BOLD Imaging in Awake Wild-Type and Mu-Opioid Receptor Knock-Out Mice Reveals On-Target Activation Maps in Response to Oxycodone.

Kelsey Moore,Jason R Yee,Praveen Kulkarni,Craig F Ferris,Brigitte L Kieffer,Emmanuel Darcq,Dan Madularu,Sade Iriah

doi:10.3389/fnins.2016.00471

Abstract

Blood oxygen level dependent (BOLD) imaging in awake mice was used to identify differences in brain activity between wild-type, and Mu (μ) opioid receptor knock-outs (MuKO) in response to oxycodone (OXY). Using a segmented, annotated MRI mouse atlas and computational analysis, patterns of integrated positive and negative BOLD activity were identified across 122 brain areas. The pattern of positive BOLD showed enhanced activation across the brain in WT mice within 15 min of intraperitoneal administration of 2.5 mg of OXY. BOLD activation was detected in 72 regions out of 122, and was most prominent in areas of high μ opioid receptor density (thalamus, ventral tegmental area, substantia nigra, caudate putamen, basal amygdala, and hypothalamus), and focus on pain circuits indicated strong activation in major pain processing centers (central amygdala, solitary tract, parabrachial area, insular cortex, gigantocellularis area, ventral thalamus primary sensory cortex, and prelimbic cortex). Importantly, the OXY-induced positive BOLD was eliminated in MuKO mice in most regions, with few exceptions (some cerebellar nuclei, CA3 of the hippocampus, medial amygdala, and preoptic areas). This result indicates that most effects of OXY on positive BOLD are mediated by the μ opioid receptor (on-target effects). OXY also caused an increase in negative BOLD in WT mice in few regions (16 out of 122) and, unlike the positive BOLD response the negative BOLD was only partially eliminated in the MuKO mice (cerebellum), and in some case intensified (hippocampus). Negative BOLD analysis therefore shows activation and deactivation events in the absence of the μ receptor for some areas where receptor expression is normally extremely low or absent (off-target effects). Together, our approach permits establishing opioid-induced BOLD activation maps in awake mice. In addition, comparison of WT and MuKO mutant mice reveals both on-target and off-target activation events, and set an OXY brain signature that should, in the future, be compared to other μ opioid agonists.

Highlights

Oxycodone (OXY) is a powerful analgesic initially prescribed for the management of acute pain, pain following surgery, and pain associated with palliative care
With a Kruskall-Wallis multiple comparison analysis WT mice treated with oxycodone showed 72 out of 122 brains areas to be significantly different from WT mice treated with saline vehicle
Post-hoc analysis showed there were no significant differences between the vehicle group and Muopioid knock-out (MuKO) mice given OXY

Summary

Introduction

Oxycodone (OXY) is a powerful analgesic initially prescribed for the management of acute pain, pain following surgery, and pain associated with palliative care. The analgesic effects of morphine are mediated through μ receptors as shown in human and animal studies using specific μ-receptor antagonists in the presence of a painful stimulus and in studies on Muopioid knock-out (MuKO) mice (Matthes et al, 1996; Sora et al, 1997). MuKO mice showed lack of morphine analgesia, rewarding effect and physical dependence, indicating that a single receptor mediates all biological effects of the prototypic opiate (Matthes et al, 1996). Whether this is true for OXY has not been tested in MuKO mice, as yet. Μ-opioid analgesics (but not endogenous μ-opioid peptides), such as morphine, fentanyl, and OXY promote inhibition of thalamic neuronal activity; the effects were not affected by μ-opioid receptor antagonist treatment, nor the deletion of the μ-opioid receptor gene (Hashimoto et al, 2009), indicating possible off-target effects for these μ-opioid agonists

Methods

Results

Conclusion