Study of the dopaminergic inhibitory system controlling the gonadotrope function in zebrafish

It was first demonstrated in a teleost fish that the stimulatory control of the gonadotrope axis by GnRH can be counterbalanced by an inhibitory control exerted by dopamine (DA). Later on, this inhibitory dopaminergic control was found in various vertebrate species. However the functional importance of this regulatory pathway varies according to the species. To deepen our knowledge on this inhibitory dopaminergic system, we used the zebrafish (Danio rerio) as a model, in which numerous molecular tools are available.

First we demonstrated that DA indeed plays a role in the neuroendocrine control of zebrafish reproduction. By injecting a dopamine receptor antagonist together with an agonist of the GnRH (GnRHa), we were able to stimulate LH expression in the pituitary, and to reactivate the spawning cycles in sexually regressed old females, an effect of which was not produced by the GnRHa alone.

We then studied the neuroanatomical basis of this inhibitory control. After observing the expression of the D2-DA receptors subtypes in LH cells, we highlighted numerous dopaminergic terminals on- or in the vicinity of- these cells. We then localized, by DiI retrograde tracing experiments in adult zebrafish, the dopaminergic cell bodies giving rise to these projections in the most antero-ventral part of the preoptic area. We have called these hypophysiotropic neurons the preoptico-hypophysial (POH) DA neurons.

We next studied the development of POHDA neurons. Taking advantage of early anatomical landmarks, we followed the embryonic development of these cells. We showed that the first POHDA neurons arise at around 72 hours post fertilization (hpf), more than 24 hours later that the DA neurons in the neighbor suprachiasmatic nucleus (SCDA). This late differentiation would explain why POHDA neurons have not been studied in the developing embryo so far. We showed that contrary to the number of the SCDA neurons, which is constant all along the fish life, that of POHDA neurons increases proportionally to the growth of the fish due to continuous neurogenesis. Finally, we examined the expression profiles of developmental genes related to the regionalization of the anterior forebrain. We showed that the genetic networks involved in the development of POHDA and SCDA populations are at least partly different.

To summarize, this work demonstrates for the first time the existence of a dopaminergic inhibitory control of gonadotrope function in zebrafish. It describes the anatomy of the preoptico-hypophyseal dopaminergic system supporting these DA actions and the setting up of these neurons during embryonic development. We show that these neuroendocrine population displays neurogenesis even during adulthood. Our findings also provide the genetic bases for future functional studies on the development of POHDA, a poorly studied neuroendocrine DA population.

Keywords: Dopamine, neuroendocrine, reproduction, development,zebrafish