Physiological modulation of the GABA receptor by convulsant and anticonvulsant barbiturates in cultured rat hippocampal neurons

Contrasting actions of a convulsant barbiturate and its anticonvulsant enantiomer on the α1 β3 γ2L GABAA receptor account for their in vivo effects

KEY POINTS

Most barbiturates are anaesthetics but unexpectedly a few are convulsants whose mechanism of action is poorly understood. We synthesized and characterized a novel pair of chiral barbiturates that are capable of photolabelling their binding sites on GABAA receptors. In mice the S-enantiomer is a convulsant, but the R-enantiomer is an anticonvulsant. The convulsant S-enantiomer binds solely at an inhibitory site. It is both an open state inhibitor and a resting state inhibitor. Its action is pH independent, suggesting the pyrimidine ring plays little part in binding. The inhibitory site is not enantioselective because the R-enantiomer inhibits with equal affinity. In contrast, only the anticonvulsant R-enantiomer binds to the enhancing site on open channels, causing them to stay open longer. The enhancing site is enantioselective. The in vivo actions of the convulsant S-enantiomer are accounted for by its interactions with GABAA receptors.

ABSTRACT

Most barbiturates are anaesthetics but a few unexpectedly are convulsants. We recently located the anaesthetic sites on GABAA receptors (GABAA Rs) by photolabelling with an anaesthetic barbiturate. To apply the same strategy to locate the convulsant sites requires the creation and mechanistic characterization of a suitable agent. We synthesized enantiomers of a novel, photoactivable barbiturate, 1-methyl-5-propyly-5-(m-trifluoromethyldiazirinyl) phenyl barbituric acid (mTFD-MPPB). In mice, S-mTFD-MPPB acted as a convulsant, whereas R-mTFD-MPPB acted as an anticonvulsant. Using patch clamp electrophysiology and fast solution exchange on recombinant human α1 β3 γ2L GABAA Rs expressed in HEK cells, we found that S-mTFD-MPPB inhibited GABA-induced currents, whereas R-mTFD-MPPB enhanced them. S-mTFD-MPPB caused inhibition by binding to either of two inhibitory sites on open channels with bimolecular kinetics. It also inhibited closed, resting state receptors at similar concentrations, decreasing the channel opening rate and shifting the GABA concentration-response curve to the right. R-mTFD-MPPB, like most anaesthetics, enhanced receptor gating by rapidly binding to allosteric sites on open channels, initiating a rate-limiting conformation change to stabilized open channel states. These states had slower closing rates, thus shifting the GABA concentration-response curve to the left. Under conditions when most GABAA Rs were open, an inhibitory action of R-mTFD-MPPB was revealed that had a similar IC50 to that of S-mTFD-MPPB. Thus, the inhibitory sites are not enantioselective, and the convulsant action of S-mTFD-MPPB results from its negligible affinity for the enhancing, anaesthetic sites. Interactions with these two classes of barbiturate binding sites on GABAA Rs underlie the enantiomers' different pharmacological activities in mice.

Positive and Negative Allosteric Modulation of an α1β3γ2 γ-Aminobutyric Acid Type A (GABAA) Receptor by Binding to a Site in the Transmembrane Domain at the γ+-β- Interface

In the process of developing safer general anesthetics, isomers of anesthetic ethers and barbiturates have been discovered that act as convulsants and inhibitors of γ-aminobutyric acid type A receptors (GABAARs) rather than potentiators. It is unknown whether these convulsants act as negative allosteric modulators by binding to the intersubunit anesthetic-binding sites in the GABAAR transmembrane domain (Chiara, D. C., Jayakar, S. S., Zhou, X., Zhang, X., Savechenkov, P. Y., Bruzik, K. S., Miller, K. W., and Cohen, J. B. (2013) J. Biol. Chem. 288, 19343-19357) or to known convulsant sites in the ion channel or extracellular domains. Here, we show that S-1-methyl-5-propyl-5-(m-trifluoromethyl-diazirynylphenyl) barbituric acid (S-mTFD-MPPB), a photoreactive analog of the convulsant barbiturate S-MPPB, inhibits α1β3γ2 but potentiates α1β3 GABAAR responses. In the α1β3γ2 GABAAR, S-mTFD-MPPB binds in the transmembrane domain with high affinity to the γ(+)-β(-) subunit interface site with negative energetic coupling to GABA binding in the extracellular domain at the β(+)-α(-) subunit interfaces. GABA inhibits S-[(3)H]mTFD-MPPB photolabeling of γ2Ser-280 (γM2-15') in this site. In contrast, within the same site GABA enhances photolabeling of β3Met-227 in βM1 by an anesthetic barbiturate, R-[(3)H]methyl-5-allyl-5-(m-trifluoromethyl-diazirynylphenyl)barbituric acid (mTFD-MPAB), which differs from S-mTFD-MPPB in structure only by chirality and two hydrogens (propyl versus allyl). S-mTFD-MPPB and R-mTFD-MPAB are predicted to bind in different orientations at the γ(+)-β(-) site, based upon the distance in GABAAR homology models between γ2Ser-280 and β3Met-227. These results provide an explanation for S-mTFD-MPPB inhibition of α1β3γ2 GABAAR function and provide a first demonstration that an intersubunit-binding site in the GABAAR transmembrane domain binds negative and positive allosteric modulators.

Stiripentol, a Putative Antiepileptic Drug, Enhances the Duration of Opening of GABAA-Receptor Channels

PURPOSE

Stiripentol (STP) is currently an efficient drug for add-on therapy in infantile epilepsies because it improves the efficacy of antiepileptic drugs (AEDs) through its potent inhibition of liver cytochromes P450. In addition, STP directly reduces seizures in several animal models of epilepsy, suggesting that it might also have anticonvulsive effects of its own. However, its underlying mechanisms of action are unknown.

METHODS

We examined the interactions of STP with gamma-aminobutyric acid (GABA) transmission by using patch-clamp methods in CA3 pyramidal neurons in the neonatal rat.

RESULTS

STP markedly increased miniature inhibitory postsynaptic current (mIPSC) decay-time constant in a concentration-dependent manner. The prolongation of mIPSC duration does not result from an interaction with GABA transporters because it persisted in the presence of GAT-1 inhibitors (SKF-89976A and NO-711). An interaction with benzodiazepine or neurosteroid binding sites also was excluded because STP-mediated increase of decay time was still observed when these sites were initially saturated (by clobazam, zolpidem, or pregnanolone) or blocked (by flumazenil or dehydroepiandrosterone sulfate), respectively. In contrast, saturating barbiturate sites with pentobarbital clearly occluded this effect of STP, suggesting that STP and barbiturates interact at the same locus. This was directly confirmed by using outside-out patches, because STP increased the duration and not the frequency of opening of GABAA channels.

CONCLUSIONS

At clinically relevant concentrations, STP enhances central GABA transmission through a barbiturate-like effect, suggesting that STP should possess an antiepileptic effect by itself.

Dissociation of postnatal hippocampal neurons for short term culture

A method is described to isolate hippocampal neurons from postnatal day 7 to postnatal day 21 rats for use in short term cultures using a combination of enzymatic and mechanical dissociation. A significant fraction of the cells (37.5 +/- 2.3%, n = 10 cultures) were labelled with anti-GABA antibodies and the neurons survived a minimum of 8 days in culture. Patch-clamp recording in the current clamp mode revealed an average membrane potential and input resistance of - 47.6 +/- 2.5 mV and 737 +/- 147 M omega respectively, after 24 h in culture. The cells exhibited normal excitability and fired multiple action potentials in response to a depolarizing pulse. The technique described here provides a good alternative to either the use of embryonic brain tissue for cultures used in electrophysiological studies which may not exhibit the mature phenotype or use of acutely isolated cells which may be unstable or have their channels and receptors modified by enzymatic treatment.

Astroglia-released factor with negative allosteric modulatory properties at the GABA A receptor

We have previously shown, using whole-cell patch-clamp techniques, that astrocytes release a negative allosteric modulator of the gamma-aminobutyric acid type A receptor (GABAA receptor) with beta-carboline-like properties, thus, likely to act at the benzodiazepine site. Here, using patch-clamp and binding techniques, we confirm that the low-molecular-weight fraction of astroglia-conditioned medium (ACM lmf) contains a factor(s) that negatively modulates GABAA-receptor function. This factor, like beta-carbolines, enhances the specific binding of [35S]t-butyl bicyclophosphorothionate (TBPS) to adult rat cortical membranes in the presence of GABA. However, it fails to interact with various ligands of the benzodiazepine (BZD) site of the GABAA receptor ([3H]flunitrazepam, [3H]Ro 15-1788 and [3H]Ro 15-4513). The question of the actual binding site of the astroglia-derived factor on the GABAA receptor, thus, remains open and can be addressed only after the purification of the active molecule(s) of ACM Imf has been completed, and a labeled form of the endogenous ligand becomes available. Taken together, however, the data suggest that type 1 astrocytes are able to modulate the effects of the main inhibitory neurotransmission in the central nervous system.

CHEB, a convulsant barbiturate, evokes calcium-dependent spontaneous glutamate release from rat cerebrocortical synaptosomes

CHEB [5-(2-cyclohexylidene-ethyl)-5-ethyl barbituric acid] is a potent convulsant barbiturate that causes direct neuronal excitation by an unknown mechanism. We have analyzed the effects of CHEB on the release of endogenous glutamate from rat cerebrocortical synaptosomes using an on-line enzyme-coupled fluorimetric assay. CHEB evoked spontaneous Ca(2+)-dependent glutamate release with an EC50 = 14.2 microM and an Emax = 3.2 mumol/min/mg. The non-convulsant barbiturates pentobarbital and phenobarbital evoked significantly less glutamate release at high concentrations. CHEB (30 microM) increased intrasynaptosomal [Ca2+] by 58 +/- 4 nM (p < 0.01; n = 4) above baseline compared to an increase of 5 +/- 4 nM (NS; n = 4) produced by pentobarbital (30 microM). CHEB-evoked glutamate release was inhibited by pentobarbital, phenobarbital, EGTA, CoCl2/CdCl2 and flunarizine, but not by local anesthetics, tetrodotoxin, nitrendipine or omega-conotoxin GVIA. These results demonstrate that CHEB acts as a potent and effective secretogogue for glutamate by a pre-synaptic mechanism that does not require activation of Na+ channels or of L-type or N-type Ca2+ channels. Stimulation of spontaneous glutamate release may contribute to the convulsant properties of CHEB.

Astroglia-released factor shows similar effects as benzodiazepine inverse agonists

Media conditioned by cultured neonatal cerebral cortex microexplants (CCM) or astrocytes (ACM) contain low molecular weight (< 1,000 Da) substance(s) which inhibits the gamma aminobutyric acid (GABA)-induced inward current recorded in cerebellar granule cells and hippocampal neurons in culture using the whole-cell patch-clamp technique. This effect is specific for CCM and ACM, as medium conditioned by PC12 cells (PC12CM) does not affect the GABA response of these cells. It is also specific for GABA-induced currents because glutamate-induced currents do not change either in amplitude or in shape in the presence of CCM or ACM. The inhibitory effect on the GABA response in cerebellar granule cells of both ACM and CCM could be suppressed by flumazenil, a specific benzodiazepine (BZD) antagonist and could be mimicked by two BZD inverse agonists. These data thus demonstrate the presence of a BZD inverse agonist-like activity in CCM and ACM. This effect of ACM on different neuronal cell types was heterogenous since no detectable effect could be observed on the GABA-induced current in GABA-responsive dorsal root ganglion (DRG) neurons, presumably reflecting a functional heterogeneity of the GABAA receptors present in these different neuronal subsets. By the release of such an endogenous BZD inverse agonist-like activity, glia cells could possibly modulate GABAA receptor-mediated responses.

Developmental alteration in GABAA receptor structure and physiological properties in cultured cerebellar granule neurons

Although we now have extensive knowledge about the GABAA receptor subunits determining benzodiazepine modulation of channel function, little is known about subunits influencing other modulatory sites on the GABAA receptor-chloride channel complex. We have identified a developmental change in subunit composition of the GABAA receptor in cultured cerebellar granule neurons that eliminates benzodiazepine-mediated enhancement of GABA responses and alters modulation by a substituted gamma-butyrolactone. Based on data from sequential PCR experiments, we mimicked the functional properties of early and mature receptors with heterologous expression of specific subunit combinations. This report describes one of the most extensive cell- and site-specific developmental changes for an ion channel seen to date.

Effects of antiepileptic drugs on 4-aminopyridine-induced epileptiform activity in young and adult rat hippocampus

Extracellular field potential recordings were used to study the effects of the antiepileptic drugs (AEDs) carbamazepine (CBZ), phenytoin (PHT), phenobarbital (PhB) and valproic acid (VPA) on the epileptiform activity evoked by 4-aminopyridine (4-AP, 50 microM) in the CA3 subfield of rat hippocampal slices obtained from young (8-23-day-old) and adult (> 60-day-old) male rats. Ictal (duration: 3-20 s; rate of occurrence: 3-12 x 10(-3) s-1) and interictal (duration: 0.2-0.8 s; rate of occurrence: 0.2-0.8 s-1) discharges were recorded in young slices, while only interictal activity (duration: 70-90 ms; rate of occurrence: 0.5-0.9 s-1) discharges were observed in adult slices. In addition, in both young and adult slices 4-AP disclosed a synchronous long-lasting potential (duration and rate of occurrence: 0.6-3 s, 7-70 x 10(-3) s-1 and 260-660 ms, 8-60 x 10(-3) s-1, respectively) that was caused by the activation of the gamma-aminobutyric acid type A (GABAA) receptor. In young slices, ictal discharges were blocked by CBZ (0.05 mM), PHT (0.1 mM), PhB (0.5 mM) and VPA (0.5 mM). With the exception of PhB, higher concentrations were necessary in these experiments for blocking the interictal activity (i.e., CBZ: 0.1 mM; PHT: > 0.2 mM; VPA: 2 mM). At these concentrations, none of the AEDs blocked the interictal activity in the adult hippocampus, but only reduced the rate of occurrence. PhB enhanced the rate of occurrence of the synchronous GABA-mediated long-lasting potentials both in young (increase: 190%) and in adult (increase: 145%) slices, while VPA increased their occurrence by 54% only in young slices. CBZ decreased the rate of occurrence of this long-lasting potential only in adult hippocampus. Our data indicate that the effects of the AEDs on 4-AP-induced epileptiform discharges are both pattern- and age-dependent. The rank order of potencies of the four AEDs was: (a) in young: CBZ > PHT > PhB > VPA; (b) in adult: CBZ > PhB > PHT > VPA.