Involvement of hormones in olfactory imprinting and homing in chum salmon

Abstract

The olfactory hypothesis for salmon imprinting and homing to their natal stream is well known, but the endocrine hormonal control mechanisms of olfactory memory formation in juveniles and retrieval in adults remain unclear. In brains of hatchery-reared underyearling juvenile chum salmon (Oncorhynchus keta), thyrotropin-releasing hormone gene expression increased immediately after release from a hatchery into the natal stream, and the expression of the essential NR1 subunit of the N-methyl-D-aspartate receptor increased during downstream migration. Gene expression of salmon gonadotropin-releasing hormone (sGnRH) and NR1 increased in the adult chum salmon brain during homing from the Bering Sea to the natal hatchery. Thyroid hormone treatment in juveniles enhanced NR1 gene activation, and GnRHa treatment in adults improved stream odour discrimination. Olfactory memory formation during juvenile downstream migration and retrieval during adult homing migration of chum salmon might be controlled by endocrine hormones and could be clarified using NR1 as a molecular marker.

Physiological Mechanisms of Imprinting and Homing Migration of Pacific Salmon

“…The increase of NR1 gene expressions in the OB show that olfactory memory retrieval abilities increase during the upstream homing migration of adult chum salmon. Interestingly, sGnRH-II gene expression in the hypothalamus of males was also high in the Bering Sea (Ueda et al 2016). These results reveal that sGnRH is involved in brain regiondependent roles on gonadal maturation and olfactory memory retrieval in adult chum salmon.…”

“…7). Ueda et al (2016) also demonstrated that NR1 expression levels showed a significant first peak correlated with larval brain development in the hatchery (Pouwels 1978), a gradual significant increase immediately after a significant TRHa/b expression surge at Site C, and a second significant peak at the mouth of their natal river at Site E (Fig. 7).…”

“…The results of in situ hybridization confirmed that NR1 mRNA localization increased prominently in the ventral telencephalon (TE) at Site C compared to the hatchery immediately before release into the river. Moreover, in experiments with the oral administration of T4 (2 mg/g pellet) to juvenile salmon, Ueda et al (2016) found increased T4 and T3 levels in the whole body and significant increases in NR1 gene expression in the whole brain for 4 days (Fig. 8).…”

“…It is interesting to note that both sGnRH levels in the TE and serum T levels in both sexes showed a coincident peak at the branch point of the Chitose River from the Ishikari River, Site I. Ueda et al (2016) showed that the mRNA expression levels of sGnRH-I/II, and NR1 in the OB and TE appeared to increase, with some differences between males and females that might be related to sex-specific gonadal maturation (Fig. 11).…”

“…The relationship between BPT hormones and natal stream odor imprinting was investigated based on whole-brain mRNA expression of TRHa/b and TSHb in the upper head that contained the pituitary gland, levels of T4 and T3 isolated in the lower jaw that contained thyroid glands, and on whole-brain mRNA expression of NR1. Ueda et al (2016) analyzed the gene expression profiles of TRHa/b and TSHb in juvenile chum salmon, and revealed that environmental changes due to the release from the hatchery to the natal stream significantly enhanced the TRHa/b expression levels and then the TSHb expression levels and T4/T3 levels increased significantly during downstream migration towards the sea (Fig. 7).…”