Nitric oxide synthase isozymes in spinally projecting PVN neurons are involved in CRF-induced sympathetic activation

In the brain, corticotropin-releasing factor (CRF) has been shown to activate the sympatho-adrenomedullary outflow, but the central mechanisms of action are still not fully understood. Previously, we reported that inducible nitric oxide synthase (iNOS) is involved in central CRF-induced elevation of plasma catecholamines in rats. Nitric oxide is mainly synthesized by neuronal NOS (nNOS) and iNOS in many areas in the brain. Of these areas, the paraventricular hypothalamic nucleus (PVN) contains neurons projecting to the intermediolateral cell column (IML) of the spinal cord, thereby directly affecting the sympathetic activity. Therefore, in the present study, we investigated the effect of intracerebroventricularly (i.c.v.) administered CRF on plasma catecholamine levels and expression of NOS isozymes (iNOS and nNOS) and Fos (a marker for neuronal activation) in the spinally projecting PVN neurons, using rats microinjected with a monosynaptic retrograde tracer into the IML. CRF (1.5 nmol/animal, i.c.v.) effectively elevated plasma catecholamine levels. The spinally projecting neurons labeled with a tracer were detected in the dorsal cap, ventral part and posterior part of the PVN. CRF significantly increased the number of spinally projecting neurons triple-labeled with Fos and iNOS in all of these PVN subnuclei. On the other hand, CRF significantly increased the number of spinally projecting neurons triple-labeled with Fos and nNOS only in the ventral part of the PVN. These results suggest that in spinally projecting PVN neurons, iNOS mainly contributes to the centrally administered CRF-induced activation of the sympatho-adrenomedullary outflow in rats.