Comparative endocrinology of piscine hypothalamic hypophysiotropic peptides: Distribution and activity

Abstract

Fishes display a variety of anatomical relationships between brain and pituitary to a degree unique among vertebrates. This group is pivotal for understanding evolution of functions of hypophysiotropic peptides. We review information concerning occurrences, distributions and physiological activities of three identified peptides in fish brain, and biological properties of fish brain extracts. Thyrotropin releasing hormone may be present universally in piscine central nervous tissue; however, this peptide has not been clearly demonstrated to have hypophysiotropic activity in fishes. Somatostatin also has been shown to occur in fish brains; studies of actions of this substance are virtually absent. Gonadotropin releasing hormone is apparently of broad occurrence in fishes; its hypophysiotropic activity is well established for several teleostean species. Anatomical relationships between brain and pituitary are particularly varied among elasmobranchs. Investigations involving additional elasmobranch representatives, as well as other fishes, are needed before generalizations can be made. Widespread extrahypothalamic distribution of hypophysiotropic peptides in lower vertebrates and neurotransmitter (or related) functions of neurones containing these peptides provide a basis for proposals concerning evolution of hypothalamic control of the pituitary gland. © 1978 by the American Society of Zoologists.