CHARACTERISTICS OF [H-3] 5-HYDROXYTRYPTAMINE BINDING TO IRIS-CILIARY BODY TISSUE OF THE RABBIT

Purpose. This study sought to identify, characterize, and localize sub types of 5-hydroxytryptamine (HT) receptors in rabbit iris-ciliary bod y. Methods. Radioligand binding assays were performed with [H-3] 5-hyd roxytryptamine on membranes prepared from rabbit iris-ciliary bodies a nd on tissue sections subsequently developed by autoradiography. Resul ts. [H-3] 5-HT appeared to bind to a single population of receptors in membrane preparations of rabbit iris-ciliary body. The apparent affin ity of the ligand (K-D) was 2.19 nM, and the density of binding sites was 58.3 fmol/mg protein. Binding of [H-3] 5-HT exhibited guanosine-5- triphosphate sensitivity. Competitive inhibition experiments were perf ormed to differentiate between 5-HT receptor subtypes. A relative pote ncy order of 5-CT > 5-HT = 8-OH-DPAT > ipsapirone > RU24969 > sumatrip tan > ritanserin > ketanserin was demonstrated. The apparent inhibitor y constants for the ligands tested fit with the profile expected of bi nding to 5-HT1A receptors. Inhibition studies with [H-3] 5-HT plus 100 nhl 8-OH-DPAT (which inhibits binding to 5-HT1A receptors only) repre senting total binding, indicated that no further displacement occurred when ligands preferentially selective for 5-HT1B, 5-HT1D alpha,1D bet a, or 5-HT2C were tested. Total binding of [H-3] 5-HT in tissue sectio ns developed by autoradiography was displaced completely by 100 nM 8-O H-DPAT. Melatonin showed little affinity for the [H-3] 5-HT binding si tes. Conclusions. A population of 5-HT1A receptors is present in rabbi t ciliary processes. There is no evidence to suggest the presence of 5 -HT1D alpha, 5-HT1D beta, 5-HT2B, or 5-HT2C receptors in the iris-cili ary body.