NITRIC OXIDE CGMP PATHWAY STIMULATES PHOSPHORYLATION OF DARPP-32, A DOPAMINE-REGULATED AND CAMP-REGULATED PHOSPHOPROTEIN, IN THE SUBSTANTIA-NIGRA/

DARPP-32, a dopamine- and cAMP-regulated phosphoprotein of M(r) 32,000 , has been shown to be phosphorylated on threonine-34, both in vitro w ith high efficiency by cAMP-dependent and cGMP-dependent protein kinas es and in vivo by dopamine acting through cAMP-dependent protein kinas e. In the present study, we investigated the nitric oxide (NO)/cGMP pa thway for its ability to regulate the state of phosphorylation of DARP P-32 in slices of rat substantia nigra. DARPP-32 was phosphorylated on threonine-34 in these slices by sodium nitroprusside (SNP), an NO don or. The effect of SNP was abolished by preincubation of the slices wit h hemoglobin, indicating that the effect of SNP was due to released NO . The same concentration of SNP produced a 4-fold elevation of the cGM P level but did not alter the level of cAMP. The effect of SNP on DARP P-32 phosphorylation was mimicked by low concentrations of 8-bromo-cGM P and 8-(4-chlorophenylthio)-guanosine 3',5'-cyclic monophosphate, act ivators of cGMP-dependent protein kinase, but not by low concentration s of 8-bromo-cAMP, an activator of cAMP-dependent protein kinase. The data indicate a physiological role for the NO/cGMP pathway in the regu lation of DARPP-32 phosphorylation in nerve terminals of striatonigral neurons. The results provide further evidence that the state of phosp horylation of DARPP-32 represents an important mechanism for integrati on of information arriving at striatonigral neurons via a variety of n euronal pathways.