Reduced thermal sensitivity in the rabbit by β-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 β-endorphin (β-E) or saline at ambient temperatures of 20 and 25 °C. Ear skin and PO/AH temperatures were recorded in loosely restrained control and β-E-pretreated rabbits while radiant heat was applied to the dorsal skin. Without β-E administration the ear skin temperature (T_ear) underwent a rapid increase during back heating. Following β-E administration there was a marked vasoconstriction along with a large reduction in responsiveness of ear skin temperature to radiant 3eat. The time to respond to radiant heat for β-E-pretreated rabbits was significantly longer than that for control rabbits. In control animals, the increase in T_ear in response to radiant heat exposure dependend upon the initial ear temperatures. However, in β-E-pretreated rabbits vasodilatation response to radiant heat exposure was nearly the same regardless of the initial T_ear. These data suggest that there is a significant reduction in passage of temperature information from cutaneous thermal receptors to the PO/AH in β-E-pretreated animals and that β-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.