A mammalian neural tissue opsin (Opsin 5) is a deep brain photoreceptor in birds

Abstract

It has been known for many decades that nonmammalian vertebrates detect light by deep brain photoreceptors that lie outside the retina and pineal organ to regulate seasonal cycle of reproduction. However, the identity of these photoreceptors has so far remained unclear. Here we report that Opsin 5 is a deep brain photoreceptive molecule in the quail brain. Expression analysis of members of the opsin superfamily identified as Opsin 5 (OPN5; also known as Gpr136, Neuropsin, PGR12, and TMEM13) mRNA in the paraventricular organ (PVO), an area long believed to be capable of phototransduction. Immunohistochemistry identified Opsin 5 in neurons that contact the cerebrospinal fluid in the PVO, as well as fibers extending to the external zone of the median eminence adjacent to the pars tuberalis of the pituitary gland, which translates photoperiodic information into neuroendocrine responses. Heterologous expression of Opsin 5 in Xenopus oocytes resulted in light-dependent activation of membrane currents, the action spectrum of which showed peak sensitivity (lambda(max)) at approximately 420 nm. We also found that short-wavelength light, i.e., between UV-B and blue light, induced photoperiodic responses in eye-patched, pinealectomized quail. Thus, Opsin 5 appears to be one of the deep brain photoreceptive molecules that regulates seasonal reproduction in birds.

Fig. 1.

Localization of Opsin 5 in the PVO and its projections to the external zone of the median eminence. (A) Schematic drawing of the quail mediobasal hypothalamus. (B) In situ hybridization of OPN5 mRNA in the PVO. (C–E) Representative image of Opsin 5-like immunoreactivity in PVO neurons that contact the CSF (C and D) and fibers in the external zone of median eminence (ME) (arrowhead) adjacent to the pars tuberalis (PT) of the pituitary gland (E). Note that the angles of the sections are slightly different between B and C (Fig. S7). (F) The neural tracer DiI was applied to the PVO (arrow) and labeled fibers in the external zone of the ME (arrowheads). (Scale bars: 10 μm, D; 100 μm, B, C, and E; and 200 μm, F.) 3V, third ventricle.

Fig. 2.

Functional characterization of Opsin 5 as a photopigment. Current recordings in Opsin 5-expressing oocytes (A) and noninjected oocytes (B) before and after light exposure. (C and D) The time course of the response is shown by plotting the current amplitudes measured at the end of the depolarizing pulses over time. Arrows indicate the timing of light exposure. (E) Irradiance-response curves for monochromatic light pulses. Bars indicate mean ± SEM (n = 3–7). (F) Action spectrum for the Opsin 5-mediated photocurrent. Half-activation values derived from sigmoidal fits of irradiance-response curves were plotted versus the wavelength and fit with a curve for a retinal1-based photopigment.

Fig. 3.

Effects of short-wavelength light on testicular growth in eye-patched, pinealectomized quail. Quail were subjected to 2 wk of photostimulation with UV-B, UV-A, blue, or white light, and testicular growth was examined. Values are mean + SEM (P < 0.01, ANOVA, F_4,37 = 72.6, n = 7–10; *P < 0.01, Scheffé post hoc test). (Scale bar, 1 cm.) LD, long-day; SD, short-day.

Fig. 4.

Model of photoperiodic signal transduction pathway in birds. Light detected by Opsin 5-positive PVO neurons that contact the CSF is transmitted to the pars tuberalis (PT) of the pituitary gland and induces thyroid-stimulating hormone (TSH) expression in the PT. PT TSH induces expression of type 2 deiodinase (DIO2) in tanycytes lining the ventrolateral walls of the third ventricle (3V) (17). DIO2 converts prohormone T₄ to bioactive T₃ (6). Long-day–induced T₃ in the MBH causes morphologic changes in GnRH nerve terminals and glial processes and induces GnRH secretion.