Serotonin receptor subtypes involved in the elevation of serum corticosterone concentration in rats by direct- and indirect-acting serotonin agonists.

The serum corticosterone concentration in rats was increased by injection of quipazine, a relatively nonselective serotonin (5-hydroxytryptamine; 5-HT) agonist, or 8-hydroxy-2-(di-n- propylamino)tetralin (8-OH-DPAT), a serotonin agonist selective for the 5-HT1A subtype of receptor. The quipazine-induced increase in serum corticosterone was antagonized by 17 different serotonin antagonists; of these, MDL 11939, pirenperone, setoperone, mianserin, LY 281067, ketanserin, ritanserin and clozapine have relatively selective affinity for the 5-HT2 subtype of receptor. The 8-OH-DPAT-induced increase in serum corticosterone was not antagonized by metergoline, the most potent antagonist of the quipazine effect, but was antagonized by pindolol or penbutolol, 5-HT1A receptor antagonists. Pindolol did not block the effect of quipazine. The results support earlier evidence that serum corticosterone concentration in rats can be increased by activation of either 5-HT1A or 5-HT2 receptors. Indirect-acting serotonin agonists - fluoxetine, L-5-hydroxytryptophan and p-chloroamphetamine - also increased serum corticosterone concentrations. The increases elicited by those agents, which earlier had been reported not to be blocked by metergoline pretreatment, also were not blocked by pretreatment with pindolol or with the combination of metergoline and pindolol. Thus, an involvement of a specific serotonin receptor subtype in the actions of these indirect agonists has not been established.