CIRCADIAN-RHYTHMS OF MELATONIN AND SEROTONIN-N-ACETYLTRANSFERASE ACTIVITY IN PROCAMBARUS-CLARKII

Invertebrates have circadian rhythms and exhibit photoperiodism and colour changes. While they lack pineal glands, those that have been investigated contain melatonin, Until now, melatonin has been shown to be present in the photoreceptor ore:ans of this species, but the presence of the rate-limiting enzyme in melatonin synthesis, serotonin-N-acetyltransferase (SNAT, EC 2.3.1.87) has not been investigated, We report here the presence of melatonin and the enzyme SNAT in the eyes (globe plus eyestalk) of the freshwater crayfish Procambarus clarkii, Both melatonin and SNAT activity exhibit circadian variations, with their acrophase during the light phase and their nadir during darkness, These rhythms have the same period, but they are 180 degrees out of phase with respect to those described in vertebrates, SNAT is seemingly different to that reported in vertebrates since EGTA, a calcium chelator, has no protective function as it does in vertebrates.