Abstract
Purpose: :
We investigated effects by mydriatic drugs on the pupil of turtle to determine if responses to light are controlled by parasympathetic and sympathetic innervations acting on the iris. Drugs were applied independently and in combination to examine interactions between innervations. Eyes of turtles were also enucleated to test if light responses by the pupil occur in absence of these innervations.
Methods: :
Turtles (Pseudemys scripta elegans) were secured in a restraint that immobilized their head movements. Heads were inserted into a light integrating sphere, and both pupils were monitored through small apertures by digital camcorders. Three turtles were examined for effects by mydriatic drugs. During each trial, drugs were topically applied, 4 times at 15 min intervals, to the corneas of each eye. One eye was randomly selected for treatment of the drug while the other, treated with saline (0.9% NaCl), was used as a control. Application doses were 0.16 mg (atropine and vecuronium bromide) and 1 mg for phenylephrine. Pupil sizes under adaptation to light (1.08 x 10 –13 J·µm–2·s–1) were then tracked after drug/saline application. Five additional turtles were used to test if light responses occur in denervated eyes. Three turtles were euthanized and their eyes enucleated; eyes in the other two turtles were left intact for comparison. Eyes were adapted to lights then lights turned off to examine recovery curves.
Results: :
Kinetics of pupil dilations in response to drugs and for eyes undergoing adaptations to dark were quantified by fitting data with linear regression analysis. Data was plotted as the logarithm percent difference, maximum pupil size minus pupil size at any particular time. In this form, time constants (τ) could be calculated from slopes of lines. Application of vecuronium bromide and phenylephrine applied in combination increased the pupil, 32% (p<0.01) and was the most rapid for the drug treatments, τ = 16 min (R2 = 0.94). Vecuronium bromide by itself dilated the pupil by 34% (p<0.01), τ = 29 min (R2 = 0.85). Phenylephrine dilated the pupil, 24% (p<0.01), τ = 28 min (R2 = 0.84). Atropine had no effect on pupil size (p = 0.99). Pupils dilated in turtles with intact eyes, 54% (p<0.01), τ = 11 min (R2 = 0.92). Pupil dilation was less but not eliminated in enucleated eyes, 13% dilation (p<0.01), τ = 22 min (R2 = 0.76).
Conclusions: :
Mydriasis by vecuronium bromide and phenylephrine suggests that parasympathetic and sympathetic innervations are involved in controlling the pupillary light response. Since the pupil remains partially responsive after enucleation of the eye, results also suggest an additional mechanism of control.
Keywords: pupil • pupillary reflex • receptors: pharmacology/physiology