Role of astrocytes and astrocytic CD38 in spinal opioid signaling

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Project Summary Opioids are commonly prescribed for the treatment of pain, and the classic mechanism of opioid signaling involves neuronal mechanisms. Chronic opioid administration results in negative effects like paradoxical hyperalgesia and opioid tolerance and can lead to dependence and addiction. These negative effects are not explained by neuronal mechanisms alone. Understanding the role of non-neuronal cells in opioid signaling during normal and pathological conditions is important for the development of novel pain interventions. Astrocytes, classically considered supportive cells, are now known to have important roles in modulating synaptic transmission and extrasynaptic glutamate a key neurotransmitter in pain and opioid analgesia. In the nucleus accumbens, an important brain region for addiction pathways, astrocytes have been shown to express µ-opioid receptors (MOR) and to respond to opioids with calcium elevations and glutamate release. Despite their critical role in modulating synaptic transmission and extrasynaptic glutamate, no studies have determined whether astrocytes play a role in µ-opioid signaling in the spinal cord, an important region for pain processing and opioid antinociception. Our preliminary behavioral experiments show that the protein CD38, an important protein responsible for calcium homeostasis, is expressed exclusively in astrocytes in the spinal cord, and animals deficient in CD38 had markedly reduced antinociceptive efficacy to morphine administered intrathecally, pointing to a role of astrocytic CD38 in spinal opioid mechanisms. Therefore, our current objectives are to define at the synaptic level: i) the role of spinal astrocytes in µ-opioid signaling and ii) the role of spinal astrocytic protein CD38, in µ-opioid signaling and antinociception. We hypothesize that astrocytes are involved in spinal opioid signaling via mechanisms that engage the astrocytic calcium protein CD38 and its metabolite cADPR. To test this hypothesis, we will perform ex vivo calcium imaging of spinal cord astrocytes using newly developed genetically encoded calcium indicators and electrophysiological readouts to measure the release of glutamate by astrocytes following opioid stimulation. To test the role of astrocytic CD38 in antinociception at the synaptic level, we will measure the ability of the opioid to inhibit optogenetic neuronal synaptic stimulation of Nav 1.8 nociceptors. Comparisons will be made between our wild type and CD38 deficient mice. Our results will characterize the involvement of astrocytes and astrocytic CD38 in µ-opioid signaling at the spinal level and would define the promising underexplored potential of astrocytes as novel targets for the development of pain interventions with reduced addiction liability.