Accepted

Presynaptic inhibition (PSI) of primary sensory neurons is implicated in controlling gain and acuity in sensory systems. Here, we define circuit mechanisms and functions of PSI of cutaneous somatosensory neuron inputs to the spinal cord. We observed that PSI can be evoked by different sensory neuron populations and mediated through at least two distinct dorsal horn circuit mechanisms. Low-threshold cutaneous afferents evoke a GABA-receptor-dependent form of PSI that inhibits similar afferent subtypes, whereas small-diameter afferents predominantly evoke an NMDA-receptor-dependent form of PSI that inhibits large-diameter fibers. Behaviorally, loss of either GABA receptors (GABARs) or NMDA receptors (NMDARs) in primary afferents leads to tactile hypersensitivity across skin types, and loss of GABARs, but not NMDARs, leads to impaired texture discrimination. Post-weaning age loss of either GABARs or NMDARs in somatosensory neurons causes systemic behavioral abnormalities, revealing critical roles of two distinct modes of PSI of somatosensory afferents in adolescence and throughout adulthood.

Neurons exhibit a limited ability of repair. Given that mechanical forces affect neuronal outgrowth, it is important to investigate whether mechanosensitive ion channels may regulate axon regeneration. Here, we show that DmPiezo, a Ca-permeable non-selective cation channel, functions as an intrinsic inhibitor for axon regeneration in Drosophila. DmPiezo activation during axon regeneration induces local Ca transients at the growth cone, leading to activation of nitric oxide synthase and the downstream cGMP kinase Foraging or PKG to restrict axon regrowth. Loss of DmPiezo enhances axon regeneration of sensory neurons in the peripheral and CNS. Conditional knockout of its mammalian homolog Piezo1 in vivo accelerates regeneration, while its pharmacological activation in vitro modestly reduces regeneration, suggesting the role of Piezo in inhibiting regeneration may be evolutionarily conserved. These findings provide a precedent for the involvement of mechanosensitive channels in axon regeneration and add a potential target for modulating nervous system repair.

Neonatal hindpaw incision primes developing spinal nociceptive circuitry, resulting in enhanced hyperalgesia following re-injury in adulthood. Spinal microglia contribute to this persistent effect and microglial inhibition at the time of adult re-incision blocks the enhanced hyperalgesia. Here, we pharmacologically inhibited microglial function with systemic minocycline or intrathecal SB203580 at the time of neonatal incision and evaluated sex-dependent differences following adult re-incision. Incision in adult male and female rats induced equivalent hyperalgesia and spinal dorsal horn expression of genes associated with microglial proliferation () and transformation to a reactive phenotype (). In control adults with prior neonatal incision, the enhanced degree and duration of incision-induced hyperalgesia and spinal microglial responses to re-incision were equivalent in males and females. However, microglial inhibition at the time of the neonatal incision revealed sex-dependent effects: the persistent mechanical and thermal hyperalgesia following re-incision in adulthood was prevented in males but unaffected in females. Similarly, re-incision induced and gene expression was downregulated in males, but not in females following neonatal incision with minocycline. To evaluate the distribution of re-incision hyperalgesia, prior neonatal incision was performed at different body sites. Hyperalgesia was maximal when the same paw was re-incised, and was increased following prior incision at ipsilateral, but not contralateral sites; supporting a segmentally restricted spinal mechanism. These data highlight the contribution of spinal microglial mechanisms to persistent effects of early-life injury in males, and sex-dependent differences in the ability of microglial inhibition to prevent the transition to a persistent pain state spans developmental stages. Following the same surgery, some patients develop persistent pain. Contributory mechanisms are not fully understood, but early-life experience and sex/gender may influence the transition to chronic pain. Surgery and painful procedural interventions in vulnerable preterm neonates are associated with long-term alterations in somatosensory function and pain that differ in males and females. Surgical injury in neonatal rodents primes the developing nociceptive system and enhances re-injury response in adulthood. Neuroimmune interactions are critical mediators of persistent pain, but sex-dependent differences in spinal neuroglial signaling influence the efficacy of microglial inhibitors following adult injury. Neonatal microglial inhibition has beneficial long-term effects on re-injury response in adult males only, emphasizing the importance of evaluating sex-dependent differences at all ages in pre-clinical studies.