Evidence suggesting that the 5-HT2 antagonist ICI 169,369 activates vagal afferents and in addition has a central hypotensive action in anaesthetized rats

Central 5-HT(2A) receptors modulate the vagal bradycardia in response to activation of the von Bezold-Jarisch reflex in anesthetized rats

Activation of 5-hydroxytryptamine (5-HT) 5-HT(1A), 5-HT(2C), 5-HT(3), and 5-HT(7) receptors modulates the excitability of cardiac vagal motoneurones, but the precise role of 5-HT(2A/2B) receptors in these phenomena is unclear. We report here the effects of intracisternal (ic) administration of selective 5-HT(2A/2B) antagonists on the vagal bradycardia elicited by activation of the von Bezold-Jarisch reflex with phenylbiguanide. The experiments were performed on urethane-anesthetized male Wistar rats (250-270 g, N = 7-9 per group). The animals were placed in a stereotaxic frame and their atlanto-occipital membrane was exposed to allow ic injections. The rats received atenolol (1 mg/kg, iv) to block the sympathetic component of the reflex bradycardia; 20-min later, the cardiopulmonary reflex was induced with phenylbiguanide (15 µg/kg, iv) injected at 15-min intervals until 3 similar bradycardias were obtained. Ten minutes after the last pre-drug bradycardia, R-96544 (a 5-HT(2A) antagonist; 0.1 µmol/kg), SB-204741 (a 5-HT(2B) antagonist; 0.1 µmol/kg) or vehicle was injected ic. The subsequent iv injections of phenylbiguanide were administered 5, 20, 35, and 50 min after the ic injection. The selective 5-HT(2A) receptor antagonism attenuated the vagal bradycardia and hypotension, with maximal effect at 35 min after the antagonist (pre-drug = -200 ± 11 bpm and -42 ± 3 mmHg; at 35 min = -84 ± 10 bpm and -33 ± 2 mmHg; P < 0.05). Neither the 5-HT(2B) receptor antagonists nor the vehicle changed the reflex. These data suggest that central 5-HT(2A) receptors modulate the central pathways of the parasympathetic component of the von Bezold-Jarisch reflex.

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

Functional studies have provided evidence that 5-HT(3) ion-channel receptors (5-HT(3)Rs) on vagal cardiopulmonary afferents mediating the Bezold-Jarisch reflex (BJR) rapidly desensitize upon repeated exposure to selective 5-HT(3)R agonists. G-protein-coupled 5-HT(2) receptors (5-HT(2)Rs) also exist on vagal afferents, although activation of these receptors does not elicit the BJR. However, there is in vivo evidence that 5-HT(2)Rs may regulate the activity of 5-HT(3)Rs. The aim of this study was to determine whether co-activation of 5-HT(2)Rs prevents desensitization of 5-HT(3)Rs mediating the BJR in conscious rats. The principal findings were that (1) tachyphylaxis rapidly developed to the BJR-mediated hemodynamic responses elicited by successive injections of 5-HT(3)R agonists and (2) co-injection of the selective 5-HT(2)R agonist, alpha-methyl-5-HT, prevented tachyphylaxis to the BJR-mediated hemodynamic responses elicited by the 5-HT(3)R agonists. Additional studies provided evidence that (1) tachyphylaxis to the 5-HT(3)R agonists was not due to impairment of the central or efferent processing of the BJR, and (2) the pressor responses elicited by alpha-methyl-5-HT were not responsible for preventing tachyphylaxis to the BJR reflex responses elicited by 5-HT(3)R agonists. These results suggest that the loss of response to 5-HT(3)R agonists is due to desensitization of 5-HT(3)Rs on vagal afferents mediating the BJR and that co-activation of 5-HT(2)Rs prevents the desensitization of these 5-HT(3)Rs.

Endogenous nitrosyl factors may inhibit the desensitization of 5-HT3 receptors on vagal cardiopulmonary afferents

The pronounced tachyphylaxis to the Bezold-Jarisch reflex (BJR) responses elicited by systemic injections of the 5-HT(3) receptor (5-HT(3)R) agonists such as phenylbiguanide (PBG) may involve desensitization and/or reduced rate of resensitization of 5-HT(3)Rs on vagal cardiopulmonary afferents. The presence of nitric oxide synthase (NOS) in vagal afferents raises the possibility that endogenous nitrosyl factors regulate the status of 5-HT(3)Rs in these afferents. Accordingly, the aim of this study was to determine whether the inhibition of NOS alters the development of tachyphylaxis to the BJR responses elicited by PBG in conscious rats. The first injection of PBG (100 microg/kg, i.v.) elicited robust reductions in heart rate (HR), diastolic arterial blood pressure (BP(D)), and cardiac output (CO) but minor changes in total peripheral resistance in saline-treated rats. Subsequent injections elicited progressively smaller responses such that the sixth injections elicited minor responses only. The first injection of PBG (100 microg/kg, i.v.) in rats treated with the NOS inhibitor, L-NAME (25 micromol/kg, i.v.) elicited similar reductions in HR, BP(D), and CO as in saline-treated rats. However, the rate of development of tachyphylaxis to PBG was markedly faster in the L-NAME-treated rats. The BJR responses elicited by 5-HT (40 microg/kg, i.v.) were markedly attenuated after the development of tachyphylaxis to PBG in saline- and in L-NAME-treated rats whereas the BJR responses elicited by the S-nitrosothiol, L-S-nitrosocysteine (5 micromol/kg, i.v.), were not attenuated in either group. These findings suggest that tachyphylaxis to PBG was not due to the loss of central or efferent processing of the BJR. Taken together, these findings suggest NOS exists in vagal cardiopulmonary afferents mediating the BJR and that nitrosyl factors influence 5-HT(3)R function.

Functional evidence for the rapid desensitization of 5-HT(3) receptors on vagal afferents mediating the Bezold-Jarisch reflex

The aim of this study was to determine whether 5-hydroxytryptamine (5-HT)(3) receptors on cardiopulmonary afferents mediating the Bezold-Jarisch reflex (BJR) desensitize upon repeated exposure to selective agonists. BJR-mediated falls in heart rate, diastolic arterial blood pressure and cardiac output elicited by the 5-HT(3)-receptor agonists, phenylbiguanide (100 microg/kg, i.v.) or 2-methyl-5-HT (100 microg/kg, i.v.), progressively diminished upon repeated injection in conscious rats. The BJR responses elicited by 5-HT (40 microg/kg, i.v.) were markedly reduced in rats which had received the above injections of phenylbiguanide or 2-methyl-5-HT whereas the BJR responses elicited by L-S-nitrosocysteine (10 micromol/kg, i.v.) were similar before and after the injections of the 5-HT(3) receptor agonists. These findings suggest that tachyphylaxis to 5-HT(3) receptor agonists may be due to the desensitization of 5-HT(3) receptors on cardiopulmonary afferents rather than the impairment of the central or peripheral processing of the BJR.

ICI 169,369, A 5-HT2/5-HT1C antagonist, that can evoke endothelium-dependent relaxation in rabbit aorta

1. The direct effects of ICI 169,369 on vascular reactivity were investigated in rabbit aortic rings with and without endothelium. 2. ICI 169,369 evoked an endothelium-dependent relaxation in aortic rings precontracted with PGF2 alpha. No direct effects on vascular reactivity were found in endothelial denuded rings. 3. The relaxations induced by ICI 169,369 were inhibited by haemoglobin, an agent known to interfere with the responses to endothelium-derived nitric oxide (EDRF) but not by the cyclo-oxygenase inhibitor, indomethacin. Inhibition of the ICI 169,369-induced relaxation by the L-arginine analogue, NG-monomethyl L-arginine (L-NMMA) confirmed that the relaxations evoked by ICI 169,369 were mediated by the endothelial L-arginine: nitric oxide pathway. 4. Studies with competitive receptor antagonists showed that in the rabbit aorta, ICI 169,369 evoked relaxations which were not elicited by the activation of any known 5-HT1, 5-HT2, 5-HT3, muscarinic, histamine, adenosine receptor or adrenoceptors. 5. The diacylglycerol kinase inhibitor, R 59022 also failed to affect these relaxations. It is concluded that ICI 169,369 has a post-receptor action, possibly by directly affecting intracellular calcium levels in the endothelial cells.