Induction and variants of neuronal nitric oxide synthase type I during synaptogenesis

In the adult central nervous system, nitric oxide (NO) is formed from L- arginine by the so-called constitutive or type I NO synthase (NOS-I₁₅₅). However, expression of NOS-1155 immunoreactivity and activity was low or not detectable in developing mouse and rat brain. NOS-I₁₅₅ was sharply induced coincident with the onset of synaptogenesis in specific brain regions. This was followed by a second phase in which total NOS-I₁₅₅ expression decreased both in specific cell populations and in the total synaptosomal subcellular fraction. Furthermore, two putative variants of NOS-I were transiently observed: an NOS-I-immunoreactive protein with increased electrophoretic mobility (NOS-I₁₄₄) and a transient hypersensitivity of NOS-I₁₅₅ to the competitive substrate inhibitor N(ω)-nitro-L-arginine. It is concluded that NOS-I expression is not constitutive but locally induced. In the central nervous system, this regionally specific, bipbasic pattern of postnatal NOS-I induction is consistent with a role for NO in synaptogenesis and synaptic plasticity.