Neurotransmission in the Sheep Middle Cerebral Artery: Modulation of Responses by 5-HT and Haemolysate

Abstract

Summary:In ring sections of the sheep middle cerebral artery, electrical field stimulation elicits a complex re sponse due to the simultaneous release of vasodilator and vasoconstrictor neurotransmitters. Haemolysate abol ishes the relaxant effects of the vasodilator neurotrans mitter and causes a marked augmentation of the contrac tile response in both the presence (448 ± 191%) and ab sence (409 ± 134%) of an intact endothelium. The haemolysate also reverses relaxation induced by sodium nitroprusside or sodium nitrite but has no effect on relax ation induced by 8-Br-cGMP. The vasodilator neuro transmitter therefore appears to act directly on the smooth muscle to cause relaxation by the stimulation of guanylate cyclase. The vasoconstrictor neurotransmitters that are released are antagonised by prazosin (100 nM), ketanserin (100 nM) and atropine (100 nM), which sug-The cerebral arteries of various species have been found to be densely innervated. Histochemical studies have shown the presence of noradrenergic, serotoninergic, cholinergic, and peptidergic nerves within the adventitia of various arteries from the cerebral vasculature (Iwayama et aI., 1970;Dahl 1970; Larsson et aI., 1976; Lee et aI., 1978; Scatton et aI., 1983; Hanko et aI., 1985). There is some in vitro evidence of functional responses produced by nerve stimulation of cerebral arteries; however, there is little functional evidence for the identity of the transmitters released (Lee et aI., 1978;Duckles, 1979; Griffith et aI., 1982; Edvinsson et aI., 1984).It has been suggested that in cerebral vasospasm, blood-borne factors such as 5-hydroxytryptamine Received July 3, 1989; revised November 28, 1989; accepted December 19, 1989.Address correspondence and reprint requests to Dr. R. M. Wadsworth at Department of Physiology and Pharmacology, University of Strathclyde, Glasgow G I lXW, Scotland.Abbreviations used: ACh, acetylcholine; EDRF, endothelium derived relaxing factor; 5-HT, 5-hydroxytryptamine; MCA, mid dle cerebral arteries; TNS, transmural nerve stimulation. 409gests that the transmitters involved are noradrenaline, 5-hydroxytryptamine (5-HT), and acetylcholine, respec tively. In the presence of these three antagonists at 10 J.1M, there was 86.9 ± 4.8% inhibition. Incubation with 5-HT (10 J.1M) causes a marked augmentation of the con tractile response (267 ± 56%) to field stimulation that can be reduced by pretreatment with either desipramine or citalopram, inhibitors of noradrenergic and serotoniner gic uptake mechanisms, respectively. The 5-HT appears to be taken up into noradrenergic nerves and released as an alternative neurotransmitter upon subsequent stimula tion. These actions of haemolysate and 5-HT may be in volved in the cerebral vasospasm observed following sub arachnoid haemorrhage.

Subarachnoid hemorrhage—induced upregulation of the 5-HT1B receptor in cerebral arteries in rats

“…36 It has previously been shown, like the results reported here, that SAH induces an increased sensitivity of cerebral arteries toward 5-HT. 22,35 The increased sensitivity, in view of the lack of clinical effect of 5-HT antagonists, could be explained as a passive secondary phenomenon to the products of fibrin degradation, 10 to hemoglobin and serum, 37 and to hemolysate; 11 however, the novelty of the present study lies in our use of more specific antagonists and more advanced molecular biological techniques. With these methods we have clearly shown which receptor is upregulated (the 5-HT 1B receptor) and that this upregulation is due to a synthesis of mRNA, which in turn is a result of increased gene transcription.…”

Relaxation of sheep cerebral arteries by vasoactive intestinal polypeptide and neurogenic stimulation: inhibition by l‐NG‐monomethyl arginine in endothelium‐denuded vessels

“…It is unlikely that the actions of L-NMMA described here can be ascribed to nonselective action since there was no functional endothelium in any of the tissues examined and the relaxant responses to CGRP were unaffected by L-NMMA. Immunohistochemical studies have shown a dense network of nerves containing VIP-like immunoreactivity in the sheep middle cerebral artery. It is not possible to state conclusively that VIP is the neurogenic mediator of relaxation from these studies; however, it does seem plausible since VIP is present in perivascular nerves and both VIP and the neurogenic mediator appear to act through a similar mechanism which is sensitive to both haemolysate (Gaw et al, 1990) and L-NMMA. It is not possible to rule out a minor contribution by CGRP to the neurogenic relaxant response, but any such contribution would have to be very small since CGRP is not affected by haemolysate and the CGRP innervation of the sheep middle cerebral artery is sparser than the VIP innervation.…”

“…The VIP-induced relaxation of the sheep middle cerebral artery, like the neurogenic mediator of relaxation (Gaw et al, 1990), was susceptible to inhibition by a haemolysate solution from feline erythrocytes. This inhibitory action of haemolysate appeared to be selective since the relaxation induced by CGRP, another vasodilator peptide, was unaffected.…”

“…Neuronal stimulation of the sheep middle cerebral artery results in a complex mechanical response (Duckles, 1979), part of which is mediated by a relaxant factor that is sensitive to inhibition by haemolysate (Gaw et al, 1990).…”