MONITORING OF CYCLIC-GMP DURING CEREBELLAR MICRODIALYSIS IN FREELY-MOVING RATS AS AN INDEX OF NITRIC-OXIDE SYNTHASE ACTIVITY

The nitric oxide synthase/cyclic GMP pathway has been studied in vivo in the adult rat cerebellum by monitoring the levels of extracellular cyclic GMP during microdialysis in conscious unrestrained animals. The basal cyclic GMP efflux was concentration-dependently reduced upon local infusion of the nitric oxide synthase inhibitor N(G)-nitro-L-arginine (10 muM - 1 mM). The nitric oxide donor S-nitroso-N-penicillamine, perfused through the dialysis probe at 1 mM, increased by about 200% the extracellular levels of cyclic GMP. The glutamate receptor agonist N-methyl-D-aspartate (500 muM) produced a cyclic GMP response which was abolished by the selective receptor antagonist D-2-amino-5-phosphonovaleric acid (500 muM) or by N(G)-nitro-L-arginine (10 muM). The elevation of cyclic GMP levels caused by local infusion of 500 muM N-methyl-D-aspartate was also abolished by parentheral administration of the N-methyl-D-aspartate channel blocker dizocilpine (0.4 mg/kg, i.p.). Local perfusion of the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (1 mM) increased by about 150% the extracellular levels of cyclic GMP. It is concluded that cyclic GMP collected during in vivo microdialysis reflects nitric oxide synthase activity in the rat cerebellum. The technique may be utilized to investigate the pathophysiology and the pharmacology of the nitric oxide/cyclic GMP pathway in the cerebellum of living animals.