Transcriptional Profiling of TGF-β Superfamily Members in Lumbar DRGs of Rats Following Sciatic Nerve Axotomy and Activin C Inhibits Neuropathic Pain

Optogenetic Activation of Peripheral Somatosensory Neurons in Transgenic Mice as a Neuropathic Pain Model for Assessing the Therapeutic Efficacy of Analgesics

Optogenetics is a novel biotechnology widely used to precisely manipulate a specific peripheral sensory neuron or neural circuit. However, the use of optogenetics to assess the therapeutic efficacy of analgesics is elusive. In this study, we generated a transgenic mouse stain in which all primary somatosensory neurons can be optogenetically activated to mimic neuronal hyperactivation in the neuropathic pain state for the assessment of analgesic effects of drugs. A transgenic mouse was generated using the advillin-Cre line mated with the Ai32 strain, in which channelrhodopsin-2 fused to enhanced yellow fluorescence protein (ChR2-EYFP) was conditionally expressed in all types of primary somatosensory neurons (advillincre/ChR2+/+). Immunofluorescence and transdermal photostimulation on the hindpaws were used to verify the transgenic mice. Optical stimulation to evoke pain-like paw withdrawal latency was used to assess the analgesic effects of a series of drugs. Injury- and pain-related molecular biomarkers were investigated with immunohistofluorescence. We found that the expression of ChR2-EYFP was observed in many primary afferents of paw skin and sciatic nerves and in primary sensory neurons and laminae I and II of the spinal dorsal horns in advillincre/ChR2+/+ mice. Transdermal blue light stimulation of the transgenic mouse hindpaw evoked nocifensive paw withdrawal behavior. Treatment with gabapentin, some channel blockers, and local anesthetics, but not opioids or COX-1/2 inhibitors, prolonged the paw withdrawal latency in the transgenic mice. The analgesic effect of gabapentin was also verified by the decreased expression of injury- and pain-related molecular biomarkers. These optogenetic mice provide a promising model for assessing the therapeutic efficacy of analgesics in neuropathic pain.

Transcriptional profiles of TGF-β superfamily members in the lumbar DRGs and the effects of activins A and C on inflammatory pain in rats

Signaling by the transforming growth factor (TGF)-β superfamily is necessary for proper neural development and is involved in pain processing under both physiological and pathological conditions. Sensory neurons that reside in the dorsal root ganglia (DRGs) initially begin to perceive noxious signaling from their innervating peripheral target tissues and further convey pain signaling to the central nervous system. However, the transcriptional profile of the TGF-β superfamily members in DRGs during chronic inflammatory pain remains elusive. We developed a custom microarray to screen for transcriptional changes in members of the TGF-β superfamily in lumbar DRGs of rats with chronic inflammatory pain and found that the transcription of the TGF-β superfamily members tends to be downregulated. Among them, signaling of the activin/inhibin and bone morphogenetic protein/growth and differentiation factor (BMP/GDF) families dramatically decreased. In addition, peripherally pre-local administration of activins A and C worsened formalin-induced acute inflammatory pain, whereas activin C, but not activin A, improved formalin-induced persistent inflammatory pain by inhibiting the activation of astrocytes. This is the first report of the TGF-β superfamily transcriptional profiles in lumbar DRGs under chronic inflammatory pain conditions, in which transcriptional changes in cytokines or pathway components were found to contribute to, or be involved in, inflammatory pain processing. Our data will provide more targets for pain research.