Effect of serotonin and its antagonist ketanserin on pial vessels.

The effect of serotonin (5-HT) and its antagonist ketanserin on the cerebral circulation was investigated in two series totaling 24 cats using the cranial window technique. 5-HT elicited a marked dilatation of small pial arteries, whereas large arteries tended to constrict. Intravenous administration of ketanserin reversed the constriction of large arteries, causing dilatation, and reduced the extent of small arteries' dilatation. In a randomized study, i.v. administration of ketanserin in its solvent versus the solvent alone revealed a strong dilatatory effect of the solvent on pial arteries (17 +/- 1.8%), which partly jeopardized a possible constrictory effect of ketanserin, as ketanserin plus solvent induced less dilatation of small pial arteries than the solvent alone. The present data support the view that serotonin exerts a dual effect on cerebral arteries, namely, dilatation of small and constriction of large vessels. The antagonist ketanserin reverses this effect, but the strong dilatatory effect of the solvent alone masks the antiserotoninergic effect.