Localization and functional effects of adenosine A1 receptors on cardiac vagal afferents in adult rats

Pseudoanginal chest pain associated with vagal nerve stimulation: a case report

Background

Vagal nerve stimulation (VNS) can be an effective therapy for patients with epilepsy refractory to anti-epileptic drugs or intracranial surgery. While generally well tolerated, it has been associated with laryngospasm, hoarseness, coughing, dyspnea, throat and atypical chest pain, cardiac symptoms such as bradycardia and occasionally asystole. We report on a patient receiving vagal nerve stimulation who experienced severe typical anginal chest pain during VNS firing without any evidence of cardiac ischemia or dysfunction. Thus, the pain appeared to be neuropathic from the stimulation itself rather than nociceptive secondary to an effect on heart function.

Case presentation

A 29-year-old man, with a history of intractable frontal lobe epilepsy refractory to seven anti-epileptic medications and subsequent intracranial surgery, underwent VNS implantation without complications. On beginning stimulation, he began to have intermittent chest pain that corresponded temporally to his intermittent VNS firing. The description of his pain was pathognomonic of ischemic cardiac chest pain. On initial evaluation, he displayed Levine’s sign and reported crushing substernal chest pain radiating to the left arm, as well as shortness of breath walking upstairs that improved with rest. He underwent an extensive cardiac workup, including 12-lead ECG, cardiac stress test, echocardiogram, 12-day ambulatory cardiac monitoring, and continuous ECG monitoring each with and without stimulation of his device. The workup was consistently negative. Inability to resolve the pain necessitated the disabling and eventual removal of the device.

Conclusion

To our knowledge, this is the first report of pseudoanginal chest pain associated with VNS. This occurrence prompted our review of the mechanisms of cardiac chest pain and suggests that vagal afferents may convey anginal pain separately or in parallel with known spinal cord pain mechanisms. These insights into the physiology of chest pain may be of general interest and important to surgeons implanting VNS devices who may potentially encounter such symptoms.

Orally active adenosine A(1) receptor agonists with antinociceptive effects in mice

Adenosine A(1) receptor (A(1)AR) agonists have antinociceptive effects in multiple preclinical models of acute and chronic pain. Although numerous A(1)AR agonists have been developed, clinical applications of these agents have been hampered by their cardiovascular side effects. Herein we report a series of novel A(1)AR agonists, some of which are structurally related to adenosine 5'-monophosphate (5'-AMP), a naturally occurring nucleotide that itself activates A(1)AR. These novel compounds potently activate A(1)AR in several orthogonal in vitro assays and are subtype selective for A(1)AR over A(2A)AR, A(2B)AR, and A(3)AR. Among them, UNC32A (3a) is orally active and has dose-dependent antinociceptive effects in wild-type mice. The antinociceptive effects of 3a were completely abolished in A(1)AR knockout mice, revealing a strict dependence on A(1)AR for activity. The apparent lack of cardiovascular side effects when administered orally and high affinity (K(i) of 36 nM for the human A(1)AR) make this compound potentially suitable as a therapeutic.

Involvement of A1 adenosine receptors and neural pathways in adenosine-induced bronchoconstriction in mice

High levels of adenosine can be measured from the lungs of asthmatics, and it is well recognized that aerosolized 5'AMP, the precursor of adenosine, elicits robust bronchoconstriction in patients with this disease. Characterization of mice with elevated adenosine levels secondary to the loss of adenosine deaminase (ADA) expression, the primary metabolic enzyme for adenosine, further support a role for this ubiquitous mediator in the pathogenesis of asthma. To begin to identify pathways by which adenosine can alter airway tone, we examined adenosine-induced bronchoconstriction in four mouse lines, each lacking one of the receptors for this nucleoside. We show, using direct measures of airway mechanics, that adenosine can increase airway resistance and that this increase in resistance is mediated by binding the A(1) receptor. Further examination of this response using pharmacologically, surgically, and genetically manipulated mice supports a model in which adenosine-induced bronchoconstriction occurs indirectly through the activation of sensory neurons.

Involvement of peripheral adenosine A2 receptors in adenosine A1 receptor-mediated recovery of respiratory motor function after upper cervical spinal cord hemisection

BACKGROUND/OBJECTIVE

In an animal model of spinal cord injury, a latent respiratory motor pathway can be pharmacologically activated through central adenosine A1 receptor antagonism to restore respiratory function after cervical (C2) spinal cord hemisection that paralyzes the hemidiaphragm ipsilateral to injury. Although respiration is modulated by central and peripheral mechanisms, putative involvement of peripheral adenosine A2 receptors in functional recovery in our model is untested. The objective of this study was to assess the effects of peripherally located adenosine A2 receptors on recovery of respiratory function after cervical (C2) spinal cord hemisection.

METHODS

Respiratory activity was electrophysiologically assessed (under standardized recording conditions) in C2-hemisected adult rats with the carotid bodies intact (H-CBI; n=12) or excised (H-CBE; n=12). Animals were administered the adenosine A2 receptor agonist, CGS-21680, followed by the A1 receptor antagonist, 1,3-dipropyl-8-cyclopentylxanthine (DPCPX), or administered DPCPX alone. Recovered respiratory activity, characterized as drug-induced activity in the previously quiescent left phrenic nerve of C2-hemisected animals in H-CBI and H-CBE rats, was compared. Recovered respiratory activity was calculated by dividing drug-induced activity in the left phrenic nerve by activity in the right phrenic nerve.

RESULTS

Administration of CGS-21680 before DPCPX (n=6) in H-CBI rats induced a significantly greater recovery (58.5 +/- 3.6%) than when DPCPX (42.6 +/- 4.6%) was administered (n=6) alone. In H-CBE rats, prior administration of CGS-21680 (n=6) did not enhance recovery over that induced by DPCPX (n=6) alone. Recovery in H-CBE rats amounted to 39.7 +/- 3.7% and 38.4 + 4.2%, respectively.

CONCLUSIONS

Our results suggest that adenosine A2 receptors located in the carotid bodies can enhance the magnitude of adenosine A1 receptor-mediated recovery of respiratory function after C2 hemisection. We conclude that a novel approach of targeting peripheral and central adenosine receptors can be therapeutically beneficial in alleviating compromised respiratory function after cervical spinal cord injury.

Hypersensitivity of pulmonary C fibers induced by adenosine in anesthetized rats

Compelling clinical evidence implicates the potential role of adenosine in development of airway hyperresponsiveness and suggests involvement of pulmonary sensory receptors. This study was carried out to determine the effect of a low dose of adenosine infusion on sensitivity of pulmonary C-fiber afferents in anesthetized open-chest rats. Infusion of adenosine (40 microg x kg-1x min-1 i.v. for 90 s) mildly elevated baseline activity of pulmonary C fibers. However, during adenosine infusion, pulmonary C-fiber responses to chemical stimulants and lung inflation (30 cmH2O tracheal pressure) were markedly potentiated; e.g., the response to right atrial injection of capsaicin (0.25 or 0.5 microg/kg) was increased by more than fivefold (change in fiber activity = 2.64 +/- 0.67 and 16.27 +/- 3.11 impulses/s at control and during adenosine infusion, n = 13, P < 0.05), and this enhanced response returned to control in approximately 10 min. The potentiating effect of adenosine infusion was completely blocked by pretreatment with 8-cyclopentyl-1,3-dipropylxanthine (100 microg/kg), a selective antagonist of the adenosine A1 receptor, but was not affected by 3,7-dimethyl-1-propargylxanthine (1 mg/kg), an A2-receptor antagonist, or 3-ethyl-5-benzyl-2-methyl-4-phenylethynyl-6-phenyl-1,4-(+/-)-dihydropyridine-3,5-dicarboxylate (2 mg/kg), an A3-receptor antagonist. This potentiating effect was also mimicked by N6-cyclopentyladenosine (0.25 microg x kg-1 x min-1 for 90 s), a selective agonist of the adenosine A1 receptor. In conclusion, our results showed that infusion of adenosine significantly elevated the sensitivity of pulmonary C-fiber afferents in rat lungs and that this potentiating effect is likely mediated through activation of the adenosine A1 receptor.

Autoradiographic visualisation of axonal transport of adenosine A1 receptors along the rat vagus nerve and characterisation of adenosine A1 receptor binding in the dorsal vagal complex of hypertensive and normotensive rats

The present study had employed in vitro receptor autoradiography with [3H]DPCPX to visualise the presence of adenosine A1 receptors on the rat nodose ganglion, which contains the perikarya of vagal afferent neurons projecting the the nucleus tractus solitarius (NTS). In addition, unilateral vagal ligation resulted in an accumulation of [3H]DPCPX binding adjacent to the ligatures, indication that adenosine A1 receptors are subject to axoplasmic flow along the rat vagus nerve. Radioligand binding assays were utilised to characterise the properties of adenosine A1 receptors in the dorsal vagal complex (NTS, area postrema and dorsal motor nucleus of the vagus) of pup and adult normotensive (Wistar Kyoto, WKY) and hypertensive (spontaneously hypertensive, SHR) rats. Saturation binding indicated that the affinity (KD) of [3H]DPCPX, and the binding site density (Bmax) were not different between the adult WKY and SHR, although the pup SHR had a lower KD value than the pup WKY rat. Competition binding assays revealed complex differences between the two rat strains; however, with respect to hypertension, the affinity of the selective adenosine A1 agonist, cyclohexyladenosine (CHA), was markedly reduced in the membranes from SHR (Ki approximately 93 nM) compared to WKY (approximately 6 nM). Such an observation is consistent with the attenuated responses of SHRs to intra-NTS injections of adenosine.