Pro- and antinociceptive actions of serotonin (5-HT)1A agonists and antagonists in rodents: relationship to algesiometric paradigm.

In mice injected with formalin into the hindpaw, the 5-HT1A receptor agonists, 8-OH-DPAT and flesinoxan, equipotently inhibited the early phase (EP) and late phase (LP) of licking. At higher doses, they provoked ataxia and inhibited the writhing elicited by intra-abdominal acetic acid. The antagonists, (-)-alprenolol, (-)-tertatolol, WAY-100,135 and S 15931 were more potent against the LP than the EP. They also inhibited writhing, and only at very high doses did they elicit ataxia. In rats, 8-OH-DPAT and flesinoxan increased the current required to elicit vocalisation upon electrical stimulation of the tail. The action of 8-OH-DPAT was blocked by WAY-100,135, which, like other antagonists, was inactive alone. Interestingly, a low dose of 8-OH-DPAT partially inhibited the antinociceptive action of the mu-opioid agonist, morphine, the action of which was dose-dependently facilitated by (-)-alprenolol and S 15931. Administered s.c., 8-OH-DPAT elicited spontaneous tail-flicks (STFs) in rats: these were abolished by WAY-100,135, (-)-tertatolol, (-)-alprenolol and S 15931. STFs were also eliminated by s.c. or i.t. administration of the alpha 2-adrenergic receptor agonist, clonidine, the GABAA agonist, muscimol or the GABAB agonist, baclofen. The mu-opioid, morphine, blocked STFs only at high doses and the kappa-opioid agonists, U 50,488 and U 69,593, even at supra-ataxic doses, were inactive. Antagonists at neurokinin (NK)1 (RP 67580), NK2 (SR 48,968) and bradykinin (BK)2 (Hoe 140) receptors, as well as aspirin, did not block STFs, though indomethacin was effective. Antagonists at the glycine B site coupled to the NMDA receptor, L 687,414, L 701,324 and (+)-HA966, blocked STFs. Furthermore, (+)-HA 966 and the competitive NMDA receptor antagonist, CPP, were active upon i.t. administration. STFs were also blocked by s.c. or i.t. administration of the AMPA antagonists, YM 900 and NBQX. In conclusion, the influence of 5-HT1A ligands upon nociception is dependent upon the algesiometric paradigm. Intriguingly, modulation of 5-HT1A receptor-mediated STFs reveals parallels to neuropathic pain.