Responses of the flexor reflex to LSD, tryptamine, 5-hydroxytryptophan, methoxamine, and d-amphetamine in acute and chronic spinal rats.

The flexor reflex of acute (40-48 h after mid-thoracic spinal transection) and chronic (at least 2 months after transection) spinal rats was evoked by tetanic electrical stimulation of both hindfeet and recorded on a polygraph using a transducer connected to the left hindfoot. The flexor reflex in the chronic spinal rat was more responsive to electrical stimulation and to the actions of drugs studied than was the flexor reflex in the acute spinal rat. In chronic spinal rats, d-amphetamine, methoxamine, LSD, tryptamine, and 5-hydroxytryptophan (5-HTP) facilitated the flexor reflex and induced spontaneous movements. These facilitative effects were seen in acute spinal rats only when much larger i.p. doses of amphetamine, methoxamine, and LSD were used. Small i.v. doses of tryptamine also produced the facilitation. The facilitation caused by LSD and tryptamine, but not 5-HTP, in chronic spinal rats was antagonized by cyproheptadine. These observations suggest that chronic spinal rats were more sensitive to the drugs than acute spinal rats and support the hypothesis that the mode of action of LSD is similar to that of tryptamine but different from that of 5-HTP since cyproheptadine antagonized the facilitative effects of LSD and tryptamine but not those of 5-HTP.