title: Opioids modulate N-methyl-D-aspartic acid (NMDA)-evoked responses of neurons in the superficial and deeper dorsal horn of the medulla (trigeminal nucleus caudalis).
abstract: Extracellular single unit recordings were made from 74 neurons in the superficial and deeper dorsal horn of the medulla (trigeminal nucleus caudalis). N-methyl-D-aspartic acid (NMDA) excited nociceptive as well as non-nociceptive neurons. NMDA receptor antagonist, DL-2-Amino-5-Phosphonovaleric acid (AP-5), blocked the NMDA-evoked excitation. Microiontophoretic application of a selective mu-opioid receptor agonist, [D-Ala2,N-Me-Phe4,Gly5-ol]enkephalin (DAMGO), reduced the NMDA-evoked responses of 100% of nociceptive specific (NS), 93% of wide dynamic range (WDR) and 86% of low threshold (LT) neurons in the superficial and deeper dorsal horn of the medulla. In contrast, application of a selective delta 1-opioid receptor agonist, [D-Pen2,5]enkephalin (DPDPE), reduced the NMDA-evoked responses of 90% of NS neurons, 72% of WDR neurons and 67% of LT neurons in the superficial and deeper dorsal horn of the medulla. DPDPE also produced excitatory or biphasic effects. The inhibitory actions of DAMGO and DPDPE were reversed by naloxone and/or 7-benzylidenenaltrexone (BNTX), mu- and delta 1-receptor antagonists. It is concluded that mu- and delta-opioid receptor agonists produce a predominantly inhibitory modulation of the NMDA-evoked responses of nociceptive and non-nociceptive neurons in the medullary dorsal horn.
authors.affiliation: Department of Physiology, Meharry Medical College, Nashville, TN 37208, USA.
authors.forename: K M
authors.initials: KM
authors.lastname: Zhang
authors.forename: X M
authors.initials: XM
authors.lastname: Wang
authors.forename: S S
authors.initials: SS
authors.lastname: Mokha
mesh_terms: D000701:Analgesics, Opioid / Q000494:pharmacology*; D000818:Animals; D020875:Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; D020881:Enkephalin, D-Penicillamine (2,5)-; D004745:Enkephalins / Q000494:pharmacology; D005071:Evoked Potentials / Q000187:drug effects; D008297:Male; D016202:N-Methylaspartate / Q000494:pharmacology*; D009474:Neurons / Q000187:drug effects*; D051381:Rats; D017207:Rats, Sprague-Dawley; D017465:Receptors, Opioid, delta / Q000819:agonists*; D017450:Receptors, Opioid, mu / Q000819:agonists*; D014275:Trigeminal Caudal Nucleus / Q000166:cytology / Q000187:drug effects*
chemical_list: D000701:Analgesics, Opioid; D004745:Enkephalins; D017465:Receptors, Opioid, delta; D017450:Receptors, Opioid, mu; D020875:Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; D016202:N-Methylaspartate; D020881:Enkephalin, D-Penicillamine (2,5)-