Comparative analysis of Met-enkephalin, galanin and GABA immunoreactivity in the developing trout preoptic–hypophyseal system

We studied the organization of Met-enkephalin-containing cells and fibers in the developing preoptic–hypophyseal system of the brown trout (Salmo trutta fario) by immunohistochemistry and determined the relationship of these cells and fibers to the galaninergic and GABAergic systems. Met-enkephalin immunoreactivity was observed in cells in the preoptic area, the hypothalamus and the pituitary of late larvae. In the hypophysis, a few Met-enkephalin-containing cells were present in all divisions of the adenohypophysis, and some immunoreactive fibers were present in the interdigitations of the neural lobe with the proximal pars distalis. Concurrently, GABAergic fibers innervated the anterior and posterior neural lobe. Galanin cells coexisted with Met-enkephalin cells in neuronal groups of the preoptic–hypophyseal system. Galaninergic and GABAergic fibers innervated the preoptic and hypothalamic areas, but GABAergic fibers containing galanin were not observed. These results indicate that Met-enkephalin, galanin and GABA may modulate neuroendocrine activities in the preoptic area, hypothalamus and pituitary during the transition from larval to juvenile period. To better know how the development of the trout preoptic–hypophyseal system takes place, we studied the patterns of cell proliferation and expression of Pax6, a conserved transcription factor involved in the hypophysis development. Pax6 expressing cells and proliferating cells were present in the Rathke’s pouch, the hypothalamus and the hypophysis of early larvae. In late larvae, Pax6 expression was no longer observed in these areas, and the density of proliferating cells largely decreased throughout development, although they remained in the hypophysis of late larvae and juveniles, suggesting that Pax6 might play an important role in the early regionalization of the pituitary in the trout.

► Met-enkephalin coexists with galanin in different nuclei of the preoptic–hypophysial system in the trout.
► Galanin and glutamic acid decarboxylase positive fibers innervate preoptic and hypothalamic areas, but both substances do not colocalize.
► Pax6 and proliferating cells are present in the Rathke’s pouch in trout embryos.
► Pax6 expression disappeared during larval period.
► Proliferating cell nuclear antigen containing cells persisted in the hypothalamus and hypophysis throughout development.