Galanin and Opioids: Molecular and Behavioral Interactions in Opioid Use Disorder Circuits Open Access

Abstract

The neuropeptide galanin has been shown to oppose the behavioral effects of opioids, particularly withdrawal and reward. The galaninergic system has therefore been identified as a possible therapeutic target for opioid use disorder (OUD). However, most studies have utilized body- and brain-wide galanin manipulations, leaving system-specific questions about galanin – which could inform targeted therapies for OUD – largely unanswered. Given that the noradrenergic system is implicated in opioid-mediated behaviors and that its major nucleus, the locus coeruleus (LC), strongly expresses galanin, noradrenergic-derived galanin is a prime target for system-specific investigation. Additionally, the mechanisms by which galanin blocks opioid reward and withdrawal remain unclear, but current theories for both implicate galanin receptor 1 (GalR1). Previous work suggests that LC-derived galanin acts on GalR1 within the LC to suppress withdrawal symptoms. In the ventral tegmental area (VTA), GalR1 forms heteromers with the mu opioid receptor (MOR), potentially allowing galanin to directly interfere with opioid signaling and attenuate reward. Therefore, the goal of this dissertation was to examine 1) whether noradrenergic galanin modulates opioid withdrawal and reward behaviors, and 2) to evaluate whether the pattern of GalR1 expression in the LC and in reward circuits provides clues about its contribution to these two aspects of OUD. We demonstrate here that manipulating noradrenergic galanin does not affect naloxone-precipitated withdrawal symptoms. We also show, for the first time, that the distribution of GalR1 in the LC is inconsistent with previous mechanistic theories of galanin-mediated suppression of withdrawal. Regarding opioid reward and reinforcement, we find that altered noradrenergic galanin levels do not affect by acute morphine-induced locomotion, morphine conditioned place preference, or intravenous remifentanil self-administration. Characterization of GalR1 and MOR mRNA co-expression indicates that the rostromedial tegmental nucleus and the VTA, two populations that exert GABAergic control over VTA dopamine neurons, both exhibit GalR1-MOR co-expression in a quarter of GABAergic neurons. Together, these findings constitute the first system-specific investigation of galanin effects on OUD-related behaviors. This work indicates that while noradrenergic galanin does not modulate opioid withdrawal or reward behaviors, enhancing GalR1-MOR heteromeric activity in the VTA may be an important area of focus for future studies.

Table of Contents

CHAPTER 1: BACKGROUND AND LITERATURE REVIEW 1

1.1. OPIOID PHARMACOLOGY 2

1.1.1. The Endogenous Opioid System 2

1.1.2. Cellular and Circuit-Level Opioid Signaling 2

1.1.3. Clinical Utility 4

1.1.4. Endogenous Opioid Reward 6

1.2. OPIOID USE DISORDER 6

1.2.1. The Opioid Epidemic 6

1.2.2. Diagnostic Criteria 8

1.2.3. Treatment 8

1.2.4. Current Challenges 9

1.3. NEURAL CIRCUITRY OF SUBSTANCE USE DISORDERS 10

1.3.1. Reward and Reinforcement 11

Circuitry 11

Animal Models 14

1.3.2. Withdrawal 19

Circuitry 19

Animal Models 22

1.3.3. Craving and Relapse 23

Circuitry 23

Animal Models 25

1.3.4. Neuropeptide Modulation of Addiction-Related Circuits 26

1.5. GALANIN 27

1.5.1. Discovery and Function 27

1.5.2. Galanin Signaling 28

1.5.3. Central Expression of Galanin and its Receptors 29

1.6. GALANINERGIC MODULATION OF OPIOID EFFECTS 32

1.6.1. Therapeutic Effects of Galanin in Rodents 32

1.6.2. Discovery of GalR1-MOR Heteromers 33

1.7. CRITICAL QUESTIONS IN THE FIELD 34

1.7.1. Effect of Noradrenergic Galanin on Opioid-Mediated Behaviors 34

1.7.2. Cell-Type Specific Expression of Galanin and its Receptors 36

1.7.3. Characterization of Central GalR1-MOR Co-Expression 36

1.8. DISSERTATION AIMS 37

1.9. FIGURES 39

 

CHAPTER 2: CELL-TYPE SPECIFIC EXPRESSION AND BEHAVIORAL IMPACT OF GALANIN AND GALR1 IN THE LOCUS COERULEUS DURING OPIOID WITHDRAWAL 44

2.1. ABSTRACT 45

2.2. INTRODUCTION 46

2.3. MATERIALS AND METHODS 48

2.4. RESULTS 56

2.5. DISCUSSION 61

2.6. FIGURES 67

 

CHAPTER 3: NORADRENERGIC GALANIN DOES NOT MODULATE OPIOID REWARD OR REINFORCEMENT 79

3.1. ABSTRACT 80

3.2. INTRODUCTION 81

3.3. MATERIALS AND METHODS 83

3.4. RESULTS 90

3.5. DISCUSSION 93

3.6. FIGURES 100

 

CHAPTER 4: CHARACTERIZATION OF GALR1 AND MOR MRNA IN OPIOID REWARD CIRCUITRY 109

4.1. ABSTRACT 110

4.2. INTRODUCTION 111

4.3. MATERIALS AND METHODS 112

4.4. RESULTS 114

4.5. DISCUSSION 116

4.6. FIGURES 121

 

CHAPTER 5: DISCUSSION 125

5.1. SUMMARY 126

5.2. CONCLUSIONS AND FUTURE DIRECTIONS 126

5.2.1. Noradrenergic Galanin and Opioid Withdrawal 126

5.2.2. Noradrenergic Galanin and Opioid Reward 131

5.2.3. Characterization of GalR1-MOR Co-Expression 135

5.3. CONTRIBUTIONS TO THE FIELD 137

5.3.1. RNA-Based Cell Soma Markers 137

5.3.2. RNAscope Image Analysis 137

5.3.3. Remifentanil IVSA in Mice 138

5.4. CONCLUDING REMARKS 139

 

REFERENCES 140

About this Dissertation

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School	Laney Graduate School
Department	Biological and Biomedical Sciences
Subfield / Discipline	Neuroscience
Degree	Ph.D.
Submission	Dissertation
Language	English
Research Field	Biology, Neuroscience
Keyword	Opioid Galanin Ventral tegmental area Withdrawal Locus coeruleus
Committee Chair / Thesis Advisor	Weinshenker, David, Emory University
Committee Members	Holmes, Philip, University of Georgia Gourley, Shannon, Emory University Dias, Brian, University of Southern California Faundez, Victor, Emory University

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