Binding of [3H]serotonin (5-HT) to membranes prepared from Arctic charr brain homogenates was most consistent with a one-site model for [3H]5-HT binding, with KD and Bmax values of 5.7±0.3 nmol l-1 and 60.7±7.3 fmol mg-1 protein, respectively. Similarly, 5-HT displacement of [3H]5-HT was best explained by a monophasic model with an apparent Ki of 4.3±0.7 nmol l-1. The ability of a number of synthetic 5-HT receptor ligands to displace [3H]5-HT was studied. 8OH-DPAT was found to interact with three [3H]5-HT binding sites, whereas buspirone, TFMPP, spiperone and mianserin all distinguish two sites. In the presence of 300 nmol l-1 buspirone, 8OH-DPAT and mianserin distinguished two [3H]5-HT binding sites, whereas spiperone interacted with only one. Moreover, 8OH-DPAT differentiated three [3H]5-HT binding sites even in the presence of 0.5 mmol l-1 GTP, making it unlikely that these sites represent different affinity states of G-protein-coupled receptors. GTP had no effect on apparent Ki values for 8OH-DPAT, but reduced the Bmax value of the high-affinity site by 60 %. GTP had a similar effect on the saturation binding curve for [3H]5-HT, reducing Bmax by approximately 50 %, whereas KD was unaffected. The results provide evidence for at least three different high-affinity [3H]5-HT binding sites, one of them showing a pharmacological profile strikingly similar to that of the mammalian 5-HT1A receptor.

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