Within the striatum, the gaseous neurotransmitter nitric oxide (NO) is produced by a subclass of interneurons containing the neuronal NO synthase (nNOS). NO promotes the second messenger cGMP through the activation of the soluble guanyl cyclase (sGC) and plays a crucial role in the integration of glutamate (GLU) and DA transmission. The aim of this study was to characterize the impact of 6-hydroxyDA (6-OHDA) lesion of the rat nigrostriatal pathway on NO/cGMP system. In vivo extracellular single units recordings were performed under urethane anesthesia to avoid any potentially misleading contributions of cortically-driven changes on endogenous NO. Hence, no electrical extrastriatal stimulation was performed and great attention was paid to the effects of 3-morpholinosydnonimine (SIN-1, a NO donor), N(G)-nitro-L-arginine methyl ester (L-NAME, a nonselective NOS inhibitor) and Zaprinast (a PDE inhibitor) delivered by iontophoresis upon the main striatal phenotypes. The latter were operationally distinguished in silent medium spiny-like neurons (MSN), with negligible spontaneous activity but displaying glutamate-induced firing discharge at rest and spontaneously active neurons (SAN), representing to a large extent nonprojecting interneurons. SANs were excited by SIN-1 and Zaprinast while MSNs showed a clear inhibition during local iontophoretic application of SIN-1 and Zaprinast. In 6-OHDA animals, SIN-1-induced excitation in SANs was significantly increased (on the contrary, the inhibitory effect of L-NAME was less effective). Interestingly, in DA-denervated animals, a subclass of MSNs (40%) displayed a peculiar excitatory response to SIN-1. These findings support the notion of an inhibitory modulatory role exerted by endogenous NO on control striatal projection cells. In addition, these findings suggest a functional cross-talk between NO, spontaneously active interneurons, and projection neurons that becomes critical in the parkinsonian state. Synapse 62:409–420, 2008. © 2008 Wiley-Liss, Inc.