Rotational behavior and electrophysiological effects induced by GABA(B) receptor activation in rat globus pallidus

Pallidal GABA B receptors: involvement in cortex beta dynamics and thalamic reticular nucleus activity

The external globus pallidus (GP) firing rate synchronizes the basal ganglia-thalamus-cortex network controlling GABAergic output to different nuclei. In this context, two findings are significant: the activity and GABAergic transmission of the GP modulated by GABA B receptors and the presence of the GP-thalamic reticular nucleus (RTn) pathway, the functionality of which is unknown. The functional participation of GABA B receptors through this network in cortical dynamics is feasible because the RTn controls transmission between the thalamus and cortex. To analyze this hypothesis, we used single-unit recordings of RTn neurons and electroencephalograms of the motor cortex (MCx) before and after GP injection of the GABA B agonist baclofen and the antagonist saclofen in anesthetized rats. We found that GABA B agonists increase the spiking rate of the RTn and that this response decreases the spectral density of beta frequency bands in the MCx. Additionally, injections of GABA B antagonists decreased the firing activity of the RTn and reversed the effects in the power spectra of beta frequency bands in the MCx. Our results proved that the GP modulates cortical oscillation dynamics through the GP-RTn network via tonic modulation of RTn activity.

Optogenetic Globus Pallidus Stimulation Improves Motor Deficits in 6-Hydroxydopamine-Lesioned Mouse Model of Parkinson's Disease

Excessive inhibition of the external globus pallidus (GPe) by striatal GABAergic neurons is considered a central mechanism contributing to motor symptoms of Parkinson's disease (PD). While electrophysiological findings support this view, behavioral studies assessing the beneficial effects of global GPe activations are scarce and the reported results are controversial. We used an optogenetic approach and the standard unilateral 6-hydroxydopamine nigrostriatal dopamine (DA) lesion model of PD to explore the effects of GPe photostimulation on motor deficits in mice. Global optogenetic GPe inhibition was used in normal mice to verify whether it reproduced the typical motor impairment induced by DA lesions. GPe activation improved ipsilateral circling, contralateral forelimb akinesia, locomotor hypoactivity, and bradykinesia in 6-OHDA-lesioned mice at ineffective photostimulation parameters (532 nm, 5 Hz, 3 mW) in normal mice. GPe photoinhibition (450 nm, 12 mW) had no effect on locomotor activity and forelimb use in normal mice. Bilateral photoinhibition (450 nm, 6 mW/side) reduced directed exploration and improved working memory performances indicating that recruitment of GPe in physiological conditions may depend on the behavioral task involved. Collectively, these findings shed new light on the functional role of GPe and suggest that it is a promising target for neuromodulatory restoration of motor deficits in PD.

Subcortical atrophy correlates with the perturbational complexity index in patients with disorders of consciousness

BACKGROUND

The complexity of neurophysiological brain responses to direct cortical stimulation, referred to as the perturbational complexity index (PCI), has been shown able to discriminate between consciousness and unconsciousness in patients surviving severe brain injury as well as several other conditions (e.g., wake, dreamless sleep, sleep and ketamine dreaming, anesthesia).

OBJECTIVE

This study asks whether, in patients with a disorder of consciousness (DOC), the complexity of the neurophysiological response to cortical stimulation is preferentially associated with atrophy within specific brain structures.

METHODS

We perform a retrospective analysis of 40 DOC patients and correlate their maximal PCI to MR-based measurements of cortical thinning and subcortical atrophy.

RESULTS

PCI was systematically and inversely associated with the degree of local atrophy within the globus pallidus, a region previously linked to electrocortical and behavioral arousal. Conversely, we fail to detect any association between variance in cortical ribbon thickness and PCI.

CONCLUSION

These findings corroborate the previously reported association between pallidal atrophy and low behavioral arousal and suggest that this region's role in maintaining the overall balance of excitation and inhibition may critically affect the emergence of complex cortical interactions in chronic disorders of consciousness. This finding thus also suggests a target for potential neuromodulatory intervention in DOC patients.

Cellular and Synaptic Dysfunctions in Parkinson's Disease: Stepping out of the Striatum

The basal ganglia (BG) are a collection of interconnected subcortical nuclei that participate in a great variety of functions, ranging from motor programming and execution to procedural learning, cognition, and emotions. This network is also the region primarily affected by the degeneration of midbrain dopaminergic neurons localized in the substantia nigra pars compacta (SNc). This degeneration causes cellular and synaptic dysfunctions in the BG network, which are responsible for the appearance of the motor symptoms of Parkinson’s disease. Dopamine (DA) modulation and the consequences of its loss on the striatal microcircuit have been extensively studied, and because of the discrete nature of DA innervation of other BG nuclei, its action outside the striatum has been considered negligible. However, there is a growing body of evidence supporting functional extrastriatal DA modulation of both cellular excitability and synaptic transmission. In this review, the functional relevance of DA modulation outside the striatum in both normal and pathological conditions will be discussed.

Orexins alleviate motor deficits via increasing firing activity of pallidal neurons in a mouse model of Parkinson's disease

Orexin is a peptide neurotransmitter released in the globus pallidus. Morphological evidence reveals that both orexin 1 receptor (OX₁R) and orexin 2 receptor (OX₂R) exist in the globus pallidus. Here we showed that bilateral microinjection of both orexin-A and orexin-B into the globus pallidus alleviated motor deficits in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced parkinsonian mice. Further in vivo extracellular single-unit recording revealed that the basal spontaneous firing rate of the globus pallidus neurons in MPTP parkinsonian mice was slower than that of normal mice. Application of orexin-A or orexin-B significantly increased the spontaneous firing rate of pallidal neurons. The influx of Ca²⁺ through the L-type Ca²⁺ channel is the major mechanism involved in orexin-induced excitation in the globus pallidus. Orexin-A-induced increase in firing rate of pallidal neurons in MPTP parkinsonian mice was stronger than that of normal mice. Orexin-A exerted both electrophysiological and behavioral effects mainly via OX₁R, and orexin-B exerted the effects via OX₂R. Endogenous orexins modulated the excitability of globus pallidus neurons mainly through OX₁R. The present behavioral and electrophysiological results suggest that orexins ameliorate parkinsonian motor deficits through increasing the spontaneous firing of globus pallidus neurons.

Combination Effects of Forced Mild Exercise and GABAB Receptor Agonist on Spatial Learning, Memory, and Motor Activity in Striatum Lesion Rats

Basal ganglia (BG) lesions cause impairments of different mammalian's movement and cognition behaviors. Motor circuit impairment has a dominant role in the movement disorders. An inhibitory factor in BG is GABA neurotransmitter, which is released from striatum. Lesions in GABAergic neurons could trigger movement and cognition disorders. Previous evidence showed that GABA_B receptor agonist (Baclofen) administration in human improves movement disorders and exercise can improve neurodegenerative and cognitive decline; however, the effects of both Baclofen and mild forced treadmill exercise on movement disorders are not well known. The main objective of this study is to investigate the combined effects of mild forced treadmill exercise and microinjection of Baclofen in the internal Globus Pallidus on striatum lesion-induced impairments of spatial learning and motor activity. We used Morris water maze and open filed tests for studying spatial learning, and motor activity, respectively. Results showed that mild exercise and Baclofen microinjection could not lonely affect the spatial learning, and motor activity impairments while the combination of them could alleviate spatial learning, and motor activity impairments in striatum-lesion animals. Our results suggest that striatum lesion-induced memory and motor activity impairments can improve with combination interaction of GABA_B receptor agonist and exercise training.

An automated behavior analysis system for freely moving rodents using depth image

A rodent behavior analysis system is presented, capable of automated tracking, pose estimation, and recognition of nine behaviors in freely moving animals. The system tracks three key points on the rodent body (nose, center of body, and base of tail) to estimate its pose and head rotation angle in real time. A support vector machine (SVM)-based model, including label optimization steps, is trained to classify on a frame-by-frame basis: resting, walking, bending, grooming, sniffing, rearing supported, rearing unsupported, micro-movements, and "other" behaviors. Compared to conventional red-green-blue (RGB) camera-based methods, the proposed system operates on 3D depth images provided by the Kinect infrared (IR) camera, enabling stable performance regardless of lighting conditions and animal color contrast with the background. This is particularly beneficial for monitoring nocturnal animals' behavior. 3D features are designed to be extracted directly from the depth stream and combined with contour-based 2D features to further improve recognition accuracies. The system is validated on three freely behaving rats for 168 min in total. The behavior recognition model achieved a cross-validation accuracy of 86.8% on the rat used for training and accuracies of 82.1 and 83% on the other two "testing" rats. The automated head angle estimation aided by behavior recognition resulted in 0.76 correlation with human expert annotation. Graphical abstract Top view of a rat freely behaving in a standard homecage, captured by Kinect-v2 sensors. The depth image is used for constructing a 3D topography of the animal for pose estimation, behavior recognition, and head angle calculation. Results of the processed data are displayed on the user interface in various forms.

The external globus pallidus: progress and perspectives

The external globus pallidus (GPe) of the basal ganglia is in a unique and powerful position to influence processing of motor information by virtue of its widespread projections to all basal ganglia nuclei. Despite the clinical importance of the GPe in common motor disorders such as Parkinson's disease, there is only limited information about its cellular composition and organizational principles. In this review, recent advances in the understanding of the diversity in the molecular profile, anatomy, physiology and corresponding behaviour during movement of GPe neurons are described. Importantly, this study attempts to build consensus and highlight commonalities of the cellular classification based on existing but contentious literature. Additionally, an analysis of the literature concerning the intricate reciprocal loops formed between the GPe and major synaptic partners, including both the striatum and the subthalamic nucleus, is provided. In conclusion, the GPe has emerged as a crucial node in the basal ganglia macrocircuit. While subtleties in the cellular makeup and synaptic connection of the GPe create new challenges, modern research tools have shown promise in untangling such complexity, and will provide better understanding of the roles of the GPe in encoding movements and their associated pathologies.

Exposure to Allergen Causes Changes in NTS Neural Activities after Intratracheal Capsaicin Application, in Endocannabinoid Levels and in the Glia Morphology of NTS

Allergen exposure may induce changes in the brainstem secondary neurons, with neural sensitization of the nucleus solitary tract (NTS), which in turn can be considered one of the causes of the airway hyperresponsiveness, a characteristic feature of asthma. We evaluated neurofunctional, morphological, and biochemical changes in the NTS of naive or sensitized rats. To evaluate the cell firing activity of NTS, in vivo electrophysiological experiments were performed before and after capsaicin challenge in sensitized or naive rats. Immunohistochemical studies, endocannabinoid, and palmitoylethanolamide quantification in the NTS were also performed. This study provides evidence that allergen sensitization in the NTS induced: (1) increase in the neural firing response to intratracheal capsaicin application, (2) increase of endocannabinoid anandamide and palmitoylethanolamide, a reduction of 2-arachidonoylglycerol levels in the NTS, (3) glial cell activation, and (4) prevention by a Group III metabotropic glutamate receptor activation of neural firing response to intratracheal application of capsaicin in both naïve and sensitized rats. Therefore, normalization of ovalbumin-induced NTS neural sensitization could open up the prospect of new treatments based on the recovery of specific brain nuclei function and for extensive studies on acute or long-term efficacy of selective mGlu ligand, in models of bronchial hyperreactivity.

GABA transporter subtype 1 and GABA transporter subtype 3 modulate glutamatergic transmission via activation of presynaptic GABA(B) receptors in the rat globus pallidus

The intra-pallidal application of γ-aminobutyric acid (GABA) transporter subtype 1 (GAT-1) or GABA transporter subtype 3 (GAT-3) transporter blockers [1-(4,4-diphenyl-3-butenyl)-3-piperidinecarboxylic acid hydrochloride (SKF 89976A) or 1-[2-[tris(4-methoxyphenyl)methoxy]ethyl]-(S)-3-piperidinecarboxylic acid (SNAP 5114)] reduces the activity of pallidal neurons in monkey. This effect could be mediated through the activation of presynaptic GABA(B) heteroreceptors in glutamatergic terminals by GABA spillover following GABA transporter (GAT) blockade. To test this hypothesis, we applied the whole-cell recording technique to study the effects of SKF 89976A and SNAP 5114 on evoked excitatory postsynaptic currents (eEPSCs) in the presence of gabazine, a GABA(A) receptor antagonist, in rat globus pallidus slice preparations. Under the condition of postsynaptic GABA(B) receptor blockade by the intra-cellular application of N-(2,6-dimethylphenylcarbamoylmethyl)-triethylammonium bromide (OX314), bath application of SKF 89976A (10 μM) or SNAP 5114 (10 μM) decreased the amplitude of eEPSCs, without a significant effect on its holding current and whole cell input resistance. The inhibitory effect of GAT blockade on eEPSCs was blocked by (2S)-3-[[(1S)-1-(3,4-dichlorophenyl)ethyl]amino-2-hydroxypropyl](phenylmethyl)phosphinic acid, a GABA(B) receptor antagonist. The paired-pulse ratio of eEPSCs was increased, whereas the frequency, but not the amplitude, of miniature excitatory postsynaptic currents was reduced in the presence of either GAT blocker, demonstrating a presynaptic effect. These results suggest that synaptically released GABA can inhibit glutamatergic transmission through the activation of presynaptic GABA(B) heteroreceptors following GAT-1 or GAT-3 blockade. In conclusion, our findings demonstrate that presynaptic GABA(B) heteroreceptors in putative glutamatergic subthalamic afferents to the globus pallidus are sensitive to increases in extracellular GABA induced by GAT inactivation, thereby suggesting that GAT blockade represents a potential mechanism by which overactive subthalamopallidal activity may be reduced in parkinsonism.

Localization and pharmacological modulation of GABA-B receptors in the globus pallidus of parkinsonian monkeys

Changes in GABAergic transmission in the external and internal segments of the globus pallidus (GPe and GPi) contribute to the pathophysiology of the basal ganglia network in Parkinson's disease. Because GABA-B receptors are involved in the modulation of GABAergic transmission in GPe and GPi, it is possible that changes in the functions or localization of these receptors contribute to the changes in GABAergic transmission. To further examine this question, we investigated the anatomical localization of GABA-B receptors and the electrophysiologic effects of microinjections of GABA-B receptor ligands in GPe and GPi of MPTP-treated (parkinsonian) monkeys. We found that the pattern of cellular and ultrastructural localization of the GABA-BR1 subunit of the GABA-B receptor in GPe and GPi was not significantly altered in parkinsonian monkeys. However, the magnitude of reduction in firing rate of GPe and GPi neurons produced by microinjections of the GABA-B receptor agonist baclofen was larger in MPTP-treated animals than in normal monkeys. Injections of the GABA-B receptor antagonist CGP55845A were more effective in reducing the firing rate of GPi neurons in parkinsonian monkeys than in normal animals. In addition, the injections of baclofen in GPe and GPi, or of CGP55845A in GPi lead to a significant increase in the proportion of spikes in rebound bursts in parkinsonian animals, but not in normal monkeys. Thus, despite the lack of changes in the localization of GABA-BR1 subunits in the pallidum, GABA-B receptor-mediated effects are altered in the GPe and GPi of parkinsonian monkeys. These changes in GABA-B receptor function may contribute to bursting activities in the parkinsonian state.

Pre-synaptic histamine H₃ receptors modulate glutamatergic transmission in rat globus pallidus

The globus pallidus, a neuronal nucleus involved in the control of motor behavior, expresses high levels of histamine H(3) receptors (H(3)Rs) most likely located on the synaptic afferents to the nucleus. In this work we studied the effect of the activation of rat pallidal H(3)Rs on depolarization-evoked neurotransmitter release from slices, neuronal firing rate in vivo and turning behavior. Perfusion of globus pallidus slices with the selective H(3)R agonist immepip had no effect on the release of [(3)H]-GABA ([(3)H]-γ-aminobutyric acid) or [(3)H]-dopamine evoked by depolarization with high (20 mM) K(+), but significantly reduced [(3)H]-d-aspartate release (-44.8 ± 2.6% and -63.7 ± 6.2% at 30 and 100 nM, respectively). The effect of 30 nM immepip was blocked by 10 μM of the selective H(3)R antagonist A-331440 (4'-[3-[(3(R)-dimethylamino-1-pyrrolidinyl]propoxy]-[1,1-biphenyl]-4'-carbonitrile). Intra-pallidal injection of immepip (0.1 μl, 100 μM) decreased spontaneous neuronal firing rate in anaesthetized rats (peak inhibition 68.8±10.3%), and this effect was reversed in a partial and transitory manner by A-331440 (0.1 μl, 1 mM). In free-moving rats the infusion of immepip (0.5 μl; 10, 50 and 100 μM) into the globus pallidus induced dose-related ipsilateral turning following systemic apomorphine (0.5 mg/kg, s.c.). Turning behavior induced by immepip (0.5 μl, 50 μM) and apomorphine was partially prevented by the local injection of A-331440 (0.5 μl, 1 mM) and was not additive to the turning evoked by the intra-pallidal injection of antagonists at ionotropic glutamate receptors (0.5 μl, 1 mM each of AP-5, dl-2-amino-5-phosphonovaleric acid, and CNQX, 6-nitro-7-sulphamoylbenzo[f]quinoxaline-2,3-dione). These results indicate that pre-synaptic H(3)Rs modulate glutamatergic transmission in rat globus pallidus and thus participate in the control of movement by basal ganglia.

Relationship between intrathecal baclofen and the central nervous system

The GABA(B) receptor agonists display a number of pharmacological effects including central muscle relaxation, decreased self-administration of cocaine and narcotic drugs, antinociception, cognitive impairment as well as enhancement of synaptic plasticity. The main relationships between intrathecal or intracerebral baclofen and the Central Nervous System (CNS) are reviewed with particular attention to actions on pain, epilepsy and basal ganglia regulation. Since baclofen may be involved in synaptic plasticity and the development of neuronal pathways, the main issues of this field are reviewed with particular attention to the effects of baclofen on the developing brain. The role of baclofen in the regulation of movement has not been clearly understood, but recent findings support its important involvement in globus pallidus and subthalamic nucleus. The neuroprotective action of baclofen in cerebral ischemia is a matter of debate. The effects of baclofen in cognition and attention are another important issue because patients with chronic intrathecal baclofen (ITB) administration often present with impairment of cognitive functions. Drug craving and its improvement after baclofen administration is also reviewed. Finally, the clinically interesting results on the regulation of food intake and blood pressure are highlighted. The preliminary experience on the effects in cortical neuron viability at different concentrations of ITB is reported.

Globus pallidus internal segment

The internal segment of the globus pallidus (GP(i)) gathers many bits of information including movement-related activity from the striatum, external segment of the globus pallidus (GP(e)), and subthalamic nucleus (STN), and integrates them. The GP(i) receives rich GABAergic inputs from the striatum and GP(e), and gamma-aminobutyric acid (GABA) receptors are distributed in the GP(i) in a specific manner. Thus, inputs from the striatum and GP(e) may control GP(i) activity in a different way. The GP(i) finally conveys processed information outside the basal ganglia. Changes in GABAergic neurotransmission have been reported in movement disorders and suggested to play an important role in the pathophysiology of the symptoms.

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.

Acute Alcohol Withdrawal is Associated with c-Fos Expression in the Basal Ganglia and Associated Circuitry: C57BL/6J and DBA/2J Inbred Mouse Strain Analyses

BACKGROUND

The DBA/2J (D2) and C57BL/6J (B6) mouse strains are the most widely studied genetic models of severe and mild acute alcohol withdrawal, respectively. Previous studies have identified quantitative trait loci and genes involved in risk for acute ethanol withdrawal using mapping populations derived from the D2 and B6 strains, but the brain region(s) and circuit(s) by which these genes and their protein products influence ethanol physiological dependence and associated withdrawal remain to be elucidated.

METHODS

B6 and D2 were administered a sedative-hypnotic dose of ethanol (4 g/kg) or saline (control) and returned to their home cages where they were left undisturbed for 7 hr, which has been shown in previous studies to correspond to peak acute ethanol withdrawal severity. The mice were then euthanized and assessed for their numbers of c-Fos immunoreactive neurons across 26 brain regions. The question addressed was whether or not ethanol-withdrawn D2 and B6 mice differed in c-Fos induction (neural activation) within circuitry that could explain the severe ethanol withdrawal of the D2 strain and the mild ethanol withdrawal in B6 strain mice.

RESULTS

At peak acute ethanol-withdrawal ethanol-withdrawn D2 and B6 mice differed in neural activation within the basal ganglia, including the subthalamic nucleus and the two major output nuclei of the basal ganglia (the medial globus pallidus and the substantia nigra pars reticulata). Genotype-dependent c-Fos induction was also apparent in associated circuitry including the lateral septum, the ventral tegmental area, the nucleus accumbens core, the dorsolateral caudate putamen, the substantia nigra pars compacta, the cingulate and entorhinal cortices, and the ventral pallidum. D2 and B6 mice showed comparable neural activation in the bed nucleus of the stria terminalis, and the nucleus accumbens shell.

CONCLUSIONS

The present studies are the first to use immediate early gene product expression to assess the pattern of neural activation associated with acute ethanol withdrawal. Our results point to the involvement of an extended basal ganglia circuit in genetically determined differences in acute ethanol withdrawal. Based on these data, we suggest that quantitative trait genes (QTGs) involved in acute ethanol withdrawal exert their effects on this phenotype via one or more of the brain regions and circuits identified. As more information becomes available that integrates neural circuit and QTG analyses, the precise mechanisms by which QTGs affect ethanol physiological dependence and associated withdrawal will become apparent.

Synaptically released GABA activates both pre- and postsynaptic GABA(B) receptors in the rat globus pallidus

The globus pallidus (GP) contains abundant GABAergic synapses and GABA(B) receptors. To investigate whether synaptically released GABA can activate pre- and postsynaptic GABA(B) receptors in the GP, physiological recordings were performed using rat brain slice preparations. Cell-attached recordings from GABA(A) antagonist-treated preparations revealed that repetitive local stimulation induced a GABA(B) antagonist-sensitive pause in spontaneous firings of GP neurons. Whole cell recordings revealed that the repetitive stimulation evoked fast excitatory postsynaptic potentials followed by a slow inhibitory postsynaptic potential (IPSP) in GP neurons. The slow IPSP was insensitive to a GABA(A) receptor antagonist, increased in amplitude with the application of ionotropic glutamate receptor antagonists, and was suppressed by the GABA(B) antagonist CGP55845. The reversal potential of the slow IPSP was close to the potassium equilibrium potential. These results suggest that synaptically released GABA activated postsynaptic GABA(B) receptors and induced the pause and the slow IPSP. On the other hand, in the neurons that were treated to block postsynaptic GABA(B) responses, CGP55845 increased the amplitudes of repetitive local stimulation-induced GABA(A)-mediated inhibitory postsynaptic currents (IPSCs) but not the ionotropic glutamate-mediated excitatory postsynaptic currents. Moreover, the GABA(B) receptor specific agonist baclofen reduced the frequency of miniature IPSCs without altering their amplitude distributions. These results suggest that synaptically released GABA also activated presynaptic GABA(B) autoreceptors, resulting in decreased GABA release in the GP. Together, we infer that both pre- and postsynaptic GABA(B) receptors may play crucial roles in the control of GP neuronal activity.

GABAergic Modulation of the Activity of Globus Pallidus Neurons in Primates: In Vivo Analysis of the Functions of GABA Receptors and GABA Transporters

Neurons in the external and internal segment of the globus pallidus (GPe and GPi, respectively) receive substantial GABAergic inputs from the striatum and through axon collaterals of neighboring pallidal neurons. The effects of GABA on pallidal activity depend on the synaptic localization of GABA receptors and the distribution and activity of GABA transporters (GATs). To explore the contribution of GABA receptors and transporters to pallidal function, we recorded the activity of single neurons in GPe or GPi before, during, and after local microinjections of GABAergic compounds in awake rhesus monkeys. Activation of GABAA or GABAB receptors with muscimol or baclofen, respectively, inhibited pallidal activity. These effects were reversed by concomitant infusion of the respective GABA receptor antagonists, gabazine and CGP-55845. Given alone, the antagonists were without consistent effect. Application of the selective GAT-1 inhibitor, SKF-89976A, and the semiselective GAT-3 blocker, SNAP-5114, decreased pallidal activity. Both GAT inhibitors increased GABA levels in the pallidum, as measured by microdialysis. Electron microscopic observations revealed that these transporters are located on glial processes and unmyelinated axonal segments, but rarely on terminals. Our results indicate that activation of GABAA and GABAB receptors inhibits neuronal activity in both segments of the pallidum. GAT-1 and GAT-3 are involved in the modulation of endogenous GABA levels and may be important in regulating the extrasynaptic levels of GABA. Together with previous evidence that a considerable proportion of pallidal GABA receptors are located outside the synaptic cleft, our experiments strongly support the importance of extrasynaptic GABAergic transmission in the primate pallidum.

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.

Tonic activation of presynaptic GABA(B) receptors on rat pallidosubthalamic terminals

AIM

The subthalamic nucleus plays a critical role in the regulation of movement, and abnormal activity of its neurons is associated with some basal ganglia motor symptoms. We examined the presence of functional presynaptic GABA(B) receptors on pallidosubthalamic terminals and tested whether they were tonically active in the in vitro subthalamic slices.

METHODS

Whole-cell patch-clamp recordings were applied to acutely prepared rat subthalamic nucleus slices. The effects of specific GABA(B) agonist and antagonist on action potential-independent inhibitory postsynaptic currents (IPSCs), as well as holding current, were examined.

RESULTS

Superfusion of baclofen, a GABA(B) receptor agonist, significantly reduced the frequency of GABA(A) receptor-mediated miniature IPSCs (mIPSCs), in a Cd2+-sensitive manner, with no effect on the amplitude, indicating presynaptic inhibition on GABA release. In addition, baclofen induced a weak outward current only in a minority of subthalamic neurons. Both the pre- and post-synaptic effects of baclofen were prevented by the specific GABA(B) receptor antagonist, CGP55845. Furthermore, CGP55845 alone increased the frequency of mIPSCs, but had no effect on the holding current.

CONCLUSION

These findings suggest the functional dominance of presynaptic GABA(B) receptors on the pallidosubthalamic terminals over the postsynaptic GABA(B) receptors on subthalamic neurons. Furthermore, the presynaptic, but not the postsynaptic, GABA(B) receptors are tonically active, suggesting that the presynaptic GABA(B) receptors in the subthalamic nucleus are potential therapeutic target for the treatment of Parkinson disease.

An electron microscope immunocytochemical study of GABA(B) R2 receptors in the monkey basal ganglia: a comparative analysis with GABA(B) R1 receptor distribution

Functional gamma-aminobutyric acid (GABA)(B) receptors are heterodimers made up of GABA(B) R1 and GABA(B) R2 subunits. The subcellular localization of GABA(B) R2 receptors remains poorly known in the central nervous system. Therefore, we performed an ultrastructural analysis of the localization of GABA(B) R2 receptor immunoreactivity in the monkey basal ganglia. Furthermore, to characterize better the neuronal sites at which GABA(B) R1 and GABA(B) R2 may interact to form functional receptors, we compared the relative distribution of immunoreactivity of the two GABA(B) receptors in various basal ganglia nuclei. Light to moderate GABA(B) R2 immunoreactivity was found in cell bodies and neuropil elements in all basal ganglia nuclei. At the electron microscope level, GABA(B) R2 immunoreactivity was commonly expressed postsynaptically, although immunoreactive preterminal axonal segments were also frequently encountered, particularly in the globus pallidus and substantia nigra, where they accounted for the third of the total number of GABA(B) R2-containing elements. A few labeled terminals that displayed the ultrastructural features of glutamatergic boutons were occasionally found in most basal ganglia nuclei, except for the subthalamic nucleus, which was devoid of GABA(B) R2-immunoreactive boutons. The relative distribution of GABA(B) R2 immunoreactivity in the monkey basal ganglia was largely consistent with that of GABA(B) R1, but some exceptions were found, most noticeably in the globus pallidus and substantia nigra, which contained a significantly larger proportion of presynaptic elements labeled for GABA(B) R1 than GABA(B) R2. These findings suggest the possible coexistence and heterodimerization of GABA(B) R1 and GABA(B) R2 at various pre- and postsynaptic sites, but also raise the possibility that the formation of functional GABA(B) receptors in specific compartments of basal ganglia neurons relies on mechanisms other than GABA(B) R1/R2 heterodimerization.

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.

GABA-B receptor activation in the rat globus pallidus potently suppresses pentylenetetrazol-induced tonic seizures

To determine the involvement of the globus pallidus in mediating epilepsy, the effects of microinjection of a GABA uptake blocker (tiagabine), a benzodiazepine agonist (zolpidem) and a GABA-B receptor agonist (baclofen) on pentylenetetrazol (PTZ)-induced tonic seizure were examined in adult rats. Administration of PTZ induced tonic seizures in all control animals, accompanied with a 100% mortality rate. Pretreatment with bilateral intrapallidal microinjection of tiagabine (1 mM) suppressed the incidence of tonic seizures to 67.7% and reduced the mortality rate to 16.7%. Furthermore, the latency to tonic seizures was 1,275 +/- 277 s, which was significantly longer than that of the corresponding control animals (319 +/- 225 s). On the other hand, administration of zolpidem (1 mM) was without significant effects on the mortality rate, the incidence and latency of the tonic seizure. In sharp contrast, microinjection of baclofen (1 mM) completely suppressed PTZ-induced tonic seizures and reduced the mortality rate to 0%. This effect was largely abolished by co-injection of the GABA-B receptor antagonist CGP55845. To elucidate the direct cellular action of baclofen, the excitability and membrane potential of globus pallidus neurons were studied by cell-attached and whole-cell patch-clamp recordings in the brain slice. Bath application of baclofen (50 microM) significantly reduced the firing of these neurons, and was often accompanied by a clear membrane hyperpolarization, in a CGP55845-sensitive manner. These data suggest that activation of GABA-B receptors in globus pallidus could significantly modulate PTZ-induced tonic seizures.

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.

Comparative cellular distribution of GABAA and GABAB receptors in the human basal ganglia: immunohistochemical colocalization of the alpha 1 subunit of the GABAA receptor, and the GABABR1 and GABABR2 receptor subunits

The GABA(B) receptor is a G-protein linked metabotropic receptor that is comprised of two major subunits, GABA(B)R1 and GABA(B)R2. In this study, the cellular distribution of the GABA(B)R1 and GABA(B)R2 subunits was investigated in the normal human basal ganglia using single and double immunohistochemical labeling techniques on fixed human brain tissue. The results showed that the GABA(B) receptor subunits GABA(B)R1 and GABA(B)R2 were both found on the same neurons and followed the same distribution patterns. In the striatum, these subunits were found on the five major types of interneurons based on morphology and neurochemical labeling (types 1, 2, 3, 5, 6) and showed weak labeling on the projection neurons (type 4). In the globus pallidus, intense GABA(B)R1 and GABA(B)R2 subunit labeling was found in large pallidal neurons, and in the substantia nigra, both pars compacta and pars reticulata neurons were labeled for both receptor subunits. Studies investigating the colocalization of the GABA(A) alpha(1) subunit and GABA(B) receptor subunits showed that the GABA(A) receptor alpha(1) subunit and the GABA(B)R1 subunit were found together on GABAergic striatal interneurons (type 1 parvalbumin, type 2 calretinin, and type 3 GAD neurons) and on neurons in the globus pallidus and substantia nigra pars reticulata. GABA(B)R1 and GABA(B)R2 were found on substantia nigra pars compacta neurons but the GABA(A) receptor alpha(1) subunit was absent from these neurons. The results of this study provide the morphological basis for GABAergic transmission within the human basal ganglia and provides evidence that GABA acts through both GABA(A) and GABA(B) receptors. That is, GABA acts through GABA(B) receptors, which are located on most of the cell types of the striatum, globus pallidus, and substantia nigra. GABA also acts through GABA(A) receptors containing the alpha(1) subunit on specific striatal GABAergic interneurons and on output neurons of the globus pallidus and substantia nigra pars reticulata.

GABA(B) receptor agonists reverse akinesia following intranigral or intracerebroventricular injection in the reserpine-treated rat

1. This study examined whether GABA(B) receptor agonists injected directly into the substantia nigra pars reticulata (SNr) and globus pallidus (GP), or given intracerebroventricularly, could reverse reserpine-induced akinesia in the rat. 2. Male Sprague-Dawley rats, stereotaxically cannulated above the SNr, GP or third ventricle, were rendered akinetic by injection of reserpine (5 mg kg(-1) s.c.). After 18 h, the locomotor effects of the GABA(B) receptor agonists, baclofen or SKF 97541 were examined. 3. Unilateral injection of baclofen (1-5 micro g in 0.5 micro l) into the GP failed to evoke any locomotor response (n=6). In contrast, unilateral intranigral injection of baclofen (0.08-1.6 micro g in 0.5 micro l) produced a dose-dependent increase in net contraversive rotations reaching a maximum of 162+/-24 turns 90 min(-1) (n=6-8). Pretreatment with the selective GABA(B) receptor antagonist, CGP 46381 (2.4 micro g in 0.5 micro l), inhibited the effects of baclofen (0.8 micro g) by 68+/-9% (n=6). 4. Following intracerebroventricular injection, baclofen (0.8-4 micro g in 2 micro l) produced a dose-dependent increase in net arbitrary locomotor units (ALUs), reaching a maximum of 447+/-154 ALUs in 35 min (n=6-7). SKF 97541 (4-32 micro g in 2 micro l) similarly reversed akinesia, reaching 129+/-69 ALUs in 15 min (n=6). 5. These data show that activation of GABA(B) receptors within the SNr, but not the GP, reverses reserpine-induced akinesia. The success of intracerebroventricular injection of baclofen suggests a potential for systemically active GABA(B) receptor agonists in the treatment of akinesia in Parkinson's disease.