Dihydropyridine-sensitive Ca2+ channels and inositol phospholipid metabolism in ethanol physical dependence

Second messengers involved in genetic regulation of the number of calcium channels in bovine adrenal chromaffin cells in culture

Bovine adrenal chromaffin cells in culture show an increased formation of [3H]inositol phosphates (after preloading with [3H]inositol) on depolarisation with increased extracellular K+. This increased breakdown of inositol lipid is further increased by the dihydropyridine Ca2+ channel activator BAY K 8644 at nM concentrations, implying that proteins which bind dihydropyridines are involved in this mechanism. Further, pretreatment of adrenal cells with pertussis toxin (100 ng ml-1) prevented the K(+)-induced breakdown of inositol lipids, arguing the involvement of a pertussis toxin-sensitive G protein in the effect. Chronic exposure of bovine adrenal chromaffin cells to a concentration of ethanol which inhibits K(+)-induced breakdown of inositol phospholipid, caused a 70-100% increase in the binding of [3H]DHP sites. In these experiments it was found that excess extracellular Ca2+ would considerably reduce this up-regulation, whereas growth of cells in pertussis toxin closely mimicked the up-regulation obtained by growth of cells in ethanol. These experiments suggest that inhibition of membrane Ca2+ flux, through a G protein-associated channel, is closely involved in the ethanol-induced regulation of [3H]dihydropyridine binding sites. The inositol lipid-protein kinase C second messenger system is also implicated in this regulation, by experiments in which inhibitors of protein kinase C (chronic treatment with phorbol myristyl acetate, or with sphingosine) up-regulated binding sites for [3H]dihydropyridine to a similar extent as that seen with growth in ethanol.

Reduced behavioral responses to intranigral muscimol following chronic ethanol

Increased biochemical measures of GABA activity are observed after acute administration of ethanol and decreased activity has sometimes been observed after chronic ethanol exposure. Since chronic alterations in neurotransmitter activity may result in changes in receptor function, it is possible that changes in GABA-receptive neurons may accompany chronic ethanol treatment. In the present study we examined the incidence of muscimol-induced motor behaviors in ethanol-naive and chronic ethanol-treated animals. Male Sprague-Dawley rats received bilateral cannula implants into substantia nigra pars reticulata for subsequent administration of muscimol or saline. After recovery from surgery, rats received chronic treatment in ethanol-vapor inhalation chambers for 15 days. Animals were then removed from the chambers and examined 10 hours after removal. Muscimol resulted in a general increase in motility in both control and ethanol-treated animals. Animals withdrawn from chronic ethanol exposure, however, exhibited significantly less muscimol-stimulated, repetitive 9 Hz movements. These results suggest that GABA receptive cells within the substantia nigra or its vicinity may be functionally less responsive to GABAergic stimulation after chronic ethanol administration.

The emerging pharmacology of ethanol

Historically, alcohol (ethanol) has been viewed as a non-specific CNS depressant, presumed to act equally on all bioelectric membranes. In contrast to this view, cellular electrophysiological studies, supported by anatomic and neurochemical evidence, support the emergence of a more specific pharmacological profile. Four regions of the rodent brain (cerebellum, hippocampus, locus coeruleus and inferior olive) have so far been examined. The effects of acute parenteral ethanol on specific identifiable neurons within these four regions are highly consistent, dose-related, and spontaneously reversible. Nevertheless, different patterns of effects are seen in each responsive region, ranging from general increased firing in inferior olive to generally depressed synaptic transmission in hippocampus, and with more subtle effects within the cerebellum and within the locus ceruleus. This survey of consistent but differing patterns of responsiveness to ethanol at specific time points after acute exposure, suggests that the global effects of ethanol on movement, on arousal and on emotions and memory must be composed of several distinct effects both within and across many cellular systems.