The serotonin autoreceptor: antagonism by quipazine

The purpose of this study was to attempt to reproduce previous findings regarding the antagonist specificity of the 5HT autoreceptor and to find additional antagonists of this receptor. Crude synaptosomal preparations of the rat hypothalamus were loaded with [3H]5HT, placed on glass microfiber filters and superfused with modified Krebs--Henseleit buffer at 37 degrees C. The release of [3H]5HT was stimulated by raising the buffer K+ concentration and was Ca2+-dependent. In the presence of 100 nM fluoxetine (a selective 5HT uptake inhibitor), exogenous 5HT inhibited the K+-induced release of [3H]5HT but did not affected basal [3H]5HT release. The K+-induced [3H]5HT release was maximally inhibited by 30 nM 5HT to a level of 66.4 +/- 4.0% of control. The concentration of 5HT required to inhibit half-maximally K+-induced [3H]5HT release was approx. 7 nM. Methiothepin and quipazine were found to block the inhibition of K+-induced [3H]5HT release by exogenous 5HT (30 nM). The IC50S for blockade of the effects of 5HT were approx. 3.8 and 670 nM for methiothepin and quipazine, respectively. Several other putative 5HT antagonists, the dopamine receptor antagonist, spiperone and the alpha receptor antagonist, phentolamine, were without effect. Thus, the 5HT autoreceptor appears to have a unique specificity for certain 5HT antagonists. In addition, blockade of 5HT autoreceptors may be one mechanism by which quipazine produces behavioral effects characteristic of a 5HT receptor agonist.