activity of another class of interneuron, which innervates the dendritic region of pyramidal cells.

Two-laser dual-immunofluorescence confocal laser scanning microscopy using Cy2- and Cy5-conjugated secondary antibodies: unequivocal detection of co-localization of neuronal markers

The ability of the confocal laser scanning microscope (CLSM) to visualize in one focal plane the fluorescence associated with multiple markers renders this instrument extremely valuable for the study of co-localization of various markers in the somata and cellular processes of neurons. In the present protocol we deal with the question whether or not co-localization exists in neurons of two different neuronal markers. The conventionally used method towards answering this type of question is double-immunofluorescence microscopy. Fundamental to this approach, independent from whether the preparations are observed in a normal fluorescence microscope or in a CLSM, is that each of the applied fluorescent labels should not chemically interact with the other label or inadvertently be visible through the illumination/filter setup designed for the other fluorophore. In the field of double-label CLSM, three types of approach are distinguished: the single-laser, two-color approach, the two-laser, two-color approach, and the time-resolved approach (Brismar and Ulfhake, 1997). Each type of approach has its own advantages and disadvantages. In the instrument in our institute (a Zeiss LSM 410), combinations of fluorophores like fluorescein isothiocyanate (FITC) and tetramethyl rhodamine isothiocyanate (TRITC) are less useful, since TRITC produces a detectable signal in the FITC illumination/filter setup. Instead of experimenting with filter sets we have chosen to take two measures to eliminate this problem. Our first measure is to use fluorophores whose absorption/emission spectra overlap as little as possible. We have selected among the recently developed carbocyanine fluorophores one fluorescing in the visible range (Cy2) (green, in the same range as FITC and with much better resistance to fading than FITC; cf. Härtig et al., 1996), and another fluorescing in the near infrared range (Cy5, infrared; cf. Mesce et al., 1993). Our second measure to ensure excellent signal separation is the adoption of a two-laser, two-color approach. Co-localization of the calcium binding protein, calretinin, and a neurotransmitter, gamma-aminobutyric acid (GABA), in interneurons in the entorhinal cortex and the hippocampus of the rat was used as the principal test model. We compare the above two-laser, two-color approach with a single-laser, two-color CLSM approach using as markers Cy2 and the red fluorophore, Texas Red (physical characteristics resembling TRITC). In this paper considerable attention is paid to control experiments to verify the reliability of the staining procedure. The results show that our two-laser, two-color CLSM approach produces a complete and unambiguous separation of the fluorescent labels, Cy2 and Cy5. We are currently using this method to determine the degree of co-localization of neurochemical substances in CNS neurons.

GABAergic presubicular projections to the medial entorhinal cortex of the rat

We characterized presubicular neurons giving rise to bilateral projections to the medial entorhinal cortex (MEA) of the rat. Retrograde labeling of presubiculo-entorhinal projections with horseradish peroxidase and subsequent GABA immunocytochemistry revealed that 20-30% of the ipsilaterally projecting neurons are GABAergic. No GABAergic projections to the contralateral MEA were observed. GABAergic projection neurons were observed only in the dorsal part of the presubiculum, which, when taking into account the topography of presubicular projections to MEA, indicates that only the dorsal part of MEA receives GABAergic input. The GABAergic projection neurons constitute approximately 30-40% of all GABAergic neurons present in the superficial layers of the dorsal presubiculum. Using double-label fluorescent retrograde tracing, we found that the ipsilateral and contralateral presubiculo-entorhinal projections originate from different populations of neurons. Anterograde labeling of presubiculo-entorhinal projections and electron microscopical analysis of labeled terminals substantiated the presence of a restricted GABAergic presubiculo-entorhinal projection. A small fraction of afferents to only ipsilateral dorsal MEA formed symmetrical synapses with dendritic shafts. No symmetrical synapses on spines were noted. Most afferents to the dorsal part of ipsilateral MEA, as well as all afferents to the remaining ipsilateral and contralateral MEA, formed asymmetrical synapses with both spines and dendritic shafts in an almost equal ratio. Thus, we conclude that the majority of the presubiculo-entorhinal projections exert an excitatory effect on both principal neurons and interneurons. The projections from the dorsal part of the presubiculum comprise a small inhibitory component that originates from GABAergic neurons and targets entorhinal interneurons.

Functional GABAA receptors on rat vagal afferent neurones

1. In the present study, in vitro electrophysiology and receptor autoradiography were used to determine whether rat vagal afferent neurones possess gamma-aminobutyric acid (GABA)A receptors. 2. GABA (1-100 microM) and isoguvacine (3-100 microM) caused a concentration-dependent depolarization of the rat isolated nodose ganglion preparation at room temperature. When applied to the tissue 20 min before the agonist, SR95531 (3 microM) and bicuculline (3 microM) caused a parallel shift to the right of the GABA and isoguvacine concentration-response curves, yielding shifts of 81 fold and 117 fold for SR95531 and 4 fold and 12 fold for bicuculline, respectively. 3. Baclofen (10 nM-100 microM) was unable to elicit a depolarization of the rat isolated nodose ganglion preparation at either room temperature or at 36 degrees C, whilst 5-aminovaleric acid (10 microM), a GABAB receptor antagonist, was unable to antagonize significantly the GABA-induced depolarization at either room temperature or at 36 degrees C. 4. [3H]-SR95531 (7.2 nM), a GABAA receptor-selective antagonist, bound topographically to sections of rat brainstem. Specific binding was highest in the medial nucleus tractus solitarius (NTS) and dorsal motor nucleus of the vagus nerve (DMVN). Binding was also observed in certain medullary reticular nuclei, in particular the parvocellular reticular nucleus. 5. Unilateral nodose ganglionectomy caused a reduction in GABAA binding site density in the medial NTS from 93 +/- 7 to 68 +/- 6 d.p.m./mm2. This procedure also caused a reduction in GABAA binding site density in the side of the NTS contralateral to the lesion, from 151 +/- 12 to 93 +/- 7 d.p.m./mm2. Sham surgery had no effect on the binding of [3H]-SR95531 in rat brainstem. 6. The present data provide evidence for the presence of GABAA receptors located on the soma and central terminals of rat vagal afferent neurones. Additionally, a population of GABAA receptors is evidenced postsynaptically in the rat NTS with respect to vagal afferent terminals. These data are discussed in relation to the functional pharmacology of GABA in this region of the NTS.

Decrease in somatostatin-immunoreactive neurons in the rat amygdaloid complex in a kindling model of temporal lobe epilepsy

In human temporal lobe epilepsy, seizures can begin in the hippocampus, amygdala, or surrounding cortical areas. Histologically, the seizure-induced selective neuronal damage and synaptic reorganization are best documented in the hippocampus. Little information is available about the damage in the other temporal lobe structures or whether the distribution of damage depends on the location of the primary seizure focus. We used an amygdala-kindling model of temporal lobe epilepsy to study whether seizures of amygdaloid origin cause damage to the amygdala and hippocampus. All rats experienced five class 5 generalized seizures. Neuronal damage was assessed by counting the density of GABA-immunoreactive (GABA-ir) and somatostatin-immunoreactive (SOM-ir) neurons in the amygdala and hilus of the dentate gyrus six months after the last seizure. We found that the density of GABA-ir neurons did not differ from that in controls in the contralateral amygdala. The density of SOM-ir neurons was, however, decreased in the lateral (69% of neurons remaining, P < 0.01), basal (67% remaining, P < 0.05), and accessory basal (68% remaining, P < 0.05) nuclei. In the hilus, the densities of GABA-ir and SOM-ir neurons were similar to that in controls. According to our data, a few seizures of amygdaloid origin may cause more severe damage to SOM-ir neurons in the amygdala than in the hilus. Such decrease in SOM-ir neurons which form one subpopulation of GABAergic inhibitory interneurons may increase the local excitability in the amygdala and, therefore, contribute to epileptogenesis.