Decreased expression of nitric oxide synthase in the colonic myenteric plexus of aged rats

Nitric oxide (NO) is a major non-adrenergic, non-cholinergic (NANC) inhibitory neurotransmitter in the gastrointestinal tract. NO released from the myenteric plexus enhances colonic transit and facilitates propulsion of the colonic contents by mediating descending relaxation. Although it has been suggested that colonic transit delays with aging, the mechanism of delayed colonic transit in aging remains unclear. We hypothesized that advanced age is associated with decreased expression of neuronal NO synthase (nNOS) and concomitant reduction in synthesis of NO in the rat colon. We studied nNOS mRNA expression, nNOS-immunohistochemistry, nNOS-immunoblotting and NOS catalytic activity in the mid-colon obtained from young (age 4-8 months) and aged (age 22-28 months) Fisher (F344xBN)F1 rats. Western blot analysis of PGP 9.5, a generic neuronal marker, of the colonic tissues were employed to study whether the total number of neurons of the myenteric plexus is reduced with aging. The number of nNOS-immunoreactive cells and nNOS synthesis in the colonic myenteric plexus were significantly reduced in aged rats. In contrast, expression of PGP 9.5 in colonic tissues was not affected in aged rats. Northern blot analysis demonstrated that the expression of neuronal nNOS mRNA was significantly reduced in the colonic tissues in aged rats. Basal and veratridine-induced release of L-[(3)H]citrulline were significantly decreased in colonic tissues from aged rats, compared to young rats. It is suggested that advanced age is associated with diminished gene expression of nNOS, nNOS synthesis and catalytic activity of NOS. This may explain the mechanism of delayed colonic transit observed in advanced age.