Do 5-HT1B/1D autoreceptors modulate dorsal raphe cell firing? In vivo electrophysiological studies in guinea pigs with GR127935.

GR127935 is a selective antagonist of release-modulating 5-HT1B/1D autoreceptors on serotonergic terminals and, as such, would be expected to produce increases in extracellular 5-HT. The changes in 5-HT observed are mixed, however, possibly due to the presence of somatodendritic 5-HT1a/1D autoreceptors. Theoretically, blockade of these autoreceptors would elevate intra-raphe 5-HT which, in turn, would activate somatodendritic 5-HT1A autoreceptors and slow firing rate. As recorded in anesthetized guinea pigs, dorsal raphe cell firing was unaffected by doses of GR127935 ranging from 20 to 20000 micrograms/kg i.v. Lower doses of GR127935 (0.002-2 micrograms/kg i.v.) yielded highly variable responses, although these were not significantly different from baseline. 8-OH-DPAT in these and similar neurons produced the robust dose-dependent inhibitory response expected of a 5-HT1A agonist; increases in extracellular 5-HT resulting from re-uptake blockade by fluoxetine also suppressed unit activity. Doses of CP-135,807, a centrally-acting 5-HT1B/1D agonist, to increase tone on the somatodendritic 5-HT1B/1D autoreceptor produced only a trend toward decreases in dorsal raphe neuronal firing. The overall weak effect of GR127935 on raphe unit activity suggests that the mechanism described previously must take into account factors such as the degree of intra-raphe 5-HT release, the endogenous tone on the autoreceptors, receptor selectivity and intrinsic activity of GR127935 and/or heterogeneity within the subtype.