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

Invertebrates have circadian rhythms and exhibit photoperiodism and co lour changes. While they lack pineal glands, those that have been inve stigated 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-acety ltransferase (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 me latonin and SNAT activity exhibit circadian variations, with their acr ophase during the light phase and their nadir during darkness, These r hythms have the same period, but they are 180 degrees out of phase wit h respect to those described in vertebrates, SNAT is seemingly differe nt to that reported in vertebrates since EGTA, a calcium chelator, has no protective function as it does in vertebrates.