NADPH diaphorase expression in superior colliculus of developing, aging and visually deafferented rats

We have studied the development of NADPH-diaphorase activity in the retinorecipient layers of the superior colliculus (SC) in rats from embryonic day 17 to adulthood, during aging, and following neonatal tetrodotoxin injection or unilateral eye removal in the neonatal or in the adult animal. In the superfi cial SC, NADPH-d activity is fi rst seen in neurons on postnatal day (P) 4; over the next two weeks, enzyme expression increases gradually, in cells as well as in the neuropil. By P12-14, around the time of eye opening, NADPH-d reactivity increases dramatically. In parallel, the dendrites of many NADPH-d-positive neurons in the superfi cial gray layer, more or less randomly distributed at fi rst, gradually align their orientation relative to the dorsoventral axis. The pattern of NADPH-d activity in the superfi cial layers of the SC (i.e. stratum griseum superfi ciale and stratum opticum) is adult-like by the fourth week of age. Deaff erentation of the superfi cial SC, both in the neonatal and adult rat, and block of retinal activity lead to reduction in the size of the SC and changes in NADPH-d-positive neurons, including dendrite misorientation, decreased cell size and reduced number. Some of these changes are seen also in the aging animal. These results document a protracted and progressive increase in the development of NADPH-d expression in the SC. Our results suggest a strong infl uence of retinal aff erents and activity on the development and maintenance of NAPHD-positive neurons in the retinorecipient layers of the SC, where NO can act as a retrograde signal to carve the terminal arbors of retinal axons.