The effects of available opioid agonist and antagonist treatments on reward processing: An exploratory thesis in patients suffering from opioid addiction

Abstract

Background and objective: Methadone and buprenorphine are the two pharmacotherapy options available for opioid addiction in Norway, but both are addictive. Extended release naltrexone (XR-NTX) renders a non-addictive pharmacotherapy option but is yet not standard treatment. Opioid modulation has been shown to affect reward sensitivity in healthy populations in previous studies, but few have investigated reward sensitivity in samples suffering from opioid addiction. The main objective of this thesis is to explore how treatment with opioid agonists and antagonists affect reward processing and subjective well-being in an opioid dependent sample. Method: Data probed in this thesis comes from two independent observational studies of reward function in pharmacotherapy for opioid addiction, with partly overlapping test batteries. I assessed subjective and objective reward sensitivity and subjective well-being in 39 patients receiving treatment with XR-NTX and 18 receiving standard treatment with buprenorphine or methadone. Objective reward sensitivity was assessed using a two-alternative forced choice signal detection task with skewed reward. Subjective reward sensitivity was measured using two versions of the Snaith-Hamilton pleasure scale (SHAPS; Visual Analogue Scale version in Study 1, four-point Likert scale in Study 2). Results: The subjective reward sensitivity measure revealed variable scores within both treatment groups. The NTX group reported significantly lower levels of subjective reward sensitivity compared to OMT in one of the samples (mean: NTX = 25.86, OMT = 22.0, Welch’s t(20.3)= -2.51, p = .02). Both treatment groups showed reward sensitivity in the behavioural task (mean: NTX = 0.16, OMT = 0.05). The NTX group showed significantly higher reward sensitivity than the OMT group (F(1, 49) = 5.64, p = .02). Conclusion: These results suggest that blocking of the endogenous opioid system does not necessarily lead to blunted reward sensitivity and anhedonia.