Cholinergic-nitrergic transmitter mechanisms in the cerebral circulation

Serotonergic Modulation of Neurovascular Transmission: A Focus on Prejunctional 5-HT Receptors/Mechanisms

5-Hydroxytryptamine (5-HT), or serotonin, plays a crucial role as a neuromodulator and/or neurotransmitter of several nervous system functions. Its actions are complex, and depend on multiple factors, including the type of effector or receptor activated. Briefly, 5-HT can activate: (i) metabotropic (G-protein-coupled) receptors to promote inhibition (5-HT₁, 5-HT₅) or activation (5-HT₄, 5-HT₆, 5-HT₇) of adenylate cyclase, as well as activation (5-HT₂) of phospholipase C; and (ii) ionotropic receptor (5-HT₃), a ligand-gated Na⁺/K⁺ channel. Regarding blood pressure regulation (and beyond the intricacy of central 5-HT effects), this monoamine also exerts direct postjunctional (on vascular smooth muscle and endothelium) or indirect prejunctional (on autonomic and sensory perivascular nerves) effects. At the prejunctional level, 5-HT can facilitate or preclude the release of autonomic (e.g., noradrenaline and acetylcholine) or sensory (e.g., calcitonin gene-related peptide) neurotransmitters facilitating hypertensive or hypotensive effects. Hence, we cannot formulate a specific impact of 5-HT on blood pressure level, since an increase or decrease in neurotransmitter release would be favoured, depending on the type of prejunctional receptor involved. This review summarizes and discusses the current knowledge on the prejunctional mechanisms involved in blood pressure regulation by 5-HT and its impact on some vascular-related diseases.

Parasympathetic innervation of vertebrobasilar arteries: is this a potential clinical target?

This review aims to summarise the contemporary evidence for the presence and function of the parasympathetic innervation of the cerebral circulation with emphasis on the vertebral and basilar arteries (the posterior cerebral circulation). We consider whether the parasympathetic innervation of blood vessels could be used as a means to increase cerebral blood flow. This may have clinical implications for pathologies associated with cerebral hypoperfusion such as stroke, dementia and hypertension. Relative to the anterior cerebral circulation little is known of the origins and neurochemical phenotypes of the parasympathetic innervation of the vertebrobasilar arteries. These vessels normally provide blood flow to the brainstem and cerebellum but can, via the Circle of Willis upon stenosis of the internal carotid arteries, supply blood to the anterior cerebral circulation too. We review the multiple types of parasympathetic fibres and their distinct transmitter mechanisms and how these vary with age, disease and species. We highlight the importance of parasympathetic fibres for mediating the vasodilatory response to sympathetic activation. Current trials are investigating the possibility of electrically stimulating the postganglionic parasympathetic ganglia to improve cerebal blood flow to reduce the penumbra following stroke. We conclude that although there are substantial gaps in our understanding of the origins of parasympathetic innervation of the vertebrobasilar arteries, activation of this system under some conditions might bring therapeutic benefits.

Overview of the Anatomy, Physiology, and Pharmacology of the Autonomic Nervous System

Comprised of the sympathetic nervous system, parasympathetic nervous system, and enteric nervous system, the autonomic nervous system (ANS) provides the neural control of all parts of the body except for skeletal muscles. The ANS has the major responsibility to ensure that the physiological integrity of cells, tissues, and organs throughout the entire body is maintained (homeostasis) in the face of perturbations exerted by both the external and internal environments. Many commonly prescribed drugs, over-the-counter drugs, toxins, and toxicants function by altering transmission within the ANS. Autonomic dysfunction is a signature of many neurological diseases or disorders. Despite the physiological relevance of the ANS, most neuroscience textbooks offer very limited coverage of this portion of the nervous system. This review article provides both historical and current information about the anatomy, physiology, and pharmacology of the sympathetic and parasympathetic divisions of the ANS. The ultimate aim is for this article to be a valuable resource for those interested in learning the basics of these two components of the ANS and to appreciate its importance in both health and disease. Other resources should be consulted for a thorough understanding of the third division of the ANS, the enteric nervous system. © 2016 American Physiological Society. Compr Physiol 6:1239-1278, 2016.

Axo-axonal interaction in autonomic regulation of the cerebral circulation

Noradrenaline (NE) and acetylcholine (ACh) released from the sympathetic and parasympathetic neurones in cerebral blood vessels were suggested initially to be the respective vasoconstricting and dilating transmitters. Both substances, however, are extremely weak post-synaptic transmitters. Compelling evidence indicates that nitric oxide (NO) which is co-released with ACh from same parasympathetic nerves is the major transmitter for cerebral vasodilation, and its release is inhibited by ACh. NE released from the sympathetic nerve, acting on presynaptic β2-adrenoceptors located on the neighbouring parasympathetic nitrergic nerves, however, facilitates NO release with enhanced vasodilation. This axo-axonal interaction mediating NE transmission is supported by close apposition between sympathetic and parasympathetic nerve terminals, and has been shown in vivo at the base of the brain and the cortical cerebral circulation. This result reveals the physiological need for increased regional cerebral blood flow in 'fight-or-flight response' during acute stress. Furthermore, α7- and α3β2-nicotinic ACh receptors (nAChRs) on sympathetic nerve terminals mediate release of NE, leading to cerebral nitrergic vasodilation. α7-nAChR-mediated but not α3β2-nAChR-mediated cerebral nitrergic vasodilation is blocked by β-amyloid peptides (Aβs). This may provide an explanation for cerebral hypoperfusion seen in patients with Alzheimer's disease. α7- and α3β2-nAChR-mediated nitrergic vasodilation is blocked by cholinesterase inhibitors (ChEIs) which are widely used for treating Alzheimer's disease, leading to possible cerebral hypoperfusion. This may contribute to the limitation of clinical use of ChEIs. ChEI blockade of nAChR-mediated dilation like that by Aβs is prevented by statins pretreatment, suggesting that efficacy of ChEIs may be improved by concurrent use of statins.

Muscarinic acetylcholine receptor modulation of mu (mu) opioid receptors in adult rat sphenopalatine ganglion neurons

The sphenopalatine ganglion (SPG) neurons represent the parasympathetic branch of the autonomic nervous system involved in controlling cerebral blood flow. In the present study, we examined the coupling mechanism between mu (mu) opioid receptors (MOR) and muscarinic acetylcholine receptors (mAChR) with Ca(2+) channels in acutely dissociated adult rat SPG neurons. Successful MOR activation was recorded in approximately 40-45% of SPG neurons employing the whole cell variant of the patch-clamp technique. In addition, immunofluorescence assays indicated that MOR are not expressed in all SPG neurons while M(2) mAChR staining was evident in all neurons. The concentration-response relationships generated with morphine and [d-Ala2-N-Me-Phe4-Glycol5]-enkephalin (DAMGO) showed IC(50) values of 15.2 and 56.1 nM and maximal Ca(2+) current inhibition of 26.0 and 38.7%, respectively. Activation of MOR or M(2) mAChR with morphine or oxotremorine-methiodide (Oxo-M), respectively, resulted in voltage-dependent inhibition of Ca(2+) currents via coupling with Galpha(i/o) protein subunits. The acute prolonged exposure (10 min) of neurons to morphine or Oxo-M led to the homologous desensitization of MOR and M(2) mAChR, respectively. The prolonged stimulation of M(2) mAChR with Oxo-M resulted in heterologous desensitization of morphine-mediated Ca(2+) current inhibition, and was sensitive to the M(2) mAChR blocker methoctramine. On the other hand, when the neurons were exposed to morphine or DAMGO for 10 min, heterologous desensitization of M(2) mAChR was not observed. These results suggest that in rat SPG neurons activation of M(2) mAChR likely modulates opioid transmission in the brain vasculature to adequately maintain cerebral blood flow.

Electrophysiological and immunofluorescence characterization of Ca(2+) channels of acutely isolated rat sphenopalatine ganglion neurons

The sphenopalatine ganglion (SPG) is the main parasympathetic ganglion that is involved in regulating cerebral vascular tone and gland secretion. SPG neurons have been implicated in some types of migraine headaches but their precise role has yet to be determined. In addition, very little information is available regarding ion channel modulation by neurotransmitters that are involved in the parasympathetic drive of SPG neurons. In this study, acute isolation of adult rat SPG neurons was developed in order to begin the electrophysiological characterization of this ganglion. Under our dissociation conditions, the average number of neurons obtained per ganglion was greater than 1200. Immunofluorescence imaging results showed positive labeling with acetylcholinesterase (AChE), confirming the parasympathetic nature of SPG neurons. On the other hand, weak tyrosine hydroxylase immunostaining was observed in these neurons. Whole-cell patch-clamp recordings revealed that most of the Ca(2+) current is carried by N-type (53%) and SNX-482 resistant R-type (30%) Ca(2+) channels. In addition, Ca(2+) currents were inhibited in a voltage-dependent manner following exposure to oxotremorine-M (Oxo-M), norepinephrine and ATP via muscarinic acetylcholine receptor 2 (M(2) AChR) subtype, adrenergic and P2Y purinergic receptors, respectively. The peptides VIP and angiotensin II failed to modulate Ca(2+) currents, suggesting that these receptors are not present on the SPG soma or do not couple to Ca(2+) channels. In summary, our data suggest that the Ca(2+) current inhibition mediated by Oxo-M, NE and ATP in adult rat SPG neurons plays an integral part in maintaining parasympathetic control of cranial functions.

Vasorelaxing effect of BAY 41-2272 in rat basilar artery: involvement of cGMP-dependent and independent mechanisms

Decreases in intrinsic NO cause cerebral vasospasms because of the dysregulation of cGMP formation by NO-mediated pathways. Because 5-cyclopropyl-2-{1-(2-fluorobenzyl)-1H-pyrazolo[3,4-b]pyridin-3-yl}pyrimidin-4-ylamine (BAY 41-2272) is a potent soluble guanylyl cyclase (sGC) stimulator in an NO-independent manner, this study aimed to investigate the mechanisms underlying the relaxant effects of BAY 41-2272 in the rat basilar artery. BAY 41-2272 (0.0001 to 1 micromol/L) induced relaxations in a concentration-dependent manner, with pEC50 values of 8.13+/-0.03 and 7.63+/-0.05 in intact and denuded rings, respectively. The sGC inhibitor 1H-[1,2,4] oxadiazolo [4,3,-a]quinoxalin-1-one (ODQ) markedly displaced the curve for BAY 41-2272 to the right in intact or denuded rings (&10-fold). The NO synthesis inhibitor NG-nitro-L-arginine methyl ester caused a rightward shift in the curve for BAY 41-2272 (4-fold), whereas the phosphodiesterase type 5 inhibitor sildenafil enhanced BAY 41-2272-induced relaxations (3- to 4-fold). The Na+-K+-ATPase inhibitor ouabain caused 3-fold rightward shifts in the curves for BAY 41-2272. Ca2+-induced contractions in K+ depolarized rings were significantly attenuated by BAY 41-2272 in an ODQ-insensitive manner. The NO donor glyceryl trinitrate and BAY 41-2272 caused rightward shifts in the contractile responses to serotonin. Their coincubation caused a synergistic inhibition of serotonin-induced contractions. BAY 41-2272 and glyceryl trinitrate increased cGMP levels (but not cAMP) by 10-fold and 4-fold above baseline, respectively, in an ODQ-sensitive manner. cGMP levels increased by 50-fold after coincubation. BAY 41-2272 potently relaxes the rat basilar artery in a synergistic fashion with NO. Targeting the sGC with selective activators, such as BAY 41-2272, may represent a new therapy to treat cerebrovascular disease.

Influence of Gender on K + -Induced Cerebral Vasodilatation

Background and Purpose— It is not known whether cerebral vasoprotective mechanisms in females include increased function of arterial K + channels. We hypothesized that vasodilator responses mediated by activation of inwardly rectifying K + (K IR ) channels are greater in cerebral arteries of female versus male rats and that this is due to the effects of estrogen.

Methods— Changes in basilar artery diameter were measured with a cranial window preparation in anesthetized Sprague-Dawley rats.

Results— K + (5 and 10 mmol/L) caused greater vasodilatation in females (percent maximum, 21±3% and 58±7%, respectively) versus males (11±1% and 37±4%, respectively; P <0.05). In contrast, vasodilator responses to aprikalim (1 and 3 μmol/L) or acetylcholine (ACh, 1 and 10 μmol/L) did not differ between the genders. The selective K IR channel inhibitor barium ion (30 μmol/L) decreased basilar artery diameter in males but not females (−7±1% versus −2±1%, P <0.05) and selectively inhibited K + -induced vasodilatation by ≈50% in both groups. Ovariectomy of female rats resulted in smaller vasodilator effects of K + , and chronic treatment of these rats with 17β-estradiol (0.01 mg/kg per day for 7 days) normalized K+-induced vasodilatation. Furthermore, the selective M2 muscarinic ACh receptor antagonist methoctramine (1 μmol/L) increased responses to K + in males to levels equivalent to responses in females but had no effect on responses to K + in females.

Conclusions— K + is a more powerful vasodilator in the female versus male cerebral circulation. This difference is estrogen dependent and could be due to a lack of M2 muscarinic ACh receptor–induced inhibition of K IR channel activation by K + in female cerebral arteries.

Neuronal Nitric Oxide Synthase as a Novel Anti-Atherogenic Factor

Nitric oxide (NO) has multiple important actions that contribute to the maintenance of vascular homeostasis. NO is synthesized by three different isoforms of NO synthase (NOS), all of which have been reported to be expressed in human atherosclerotic vascular lesions. Although the regulatory roles of endothelial NOS (eNOS) and inducible NOS (iNOS) on the development of atherosclerosis have been described, little is known about the role of neuronal NOS (nNOS). Recent studies have demonstrated that nNOS also exerts important vasculoprotective effects in vivo. In a carotid artery ligation model, nNOS-knockout mice exhibited accelerated neointimal formation and constrictive vascular remodeling caused by blood flow disruption. In a rat balloon injury model, the selective inhibition of nNOS activity potently enhanced vasoconstrictor responses to a variety of calcium-mobilizing stimuli, and exacerbated neointimal formation. Moreover, in apolipoprotein E-knockout mice, deficiency of nNOS induced progression of aortic vascular lesion formation. In these models, nNOS was up-regulated in vascular lesions, and was predominantly expressed in the neointima and medial smooth muscle cells. These results provide the first direct evidence that nNOS plays important roles in suppressing arteriosclerotic vascular lesion formation. Thus, nNOS could be regarded as a novel anti-atherogenic factor.

Localization of the m5 muscarinic cholinergic receptor in rat circle of Willis and pial arteries

The expression and microanatomical localization of the muscarinic cholinergic m5 receptor subtype was investigated in rat circle of Willis and pial arteries by in situ hybridization, immunoblotting and immunohistochemistry. In situ hybridization histochemistry revealed a strong signal in the endothelium of circle of Willis and pial arteries and a moderate signal in the tunica media of the same arteries, within smooth muscle. Exposure of membranes of arteries to anti-m5 receptor protein antibodies caused the development of a band of approximately 81 kDa. Immunohistochemistry revealed the accumulation of m5 receptor protein immunoreactivity primarily within endothelium of circle of Willis and cerebral arteries and to a lesser extent in the tunica media, within smooth muscle. Medium (external diameter 200-100 microm) and small-sized (external diameter smaller than 100 microm) pial arteries displayed a significantly higher immune staining than large-sized pial arteries or circle of Willis arteries. The above data that are consistent with recent functional studies reporting cholinergic dilation of cerebral blood vessels mediated via a m5 receptor, have shown that both endothelial and muscular components of cerebral arteries synthesize and express a muscarinic m5 receptor. In view of the peculiar localization in cerebral vessels, handling of the muscarinic m5 receptor may be considered as an approach in the treatment of cerebrovascular disease.

The vasorelaxant effect of pituitary adenylate cyclase activating polypeptide and vasoactive intestinal polypeptide in isolated rat basilar arteries is partially mediated by activation of nitrergic neurons

The structurally related neuropeptides pituitary adenylate cyclase activating polypeptide (PACAP) and vasoactive intestinal polypeptide (VIP) are recognised by two G protein-coupled receptors, termed VPAC(1)-R and VPAC(2)-R, with equal affinity. PACAP and VIP have previously been shown to relax cerebral arteries in an endothelium-independent manner. The aim of the present study was to test if intramural neurons are involved in the mediation of PACAP/VIP-induced vasodilatory responses. Therefore, the vascular tone of isolated rat basilar arteries was measured by means of a myograph. The vasorelaxing effect of PACAP was assessed in arteries precontracted by serotonin in the absence or presence of different test compounds known to selectively inhibit certain signaling proteins. The vasorelaxant effect of PACAP could be significantly reduced by the inhibitor of neuronal N-type calcium channels omega-conotoxin GVIA (omega-CgTx), as well as by 3-bromo-7-nitroindazole (3Br-7-Ni), an inhibitor of the neuronal nitric oxide-synthase (nNOS). The localization of N-type calcium channels and VPAC-Rs within the rat basilar artery was investigated by confocal laser scanning microscopy using omega-CgTx- and VIP-analogs labelled with fluorescent dyes. These findings suggest that activation of intramural neurons may represent an important effector mechanism for mediation of the vasorelaxant PACAP-response.

Alpha7-nicotinic acetylcholine receptors on cerebral perivascular sympathetic nerves mediate choline-induced nitrergic neurogenic vasodilation

It has been suggested in isolated porcine cerebral arteries that stimulation by nicotine of alpha7-nicotinic acetylcholine receptors (alpha7-nAChRs) on sympathetic nerves, but not direct stimulation of parasympathetic nitrergic nerves, caused nitrergic neurogenic dilation. Direct evidence supporting this hypothesis has not been presented. The present study, which used in vitro tissue bath and confocal microscopy techniques, was designed to determine whether choline, a selective agonist for alpha7-nAChRs, induced sympathetic-dependent nitrergic dilation of porcine basilar arterial rings. Choline and several nAChR agonists induced exclusive relaxation of basilar arterial rings without endothelium. The relaxation was blocked by tetrodotoxin, nitro-L-arginine, guanethidine, and beta2-adrenoceptor antagonists. Furthermore, the relaxation was blocked by methyllycaconitine and alpha-bungarotoxin (preferential alpha7-nAChR antagonists) and mecamylamine but was not affected by dihydro-beta-erythroidine (a preferential alpha4-nAChR antagonist). Confocal microscopic study demonstrated that choline and nicotine induced significant calcium influx in cultured porcine superior cervical ganglionic cells but failed to affect calcium influx in cultured sphenopalatine ganglionic cells, providing direct evidence that choline and nicotine did not act directly on the parasympathetic nitrergic neurons. The increased calcium influx in superior cervical ganglionic cells was attenuated by alpha-bungarotoxin and methyllycaconitine but not by dihydro-beta-erythroidine. These results support our hypothesis that activation of alpha7-nAChRs on cerebral perivascular sympathetic nerves causes calcium influx and the release of norepinephrine, which then act on presynaptic beta2-adrenoceptors located on the neighboring nitrergic nerve terminals, resulting in NO release and vasodilation. Endogenous choline may play an important role in regulating cerebral sympathetic activity and vascular tone.