Secretin
Secretin was the first hormone to be identified. In 1902, William Bayliss and Ernest Starling were studying how the nervous system controls the process of digestion. It was known that the pancreas secreted digestive juices in response to the passage of food (chyme) through the pyloric sphincter into the duodenum. They discovered (by cutting all the nerves to the pancreas in their experimental animals) that this process was not, in fact, governed by the nervous system. They determined that a substance secreted by the intestinal lining stimulates the pancreas after being transported via the bloodstream. They named this intestinal secretion "secretin". Secretin was the first such "chemical messenger" identified. This type of substance is now called a hormone, a term coined by Bayliss in 1905.
Autopharmacology
Historically, the first approach to the concept of autopharmacology began with British physiologist and pharmacologist Henry Dale in the 1910s, discovered the role of acetylcholine in synaptic transmission, and later proved by Austrian physiologist Otto Loewi, to be the neurotransmitter involved in the proximal synapses of the autonomic nervous system (initially named "Vagusstoff" by Loewi, and later identified as acetylcholine). The same happened to another autonomic neurotransmitter, noradrenaline ("Akzeleransstoff" by Loewi), which later proved to be chemically similar to a long used pharmacological agent, adrenaline, a hormone secreted by the adrenal glands. Both scientists were awarded the 1936 Nobel Prize for Physiology or Medicine for their pioneering and important contributions.