MULTIPLE HIGH-AFFINITY BINDING-SITES FOR [H-3] SEROTONIN IN THE BRAINOF A TELEOST FISH, THE ARCTIC CHARR (SALVELINUS-ALPINUS)

Binding of [H-3]serotonin (5-HT) to membranes prepared from Arctic cha rr brain homogenates was most consistent with a one-site model for [H- 3]5-HT binding, with K-D and B-max values of 5.7+/-0.3 nmol l(-1) and 60.7+/-7.3 fmol mg(-1) protein, respectively. Similarly, 5-HT displace ment of [H-3]5-HT was best explained by a monophasic model with an app arent K-i of 4.3+/-0.7 nmol l(-1). The ability of a number of syntheti c 5-HT receptor ligands to displace [H-3]5-HT was studied. 80H-DPAT wa s found to interact with three [H-3]5-HT binding sites, whereas buspir one, TFMPP, spiperone and mianserin all distinguish two sites. In the presence of 300 nmol l(-1) buspirone, 80H-DPAT and mianserin distingui shed two [H-3]5-HT binding sites, whereas spiperone interacted with on ly one. Moreover, 80H-DPAT differentiated three [H-3]5-HT binding site s 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 r eceptors. GTP had no effect on apparent K-i values for 80H-DPAT, but r educed the B-max value of the high-affinity site by 60%, GTP had a sim ilar effect on the saturation binding curve for [H-3]5-HT, reducing B- max by approximately 50%, whereas K-D was unaffected. The results prov ide evidence for at least three different high-affinity [H-3]5-HT bind ing sites, one of them showing a pharmacological profile strikingly si milar to that of the mammalian 5-HT1A receptor.