Evaluating the Role of the Kappa Opioid Receptor Nalfurafine in Preclinical Models of Demyelination

<p><strong>Multiple sclerosis (MS) is a neurodegenerative autoimmune disease affecting the central nervous system (CNS), impacting more than 2.9 million people worldwide with increasing prevalence. Characterised by inflammatory demyelination, MS involves immune-mediated destruction of the myelin sheath surrounding axons, disrupting saltatory conduction and ultimately leading to axonal degeneration. A critical factor in MS progression is the failure of remyelination, primarily due to the inability of oligodendrocyte precursor cells (OPCs) to effectively differentiate into mature myelinating oligodendrocytes (OLs). Current treatments primarily target inflammation but fail to promote repair, underscoring the urgent need for therapies that enhance remyelination.</strong></p><p>The kappa opioid receptor (KOR) has been established as a promising target for promoting CNS remyelination. Traditional KOR agonists like U50,488 have shown preclinical remyelination efficacy, however their clinical use is limited by adverse effects such as sedation, aversion, and dysphoria. Nalfurafine, a selective KOR agonist, already approved for clinical use in Japan, has emerged as a promising alternative with an established safety profile. Nalfurafine has demonstrated remyelinating capabilities, yet the exact mechanisms and how these effects develop across different regions of the CNS and the timing of this remain to be fully characterised. This thesis investigates KOR expression in OL lineage cells and evaluates nalfurafine's potential as a remyelinating agent in preclinical models of demyelination mimicking MS.</p><p>To examine KOR expression in OL lineage cells, we employed RNAScope® in situ hybridisation combined with immunohistochemistry in PdgfRα-H2B-eGFP transgenic mice. Our results demonstrated approximately 56% of OPCs and 36% of mature OLs express Oprk1 mRNA (encoding KOR), indicating potential downregulation during OL maturation. Anatomical mapping revealed high Oprk1 expression in specific brain regions, including the claustrum, major island of Calleja, and endopiriform nucleus, providing valuable insights into potential region-specific therapeutic responses.</p><p>We subsequently evaluated nalfurafine's therapeutic efficacy in the experimental autoimmune encephalomyelitis (EAE) model, which mimics immune driven demyelination. Treatment with nalfurafine (0.01 mg/kg) significantly reduced paralysis disease scores compared to vehicle controls and enhanced the differentiation of OL lineage cells into mature OLs. Specifically, nalfurafine increased the numbers of immature OPCs (Mash1+/PdgfRαeGFP+) and mature OLs (GSTπ+), resulting in elevated myelin levels in the ventral spinal cord. These findings support the role of nalfurafine in promoting remyelination through enhancing OPC differentiation.</p><p>To further elucidate the mechanisms underlying nalfurafine's remyelinating effects, we employed the cuprizone model of toxin-induced demyelination. This model, with limited peripheral immune involvement, provided a clearer window into the direct effects of nalfurafine on OL biology. Our results confirmed nalfurafine primarily enhanced the differentiation of OPCs into mature OLs rather than stimulating proliferation, with effects most pronounced in the lateral aspect of the corpus callosum and progressively developing over time. The absence of significant BrdU incorporation following nalfurafine treatment supports that KOR agonism primarily promotes differentiation rather than proliferation of OL lineage cells.</p><p>Collectively, these findings establish KOR as a therapeutic target for enhancing remyelination through promoting OPC differentiation. The consistent efficacy of nalfurafine across distinct experimental models of demyelination, coupled with its existing clinical approval, positions KOR agonism as a viable approach for translation into the clinic. This work provides a strong foundation for advancing KOR-targeted interventions toward clinical application, potentially offering new hope for MS patients suffering from progressive disability due to impaired remyelination.</p>