Distribution of gonadotropin-inhibitory hormone (GnIH)-like immunoreactivity in the brain and pituitary of the frog (Pelophylax esculentus) during development

Gonadotropin-inhibitory hormone (GnIH) is a newly discovered hypothalamic RFamide peptide that influences reproduction by regulating brain and pituitary neuroendocrine functions in vertebrates. We report here for the first time, the ontogenetic description of GnIH-like immunoreactivity in the brain, olfactory system, and pituitary of the frog, Pelophylax esculentus. GnIH-like immunoreactive (GnIH-ir) elements were first observed in larvae at stage 24 in the olfactory mucosa, ventral telencephalon, and diencephalon. GnIH-ir-positive staining progressively increased in frequency and intensity during larval growth and other ir perikarya appeared in the medial septum, anterior commissure, dorsal hypothalamus, and posterior tuberculum. A decline in GnIH-ir neurons was seen along the olfactory/vomeronasal/terminal nerve complex in the stages following the pre- and prometamorphosis, while other GnIH-ir neurons showed positivity in the ventromedial surface of the olfactory bulbs and into the habenular nuclei, but the latter are no longer observed in the following stages of development. The anterior-posterior axis in several brain areas, along with the median eminence and pars intermedia of the hypophysis had the appearance of GnIH-ir fibers from early stages, with a progressive increase in the number till metamorphosis in all major subdivisions of the brain. After premetamorphosis, GnIH-ir fibers arising from labeled neurons in the suprachiasmatic nucleus could be seen contacting the ventricular lumen. The transient appearance of GnIH-ir elements in the olfactory system may hint at an olfactory placode origin in the extracranial region. The distribution of GnIH in several brain regions throughout development suggests important involvement of GnIH in multiple brain functions during development.