Coexpression of the mu-opioid receptor splice variant MOR_1c and the vesicular glutamate transporter 2 (VGLUT2) in rat central nervous system

It has been reported that mu-opioid agonists depress glutamate release in some neurons but the specific receptor subtype mediating this effect is unclear. The purpose of the present study was to examine whether a particular mu-opioid receptor (MOR) splice-variant, MOR_1C, is expressed in rat central nervous system (CNS) by terminals expressing the vesicular glutamate transported (VGLUT2), a marker of glutamatergic neurons. Several MOR splice variants have been identified in mice and MOR_1C appears mainly to be localized to fibers and terminals, from which most neurotransmitter release would be expected. In addition, VGLUT2 has been found in the CNS and antibodies to it are reliable markers for glutamatergic terminals. Using fluorescence immunohistochemistry and confocal microscopy to examine spatial relationships between MOR_1C and VGLUT2, we found that MOR_1C and VGLUT2 puncta were widely distributed throughout the rat CNS; moreover, many regions contained terminals that expressed both. Thus, it appears that coexpression of MOR_1C and VGLUT2 is common in the rat CNS. We hypothesize that activation of MOR_1C by mu-opioid agonists at some glutamatergic terminals may be a mechanism by which glutamate release is inhibited.