Teleost eyes grow throughout life by adding neurons and stretching extant tissue. New retinal neurons of all types are added at the ciliary margin and new rod photoreceptors are inserted throughout retina in the outer nuclear layer (ONL). New rod photoreceptors result from the division of progenitor cells located in the ONL amidst functioning rod photoreceptor cell nuclei, but it is not known how new rod addition is regulated. Previous experiments using an organotypic retinal slice preparation revealed that insulin-like growth factor 1 (IGF-1) up-regulates the division of the rod progenitor cells [Dev. Brain Res. 76 (1993) 183], but the site of IGF-1 action was unknown. Here, we show where in the retina IGF-1 is made, where IGF receptors are located, and we identify the role of IGF-1 in adult retinal rod neurogenesis with both gain-and loss-of-function experiments. We found that IGF-1 is expressed by cone photoreceptor cells and its abundance varies with a daily rhythm, being significantly higher at night. In vivo application of exogenous IGF-1 increases rod progenitor cell division, an effect that is greater at night than during the day. We also show that inhibiting the function of IGF receptors decreases proliferation of rod progenitor cells. Finally, we show that IGF receptors are located on rod progenitor cells as well as on cone and rod photoreceptors. Taken together, these data suggest that the rhythmic production and release of IGF-1 plays a role in regulating the insertion of new rod photoreceptors into the retina. The diurnal change in IGF-1 abundance and effects of exogenous IGF-1 are consistent with the previous demonstration that rod progenitor cell division is threefold greater at night than in the day [Brain Res. 673 (1995) 119; Brain Res. 712 (1996) 40]. We also show that the insertion of new rod photoreceptors at the central edge of the ciliary neurogenic zone very likely also depends on IGF-1 production by cone photoreceptors. We propose that addition of new rod photoreceptors into the functioning retina is regulated through a feedback mechanism mediated at least in part via the IGF-1 produced in the cone photoreceptors.
