Stress-induced changes in the analgesic and thermic effects of morphine administered centrally

Stress (e.g. restraint) potentiates analgesia and changes in body temperature induced by morphine administered systemically to rats. In order to determine if stress-induced potentiation of these effects of morphine are mediated within the central nervous system, restrained and unstressed groups of rats were injected in the lateral ventricle (i.c.v.) with graded doses of morphine, and their analgesic and body temperature responses were measured. Compared to unstressed animals, restrained rats had a greater analgesic response at each dose of morphine, characterized by an increase in both the magnitude and duration of the drug effect. The unstressed group of rats responded consistently to 1.0-100 micrograms of morphine with a 1.5-2.0 degrees C increase in core temperature. Restrained rats had either a smaller increase in body temperature or a hypothermia at these doses of morphine. Thus, restraint stress can modify the effects of morphine administered i.c.v. on analgesia and body temperature in a manner similar to that seen after systemic administration of morphine, indicating that this phenomenon is mediated centrally.