Differential effects of morphine and LiCl on schedule-induced polydipsia

Lithium chloride (LiCl) and morphine both produce a conditioned taste avoidance response, while only LiCl is able to elicit a conditioned rejection response (taste reactivity), indicating that the effects of conditioning are drug and preparation dependent. The present experiments extend this assessment to another behavioral preparation, schedule-induced polydipsia (SIP), by examining the ability of LiCl and morphine to produce conditioned suppression of nonregulatory drinking. In Experiment 1, schedule-induced saccharin consumption was followed by LiCl or morphine (at doses comparably effective in conditioning taste avoidance under water deprivation) or by the distilled water vehicle. Although both LiCl and morphine suppressed SIP, morphine produced a significantly weaker suppression than did LiCl. Using a massed feeding design in which animals received all their food pellets in a single meal, Experiment 2 determined that LiCl and morphine were equally effective in suppressing consumption, indicating that the differential effects seen under SIP were due to the schedule of spaced food pellet deliveries. The basis for the differential effects of LiCl and morphine on SIP may be a function of an increase in the reinforcing properties of drugs of abuse (such as morphine) within this procedure that mask the acquisition and/or display of the conditioned suppression. If so, then this procedure may be useful in assessing the reinforcing properties of such drugs.