Reassessing morphine effects in cats: III. Responses of intact, caudate nuclei-lesioned and hemispherectomized animals following chronic administration and precipitated withdrawal.

Behavioral response to low doses of morphine (2.0 to 3.0 mg/kg, IP) administered for up to 15 days, and responses to subsequent naloxone challenges, were measured in intact, unilaterally and bilaterally caudate-lesioned (acaudate) cats, and in hemispherectomized cats using a video time-sampling method. For all groups minor tolerance to posture and movement activation patterns was seen, with a reciprocal increase in motor relaxation, which was somewhat more marked for acaudate cats. In contrast to this weak tolerance, all cats showed strong, typical withdrawal manifestations at the beginning of abstinence and a "mini withdrawal" could still be precipitated 15-30 days later when morphine was no longer detectable in the blood. The cats with the unilateral lesions showed whole body turning toward the lesioned side after morphine and away from the lesioned side following naloxone. Only hemispherectomized and acaudate animals showed significant physical deterioration (e.g., weight loss, decreased activity). The comparisons between weak tolerance development versus strong physical dependence and the possible mechanisms involved in shifting the turning biases are discussed. The potential of the cat as a model for studying opiate effects is stressed.