sentence: Together , the above studies confirm that both D1 - and D2-like receptor activation strongly regulates striatal nNOS activity , albeit in opposing manners ( i.e , D1 receptor activation is facilitatory , D2 receptor activation is inhibitory ) , and identify a critical role of nNOS – NO signaling in the generation of striatal cGMP.Consistent with the above studies of striatal cGMP synthesis , our laboratory has reported that electrical and chemical stimulation of the substantia nigra and systemic administration of the D1 receptor agonist SKF 81297 all robustly increase amperometric measures of striatal NO efflux via nNOS and D1-like receptor-dependent mechanisms ( Sammut et al ., 2006 , 2007a ; Park and West , 2009 ) .