Differential engagements of glutamate and GABA receptors in cardiovascular actions of endogenous nNOS or iNOS at rostral ventrolateral medulla of rats.

1. We evaluated in Sprague-Dawley rats anaesthetized with propofol the engagement of soluble guanylyl cyclase (sGC)/cGMP cascade, glutamatergic and GABAergic neurotransmission in the cardiovascular actions of endogenous nitric oxide (NO) at the rostral ventrolateral medulla (RVLM). 2. Microinjection bilaterally into the RVLM of a selective iNOS inhibitor, S-methylisothiourea (SMT, 250 pmoles), or a selective nNOS inhibitor, 7-nitroindazole (7-NI, 5 pmoles), induced respectively an enhancement or a reduction in systemic arterial pressure, heart rate and power density of the vasomotor components in the spectrum of arterial blood pressure signals, our experimental index for sympathetic neurogenic vasomotor tone. 3. The cardiovascular actions of SMT or 7-NI in the RVLM were significantly antagonized by co-administration into the RVLM of the sGC inhibitor, 1H-[1,2,4]Oxadiazole[4,3-alpha]quinoxalin-1-one (ODQ, 250 or 500 pmoles). 4. The cardiovascular excitatory effects after blockade of endogenous iNOS activity were significantly attenuated when N-methyl-D-aspartate (NMDA) receptor antagonist, dizocilpine (20 or 50 pmoles), or non-NMDA receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (250 or 500 pmoles), was co-microinjected bilaterally into the RVLM. 5. On the other hand, the cardiovascular depressive responses to blockade of endogenous nNOS activity were significantly antagonized on co-administration of GABA(A) receptor antagonist, bicuculline methiodine (5 or 10 pmoles), but not GABA(B) receptor antagonist, 2-hydroxy saclofen (50 or 100 pmoles). 6. We conclude that the cardiovascular actions of endogenous NO in the RVLM engage the sGC/cGMP pathway. In addition, whereas NO derived from nNOS induced sympathoexcitation via both NMDA and non-NMDA receptors in the RVLM, NO generated by iNOS elicited sympathoinhibition via GABA(A) receptors.