THE ROLE OF 5-HT3 RECEPTORS IN PERIAQUEDUCTAL GRAY-INDUCED INHIBITIONOF NOCICEPTIVE DORSAL HORN NEURONS IN RATS

Electrical stimulation in the periaqueductal gray (PAG) can inhibit do rsal horn cell responses to both innocuous and noxious cutaneous stimu li. This inhibition is believed to be due to the release of serotonin (5-HT) into the dorsal horn of the spinal cord from descending axons o f the nucleus raphe magnus and the adjacent reticular formation. It is still not clearly known which subtypes of 5-HT receptors are involved in the PAG-induced inhibition. Extracellular single-unit recordings o f dorsal horn cell activity, in combination with drug administration t hrough a microdialysis fiber, were used to test the role of 5-HT3 rece ptors in FAG-induced inhibition. The responses of the cells to mechani cal stimulation of the skin (BRUSH, PRESS and PINCH) and to the same s timuli while stimulating FAG were recorded. When the 5-HT3 antagonist, ondansetron, was perfused through the microdialysis fiber, not only w as the background activity of the cell increased, but also the respons es to BRUSH, PRESS and PINCH stimuli. The FAG-induced inhibition of re sponses to the same stimuli was partially or completely blocked by ond ansetron. Another 5-HT, antagonist, zacopride, did not increase the ba ckground activity or responses to PRESS and PINCH, yet this agent, lik e ondansetron, blocked FAG inhibition. The 5-HT, agonist, phenylbiguan ide, inhibited the background activity and the responses to mechanical stimuli, These results suggest that 5-HT released in tile dorsal horn by stimulation in the FAG excites inhibitory interneurons through 5-H T, receptors, resulting in inhibition of dorsal horn neurons.