Differential opioid receptor expression and biochemical coupling profiles on glia

The opioid receptor family comprises classical; MOP (mu/µ), KOP (kappa/k), DOP(delta/δ) receptors along with non-classical Nociceptin/Orphanin FQ (N/OFQ) peptide receptor (NOP). Expression on glial cells is controversial where there is a role for glia and opioids in pain processing and immunomodulation. Here we detail expression and function of opioid receptors in a wide range of established and primary glia. Namely, 1321N1 human astrocytoma, C6 rat astrocytoma, mouse primary astrocytes, human MO3.13 oligodendrocyte like cells, human HOG oligodendrocytoma, mouse EOC-20 microglia and mouse primary microglia. We used (i)-PCR for mRNA, (ii)-radioligand binding and (iii)-fluorescent probe binding for expression, (iv)-MAPK for activation and (v)-scratch assay for migration. MOP mRNA was detected in C6 and mouse primary astrocytes only. NOP mRNA was found in 1321N1, C6 and mouse primary astrocytes along with MO3.13 and HOG cells. There was variable expression of DOP and KOP; this was not probed further. Surprisingly, microglia (EOC-20 or primary mouse) did not express mRNA for opioid receptors unless treated with media from astrocytes. mRNA expression profile was generally matched by [3H]-DPN binding to classical and [3H]-N/OFQ binding to non-classical opioid receptors. In addition, C6, mouse primary and 1321N1 astrocytes along with MO3.13 and HOG cells bound the fluorescent NOP probe N/OFQATTO594. C6 and primary mouse astrocytes also bound the MOP fluorescent probe DermorphinATTO488. Where expressed both MOP and NOP supported Endomorphin-1 and N/OFQ-induced phosphorylation of ERK1/2 and reduced scratch migration. In microglia, astrocyte modulated opioid receptor expression could influence central opioid-immunomodulation and pain processing; further studies are required.