Refinement of the ipsilateral retinocollicular projection is disrupted in double endothelial and neuronal nitric oxide synthase gene knockout mice

Development of retinal connections to the superior colliculus (SC) requires an activity dependent refinement process in which axons gradually become r estricted to appropriate retinotopic locations. Nitric oxide has been impli cated in this process. We tested this possibility by studying the refinemen t of the ipsilateral retinocollicular projections (IRP) in normal C57-BL/6 mice and in double knockout mice in which the genes for the edothelial and neuronal isoforms of nitric oxide synthase (e, nNOS) were disrupted. Mice a ged between P19 and adulthood were perfused 44-48 h after anterograde injec tions of WGA-HRP into one eye in order to measure the distribution of the l abeled IRP. In normal mice, segregation of the IRP was complete at P21, wit h the ipsilateral projection restricted to the rostro-medial SC. By contras t, the ipsilateral projection was spread over much more of the SC in double e, nNOS knockouts at P21 with patches of label distributed across the enti re medio-lateral axis of the rostral 700 mu m. Although the distribution of the ipsilateral projection became more restricted in knockout animals at l ater ages, it was still more extensive than that of normal mice of the same age at P28 and P42. In the adult, the distribution of axons was similar in both normal and double knockout animals. These results show that refinemen t of the IRP is delayed when expression of eNOS and nNOS is disrupted, pres umably to axons with uncorrelated activity because nitric oxide serves as a repellant molecule during normal development. (C) 2000 Elsevier Science B. V. All rights reserved.