Stereoselectivity of the anxiolytic effect of propranolol microinjected into the dorsal midbrain central gray

In a previous study we have shown that microinjection of d,1-propranolol into the dorsal midbrain central gray of the rat causes an anxiolytic effect in the elevated plus-maze model which is likely to be mediated by endogenous 5-hydroxytryptamine. In the present experiment, the effects of 1- and d,1-propranolol were compared under the same experimental conditions. Both the 1-isomer and the racemic mixture increased the percentage of open arm entries without affecting the total number of entries into either open or enclosed arms of the maze, thus reproducing the selective anxiolytic effect previously described. The doses of 5 nmol 1-propranolol and 10 nmol d,1-propranolol caused anxiolytic effects of comparable magnitude, while the doses of 2.5 nmol of the former and 5 nmol of the latter were ineffective. Therefore, the 1-isomer was nearly twice as potent as the racemic mixture, thus being responsible for the pharmacological activity observed. These results are compatible with the proposal that propranol blocks stereospecific autoreceptors in serotonergic nerve endings that inhibit neurotransmitter release.