Neuropeptide

Neuropeptides are "peptides released by neurons as intercellular messengers. Many neuropeptides are also hormones released by non-neuronal cells." In general, neuropeptides are synthesised as part of a much larger precursor molecule which is then cleaved enzymatically to yield various peptide fragments. In neurons, neuropeptides are generally packaged within large neurosecretory vesicles, that may be found in any region of the neuron - soma, dendrites, axons and nerve endings. The vesicle contents are released into the extracellular fluid when the vesicle membrane fuses with the neuronal plasma membrane in a prosess of calcium-dependent exocytosis. For example, vasopressin is made by magnocellular neurons in the supraoptic nucleus and paraventricular nucleus of the hypothalamus; it is a nine-amino acid fragment of a large (~30kD) precursor molecule which is packaged into large membrane-bound vesicles that are transported to all parts of the cell but especially to the neurosecretory nerve endings in the posterior pituitary gland. During transport, the precursor molecule is cleaved to yield a signal peptide, vasopressin, vasopressin-related neurophysin (neurophysin II), and a glycoprotein. The glycoprotein has no known function; neurophysin is important for the correct folding of the precursor hormone to allow it to be packaged in vesicles. Each vesicle contains about 85,000 molecules of vasopressin.

Types of neuropeptides

More than 100 different peptides have been shown to be expressed by different subpopulations of neurons in the CNS. Amongst them are the hypothalamic releasing hormones that control secretion of hormones from the anterior pituitary gland; for example, somatostatin is made in many neurons, including in interneurons in the hippocampus, but is also released from the neurosecretory nerve endings of neuroendocrine neurons in the periventricular nucleus of the hypothalamus to inhibit the secretion of growth hormone, while another population of neuroendocrine neurons in the arcuate nucleus of the hypothalamus synthesises and releases growth hormone releasing hormone which stimulates the secretion of growth hormone.

Amongst the best known and most widely distributed families of neuropeptides are the endogenous opioids: the dynorphins, the endorphins, the enkephalins and nociceptin.

Many neuropeptides have specific roles in particular behaviors or physiological functions, though often their names are misleading. For example, orexin was named because of its apparent ability to stimulate eating, but is now recognised as having its major role in arousal mechanisms. Conversely, important appetite regulating neuropeptides include alpha melanocyte stimulating hormone, melanin concentrating hormone, cocaine and amphetamine related transcript, and prolactin releasing peptide. Vasoactive intestinal peptide, released by neurons of the suprachiasmatic nucleus is a major controller of circadian rhythms.