Cerebrovascular responses to subarachnoid blood and serotonin in the monkey

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Abstract
In vitro experiments were performed to determine the serum concentration of serotonin in the monkey Macaca mulatta and the ability of cyproheptadine to block serotonin and serum-induced contractions in monkey cerebral arteries. Cynomolgus monkeys (34) were used to study changes in regional cerebral blood flow (CBF) measured by the intracarotid 133Xe technique, and angiographically determined cerebral arterial diameter by subarachnoid injection of artificial CSF, blood and serotonin. Animals (5) in each group were given in 1.0 mg/kg i.v. cyproheptadine (a serotonin-blocking agent) during the post-injection period. Subarachnoid injection of artificial CSF produced no change in CBF or arterial diameter. Post-injection administration of cyproheptadine had no effect on these parameters. Subarachnoid injection of fresh autogenous blood produced a significant but transient (< 1 h) decrease in CBF and moderate vasospasm which lasted at least 3 h and was essentially unaffected by i.v. cyproheptadine. CBF and arterial diameters were unchanged following subarachnoid injection of serotonin at concentrations (5 .times. 10-6 M) present in normal monkey serum. Serotonin at 5 .times. 10-6 M invariably produced near maximal contractions in the in vitro cerebral artery preparations. Higher (.times. 10) serotonin concentrations caused a transient CBF response similar to that obtained with blood. The induced cerebral vasospasm was of shorter duration than that obtained with blood. A major role for serotonin in the production of post-subarachnoid hemorrhage vasospasm is not supported. In vitro experiments do not reflect the ability of serotonin to constrict cerebral arteries in the intact animal.