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Are 5-hydroxytryptamine7 receptors involved in [3H]5-hydroxytryptamine binding to 5-hydroxytryptamine 1nonA-nonB receptors in rat hypothalamus?

M Gobbi, L Parotti and T Mennini

Istituto di Ricerche Farmacologiche "Mario Negri," Milano, Italy.

We assayed [3H]5-hydroxytryptamine ([3H]5-HT) binding in rat hypothalamic membranes to confirm the possible of measuring 5-HT7 receptors. Binding was tested in the presence of 3 microM (+/-)- pindolol, a concentration higher than previously suggested for the same purpose (0.1 micron). This higher concentration was, however, needed to fully saturate 5-HT1A and 5-HT1B receptors without interaction with 5- HT7 receptors. Under these conditions, [3H]5-HT binding could be further inhibited with methiothepin (used to determine nonspecific binding) and with 5-HT, with an IC50 of 1.4 nM and a slope of 1. The inhibition curves of (+/-)-8-hydroxy-dipropylaminotetralin, ritanserin, and mianserin were shallow (slopes, 0.35-0.58) and could be better analyzed with the two-site model, indicating that the pindolol- insensitive [3H]5-HT binding sites in rat hypothalamic membranes are heterogeneous. Although the IC50 of the compounds tested suggests that one population of sites is actually associated with 5-HT7 receptors, our data clearly indicate that this binding assay does not selectively label 5-HT7 receptors in native tissues. These results challenge a previous report and suggest that the proposed down-regulation of 5-HT7 receptors after fluoxetine treatment should be considered with caution. The development of more selective and sensitive binding assays will probably offer significant advantage.

Volume 49, Issue 3, pp. 556-559, 03/01/1996
Copyright © 1996 by American Society for Pharmacology and Experimental Therapeutics




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