Mapping the pattern of essential neuroendocrine cells related to puberty and VA opsin expression provides further insight in the photoreceptive regulation of the BPG 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) are 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 gonadotrophin releasing hormone (GNRH) is located in the same neurons as vertebrate ancient (VA) opsin suggesting a direct photoreceptive regulation for onset of sexual maturity. This study presents a comprehensive topographic mapping of gnrh2, gnrh3, kisspeptin 2 (kiss2) gonadotrophin 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 midbrain tegmentum. 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 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 Atlantic salmon brain show that diencephalic (thalamus, hypothalamus) and midbrain (tegmentum) regions seem central for controlling sexual maturation.