Ultrastructural relationship between the AMPA-GluR2 receptor subunit and the mu-opioid receptor in the mouse central nucleus of the amygdala

Abstract

Activation of GluR2 expressing non-calcium permeable AMPA-type glutamate receptors in the central nucleus of the amygdala (CeA) may play an important role in integrating emotion and memory with goal directed behaviors involved in opioid addiction. The location of non-calcium permeable AMPA receptors within distinct neuronal compartments (i.e. soma, dendrite, or axon) is an important functional feature of these proteins, however, their ultrastructural location and subcellular relationship with mu-opioid receptors (μOR) in the CeA are unknown. Immunocytochemical electron microscopy was used to characterize the ultrastructural distribution of GluR2 and its association with μOR in the mouse CeA. A single labeling analysis of GluR2 distribution employing immunoperoxidase or immunogold markers revealed that this protein was frequently affiliated with intracellular vesicular organelles, as well as the plasma membrane of CeA neuronal profiles. Among all GluR2 labeled neuronal structures, over 85% were dendrites or somata. Unlabeled axon terminals frequently formed asymmetric excitatory-type synaptic junctions with GluR2 labeled dendritic profiles. Dual labeling immunocytochemical analysis showed that GluR2 and μOR were co-localized in neuronal compartments. Among all dual labeled structures, approximately 80% were dendritic. Synaptic inputs to these dual labeled dendrites were frequently from unlabeled axon terminals forming asymmetric excitatory-type synapses. The presence of GluR2 in dendritic profiles receiving asymmetric synapses suggests that activation of the non-calcium permeable AMPA receptor plays a role in the postsynaptic modulation of excitatory signaling involving CeA neuronal circuits that coordinate sensory, affective, and behavioral processes involved in drug addiction. Given the critical role of non-calcium permeable AMPA receptor function in neural and behavioral adaptability, their dendritic association with μOR in CeA dendrites provides a neuronal substrate for opioid-mediated plasticity.

Endogenous opioids regulate moment-to-moment neuronal communication and excitability

“…However, it is not known whether endogenously released opioids can activate this potassium conductance. Given the strong endogenous opioid regulation in Im neurons (current findings), their very high MOR expression21 and that 60% of MOR receptors are in postsynaptic dendritic compartments3435, we wondered whether endogenously released opioids could directly regulate Im neurons. We have already shown ME is well placed to regulate synaptic transmission (Fig.…”

“…For example, the glutamatergic synaptic input from the BLA could be targeted by endogenously released opioids, as BLA pyramidal neurons express both MOR and DOR2134. Further, in other subdivisions of the amygdala, at least 60% of MOR is found in postsynaptic dendritic compartments, rather than in axons or terminals, suggesting potential for postsynaptic regulation3435. To date, endogenously released opioids have not been shown to directly activate a postsynaptic conductance in any brain region.…”

Unraveling glutamate–opioid receptor interactions using high-resolution electron microscopy: Implications for addiction-related processes

“…The pre-embedding immunogold-silver technique used in the study by Beckerman and Glass (2010) has several advantages. This approach maintains morphological preservation while preserving discrete subcellular localization of the antigen of interest (Leranth and Pickel, 1989).…”

“…Triton X-100) can be considered (Leranth and Pickel, 1989). In summary, the quantitative approach used by Beckerman and Glass (2010) has been extensively validated (Leranth and Pickel, 1989) and, under carefully controlled conditions, can allow for the quantitative evaluation of the co-localization of neurotransmitters and receptors within the neuropil.…”

“…The review begins by highlighting the advantages of high-resolution electron microscopic immunohistochemistry for unraveling receptor interactions at the synapse. With an emphasis on a recent publication describing the anatomical relationship between the μ-opioid receptor (MOR) and the AMPA-GluR2 subunit by Beckerman and Glass (2010), we review the anatomical evidence for opioid-induced neural plasticity of glutamate receptors in selected brain circuits that are key integrative substrates in the brain's motivational system. The findings stress the importance of glutamate-opioid interactions as important neural mediators of adaptations to chronic use of abused drugs, particularly within the amygdaloid complex.…”