Photomicrographs illustrating the presence of quiescent nNOS in mice. The density of cit-IR neurons in the striatum (a) is less than the density of nNOS-IR in a contiguous section (b). NMDA administration markedly increases cit-IR (c) to levels comparable with nNOS-IR. Similarly, in the striatum of mice subjected to 30 min of unilateral MCAo, contralateral cit-IR density (d) is less than nNOS-IR (e), although ipsilateral (f), they are comparable.Cit, Citrulline; ip, ipsilateral;co, contralateral; −/−, mouse deficient in nNOS expression; ctrl, control. a–e, Bright-field, 100×. Data are from typical immunohistochemistry that was repeated at least three times for each condition. g, Quantification of citrulline and nNOS-IR in the striatum demonstrates the presence of a large pool of quiescent nNOS neurons that can be activated by either NMDA application or 30 min of MCAo. Each data point is mean ± SEM. Control, n = 8; NMDA,n = 3; ipsilateral MCAo, n = 6; contralateral MCAo, n = 6. p < 0.001 for either NMDA or ipsilateral MCAo compared with either control or contralateral MCAo using one-way ANOVA with Bonferroni’spost hoc test; there is no statistically significant difference between either control and contralateral, or NMDA and ipsilateral.