Tachyphylaxis to 5-HT3-receptor-mediated activation of vagal afferents is prevented by co-activation of 5-HT2 receptors

Infrared light elicits endothelium-dependent vasodilation in isolated occipital arteries of the rat via soluble guanylyl cyclase-dependent mechanisms

The left and right occipital arteries provide blood supply to afferent cell bodies in the ipsilateral nodose and petrosal ganglia. This supply is free of an effective blood-ganglion barrier, so changes in occipital artery blood flow directly affect the access of circulating factors to the afferent cell bodies. The application of infrared (IR) light to modulate neural and other cell processes has yielded information about basic biological processes within tissues and is gaining traction as a potential therapy for a variety of disease processes. To address whether IR can directly modulate vascular function, we performed wire myography studies to determine the actions of IR on occipital arteries isolated from male Sprague-Dawley rats. Based on our previous research that functionally-important differences exist between occipital artery segments close to their origin at the external carotid artery (ECA) and those closer to the nodose ganglion, the occipital arteries were dissected into two segments, one closer to the ECA and the other closer to the nodose ganglion. Segments were constricted with 5-hydroxytryptamine to a level equal to 50% of the maximal response generated by the application of a high (80 mM) concentration of K+ ions. The direct application of pulsed IR (1,460 nm) for 5 s produced a rapid vasodilation in occipital arteries that was significantly more pronounced in segments closest to the ECA, although the ECA itself was minimally responsive. The vasodilation remained for a substantial time (at least 120 s) after cessation of IR application. The vasodilation during and following cessation of the IR application was markedly diminished in occipital arteries denuded of the endothelium. In addition, the vasodilation elicited by IR in endothelium-intact occipital arteries was substantially reduced in the presence of a selective inhibitor of the nitric oxide-sensitive guanylate cyclase, 1H-[1,2,4]oxadiazolo [4,3-a]quinoxalin-1-one (ODQ). It appears that IR causes endothelium-dependent, nitric-oxide-mediated vasodilation in the occipital arteries of the rat. The ability of IR to generate rapid and sustained vasodilation may provide new therapeutic approaches for restoring or improving blood flow to targeted tissues.

Thyrotropin-releasing hormone induces Ca2+ increase in a subset of vagal nodose ganglion neurons

Thyrotropin-releasing hormone (TRH) plays a central role in metabolic homeostasis, and single-cell sequencing has recently demonstrated that vagal sensory neurons in the nodose ganglion express thyrotropin-releasing hormone receptor 1 (TRHR1). Here, in situ hybridization validated the presence of TRHR1 in nodose ganglion (NG) neurons and immunohistochemistry showed that the receptor is expressed at the protein level. However, it has yet to be demonstrated whether TRHR1 is functionally active in NG neurons. Using NG explants transduced with a genetically encoded Ca²⁺ indicator (GECI), we show that TRH increases Ca²⁺ in a subset of NG neurons. TRH-induced Ca²⁺ transients were briefer compared to those induced by CCK-8, 2-Me-5-HT and ATP. Blocking Na⁺ channels with TTX or Na⁺ substitution did not affect the TRH-induced Ca²⁺ increase, but blocking Gq signaling with YM-254890 abolished the TRH-induced response. Field potential recordings from the vagus nerve in vitro showed an increase in response to TRH, suggesting that TRH signaling produces action potentials in NG neurons. These observations indicate that TRH activates a small group of NG neurons, involving Gq pathways, and we hypothesize that these neurons may play a role in gut-brain signaling.

Relationship between male hormonal status, Bezold–Jarisch reflex function, and ACE activity (cardiac and plasmatic)

The negative relationship between androgens and the Bezold–Jarisch reflex (BJR) has been demonstrated, but no studies evaluated the physiological influence of testosterone on this reflex. We evaluated the influence of male rat castration on the BJR, cardiac morphometric parameters, and the plasmatic and the cardiac angiotensin converting enzyme (ACE) activity. After castration (CAS), the rats were divided into 24 and 72 h (CAS24H, CAS72H), and 7 and 21 days (CAS7D, CAS21D) groups. The BJR was studied by administering increasing doses of phenylbiguanide (PBG; 1.5–24 μg/kg) at different times after castration. Castration results in the following: (i) reduction in testosterone levels (SHAM: 238.7 ± 15.1; CAS24H: 9.0 ± 0.5; CAS72H: 6.7 ± 0.4; CAS7D: 5.2 ± 0.2; and CAS21D: 2.2 ± 0.3 ng/dL; p < 0.05); (ii) no changes in 17β-estradiol; (iii) a reduced BJR sensitivity (PBG 6 μg/kg; SHAM: 77 ± 7; CAS24H: 63 ± 10; CAS72H: 55 ± 6; CAS7D: 54 ± 4; and CAS21D: 35 ± 2%; p < 0.01); (iv) a decrease in cardiac (SHAM: 107 ± 6; CAS24H: 92 ± 2; CAS72H: 82 ± 3; CAS7D: 54 ± 3; and CAS21D: 43 ± 4%; p < 0.05) and plasmatic (SHAM: 135 ± 8; CAS24H: 102 ± 5; CAS72H: 99 ± 3; CAS7D: 89 ± 4; and CAS21D: 56 ± 6%; p < 0.05) ACE activity. No changes were observed in cardiac morphometry and hemodynamic parameters. Therefore, castration leads to decrease in testosterone levels as early as 24 h, reduction in ACE activity and loss of BJR sensitivity 7 days after castration. The loss of BJR sensitivity was not related to cardiac morphometric changes and cardiovascular hemodynamics.

Coronary Vasospasm and Atrial Fibrillation Associated with Ondansetron Therapy

Objective:

To provide further evidence of cardiovascular adverse effects of ondansetron, including new-onset atrial fibrillation, ST segment elevation, and chest pain subsequent to ondansetron administration, and to review cardiovascular adverse events related to several 5-hydroxytryptamine type 3 (5-HT3) receptor antagonists.

Case Summary:

A 51-year-old male with an uncomplicated past medical history was admitted for an elective inguinal hernia repair and septoplasty. His maintenance medications were discontinued prior to surgery. After a second 4-mg dose of intravenous ondansetron was administered, he developed nausea and diaphoresis. His electrocardiograph revealed new-onset atrial fibrillation and inferolateral ST segment elevation with ST segment alternans. During emergent cardiac catheterization, no obstructive coronary artery disease was evident. The patient's heart rhythm was electrically converted to normal sinus rhythm. During 3 years of follow-up, he has had no return of chest pain or hypotension.

Discussion:

Although considered a safe class of medications by many clinicians, several of the 5-HT3 receptor antagonists have been associated with serious cardiovascular effects. Three case reports described cardiac dysrhythmias and 9 documented coronary vasospasm and chest pain, possibly resulting from ondansetron. This is the first reported case of a combination of hypotension, atrial fibrillation, ST segment elevation, and chest pain following ondansetron administration after elective surgery in a healthy adult male with a nonconfounding medication profile. The Naranjo probability scale indicated that ondansetron was the probable cause of these cardiovascular events.

Conclusions:

This case report supports the concern regarding cardiovascular adverse effects of ondansetron. Clinicians should be aware of cardiovascular adverse reactions that may be associated with intravenous ondansetron and monitor for electrocardiographic changes as indicated. Further investigation is needed to delineate the actual incidence of cardiovascular effects associated with ondansetron and whether the intravenous rate of administration is a contributing factor.

Flavin adenine dinucleotide may release preformed stores of nitrosyl factors from the vascular endothelium of conscious rats

This study determined whether flavin adenine dinucleotide (FAD) may elicit vasodilation in conscious rats via release of preformed endothelium-derived nitrosyl factors. Injections 1-6 (inj(1-6)) of FAD (2.5 micromol/kg, IV) elicited pronounced and equivalent vasodilator responses in saline-treated rats. Inj(1) of FAD elicited pronounced vasodilation in L-NAME-treated rats pretreated with the nitric oxide (NO) synthesis inhibitor, NG-nitro-L-arginine (L-NAME; 50 micromol/kg, IV), whereas Inj(2-6) elicited progressively smaller responses such that inj(6) elicited minor responses. The vasodilator responses elicited by the endothelium-dependent agonist, acetylcholine, were markedly attenuated in L-NAME-treated rats that had received inj(1-6) of FAD but not in saline-treated rats that had received inj(1-6) of FAD. The vasodilator actions of L-S-nitrosocysteine and the NO donor, sodium nitroprusside, were not diminished after the injections of FAD in saline- or in L-NAME-treated rats. Binding studies demonstrated that the densities of muscarinic M3 receptors were increased in thoracic aorta endothelium of rats treated with L-NAME + inj(1-6) of saline or L-NAME + inj(1-6) of FAD as compared to rats treated with saline + inj(1-6) of saline or saline + inj(1-6) of FAD. The progressive loss of response to injections of FAD in L-NAME-treated rats coupled with the loss of response to acetylcholine suggests that FAD elicits the use-dependent depletion of vesicular pools of nitrosyl factors in endothelial cells that cannot be replenished in the absence of NO synthesis.

5-HT activates vagal afferent cell bodies in vivo: Role of 5-HT2 and 5-HT3 receptors

Occipital artery (OA) injections of 5-HT elicit pronounced reductions in heart rate and mean arterial blood pressure (MAP) in urethane-anesthetized rats by activation of vagal afferent cell bodies in the ipsilateral nodose ganglion. In contrast, internal carotid artery (ICA) and i.v. injections elicit similar cardiovascular responses by activation of peripheral vagal afferent terminals. The aim of this study was to examine the roles of 5-HT3 and 5-HT2 receptors in the 5-HT-induced activation of vagal afferent cell bodies and peripheral afferent terminals in urethane-anesthetized rats. OA, ICA and i.v. injections of 5-HT elicited dose-dependent reductions in heart rate and MAP that were virtually abolished after i.v. administration of the 5-HT3 receptor antagonists, MDL 7222 or ICS 205-930. The responses elicited by the OA injections of 5-HT were markedly diminished after i.v. injection of the 5-HT2 receptor antagonists, xylamidine or ketanserin, whereas the responses elicited by i.v. or ICA injections of 5-HT were not affected. The present findings suggest that (1) 5-HT3 and 5-HT2 receptor antagonists gain ready access to nodose ganglion cells upon i.v. administration, and (2) functional 5-HT3 and 5-HT2 receptors exist on the cell bodies of vagal afferent neurons mediating the cardiovascular responses elicited by OA injections of 5-HT. These findings also support a wealth of evidence that 5-HT3 receptors exist on the peripheral terminals of vagal afferents, and although they do not discount the possibility that 5-HT2 receptors exist on peripheral vagal afferent terminals, it appears that activation of these receptors does not have pronounced effects on 5-HT3 receptor activity on terminals that mediate the hemodynamic responses to 5-HT.