An implanted reservoir of morphine solution for rapid induction of physical dependence in rats.

1. Rats were dosed continuously with morphine hydrochloride by giving a daily dose through tubes connected to small, subcutaneously implanted reservoirs. Morphine was withdrawn by washing out the reservoir with drug vehicle. The daily dose of morphine, or substitute drug received by each rat was determined by difference by estimating the drug remaining in reservoir washings.2. Withdrawal symptoms were more pronounced after 9 days than after 4 days of dosing with morphine.3. Body weight loss, maximal at 24 h, and increased defaecation during the first 7 h were the chief physiological signs of morphine withdrawal. The body weight loss was the result of hypodipsia and anorexia exacerbated by increased defaecation.4. When substituted for morphine in the reservoir, methadone and codeine completely prevented body weight loss and increased defaecation, while pethidine was effective against increased defaecation, but not against 24 h body weight loss. The opiate-antagonist analgesics pentazocine, nalorphine and cyclazocine either had no effect on withdrawal symptoms or increased their severity.5. In morphine dependent rats under continued morphine administration subcutaneous doses of the opiate-antagonists nalorphine, cyclazocine and naloxone all precipitated the withdrawal symptoms of body weight loss and increased defaecation. The weak antagonist pentazocine caused a significantly increased defaecation, but no significant change in body weight, while the opiates pethidine, codeine and methadone had no significant effect on body weight or defaecation.6. The advantages of inducing dependence by this method of dosing are discussed.