SYMPATHOMIMETIC ACTIONS OF MONOAMINE OXIDASE INHIBITORS IN THE ISOLATING NICITATING MEMBRANE OF THE CAT

Serotonin-containing neurons in brain: depression of firing by monoamine oxidase inhibitors

Monoamine oxidase inhibitors were administered to rats while the activities of single, serotonin-containing neurons of the midbrain raphe nuclei were being monitored with microelectrodes. All the inhibitors tested (pargyline, tranylcypromine, phenelzine, iproniazid) caused depression of raphe unit firing rate. The ability of monoamine oxidase inhibitors to depress raphe units was impaired by prior treatment with p-chlorophenylalanine, an inhibitor of serotonin synthesis.

Acute effects of a monoamine oxidase inhibitor, tranylcypromine, on thermoregulation in the conscious rabbit

1. The effect of a single injection of the monoamine oxidase inhibitor, tranylcypromine, administered intravenously (20 mg/kg) or into the lateral cerebral ventricle (5-10 mg), on hypothalamic and rectal temperature, has been investigated.2. Intravenous tranylcypromine causes a significant rise in body temperature, which is due, at least in part, to cutaneous vasoconstriction; this vasoconstriction is augmented by sympathectomy. It is concluded that this vasoconstrictor effect is not mediated via the central nervous system.3. Intraventricular tranylcypromine caused a transient but significant fall in core temperature. This is interpreted as being compatible with selective augmentation of hypothalamic levels of 5-hydroxytryptamine.

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.

Monoamine oxidase inhibition: effect on 5-hydroxytryptamine output from perfused third ventricle and body temperature

1. In cats anaesthetized with pentobarbitone sodium the third ventricle was perfused with artificial c.s.f., the effluent was collected in 30 min samples and assayed for 5-hydroxytryptamine (5-HT) on the rat stomach-strip preparation. Rectal temperature was monitored continuously.2. On perfusion of artificial c.s.f. through the third ventricle, small amounts of 5-HT appeared in the effluent; the amounts decreased with successive samples.3. When tranylcypromine (Parnate), an inhibitor of amine oxidase, was added to the perfusion fluid and perfusion was continued, the 5-HT output increased. This increase was associated with shivering and a rise in temperature which was not, however, maintained.4. When tranylcypromine was injected intraperitoneally, during the fall in temperature produced by the pentobarbitone sodium anaesthesia, the 5-HT output also increased, shivering occurred and the fall in temperature was halted or reversed. The effect on temperature was maintained.5. When the cat was killed and perfusion was continued, the 5-HT output, already elevated by the tranylcypromine before death, increased even further in the next few samples.