vasoactive intestinal peptide is typically co-released with acetylcholine. Neuropeptide release can also be specific. In Drosophila larvae, for example, eclosion hormone is expressed in just two neurons. Discovery The first neuropeptide, Substance P, was discovered by Ulf von Euler and John Gaddum in 1931. In the early 1900s, chemical messengers were crudely extracted from whole animal brains and tissues and studied for their physiological effects. In an effort to isolate and study acetylcholine, von Euler and Gaddum made a crude powder extract from whole equine brain and intestine and found that it induced muscle contractions and depressed blood pressure. The effects were not abolished by atropine and thus could not solely be attributed to acetylcholine. Substance P was first purified and sequenced in 1971 by Michael Chang and Susan Leeman, revealing its 11 amino-acid peptide chain. Similar methods were used to identify other neuropeptides in the early 1950s, such as vasopressin and oxytocin. In insects, proctolin was the first neuropeptide to be isolated and sequenced. In 1975, Alvin Starratt and Brian Brown extracted the pentapeptide from hindgut muscles of the cockroach and found that its application enhanced muscle contractions. While Starratt and Brown initially thought of proctolin as an excitatory neurotransmitter, proctolin was later confirmed as a neuromodulatory peptide. The term “neuropeptide” was first used in the 1970s by David de Wied, who studied the effects of the peptide hormones ACTH, MSH, and vasopressin on learning and memory. Receptor targets Most neuropeptides act on G-protein coupled receptors (GPCRs). Neuropeptide-GPCRs fall into two families: rhodopsin-like and the secretin class. Most peptides activate a single GPCR, while some activate multiple GPCRs (e.g. AstA, AstC, DTK). Peptide-GPCR binding relationships are highly conserved across animals. Aside from conserved structural relationships, some peptide-GPCR functions are also conserved across the animal kingdom. For example, neuropeptide F/neuropeptide Y signaling is structurally and functionally conserved between insects and mammals. Although peptides mostly target metabotropic receptors, there is some evidence that neuropeptides bind to other receptor targets. Peptide-gated ion channels (FMRFamide-gated sodium channels) have been found in snails and Hydra. Other examples of non-GPCR targets include: insulin-like peptides and tyrosine-kinase receptors in Drosophila and atrial natriuretic peptide and eclosion hormone with membrane-bound guanylyl cyclase receptors in mammals and insects. Actions Neuropeptides are extremely ancient and highly diverse chemical messengers. Indeed, placozoans such as Trichoplax, extremely basal animals which do not yet possess neurones, use peptides for cell-to-cell communication in