Vestibulo-ocular pathways modulate extraocular muscle myosin expression patterns.

The genetic and epigenetic influences that establish and maintain the unique phenotype of the extraocular muscles (EOMs) are poorly understood. The vestibulo-ocular reflex (VOR) represents an important input into the EOMs, as it stabilizes eye position relative to the environment and provides a platform for function of all other eye movement systems. A role for vestibular cues in shaping EOM maturation was assessed in these studies using the ototoxic nitrile compound 3',3'-iminodipropionitrile (IDPN) to eliminate the receptor hair cells that drive the vestibulo-ocular reflex. Intraperitoneal injections of IDPN were followed by a 2-week survival period, after which myosin heavy chain (MyHC) analysis of the EOMs was performed. When IDPN was administered to juvenile rats, the proportion of eye muscle fibers expressing developmental and fast myosins was increased, while EOM-specific MyHC mRNA levels were downregulated. By contrast, IDPN treatment in adult rats affected only the proportion of fibers expressing developmental MyHC isoforms, leaving the EOM-specific MyHC mRNA unaltered. These data provide evidence that the VOR modulates EOM-specific MyHC expression in development. The lack of significant changes in EOM-specific MyHC expression in adult EOM following IDPN administration suggests that there may be a critical period during development when alterations in vestibular activity have significant and permanent consequences for the eye muscles.