Kisspeptin-2 stimulates testicular function in adult pejerrey (Odontesthes bonariensis): Does it act directly on the testes?

Kisspeptin is a neuropeptide that regulates GnRHergic neurons in mammals; however, its function in teleost fish remains less understood. This study examined the role of kisspeptin2 (Kiss2) in the regulation of testicular function in adult pejerrey (Odontesthes bonariensis), with a particular focus on differentiating between central and direct gonadal effects. Spermiating males were intraperitoneally injected with homologous Kiss2 (pjKiss2) at 0, 50, or 250 ng/g body weight. Administration of pjKiss2 at 50 ng/g induced a significant decrease in the testicular lobular area occupied by sperm, indicative of sperm release, associated with a trend of increased plasma androgen levels. Moreover, in vitro incubation of testicular explants with pjKiss2 (100 nM) led to an increase in androgens levels in the incubation media after 6 h. Immunohistochemical analyses revealed the presence of Kiss2-expressing neurons in the hypothalamus, consistent with previous reports in other teleosts. Also, a few Kiss2-immunoreactive fibers were observed in the pituitary gland, as well as Kiss2-expressing cells in the proximal pars distalis (PPD) of this gland. Notably, Kiss2 immunoreactivity was detected in the spermatogonia in the testis. Taken together, these results demonstrate that Kiss2 could directly modulate testicular function and promote spermiation in pejerrey, suggesting a local intragonadal role of kisspeptin beyond its classical neuroendocrine actions.