Exp. Eye Res. (1998), 66, 183–193 The Circadian Rhythm in Intraocular Pressure and its Relation to Diurnal Ocular Growth Changes in Chicks DEBORA L. NICKLA a *, CHRISTINE WILDSOET b  JOSH WALLMAN a a The City College of CUNY, New York, NY 10031 U.S.A. b Queensland University of Technology, Brisbane, Australia (Received Columbia 5 March 1997 and accepted in revised form 25 September 1997) Recent investigations have shown that growing chicken eyes elongate during the day and shorten during the night. We asked whether the chick, like a number of other animals, exhibits a rhythm in intraocular pressure (IOP) and whether this rhythm might be associated with this rhythm in elongation. We find that the intraocular pressure in normal eyes is high during the day and low in the middle of the night, similar to the rhythm in ocular elongation. The amplitude of this rhythm in IOP is approximately 8 mm Hg; it persists in constant darkness, albeit with a reduced amplitude, implying that the rhythm has a circadian component. Form deprivation by translucent diffusers does not affect the amplitude of the rhythm in IOP, but makes the phase of the rhythm more variable, such that the trough no longer consistently occurs at night. We find that the magnitude of the ocular compliance (the change in length induced by change in intraocular pressure) is consistent with the possibility that the diurnal changes in IOP might, through mechanical stretch, account for much of the diurnal changes in length. However, in individual eyes, we find consistent phase differences between the rhythms in IOP and ocular elongation. Therefore, we propose that the rhythm in IOP influences ocular elongation in ways other than by simply inflating the eye, for example, by influencing underlying rhythms in scleral extracellular matrix production. We conclude that the rhythm in IOP plays a role in the regulation of the growth of the eye.  1998 Academic Press Limited Key words : intraocular pressure ; diurnal rhythms ; circadian ; chicken ; form-deprivation ; myopia ; ocular compliance ; eye growth. 1. Introduction The eyes of chickens show a diurnal rhythm in elongation, elongating during the day and shrinking at night (Weiss and Schaeffel, 1993). What might account for the rhythm in ocular elongation ? It has long been known that intraocular pressure (IOP) shows a diurnal rhythm in some species, including humans (Drance, 1960 ; Henkind, Leitman and Weitz- man, 1973 ; Frampton, Rin and Brown, 1987), rabbits (Rowland, Potter and Reiter, 1981 ; Liu and Shieh, 1995 ; Schnell, Debon and Percicot, 1996), and rats (Krishna et al., 1995). We undertook the study reported here to answer two questions in relation to normal eye growth. First, does the chick eye show a rhythm in IOP? Second, is the rhythm in ocular elongation a mechanical consequence of the rhythm in intraocular pressure ? To these ends, we measured IOP around the clock, both in chickens reared in normal lightdark conditions as well as in those in constant darkness, as a way of distinguishing whether the rhythm was light driven or circadian. In addition, we measured the compliance of the eye to determine whether this was consistent with the notion that changes in IOP could directly produce the diurnal changes in length observed by changes in the degree * Reprint requests : Debora L. Nickla, New England College of Optometry, 424 Beacon St., Boston, MA 02115, U.S.A. of inflation. Finally, we measured IOP and axial length simultaneously at 6 h intervals over 24 hr to deter- mine whether these two rhythms were in phase, as would be expected if the IOP rhythm were the sole cause of the axial length rhythm. In chicks, form deprivation, e.g. by translucent diffusers, results in increased ocular elongation and myopia (Wallman, Turkel and Trachtman, 1978 ; Hodos and Kuenzel, 1984). Might the abnormal rate of ocular elongation in form-deprived eyes be associ- ated with alterations in ocular circadian rhythms ? The daytime rise in retinal dopamine, a neuro- modulator, is reduced in form-deprived eyes compared to normal eyes (Iuvone et al., 1989; Stone et al., 1989), suggesting that circadian rhythms might have been altered. Other studies have shown that elimin- ating temporal cues by rearing animals under constant light or darkness results in excessive eye growth (Lauber, Shutze and McGinnis, 1961 ; Gottlieb, Wentzek and Wallman, 1987). To look for alterations in rhythms in form-deprived eyes, we measured IOP and axial length at the same times and under the same conditions as in normal eyes. Although it was initially reported that form-deprived eyes lack a rhythm in elongation (Weiss and Schaeffel, 1993), we found that these eyes showed a diurnal rhythm in elongation that was similar to normal (NIckla, Wildsoet and Wallman, 1988 ; Nickla and Wallman, 1995a). We find that the IOP of normal chick eyes undergoes 0014–48359802018311 $25.000ey970425  1998 Academic Press Limited