It has been well established that cerebral blood vessels in several species receive vasoconstrictor and dilator nerves. 1,2 Norepinephrine (NE) released from sympathetic adrenergic nerves originating from the superior cervical ganglion (SCG) and acetylcholine (ACh) released from parasympathetic cholinergic nerves originating from the sphenopalatine ganglion were first suggested to be the respective transmitters for vasoconstriction and dilation. 3 Pharmacological and morphological studies have shown that activation of nicotinic acetylcholine receptors (nAChRs) located on the perivascular sympathetic nerve terminals of basilar arteries (BAs) increases the release of NE. This released NE then acts on β 2 -adrenoceptors located on neighboring parasympathetic nitrergic nerves to facilitate nitric oxide (NO) release, resulting in vasorelaxation 4,5 and increased blood flow. 6 These findings indicate that activation of cerebral perivascular sympathetic nerves of BAs causes dilation via a sympathetic/parasympathetic interaction mechanism (Figure 6). However,
Previous studies have demonstrated that nicotine can induce relaxation of the middle cerebral artery (MCA). However, whether this relaxation is associated with the activity of sensory calcitonin gene–related peptide (CGRP) nerves and whether this is modulated by hydrogen protons (H+), facilitating the release of CGRP from sensory CGRPergic nerve terminals in the MCA, remains unclear. In this study, we examined the role of H+ in the modulation of neurogenic vasomotor responses in the rat-isolated endothelium-denuded MCA. Wire myography was used to measure vasoreactivity and indicated that nicotine-induced relaxation was sensitive to tetrodotoxin and lidocaine and drastically reduced levels of guanethidine (an adrenergic neuronal blocker), NG-nitro-L-arginine (L-NNA), CGRP8-37, vasoactive intestinal polypeptide (VIP)6-28, capsaicin, capsazepine (a transient receptor potential vanilloid-1 inhibitor), and tetraethylammonium. However, this nicotine-induced relaxation was not sensitive to propranolol. Lowering the pH of the buffer solution with HCl caused pH-dependent vasorelaxation and deceased intracellular pH in the MCA rings, which was sensitive to L-NNA, CGRP8-37, VIP6-28, capsazepine, 4-aminopyridine (a voltage-gated potassium channel antagonist), and paxilline (a large conductance Ca2+-activated K+ channel antagonist). However, HCl-induced relaxation was not inhibited by glibenclamide (an ATP-sensitive K+ channel blocker). These results suggested that electrical and chemical activation of cerebral perivascular adrenergic nerves led to the release of H+, which then facilitated the release of NO, VIP, and CGRP, resulting in vasorelaxation. Lowering the pH of the buffer solution caused potassium channels of vascular smooth muscle cells and perivascular nerves to open. In conclusion, our results demonstrated that H+ may act as a modulator on MCA perivascular nerves and/or smooth muscles.
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