Reduced thermal sensitivity in the rabbit by beta-endorphin injection into the preoptic/anterior hypothalamus

Male New Zealand White rabbits, Oryctolagus cuniculus, were stereotaxically implanted with a guide tube above the preoptic/anterior hypothalamus (PO/AH) for the injection of beta-endorphin (beta-E) or saline at ambient temperatures of 20 and 25 degrees C. Ear skin and PO/AH temperatures were recorded in loosely restrained control and beta-E-pretreated rabbits while radiant heat was applied to the dorsal skin. Without beta-E administration the ear skin temperature (Tear) underwent a rapid increase during back heating. Following beta-E administration there was a marked vasoconstriction along with a large reduction in responsiveness of ear skin temperature to radiant heat. The time to respond to radiant heat for beta-E-pretreated rabbits was significantly longer than that for control rabbits. In control animals, the increase in Tear in response to radiant heat exposure depended upon the initial ear temperatures. However, in beta-E-pretreated rabbits vasodilatation response to radiant heat exposure was nearly the same regardless of the initial Tear. These data suggest that there is a significant reduction in passage of temperature information from cutaneous thermal receptors to the PO/AH in beta-E-pretreated animals and that beta-E-induced reduction in sensitivity of the vasomotor system to radiant heat may account for the effectiveness of this opioid peptide to promote hyperthermia in the rabbit.