A mammalian melanopsin in the retina of a fresh water turtle, the red-eared slider (Trachemys scripta elegans)

Abstract

A mammalian-like melanopsin (Opn4m) has been found in all major vertebrate classes except reptile. Since the pupillary light reflex (PLR) of the fresh water turtle takes between 5 and 10 min to achieve maximum constriction, and since photosensitive retinal ganglion cells (ipRGCs) in mammals use Opn4m to control their slow sustained pupil responses, we hypothesized that a Opn4m homolog exists in the retina of the turtle. To identify its presence, retinal tissue was dissected from seven turtles, and total RNA extracted. Reverse transcriptase-polymerase chain reactions (RT-PCRs) were carried out to amplify gene sequences using primers targeting the highly conserved core region of Opn4m, and PCR products were analyzed by gel electrophoresis and sequenced. Sequences derived from a 1004-bp PCR product were compared to those stored in GenBank by the basic local alignment search tool (BLAST) algorithm and returned significant matches to several Opn4ms from other vertebrates including chicken. Quantitative real-time PCR (qPCR) was also carried out to compare expression levels of Opn4m in different tissues. The normalized expression level of Opn4m in the retina was higher in comparison to other tissue types: iris, liver, lung, and skeletal muscle. The results suggest that Opn4m exists in the retina of the turtle and provides a possible explanation for the presence of a slow PLR. The turtle is likely to be a useful model for further understanding the photoreceptive mechanisms in the retina which control the dynamics of the PLR.

The pupillary light responses of animals; a review of their distribution, dynamics, mechanisms and functions

“…However, direct stimulation of the efferent pathway results in much faster pupillary constriction (Dearworth et al, 2009), suggesting the cause of the sluggish PLR of turtles is to be found within the afferent part of the reflex arc. In the turtle, pupillary constriction may, for example, be more reliant than mammals on melanopsin-containing ipRGCs, whose dynamics are slow (Dearworth et al, 2011; Section 5.1).…”

Seeing the light to change colour: An evolutionary perspective on the role of melanopsin in neuroendocrine circuits regulating light‐mediated skin pigmentation

“…Nonetheless, parietalectomy affects melatonin biosynthesis by the pineal gland, and the circadian rhythm in iguanid ( Sceloporus occidentalis; Bethea & Walker, ) and scincid ( Trachydosaurus rugosus ; Firth & Kennaway, ) lizards, reflecting a neuronal connection between the parietal eye and pineal gland. The fact that melanopsin is absent from the reptilian pineal complex (Dearworth et al., ; Frigato, Vallone, Bertolucci, & Foulkes, ; Figure d), suggests that either a melanopsin‐activated eye‐pineal gland neural circuit or other photopigments within the pineal complex regulate melatonin secretion. Data from zebrafish support the idea that photopigments other than melanopsin regulate melatonin secretion.…”

Evaluation of rebound tonometry in red‐eared slider turtles (Trachemys scripta elegans)

“…This species is also used as a laboratory animal model for basic investigation in vision, particularly for pupillary light reflex and retinal research. [1][2][3][4] Normal mean intraocular pressure (IOP) values have been reported for a number of healthy, conscious and chemically restrained exotic and wild animal species. [5][6][7][8][9] While limited information is available on IOP in some terrestrial reptile species, 10 in aquatic freshwater turtles, such as red-eared slider turtles, normal IOP values have not been reported [11][12][13][14] and ocular anatomy and normal ophthalmic reference data for this species are scarce.…”