The effects of food schedule adaptation on the ability of naloxone to suppress the acquisition of schedule-induced polydipsia

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.

Schedule-induced polydipsia: gender-specific effects and consequences of prenatal cocaine and postnatal handling

The impact of gestational cocaine in conjunction with postnatal handling on schedule-induced polydipsia (SIP) was examined. Rat offspring were derived from Sprague-Dawley dams injected subcutaneously with 40 mg/kg/3 cc cocaine hydrochloride (C40) on gestational days 8-20, dams injected with vehicle and pair fed 4 (PF4) days to mimic the acute anorexic effects of cocaine administration, and nontreated (NT) control dams. In adulthood, offspring were food deprived and given 13 daily 30-min SIP sessions, with water intake recorded during the scheduled (fixed time 60 s-FT60) food delivery. For 4 days thereafter, animals received saline, 5 or 10 mg/kg of cocaine in counterbalanced order prior to SIP testing. Acquisition and maintenance of SIP, but not cocaine-induced suppression of SIP performance, were observed to be dependent upon prenatal treatment, handling, and gender. Females acquired SIP faster and exhibited notably higher levels of polydipsia than males. Early handling increased levels of established SIP in NT offspring, while enhancing SIP acquisition in both PF4 and C40 offspring. In nonhandled animals, NT offspring exhibited less SIP than PF4 and C40 offspring, differences that were attenuated by early handling. These effects are discussed in relation to previously reported neurohormonal characteristics of these gender and treatment variables.

Antagonism of drug discrimination learning within the conditioned taste aversion procedure

Animals injected with morphine prior to the presentation of a saccharin-LiCl pairing and the morphine vehicle prior to saccharin alone rapidly acquired the drug discrimination, avoiding saccharin following the administration of morphine and consuming saccharin following its vehicle after only four conditioning trials. Once stimulus control was established, the opiate antagonist naloxone (1 mg/kg) was administered prior to morphine in a test of its ability to antagonize the morphine stimulus. Pretreatment times ranged from 10 to 180 min. Naloxone antagonized the stimulus properties of morphine for all subjects, although there were individual differences in the onset, duration (time course) and degree of antagonism. Together with the rapid acquisition typically reported in this design, the fact that antagonism was demonstrated in the present study suggests that the conditioned taste aversion procedure may be useful in the general assessment of drug discriminations.

Endogenous opiates: 1990

This paper, an examination of works published during 1990, is thirteenth in a series of our annual reviews of the research involving the behavioral, nonanalgesic, effects of the endogenous opiate peptides. The specific topics this year include stress; tolerance and dependence, eating; drinking; gastrointestinal, renal, and hepatic functions; mental illness; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; locomotor activity; sex, pregnancy, development, and aging; immunological responses; and other behavior.