Functional characterization of the 5-HT terminal autoreceptor in the guinea-pig brain cortex

1 In guinea-pig cerebral cortical slices in vitro we have shown that the rank order of potency of 5-hydroxytrptamine (5-HT), 5-carboxamidotryptamine and sumatriptan for inhibition of electrically stimulated [3H]-5-HT release correlates well with published data on their 5-HT1D receptor binding affinities. 2 Both the non-selective 5-HT1D receptor antagonist, methiothepin and the selective 5-HT1D receptor antagonist, N-[4-methoxy-3-(4-methyl-1-piperazinyl]phenyl]-2'-methyl-4'- (5-methyl-1,2,4-oxadiazole-3-yl) [1,1-biphenyl]4-carboxamide (GR127935) increased stimulated [3H]-5-HT release per se and also attenuated agonist-induced inhibition of [3H]-5-HT release. GR127935 (10 nM-100 nM) produced a pA2 of 9.0 against 5-HT, which is consistent with its 5-HT1D receptor binding affinity. 3 From these findings we conclude that, in guinea-pig cerebral cortex, the 5-HT terminal autoreceptor is of the 5-HT1D receptor subtype. However, three observations suggest the presence of multiple terminal autoreceptors: shallow inhibition curves to the agonists; a shallow Schild slope of GR127935 antagonism and differences in the maximal responses to 5-HT between whole cortex and frontal cortex.