Characterization of serotonin receptors in the iris + ciliary body of the albino rabbit

Experiments were undertaken to determine if serotonin (5-HT) radioligand binding sites were present in a membrane fraction of iris + ciliary body from adult, albino rabbits. The total binding of 3H-5-HT, 3H-spiroperidol (SPI) and 3H-ketanserin (KET), all at 2 nM, was determined in the absence and in the presence of ketanserin, mianserin, methysergide or 5-methoxytryptamine (5-MT), all at 1 μM. Except for ketanserin, which displaced 15% of 3H-KET binding, none of the agents altered 3H-SPI or 3H-KET binding. Reductions of 12% and 14% in 3H-5-HT binding were achieved by ketanserin and mianserin, respectively. In contrast, methysergide and 5-MT displaced 3H-5-HT binding by 57% and 56%, respectively. 5-HT (1 μM) also displaced 3H-5-HT binding by approximately 60% and this was used to measure nonspecific binding. Specific binding of 3H-5-HT was saturable and of high affinity with one population of binding sites being labelled. Kd and Bmax values of 1.1 nM and 57.8 fmoles/mg protein were obtained. Seven 5-HT antagonists possessing various affinities for 5-HT1 and 5-HT2 binding sites were assessed for their ability to displace specific 3H-5-HT binding. Ketanserin was the least potent (Ki > 3 μM). In contrast, the respective Ki values for metergoline and methysergide were 13 nM and 20 nM. These observations indicate the presence of 5-HT1 binding sites. The indole-containing agonists 5-HT, 5-MT and 5-methoxy-N, N-dimethyltryptamine were potent displacers of specific 3H-5-HT binding, the observed Ki values being 0.6 nM, 7.0 nM and 3.6 nM, respectively. In contrast, weaker affinities were displayed by piperazine substituted 5-HT agonists, e. g. a Ki value of 757 nM for trifluoro-m-tolyl-piperazine. Findings for 5-HT agonists suggest that 5-HT1A receptors are present in the rabbit iris + ciliary body. The high affinity (Ki of 2.8 nM) observed for 8-OH-DPAT in this study is in harmony with this concept.