Nitric oxide produces different actions in different areas of the periaqueductal grey in cats

In 20 urethane-anaesthetised cats, microinjection of N-methyl-D-aspartate (NMDA) into the intermediate-lateral (IL-) or the dorsolateral (DL-) periaqueductal grey areas (PAG) of the midbrain elicited similar patterns of cardiovascular responses: increases in mean systemic arterial pressure (MSAP), heart rate (HR) and mean blood flows (F) of the common carotid and femoral arteries, accompanied with a 'hissing-howling' response. Similar increases in MSAP and Fs were induced by microinjections of S-nitroso-N-acetylpenicillamine (SNAP), a potent nitric oxide (NO) donor, on the IL-PAG (A2.5-A0.5). In contrast, microinjections of N(G)-nitro-L-arginine methyl ester (L-NAME), a NO synthase inhibitor, in the same area elicited a decrease in MSAP and Fs. On the other hand, microinjections of either SNAP or L-NAME reversed the original IL-PAG responses when injected into the DL-PAG. Pretreatments with SNAP significantly inhibited NMDA-induced responses in the DL-PAG but potentiated such responses in the IL-PAG. In contrast, pretreatments with L-NAME potentiated the NMDA-induced responses in the DL-PAG but inhibited such responses in the IL-PAG. These data suggest that NO may be a neurotransmitter or neuromodulator that exerts differential function in different defense areas, namely the IL- and the DL-PAG.