Over the years, a number of neuropeptides have been identified which play a variety of functional roles in the nervous system. Several have well-known endocrine roles such as ACTH, oxytocin, and vasopressin from the pituitary gland. Also included are the hypothalamic factors which control the release of certain pituitary hormones. These are somatostatin (growth hormone-inhibiting factor), thyrotropin-releasing factor (TRF), and luteinizing hormone-releasing factor (LHRF).

Other neuropeptides appear to function as neurotransmitters. One of these is substance P, found in certain pathways in the brain and in terminal endings of specific primary sensory fibers of spinal nerves. The latter are represented by those fibers which synapse on secondary spinal cord neurons responding most readily to pain. Thus it is hypothesized to operate as a transmitter for painful stimuli from the periphery to the CNS.

Perhaps the most interesting group of neuropeptides are the enkephalins and endorphins. The morphinelike enkephalins have been found in inter­neurons in the same regions of the spinal cord where substance P is released. and there is evidence to suggest that they inhibit the release of substance P. Thus, enkephalin-containing neurons may work to suppress the transmission of painful information between primary and secondary neurons. Enkephalins probably operate by presynaptically inhibiting the release of substance P from primary neurons, giving them a modulatory role at these synapses.