Eyes possess their own biological clocks.

In the eye, many daily cycles, such as the regeneration of light receptors and increased sensitivity to light, appear timed to the cadence of a unique drummer. Now, two biologists have shown that the retinas of hamsters carry their own circadian timepieces to maintain these rhythms on a roughly 24-hour cycle.

The eye timepieces are distinct from the hub of tissue in the brain's hypothalamus that serves as an orchestral conductor, pacing the symphony of interwoven daily rhythms, from sleep-wake cycles and pain sensitivity to hormone production. Until about 20 years ago, the brain's suprachiasmatic nucleus (SCN) was accepted as the one and only body clock. Then, biologists found they could throw the eyes' rhythms out of sync with other circadian cycles. Since that time, several researchers have confirmed that eye rhythms persist-and can be reset-even after the SCN, or the eyes' ability to communicate with it, is destroyed.

From such findings, Michael Terman of Columbia University and his colleagues published a hypothesis in 1991 arguing that the eye must possess its own clock, independent of the SCN. However, Terman now notes, because the studies at that time used whole animals, 'there was always a nagging suspicion in the field that these results were artifactual'-that they reflected some indirect influence by other systems in the body.

Gianluca Tosini and Michael Menaker of the University of Virginia in Charlottesville have now erased such doubts by finding resettable circadian rhythms in retinas removed from hamsters and maintained in culture in the lab for several days. In the April 19 Science, they report that these retinas produce the hormone melatonin in quantities that ebb and flow daily. The scientists reset the synthesis cycle of this hormone (SN: 5/13/95, p. 300), normally produced only at night, by illuminating the retinas at different times.

Menaker believes the retinal clock will turn up in other mammals, including humans. 'Now, if we can discover what this [clock] is doing for the eye,' Menaker says, 'we may get a handle on some pathologies that occur there.'

This demonstration of separate clocks in the retina and the brain, adds Terman, means 'one has to ask how they talk to each other,' because they usually work in synchrony.

Al Lewy of the Oregon Health Sciences University in Portland describes the new study as 'just elegant' but expressed deep concern over the implications of its melatonin findings. 'If we knew with certainty that melatonin has an important function in the human eye, this hormone [now available as a nonprescription sleep aid] should be taken off the market immediately-no questions asked' until follow-up studies determine whether the supplement harms the eyes.

Ophthalmologist Charlotte Reme of the University of Zurich agrees. Melatonin supplements could change the timing of hormone peaks in the eye. 'And when we have a high melatonin level in the retina and are exposed to very bright light,' she warns, 'we risk light-induced damage.'