First discovered by William Bayliss and Ernest Starling in 1902, and named by the latter 1905, they are an extensive group of molecules with a small mass secreted by the endocrine glands of vertebrate species, but which are also produced by many other organ systems and tissue types. The molecules are secreted directly into the bloodstream, as well as into adjacent tissues and other body fluids, and their effects are very varied: some act to stimulate growth (e.g., of secondary sexual characteristics), others inhibit growth, while still others can induce or suppress apoptosis. They can also activate or inhibit the immune system and regulate metabolism. The majority of hormones are peptide hormones or steroid hormones, and also include androgens, corticosteroid hormones, gonadtrophins, neuropeptides, sex hormones, and trophic hormones. Many hormones also function as neurotransmitters (e.g., endorphins, norepinephrine). A multifunctional hormone attracting increasing attention currently is ghrelin because it can potentially serve as a neuroprotective factor in the hippocampus, hypothalmus and cortex, as well as promoting adiposity and appetite. It has also been claimed that ghrelin activates the mesolimbic cholinergic-dopminergic reward link that communicates the hedonic rewards such as foods, as well as addictive drugs such as ethanol. Mainly produced in the stomach, but also in the arcuate nucleus, it was first reported by Masayasu Kojima and colleagues in 1999, and it is now thought to be essential for cognitive adjustments to new environments and for the process of leaning in general.
See Adrenal cortex, Adrenal glands, Adrenal medulla, Androgen, Apoptosis (or cell death), Arcuate nucleus, Cerebral cortex (or pallium), Cholesterol, Corticosteroids, Corticotropin-releasing hormone (CRH), Endorphins, Estradiol, Estrogen, Epinephrine (or adrenaline), Fetal programming, Gonadotropin releasing hormone (GnRH), Growth hormone (or somatotropin), Hippocampus, Hormones, Hypothalamic-pituitary-adrenal (HPA) axis, Hypothalamus, Ligands, Lipids, Menarche, Metabolism, Norephinephrine (or noradrenaline, Oxytocin, Pancreas, Peptides, Pituitary gland, Progesterone, Prolactin, Protein-folding problem, Secondary sexual characteristics, Smooth (or involuntary) muscle, Steroid hormones, Testosterone