THE EFFECTS OF MONOAMINE OXIDASE INHIBITORS ON SPINAL SYNAPTIC ACTIVITY

¹ Department of Pharmacology, University of Illinois, College of Medicine, Chicago, Illinois

The effect of several monoamine oxidase (MAO) inhibitors was investigated on spinal neuronal potentials recorded from an L₇ ventral root and evoked by stimulation of an L₇ dorsal root. Within 3 to 4 hr after the injection of the MAO inhibitors, pargyline, nialamide, tranylcypromine and -ethyltryptamine, the monosynaptic spike height was selectively increased whereas the polysynaptic potentials remained unchanged. The increase in the monosynaptic spike height was statistically significant with each compound. The following results indicated that this action was associated with MAO inhibition and the accumulation of 5-hydroxytryptamine (5-HT). Four hours after the injection of pargyline, cord levels of 5-HT and norepinephrine (NE) were increased by 70 and 17%, respectively. Four hours after pretreatment with a MAO inhibitor the actions of 5-hydroxytryptophan (5-HTP) and dihydroxyphenylalanine (dopa) on spinal reflexes were strongly potentiated. Chronic reserpine treatment did not prevent the effects of pargyline or 5-HTP but strongly depressed the response to dopa. The increase of the monosynaptic spike height induced by the MAO inhibitors was immediately antagonized by the 5-HT antagonist methysergide but was not prevented by the NE antagonist phenoxybenzamine.