Spinal cord injury-specific depression of monosynaptic spinal reflex transmission by l-5-hydroxytryptophan results from loss of the 5-HT uptake system and not 5-HT receptor supersensitivity

We studied changes in the spinal segmental reflex and serotonergic (5-HT) responses in rats after spinal cord injury (SCI) produced by the weight-dropping method at the T8 level. The spinal monosynaptic reflex amplitude (MSR) was recorded from the L5 ventral root following stimulation of the ipsilateral L5 dorsal root. The 5-HT precursor l-5-hydroxytryptophan (L-5-HTP) depressed MSR in the spinal cord injured rats but not in normal rats. We investigated whether the SCI-specific depression of MSR by L-5-HTP was attributable to postsynaptic supersensitivity of 5-HT receptors or presynaptic loss of the 5-HT uptake system. Sumatriptan, a selective 5-HT1B/1D receptor agonist that is not taken up by 5-HT transporters, depressed the MSR similarly in both SCI and normal rats, suggesting that SCI resulted in the loss of 5-HT terminals and not postsynaptic supersensitivity of 5-HT receptors.