Antihypertensive responses elicited by central moxonidine in rats: possible role of nitric oxide

Centrally acting drug moxonidine decreases reactive oxygen species via inactivation of the phosphoinositide-3 kinase signaling in the rostral ventrolateral medulla in hypertensive rats

OBJECTIVE

Centrally acting antihypertensive action of moxonidine is a result of activation of Imidazoline-1 receptor (I1R) in the rostral ventrolateral medulla (RVLM). Hypertension shows an increase in reactive oxygen species (ROS) in the RVLM. The present objective was to determine the phosphoinositide-3 kinase (PI3K) signaling pathway involved in the effect of moxonidine on ROS generation in the RVLM of spontaneously hypertensive rat (SHR).

METHODS

Wistar-Kyoto rats and SHR received intracisternal infusion (2 weeks) of tested agents which were subjected to subsequent experiments. In-situ ROS in the RVLM was evaluated by the oxidative fluorescence dye. Western blot and PCR analysis were performed to detect the expression levels of PI3K signaling pathway. Lentivirus was injected bilaterally into the RVLM for silencing PI3K signaling.

RESULTS

ROS production in the RVLM was dose-dependently reduced in SHRs treated with infusion of moxonidine (20 nmol/day), which was prevented by the I1R antagonist efaroxan but not by the α2-adrenoceptor antagonist yohimbine. Moxonidine pretreatment significantly blunted cardiovascular sensitivity to injection of tempol (5 nmol) or angiotensin II (10 pmol) into the RVLM in SHR. Expression levels of PI3K/Akt, nuclear factor kappa-B (NFκB), NADPHase (NOX4), and angiotensin type I receptor (AT1R) in the RVLM were markedly decreased in SHR treated with moxonidine. Infection of lentivirus containing PI3K shRNA in the RVLM effectively prevented effects of moxonidine on cardiovascular activity and expression levels of Akt, NFκB, NOX4, and AT1R.

CONCLUSION

The centrally antihypertensive drug moxonidine decreases ROS production in the RVLM through inactivation of the PI3K/Akt signaling pathway in hypertension.

Sympathoinhibitory mechanism of moxonidine: role of the inducible nitric oxide synthase in the rostral ventrolateral medulla

AIMS

The central antihypertensive drug moxonidine lowers blood pressure (BP) through stimulating an imidazoline receptor within the rostral ventrolateral medulla (RVLM). Nitric oxide (NO) generated by the inducible NO synthase (iNOS) in the RVLM has been suggested to be involved in tonic sympathetic inhibition. The aim of this study was to determine the role of NO generated by iNOS in mediating moxonidine-induced cardiovascular inhibition in rats.

METHODS AND RESULTS

In anaesthetized rats, the cardiovascular response to local or systemic injection of moxonidine was observed after treatment with the selective iNOS inhibitor S-methylisothiourea (SMT) in the brain. Using immunohistochemical staining and western blot techniques, the protein expression of iNOS in the RVLM was measured in the moxonidine-infused rats. Intracerebroventricular (ICV) injection of SMT (1-100 nmol) dose-dependently attenuated the moxonidine (20 nmol, ICV)-induced decrease in BP and heart rate. Prior injection of SMT (20 and 200 pmol) into the RVLM also dose-dependently prevented the decrease in BP and renal sympathetic nerve activity evoked by RVLM microinjection of moxonidine (5 nmol) or intravenous injection of moxonidine (50 microg/kg). We further found that expression of iNOS protein following chronic ICV infusion of moxonidine (20 nmol, 2 weeks) is selectively upregulated in the RVLM but not in the nucleus tractus solitarius.

CONCLUSION

The present data suggest that an NO mechanism generated by iNOS in the RVLM plays an important role in mediating the sympathetic inhibition of the centrally acting drug moxonidine.

Involvement of central α1- and α2-adrenoceptors on cardiovascular responses to moxonidine

In the present study we compared the effects produced by moxonidine (alpha2-adrenoceptor/imidazoline agonist) injected into the 4th cerebral ventricle and into the lateral cerebral ventricle on mean arterial pressure, heart rate and on renal, mesenteric and hindquarter vascular resistances, as well as the possible action of moxonidine on central alpha1- or alpha2-adrenoceptors to produce cardiovascular responses. Male Holtzman rats (n=7-8) anesthetized with urethane (0.5 g/kg, intravenously-i.v.) and alpha-chloralose (60 mg/kg, i.v.) were used. Moxonidine (5, 10 and 20 nmol) injected into the 4th ventricle reduced arterial pressure (-19+/-5, -30+/-7 and -43+/-8 mmHg vs. vehicle: 2+/-4 mmHg), heart rate (-10+/-6, -16+/-7 and -27+/-9 beats per minute-bpm, vs. vehicle: 4+/-5 bpm), and renal, mesenteric and hindquarter vascular resistances. Moxonidine (5, 10 and 20 nmol) into the lateral ventricle only reduced renal vascular resistance (-77+/-17%, -85+/-13%, -89+/-10% vs. vehicle: 3+/-4%), without changes on arterial pressure, heart rate and mesenteric and hindquarter vascular resistances. Pre-treatment with the selective alpha2-adrenoceptor antagonist yohimbine (80, 160 and 320 nmol) injected into the 4th ventricle attenuated the hypotension (-32+/-5, -25+/-4 and -12+/-6 mmHg), bradycardia (-26+/-11, -23+/-5 and -11+/-6 bpm) and the reduction in renal, mesenteric and hindquarter vascular resistances produced by moxonidine (20 nmol) into the 4th ventricle. Pre-treatment with yohimbine (320 nmol) into the lateral ventricle did not change the renal vasodilation produced by moxonidine (20 nmol) into the lateral ventricle. The alpha1-adrenoceptor antagonist prazosin (320 nmol) injected into the 4th ventricle did not affect the cardiovascular effects of moxonidine. However, prazosin (80, 160 and 320 nmol) into the lateral ventricle abolished the renal vasodilation (-17+/-4, -6+/-9 and 2+/-11%) produced by moxonidine. The results indicate that the decrease in renal vascular resistance due to moxonidine action in the forebrain is mediated by alpha1-adrenoceptors, while the cardiovascular effects produced by moxonidine acting in the brainstem depend at least partially on the activation of alpha2-adrenoceptors.