Nitric oxide synthase and NADPH diaphorase distribution in the bullfrog (Rana catesbeiana) CNS: pathways and functional implications.

The gas nitric oxide (NO) is emerging as an important regulator of normal physiology and pathophysiology in the central nervous system (CNS). The distribution of cells releasing NO is poorly understood in non-mammalian vertebrates. Nitric oxide synthase immunocytochemistry (NOS ICC) was thus used to identify neuronal cells that contain the enzyme required for NO production in the amphibian brain and spinal cord. NADPH-diaphorase (NADPHd) histochemistry was also used because the presence of NADPHd serves as a reliable indicator of nitrergic cells. Both techniques revealed stained cells in all major structures and pathways in the bullfrog brain. Staining was identified in the olfactory glomeruli, pallium and subpallium of the telencephalon; epithalamus, thalamus, preoptic area, and hypothalamus of the diencephalon; pretectal area, optic tectum, torus semicircularis, and tegmentum of the mesencephalon; all layers of the cerebellum; reticular formation; nucleus of the solitary tract, octaval nuclei, and dorsal column nuclei of the medulla; and dorsal and motor fields of the spinal cord. In general, NADPHd histochemistry provided better staining quality, especially in subpallial regions, although NOS ICC tended to detect more cells in the olfactory bulb, pallium, ventromedial thalamus, and cerebellar Purkinje cell layer. NOS ICC was also more sensitive for motor neurons and consistently labeled them in the vagus nucleus and along the length of the rostral spinal cord. Thus, nitrergic cells were ubiquitously distributed throughout the bullfrog brain and likely serve an essential regulatory function.