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General Considerations
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.
Enkephalin is also found in several areas of the brain
and brainstem, paralleling the distribution of opiate
receptors. The highest concentration occurs in the
globus pallidus with lesser amounts in the caudate
nucleus, hypothalamus, periaqueductal gray matter, and
amygdala. The intriguing possibility exists that
enkephalins may be naturally occurring analgesics
operating as modulating neurotransmitters in various
pain-mediating pathways |
