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Introduction

Calcitonin gene-related peptide (CGRP) is the first neuropeptide synthesized by gene recombination and biosynthesis that consists of 37 amino acids and is classified into two subtypes, α-CGRP and β-CGRP, which are encoded by the calcitonin/α-CGRP and β-CGRP genes, respectively (1,2). CGRP is a neurotransmitter of the capsaicin-sensitive sensory nerve, which is synthesized in neuronal cells and stored at the nerve endings (1). The transient receptor potential channel vanilloid type 1 (TRPV1), also known as vanilloid receptor subtype 1 (VR1), is a critical receptor that regulates the synthesis and release of CGRP (3). Given that the binding site of TRPV1 is located in the inner side of the cell membrane, its endogenous ligand anandamide is only active following intracellular transport (4). CGRP has exhibited extensive and complex biological activity. For example, in addition to its involvement in neuralgia and migraine, CGRP can regulate vascular tone, maintain the balance of circulation, attenuate ischaemic injury and inhibit cardiac fibroblast proliferation, as well as cardiac remodelling (5). Furthermore, CGRP plays a positive role in protecting the gastric mucosa in the gastrointestinal system (6,7). It has been reported that CGRP participates in the pathophysiological processes of several cardiovascular diseases, including hypertension, myocardial infarction, heart failure and pulmonary hypertension (5). Exogenous CGRP has been proven to be effective in the treatment of hypertension, pulmonary hypertension, acute lung injury, cerebral/cardiac ischaemia-reperfusion injury and chronic heart failure, suggesting that promoting endogenous synthesis and release of CGRP may be a novel direction of drug research and development (8–11). The present review summarizes the existing studies focusing on CGRP-mediated pharmacological effects of drugs, as presented in Table I.

Table I. Pharmacological effects of drugs that regulate CGRP function.

Table I.

Pharmacological effects of drugs that regulate CGRP function.

Drug	Pharmacological effect	Molecular mechanism	(Refs.)
Nitroglycerine	Anti-angina	ALDH→ NO↑→ CGRP↑	(16,17,23)
	Anti-myocardial ischaemia		(30)
	Treatment for chronic cardiac insufficiency		(34)
	Tolerance of nitroglycerine	ROS→ALDH2↓→NO↓→CGRP↓	(36)
Sartan	Anti-hypertension	Ang-II activating AT1↓→CGRP↑	(45–47)
Rutaecarpine	Anti-hypertension	TRPV1→Ca2+↑→CGRP↑	(52,54,55)
	Anti-pulmonary hypertension		(51)
	Anti-ventricular remodeling		(51,62)
	Protection of the gastric mucosa	TRPV1→CGRP↑→Gastric acid↓/K-ATP↑	(49,70,71)

[i] ALDH, aldehyde dehydrogenase; NO, nitric oxide; CGRP, calcitonin gene-related peptide; ROS, reactive oxygen species; ALDH2, aldehyde dehydrogenase 2; Ang-II, angiotensin II; AT1, angiotensin receptor 1; TRPV1, transient receptor potential channel vanilloid type 1; K-ATP, ATP-sensitive potassium.

CGRP mediates the pharmacological effect of nitroglycerine

Nitroglycerine, a classic anti-angina drug that is extensively applied in clinical settings, can be used for treating and alleviating chronic cardiac insufficiency (12). Previous studies have demonstrated that CGRP is involved in nitroglycerine treatment of myocardial ischaemia and chronic cardiac insufficiency (13,14).

Anti-angina effect of nitroglycerine

Nitroglycerine decreases cardiac blood volume, ventricular wall tension and myocardial oxygen consumption by dilating venules, which also dilates the coronary artery and increases the blood supply of the myocardial ischaemic area to attenuate angina pectoris (15). Nitroglycerine produces nitric oxide (NO) under the catalysis of aldehyde dehydrogenase (ALDH), and NO mediates the vasodilation of nitroglycerine (16). Previous studies have confirmed the key role of CGRP in mediating the inhibitory effect of nitroglycerine on angina. The results of the vessel ring ex vivo experiment demonstrated that the vasodilation effect of nitroglycerine can be reversed by CGRP receptor antagonists and capsaicin (17–19). In vivo, pre-treatment with capsaicin can also neutralize the hypotensive effect of nitroglycerine as it can deplete CGRP (20). An experiment was designed to assess whether the vasodilation effect associated with nitroglycerine was mediated by CGRP. Based on the polymorphisms of the ALDH gene, 40 unrelated male healthy volunteers with a mean age of 28 years (25 to 32 years) were enrolled, screened and stochastically divided into two groups according to their ALDH genotype, as follows: The ALDH2*1/*1 group (nine individuals, normal wild-type homozygote, ALDH 504 is glutamic acid, with high enzyme activity) and the ALDH2*2 group (nine individuals, carrying a mutant allele, ALDH 504 is lysine, resulting in reduced enzyme activity). Following treatment with nitroglycerine, the ALDH2*1/*1 group exhibited a distinct blood pressure drop and a significant increase in the concentration of CGRP in the blood, while the ALDH2*2 group displayed less changes in these two parameters (21). An additional study suggested that increased CGRP expression ultimately accelerates the antithrombotic effect of nitroglycerine (22). Collectively, these findings support the hypothesis that nitroglycerine releases NO to increase the concentration of CGRP, and subsequently plays an antagonistic role in the occurrence and development of angina (23).

Protective effects of nitroglycerine on ischaemic myocardium

Ischaemia-reperfusion injury is regarded as an aggravated injury in an ischaemic heart resulting from reperfusion following a period of ischaemia (24). Ischaemic preconditioning refers to the phenomenon by which transient ischaemia, prior to heart ischaemia, partially alleviates cardiac damage for 5–30 min (25). The protection of ischaemic preconditioning is mediated by endogenous active substances expressed due to ischaemic stimulation, such as adenosine and CGRP, which are also considered endogenous myocardial protective substances since they are released by the heart (26). It has been reported that transient cardiac ischaemia increases CGRP expression, while pretreatment with either a CGRP receptor blocker or capsaicin can offset cardiac defence mediated by ischaemic preconditioning, including the improvement of cardiac function, reduction in the myocardial infarction area and decreased creatine kinase release (27). The protective effect of CGRP on ischaemic myocardium may be associated with the repair of injured cardiomyocytes, the suppression of active oxygen species induced by hypoxia/reoxygenation and the balance of the mitochondrial membrane potential (28). CGRP induced by aerobic exercise has also been demonstrated to be protective in ischaemic myocardium (29). Considering that TRPV1 is temperature-sensitive, previous studies assessed whether the protection of ischaemic myocardium induced by high-temperature pre-adaptation was potentially associated with the induction of CGRP release. The experimental results demonstrated that both a CGRP receptor blocker and capsaicin can counteract the ischaemic myocardial protection associated with high-temperature pre-adaptation (19). Given that nitroglycerine can release GCRP, the hypothesis that pretreatment with nitroglycerine can imitate ischaemic preconditioning and serve a protective role in the ischaemic heart was investigated. These processes have only been demonstrated in vivo and in vitro (30). The results demonstrated that CGRP serum levels are significantly decreased in diabetic rats with acute myocardial ischaemia-reperfusion, and exogenous CGRP can recover myocardial cell damage from high glucose and ischaemia-reperfusion (31). Paeoniflorin, a pinane monoterpene glucoside extracted from the root of paeonia lactiflora pall, has been demonstrated to provide heart protection in diabetic mice by activating the TRPV1/calmodulin-dependent protein kinase/cAMP response element binding protein/CGRP signaling pathway (32).

Treatment of chronic cardiac insufficiency with nitroglycerine

Chronic cardiac insufficiency, also known as chronic heart failure, is a complicated syndrome induced by either ventricular filling or ejection capacity insufficiency, which stems from organic causes or functional cardiac diseases. Given that the pathological process of chronic cardiac insufficiency is partly associated with haemodynamic changes, drugs can improve cardiac function by dilating blood vessels (arteries and veins) and affecting haemodynamics (33). Nitrate esters currently play a significant role in the treatment of chronic cardiac insufficiency. By dilating the venules, nitroglycerine decreases the cardiac blood volume and ventricular wall tension, and strengthens the left ventricular compliance, myocardial contractility and cardiac output, ultimately improving cardiac function (17,34). A previous study demonstrated that nitroglycerine can be used in the treatment of chronic cardiac insufficiency by stimulating the CGRP signaling pathway. According to the gene type, patients were divided into two groups, as follows: The ALDH2*1/*1 group and the ALDH2*2 group. Following treatment with nitroglycerine, the experimental data indicated that the levels of CGRP in the ALDH2*1/*1 homozygote group were significantly increased, whereas the patients in this group exhibited apparent improvements in both blood pressure and left ventricular ejection fraction compared with those in the ALDH2*2 mutant group (34).

Tolerance to nitroglycerine

The underlying molecular mechanism of the tolerance to nitroglycerine following its continuous application remains unclear. Previous studies have concluded that nitroglycerine tolerance is associated with thiol (-SH) consumption of tissue cells, since the application of drugs containing -SH, such as captopril or N-acetylcysteine, can partially reverse the tolerance to nitroglycerine (35). Previous studies have reported that the tolerance may be associated with nitroglycerine-induced oxidative stress (generation of active oxygen species), which restrains the activity of ALDH2, the release of NO and eventually attenuates the vasomotor effect (36). In addition, CGRP mediates the vasodilation effect of nitroglycerine, resulting in tolerance from a secondary decrease in CGRP caused by NO reduction (37). Previous in vivo and in vitro experiments have demonstrated that when nitroglycerine is tolerated, the levels of CGRP and the vasodilation effect decrease simultaneously, whereas pretreatment with drugs containing -SH can markedly increase the levels of NO and CGRP, partially reversing the tolerance (23,36,37). A recent study demonstrated that nitroglycerine tolerance may intensify the anti-ischaemic effect and cardiovascular mortality in rats, whereas exposure to a CGRP antagonist (CGRP8-37) may improve this condition (38).

CGRP-mediated antihypertensive effect of sartans

It is widely accepted that the occurrence and development of hypertension is strongly associated with peripheral vascular resistance. CGRP induces vascular relaxation by decreasing the peripheral vascular resistance. Thus, suppressing CGRP synthesis and release can promote the development of hypertension (39). A previous study demonstrated that α-CGRP knockout animals exhibit higher basal blood pressure (40). In addition, both phenol-induced hypertension rats and spontaneous hypertension rats display lower levels of CGRP in the blood (4,41). Clinical studies have reported that CGRP levels in the blood of patients with essential hypertension and pregnancy hypertension are significantly lower than those in healthy subjects (4). Recently, it has been demonstrated that endogenous α-CGRP induced by physical activity can protect cardiac function during treatment of mice with chronic hypertension (42).

Released CGRP is regulated by several endogenous active substances, including NO, bradykinin, neuropeptide Y and angiotensin II (Ang II). Ang II inhibits CGRP release by activating anterior membrane angiotensin receptor 1 (AT1) (43). Sartans are AT1 receptor blockers, which can prevent Ang II from contracting blood vessels leading to the stimulation of aldosterone secretion. In addition, they can decrease peripheral vascular resistance and lower blood pressure (44). Thus, it was hypothesized that sartans can promote CGRP release by blocking the AT1 receptor since Ang II can activate the AT1 receptor and suppress the release of CGRP.

Previous studies have confirmed that while sartans decrease the blood pressure of spontaneously hypertensive rats, CGRP expression is significantly increased in the blood, heart, kidney and dorsal root ganglion (45,46). In addition, a clinical trial demonstrated that following treatment with the Ang II type 1 receptor blocker olmesartan, the systolic and diastolic blood pressure levels of patients with hypertension were normalized, and CGRP levels also increased (47). Taken together, these findings support the hypothesis that the molecular mechanism of blood pressure reduction of sartans may also be associated with the CGRP pathway.

CGRP-mediated pharmacological effect of rutaecarpine

Rutaecarpine is an active component extracted from the fruit of Euodia rutaecarpa, a traditional Chinese medicine used for the treatment of hypertension. It mediates an extensive range of pharmacological effects, including vasodilation, reduction in myocardial ischaemic injury, inhibition of thrombosis and protection of the gastric mucosa (48,49). A previous study suggested that rutaecarpine stimulates the synthesis and release of CGRP by activating the capsaicin receptor (50).

CGRP-mediated antihypertensive effect of rutaecarpine

A previous demonstrated that the CGRP receptor blocker, CGRP8-37 can eliminate the positive inotropic force and the positive frequency effect of rutaecarpine in the isolated cavy heart (51). In addition, it was indicated that rutaecarpine promotes the release of CGRP in a concentration-dependent manner and that exposure to capsazepine, a TRPV1 receptor antagonist, suppresses CGRP expression (52). TRPV1 receptor blockers have been demonstrated to counteract the vasodilation effects of rutaecarpine, suggesting that rutaecarpine protects the cardiovascular system by releasing CGRP through the activation of the TRPV1 receptor (3). As a vasodilator neurotransmitter, CGRP is tightly associated with the occurrence and development of hypertension (53). Thus, it is reasonable to assume that CGRP can mediate the antihypertensive effect of rutaecarpine since the latter promotes the release of CGRP and the stimulation of the TRPV1 receptor (52). Preclinical experiments have demonstrated that rutaecarpine elevates the synthesis and release of CGRP in both spontaneously hypertensive rats and phenol-induced hypertensive rats, whereas pretreatment with capsaicin eliminates the hypotensive effect of rutaecarpine by excessive binding to CGRP (54,55).

CGRP-mediated anti-pulmonary hypertension effect of rutaecarpine

Pulmonary hypertension is a chronic respiratory disease characterized by persistent pulmonary hypertension caused by pulmonary capillary occlusion. Despite that the pathogenesis of pulmonary hypertension remains unclear, it is universally accepted that the vascular remodeling caused by smooth muscle cell proliferation and migration is the main contributing factor to the development of this disease (56). CGRP expression in the blood has been decrease in both mature and newborn rats suffering from pulmonary hypertension (57). It has been reported that continuous hypoxia for 3 weeks can cause persistent pulmonary hypertension in neonatal rats, with a decrease in plasma CGRP expression (58). In addition, targeted knockout of the CGRP receptor gene has been demonstrated to aggravate hypoxia-induced pulmonary hypertension (59). CGRP suppresses the extracellular signaling-regulated kinase 1/2 pathway, which contributes to the inhibition of vascular remodeling through a vasodilation effect, and the control of the proliferation of vascular smooth muscle cells (5). A previous study demonstrated that exogenous CGRP can inhibit the proliferation of smooth muscle cells induced by hypoxia in a concentration-dependent manner (60). Adenoviral transfection of the CGRP gene significantly decreases the pulmonary vascular resistance of rats with hypoxia-induced pulmonary hypertension, improving pulmonary vascular remodelling (53). It has also been reported that transfection of CGRP into endothelial progenitor cells mitigates pulmonary hypertension and vascular remodelling (57). Given that the release of CGRP alleviates pulmonary hypertension, rutaecarpine may also relieve pulmonary hypertension due to its activating effect on CGRP. It has been demonstrated that rutaecarpine inhibits the pulmonary hypertension induced by monocrotaline or hypoxia in rats by elevating CGRP levels in the blood (51). A recent study discovered that rutaecarpine inhibits the Notch1/eIF3a signaling pathway in order to improve the synthesis and release of CGRP, alleviating pulmonary fibrosis and the epithelial-to-mesenchymal transition process (61).

CGRP-mediated inhibitory effect of rutaecarpine on cardiac remodeling

As a compensatory response to the increase in ventricular pressure, cardiac remodeling is usually accompanied by pathological changes, including cardiac hypertrophy and cardiac fibroblast proliferation. Cardiac fibroblasts account for 60–70% of the total number of cardiac cells and play an important role in cardiac remodeling. Under pathological conditions, cardiac fibroblasts proliferate and differentiate into myofibroblasts (62). However, this process acts in an autocrine manner and several active substances, such as Ang II, endothelin and inflammatory factors can further promote the proliferation of cardiac fibroblasts (63). CGRP has been demonstrated to inhibit the proliferation of cardiac fibroblasts and cardiac remodelling (51). In addition, exogenous CGRP has been reported to inhibit the senescence of cardiac fibroblasts and the development of cardiac fibrosis by increasing the expression levels of klotho (64). A previous study demonstrated that pressure loading (coarctation of the aortic arch) aggravates the left ventricular hypertrophy and fibrosis of calcitonin/α-CGRP gene knockout mice (65). TRPV1 gene knockout also exacerbates cardiac remodeling in mice (66). Another study demonstrated that rutaecarpine alleviates left ventricular remodeling induced by isoproterenol by activating CGRP, which regulates the expression of apoptosis-related genes, including Bax and Bcl2 (62). To investigate whether the potential molecular mechanism of rutaecarpine-mediated inhibition of cardiac remodeling acts via the CGRP signaling pathway, an experiment was designed to construct a right ventricular remodeling model in rats with pulmonary hypertension induced by monocrotaline and hypoxia. The results demonstrated that rutaecarpine affects the eIF3a/p27 signaling pathway by promoting CGRP release, eventually suppressing cardiac remodelling (51).

Protective effects of rutaecarpine, curcumin and capsaicin derivatives on gastric mucosa are mediated via CGRP

Gastrointestinal function is regulated by visceral sensory nerves (67). CGRP may play an important role in protecting the gastric mucosa, and is considered an essential neurotransmitter among the dorsal root ganglion and vagal ganglion of the splanchnic nerves (6,7,68). A previous study indicated that CGRP can significantly inhibit gastric acid secretion induced by gastrin in rats (69). In addition, CGRP improves gastric mucosal blood flow, which is partially associated with the ATP-sensitive potassium channel (70,71). It has been reported that CGRP alleviates mucosal injury induced by ischaemia-reperfusion by decreasing inflammation and apoptosis (7). In addition, the CGRP receptor antagonist, CGRP8-37 has been demonstrated to aggravate gastric mucosal injury induced by indomethacin (indomethacin) or ethanol (72,73). Notably, the increase in the healing time of gastric ulcer by the use of acetic acid and ethanol exacerbates gastric mucosal injury during stress in CGRP gene knockout animal models (74,75). Based on this evidence, TRPV1 may be used as a novel target to investigate drugs that can prevent and treat gastric mucosal injury, as demonstrated by the protective effect of CGRP on the gastric mucosa and the activation effect of TRPV1 on the synthesis and release of CGRP. Currently, several studies have reported that TRPV1 agonists, such as capsaicin, capsaicin ester, cannabinoid, curcumin and rutaecarpine can decrease gastric mucosal damage by promoting the release of CGRP (3,49,76–81). Indirect activation of CGRP induced by curcumin can mitigate gastrorrhagia in rats caused by 75% ethanol, which may be associated with the suppression of HIF-1α and Cdx-2, and the activation of HO-1 and SOD2 (82). A previous study demonstrated that pretreatment with rutaecarpine enhances both CGRP mRNA and protein expression, ameliorating ulcerative colitis in mice (83). An epidemiological study have demonstrated that Chinese subjects are three times more likely to suffer from gastric ulcers compared with Malaysian or Indian subjects, which may be associated with their habits of consuming higher amounts of pepper compared to those of other Asian populations (84). Rutaecarpine has been reported to upregulate endogenous CGRP expression by activating the VR1 signaling pathway, which results in the protection of rats with acute pancreatitis (85).

Discussion and perspective

Aforementioned studies have focused on neuronal CGRP, which acts as the transmitter of sensory nerves (Fig. 1) (39,43). However, it has been reported that CGRP also exists in non-nerve tissue cells, such as endothelial cells, endothelial progenitor cells, lymphocytes, bronchial epithelial cells and adipocytes, and plays an important role in regulating local tissues (86). The α- and β-subtypes of CGRP have been observed in endothelial cells and are regulated by TRPV1 to maintain physiological function of endothelial cells (87). A recent study suggested that cardiac fibroblasts synthesize and secrete CGRP, which suppresses their activation in an autocrine manner (88). Drugs such as clonidine, which is an α-receptor agonist regulating CGRP through a non-nerve pathway, have been demonstrated to influence the secretion of CGRP in endothelial cells by affecting its synthesis and secretion in local tissues (89). In addition, rutaecarpine delays cell senescence by promoting the expression and secretion of CGRP in endothelial progenitor cells (90). Thus, whether drugs can affect CGRP synthesis and secretion of other non-nerve cells remains to be investigated.

Figure 1. Schematic diagram describing the role of CGRP in the pharmacological effects of specific drugs. CGRP, calcitonin gene-related peptide; ROS, reactive oxygen species; ALDH, aldehyde dehydrogenase; NO, nitric oxide; TRPV1, transient receptor potential channel vanilloid type 1; Ang-II, angiotensin II; AT1, angiotensin receptor 1; CaMK, camodulin dependent protein kinase; CREB, cAMP-response element binding protein.

As the pivotal receptor affecting the release of CGRP, the TRP receptor contains several subtypes, including TRPC, TRPV, TRPM, TRPML, TRPP, TRPA and TRPN (91). Previous studies that assessed the regulation of CGRP synthesis and release focused on TRPV1, which is considered a significant target for drug discovery (54,91). Transient receptor potential ankyrin 1 (TRPA1) is localized in primary sensory dorsal root ganglion neurons. It was confirmed that H2S induced NO production and the subsequent activation of the neuroendocrine HNO-TRPA1-CGRP signaling pathway is the main cause of vasodilatory effects (92). A previous study demonstrated that cinnamaldehyde activates TRPA1 and inhibits hypoxia-induced cardiac fibrosis through a molecular mechanism that involves the upregulation of cardiac fibroblast-derived CGRP . Collectively, these studies suggest that TRPA1 may also be considered a novel target for drug discovery.

The data reported in the present review demonstrates that CGRP participates in the regulation of the function of multiple organs. It also mediates the pharmacological effects of marketed drugs, such as nitroglycerine and sartans. These studies not only improve our understanding on the therapeutic effects of marketed drugs, but also provide novel targets for drug discovery. Thus, targeting endogenous CGRP synthesis and release may be considered a novel direction for drug research and development.

Acknowledgements

Not applicable.

Funding

The present study was supported by grants from the National Natural Scientific Foundation of China (grant nos. 81703518 and 81973406), the Hunan Provincial Natural Scientific Foundation (grant nos. 2019JJ50849 and 2020JJ4823), the Scientific Research Project of Hunan Provincial Health and Family Planning Commission (grant no. B20180253) and the Fundamental Research Funds for the Central Universities of Central South University (grant no. 2020zzts822).

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Authors' contributions

WJM and YCY contributed equally in classifying the literature and drafting the manuscript. BKZ and YJL collected and analyzed the literature. WQL designed the framework of this article and critically revised the manuscript. All authors have read and approved the final manuscript.

Ethics approval and consent to participate

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Patients consent for publication

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Competing interests

The authors declare that they have no competing interests.

Glossary

Abbreviations

Abbreviations:

References