(Na+,K+)-ATPase and noradrenergic function: effects of chronic ethanol

Hormetic acute response and chronic effect of ethanol on adenine nucleotide hydrolysis in rat platelets

The objective of this study was to verify the acute and chronic effects of ethanol on platelet NTPDase and 5'-nucleotidase activities. These enzymes modulate platelet function by regulating adenine nucleotide bioavailability and adenosine production. In the acute treatment, doses of 0.8, 2.0, 4.0, 6.0 and 8.0 g/kg ethanol were administered via orogastric tube, and induced a biphasic or hormetic effect on ATP, ADP and AMP platelet hydrolysis. Ethanol at a dose of 0.8 and 2.0 g/kg increased NTPDase activity (44 and 35%, P < 0.0001) with ATP as substrate, whereas when ADP was used there was only a tendency for NTPDase activity to increase. ATP and ADP hydrolysis decreased by 31-77% (P < 0.0001) in 4.0, 6.0 and 8.0 g/kg of ethanol compared to the control. AMP hydrolysis showed a tendency to increase at ethanol doses of 0.8 and 2.0 g/kg, but was inhibited by 45-100% (P < 0.0001) at the higher doses. Chronic treatment consisted of the oral administration of 20% ethanol solution during 31 weeks as the only source of liquid and inhibited NTPDase activity (15 and 20%, P < 0.05) with ATP and ADP as substrate, respectively. However, AMP hydrolysis by 5'-nucleotidase increased by 40% (P < 0.05). Thus, we speculate that the effects of ethanol on NTPDase and 5'-nucleotidase activities could be related with the platelets alterations commonly observed in alcohol users.

Research on tolerance: what can we learn from history?

The concept of tolerance to ethanol has evolved gradually over the past two centuries, and all of the basic clinical features, as they are now understood, have been clearly recognized for nearly 100 years. The basic mechanisms involved in central nervous system tolerance, however, have been elucidated only in the past 20 to 30 years. Little progress was made as long as tolerance was viewed as a purely cellular or physiological adaptation to alcohol, and researchers used overly simple paradigms based on mere exposure to the drug. With the recognition that learning, both operant and associative, can play a major role in the development of tolerance to alcohol and cross-tolerance to other drugs, a radical change in research approaches became possible. Most of the neural mechanisms related to learning and memory are now known to be involved in the development and retention of tolerance, and the simplistic models used in earlier research must now be abandoned. Nevertheless, a review of the history of past research points to a number of important lessons for future work, including the following: (1) many of the present concepts were enunciated by astute observers many decades ago, and research was hindered because this older literature was forgotten; (2) for many decades progress was slow because of a narrow focus on specific techniques, questions, and hypotheses that overlooked important research in related disciplines; (3) the course of research is often irregular, and past questions may have to be revisited with new approaches--but these are more likely to be fruitful if based on knowledge of past history; and (4) excellent researchers often obtain apparently contradictory findings, but the disagreements may hold the key to deeper understanding of the phenomena, and should not be brushed over by ignoring the minority findings and interpretations. As in all scientific research, the most important requirement for major progress is the formulation of good questions or hypotheses: the results yielded by the best available techniques can be only as good as the questions they are meant to answer.

Dexamethasone and adrenalectomy alter brain (Na+,K+)-ATPase responses to noradrenergic stimulation or depletion

We investigated the effects of adrenalectomy or dexamethasone treatment on the regulation of brain (Na+,K+)-ATPase by noradrenaline. Noradrenergic stimulation was produced by repeated injections of yohimbine, and noradrenaline depletion by an injection of the selective toxin DSP4 (N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine). Adrenalectomy had no effect on the number of ouabain binding sites in cerebral cortex, but increased the number of sites synergistically with noradrenergic stimulation. Dexamethasone prevented the decrease in ouabain binding in rats treated with DSP4, but did not itself alter ouabain binding. Neither dexamethasone nor adrenalectomy altered the changes in beta-receptor binding associated with the noradrenergic manipulations. Changes in exposure to corticosteroids may alter the coupling between adrenoceptor binding and second messenger activation in a way that affects (Na+,K+)-ATPase regulation.

The mode of action of ethanol on batrachotoxinin-A benzoate binding to sodium channels in mouse brain cortex

Since ethanol has local anesthetic activity its effect was examined in vitro on the scorpion toxin-enhanced specific binding of [3H]batrachotoxinin A 20-alpha-benzoate ([3H]BTX-B) to the gating complex in sodium channel preparations from mouse brain cortex by equilibrium and kinetic experiments. Ethanol inhibited the specific binding of [3H]BTX-B in a vesicular preparation with an IC50 value of 310 mM and a Hill number of 1.0. Ethanol increased the equilibrium dissociation constant of batrachotoxin in a concentration dependent manner without changing the maximal binding capacity and decreased the half-time of the aconitine-induced dissociation of [3H]BTX-B. Thus, ethanol acts as an apparent competitive inhibitor, allosterically affecting the [3H]BTX-B binding, like other local anesthetics. Results of competition experiments in the presence of different concentrations of ethanol and fixed concentration of tetracaine (and vice versa) are consonant with an interaction of ethanol and tetracaine with the same binding sites. Experiment carried out at 32, 37 and 42 degrees C indicated that the effect of ethanol is not mimicked by increasing the temperature.