Ontogenetic development and neuroanatomical localization of growth hormone‐releasing hormone (GHRH) in the brain and pituitary gland of pejerrey fish Odontesthes bonariensis

The presence and distribution of growth hormone‐releasing hormone (GHRH) were studied by immunocytochemistry in adult and developing pejerrey fish, Odontesthes bonariensis (Atheriniformes). A few perikarya and fibers with immunoreactivity to GHRH (ir‐GHRH) were identified in the olfactory bulbs at hatching. One week later, scattered ir‐GHRH cell bodies were observed in the preoptic area and some fibers were detected entering the pituitary gland. Isolated ir‐GHRH perikarya were revealed in the hypothalamus and in the medulla oblongata (MO) 3 weeks after hatching. Seven weeks after hatching, ir‐GHRH cells were also identified in the nucleus of the lateral lemniscus and the cerebellum . Both nuclei presented strong ir‐GHRH projections extending rostro‐ventrally. At 11 weeks after hatching another group of ir‐GHRH cells were revealed in the midbrain tegmentum . After that time the pattern of distribution of ir‐GHRH structures remained unchanged. At 1 week after hatching and later, the pituitary gland consistently revealed ir‐GHRH cells and fibers mainly in the proximalis pars distalis and in a minor proportion of the pars intermedia since week 1. The pineal gland showed ir‐GHRH cells projecting into the pineal lumen, at week 6 after hatching and later. The pineal stalk and the subcomissural organ also presented ir‐GHRH structures. Additionally, ir‐GHRH material was found from week 3 to the adult stage in the following extraneural organs: gills, gut, kidney and hepatopancreas. These results represent the anatomical substrate for understanding the physiology of GHRH peptide in pejerrey, adding information on the ontogeny of neural structures expressing GHRH.