Neuroprotective Effect of Naloxone in Inflammation-Mediated Dopaminergic Neurodegeneration: Dissociation from the Involvement of Opioid Receptors

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 β-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 ).