Modification of vasoconstrictor responses in cerebral blood vessels by lesioning of the trigeminal nerve: possible involvement of CGRP.

The functional role of the trigeminal system has been addressed in experiments on the cortical surface of alpha-chloralose anaesthetized cats. Application of calcitonin gene-related peptide (CGRP) caused a concentration-dependent increase in arteriolar calibre by 38 +/- 5% (n = 8) with an IC50 of 2 nM. Cerebral veins did not relax upon CGRP administration (n = 12). Substance P (SP) was less potent but showed dilatation of both arterioles (21 +/- 4%) and veins (16 +/- 4%). The cerebrovascular trigeminal system was investigated after chronic (14 days) surgical lesion of the trigeminal nerve with the concomitant disappearance of perivascular CGRP/SP immunoreactive nerves. The cortical arteriolar responses to subarachnoid microinjections of acidic (pH 6.8) and basic CSF (pH 7.6) as well as noradrenaline (10(-4) M), neuropeptide Y (10(-7) M), prostaglandin F2x (10(-6 M), barium chloride (10(-4) M), and autologous blood (5 microl) were examined in anaesthetized cats with lesions of the trigeminal nerve, and were compared with their effects in sham-operated animals. The magnitude of the vasodilator and vasoconstrictor responses to these agents was unaffected by trigeminal lesions. However, duration of the vasoconstriction produced by basic CSF, but not the vasodilitation to acidic CSF, was markedly prolonged by trigeminal lesions (from 0.8 +/- 0.1 min to 2.2 +/- 0.3 min, p < 0.01). Also, the vasoconstrictor responses to noradrenaline, prostaglandin F2x, barium chloride, and autologous blood were significantly prolonged, while the maximum contractile effect to each agent was similar in lesioned as in sham-operated controls. The effects of CGRP, SP, and neurokinin A (NKA) have been examined on isolated cerebral arteries in vitro. Different CGRP analogues induced a strong relaxation with no difference in Imax (85-96%) or pD2 values (8.65 - 9.12). NKA induced a stronger relaxation than SP (Imax: 33% and 13%, respectively). SP was more potent than NKA (pD2:8.7 and 7.7, respectively). Capsaicin, a substance which selectively causes the release of stored sensory neuropeptides (CGRP, SP, NKA), caused in vitro relaxation of precontracted arteries. This relaxation was not affected by the neurokinin blocker spantide, but shifted towards higher capsaicin concentrations by the CGRP antagonist (CGRP 8-37. Thus, in this preparation CGRP rather than a neurokinin (SP/NKA) is responsible for the capsaicin-induced dilatations.