Pharmacological characterization of the 5-HT1A, 5-HT2 and 5-HT3 receptors in the bovine ciliary muscle

We aimed to investigate the effects of serotonin (5-hydroxytryptamine, 5-HT) on the bovine ciliary muscle and subsequently to characterize and identify the subtypes of 5-HT receptors involved in the serotonin-evoked contractility muscle. The binding of 3H]ketanserin, 3H]granisetron and 3H]8-hydroxy-2-(di-n-propylamino)tetralin (3H]8-OH-DPAT) was analyzed. All labelled compounds bound with high affinity to a single site in the membrane preparations studied. The affinity (K(d)) of the binding site was 7.5+/-1.2 nM for 3H]ketanserin, 6.9+/-0.8 nM for 3H]granisetron and 4.4+/-0.31 nM for 3H]8-OH-DPAT. The density of receptors (B(max)) was 1062+/-43.0 fmol/mg protein for 3H]ketanserin, 566+/-2.32 fmol/mg protein for 3H]granisetron and 205+/-4.63 fmol/mg protein for 3H]8-OH-DPAT. The serotonin-induced contraction appeared to be competitively antagonized by ketanserin (0.1, 1 and 10 microM) and ondansetron (0.1, 10 and 100 microM) which produced a pA(2) value of 8.5+/-0.12 and 8.0+/-0.19, respectively. 8-OH-DPAT and 5-carboxamidotryptamine (5-CT) proved to be completely ineffective. We conclude that serotonin induces bovine ciliary muscle contraction via 5-HT(2) and 5-HT(3) receptors while the 5-HT(1A) receptors, although present, do not mediate the contractile response.