Alterations in the behavioral effects of LSD by motivational and neurohumoral variables.

Forty naive male albino rats were trained to press a bar on a fixed-ratio (FR 32) schedule of water reinforcement. They were then divided into two groups, one of which (N = 20) received 5 min of extra water 12 hr before each experimental session; the other group (N = 20) received no extra water. Half of the animals in each group was given three daily doses (100 mg/kg) of the trytophan hydroxylase inhibitor p-chlorophenylalanine methyl ester (PCPA) while the remaining animals were given control injections of the PCPA vehicle. Ten days following the last administration of PCPA (or vehicle) all animals were given a low dose of LSD (20 mug/kg). Bar-pressing behavior was significantly disrupted only in those animals receiving both PCPA and extra water. Central (whold brain) concentrations of serotonin (5-HT) were significantly lower in all animals which had been treated with PCPA. These results, along with those previously reported, suggest that amount of deprivation can be an important determinant of both the ability of drugs to alter behavior and the dependence of such alterations upon underlying neuronal activity.