Apparent hyperalgesic action of the 5-HT1A agonist, 8-OH-DPAT, in the rat reflects induction of spontaneous tail-flicks.

The 5-HT1A agonist, 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT), induced a dose-dependent reduction in latency to withdraw the tail from noxious hot water (48 degrees C). However, a similar apparent 'hyperalgesia' was seen at a non-noxious temperature of 38 degrees C. Indeed, 8-OH-DPAT induced spontaneous 'tail-flicks' in the absence of external stimulation. This property was shared by lisuride and LSD, which also have high intrinsic activity at 5-HT1A sites. Agonists at other serotonin (5-HT) receptor types (5-HT1B, 5-HT1C, 5-HT2, 5-HT3) were inactive. Tail-flicks induced by 8-OH-DPAT could be antagonised by the 5-HT1 2 antagonist, methiothepin, but not by ritanserin or GR-38032F, which are antagonists at 5-HT2 and 5-HT3 sites, respectively. Ipsapirone and buspirone, partial 5-HT1A agonists, acted as antagonists. Further, BMY 7378, a proposed selective antagonist at 5-HT1A sites, also blocked the tail-flicks. Thus, the apparent 'hyperalgesia' induced by 8-OH-DPAT may reflect induction of spontaneous tail-flicks. These flicks appear to be mediated by 5-HT1A receptors and represent a novel model of 5-HT1A function in the rat.