5-Hydroxytryptamine reduces substance P responses on dorsal horn interneurones: a possible interaction of neurotransmitters.

Responses of dorsal horn neurons to iontophoretically administered substance P were attenuated by iontophoretically applied 5-hydroxytryptamine (5-HT). This occurred even when 5-HT alone had no effect on the excitability of the cell. 5-HT did not attenuate responses to iontophoretic application of glutamate. It has been suggested that 5-HT presynaptically inhibits the nociceptive afferents but direct application of 5-HT whilst recording the polarisation of primary afferents in vivo failed to confirm this postulate. These results cannot be explained by a presynaptic inhibition of primary afferents by 5-HT and it is suggested that 5-HT is either acting directly on the substance P receptor or on the receptor-excitation coupling mechanism.