Characterization of functional μ opioid receptor turnover in rat locus coeruleus: an electrophysiological and immunocytochemical study
British Journal of Pharmacology. 2017-07-07; 174(16): 2758-2772
DOI: 10.1111/bph.13901
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Medrano MC(1), Santamarta MT(1), Pablos P(1), Aira Z(2), Buesa I(2), Azkue JJ(2), Mendiguren A(1), Pineda J(1).
Author information:
(1)Department of Pharmacology, Faculty of Medicine and Nursing, University of the
Basque Country (UPV/EHU), Leioa, Bizkaia, Spain.
(2)Department of Neuroscience, Faculty of Medicine and Nursing, University of the
Basque Country (UPV/EHU), Leioa, Bizkaia, Spain.
BACKGROUND AND PURPOSE: Regulation of μ receptor dynamics such as its trafficking
is a possible mechanism underlying opioid tolerance that contributes to
inefficient recycling of opioid responses. We aimed to characterize the
functional turnover of μ receptors in the noradrenergic nucleus locus coeruleus
(LC).
EXPERIMENTAL APPROACH: We measured opioid effect by single-unit extracellular
recordings of LC neurons from rat brain slices. Immunocytochemical techniques
were used to evaluate μ receptor trafficking.
KEY RESULTS: After near-complete, irreversible μ receptor inactivation with
β-funaltrexamine (β-FNA), opioid effect spontaneously recovered in a rapid and
efficacious manner. In contrast, α2 -adrenoceptor-mediated effect hardly
recovered after receptor inactivation with the irreversible antagonist EEDQ. When
the recovery of opioid effect was tested after various inactivating time
schedules, we found that the longer the β-FNA pre-exposure, the less efficient
and slower the functional μ receptor turnover became. Interestingly, μ receptor
turnover was slower when β-FNA challenge was repeated in the same cell,
indicating constitutive μ receptor recycling by trafficking from a depletable
pool. Double immunocytochemistry confirmed the constitutive nature of μ receptor
trafficking from a cytoplasmic compartment. The μ receptor turnover was slowed
down when LC neuron calcium- or firing-dependent processes were prevented or
vesicular protein trafficking was blocked by a low temperature or transport
inhibitor.
CONCLUSIONS AND IMPLICATIONS: Constitutive trafficking of μ receptors from a
depletable intracellular pool (endosome) may account for its rapid and efficient
functional turnover in the LC. A finely-tuned regulation of μ receptor
trafficking and endosomes could explain neuroadaptive plasticity to opioids in
the LC.
© 2017 The British Pharmacological Society.
DOI: 10.1111/bph.13901
PMCID: PMC5522999
PMID: 28589556 [Indexed for MEDLINE]