Opioid Modulation of Retinal Ganglion Cells Providing Photoentrainment of the Circadian Clock

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Abstract Opiates are the cornerstone of analgesic therapy, but also produce numerous side effects. Besides their high propensity for addiction, repeated opioid administration leads to progressive sleep disorders, expressed as worsening insomnia and daytime sleepiness/sleeping. Opioid- induced sleep disorders (OISDs) are strong predictors of multi-substance use disorders, and psychiatric comorbidities including suicidal ideation. The primary target of opioid analgesic drugs is the µ-opioid receptor (MOR). MORs are widely expressed within the sleep/wake circuitry, therefore systemically delivered opioids might interfere with sleep at multiple sites of action. Importantly, no consensus has been reached on which specific CNS sites therapeutic or abused opioids act upon to trigger OISD, nor do current therapies specifically address OISD. Melanopsin-expressing, intrinsically photosensitive retinal ganglion cells (ipRGCs) synchronize the sleep/wake schedule to environmental cycles of light/darkness (“photoentrainment”) by sending light-evoked spike trains to the brain in humans and most mammals. Our recent work demonstrated that ipRGCs express MORs by which MOR-selective agonists directly inhibit the light responses of ipRGCs. Furthermore, we have shown that upon chronic, systemic delivery, morphine accumulates in the eye, and acts on MORs expressed by ipRGCs to alter the regular rhythm of sleep/wake while contributing to the chronic morphine-triggered behavioral sensitization. The objectives of the current proposal are to analyze whether kinetics of opioid deposition in the eye along with specifics of MOR signaling in ipRGCs directly contribute to the gradually worsening OISD. We will also test if antagonist of MORs delivered in engineered nanoparticles into the eye can reduce OISDs. The results will provide a mechanistic description of a novel neural pathway by which systemically administered opioids alter sleep/wake cycle. Additionally, the results will show the feasibility of using intravitreal MOR selective antagonists to reduce the severity and inherent comorbidities of sleep disorders in patients receiving long-term opioid therapies.