Endogenous circadian rhythm of basal pupil size in rabbits

The authors examined the circadian change of basal pupil size in light-dark entrained normal rabbits and in entrained rabbits with unilaterally decentralized ocular sympathetic nerves. The pattern of the circadian pupillary rhythm was compared with the circadian patterns of intraocular pressure (IOP) and body temperature in the same rabbits.

Adult male New Zealand albino rabbits were entrained to a daily 12-hour light-12-hour dark cycle. Using an infrared camera recording system, basal pupil sizes in seven entrained rabbits were videotaped under a constant dark environment at 1-hour intervals for 24 hours. Hourly IOP and rectal temperature in the same rabbits were measured separately under both the regular light-dark cycle and constant dark. In 10 entrained rabbits that underwent unilateral transection of the cervical sympathetic trunk, measurements of pupil size and IOP in constant dark were performed.

In normal rabbits, a smaller basal pupil size in the subjective light phase and a consistently larger pupil size in the subjective dark phase were observed. Notably, pupil size increased sharply at the beginning of the subjective dark phase and peaked shortly. Circadian rhythms of IOP and body temperature were present under the regular light-dark cycle and in constant dark. As did pupillary enlargement, IOP rose sharply at the beginning of the subjective dark phase and peaked shortly thereafter. Body temperature, however, increased gradually and peaked in the late subjective dark phase. In rabbits with unilaterally decentralized ocular sympathetic nerves, the circadian rhythm of pupil size was present only in the intact eye. In addition, the circadian IOP elevation in the decentralized eye was reduced significantly.

In light-dark entrained rabbits, basal pupil size changes in a circadian pattern and peaks at the beginning of the dark phase. The circadian pupillary rhythm disappears after ocular sympathetic decentralization. There are similar characteristics in the circadian rhythm of IOP. The increase of basal pupil size in the early dark phase is not related to the nocturnal increase of body temperature.