We have previously shown that serotonin (5-hydroxytryptamine, 5-HT) stimulates corticosterone and aldosterone secretion from perifused frog adrenal gland in vitro through activation of 5-HT4 receptors. In the present study, we have used this model to investigate the effect of newly discovered 5-HT4 receptor agonists and antagonists on corticosteroid secretion. Serotonin, the benzamide derivatives (R,S)-zacopride ((R,S)-4-amino-N-(1-azabicyclo[2.2.2]oct-3-yl)-5-chloro-2-methoxybenzamide, HCl) and its enantiomers, the azabicycloalkyl benzimidazolone derivatives BIMU 1 (endo-N-(8-methyl-8-azabicyclo-[3.2.1]oct-3-yl)-2,3-dihydro-3-ethyl-2-oxo-1H-benzimidazole-1-carboxamide, HCl) and BIMU 8 (endo-N-(8-methyl-8-azabicyclo-[3.2.1]oct-3-yl)-2,3-dihydro-(1-methyl)ethyl-2-oxo-1H-benzimidazole-1-carboxamide, HCl) were all capable of enhancing corticosterone and aldosterone secretion in a dose-dependent manner. Serotonin was the most potent stimulator of steroidogenesis (EC(50) = 1.5 x 10(-7) M) while the potency of the benzamide and the benzimidazolone derivatives was approximately 10 times lower. The rank order of efficacy of the different 5-HT4 receptor agonists was: (S)-zacopride > BIMU 8 = (R,S)-zacopride > BIMU 1 = (R)-zacopride = 5-HT. The stimulatory effects of 5-HT and the benzimidazolone derivatives on corticosteroid secretion were not additive, suggesting that they activated the same receptor. The indoleamine derivatives ICS 205930 ((3 alpha-tropanyl)-1H-indole-3-carboxylic acid, ester) and GR 113808 ([1-[2-(methylsulphonylamino)ethyl]-4-piperidinyl]methyl 1-methyl-1H-indole-3-carboxylate, maleate), and the benzimidazolone derivative DAU 6285 (endo-8-methyl-8-azabicyclo-[3.2.1]oct-3-yl)-2,3-dihydro-6-methoxy-2-oxo-1H-benzimidazole-1-carboxylate, HCl), all induced a dose-dependent inhibition of (R,S)-zacopride-induced stimulation of corticosteroid secretion. The affinity of GR 113808 (pK(i) = 10.34) was higher than that of DAU 6285 and ICS 205930 (pK(i) = 7.84 and 6.20, respectively). Together, these data indicate that the pharmacological profile of the 5-HT4 receptor in the frog adrenal gland is very similar to those recently characterized in the brain and peripheral organs of mammals. It thus appears that the frog adrenal gland is a valuable model for the investigation of the biochemical characteristics and mode of action of novel 5-HT4 receptor ligands.
