A Tale of Two Opioids: Comparing the Potency of Methadone and Morphine in Analgesia and Abuse Liability in Female Mice Open Access

Abstract

The potential use of methadone as an analgesic has long been debated due to its potent antinociceptive properties and lower abuse liability when compared to canonical opioids such as morphine, the current gold standard of first-line analgesia. While both morphine and methadone induce activation of the central analgesic and reward circuits through stimulation of the mu-opioid receptor (MOR), recent research has suggested that differences in the abuse liability between methadone and morphine may be mediated by a heteromer formed by MOR and the galanin 1 receptor (GalR1). Methadone and morphine have similar potency for the MOR alone, but methadone is much less potent at activating the heteromer. Importantly, GalR1 and MOR co-expression appears to be mostly limited to the reward circuitry, whereas neurons that comprise the central analgesic circuits express either MOR or GalR1, suggesting a potential mechanism for the lower abuse liability of methadone. However, very few studies have directly compared morphine- and methadone-induced reward and analgesia. The purpose of this study was to compare the potencies of methadone and morphine in inducing analgesia (using von Frey and hot plate assays) and abuse liability (using conditioned place preference; CPP) in female mice. RNAScope in situ hybridization was used to characterize the co-expression of MOR and GalR1 mRNA in GABAergic neurons in the rostromedial tegmental nucleus (RMTg) following the CPP paradigm to determine whether intermittent opioid exposure potentially alters heteromer abundance in a key node of the brain reward circuit. We found that while methadone required a higher dose than morphine to induce a CPP, both drugs required similar doses to induce analgesia in both mechanical and thermal antinociception tests. Neither drug induced changes in MOR or GalR1 mRNA, nor the degree of co-expression, following CPP administration. These results are consistent with the idea that the MOR-GalR1 heteromer is critical for opioid-induced reward but not analgesia and support further consideration of methadone as a first line analgesic and the MOR-GalR1 heteromer as a target for opioid misuse therapies.

Table of Contents

Table of Contents

Introduction 1

Background on Opioid Epidemic 1

The Effects of Morphine and Other Opioids on the Reward and Analgesic Circuits 2

Methadone as an Alternative Opioid Analgesic 5

The Galanin System May Mediate Methadone’s Low Abuse Liability 6

Signal Transduction and Expression following Opioid Activation 9

Studying Abuse Liability via Conditioned Place Preference 12

Studying Analgesia via Antinociception Assays 13

Figure 1: Sites of MOR expression and GalR1 expression in the mouse brain 15

Study Aims 15

Prediction 16

Methods 16

Animals 16

Conditioned Place Preference (CPP) 17

Figure 2: CPP apparatus and chamber cues 18

Nociception Assays 18

Figure 3: Thermal and Mechanical Antinociception Assays 19

Von Frey Filaments Test 19

Hot Plate Test 20

Tissue Collection 20

RNAScope Assay of RMTg 20

Image Analysis 22

Statistical Analysis 22

Timeline 22

Results and Discussion 23

Figure 4. Methadone induces preference in female mice starting at 1 mg/kg. 25

Figure 5. Morphine induces preference in female mice starting at 0.3 mg/kg. 28

Figure 6. Methadone and morphine induce analgesia in female mice starting at 10 mg/kg. 31

Figure 7. Identifying Location of RMTg using TH and GAD expression. 34

Figure 8. Intermittent morphine or methadone exposure during CPP does not induce changes in GalR1 or MOR mRNA expression. 37

Figure 9. Intermittent morphine or methadone exposure during CPP does not induce changes in GalR1 or MOR puncta that is dependent on the level of expression. 39

Conclusion 42

Works Cited 43

About this Honors Thesis

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School	Emory College
Department	Neuroscience and Behavioral Biology
Degree	B.S.
Submission	Honors Thesis
Language	English
Research Field	Biology, Neuroscience Health Sciences, Medicine and Surgery Biology, Genetics
Keyword	Methadone Morphine Opioid MOR-GalR1 Heteromer Neuroscience
Committee Chair / Thesis Advisor	David Weinshenker, Emory University
Committee Members	Rohan Palmer, Emory University Shannon Gourley, Emory University Arri Eisen, Emory University

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