In vitro tolerance to inhibition by ethanol of N-methyl-D-aspartate-induced depolarization in locus coeruleus neurons of behaviorally ethanol-tolerant rats

Intracellular recordings were made in pontine slice preparations of the rat brain containing the locus coeruleus (LC). Ethanol at 100 mM, but not at 1 0 or 30 mM inhibited depolarizing responses to pressure-applied N-methyl-D- aspartate (NMDA) in LC neurons of ethanol-naive rats. Ethanol (100 mM) had a similar effect in LC neurons of ethanol-naive rats, of rats treated with ethanol for 14 days (3 g/kg daily, i.p.) and of rats treated with equicalor ic amounts of saccharose (5 g/kg daily, i.p.). The blood concentration of e thanol was markedly decreased at 4 h, and was below the detection limit at 24 h after the last injection. Behavioral measurements in the open-field sy stem demonstrated the development of tolerance in rats receiving ethanol fo r 14 days. Moreover, an anxiety-related reaction was shown to develop when the acute effect of the last ethanol injection vanished. Therefore, in subs equent in vitro experiments, ethanol (10 mM) was continuously present in th e superfusion medium in order to mimic a steady blood concentration and to prevent a withdrawal-like situation. Under these conditions, ethanol (100 m M) still continued to inhibit the NMDA-induced depolarization in slices of untreated rats, but became ineffective in slices of ethanol-treated rats at 4 h after the last injection. By contrast, a supersensitivity to ethanol d eveloped in brain slices at 24 h after the last ethanol injection. In concl usion, in vitro tolerance between systemically and locally applied ethanol at LC neurons could only be demonstrated when a low concentration of ethano l was added to the superfusion medium to simulate the blood concentration o f this compound. (C) 2001 Elsevier Science Ltd. All rights reserved.