Laminar distribution and morphology of NADPH-diaphorase containing neurons in the optic tectum of the pigeon.

: The laminar distribution and morphology of nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d) containing neurons is studied in the optic tectum (OT) of the pigeon. NADPH-d positive cells are arranged in three bands: the first band, in the stratum griseum et fibrosum superficiale, sublayer II a, is formed by small marginal and horizontal neurons; the second band is broad and extends throughout layers II i-j and III (stratum griseum centrale); the third band lies around the tectal ventricle, and consists of neurons in the stratum griseum profundum (SGP) and tanycytes. A comparison with Golgi-impregnated sections suggests that most NADPH-d positive cells in the OT have local or intratectal axons, while the large efferent neurons in layers IIj and III are NADPH-d negative. Similarities and differences between NADPH-d containing neurons in the avian OT and the mammalian superior colliculus are discussed. The processes of NADPH-d positive neurons in the SGP and of the tanycytes of the tectal ventricle form dense fascicles traversing the stratum album centrale. They surround and contact the radial blood vessels that originate in the SGP. We discuss the possibility that NADPH-d, or nitric oxide synthase, plays a role in the regulation of local blood flow in the deep tectal layers.