THE EFFECTS OF 5-HYDROXYTRYPTOPHAN AND l-TRYPTOPHAN ON SPINAL SYNAPTIC ACTIVITY

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

The potentials evoked by supramaximal stimulation of an L₇ dorsal root or peripheral nerve and recorded from the L₇ ipsilateral ventral moot and a dorsal rootlet were studied after the injection of 5-hydroxytryptophan (5-HTP) and l-tryptphan. The injection of 5-HTP produced a marked increase in the monosynaptic spike height and depression of the polysynaptic and dorsal root reflexes. This was accompanied by spontaneous motoneuronal discharge and an average decrease of 0.2 msec in the stimulus-response latency. An increase in the monosynaptic spike height averaging 310% ± 30 (S.E.) but the polysynaptic and dorsal root reflexes exhibited only a This action was slow in onset, reaching a maximum about 90 min after injection. A the time of maximal effect, cord levels of 5-hydroxytryptamine (5-HT) Were markedly elevated. l-Tryptophan in doses of 100 mg/kg produced an average increase in the monosynaptic spike height of 172% ± 31 (S.E.), but the polysynaptic and dorsal root reflexes exihibited only a moderate increase. It is postulated that the increase in 5-HT produced by 5-HTP and l-tryptophan occurs largely in descending fibers in the cord, and that this excess 5-HT induces, either directly or indirectly via interneurons, an increase in motoneuronal excitability.