BUSPIRONE, BUT NOT SUMATRIPTAN, INDUCES MIOSIS IN HUMANS - RELEVANCE FOR A SEROTONINERGIC PUPIL CONTROL

Background and objective: Drugs that act on the serotoninergic system have been shown to influence the pupil size, However, the 5-hydroxytry ptamine (5-HT) receptor type or subtype that affects pupil diameter ha s not been defined in humans, With a placebo-conet olled, double-blind randomized design, we investigated in healthy volunteers the effect o n pupil size of buspirone and sumatriptan, which mainly act on 5-HT1A- and the 5-HT1-like receptors, respectively. Methods: The pupil area w as measured by means of a videopupillometer before and after a single oral administration of placebo or of three different doses of active d rugs. Heart rate and arterial blood pressure were recorded after pupil area measurement. Results: Buspirone (5, 10, and 20 mg) caused a dose -dependent miosis. Sumatriptan (50, 100, and 200 mg) did not affect th e pupil size. Twenty milligrams of buspirone reduced the mydriasis ind uced by pretreatment with homatropine eyedrops. A 20 mg dose of buspir one reduced blood pressure without change in heart rate, whereas buspi rone, at doses lower than 20 mg, and sumatriptan did not affect heart rate and blood pressure. Conclusions: This study suggests that buspiro ne, but not sumatriptan, the selective agonist of 5-HT1-like receptors , causes miosis in humans by activation of 5-HT1A receptors, possibly located in the central nervous system where they inhibit iris sympathe tic pathways. Measurement of pupil size seems to provide a valuable an d sensitive index of 5-HT1A receptor function in humans.