Differences among effects of sedative-hypnotic drugs on GABA-mediated chloride flux: quench flow studies

Ethanol modulation of GABAergic transmission: the view from the slice

For almost three decades now, the GABAergic synapse has been the focus of intense study for its putative role in mediating many of the behavioral consequences associated with acute and chronic ethanol exposure. Although it was initially thought that ethanol interacted solely with the postsynaptic GABAA receptors that mediate the majority of fast synaptic inhibition in the mammalian central nervous system (CNS), a number of recent studies have identified novel pre- and postsynaptic mechanisms that may contribute to the acute and long-term effects of ethanol on GABAergic synaptic inhibition. These mechanisms appear to differ in a brain region specific manner and may also be influenced by a variety of endogenous neuromodulatory factors. This article provides a focused review of recent evidence, primarily from in vitro brain slice electrophysiological studies, that offers new insight into the mechanisms through which acute and chronic ethanol exposures modulate the activity of GABAergic synapses. The implications of these new mechanistic insights to our understanding of the behavioral and cognitive effects of ethanol are also discussed.

Ethanol and amino acids in the central nervous system: assessment of the pharmacological actions of acamprosate

Ethanol induces alterations in the central nervous system by differentially interfering with a number of neurotransmitter systems, although the mechanisms by which such effects are executed are not well understood. The present review therefore, is designed to ascertain the effect of ethanol on both excitatory and inhibitory amino acid neurotransmitters, as well as the sulphonated amino acid taurine, assayed by the microdialysis technique within specific brain regions of rat during different types of alcohol intoxication, acute and chronic, as well as during the withdrawal period. Such an understanding of these pharmacological actions of ethanol on neurotransmitters is essential in order to provide the impetus for the development of appropriate therapeutic intervention to ameliorate the multitude of neurochemical disorders induced by ethanol. In addition the possible mode of action of a new therapeutic drug for the treatment of alcoholism, acamprosate will be discussed. The first part of this review will be limited to studies of the effect of ethanol on both amino acid neurotransmitters and the sulphonated amino acid taurine, a possible neuromodulator. While, the second part will seek to establish the possible mechanism of action of a new therapeutic drug, acamprosate, which is used to combat the effects of ethanol, particularly during the craving period, as well as maintaining abstinence in weaned alcoholics.

Neurochemical bases of locomotion and ethanol stimulant effects

The locomotor stimulant effect produced by alcohol (ethanol) is one of a large number of measurable ethanol effects. Ethanol-induced euphoria in humans and locomotor stimulation in rodents, a potential animal model of human euphoria, have long been recognized and the latter has been extensively characterized. Since the euphoria produced by ethanol may influence the development of uncontrolled or excessive alcohol use, a solid understanding of the neurochemical substrates underlying such effects is important. Such an understanding for spontaneous locomotion and for ethanol's stimulant effects is beginning to emerge. Herein we review what is known about three neurochemical substrates of locomotion and of ethanol's locomotor stimulant effects. Several lines of research have implicated dopaminergic, GABAergic, and glutamatergic neurotransmitter systems in determining these behaviors. A large collection of work is cited, which strongly implicates the above-mentioned neurotransmitter substances in the control of spontaneous locomotion. A smaller, but persuasive, body of evidence suggests that central nervous system processes utilizing these transmitters are involved in determining the effects of ethanol on locomotion. Particular emphasis has been placed on the mesolimbic ventral tegmental area to nucleus accumbens dopaminergic pathway, and on the ventral pallidum/substantia innominata, where GABA and glutamate have been found to play a role in altering the activity of this dopaminergic pathway. Research on ethanol and drug locomotor sensitization, increased responsiveness to the substance with repeated administration, is also reviewed as a process that may be important in the development of drug addiction.

Interaction of ethanol and allosteric modulators with GABAA-activated currents in adult medial septum/diagonal band neurons

Behavioral and electrophysiological studies suggest that neurons in the medial septum may express ethanol sensitive GABAA receptors. In the present study, patch-clamp recordings of whole-cell currents were used to directly characterize the ethanol sensitivity of GABAA receptors on acutely dissociated neurons, isolated from the medial septum/nucleus of the diagonal band (MS/nDB) of the adult rat brains. MS/nDB neurons displayed inward currents in response to GABA applied rapidly with a large-bore dual pipette system. The currents were mediated by the activation of GABAA receptors, since they reversed near the calculated reversal potential for chloride and were completely blocked by bicuculline. GABA responses were concentration dependent with an EC50 of 8.7 microM GABA and a slope of 1.35 suggesting cooperativity. Pharmacologically relevant concentrations of ethanol (3-300 mM) neither significantly increased nor decreased mean responses to GABA in neurons from Sprague Dawley or High Alcohol Sensitivity (HAS) rats. Mean GABA currents were significantly increased by 300 mM ethanol in neurons from 'ethanol sensitive' Fischer 344, ACI and Wistar Kyoto inbred rats. In subsets of neurons, 12.5 to 57.1% of those tested from these 5 rats strains, ethanol (30-300 mM) significantly increased GABA currents by > or = 20%. An additional, 10 percent of cells from Sprague Dawley rats showed ethanol-induced inhibition of GABA-activated current by < or = 20%. Allosteric modulators pentobarbital (10 microM), midazolam (1 microM) and lanthanum (300 microM), enhanced, while zinc (30 microM) decreased GABA-activated currents in all neurons, consistent with the well-known actions of these agents. These results suggest that GABAA receptors on MS/dDB neurons are pharmacologically similar to those on other neurons with respect to regulation by allosteric modulators. On the other hand, ethanol sensitivity of GABAA receptors varies considerably from cell to cell ranging from significant enhancement to inhibition of GABA-activated current.

GABA and pentobarbital potentiation of chloride influx into microsacs is influenced by incubation time

GABA-mediated chloride influx into rat cerebral cortical microsacs, and its potentiation by pentobarbital (PB), was studied in incubations lasting 50 or 3000 ms. The 3000 ms assays were carried out manually, while the 50 ms assays were performed in a quench flow machine. The EC50 of the GABA effect and the Hill coefficient were both lower at the longer incubation times. Percent increase of the effect of GABA (at EC30 concentration) by pentobarbital was greater in the 3000 ms than in the 50 ms incubations. Preincubation of microsacs with PB for 5 s before the 50 ms incubation with GABA, did not increase the effect of PB beyond that obtained by adding PB into the incubation only. This finding is consistent with earlier ones cited, which imply that only PB binding that occurs after GABA has bound can potentiate the effect of GABA, and suggests that the concentrations of PB and GABA required to enhance chloride influx in vivo may be higher than those commonly reported in the literature.

Morphine physical dependence in the hibernator: central nervous system mechanisms underlying the development of dependence remain functional during depression induced by pentobarbital anesthesia

Earlier work suggested that adaptive mechanisms of the hibernating brain may also block the development of morphine physical dependence. To test an alternate view that CNS depression itself might be the major factor in the failure of dependence to develop, we compared the strength of dependence produced by morphine (two 75-mg pellets, s.c.) given for 12 hr in the presence, versus in the absence, of continuous pentobarbital anesthesia in nonhibernating ground squirrels (Citellus lateralis) and, in addition, in rats. Dependence was measured by the naloxone (5 mg/kg, s.c.) evoked abstinence syndrome in the awake state. The results demonstrated that pentobarbital-induced general anesthesia does not significantly reduce the development of morphine dependence in either species. We conclude that CNS depression alone does not account for the hibernation-related reduction in morphine physical dependence.