Hibernation triggers and cryogens: do they play a role in hibernation?

A survey of the literary evidence on cryogens and hibernation induction triggers is given and the results of experiments on the effect of hypothalamic or i.v. injections of opioids and plasma from hibernating European hamsters on body temperature control of rabbits are presented. Pharmacological doses of a delta opioid--DADLE (25 or 50 micrograms), when injected into the anterior hypothalamus, induce a small and short-lasting hypothermic effect in cold exposed rabbits, due to the downward shift of the temperature threshold for shivering. Lower doses (5 micrograms) are without effect, similarly as i.v. administrations (500 micrograms/kg) of this substance. Intrahypothalamic injections of met-enkephalin (0.1-1 microgram) induce a slight hyperthermia due to the shift of all thermoregulatory effectors to higher body temperatures. Intrahypothalamic injections of plasma from hibernating European hamsters do not influence the body temperature control in rabbits.