THE ACTIONS OF 5-HT1 AGONISTS AND ANTAGONISTS ON NOCICEPTIVE PROCESSING IN THE RAT SPINAL-CORD - RESULTS FROM BEHAVIORAL AND ELECTROPHYSIOLOGICAL STUDIES

We have developed a technique which allows drugs to be microinjected i ntrathecally in anaesthetised rats whilst single unit recordings are m ade from dorsal horn neurones. Using this technique together with reco rdings of tail flick latency (TFL) elicited from lightly anaesthetised rats we have found that the specific 5-HT1a agonist 8-OH DPAT (15, 15 0, 300 nmol) increases nociceptive responses recorded from single dors al horn neurones and decreases TFL. The non-specific 5-HT1b agonist TF MPP (300 nmol) and the general 5-HT1 agonist 5-CT (0.3, 3.0, 30 nmol) both decreased nociceptive responses and has inconsistent effects on T FL. Intrathecally applied 5-HT (130, 260 nmol) generally reduced nocic eptive neuronal responses and increased TFL. In a minority of experime nts, however, 5-HT increased nociceptive responses and it is suggested that this effect is associated with activation of 5-HT1a receptors. A ctivity at 5-HT1b receptors has the effect of suppressing or reducing responsiveness. The increased responsiveness of dorsal horn neurones t o noxious stimulation associated with activity at 5-HT1a receptors may be associated either with increases in receptive field size, promotio n of spinal nocifensive reflexes or the facilitation of the rostral tr ansmission to specific brainstem sites.