ELECTROPHYSIOLOGICAL AND IMMUNOCYTOCHEMICAL EVIDENCE FOR A CGMP-MEDIATED INHIBITION OF SUBFORNICAL ORGAN NEURONS BY NITRIC-OXIDE

The activation of neurons in the subfornical organ (SFO) by angiotensi n II (AngII) is well established and is widely regarded as the basis f or the AngII-induced increase in water intake. Application of the nitr ic oxide (NO) donor sodium nitroprusside (SNP) led to an inhibition of the spontaneous electrical activity in 96% of the neurons sensitive f or SNP (n = 50), In addition, the firing rate in 60% of the neurons in hibited by SNP decreased in response to superfusion with the natural s ubstrate of the NO synthase (NOS) L-arginine whereas 70% increased the ir frequency after application of the NOS blocker N-G-monomethyl-L-arg inine (L-NMMA; n = 10), The inhibitory effect of SNP could be mimicked by application of membrane-permeable 8-Br-cGMP. The presence of nNOS, the neuronal isoform of NOS, was demonstrated immunocytochemically an d using the NADPH-diaphorase technique on SFO slices. Using a highly s elective antibody against cGMP in formaldehyde-fixed tissue, the NO do nors SNP, 3-morpholinosydnonimine (SIN-1), and S-nitroso-N-acetyl-DL-p enicillamine (SNAP) caused a strong increase in cGMP formation when ap plied under the same conditions as used for the electrophysiological r ecordings, These electrophysiological results suggest an important rol e for NO in SFO-mediated responses and offer a plausible explanation f or the in vivo-observed opposite effects of AngII and NO on water inta ke.