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

Glycine receptors in the striatum, globus pallidus, and substantia nigra of the human brain: An immunohistochemical study

Glycine receptors (GlyRs) are heteropentameric chloride ion channels that facilitate fast-response, inhibitory neurotransmission in the mammalian spinal cord and brain. GlyRs have four functional subunits, alpha1-3 and beta, which likely exist in heteromeric alphabeta combinations. Mutations in GlyR alpha1 and beta subunits are well known for their involvement in hyperekplexia, a paroxysmal motor disorder. In this study we present the first detailed immunohistochemical investigation at the regional, cellular, and subcellular levels of GlyRs in the human basal ganglia. The results show that GlyRs are present at the regional level in low concentrations in the striatum and globus pallidus and are present in the highest concentrations in the substantia nigra. At the cellular level, GlyRs are present only in discrete populations of neurons immunoreactive for choline acetyltransferase (ChAT), parvalbumin, and calretinin in the human striatum, on a subpopulation of parvalbumin- and calretinin-positive neurons in the globus pallidus, and in the substantia nigra GlyRs are present on approximately three-fourths of all pars compacta and one-third of all pars reticulata neurons. They also form a distinct band of immunoreactive neurons in the intermedullary layers of the globus pallidus. At the subcellular level in the substantia nigra pars reticulata (SNr), GlyRs show a localized distribution on the soma and dendrites that partially complements but does not overlap with the distribution of gamma-aminobutyric acid (GABA)A receptors. Our results demonstrate the precise cellular and subcellular localization of GlyRs in the human basal ganglia and suggest that glycinergic receptors may play an important complementary role to other inhibitory receptors in modulating cholinergic, dopaminergic, and GABAergic neuronal pathways in the basal ganglia.

DHEA improves symptomatic treatment of moderately and severely impaired MPTP monkeys

The steroid dehydroepiandrosterone (DHEA) is abundant in men and women and decreases rapidly during aging. Parkinson's disease (PD) is the second most common neurodegenerative disorder just behind Alzheimer. l-3,4-Dihydroxyphenylalanine (l-Dopa) therapy remains the most effective treatment but many patients develop motor complications. This study investigated the acute effect of DHEA alone and with l-Dopa in 12 females monkeys lesioned with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) to model PD. DHEA administration alone improved the mean parkinsonian score at 1, 5 and 15mg/kg in moderately and severely impaired MPTP monkeys and increased blood DHEA concentrations. DHEA with a low dose of l-Dopa increased the l-Dopa effect in moderately and severely impaired MPTP monkeys. DHEA lengthened duration of the effect of the low dose of l-Dopa by 15-45min. DHEA at 1, 5 and 15mg/kg combined with a high dose of l-Dopa did not increase dyskinesias. DHEA could act by reducing inhibitory GABAergic activity in the striatal output pathways. DHEA could also be metabolized into estradiol in the brain and increase acutely dopamine activity.

Dopamine presynaptically and heterogeneously modulates nucleus accumbens medium-spiny neuron GABA synapses in vitro

Background

The striatal complex is the major target of dopamine action in the CNS. There, medium-spiny GABAergic neurons, which constitute about 95% of the neurons in the area, form a mutually inhibitory synaptic network that is modulated by dopamine. When put in culture, the neurons reestablish this network. In particular, they make autaptic connections that provide access to single, identified medium-spiny to medium-spiny neuron synaptic connections.

Results

We examined medium-spiny neuron autaptic connections in postnatal cultures from the nucleus accumbens, the ventral part of the striatal complex. These connections were subject to presynaptic dopamine modulation. D1-like receptors mediated either inhibition or facilitation, while D2-like receptors predominantly mediated inhibition. Many connections showed both D1 and D2 modulation, consistent with a significant functional colocalization of D1 and D2-like receptors at presynaptic sites. These same connections were subject to GABA_A, GABA_B, norepinephrine and serotonin modulation, revealing a multiplicity of modulatory autoreceptors and heteroreceptors on individual varicosities. In some instances, autaptic connections had two components that were differentially modulated by dopamine agonists, suggesting that dopamine receptors could be distributed heterogeneously on the presynaptic varicosities making up a single synaptic (i.e. autaptic) connection.

Conclusion

Differential trafficking of dopamine receptors to different presynaptic varicosities could explain the many controversial studies reporting widely varying degrees of dopamine receptor colocalization in medium-spiny neurons, as well as more generally the diversity of dopamine actions in target areas. Longer-term changes in the modulatory actions of dopamine in the striatal complex could be due to plasticity in the presynaptic distribution of dopamine receptors on medium-spiny neuron varicosities.

Immunocytochemical colocalization of GABA-B receptor subunits in gonadotropin-releasing hormone neurons of the sheep

GABA has been shown to play an important role in the control of gonadotropin-releasing hormone (GnRH) and luteinizing hormone secretion in many mammals. In sheep, seasonal differences in the ability of GABA-B receptor antagonists to alter pulsatile luteinizing hormone secretion have led to the hypothesis that this receptor subtype mediates the increased inhibitory effects of estradiol on GnRH and luteinizing hormone pulse frequency seen during the non-breeding season (anestrus). The aim of the present study was to use multiple-label immunocytochemistry to determine if ovine GnRH neurons contain the GABA-B receptor subunits R1 and/or R2, and to determine whether there are seasonal differences in the colocalization of these subunits in GnRH neurons. A majority of GnRH cells in the preoptic area, anterior hypothalamic area, and medial basal hypothalamus of both breeding season and anestrous ewes contained either GABA-B R1 or R2 subunits; a subset of GnRH neurons in breeding season (42%) and anestrous ewes (60%) contained both subunits. In contrast to colocalization within cell bodies, GnRH fibers in the median eminence did not colocalize GABA-B receptor subunits. Although the percentage of GnRH neurons expressing GABA-B receptor subunits tended to be higher in anestrus than in the breeding season, there were no significant seasonal differences in R1 and R2 subunit colocalization in GnRH cell bodies. Thus, while GABA may act directly on GnRH cell bodies via GABA-B receptors in the sheep, any role that GABA-B receptors may play in seasonal reproductive changes is likely mediated by other neurons afferent to GnRH cells.

GABA(B) receptors in the centromedian/parafascicular thalamic nuclear complex: an ultrastructural analysis of GABA(B)R1 and GABA(B)R2 in the monkey thalamus

Strong gamma-aminobutyric acid type B (GABA(B)) receptor binding has been shown throughout the thalamus, but the distribution of the two GABA(B) receptor subunits, GABA(B) receptor subunit 1 (GABA(B)R1) and GABA(B) receptor subunit 2 (GABA(B)R2), remains poorly characterized. In primates, the caudal intralaminar nuclei, centromedian and parafascicular (CM/PF), are an integral part of basal ganglia circuits and a main source of inputs to the striatum. In this study, we analyzed the subcellular and subsynaptic distribution of GABA(B) receptor subunits by using light and electron microscopic immunocytochemical techniques. Quantitative immunoperoxidase and immunogold analysis showed that both subunits display a similar pattern of distribution in CM/PF, being expressed largely at extrasynaptic and perisynaptic sites in neuronal cell bodies, dendrites, and axon-like processes and less abundantly in axon terminals. Postsynaptic GABA(B)R1 labeling was found mostly on the plasma membrane (70-80%), whereas GABA(B)R2 was more evenly distributed between the plasma membrane and intracellular compartments of CM/PF neurons. A few axon terminals forming symmetric and asymmetric synapses were also labeled for GABA(B)R1 and GABA(B)R2, but the bulk of presynaptic labeling was expressed in small axon-like processes. About 20% of presynaptic vesicle-containing dendrites of local circuit neurons displayed GABA(B)R1/R2 immunoreactivity. Vesicular glutamate transporters (vGluT1)-containing terminals forming asymmetric synapses expressed GABA(B)R1 and/or displayed postsynaptic GABA(B)R1 at the edges of their asymmetric specialization. Overall, these findings provide evidence for multiple sites where GABA(B) receptors could modulate GABAergic and glutamatergic transmission in the primate CM/PF complex.

Immunohistochemical staining of post-mortem adult human brain sections

One of the challenges for modern neuroscience is to understand the basis of coordinated neuronal function and networking in the human brain. Some of these questions can be addressed using low- and high-resolution imaging techniques on post-mortem human brain tissue. We have established a versatile protocol for fixation of post-mortem adult human brain tissue, storage of the tissue in a human brain bank, and immunohistochemical analysis in order to understand human brain functions in normal controls and in neuropathological conditions. The brains are fixed by perfusion through the internal carotid and basilar arteries to enhance the penetration of fixative throughout the brain, then blocked, postfixed, cryoprotected, snap-frozen and stored at -80 degrees C. Sections are processed for immunohistochemical single- or double-label staining and conventional-, electron- or confocal laser scanning-microscopy analysis. The results gained using this tissue and protocol are vital for determining the localization of neurochemicals throughout the human brain and to document the changes that occur in neurological diseases.

GABA(B) receptors at glutamatergic synapses in the rat striatum

Although multiple effects of GABA(B) receptor activation on synaptic transmission in the striatum have been described, the precise locations of the receptors mediating these effects have not been determined. To address this issue, we carried out pre-embedding immunogold electron microscopy in the rat using antibodies against the GABA(B) receptor subunits, GABA(B1) and GABA(B2). In addition, to investigate the relationship between GABA(B) receptors and glutamatergic striatal afferents, we used antibodies against the vesicular glutamate transporters, vesicular glutamate transporter 1 and vesicular glutamate transporter 2, as markers for glutamatergic terminals. Immunolabeling for GABA(B1) and GABA(B2) was widely and similarly distributed in the striatum, with immunogold particles localized at both presynaptic and postsynaptic sites. The most commonly labeled structures were dendritic shafts and spines, as well as terminals forming asymmetric and symmetric synapses. In postsynaptic structures, the majority of labeling associated with the plasma membrane was localized at extrasynaptic sites, although immunogold particles were also found at the postsynaptic specialization of some symmetric, putative GABAergic synapses. Labeling in axon terminals was located within, or at the edge of, the presynaptic active zone, as well as at extrasynaptic sites. Double labeling for GABA(B) receptor subunits and vesicular glutamate transporters revealed that labeling for both GABA(B1) and GABA(B2) was localized on glutamatergic axon terminals that expressed either vesicular glutamate transporter 1 or vesicular glutamate transporter 2. The patterns of innervation of striatal neurons by the vesicular glutamate transporter 1- and vesicular glutamate transporter 2-positive terminals suggest that they are selective markers of corticostriatal and thalamostriatal afferents, respectively. These results thus provide evidence that presynaptic GABA(B) heteroreceptors are in a position to modulate the two major excitatory inputs to striatal spiny projection neurons arising in the cortex and thalamus. In addition, presynaptic GABA(B) autoreceptors are present on the terminals of spiny projection neurons and/or striatal GABAergic interneurons. Furthermore, the data indicate that GABA may also affect the excitability of striatal neurons via postsynaptic GABA(B) receptors.

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.

Differential sensitivity to the motor and hypothermic effects of the GABA B receptor agonist baclofen in various mouse strains

RATIONALE

Comparison of different mouse strains can provide valuable information about the genetic control of behavioural and molecular phenotypes. Recent evidence has demonstrated the importance of GABA B receptors in anxiety and depression. Investigation of the phamacogenetics of GABA B receptor activation may aid in the understanding of mechanisms underlying the role of GABA B in affect.

OBJECTIVES

The aim of current study was to determine the relative sensitivity of different mouse strains to GABA B receptor agonism in two models of GABA B receptor function, namely hypothermia and motor incoordination.

METHODS

Mice each from 11 strains (BALB/cByJIco, DBA/2JIco, OF1, FVB/NIco, CD1, C3H/HeOuJIco, 129/SvPasIco, NMRI, C57BL/6JIco, A/JOlaHsd and Swiss) were trained to walk on a rotarod for 300 s. On the following day, mice received 0, 3, 6 or 12 mg/kg of L: -baclofen PO. Rectal temperature and rotarod performance were measured at 0, 1, 2 and 4 h after drug application.

RESULTS

L: -Baclofen produced a significant dose-dependent hypothermia and ataxia in most, but not all, mouse strains examined. The magnitude and duration of response was influenced by strain, with mice of the 129/SvPasIco strain showing largest hypothermic response to 12 mg/kg l-baclofen and C3H/HeOuJIco the lowest, whereas the BALB/cByJIco strain demonstrated greatest ataxic response on the rotarod, and NMRI the least. Interestingly, some strains (notably C3H/HeOuJIco) had marked differential hypothermic and ataxic responses, with minimal body temperature responses to L: -baclofen but significant ataxia on the rotarod observed.

CONCLUSION

There is differential genetic control on specific GABA B receptor populations that mediate hypothermia and ataxia. Further, these studies demonstrate that background strain is an important determinant of GABA B receptor mediated responses, and that hypothermic and ataxic responses may be influenced by independent genetic loci.

Medial preoptic area interactions with the nucleus accumbens–ventral pallidum circuit and maternal behavior in rats

Several experiments explored the roles of nucleus accumbens (NA), ventral pallidum (VP) and medial preoptic area (MPOA) in the regulation of maternal behavior in rats. A preliminary experiment found that bilateral radiofrequency lesions of medial NA did not disrupt maternal behavior. Experiment 1 found that bilateral infusions of muscimol into VP, but not into medial NA, reversibly disrupted maternal behavior. Experiment 2 found that unilateral muscimol injections into VP disrupted maternal behavior to a greater extent when paired with a contralateral N-methyl-d-aspartic acid (NMDA) MPOA lesion than when paired with a sham MPOA lesion. Experiment 3 showed that a unilateral NMDA MPOA lesion paired with a contralateral NMDA VP lesion (Contra group) disrupted maternal behavior to a much greater extent than did sham NMLA lesions or NMDA lesions of MPOA and VP ipsilateral to one another. Experiment 3 focused on the specificity of the maternal behavior disruptions and found that the primary maternal deficit in the Contra females was a severe deficit in retrieval behavior. Importantly, these females showed normal hoarding behavior, home cage activity, and elevated plus maze activity. Experiment 3 used Neu N immunohistochemistry to define the extent of MPOA and VP excitotoxic lesions. It is hypothesized that MPOA acts to facilitate the active components of maternal behavior by inhibiting NA, which then releases VP from GABAergic inhibition, and such disinhibition of VP allows pup stimuli to trigger appropriate maternal responses.

Unravelling the unusual signalling properties of the GABA(B) receptor

GABA(B) receptors are the cornerstone receptors in the modulation of inhibitory signalling in the central nervous system and continue to be targets for the amelioration of a number of neuropsychiatric and neurological disorders. Unravelling the molecular identity of this receptor has spurred much research over the past five or so years and generated a renewed interest and excitement in the field. Many questions are being answered and lessons learnt, not only about GABA(B) receptor function but also about general mechanisms of G-protein-coupled receptor signalling. However, as questions are being answered as many new questions are being raised and many GABA(B)-related conundrums continue to remain unanswered. In this report, we review some of the most recent work in the area of GABA(B) receptor research. In particular, we focus our attentions on the emerging mechanisms thought to be important in GABA(B) receptor signalling and the growing complex of associated proteins that we consider to be part of the GABA(B) receptor "signalosome."

Morphological features, distribution and compartmental organization of the nicotinamide adenine dinucleotide phosphate reduced-diaphorase interneurons in the human striatum

Striatal nicotinamide adenine dinucleotide phosphate reduced-diaphorase (NADPH-d)-positive (+) cells are one of the major classes of striatal interneurons. The present study analyzes their somatodendritic morphology, distribution pattern, and compartmental organization in the caudate nucleus (CN) and putamen (Put) of nine normal human brains. The following striatal territories are examined: 1) the precommissural head of the CN; 2) the postcommissural head of the CN; 3) the body of the CN; 4) the gyrus of the CN; 5) the tail of the CN; 6) the precommissural Put; and 7) the postcommissural Put. Three morphologically distinct types of NADPH-d+ neurons were found in each of these territories. The two most common NADPH-d+ neurons displayed an ovoid or triangular perikaryon from which several thick primary dendrites emerged, although much less numerous, bipolar-shaped NADPH-d+ cells were also observed. The highest density of NADPH-d+ neurons was found in the gyrus of the CN, followed by the body of the CN, tail of the CN, postcommissural head of the CN, postcommissural Put, precommissural head of the CN, and precommissural Put. The matrix was the striatal compartment with the densest NADPH-d+ neuronal population. Some of these cells also occurred in the center and peripheral regions of the striosomes located in the head of the CN and in the Put. In the body and gyrus of the CN, the striosomes were largely devoid of these striatal interneurons. Knowledge of the density and distribution of these interneurons should advance our understanding of the organization of the normal human striatum and help to evaluate the effects of neurodegenerative processes on cell density.