Postnatal development of nitric oxide synthase expression in the mouse superior colliculus

Since nitric oxide has a role in the refinement of the retinal projection t o the superior colliculus (SC), we studied the onset of neuronal nitric oxi de synthase (nNOS) expression in the mouse SC in order to compare its devel opment with that of the refinement process. Sections from animals at ages P 1, P5, P8, P1Y, P15, and P21 and adults were examined with nicotinamide ade nine dinucleotide phosphate diaphorase (NADPHd) histochemistry or immunocyt ochemistry using an antibody directed against nNOS. At all ages there was a wedge of labeled neurons in the dorsolateral periaqueductal gray extending into the deep layers of the SC. At P1 there was also a single superficial band of labeled neurons within the region that will become the intermediate gray layer (IGL). By P5, labeled neurons were also seen in what will becom e the superficial gray layer. There was a ventral to dorsal progression in nNOS expression with substantial changes in the numbers of labeled neurons in the different laminae between P5 and adulthood. The number of labeled ne urons in the IGL peaked at P15, whereas in the superficial layers the numbe rs continued to increase through P21 and then declined in adults. At all ag es these neurons represented a variety of morphological cell types. The ons et of nNOS expression in the different laminae is earlier than has been rep orted in studies using NADPHd as a marker for nNOS. The temporal and spatia l patterns of nNOS expression reported here match more closely the time cou rse of pathway refinement in the SC, providing additional evidence for the involvement of nitric oxide in this process. J. Comp. Neurol. 427:581-592, 2000. (C) 2000 Wiley-Liss, Inc.