Understanding the genomic basis and environmental factors associated with opioid use disorder

Opioid use disorder is a multifactorial neuropsychiatric condition characterised by compulsive drug-seeking behaviour despite adverse consequences, and it remains a significant global public health concern. The widespread prescription of opioids for chronic pain has contributed to escalating rates of misuse, dependence, and overdose. This doctoral thesis investigates the genetic, molecular, and environmental factors involved in opioid use disorder, with the aim of elucidating mechanisms of vulnerability, resilience, and therapeutic potential through an integrative, multi-level approach. The thesis begins with a comprehensive examination of opioid pharmacology, receptor subtypes, and the neuroplastic adaptations induced by chronic opioid exposure. Particular emphasis is placed on dysregulated dopaminergic signalling and synaptic remodelling within the brain’s reward circuitry, highlighting the limitations of current pharmacotherapies and the promise of novel agents. Chapter 2 presents a systematic review of genetic and transcriptomic studies in both human and animal models, synthesising evidence from candidate gene research, genome-wide association studies, and RNA sequencing. Consistent risk markers emerge in opioid receptor genes such as OPRM1 and in dopaminergic pathways including DRD1 and DRD2, while transcriptomic data reveal disruptions in synaptic plasticity, neuroinflammation, and neurotransmission. Methodological gaps and the need for multi-omic integration are critically discussed. Chapters 3 and 4 detail original transcriptomic investigations conducted at the University of Camerino and the Medical University of South Carolina form the empirical core of the thesis. Using heterogeneous stock rats exposed to chronic heroin, animals were phenotyped into vulnerable, resilient, and intermediate groups. Transcriptomic profiling across five brain regions—the nucleus accumbens core and shell, prefrontal cortex, infralimbic cortex, and ventral tegmental area—revealed sex-specific and region-specific gene expression signatures associated with addiction phenotypes. Notably, convergent findings across sites implicated MAPK/ERK signalling, ion channel dysregulation, immune modulation, and chromatin remodelling as central mechanisms. Chapter 5 introduces a novel dimension by systematically incorporating environmental variables into the molecular analysis of opioid use disorder. Factors such as cage size, social housing, bedding, air quality, and ambient noise were evaluated for their impact on addictionrelated behaviours and gene expression. This integrative study demonstrates that microenvironmental variation significantly modulates neuroinflammatory, synaptic, and metabolic pathways, with distinct sex-dependent effects—establishing a new benchmark for future research on gene–environment interactions in substance abuse vulnerability. The final synthesis of findings emphasises the dynamic interaction between genetic susceptibility, environmental influences, and biological sex in determining individual directions of vulnerability or resilience to opioid use disorder. Protective adaptations are characterised by upregulated mitochondrial efficiency, increased ribosomal activity, and finely tuned neuroimmune responses. In contrast, susceptibility is marked by disruptions in neurotransmitter systems, compromised synaptic integrity, and heightened neuroinflammatory signalling. Building on these insights, a set of actionable therapeutic targets offers a foundation for precision-based approaches to treatment. The most novel aspect of the thesis lies in its rigorous evaluation of environmental factors and their influence on addiction-related behaviours and gene expression. This integrative analysis demonstrates that subtle differences in laboratory microenvironments can significantly modulate neuroinflammatory, synaptic, and metabolic pathways, with distinct effects observed between sexes. By establishing a direct link between environmental context and molecular vulnerability to opioids, the thesis sets a new precedent for how preclinical addiction studies should be designed and interpreted. Taken together, this thesis advances a biologically and environmentally integrated framework for understanding opioid abuse. By combining neurobiological insight, genetic precision, and environmental analysis, it identifies mechanistic pathways of vulnerability and resilience, and proposes sex-specific, precision medicine strategies to mitigate the burden of opioid use disorder. It makes a distinctive and innovative contribution to the neuroscience field by integrating genomic, transcriptomic, behavioural, and environmental dimensions in the study of opioid use disorder. While previous research has largely concentrated on genetic predisposition and neurobiological mechanisms in isolation, this study pioneers a multilayered approach that systematically integrates environmental variability and sex as biological variables—two factors frequently neglected in preclinical addiction research. Notably, it also encompasses the analysis of both vulnerability and resilience to opioid exposure, a dual perspective that, to the best of our knowledge, represents a novel and underexplored dimension in the field.