Endogenous N-acetylaspartylglutamate (NAAG) inhibits synaptic plasticity/transmission in the amygdala in a mouse inflammatory pain model.
BACKGROUND
The peptide neurotransmitter N-acetylaspartylglutamate (NAAG) is widely expressed throughout the vertebrate nervous system, including the pain processing neuraxis. Inhibitors of NAAG peptidases are analgesic in animal models of pain. However, the brain regions involved in NAAG's analgesic action have not been rigorously defined. Group II metabotropic glutamate receptors (mGluR2/3) play a role in pain processing in the laterocapsular part of the central nucleus of the amygdala (CeLC). Given the high concentration of NAAG in the amygdala and its activation of group II mGluRs (mGluR3 > mGluR2), this study was undertaken using the mouse formalin model of inflammatory pain to test the hypothesis that NAAG influences pain processing in the amygdala. Evoked excitatory postsynaptic currents (eEPSCs) were studied in neurons in the CeLC of mouse brain slices following stimulation of the spinoparabrachial amygdaloid afferents.


RESULTS
Application of a NAAG peptidase inhibitor, ZJ43, dose dependently inhibited the amplitude of the eEPSCs by up to 50% in control CeLC demonstrating the role of NAAG in regulation of excitatory transmission at this synapse. A group II mGluR agonist (SLx-3095-1) similarly inhibited eEPSC amplitude by about 30%. Both effects were blocked by the group II mGluR antagonist LY341495. ZJ43 was much less effective than SLx in reducing eEPSCs 24 hours post inflammation suggesting an inflammation induced reduction in NAAG release or an increase in the ratio of mGluR2 to mGluR3 expression. Systemic injection of ZJ43 proximal to the time of inflammation blocked peripheral inflammation-induced increases in synaptic transmission of this pathway 24 hrs later and blocked the induction of mechanical allodynia that developed by this time point.


CONCLUSIONS
The main finding of this study is that NAAG and NAAG peptidase inhibition reduce excitatory neurotransmission and inflammation-induced plasticity at the spinoparabrachial synapse within the pain processing pathway of the central amygdaloid nucleus.
Adedoyin
Mary O
MO
Department of Biology, Georgetown University, Biology Reiss Building 37th and O St. NW, Washington, D.C. 20057, USA.
Vicini
Stefano
S
Neale
Joseph H
JH
D000679:Amygdala / Q000187:drug effects / Q000503:physiopathology*; D000818:Animals; D001522:Behavior, Animal / Q000187:drug effects; D004151:Dipeptides / Q000378:metabolism* / Q000494:pharmacology; D004195:Disease Models, Animal; D019706:Excitatory Postsynaptic Potentials / Q000187:drug effects; D005557:Formaldehyde; D043425:Glutamate Carboxypeptidase II / Q000037:antagonists & inhibitors / Q000378:metabolism; D006930:Hyperalgesia / Q000150:complications / Q000188:drug therapy / Q000503:physiopathology / Q000517:prevention & control; D066298:In Vitro Techniques; D007249:Inflammation / Q000150:complications* / Q000503:physiopathology; D051379:Mice; D008954:Models, Biological; D009473:Neuronal Plasticity / Q000187:drug effects / Q000502:physiology*; D009619:Nociceptors / Q000187:drug effects / Q000378:metabolism; D010146:Pain / Q000150:complications* / Q000503:physiopathology*; D018091:Receptors, AMPA / Q000378:metabolism; D018094:Receptors, Metabotropic Glutamate / Q000819:agonists / Q000378:metabolism; D016194:Receptors, N-Methyl-D-Aspartate / Q000378:metabolism; D009435:Synaptic Transmission / Q000187:drug effects / Q000502:physiology*; D014508:Urea / Q000031:analogs & derivatives / Q000494:pharmacology / Q000627:therapeutic use
D004151:Dipeptides; D018091:Receptors, AMPA; D018094:Receptors, Metabotropic Glutamate; D016194:Receptors, N-Methyl-D-Aspartate; C486211:ZJ43; D005557:Formaldehyde; C027172:isospaglumic acid; D014508:Urea; D043425:Glutamate Carboxypeptidase II