GABAA receptor pharmacology

Aminomethyl-2,6-difluorophenols as a novel class of increased lipophilicity GABA(C) receptor antagonists

3- and 4-(Aminomethyl)-2,6-difuorophenols were tested for activity against the three major classes of GABA receptors. 4-(Amninomethyl)-2,6difluorophenol was shown to be a competitive and somewhat selective antagonist at p1 GABA(C) receptors expressed in Xenopus oocytes (K(B) = 75.5 microM with a 95% Confidence Interval range of 75.2 microM to 75.8 microM). This is the first in a novel class of increased lipophilicity GABA(C) receptor antagonists with little activity at alpha1beta2gamma2 GABA(A) and GABA(B) receptors.

The 'ABC' of GABA receptors: a brief review

1. In the mammalian central nervous system, GABA is the main inhibitory neurotransmitter. GABA is a highly flexible molecule and, thus, can exist in many low-energy conformations. Conformationally restricted analogues of GABA have been used to help identify three major GABA receptors, termed GABAA, GABAB and GABAC receptors. 2. GABAA and GABAC receptors are members of a super-family of transmitter-gated ion channels that include nicotinic acetylcholine, strychnine-sensitive glycine and 5HT3 receptors. GABAA receptors are hetero-oligomeric Cl- channels that are selectively blocked by the alkaloid bicuculline and modulated by steroids, barbiturates and benzodiazepines. To date, 16 human GABAA receptor cDNA have been cloned. 3. GABAB receptors are seven transmembrane receptors that are coupled to G-proteins and activate second messenger systems and Ca2+ and K+ ion channels. To date, three GABAB receptor proteins have been cloned and these resemble metabotropic glutamate receptors. GABAB receptors are hetero-oligomeric receptors made up of a mixture of a combination of the subunits. These receptors are selectively activated by (-)-baclofen and CCGP27492 and are blocked by phaclofen, the phosphonic acid analogue of baclofen. 4. In contrast, GABAC receptors represent a relatively simple form of transmitter-gated Cl- channel made up of a single type of protein subunit. Two human GABAC receptor cDNA have been cloned. These receptors are not blocked by bicuculline nor are they modulated by steroids, barbiturates or benzodiazepines. Instead, GABAC receptors are selectively activated by the conformationally restricted analogues of GABA in the folded conformation cis-4-aminocrotonic acid and (1s,2R)-2-(aminomethyl)-1-carboxycyclopropane. (1,2,5,6-Tetrahydropyridine-4-yl)methylphosphinic acid, a methylphosphinic acid analogue of GABA in a partially folded conformation, is a selective antagonist at GABAC receptors.

Irreversible site-directed labeling of the 4-aminobutyrate binding site by tritiated meta-sulfonate benzene diazonium. Contribution of a nucleophilic amino acid residue of the alpha1 subunit

Tritiated meta-sulfonate benzene diazonium ([3H]MSBD), a molecule structurally related to 4-aminobutyrate (GABA), which presents a reactivity toward nucleophilic amino acid residues, was synthesized to investigate the GABA binding site on the GABAA receptor. Irreversible labeling reactions using [3H]MSBD were performed on purified GABAA receptors isolated from cow brain membranes and labeled receptors were analyzed by SDS/PAGE. [3H]MSBD was found to be specifically incorporated into proteins in the 45-60 kDa molecular mass range which were identified as alpha1 subunits and beta2/beta3 subunits by immunoprecipitation with subunit-specific antibodies. The specific immunoprecipitation of alpha and beta subunits confirms that binding of [3H]MSBD occurs at the boundary of these subunits. These labeling results confirm the involvement of nucleophilic residues from the beta subunit but reveal also the contribution of yet unidentified nucleophilic residues on the alpha subunit for the GABA binding site.

Interaction of non-competitive blockers within the gamma-aminobutyric acid type A chloride channel using chemically reactive probes as chemical sensors for cysteine mutants

Selected channel-lining cysteine mutants from the M2 segment of rat alpha1 gamma-aminobutyric acid (GABA) type A receptor subunit, at positions 257, 261, 264, and 272 were co-expressed with beta1 and gamma2 subunits in Xenopus oocytes. They generated functional receptors displaying conductance and response to both GABA and picrotoxinin similar to the wild type alpha1beta1gamma2 receptor. Three chemically reactive affinity probes derived from non-competitive blockers were synthesized to react with the engineered cysteines: 1) dithiane bis-sulfone derivative modified by an isothiocyanate function (probe A); 2) fiprole derivatives modified by an alpha-chloroketone (probe B) and alpha-bromoketone (probe C) moiety. These probes blocked the GABA-induced currents on all receptors. This blockade could be fully reversed by a washing procedure on the wild type, the alpha1T261Cbeta1gamma2 and alpha1L264Cbeta1gamma2 mutant receptors. In contrast, an irreversible effect was observed for all three probes on both alpha1V257Cbeta1gamma2 and alpha1S272Cbeta1gamma2 mutant receptors. This effect was probe concentration-dependent and could be abolished by picrotoxinin and/or t-butyl bicyclophosphorothionate. These data indicate a major interaction of non-competitive blockers at position 257 of the presumed M2 segment of rat alpha1 subunit but also suggest an interaction at the more extracellular position 272.

Stereoselective interaction of thiopentone enantiomers with the GABA(A) receptor

1. As pharmacokinetic differences between the thiopentone enantiomers seem insufficient to explain the approximately 2 fold greater potency for CNS effects of (-)-S- over (+)-R-thiopentone, this study was performed to determine any enantioselectivity of thiopentone at the GABA(A) receptor, the primary receptor for barbiturate hypnotic effects. 2. Two electrode voltage clamp recording was performed on Xenopus laevis oocytes expressing human GABA(A) receptor subtype alpha1beta2gamma2 to determine relative differences in potentiation of the GABA response by rac-, (+)-R- and (-)-S-thiopentone, and rac-pentobarbitone. Changes in the cellular environment pH and in GABA concentrations were also evaluated. 3. With 3 microM GABA, the EC50 values were (-)-S-thiopentone (mean 26.0+/-s.e.mean 3.2 microM, n=9 cells) >rac-thiopentone (35.9+/-4.2 microM, n=6, P=0.1) >(+)-R-thiopentone (52.5+/-5.0 microM, n=8, P<0.02) >rac-pentobarbitone (97.0+/-11.2 microM, n=11, P<0.01). Adjustment of environment pH to 7.0 or 8.0 did not alter the EC50 values for (+)-R- or (-)-S-thiopentone. 4 Uninjected oocytes responded to >100 microM (-)-S- and R-thiopentone. This direct response was abolished by intracellular oocyte injection of 1,2-bis(2-aminophenoxy)ethane-N, N,N1,N1-tetraacetic acid (BAPTA), a Ca2+ chelating agent. With BAPTA, the EC50 values were (-)-S-thiopentone (20.6+/-3.2 microM, n=8) <(+)-R-thiopentone (36.2+/-3.2 microM, n=9, P<0.005). 5 (-)-S-thiopentone was found to be approximately 2 fold more potent than (+)-R-thiopentone in the potentiation of GABA at GABA(A) receptors expressed on Xenopus oocytes. This is consistent with the differences in potency for CNS depressant effects found in vivo.

Interaction of anisatin with rat brain gamma-aminobutyric acidA receptors: allosteric modulation by competitive antagonists

Anisatin, a toxic sesquiterpene isolated from the Japanese star anise (Illicium anisatum L.), competitively inhibited the specific binding of [3H]4'-ethynyl-4-n-propylbicycloorthobenzoate ([3H]EBOB), a non-competitive antagonist of gamma-aminobutyric acid (GABA)A receptors, to rat brain membranes with an IC50 value of 0.43 microM. R 5135, a competitive GABA antagonist, decreased the potency of anisatin in inhibiting [3H]EBOB binding in a negatively cooperative manner. Two other competitive antagonists, SR 95531 (gabazine) and (-)-bicuculline methiodide, had similar effects. On the other hand, R 5135 exerted little influence on the potencies of the other non-competitive antagonists tested: EBOB, picrotoxinin, isopropylbicyclophosphate, and dieldrin. Thus, anisatin was clearly different from the other non-competitive antagonists in responding to the action of competitive antagonists on (GABA)A receptors. These findings suggest that the binding region of anisatin might overlap with that of the other non-competitive antagonists, but that anisatin must interact with other specific region(s).

General anaesthetic actions on ligand-gated ion channels

The molecular mechanisms of general anaesthetics have remained largely obscure since their introduction into clinical practice just over 150 years ago. This review describes the actions of general anaesthetics on mammalian neurotransmitter-gated ion channels. As a result of research during the last several decades, ligand-gated ion channels have emerged as promising molecular targets for the central nervous system effects of general anaesthetics. The last 10 years have witnessed an explosion of studies of anaesthetic modulation of recombinant ligand-gated ion channels, including recent studies which utilize chimeric and mutated receptors to identify regions of ligand-gated ion channels important for the actions of general anaesthetics. Exciting future directions include structural biology and gene-targeting approaches to further the understanding of general anaesthetic molecular mechanisms.

Postnatal ethanol exposure blunts upregulation of GABAA receptor currents in Purkinje neurons

Recently, we found that early postnatal ethanol exposure inhibits the maturation of GABAA receptors (GABAARs) in developing medial septum/diagonal band (MS/DB) neurons, suggesting that these receptors may represent a target for ethanol related to fetal alcohol syndrome (FAS). To determine whether GABAARs on other neurons are also sensitive to a postnatal ethanol insult, postnatal day (PD) 4-9, rat pups were artificially reared and exposed to ethanol (4.5 g kg-1 day-1, 10.2% v/v). The pharmacological profile of acutely dissociated cerebellar Purkinje cell GABAARs from untreated, artificially reared controls and ethanol-treated animals was examined with conventional whole-cell patch clamp recordings during PD 12-16 (juveniles) and PD 25-35 (young adults). For untreated animals, GABA (0.3-100 microM) consistently induced inward Cl- currents in a concentration-dependent manner showing an age-related increase in maximum response without change in EC50 or slope value. Acute ethanol (100 mM) consistently inhibited 3 microM GABA currents (10-20%); positive modulators, pentobarbital (10 microM), midazolam (1 microM) and loreclezole (10 microM), consistently potentiated; the negative modulator, Zn2+ (30 microM), inhibited GABA currents across both juvenile and young adult groups. Loreclezole potentiation increased while Zn2+ inhibition decreased with age in untreated Purkinje neurons. Postnatal ethanol exposure (PD 4-9) decreased GABAAR maximum current density in young adult Purkinje cells but not in juvenile neurons. However, sensitivity to allosteric modulators did not change after ethanol. These data are consistent with the hypothesis that postnatal ethanol exposure during the brain growth spurt can disturb GABAAR development across the brain, although the mechanism(s) underlying this action remains to be determined.

Potential anxiolytic agents. 3. Novel A-ring modified pyrido[1,2-a]benzimidazoles

A variety of pyrido[1,2-a]benzimidazoles (PBIs) modified on the A-ring were prepared and evaluated for affinity to the benzodiazepine binding site on the GABA-A receptor and in animal models predictive of anxiolytic activity in humans. A-ring benzo-fused derivative 7 exhibited potent activity, as did the 6- and 7-pyrido compounds 3 and 4.

Substance P release in the dorsal horn assessed by receptor internalization: NMDA receptors counteract a tonic inhibition by GABA(B) receptors

Inhibitory amino acids have antinociceptive actions in the spinal cord that may involve inhibition of neurotransmitter release from primary afferents. Rat spinal cord slices with dorsal roots were used to study the effect of GABA and glycine on substance P release, assessed by the internalization of neurokinin 1 receptors. After electrical stimulation of the dorsal root at 100 Hz, about half of neurokinin 1 receptor-immunoreactive neurons in laminae I-IIo showed internalization. This internalization was inhibited by GABA (100 microM) and the GABA(B) agonist R-baclofen (10 microM), but not by the GABA(A) agonist muscimol (20 microM) or glycine (100 microM). The GABA(B) antagonist 2-hydroxysaclofen (100 microM) reversed the inhibitory effect of GABA, but not the GABA(A) antagonist bicuculline (100 microM). These findings demonstrate that GABA(B) receptors, but not GABA(A) or glycine receptors, inhibit substance P release induced by dorsal root stimulation. In contrast, R-baclofen did not inhibit the internalization produced by NMDA (100 microM), indicating that the stimulatory effect of NMDA receptors on substance P release is able to surmount the inhibitory effect of GABA(B) receptors. In the presence of the GABA(B) antagonist 2-hydroxysaclofen (100 microM), but not in its absence, stimulation of the dorsal root at 1 or 10 Hz was able to elicit internalization, which was not inhibited by the NMDA receptor antagonist AP-5 (50 microM) or the channel blocker MK-801 (10 microM). Therefore, inhibition of substance P release by GABA(B) receptors is tonic, and in its absence SP release no longer requires NMDA receptor activation.

Development of GABAA receptors on medial septum/diagonal band (MS/DB) neurons after postnatal ethanol exposure

The impact of 'binge-like' ethanol exposure on postnatal days (PD) 4-9 was examined on development of gamma-aminobutyric acid type A receptors (GABAAR) during the first month of life in the rat. Whole-cell patch-clamp recordings in acutely isolated medial septum/diagonal band (MS/DB) neurons were used to define effects of rapidly applied ethanol and other allosteric modulators on bicuculline-sensitive GABA currents. Three age groups were examined including 'pups' (PD 4-10), 'juveniles' (PD 11-16) and 'young adults' (PD 25-35). In untreated neurons, maximum responses to GABA and the apparent GABA EC50 increased approximately 2-fold during the first month of life. Potentiation of GABA responses by pentobarbital, midazolam, and loreclezole all increased with age, while Zn2+ inhibition declined. Initial inhibition by ethanol switched to potentiation of GABA responses during this time. In vivo, binge-like ethanol treatment (4.5 g kg-1 day-1 divided into two doses, 2 h apart on PD 4-9) reduced both the GABA maximal response and GABA EC50 measured on PD 11-16. These measures returned to control levels by PD 25-35. After binge-like postnatal ethanol exposure, age-dependent loss of Zn2+ inhibition of GABA responses was increased, while potentiating actions of in vitro ethanol were blocked. GABAAR modulation by other drugs was unaffected. These data suggest that early postnatal ethanol exposure disrupts the expected developmental pattern of GABAAR function in MS/DB neurons, an action that could contribute to neurobehavioral deficits associated with the fetal alcohol syndrome. Whether these changes are due to cellular damage, delayed gene expression or post-translational modification needs to be determined.

Modulation of GABA(A) receptors and inhibitory synaptic currents by the endogenous CNS sleep regulator cis-9,10-octadecenoamide (cOA)

1. Cis-9,10-octadecenoamide (cOA) accumulates in the CSF of sleep-deprived cats and may represent a novel signalling molecule. Synthetic cOA has been shown to induce physiological sleep when injected into laboratory rats. Here we assess the cellular mode of action of cOA in vitro. 2. In all rat cultured cortical neurones (pyramidal cells) examined, the synthetic brain lipid (3.2-64 microM) enhanced the responses to subsaturating GABA concentrations (up to circa 2x) in a concentration-dependent manner (EC50, circa 15 microM). 3. (20 microM) cOA significantly enhanced the affinity of exogenous GABA for its receptor without changing the Hill slope or the maximal response. These effects were not voltage-dependent or secondary to shifts in E(Cl). 4. In the absence of GABA, cOA directly evoked small inhibitory currents in a subpopulation (<7%) of sensitive cells. 5. 20 microM cOA reversibly enhanced the duration of spontaneous inhibitory post synaptic currents (circa 2 fold) without significantly altering their amplitude. 6. At 32-64 microM, cOA reversibly reduced the incidence and amplitude of both inhibitory post synaptic currents (i.p.s.cs) and excitatory post synaptic currents (e.p.s.cs) in the cultured neuronal circuits in common with other depressant drugs acting at the GABA(A) receptor. 7. 32 microM Oleic acid did not modulate exogenous GABA currents or synaptic activity suggesting that cOAs actions are mediated through a specific receptor. 8. A specific, protein-dependent interaction with GABA(A) receptors was confirmed in Xenopus oocytes. Recombinant human receptors were modulated by 10 microM cOA (and diazepam) only when a gamma2 subunit was co-expressed with alpha1beta2: the cOA response was not sensitive to the specific benzodiazepine antagonist flumazenil (1 microM). 9. cOA may represent an endogenous ligand for allosteric modulatory sites on isoforms of GABA(A) receptors which are crucial for the regulation of arousal and have recently been implicated in the circadian control of physiological sleep.

Picrodendrin and related terpenoid antagonists reveal structural differences between ionotropic GABA receptors of mammals and insects

Twenty-eight picrotoxane terpenoids, including picrodendrins isolated from the Euphorbiaceae plant, Picrodendron baccatum (L.) Krug and Urban, have been evaluated for their ability to inhibit the specific binding of [3H]EBOB, the noncompetitive antagonist of ionotropic GABA receptors, to rat-brain and housefly (Musca domestica L.)-head membranes. Picrodendrin Q was the most potent competitive inhibitor of [3H]EBOB binding, with IC50 values of 16 nM (rat) and 22 nM (Musca). We find that the spiro gamma-butyrolactone moiety at the 13-position, which contains a carbonyl group conjugated with an unsaturated bond, and the substituents at the 4-position play important roles in the interaction of picrodendrins with their binding site in rat receptors. In contrast, such structural features are not strictly required in the case of the interaction with Musca receptors; the spiro saturated gamma-butyrolactone moiety at the 13-position, which bears the 16-sp3 carbon atom, and the hydroxyl groups at various positions are somewhat tolerated. Quantitative structure-activity studies have clearly shown that the electronegativity of the 16-carbon atom and the presence or absence of the 4- and 8-hydroxyl groups are important determinants of the potency of nor-diterpenes in Musca receptors, while the negative charge on the 17-carbonyl oxygen atom is likely important in the case of rat receptors. These findings indicate that there are significant differences between the structures of the complementary binding sites in rat GABA receptors and Musca GABA receptors. We also infer differences between native Musca GABA receptors and the Drosophila Rdl subunit-containing homo-oligomeric GABA receptors in the structures of their binding sites.

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.

Actions of cyclic esters, S-esters, and amides of phenyl- and phenylthiophosphonic acids on mammalian and insect GABA-gated chloride channels

Cyclic esters, S-esters, and amides of phenyl(thio)phosphonic acid were synthesized to probe the interaction between noncompetitive antagonists of ionotropic gamma-aminobutyric acid (GABA) receptors and their binding site. Some of these compounds competitively inhibited the specific binding of [3H]EBOB, a noncompetitive GABA antagonist, to rat-brain and housefly-head membranes. The trans isomer of the ester bearing a tert-butyl group at the 5-position and a bromine atom at the p-position (5t) was most potent in rat receptors with an IC50 value of 40 nM, while the trans isomer of the S-ester bearing the same substituents (10t) was most potent in housefly receptors with an IC50 value of 55 nM. In both cases, the corresponding amide analogue (12t) was less potent. The potencies of 5t and 12t tended to decrease in the presence of GABA, particularly in housefly receptors, while that of 10t remained unchanged. The rank order of activity in inhibiting [3H]EBOB binding to housefly-head membranes in the presence of GABA (10t > 5t > 12t) was in accord with that of insecticidal activity. S-Ester 10t depressed 10 microM and 300 microM GABA-induced 36Cl- influx into mouse cerebral synaptoneurosomes, whereas ester 5t depressed 10 microM GABA-induced 36Cl- influx but not 300 microM GABA-induced flux. Amide 12t was inactive at both GABA concentrations. These findings indicate that six-membered cyclic phenylthiophosphonic acid derivatives act as noncompetitive antagonists of GABA receptors and suggest that 10t is able to bind to the receptor in the open, desensitized, and closed states, whereas the affinity of 5t and 12t is lower in the open and desensitized states than in the closed state. The derivatives have similar structures except for the heteroatoms at the 1- and 3-positions, so that the heteroatoms may play a unique role when antagonists bring the open state of the GABA-gated channel to the desensitized or closed state.

Diazepam enhancement of GABA-gated currents in binary and ternary GABAA receptors: relationship to benzodiazepine binding site density

Although the predominant GABAA receptor isoform in the adult rodent central nervous system is a ternary complex composed of alpha 1 beta 2/3 gamma 2-subunits, small populations of binary receptors lacking beta-subunits (i.e., complexes containing alpha gamma-subunits) have also been identified. When expressed in HEK 293 cells, recombinant GABAA receptors composed of either alpha 1 beta 2/3 gamma 2- or alpha 1 gamma 2-subunits form benzodiazepine-responsive, GABA-gated chloride channels. The objective of this study was to compare the ability of a prototypic benzodiazepine (diazepam) to augment GABA-gated chloride currents in these binary and ternary receptor isoforms. The potency of GABA was characteristically increased by diazepam (1 microM) in both receptor isoforms, but this increase was significantly greater (p < 0.05) in receptors composed of alpha 1 beta 2 gamma 2-subunits (approximately five- to sixfold) compared to alpha 1 gamma 2-subunits (approximately 2.2-fold). At GABA concentrations approximating its EC50 value (5 microM), the greater augmentation observed in ternary receptors was attributable to a higher efficacy of diazepam. Radioligand binding studies revealed that the Bmax of [3H]flunitrazepam was increased approximately 1.8- and 3.5-fold in cells expressing alpha 1 beta 2 gamma 2- and alpha 1 beta 3 gamma 2-subunits, respectively, compared to cells expressing alpha 1 gamma 2-subunits. A similar increase (approximately 3.8-fold) in the Bmax of [3H]Ro 15-4513 was observed in HEK 293 cells transiently transfected with cDNAs encoding alpha 6 beta 3 gamma 2-compared to alpha 6 gamma 2-subunits. The Kd values of these radioligands were not different in binary and ternary receptor isoforms. It is hypothesized that the greater efficacy of diazepam in alpha 1 beta 2 gamma 2 compared to alpha 1 gamma 2 GABAA receptors results from the higher benzodiazepine binding site density produced by the formation of a ternary complex.

Analogues of gamma-aminobutyric acid (GABA) and trans-4-aminocrotonic acid (TACA) substituted in the 2 position as GABAC receptor antagonists

1. gamma-Aminobutyric acid (GABA) and trans-4-aminocrotonic acid (TACA) have been shown to activate GABAC receptors. In this study, a range of C2, C3, C4 and N-substituted GABA and TACA analogues were examined for activity at GABAC receptors. 2. The effects of these compounds were examined by use of electrophysiological recording from Xenopus oocytes expressing the human rho 1 subunit of GABAC receptors with the two-electrode voltage-clamp method. 3. trans-4-Amino-2-fluorobut-2-enoic acid was found to be a potent agonist (KD = 2.43 microM). In contrast, trans-4-amino-2-methylbut-2-enoic acid was found to be a moderately potent antagonist (IC50 = 31.0 microM and KB = 45.5 microM). These observations highlight the possibility that subtle structural substitutions may change an agonist into an antagonist. 4. 4-Amino-2-methylbutanoic acid (KD = 189 microM), 4-amino-2-methylenebutanoic acid (KD = 182 microM) and 4-amino-2-chlorobutanoic acid (KD = 285 microM) were weak partial agonists. The intrinsic activities of these compounds were 12.1%, 4.4% and 5.2% of the maximal response of GABA, respectively. These compounds more effectively blocked the effects of the agonist, GABA, giving rise to KB values of 53 microM and 101 microM, respectively. 5. The sulphinic acid analogue of GABA, homohypotaurine, was found to be a potent partial agonist (KD = 4.59 microM, intrinsic activity 69%). 6. It was concluded that substitution of a methyl or a halo group in the C2 position of GABA or TACA is tolerated at GABAC receptors. However, there was dramatic loss of activity when these groups were substituted at the C3, C4 and nitrogen positions of GABA and TACA. 7. Molecular modelling studies on a range of active and inactive compounds indicated that the agonist/competitive antagonist binding site of the GABAC receptor may be smaller than that of the GABAA and GABAB receptors. It is suggested that only compounds that can attain relatively flat conformations may bind to the GABAC receptor agonist/competitive antagonist binding site.

Modulation of a recombinant invertebrate gamma-aminobutyric acid receptor-chloride channel complex by isoflurane: effects of a point mutation in the M2 domain

1. Inhalational anaesthetics modulate ligand-gated ion channels at clinical concentrations. In this paper we address submolecular mechanisms for gamma-aminobutyric acid (GABA) receptor modulation by isoflurane. 2. Wild-type Drosophila melanogaster homo-oligomeric GABA receptors were characterized and compared with an ion-channel mutant (alanine substituted to a serine in M2) by means of two-electrode voltage-clamp in membrane-invariant Xenopus oocytes. 3. Both channel receptor isoforms generated outwardly rectifying, bicuculline-insensitive currents with reversal potentials characteristic of a chloride current. 4. As previously shown, the point mutation in the M2 domain conferred a profound resistance to the blocking action of 10 microM picrotoxinin (PTX): circa 7 fold reduction at the GABA EC20. 5. Isoflurane, 195-389 microM, enhanced GABA conductance in both receptor variants by significantly increasing the affinity of the agonist for its receptor without changing Hill slope or maximal response. Relative potencies were statistically indistinguishable. 6. Isoflurane concentration-response curves (on circa GABA EC25) demonstrated that enhancement was effected at around 100-195 microM for both receptor subtypes, but a dramatic divergence was evident at concentrations above 400 microM: wild-type receptors exhibited concentration-dependent block, whilst mutant conductances continued to increase over the same concentration range, showing no tendency to saturate (up to 3330 microM). 7. The above divergence was not attributable to differential desensitization: neither wild-type nor mutant conductance desensitized significantly (P > 0.05) in the absence or presence of anaesthetic. 8. This work demonstrates that modulatory sites for anaesthetic are present on a relatively primitive insect ion channel. 9. The depression of GABA response at high isoflurane concentrations, in WT receptors, (typical of a variety of anaesthetic agents) may reflect low affinity channel block via the PTX site. 10. The non-saturable enhancement of chloride conductances, when the PTX site is mutated, is not consistent with topical proposals that inhalational anaesthetics (stereoselectively) occupy a finite number of sites on these membrane spanning proteins.

Effects of 5-(4-piperidyl) isoxazol-3-ol (4-PIOL), a GABA(A) receptor partial agonist, on recombinant human GABA(A) receptors

gamma-Aminobutyric acidA (GABA(A)) gated chloride ion channels were expressed from human recombinant cDNA using the baculovirus/Sf-9 insect cell expression system. The electrophysiological effects in whole-cell currents of 5-(4-piperidyl) isoxazol-3-ol (4-PIOL), a GABA(A) receptor partial agonist, were investigated on GABA(A) receptor complexes of alpha1beta2gamma2S subunits as well as a slightly modified construct of alpha1(valine 121)beta2gamma2S subunits. Here we report that (1)4-PIOL induces an inward whole-cell current in a concentration-dependent manner in both alpha1(val 121)beta2gamma2S and alpha1(ile 121)beta2gamma2S receptor subunit combinations. (2) The 4-PIOL induced whole-cell currents were more pronounced in alpha1(val 121)beta2gamma2S than in alpha1(ile 121)beta2gamma2S receptor subunit combinations. (3) 4-PIOL inhibited GABA-induced responses on alpha1(ile 121)beta2gamma2S and alpha1(val 121)beta2gamma2S receptor combinations with similar potency.

Characterisation of the furanocoumarin phellopterin as a rat brain benzodiazepine receptor partial agonist in vitro

Phellopterin, a naturally occurring furanocoumarin found in the roots of Angelica dahurica, inhibits [3H]diazepam and ethyl 8-fluoro-5,6-dihydro-5-methyl-6-oxo-4H-imidazo[1,5-a][1,4] benzodiazepine-3-carboxylate ([3H]Ro 15-1788) binding to the benzodiazepine site of the rat brain gamma-aminobutyric acidA (GABAA) receptor in vitro with IC50 values of 400 and 680 nM, respectively. Two other naturally occurring furanocoumarins, byakangelicol and imperatorin were significantly less potent, with IC50 values for inhibition of [3H]diazepam binding of 8.0 and 12.3 microM, respectively. Scatchard plot analysis showed that the inhibitory activity of phellopterin was due to competitive inhibition of the benzodiazepine ligand binding. The results of GABA- and t-butylbicyclophosphorothionate (TBPS)-shift assays suggest that phellopterin is a partial agonist of the central benzodiazepine receptors in vitro.