Neuropeptides in the Amphibian Brain

This chapter focuses on neuropeptides in the amphibian brain emphasizing neuroendocrine relations. Neuropeptides are synthesized as multiple molecular variants, and they may play different roles in various organisms. An analysis of the distribution of neuropeptides in vertebrates and their phenotypic plasticity, especially during development, leads to an understanding of the basic neurochemical organization but, at the same time, gives an appreciation of the wide diversity among species. The neuroendocrine and immunological functions exerted by neuropeptides constitute a fundamental link with other homeostatic systems that permit adaptation to a changing environment. Gonadotropin-releasing hormone (GnRH) is a decapeptide amide originally identified in the brain of a pig for its ability to stimulate the release of gonadotropins, luteinizing hormone (LH), and follicle-stimulating hormone (FSH) from the anterior pituitary. The main groups of GnRH-secreting perikarya are located within the telencephalon from the olfactory bulb to the septum. The distribution of GnRH cell bodies and fibers corresponds to the course of the olfactory nervus terminalis, which is a small cranial nerve made of fibers and associated cells that form glanglion-like clusters. In addition, GnRH stimulates the synthesis and release of gonadotropins from the amphibian pituitary. Mammalian GnRH—added in vitro to interrenal gland from male and female Rana esculenta—induces a significant increase in prostaglandin F₂ concentrations.