Skip to main page content
U.S. flag

An official website of the United States government

Dot gov

The .gov means it’s official.
Federal government websites often end in .gov or .mil. Before sharing sensitive information, make sure you’re on a federal government site.

Https

The site is secure.
The https:// ensures that you are connecting to the official website and that any information you provide is encrypted and transmitted securely.

Access keys NCBI Homepage MyNCBI Homepage Main Content Main Navigation
. 2012 Dec;36(12):3636-42.
doi: 10.1111/ejn.12003. Epub 2012 Sep 24.

μ-Opioid receptor desensitization: homologous or heterologous?

Javier Llorente  1 , Janet D Lowe, Helen S Sanderson, Elena Tsisanova, Eamonn Kelly, Graeme Henderson, Chris P Bailey
Affiliations
Free PMC article

μ-Opioid receptor desensitization: homologous or heterologous?

Javier Llorente et al. Eur J Neurosci. 2012 Dec.
Free PMC article

Abstract

There is considerable controversy over whether μ-opioid receptor (MOPr) desensitization is homologous or heterologous and over the mechanisms underlying such desensitization. In different cell types MOPr desensitization has been reported to involve receptor phosphorylation by various kinases, including G-protein-coupled receptor kinases (GRKs), second messenger and other kinases as well as perturbation of the MOPr effector pathway by GRK sequestration of G protein βγ subunits or ion channel modulation. Here we report that in brainstem locus coeruleus (LC) neurons prepared from relatively mature rats (5-8 weeks old) rapid MOPr desensitization induced by the high-efficacy opioid peptides methionine enkephalin and DAMGO was homologous and not heterologous to α(2)-adrenoceptors and somatostatin SST(2) receptors. Given that these receptors all couple through G proteins to the same set of G-protein inwardly rectifying (GIRK) channels it is unlikely therefore that in mature neurons MOPr desensitization involves G protein βγ subunit sequestration or ion channel modulation. In contrast, in slices from immature animals (less than postnatal day 20), MOPr desensitization was observed to be heterologous and could be downstream of the receptor. Heterologous MOPr desensitization was not dependent on protein kinase C or c-Jun N-terminal kinase activity, but the change from heterologous to homologous desensitization with age was correlated with a decrease in the expression levels of GRK2 in the LC and other brain regions. The observation that the mechanisms underlying MOPr desensitization change with neuronal development is important when extrapolating to the mature brain results obtained from experiments on expression systems, cell lines and immature neuronal preparations.

PubMed Disclaimer

Figures

FIG. 1
FIG. 1
GIRK channel currents in mature rat LC neurons evoked by maximally effective concentrations of ME, NA and SRIF do not add together. (A,B) Outward potassium currents recorded from single LC neurons in each case in response to application of maximally effective concentrations of ME, NA and SRIF. (C, D) Pooled data from experiments as illustrated in A and B, showing that the current evoked by ME (n = 6) and NA (n = 6) or ME (n = 4) and SRIF (n = 4) in combination was not greater than that evoked by ME alone. The estimated level of current if the responses had summated is indicated by the dashed line (sum).
FIG. 2
FIG. 2
Desensitization of GPCR-evoked GIRK channel currents in mature rat LC neurons: desensitization of one GPCR type does not influence the desensitization of another. (A) Outward potassium current recorded from a single LC neuron in response to application of maximally effective concentrations of somatostatin (SRIF; 3 μm) and Met-enkephalin (ME; 30 μm). Drugs were applied for the periods indicated by the bars. The response to SST desensitized in the continued presence of the drug but application of ME in the presence of SST (shown in green) still evoked the maximal current and the response still desensitized. Scale bars represent 50 pA and 5 min. (B) Recording from another LC neuron showing desensitization of the ME response when it was applied alone. (C) A maximally effective concentration of noradrenaline (NA; 100 μm) evoked an outward current that desensitized less than that resulting from SST or ME application (compare C with A or B). (D) Application of ME in the presence of NA. The maximum NA current was slightly less than that evoked by ME (see also Bailey et al., 2009) so application of ME (shown in red) ‘on top’ of NA evoked a small additional current that desensitized in the continued presence of ME. On washout of ME the NA-evoked current returned to the pre-ME level. (E) The desensitization of the NA-evoked current was unaffected by the application of ME. On washout of ME the NA current (in blue) was of a similar amplitude as in cells not exposed to ME (in black) (n = 4 for each). (F) When considered in isolation from any underlying current the ME-evoked current desensitized with the same kinetics and to the same extent in the absence (in black) and presence of SRIF (green) or NA (red) (n = 4 for each).
FIG. 3
FIG. 3
MOPr desensitization in mature LC neurons is not at the level of the GIRK channel. (A) In the continued presence of the GIRK blocker rTertiapinQ (100 nm), outward potassium current was recorded from a single LC neuron in a brain slice from a mature rat in response to application of a submaximal concentration of noradrenaline (NA; 5 μm) or a maximally effective concentration of DAMGO (10 μm), reversed with the MOPr antagonist naloxone (Nal; 5 μm). The response to DAMGO desensitized in the continued presence of the drug, whereas the response to NA which was applied both before and after the DAMGO treatment was unchanged. Pooled data from experiments as in A show that (B) the mean peak amplitudes of DAMGO-elicited potassium currents were significantly inhibited by the presence of rTertiapinQ (*P < 0.05 vs. control, Student's t-test; n = 5–7) and that (C) whereas the DAMGO response rapidly desensitized [**P < 0.001, one-sample Student's t-test (t6 = 26.08, P < 0.0001), n = 7], there was no significant decline in the NA response after DAMGO-induced MOPr desensitization (P > 0.05, n = 7).
FIG. 4
FIG. 4
MOPr desensitization in mature LC neurons is homologous, whereas in immature LC neurons it is heterologous. (A) Outward potassium current recorded from a single LC neuron in a brain slice from a rat (> 5 weeks old) in response to application of a submaximal concentration of noradrenaline (NA; 5 μm) or a maximally effective concentration of the MOPr agonist DAMGO (10 μm), reversed with the MOPr antagonist naloxone (Nal; 5 μm). The response to DAMGO desensitized in the continued presence of the drug, whereas the response to NA which was applied both before and after the DAMGO treatment was unchanged. (B) Potassium currents recorded from an LC neuron from an immature rat (< P20) also showed rapid desensitization to DAMGO (10 μm), whereas the response to NA (5 μm) was decreased after DAMGO treatment. (C) Pooled data from experiments as illustrated in A and B showing that in LC neurons from both older rats (n = 5) and young rats (n = 6) the DAMGO response exhibited similar levels of desensitization. (D) Pooled data from experiments as illustrated in A and B showing that in LC neurons from older rats (n = 5) there was no significant desensitization of the NA response (P > 0.05) following exposure to DAMGO, whereas in neurons from young rats (n = 6) significant heterologous desensitization of the response to NA after DAMGO treatment was observed (***P < 0.001, Student's t-test). (E) Pooled data from LC neurons from < P20 rats as illustrated in B. Rapid desensitization to DAMGO (10 μm for 10 min) was unchanged by the combined presence of the PKC inhibitor GF109203x (1 μm) and the JNK inhibitor SP600125 (30 μm) (n = 6). (F) Pooled data from LC neurons from < P20 rats as illustrated in B. In < P20 rats, the response to NA (5 μM) was decreased after DAMGO treatment, an effect that was unchanged by the combined presence of GF109203x (GF) and SP600125 (SP) (n = 5–6).
FIG. 5
FIG. 5
GRK2 expression levels in brain regions from young and mature rats. Protein obtained from relevant brain regions was subjected to SDS-PAGE and Western blotting with a GRK2 antibody. Illustrated for each brain region are representative blots of GRK2 from young (Y; < P20) and older (O; > 5 weeks) rat brains. Relative GRK2 levels were obtained by densitometry and normalized to the corresponding tubulin value (LC n = 5, cortex n = 9, striatum n = 7, hippocampus n = 4; GRK2 expression level in younger animals taken as 100%). *P < 0.05.
See this image and copyright information in PMC

Similar articles

See all similar articles

Cited by

See all "Cited by" articles

References

    1. Alvarez VA, Arttamangkul S, Dang V, Salem A, Whistler JL, Von Zastrow M, Grandy DK, Williams JT. μ-Opioid receptors: ligand-dependent activation of potassium conductance, desensitization, and internalization. J. Neurosci. 2002;22:5769–5776. - PMC - PubMed
    1. Arttamangkul S, Lau EK, Lu HW, Williams JT. Desensitization and trafficking of μ-opioid receptors in locus ceruleus neurons: modulation by kinases. Mol. Pharmacol. 2012;81:348–355. - PMC - PubMed
    1. Bailey CP, Kelly E, Henderson G. Protein kinase C activation enhances morphine-induced rapid desensitization of μ-opioid receptors in mature rat locus ceruleus neurons. Mol. Pharmacol. 2004;66:1592–1598. - PubMed
    1. Bailey CP, Smith FL, Kelly E, Dewey WL, Henderson G. How important is protein kinase C in μ-opioid receptor desensitization and morphine tolerance? Trends Pharmacol. Sci. 2006;27:558–565. - PubMed
    1. Bailey CP, Oldfield S, Llorente J, Caunt CJ, Teschemacher AG, Roberts L, McArdle CA, Smith FL, Dewey WL, Kelly E, Henderson G. Involvement of PKC alpha and G-protein-coupled receptor kinase 2 in agonist-selective desensitization of μ-opioid receptors in mature brain neurons. Br. J. Pharmacol. 2009;158:157–164. - PMC - PubMed

Publication types

MeSH terms

Substances