(-)-Propranolol and (+/-)-cyanopindolol are mixed agonists-antagonists at serotonin autoreceptors in the hippocampus of the rat brain.

The effects of (-)-propranolol, (+)-propranolol and (+/-)-cyanopindolol on the release of [3H]5-hydroxytryptamine [( 3H]5-HT) were investigated in synaptosomes from the hippocampus of the rat, depolarized in superfusion with 15 mM KCl. (-)-Propranolol, but not (+)-propranolol, inhibited in a concentration-dependent way the K+-evoked release of [3H]5-HT. (+/-)-Cyanopindolol behaved similarly but was about 10 times more potent than (-)-propranolol. The inhibitory effects of (-)-propranolol and (+/-)-cyanopindolol were prevented by the autoreceptor antagonist methiothepin. Both beta-adrenoceptor antagonists antagonized the inhibition by exogenous 5-HT of the K+-evoked release of [3H]5-HT. The data suggest that some beta-adrenoceptor antagonists may behave as mixed agonists-antagonists at the 5-HT autoreceptor. Synaptosomes in superfusion appear to be particularly suitable to study separately agonistic compared to antagonistic activity of compounds having a mixed agonist-antagonist profile.