Physiologic evidence for descending tryptaminergic pathways in the spinal cord.

LSD was more effective in facilitating the flexor reflex in the chronic than in the acute spinal dog. On the other hand, $\text{1}$-tryptophan produced a significant facilitation of the flexor reflex in the acute spinal dog but not in the chronic spinal dog. These data are consistent with the hypothesis that there are long descending tryptaminergic axons which have the capacity of converting tryptophan to tryptamine and releasing it in the vicinity of post-synaptic facilitatory receptors which degenerate in the chronic spinal dog.