Probes for INS

Chronic pain, which affects around 1/3 of the world population and is often comorbid with memory deficit and mood depression, is a leading source of suffering and disability. Studies in past decades have shown that hyperexcitability of primary sensory neurons resulting from abnormal expression of ion channels and central sensitization mediated pathological synaptic plasticity, such as long-term potentiation in spinal dorsal horn, underlie the persistent pain. The memory/emotional deficits are associated with impaired synaptic connectivity in hippocampus. Dysregulation of numerous endogenous proteins including receptors and intracellular signaling molecules is involved in the pathological processes. However, increasing knowledge contributes little to clinical treatment. Emerging evidence has demonstrated that the neuroinflammation, characterized by overproduction of pro-inflammatory cytokines and glial activation, is reliably detected in humans and animals with chronic pain, and is sufficient to induce persistent pain and memory/emotional deficits. The abnormal expression of ion channels and pathological synaptic plasticity in spinal dorsal horn and in hippocampus are resulting from neuroinflammation. The neuroinflammation is initiated and maintained by the interactions of circulating monocytes, glial cells and neurons. Obviously, unlike infectious diseases and cancer, which are caused by pathogens or malignant cells, chronic pain is resulting from alterations of cells and molecules which have numerous physiological functions. Therefore, normalization (counterbalance) but not simple inhibition of the neuroinflammation is the right strategy for treating neuronal disorders. Currently, no such agent is available in clinic. While experimental studies have demonstrated that intracellular Mg2+ deficiency is a common feature of chronic pain in animal models and supplement Mg2+ are capable of normalizing the neuroinflammation, activation of upregulated proteins that promote recovery, such as translocator protein (18k Da) or liver X receptors, has a similar effect. In this article, relevant experimental and clinical evidence is reviewed and discussed.

While depression and chronic pain are frequently comorbid, underlying neuronal circuits and their psychopathological relevance remain poorly defined. Here we show in mice that hyperactivity of the neuronal pathway linking the basolateral amygdala to the anterior cingulate cortex is essential for chronic pain-induced depression. Moreover, activation of this pathway in naive male mice, in the absence of on-going pain, is sufficient to trigger depressive-like behaviors, as well as transcriptomic alterations that recapitulate core molecular features of depression in the human brain. These alterations notably impact gene modules related to myelination and the oligodendrocyte lineage. Among these, we show that Sema4a, which was significantly upregulated in both male mice and humans in the context of altered mood, is necessary for the emergence of emotional dysfunction. Overall, these results place the amygdalo-cingulate pathway at the core of pain and depression comorbidity, and unravel the role of Sema4a and impaired myelination in mood control.

Painful diabetic neuropathy (PDN) is one of the most common and intractable complications of diabetes. PDN is characterized by small-fiber degeneration, which can progress to complete loss of cutaneous innervation and is accompanied by neuropathic pain. Uncovering the mechanisms underlying axonal degeneration in PDN remains a major challenge to finding effective and disease-modifying therapies. Sensory nerve afferents normally extend into the epidermis in close juxtaposition to keratinocytes but degenerate in diabetic skin. Our aim is to identify the changes in gene expression profiles and the interactions between dorsal root ganglion (DRG) neurons and keratinocytes to explore the mechanisms by which keratinocytes communicate with cutaneous afferents and how this communication impacts axonal degeneration underlying neuropathic pain in PDN. We used a mouse model of PDN where mice were fed a regular diet (RD, 11% fat) or a high-fat diet (HFD, 42% fat) for 10 weeks during which these mice develop glucose intolerance, mechanical allodynia, small fiber neuropathy. Using a single-cell RNA (scRNA-seq) sequencing approach we captured DRG and keratinocytes gene expression profiles and generated interactome maps. scRNA-seq identified both neuronal and non-neuronal clusters and several differentially expressed genes between RD and HFD from the DRG. We were able to identify several clusters of immune cells and keratinocytes at different stages of differentiation. scRNA-seq results were validated using RNAscope on DRG and skin frozen sections. Moreover, we generated interactome maps between DRG neurons and the peripheral cells to highlight ligand-receptor interactions and we looked to identify genes that were differentially expressed in these interactions. Taken together our data highlights the importance of studying neurons in conjunction with the cells in the tissues with which they interact to identify ligand-receptor interactions that may lead to the identification of neuron signaling in a chronic pain state such as PDN. Grant support from 1R01AR77691-01.

Chronic pain remains a disabling disease with limited therapeutic options. Pyramidal neurons in the prefrontal cortex (PFC) express excitatory Gq-coupled 5-HT2A receptors (5-HT2AR) and their effector system, the inhibitory Kv7 ion channel. While recent publications show these cells innervate brainstem regions important for regulating pain, the cellular mechanisms underlying the transition to chronic pain are not well understood. The present study examined whether local blockade of 5-HT2AR or enhanced Kv7 ion channel activity in the PFC would attenuate mechanical allodynia associated with spared nerve injury (SNI) in rats. Following SNI, we show that inhibition of PFC 5-HT2ARs with M100907 or opening of PFC Kv7 channels with retigabine reduced mechanical allodynia. Parallel proteomic and RNAScope experiments evaluated 5-HT2AR/Kv7 channel protein and mRNA. Our results support the role of 5-HT2ARs and Kv7 channels in the PFC in the maintenance of chronic pain.