Biphasic Renal Sympathetic Response to Hemorrhagic Hypotension in Mice

Differential engagement of inhibitory and excitatory cardiopulmonary reflexes by capsaicin and phenylbiguanide in C57BL/6 mice

The Bezold-Jarisch reflex is a powerful inhibitory reflex initiated by activation of cardiopulmonary vagal nerves during myocardial ischemia, hemorrhage, and orthostatic stress leading to bradycardia, vasodilation, hypotension, and vasovagal syncope. This clinically relevant reflex has been studied by measuring heart rate (HR) and mean arterial pressure (MAP) responses to injections of a variety of chemical compounds. We hypothesized that reflex responses to different compounds vary due to differential activation of vagal afferent subtypes and/or variable coactivation of excitatory afferents. HR and MAP responses to intravenous injections of the transient receptor potential vanilloid-1 (TRPV1) agonist capsaicin and the serotonin 5-HT3 receptor agonist phenylbiguanide (PBG) were measured in anesthetized C57BL/6 mice before and after bilateral cervical vagotomy. Capsaicin and PBG evoked rapid dose-dependent decreases in HR and MAP followed by increases in HR and MAP above baseline. Bezold-Jarisch reflex responses were abolished after vagotomy, whereas the delayed tachycardic and pressor responses to capsaicin and PBG were differentially enhanced. The relative magnitude of bradycardic versus depressor responses (↓HR/↓MAP) in vagus-intact mice was greater with capsaicin. In contrast, after vagotomy, the magnitude of excitatory tachycardic versus pressor responses (↑HR/↑MAP) was greater with PBG. Although capsaicin-induced increases in MAP and HR postvagotomy were strongly attenuated or abolished after administration of the ganglionic blocker hexamethonium, PBG-induced increases in MAP and HR were mildly attenuated and unchanged, respectively. We conclude that responses to capsaicin and PBG differ in mice, with implications for delineating the role of endogenous agonists of TRPV1 and 5-HT3 receptors in evoking cardiopulmonary reflexes in pathophysiological states.

Transient receptor potential vanilloid 1 mediates cocaine reinstatement via the D1 dopamine receptor in the nucleus accumbens

PURPOSE

The transient receptor potential vanilloid 1 (TRPV1) is a nonselective cation channel that mediates synaptic modification in the nucleus accumbens (NAc). However, no study has yet examined the mechanism of TRPV1 in the NAc on cocaine reinstatement. We investigated the mechanism of TRPV1 in NAc on cocaine reinstatement using the conditioned place preference (CPP) test in mice.

METHODS

We examined the effect of capsazepine (5 mg/kg, a TRPV1 antagonist, administered intraperitoneally (i.p.)), capsaicin (0.3 mg/kg, a TRPV1 agonist, administered i.p.), and genetic deletion of TRPV1 on the reinstatement of cocaine-induced CPP (15 mg/kg, administered i.p.). The expression of TRPV1 and Ca2⁺/calmodulin-mediated kinase II (CaMKII) in the NAc were determined after cocaine reinstatement. Microinjection of SB366791 (0.2 ng, a selective TRPV1 antagonist) in the NAc was assessed on SKF-81297 (1 µg, D1-like dopamine (DA) receptor agonist) primed cocaine reinstatement.

RESULTS

Capsazepine suppressed and capsaicin potentiated cocaine CPP in the reinstatement phase. In addition, genetic deletion of TRPV1 inhibited cocaine-priming reinstatement. Cocaine reinstatement was mediated by increased TRPV1 expression in the NAc, which involves CaMKII. Microinjection of SB366791 in the NAc prevented the cocaine reinstatement evoked by microinjection of SKF-81297 in the NAc.

CONCLUSIONS

These findings suggest that activation of TRPV1 mediates the stimulation of D1-like DA receptors and CaMKII in the NAc, resulting in the facilitation of cocaine reinstatement behaviors. Thus, our findings reveal a previously unknown TRPV1 mechanism in the reinstatement to drugs of abuse.

Involvement of median preoptic nucleus and medullary noradrenergic neurons in cardiovascular and sympathetic responses of hemorrhagic rats

The infusion of hypertonic saline solution (HSS) is known to be beneficial to the treatment of hypovolemic hemorrhage (HH). The central mechanism of HSS-induced cardiovascular and autonomic recovery of animals subjected to HH remains unclear. Hence, the present study evaluated the involvement of median preoptic nucleus (MnPO) and medullary noradrenergic neurons (A1 and A2) in HSS-induced cardiovascular and sympathetic responses in hemorrhagic rats. The wistar rats were subjected to specific lesion of noradrenergic neurons through the nanoinjections of anti-DβH-saporin into caudal ventrolateral medulla (A1 neurons) and nucleus of the solitary tract (A2 neurons). After recovery, mean arterial pressure (MAP) and renal sympathetic nervous activity were recorded. The HH was performed through blood withdrawal until a MAP of 60 mmHg was attained. In sham rats, HSS infusion (3M NaCl) reestablished MAP without change in HH-induced sympathoinhibition. The muscimol (agonist of GABAA receptor) was nanoinjected in MnPO during HH and MnPO inhibition abolished the recovery of MAP and HSS-induced sympathoinhibition. Simultaneous lesions of A1 and A2 abolished MAP restoration and sympathoinhibition after HSS infusion. These results suggest that the recovery of MAP and HSS-induced sympathoinhibition in hemorrhaged rats depend on intact neural projections from A1 and A2 to MnPO.

Mouse Anaphylactic Hypotension Is Characterized by Initial Baroreflex Independent Renal Sympathoinhibition Followed by Sustained Renal Sympathoexcitation

Aim: The hemodynamic response to mouse systemic anaphylaxis is characterized by an initial hypertension followed by sustained hypotension. However, the defense mechanisms of the sympathetic nervous system against this circulatory disturbance is not known. Here, we investigated the renal sympathetic nerve activity (RSNA) response to mouse systemic anaphylaxis, along with the roles of carotid sinus baroreceptor, vagal nerves and the transient receptor potential vanilloid type 1 channel (TRPV1). Methods: Male ovalbumin-sensitized C57BL/6N mice were used under pentobarbital anesthesia. RSNA, systemic arterial pressure (SAP) and heart rate (HR) were continuously measured for 60 min after the antigen injection. Results: Within 3 min after antigen injection, RSNA decreased along with a transient increase in SAP. Thereafter, RSNA showed a progressive increase during sustained hypotension. In contrast, HR continuously increased. Sinoaortic denervation, but not vagotomy, significantly attenuated the renal sympathoexcitation and tachycardia from 30 and 46 min, respectively, after antigen. The responses of RSNA, SAP and HR to anaphylaxis were not affected by pretreatment with a TRPV1 inhibitor, capsazepine, or by genetic knockout of TRPV1. Conclusion: The mouse systemic anaphylaxis causes a biphasic RSNA response with an initial baroreflex-independent decrease and secondary increase. The antigen-induced sympathoexcitation and tachycardia at the late stage are partly mediated by carotid sinus baroreceptors. Either vagal nerve or TRPV1 does not play any significant roles in the RSNA and HR responses in anesthetized mice.