Abstract
The neuroplastic mechanisms in the fish brain that underlie sex reversal remain unknown. Gonadotropin-releasing hormone 3 (GnRH3) neurons control male reproductive behaviours in Mozambique tilapia and show sexual dimorphism, with males having a greater number of GnRH3 neurons. Treatment with androgens such as 11-ketotestosterone (KT), but not 17β-estradiol, increases the number of GnRH3 neurons in mature females to a level similar to that observed in mature males. Compared with oestrogen, the effect of androgen on neurogenesis remains less clear. The present study examined the effects of 11-KT, a non-aromatizable androgen, on cellular proliferation, neurogenesis, generation of GnRH3 neurons and expression of cell cycle-related genes in mature females. The number of proliferating cell nuclear antigen-positive cells was increased by 11-KT. Simultaneous injection of bromodeoxyuridine and 11-KT significantly increased the number of newly-generated (newly-proliferated) neurons, but did not affect radial glial cells, and also resulted in newly-generated GnRH3 neurons. Transcriptome analysis showed that 11-KT modulates the expression of genes related to the cell cycle process. These findings suggest that tilapia could serve as a good animal model to elucidate the effects of androgen on adult neurogenesis and the mechanisms for sex reversal in the fish brain.
Highlights
Sex reversal is a well-known phenomenon in fishes, with some naturally changing their phenotype during their lifetime and others switching sexes in response to environmental factors or hormone treatment
To evaluate the androgen effects on cellular proliferation, we focused on transcripts that were clustered within “Cyclins and Cell Cycle Regulation” pathway in PathCards and “Cell Cycle” pathway according to the Kyoto Encyclopedia of Genes and Genomes (KEGG)
This study revealed the promotive activity of androgens on cellular proliferation, adult neurogenesis, and the production of Gonadotropin-releasing hormone 3 (GnRH3) neurons, partly via adult neurogenesis, in the female tilapia brain
Summary
11-KT-induces increase in proliferating cells (PCNA-positive) in the periventricular regions of mature females. 11-KT-treated female fish displayed many Hu-positive cells with a BrdU-labelled nucleus (newly-generated/proliferated neurons) along the ventricle and in substantial areas of the dorsal and ventral parts of the brain (red cells with green nuclei in Fig. 2a-J and L). The number of newly-generated neurons (Hu-positive cells with a BrdU-labelled nucleus) in the dorsal and ventral periventricular regions, in a frontal section cutting through the TN, were markedly increased in 11-KT-injected females, compared with controls (p < 0.01 or p < 0.001; controls vs 11-KT-treated females, unpaired Student’s t-test). We found many GFAP-positive cells in the dorsal and ventral areas around the ventricle in both control and 11-KT-treated females (red cells in Fig. 3); no GFAP-positive cells with a BrdU-labelled nucleus were observed (Fig. 3A’–D’) These findings indicated that 11-KT did not increase the proliferation of ventricular radial glial cells around the TN in the female brain.
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