Pharmacogenetic inhibition of TrkB signaling in adult mice attenuates mechanical hypersensitivity and improves locomotor function after spinal cord injury

TrkB Agonist (7,8-DHF)-Induced Responses in Dorsal Root Ganglia Neurons Are Decreased after Spinal Cord Injury: Implication for Peripheral Pain Mechanisms

Brain-derived neurotrophic factor (BDNF) and tropomyosin receptor kinase B (TrkB) are known to contribute to both protective and pronociceptive processes. However, their contribution to neuropathic pain after spinal cord injury (SCI) needs further investigation. In a recent study utilizing TrkBF616A mice, it was shown that systemic pharmacogenetic inhibition of TrkB signaling with 1NM-PP1 (1NMP) immediately after SCI delayed the onset of pain hypersensitivity, implicating maladaptive TrkB signaling in pain after SCI. To examine potential neural mechanisms underlying the behavioral outcome, patch-clamp recording was performed in small-diameter dissociated thoracic (T) dorsal root ganglia (DRG) neurons to evaluate TrkB signaling in uninjured mice and after T10 contusion SCI. Bath-applied 7,8-dihydroxyflavone (7,8-DHF), a selective TrkB agonist, induced a robust inward current in neurons from uninjured mice, which was attenuated by 1NMP treatment. SCI also decreased 7,8-DHF-induced current while increasing the latency to its peak amplitude. Western blot revealed a concomitant decrease in TrkB expression in DRGs adjacent to the spinal lesion. Analyses of cellular and membrane properties showed that SCI increased neuronal excitability, evident by an increase in resting membrane potential and the number of spiking neurons. However, SCI did not increase spontaneous firing in DRG neurons. These results suggest that SCI induced changes in TrkB activation in DRG neurons even though these alterations are likely not contributing to pain hypersensitivity by nociceptor hyperexcitability. Overall, this reveals complex interactions involving TrkB signaling and provides an opportunity to investigate other, presumably peripheral, mechanisms by which TrkB contributes to pain hypersensitivity after SCI.

Diet-microbiome interactions promote enteric nervous system resilience following spinal cord injury

Spinal cord injury (SCI) results in numerous systemic dysfunctions, including intestinal dysmotility and enteric nervous system (ENS) atrophy. The ENS has capacity to recover following perturbation, yet intestinal pathologies persist. With emerging evidence demonstrating SCI-induced alterations to gut microbiome composition, we hypothesized that microbiome modulation contributes to post-injury enteric recovery. Here, we show that intervention with the dietary fiber, inulin, prevents SCI-induced ENS atrophy and dysmotility in mice. While SCI-associated microbiomes and specific injury-sensitive gut microbes are not sufficient to modulate intestinal dysmotility after injury, intervention with microbially-derived short-chain fatty acid (SCFA) metabolites prevents ENS dysfunctions in injured mice. Notably, inulin-mediated resilience is dependent on IL-10 signaling, highlighting a critical diet-microbiome-immune axis that promotes ENS resilience post-injury. Overall, we demonstrate that diet and microbially-derived signals distinctly impact ENS survival after traumatic spinal injury and represent a foundation to uncover etiological mechanisms and future therapeutics for SCI-induced neurogenic bowel.

Diet-microbiome interactions promote enteric nervous system resilience following spinal cord injury

Spinal cord injury (SCI) results in a plethora of physiological dysfunctions across all body systems, including intestinal dysmotility and atrophy of the enteric nervous system (ENS). Typically, the ENS has capacity to recover from perturbation, so it is unclear why intestinal pathophysiologies persist after traumatic spinal injury. With emerging evidence demonstrating SCI-induced alterations to the gut microbiome composition, we hypothesized that modulation of the gut microbiome could contribute to enteric nervous system recovery after injury. Here, we show that intervention with the dietary fiber, inulin prevents ENS atrophy and limits SCI-induced intestinal dysmotility in mice. However, SCI-associated microbiomes and exposure to specific SCI-sensitive gut microbes are not sufficient to modulate injury-induced intestinal dysmotility. Intervention with microbially-derived short-chain fatty acid (SCFA) metabolites prevents ENS dysfunctions and phenocopies inulin treatment in injured mice, implicating these microbiome metabolites in protection of the ENS. Notably, inulin-mediated resilience is dependent on signaling by the cytokine IL-10, highlighting a critical diet-microbiome-immune axis that promotes ENS resilience following SCI. Overall, we demonstrate that diet and microbially-derived signals distinctly impact recovery of the ENS after traumatic spinal injury. This protective diet-microbiome-immune axis may represent a foundation to uncover etiological mechanisms and future therapeutics for SCI-induced neurogenic bowel.

Sodium Bicarbonate Decreases Alcohol Consumption in Mice

We previously reported that mice with low neuronal pH drink more alcohol, demonstrating the importance of pH for alcohol reward and motivation. In this study, we tested whether systemic pH affects alcohol consumption and if so, whether it occurs by changing the alcohol reward. C57BL/6J mice were given NaHCO3 to raise their blood pH, and the animals' alcohol consumption was measured in the drinking-in-the-dark and two-bottle free choice paradigms. Alcohol consumption was also assessed after suppressing the bitterness of NaHCO3 with sucrose. Alcohol reward was evaluated using a conditioned place preference. In addition, taste sensitivity was assessed by determining quinine and sucrose preference. The results revealed that a pH increase by NaHCO3 caused mice to decrease their alcohol consumption. The decrease in high alcohol contents (20%) was significant and observed at different ages, as well as in both males and females. Alcohol consumption was also decreased after suppressing NaHCO3 bitterness. Oral gavage of NaHCO3 did not alter quinine and sucrose preference. In the conditioned place preference, NaHCO3-treated mice spent less time in the alcohol-injected chamber. Conclusively, the results show that raising systemic pH with NaHCO3 decreases alcohol consumption, as it decreases the alcohol reward value.

Brain-Derived Neurotrophic Factor, Nociception, and Pain

This article examines the involvement of the brain-derived neurotrophic factor (BDNF) in the control of nociception and pain. BDNF, a neurotrophin known for its essential role in neuronal survival and plasticity, has garnered significant attention for its potential implications as a modulator of synaptic transmission. This comprehensive review aims to provide insights into the multifaceted interactions between BDNF and pain pathways, encompassing both physiological and pathological pain conditions. I delve into the molecular mechanisms underlying BDNF's involvement in pain processing and discuss potential therapeutic applications of BDNF and its mimetics in managing pain. Furthermore, I highlight recent advancements and challenges in translating BDNF-related research into clinical practice.

A review of dorsal root ganglia and primary sensory neuron plasticity mediating inflammatory and chronic neuropathic pain

Pain is a sensory state resulting from complex integration of peripheral nociceptive inputs and central processing. Pain consists of adaptive pain that is acute and beneficial for healing and maladaptive pain that is often persistent and pathological. Pain is indeed heterogeneous, and can be expressed as nociceptive, inflammatory, or neuropathic in nature. Neuropathic pain is an example of maladaptive pain that occurs after spinal cord injury (SCI), which triggers a wide range of neural plasticity. The nociceptive processing that underlies pain hypersensitivity is well-studied in the spinal cord. However, recent investigations show maladaptive plasticity that leads to pain, including neuropathic pain after SCI, also exists at peripheral sites, such as the dorsal root ganglia (DRG), which contains the cell bodies of sensory neurons. This review discusses the important role DRGs play in nociceptive processing that underlies inflammatory and neuropathic pain. Specifically, it highlights nociceptor hyperexcitability as critical to increased pain states. Furthermore, it reviews prior literature on glutamate and glutamate receptors, voltage-gated sodium channels (VGSC), and brain-derived neurotrophic factor (BDNF) signaling in the DRG as important contributors to inflammatory and neuropathic pain. We previously reviewed BDNF's role as a bidirectional neuromodulator of spinal plasticity. Here, we shift focus to the periphery and discuss BDNF-TrkB expression on nociceptors, non-nociceptor sensory neurons, and non-neuronal cells in the periphery as a potential contributor to induction and persistence of pain after SCI. Overall, this review presents a comprehensive evaluation of large bodies of work that individually focus on pain, DRG, BDNF, and SCI, to understand their interaction in nociceptive processing.

C-low threshold mechanoreceptor activation becomes sufficient to trigger affective pain in spinal cord-injured mice in association with increased respiratory rates

The mechanisms of neuropathic pain after spinal cord injury (SCI) are not fully understood. In addition to the plasticity that occurs within the injured spinal cord, peripheral processes, such as hyperactivity of primary nociceptors, are critical to the expression of pain after SCI. In adult rats, truncal stimulation within the tuning range of C-low threshold mechanoreceptors (C-LTMRs) contributes to pain hypersensitivity and elevates respiratory rates (RRs) after SCI. This suggests that C-LTMRs, which normally encode pleasant, affiliative touch, undergo plasticity to transmit pain sensation following injury. Because tyrosine hydroxylase (TH) expression is a specific marker of C-LTMRs, in the periphery, here we used TH-Cre adult mice to investigate more specifically the involvement of C-LTMRs in at-level pain after thoracic contusion SCI. Using a modified light-dark chamber conditioned place aversion (CPA) paradigm, we assessed chamber preferences and transitions between chambers at baseline, and in response to mechanical and optogenetic stimulation of C-LTMRs. In parallel, at baseline and select post-surgical timepoints, mice underwent non-contact RR recordings and von Frey assessment of mechanical hypersensitivity. The results showed that SCI mice avoided the chamber associated with C-LTMR stimulation, an effect that was more pronounced with optical stimulation. They also displayed elevated RRs at rest and during CPA training sessions. Importantly, these changes were restricted to chronic post-surgery timepoints, when hindpaw mechanical hypersensitivity was also evident. Together, these results suggest that C-LTMR afferent plasticity, coexisting with potentially facilitatory changes in breathing, drives at-level affective pain following SCI in adult mice.