title: Endomorphin-1 and endomorphin-2 modulate responses of trigeminal neurons evoked by N-methyl-D-aspartic acid and somatosensory stimuli.
abstract: The present study investigated the modulation of N-methyl-D-aspartate (NMDA)-evoked and peripheral cutaneous stimulus-evoked responses of trigeminal neurons by endomorphins, endogenous ligands for the mu-opioid receptor. Effects of endomorphins, administered microiontophoretically, were tested on the responses of nociceptive neurons recorded in the superficial and deeper dorsal horn of the medulla (trigeminal nucleus caudalis) in anesthetized rats. Endomorphin-1 and endomorphin-2 predominantly reduced the NMDA-evoked responses, producing an inhibitory effect of 54.1 +/- 2.96% (mean +/- SE; n = 34, P < 0.001) in 92% (34/37) of neurons and 63.6 +/- 3.61% (n = 32, P < 0.001) in 91% (32/35) of neurons, respectively. The inhibitory effect of endomorphins was modality specific; noxious stimulus-evoked responses were reduced more than nonnoxious stimulus-evoked responses. Naloxone applied at iontophoretic current that blocked the inhibitory effect of [D-Ala(2), N-Me-Phe(4), Gly(5)-ol]-enkephalin, reduced the peak inhibitory effect of endomorphins on the NMDA- and natural stimulus-evoked responses. We suggest that endomorphins by acting at micro-opioid receptor selectively modulate noxious stimulus-evoked responses in the medullary dorsal horn.
authors.affiliation: Department of Anatomy and Physiology, Meharry Medical College, Nashville, Tennessee 37208, USA.
authors.forename: X M
authors.initials: XM
authors.lastname: Wang
authors.forename: K M
authors.initials: KM
authors.lastname: Zhang
authors.forename: L O
authors.initials: LO
authors.lastname: Long
authors.forename: C A
authors.initials: CA
authors.lastname: Flores
authors.forename: S S
authors.initials: SS
authors.lastname: Mokha
mesh_terms: D000701:Analgesics, Opioid / Q000494:pharmacology*; D000818:Animals; D004558:Electric Stimulation; D020875:Enkephalin, Ala(2)-MePhe(4)-Gly(5)- / Q000494:pharmacology; D018690:Excitatory Amino Acid Agonists / Q000008:administration & dosage / Q000494:pharmacology*; D007478:Iontophoresis; D008297:Male; D008526:Medulla Oblongata / Q000187:drug effects / Q000502:physiology; D016202:N-Methylaspartate / Q000008:administration & dosage / Q000494:pharmacology*; D009270:Naloxone / Q000494:pharmacology; D009292:Narcotic Antagonists / Q000494:pharmacology; D009474:Neurons / Q000187:drug effects / Q000502:physiology*; D009619:Nociceptors / Q000187:drug effects / Q000502:physiology; D009842:Oligopeptides / Q000494:pharmacology / Q000502:physiology*; D010812:Physical Stimulation; D051381:Rats; D017207:Rats, Sprague-Dawley; D014276:Trigeminal Nerve / Q000166:cytology / Q000187:drug effects / Q000502:physiology*
chemical_list: D000701:Analgesics, Opioid; D018690:Excitatory Amino Acid Agonists; D009292:Narcotic Antagonists; D009842:Oligopeptides; C105293:endomorphin 1; D020875:Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; D009270:Naloxone; C105294:endomorphin 2; D016202:N-Methylaspartate