R. Zimmerman, M. Grahek, A. Ptak, J. Hagen, J.P. Houchins, S. Stoesz, K. Reagan, and A. Kalyuzhny
Glutamate is the major excitatory neurotransmitter in central nervous system (CNS) and acts via ionotropic (NMDA, kainate, and AMPA) and metabotropic glutamate receptors (mGluRs). During neuropathic pain, glutamatergic neurotransmission becomes elevated and impairs the antinociceptive effects of opioid drugs. It has been reported that spinal mGluRs can modulate nociception, and mGluR antagonists can potentiate the effects of morphine. To analyze the anatomical substrate underlying the interactions between mGluRs and μ opioid receptors, we employed multicolor immunofluorescence histochemistry on tissue sections of rat spinal cord and dorsal root ganglia (DRG). Polyclonal and mouse monoclonal antibodies were raised against recombinant mGluR1 (Group I), mGluR2 and mGluR3 (Group II), and mGluR8 (Group III) proteins. Rabbit monoclonal antibodies against the rat μ Opioid Receptor (OPRM1) were raised against a peptide immunogen from the N-terminal portion of the receptor.
There was an overlap in mGluRs and OPRM1 expression in spinal cord dorsal horn, and we observed colocalization in some punctate profiles. In the spinal cord dorsal horn, mGluRs were distributed to lamina I – III with more intense labeling in lamina II, whereas OPRM1 expression was observed mostly in lamina I – II, with higher staining density in lamina I. Labeling of mGluR1, mGluR2, mGluR3, and mGluR8 in the DRG was more abundant in medium- and large-sized neurons as opposed to small-sized neurons, whereas labeling for OPRM1 was detected predominantly in small-sized neurons. In the DRG, we observed colocalization of OPRM1 and mGluRs in small-sized neurons, but the frequency of this occurrence was different for the different mGluR subtypes.
Our data indicate that there is an anatomical overlap in distribution of OPRM1 and mGluRs in the spinal cord and DRG. Additionally, colocalization of OPRM1 and mGluRs in small-sized DRG neurons suggest there may be direct interactions between these two receptors.