Developmental changes of NADPH-diaphorase positive structures in the isthmic nuclei of the chick

Nicotine adenine dinucleotide phosphate-diaphorase staining was used to stu dy nitric oxide synthase activity and distribution in the midbrain visual s tructures of white leghorn chick embryos and post-hatched chickens. Enzyme staining first appeared in the isthmic region at the tenth embryonic day (E 10) in the neuropil of the nucleus isthmi, pars parvocellularis. At Ell fai nt enzyme positivity appeared also in the nucl. isthmi pars magnocellularis , the nucl. semilunaris and the nucl. isthmo-opticus. The staining intensit y of the isthmic nuclei dramatically increased between the 12.5th and the 1 3th days of incubation. The nucl. isthmi, pars parvocellularis showed the s trongest enzyme reaction throughout embryonic life. A day before hatching a ll the isthmic nuclei were heavily stained, however, nicotine adenine dinuc leotide phosphate-diaphorase-positive cells occurred exclusively in the nuc leus isthmo-opticus. In the tectum opticum, intensely stained cells occupie d the stratum fibrosum et griseum superficiale. The layer containing the pr ojection neurons to the isthmo-optic nucleus was unstained. In the isthmic region, the intensity of staining surpassed that of the tectum and reached its maximum at E17 and then slowly decreased till the end of the experiment al period (120 days post-hatched). The tractus isthmo-opticus showed nicoti ne adenine dinucleotide phosphate-diaphorase activity throughout the invest igated period of life of the chicken, but the tractus tectoisthmo-opticus w as unstained. Our results suggest that in the isthmic nuclei, nicotine aden ine dinucleotide phosphate-diaphorase-positive neurons occur only in the is thmo-optic nucleus and optic tectum. The other positively stained structure s are the fibers and terminals of tectal cells. In most brain areas nicotin e adenine dinucleotide phosphate-diaphorase indicates nitric oxide synthase that produces nitric oxide. The transient appearance of this molecule is p robably necessary for neuronal differentiation or the establishment of syna ptic connections in the isthmic nuclei, and these developmental changes are under the control of the optic tectum.