Body temperature responses in cats and rabbits to the monoamine oxidase inhibitor tranylcypromine.

1. In unanaesthetized cats tranylcypromine (1-10 mg/kg) had scarcely any effect on rectal temperature when injected intraperitoneally, yet such injections prevented the deep and long-lasting fall in rectal temperature which normally occurs when the cat is anaesthetized by intraperitoneal pentobarbitone sodium or intravenous chloralose. The anaesthesia itself, however, was not affected. In some of the experiments with pentobarbitone sodium rectal temperature even rose to fever level.2. In anaesthetized as well as in unanaesthetized cats injections of tranylcypromine (0.1-1 mg) into the cerebral ventricles caused a rise in rectal temperature.3. In rabbits, rectal temperature was scarcely affected when surgical anaesthesia was produced by intravenous infusions of pentobarbitone sodium under the same condition in which, in cats, intraperitoneal pentobarbitone sodium produced a deep and long-lasting fall in temperature, i.e. when no external heat was applied but excessive dissipation of heat was prevented by placing the rabbit on a cotton-wool pad. However, when it was placed on the metal surface of an operating table, the anaesthesia was associated with a deep fall in rectal temperature.5. In anaesthetized and unanaesthetized rabbits tranylcypromine had no effect on rectal temperature when injected intraperitoneally (10 mg/kg) or into the cerebral ventricles (1 mg).5. These results are discussed in relation to the theory that the three monoamines in the hypothalamus, 5-hydroxytryptamine (5-HT), adrenaline and noradrenaline, act as central transmitters in temperature regulation.