Structure and Function of the Head Activator in Hydra and in Mammals

Abstract

Publisher Summary This chapter focuses on the structure and function of the head activator (HA) in hydra and in mammals. In normal hydra, the HA is produced by nerve cells. Within the nerve cells, HA is stored in neurosecretory granules. The chapter discusses the effects of the HA on cellular growth and differentiation in hydra. The HA is also present in other animals, such as artemia, drosophila, frogs, and mammals. In the mammalian system, the HA occurs in three main locations—the hypothalamus, the retina, and the gastrointestinal tract. It is also present in smaller amounts in the other parts of the neuronal, neuroendocrine, or paracrine system. The HA occurs also in body fluids, such as the blood and cerebrospinal fluid, and in tumors or tumor cell lines predominantly of neuronal or endocrine origin. The physiological role of this peptide in mammals is that (1) it is involved as a modulator of brain function possibly as an information intermediary between hypothalamus and hypophysis, (2) it is involved in digestion control processes as indicated by elevated levels in the blood after a meal and by its stimulatory effect on pancreas secretion, and (3) as a possible growth-promoting agent in autocrine control processes.