In many vertebrates, the stimulatory control of the gonadotrope axis by GnRH is opposed by an inhibitory control exerted by dopamine (DA). However, the anatomical support of this control has been poorly studied. To get a deeper knowledge of the DA neurons projecting to the pituitary and involved in this control, we used the zebrafish (Danio rerio).
We first determined the origin of the DA fibers innervating gonadotrope cells, by retrograde tracing experiments. We thus injected DiI microcrystals in the pituitary attached to the brain of adult fish and performed tyrosine hydroxylase (TH) immunofluorescence (IF) on their brain after DiI migration. We observed that the DA neurons projecting to the pituitary originated in the most antero-ventral part of the preoptic area, just anterior to the optic recess (OR). We called these neurons preoptico-hypophyseal DA neurons (POHDA). Since in teleosts, two non-allelic genes (th1 and th2) encode the key-enzyme in DA biosynthesis, wecharacterized th geneexpression in these neurons by double Fluorescent In Situ Hybridization (FISH). We showed that all the POHDA neurons express th1, and some of them co-express th2.
The development of POHDA neurons was then studied, and compared with that of the suprachiasmatic DA (SCDA) neurons, which are just posterior to the OR. Firstly we examined the schedule of TH expression using TH IF and noticed that POHDA neurons express TH relatively late (72hpf) as compared to all other DA populations, and that their terminals reach the pituitary at around 84hpf. Secondly, we examined the expression patterns of several developmental genes (foxg1, otpa/b, nurr1a/b, lhx5, dlx2a, and six3b), by FISH combined with TH IF.
This is, to our knowledge, the first study characterizing the DA neuroendocrine neurons in zebrafish and providing some insights about the development of the DA neurons exerting an inhibitory action on gonadotropin secretion.