Biomedical Engineering Reference
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Neuroprotective Effect of Naloxone
in Infl ammation-Mediated Dopaminergic
Neurodegeneration
Dissociation from the Involvement of Opioid Receptors
Bin Liu and Jau-Shyong Hong
1. The Opioid System
Historically, the nociceptive/analgesic effect of naturally occurring opiates
such as morphine has long been recognized by humans. Advances in research in
the last several decades have revealed the existence of the so-called endogenous
opioid peptides, can be divided into three classes: dynorphins, enkephalins, and
-endorphins. Contrary to the initial understanding, in addition to the cells of
the central nervous system, those of peripheral tissues such as cardiac myocytes
and heart tissues also express opioid peptides
(1-3)
. The wide distribution of
opioid peptides throughout the body underscores their involvement in a variety
of cellular activities including pain regulation, respiration, immune responses,
and ion channel activity
(4)
as well as possibly pathophysiological conditions
such as asthma, alcoholism, and eating disorders
(5-7)
.
Endogenous opioid peptides are synthesized as biologically inactive precur-
sor polypeptides termed preprodynorphin, preproenkephalin, and preproopio-
imelanocortin, the last being a precursor of
-endorphins
(8)
. Precise processing
by the action of highly regulated proteolytic enzymes converts the inert
polypeptides into active fragments of varying lengths and bioactivity. It is now
well known that endogenous opioid peptides exert their bioactivity through
binding to cell surface receptors. Intensive research by means of ligand binding
and molecular cloning studies in the last 40 yr has identifi ed at least three
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