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Assessment of Morphine-induced Hyperalgesia and Analgesic Tolerance in Mice Using Thermal and Mechanical Nociceptive Modalities
Published on: July 29, 2014
Opioid tolerance in periaqueductal gray neurons isolated from mice chronically treated with morphine
Elena E Bagley1, Billy C H Chieng, MacDonald J Christie
1Pain Management Research Institute, E25, Kolling Institute, University of Sydney at Royal North Shore Hospital, St Leonards, Australia.
Abstract:
The midbrain periaqueductal gray (PAG) is a major site of opioid analgesic action, and a significant site of cellular adaptations to chronic morphine treatment (CMT). We examined mu-opioid receptor (MOP) regulation of voltage-gated calcium channel currents (I(Ca)) and G-protein-activated K channel currents (GIRK) in PAG neurons from CMT mice. Mice were injected s.c. with 300 mg kg(-1) of morphine base in a slow release emulsion three times over 5 days, or with emulsion alone (vehicles). This protocol produced significant tolerance to the antinociceptive effects of morphine in a test of thermal nociception. Voltage clamp recordings were made of I(Ca) in acutely isolated PAG neurons and GIRK in PAG slices. The MOP agonist DAMGO (Tyr-D-Ala-Gly-N-Me-Phe-Gly-ol enkephalin) inhibited I(Ca) in neurons from CMT mice (230 nM) with a similar potency to vehicle (150 nM), but with a reduced maximal effectiveness (37% inhibition in vehicle neurons, 27% in CMT neurons). Inhibition of I(Ca) by the GABA(B) agonist baclofen was not altered by CMT. Met-enkephalin-activated GIRK currents recorded in PAG slices were significantly smaller in neurons from CMT mice than vehicles, while GIRK currents activated by baclofen were unaltered. These data demonstrate that CMT-induced antinociceptive tolerance is accompanied by homologous reduction in the effectiveness of MOP agonists to inhibit I(Ca) and activate GIRK. Thus, a reduction in MOP number and/or functional coupling to G proteins accompanies the characteristic cellular adaptations to CMT previously described in PAG neurons.
Insights
Chronic morphine treatment reduces the effectiveness of mu-opioid receptors (MOP) in the periaqueductal gray (PAG). This cellular adaptation in PAG neurons contributes to morphine tolerance.
Area of Science:
- Neuroscience
- Pharmacology
- Cellular Biology
Background:
- The periaqueductal gray (PAG) is crucial for opioid analgesia.
- Chronic morphine treatment (CMT) induces cellular adaptations in the PAG.
- Understanding these adaptations is key to addressing opioid tolerance.
Purpose of the Study:
- To investigate mu-opioid receptor (MOP) regulation of ion channels in PAG neurons from mice undergoing CMT.
- To determine how CMT affects voltage-gated calcium channel (I(Ca)) and G-protein-activated potassium channel (GIRK) currents in the PAG.
- To correlate cellular changes with observed antinociceptive tolerance.
Main Methods:
- Mice received chronic morphine treatment (CMT) or vehicle.
- Electrophysiological recordings (voltage clamp) were used to measure I(Ca) in isolated PAG neurons.
- GIRK currents were measured in PAG slices using voltage clamp.
- Responses to MOP agonists (DAMGO, Met-enkephalin) and a GABA(B) agonist (baclofen) were assessed.
Main Results:
- CMT induced tolerance to morphine's antinociceptive effects.
- MOP agonist inhibition of I(Ca) showed reduced maximal effectiveness in CMT neurons.
- Met-enkephalin-activated GIRK currents were smaller in CMT neurons.
- GABA(B) agonist-induced currents were unaffected by CMT.
Conclusions:
- CMT leads to a homologous reduction in MOP effectiveness to inhibit I(Ca) and activate GIRK in the PAG.
- These findings indicate a decrease in MOP number and/or G-protein coupling in PAG neurons following CMT.
- This cellular adaptation contributes to the development of antinociceptive tolerance to morphine.
