Differential Expression of the GABAA Receptor Complex in the Dorsal Thalamus and Reticular Nucleus: An Immunohistochemical Study in the Adult and Developing Rat

Thalamic inhibitory circuits and network activity development

Inhibitory circuits in thalamus and cortex shape the major activity patterns observed by electroencephalogram (EEG) in the adult brain. Their delayed maturation and circuit integration, relative to excitatory neurons, suggest inhibitory neuronal development could be responsible for the onset of mature thalamocortical activity. Indeed, the immature brain lacks many inhibition-dependent activity patterns, such as slow-waves, delta oscillations and sleep-spindles, and instead expresses other unique oscillatory activities in multiple species including humans. Thalamus contributes significantly to the generation of these early oscillations. Compared to the abundance of studies on the development of inhibition in cortex, however, the maturation of thalamic inhibition is poorly understood. Here we review developmental changes in the neuronal and circuit properties of the thalamic relay and its interconnected inhibitory thalamic reticular nucleus (TRN) both in vitro and in vivo, and discuss their potential contribution to early network activity and its maturation. While much is unknown, we argue that weak inhibitory function in the developing thalamus allows for amplification of thalamocortical activity that supports the generation of early oscillations. The available evidence suggests that the developmental acquisition of critical thalamic oscillations such as slow-waves and sleep-spindles is driven by maturation of the TRN. Further studies to elucidate thalamic GABAergic circuit formation in relation to thalamocortical network function would help us better understand normal as well as pathological brain development.

Synaptic development of the mouse dorsal lateral geniculate nucleus

The dorsal lateral geniculate nucleus (dLGN) of the mouse has emerged as a model system in the study of thalamic circuit development. However, there is still a lack of information regarding how and when various types of retinal and nonretinal synapses develop. We examined the synaptic organization of the developing mouse dLGN in the common pigmented C57/BL6 strain, by recording the synaptic responses evoked by electrical stimulation of optic tract axons, and by investigating the ultrastructure of identified synapses. At early postnatal ages (<P12), optic tract evoked responses were primarily excitatory. The full complement of inhibitory responses did not emerge until after eye opening (>P14), when optic tract stimulation routinely evoked an excitatory postsynaptic potential/inhibitory postsynaptic potential (EPSP/IPSP) sequence, with the latter having both a GABA(A) and GABA(B) component. Electrophysiological and ultrastructural observations were consistent. At P7, many synapses were present, but synaptic profiles lacked the ultrastructural features characteristic of the adult dLGN, and little gamma-aminobutyric acid (GABA) could be detected by using immunocytochemical techniques. In contrast, by P14, GABA staining was robust, mature synaptic profiles of retinal and nonretinal origin were easily distinguished, and the size and proportion of synaptic contacts were similar to those of the adult. The emergence of nonretinal synapses coincides with pruning of retinogeniculate connections, and the transition of retinal activity from spontaneous to visually driven. These results indicate that the synaptic architecture of the mouse dLGN is similar to that of other higher mammals, and thus provides further support for its use as a model system for visual system development.

Interference of ethanol and methylmercury in the developing central nervous system

Studies involving alcohol and its interactions with other neurotoxicants represent the focus of several works of research due to the fact that the use of alcohol can sometimes leads to serious health problems. Fetal exposure to alcohol and mercury has a high incidence in some regions of Brazil, where there are pregnant women who are alcoholics and live in mining areas. This work was conducted to examine the effects of combined exposure to ethanol (EtOH) and methylmercury (MeHg) in rats during the development of the central nervous system (CNS). Experimental behavioral animal models/tests were used in order to examine locomotion, anxiety, depression and memory. Pregnant rats received tap water or EtOH 22.5% w/v (6.5 g/kg per day), by gavage) during pregnancy and breast-feeding. On the 15th day of pregnancy, some groups received 8 mg/kg of MeHg (by gavage). The groups were as follows: control, EtOH, MeHg and EtOH+MeHg. The experimental results showed that the EtOH, MeHg and EtOH+MeHg groups reduced the percentage of frequency and time spent in the open arms entries of the elevated plus-maze (EPM) test, when compared to the control group. This result suggests an anxiogenic behavioral response. The MeHg group increased locomotor activity in the arena and the immobility time in the forced swimming test, suggestive of depression-like behavior. The EtOH+MeHg group showed greater reductions in the percentages of frequency and time spent in the open arms entries in the EPM test, suggesting a sedative-behavior since the frequency of enclosed arm entries was affected. In the inhibitory avoidance task, the EtOH+MeHg group reduced the latency of the step-down response onto the grid floor, suggesting a cognitive and behavior dysfunctions. Taken together, the results suggest that EtOH and/or MeHg intoxication during the developing CNS may be a risk for deficits related to locomotor impairment, anxiety, depression and neurocognitive functions. There is a possibility that EtOH may prevent some of the MeHg responses, but the precise mechanism of action involved in this process needs to be considered for future research.

GABA immunoreactivity in the developing rat thalamus and Otx2 homeoprotein expression in migrating neurons

We investigated the expression of gamma-aminobutyric acid (GABA) in the developing rat thalamus by immunohistochemistry, using light, confocal and electron microscopy. We also examined the relationship between the expression of the homeoprotein Otx2, a transcription factor implicated in brain regionalization, and the radial and non-radial migration of early generated thalamic neurons, identified by the neuronal markers calretinin (CR) and GABA. The earliest thalamic neurons generated between embryonic days (E) 13 and 15 include those of the reticular nucleus, entirely composed by GABAergic neurons. GABA immunoreactivity appeared at E14 in immature neurons and processes laterally to the neuroepithelium of the diencephalic vesicle. The embryonic and perinatal periods were characterized by the presence of abundant GABA-immunoreactive fibers, mostly tangentially oriented, and of growth cones. At E15 and E16, GABA was expressed in radially and non-radially oriented neurons in the region of the reticular thalamic migration, between the dorsal and ventral thalamic primordia, and within the dorsal thalamus. At these embryonic stages, some CR- and GABA-immunoreactive migrating-like neurons, located in the migratory stream and in the dorsal thalamus, expressed the homeoprotein Otx2. In the perinatal period, the preponderance of GABAergic neurons was restricted to the reticular nucleus and several GABAergic fibers were still detectable throughout the thalamus. The immunolabeling of fibers progressively decreased and was no longer visible by postnatal day 10, when the adult configuration of GABA immunostaining was achieved. These results reveal the spatio-temporal features of GABA expression in the developing thalamus and suggest a novel role of Otx2 in thalamic cell migration.

Alteration of GABAergic synapses and gephyrin clusters in the thalamic reticular nucleus of GABAA receptor alpha3 subunit-null mice

Multiple GABAA-receptor subtypes are assembled from alpha, beta and gamma subunit variants. GABAA receptors containing the alpha3 subunit represent a minor population with a restricted distribution in the CNS. In addition, they predominate in monoaminergic neurons and in the nucleus reticularis thalami (nRT), suggesting a role in the regulation of cortical function and sleep. Mice with a targeted deletion of the alpha3 subunit gene (alpha3(0/0)) are viable and exhibit a subtle behavioural phenotype possibly related to dopaminergic hyperfunction. Here, we investigated immunohistochemically the consequences of the loss of alpha3 subunit for maturation of GABAA receptors and formation of GABAergic synapses in the nRT. Throughout postnatal development, the regional distribution of the alpha1, alpha2, or alpha5 subunit was unaltered in alpha3(0/0) mice and the prominent alpha3 subunit staining of nRT neurons in wildtype mice was not replaced. Subcellularly, as seen by double immunofluorescence, the alpha3 and gamma2 subunit were clustered at postsynaptic sites in the nRT of adult wildtype mice along with the scaffolding protein gephyrin. In alpha3(0/0) mice, gamma2 subunit clustering was disrupted and gephyrin formed large aggregates localized at the cell surface, but unrelated to postsynaptic sites, indicating that nRT neurons lack postsynaptic GABAA receptors in mutant mice. Furthermore, GABAergic terminals were enlarged and reduced in number, suggesting a partial deficit of GABAergic synapses. Therefore, GABAA receptors are required for gephyrin clustering and long-term synapse maintenance. The absence of GABAA-mediated transmission in the nRT may have a significant impact on the function of the thalamo-cortical loop of alpha3(0/0) mice.

Influence of age and of pre-treatment with D-cycloserine on the behavior of ethanol-treated rats tested in the elevated plus-maze apparatus

There is evidence that ethanol is able to influence central functions through the antagonism of the NMDA-receptor system. It has been shown that this system is also involved in the modulation of anxiety-related behavior in rats. Recently, we observed gender- and age-related behavioral influences in rats tested on the elevated plus-maze apparatus The present study was undertaken in order to investigate: (1) the effects of ethanol (0.8, 1.0 or 1.2 g/kg, i.p.) on the behavior of male and female rats tested on the elevated plus-maze at 2, 3, 4 or 5 months of age; (2) the effect of the pre-treatment with D-cycloserine (3.0 or 6.0 mg/kg), an agonist of the glutamate NMDA-receptor system, 30 min before the ethanol (1.2 g/kg) injections, in rats tested in the elevated plus-maze at 2 months or 4 months of age. The results demonstrated that ethanol did not affect the time spent and the frequency of entries on the open arms of the elevated plus-maze in rats tested at 2 months of age, but increased these parameters in older animals. Moreover, the results showed that D-cycloserine, at doses that did not affect the behavior of control animals, antagonized the increased frequency of entries and time spent on open arms produced by ethanol in rats tested at 4 months of age. Our results suggest an age-related influence on the anxiolytic action of ethanol in rats tested in the elevated plus-maze. Moreover, the results suggest that the NMDA-receptor system can be involved in this effect, and strengthens the evidence for the participation of the NMDA-receptor system in anxiety-related behavior.

Nature of inhibitory postsynaptic activity in developing relay cells of the lateral geniculate nucleus

Using intracellular recordings in an isolated (in vitro) brain stem preparation, we examined the inhibitory postsynaptic responses of developing neurons in the dorsal lateral geniculate nucleus (LGN) of the rat. As early as postnatal day (P) 1-2, 31% of all excitatory postsynaptic (EPSP) activity evoked by electrical stimulation of the optic tract was followed by inhibitory postsynaptic potentials (IPSPs). By P5, 98% of all retinally evoked EPSPs were followed by IPSP activity. During the first postnatal week, IPSPs were mediated largely by GABA(A) receptors. Additional GABA(B)-mediated IPSPs emerged at P3-4 but were not prevalent until after the first postnatal week. Experiments involving the separate stimulation of each optic nerve indicated that developing LGN cells were binocularly innervated. At P11-14, it was common to evoke EPSP/IPSP pairs by stimulating either the contralateral or ipsilateral optic nerve. During the third postnatal week, binocular excitatory responses were encountered far less frequently. However, a number of cells still maintained a binocular inhibitory response. These results provide insight about the ontogeny and nature of postsynaptic inhibitory activity in the LGN during the period of retinogeniculate axon segregation.

Kinetic and pharmacological properties of GABA(A) receptors in single thalamic neurons and GABA(A) subunit expression

Synaptic inhibition in the thalamus plays critical roles in sensory processing and thalamocortical rhythm generation. To determine kinetic, pharmacological, and structural properties of thalamic gamma-aminobutyric acid type A (GABA(A)) receptors, we used patch-clamp techniques and single-cell reverse transcriptase polymerase chain reaction (RT-PCR) in neurons from two principal rat thalamic nuclei-the reticular nucleus (nRt) and the ventrobasal (VB) complex. Single-channel recordings identified GABA(A) channels with densities threefold higher in VB than nRt neurons, and with mean open time fourfold longer for nRt than VB [14.6 +/- 2.5 vs. 3.8 +/- 0.7 (SE) ms, respectively]. GABA(A) receptors in nRt and VB cells were pharmacologically distinct. Zn(2+) (100 microM) reduced GABA(A) channel activity in VB and nRt by 84 and 24%, respectively. Clonazepam (100 nM) increased inhibitory postsynaptic current (IPSC) decay time constants in nRt (from 44.3 to 77.9 ms, P < 0.01) but not in VB. Single-cell RT-PCR revealed subunit heterogeneity between nRt and VB cells. VB neurons expressed alpha1-alpha3, alpha5, beta1-3, gamma2-3, and delta, while nRt cells expressed alpha3, alpha5, gamma2-3, and delta. Both cell types expressed more subunits than needed for a single receptor type, suggesting the possibility of GABA(A) receptor heterogeneity within individual thalamic neurons. beta subunits were not detected in nRt cells, which is consistent with very low levels reported in previous in situ hybridization studies but inconsistent with the expected dependence of functional GABA(A) receptors on beta subunits. Different single-channel open times likely underlie distinct IPSC decay time constants in VB and nRt cells. While we can make no conclusion regarding beta subunits, our findings do support alpha subunits, possibly alpha1 versus alpha3, as structural determinants of channel deactivation kinetics and clonazepam sensitivity. As the gamma2 and delta subunits previously implicated in Zn(2+) sensitivity are both expressed in each cell type, the observed differential Zn(2+) actions at VB versus nRt GABA(A) receptors may involve other subunit differences.

Distribution of GABA(B) receptor protein in somatosensory cortex and thalamus of adult rats and during postnatal development

In the present study we report the immunolocalisation of gamma-aminobutyric acid (GABA)(B) receptors within the cerebral somatosensory cortex (S1) and thalamus of adult and young (1-22 postnatal days) rats. The antibody used recognises a peptide in the carboxy-terminal domain and therefore did not distinguish between the different isoforms GABA(B)1a or GABA(B)1b. The results showed that GABA(B) receptor protein was widely distributed in the brain of both adult and young rats, with different degrees of labelling in separate cerebral nuclei. Antibody labelling was localised both on cells and the neuropil. In the cerebral cortex of adult animals the highest immunolabelling was evident in layers V and VIb, although immunoreactivity was also present in the superficial layers. The strongest signal was evident in the medial habenula.The thalamus showed labelling in the reticular, ventrobasal and geniculate nuclei. In the first postnatal days GABA(B) expression was evident in the cortical cells of layer V, VIb and in the cortical plate. The pattern of labelling in the cerebral cortex of young rats became indistinguishable from that of adult rats by day 12. In the thalamus, the main difference compared to the adult pattern was observed in the mediodorsal nucleus which, in early development, showed a high immunosignal, however, by postnatal day 22 the immunoreactivity decreased with only some scattered cells labelled in the adult brain.

Mature intrastriatal striatal grafts revert the changes in the expression of pallidal and thalamic alpha 1, alpha 2 and beta 2/3 GABAA receptor subunit induced by ibotenic acid lesions in the rat striatum

A between-side comparison of GABAA receptor subunit expression levels in the globus pallidus and anterior-pole motor thalamic nuclei of rats with an ibotenate lesion of the striatum, and rats receiving a fetal striatal graft in the lesioned area was made by using immunocytochemistry with subunit-specific antibodies, at different times post-lesion or different times post-grafting. At 10 days post-lesion, there was already an increase in the labeling of the alpha 1- and beta 2/3-subunits in the globus pallidus, entopeduncular nucleus and ventrolateral nucleus ipsilateral to the lesion when compared with the contralateral side, while there were no significant changes at the level of the ventromedial nucleus. Labeling of the alpha 2-subunit showed a clear increase in the entopeduncular nucleus compared with the contralateral side at 10 days post-lesion. Similar changes were also observed for the different subunits studied at 30 and 120 days after lesioning. Rats with 20-day old transplants of fetal striatal neurons that were implanted in the ibotenate lesioned striatum at 10 days post-lesioning, continued to show changes in the expression of GABAA receptor subunits, albeit at a lower level than those of ibotenate lesioned rats at similar age post-lesion. However, when examining rats with 70-day old transplants, the ibotenate-lesion induced between-side changes were almost completely compensated. These findings suggest a correlation between the maturation of the grafts and their capability to function in reestablishing neuronal circuits as shown by the reduction of changes in GABAergic transmission induced by ibotenate lesions, as indicated by the reversal of changes in GABAA receptor subunit in several areas of the basal ganglia circuit.

Immunoreactivity for the GABA transporter-1 and GABA transporter-3 is restricted to astrocytes in the rat thalamus. A light and electron-microscopic immunolocalization

GABA plasma membrane transporters mediate GABA uptake into presynaptic terminals and surrounding glial processes and thus play a key role in shaping the time course and spatial extent of GABA's action. In the present study we have investigated the cellular and subcellular localization of two GABA transporters (1 and 3) in the rat thalamus using affinity-purified polyclonal antibodies. GABA transporter-1 and -3 immunoreactivity, detected with immunoperoxidase and immunofluorescence methods, is present throughout the thalamus in small punctate structures scattered in the neuropil among unlabelled neuronal perikarya. Labelling for GABA transporter-3 is always more intense than that for GABA transporter-1. Astrocytic processes, identified by their immunoreactivity for glial fibrillary acidic protein, express both GABA transporters. Ultrastructural investigations confirm that GABA transporter-1 and -3 labelling is restricted to astrocytes. Labelled astrocytes are adjacent to terminals making either symmetric or asymmetric synaptic contacts, and are close to neuronal profiles that do not form synaptic contacts in the plane of the section. In double-labelled thin sections some GABA transporter-1- or -3-positive astrocytic processes, detected with immunoperoxidase labelling, surround GABA-positive terminals, detected with antibodies to GABA and immunogold labelling. These findings demonstrate that in rat thalamus the GABA uptake system mediated by GABA transporter-1 and -3 is localized exclusively to astrocytes near the synapses and in the neuropil, and absent from GABAergic terminals. Astrocytes play therefore an important role in mediating GABA transmission in the thalamus, compared to cortical regions.

Afferent arrival and onset of functional activity in the trigeminothalamic pathway of the rat

In this study, a novel in vitro slice preparation has been used to study the anatomical and physiological development of the trigeminothalamic pathway in the prenatal and neonatal rat. Anterograde tracing studies showed that the most rostral trigeminal fibres had reached the cephalic flexure by embryonic day (E)15, and entered the diencephalon by E16. By E17 the first few fibres had reached the ventroposteromedial thalamic nucleus (VPM) where they terminated in growth cones. The projection was more substantial and fibres had begun branching by E18, and arbors were more elaborate by E19. The fibres densely filled the nucleus by the day of birth (PO). The physiological studies showed that postsynaptic responses to stimulation of the trigeminal nerve or principal sensory nucleus (Pr5) could first be recorded at E17. Reliable responses to stimulation of either the nerve or Pr5 were recorded from E18 on. Stimulation of Pr5 enabled both axonal and synaptic signals to recorded in VPM. A GABAergic influence was acting to decrease the overall level of excitability in the thalamus from E18. In prenatal animals, the excitatory response was primarily mediated by NMDA receptors, and by P1 a non-NMDA mediated component was beginning to appear. These results demonstrate that the capacity for axonal conduction in the trigeminothalamic fibres and synaptic transmission in the thalamus are present from the time that anatomical connections are first established.

The effect of ageing on RNA content in neurons from the thalamic reticular nucleus visual sector

In this paper we investigate nucleic acid content in neurons from the dorsocaudal region of the thalamic reticular nucleus in ageing Wistar rats. Nucleic acid per surface unit was analysed by calculating mean extinction using cytophotometric methods. Once the mean extinction and nuclear and cytoplasmic areas were known, nucleic acid total content was calculated. There was an increase in nucleic acid total content and in nuclear and cytoplasmic areas from the age of 3 months onwards. We interpreted these findings as a compensatory response, by 'neuronal hypertrophy', to the deterioration process occurring in the ageing rats. Between the 24th and 30th month, i.e. old age, nucleic acid per surface unit and total content in the cytoplasm exhibited a considerable decrease.

Postnatal development of GABA-immunoreactive terminals in the reticular and ventrobasal nuclei of the rat thalamus: a light and electron microscopic study

The postnatal development of inhibitory GABAergic circuits in the thalamic reticular and ventrobasal nuclei was studied in rats ranging from the day of birth to the end of the third postnatal week by means of a postembedding immunogold staining procedure to visualize GABA. In the reticular nucleus, GABA labeling was present from birth in cell bodies, dendrites, growth cones and a few synaptic terminals, whereas in the ventrobasal nucleus it was exclusively in axonal processes identifiable as growth cones, vesicle-rich profiles and synaptic terminals. In both nuclei, GABA-labeled synaptic terminals were, however, very scarce and immature in neonatal animals and they became numerous and morphologically mature only after the end of the second postnatal week. These findings suggest that inhibitory synaptic responses in the somatosensory thalamus are not yet fully mature throughout the first two postnatal weeks and support the hypothesis that GABA may initially play trophic roles. The relatively late maturation of the thalamic GABAergic system may have important functional consequences, as the reticulothalamic circuits are responsible for the generation of spindle wave oscillations whose cellular mechanisms are also involved in the generation of spike-and-wave (absence) seizures in humans and in animal models.

Individual housing from rearing modifies the performance of young rats on the elevated plus-maze apparatus

Previous studies from our laboratory have demonstrated that male and female rats exhibit a differential pattern of behavior in the elevated plus-maze as a function of age. In the present study, the influence of individual housing conditions on young animals treated with one of two classical anxioselective drugs, diazepam or pentylenetetrazole, was investigated in the elevated plus-maze. In Experiment I, males and females were housed for 30 days after weaning either individually or in groups, and tested in the elevated plus-maze at 60 days of age. In Experiment 2, the effects of diazepam (0.75 or 1.0 mg/kg) or of pentylenetetrazole (20 or 30 mg/kg) on the behavior of isolated or grouped rats were studied at 60 days of age in the elevated plus-maze. The results show that isolated housed animals tested with diazepam at 60 days of age exhibited increased frequency and time spent on the open arms of the apparatus compared to control rats. The effect of diazepam was not observed in grouped animals tested at 60 days of age. Pentylenetetrazole produced a decrease in the frequency and time spent on the open arms. This effect was more prominent in grouped animals. The results suggest that 60-day-old rats deprived of playfighting experience present high basal anxiety levels and also that rearing conditions (isolated or grouped) are able to interact with both anxiolytic and anxiogenic effects of experimental drugs.

Nucleus-specific expression of GABA(A) receptor subunit mRNAs in monkey thalamus

Expression of 10 GABAA receptor subunit genes was examined in monkey thalamus by in situ hybridization using cRNA probes specific for alpha 1, alpha 2, alpha 3, alpha 4, alpha 5, beta 1, beta 2, beta 3, gamma 1, and gamma 2 subunit mRNAs. These displayed unique hybridization on patterns with significant differences from rodents. Alpha 1, beta 2, and gamma 2 transcripts were expressed at high levels in all dorsal thalamic nuclei, but expression was significantly higher in sensory relay nuclei-especially the dorsal lateral geniculate nucleus. Other transcripts showed nucleus-specific differences in levels of expression and in the range expressed. Alpha 5 and alpha 4 subunit transcripts were expressed in all nuclei except the intralaminar nuclei. Levels of alpha 2, alpha 3, beta 1, beta 3, and gamma 1 expression were very low, except in intralaminar nuclei. In the reticular nucleus, most subunit transcripts were not expressed, and only gamma 2 transcripts were consistently detected at modest levels. Thalamic GABAA receptors may be assembled from nucleus-specific groupings of subunit polypeptides.

Development of firing patterns and electrical properties in neurons of the rat ventrobasal thalamus

The electrophysiological properties of thalamic neurons of the rat ventrobasal complex (VB) in vitro were studied during early postnatal development. Current clamp recordings using the whole cell patch clamp method revealed that immature thalamic neurons had less negative membrane potential and higher input resistance than mature neurons. One of the most remarkable differences was the absence of spike bursts riding on the low threshold calcium spike (LTS) in VB neurons before postnatal day 12 (P12). Action potentials recorded from immature neurons had longer duration than those of mature cells and were followed by a longer afterhyperpolarization (spike-ahp). The spike-ahp became shorter as maturation progressed, reaching mature characteristics around P12, coinciding with the appearance of spike bursting on the LTS. The calcium activated potassium conductance, IC, played a prominent role in the spike-ahp in immature neurons. In conclusion, the major differences in intrinsic membrane properties of VB neurons occur during the first 12 postnatal days. The appearance of spike bursting riding on the LTS at P12 is consistent with the emergence of synchronized thalamocortical oscillations in rats around that age.

Brain GABAA receptors studied with subunit-specific antibodies

Brain GABAA/benzodiazepine receptors are highly heterogeneous. This heterogeneity is largely derived from the existence of many pentameric combinations of at least 16 different subunits that are differentially expressed in various brain regions and cell types. This molecular heterogeneity leads to binding differences for various ligands, such as GABA agonists and antagonists, benzodiazepine agonists, antagonists, and inverse agonists, steroids, barbiturates, ethanol, and Cl- channel blockers. Different subunit composition also leads to heterogeneity in the properties of the Cl- channel (such as conductance and open time); the allosteric interactions among subunits; and signal transduction efficacy between ligand binding and Cl- channel opening. The study of recombinant receptors expressed in heterologous systems has been very useful for understanding the functional roles of the different GABAA receptor subunits and the relationships between subunit composition, ligand binding, and Cl- channel properties. Nevertheless, little is known about the complete subunit composition of the native GABAA receptors expressed in various brain regions and cell types. Several laboratories, including ours, are using subunit-specific antibodies for dissecting the heterogeneity and subunit composition of native (no reconstituted) brain GABAA receptors and for revealing the cellular and subcellular distribution of these subunits in the nervous system. These studies are also aimed at understanding the ligand-binding, transduction mechanisms, and channel properties of the various brain GABAA receptors in relation to synaptic mechanisms and brain function. These studies could be relevant for the discovery and design of new drugs that are selective for some GABAA receptors and that have fewer side effects.

Cerebellar-responsive neurons in the thalamic ventroanterior-ventrolateral complex of rats: in vivo electrophysiology

In vivo intracellular recordings were obtained from identified thalamocortical neurons in the ventroanterior-ventrolateral complex in urethane-anesthetized rats. This thalamic nucleus has few interneurons. Neurons that responded to cerebellar stimulation were injected intracellularly with horseradish peroxidase or biocytin and examined with light and electron microscopy (see companion paper). Intrinsic membrane properties and voltage-dependent rhythmic activity of cerebellar-responsive ventroanterior-ventrolateral neurons were similar to those described previously for thalamic neurons. Thus, in addition to conventional "fast" Na(+)-dependent spikes, rat ventroanterior-ventrolateral neurons had "slow" Ca(2+)-mediated low-threshold spikes and membrane conductances that supported rhythmic oscillations. Two modes of spontaneous activity were observed: (i) a tonic firing pattern that consisted of irregularly occurring fast spikes that predominated when the membrane potential was more positive than about -60 mV, and (ii) a rhythmic firing pattern, observed when the membrane potential was more negative than about -65 mV, composed of periodic (4-8 Hz) membrane hyperpolarizations and ramp depolarizations that often produced a low-threshold spike and a burst of fast spikes. In some neurons, spontaneous fast prepotentials were also observed, often with a relatively constant rate (up to 70 Hz). Cerebellar stimulation elicited excitatory postsynaptic potentials that in some cases appeared to be all-or-none and were similar in form to fast prepotentials. Stimulation of ipsilateral motor cortex elicited a short-latency antidromic response followed by a monosynaptic excitatory postsynaptic potential, which had a slower rise time than excitatory postsynaptic potentials evoked from cerebellum, suggesting that cortical inputs were electrotonically distal to cerebellar inputs. In the presence of moderate membrane hyperpolarization, the cortically evoked excitatory postsynaptic potential was followed by a long-lasting hyperpolarization (100-400 ms duration), a rebound depolarization and one or two cycles resembling spontaneous rhythmic activity. Membrane conductance was increased during the initial component of the long hyperpolarization, much of which was probably due to an inhibitory postsynaptic potential. In contrast, membrane conductance was unchanged or slightly decreased during the latter three-quarters of the long hyperpolarization. The amplitude of this component of the long hyperpolarization usually decreased when the membrane was hyperpolarized with intracellular current injection. Thus, both disfacilitation and an inhibitory postsynaptic potential may have contributed to the latter portion of the cortically-evoked long hyperpolarization. The cortically-evoked inhibitory postsynaptic potentials likely originated predominantly from feedforward activation of GABAergic neurons in the thalamic reticular nuclei.(ABSTRACT TRUNCATED AT 400 WORDS)

Distribution of AMPA selective glutamate receptors in the thalamus of adult rats and during postnatal development. A light and ultrastructural immunocytochemical study

The regional, cellular and subcellular distribution of AMPA receptors was demonstrated immunocytochemically within the thalamus of adult and young (from 1 to 20 days postnatal, P1-P20) rats. The antipeptide antibodies used recognize individual subunit proteins of the AMPA-preferring glutamate receptor, i.e., GluR1, GluR2-3 and GluR4. Our results demonstrate that these AMPA receptor subunits are generally not highly expressed in the thalamus, as compared to other brain areas and that they are enriched differentially within different thalamic nuclei. GluR1 is mostly found in intralaminar and midline nuclei throughout life, whereas GluR2-3 is moderately expressed in the thalamus, with no major developmental changes. GluR4 is the predominant subunit expressed in the reticular nucleus in adult rats, but not in young animals, where until P9 it is instead present in the ventrobasal complex. Samples of paraventricular and lateral geniculate nuclei stained with GluR1 and of reticular nucleus as well as ventrobasal complex stained with GluR4 were used for the ultrastructural study. In all the samples, labelling was in the somatic and dendritic cytoplasm, with dense patches of reaction product apposing post-synaptic densities of terminals with round clear vesicles and asymmetric specializations. Glial staining was observed only with the GluR1 antiserum and there was no evidence of labelled synaptic terminals. The differential distribution of GluR subunits in the thalamus suggests that certain subunits may participate more than others in mediating post-synaptic responses in distinct neuronal populations and also that other GluR types may be involved in the thalamic networks.

Acetylcholinesterase as an early marker of the differentiation of dorsal thalamus in embryonic rats

The enzyme acetylcholinesterase (AChE) is transiently expressed in rats by neurons of the principal sensory thalamic nuclei, although these neurons do not use acetylcholine as a neurotransmitter. Reports that AChE expression begins at late embryonic stages led to the proposal that AChE may function in the establishment of connections, but not in earlier events. However, we find AChE reactivity in rat dorsal thalamus 5 days earlier than previously described. Cells that form the ventrobasal complex (VB), the dorsal lateral geniculate nucleus (dLG) and the medial geniculate nucleus, express AChE as they migrate and aggregate into definitive nuclei. AChE-positive cells are occasionally observed in the dorsal thalamic neuroepithelium, but are more common in others regions of the diencephalic neuroepithelium. AChE reactivity delineates VB and dLG earlier than Nissl-stained cytoarchitecture. These findings indicate that AChE is an early marker of neuronal differentiation. Certain properties of AChE, together with its early detection, are consistent with a proposed role in the migration of principal sensory neurons and their organization into discrete nuclei.

GABAA-receptor immunoreactivity in the rat dorsal thalamus: an ultrastructural investigation

The ultrastructural localization of GABAA-receptor (GABAA-R) immunoreactivity (ir) in representative nuclei of the rat dorsal thalamus was investigated using the monoclonal antibody 62-3G1 to the beta 2 and beta 3 subunits of the GABAA-R [8]. The pattern of distribution and the subcellular localization of ir were similar in all the thalamic nuclei examined, with the exception of the reticular nucleus that was unlabeled. The reaction product was present along somatic and dendritic plasma membranes of thalamic neurons and on their intracellular membranes. No labelling was observed in glial cells. The ir was present in areas of plasma membranes related and non related to terminals containing flat vesicles, and also on invaginated plasma membranes suggesting a recycling process of the receptor complex. The distribution and mismatches between GABA neurotransmitter and its receptor localization are discussed.