THE EFFECTS OF 5-HYDROXYTRYPTAMINE ANTAGONISTS ON SPINAL NEURONAL ACTIVITY

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

The effects of several 5-hydroxytryptamine (5-HT) antagonists were studied on potentials evoked in acute spinal cats by supramaximal stimulation of an L₇ dorsal root or peripheral nerve and recorded from the ipsilateral L₇ ventral root. The effects of the different 5-HT antagonists on the evoked potentials varied from marked depression to slight facilitation. Since these actions still appeared in animals depleted of endogenous 5-HT, they apparently are not associated with the antagonism of 5-HT. The 5-hydroxytryptophan (5-HTP)-induced increase in the monosynaptic spike amplitude and spontaneous motoneuronal discharge and decreased stimulus-response latency were immediately antagonized by the injection of the 5-HT antagonists, methysergide, 2-brom-d-lysergic acid diethylamide, d-lysergic acid diethylamide, cinanserin and cyproheptadine. The amount of antagonist necessary for complete reversal was directly proportional to the dose of 5-HTP. None of the 5-HT antagonists effectively antagonized the depressant actions of 5-HTP on polysynaptic and dorsal root reflexes. Pretreatment with a 5-HT antagonist also prevented the excitatory effects of 5-HTP. It is concluded that the 5-HT antagonists will effectively antagonize the excitatory, but not the depressant, effects of 5-HTP on evoked potentials in the spinal cord.