Effects of intraventricular brain injections of neurotransmitters on colonic temperature in morphine-tolerant rats

The possible sensitivity changes caused by morphine tolerance in the postsynaptic receptors of putative neurotransmitters involved in the central thermoregulatory mechanisms were studied by intraventricular brain injections of 5-HT, NA, DA, ACh, physostigmine, pilocarpine and carbachol into 20-day old morphine-tolerant and control rats. Temperature acclimation effects were ruled out by keeping the animals in a thermoneutral environment (32°C) during the periods in which morphine would ordinarily have caused hypothermia at lower environmental temperatures. At 22–24°C, the hypothermia produced by ACh, physostigmine and both in combination was significantly greater in morphine-tolerant animals, while there were no significant differences between tolerant and control animals in the colonic temperature responses to the other substances tested. It is suggested that cholinergic receptors of pronounced ACh specificity are induced in the central thermoregulatory neurones of immature rats by repeated administration of morphine for 6 days.