VISUAL-SYSTEM MALDEVELOPMENT DISRUPTS EXTRAOCULAR MUSCLE-SPECIFIC MYOSIN EXPRESSION

The genetic and epigenetic influences that are responsible for the est ablishment and maintenance of the unique phenotype of the extraocular muscles (EOMs) are poorly understood. A role for visual cues in shapin g EOM maturation was assessed in rats by using two visual deprivation paradigms, dark rearing and monocular deprivation. Isoforms of the con tractile protein myosin heavy chain (MHC) were used as an index of phe notypic change in developing and adult EOMs after these visual insults . In rats that were dark reared during the visual critical period, the proportion of EOM fibers expressing either fast or slow MHCs was decr eased significantly. EOM-specific myosin was also sensitive to dark re aring during the critical period, as evidenced by a significant decrea se in its mRNA in EOMs fi om these rats. EOM-specific MHC did not chan ge in either dark-reared rats returned to normally illuminated conditi ons or in adult rats denied visual experience for a similar time perio d. These data suggest that there maybe a critical period during develo pment when alterations in visual activity have significant consequence s for the eye muscle phenotype. In contrast to dark rearing, monocular deprivation had a minimal effect on expression of the typical myosin isoforms and no effect on EOM-specific myosin expression. Collectively , these data confirm the hypothesis that visual input to the oculomoto r system during development modulates EOM-specific MHC expression.