Abstract
Endomorphin (Endo) 1 and 2, two tetrapeptides isolated from the bovine and human brain, have been proposed to be the endogenous ligand for the μ-opiate receptor. A multi-disciplinary study was undertaken to address the issues of localization, release and biological action of Endo with respect to the rat dorsal horn. First, immunohistochemical studies showed that Endo-1- or Endo-2-like immunoreactivity (Endo-1- or Endo-2-LI) is selectively expressed in fiber-like elements occupying the superficial layers of the rat dorsal horn, which also exhibit a high level of μ-opiate receptor immunoreactivity. Second, release of immunoreactive Endo-2-like substances (irEndo) from the in vitro rat spinal cords upon electrical stimulation of dorsal root afferent fibers was detected by the immobilized antibody microprobe technique. The site of release corresponded to laminae I and II where the highest density of Endo-2-LI fibers was localized. Lastly, whole-cell patch clamp recordings from substantia gelatinosa (SG) neurons of rat lumbar spinal cord slices revealed two distinct actions of exogenous Endo-1 and Endo-2: (1) depression of excitatory and/or inhibitory postsynaptic potentials evoked by stimulation of dorsal root entry zone, and (2) hyperpolarization of SG neurons. These two effects were prevented by the selective μ-opiate receptor antagonist β-funaltrexamine. The localization of endomorphin-positive fibers in superficial layers of the dorsal horn and the release of irEndo upon stimulation of dorsal root afferents together with the observation that Endo inhibits the activity of SG neurons by interacting with μ-opiate receptors provide additional support of a role of Endo as the endogenous ligand for the μ-opiate receptor in the rat dorsal horn.
This is a preview of subscription content, log in via an institution to check access.
Similar content being viewed by others
References
Besse D, Lombard MC, Zajac JM, Roques BP, Besson JM. Pre- and postsynaptic distribution of μ, δ and κ opioid receptors in the superficial layers of the cervical dorsal horn of the rat spinal cord. Brain Res 521:15–22;1990.
Ding YQ, Keniko T, Nomura S, Mizuno N. Immunohistochemical localization of μ-opiate receptors in the central nervous system of the rat. J Comp Neurol 367:375–402;1996.
Duggan AW, Hendry IA, Green JL, Morton CR, Hutchison WD. The preparation and use of antibody microprobes. J Neurosci Methods 23:241–247;1988.
Dun NJ, Dun SL, Wu SY, Forstermann U, Schmidt HHHW, Tseng LF. Nictric oxide synthase immunoreactivity in the rat, mouse, cat and squirrel monkey spinal cord. Neuroscience 54:845–857;1993.
Hackler L, Zadina JE, Ge LJ, Kastin AJ. Isolation of relatively large amounts of endomorphin-1 and endomorphin-2 from human brain cortex. Peptides 18:1635–1639;1997.
Hendry IA, Morton CR, Duggan AW. Analysis of antibody microprobe autoradiography by computerized image processing. J Neurosci Methods 23:249–256;1988.
Lai CC, Wu SY, Dun SL, Dun NJ: Nociceptin-like immunoreactivity in the rat dorsal horn and inhibition of substantia gelatinosa neurons. Neuroscience 81;887–891;1997.
Martin-Schild S, Gerall AA, Kastin AJ, Zadina JE. Endomorphin-2 is an endogenous opioid in primary sensory afferent fibers. Peptides 19:1783–1789;1998.
Martin-Schild S, Gerall AA, Kastin AJ, Zadina JE. Differential distribution of endomorphin 1- and endomorphin 2-like immunoreactivities in the CNS of the rodent. J Comp Neurol 450:450–471;1999.
Mollereau C, Pamentier M, Maileux R, Butour JL, Moisand C, Chalon P, Caput D, Vassart G, Meunier JC. ORL1, a novel member of the opioid receptor family. Fed Eur Biochem Soc Lett 341:33–38;1994.
North RA. Opioid actions on membrane ion channels. In: Opioids 1. Handbook of Experimental Pharmacology. Berlin, Springer, 104:773–795;1993a.
North RA. Presynaptic actions of opioids. In: Dunwiddie TV, Lovinger DV, eds. Presynaptic Receptors in the Mammalian Brain. Boston, Birkhäuser, 71–86;1993b.
Pierce TL, Grahek MD, Wessendorf MW. Immunoreactivity for endomorphin-2 occurs in primary afferents in rats and monkey. Neuroreport 9:385–389;1998.
Schreff M, Schulz S, Wiborny D, Holt V. Immunofluorescent identification of endomorphin-2 containing nerve fibers and terminals in the rat brain and spinal cord. Neuroreport 9:1031–1034;1998.
Stone LS, Fairbanks CA, Laughlin TM, Nguyen HO, Bushy TM, Wessendorf MW, Wilcox GL. Spinal analgesic actions of the new endogenous opioid peptides endomorphin-1 and -2. Neuroreport 8:3131–3135;1997.
Wick MJ, Minnerath SR, Lin X, Elde R, Law PY, Loh HH. Isolation of a novel cDNA encoding a putative membrane receptor with high homology to the cloned μ, δ and κ opioid receptor. Mol Brain Res 117:1609–1611;1994.
Williams CA, Wu SY, Cooke J, Dun NJ. Release of nociceptin-like substances from the rat spinal cord dorsal horn. Neurosci Lett 244:141–144;1998.
Williams CA, Wu SY, Dun SL, Kwok EH, Dun NJ. Release of endomorphin-2 like substances from the rat spinal cord. Neurosci Lett 273:25–28;1999.
Wu SY, Dun SL, Wright MT, Chang JK, Dun NJ. Endomorphin-like immunoreactivity in the rat dorsal horn and inhibition of substantia gelatinosa neurons in vitro. Neuroscience 89:317–321;1999.
Zadina JE, Hackler L, Ge L-J, Kastin AJ. A potent and selective endogenous agonist for the μ-opiate receptor. Nature 386:499–502;1997.
Author information
Authors and Affiliations
Rights and permissions
About this article
Cite this article
Dun, N.J., Dun, S.L., Wu, S.Y. et al. Endomorphins: Localization, release and action on rat dorsal horn neurons. J Biomed Sci 7, 213–220 (2000). https://doi.org/10.1007/BF02255468
Issue Date:
DOI: https://doi.org/10.1007/BF02255468