Expression of penile neuronal nitric oxide synthase variants in the rat and mouse penile nerves.

Penile erection is mediated by nitric oxide (NO) synthesized by the neuronal nitric oxide synthase (nNOS). In the rat penis, the main nNOS mRNA variant, PnNOS, differs from cerebellar nNOS (CnNOS) by a 102 base pair insert encoding a 34-amino acid sequence. In the mouse, two nNOS mRNAs have been identified: nNOSalpha, encoding a 155-kDa protein, and an exon 2-deletion variant, nNOSbeta, encoding a 135-kDa protein that lacks a domain where a protein inhibitor of nNOS (PIN) binds. We wished to determine whether PnNOSalpha and beta are expressed in the rat penis and are located in the nerves and whether the beta form persists in the potent nNOS knock-out mouse (nNOS( big up tri, open big up tri, open)). A PnNOS antibody against the insert common to both PnNOSalpha and beta detected the expected 155-kDa protein in PnNOSalpha-transfected cells. This antibody, and the one common to PnNOS/CnNOS, showed (on Western blots) the 155- and 135-kDa nNOS variants in rat penile tissue during development and aging. PnNOSalpha mRNA and its subvariants were found as the main nNOS in the penile corpora, the cavernosal nerve, and the pelvic ganglia, with lower levels of PnNOSbeta mRNA. In tissue sections, PnNOS protein was immunodetected in the penile nerve endings in the rat and in the nNOS wild-type and nNOS( big up tri, open big up tri, open) mice. An antibody against the sequence encoded by exon 2 did not react (on Western blots) with the 135-kDa band, which confirms that this protein is the beta form. In conclusion, both PnNOSalpha and beta are expressed in the rat penis at all ages and are located in the nerves. The beta form may allow nitric oxide synthesis during erection to be partially insensitive to PIN. The residual expression of PnNOS, and possibly CnNOS, in the penis of the nNOS( big up tri, open big up tri, open) mouse occurs through transcription of the beta mRNA, and this may explain the retention of erectile function when the expression of nNOSalpha is disrupted.