N-Methyl- d-Aspartate Receptors Mediate Endogenous Opioid Release in Enteric Neurons After Abdominal Surgery

Background & Aims: We tested the hypothesis that N-methyl- d-aspartate (NMDA) receptors mediate surgery-induced opioid release in enteric neurons. Methods: We used μ opioid receptor (μOR) internalization as a measure of opioid release with immunohistochemistry and confocal microscopy. μOR internalization was quantified in enteric neurons from nondenervated and denervated ileal segments of guinea pig after abdominal laparotomy with and without pretreatment with NMDA-receptor antagonists acting at different recognition sites (+)-5-methyl-10, 11-dihydro-5H-dibenzo [ a, b] cyclohepten-5, 10-imine (MK-801) or (D) 2-amino-5-phosphopenoic acid (AP-5) at .5, 1 mg/kg; 8-chloro-4-hydroxy-1-oxo-1, 2-dihydropyridazinol [4,5-]quinoline-5-oxide choline (MRZ 2/576) or 8-chloro-1, 4-dioxo-1,2,3,4-tetrahydropyridazinol [4,5-]quinoline choline salt (MRZ 2/596) at .3, 1 mg/kg, or with an antagonist for the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors, 6-cyano-7-nitroquinoxaline-2, 3-dione (1, 3 mg/kg). To determine whether NMDA stimulation induces opioid release, (1) ilea were exposed to NMDA (100 μmol/L) and D-serine (10 μmol/L) with or without the antagonist MK-801 or AP-5 (50 μmol/L); and (2) neuromuscular preparations of the ileum were stimulated electrically (20 Hz, 20 min) with or without MK-801 or AP-5 (50 μmol/L). Results: μOR endocytosis induced by abdominal laparotomy was inhibited significantly by NMDA-receptor antagonists in nondenervated and denervated ileal segments, but not by the AMPA-receptor antagonist. μOR endocytosis in neurons exposed to NMDA or electrical stimulation was prevented by NMDA-R antagonists. Conclusions: Abdominal laparotomy evokes local release of glutamate that results in endogenous opioid release through the activation of peripheral NMDA receptors. This suggests an interaction between the glutamatergic and opioid systems in response to the noxious and perhaps mechanosensory stimulation of surgery.