Pharmacological properties of recombinant "diazepam-insensitive" GABAA receptors

Immunohistochemical distribution of 10 GABAA receptor subunits in the forebrain of the rhesus monkey Macaca mulatta

GABAA receptors are composed of five subunits arranged around a central chloride channel. Their subunits originate from different genes or gene families. The majority of GABAA receptors in the mammalian brain consist of two α-, two β- and one γ- or δ-subunit. This subunit organization crucially determines the physiological and pharmacological properties of the GABAA receptors. Using immunohistochemistry, we investigated the distribution of 10 GABAA receptor subunits (α1, α2, α3, α4, α5, β1, β2, β3, γ2, and δ) in the fore brain of three female rhesus monkeys (Macaca mulatta). Within the cerebral cortex, subunits α1, α5, β2, β3, and γ2 were found in all layers, α2, α3, and β1 were more concentrated in the inner and outer layers. The caudate/putamen was rich in α1, α2, α5, all three β-subunits, γ2, and δ. Subunits α3 and α5 were more concentrated in the caudate than in the putamen. In contrast, α1, α2, β1, β2, γ2, and δ were highest in the pallidum. Most dorsal thalamic nuclei contained subunits α1, α2, α4, β2, β3, and γ2, whereas α1, α3, β1, and γ2 were most abundant in the reticular nucleus. Within the amygdala, subunits α1, α2, α5, β1, β3, γ2, and δ were concentrated in the cortical nucleus, whereas in the lateral and basolateral amygdala α1, α2, α5, β1, β3, and δ, and in the central amygdala α1, α2, β3, and γ2 were most abundant. Interestingly, subunit α3-IR outlined the intercalated nuclei of the amygdala. In the hippocampus, subunits α1, α2, α5, β2, β3, γ2, and δ were highly expressed in the dentate molecular layer, whereas α1, α2, α3, α5, β1, β2, β3, and γ2 were concentrated in sector CA1 and the subiculum. The distribution of GABAA receptor subunits in the rhesus monkey was highly heterogeneous indicating a high number of differently assembled receptors. In most areas investigated, notably in the striatum/pallidum, amygdaloid nuclei and in the hippocampus it was more diverse than in the rat and mouse indicating a more heterogeneous and less defined receptor assembly in the monkey than in rodent brain.

Sex differences in the burying behavior test in middle-aged rats: effects of diazepam

The full behavioral profile displayed during the burying behavior test was studied in middle aged (11-14 months) males, females with irregular estrous cycles, and females in persistent diestrus, with and without diazepam (0.5-2.0mg/kg). Ambulation and motor coordination were also tested to discern behavioral changes from general motor alterations. Without diazepam treatment, middle-aged males showed longer burying behavior latencies, more prod explorations and less freezing than both groups of females. Untreated middle aged males also showed less cumulative burying and more immobility compared to females with irregular cycles. None of the parameters showed any difference between the female groups. Diazepam (0.5 and 1.0mg/kg) increased burying behavior latency in females, but had no effect on any parameter in middle aged males. However, a higher dose (2.0mg/kg) of diazepam increased immobility, freezing and the number of prod shocks and decreased prod explorations and groomings, but impaired motor coordination in males. In contrast with young males and females, diazepam at any dose reduced cumulative burying. Data are discussed on the bases of (1) sex and age differences in burying behavior and on (2) the anxiolytic-like action of diazepam and its side effects.

Selective GABAA ??5 Benzodiazepine Inverse Agonist Antagonizes the Neurobehavioral Actions of Alcohol

BACKGROUND

Previous research has implicated the alpha5-containing GABAA receptors of the hippocampus in the reinforcing properties of alcohol. In the present study, a selective GABAA alpha5 benzodiazepine inverse agonist (e.g., RY 023) was used in a series of in vivo and in vitro studies to determine the significance of the alpha5-receptor in the neurobehavioral actions of alcohol.

METHODS

In experiment one, systemic injections of RY 023 (1 to 10 mg/kg IP) dose-dependently reduced ethanol-maintained responding by 52% to 86% of controls, whereas bilateral hippocampal infusions (0.3 to 20 microg) reduced responding by 66% to 84% of controls. Saccharin responding was reduced only with the highest intraperitoneal (e.g., 10 mg) and microinjected (e.g., 20 microg) doses. In experiment two, RY 023 (3.0 to 15 mg/kg IP) reversed the motor-impairing effects of a moderate dose of alcohol (0.75 g/kg) on an oscillating bar task in the absence of intrinsic effects. In the open field, RY 023 (3.0 to 7.5 mg/kg) produced intrinsic effects alone but attenuated the suppression of the 1.25 g/kg ethanol dose. Because the diazepam-insensitive receptors (e.g., alpha4 and alpha6) have been suggested to play a role in alcohol motor impairing and sedative actions, experiment three compared the efficacy of RY 023 with Ro 15-4513 and two prototypical benzodiazepine antagonists (e.g., flumazenil and ZK 93426) across the alpha4beta3gamma2-, alpha5beta3gamma2-, and alpha6beta3gamma2-receptor subtypes in Xenopus oocytes.

RESULTS

RY 023 produced classic inverse agonism at all receptor subtypes, whereas Ro15-4513 and the two antagonists displayed a neutral or agonistic profile at the diazepam-insensitive receptors.

CONCLUSIONS

Overall, the results extend our previous findings by demonstrating that an alpha5-subtype ligand is capable of attenuating not only the rewarding action of alcohol but also its motor impairing and sedative effects. We propose that these actions are mediated in part by the alpha5-receptors of the hippocampus. The hippocampal alpha5-receptors could represent novel targets in understanding the neuromechanisms regulating the neurobehavioral actions of alcohol in humans.

Identification of a residue in the γ-aminobutyric acid type A receptor α subunit that differentially affects diazepam-sensitive and -insensitive benzodiazepine site binding

GABAA receptors that contain either the alpha4- or alpha6-subunit isoform do not recognize classical 1,4-benzodiazepines (BZDs). However, other classes of BZD site ligands, including beta-carbolines, bind to these diazepam-insensitive receptor subtypes. Some beta-carbolines [e.g. ethyl beta-carboline-3-carboxylate (beta-CCE) and methyl 6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM)] display a higher affinity for alpha4- compared to alpha6-containing receptors. In order to identify the structural determinants that underlie these affinity differences, we constructed chimeric alpha6/alpha4 subunits and co-expressed these with wild-type rat beta2 and gamma2L subunits in tsA201 cells for radioligand binding analysis. After identification of candidate regions, site-directed mutagenesis was used to narrow the ligand selectivity to a single amino acid residue (alpha6N204/alpha4I203). Substitutions at alpha6N204 did not alter the affinity of the imidazobenzodiazepine Ro15-4513. A homologous mutation in the diazepam-sensitive alpha1 subunit (S205N) resulted in a 7-8-fold reduction in affinity for the beta-carbolines examined. Although the binding of the classical agonist flunitrazepam was relatively unaffected by this mutation in the alpha1 subunit, the affinity for Ro15-1788 and Ro15-4513 was decreased by approximately 19-fold and approximately 38-fold respectively. The importance of this residue, located in the Loop C region of the extracellular N-terminus of the subunit protein, emphasizes the differential interaction of ligands with the alpha subunit in diazepam-sensitive and -insensitive receptors.

Diazepam fails to potentiate GABA-induced chloride uptake and to produce anxiolytic-like action in aged rats

The pharmacological response to benzodiazepines has been demonstrated to be different in aged individuals in comparison to adults. We studied the age-dependent changes in some of the in vitro and behavioral effects of diazepam in aged (24 months old) rats, comparing them to adults (3 months old). We evaluated the in vitro gamma-aminobutyric acid (GABA)-induced 36Cl- uptake and the diazepam potentiation of GABA-stimulated 36Cl- uptake in microsacs from cerebral cortex of both groups of animals. We found no differences in the GABA-stimulated 36Cl- uptake between adult and aged animals, and diazepam failed to potentiate GABA-induced 36Cl- flux in the aged cortical microsacs. We also examined the effect of 0.03-10 mg of diazepam on locomotor activity in an open-field test and the anxiolytic-like action of diazepam in doses ranging from 0.03 to 1 in a dark-light transition test. We observed no anxiolytic-like action of the drug in the dark-light transition test in the aged rats, while there was a shift to the left in the diminution of locomotor activity evaluated by the open-field test. We conclude that the pharmacodynamic changes observed in cortical GABA(A) receptors in aged rats could partially explain the lack of anxiolytic-like action but not the oversedation evidenced in this group of animals.

Harmaline-induced changes in gamma aminobutyric acid(A) receptor subunit mRNA expression in murine olivocerebellar nuclei

Increased CNS activity in the form of electrically or chemically induced seizures is known to alter the properties of GABA(A) receptors. The tremorgen, harmaline, causes a bursting pattern of activity in inferior olivary neurons, the effects of which are transmitted throughout the olivocerebellar circuit to other regions of the CNS. In situ hybridization was used to determine the effect of this increased activity on gamma aminobutyric acid(A) (GABA(A)) receptor subunit gene expression in the cerebellar Purkinje cell layer, deep cerebellar nuclei and inferior olivary complex of adult mice. In Purkinje cells, the expression of alpha(1), beta(2), and gamma(2) mRNAs was increased only slightly (<5%) by harmaline administration, while in deep cerebellar neurons, beta(2) transcript levels were initially elevated (26%), but dropped to control levels immediately thereafter. The expression of alpha(2), alpha(4), beta(3) and gamma(1) mRNAs in olivary neurons was affected differentially by harmaline administration. The alpha(4) transcript was increased, reaching >60% above control levels at 6 h post-injection. A smaller increase was observed for alpha(2) mRNA, while beta(3) and gamma(1) transcripts dropped below control levels during the same period. The expression of corticotropin-releasing factor mRNA was also elevated in the olivary complex. These data indicate that while Purkinje cells and deep cerebellar neurons are only minimally affected, harmaline induced changes in cellular properties may result in increased numbers of alpha(4)-containing, diazepam-insensitive, GABA(A) receptors in olivary neurons.

A single amino acid residue on the alpha(5) subunit (Ile215) is essential for ligand selectivity at alpha(5)beta(3)gamma(2) gamma-aminobutyric acid(A) receptors

Imidazobenzodiazepines such as RY-80 have been reported to exhibit both high affinity and selectivity for GABA(A) receptors containing an alpha(5) subunit. A single amino acid residue (alpha(5)Ile215) has been identified that plays a critical role in the high-affinity, subtype-selective effects of RY-80 and structurally related ligands. Thus, substitution of alpha(5)Ile215 with the cognate amino acid contained in the alpha(1) subunit (Val211) reduced the selectivity of RY-80 for alpha(5)beta(3)gamma(2) receptors from approximately 135- to approximately 8-fold compared with alpha(1)beta(3)gamma(2) receptors. This mutation produced a comparable reduction in the selectivity of RY-24 (a structural analog of RY-80) for alpha(5)beta(3)gamma(2) receptors but did not markedly alter the affinities of ligands (e.g., flunitrazepam) that are not subtype-selective. Conversely, substitution of the alpha(1) subunit with the cognate amino acid contained in the alpha(5) subunit (i.e., alpha(1)V211I) increased the affinities of alpha(5)-selective ligands by a approximately 20-fold and reduced by 3-fold the affinity of an alpha(1)-selective agonist (zolpidem). Increasing the lipophilicity (e.g., by substitution of Phe) of alpha(5)215 did not significantly affect the affinities (and selectivities) of RY-80 and RY-24 for alpha(5)-containing GABA(A) receptors. However, the effect of introducing hydrophilic and or charged residues (e.g., Lys, Asp, Thr) at this position was no greater than that produced by the alpha(5)I215V mutation. These data indicate that residue alpha(5)215 may not participate in formation of the lipophilic L(2) pocket that has been proposed to contribute to the unique pharmacological properties of alpha(5)-containing GABA(A) receptors. RY-80 and RY-24 acted as inverse agonists in both wild-type alpha(5)beta(3)gamma(2) and mutant alpha(5)I215Kbeta(3)gamma(2) receptors expressed in Xenopus laevis oocytes. However, both RY-24 and RY-80 acted as antagonists at mutant alpha(5)I215Vbeta(3)gamma(2) and alpha(5)I215Tbeta(3)gamma(2) receptors, whereas the efficacy of flunitrazepam was similar at all three receptor isoforms. The data demonstrate that amino acid residue alpha(5)215 is a determinant of both ligand affinity and efficacy at GABA(A) receptors containing an alpha(5) subunit.

Effects of treatment with GABA(A) receptor subunit antisense oligodeoxynucleotides on GABA-stimulated 36Cl- influx in the rat cerebral cortex

GABA(A) receptor function was studied in cerebral cortical vesicles prepared from rats after intracerebroventricular microinjections of antisense oligodeoxynucleotides (aODNs) for alpha1, gamma2, beta1, beta2 subunits. GABA(A) receptor alpha1 subunit aODNs decreased alpha1 subunit mRNA by 59+/-10%. Specific [3H]GABA binding was decreased by alpha1 or beta2 subunit aODNs (to 63+/-3% and 64+/-9%, respectively) but not changed by gamma2 subunit aODNs (94+/-5%). Specific [3H]flunitrazepam binding was increased by alpha1 or beta2 subunit aODNs (122+/-8% and 126+/-11%, respectively) and decreased by gamma2 subunit aODNs (50+/-13%). The "knockdown" of specific subunits of the GABA(A )receptor significantly influenced GABA-stimulated 36Cl- influx. Injection of alpha1 subunit aODNs decreased basal 36Cl- influx and the GABA Emax; enhanced GABA modulation by diazepam; and decreased antagonism of GABA activity by bicuculline. Injection of gamma2 subunit aODNs increased the GABA Emax; reversed the modulatory efficacy of diazepam from enhancement to inhibition of GABA-stimulation; and reduced the antagonist effect of bicuculline. Injection of beta2 subunit aODNs reduced the effect of diazepam whereas treatment with beta1 subunit aODNs had no effect on the drugs studied. Conclusions from our studies are: (1) alpha1 subunits promote, beta2 subunits maintain, and gamma2 subunits suppress GABA stimulation of 36Cl- influx; (2) alpha1 subunits suppress, whereas beta2, and gamma2 subunits promote allosteric modulation by benzodiazepines; (3) diazepam can act as an agonist or inverse agonist depending on the relative composition of the receptor subunits: and (4) the mixed competitive/non-competitive effects of bicuculline result from activity at alpha1 and gamma2 subunits and the lack of activity at beta1 and beta2 subunits.

The effects of the novel benzodiazepine receptor inverse agonist Ru 34000 on ethanol-maintained behaviors

Ru 34000 [5-ethyl-7-methoxy-imidazo (1,2-a) pyrimidin-2-yl cyclopropyl methanone] is a novel imidazopyrimidine benzodiazepine inverse agonist that exhibits low affinity for central benzodiazepine receptors (Ki approximately 0.98 microM). The present study examined the in vivo actions of Ru 34000 (0.5-5 mg/kg) following intraperitoneal (i.p.), subcutaneous (s.c), oral (p.o.), and intraventral tegmental administration in alcohol-preferring (P) rats trained under a concurrent operant schedule (FR4-FR4) for ethanol (10% v/v) and a palatable saccharin (0.025% or 0.75% w/v) reinforcer. Ru 34000 (i.p., s.c., p.o.) markedly reduced ethanol responding by 28-96% of control levels without affecting saccharin responding, except for the highest dose level. Clear dose-dependent suppressant effects were observed with all routes of administration on ethanol responding. Flumazenil [ethyl-8-fluro-5, 6-dihydro-5-methyl-6-4H-imidazo [1,5-a]-[1,4]-benzodiazepine-3-carboxylate] (6 mg/kg; i.p.), a benzodiazepine receptor antagonist reversed the Ru 34000-reduction of ethanol responding, suggesting that the effects were mediated at the benzodiazepine receptor. Bilateral microinjections of Ru 34000 (50, 100, 200 ng) into the ventral tegmental area dose-dependently reduced ethanol responding by as much as 97% of control levels. The results suggest that the in vivo actions of Ru 34000 are determined not only by its binding affinity, but also by its bioavailability at active benzodiazepine sites and route of drug administration. Low affinity imidazopyrimidines may be useful pharmacological probes to further understand the role of the GABA(A)-benzodiazepine receptor complex in ethanol motivated behaviors.

GABAA-benzodiazepine receptors in the striatum are involved in the sedation produced by a moderate, but not an intoxicating ethanol dose in outbred Wistar rats

The role of the dorsal striatum in mediating the sedation produced by a moderate (0.75 g/kg) and an intoxicating (1.25 g/kg) EtOH dose was investigated in the open field by determining the capacity of direct intrastriatal injections of RY 008, a partial inverse agonist of the benzodiazepine (BDZ) receptor, to antagonize EtOH's effects. SR 95531, the competitive high-affinity GABAA antagonist was used as a reference compound. Intrastriatal RY 008 (50, 500 ng) and SR 95531 (50 ng) antagonized the sedation produced by the 0.75 g/kg EtOH dose. However, RY 008 did not alter the sedation produced by the 1.25 g/kg dose. RY 008 alone was without effect. RY 008 also failed to negatively modulate GABAergic function at alpha1beta2gamma2 or alpha6beta2gamma2 receptor subtypes expressed in Xenopus oocytes. Intrastriatal modulation of the moderate EtOH dose was site specific: no antagonism by RY 008 after intraaccumbens infusions was observed. The results suggest that central GABAA-BDZ receptors in the dorsal striatum play an important role in mediating the sedation produced by a moderate EtOH dose in the open field.

Anomalous rectifying properties of 'diazepam-insensitive' GABA(A) receptors

Studies using recombinant systems indicate that 'diazepam-insensitive' GABA(A) receptors in the central nervous system contain alpha4 and alpha6 subunits while 'diazepam-sensitive' GABA(A) receptors contain alpha1, alpha2, alpha3 and alpha5 subunits. Both native and recombinant diazepam-sensitive GABA(A) receptors typically exhibit large, outwardly rectifying currents. For example, in patch clamp studies, Human Embryonic Kidney (HEK) 293 cells transfected with cDNAs encoding alpha1beta2gamma2 subunits exhibit a rectification ratio (I +60 mV/I -60 mV) of 1.95 +/- 0.21. However, anomalous rectification was observed in recombinant diazepam-insensitive GABA(A) receptors composed of either alpha4beta2gamma2 (rectification ratio, 0.74 +/- 0.09) or alpha6beta2gamma2 (rectification ratio, 0.67 +/- 0.11) subunits. Based on sequence differences between diazepam-sensitive and -insensitive GABA(A) receptor alpha subunits in the vicinity of the putative channel lining, a point mutation was introduced at His273 on the alpha4 subunit. The rectification ratio in cells expressing a mutated alpha4(Asn273)beta2gamma2 receptor increased to 1.92 +/- 0.17. Moreover, mutation of the homologous residue in the alpha1 subunit to histidine reduced the rectification ratio of alpha1(His274)beta2gamma2 to 1.02 +/- 0.12. The affinities of benzodiazepine site ligands at diazepam-sensitive and -insensitive GABA(A) receptors were unaffected by these mutations. Thus, the electrophysiological properties of diazepam-sensitive and -insensitive GABA(A) receptors may be as divergent as their pharmacological characteristics.

The novel benzodiazepine inverse agonist RO19-4603 antagonizes ethanol motivated behaviors: neuropharmacological studies

The novel imidazothienodiazepine inverse agonist RO19-4603 has been reported to attenuate EtOH intake in home cage drinking tests for at least 24 h post-drug administration after systemic administration. In the present study, selectively bred alcohol-preferring (P) rats were trained under a concurrent (FR4-FR4) operant schedule to press one lever for EtOH (10% v/v) and another lever for saccharin (0.05% or 0.75% g/v), then dose-response and timecourse effects of RO19-4603 were evaluated. Systemic RO19-4603 injections (0.0045-0.3 mg/kg; i.p.) profoundly reduced EtOH responding by as much as 97% of vehicle control on day 1. No effects were seen on saccharin responding except with the highest dose level (0.3 mg/kg). In a second experiment, microinjections of RO19-4603 (2-100 ng) directly into the nucleus accumbens (NA) suppressed EtOH responding on day 1 by as much as 53% of control: Control injections dorsal to the NA or ventral tegmental area did not significantly alter EtOH or saccharin responding. On day 2, rats in both experiments received no RO19-4603 treatments; however, all 7 of the i.p. doses, and all 3 of the intra-NA infusions continued to significantly suppress EtOH responding by 43-85% of vehicle control levels. In addition, i.p. injections of RO19-4603 produced a dose-dependent decrease in the slope of the cumulative record for EtOH responding, while concomitantly producing a dose-dependent increase in the slope for saccharin responding. RO19-4603's actions appear to be mediated via recognition sites at GABAA-BDZ receptors which regulate EtOH reinforcement, and not via mechanisms regulating ingestive behaviors. Based on recent in situ hybridization studies in our laboratory, we hypothesize that occupation of alpha4 containing GABAA diazepam insensitive (DI) receptors in the NA, may mediate in part, the RO19-4603 suppression of EtOH responding in EtOH-seeking P rats.

Benzodiazepine receptor antagonists modulate the actions of ethanol in alcohol-preferring and -nonpreferring rats

The pyrazoloquinoline CGS 8216 (2-phenylpyrazolo-[4,3-c]-quinolin-3 (5H)-one, 0.05-2 mg/kg) and the beta-carboline ZK 93426 (ethyl-5-isopropyl-4-methyl-beta-carboline-3-carboxylate, 1-10 mg/kg) benzodiazepine receptor antagonists were evaluated for their capacity to modulate the behavioral actions of ethanol in alcohol preferring and -nonpreferring rats. When alcohol-preferring rats were presented with a two-bottle choice test between ethanol (10% v/v) and a saccharin (0.0125% g/v) solution, both antagonists dose-dependently reduced intake of ethanol by 35-92% of control levels on day 1 at the initial 15 min interval of the 4 h limited access. Saccharin drinking was suppressed only with the highest doses. CGS 8216 (0.25 mg/kg) and ZK 93426 (4 mg/kg) unmasked the anxiolytic effects of a hypnotic ethanol dose (1.5 g/kg ethanol) on the plus maze test in alcohol-preferring rats, but potentiated the ethanol-induced suppression in alcohol-nonpreferring rats. CGS 8216 (0.25 mg/kg) and ZK 93426 (4 mg/kg) attenuated the ethanol (0.5 and 1.5 g/kg)-induced suppression in the open field in alcohol-nonpreferring rats; however, CGS 8216 potentiated the depressant effects of the lower ethanol dose (0.5 g/kg) in alcohol-preferring rats. These findings provide evidence that benzodiazepine receptor antagonists may differentially modulate the behavioral actions of ethanol in alcohol-preferring and-nonpreferring rats. It is possible that the qualitative pharmacodynamic differences seen in the present study may be related to selective breeding for alcohol preference. The findings indicate the potential for development of receptor specific ligands devoid of toxic effects which may be useful in the treatment of alcohol abuse and alcoholism.