Melatonin increases pial artery tone and decreases the lower limit of cerebral blood flow autoregulation

We studied a possible link between the melatonin‐induced increase in cerebral arteriolar tone and the melatonin‐induced shift in cerebral blood flow (CBF) autoregulation to a lower pressure level. Using the cranial window technique, we showed that intravenous infusion of melatonin constricted pial arterioles (–5.1 ± 1.3 and –5.4 ± 0.7 μm at 60 and 600 ng/kg/h, respectively). Perivascular application of luzindole alone had no significant effect but abolished vasoconstriction induced by melatonin (–0.5 ± 0.7 and + 3.0 ± 1.2 μm at 60 and 600 ng/kg/h respectively). Using a combination of the hydrogen clearance and cranial window techniques, we showed that intravenous infusion of melatonin had no effect on baseline CBF but shifted the lower limit (LL) of CBF autoregulation (stepwise hypotension) to a lower pressure (90 ± 2 mmHg in vehicle vs. 71 ± 3 and 51 ± 5 mmHg, both P