Serotonergic mediation of the anxiolytic effect of intracerebrally injected propranolol measured in the elevated plus-maze.

The effect of intracerebrally injected propranolol was measured in the elevated plus-maze, an animal model of anxiety. Microinjection of 10 nmol of propranolol into the dorsal midbrain central gray of the rat increased the percentage of open arm entries, without affecting the total number of arm entries. This selective anxiolytic effect of propranolol was antagonized by 10 nmol of ritanserin, also injected into the dorsal midbrain. The same dose of ritanserin, given alone, did not affect the percentage of open arm entries, though it tended to decrease the total number of entries, an indication of unspecific behavioral depression. Since propranolol is a stereospecific antagonist of presynaptic serotonin (5-HT) autoreceptors and ritanserin is a selective blocker of type 2 5-HT receptors, the present results suggest that the anxiolytic action of propranolol in the midbrain central gray is due to release of endogenous 5-HT acting upon 5-HT2 receptors.