Morphine Induces AMPA Receptor Internalization in Primary Hippocampal Neurons via Calcineurin-Dependent Dephosphorylation of GluR1 Subunits

Abstract

Chronic morphine treatment resulting in the alteration of postsynaptic levels of AMPA receptors, thereby modulating synaptic strength, has been reported. However, the mechanism underlying such drug-induced synaptic modification has not been resolved. By monitoring the GluR1 trafficking in primary hippocampal neurons using the pHluorin-GluR1 imaging and biotinylation studies, we observed that prolonged morphine exposure significantly induced loss of synaptic and extrasynaptic GluR1 by internalization. The morphine-induced GluR1 endocytosis was independent of neural network activities or NMDA receptor activities, as neither blocking the sodium channels with tetrodotoxin nor NMDA receptors with DL-APV altered the effects of morphine. Instead, morphine-induced GluR1 endocytosis is attributed to a change in the phosphorylation state of the GluR1 at Ser 845 as morphine significantly decreased the dephosphorylation of GluR1 at this site. Such changes in Ser 845 phosphorylation required morphine-induced activation of calcineurin, based on the observations that a calcineurin inhibitor, FK506, completely abrogated the dephosphorylation, and morphine treatment led to an increase in calcineurin enzymatic activity, even in the presence of DL-APV. Importantly, pretreatment with FK506 and overexpression of the GluR1 mutants, S845D (phospho-mimic) or S845A (phospho-blocking) attenuated the morphine-induced GluR1 endocytosis. Therefore, the calcineurin-mediated GluR1-S845 dephosphorylation is critical for the morphine-induced changes in the postsynaptic AMPA receptor level. Together, these findings reveal a novel molecular mechanism for opioid-induced neuronal adaptation and/or synaptic impairment.

FK506-Binding Protein 12 Modulatesμ-Opioid Receptor Phosphorylation and Protein Kinase Cε–Dependent Signaling by Its Direct Interaction with the Receptor

“…These data strongly indicate that FKBP12 serves as an adaptor for the recruitment of PKC« and calcineurin. The fact that morphine can regulate the synaptic GluA1 subunit level of the a-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor via its activation of calcineurin (Kam et al, 2010) suggests functions of these molecules in OPRM1 signaling beyond those that were explored in our study.…”

Differential Modulation of Drug-Induced Structural and Functional Plasticity of Dendritic Spines

“…The strength of synaptic responses can be altered by changes in either quantal content (releasing probability multiplied by number of releasing sites) or quantal size (postsynaptic response per synaptic vesicle), which is often estimated by measuring the amplitude and frequency of miniature excitatory postsynaptic potential or mEPSC responses (Del Castillo and Katz, 1954). Although both calcineurin and CaMKII have been reported to play roles in AMPA receptor trafficking during activity-dependent synaptic plasticity (Silva et al, 1992;Mulkey et al, 1994;Hayashi et al, 2000;Dell'Acqua et al, 2006;Kam et al, 2010), surprisingly, only calcineurin is necessary for morphine-induced reductions in the amplitude of AMPA receptor-mediated mEPSC responses (Fig. 7).…”

“…Our previous studies have demonstrated that chronic postsynaptic activation of MORs leads to two forms of plasticity of dendritic spines: loss of preexisting spines and loss of synaptic AMPA receptors (Liao et al, , 2007aKam et al, 2010). Here, we used our time-lapse live imaging system to clarify the signaling pathway that mediates morphine-induced spine loss (Fig.…”

“…The morphine-induced reduction in mEPSC amplitude may result from loss of AMPA receptors in dendritic spines through endocytosis Kam et al, 2010). To determine the roles of calcineurin and CaMKII in morphine-induced loss of AMPA receptors in dendritic spines, we examined the morphine-induced cellular redistribution of GluR1 and GluR2 subunits of AMPA receptors in the presence of either FK506 or KN-62 (Figs.…”

Phosphorylation of the N-methyl-d-aspartate receptor is increased in the nucleus accumbens during both acute and extended morphine withdrawal

“…Calcineurin can also alter opioidspecific behaviors, because its inhibition decreases naloxoneinduced withdrawal symptoms in mice (Dougherty et al, 1987;Dougherty and Dafny, 1988;Homayoun et al, 2003) and can block conditioned place preference for morphine in mice (Suzuki et al, 1993;Motiei Langroudi et al, 2005). Calcineurin has also been linked to glutamate receptors after opioid exposure previously, because it can dephosphorylate AMPA receptors in chronic morphine-exposed primary hippocampal neurons that have undergone no withdrawal period (Kam et al, 2010). Because calcineurin appears to be able to alter NMDA receptors in models of withdrawal and reward, it too may be a component in long-term opioid-induced neural plasticity.…”