Abstract

The superficial peroneal nerve of the acute decerebrate spinal (L₁) cat was stimulated with a voltage and duration maximal for C-fiber activation. Short latency polysynaptic reflexes (PSR) and long latency C-fiber reflexes (CFR) were recorded from an ipsilateral S₁ ventral root. Both the PSR and the integral of the CFR were recorded on tape and averaged using a PDP-12 computer. Infusions of tryptamine at a rate of 0.5 mg/kg/min for 10 minutes increased the CFR to a mean of 308% (S.E.M.±37.3%) of control and the PSR to a mean of 125% (S.E.M.± 5.7%) of control. The facilitation by tryptamine was antagonized by cyproheptadine 0.5 mg/kg. Lysergic acid diethylamide (LSD) infused at a rate of 0.375 µg/kg/min for 40 minutes produced facilitation of the CFR which reached a maximum of 297% of control (S.E.M.±43.7%) 60 minutes postinfusion. Facilitation of the PSR by LSD was significant only immediately postinfusion (110% control). The facilitation of the CFR by LSD was partially antagonized by cyproheptadine, 0.5 mg/kg. Methysergide, 0.4 mg/kg, had a biphasic action on the CFR, producing an initial depression followed by facilitation. Tryptamine infusion produced a significant depression of the CFR after methysergide pretreatment. LSD produced no further increase in the CFR after facilitation produced by methysergide. These studies demonstrated that tryptamine and LSD have a similar mode of action on spinal cord CFR, and that tryptamine and LSD produce a greater facilitation of the CFR than the short-latency PSR.