Biochemical evaluations of the effects of loreclezole and propofol on the GABAA receptor in rat brain

Effects of bilobalide, ginkgolide B and picrotoxinin on GABAA receptor modulation by structurally diverse positive modulators

Anxiolytics and anticonvulsants generally positively modulate the action of GABA, whereas many convulsants (including the chloride channel blocker picrotoxinin) negatively modulate the action of GABA on GABA_A receptors. Like picrotoxinin, bilobalide and ginkgolide B, active constituents of Ginkgo biloba, have been shown to negatively modulate the action of GABA at α₁β₂γ_2L GABA_A receptors. However, unlike picrotoxinin, bilobalide and ginkgolide B are not known to cause convulsions. We have assessed the action of bilobalide, ginkgolide B and picrotoxinin on a range of GABA_A modulators (etomidate, loreclezole, propofol, thiopentone sodium, diazepam, and allopregnanolone), using two-electrode voltage clamp electrophysiology at recombinant α₁β₂γ_2L GABA_A receptors expressed in Xenopus oocytes. The results indicate that bilobalide and ginkgolide B differ from picrotoxinin in their ability to inhibit the actions of a range of these structurally diverse GABA_A positive modulators consistent with these modulators acting on a multiplicity of active sites associated with GABA_A receptors. In the presence GABA, ginkgolide B was more potent than bilobalide in inhibiting the GABA-potentiating effect of propofol, equipotent against loreclezole and allopregnanolone, and less potent against etomidate, diazepam, and thiopentone sodium. This indicates that in comparison to picrotoxinin, bilobalide and ginkgolide B differ in their effects on the different modulators.

A Multifaceted GABAA Receptor Modulator: Functional Properties and Mechanism of Action of the Sedative-Hypnotic and Recreational Drug Methaqualone (Quaalude)

In the present study, we have elucidated the functional characteristics and mechanism of action of methaqualone (2-methyl-3-o-tolyl-4(3H)-quinazolinone, Quaalude), an infamous sedative-hypnotic and recreational drug from the 1960s-1970s. Methaqualone was demonstrated to be a positive allosteric modulator at human α1,2,3,5β2,3γ2S GABAA receptors (GABAARs) expressed in Xenopus oocytes, whereas it displayed highly diverse functionalities at the α4,6β1,2,3δ GABAAR subtypes, ranging from inactivity (α4β1δ), through negative (α6β1δ) or positive allosteric modulation (α4β2δ, α6β2,3δ), to superagonism (α4β3δ). Methaqualone did not interact with the benzodiazepine, barbiturate, or neurosteroid binding sites in the GABAAR. Instead, the compound is proposed to act through the transmembrane β((+))/α((-)) subunit interface of the receptor, possibly targeting a site overlapping with that of the general anesthetic etomidate. The negligible activities displayed by methaqualone at numerous neurotransmitter receptors and transporters in an elaborate screening for additional putative central nervous system (CNS) targets suggest that it is a selective GABAAR modulator. The mode of action of methaqualone was further investigated in multichannel recordings from primary frontal cortex networks, where the overall activity changes induced by the compound at 1-100 μM concentrations were quite similar to those mediated by other CNS depressants. Finally, the free methaqualone concentrations in the mouse brain arising from doses producing significant in vivo effects in assays for locomotion and anticonvulsant activity correlated fairly well with its potencies as a modulator at the recombinant GABAARs. Hence, we propose that the multifaceted functional properties exhibited by methaqualone at GABAARs give rise to its effects as a therapeutic and recreational drug.

Isobolographic analysis of interactions between loreclezole and conventional antiepileptic drugs in the mouse maximal electroshock-induced seizure model

This study examined the interaction characteristics between loreclezole (LCZ) and various conventional antiepileptic drugs (phenytoin--PHT, carbamazepine--CBZ, valproate--VPA and phenobarbital--PB) in the mouse maximal electroshock (MES)-induced seizure model using isobolographic analysis. Drug-related adverse effects were ascertained by use of the chimney test (motor impairment) and the step-through passive avoidance task (learning and retrieval). It was observed that the combination of LCZ with VPA or PB, at the fixed ratio of 1:1, was supra-additive (synergistic) and the combination of LCZ with CBZ, at all fixed ratios tested (1:3, 1:1 and 3:1), was supra-additive against electroconvulsions. The remaining combinations evaluated, i.e., LCZ with PB or VPA at fixed ratios of 1:3 and 3:1, as well as all fixed-ratio combinations between LCZ and PHT, were additive in the MES test in mice. Pharmacokinetic characterization revealed that LCZ significantly increased both free plasma and brain concentrations of CBZ and PHT, but was without effect on PB. Moreover, a bi-directional pharmacokinetic interaction between LCZ and VPA was observed in that while LCZ increased free plasma, but not total brain VPA concentrations, VPA increased the total brain, but not free plasma LCZ concentrations. Adverse-effect testing revealed that for all antiepileptic drug combinations neither motor performance nor long-term memory was altered. Of the drug combinations investigated, only that of LCZ and PB at the fixed ratio of 1:1 was not associated with any pharmacokinetic interactions, and thus it may be concluded that the supra-additive (synergistic) isobolographic interaction was pharmacodynamic in nature. Furthermore, the fact that LCZ and PB have similar mechanisms of action would suggest that drugs with similar mechanisms of action may provide rational polytherapy regimens.

Central nervous system depressant action of flavonoid glycosides

The pharmacological effects on the central nervous system (CNS) of a range of available flavonoid glycosides were explored and compared to those of the glycosides 2S-hesperidin and linarin, recently isolated from valeriana. The glycosides 2S-neohesperidin, 2S-naringin, diosmin, gossipyn and rutin exerted a depressant action on the CNS of mice following i.p. injection, similar to that found with 2S-hesperidin and linarin. We demonstrate in this work that these behavioural actions, as measured in the hole board, thiopental induced sleeping time and locomotor activity tests, are unlikely to involve a direct action on gamma-aminobutyric acid type A (GABA(A)) receptors. The corresponding aglycones were inactive, pointing to the importance of the sugar moieties in the glycosides in their CNS depressant action following systemic administration. The pharmacological properties of the flavonoid glycosides studied here, in addition to our previous results with hesperidin and linarin, opens a promising new avenue of research in the field.

Radioligand binding studies of caloporoside and novel congeners with contrasting effects upon [35S] TBPS binding to the mammalian GABAA receptor

Caloporoside is a natural active fungal metabolite, which was isolated from Caloporous dichrous and was described to exhibit antibacterial, antifungal and phospholipase C inhibitory activity. We have previously reported evidence that related beta-linked compounds, lactose and octyl-beta-d-mannoside, bind and functionally modulate rodent GABA(A) receptors, respectively. We have characterized the binding pharmacology of synthetic caloporoside and two further congeners, 2-hydroxy-6-([(16R)-(beta-d-mannopyranosyloxy)heptadecyl]) benzoic acid and octyl-beta-d-glucoside on GABA(A) receptors using a [35S]-t-butylbicyclophosphoorothionate (TBPS) radioligand binding assay. Caloporoside and 2-hydroxy-6-([(16R)-(beta-d-mannopyranosyloxy)heptadecyl]) benzoic acid produced concentration-dependent complete inhibition of specific [35S] TBPS binding with overall apparent IC50 values of 14.7+/-0.1 and 14.2+/-0.1 microM, respectively. In contrast, octyl-beta-d-glucoside elicited a concentration-dependent stimulation of specific [35S] TBPS binding (E(max)=144+/-4%; EC50=39.2+/-22.7 nM). The level of stimulation was similar to that elicited by diazepam (E(max)=147+/-6%; EC50=0.8+/-0.1 nM), and was occluded by GABA (0.3 microM). However, the three test compounds failed to elicit any significant effect (positive or negative) upon [3H] flunitrazepam or [3H] muscimol binding, indicating that they did not bind directly, or allosterically couple, to the benzodiazepine or agonist binding site of the GABA(A) receptor, respectively. The constituent monosaccharide, glucose, and both the closely related congeners octyl-beta-d-glucoside or hexyl-beta-d-glucoside have no significant effect upon [35S] TBPS binding. These data, together, provide strong evidence that a beta-glycosidic linkage and chain length are crucial for the positive modulation of [35S] TBPS binding to the GABA(A) receptor by this novel chemical class.

Intravenous Subhypnotic Propofol in Central Pain

OBJECTIVE

To validate IV subhypnotic propofol, a gamma-aminobutyric acid A (GABA-A) agonist, as a diagnostic test for central pain.

METHODS

The efficacy of systemic propofol (0.2 mg/kg IV bolus) was evaluated in a double-blind, placebo-controlled and crossover fashion on both spontaneous ongoing pain and allodynia in 44 patients with chronic central pain of both brain and cord origin.

RESULTS

Propofol was significantly superior to the placebo (Intralipid, Kabi Pharmacia) in reducing the intensity of spontaneous ongoing pain for up to 1 hour after the injection: 24 of 44 patients (55%) receiving propofol showed a significant reduction in spontaneous pain, whereas only 6 patients showed this after the placebo. Propofol also significantly reduced the intensity of both mechanical and cold allodynia. In a few cases, only the evoked components were abolished but not the spontaneous pain. In general, the side effects were minimal and consisted mainly of transitory burning upon injection of both propofol and placebo and slight lightheadedness in a few cases.

CONCLUSIONS

Systemic propofol induces analgesic effects on all studied components of central pain and highlights the key role of GABA modulation in central pain.

Modulation of [3H] TBOB binding to the rodent GABAA receptor by simple disaccharides

We have recently reported evidence that a simple beta-linked alkylated mannose reversibly increased the magnitude of GABA(A) receptor currents evoked in cultured rat pyramidal neurons whilst concomitantly reducing the incidence of spontaneous synaptic activity. In this present study, the effects of the simple beta-linked disaccharide, lactose was investigated using a [3H] TBOB (t-[3H] butylbicycloorthobenzoate) binding assay in adult rat forebrain and cerebellum membranes. Lactose elicited a significant potentiation of [3H] TBOB binding to well-washed forebrain and cerebellar membranes (mean E(max) values=367 and 287%; mean EC(50) values=1.5 and 30 microM, respectively, N=4). The alpha-linked disaccharides, maltose and sucrose also potentiated [3H] TBOB binding, but with 100-600-fold higher EC(50) values than lactose. The lactose-mediated potentiation of [3H] TBOB in the forebrain and cerebellum was completely abolished in the presence of 0.3 microM GABA. Over the concentration range in which significant potentiation of [3H] TBOB binding was detected, lactose elicited no significant effect upon [3H] flunitrazepam binding. This study demonstrated that lactose can modulate the GABA(A) receptor channel, allosterically coupled to the agonist site, but independent of the benzodiazepine site. Furthermore, lactose displayed differential effects upon forebrain and cerebellar GABA(A) receptors indicating that it may be a novel subtype selective agent.

Comparison of the intravenous reinforcing effects of propofol and methohexital in baboons

Propofol is a widely used intravenous anesthetic that can directly activate and positively modulate the GABA(A)-receptor. Propofol is not currently regulated under the USA Controlled Substances Act. The present study evaluated the intravenous reinforcing effects of propofol compared to the intravenous barbiturate anesthetic methohexital in baboons using a procedure in which doses of the test drug were substituted for a standard cocaine dose. Drug or vehicle was available for self-injection during daily 5.5-h sessions under a fixed-ratio 120 or 160 schedule of reinforcement. A 40-min timeout after each injection limited the maximum of injections per session to eight. Food pellets were available continuously during the session under a fixed ratio 10 schedule of reinforcement. Self-injection of cocaine (0.001-0.32 mg/kg/injection) and vehicle was characterized first. Cocaine maintained self-injection in a dose-dependent manner, with peak injections maintained by 0.32 mg/kg. Vehicle and each dose of propofol (0.1-1.0 mg/kg/injection) and methohexital (0.01-1.0 mg/kg/injection) were substituted for 0.32 mg/kg cocaine for at least 10 sessions. Propofol and methohexital maintained self-injection greater than vehicle in all three baboons, and these effects were dose dependent. Methohexital maintained peak mean levels of self-injection that were >6 injections/day at doses of 0.56 and 1.0 mg/kg, and did not alter food intake systematically. Propofol maintained peak mean levels of self-injection at 1.0 mg/kg that ranged from 2.2 to >6 injections/day across the baboons. Food intake was increased slightly above vehicle levels by propofol self-injection in two baboons, and was decreased in the third baboon. These data indicate that propofol, like methohexital, can function as a positive reinforcer.

Propofol's biphasic effect on GABA(A)-receptor-mediated response of the isolated guinea pig ileum

This study examines the effect of the anaesthetic agent propofol on the guinea pig ileum and whether this effect derives from an interaction of the drug with GABA receptors. Propofol produced a biphasic effect consisting of a dose-dependent contractile effect (EC(50)=2.2x10(-5)m) followed by an 'after relaxation'. This propofol effect was similar to the one produced by GABA and was bicuculline-sensitive (ID(50)=3.2x10(-7)m). Propofol, at concentrations of 10(-7)and 10(-6)m, potentiated the ileum contractile responses to GABA, but only at the lower dose range of applied GABA, while at a concentration of 10(-5)m, it inhibited the contractile effect over the entire dose range of applied GABA. In addition, while the contractile response of the ileum to exogenously applied acetylcholine was not influenced by propofol at concentrations of up to 7x10(-6)m, it was antagonised by higher concentrations of propofol. In conclusion, the above data permit us to suggest that propofol's contractile effect on the guinea pig ileum is mediated by an interaction of the drug with GABA(A)-receptors.

Modulatory and direct effects of propofol on recombinant GABAA receptors expressed in xenopus oocytes: influence of alpha- and gamma2-subunits

Propofol (2,6-diisopropylphenol) is an intravenous general anaesthetic which can directly activate and positively modulate the GABAA receptor. The effects of propofol on human recombinant GABAA receptors were studied in Xenopus oocytes expressing either alpha1beta2, alpha1beta2gamma2L, or alpha2beta2gamma2L receptor isoforms. In all receptor isoforms tested, propofol was able to potentiate the GABA-activated currents in a concentration-dependent manner. Although propofol potentiated both alpha1beta2 and alpha1beta2gamma2L receptor isoforms with equal affinity, the efficacy of propofol potentiation was markedly greater in the alpha1beta2 receptor isoform. In contrast, potentiation of the alpha2beta2gamma2L receptor isoform by propofol occurred with higher affinity and lower efficacy than in the alpha1beta2gamma2L receptor isoform. Propofol directly activated all three receptor isoforms in a concentration dependent manner. Addition of the gamma2L subunit subtype to the alpha1beta2 receptor isoform decreased receptor sensitivity to direct activation by propofol. Replacement of the alpha1-subunit subtype with the alpha2-subunit subtype increased receptor sensitivity to propofol's direct effects. These results suggest that the alpha-and gamma2L-subunit subtypes each have the ability to influence both the direct and modulatory actions of propofol on GABAA receptor function.

Direct activation of GABAA receptors by loreclezole, an anticonvulsant drug with selectivity for the beta-subunit

Loreclezole, an anticonvulsant and antiepileptic compound, potentiates gamma-aminobutyric acid (GABA) type A receptor function, by interacting with a specific allosteric modulatory site on receptor beta-subunits. A similar selectivity for GABAA receptor beta-subunits is apparent for the direct activation of receptor-operated Cl- channels, by the general anesthetics propofol and pentobarbital. The ability of loreclezole to activate GABAA receptors directly has now been compared, biochemically and electrophysiologically, with that of propofol. In well-washed rat cortical membranes (devoid of endogenous GABA), loreclezole and propofol increased t-[35S]butylbicyclophosphorothionate ([35S]TBPS) binding by up to 28% (at 5 microM) and 80% (at 10 microM), respectively. Higher concentrations (50-100 microM) of both compounds inhibited [35S]TBPS binding with great efficacy, an effect mimicked by GABA. In contrast, the benzodiazepine diazepam increased [35S]TBPS binding, but failed to inhibit this parameter, even at high concentrations. At concentrations of 50-100 microM, loreclezole induced inward Cl- currents in the absence of GABA, in Xenopus oocytes expressing human recombinant GABAA receptors, comprised of alpha 1-, beta 2- and gamma 2S-subunits. At 100 microM, the current evoked by loreclezole was 26% of that induced by 5 microM GABA. The current evoked by 100 microM propofol was 98% of that induced by 5 microM GABA. Currents induced by loreclezole, like those evoked by propofol, were potentiated by diazepam in a flumazenil-sensitive manner and blocked by either bicuculline or picrotoxin. These data suggest that loreclezole shares, with propofol, an agonistic action at GABAA receptors containing the beta 2-subunit and that the different efficacies of the two compounds in this regard, may underlie the difference in their pharmacological profiles. The failure of loreclezole to activate GABAA receptors containing the beta 1-subunit may be responsible for its lack of hypnotic effect.