Twenty-four-hour change in axial length in the rabbit eye.

PURPOSE To study the 24-hour changes in axial length, lens thickness, and anterior chamber depth in rabbits and to examine the role of ocular sympathetic activity on these changes. METHODS Young adult rabbits were entrained to a daily 12-hour light-12-hour dark cycle. Axial length, lens thickness, and anterior chamber depth were measured using ultrasonic techniques. In the first group of 12 rabbits, measurements were taken in the middle light phase and in the early dark phase. In the second group of 12 rabbits, measurements were taken in constant dark every 2 hours for a period of 24 hours. The latter group of rabbits underwent unilateral transection of the cervical sympathetic trunk. Three to four weeks later, axial length, lens thickness, and anterior chamber depth were measured again in constant dark every 2 hours for 24 hours. RESULTS Under the light- dark condition, axial length and anterior chamber depth were larger in the early dark phase than in the middle light phase. Lens thickness changed in the opposite direction. Under the constant-dark condition, axial length and anterior chamber depth changed gradually during the 24-hour period. The trough appeared in the late subjective light phase, and the peak appeared in the late subjective dark phase. Lens thickness remained relatively constant. In the eyes with decentralized ocular sympathetic nerves, 24-hour changes in axial length and anterior chamber depth occurred. However, magnitudes of nocturnal enlargement were relatively smaller than those in the intact eyes. Although larger in the decentralized eyes, lens thickness appeared unchanged in either eye for 24 hours. CONCLUSIONS Consistent 24-hour changes in axial length and anterior chamber depth occur in young adult rabbits. These changes are driven endogenously. Significant portions of the nocturnal enlargements of axial length and anterior chamber depth are unrelated to ocular sympathetic activity.