The effect of repeated early injury on reward-related processing in the adult rat Lucie Alanna Low University College London 2010 Thesis presented for the degree of Doctor of Philosophy 1 Declaration: I, Lucie Low, confirm that the work presented in this thesis is my own. Where information has been derived from other sources, I confirm that this has been indicated in the thesis. Lucie Low April 2010 2 Abstract Pain during early life can affect the developing central nervous system, leading to altered neural function in the adult organism. In this thesis, I investigate the long-term effects of repeated early pain on reward-related processing in the adult rat. I hypothesised that the reward system was likely to be sensitive to early activation of pain pathways, as the brain systems involved in both pain and reward overlap extensively, and virtually all centrally acting analgesic drugs are also drugs of reward. To begin, I investigate the extent to which the developing reward system is activated by a classic analgesic and drug of abuse, morphine. Comparing neonatal and adult activation of the dopaminergic system, results show that a single morphine challenge activates neonatal reward pathways, but that there are qualitative differences in the neonatal response to repeated morphine. Next, I show how reward-related behaviours of adult animals repeatedly injured as neonates differ from those of uninjured littermates, and finally propose the lateral hypothalamic orexin system as a biomarker reflecting this behaviour. The results provide evidence that neonatal injury interferes with the normal development of reward systems during a critical period of development, resulting in characteristic changes in reward behaviour and cell signalling in the adult animal. 3 Acknowledgements I firstly want to thank Maria for being such a fantastic supervisor. I’ve learnt such a huge amount from her guidance and wisdom, and am immensely grateful for her tutelage. I also want to thank Amanda Williams for being an excellent second supervisor, and for statistical help and advice. Plus thanks go to Steve Hunt, for his encyclopaedic knowledge, help and advice, and amusing sarcasm at times! Thanks to everyone in the Hunt/Fitzgerald lab for being such great colleagues over the past 4 years – special thanks to Gareth Hathway, Rebeccah Slater, Suellen Walker, David Vega-Avelaira, Stéphanie Koch, Keri Tochiki and Julia Slone-Murphy for help, ideas, support, friendship and many fun times. Thanks also to the Wellcome Trust and London Pain Consortium for funding me and enabling me to complete this research, and to Kate Jeffery and Brian Bye, plus all the people at the Institute of Behavioural Neuroscience, for their support and patience with my endless requests for ‘quiet on the corridor’. Finally, immense thanks go to my best mate Alex Kaye, and especially to my wonderful parents. I can’t thank them enough for their proof-reading skills, help and support over the last 27 years, and the course of this thesis – I really couldn’t have done it without you. X 4 1 INTRODUCTION .......................................................................................18 1.1 The problem of early pain......................................................................................................... 18 1.1.1 Pain management in neonates ................................................................................................. 19 1.1.2 Early pain in humans causes long-term alterations in pain circuitry and sensitivity............... 20 1.1.2.1 Effects are mediated by alterations in central nociceptive processing........................... 22 1.1.2.2 Prematurity can affect the stress response .....................................................................23 1.1.2.3 NICU experience can alter long-term cognitive development....................................... 23 1.1.3 Animal models provide information on the development of nociceptive processing ............. 24 1.1.3.1 Nociceptive transmission pathways............................................................................... 25 1.1.3.2 Spinal nociceptive processing develops over the first few weeks of an animal’s life ... 26 1.1.3.3 Descending inhibitory control from the brainstem is not mature in the neonatal rat..... 28 1.1.3.4 Long-term effects of neonatal noxious stimuli in rodents .............................................28 1.1.3.5 Critical periods in development and long-term effects.................................................. 30 1.2 Pain and reward processing are linked....................................................................................31 1.2.1 Areas of the brain involved in pain and reward processing overlap........................................31 1.2.1.1 NAcc.............................................................................................................................. 32 1.2.1.2 Amygdala ...................................................................................................................... 32 1.2.1.3 Anterior cingulate cortex............................................................................................... 32 1.2.2 Analgesia and relief from pain are rewarding.........................................................................33 1.2.2.1 Termination of a pain stimulus is rewarding .................................................................33 1.2.2.2 The placebo effect relies on reward processing.............................................................34 1.2.2.3 Rewarding stimuli induce analgesia .............................................................................. 34 1.2.2.4 Chronic pain states reduce reward responses.................................................................35 1.2.3 Endogenous opioids modulate pain and reward......................................................................35 1.2.4 Motivation-decision model ..................................................................................................... 36 1.2.5 Does early pain lead to altered reward processing in later life?.............................................. 38 1.3 The aims of this thesis................................................................................................................ 39 2 CHAPTER 2 – INVESTIGATING THE EFFECT OF MORPHINE ON NEONATAL REWARD PATHWAYS ...............................................................41 2.1 Introduction................................................................................................................................ 41 2.1.1 Dopamine (DA)....................................................................................................................... 41 2.1.1.1 Dopamine in the brain ................................................................................................... 42 2.1.2 Mesolimbic dopaminergic system........................................................................................... 44 2.1.2.1 Genetic manipulation of the DA system........................................................................45 2.1.3 Ventral Tegmental Area.......................................................................................................... 45 2.1.3.1 Firing properties of VTA DA cells................................................................................46 5 2.1.3.2 Behavioural studies........................................................................................................ 47 2.1.3.3 Human imaging studies ................................................................................................. 47 2.1.4 Ontogeny of the dopaminergic system.................................................................................... 48 2.1.4.1 Cell differentiation......................................................................................................... 48 2.1.4.2 Monoamine levels and receptor expression................................................................... 48 2.1.5 Opioids.................................................................................................................................... 49 2.1.5.1 Morphine ....................................................................................................................... 50 2.1.5.2 The developmental regulation of MOR expression and morphine function.................. 51 2.1.5.2.1 Ontogeny of MOR expression .................................................................................. 51 2.1.5.2.2 Binding to opioid receptors is seen prenatally .......................................................... 51 2.1.5.2.3 Morphine analgesia in the neonate............................................................................ 52 2.1.5.2.4 Chronic morphine administration in the neonate alters opioid signalling................. 52 2.1.5.2.5 Modulation of opioid signalling by external influences............................................ 53 2.1.5.3 Morphine is rewarding................................................................................................... 53 2.1.5.4 Morphine and dopaminergic signalling ......................................................................... 54 2.1.6 Reward processing in the neonate – the role of dopamine......................................................55 2.1.6.1 Bonding behaviour is mediated by dopaminergic pathways..........................................55 2.1.7 Morphine is rewarding in the neonate.....................................................................................56