Convulsant properties of GABA antagonists and anticonvulsant properties of ethanol in selectively bred long- and short-sleep mice

Acute and chronic effects of melatonin as an anticonvulsant in male gerbils

Melatonin, a hormone produced in the pineal gland and released into the general circulation on a diurnal basis, has been implicated in many behavioral processes, where it has been shown to have anxiolytic, sedative, and anticonvulsant effects. Male gerbils (Meriones unguiculatus) injected daily with melatonin (25 micrograms, s.c.) exhibited a reduced seizure response to pentylenetetrazol (PTZ, 60 mg/kg, s.c.). The present studies determined 1) whether melatonin's effect was related to the time of day that it was administered and 2) whether a single acute injection of melatonin at various doses could produce anticonvulsant activity. Gerbils provided with 13 weeks of daily melatonin injections (25 micrograms, s.c.) exhibited fewer convulsions after PTZ treatment irrespective of the time of day melatonin was injected. In addition, the melatonin-treated gerbils had lower mortality rates (1/12) than the untreated or vehicle-injected gerbils (5/12). On the other hand, single acute injections of melatonin (0.1-10 mg/kg, i.p.) produced no anticonvulsant activity. It appears that the anticonvulsant effects of melatonin occur only after the animals are chronically exposed to the indole. In addition, melatonin's anticonvulsant ability may utilize a different mechanism than those involved in its endocrine effects, since no diurnal difference in melatonin's anticonvulsant activity was observed.

Low-level hyperbaric antagonism of ethanol's anticonvulsant property in C57BL/6J mice

This study investigated the ability of hyperbaric exposure to antagonize ethanol's anticonvulsant effect on isoniazid (INH)-induced seizures. Drug-naive, male C57BL/6 mice were injected intraperitoneally with saline, 1.5, 2.0, or 2.5 g/kg ethanol followed immediately by an intramuscular injection of 300 mg/kg of INH. The mice were then exposed to either 1 atmosphere absolute (1 ATA) air, 1 ATA helium-oxygen gas mixture (heliox), or 12 ATA heliox at temperatures that offset the hypothermic effects of helium. Ethanol increased the latency to onset of myoclonus in a dose-dependent manner. Exposure to 12 ATA heliox antagonized ethanol's anticonvulsant effect at 2.0 and 2.5 g/kg, but not at 1.5 g/kg. Ethanol also increased the latency to onset of clonus in a dose-dependent manner beginning at 2.0 g/kg. Exposure to 12 ATA heliox antagonized this anticonvulsant effect. When exposed to 12 ATA heliox, the blood ethanol concentrations at time to onset of myoclonus were significantly higher in mice treated with 2.5 g/kg of ethanol as compared with blood ethanol concentrations of mice exposed to 1 ATA air. These findings extend the acute behavioral effects of ethanol known to be antagonized by hyperbaric exposure and support the hypothesis that low-level hyperbaric exposure blocks or reverses the initial action(s) of ethanol leading to its acute behavioral effects.

Changes in GABAergic and non-GABAergic synapses during chronic ethanol exposure and withdrawal in the dentate fascia of LS and SS mice

Ethanol-sensitive LSIBG and ethanol-insensitive SSIBG mice were exposed to ethanol (23.5% ethanol-derived calories) for 4 months. Half of the animals was sacrificed at this time and the other half was withdrawn from the ethanol diet for 1 month. GABA immunoelectron microscopy was used to study the impact of the treatments on synaptic contacts in the dentate molecular layer. In the LS mice a significant loss of non-GABAergic axospinous synapses (26.7%; p < 0.05) was observed during ethanol exposure which was followed by a loss of GABAergic synapses on dendritic shafts (54.7%; p < 0.01) during withdrawal. In the SS mice there was a significant decrease in the non-GABAergic axospinous synapses (23.5%; p < 0.05) and a significant increase in axodendritic synapses (63.3%; p < 0.05) during ethanol exposure. The observed changes in the GABAergic and non-GABAergic innervation of the dentate fascia induced by ethanol were observed in the projection zone of the perforant path. They could adversely affect the hippocampal physiology with a consequent impairment of mnemonic functions.

Ethanol effects on 2-deoxyglucose uptake into tissues obtained from LS and SS mice

The uptake of 2-deoxy-D-[14C]glucose (2-DG) has been used with some success to identify neuronal sites of drug action, and has also been useful in identifying brain regions that are differentially sensitive to ethanol in selectively bred rat lines, the alcohol tolerant (AT) and alcohol nontolerant (ANT) lines. The studies reported here utilized the 2-DG method in an attempt to ascertain whether the LS and SS mouse lines, which were selectively bred for differences in sensitivity to high dose, anesthetic effects of ethanol, do so because of disruption of specific neuronal sites. LS and SS mice were injected with saline or a 4.0-g/kg dose of ethanol 15 min before injection with 2-deoxy-D-[14C]glucose, and uptakes into blood, eight brain regions, the liver, and adrenal tissues were measured 2-60 min afterwards. Ethanol produced statistically significant decreases in 2-DG uptake into every region of the LS mouse brain, but only three brain regions showed significant decreases in uptake in the SS brain. Other SS brain regions showed a trend towards decreased uptake, but these trends were not significant. A comparison of percent decrease in 2-DG uptake across all brain regions showed that ethanol decreased 2-DG uptake approximately twice as much in LS brain regions as in SS brain regions. Since 2-DG uptake into adrenal and hepatic tissue was not affected by ethanol injection in either mouse line, it seems likely that the decreased 2-DG uptake, seen more readily in the LS brain, is due to ethanol-induced central nervous system (CNS) depression.(ABSTRACT TRUNCATED AT 250 WORDS)

Adrenalectomy increases bicuculline-induced seizure sensitivity in long-sleep and short-sleep mice

Susceptibility to bicuculline-induced seizure onset and tonus was increased in LS and SS mice after adrenalectomy (ADX). Replacement with 10% corticosterone (CCS) in ADX animals resulted in a return to seizure latencies equal to those of sham-operated (SHAM) mice. In SS mice, dexamethasone (DEX) and cholesterol-control replacement was as effective as 10% CCS in returning seizure thresholds to SHAM values. In LS mice, DEX was only effective at a low bicuculline dose. Within the sham-operated group SS mice were more susceptible to bicuculline-induced seizure onset than LS mice; however, after ADX latencies did not differ between the two lines. These results suggest that seizure thresholds are regulated to some extent by the hypothalamic-pituitary-adrenal (HPA) axis. The effects of ADX on GABA-related seizure activity may also be influenced by genotype, such that genetic differences in GABAA receptor function and adrenocortical responses in LS and SS mice may be responsible for the differential seizure latencies observed in sham-operated mice.

Selected mouse lines, alcohol and behavior

The technique of selective breeding has been employed to develop a number of mouse lines differing in genetic sensitivity to specific effects of ethanol. Genetic animal models for sensitivity to the hypnotic, thermoregulatory, excitatory, and dependence-producing effects of alcohol have been developed. These genetic animal models have been utilized in numerous studies to assess the bases for those genetic differences, and to determine the specific neurochemical and neurophysiological bases for ethanol's actions. Work with these lines has challenged some long-held beliefs about ethanol's mechanisms of action. For example, lines genetically sensitive to one effect of ethanol are not necessarily sensitive to others, which demonstrates that no single set of genes modulates all ethanol effects. LS mice, selected for sensitivity to ethanol anesthesia, are not similarly sensitive to all anesthetic drugs, which demonstrates that all such drugs cannot have a common mechanism of action. On the other hand, WSP mice, genetically susceptible to the development of severe ethanol withdrawal, show a similar predisposition to diazepam and phenobarbital withdrawal, which suggests that there may be a common set of genes underlying drug dependencies. Studies with these models have also revealed important new directions for future mechanism-oriented research. Several studies implicate brain gamma-aminobutyric acid and dopamine systems as potentially important mediators of susceptibility to alcohol intoxication. The stability of the genetic animal models across laboratories and generations will continue to increase their power as analytic tools.

Genotype-dependent effects of GABAergic agents on sedative properties of ethanol

Two lines of mice, selectively bred for differential sensitivity to the soporific effects of ethanol (ETOH), were administered GABAergic drugs in an effort to evaluate a role for GABA in ETOH sensitivity. ETOH sensitive Long-Sleep mice (LS) showed potentiated ETOH sedation when administered bicuculline, muscimol and aminooxyacetic acid (AOAA). ETOH-insensitive SS mice exhibited reduced ETOH sedation in the presence of the antagonists, bicuculline and picrotoxin, and potentiated sedation in the presence of muscimol and AOAA. These changes in narcosis duration were interpreted as central effects, since blood ethanol levels at waking from ETOH sedation varied with GABAergic drug treatment. Picrotoxin antagonized pentobarbital-induced narcosis in both lines, but to a greater extent in SS mice. These and other experiments with a genetically heterogeneous stock suggest GABA involvement in genotype-dependent ETOH sensitivity, but do not support a simple role of GABA receptor involvement.

Modification of ethanol effects by bicuculline: genotype-dependent responses and inheritance

Genetic influences on the interaction between ethanol (ETOH) and gamma-aminobutyric acid (GABA) neurotransmitter systems were evaluated with a survey of responses to coadministration of ETOH and a GABA antagonist, bicuculline, in a battery of inbred mouse strains. The selectively bred ETOH-sensitive Long-Sleep (LS) mice, the relatively ETOH-resistant Short-Sleep (SS) mice, and a genetically heterogeneous stock (GHS) were also evaluated. The effect of bicuculline on ETOH-induced sedation, hypothermia, and blood ethanol content upon recovery from sedation was assessed. Inheritance of these responses was also examined using F1 hybrids. The effect of bicuculline on ETOH-produced narcosis varied widely among stocks and included antagonism, potentiation, and no effect. Changes in ETOH-induced narcosis produced by bicuculline were accompanied by changes in blood ethanol concentrations consistent with an hypothesis of altered central nervous system sensitivity to ETOH. Knowledge of a strain's seizure susceptibility to the GABA antagonist or of its sensitivity to the hypnotic effects of ETOH were of no predictive value in estimating the outcome of coadministration studies, suggesting at least partially separate genetic influences on each phenotype. In cross-breeding studies there was commonly dominance toward a profile of bicuculline antagonism of ETOH narcosis but different patterns of dominance were observed for seizure susceptibility, again indicating separate genetic control. The results suggest considerable complexity of GABAergic involvement in genotype-dependent ETOH sensitivity.