neurohormone, any of a group of substances that are produced by specialized neurosecretory cells of the nervous system, and that are released into the blood, similar to hormones of the endocrine system. Neurohormones pass along axons (portions of neurons that carry impulses away from the neuron cell body) and are released into the bloodstream at special regions known as neurohemal organs. Neurohormones thus constitute a linkage between sensory stimuli (events or conditions perceived by the nervous system) and chemical responses (endocrine secretions that act on other tissues of the endocrine system or on tissues of other systems, such as those involved with excretion or reproduction).
Neurohormones may be classified in any of several ways, including by composition and action. Examples of short neurohormones, those with relatively short polypeptide chains, include oxytocin and vasopressin. Both oxytocin and vasopressin are produced in the hypothalamus of the brain and are then secreted into the blood by the neurohypophysis (part of the pituitary gland). For this reason, the two hormones are sometimes also classified as so-called neurohypophysial hormones.
Another group of neurohormones, known as releasing hormones (the first of which, thyrotropin-releasing hormone, was chemically identified in 1969), also originates in the hypothalamus. The members of this group are transmitted within the neural cells to a second locus in the brain, from which they pass in the bloodstream to the adenohypophysis, which also is a part of the pituitary gland. There, they either stimulate or inhibit the release of the various adenohypophysial hormones.
Enkephalins and other endorphins, first observed in 1975 in the course of investigations of the mechanism of action of morphine and other analgesics (pain relievers), act as inhibitory neurotransmitters. Their neurohormonal activity is manifested by their stimulation of the secretion of growth hormone and vasopressin by an indirect process involving a site (other than the secretory neuron) in the central nervous system.