Visual photoreceptor subtypes in the chicken retina: melatonin-synthesizing activity and in vitro differentiation

Abstract

The chicken retina contains five visual photoreceptor subtypes, based on the specific opsin gene they express. In addition to the central role they play in vision, some or all of these photoreceptors translate photoperiodic information into a day-night rhythm of melatonin production. This indolic hormone plays an important role in the photoperiodic regulation of retinal physiology. Previous studies have stopped short of establishing whether melatonin synthesis takes place in all the photoreceptor spectral subtypes. Another issue that has been left unsettled by previous studies is when during development are retinal precursor cells committed to a specific photoreceptor subtype and to a melatoninergic phenotype? To address the first question, in situ hybridization of the five opsins was combined with immunofluorescent detection of the melatonin-synthesizing enzyme hydroxyindole O-methyltransferase (HIOMT, EC.2.1.1.4). Confocal microscopy clearly indicated that all photoreceptor spectral subtypes are involved in melatonin synthesis. To tackle the second question, retinal precursor cells were dissociated between embryonic day 6 (E6) and E13 and cultured in serum-free medium for 4 days to examine their ability to autonomously activate the expression of opsins and HIOMT. Real-time PCR on cultured precursors indicated that red-, green- and violet-sensitive cones are committed at E6, rods at E10 and blue-sensitive cones at E12. HIOMT gene expression was programmed at E6, probably reflecting the differentiation of early cones. The present study provides a better characterization of photoreceptor subtypes in the chicken retina and describes a combination of serum-free culture and real-time PCR that should facilitate further developmental studies.

Unique Regulation of the Melatonin Synthetic Pathway in the Retina of Diurnal FemaleArvicanthis ansorgei(Rodentia)

“…Previous studies in rat retina have revealed very faint expression of Hiomt (28), and it was virtually undetectable in bovine (51) and macaque retinas (26). Previous studies showed HIOMT immunoreactivity in IS of several mammalian [rat, bovine and human (53)] and non mammalian species [chicken (30,54) and lizard (53)] ( Table 2). In situ hybridization studies have been hampered by the lack of specific probes due to the highly divergent Hiomt sequences as demonstrated between mouse strains (52).…”

“…These differences indicate that control of melatonin synthesis occurs at the level of transcription in rodents, while in primates this control is principally post-translational (26). However, chicken retinas express hiomt mRNA in photoreceptors (30). HIOMT/hiomt biology is still poorly understood, partly because the level of hiomt gene expression is low and constitutive in rat retina (28), partly because HIOMT has not been detected in nonhuman primate eyes (26), and also because its sequence is extremely divergent between species (29).…”

Activation of Phospholipase C Mimics the Phase Shifting Effects of Light on Melatonin Rhythms in Retinal Photoreceptors

“…Photoreceptors are the primary source of melatonin In the retinas of vertebrates [1], [33], [43], [44] and its synthesis in these cells is controlled by the clocks harbored by these cells [1], [4], [45], [46]. We prepared re-aggregate cultures from E6 chicken retinas and exposed them to a 12L:12D cycle for six days.…”

Activation of rhodopsin gene transcription in cultured retinal precursors of chicken embryo: role of Ca2+ signaling and hyperpolarization‐activated cation channels

“…; Voisin et al . ). The nature of this long time lag between commitment to a photoreceptor fate and opsin gene expression has not been elucidated.…”

“…More recently, we observed that chicken retinal precursors isolated in culture after E10 spontaneously activate rhodopsin gene transcription (Voisin et al . ). Therefore, this study sought to examine the possible role of cAMP‐ and Ca 2+ ‐dependent signaling pathways in the spontaneous activation of rhodopsin gene transcription in chicken retinal precursors cultured after E10.…”