130 - Eye Movements Are Crucial for the Development of Palisade Endings

Poster Board Number: 130
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Abstract Body : Introduction and Objective: Classical proprioceptor organs (muscle spindles and Golgi tendon organs) are absent in most mammalian species' extraocular muscles (EOMs). Instead, unique nervous end organs, termed palisade endings are present in the EOMs of most mammals. Palisade endings are regularly present in the EOMs of frontal-eyed species (humans, monkeys, dogs, cats, and ferrets), whereas they are infrequent in lateral-eyed species. For decades, palisade endings have been considered to be sensory structures. This was put into question when recent studies have shown that palisade endings have a cholinergic phenotype and originate from the oculomotor system. In a previous study, we demonstrated in cats that palisade endings develop postnatally in a heterochronic sequence. In the current study, we tested if visual deprivation and/or eye immobilization affect the palisade endings development.
Materials and Methods: We performed unilateral and bilateral visual deprivation. For unilateral visual deprivation, the right eye of 3 newborn cats was covered until postnatal (P) day 30 starting from P7 (just before eye-opening). For bilateral visual deprivation, both eyes of 3 newborn cats were covered until P45 starting from P7. To block eye movements, 7 newborn cats received an injection of Botulinum Neurotoxin-A (BoNT-A) into the retrobulbar space of the left orbit at birth. The survival time was 45 days for 3 cats and 95 days for 4 cats. For palisade endings analysis, EOM whole-mount preparations were fluorescently labeled with anti-neurofilament (a general marker for axons), anti-synaptophysin (a general marker of synaptic boutons), or anti-growth associated protein 43 (GAP43, a marker for axons during development and regeneration). Muscle fibers were counterstained with phalloidin. Immunolabeled specimens were examined with a confocal laser-scanning microscope.
Results: Following unilateral and bilateral visual deprivation, palisade endings did not exhibit qualitative and quantitative differences with palisade endings from age-matched controls. However, after BoNT-A-induced ocular immobilization (for 45 and 95 days), palisade endings were absent in the superior rectus and lateral rectus muscles, and they were only present in the inferior and medial rectus muscles. BoNT-A treated palisade endings showed a rudimentary structure, and their number was reduced. In addition, the expression of GAP43 was significantly reduced in the BoNT-A-treated palisade endings.
Conclusions: The results of this study demonstrate that the development of palisade endings is delayed by eye immobilization but not by visual deprivation. Since the motor stimulus drives the proper development of palisade endings, this should be taken into account when designing therapies for oculomotor dysfunctions.
Significance/Implication: The present study shows that eye movements are essential for the maturation of palisade endings. The next step in the future will be to carry out molecular and physiological experiments to study the function of palisade endings.