Abstract
Summary
A 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 prolonged early morphine exposure in young rats may be used as an experimental model for the study of long-range effects of neonatal exposure to narcotics.
We wish to thank Mr. Andrew Baldwin for his valuable technical assistance. This work was supported in part by USPHS Grant No. DA 826 and NIH Biomedical Research Support Grant No. RR-07055.
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