Effects of anticonvulsant and convulsant gamma-butyrolactones and thiobutyrolactones on GABA-mediated chloride uptake

Alteration of GLUR2 expression in the rat brain following absence seizures induced by gamma-hydroxybutyric acid

We explored the involvement of the glutamate receptor subunit B (GluR2) in the mechanism of absence seizures induced by gamma-hydroxybutyric acid (GHB). The expression and distribution of GluR2 protein in rat brain were examined during and after GHB-induced absence seizures. The data indicate that GluR2 protein expression significantly decreases following the onset of absence seizures. The suppression of GluR2 expression was prolonged and it outlasted the duration of the continuous absence seizure activity. The alteration of GluR2 protein levels was accompanied by a re-distribution of GluR2 expression from laminae V to IV in cerebral cortex. We also analyzed the duration and latency of absence seizures induced by GHB 72 h following an initial GHB-induced absence seizure, a time when suppression of GluR2 protein was maximal. The second absence seizure was significantly more prolonged than the first. These data may indicate that the putative down-regulation of GluR2 following GHB-induced absence seizure could have contributed to the potentiation of subsequent seizures in animals. A related hypothesis posed by the data is that down-regulation of GluR2 is involved in the mechanisms of the maintenance of recurrent absence seizure activity once it is initiated and therefore, may contribute to the chronicity of seizures in absence epilepsy.

Age-related changes in GABA(A) receptor subunit composition and function in rat auditory system

A decline in the ability to discriminate speech from noise due to age-related hearing loss (presbycusis) may reflect impaired auditory information processing within the central nervous system. Presbycusis may result, in part, from functional loss of the inhibitory neurotransmitter GABA. The present study assessed age-related changes of the GABA(A) receptor in the inferior colliculus of young-adult, middle-aged, and aged rats related to: (i) receptor subunit composition and (ii) receptor function. Western blotting was used to measure protein levels of selected GABA(A) receptor subunits in preparations obtained from the inferior colliculus of Fischer 344 and Fischer 344/Brown-Norway F1 hybrid rats. In both strains, the aged group exhibited significant increases in gamma1 subunit protein and a decrease in alpha1 subunit protein. To examine the functional consequence of this putative age-related subunit change, we measured the ability of exogenous GABA to flux/translocate chloride ions into microsac preparations derived from Fischer 344 inferior colliculus. GABA-mediated chloride influx was significantly increased in samples prepared from the inferior colliculus of aged animals. Together with previous studies, these results strongly suggest an age-related change in GABA(A) receptor composition. These changes may reflect a compensatory up-regulation of inhibitory function in the face of significant loss of presynaptic GABA release. These findings provide one example of plastic neurotransmitter receptor changes which can occur during the ageing process.

Effect of gamma-crotonolactone in experimental models of generalized absence seizures

The effect of the unsaturated lactone, gamma-crotonolactone (GCL), against spike wave discharges (SWD) in two pharmacological models of generalized absence seizures in rat was investigated. The models used were the gamma-hydroxybutyrate (GHB) model and the low dose pentylenetetrazole model. GCL pretreatment resulted in a significant decrease in duration of SWD in both models. In addition, this compound reduced the ability of the agonist, muscimol, to enhance SWD duration in the GHB model and was effective in developing animals as well as in adult animals with GHB-induced SWD. These data suggest that GCL may be a useful tool in the study of basic mechanisms of SWD generation in experimental absence seizures.

Convulsant gamma-butyrolactones block GABA currents in cultured chick spinal cord neurons

The gamma-aminobutyric acid (GABA) inhibiting properties of the beta-substituted gamma-butyrolactone convulsant, beta-isopropyl-gamma-butyrolactone (beta IPGBL), were studied using gigaseal recording techniques in cultured chick spinal cord neurons. beta IPGBL produced a dose-dependent inhibition of GABA currents with half maximal effect at 92 microM. The effects of beta IPGBL were immediate and completely reversible within minutes after exposure. The inhibition by beta IPGBL showed mixed competitive and non-competitive features with little voltage-dependence. beta IPGBL did not alter the GABA reversal potential nor the degree of GABA desensitization. At a single-channel level, beta IPGBL markedly diminished the opening of GABA channels and decreased the mean channel open time by 30-40% without affecting the amplitude of the single-channel current.

Alpha-substituted thiobutyrolactones potentiate GABA currents in voltage-clamped chick spinal cord neurons

The effect of the experimental anticonvulsant, alpha-ethyl,methyl thiobutyrolactone (alpha-EMTBL), on gamma-aminobutyric acid (GABA) currents was examined in cultured chick spinal cord neurons using gigaseal recording techniques. alpha-EMTBL potentiated GABA responses in a dose-dependent fashion with half-maximal effect at 7 microM. alpha-EMTBL also augmented the response to maximal doses of GABA. In competition experiments, alpha-EMTBL relieved the block of GABA currents produced by saturating concentrations of picrotoxinin whereas diazepam and phenobarbital did not. At a single channel level, alpha-EMTBL increased the probability of opening GABA channels without significantly altering the mean channel open time or the single channel conductance. These results indicate that alpha-EMTBL potentiates GABA responses and has properties which distinguish it from benzodiazepines or barbiturates.