Effects of lithium dose (UCS) on the acquisition and extinction of a discriminated morphine aversion: tests with morphine and Delta(9)-THC

The effects of varying the lithium dose (unconditioned stimulus [UCS]; LiCl range 30-180 mg/kg) on the acquisition and extinction of stimulus control by 5.6 mg/kg of morphine in a discriminated taste aversion (DTA) procedure were examined in rats. In addition, pharmacological specificity was examine d by substituting (-)-delta-9-tetrahydrocannabinol (Delta(9)-THC) for morph ine during a test phase intervening between acquisition and extinction. DTA acquisition was more rapid at higher LiCl doses. The lowest dose of LiCl, 30 mg/kg, did not robustly maintain a DTA. Two groups treated with 60 mg/kg LiCl, differing only in the type of drinking nozzle used (ball-bearing vs standard non-ball-bearing), behaved similarly. Suppression of drinking was related to the morphine dose, in an orderly manner (dose range 0.3-10 mg/kg ), in rats for which morphine was followed by LiCl. No significant decline in drinking occurred for rats for which morphine was followed by saline, ex cept perhaps at the 10 mg/kg test dose of morphine. The control of drinking was pharmacologically specific; both experimental and control animals were equally affected in tests with Delta(9)-THC (0.3-10 mg/kg). Low doses of D elta(9)-THC increased water consumption; this did not occur with morphine. During extinction the reinstitution of drinking was similar across groups t hat had been effectively conditioned, i.e. there was no apparent effect of lithium dose on extinction. After extinction, a much attenuated reaction oc curred to morphine in tests with 3 and 10 mg/kg. These doses of morphine ha d significantly suppressed drinking before the extinction phase. Collective ly, these data add to the formal similarities between sensory and drug disc riminative stimuli. (C) 1999 Lippincott Williams & Wilkins.