Enhancement of GABA-mediated postsynaptic inhibition in cultured mammalian spinal cord neurons: a common mode of anticonvulsant action

Interaction of anticonvulsants with the barbiturate-benzodiazepine-GABA receptor complex

Unlike the anesthetic barbiturate pentobarbital and the anxiolytic pyrazolopyridine etazolate, which enhance [3H]diazepam binding to rat brain membranes, the anticonvulsant barbiturates phenobarbital and metharbital, and also chlormethiazole, at therapeutic concentrations (10-1000 muM), do not stimulate [3H]diazepam binding, but instead block the enhancement by both pentobarbital and etazolate. The same anticonvulsants at similar concentrations inhibit [3H]alpha-dihydropicrotoxinin (DHP) binding suggesting that these anticonvulsants compete for the same receptor sites as pentobarbital and etazolate, designated the barbiturate-picrotoxinin receptor component of the benzodiazepine-GABA receptor complex.

Pentobarbital: dual actions to increase brain benzodiazepine receptor affinity

The binding of [3H]diazepam to benzodiazepine receptors was studied in extensively washed membranes of rat cerebral cortex in the presence of the depressant barbiturate, pentobarbital. Pentobarbital, like the endogenous neurotransmitter gamma-aminobutyric acid (GABA), increased the basal binding and also potentiated the GABA-enhanced binding of [3H]diazepam to benzodiazepine receptors by increasing the apparent affinity of [3H]diazepam for the benzodiazepine receptor. The concentrations of pentobarbital necessary to elicit these effects in vitro are the same as those observed after treatment with pharmacologically relevant doses, suggesting that a common neurochemical association may exist between these types of compounds.

Barbiturate enhancement of GABA-mediated inhibition and activation of chloride ion conductance: correlation with anticonvulsant and anesthetic actions

The anesthetic-sedative barbiturate pentobarbital (PB) and the anticonvulsant barbiturate phenobarbital (PhB) were applied to mammalian spinal cord neurons in primary dissociated cell culture to assess their effects on: (1) postsynaptic GABA-responses; (2) paroxysmal activity produced by the convulsant bicuculline; (3) resting membrane properties; and (4) spontaneous neuronal activity. The results demonstrated that: (1) anticonvulsant actions occurred at barbiturate concentrations which augmented GABA-responses; (2) anesthetic actions occurred at barbiturate concentrations which produced direct increases in chloride conductance; (3) both anticonvulsant and anesthetic actions occurred at clinically relevant concentrations; and (4) concentrations of PhB, but not PB, which produced GABA-augmentation and direct conductance changes were widely separated. These findings support the hypotheses that augmentation of GABA-mediated inhibition and possibly reduction of glutamate (GLU)-mediated excitation form the basis at least in part for barbiturate anticonvulsant action and that addition of direct increases in chloride conductance to augmentation of GABA-mediated inhibition and reduction of GLU-mediated excitation may partially underlie anesthetic-sedative barbiturate action.

Effects of phenytoin on pyramidal neurons of the rat hippocampus

The effects of phenytoin (35 micrograms/ml) on membrane properties and inhibitory postsynaptic potential (IPSPs) in CA1 and CA3 pyramidal neurons of the in vitro rat hippocampus were examined. No significant change was observed on input resistance or resting membrane potential. Action potential amplitude, overshoot, rate of rise and rate of decay were decreased. IPSP conductance increase and reversal potential, evoked in CA3 cells through mossy fiber stimulation and in CA1 cells through recurrent and Schaffer's collateral stimulation, were unaffected.

Differential effects of pentobarbital and ether on the synaptic transmission of the hippocampal CA1 region in the rat

The effect of ether and sodium pentobarbital on the synaptic transmission of the hippocampal CA1 region was studied in chronically implanted rats. Animal behavior, EEG, and the average evoked potentials (AEPs) following electrical stimulation of the alveus or the stratum radiatum in the CA1 region were recorded. Components of the AEPs, interpreted previously as generated by population excitatory postsynaptic potentials (EPSPs), population inhibitory postsynaptic potentials (IPSPs) (Leung 1979a, b, c) or population postsynaptic spikes (Andersen et al. 1971), were differentially sensitive to ether or pentobarbital. Ether reduced the population EPSPs and population spike evoked at all intensities tested (1-4 X threshold); the population IPSP was slightly enhanced at intermediate stimulus intensities. Pentobarbital suppressed the population EPSP evoked by alvear stimulation but not that by radiatum stimulation, reduced the population spike and greatly enhanced and prolonged the population IPSP evoked at low stimulus intensities. At high stimulus intensities, the IPSP was interpreted to be smaller after pentobarbital but neuronal output from the hippocampal CA1 region, as seen from the evoked population spike, remained attenuated. It is concluded that ether and pentobarbital both suppress hippocampal neuronal excitability but the effect of anesthesia differs for different anesthetics, for different synapses and for different levels of activity in the input fibers.

Effect of anticonvulsant drugs on inhibitory and excitatory pathways

A conditioning stimulus to the periventricular gray matter inhibits the response of spinal trigeminal neurons to maxillary nerve stimulation. Sodium valproate and ethosuximide decrease the periventricular inhibition without significantly affecting the response of these neurons to the unconditioned maxillary nerve stimulus. We have now found that carbamazepine and phenytoin decrease the response to the unconditioned maxillary nerve stimulus, and only depress the periventricular inhibition secondarily. These results further support the hypothesis that the ability to depress selectively inhibitory pathways in the CNS is an important characteristic of antiabsence drugs, and that absence seizures may represent paroxysmal discharges in inhibitory pathways.

The effects of pentobarbital and related compounds on frog motoneurons

The effect of pentobarbital (PB) and related compounds on frog motoneurons was examined with sucrose gap recording from the ventral roots. PB was found to: (1) depress the action of glutamate, (2) selectively enhance the action of GABA, (3) reverse the non-competitive picrotoxin antagonism of GABA and the competitive strychnine antagonism of beta-alanine, but not the competitive bicuculline methiodide antagonism of GABA, and (4) elicit a GABAmimetic hyperpolarization. The first three actions had a threshold concentration of 10 microM, while the GABAmimetic action required a 10-fold higher concentration. The reversal of picrotoxin's action by PB suggests that PB might modify GABA mechanisms by combining to the picrotoxin recognition site. Phenobarbital shared all of the properties of PB but was approximately one-fifth as potent. The only property that phenytoin shared with PB was a weak depression of glutamate responses. Chlordiazepoxide selectively enhanced GABA responses but was devoid of the other actions of PB. These results suggest that the GABAmimetic effect of PB may be an important feature in the depressant and anesthetic properties of PB. The anesthetic chloralose, which is structurally unrelated to PB, nevertheless shared all of the actions of PB. This finding suggests that the properties described for PB may also be found in other general anesthetics.

Characterization of benzodiazepine receptors in primary cultures of fetal mouse brain and spinal cord neurons

Primary cultures of fetal mouse brain and spinal cord were examined for the presence of binding sites for [3H]diazepam. Both brain and spinal cord cultures contain high affinity binding sites which resemble benzodiazepine receptors found in mammalian CNS with respect to both pharmacologic profile and response to exogenously applied GABA. These observations, coupled with the electrophysiologic properties of these cells suggest that primary cultures of fetal mouse brain and spinal cord may be valid models for studying the role and regulation of the benzodiazepine receptor.

Water soluble benzodiazepines with agonistic and antagonistic actions on GABA-induced inhibition in cultured hypothalamus

The interaction of two benzodiazepine compounds, flurazepam (a sedative and RO 5-3663 (a convulsant), with amino acid depressions of spontaneous neuronal activity in hypothalamic tissue cultures has been examined. Flurazepam selectively potentiated, GABA-induced inhibition, while RO 5-3663 selectively reduced GABA-induced inhibtion. These results are interpreted as supporting the presence of two types of benzodiazepine binding sites in the brain which would modulate endogenous GABA-mediated inhibition and behavior.