Impaired development of tolerance to morphine analgesia in rats with hereditary diabetes insipidus

Recently it was reported that vasopressin facilitates the development of resistance to the analgesic action of morphine. Therefore, the development of tolerance to daily administration of morphine-HCl (10 mg/kg i.p.) was studied in a series of trials on a hot plate using rats with hereditary diabetes insipidus (DI), which lack the ability to synthesize vasopressin. In contrast to heterozygous DI rats, who developed full tolerance after the fifth injection, homozygous DI rats showed a delayed development of tolerance. Substitution of HO-DI rats with either arginine-8-vasopressin (3 g/rat, s.c. daily) or the endocrinologically inert fragment of vasopressin desglycinamide lysine-8-vasopressin (5 g/100 g, s.c. daily) restored the impaired development of tolerance towards normal. The data support the notion that vasopressin is important to the development of tolerance to narcotic analgesics and that its mechanism of action is dissociated from its endocrine effect but rather resembles that of its known influence on memory consolidation.