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.

Strain-dependent differences in schedule-induced polydipsia: an assessment in Lewis and Fischer rats.

Strain-dependent differences have been used to highlight unknown genetic contributions to important behavioral and physiological end points. In this regard, the Fischer (F344) and Lewis (LEW) rat strains have often been studied because they exhibit a myriad of behavioral and physiological differences. Recently, schedule-induced polydipsia (SIP), a potential model of stress and drug abuse, has been reported to differ between the two strains (see [Pharmacol. Biochem. Behav. 67 (2002) 809]) with F344 rats displaying greater levels of consumption than LEW rats. Given the importance of SIP as a behavioral model of stress and of drug abuse, the present study further explored SIP in F344 and LEW strains by assessing the acquisition and steady-state performance of SIP (under a fixed-time 30 schedule of food delivery; FT30), its characteristic postprandial temporal licking pattern and its modulation by variations in the food delivery schedule (FT15, FT30 and FT60). F344 rats acquired SIP at a faster rate and drank at a higher asymptotic level than LEW rats. Both strains displayed the typical inverted U-shaped post-pellet pattern of drinking and changes in levels of consumption (and displacement of the initiation of post-pellet drinking) with changes in the FT value, supporting the position that the increased drinking seen in both groups was schedule induced. These strain differences in SIP are consistent with the fact that the F344 and LEW strains differ on other behavioral and physiological indices of stress and raise the issue of the use of this model in the assessment of differential drug intake between the two strains.