Plasma melatonin rhythm lost in preglaucomatous chicks.

Domestic chicks (Gallus domesticus) reared under continuous light, and thus developing light-induced avian glaucoma (LIAG), were tested at 4-hourly intervals around the clock, for evidence of hormonal or other rhythms which might be related to the eye effects of LIAG. Plasma melatonin, corticosterone and thyroxine (T4) all displayed daily rhythms in the young chick reared under control (diurnal) lighting conditions. These daily hormone cycles were somewhat damped under either continuous light or constant darkness, though there were hints of a surviving rhythm, albeit phase-shifted or free-running, in plasma corticosterone and T4. In the eye, mitotic rate in the corneal epithelium displayed a prominent rhythm, high in the dark and low in the light. This rhythm was suppressed by constant darkness, and also in continuous light, the latter apparently related to the impaired corneal growth of LIAG. The daily pattern of corneal mitotic activity and the diurnal curve for plasma melatonin showed remarkable similarities, suggesting the possibility of a causal relationship between the two phenomena.