Developmental changes of GABAA receptor-chloride channels in rat Meynert neurons

The developmental changes of GABA_A receptors were investigated in Meynert neurons freshly dissociated from day 0, 2 week-, and 6 month-old rats using both nystatin and gramicidin perforated patch recording modes under voltage-clamp conditions. The age-related changes in the current amplitude and threshold concentration in the concentration–response relationships for GABA indicated the developmental alteration of the GABA_A receptor subunits and the channel density. The GABA-induced

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measured by the gramicidin perforated patch mode shifted to more negative with development. The decay time constant of GABAergic inhibitory postsynaptic spontaneous currents (sIPSCs) in the synaptic active zone accelerated with aging. The GABA-induced currents were potentiated in a concentration dependent manner in the presence of benzodiazepine (BZP) agonists, diazepam (DZP) and zolpidem (ZPM). The potentiation rate of DZP on the GABA_A response decreased with aging, but not in the case of ZPM, which demonstrated a stronger action in the aging rat neurons. These results suggested that the GABA_A receptor·Cl⁻ channel complexes may thus change both the assembly and interaction of subunits as well as their functional roles with aging.     

Localization of GABAA and GABAB receptor subtypes on serotonergic neurons

The effect of selective destruction of serotonin (5-HT)-containing neurons with 5,7-dihydroxytryptamine (5,7-DHT) on [3H] muscimol and (-)-[3H]baclofen binding was investigated in various rat brain regions. Ten days after intracerebroventricular 5,7-DHT, serotonin levels and [3H]imipramine binding were markedly decreased. 5,7-DHT reduced [3H]muscimol binding only in the mesencephalon, and (-)-[3H]baclofen binding was unmodified in all the areas considered. These results suggest that except in the mesencephalon GABA receptors may not be localized on serotonergic nerve terminals.     

Benzodiazepine- and barbiturate-interactions with GABAA receptor responses on lactotrophes

Modulation of the biphasic effect of muscimol on prolactin secretion by benzodiazepines and secobarbital was investigated, using an in vitro superfusion system. The stimulatory effect of low concentrations of muscimol was potentiated by both classes of drugs, and the effect of benzodiazepines appeared to be mediated by central-type benzodiazepine receptors. Neither benzodiazepines nor secobarbital affected the inhibitory response to muscimol. Clonazepam reduced the potency of bicuculline methiodide as an antagonist of the stimulatory effect, but did not alter the potency of picrotoxinin. These results demonstrate a selective potentiation of one component of the GABAA receptor effect on lactotrophs by benzodiazepines and barbiturates and provide evidence for a functional effect of these drugs at a site without the CNS.     

Changes in neuroactive steroid content during social isolation stress modulate GABAA receptor plasticity and function

Rats deprived of social contact with other rats at a young age experience a form of prolonged stress that leads to long-lasting alteration in their behavior profile. This chronic stress paradigm is thus thought to be anxiogenic for these normally gregarious animals and their abnormal reactivity to environmental stimuli, when reared under this condition, is thought to be a product of prolonged stress. Neurochemical, molecular, and electrophysiological evidences demonstrate that social isolation is associated with alteration in the structure and function of GABA_A receptors and suggest that endogenous content of the progesterone metabolite 3α,5α-TH PROG may be an important determinant in regulating brain excitability and sensitivity to stimuli and point out its possible role in psychiatric and neurological disorder.     

Baclofen reduces GABAA receptor responses in acutely dissociated neurons of bullfrog dorsal root ganglia

The effect of baclofen on the function of the γ-aminobutyric acid_A (GABA_A) receptor was examined in acutely dissociated neurons of bullfrog dorsal root ganglia (DRG) by using the whole-cell coltage-clamp method. Baclofen (0.1–100 μM) depressed the inward currents produced by GABA (100 μM) and muscimol (100 μM). Baclofen shifted the concentration-response curve for GABA (1 μM-1 mM) downward. Baclofen decreased the maximum response (V_max) to GABA without changing the apparent dissociation constant (K_d), suggesting a noncompetitive antagonism. The effect of baclofen on the GABA current was blocked by antagonists for the GABA_B receptor; the rank order of potency was P-[3-Aminopropyl]-P-diethoxymethylphosphinic acid (CGP 55845A) ≫ 3-N[1-(S)-(3,4-dichlorophenyl)ethyl]amino-2-(S)-hydroxypropyl-P-benzylphosphinic acid (CGP 35348) > saclofen ≫ phaclofen. Baclofen produced an irreversible depression of the GABA current in neurons dialyzed with an internal solution containing guanosine (5′-O-(3-thiotriphosphate) (GTPγS, 100 μM). Intracellular guanosine 5′-O-(2-thiodiphosphate) (GDPβS, 100 μM) blocked the inhibitory effect of baclofen on the GABA current. Forskolin (10 μM) and dibutyryl N⁶, 2′-O-dibutyryladenosine 3′:5′-cyclic monophophate (db-cyclic AMP) (200 μM) depressed the GABA current. N-(2-aminoethyl)-5-isoquinolinesulfonamide (H-9, 40 μM) and N-(2-guanidinoethyl)-5-isoquinolinesulfonamide (HA-1004, 50 μM), protein kinase A (PKA) inhibitors, reduced the depressant effect of baclofen on the GABA current. The baclofen-induced depression of the GABA current was blocked by PKI(5–24), a specific PKA inhibitor, but not by PKC(19–36), a specific protein kinase C(PKC) inhibitor. We suggest that GABA_B receptors regulate the GABA_A receptor function through a G-protein linked to the adenylyl cyclase-PKA pathway in bullfrog DRG neurons. Synapse 26:165–174, 1997. © 1997 Wiley-Liss, Inc.     

Glucocorticoid modulates the sensitivity of the GABAA receptor on primary afferent neurons of bullfrogs

With intracellular and voltage-clamp recording techniques, we have demonstrated that the glucocorticoids, prednisolone and hydrocortisone at a concentration of 5 microM to 1 mM, reversibly depressed gamma-aminobutyric acid (GABA)-induced responses on primary afferent neurons of bullfrogs. An analysis with dose-response curves revealed that the glucocorticoids decreased the sensitivity of the GABAA receptor in a non-competitive manner. We suggest that glucocorticoids act as an antagonist of the GABAA receptor on primary afferent neurons, probably by reducing the number of functional GABAA receptor ionic channel complexes.     

Temperature-dependent effect of zolpidem on the GABAA receptor-mediated response at recombinant human GABAA receptor subtypes

The effects of zolpidem on the two forms of recombinant human GABA_A receptors (α1β2γ2s and α3β2γ2s) at different temperatures were functionally investigated, using the whole-cell patch recording configuration. In both forms, zolpidem potentiated the response to GABA in a concentration-dependent manner. At 16°C, the apparent dissociation constant (K_D) values for the α1β2γ2s and α3β2γ2s forms were 3.7×10⁻⁸ and 5.6×10⁻⁷ M, respectively. When the temperature was increased to 36°C, the K_D values for the α1β2γ2s and α3β2γ2s forms were 2.1×10⁻⁷ and 1.5×10⁻⁶ M, respectively. Although the affinity ratio was reduced from 15.1 to 7.1-fold the selectivity of zolpidem for the α1β2γ2s still remained at 36°C.     

Calcineurin-mediated GABAA receptor dephosphorylation in rats after kainic acid-induced status epilepticus

Calcineurin (CaN) is a neuronally enriched, calcium-dependent phosphatase, which plays an important role in a number of neuronal processes including development of learning and memory, and modulation of receptor's function and neuronal excitability as well as induction of apoptosis. It has been established in kindling model that the status epilepticus (SE)-induced increase in CaN activity is involved in the development of seizures through down-regulation of γ-aminobutyric acid A receptor (GABA_AR) activation. However, the mechanism by which CaN mediates GABA_A receptor dephosphorylation in SE is not fully understood. Here, using a model of kainic acid (KA)-induced SE and CaN inhibitor FK506, we observed the behaviors induced by KA and levels of CaN activity and CaN expression in hippocampus by immunobloting. The results showed that the SE-induced CaN activity was time-dependent, with a peak at 2 h and a return to basal level at 24 h, whereas a significant increase in CaN expression was seen at 24 h after SE. It is proposed that the rapid elevation in CaN activity after KA-induced SE is not likely due to an increase in CaN expression but rather an increase in CaN activation state or kinetics. In addition, we also demonstrated that pre-treatment with FK506 remarkably suppressed the SE-induced CaN activity and its expression, and reversed the SE-induced dephosphorylation of GABA_AR 2/3 subunits. Taken together, our data suggest that down-regulation in inhibition of GABA_AR 2/3 by CaN activity contributes to an elevation in neuronal excitability of hippocampus, which may be involved in development of chronic processes of seizures.     

Chronic neurosteroid treatment attenuates single cell GABAA response and its potentiation by modulators in cortical neurons

In previous studies we have observed that chronic neurosteroid 5α-pregnan-3α-ol-20-one (5α3α) treatment produced downregulation of the GABA_A receptors, heterologous uncoupling, and decreased heterologous efficacy at the GABA_A receptor complex in cultured mammalian cortical neurons. In this study, using whole cell recording, we examined the consequence of chronic 5α3α (1 μM; 5 days) treatment on GABA-induced currents in isolated cortical neurons. We observed that the GABA current was decreased by 78% after 5 days treatment of cortical cells with 1 μM; 5α3α. We also observed decreased pentobarbital, and 5α3α potentiation of GABA currents after chronic 5α3α treatment. These findings support the notion that GABA response, and its potentiation by pentobarbital, and neurosteroid, 5α3α, are attenuated after chronic 5α 3α treatment.     

Clathrin-coated vesicles from bovine brain contain uncoupled GABAA receptors

Clathrin-coated vesicles are thought to be a vehicle for the sequestration of GABA_A receptors. For coated vesicles from bovine cerebrum, we examined the binding properties of [³H]muscimol, a GABA_A-specific agonist, [³H]flunitrazepam, a benzodiazepine agonist, and [³⁵S]t-butylbiocyclophosphorthionate (TBPS), a ligand for GABA_A receptor channels. Under standard conditions, the binding level of [³H]muscimol, [³H]flunitrazepam, and [³⁵S]TBPS to coated vesicles represented 12.3±1.8%, 7.9±1%, and 10.2±1.8%, respectively, of that in crude synaptic membranes. Coated vesicles showed a single [³H]flunitrazepam binding site with a K_D value (12 nM) which was 9-fold that for synaptic membranes. The allosteric coupling between binding sites was measured by the addition of GABA to [³H]flunitrazepam and [³⁵S]TBPS binding assays. For [³H]flunitrazepam binding to synaptic membranes, GABA gave an EC₅₀=2.0 μM and at saturation (100 μM) an enhancement of 122%. This stimulation was completely blocked by the GABA antagonist SR95531. In contrast, neither GABA nor SR95531 had a significant effect on [³H]flunitrazepam binding to CCVs, indicating that the allosteric interaction between GABA and benzodiazepine binding sites is abolished. Likewise, GABA displaced nearly all of the [³⁵S]TBPS binding to synaptic membranes but had no effect on binding to coated vesicles, indicating that coupling between the GABA binding sites and chloride channel is also impaired. Thus GABA_A receptors appear to be uncoupled during normal intracellular trafficking via coated vesicles. The presence of major GABA_A receptor subunits on these particles was verified by quantitative immunoblotting. Relative to the levels in synaptic membranes, CCVs contained 110±14% and 29.5±3.8%, respectively, of the immunoreactivity for GABA_A receptor β2 and α1 subunits. Thus, in comparison to GABA_A receptors on synaptic membranes, those on CCVs have a reduced α1/β2-subunit ratio. It may be suggested that a selective decline in the content of α1 subunits in coated vesicles could in part account for GABA_A receptor uncoupling.     

Functional analysis of GABAA receptors in nucleus tractus solitarius neurons from neonatal rats

To gain insight into specific GABA_A receptor configurations functionally expressed in the nucleus tractus solitarius (NTS), we conducted several physiological and pharmacological assessments. NTS neurons were characterized in thin brain slices from 1–14 day old rats using whole-cell patch clamp recordings. GABA_A− receptor-mediated currents were detected in all neurons tested, with an average EC₅₀ of 22.2 μM. GABA currents were consistently stimulated by diazepam (EC₅₀=63 nM), zolpidem (EC₅₀=85 nM), loreclezole (EC₅₀=10.1 μM) and the neurosteroid 5α-pregnan-3α-hydroxy-20-one (3α-OH-DHP). In contrast, GABA-gated currents of the NTS were inhibited by the divalent cation Zn²⁺ (IC₅₀=33.6 μM) picrotoxin (IC₅₀=2.4 μM) and blockade of endogenous protein tyrosine kinase. GABA-activated currents were insensitive to furosemide (10–1000 μM) in all NTS neurons tested. Collectively, the data suggest that in neonatal rats, the predominant α subunit isoform present in GABA_A receptors of the NTS appears to be the α1 and/or α2 subunit. β2 and/or β3 subunits are the major β isoform, while the predominant γ subunit is likely γ2. Our data suggest the contribution to NTS GABA currents by α3–α6, β1, γ1 and δ subunits, if present, is minor by comparison.     

Propofol increases agonist efficacy at the GABAA receptor

Using the whole-cell patch-clamp technique, we have determined that propofol, but not midazolam, increases the efficacy of piperidine-4-sulphonic acid (P4S), a partial agonist at α₁β₁γ_2s GABA_A receptors expressed in HEK 293 cells. These findings are consistent with the idea that propofol facilitates receptor gating, while midazolam increases receptor occupancy by the agonist.     

Modulation of acetylcholine release via GABAA and GABAB receptors in rat striatum

In order to investigate whether changes in acetylcholine (ACh) release induced by GABA receptors are due to a direct or indirect effect on cholinergic neurons in the striatum, GABA_A and GABA_B receptor bindings were assayed in the striatum microinjected with ethylcholine mustard aziridinium ion (AF64A), a cholinergic neurotoxin. Intra-striatal injection of a selective concentration of AF64A (10 nmol) reduced GABA_A receptor binding without significantly altering GABA_B receptor binding. Treatment with a higher, less selective concentration of AF64A (20 nmol) reduced all markers examined. These results suggest that GABA_A, but not GABA_B receptors, are located on cholinergic neurons in the striatum, and that GABA can directly modulate ACh release through stimulation of GABA_A receptors. Findings further suggest that GABA can also indirectly modulate ACh release through stimulation of GABA_B receptors located on non-cholinergic neuronal elements in the striatum.     

Subunit dependent modulation of GABAA receptor function by neuroactive steroids

Neurosteroids are potent, endogenous modulators of GABA_A receptor function in the central nervous system. The endogenous progesterone metabolite allopregnanolone (ALP) and the synthetic steroid compound alphaxalone (AFX) have been shown to both directly activate and potentiate GABA_A receptor-activated membrane current (I_GABA). The role of different α and γ subunit subtypes in modulation of I_GABA by ALP and AFX was investigated using recombinant GABA_A receptor isoforms expressed in Xenopus oocytes. Changing or removal of the α subunit subtype altered the efficacy of both ALP and AFX (α2β1γ2L>α1β1γ2Lβ1γ2L) to potentiate I_GABA, but did not alter the potency of the neuroactive steroids at these receptor isoforms. The efficacy of ALP to enhance I_GABA was also dependent on the γ subunit subtype (α1β1γ3>α1β1γ2L=α1β1γ1). AFX also had higher efficacy in the α1β1γ3 receptor isoform compared to α1β1γ1. In contrast to ALP, the potency of AFX was greater in the α1β1γ3 and α1β1γ1 receptor isoforms compared to α1β1γ2L. This study provides evidence that the α subunit subtype determines the efficacy, but not the potency, of these neuroactive steroids to potentiate I_GABA. The γ3 subunit subtype increases the maximal efficacy of neuroactive steroids compared to other γ subunit subtypes. These results suggest that the heteromeric assembly of different GABA_A receptor isoforms containing different subunit subtypes results in multiple steroid recognition sites on GABA_A receptors that in turn produce distinctly different modulatory interactions between neuroactive steroids acting at the GABA_A receptor.     

Effects of repeated amphetamine treatment on regional GABAA receptor binding

The present study examined the effects of repeated exposure to amphetamine on GABA_A receptor binding in cortical and subcortical areas. The goal of the study was to determine whether changes in specific binding were related to behavioral sensitization. Animals were exposed to either saline (0.3 ml, s.c.; n=12) or -amphetamine (2.5 mg/kg, s.c.; n=12) for 6 consecutive days and sacrificed after a 14-day withdrawal period. Differences in GABA_A receptor binding in these two groups of animals were assessed using the GABA_A receptor antagonist [³H]SR 95531. To verify that the preceding treatment regimen led to the development of behavioral sensitization, a separate set of animals (n=8/group) was exposed to the same regimen and challenged with -amphetamine (2.5 mg/kg, s.c.) after the 14-day withdrawal period. As expected, preexposure to amphetamine led to the development of amphetamine sensitization. There were no differences in GABA_A receptor binding in animals preexposed to saline and amphetamine in the prefrontal cortex, caudate-putamen, hypothalamus, or cerebellum. These findings do not provide support for the idea that changes in GABA_A receptor binding in the medial prefrontal cortex or various subcortical areas are related to the development of behavioral sensitization.     

Neonatal allopregnanolone increases novelty-directed locomotion and disrupts behavioural responses to GABAA receptor modulators in adulthood

Recent findings indicate that neurosteroids could act as important modulators during brain development. The aim of the present work is to screen whether developmentally altered AlloP levels may have long-lasting effects on behaviour and influence the emotional response to several GABA_A receptor modulating drugs in adulthood. Acute allopregnanolone administration (10 mg/kg) in the fifth postnatal day: (1) provoked long-term effects, as an increase of the novelty-directed locomotor activity and a decrease of its habituation in the open field in adult rats; (2) altered GABA_A receptor response in adulthood, as reflected by the disruption of the effects of midazolam (1 mg/kg) and flumazenil (10 mg/kg) on the locomotor habituation in adulthood. Whereas the behavioural responses to 0.75 mg/kg of lorazepam or 3 mg/kg of 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP) were not affected, although lorazepam decreased locomotor activity in both neonatal AlloP and control rats, probably related to the sedative properties of the dose tested. Also, in the elevated plus-maze, the anxiolytic effects of lorazepam were observed in controls, but not in neonatal allopregnanolone-treated rats. This suggests that neonatal allopregnanolone decreases sensitivity to the anxiolytic effects of lorazepam at a dose of 0.75 mg/kg. Results suggest that alterations in neonatal allopregnanolone could result in an altered GABA_A receptor response in adulthood that is evident behaviourally. These results point out the importance of the maturation of the endogenous neurosteroid mechanisms in the brain related to locomotor response to novelty and the responses to GABA_A modulators in adulthood. This work opens future directions focused on the effects of acute and long-lasting neonatal alterations of AlloP levels on vulnerability to psychopathology in adulthood.     

The expression of GABAA receptor subunits in the substantia nigra is developmentally regulated and region-specific

The substantia nigra pars reticulata (SNR) controls the spread of seizures. GABA_Aergic drug (agonist or antagonist) infusions into the SNR have age-specific and site-specific effects on flurothyl-induced seizures. Developmental and cell-specific regulation of GABA_A receptor subunit expression may be responsible for these specific effects. To test this hypothesis,in situ hybridization was used to examine regional expression of α1 and γ2L GABA_A receptor subunit mRNAs in the SNR during development. Distinct temporal and spatial patterns of expression were observed. In rats at postnatal days (PN) 21–60, fewer neurons were labeled with probes directed to α1 and γ2L subunits in SNR_anterior compared with SNR_posterior. In addition, neurons in SNR_anterior contained higher amounts of hybridization grains than in SNR_posterior. In PN 15 rats, the labeling of neurons was relatively diffuse throughout the anterior and posterior SNR regions with moderate amounts of hybridization grains for both subunits. The finding of age-related differential distribution of α1 and γ2L subunit mRNAs in the SNR suggests that GABA_A receptor heterogeneity may play a role in the age-specific and site-specific effects of GABA_Aergic agents on seizures in the SNR.

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Sommario La sostanza nera pars reticulata (SNR) controlla la propagazione delle crisi. Infusioni di GABA agonisti o antagonisti nella SNR hanno effetti specifici in rapporto all’età e alla sede sulle crisi indotte con flurothyl, per la probabile influenza di fattori regolatori dell’espressione di subunità del receptore GABA_A specificamente legati allo sviluppo e al tipo cellulare. L’ipotesi è stata verificata ana-lizzando l’ espressione regionale del mRNA delle subunità recettoriali GABA_A α1 e γ2L durante lo sviluppo. Diverse modalità di espressione temporale e spaziale sono state osservate. Nel ratto ad una età postnatale (PN) di 21–60 giorni, il numero di neuroni marcati con sonde dirette alle subunità α1 e γ2L è risultato inferiore rispetto alla posteriore. Inoltre, i neuroni di SNR anteriore contenevano una quantità più elevata di granuli di ibridizzazione rispetto a quelli di SNR posteriore. In ratti PN 15, invece, la marcatura di neuroni è risultata relativamente diffusa nell’ambito delle regioni anteriori e posteriori di SNR con una quantità moderata di granuli di ibridizzazione per entrambe le subunità. Il rilievo di una distribuzione differenziale età dipendente dell’mRNA delle subunità α1 e γ2L nella SNR suggerisce che un’eterogeneità recettoriale possa giocare uno specifico ruolo nel determinare gli effetti età e sito-specifici sulle crisi di agenti GABAergici iniettati nella SNR.

     

GABAA and GABAB receptors modulating basal and footshock-induced nitric oxide releases in rat prefrontal cortex

Using an in vivo brain microdialysis technique, we measured extracellular levels of nitric oxide (NO) metabolites (NO_x⁻) in the medial prefrontal cortex (mPFC) upon perfusion of γ-aminobutyric acid (GABA) receptor antagonists as well as agonists, and also examined the effects of GABA receptor agonists on mild intermittent footshock-induced NO releases in the mPFC in conscious rats. Perfusion of either bicuculline methiodide, a GABA_A receptor antagonist, or saclofen, a GABA_B receptor antagonist, through a microdialysis probe resulted in dose-dependent increases in NO_x⁻ levels. Higher-dose perfusion of either muscimol (50 μM), a GABA_A receptor agonist, or baclofen (250 μM), a GABA_B receptor agonist resulted in a significant decrease in NO_x⁻ levels. The elevated levels of NO_x⁻ after mild intermittent footshock were attenuated by perfusion of either muscimol (10 μM) or baclofen (50 μM), either of which alone did not affect basal NO_x⁻ levels. These findings are likely to provide helpful clues to our understanding of the inhibitory modulation of basal and footshock-induced NO metabolites releases by GABA_A and GABA_B receptors in the mPFC.     

Freeze-fracture immunocytochemical study of the expression of native and recombinant GABAA receptors

To assess the density and distribution of native and recombinant GABA_A receptors we used label-fracture and fracture-flip technologies combined with immunocytochemistry using monoclonal and polyclonal Abs directed against the extracellular domain of the GABA_A receptor protein located in the freeze-fracture replicas. In cortical neurons there is a high density of GABA_A receptors on both soma and dendrites with some areas were the density of receptors is higher, but there are no well defined clusters. In cerebellar granule cells most of the receptors are distributed in round clusters both in neurites and soma. In astroglial cells the receptor density is lower than in neurons and only occasionally they appear in clusters. In cells transfected with cDNAs encoding for various molecular forms of GABA_A receptor subunits, the receptor density is moderate when cDNAs for α, β and γ subnits are cotransfected; however, on cells cotransfected with cDNAs for β and γ subunits the receptor density is significantly lower. Recombinant receptors appear randomly distributed and occasionally they aggregate in small groups.     

Peptide heterogeneity of GABAA/benzodiazepine receptors in bovine cerebral cortex and cerebellum

The GABA_A/benzodiazepine receptor complex has been purified from both bovine cerebral cortex and cerebellum by immunoaffinity chromatography on immobilized monoclonal antibody 62-3G1. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the purified receptor from either cerebral cortex or cerebellum revealed 3 main bands corresponding to 51 000, 55 000 and 57 000M_r silver-stained peptides In addition, a minor band corresponding to a 53 000M_r peptide was also found. The difference between the two receptor preparations were: (1) that the main silver-stained 55 000M_r subunit was present in a relative smaller quantity in cerebellum than in cerebral cortex, and (2 when the membrane-bound receptor was photoaffinity-labeled with [³H]flunitrazepam and subsequently immunoaffinity-purified, two photolabeled peptide bands of 51 000 and 57 000M_r were found in cerebral cortex while only the 51 000M_r photolabeled peptide was detected cerebellum following one-dimension sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Peptide maps of the 57 000M_r [³H]flunitrazepam photoaffinity-labeled peptide indicated that it was composed of two closely migrating photolabeled peptides of 55 000M_r and 57 000M_r 0899 Peptide mapping and deglycosylation experiments using the [³H]flunitrazepam photolabeled receptor suggested that the photolabeled peptides commonly present in cerebellum and cerebral cortex are qualitatively similar if not identical. The results suggest that there are subunits of some type(s) of GABA_AR/BZDR complex(es) which are more abundant in cerebral cortex than in cerebellum. Photoaffinity labeling with [³H]muscimol showed similar photolabeled peptides in both cerebral cortex and cerebellum: two main peptides of 54 000 and 57 000M_r wer photolabeled with [³H]muscimol to a similar extent in both receptor preparations. Following deglycosylation, the mobility shifts of the peptides that were photolabeled with [³H]flunitrazepam or [³H]muscimol were different, suggesting that the co-migrating 54 000 – 57 000M_r peptides that have high affinity binding sites for [³H]flunitrazepam or [³H]muscimol are different receptor subunits.