Neuronal nitric oxide synthase alternatively spliced forms: Prominent functional localizations in the brain

Neuronal nitric-oxide synthase (nNOS) is subject to alternative splicing. In mice with targeted deletions of exon 2 (nNOS(Δ/Δ)), two alternatively spliced forms, nNOSβ and γ, which lack exon 2, have been described. We have compared localizations of native nNOSα and nNOSβ and γ by in situ hybridization and immunohistochemistry in wild-type and nNOS(Δ/Δ) mice. To assess nNOS catalytic activity in intact animals we localized citrulline, which is formed stoichiometrically with NO, by immunohistochemistry. nNOSβ is prominent in several brain regions of wild- type animals and shows 2-to 3-fold up-regulation in the cortex and striatum of nNOS(Δ/Δ) animals. The persistence of much nNOS mRNA and protein, and distinct citrulline immunoreactivity (cit-IR) in the ventral cochlear nuclei and some cit-IR in the striatum and lateral tegmental nuclei, indicate that nNOSβ is a major functional form of the enzyme in these regions. Thus, nNOSβ, and possibly other uncharacterized splice forms, appear to be important physiological sources of NO in discrete brain regions and may account for the relatively modest level of impairment in nNOS(Δ/Δ) animals.