Chapter 185 - Endomorphins as Endogenous Peptides for μ-Opioid Receptor: Their Differences in the Pharmacological and Physiological Characters

Abstract

Endomorphins (EM) are endogenous μ-opioid peptides found in the mammalian brains. Although both peptides are broadly localized in the central nervous system, there are some differences in their distributions. EMs have several pharmacological and physiological characters that are identical with other endogenous opioid peptides and δ-opioid receptor (MOP-R) agonists. Like other endogenous opioid peptides or selective MOP-R agonists, EMs have a variety of biological actions. As expected, a major biological action of EMs is its remarkable antinociceptive effect. After intrathecal and intracerebroventricular injections, EMs show as potent antinociception as the other endogenous opioid peptides. Interestingly, EMs, which have only one difference in amino acid residue at position 3, show some characteristic differences from one another in their pharmacological and physiological effects. In a neuropathic pain model in which the sciatic nerve is ligated, EM-2-LI is dramatically decreased in the superficial layer of the dorsal horn. This evidence strongly suggests that EM-2 can regulate the transmission of nociceptive stimulati in the spinal cord, and the reduction of EM-2-containing neurons may be associated with the nerve ligation-induced neuropathic pain. In contrast, EM-2-LI in the periaqueductal gray matter is greatly increased by spinal nerve ligation.