Endomorphin peptides: Pharmacological and functional implications of these opioid peptides in the brain of mammals. Part One

The present paper describes several aspects of the biological activities, physiological and behavioral responses displayed by the most recent discovered opioid peptides: endomorphins. Endormorphins comprise two endogenous C-terminal amide tetrapeptides, named as endomorphin-1 (EM1; Tyr-Pro-Trp-Phe-NH₂) and endomorphin-2 (EM2; Tyr-Pro-Phe-Phe-NH₂), which were discovered a decade ago (1997) by Zadina's group. Initially, they reported the identification of two endogenous opioid peptides that displayed high binding affinities and selectivities for the μ-opioid receptor among other identified and cloned opioid receptors. These led authors to support the hypothesis that endomorphin peptides represent the endogenous ligand agonists for the μ-opioid receptor. Both peptides were identified and isolated from bovine and human brains. They consist of four amino acids that share a 75% structural homology among amino acids, and which display the structural α-amidated form of C-terminal -Phe- residue, as demonstrated for many other bioactive neuropeptides. These peptides are structurally distinct from other endogenous opioid substances identified in the brain of mammals, although they share some similarities with other amide terapeptides such as Tyr-W-MIF-1, found also in the mammalian brain. Here, we review the structure-relationship activity of both endomorphin molecules comparing their binding properties to different opioid receptors. Both EM1/EM2 peptides appear to be vulnerable to enzymatic degradation when exposed to the activities of different proteolytic enzymes, as occurs with many other neuroactive peptides found in the SNC of mammals. Immunohistochemical studies showed the wide and asymmetric distribution of both EM1-2 peptides in the brain, leading to the extensive pharmacological, cellular, and physiological studies that demonstrated the wide and varied bioactivities displayed by these peptides at both central and peripheral tissues. These studies led several authors to suggest the potential endogenous role of these peptides in major physiological processes (e.g. analgesia or antinociception). Based on the generation of specific (rabbit) polyclonal antibodies and the use of combined radioimmunoassay (RIA) techniques and immunohistochemical procedures, it was shown the wide distribution of EM1-2-LI (endomorphin1-2-like immunoreactivities) throughout the brain of different species (e.g. rat, primate, human), particularly co-localized in specific areas where μ-opioid receptor has been shown to be expressed. IHC mapping of endomorphin material in the CNS showed a parallelism with the neuroanatomical distribution of other endogenous opioid peptides (e.g. Met/Leu-enk, Dynorphin A, β-endorphin) previously reported. These studies showed for instance that, whereas EM1-LI was shown to be widely and densely distributed throughout the brain, particularly in forebrain structures (e.g. nucleus accumbens [NAc]; cortex [Cx]; amygdale [AMG]; thalamus [Th], the hypothalamus [Hyp], the striatum [CPu]), including the upper brainstem (BS); and dorsal root ganglia (DRG); EM2-LI is highly expressed in spinal cord and lower brainstem. Interesting enough is the demonstration of the expression of EM1-2-LI outside the CNS (e.g. spleen, thymus and blood), and detected in immune cells (e.g. macrophages/monocytes, lymphocytes, and polymophonuclear leucocytes) surrounding inflammatory foci. Pharmacological studies showed that these peptides displace with high potency several μ-opioid receptor ligands agonists in a concentration-dependent manner. Moreover, EM1-2 peptides have been shown to modulate the release of several conventional transmitters from neurons (e.g. DA, NA, 5-HT, ACh) besides on active neurohormones. Additionally, in vitro and in vivo studies showed that both EM-1/EM-2 peptides produce their pharmacological and biological effects by stimulating either μ₁ or μ₂-opioid receptors, which mediate the distinct pharmacological activities detected for each peptide. Cellular studies showed that both EM-1/EM-2 peptides induce a potent granule/vesicle endocytosis and trafficking of μ-opioid receptor in cells transfected with the μ-opioid receptor cDNA; following some endocytosis responses and μ-opioid receptor trafficking mechanisms shown in enteric neurons; cells previously reported to express naturally μ-opioid binding sites on cells. Endomorphins have been shown to induce potent antinociceptive responses after ICV or IT administration into mice; to modulate nociceptive transmission and pain sensation into the brain after stimulating peripheral nociceptors on primary neuronal afferents; and to generate cross-tolerance between endomorphin peptides and between EM1 and opiate compounds, such as morphine.