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Neither Functional Rod Photoreceptors nor Rod or Cone Outer Segments Are Required for the Photic Inhibition of Pineal Melatonin¹

Department of Biology, Imperial College of Science Technology and Medicine, London, United Kingdom SW7 2AZ

Address all correspondence and requests for reprints to: Dr. Robert Lucas, Department of Biology, Sir Alexander Fleming Building, Imperial College Road, Imperial College of Science Technology and Medicine, London, United Kingdom SW7 2AZ. E-mail: r.j.lucas{at}ic.ac.uk

Pineal melatonin production is rapidly suppressed by light. In mammals, the photoreceptors mediating this response are ocular; however, definitive information regarding their nature and precise location is absent. In an attempt to define these photoreceptors, we examined the sensitivity of pineal melatonin production to inhibition by controlled irradiance monochromatic green light (max 509 nm) in C3H mice bearing either of two mutations affecting the retina: retinal degeneration (rd), a disruption of rod phototransduction, and retinal degeneration slow (rds), an ablation of photoreceptor outer segments. Diurnal profiles of pineal melatonin content were similar in both mutant genotypes and in wild-type mice; melatonin peaked between 3–5 h before lights on. All three genotypes exhibited irradiance dependent inhibition of pineal melatonin content; 2.6 x 10^-2 microwatts/cm² 509 nm light induced complete suppression in all three genotypes, whereas lower irradiances were ineffective in all cases. Bilateral enucleation abolished responses even to 6 microwatts/cm² 509 nm light. These results demonstrate that the process of irradiance detection for pineal melatonin inhibition is buffered against considerable loss of photoreceptive capacity and that neither rod photoreceptors nor rod or cone outer segments are required for mediating this response in mice.

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