This electronic thesis or dissertation has been downloaded from Explore Bristol Research, http://research-information.bristol.ac.uk Author: Santhakumar, Rakulan Title: Studies in opioid induced respiratory depression General rights Access to the thesis is subject to the Creative Commons Attribution - NonCommercial-No Derivatives 4.0 International Public License. A copy of this may be found at https://creativecommons.org/licenses/by-nc-nd/4.0/legalcode This license sets out your rights and the restrictions that apply to your access to the thesis so it is important you read this before proceeding. Take down policy Some pages of this thesis may have been removed for copyright restrictions prior to having it been deposited in Explore Bristol Research. 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This electronic thesis or dissertation has been downloaded from Explore Bristol Research, http://research-information.bristol.ac.uk Author: Santhakumar, Rakulan Title: Studies in opioid induced respiratory depression General rights Access to the thesis is subject to the Creative Commons Attribution - NonCommercial-No Derivatives 4.0 International Public License. A copy of this may be found at https://creativecommons.org/licenses/by-nc-nd/4.0/legalcode This license sets out your rights and the restrictions that apply to your access to the thesis so it is important you read this before proceeding. Take down policy Some pages of this thesis may have been removed for copyright restrictions prior to having it been deposited in Explore Bristol Research. However, if you have discovered material within the thesis that you consider to be unlawful e.g. breaches of copyright (either yours or that of a third party) or any other law, including but not limited to those relating to patent, trademark, confidentiality, data protection, obscenity, defamation, libel, then please contact [email protected] and include the following information in your message: •Your contact details •Bibliographic details for the item, including a URL •An outline nature of the complaint Your claim will be investigated and, where appropriate, the item in question will be removed from public view as soon as possible. Studies in opioid induced respiratory depression Rakulan Santhakumar A dissertation submitted to the University of Bristol in accordance with the requirements for award of the degree of MSc by Research in Physiology and Pharmacology in the Faculty of Life Sciences and the school of School of Physiology, Pharmacology and Neuroscience September 2020 Word Count: 31,028 i Abstract The main cause of fatality from opioid overdose is opioid induced respiratory depression (OIRD), with much of the street heroin being cut with fentanyl. Heroin habits often follow drug abstinence-relapse cycles. Currently, overdose is reverse by antagonising the μ-opioid receptor with naloxone. This may lead to opioid withdrawal syndrome, which can induce aggression, a problem to first responders and clinicians. Clinically, balancing analgesia and respiratory depression can present itself to be difficult. AMPA receptor mediators show promise in reversing respiratory depression without affecting analgesia. Heroin and fentanyl dose-dependently depressed respiration in male CD1 mice with heroin doing so through depressing respiratory rate and fentanyl through depressing respiratory rate and tidal volume. The idea that 6-MAM over morphine is the active metabolite of heroin is relatively new. In this study, 6-MAM induced OIRD. It has been previously shown that heroin fentanyl mixtures act synergistically in decreasing oxygen saturation within the brain. A heroin- fentanyl mixture failed to show additivity or synergy in causing respiratory depression as a ceiling effect in response likely occurred. Investigating alternative OIRD reversing agents and the role of the AMPA receptor during opioid induced respiratory depression. Ketamine has been shown to reverse opioid induced respiratory depression to some of the fentanyls but has yet been shown to reverse heroin induced respiratory depression (HIRD). Ketamine (10, 30mg/kg i.p.) and the metabolite, 2R,6R-hydroxynorketmaine (10, 30mg/kg i.p.) failed to reverse HIRD. Tianeptine 30mg/kg i.p. induced OIRD equipotent to heroin 10mg/kg i.p. and showed to induce analgesia (measured in tail flick latency). The decrease in potency could be attributed to AMPA receptor potentiation. Respiratory depression and analgesia showed reversal by naloxone (0.3 mg/kg i.p.), indicating μ-opioid receptor agonism. Many users claim that they perceive an enhanced rate of tolerance to euphoria following periods of abstinence. After heroin pre-treatment using osmotic mini-pumps an enhanced rate of tolerance development to respiratory depression after a 6-day abstinence period is indicated. However, robust tolerance was not seen with the current protocol. When mice were pre-treated with methadone, this enhanced rate was not present. Indicating a PKC mediated mechanism as heroin metabolites are said to induce desensitisation of the μ-opioid receptor. This thesis reports a variety of emerging novel fields within the field of opioid induced respiratory depression and provides an insight into a possible future direction of opioid induced respiratory depression mediating agents and the process of understanding the real- world use of heroin and incorporating behaviours into experimental design. i Acknowledgements I would like to thank my supervisors Professor Graeme Henderson and Professor Eamonn Kelly for their help, advice and the opportunity to conduct this research. Thank you Dr Rob Hill for passing your experimental knowledge and practical abilities onto me and for succumbing to my minesweeper high score. I’d also like to thank Dr Joanna Kesten and those at the Bristol Drug’s Project for your expertise and input. I’d like to thank my friends who stayed on for a fourth year in Bristol, my school friends for keeping me sane, in addition to my Cambridge housemates at The Cottages. Finally, a thank you to my family, especially my late grandmother, for their copious amounts of support. ii Author’s declaration I declare that the work in this dissertation was carried out in accordance with the requirements of the University's Regulations and Code of Practice for Research Degree Programmes and that it has not been submitted for any other academic award. Except where indicated by specific reference in the text, the work is the candidate's own work. Work done in collaboration with, or with the assistance of, others, is indicated as such. Any views expressed in the dissertation are those of the author. SIGNED: .... ........................ DATE: ....................................... iii Table of Contents Abstract ................................................................................................................................ i Acknowledgements ............................................................................................................ ii Author’s declaration .......................................................................................................... iii Table of Contents ............................................................................................................... iv Table of Figures ................................................................................................................ vii Table of Tables ................................................................................................................... ix Table of Abbreviation ......................................................................................................... x 1.0 Introduction ................................................................................................................... 1 1.1 Background to Opioid Use ....................................................................................................... 1 1.2 Heroin .......................................................................................................................................... 2 1.3 Fentanyl ...................................................................................................................................... 2 1.4 Opioid Receptors ....................................................................................................................... 3 1.4.1 Activation of Opioid Receptors ......................................................................................... 3 1.5 Desensitisation of the MOPr and Tolerance .......................................................................... 4 1.6 Opioid Respiratory control ........................................................................................................ 6 1.7 AMPA Receptors in Respiratory Control ................................................................................ 8 1.8 Ketamine ....................................................................................................................................