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

himera monstrosa (rabbit fish) like other holocephalans is a rare, delicate deep sea fish. Owing to the dif- ficulty of sampling individuals in good shape, there is a paucity of information available on the morphol- ogy 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 trig- ger 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 3b-hydroxysteroid- dehydrogenase (3b-HSD; in the inner thecal layer) and aromatase cytochrome (P450; granulosa layer). Our results showed that 3b-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 dien- cephalon (boundary between ventral thalamus and tegmentum of the mesencephalon). This form of GnRH is probably correlated with sexual behaviour.