Neither functional rod photoreceptors nor rod or cone outer segments are required for the photic inhibition of pineal melatonin

Pineal melatonin production is rapidly suppressed by light. In mammals, the photoreceptors mediating this response are ocular; however, definitive inf ormation regarding their nature and precise location is absent. In an attem pt to define these photoreceptors, we examined the sensitivity of pineal me latonin production to inhibition by controlled irradiance monochromatic gre en light (lambda max 509 nm) in C3H mice bearing either of two mutations af fecting the retina: retinal degeneration (rd), a disruption of rod phototra nsduction, and retinal degeneration slow (rds), an ablation of photorecepto r 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 i nhibition of pineal melatonin content; 2.6 x 10(-2) microwatts/cm(2) 509 nm light induced complete suppression in all three genotypes, whereas lower i rradiances were ineffective in all cases. Bilateral enucleation abolished r esponses even to 6 microwatts/cm2 509 nm light. These results demonstrate t hat the process of irradiance detection for pineal melatonin inhibition is buffered against considerable loss of photoreceptive capacity and that neit her rod photoreceptors nor rod or cone outer segments are required for medi ating this response in mice.