Mapping the Pattern of Essential Neuroendocrine Cells Related to Puberty and VA Opsin Expression Provides Further Insight in the Photoreceptive Regulation of the Brain-Pituitary-Gonadal Axis in Atlantic Salmon (Salmo salar)

In Atlantic salmon (Salmo salar), seasonal photoperiod is shown to regulate the onset of sexual maturation, yet which brain region(s) is involved, and how light information impacts the neuroendocrine system are still not fully understood in teleosts. Detailed knowledge about the photoperiodic regulation of maturation in fish is still missing. In birds, it is shown that gonadotropin-releasing hormone (Gnrh) is located in the same neurons as vertebrate ancient (VA) opsin, suggesting a direct photoreceptive regulation for the onset of sexual maturity. This study presents a comprehensive topographic mapping of gnrh2, gnrh3, kisspeptin 2 (kiss2), gonadotropin-inhibiting hormone (gnih), and VA opsin using in situ hybridization on mature Atlantic salmon brains. Neurons positive for gnrh3 are expressed in the olfactory bulb and ventral telencephalon, while gnrh2-positive neurons are located dorsally in the midbrain tegmentum. Gonadotropin-inhibiting hormone (Gnih)-expressing cell bodies are present in the ventral thalamus and extend caudally to the hypothalamus with kiss2-expressing cells appearing in a lateral position. VA opsin-positive cells are present in the telencephalon, the rostro-dorsal ring of the left habenula, the ventral thalamus, and the midbrain tegmentum. The results show no similar co-location as found in birds, hypothesizing that the photoreceptive modulation of Gnrh in salmon may interact through neuronal networks. The topography analyses of the essential neuroendocrine cells related to sexual maturation in the Atlantic salmon brain show that diencephalic (thalamus, hypothalamus) and midbrain (tegmentum) regions seem central for controlling sexual maturation.

Transient photoreception in the hindbrain is permissive to the life history transition of hatching in Atlantic halibut

In nonmammalian vertebrates, photoreception takes place in the deep brain already early in development, but knowledge is lacking about the functions of these nonvisual photoreceptive systems. Prior to hatching, Atlantic halibut has a transient bilateral cluster of photoreceptive cells in the hindbrain. The cluster is imbedded in a neuronal network projecting to the narrow belt of hatching glands in the yolk sac. In halibut, hatching is inhibited in light and activated by transfer to darkness and c-fos analysis during hatching shows that the hindbrain cluster and hatching glands have neural activation. Unexpectedly, the hindbrain cluster expresses dual photopigments, vertebrate ancient opsin and melanopsin. Evolutionarily, these opsins are believed to belong to different classes of photopigments found in rhabdomeric and ciliary photoreceptors. The concept that an organism develops transient light sensitivity to target critical aspects of life history transitions as hatching provides a fascinating landscape to investigate the timing of other biological events.

Immunolesion of melanopsin neurons causes gonadal regression in Pekin drakes (Anas platyrhynchos domesticus)

Several light sensitive receptors have been described in the avian brain that are thought to regulate the reproductive axis independently from the eyes and pineal gland. Recently, our lab has described the presence of three of these photoneuroendocrine systems in the Pekin duck: opsin, opsin 5, & melanopsin. We set out to test the hypothesis that melanopsin receptive neurons are necessary to maintain seasonal reproductive status along with growth and development in the Pekin drake. To accomplish these goals we first investigated 50-week-old Pekin drakes that were housed in the aviary at Hope College under long day length (18h lights on) conditions in floor pens. To specifically lesion melanopsin-receptive neurons, 3μl of an anti-melanopsin-saporin conjugate (MSAP, 100ng/ul) was injected into the lateral ventricle (n=10). Control drakes were injected with 3μl of equimolar unconjugated anti-melanopsin and saporin (SAP, n=10). Reproductive behaviors were analyzed weekly in a test pen with adult hens and MSAP drakes showed a significant (p<0.01) reduction in reproductive behaviors after week 2. After 5weeks, drakes were euthanized and body weights were measured, and brains, pituitaries, and testes collected and stored for analyses. Mature MSAP-treated drakes had significantly (p<0.001) reduced relative teste weights compared to SAP controls. qRT-PCR analyses of hypothalamus showed a significant reduction (p<0.001) in GnRH and melanopsin mRNA levels, but not opsin 5, vertebrate ancient opsin, or opsin 2 (rhodopsin). Immunocytochemical analyses showed a significant reduction (p<0.01) in tyrosine hydroxylase-immunoreactivity in the PMM. These data suggest that although blue light alone is not able to maintain testicular function, the blue-light sensitive melanopsin activity is critical to maintain gonadal function.