Unique Regulation of the Melatonin Synthetic Pathway in the Retina of Diurnal Female<i>Arvicanthis ansorgei</i>(Rodentia)

Abstract

Knowledge about melatonin synthesis and its potential roles within the retina remains fragmented, especially in mammals where studies have focused on the penultimate enzyme of melatonin synthesis arylalkylamine N-acetyltransferase (AA-NAT), whereas the final enzyme necessary for melatonin production is hydroxyindole-O-methytransferase (HIOMT). We explored multiple parameters of the melatonin synthetic pathway in the cone-rich retina of a diurnal rodent, Arvicanthis ansorgei, cones being previously implicated as probable reservoirs of melatonin production. We analyzed the temporal and spatial expression of Aa-nat mRNA and AA-NAT protein and enzymatic activity of AA-NAT, HIOMT, as well as the melatonin receptor type 1 and melatonin itself. We report that Aa-nat mRNA was localized principally to cones and ganglion cells (retinal ganglion cell [RGC]) with opposing cyclic expression, being maximal in cones during the night, and maximal in RGC in the daytime. AA-NAT protein was also immunolocalized to these same populations, and was present and active throughout the 24-hour period. HIOMT immunolocalization mirrored that of AA-NAT, but expression levels and activity were extremely low and remained uniform throughout the 24-hour period. MT1 showed a complementary expression pattern to the synthetic enzymes, present in rod photoreceptors, some inner retinal neurons and RGC. Surprisingly, melatonin levels were consistently low throughout the day/night cycle, in accordance with the low activity levels of HIOMT. These data demonstrate that the melatonin synthetic pathway in a diurnal rodent differs from that described for other tissues and species (nocturnal and diurnal), the contrasting phase expression in photoreceptors and RGC, suggesting distinct roles in these populations. (Endocrinology 156: 3292-3308, 2015) M elatonin is a ubiquitous molecule retained through- out evolution from bacteria to humans acting as a substrate for multiple functions ranging from antioxidant properties to circadian clock output (1, 2). In mammals, melatonin is a neurohormone synthesized principally at night by the pineal gland (3). Circadian regulation of this indolamine is controlled by a polysynaptic pathway com-mencing with retinal photoreception, neural transmission to the hypothalamus and relays within the autonomic nervous system. Then, transduction of the neural message into a biochemical output occurs within the pineal gland Melatonin, as a prominent output of the central circadian clock, modulates a wide array of physiological functions such as the sleep/wake cycle (5), reproduction