5-Hydroxytryptamine as a sensitizer of somatic nociceptors for pain-producing substances.

Experiments were performed on loosely restrained, conscious dogs in the femoral artery of which a chronic catheter was implanted. All substances were administered through the catheter and vocalization responses to them were used as a principal measure of nociception. In 10 experiments done on 7 dogs, after a single injection of 3 or 10 mumol of 5-hydroxytryptamine, threshold doses (about 0.5 mumol) of acetylcholine for vocalization produced vocalization responses comparable with those to about 3 times threshold doses. In 5 experiments on 5 dogs, after 10 mumol of 5-hydroxytryptamine, threshold doses (about 0.3 nmol) of bradykinin for vocalization produced vocalization responses comparable with those to about twice threshold doses. These doses of 5-hydroxytryptamine produced small vocalization responses in only 3 of the 12 dogs. These results indicate that 5-hydroxytryptamine, although almost devoid of the activity producing noceceptive responses, sensitizes somatic nociceptors for pain-producing substances.