Sympathetic influence on the pupillary light response in three red-eared slider turtles (Trachemys scripta elegans)

We investigated the effects of phenylephrine and its combination with vecuronium bromide on the iris of turtles to determine if the pupillary light response is affected by sympathetic innervation.

Three red-eared slider turtles, Trachemys scripta elegans.

Diameters of light-adapted pupils were tracked before and after topical application of drugs to eyes. Phenylephrine was applied independently; in a second group of trials, vecuronium bromide was applied with phenylephrine.

Rates of pupil dilation in response to drugs were quantified by fitting data with time constant (tau) equations. Phenylephrine dilated the pupil 24%, tau = 29 min. Combination of phenylephrine with vecuronium bromide increased the pupil size 35%, and dilation was more rapid, tau = 14 min. We also were able to predict these time constants by performing different mathematical operations with an equation developed from a prior study using only vecuronium bromide. When this equation was subtracted from the equation for eyes treated with both vecuronium bromide and phenylephrine, the difference gave the observed tau for phenylephrine; when added to phenylephrine, the sum closely matched the tau for eyes treated with vecuronium bromide and phenylephrine. Further, the tau for vecuronium bromide treated eyes was predicted by subtracting the equation for phenylephrine from that of eyes treated with both vecuronium bromide and phenylephrine.

Our results suggest that sympathetic innervation interacts with the parasympathetic pathway to control the pupillary light response in turtles.