Nociceptors in cardiovascular functions: complex interplay as a result of cyclooxygenase inhibition

Prostaglandins (PGs) are requisite components of inflammatory pain as indicated by the efficacy of cyclooxygenase 1/2 (COX1/2) inhibitors. PGs do not activate nociceptive ion channels directly, but sensitize them by downstream mechanisms linked to G-protein coupled receptors. Antiinflammatory effects are purported to arise from inhibition of synthesis and/or release of proinflammatory agents. Release of these agents from peripheral and central terminals of sensory neurons modulates nociceptive input from the periphery and synaptic transmission at the first sensory synapse, respectively. Heart and blood vessels are densely innervated by sensory nerve endings that express chemo-, mechano-, and thermo-sensitive receptors. Activation of these receptors mediates synthesis and/or release of vasoactive agents by virtue of their Ca2+permeability. In this article, we discuss that inhibition of COX2 reduces PG synthesis and renders beneficial effects by preventing sensitization of nociceptors, but at the same time, it might contribute to deleterious cardiovascular effects by compromising the synthesis and/or release of vasoactive agents.


Synthesis and functions of arachidonic acid and its metabolites
Arachidonic acid (AA) and its metabolites are involved in several important cardiovascular functions. In this article, we address the adverse cardiovascular effects that arise as a result of block of PG mediated modulation of nociceptive ion channels. AA is produced from membrane phospholipids by phospholipase A 2 (PLA 2 ), a calciumdependent enzyme, which is activated by proinflammatory agents and shear stress exerted on the vessel wall. Activation of phospholipase C (PLC) hydrolyzes phosphatidyl inositol 4, 5 bisphosphate (PIP 2 ) to inositol 1, 4, 5 trisphosphate (IP 3 ) and diacyl glycerol (DAG). DAG activates protein kinase C (PKC) and DAG lipase, activation of DAG lipase can in turn produce AA. Activation of phospholipase D produces anandamide, which can subsequently be converted to AA by fatty acid amide hydrolase [1].

Role of sensory innervation in the cardiovascular system
Noxious stimuli are transduced by peripheral nociceptors, which transmit nociceptive information to pain processing centers in the brain via the spinal cord. Heart and blood vessels are densely innervated by sensory nerve endings that express chemo-, mechano-, and thermo-sen-sitive receptors, which include acid sensitive ion channels (ASIC), degenerin/epithelial sodium channels (DEG/ ENAC), purinergic ATP gated ion channels (P2X), and transient receptor potential (TRP) channels [3][4][5][6][7]. Activation of nociceptive ion channels, particularly ASIC3 and TRPV1, has been implicated in ischemic cardiac pain [5]. Both these channels can be activated by acidic pH and sensitized by proinflammatory agents synthesized and/or released during ischemia.
Activation of Ca 2+ permeant nociceptive ion channels on the peripheral and central terminals of sensory neurons leads to the synthesis and/or release of a variety of proinflammatory agents and neuropeptides, like bradykinin (BK), PGs, calcitonin gene-related peptide (CGRP), substance P (SP), vasoactive intestinal peptide (VIP) and adenosine triphosphate (ATP) etc. [8,9]. Increases in intracellular Ca 2+ initiate several second messenger pathways, including activation of PLA 2 , PLC and Ca 2+ -depend-Schematic diagram showing the pathways involved in synthesis and metabolism of AA  ent kinases, which can lead to the generation of AA and its metabolites, release of Ca 2+ from intracellular stores, and phosphorylation of nociceptive receptors, respectively. BK is thought to be synthesized and released on demand from sympathetic nerve endings [11]. BK initiates prostanoid synthesis and mediates release of vasoactive neuropeptides [10,11]. PGE 2 and PGI 2 are produced in response to nociceptive stimuli and lead to inflammation and pain by sensitization of nociceptors. PGI 2 is a potent vasodilator and platelet deaggregator [12]. In blood vessels, activation of nociceptive receptors results in an endothelium independent vasodilatory response, which is mediated mainly by the release of CGRP [13]. CGRP is a potent vasodilator (coronary vasculature is particularly sensitive) that increases both heart rate and contractile force [13,14]. SP and VIP released from sensory nerve terminals induce vasodilation and positive chronotropic effect [15]. ATP is released ubiquitously along with neurotransmitters and induces vasoconstriction by activation of P2X receptors, however, its breakdown product adenosine is a potent vasodilator and also inhibits neurotransmitter/ neuropeptide release [16]. Relatively less prominent vasoactive agents are also released from the nociceptive nerve endings including galanin, corticotrophin-releasing factor, arginine, cholecystokinin-octapeptide, neuropeptide K, eledoisin-like peptide and bombesin-like peptides [14]. Nociceptor stimulation not only serves as a sensoryafferent, but also plays a significant role in sensory-efferent functions [8]. It has also been postulated that vascular regulation via an efferent mechanism could be independent of the sensory afferent function [17] and the selective synthesis and/or release of specific vasoactive agents could arise from the nature of the stimulus and/or its intensity [18]. Thus, activation of Ca 2+ permeable nociceptive ion channels at the peripheral and central terminals of sensory neurons can play an important role in the synthesis and/or release of vasoactive agents.

Nociceptive ion channels in cardiovascular system
Several nociceptive ion channels have been cloned. Most of these channels are modulated by PKA and PKC mediated phosphorylation. Significantly, PGE 2 and PGI 2 mediate their effects by activation of PKA and PKC pathways. The Transient Receptor Potential (TRP) channels (TRPVanilloid, TRPAnkyrin, TRPClassical, and TRPMelastatin) are chemo-, mechano-, and thermo-sensitive. TRPV1 is a well-characterized channel, which transduces heat in the noxious temperature range (>42°C) and is critical for inflammatory thermal sensation [19]. It is a Ca 2+ permeant polymodal receptor activated by protons, anandamide, lipoxygenase metabolites of AA, N-arachidonyl dopamine, capsaicin (an active ingredient in hot chilli peppers) and resiniferatoxin (RTX, an ultrapotent agonist obtained from the cactus, Euphorbia resinifera) [20]. TRPV1 is distributed in the heart and blood vessels and is sensitized by PGs via PKA and PKC mediated phosphorylation [21]. Importantly, in the phosphorylated state, the activation threshold of TRPV1 is reduced below body temperature rendering the channel constitutively active [20]. Furthermore, phosphorylation also promotes translocation of TRPV1 from the cytosol to the plasma membrane [22,23]. Activation of TRPV1 in sensory nerve endings supplying heart and blood vessels releases multiple vasoactive agents [14]. In diabetes, TRPV1 has been shown to be downregulated, which might contribute to the cardiovascular complications [23].
The role of TRPV1 in the cardiovascular system has been addressed: 1) Infusion of TRPV1 agonists significantly alters blood pressure, which could be mostly reversed by selective TRPV1 antagonists [24,25]; 2) Ablation of TRPV1 expressing C fiber terminals by capsaicin or resiniferatoxin (RTX) results in the loss of CGRP release, increased plasma renin activity, and an inability to control salt loading by the kidneys [14]; 3) Activation of TRPV1 or ASIC3 by protons during ischemia mediates a sympathoexcitatory reflex that is abolished by RTX treatment [5,26].

Inhibition of COX leads to increased metabolism of AA via LOX and CYP pathways. Products of LOX pathway (12-and 15-(S)-HPETE, 5-and 15-(S)-HETEs and LB 4 )
can directly gate TRPV1 [20]. Myogenic constriction in response to increased pressure on the intraluminal surface of blood vessels is mediated by the CYP byproduct 20-HETE, which directly activates TRPV1 and releases SP [27].
We propose that reduction of PG levels may contribute to deleterious vascular effects by decreasing sensitization of TRPV1 and subsequent reduction of CGRP and SP release. This possibility is supported by the finding that recovery from myocardial ischemia is compromised in TRPV1 knockout mice [28] and proton mediated CGRP release from the heart is mediated exclusively by TRPV1 [29,30]. From these studies it is clear that several nociceptive ion channels are modulated by activation of PKA and PKC, therefore, it is reasonable to expect that PGs coupled to these pathways would be able to sensitize the nociceptive ion channels. Thus, in our opinion, it is highly probable that the block of PG synthesis by COX inhibitors affects the cardiovascular functions mediated by nociceptive ion channels (Fig. 2).

Advantages and disadvantages of selective inhibition of COX2
Although COX2 inhibitors have become popular, their analgesic effects are comparable to non-specific COX inhibitors [51]. The selectivity of COX2 inhibitors has a significant advantage of avoiding gastrointestinal side effects (VIGOR study) due to the preservation of PGE 2 levels and a reduction in the incidence of colon cancer by inhibition of PG-mediated angiogenesis [52][53][54]. The inducible nature of COX2 is claimed to have significant advantages because it is activated only at the sites of inflammation. In this regard, it is significant to note that atherosclerotic lesions are inflammatory in nature [55] and PGI 2 (vasodilator, platelet deaggregator and sensitizer of nociceptive receptors) is synthesized via COX2 activation as a necessary protective mechanism. Nonspecific COX inhibitors decrease production of both, PGI 2 and TxA 2 (platelet aggregator), thereby avoiding an imbalance between PGI 2 and TxA 2 levels [56]. In contrast, when COX2 is inhibited selectively, platelet aggregation by TxA 2 is intact, but at the same time PGI 2 induced platelet deaggregation is compromised, resulting in enhanced platelet aggregation [57]. Here, we propose that inhibition of PGE 2 and PGI 2 could also reduce sensitization of nociceptors and compromise release of potent vasodilators in response to ischemia, which could be critical in reversing hypoperfusion in conditions like myocardial ischemia. Indeed, injury-induced platelet activation is enhanced in PGI 2 receptor (IP) knock-out mice [58], whereas it is reduced in TxA 2 receptor (TP) knock-out mice [58]. These findings are consistent with patients treated with COX2 inhibitors suffering from higher incidence of MI and stroke as compared to naproxen treated patients [53,59,60]. A combination of a COX2 and a low dose of COX1 inhibitors (for example, 80 mgs of aspirin) may be Second messenger pathways that modulate nociceptive ion channels Figure 2 Second messenger pathways that modulate nociceptive ion channels. a beneficial strategy to prevent TxA 2 -mediated platelet aggregation. Furthermore, the need for platelet deaggregation becomes even more critical, given the lifetime risk of developing atrial fibrillation significantly increases over 40 years of age [61], which can initiate thromboembolism.

Concluding remarks and future directions
The beneficial effects of COX inhibitors are derived from their ability to inhibit synthesis of PGs. However, several important cardiovascular functions mediated by PGs are compromised, including direct vasodilation, vasoconstriction, and platelet aggregation/deaggregation. Herein, we propose that the ability of PGs to sensitize nociceptive ion channels involved in the release of potent vasoactive agents could also be compromised. A well-characterized receptor in this context is TRPV1, which is sensitized by PGs and its activation mediates the synthesis and/or release of vasoactive agents by virtue of its high Ca 2+ permeability. TRPV1 is currently being pursued as a potential target for the next generation of analgesics [31]. Use of COX inhibitors should be dictated objectively by understanding the mechanisms by which cardiovascular complications are induced, instead of being swayed by emotional testimonies in congressional inquires. Drug industries would be better advised to invest in research rather than spending billions (3 billion in 2004) in advertising and direct marketing to patients. Judicious use of these drugs with open dialogue between drug industries, physicians and patients must be encouraged, so that all the parties involved can make an informed decision, fully aware of the consequences. Patients who are in the right category would benefit from these drugs, while sparing others who are at a risk for cardiovascular complications. This strategy/approach will also avoid expensive class action lawsuits and prevent driving the cost of medication higher; otherwise, patients who need the medication most may not be able to afford.