System Interactions in the Regulation of Appetite University of Leeds Fiona Louise Wright BSc (Hons) Submitted in accordance with the requirements of the degree of Doctor of Philosophy The University of Leeds Institute of Psychological Sciences September 2013 ii The candidate confirms that the work submitted is her own, except where work which has formed part of jointly-authored publications has been included. The contribution of the candidate and the other authors to this work has been explicitly indicated below. The candidate confirms that appropriate credit has been given within the thesis where reference has been made to the work of others. (see page xxii) This copy has been supplied on the understanding that it is copyright material and that no quotation from the thesis may be published without proper acknowledgement © 2013 The University of Leeds and Fiona Louise Wright iii Acknowledgements First and foremost, I would like to sincerely thank Professor John Rodgers, my PhD supervisor. I could never give enough thanks for his guidance and support over the last four years. He is a truly wonderful supervisor, and his professional knowledge and skill has been continually inspiring. A thank you, as well, to my secondary supervisor, Amanda Harrison, for her supervision and encouragement during John’s absence at the start of this journey. Lisa Broadhead also deserves a special thank you for her company and help in the laboratory during the experimental parts of the thesis. I’d also like to acknowledge Neil Lowley for his wonderful technical help down in the lab. To my funding body, my parents, I owe more thanks than can be put into words. Without their encouragement and financial support I would never have even started this doctoral journey. I am forever indebted to them for the sacrifices they have made to make my dream a reality. Thanks also to my lovely younger sister, Claire, who has supported me in everything I’ve done. Thank you to Nicola, Lucy, Sarah and Michelle, who have been continually emphathetic and encouraging. Also to Faye and Sarah, who made a pleasant and welcome break from the isolation down in the lab. And finally, a huge thank you goes to Will, without whom I’d have surely gone insane. Words could never describe his continued love, support, patience and tolerance. I am endlessly grateful for his unfaltering belief that I could do this and it would “all be fine”. I would like to dedicate this thesis to my dad, who always said his aim in life was for his children to achieve what he has yet to. iv Abstract Despite the huge range of neurobiological targets within the appetite system, the development of pharmacological anti-obesity agents is making slow progress as a result of limitations on maximal weight loss, adverse side-effects, and/or long-term resistance. However, in principle, the use of drug polytherapy allows for the use of lower doses, possible synergistic/additive weight loss, fewer and less serious side- effects and reduced potential for counter-regulation. Although food intake and/or bodyweight have been and are being researched following co-treatment with a range of agents, there is a distinct gap in the literature regarding the behavioural specificity of the anorectic effects for recently approved and upcoming anti-obesity therapies. The present thesis therefore characterised the acute effects of individual systemic (i.p.) treatment with the general opioid receptor antagonist naltrexone (0.1, 1.0 and 3.0mg/kg), the noradrenaline and dopamine reuptake inhibitor bupropion (10, 20 and 40mg/kg), the serotonin 5-HT1B/2C receptor agonist mCPP (0.1, 1.0 and 3.0mg/kg), and the GLP-1R mimetic exendin-4 (0.025, 0.25, and 2.5µg/kg), on food intake, feeding and non-feeding behaviour, the behavioural satiety sequence (BSS), and weight gain. In addition, the acute anorectic response to co-treatment with sub- maximal doses of each non-opioid compound plus an opioid antagonist (naloxone or naltrexone) was assessed. The results suggested that, while the anorectic effects of naloxone and naltrexone were behaviourally-selective, those of rimonabant, bupropion, mCPP and exendin-4 may have largely resulted from competing behaviour. The co-treatment studies highlighted concurrent anorexia and an undesirable behaviour for rimonabant, bupropion and, potentially, mCPP. However, the anorectic action of mCPP and exendin-4 may have largely resulted from malaise. The results further showed that, while only the combination of bupropion and naltrexone produced an additive effect on food intake, co-treatment with an opioid antagonist reduced/eliminated unwanted effects normally associated with higher doses of rimonabant, bupropion and, potentially, mCPP. The search for efficacious and safe anti-obesity agents should therefore focus, to an even greater extent than at present, on the therapeutic potential of targeting multiple systems (polytherapy). Overall, current findings have emphasised the value of detailed behavioural analysis of drug effects on appetite. As such, novel treatment combinations may well produce a successful anti-obesity agent, if clinical trials are prefaced by adequate preclinical testing. v Table of Contents Acknowledgements ..............................................................................................................iii Abstract .................................................................................................................. iv Contents ....................................................................................................................v List of Figures ................................................................................................................ xvi List of Tables ................................................................................................................. xx Publications ............................................................................................................... xxii List of Abbreviations ....................................................................................................... xxiii Abbreviations for Treatment Conditions ........................................................................ xxv Chapter 1 Neurobiology of Appetite ........................................................... 1 1.1 Historical Overview of Theories and Models of Appetite Regulation ...... 2 1.1.1 Set-point theories .......................................................................... 2 1.1.2 The dual centre model .................................................................. 4 1.1.3 The Hypothalamus Revisited ........................................................ 6 1.2 Adiposity Signals ................................................................................... 8 1.2.1 Insulin ........................................................................................... 8 1.2.2 Leptin .......................................................................................... 10 1.3 The Melanocortin System .................................................................... 13 1.3.1 POMC ......................................................................................... 13 1.3.2 α-Melanocyte Stimulating Hormone ............................................ 14 1.3.3 Cocaine and Amphetamine-Regulated Transcript ....................... 15 1.4 The ARC NPY/AgRP System .............................................................. 16 1.4.1 NPY 16 1.4.2 Agouti-Related Peptide ............................................................... 17 1.4.3 Orexins ....................................................................................... 18 1.5 Other Systems ..................................................................................... 20 1.5.1 Anorectic –Peripheral .................................................................. 21 1.5.1.1 Cholecystokinin ............................................................ 22 1.5.1.2 Oxyntomodulin.............................................................. 23 1.5.1.3 Bombesin ..................................................................... 24 1.5.1.4 Somatostatin ................................................................ 25 1.5.1.5 Fat-Specific Satiation Peptides ..................................... 26 1.5.1.6 Amylin .......................................................................... 27 vi 1.5.1.7 Peptide YY ................................................................... 28 1.5.1.8 Pancreatic Polypeptide ................................................. 29 1.5.2 Anorectic – Central ..................................................................... 30 1.5.2.1 Neuropeptide W ............................................................ 30 1.5.2.2 Neurotensin and Neuromedin ....................................... 31 1.5.2.3 Oxytocin ....................................................................... 32 1.5.2.4 Corticotropin-releasing factor ........................................ 33 1.5.3 Orexigenic – Peripheral .............................................................. 34 1.5.3.1 Ghrelin .......................................................................... 34 1.5.4 Orexigenic - Central .................................................................... 36 1.5.4.1 Galanin and Galanin-like peptide (GALP) ..................... 36 1.5.4.2 Melanin-concentrating hormone ................................... 37 1.5.5 Summary ...................................................................................