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.