Body temperature effects of opioids in rats: intracerebroventricular administration.

A variety of opioids were injected intracerebroventricularly into unrestrained rats, and rectal temperatures were monitored over several hours. Complete dose-response data for morphine, heroin, etorphine, d- and l-ethylketazocine, d- and l-pentazocine, and d- and l-N-allylnormetazocine revealed a predominant response of hyperthermia. Only etorphine in high doses caused a pronounced decrease in body temperature. These results differed considerably from those obtained previously with subcutaneously administered opioids but can be explained in terms of a dual-receptor theory of temperature control.