The role of habenular hyperactivity in the action of tianeptine, a novel treatment for neuropathic pain

Project Summary/Abstract Neuropathic pain (NP) is defined as chronic pain following injury or disease in the somatosensory nervous system resulting in pain and affective symptoms. The primary treatments are monoamine targeting antidepressants, which roughly half of patients do not respond to. An atypical antidepressant called tianeptine (TIA) was discovered by our lab to be an MOR agonist, but differs from other opioids by lacking euphoric quality, metabolizing rapidly out of the body, and not showing tolerance/withdrawal effects all at clinical doses. Preliminary data indicates that TIA reduces allodynia from NP, without rapid tolerance as observed in morphine. How and where TIA is acting to relieve these symptoms is unclear. One candidate region is the habenula (Hb), which is dysregulated in chronic pain, and has inhibitory outputs to several monoamine regulating regions. Typically, neurons in the Hb fire in response to aversive stimuli, inhibiting those downstream reward pathways. Under chronic pain, the region becomes hyperactive. One option for how this happens relates to the habenula having the densest MOR expressing in the brain. MORs are typically inhibitory, and activated by the release of endogenous opioids. However, during chronic pain this system becomes dysregulated and, in many regions, MORs decrease in expression or availability. I hypothesize that pathological hyperactivity in the Hb, which is associated with an increase in allodynia and anhedonia, is at least partially due to a decrease in MOR expression or availability under conditions of chronic pain, and that chronic TIA administration will normalize activity in the region, correlating with decreased NP behaviors. In order to address this, I will use the spared nerve injury model (SNI) of NP, which induces both painful and affective symptoms, including allodynia and anhedonia. In Aim 1, I will test the hypothesis that habenula hyperactivity is related to changes in MOR activity, and that tianeptine normalizes habenular activity in NP. In 1.A, I will perform fiber photometry in SNI and sham control male and female MOR-Cre mice. I will measure calcium activity in both global and MOR expressing cells in the Hb, during measures of allodynia and anhedonia. In 1.B, I will chronically treat animals with TIA, and observe if this results in reversal of neural and/or behavioral disparities between SNI and control mice. In Aim 2, I will examine the molecular and transcriptional changes caused by NP and following chronic TIA treatment by comparing habenular tissue from sham/vehicle, SNI/vehicle, sham/TIA, and SNI/TIA mice. In 2.A, I will use a protein ligation assay to examine changes in MOR expression with regional detail, and in 2.B, I will analyze scRNAseq data for both broad transcriptional changes induced by pain and tianeptine, how co-expression patterns change between conditions. This will allow me to observe if the ratio of MOR co-expression shifts, as it did in preliminary re-analysis of published data looking at the habenula following acute unavoidable shock. To date, no studies have looked at habenular MORs during neuropathic pain.