EXCITATORY EFFECTS OF NITRIC-OXIDE WITHIN THE ROSTRAL VENTROLATERAL MEDULLA OF FREELY MOVING RATS

The aim of the present study was to examine the participation of NO in the rostral ventrolateral medulla (RVLM) of freely moving rats. We ut ilized NO donors and L-arginine, which were microinjected into the RVL M. Unilateral microinjection (100 nL) of 2.5 nmol sodium nitroprusside produced a biphasic response consisting of an initial, rapid increase in arterial pressure (AP) from 125+/-5 to 161+/-8 mm Hg (P<.01) and a second, long-lasting response with a progressive increase in AP (maxi mum Delta peak, 34+/-9 mm Hg; P<.01). Another NO donor, S-nitroso-N-ac etylpenicillamine (SNAP; 2.5 nmol), also produced immediate hypertensi on from 118+/-5 mm Hg to 168+/-7 mm Hg (P<.01) but without the second, long-lasting response. L-Arginine (5, 24, and 140 nmol) produced a gr adual increase in AP. L-Glutamate (5 nmol) microinjected into the RVLM produced an increase in AP from 122+/-9 mm Hg to 171+/-8 mm Hg (P<.01 ) and bradycardia from 342+/-10 to 315+/-8 beats/min. This AP response was significantly attenuated, from 115+/-7 to 128+/-9 mm Hg (P<.05), after microinjection of methylene blue (3 nmol) without alterations In heart rate. These results indicate that NO may have an excitatory eff ect on the RVLM of freely moving rats, probably in association with gl utamatergic synapses via cGMP mechanisms.