GnRH immunodetection in the brain of the holocephalan fish Chimaera monstrosa L.: Correlation to oocyte maturation

Chimera monstrosa (rabbit fish) like other holocephalans is a rare, delicate deep sea fish. Owing to the difficulty of sampling individuals in good shape, there is a paucity of information available on the morphology and physiology of this species especially concerning reproduction. In holocephalans, a hypothalamus-pituitary-gonadal axis has been postulated and a GnRH molecule identical to cGnRH II has been identified. The aim of the present study was to correlate the presence of steroidogenic enzymes in the ovarian follicles with the presence of GnRH in the hypothalamus. Estrogens, the steroids that trigger the accumulation of yolk (vitellogenesis) in the oocytes are synthesized by the somatic cells of the follicle in the vitellogenic stages via a cascade of steroid dehydrogenases involving 3β-hydroxysteroid-dehydrogenase (3β-HSD; in the inner thecal layer) and aromatase cytochrome (P450; granulosa layer). Our results showed that 3β-HSD is present concomitant with the presence of cGnRH II in the preoptic area and in the ventral hypothalamus. Another form of immunoreactive GnRH, mGnRH is also present in the brain of C. monstrosa. It is localized in the ventral telencephalon and in the midbrain caudal diencephalon (boundary between ventral thalamus and tegmentum of the mesencephalon). This form of GnRH is probably correlated with sexual behaviour.