DIFFERENCES IN MORPHINE REINFORCEMENT PROPERTY IN 2 INBRED RAT STRAINS - ASSOCIATIONS WITH CORTICAL RECEPTORS, BEHAVIORAL ACTIVITY, ANALGESIA AND THE CATALEPTIC EFFECTS OF MORPHINE

The purpose of the current study was to investigate genetic difference s between two inbred strains of rats, Fisher-344 (F344/N) and Wistar A lbino Glaxo (WAG/GSto), in a number of drug-naive and drug-related beh aviors, including oral and intravenous morphine self-administration. F 344/N and WAG/GSto rats differed in drug-naive behaviors such as nocic eption, rearing and sensitivity to lick suppression tests but did not differ in locomotor activity, ambulation or grooming behavior. F344/N rats were less sensitive to thermal stimuli as measured via tail-flick response, and more sensitive to the suppressive effects of intermitte nt shock in a lick suppression test. The F344/N rats demonstrated a si gnificantly greater amount of rearing in open field tests but did not differ from WAG/GSto rats in locomotor activity, ambulation or groomin g behavior. In addition to the behavioral results, naive F344/N and WA G/GSto rats were found to differ in mu and alpha2 receptor concentrati ons (F344/N > WAG/GSto) and in 5HT2 and D2 affinity constants (WAG/GSt o > F344/N). These two inbred rat strains also differed in drug-relate d behaviors. F344/N rats showed significantly greater depression of lo comotor activity at morphine 3 mg/kg than WAG/GSto rats. In addition, F344/N rats consumed significantly greater amounts of morphine in a tw o-bottle choice procedure and morphine maintained significantly greate r amounts of behavior during intravenous self-administration sessions. Importantly, drug maintained behavior was significantly greater than with vehicle only in the F344/N rats during operant self-administratio n sessions.