Thermoregulatory responses to the injection of monoamines acetylcholine and prostaglandins into a lateral cerebral ventricle of the echidna

摘要

1. The putative neurotransmitter substances 5-hydroxytryptamine (5-HT), noradrenaline (NA), acetylcholine (ACh) and prostaglandins E1 (PGE1) and E2 (PGE2) have been injected into a lateral cerebral ventricle of the conscious echidna (Tachyglossus aculeatus); thermoregulatory responses in thermoneutral (ambient dry bulb temperature, Tdb, of approximately 22° C), cool (Tdb of approximately 14° C) and warm (Tdb of approximately 26° C) environments were determined.2. Under all conditions, all of the drugs tested either caused deep body temperature to fall or else had no effect; the fall was brought about by peripheral vasodilatation and/or reduced metabolic rate due to a decrease in shivering or to general relaxation.3. Responses of the many placental mammals to 5-HT, NA and ACh vary widely, and the echidna, a monotreme, appears to exhibit responses most like those of the rat.4. Placental mammals tested to date invariably exhibit a hyperthermic response to prostaglandins, and the hypothermic responses of this monotreme is therefore unique. The present study included the confirmation of a hyperthermic response to PGE1 and PGE2 in cats and rats.5. It is concluded that the concept of thermoregulation by amines and other substances in the hypothalamus of placental mammals may also be applicable to the monotremes which have evolved separately from the marsupials and placental mammals for about 180 million years.