Alterations in brain monoamine oxidase kinetics after chronic ethanol administration and withdrawal

Abstract

Chronic ethanol administration to rats can produce a number of biochemical changes associated with brain neurotransmitter function (Ellingboe, 1978; Holman, 1983). For example, 5-HT* turnover is increased after 3 weeks of ethanol treatment, but 1 week after withdrawal this is reversed (Badawy et al., 1980). These findings are of interest as 5-HT has been implicated in ethanol self-administration (Rockman et al., 1982) and in depression, which may be similar to the mood disturbances seen in alcoholics (Coppen & Wood, 1982). In the brain, the major catabolic route for 5-HT is oxidative deamination by M A 0 [monoamine: 0, oxidoreductase (deaminating), EC 1.4.3.41, and in particular the A form of this enzyme (Youdim & Ashkenazi, 1982). Thus it is conceivable that changes in 5-HT metabolism are the result of alterations in the activity of MAO. In the present study, the effects of chronic ethanol administration and withdrawal on M A 0 kinetics in the brain stem (pons plus medulla oblongata), corpus striatum and hippocampus were examined. Male Sprague-Dawley rats (15@-180g) bred at NESCOT were housed three to a cage with free access to Labsure CRM diet [RHM (South) Ltd., Poole, Dorset, U.K.]. Ethanol was diluted in the drinking water to give a daily intake of about lOg/kg body wt. for 28 days (Badawy et al., 1980). At I days before the end of the experiment, half the ethanol group were withdrawn by replacing their drug solution with water. M A 0 was assayed at 37OC with air as the gas phase, by a modification of the meihod of Fowler & Tipton (1982). Each incubation contained supernatant (400-500 pg of protein), 13H]-5-HT (100-1OOop~) and 10mM-potassium phosphate buffer, pH7.2, in a final volume of lml. The reaction was stopped after 20min by the addition of 500 pl of 2 M-HCI, and the acid metabolite extracted into 4ml of ethyl acetate by vigorous shaking for 5 min. The radioactivity was counted at an efficiency of 48% in a Searle-Nuclear Chicago Mk I1 liquid-scintillation spectrometer. Under the assay conditions described, M A 0 activity was equivalent to the initial velocity, and most of the 3H-labelled product extracted with a single portion of ethyl acetate (i.e. 8 1 k 2%; n = 18). Vma. was unaltered after chronic ethanol treatment or withdrawal in any of the brain regions studied (Table 1). However, on withdrawal, K , was increased 2.5 times in the brain stem and corpus striatum, suggesting competitive inhibition (Table 1). Ethanol (100mM) in uitro had no effect on the enzyme, whereas the IC,, values for the MAO-A inhibitor