Abstract
BackgroundMetabotropic glutamate receptors (mGluRs) have been identified as significant analgesic targets. Systemic treatments with inhibitors of the enzymes that inactivate the peptide transmitter N-acetylaspartylglutamate (NAAG), an mGluR3 agonist, have an analgesia-like effect in rat models of inflammatory and neuropathic pain. The goal of this study was to begin defining locations within the central pain pathway at which NAAG activation of its receptor mediates this effect.ResultsNAAG immunoreactivity was found in neurons in two brain regions that mediate nociceptive processing, the periaqueductal gray (PAG) and the rostral ventromedial medulla (RVM). Microinjection of the NAAG peptidase inhibitor ZJ43 into the PAG contralateral, but not ipsilateral, to the formalin injected footpad reduced the rapid and slow phases of the nociceptive response in a dose-dependent manner. ZJ43 injected into the RVM also reduced the rapid and slow phase of the response. The group II mGluR antagonist LY341495 blocked these effects of ZJ43 on the PAG and RVM. NAAG peptidase inhibition in the PAG and RVM did not affect the thermal withdrawal response in the hot plate test. Footpad inflammation also induced a significant increase in glutamate release in the PAG. Systemic injection of ZJ43 increased NAAG levels in the PAG and RVM and blocked the inflammation-induced increase in glutamate release in the PAG.ConclusionThese data demonstrate a behavioral and neurochemical role for NAAG in the PAG and RVM in regulating the spinal motor response to inflammation and that NAAG peptidase inhibition has potential as an approach to treating inflammatory pain via either the ascending (PAG) and/or the descending pain pathways (PAG and RVM) that warrants further study.
Highlights
IntroductionMetabotropic glutamate receptors (mGluRs) have been identified as significant analgesic targets
Metabotropic glutamate receptors have been identified as significant analgesic targets
Using highly specific, multi-stage affinity purified antibodies, NAAG immunoreactivity was observed in neurons and axons in the rat periaqueductal gray (PAG) and rostral ventromedial medulla (RVM) (Figure 1a,b)
Summary
Metabotropic glutamate receptors (mGluRs) have been identified as significant analgesic targets. The heterotropic group II metabotropic glutamate receptor (mGluR2 and mGluR3) agonists have shown analgesic efficacy in animal models of inflammatory and neuropathic pain [1]. The enzymes that inactivate synaptically released NAAG have been cloned [21,22,23,24,25] and potent inhibitors of these enzymes have been developed [26,27] Systemic administration of these inhibitors, including ZJ43, reduces pain responses in animal models [2,4,5,6,7,8,9,28,29]
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