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

1. The putative neurotransmitter substances 5-hydroxytryptamine (5-HT), noradrenaline (NA), acetylcholine (ACh) and prostaglandins E(1) (PGE(1)) and E(2) (PGE(2)) have been injected into a lateral cerebral ventricle of the conscious echidna (Tachyglossus aculeatus); thermoregulatory responses in thermoneutral (ambient dry bulb temperature, T(db), of approximately 22 degrees C), cool (T(db) of approximately 14 degrees C) and warm (T(db) of approximately 26 degrees 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 PGE(1) and PGE(2) 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.