http://researchcommons.waikato.ac.nz/ Research Commons at the University of Waikato Copyright Statement: The digital copy of this thesis is protected by the Copyright Act 1994 (New Zealand). The thesis may be consulted by you, provided you comply with the provisions of the Act and the following conditions of use: Any use you make of these documents or images must be for research or private study purposes only, and you may not make them available to any other person. Authors control the copyright of their thesis. You will recognise the author’s right to be identified as the author of the thesis, and due acknowledgement will be made to the author where appropriate. You will obtain the author’s permission before publishing any material from the thesis. Oxytocin as an appetite suppressant that reduces feeding reward A thesis submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Biological Sciences at The University of Waikato by Florence M. Herisson _________ The University of Waikato 2016 2 Abstract In the environment in which palatable and highly caloric foods are readily available, eating behavior is oftentimes not dictated by the necessity to replenish lacking energy, but rather by the pleasure of consumption. Centrally acting oxytocin (OT) is known to promote termination of feeding to protect internal milieu by preventing excessive stomach distension, hyperosmolality and ingestion of toxins. Initial evidence suggests that another possible role for OT in mechanisms governing food intake is to reduce consumption of select palatable tastants. This thesis explores the question whether OT is as an appetite suppressant that reduces feeding reward. The first set of experiments addresses whether OT affects intake of (a) all carbohydrates, (b) only sweet carbohydrates or (c) sweet non-carbohydrate saccharin in mice. In those studies, generalized injection of a blood brain barrier penetrant OT receptor antagonist, L-368,899, significantly increased the intake of sweet (sucrose, glucose, fructose, polycose) and non-sweet (cornstarch) carbohydrates and promoted a trend approaching significance in saccharin consumption. Consumption of carbohydrate-enriched foods led to an increase in OT mRNA levels in the hypothalamus. The second set of studies identifies the nucleus accumbens core (AcbC), a key component of the reward system, as a site that mediates anorexigenic effects of OT. Rats injected with OT directly in the AcbC showed a decreased intake of 3 sucrose and saccharin solutions as well as of standard chow. This treatment did not cause taste aversion, hence the outcome was not due to sickness/malaise. The effects of AcbC OT on feeding could be observed only in animals offered a meal in a non-social environment. Once a social setting (devoid of direct antagonistic interactions between individuals) of a meal was introduced, AcbC OT failed to reduce feeding. AcbC levels of OT receptor transcript were affected by exposure to palatable food as well as by food deprivation. The third and final set of studies shows that aberrant integrity of neuronal circuitry within the neuroendocrine and reward systems due to genetic deletion of connexin 36 (Cx36) gap junctions leads to dysregulation of the OT system’s functioning in the Cx36 KO mouse. This dysregulation is associated with hypersensitivity to aversive properties of foods, reduced interest in feeding for reward (palatable carbohydrates and saccharin) and abnormal ingestion of energy. Overall, the findings suggest that OT diminishes feeding for reward, particularly the intake of palatable carbohydrates and saccharin, by acting – at least in part – via the reward system. OT appears to be part of central mechanisms that cross- link homeostasis-driven and palatability-related (i.e., flavor- and macronutrient- specific) termination of consumption. 4 A Jean-jacques “On n’existe pas sans faire” De Beauvoir 5 6 List of papers This thesis is based on the following papers, which are referred to in the text by their Roman numerals. I. Herisson FM, Brooks LL, Waas JR, Levine AS, Olszewski PK. Functional relationship between oxytocin and appetite for carbohydrates versus saccharin. Neuroreport. 2014 Aug 20;25(12):909-14. (published) II. Herisson FM, Waas JR , Fredriksson R, Schiöth HB, Levine AS, Olszewski PK. Oxytocin acting in the nucleus accumbens core decreases food intake. Journal of Neuroendocrinology, 2016, 28, 10.1111/jne.12381 (published) III. Herisson FM, Bird S, Sleigh JW, Levine AS, Olszewski PK. Connexin 36 deletion diminishes feeding for reward. ( submitted) 7 8 List of papers not included in this thesis Eriksson A, Williams MJ, Voisin S, Hansson I, Krishnan A, Philippot G, Yamskova O, Herisson FM, Dnyansagar R, Moschonis G, Manios Y, Chrousos GP, Olszewski PK, Frediksson R, Schiöth HB. Implication of coronin 7 in body weight regulation in humans, mice and flies. BMC Neurosci. 2015 Mar 14;16:13 Olszewski PK, Waas JR, Brooks LL, Herisson FM, Levine AS. Oxytocin receptor blockade reduces acquisition but not retrieval of taste aversion and blunts responsiveness of amygdala neurons to an aversive stimulus. Peptides. 2013 Dec;50:36-41. 9 10 Content List of figures ..................................................................................................................... 15 List of tables ...................................................................................................................... 18 Abbreviations .................................................................................................................... 20 Chapter 1 ........................................................................................................................... 23 Introduction and Aims ...................................................................................................... 23 1.1. Oxytocin and receptors .......................................................................................... 24 1.1.1 Oxytocin ........................................................................................................... 24 1.1.2 Oxytocin receptor ............................................................................................ 27 1.2. Main behavioral and physiological effects of oxytocin .......................................... 32 1.3 Mechanisms controlling food intake: focus on the role of oxytocin ...................... 34 1.3.1. Oxytocin and homeostatic control of food intake .......................................... 35 1.4 Oxytocin as an appetite suppressant acting in the wide network of central sites: functional relationship between oxytocin and gap junction protein, connexin 36? .... 45 1.4.1. Cx36 gap junction ............................................................................................ 46 Aims .................................................................................................................................. 48 References .................................................................................................................... 49 Chapter 2 ........................................................................................................................... 73 Oxytocin receptor blockade enhances appetite for carbohydrates, particularly sucrose 73 Abstract ......................................................................................................................... 73 2.1. Introduction ........................................................................................................... 74 2.2. Methods ................................................................................................................. 76 2.2.1. Animals ............................................................................................................ 76 2.2.2. Effect of OTr blockade on consumption in the no-choice single-bottle paradigm ................................................................................................................... 76 2.2.3. Effect of OTr blockade on sugars vs saccharin preference ............................. 77 2.2.4. OT mRNA levels in mice consuming sucrose, CS or saccharin ........................ 77 2.3. Results .................................................................................................................... 79 2.4. Discussion ............................................................................................................... 82 References .................................................................................................................... 85 11 Chapter 3 ........................................................................................................................... 89 Oxytocin acting via the nucleus accumbens core decreases food intake driven by hunger and by reward in rats offered a meal in a non-social setting ........................................... 89 Abstract ......................................................................................................................... 89 3.1. Introduction ........................................................................................................... 90 3.2. Materials and Methods .......................................................................................... 92 3.2.1. Animals. ........................................................................................................... 92 3.2.2. Surgeries. .......................................................................................................