Nitric oxide modulates ventilatory responses to hypoxia in the developing rat.

Nitric oxide (NO) is an important excitatory neurotransmitter in the central nervous system. In the adult rat, both selective and nonselective blockers of constitutive nitric oxide synthase (NOS) induce marked ventilatory reductions during sustained hypoxia, thereby enhancing ventilatory roll-off. Since hypoxic ventilatory depression is greater in developing mammals during the late phases of hypoxic exposure, we hypothesized that limited NOS activity may play a role in the late arm of the ventilatory response. To test our hypothesis, 5-d-, 10-d-, and 15-d-old rat pups underwent a 30-min hypoxic challenge (10% O2) before and after administration of 100 mg/kg N-nitro-L-arginine methyl ester (L-NAME), a competitive NOS inhibitor. Minute ventilation (VE) was measured using whole-body plethysmography. In 5-d-old pups, early VE hypoxic responses were enhanced, and late VE were similar after administration of L-NAME. In contrast, in 15-d-old hypoxic pups, L-NAME administration was associated with smaller early VE increments and significantly larger VE reductions when compared with pretreatment conditions. The role of central nervous system NO in the development of these ventilatory changes was further assessed by Western blots of protein equivalents from the nucleus tractus solitarius (NTS), the first central relay for peripheral chemoreceptor afferent input, which revealed increasing neuronal NOS expression with age. Furthermore, NADPH-diaphorase immunohistochemical staining of neurons in the NTS revealed increased positively labeled neuronal populations within subnuclei of this structure with advancing postnatal age. Current findings suggest that NOS activity mediates both excitatory and inhibitory components of the hypoxic ventilatory response. Furthermore, in brainstem respiratory regions, NO may play a role in modulating the prominent second phase of the biphasic response to hypoxia typically seen in early postnatal life.