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Anand C. Joshi, Ernest Baskin, Vallabh E. Das; Muscimol Inactivation Of The Cerebellar Fastigial Oculomotor Region In Strabismic Monkeys Changes Strabismus Angle. Invest. Ophthalmol. Vis. Sci. 2012;53(14):6337. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
We have shown that neurons in the supraoculomotor area (SOA), known to encode vergence angle in normal monkeys, encode the horizontal strabismus angle in monkeys with a sensory-strabismus. The SOA receives strong afferent projects from the caudal fastigial nucleus in the cerebellum (fastigial oculomotor region - FOR) and therefore the objective of the current study was to investigate the role of the FOR in maintaining the state of eye misalignment in strabismic monkeys.
We identified the FOR in two strabismic monkeys (one exotrope, one esotrope) by the presence of saccade related neuronal activity and by the generation of saccadic eye movements to low current electrical stimulation (10-30uA, 600Hz). Microinjections of the GabaA agonist muscimol (0.5-1ul; S1- 4 injections; S2- 4 injections) was used to reversibly inactivate the FOR unilaterally. Horizontal and vertical saccades, horizontal sinusoidal smooth-pursuit and strabismus angle were compared in pre- and post-injection conditions.
Unilateral injections induced horizontal saccade dysmetria in all experiments. Mean pre-injection horizontal saccade gain for both monkeys was 0.88±0.14; post-injection gain for ipsilesional saccades was 1.34±0.28 (52% increase) and contralesional saccades was 0.63±0.22 (28% decrease; paired t-test p<0.001). Post-injection, there was an 8% decrease for upward saccades and no significant difference for downward saccades; ipsilesional pursuit gains were increased by 8% while contralesional gains were reduced by 33%. In both monkeys, muscimol inactivation of the FOR induced a significant divergent change in strabismus angle. In the exotrope, the strabismus angle increased from 3.28±2.44° to 12.54±1.98° (paired t-test; p<0.001). In the esotrope, the strabismus angle decreased from 32.03±3.49° to 22.17±4.74° (paired t-test; p<0.001).
The Fastigial Oculomotor Region, known to be involved in vergence, saccade and smooth-pursuit eye movements, also plays a role in maintenance of the strabismus angle. We hypothesize that a circuit involving the SOA and the midline cerebellum is important in the development of proper eye alignment in infancy.
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