Alterations in adrenergic sensitivity of the rabbit iris after variation of environmental lighting conditions

After variation of the environmental lighting conditions, pupillary diameters of albino rabbits in response to progressively increasing doses of the sympathomimetics norepinephrine, epinephrine, and isoproterenol were measured. After maintenance of the rabbits under conditions of constant light for one week, an increased sensitivity of the iris dilator to the mydriatic effects of these adrenergic agents became evident. After maintenance of the rabbits in constant dark for one week, on the other hand, there was no change in the sensitivity of the iris upon administration of the same sympathomimetics. Norepinephrine levels measured in iris and ciliary processes after one week of either constant light of constant dark, moreover, did not differ significantly from those determined under normal lighting conditions. These results indicate that the sensitivity of the iris dilator can be altered by variation of the physiologic stimulus, light. In addition, the chronic interruption of contact between the normal neurotransmitter norepinephrine and its effector cells leading to supersensitivity in the dilator appears to be due to the removal of a trophic influence of norepinephrine, rather than its absolute loss.