The pharmacology of the benzodiazepine site of the GABA-A receptor is dependent on the type of gamma-subunit present

Exploring the Utility of remimazolam in cesarean sections under general anesthesia: A preliminary retrospective analysis and Implications for future study

Background

Remimazolam has recently been introduced as a maintenance agent for general anesthesia. However, the effect of remimazolam on peripartum prognosis has not been reported. Therefore, this study aimed to compare the effects of remimazolam and propofol for uterotonic drugs following cesarean section.

Methods

The electronic medical records of 51 adult women who underwent elective cesarean sections by single obstetrician under general anesthesia were collected. Participants were categorized into two groups: the propofol group and the remimazolam group. General anesthesia was maintained by continuous infusion of propofol or remimazolam after delivery. The number of uterotonic drugs administered during the cesarean section, the estimated blood loss (EBL), and length of hospital stay (LOS) after delivery were assessed.

Results

Of the 51 patients included in the study, 35 were in the propofol group and 16 in the remimazolam group. In the remimazolam group, five patients (31.3%, 5/16) received more uterotonics than the standard regimen. Conversely, in the propofol group, 19 patients (54.3%, 19/35) were injected with more uterotonics than the standard regimen. Logistic regression analysis showed that abnormal positioning of the placenta (P = 0.079) and not using remimazolam (P = 0.100) were the most relevant factors associated with the increased use of uterotonics. There was no significant difference in EBL between the two groups. The use of remimazolam was clinically relevant with a shorter LOS (P = 0.059).

Conclusions

The use of remimazolam as a maintenance agent did not result in significantly higher use of intrapartum uterotonics compared to the use of propofol. These results cannot exclude all adverse effects of remimazolam during cesarean delivery. Further randomized controlled trials must be conducted to obtain high-quality evidence.

The Z-Drugs Zolpidem, Zaleplon, and Eszopiclone Have Varying Actions on Human GABA A Receptors Containing γ1, γ2, and γ3 Subunits

γ-Aminobutyric-acid type A (GABA A ) receptors expressing the γ1 or γ3 subunit are only found within a few regions of the brain, some of which are involved in sleep. No known compounds have been reported to selectively target γ1- or γ3-containing GABA A receptors. Pharmacological assessments of this are conflicting, possibly due to differences in experimental models, conditions, and exact protocols when reporting efficacies and potencies. In this study, we evaluated the modulatory properties of five non-benzodiazepine Z-drugs (zaleplon, indiplon, eszopiclone, zolpidem, and alpidem) used in sleep management and the benzodiazepine, diazepam on human α1β2γ receptors using all three γ subtypes. This was accomplished using concatenated GABA A pentamers expressed in Xenopus laevis oocytes and measured via two-electrode voltage clamp. This approach removes the potential for single subunits to form erroneous receptors that could contribute to the pharmacological assessment of these compounds. No compound tested had significant effects on γ1-containing receptors below 10 μM. Interestingly, zaleplon and indiplon were found to modulate γ3-containing receptors equally as efficacious as γ2-containing receptors. Furthermore, zaleplon had a higher potency for γ3- than for γ2-containing receptors, indicating certain therapeutic effects could occur via these γ3-containing receptors. Eszopiclone modulated γ3-containing receptors with reduced efficacy but no reduction in potency. These data demonstrate that the imidazopyridines zaleplon and indiplon are well suited to further investigate potential γ3 effects on sleep in vivo.

Inhibition of the GABAA Receptor by Sulfated Neurosteroids: A Mechanistic Comparison Study between Pregnenolone Sulfate and Dehydroepiandrosterone Sulfate

The γ-aminobutyric acid type A receptor (GABAAR) is negatively modulated by two structurally similar neurosteroids, pregnenolone sulfate (PS) and dehydroepiandrosterone sulfate (DHEAS). This study attempted to ascertain the molecular mechanisms of inhibition of the GABA-ergic current by neurosteroids. We demonstrated that the presence of the γ subunit in GABAAR enhances the efficacy of DHEAS without altering its binding affinity. A saturating concentration of DHEAS blocked approximately 75 % of currents mediated by GABAAR, which is composed of human α1, β1, and γ2S subunits, whereas the inhibition was only 35 % in GABAAR containing only α1 and β1 subunits. The IC50 values of DHEAS with and without the γ subunit were almost identical. In contrast to DHEAS, neither the affinity nor the efficacy of PS was altered by the γ subunit. When Val256 of α1 subunit was mutated to Ser, the mutant channel became resistant to inhibition by both DHEAS and PS. PS exerted its inhibitory effect by enhancing the desensitization kinetics of GABAAR possibly through promoting the interaction between the M2-M3 linker and extracellular loop 7/loop 2. Mutant α1, containing double Cys in loop 2/loop 7 and the M2-M3 linker, formed disulfide bonds three times as much fast, when treated with saturating GABA+PS, compared with GABA alone or with GABA+DHEAS. We demonstrated that PS, but not DHEAS, mediates GABA-ergic inhibition by promoting collisions between the structural elements involved in receptor desensitization, i.e., loop 2, loop 7, and the M2-M3 linker, thus following different inhibitory mechanisms.

Negative modulation of GABAA α5 receptors by RO4938581 attenuates discrete sub-chronic and early postnatal phencyclidine (PCP)-induced cognitive deficits in rats

RATIONALE

A growing body of evidence suggests that negative modulation of γ-aminobutyric acid (GABA) GABA(A) α5 receptors may be a promising strategy for the treatment of certain facets of cognitive impairment; however, selective modulators of GABA(A) α5 receptors have not yet been tested in "schizophrenia-relevant" cognitive assay/model systems in animals.

OBJECTIVES

The objectives of this study were to investigate the potential of RO4938581, a negative modulator of GABA(A) α5 receptors, and to attenuate cognitive impairments induced following sub-chronic (sub-PCP) and early postnatal PCP (neo-PCP) administration in the novel object recognition (NOR) and intra-extradimensional shift (ID/ED) paradigms in rats. Complementary in vitro, ex vivo and in vivo studies were performed to confirm negative modulatory activity of RO4938581 and to investigate animal model validity, concept validity and potential side effect issues, respectively.

RESULTS

In vitro studies confirmed the reported negative modulatory activity of RO4938581, whilst immunohistochemical analyses revealed significantly reduced parvalbumin-positive cells in the prefrontal cortex of sub-PCP- and neo-PCP-treated rats. RO4938581 (1 mg/kg) ameliorated both sub-PCP- and neo-PCP-induced cognitive deficits in NOR and ID/ED performance, respectively. In contrast, QH-II-066 (1 and 3 mg/kg), a GABA(A) α5 receptor positive modulator, impaired cognitive performance in the NOR task when administered to vehicle-treated animals. Additional studies revealed that both RO4938581 (1 mg/kg) and QH-II-066 (1 and 3 mg/kg) attenuated amphetamine-induced hyperactivity in rats.

CONCLUSIONS

Taken together, these novel findings suggest that negative modulation of GABA(A) α5 receptors may represent an attractive treatment option for the cognitive impairments, and potentially positive symptoms, associated with schizophrenia.

Upregulation of high-affinity GABA(A) receptors in cultured rat dorsal root ganglion neurons

Despite evidence that high-affinity GABA(A) receptor subunit mRNA and protein are present in dorsal root ganglia (DRG), low-affinity currents dominate those detected in acutely dissociated DRG neurons in vitro. This observation raises the possibility that high-affinity receptors are normally trafficked out of the DRG toward central and peripheral terminals. We therefore hypothesized that with time in culture, there would be an increase in high-affinity GABA(A) currents in DRG neurons. To test this hypothesis, we studied dissociated DRG neurons 2 h (acute) and 24 h (cultured) after plating with whole-cell patch-clamp techniques, Western blot, and semiquantitative reverse transcriptase polymerase chain reaction (sqRT-PCR) analysis. GABA(A) current density increases dramatically with time in culture in association with the emergence of two persistent currents with EC50's of 0.25±0.01 μM and 3.2±0.02 μM for GABA activation. In a subpopulation of neurons, there was also an increase in the potency of GABA activation of the transient current from an EC50 of 78.16±10.1 μM to 9.56±1.3 μM with time in culture. A fraction of the high-affinity current was potentiated by δ-subunit agonist 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridine-3-ol (THIP). δ-subunit immunoreactivity was largely restricted to the cytosolic fraction in acute, but the membrane fraction in cultured, DRG neurons, with no detectable change in δ-subunit mRNA. However, the emergence of a high-affinity current blocked by THIP and insensitive to bicuculline was detected in a subpopulation of cultured neurons as well in association with an increase in ρ2- and ρ3-subunit mRNA in cultured DRG neurons. Our results suggest that high-affinity δ-subunit-containing GABA(A) receptors are normally trafficked out of the DRG where they are targeted to peripheral and central processes. They also highlight that the interpretation of data obtained from cultured DRG neurons should be made with caution.

The effects of midazolam and sevoflurane on the GABA(A) receptors with alternatively spliced variants of the γ2 subunit

Background

Emergence agitation after sevoflurane anesthesia in children can be prevented by midazolam. Alternative splicing of the GABA_A receptor changes with age. Therefore, we hypothesized that alternative splicing of the γ2 subunit affects the GABA current when applying sevoflurane and midazolam.

Methods

We performed the whole-cell patch clamp technique on human embryonic kidney 293 cells that were transfected with α1β2γ2L or α1β2γ2S. The concentration-response relations were recorded for midazolam and sevoflurane, and the co-application responses were measured at concentrations of 1.5 nM, 15 nM and 300 nM of midazolam and 0.5%, 2.0% and 4.0% of sevoflurane. Each GABA current was compared with that produced by 5 µM of GABA.

Results

The concentration-response relationships for midazolam and sevoflurane were dose-dependent without any differences between the α1β2γ2L and α1β2γ2S subtypes. 1.5 nM and 15 nM of midazolam did not significantly enhance the current after treatment with 0.5% sevoflurane for both subtypes. The current after treatment with 2.0% sevoflurane was enhanced by 1.5 nM midazolam for the α1β2γ2S subtype, but not for the α1β2γ2L subtype. In the case of 2.0% sevoflurane with 15 nM of midazolam, and 4.0% sevoflurane with 300 nM of midazolam, the GABA currents were significantly enhanced for both subtypes.

Conclusions

These results show that the difference in the γ2 subunit cannot explain the emergence agitation after sevoflurane anesthesia in children in vitro. This suggests that co-application of sevoflurane and midazolam enhances the GABA current according to the alternative splicing of the γ2 subunit and the concentration of both drugs.

Agonist function of the recombinant alpha 4 beta 3 delta GABAA receptor is dependent on the human and rat variants of the alpha 4-subunit

1. It is known that the alpha(4)-subunit is likely to occur in the brain predominantly in alpha(4)beta(3)delta receptors at extrasynaptic sites. Recent studies have revealed that the alpha(1)-, alpha(4)-, gamma(2)- and delta-subunits may colocalize extrasynaptically in dentate granule cells of the hippocampus. In the present study, we characterized a series of recombinant GABA(A) receptors containing human (H) and rat (R) alpha(1)/alpha(4)-, beta(2)/beta(3)- and gamma(2S)/delta-subunits in Xenopus oocytes using the two-electrode voltage-clamp technique. 2. Both H alpha(1)beta(3)delta and H alpha(4)beta(3)gamma(2S) receptors were sensitive to activation by GABA and pentobarbital. Contrary to earlier findings that the alpha(4)beta(3)delta combination was more sensitive to agonist action than the alpha(4)beta(3)gamma(2S) receptor, we observed extremely small GABA- and pentobarbital-activated currents at the wild-type H alpha(4)beta(3)delta receptor. However, GABA and pentobarbital activated the wild-type R alpha(4)beta(3)delta receptor with high potency (EC(50) = 0.5 +/- 0.7 and 294 +/- 5 micromol/L, respectively). 3. Substituting the H alpha(4) subunit with R alpha(4) conferred a significant increase in activation on the GABA and pentobarbital site in terms of reduced EC(50) and increased I(max). When the H alpha(4) subunit was combined with the R beta(3) and R delta subunit in a heteropentameric form, the amplitude of GABA- and pentobarbital-activated currents increased significantly compared with the wild-type H alpha(4)beta(3)delta receptor. 4. Thus, the results indicate that the R alpha(4)beta(3)delta, H alpha(1)beta(3)delta and H alpha(4)beta(3)gamma(2S) combinations may contribute to functions of extrasynaptic GABA(A) receptors. The presence of the R alpha(4) subunit at recombinant GABA(A) receptors containing the delta-subunit is a strong determinant of agonist action. The recombinant H alpha(4)beta(3)delta receptor is a less sensitive subunit composition in terms of agonist activation.

Synthesis, pharmacological studies and molecular modeling of some tetracyclic 1,3-diazepinium chlorides

Seven new 1,3-diazepinium chlorides exhibiting some structural similarities to the 1,4-benzodiazepines were synthesized. In a Hippocratic screen using mice, three of these salts, 3-methoxy-6-oxo-7,13-dihydro-6H-benzofuro[2,3-e]pyrido[1,2-a][1,3]diazepin-12-ium chloride (8a), 3-methoxy-9-methyl-6-oxo-7,13-dihydro-6H-benzofuro[2,3-e]pyrido[1,2-a][1,3]diazepin-12-ium chloride (8c) and 3-methoxy-11-methyl-6-oxo-7,13-dihydro-6H-benzofuro[2,3-e]pyrido[1,2-a][1,3]diazepin-12-ium chloride (8e) were examined for their effect on the central nervous system, and their activities compared to that of diazepam. On their own, salts 8a, 8c and 8e solicited no sedative effects on the behaviour of the animals. However, they elicited significant effects in combination with diazepam on diazepam-induced activities such as decreased motor activity, ataxia and loss of righting reflex. Compounds 8a and 8c were fitted into the pharmacophore/receptor model developed by Cook et al. with interaction at the L(1), H(1) and A(2) sites indicating that they are potential inverse agonists of the Bz receptor. The compounds displayed some affinity for the alpha1 isoform of the GABA(A)/BzR (L(Di) interaction) but are non-selective for alpha5 (no L(2) interaction). Results of binding affinity studies showed that compound 8a is mildly selective for the alpha1 receptor although not very potent (K(i)=746.5nM). The significant potentiation of diazepam-induced ataxia and decreased motor activity by compounds 8a and 8c in the Hippocratic screen may be associated with alpha1 selectivity.

Docking of 1,4-benzodiazepines in the alpha1/gamma2 GABA(A) receptor modulator site

Positive allosteric modulation of the GABA(A) receptor (GABA(A)R) via the benzodiazepine recognition site is the mechanism whereby diverse chemical classes of therapeutic agents act to reduce anxiety, induce and maintain sleep, reduce seizures, and induce conscious sedation. The binding of such therapeutic agents to this allosteric modulatory site increases the affinity of GABA for the agonist recognition site. A major unanswered question, however, relates to how positive allosteric modulators dock in the 1,4-benzodiazepine (BZD) recognition site. In the present study, the X-ray structure of an acetylcholine binding protein from the snail Lymnea stagnalis and the results from site-directed affinity-labeling studies were used as the basis for modeling of the BZD binding pocket at the alpha(1)/gamma(2) subunit interface. A tethered BZD was introduced into the binding pocket, and molecular simulations were carried out to yield a set of candidate orientations of the BZD ligand in the binding pocket. Candidate orientations were refined based on known structure-activity and stereospecificity characteristics of BZDs and the impact of the alpha(1)H101R mutation. Results favor a model in which the BZD molecule is oriented such that the C5-phenyl substituent extends approximately parallel to the plane of the membrane rather than parallel to the ion channel. Application of this computational modeling strategy, which integrates site-directed affinity labeling with structure-activity knowledge to create a molecular model of the docking of active ligands in the binding pocket, may provide a basis for the design of more selective GABA(A)R modulators with enhanced therapeutic potential.

Abuse and dependence liability of benzodiazepine-type drugs: GABA(A) receptor modulation and beyond

Over the past several decades, benzodiazepines and the newer non-benzodiazepines have become the anxiolytic/hypnotics of choice over the more readily abused barbiturates. While all drugs from this class act at the GABA(A) receptor, benzodiazepine-type drugs offer the clear advantage of being safer and better tolerated. However, there is still potential for these drugs to be abused, and significant evidence exists to suggest that this is a growing problem. This review examines the behavioral determinants of the abuse and dependence liability of benzodiazepine-type drugs. Moreover, the pharmacological and putative biochemical basis of the abuse-related behavior is discussed.

Contribution of specific binding to the central benzodiazepine site to the brain concentrations of two novel benzodiazepine site ligands

The in vivo occupancy of brain benzodiazepine binding sites by compounds A and B was measured using a [(3)H]Ro 15-1788 binding assay and related to plasma and brain drug concentrations. The plasma concentration associated with 50% occupancy was higher for compound A than compound B (73 and 3.7 nM, respectively), however, there was little difference in the brain concentrations required (73 and 63 nM). Both compounds showed a non-linear relationship between plasma and brain concentrations such that above brain concentrations of approximately 100 nM increasing plasma concentrations did not result in a concomitant increase in brain concentrations. This is consistent with brain concentrations being dependent on a saturable compartment which was postulated to be the benzodiazepine binding site-containing GABA(A) receptors. This hypothesis was tested in alpha1H101R mice, in which the alpha1 subunit of the GABA(A) receptor is rendered insensitive to benzodiazepine binding resulting in an approximate 50% reduction in the total benzodiazepine-containing GABA(A) receptor population. It was shown that the Occ(50) brain concentrations in the alpha1H101R animals was lower (17 nM) than in wild type mice (63 nM), as was the plateau concentration in the brain (105 and 195 nM, respectively). These data suggest measured concentrations of compounds A and B in brain tissue are dependent on receptor expression with a minimal contribution from unbound and non-specifically bound compound.

Characterization of two classes of benzodiazepine binding sites in Schistosoma mansoni

As we have recently shown that GABA should be considered a putative neurotransmitter inSchistosoma mansoni, the present work aimed to search for GABAAreceptors in adult worms using [3H]-flunitrazepam to label the allosteric benzodiazepine binding site which is classically present on GABAAreceptor complexes. We detected a large population (Bmax=8·25±1·1 pmol . mg protein−1) of high affinity (Kd=33·6±1·5 nm) binding sites for flunitrazepam. These sites harboured a singular pharmacological modulation that does not fit well with a mammalian central benzodiazepine receptor, mainly due to a very high affinity for Ro5-4864 and a very low affinity for clonazepam. We also detected a second population of benzodiazepine binding sites labelled with high affinity (IC50=85 nm) by [3H]-PK11195, a selective ligand of the mammalian peripheral benzodiazepine receptor. In conclusion, this work describes the pharmacological properties of a large population of central-like benzodiazepine receptors supporting their study as putative new targets for the development of anti-parasitic agents. We also describe, for the first time, the presence of peripheral benzodiazepine receptors in this parasite.

Agonistic effects of the β-carboline DMCM revealed in GABAA receptor γ2 subunit F77I point-mutated mice

Affinity of the inverse agonist methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM) to the benzodiazepine binding site of the GABA(A) receptor is abolished by a phenylalanine (F) to isoleucine (I) substitution at position 77 of the gamma2 subunit. We tested the effects of DMCM in gene knockin gamma2I77 mice carrying this mutation. Unlike in wild-type mice, DMCM was not able to reverse the GABA-induced reduction of the picrotoxin-sensitive t-butylbicyclophosphoro-[35S]thionate ([35S]TBPS) binding to GABA(A) receptor channels in the forebrain sections of gamma2I77 mice. Accordingly, DMCM was not convulsant in the mutant mice even at doses 20-fold higher (60mg/kg, i.p.) than those producing convulsions in wild-type littermate controls (3 mg/kg, i.p.). Neither did DMCM raise the c-Fos levels in gamma2I77 mouse brain. DMCM additionally exhibits a less well described agonistic effect on GABA(A) receptors that is normally masked by its strong inverse agonist effect. DMCM agonistically enhanced the GABA-induced reduction in [35S]TBPS binding to the cerebellar granule cell layer in control and mutant mice. In vivo DMCM (20-60 mg/kg i.p.) produced modest anxiolytic-like effects in gamma2I77 mice as assessed by elevated plus maze and staircase tests, but no motor impairment was found in the rotarod test. The results suggest only minor agonistic efficacy for the beta-carboline DMCM.

Differential effects of ethanol on gamma-aminobutyric acid-A receptor-mediated synaptic currents in congenic strains of inbred long and short-sleep mice

BACKGROUND

Ethanol enhances gamma-aminobutyric acid (GABA)A receptor-mediated responses in the brain, and this enhancement is greater in a mouse line behaviorally more sensitive to ethanol (long sleep) than in a line (short sleep) behaviorally less ethanol sensitive (assayed by loss of righting; sleep time). Quantitative trait locus (QTL) analysis of inbred long sleep (ILS) and inbred short sleep (ISS) phenotypes revealed four chromosomal regions (Lore1, Lore2, Lore4, and Lore5) that together account for approximately 50% of ethanol-induced sleep-time variance. Congenic strains were generated, each of which is homozygous for one of four ISS Lore QTLs on the ILS background. These congenic mouse strains are ideally suited for asking which QTL regions might correlate with other phenotypes that differ between ILS and ISS mice. Here we used the congenics to investigate altered GABAA responses to ethanol.

METHODS

Evoked GABAA receptor-mediated inhibitory postsynaptic currents (IPSCs) were measured by whole-cell voltage-clamp recording procedures in CA1 pyramidal neurons in hippocampal brain slices.

RESULTS

GABAA IPSC responses in hippocampal brain slices from ILS mice were significantly enhanced by 80 mM ethanol, whereas those from ISS mice were not affected. ILS.Lore2 and ILS.Lore5 congenic strains were significantly enhanced by 80 mM ethanol, similar to the background (control) ILS mice. However, ethanol had no significant effect on GABAA responses in ILS.Lore1 and ILS.Lore4 congenic mice, similar to the ISS mice, thus reflecting the influence of ISS alleles on the ILS phenotype.

CONCLUSIONS

Our results suggest that alleles located in the Lore1 and Lore4 QTL regions confer ethanol sensitivity of GABAA receptor-mediated IPSCs. Thus, for these QTLs, GABAA IPSCs may represent an endophenotype of sedative/hypnotic sensitivity to ethanol. Although the Lore2 and Lore5 QTL regions have a significant effect on sleep time, they do not play a significant role in the differential ethanol enhancement of GABAA IPSCs between ILS and ISS mice.

Does amygdaloid corticotropin-releasing hormone (CRH) mediate anxiety-like behaviors? Dissociation of anxiogenic effects and CRH release

The brain corticotropin-releasing hormone (CRH) circuits are activated by stressful stimuli, contributing to behavioral and emotional responses. The present study assessed anxiety-like responses and in vivo neurochemical alterations at the central nucleus of the amygdala (CeA) evoked by exposure to an unfamiliar (anxiogenic) environment. Also, the impact of anxiolytic treatments and those that affect CRH were assessed in this paradigm. Novel environment (new cage) markedly suppressed ingestion of a palatable snack. This effect was dose-dependently antagonized by diazepam and was utilized as an index of anxiety in the rodent. Although exposure to a novel environment also stimulated the in vivo release of CRH and glutamate at the CeA, various CRH antagonists (e.g. alphah-CRH, Calpha-MeCRH, CP-154,526, antisauvagine-30, preproTRH178-199) did not attenuate the stressor-elicited behavioral suppression, although Calpha-MeCRH was found to attenuate the freezing response elicited by contextual stimuli that were associated with previously administered footshock. Moreover, central infusion of CRH failed to suppress snack consumption in the home cage. Although diazepam had potent anxiolytic effects in this paradigm, this treatment did not prevent the stressor-associated release of CRH and glutamate at the CeA. Thus, while neural circuits involving CRH and/or glutamatergic receptors at the CeA may be activated by an unfamiliar environment, the data challenge the view that activation of these receptors is necessary for the expression of anxiety-like behavioral responses. Rather than provoking anxiety, these systems might serve to draw attention to events or cues of biological significance, including those posing a threat to survival.

Inhibition of type A GABA receptors by L-type calcium channel blockers

Modulation of type A GABA receptors (GABAA) by L-type Ca++ channel blockers was investigated. The dihydropyridines nifedipine and nitrendipine, and the phenylalkylamine verapamil inhibited recombinant rat alpha1beta2gamma2 receptors recorded from human embryonic kidney (HEK) 293 cells; nifedipine at low concentrations also elicited modest stimulatory effects on GABA-gated current. The IC50 for GABA current inhibition was lowest for nitrendipine (17.3 +/- 1.3 microM), so subsequent studies were focused on further exploring its mechanism and possible site of action. When co-applied with GABA, nitrendipine had minimal effects on initial current amplitude, but significantly enhanced current decay rate. Nitrendipine-mediated inhibition was subunit-selective, as its IC50 was 10-fold lower in alpha1beta2 receptors. Nitrendipine's effect in recombinant human alpha1beta2gamma2 receptors was similar (IC50=23.0 +/- 1.3 microM) to that observed in rat receptors of the same configuration, indicating the site of action is conserved in the two species. The inhibitory effects were dependent on channel gating, were independent of transmembrane voltage, and were also observed in GABAA receptors recorded from hypothalamic brain slices. The pharmacologic mechanism of inhibition by nitrendipine was non-competitive, indicating it does not act at the GABA binding site. Nitrendipine block was retained in the presence of the benzodiazepine antagonist flumazenil, indicating it does not interact at the benzodiazepine site. The actions of nitrendipine were not affected by a mutation (beta2T246F) that confers resistance to the channel blocker picrotoxin, and they were not altered in the presence of the picrotoxin site antagonist alpha-isopropyl-alpha-methyl-gamma-butyrolactone, demonstrating nitrendipine does not act at the picrotoxin site of the GABAA receptor. Possible interaction of nitrendipine with the Zn++ site was also eliminated, as mutation of beta2 H267 to A, which confers resistance to Zn++, had no effect on nitrendipine-mediated inhibition. Our data suggest some of the central effects of dihydropyridines may be due to actions at GABAA receptors. Moreover, the effects may be mediated through interaction with a novel modulatory site on the GABAA receptor.

Deletion of the alpha1 or beta2 subunit of GABAA receptors reduces actions of alcohol and other drugs

Enhancement of the activation of GABAA receptors is a common feature of many sedative and hypnotic drugs, and it is probable that the GABAA receptor complex is a molecular target for these drugs in the mammalian central nervous system. We set out to elucidate the role of the two predominant (alpha1 and beta2) subunits of GABAA receptor in sedative drug action by studying mice lacking these two subunits. Both alpha1 (-/-) and beta2 (-/-) null mutant mice showed markedly decreased sleep time induced by nonselective benzodiazepine, flurazepam, and GABAA agonist, 4,5,6,7-tetrahydroisoxazolo(5,4-c)pyridin-3-ol. The sleep time induced by the beta-selective drug etomidate was decreased only in beta2 (-/-) knockout mice. In contrast, alpha1 (-/-) mice were more resistant to the alpha1-selective drug zolpidem than beta2 (-/-) or wild-type animals. Knockout mice of both strains were similar to wild-type mice in their responses to pentobarbital. The duration of loss of the righting reflex produced by ethanol was decreased in male mice for both null alleles compared with wild-type mice, but there were no differences in ethanol-induced sleep time in mutant females. Deletion of either the alpha1 or beta2 subunits reduced the muscimol-stimulated 36Cl36 influx in cortical microsacs suggesting that these mutant mice have reduced number of functional brain GABAA receptors. Our results show that removal of either alpha1 or beta2 subunits of GABAA receptors produce strong and selective decreases in hypnotic effects of different drugs. Overall, these data confirm the crucial role of the GABAA receptor in mechanisms mediating sedative/hypnotic effects.

Functional characterization of GABA(A) receptors in neonatal hypothalamic brain slice

The hypothalamus influences a number of autonomic functions. The activity of hypothalamic neurons is modulated in part by release of the inhibitory neurotransmitter GABA onto these neurons. GABA(A) receptors are formed from a number of distinct subunits, designated alpha, beta, gamma, delta, epsilon, and theta, many of which have multiple isoforms. Little data exist, however, on the functional characteristics of the GABA(A) receptors present on hypothalamic neurons. To gain insight into which GABA(A) receptor subunits are functionally expressed in the hypothalamus, we used an array of pharmacologic assessments. Whole cell recordings were made from thin hypothalamic slices obtained from 1- to 14-day-old rats. GABA(A) receptor-mediated currents were detected in all neurons tested and had an average EC(50) of 20 +/- 1.6 microM. Hypothalamic GABA(A) receptors were modulated by diazepam (EC(50) = 0.060 microM), zolpidem (EC(50) = 0.19 microM), loreclezole (EC(50) = 4.4 microM), methyl-6,7-dimethoxy-4-ethyl-beta-carboline (EC(50) = 7.7 microM), and 5alpha-pregnan-3alpha-hydroxy-20-one (3alpha-OH-DHP). Conversely, these receptors were inhibited by Zn(2+) (IC(50) = 70.5 microM), dehydroepiandrosterone sulfate (IC(50) = 16.7 microM), and picrotoxin (IC(50) = 2.6 microM). The alpha4/6-selective antagonist furosemide (10-1,000 microM) was ineffective in all hypothalamic neurons tested. The results of our pharmacological analysis suggest that hypothalamic neurons express functional GABA(A) receptor subtypes that incorporate alpha1 and/or alpha2 subunits, beta2 and/or beta3 subunits, and the gamma2 subunit. Our results suggest receptors expressing alpha3-alpha6, beta1, gamma1, and delta, if present, represent a minor component of functional hypothalamic GABA(A) receptors.

Drug interactions at GABA(A) receptors

Neurotransmitter receptor systems have been the focus of intensive pharmacological research for more than 20 years for basic and applied scientific reasons, but only recently has there been a better understanding of their key features. One of these systems includes the type A receptor for the gamma-aminobutyric acid (GABA), which forms an integral anion channel from a pentameric subunit assembly and mediates most of the fast inhibitory neurotransmission in the adult vertebrate central nervous system. Up to now, depending on the definition, 16-19 mammalian subunits have been cloned and localized on different genes. Their assembly into proteins in a poorly defined stoichiometry forms the basis of functional and pharmacological GABA(A) receptor diversity, i.e. the receptor subtypes. The latter has been well documented in autoradiographic studies using ligands that label some of the receptors' various binding sites, corroborated by recombinant expression studies using the same tools. Significantly less heterogeneity has been found at the physiological level in native receptors, where the subunit combinations have been difficult to dissect. This review focuses on the characteristics, use and usefulness of various ligands and their binding sites to probe GABA(A) receptor properties and to gain insight into the biological function from fish to man and into evolutionary conserved GABA(A) receptor heterogeneity. We also summarize the properties of the novel mouse models created for the study of various brain functions and review the state-of-the-art imaging of brain GABA(A) receptors in various human neuropsychiatric conditions. The data indicate that the present ligands are only partly satisfactory tools and further ligands with subtype-selective properties are needed for imaging purposes and for confirming the behavioral and functional results of the studies presently carried out in gene-targeted mice with other species, including man.

Differences in potency and efficacy of a series of phenylisopropylamine/phenylethylamine pairs at 5-HT(2A) and 5-HT(2C) receptors

The pharmacological profile of a series of (+/-)-2,5-dimethoxy-4-(X)-phenylisopropylamines (X=I, Br, NO(2), CH(3), or H) and corresponding phenylethylamines, was determined in Xenopus laevis oocytes injected with cRNA coding for rat 5-HT(2A) or 5-HT(2C) receptors. The efficacy and relative potency of these drugs were determined and compared to classical 5-HT(2) receptor agonists and antagonists. The rank order of agonist potency at the 5-HT(2A) receptor was: alpha-methyl-5-HT=5-HT>m-CPP>MK-212; at the 5-HT(2C) receptor the order was: 5-HT>alpha-methyl-5-HT>MK-212>m-CPP. All these compounds were full agonists at the 5-HT(2C) receptor, but alpha-methyl-5-HT and m-CPP showed lower efficacy at the 5-HT(2A) receptor. 4-(4-Fluorobenzoyl)-1-(4-phenylbutyl)piperidine (4F 4PP) was 200 times more potent as a 5-HT(2A) antagonist than at 5-HT(2C) receptors. Conversely, RS 102221 was 100 times more potent as a 5-HT(2C) antagonist, confirming their relative receptor selectivities. The phenylisopropylamines were partial agonists at the 5-HT(2A) receptor, with I(max) relative to 5-HT in the 22+/-7 to 58+/-15% range; the corresponding phenylethylamines had lower or undetectable efficacies. All these drugs had higher efficacies at 5-HT(2C) receptors; DOI was a full 5-HT(2C) agonist. 2C-I and the other phenylethylamines examined showed relative efficacies at the 5-HT(2C) receptor ranging from 44+/-10% to 76+/-16%. 2C-N was a 5-HT(2) receptor antagonist; the mechanism was competitive at the 5-HT(2A), but non-competitive at the 5-HT(2C) receptor. The antagonism was time-dependent at the 5-HT(2C) receptor but independent of pre-incubation time at the 5-HT(2A) receptor subtype. The alpha-methyl group determines the efficacy of these phenylalkylamines at the 5-HT(2A) and 5-HT(2C) receptors.

GABAAreceptor subunit interactions important for benzodiazepine and zinc modulation: a patch-clamp and single cell RT-PCR study

The expression of mRNAs for the GABAA receptor subunits alpha1, alpha6, beta2, beta3, gamma2 and delta in single mouse cerebellar granule cells and cortical interneurons were analysed by RT-PCR and correlated to their midazolam and zinc modulation of agonist-induced receptor currents. The registration of molecular and electrophysiological data from each cell allowed us to estimate the significance of individual subunits and their two-factor interaction for modulation. The presence of alpha6 decreased midazolam modulation, but statistical analysis also suggested interactions of alpha6 with beta3 and gamma2 with respect to midazolam modulation. Zinc modulation was decreased by the presence of gamma2, and analysis points to an beta3 effect as well as an interaction between gamma2 and delta in zinc modulation. Thus, our model confirmed, in single native cells, the known effects of alpha6 in midazolam and gamma2 in zinc modulation, and additionally pointed to significant subunit interactions that need to be further tested in recombinant receptors. The present study offers a method to identify subunit interactions in heteromeric receptor complexes.

Functional analysis of GABA(A) 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 EC(50) of 22.2 microM. GABA currents were consistently stimulated by diazepam (EC(50)=63 nM), zolpidem (EC(50)=85 nM), loreclezole (EC(50)=10.1 microM) and the neurosteroid 5alpha-pregnan-3alpha-hydroxy-20-one (3alpha-OH-DHP). In contrast, GABA-gated currents of the NTS were inhibited by the divalent cation Zn(2+) (IC(50)=33.6 microM) picrotoxin (IC(50)=2.4 microM) and blockade of endogenous protein tyrosine kinase. GABA-activated currents were insensitive to furosemide (10-1000 microM) in all NTS neurons tested. Collectively, the data suggest that in neonatal rats, the predominant alpha subunit isoform present in GABA(A) receptors of the NTS appears to be the alpha1 and/or alpha2 subunit. beta2 and/or beta3 subunits are the major beta isoform, while the predominant gamma subunit is likely gamma2. Our data suggest the contribution to NTS GABA currents by alpha3-alpha6, beta1, gamma1 and delta subunits, if present, is minor by comparison.

Differential expression of GABA(B)R1 and GABA(B)R2 receptor immunoreactivity in neurochemically identified neurons of the rat neostriatum

Gamma-aminobutyric acid (GABA) is the major inhibitory neurotransmitter in the neostriatum. Functions of GABA are known to mediate GABA(A) and GABA(B) receptors. A functional GABA(B) receptor is known to compose of heteromeric subunits, namely the GABA(B)R1 and GABA(B)R2 subunits. Our previous report (Yung et al. [1999] Brain Res. 830:345-352) has demonstrated that all major subpopulations of striatal neurons express GABA(B)R1 immunoreactivity. The cellular localization of the second subunit of GABA(B) receptor protein, i.e., GABA(B)R2 immunoreactivity, in the rat neostriatum is not yet known. By using a new commercially available specific antibody against GABA(B)R2, immunofluorescence was performed to investigate the cellular expression of GABA(B)R2 in neurochemically identified subpopulations of neurons in the rat neostriatum. Immunoreactivity for GABA(B)R2 was primarily found in the neuropil of the rat neostriatum. Double labeling revealed that those perikarya that expressed immunoreactivity for parvalbumin, choline acetyltransferase, nitric oxide synthase, glutamate receptor two, N-methyl-D-aspartate receptor one, or GABA(A)alpha1 receptor, respectively, did not express GABA(B)R2 immunoreactivity. In addition, perikarya and most of the neuropilar elements in the neostriatum that expressed glutamic acid decarboxylase 67 immunoreactivity were found to be GABA(B)R2-negative. In contrast, immunoreactivity for GABA(B)R1 was found to be expressed by all of the above neuronal subpopulations. Moreover, a vast number of SV2-immunoreactive profiles and a number of tyrosine hydroxylase-immunoreactive profiles in the neuropil of the neostriatum were found to display GABA(B)R2 immunoreactivity. The present results indicate that there is a differential expression of GABA(B)R2 and GABA(B)R1 immunoreactivity in different subpopulations of striatal neurons that are identified by their specific neurochemical markers. Immunoreactivity for GABA(B)R2 is likely to localize in neuropilar elements of the neostriatum that may belong to non-GABAergic elements. These findings provide anatomical evidence of GABA(B)R2 receptor localization in the neostriatum that may have an important functional implication of the GABA(B)-mediated functions in neurons of the neostriatum.

Stable GABA(A) receptor intermediates in SF-9 cells expressing alpha1, beta2 and gamma2 subunits

Spodoptera frugiperda insect cells (Sf-9 cells) were used to study GABA(A) receptor assembly. Time courses of the expression level of alpha1beta2 and alpha1beta2gamma2 receptor protein showed [(3)H]muscimol binding to appear 2 hr before [(3)H]flunitrazepam and [(35)S]TBPS binding. This indicates that muscimol may bind to pentamers with an immature conformation or to molecules smaller than the pentamer. Binding studies performed on fractions from sucrose gradients loaded with solubilized alpha1beta2 or alpha1beta2gamma2 containing membranes revealed no binding other than to the pentameric fractions. Western blotting on fractionated sucrose gradients, however, clearly revealed the existence of GABA(A) receptor intermediates. The alpha1 subunit was seen in fractions corresponding to molecules smaller than the pentamer only when co-expressed with gamma2, indicating that the gamma2 subunit is needed for the alpha1 to form relatively long lasting intermediates. Moreover, Western blots revealed multiple isoforms for each subunit. In general, it was primarily the lower molecular weight forms that were detected in the pentameric fractions. The exception being for the alpha1 and gamma2 forms in subunit combinations that did not contain both of these subunits (i.e., alpha1, gamma2, alpha1beta2, beta2gamma2), where higher molecular weight forms were strongly represented. These findings show that alpha1 and gamma2 prefer specific protein forms when expressed together.

Subunit composition and pharmacological characterization of gamma-aminobutyric acid type A receptors in frog pituitary melanotrophs

The frog pars intermedia is composed of a single population of endocrine cells directly innervated by gamma-aminobutyric acid (GABA)ergic nerve terminals. We have previously shown that GABA, acting through GABA(A) receptors, modulates both the electrical and secretory activities of frog pituitary melanotrophs. The aim of the present study was to take advantage of the frog melanotroph model to determine the relationship between the subunit composition and the pharmacological properties of native GABA(A) receptors. Immunohistochemical labeling revealed that in situ and in cell culture, frog melanotrophs were intensely stained with alpha2-, alpha3-, gamma2-, and gamma3-subunit antisera and weakly stained with a gamma1-subunit antiserum. Melanotrophs were also immunolabeled with a monoclonal antibody to the beta2/beta3-subunit. In contrast, frog melanotrophs were not immunoreactive for the alpha1-, alpha5-, and alpha6-isoforms. The effects of allosteric modulators of the GABA(A) receptor on GABA-activated chloride current were tested using the patch-clamp technique. Among the ligands acting at the benzodiazepine-binding site, clonazepam (EC50, 5 x 10(-9) M), diazepam (EC50, 10(-8) M), zolpidem (EC50, 3 x 10(-8) M), and beta-carboline-3-carboxylic acid methyl ester (EC50, 10(-6) M) were found to potentiate the whole cell GABA-evoked current in a dose-dependent manner. Methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (IC50, 3 x 10(-5) M) inhibited the current, whereas Ro15-4513 had no effect. Among the ligands acting at other modulatory sites, etomidate (EC50, 2 x 10(-6) M) enhanced the GABA-evoked current, whereas 4'-chlorodiazepam (IC50, 4 x 10(-7) M), ZnCl2 (IC50, >5 x 10(-5) M), and furosemide (IC50, >3 x 10(-4) M) depressed the response to GABA. PK 11195 did not affect the GABA-evoked current or its inhibition by 4'-chlorodiazepam. The results indicate that the native GABA(A) receptors in frog melanotrophs are formed by combinations of alpha2-, alpha3-, beta2/3-, gamma1-, gamma2-, and gamma3-subunits. The data also demonstrate that clonazepam is the most potent, and zolpidem is the most efficient positive modulator of the native receptors. Among the inhibitors, 4'-chlorodiazepam is the most potent, whereas ZnCl2 is the most efficient negative modulator of the GABA(A) receptors. The present study provides the first correlation between subunit composition and the functional properties of native GABA(A) receptors in nontumoral endocrine cells.

Pharmacophore/receptor models for GABA(A)/BzR subtypes (alpha1beta3gamma2, alpha5beta3gamma2, and alpha6beta3gamma2) via a comprehensive ligand-mapping approach

Pharmacophore/receptor models for three recombinant GABA(A)/BzR subtypes (alpha1beta3gamma2, alpha5beta3gamma2, and alpha6beta3gamma2) have been established via an SAR ligand-mapping approach. This study was based on the affinities of 151 BzR ligands at five distinct (alpha1-3,5,6beta3gamma2) recombinant GABA(A)/BzR receptor subtypes from at least nine different structural families. Examination of the included volumes of the alpha1-, alpha5-, and alpha6-containing subtypes indicated that region L(2) for the alpha5-containing subtype appeared to be larger in size than the analogous region of the other receptor subtypes. Region L(Di), in contrast, appeared to be larger in the alpha1 subtype than in the other two subtypes. Moreover, region L(3) in the alpha6 subtype is either very small or nonexistent in this diazepam-insensitive subtype (see Figure 16 for details) as compared to the other subtypes. Use of the pharmacophore/receptor models for these subtypes has resulted in the design of novel BzR ligands (see 27) selective for the alpha5beta3gamma2 receptor subtype. alpha5-Selective ligand 27 when injected directly into the hippocampus did enhance memory in one paradigm (Bailey et al., unpublished observations); however, systemic administration of either 9 or 27 into animals did not provide an observable enhancement. This result is in complete agreement with the observation of Liu (1996). It has been shown (Liu, 1996; Wisden et al., 1992) that in the central nervous system of the rat (as well as monkeys and pigeons) there are several native subtypes of the GABA(A) receptor which exhibit different functions, regional distributions, and neuronal locations. Although 27 binds more potently at alpha5beta3gamma2 receptor subtypes and is clearly an inverse agonist (Liu et al., 1996; Liu, 1996), it is possible that this ligand acts as an agonist at one or more subtypes. Liu (1996) clearly showed that a number of imidazobenzodiazepines were negative modulators at one subtype and agonists at another. Therefore, selectivity for a particular subtype at this point is not sufficient to rule out some physiological effect at other GABA(A)/BzR subtypes. The inability of 27 to potentiate memory when given systemically is again in support of this hypothesis, especially since alpha1beta2gamma2 subtypes are distributed throughout the brain (Wisden et al., 1992). A drug delivered systemically is far more likely to interact with all subtypes than one delivered to a specific brain region. This observation (systemic vs intrahippocampal) provides further support for the design of more subtype-specific ligands at the BzR to accurately define their pharmacology, one key to the design of new drugs with fewer side effects.

Modulation of bistratified cell IPSPs and basket cell IPSPs by pentobarbitone sodium, diazepam and Zn2+: dual recordings in slices of adult rat hippocampus

Simultaneous intracellular recordings from presynaptic Stratum pyramidale interneurons and postsynaptic pyramidal cells in adult rat hippocampal slices were performed to investigate the strength of the modulation of single-axon inhibitory postsynaptic potentials (IPSPs) by the GABAA receptor modulators pentobarbitone, diazepam and zinc. The processing of biocytin-filled interneurons for light microscopy revealed that these single-axon IPSPs were generated by basket cells (n = 33), bistratified cells (n = 18) and axo-axonic cells (n = 2). The IPSPs generated by these three groups of interneurons had amplitudes and widths at half amplitude with similar ranges, but when bistratified cell IPSPs were compared with basket cell IPSPs with similar half widths their rise times were slower. Pentobarbitone sodium (250 microM) powerfully enhanced 13 tested IPSPs generated by all three cell types. Amplitudes were enhanced by 82 +/- 56%, 10-90% rise times by 150 +/- 101% and the widths at half amplitude by 71 +/- 29%. Diazepam (1-2 microM) also increased all IPSPs tested, although the changes were more moderate in basket cell IPSPs (amplitudes increased by 19 +/- 11%, n = 8) than in bistratified cell IPSPs (amplitudes increased by 66 +/- 48%, n = 5). Basket cell IPSP 10-90% rise times and widths at half amplitude were not significantly increased. Bistratified cell IPSP 10-90% rise times were increased by 44 +/- 24% and the widths at half amplitude by 32 +/- 35%. The one tested IPSP generated by an axo-axonic cell was also diazepam-sensitive. Zinc, 250 microM, decreased four out of 10 IPSPs generated by basket cells and four out of five IPSPs generated by bistratified cells. The one tested axo-axonic cell IPSP was zinc-insensitive. These data suggest that IPSPs generated in CA1 pyramidal cells by basket and bistratified cells display different pharmacologies and may be mediated by different receptors or receptor combinations.

Incremental conductance levels of GABAA receptors in dopaminergic neurones of the rat substantia nigra pars compacta

1. Molecular and biophysical properties of GABAA receptors of dopaminergic (DA) neurones of the pars compacta of the rat substantia nigra were studied in slices and after acute dissociation. 2. Single-cell reverse transcriptase-multiplex polymerase chain reaction confirmed that DA neurones contained mRNAs encoding for the alpha3 subunit of the GABAA receptor, but further showed the presence of alpha4 subunit mRNAs. alpha2, beta1 and gamma1 subunit mRNAs were never detected. Overall, DA neurones present a pattern of expression of GABAA receptor subunit mRNAs containing mainly alpha3/4beta2/3gamma3. 3. Outside-out patches were excised from DA neurones and GABAA single-channel patch-clamp currents were recorded under low doses (1-5 microM) of GABA or isoguvacine, a selective GABAA agonist. Recordings presented several conductance levels which appeared to be integer multiples of an elementary conductance of 4-5 pS. This property was shared by GABAA receptors of cerebellar Purkinje neurones recorded in slices (however, with an elementary conductance of 3 pS). Only the 5-6 lowest levels were analysed. 4. A progressive change in the distribution of occupancy of these levels was observed when increasing the isoguvacine concentration (up to 10 microM) as well as when adding zolpidem (20-200 nM), a drug acting at the benzodiazepine binding site: both treatments enlarged the occupancy of the highest conductance levels, while decreasing that of the smallest ones. Conversely, Zn2+ (10 microM), a negative allosteric modulator of GABAA receptor channels, decreased the occupancy of the highest levels in favour of the lowest ones. 5. These properties of alpha3/4beta2/3gamma3-containing GABAA receptors would support the hypothesis of either single GABAA receptor channels with multiple open states or that of a synchronous recruitment of GABAA receptor channels that could involve their clustering in the membranes of DA neurones.

Functional GABAA receptor heterogeneity of acutely dissociated hippocampal CA1 pyramidal cells

CA1 pyramidal cells were voltage clamped, and GABA was applied to individual cells with a modified U-tube, rapid drug application system. With Vh = -50 mV, inward currents elicited by 10 microM GABA were inhibited by GABAA receptor (GABAR) antagonists and were baclofen insensitive, suggesting that GABA actions on isolated CA1 pyramidal cells were GABAR mediated. GABA concentration-response curves averaged from all cells were fitted best with a two-site equation, indicating the presence of at least two GABA binding sites, a higher-affinity site (EC50-1 = 11.0 microM) and a lower-affinity site (EC50-2 = 334.2 microM), on two or more populations of cells. The effects of GABAR allosteric modulators on peak concentration-dependent GABAR currents were complex and included monophasic (loreclezole) or multiphasic (diazepam) enhancement, mixed enhancement/inhibition (DMCM, zolpidem) or multiphasic inhibition (zinc). Monophasic (70% of cells) or biphasic (30% of cells) enhancement of GABAR currents by diazepam suggested three different sites on GABARs (EC50-1 =1.8 nM; EC50-2 = 75.8 nM; EC50-3 = 275.9 nM) revealing GABAR heterogeneity. The imidazopyridine zolpidem enhanced GABAR currents in 70% of cells with an EC50 = 222.5 nM, suggesting a predominance of moderate affinity alpha2 (or alpha3-) subtype-containing BZ Type IIA receptors. A small fraction of cells (10%) had a high affinity for zolpidem, something that is suggestive of alpha1 subtype-containing BZ Type I receptors. The remaining 30% of cells were insensitive to or inhibited by zolpidem, suggesting the presence of alpha5 subtype-containing BZ Type IIB receptors. Whether BZ Type I and Type II receptors coexist could not be determined. The beta-carboline methyl 6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM) inhibited GABAR currents in all cells at midnanomolar concentrations, but in addition, potentiated GABAR currents in some cells at low nanomolar concentrations, characterizing two groups of cells, the latter likely due to functional assembly of alpha5betaxgamma2GABARs. In all cells, GABAR currents were moderately sensitive (EC50 = 9 microM) to loreclezole, consistent with a relatively greater beta3 subtype, than beta1 subtype, subunit mRNA expression. Two populations of cells were identified based on their sensitivities to zinc(IC50 = 28 and 182 microM), suggesting the presence of at least two GABAR isoforms including alpha5beta3gamma2 GABARs. Consistent with the heterogeneity of expression of GABAR subunit mRNA and protein in the hippocampus and based on their differential responses to GABA and to allosteric modulators, distinct populations of CA1 pyramidal cells likely express multiple, functional GABAR isoforms.

The effects of a point mutation of the beta2 subunit of GABA(A) receptor on direct and modulatory actions of general anesthetics

The gamma-aminobutyric acid type A receptor (GABA(A) receptor) sites involved in the direct and modulatory actions of general anesthetics remain to be elucidated. The mutation of tyrosine at position 157 in the beta2 GABA(A) receptor subunit was reported to reduce sensitivity to activation by GABA, but not pentobarbital. We examined whether this mutation of the beta2 subunit (Tyr157-->Phe) affects the direct and modulatory actions of other general anesthetics such as propofol and etomidate. Using the two-electrode voltage clamp method, we recorded Cl- current in Xenopus oocytes expressing alpha1beta2gamma2s and alpha1-mutated beta2gamma2s subunits. The mutation of the beta2 subunit reduced the apparent affinity for propofol. However, the mutation had no effect on both the direct actions of pentobarbital and etomidate or on the modulatory actions of pentobarbital, propofol and etomidate. These results suggest that unique loci may exist for the direct action of propofol and that the GABA binding site may not mediate the modulatory actions of general anesthetics at GABA(A) receptors.

Zolpidem-associated hallucinations and serotonin reuptake inhibition: a possible interaction

BACKGROUND

Zolpidem (Ambien) is a sedative believed to act exclusively at the benzodiazepine omega 1 receptor. Sporadic case reports of zolpidem-associated hallucinations have appeared over the past 5 years, and over the past 2 years, the Washington Poison Center received five reports of prolonged visual hallucinations associated with zolpidem.

CASE REPORTS

All five patients reported experiencing visual hallucinations lasting from 1-7 hours soon after taking zolpidem. Most had been taking zolpidem for less than a week and all five were concurrently taking an antidepressant: sertraline, desipramine, fluoxetine, bupropion, or venlafaxine; two sought assistance at a hospital.

DISCUSSION

The precise mechanism of zolpidem-associated hallucinations remains unknown. In some previously published cases, the zolpidem-associated hallucinations have been short in duration, lasting at most 30 minutes. In contrast, the five patients in our series and in five previously reported cases, the hallucinations were more persistent, lasting up to 7 hours. Of these ten cases with persistent symptoms, nine were concurrently taking antidepressants that inhibit serotonin-reuptake, despite the fact that zolpidem has no known serotonin-mediated mechanisms.

CONCLUSIONS

These cases, plus prior case reports, suggest that a pharmacodynamic interaction between serotonin reuptake inhibition and zolpidem may lead to prolonged zolpidem-associated hallucinations in susceptible individuals.

Key amino acids in the gamma subunit of the gamma-aminobutyric acidA receptor that determine ligand binding and modulation at the benzodiazepine site

Pharmacological analyses of gamma-aminobutyric acidA (GABAA) receptor subtypes have suggested that both the alpha and gamma subunits, but not the beta subunit, contribute to the benzodiazepine binding site. We took advantage of the different pharmacological properties conferred by the inclusion of different gamma subunits in the receptor macromolecule to identify amino acids gamma2Phe77 and gamma2Met130 as key determinants of the benzodiazepine binding site. gamma2Phe77 was required for high affinity binding of the benzodiazepine site ligands flumazenil, CL218,872, and methyl-beta-carboline-3-carboxylate but not flunitrazepam. This amino acid was, however, required for allosteric modulation by flunitrazepam, as well as other benzodiazepine site ligands. In contrast, gamma2Met130 was required for high affinity binding of flunitrazepam, clonazepam, and triazolam but not flumazenil, CL218, 872, or methyl-beta-carboline-3-carboxylate and did not affect benzodiazepine efficacy. Introduction of the phenylalanine and methionine into the appropriate positions of gamma1 was not sufficient to confer high affinity for the benzodiazepine site ligand zolpidem. These data show that gamma2Phe77 and gamma2Met130 are necessary for high affinity binding of a number of benzodiazepine site ligands. Although most previous studies have focused on the contribution of the alpha subunit, we demonstrated a critical role for the gamma subunit at the benzodiazepine binding site, indicating that this modulatory site is located at the interface of these two subunits. Furthermore, gamma2Phe77 is homologous to alpha1Phe64, which has been previously shown to be a key determinant of the GABA binding site, suggesting a conservation of motifs between different ligand binding sites on the GABAA receptor.

GABAA receptor subtypes differentiated by their gamma-subunit variants: prevalence, pharmacology and subunit architecture

Native GABAA receptors containing different gamma-subunit variants were distinguished immunobiochemically with antisera selectively recognizing the gamma 1-, gamma 2- and gamma 3-subunits. While GABAA receptors containing the gamma 2-subunits were confirmed to be rather ubiquitous in the adult brain, receptors characterized by the gamma 1- or gamma 3-subunit were of low abundance, as shown by immunoprecipitation. The three receptor populations differed strikingly in their benzodiazepine (BZ) site ligand binding profiles. The gamma 3-receptor population displayed reduced affinity for the full agonists clonazepam flunitrazepam and virtually lacked sensitivity to zolpidem. The gamma 1-receptor population displayed low affinity for all benzodiazepine site ligands tested, except for flunitrazepam, and could be differentiated from the gamma 2- and gamma 3-receptors by its low affinity for the inverse agonist beta CCM and its lack of affinity for the partial inverse agonist Ro 15-4513 and the antagonist flumazenil. Since flumazenil antagonizes all major effects of BZ agonists, gamma 1-receptors are not involved in mediating these actions in vivo. In immunopurified receptors, the gamma-subunit variants were found to be assembled with different variants of alpha- and beta-subunits, indicating that not only the gamma 2-subunit gamma 1- and gamma 3-subunits are part of various receptor subtypes. In addition, the gamma 2- and gamma 3-subunits can be co-assembled in native receptors, consistent with the subunit stoichiometry of two alpha-, one beta- and two gamma-subunits proposed previously for recombinant receptors.

[3H]L-655,708, a novel ligand selective for the benzodiazepine site of GABAA receptors which contain the alpha 5 subunit

A compound (L-655,708) has been identified which has at least 50-fold selectivity for the benzodiazepine site on GABAA receptors containing an alpha 5 subunit over those containing an alpha 1, alpha 2, alpha 3 or alpha 6 subunit in combination with beta 3 and gamma 2. The compound was radiolabelled with tritium and investigated as a novel radioligand which recognizes the benzodiazepine site of GABAA receptors which contain the alpha 5 subunit. [3H]L-655,708 labels one saturable and specific population of binding sites in rat hippocampus with a Kd of 2.4 +/- 0.7 nM and a Bmax of 256 +/- 42 fmol/mg protein. The pharmacology of the binding site labelled was consistent with that of receptors present in cells transfected with alpha 5, beta 2 and gamma 2 and with receptors immunoprecipitated from rat brain with an alpha 5-selective antiserum. It is concluded that [3H]L-655,708 is the first radioligand to date which is selective for any BZ2 subtype of the GABAA receptor and should provide a valuable tool for elucidating the structure and function of the alpha 5-containing GABAA receptor subtype.