Effects of morphine pellet implantation in neonatal rats.

Abstract

SummaryA morphine pellet implantation method has been modified for use in very young rats. Compared to P-implanted controls, implantation of an M pellet on either Day 5 or 11 results in diminished body weight for periods at least up to 15 weeks. Such treatment on Day 11 is associated with reduced sensitivity to the analgesic action of morphine (10 mg/kg) when tested on Day 48 using the hot-plate technique. Resting pituitary-adrenal activity in animals implanted with morphine on Day 5 or 11 was within normal limits when examined on Day 43. Corticosterone responses to a challenge dose of morphine (40 mg/kg) on Day 68 failed to demonstrate tolerance unequivocally. These findings are consistent with those obtained earlier with daily injections of morphine and demonstrate that the long-lasting ponderal effect and tolerance to the analgesic actions of morphine may be produced without the confounding effects of daily handling and stress of abrupt withdrawal in neonatally addicted rats. This convenient method of ...