Effects of met-enkephalin on body temperature of normal and morphine-tolerant rats

Abstract

The endogenous opioid met-enkephalin intraventricularly administered to the rat at the dose of 100 μg raised rectal temperature, whereas 400 μg of the pentapeptide caused a diphasic effect, i.e., hypothermia followed by hyperthermia. Met-enkephalin was ineffective when administered i.p. The effects on temperature were substantially similar to those elicited, for both routes of administration, by morphine, which may either raise or lower rat temperature depending on the dose. More naloxone was required to antagonize thermic effects of met-enkephalin than morphine. Finally, there was a lack of effects on temperature for metenkephalin centrally administeted to morphine-tolerant animals, thus providing further evidence, in vivo, of cross tolerance between opiates and naturally occurring ligands of opiate receptors.