1We describe the actions of GR43175, a 5-hydroxytryptamine1 (5-HT,)-like receptor agonist, on neurogenically-mediated plasma protein extravasation within an important pain-sensitive intracranial tissue, the dura mater. 2 GR43175 markedly attenuated extravasation of '25I-albumin from blood vessels within ipsilateral dura mater when administered to rats (100 pgkg-1) fifteen minutes before unilateral electrical trigeminal stimulation (1.2 mA, 5Hz, Sims, S min.); the ratio (stimulated/unstimulated sides) decreased from 1.81 to 1.23, P < 0.005).3 GR43175 (l00pgkg-1, i.v., rats; 30pgkg-, i.v., guinea-pigs) decreased the leakage of radiolabelled albumin from 163% to 119% (P < 0.005, guinea-pig) or from 174 to 118% (P < 0.05, rat) above vehicletreated controls when injected ten minutes before systemic capsaicin treatment (0.5 or 1 umol kg-1, i.v.).4 GR43175 (30-300pgkg-1) did not block plasma protein extravasation within extracranial tissues of rats and guinea-pigs innervated by the trigeminal nerve (conjunctiva, eyelid and lip).5 The protein leakage which followed the i.v. administration of 5-HT (1 ymol kg 1) or neuropeptides which mediate neurogenic plasma extravasation, substance P (0.3 nmol kg-1 or 1 nmol kg 1) and neurokinin A (1 nmol kg-'), was not blocked by GR43175 (100, 300pgkg-1) despite the presence of leakage in amounts equivalent to that following neurogenic stimulation.6 GR43175 (l00gkg-1) decreased bradykinin (1Opmolkg-')-induced extravasation from 142 to 115% above vehicle-treated animals (P < 0.05). 7 These results demonstrate an important action of GR43175 on neurogenic mechanisms in dural blood vessels. Since the ergot alkaloids possess a similar profile of drug activity, it is suggested that drugs useful in the treatment of acute vascular headaches may share a similar mechanism of action.

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