Uptake and release of serotonin in rat cerebrovascular nerves after subarachnoid hemorrhage.

BACKGROUND AND PURPOSE Serotonin released from platelets has been suggested as one substance causing the vasospasm following subarachnoid hemorrhage. We studied whether such serotonin is able to constrict pial vessels. METHODS We studied the uptake of serotonin in pial perivascular nerves by immunohistochemistry. We measured the contractile response in rat basilar artery after in vitro incubation with serotonin and during electrical field stimulation of perivascular nerves following experimental subarachnoid hemorrhage. RESULTS After incubation with serotonin, electrical field stimulation caused a tetrodotoxin- and ketanserin-blockable contractile response. We observed no such response in vessels from rats treated with 6-hydroxydopamine or after blockade of serotonin uptake. After subarachnoid hemorrhage, a pronounced network of serotonin-immunoreactive nerve fibers was demonstrated in the vessel wall. In vessels from control rats, no serotonin fibers were seen, and in vessels from 6-hydroxydopamine-treated animals with subarachnoid hemorrhage only a few such fibers were seen. Electrical field stimulation of the basilar artery from rats tested 2 or 16 hours (but not 10 minutes or 24 hours) after subarachnoid hemorrhage showed contractile responses that were prevented by tetrodotoxin, ketanserin, and prior 6-hydroxydopamine treatment. CONCLUSIONS Our study demonstrates a capacity of the perivascular sympathetic nerves to take up serotonin both in vitro and during the early phase of subarachnoid hemorrhage. Such uptake may help to remove excess serotonin from the subarachnoid space. Only if serotonin is subsequently released upon nerve activation may minor smooth muscle contraction develop.