Pre- and postsynaptic regulation of locus coeruleus neurons after chronic morphine treatment: a study of GIRK-knockout mice - PubMed (original) (raw)

Pre- and postsynaptic regulation of locus coeruleus neurons after chronic morphine treatment: a study of GIRK-knockout mice

Maria Torrecilla et al. Eur J Neurosci. 2008 Aug.

Abstract

While the acute inhibitory effect of opioids on locus coeruleus (LC) neurons is mediated primarily by the activation of G protein-gated inwardly-rectifying K(+) (GIRK) channels, the 3'-5'-cyclic adenosine monophosphate (cAMP) system has been implicated in the effects of chronic morphine exposure. Presently, the impact of chronic morphine treatment on GIRK-dependent and GIRK-independent mechanisms underlying the opioid-induced inhibition of LC neurons is unclear. Here, opioid-induced postsynaptic inhibition was studied in LC neurons from wild-type and GIRK2/GIRK3(-/-) mice at baseline and following chronic morphine treatment. The postsynaptic inhibition of LC neurons caused by the opioid agonist [Met](5) enkephalin (ME) was unaffected by chronic morphine treatment in mice of either genotype. Furthermore, chronic morphine treatment had no effect on the forskolin augmentation of the ME-induced current in wild-type LC neurons and only a minor effect on the ME-induced current in LC neurons from GIRK2/GIRK3(-/-) mice. Chronic morphine treatment did, however, lead to an increased frequency of spontaneous excitatory postsynaptic currents (EPSCs) in the LC. Interestingly, while forskolin augmented the EPSC frequency similarly in untreated and morphine-treated wild-type mice, as well as untreated GIRK2/GIRK3(-/-) mice, it failed to increase the frequency of EPSCs in morphine-treated GIRK2/GIRK3(-/-) mice. Altogether, the findings suggest that chronic morphine treatment exerts little impact on ion channels and signaling pathways that mediate the postsynaptic inhibitory effects of opioids but does enhance excitatory neurotransmission in the mouse LC.

PubMed Disclaimer

Figures

Figure 1

Figure 1. Opioid-induced outward currents in LC neurons from wild type and GIRK2/GIRK3-/- mice

A) Current traces showing the outward current induced by ME (30 μM) in the absence and presence of forskolin (Fsk, 30 μM) in slices from wild-type mouse (top trace) and a GIRK2/GIRK3-/- mouse (bottom trace). The holding potential (Vhold) was -60 mV. Forskolin (Fsk) induced a small inward current and increased spontaneous excitatory input. B) Summary showing the amplitude of the current induced by ME (30 μM) in slices from wild-type (white bars) and GIRK2/GIRK3-/- (black bars) mice, and the impact of chronic morphine treatment and Fsk on the ME-induced current (Vhold = -60 mV). In the interest of clarity, only within-genotype differences are noted on the plot; genotype-dependent differences were clearly evident and noted in the text. Symbols: * p<0.05 vs. Fsk, within genotype.

Figure 2

Figure 2. Current-voltage relationships in LC neurons from untreated and morphine-treated mice

The voltage dependence of the whole-cell currents measured following application of ME (30 μM; white circles), Fsk alone (gray circles), or ME in the presence of Fsk (ME+Fsk; black circles) are plotted for LC neurons from untreated and morphine-treated wild-type (A,B) and GIRK2/GIRK3-/- (C,D) mice. While the small Fsk-induced current measured at Vhold = -60 mV was similar across the four groups tested, some differences in the I-V profiles were evident (e.g., compare C and D). Also note that the I-V plots for the ME-induced current measured in the absence or presence of Fsk were largely overlapping in panel D, reflecting the lack of effect of Fsk on ME-induced current in LC neurons from morphine-treated GIRK2/GIRK3-/- mice.

Figure 3

Figure 3. ME-induced hyperpolarization in LC neurons from untreated and morphine-treated mice

A) The superimposed traces on the left show the hyperpolarization induced by 30 μM ME in LC neurons from a wild-type (black trace) and GIRK2/GIRK3-/- mouse (grey trace). The superimposed traces on the right show the hyperpolarization induced by 3 μM UK-14304 in LC neurons from wild-type (black trace) and GIRK2/GIRK3-/- (grey trace) mice. B) Summary plot showing the hyperpolarization induced by ME (30 μM) and UK-14304 (UK, 3 μM) in LC neurons from untreated and morphine-treated wild-type (white bar) and GIRK2/GIRK3-/- (black bar) mice. C) Concentration-response curves describing the hyperpolarization induced by ME in LC neurons from untreated (white symbols) and morphine-treated (black symbols) wild-type and GIRK2/GIRK3-/- mice. Symbols: * p<0.05 vs. GIRK2/GIRK3-/- mice.

Figure 4

Figure 4. Excitatory input to LC neurons in untreated and morphine-treated wild-type mice

A) Representative current traces showing spontaneous EPSCs in slices from untreated (top) and morphine-treated (bottom) wild-type mice (Vhold = -60 mV). B) Cumulative histograms of frequency (left panel, >2000 EPSCs; 13 slices from 7 animals) and amplitude (right panel, >2000 EPSCs; 15 slices from 7 animals) showing that the frequency of spontaneous EPSCs is increased in slices from morphine-treated animals, while there was no significant change in the amplitude distribution.

Figure 5

Figure 5. Excitatory input to LC neurons in untreated and morphine-treated GIRK2/GIRK3-/- mice

A) Representative current traces showing spontaneous EPSCs in slices from untreated (top) and morphine-treated (bottom) GIRK2/GIRK3-/- mice (Vhold = -60 mV). B) Cumulative histograms of frequency (left panel, >2000 EPSCs; 20 slices from 8 animals) and amplitude (right panel, >2000 EPSCs; 18 slices from 8 animals) showing that the frequency of spontaneous EPSCs is increased in slices from morphine-treated animals, while there was no significant change in the amplitude distribution.

Figure 6

Figure 6. Forskolin increases the frequency of spontaneous EPSCs

A) Representative traces from morphine-treated wild-type and GIRK2/GIRK3-/- mice (Vhold = -60 mV). The traces show the increase in EPSC frequency induced by Fsk in the slice from a morphine-treated wild-type mouse (top two traces) and the lack of action of Fsk in a slice taken from a morphine-treated GIRK2/GIRK3-/- mouse (lower two traces). B) Summary plot of the frequencies (in Hz) of spontaneous EPSCs measured in LC neurons from untreated and morphine-treated wild-type (left panel) and GIRK2/GIRK3-/- (right panel) mice, in the absence (black rectangle, con) or presence (white rectangle) of Fsk (10 μM). Symbols: * p<0.05 vs. con group, within genotype and morphine-treatment groups; ** p<0.01 vs. untreated group, within Fsk treatment group.

References

    1. Aghajanian GK, Kogan JH, Moghaddam B. Opiate withdrawal increases glutamate and aspartate efflux in the locus coeruleus: an in vivo microdialysis study. Brain Res. 1994;636:126–130. - PubMed
    1. Akaoka H, Aston-Jones G. Opiate withdrawal-induced hyperactivity of locus coeruleus neurons is substantially mediated by augmented excitatory amino acid input. J Neurosci. 1991;11:3830–3839. - PMC - PubMed
    1. Alreja M, Aghajanian GK. Opiates suppress a resting sodium-dependent inward current and activate an outward potassium current in locus coeruleus neurons. J Neurosci. 1993;13:3525–3532. - PMC - PubMed
    1. Alreja M, Aghajanian GK. Use of the whole-cell patch-clamp method in studies on the role of cAMP in regulating the spontaneous firing of locus coeruleus neurons. J Neurosci. 1995;59:67–75. - PubMed
    1. Bie B, Peng Y, Zhang Y, Pan ZZ. cAMP-mediated mechanisms for pain sensitization during opioid withdrawal. J Neurosci. 2005;25:3824–3832. - PMC - PubMed

Publication types

MeSH terms

Substances

Grants and funding

LinkOut - more resources