THE INTERCELLULAR COMMUNICATION VIA NITRIC-OXIDE AND ITS REGULATION IN COUPLING OF CYCLIC-GMP SYNTHESIS UPON STIMULATION OF MUSCARINIC CHOLINERGIC RECEPTORS IN RAT SUPERIOR CERVICAL SYMPATHETIC-GANGLIA

Cyclic GMP (cGMP) production in rat superior cervical sympathetic gang lia (SCG) was markedly increased (ca. 7-9-foId) by the addition of eit her acetylcholine (ACh; 0.1 mM) or a muscarinic agonist, carbachol (Ca rb; 0.1 mM), in the presence of an inhibitor (3-isobutyl-1-methylxanth ine) for cGMP hydrolytic enzyme during in vitro aerobic incubation at 37 degrees C for 5 min. The ACh-induced accumulation of cGMP in SCG wa s effectively blocked (-73%) by the further addition of atropine (10 m u M), a muscarinic antagonist, whereas a nicotinic blocker, hexamethon ium (10 mu M) partially antagonized (-41%) this ACh stimulation. The i nhibitory effect of hexamethonium on ACh-evoked ganglionic cGMP produc tion was effectively augmented (-83%) by addition of N-G-monomethyl-L- arginine (L-NMMA, 50 mu M), a compound that inhibits nitric oxide (NO) synthesis from L-arginine. Comparable inhibition of cGMP formation wa s observed following application of L-NMMA to the SCG upon stimulation of Carb. In contrast, L-NMMA had no effect on the decreased level of ACh-evoked cGMP production caused by the muscarinic antagonist. The Ca rb-induced elevation of ganglionic cGMP synthesis was significantly re duced within 1 min of incubation in the medium containing hemoglobin ( Hb; 20 mu M)) an agent that scavenges only the extracellular fraction of NO. Thereafter, the tissue cGMP formation attenuated to the central level by subsequent incubation for several minutes. Addition of prote in kinase C (PKC) activator, 12-O-tetradecanoylphorboI 13-acetate (TPA ; 1 mu M) to the medium significantly decreased Carb-evoked cGMP synth esis (-61%) in SCG, whereas superoxide dismutase (SOD; 30 U/ml) only s lightly suppressed the Carb stimulation. This finding supports the ide a that PKC might play a role in dampening the muscarinic receptor-medi ated increase in NO release within ganglionic tissue. In axotomized SC G one week prior to examination, where sympathetic neurons were degene rated and reactive proliferation of glial cells was in progress, no st imulatory effect of Carb-induced cGMP production via NO release was se en. These results provide evidence that a large fraction of NO generat ed upon stimulation of muscarinic receptors in sympathetic neuronal ce lls can possibly freely diffuse in extracellular space, and then be ta ken back into the same group of surrounding cells to stimulate cGMP pr oduction.