Changes in morphine self-administration and morphine dependence after lesions of the caudate nucleus in rats

Rats were trained to bar press for intravenous infusions of morphine sulfate during 1-hr daily test sessions. Small, centrally placed bilateral lesions of the caudate nucleus reduced rates of morphine self-administration to approximately one seventh of preoperative levels; postoperative rates were similar to preoperative rates when the postoperative unit infusion dose of morphine was one tenth of the preoperative dose. Caudate lesions also lowered the threshold dose at which morphine's rewarding property could be detected. Physical dependence was studied in other rats receiving a 3-day continuous infusion of morphine sulfate via implanted subcutaneous silicone reservoirs. Caudate lesions ameliorated withdrawal-induced weight loss and naloxone-induced “wet dog shakes”. Both the self-administration and dependence data are consistent with the idea that morphine blocks dopaminergic transmission in the striatum.