Gonadotropin-inhibitory hormone (GnIH), a neuropeptide, suppresses gonadotropin (GTH) secretion in birds and mammals. In fish, the GnIH homolog LPXRFamide (LPXRFa) produces mature peptides with species-dependent effects on sexual reproduction. Here, we investigated the effects of LPXRFa on GTH secretion in the chub mackerel (cm; Scomber japonicus). We cloned cmlpxrfa (603 bp) and cmlpxrfa-r (1416 bp). Additionally, we isolated lpxrfa from the bluefin tuna (Thunnus orientalis) to confirm the conservation of the LPXRFa mature sequence. Phylogenetic analysis showed that the LPXRFa precursor protein produces three mature peptides, LPXRFa-1, -2, and -3, in both species. Reverse transcription-quantitative polymerase chain reaction revealed that cmlpxrfa is expressed in the hypothalamus and thalamus and midbrain (T.MB), and sexual differences were observed. Receptor expression was observed in the preoptic area, hypothalamus, T.MB, and pituitary. Female hypothalamic lpxrfa expression did not change during puberty. Reporter gene assay showed that LPXRFa induced receptor activation via the CRE and SRE signaling pathways. However, in the presence of forskolin, an intracellular cyclic AMP enhancer, none of the LPXRFa could suppress receptor activity. The in vitro bioassay results showed that gonadotropin-releasing hormone-1 (GnRH1) had no effect on follicle-stimulating hormone (FSH) secretion, whereas the three LPXRFa significantly increased FSH secretion in pituitary cells from male chub mackerel. Contrarily, GnRH1 and three LPXRFa significantly increased luteinizing hormone secretion. The in vivo administration of LPXRFa had no effect on fshb and lhb expression in prepubertal and mature male chub mackerel. Overall, cmLPXRFa lacks the ability to suppress GTH secretion but can promote GTH secretion.

Summary sentence

The chub mackerel LPXRFamide gene produces three mature peptides and not have the ability to suppress gonadotropin secretion but may promote gonadotropin secretion.