1. In cats, the effects of
tranylcypromine and
pheniprazine, two
monoamine oxidase (
MAO) inhibitors with strong
amphetamine-like actions, of
pargyline, an inhibitor without
amphetamine-like actions, and of
amphetamine itself, were examined on the
hypothermia produced by a 2 hr period of
halothane inhalation.2. The
hypothermia was prevented by
intraperitoneal injections of the three
MAO inhibitors.
Tranylcypromine and
pheniprazine acted in doses of a few milligrams,
pargyline in doses of over 100 mg.3. The
hypothermia was prevented by
injections into the cerebral ventricles of
tranylcypromine and
pheniprazine, in doses which were effective also on
intraperitoneal injection;
intraperitoneal injections were sometimes more effective. The large doses of
pargyline needed to prevent the
hypothermia when injected intraperitoneally were not tested by the intraventricular route, as the
injections had to be made in a volume of 0.1 ml. In smaller doses intraventricular
pargyline was not effective.4. The
hypothermia was prevented by an intraperitoneal or
intraventricular injection of
amphetamine in a dose as little as 1 mg;
intraperitoneal injections were sometimes more effective.5. The effects of
tranylcypromine and
pargyline given intraperitoneally, and of
amphetamine given intraventricularly as well, were also examined on the
hypothermia produced by an
intraventricular injection of 200 mug
noradrenaline. The two
MAO inhibitors and
amphetamine prevented the
hypothermia, or greatly reduced it.6. It is concluded (a) that even on
intraventricular injection the
MAO inhibitors must first be absorbed into the blood stream before they can prevent the
hypothermia of a
halothane anaesthesia; (b) that their action may not be solely on the anterior hypothalamus; and (c) that they may not act only through
MAO inhibition.