INFLUENCE OF INHERITANCE AND FOSTERING ON SENSITIVITY TO EFFECTS OF MORPHINE ON NOCICEPTION AND LOCOMOTOR-ACTIVITY IN 2 INBRED RAT STRAINS

There are differences between individuals in their pain threshold, loc omotor activity, and sensitivity to drugs of abuse which affect these processes. We have investigated the role of genetic differences and fo stering in determining sensitivity to the action of morphine on nocice ption and locomotor activity in two strains of rats [Fischer-344 (F) a nd WAG/G (W)], using methods of reciprocal cross and cross-fostering. Nine-week old rats were used in the experiments. It was found that FF rats were more sensitive to the action of morphine on locomotor activi ty in open field apparatus than WW rats. Both types of Fl hybrids inhe rited a low sensitivity to the depressive action of morphine, their ac tivity significantly differed from FF activity and had no distinction from that of WW rats. However, there was a strong maternal effect: if WW rats were fostered by a FF mother, the depression of their locomoto r activity was similar to that of FF inbred rats, and vice versa. Ther e were significant differences between normal and cross-fostered rats in all groups. WW rats had a higher sensitivity to thermal stimuli tha n FF rats. Both types of hybrids had intermediate nociceptive latencie s. The strain difference between WW and FF rats in the baseline latenc ies of tail withdrawal disappeared after cross-fostering-the latencies in FF rats were significantly decreased. Administration of morphine p roduced greater antinociception in FF rats than in WW. Cross-fostering of FF rats by WW mothers dramatically decreased their sensitivity to the analgesic action of morphine, while WW rats fostered by FF mothers did not change their resistance to morphine induced antinociception. Thus, the sensitivity of young rats to morphine-induced analgesia or d epression of locomotor activity is mostly determined by fostering duri ng the first 21 days. Copyright (C) 1996 Elsevier Science Ltd.