Synaptic localization of GABA(A) receptor subunits in the substantia nigra of the rat: effects of quinolinic acid lesions of the striatum

Pre- and Postembedding Immunoelectron Microscopy to Analyze the Central Nervous System

Immunocytochemistry, a method of delineating the subcellular localization of target proteins, was developed from immunohistochemistry. In principle, proteins are labeled using an antigen-antibody reaction. In order to observe under an electron microscope, the reaction product must scatter the electron beam with sufficient contrast while it is necessary to have an amplifying label that can withstand the observation. We have some detailed tips on making electron microscope samples to achieve this objective, and we would be happy to help you.

Role of the striatal dopamine, GABA and opioid systems in mediating feeding and fat intake

Food intake, which is a highly reinforcing behavior, provides nutrients required for survival in all animals. However, when fat and sugar consumption goes beyond the daily needs, it can favor obesity. The prevalence and severity of this health problem has been increasing with time. Besides covering nutrient and energy needs, food and in particular its highly palatable components, such as fats, also induce feelings of joy and pleasure. Experimental evidence supports a role of the striatal complex and of the mesolimbic dopamine system in both feeding and food-related reward processing, with the nucleus accumbens as a key target for reward or reinforcing-associated signaling during food intake behavior. In this review, we provide insights concerning the impact of feeding, including fat intake, on different types of receptors and neurotransmitters present in the striatal complex. Reciprocally, we also cover the evidence for a modulation of palatable food intake by different neurochemical systems in the striatal complex and in particular the nucleus accumbens, with a focus on dopamine, GABA and the opioid system.

Blockade of Intranigral and Systemic D3 Receptors Stimulates Motor Activity in the Rat Promoting a Reciprocal Interaction among Glutamate, Dopamine, and GABA

In vivo activation of dopamine D3 receptors (D3Rs) depresses motor activity. D3Rs are widely expressed in subthalamic, striatal, and dendritic dopaminergic inputs into the substantia nigra pars reticulata (SNr). In vitro studies showed that nigral D3Rs modulate their neurotransmitter release; thus, it could be that these changes in neurotransmitter levels modify the discharge of nigro-thalamic neurons and, therefore, motor behavior. To determine how the in vitro responses correspond to the in vivo responses, we examined the effect of intra-nigral and systemic blockade of D3Rs in the interstitial content of glutamate, dopamine, and GABA within the SNr using microdialysis coupled to motor activity determinations in freely moving rats. Intranigral unilateral blockade of D3R with GR 103,691 increased glutamate, dopamine, and GABA. Increments correlated with increased ambulatory distance, non-ambulatory activity, and induced contralateral turning. Concomitant blockade of D3R with D1R by perfusion of SCH 23390 reduced the increase of glutamate; prevented the increment of GABA, but not of dopamine; and abolished behavioral effects. Glutamate stimulates dopamine release by NMDA receptors, while blockade with kynurenic acid prevented the increase in dopamine and, in turn, of GABA and glutamate. Finally, systemic administration of D3R selective antagonist U 99194A increased glutamate, dopamine, and GABA in SNr and stimulated motor activity. Blockade of intra-nigral D1R with SCH 23390 prior to systemic U 99194A diminished increases in neurotransmitter levels and locomotor activity. These data highlight the pivotal role of presynaptic nigral D3 and D1R in the control of motor activity and help to explain part of the effects of the in vivo administration of D3R agents.

Dopamine and Acetylcholine, a Circuit Point of View in Parkinson's Disease

Data from the World Health Organization (National Institute on Aging, 2011) and the National Institutes of Health (He et al., 2016) predicts that while today the worldwide population over 65 years of age is estimated around 8.5%, this number will reach an astounding 17% by 2050. In this framework, solving current neurodegenerative diseases primarily associated with aging becomes more pressing than ever. In 2017, we celebrate a grim 200th anniversary since the very first description of Parkinson’s disease (PD) and its related symptomatology. Two centuries after this debilitating disease was first identified, finding a cure remains a hopeful goal rather than an attainable objective on the horizon. Tireless work has provided insight into the characterization and progression of the disease down to a molecular level. We now know that the main motor deficits associated with PD arise from the almost total loss of dopaminergic cells in the substantia nigra pars compacta. A concomitant loss of cholinergic cells entails a cognitive decline in these patients, and current therapies are only partially effective, often inducing side-effects after a prolonged treatment. This review covers some of the recent developments in the field of Basal Ganglia (BG) function in physiology and pathology, with a particular focus on the two main neuromodulatory systems known to be severely affected in PD, highlighting some of the remaining open question from three main stand points:

- Heterogeneity of midbrain dopamine neurons.

- Pairing of dopamine (DA) sub-circuits.

- Dopamine-Acetylcholine (ACh) interaction.

A vast amount of knowledge has been accumulated over the years from experimental conditions, but very little of it is reflected or used at a translational or clinical level. An initiative to implement the knowledge that is emerging from circuit-based approaches to tackle neurodegenerative disorders like PD will certainly be tremendously beneficial.

Habenula-Induced Inhibition of Midbrain Dopamine Neurons Is Diminished by Lesions of the Rostromedial Tegmental Nucleus

Neurons in the lateral habenula (LHb) are transiently activated by aversive events and have been implicated in associative learning. Functional changes associated with tonic and phasic activation of the LHb are often attributed to a corresponding inhibition of midbrain dopamine (DA) neurons. Activation of GABAergic neurons in the rostromedial tegmental nucleus (RMTg), a region that receives dense projections from the LHb and projects strongly to midbrain monoaminergic nuclei, is believed to underlie the transient inhibition of DA neurons attributed to activation of the LHb. To test this premise, the effects of axon-sparing lesions of the RMTg were assessed on LHb-induced inhibition of midbrain DA cell firing in anesthetized rats. Quinolinic acid lesions decreased the number of NeuN-positive neurons in the RMTg significantly while largely sparing cells in neighboring regions. Lesions of the RMTg reduced both the number of DA neurons inhibited by, and the duration of inhibition resulting from, LHb stimulation. Although the firing rate was not altered, the regularity of DA cell firing was increased in RMTg-lesioned rats. Locomotor activity in an open field was also elevated. These results are the first to show that RMTg neurons contribute directly to LHb-induced inhibition of DA cell activity and support the widely held proposition that GABAergic neurons in the mesopontine tegmentum are an important component of a pathway that enables midbrain DA neurons to encode the negative valence associated with failed expectations and aversive stimuli.

SIGNIFICANCE STATEMENT

Phasic changes in the activity of midbrain dopamine cells motivate and guide future behavior. Activation of the lateral habenula by aversive events inhibits dopamine neurons transiently, providing a neurobiological representation of learning models that incorporate negative reward prediction errors. Anatomical evidence suggests that this inhibition occurs via the rostromedial tegmental nucleus, but this hypothesis has yet to be tested directly. Here, we show that axon-sparing lesions of the rostromedial tegmentum attenuate habenula-induced inhibition of dopamine neurons significantly. These data support a substantial role for the rostromedial tegmentum in habenula-induced feedforward inhibition of dopamine neurons.

Systematic Analysis of Drug Targets Confirms Expression in Disease-Relevant Tissues

It is commonly assumed that drug targets are expressed in tissues relevant to their indicated diseases, even under normal conditions. While multiple anecdotal cases support this hypothesis, a comprehensive study has not been performed to verify it. We conducted a systematic analysis to assess gene and protein expression for all targets of marketed and phase III drugs across a diverse collection of normal human tissues. For 87% of gene-disease pairs, the target is expressed in a disease-affected tissue under healthy conditions. This result validates the importance of confirming expression of a novel drug target in an appropriate tissue for each disease indication and strengthens previous findings showing that targets of efficacious drugs should be expressed in relevant tissues under normal conditions. Further characterization of the remaining 13% of gene-disease pairs revealed that most genes are expressed in a different tissue linked to another disease. Our analysis demonstrates the value of extensive tissue specific expression resources.both in terms of tissue and cell diversity as well as techniques used to measure gene expression.

Signaling mechanisms downstream of quinolinic acid targeting the cytoskeleton of rat striatal neurons and astrocytes

The studies of signaling mechanisms involved in the disruption of the cytoskeleton homeostasis were performed in a model of quinolinic acid (QUIN) neurotoxicity in vitro. This investigation focused on the phosphorylation level of intermediate filament (IF) subunits of astrocytes (glial fibrillary acidic protein - GFAP) and neurons (low, medium and high molecular weight neurofilament subunits - NFL, NFM and NFH, respectively). The activity of the phosphorylating system associated with the IFs was investigated in striatal slices of rat exposed to QUIN or treated simultaneously with QUIN plus glutamate receptor antagonists, calcium channel blockers or kinase inhibitors. Results showed that in astrocytes, the action of 100 μM QUIN was mainly due to increased Ca(2+) influx through NMDA and L-type voltage-dependent Ca(2+) channels (L-VDCC). In neuronal cells QUIN acted through metabotropic glutamate receptor (mGluR) activation and influx of Ca(2+) through NMDA receptors and L-VDCC, as well as Ca(2+) release from intracellular stores. These mechanisms then set off a cascade of events including activation of PKA, PKCaMII and PKC, which phosphorylate head domain sites on GFAP and NFL. Also, Cdk5 was activated downstream of mGluR5, phosphorylating the KSP repeats on NFM and NFH. mGluR1 was upstream of phospholipase C (PLC) which, in turn, produced diacylglycerol (DAG) and inositol 3,4,5 triphosphate (IP3). DAG is important to activate PKC and phosphorylate NFL, while IP(3) contributed to Ca(2+) release from internal stores promoting hyperphosphorylation of KSP repeats on the tail domain of NFM and NFH. The present study supports the concept of glutamate and Ca(2+) contribution in excitotoxic neuronal damage provoked by QUIN associated to dysfunction of the cytoskeleton homeostasis and highlights the differential signaling mechanisms elicited in striatal astrocytes and neurons.

Dimeric states of neural- and epithelial-cadherins are distinguished by the rate of disassembly

Epithelial- and neural-cadherins are specifically localized at synapses in neurons which can change the shape and contact surface on a time scale of seconds to months. We have focused our studies on the role of the extracellular domains of cadherins in the dynamics of synapses. The kinetics of dimer disassembly of the first two extracellular domains of E- and N-cadherin, ECAD12 and NCAD12, were studied with analytical size exclusion chromatography and sedimentation velocity. NCAD12 forms three different dimers that are distinguished by assembly conditions and kinetics of dissociation. ECAD12 dimer disassembles rapidly regardless of the calcium concentration, whereas the disassembly of NCAD12 dimers was strongly dependent on calcium concentration. In addition to the apo- and saturated-dimeric forms of NCAD12, there is a third dimeric form that is a slow exchange dimer. This third dimeric form for NCAD12, formed by decalcification of the calcium-saturated dimer, was kinetically trapped in apo-conditions and did not disassemble over a period of months. Sedimentation velocity experiments showed that this dimer, upon addition of calcium, had similar weighted averages as a calcium-saturated dimer. These studies provide evidence that the kinetics of dimer disassembly of the extracellular domains may be a major contributor to the morphological dynamics of synapses in vivo.

γ-Aminobutyric acid type B receptor changes in the rat striatum and substantia nigra following intrastriatal quinolinic acid lesions

Changes in the regional distribution of the metabotropic GABA type B receptors (GABA(B)) were investigated in a rat model of Huntington's disease. Animals received a unilateral intrastriatal injection of quinolinic acid (QA), and GABA(B) immunoreactivity was monitored 3, 11, and 21 days postinjection in the striatum and substantia nigra (SN). Two antibodies, recognizing either the GABA(B1) or the GABA(B2) receptor subtypes, were used. QA injection rapidly induced a protracted increase in GABA(B1) or GABA(B2) immunoreactivity in the lesioned striatum, despite the neuronal loss. In the SN, a continuous increase in GABA(B1) and GABA(B2) immunoreactivity was observed at all time points in the ipsilateral pars reticulata (SNr), whereas the pars compacta (SNc) was unaffected by this phenomenon. This increase was supported by a densitometric analysis. At day 21 postlesion induction, intensely labeled stellate cells and processes were found in the ipsilateral SNr, in addition to immunoreactive neurons. Double labeling of GABA(B1) and glial fibrillary acidic protein (GFAP) showed that the stellate cells were reactive astrocytes. Hence, part of the sustained increase in GABA(B) immunoreactivity that takes place in the SNr and possibly the striatum may be ascribed to reactive astrocytes. It is suggested that GABA(B) receptors are up-regulated in these reactive astrocytes and that agonists might influence the extent of this astroglial reaction.

A dynamic role for GABA receptors on the firing pattern of midbrain dopaminergic neurons

Dopaminergic neurons are subject to a significant background GABAergic input in vivo. The presence of this GABAergic background might be expected to inhibit dopaminergic neuron firing. However, dopaminergic neurons are not all silent but instead fire in single-spiking and burst firing modes. Here we present evidence that phasic changes in the tonic activity of GABAergic afferents are a potential extrinsic mechanism that triggers bursts and pauses in dopaminergic neurons. We find that spontaneous single-spiking is more sensitive to activation of GABA receptors than phasic N-methyl-D-aspartate (NMDA)-mediated burst firing in rat slices (P15-P31). Because tonic activation of GABA(A) receptors has previously been shown to suppress burst firing in vivo, our results suggest that the activity patterns seen in vivo are the result of a balance between excitatory and inhibitory conductances that interact with the intrinsic pacemaking currents observed in slices. Using the dynamic clamp technique, we applied balanced, constant NMDA and GABA(A) receptor conductances into dopaminergic neurons in slices. Bursts could be produced by disinhibition (phasic removal of the GABA(A) receptor conductance), and these bursts had a higher frequency than bursts produced by the same NMDA receptor conductance alone. Phasic increases in the GABA(A) receptor conductance evoked pauses in firing. In contrast to NMDA receptor, application of constant AMPA and GABA(A) receptor conductances caused the cell to go into depolarization block. These results support a bidirectional mechanism by which GABAergic inputs, in balance with NMDA receptor-mediated excitatory inputs, control the firing pattern of dopaminergic neurons.

Hearing loss alters the subcellular distribution of presynaptic GAD and postsynaptic GABAA receptors in the auditory cortex

We have shown previously that auditory experience regulates the maturation of excitatory synapses in the auditory cortex (ACx). In this study, we used electron microscopic immunocytochemistry to determine whether the heightened excitability of the ACx following neonatal sensorineural hearing loss (SNHL) also involves pre- or postsynaptic alterations of GABAergic synapses. SNHL was induced in gerbils just prior to the onset of hearing (postnatal day 10). At P17, the gamma-aminobutyri acid type A (GABA(A)) receptor's beta2/3-subunit (GABA(A)beta2/3) clusters residing at plasma membranes in layers 2/3 of ACx was reduced significantly in size (P < 0.05) and number (P < 0.005), whereas the overall number of immunoreactive puncta (intracellular + plasmalemmal) remained unchanged. The reduction of GABA(A)beta2/3 was observed along perikaryal plasma membranes of excitatory neurons but not of GABAergic interneurons. This cell-specific change can contribute to the enhanced excitability of SNHL ACx. Presynaptically, GABAergic axon terminals were significantly larger but less numerous and contained 47% greater density of glutamic acid decarboxylase immunoreactivity (P < 0.05). This suggests that GABA synthesis may be upregulated by a retrograde signal arising from lowered levels of postsynaptic GABA(A)R. Thus, both, the pre- and postsynaptic sides of inhibitory synapses that form upon pyramidal neurons of the ACx are regulated by neonatal auditory experience.

Excitation Control: Balancing PSD-95 Function at the Synapse

Excitability of individual neurons dictates the overall excitation in specific brain circuits. This process is thought to be regulated by molecules that regulate synapse number, morphology and strength. Neuronal excitation is also influenced by the amounts of neurotransmitter receptors and signaling molecules retained at particular synaptic sites. Recent studies revealed a key role for PSD-95, a scaffolding molecule enriched at glutamatergic synapses, in modulation of clustering of several neurotransmitter receptors, adhesion molecules, ion channels, cytoskeletal elements and signaling molecules at postsynaptic sites. In this review we will highlight mechanisms that control targeting of PSD-95 at the synapse, and discuss how this molecule influences the retention and clustering of diverse synaptic proteins to regulate synaptic structure and strength. We will also discuss how PSD-95 may maintain a balance between excitation and inhibition in the brain and how alterations in this balance may contribute to neuropsychiatric disorders.

Excitation Control: Balancing PSD-95 Function at the Synapse

Excitability of individual neurons dictates the overall excitation in specific brain circuits. This process is thought to be regulated by molecules that regulate synapse number, morphology and strength. Neuronal excitation is also influenced by the amounts of neurotransmitter receptors and signaling molecules retained at particular synaptic sites. Recent studies revealed a key role for PSD-95, a scaffolding molecule enriched at glutamatergic synapses, in modulation of clustering of several neurotransmitter receptors, adhesion molecules, ion channels, cytoskeletal elements and signaling molecules at postsynaptic sites. In this review we will highlight mechanisms that control targeting of PSD-95 at the synapse, and discuss how this molecule influences the retention and clustering of diverse synaptic proteins to regulate synaptic structure and strength. We will also discuss how PSD-95 may maintain a balance between excitation and inhibition in the brain and how alterations in this balance may contribute to neuropsychiatric disorders.

GABAergic circuits in the basal ganglia and movement disorders

GABA is the major inhibitory neurotransmitter in the basal ganglia, and GABAergic pathways dominate information processing in most areas of these structures. It is therefore not surprising that abnormalities of GABAergic transmission are key elements in pathophysiologic models of movement disorders involving the basal ganglia. These include hypokinetic diseases such as Parkinson's disease, and hyperkinetic diseases, such as Huntington's disease or hemiballism. In this chapter, we will briefly review the major anatomic features of the GABAergic pathways in the basal ganglia, and then describe in greater detail the changes of GABAergic transmission, which are known to occur in movement disorders.

GABA(A) receptors: structure and function in the basal ganglia

gamma-Aminobutyric acid type A (GABA(A)) receptors, the major inhibitory neurotransmitter receptors responsible for fast inhibition in the basal ganglia, belong to the superfamily of "cys-cys loop" ligand-gated ion channels. GABA(A) receptors form as pentameric assemblies of subunits, with a central Cl(-) permeable pore. On binding of two GABA molecules to the extracellular receptor domain, a conformational change is induced in the oligomer and Cl(-), in most adult neurons, moves into the cell leading to an inhibitory hyperpolarization. Nineteen mammalian subunit genes have been identified, each showing distinct regional and cell-type-specific expression. The combinatorial assembly of the subunits generates considerable functional diversity. Here we place the focus on GABA(A) receptor expression in the basal ganglia: striatum, globus pallidus, substantia nigra and subthalamic nucleus, where, in addition to the standard alpha1beta2/3gamma2 receptor subtype, significant levels of other subunits (alpha2, alpha3, alpha4, gamma1, gamma3 and delta) are expressed in some nuclei.

Localization of GABA receptors in the basal ganglia

The majority of neurons in the basal ganglia utilize GABA as their principal neurotransmitter and, as a consequence, most basal ganglia neurons receive extensive GABAergic inputs derived from multiple sources. In order to understand the diverse roles of GABA in the basal ganglia it is necessary to define the precise localization of GABA receptors in relation to known neuron subtypes and known afferents. In this chapter, we summarize data on the ultrastructural localization of ionotropic GABA(A) receptors and metabotropic GABA(B) receptors in the basal ganglia. In each of the regions of the basal ganglia that have been studied, GABA(A) receptor subunits are located primarily at symmetrical synapses formed by GABAergic boutons, where they display a several-hundred-fold enrichment over extrasynaptic sites. In contrast, GABA(B) receptors are widely distributed at synaptic and extrasynaptic sites on both presynaptic and postsynaptic membranes. Presynaptic GABA(B) receptors are localized on striatopallidal, striatonigral and pallidonigral afferent terminals, as well as glutamatergic terminals derived from the cortex, thalamus and subthalamic nucleus. It is concluded that fast GABA transmission mediated by GABA(A) receptors in the basal ganglia occurs primarily at synapses whereas GABA transmission mediated by GABA(B) receptors is more complex, involving receptors located at presynaptic, postsynaptic and extrasynaptic sites.

GABAergic structures in the ventral part of the oral pontine reticular nucleus: An ultrastructural immunogold analysis

GABA mediates inhibitory effects in neurons of the ventral part of the oral pontine reticular nucleus (vRPO). Evidence increasingly suggests that GABA plays an important role in the modulation of rapid eye movement (REM) sleep generation in the cat vRPO. Here, we investigate the anatomical substrate of this modulation using GABA immunocytochemistry. Immunoperoxidase labeling revealed a few small GABA-immunoreactive cell bodies scattered throughout the vRPO. The numerical densities of all vRPO synapses and the GABA-immunoreactive synapses were estimated, at the electron microscopical level, by using a combination of the physical disector and the post-embedding immunogold techniques. We estimated that 30% of all vRPO synaptic terminals were immunoreactive to GABA. Our findings support the hypothesis that vRPO neuron activity is significantly controlled by inhibitory GABAergic terminals that directly target somata and the different parts of the dendritic tree, including distal regions. GABAergic input could inhibit vRPO REM sleep-inducing neurons during other states of the sleep-wakefulness cycle such as wakefulness or non-REM sleep.

Glutamate and GABA receptors and transporters in the basal ganglia: what does their subsynaptic localization reveal about their function?

GABA and glutamate, the main transmitters in the basal ganglia, exert their effects through ionotropic and metabotropic receptors. The dynamic activation of these receptors in response to released neurotransmitter depends, among other factors, on their precise localization in relation to corresponding synapses. The use of high resolution quantitative electron microscope immunocytochemical techniques has provided in-depth description of the subcellular and subsynaptic localization of these receptors in the CNS. In this article, we review recent findings on the ultrastructural localization of GABA and glutamate receptors and transporters in monkey and rat basal ganglia, at synaptic, extrasynaptic and presynaptic sites. The anatomical evidence supports numerous potential locations for receptor-neurotransmitter interactions, and raises important questions regarding mechanisms of activation and function of synaptic versus extrasynaptic receptors in the basal ganglia.

Synaptic and extrasynaptic GABA-A and GABA-B receptors in the globus pallidus: an electron microscopic immunogold analysis in monkeys

GABA-A and GABA-B receptors mediate differential effects in the CNS. To better understand the role of these receptors in regulating pallidal functions, we compared their subcellular and subsynaptic localization in the external and internal segments of the globus pallidus (GPe and GPi) in monkeys, using pre- and post-embedding immunocytochemistry with antibodies against GABA-A (alpha1, beta2/3 subunits) and GABA-BR1 receptor subtype. Our results demonstrate that GABA-A and GABA-B receptors display a differential pattern of subcellular and subsynaptic localization in both segments of the globus pallidus. The majority of GABA-BR1 immunolabeling is intracellular, whereas immunoreactivity for GABA-A receptor subunits is mostly bound to the plasma membrane. A significant proportion of both GABA-BR1 and GABA-A receptor immunolabeling is extrasynaptic, but GABA-A receptor subunits also aggregate in the main body of putative GABAergic symmetric synapses established by striatal- and pallidal-like terminals. GABA-BR1 immunoreactivity is expressed presynaptically in putative glutamatergic terminals, while GABA-A alpha1 and beta2/3 receptor subunits are exclusively post-synaptic and often coexist at individual symmetric synapses in both GPe and GPi. In conclusion, our findings corroborate the concept that ionotropic and metabotropic GABA receptors are located to subserve different effects in pallidal neurons. Although the aggregation of GABA-A receptors at symmetric synapses is consistent with their role in fast inhibitory synaptic transmission, the extrasynaptic distribution of both GABA-A and GABA-B receptors provides a substrate for complex modulatory functions that rely predominantly on the spillover of GABA.

Subcellular localization of GABAB receptor subunits in rat globus pallidus

The inhibitory amino acid gamma-aminobutyric acid (GABA) is the major neurotransmitter in the globus pallidus. Although electrophysiological studies have indicated that functional GABA(B) receptors exist in rat globus pallidus, the subcellular localization of GABA(B) receptor subunits and their spatial relationship to glutamatergic and GABAergic synapses are unknown. Here, we use pre-embedding immunogold labeling to study the subcellular localization of GABA(B) receptor subunits, GABA(B1) and GABA(B2), in globus pallidus neurons and identified populations of afferent terminals. Immunolabeling for GABA(B1) and GABA(B2) was observed throughout the globus pallidus, with GABA(B1) more strongly expressed in perikarya and GABA(B2) mainly expressed in the neuropil. Electron microscopic analysis revealed that the majority of GABA(B1) labeling was localized within the cytoplasm, whereas most of GABA(B2) labeling was associated with the plasma membrane. At the subcellular level, both the GABA(B1) and GABA(B2) immunogold labeling was localized at pre- and postsynaptic sites. At asymmetric, putative excitatory, synapses, GABA(B1) and GABA(B2) immunogold labeling was found at perisynaptic sites of both pre- and postsynaptic specializations. Double immunolabeling, using the vesicular glutamate transporter 2 (VGLUT2), revealed the glutamatergic nature of most immunogold-labeled asymmetric synapses. At symmetric, putative GABAergic, synapses, including those formed by anterogradely labeled striatopallidal terminals, GABA(B1) and GABA(B2) immunogold labeling was found in the main body of both pre- and postsynaptic specializations. These results demonstrate the existence of presynaptic GABA(B) auto- and heteroreceptors and postsynaptic GABA(B) receptors, which may be involved in modulating synaptic transmission in the globus pallidus.

Electrophysiological and behavioral effects of zolpidem in rat globus pallidus

The globus pallidus is believed to play a critical role in the normal function of the basal ganglia, and abnormal activity of its neurons may underlie some basal ganglia motor symptoms. A high density of benzodiazepine binding sites on GABAA receptors has been reported in the rat globus pallidus. The present study investigates the effect of activating the benzodiazepine site by the agonist zolpidem. In in vitro slices, 100 nM of zolpidem significantly prolonged the half decay time of both miniature and spontaneous inhibitory postsynaptic currents by 30.1 +/- 3.0% (n=12) and 17.8 +/- 2.4% (n=16), respectively, with no effect on their amplitudes and frequencies. In the behaving animal, when zolpidem was microinjected into the globus pallidus unilaterally, it caused a robust ipsilateral rotation (26.4 +/- 2.4 turns/30 min, n=8), significantly higher than that of control animals receiving vehicle injection (1.3 +/- 1.6 turns/30 min, n=6). This effect was in agreement with the in vitro effect of zolpidem in enhancing the action of GABA on postsynaptic GABAA receptors. All the effects of zolpidem, in vitro or in vivo, were sensitive to the benzodiazepine antagonist flumazenil, confirming the specificity on the benzodiazepine site. This finding on the effect of zolpidem on motor behavior provides a rationale for further investigations into its potential in the treatment of motor disorders originating from the basal ganglia.

The subcellular localization of GABA(B) receptor subunits in the rat substantia nigra

The inhibitory effects of GABA within the substantia nigra (SN) are mediated in part by metabotropic GABA(B) receptors. To better understand the mechanisms underlying these effects, we have examined the subcellular localization of the GABA(B) receptor subunits, GABA(B1) and GABA(B2), in SN neurons and afferents using pre-embedding immunocytochemistry combined with anterograde or retrograde labelling. In both the SN pars compacta (SNc) and pars reticulata (SNr), GABA(B1) and GABA(B2) showed overlapping, but distinct, patterns of immunolabelling. GABA(B1) was more strongly expressed by putative dopaminergic neurons in the SNc than by SNr projection neurons, whereas GABA(B2) was mainly expressed in the neuropil of both regions. Immunogold labelling for GABA(B1) and GABA(B2) was localized in presynaptic and postsynaptic elements throughout the SN. The majority of labelling was intracellular or was associated with extrasynaptic sites on the plasma membrane. In addition, labelling for both subunits was found on the presynaptic and postsynaptic membranes at symmetric, putative GABAergic synapses, including those formed by anterogradely labelled striatonigral and pallidonigral terminals. Labelling was also observed on the presynaptic membrane and at the edge of the postsynaptic density at asymmetric, putative excitatory synapses. Double immunolabelling, using the vesicular glutamate transporter 2, revealed the glutamatergic nature of many of the immunogold-labelled asymmetric synapses. The widespread distribution of GABA(B) subunits in the SNc and SNr suggests that GABA(B)-mediated effects in these regions are likely to be more complex than previously described, involving presynaptic autoreceptors and heteroreceptors, and postsynaptic receptors on different populations of SN neurons.

Formation and plasticity of GABAergic synapses: physiological mechanisms and pathophysiological implications

gamma-Aminobutyric acid(A) (GABA(A)) receptors mediate most of the fast inhibitory neurotransmission in the CNS. They represent a major site of action for clinically relevant drugs, such as benzodiazepines and ethanol, and endogenous modulators, including neuroactive steroids. Alterations in GABA(A) receptor expression and function are thought to contribute to prevalent neurological and psychiatric diseases. Molecular cloning and immunochemical characterization of GABA(A) receptor subunits revealed a multiplicity of receptor subtypes with specific functional and pharmacological properties. A major tenet of these studies is that GABA(A) receptor heterogeneity represents a key factor for fine-tuning of inhibitory transmission under physiological and pathophysiological conditions. The aim of this review is to highlight recent findings on the regulation of GABA(A) receptor expression and function, focusing on the mechanisms of sorting, targeting, and synaptic clustering of GABA(A) receptor subtypes and their associated proteins, on trafficking of cell-surface receptors as a means of regulating synaptic (and extrasynaptic) transmission on a short-time basis, on the role of endogenous neurosteroids for GABA(A) receptor plasticity, and on alterations of GABA(A) receptor expression and localization in major neurological disorders. Altogether, the findings presented in this review underscore the necessity of considering GABA(A) receptor-mediated neurotransmission as a dynamic and highly flexible process controlled by multiple mechanisms operating at the molecular, cellular, and systemic level. Furthermore, the selected topics highlight the relevance of concepts derived from experimental studies for understanding GABA(A) receptor alterations in disease states and for designing improved therapeutic strategies based on subtype-selective drugs.