Z. David Luo

The spinal dorsal horn is the first relay structure coding for pain transmission and modulation. Previous anatomical and electrophysiological studies have examined spinal dorsal horn circuit connections and network activity. Further work is required to understand spinal cord sensory information processing that underlies pathological neuropathic pain states. Our previous studies suggest that peripheral nerve injury enhances presynaptic excitatory input onto spinal superficial dorsal horn neurons, which in turn contributes to pathologic nociception. The potential changes in local postsynaptic circuits in the dorsal horn that lead to pathologically heightened behavioral responses to pain remain largely unexplored. We combined whole-cell electrophysiological recordings with laser-scanning photostimulation to test whether peripheral nerve injury in the spinal nerve ligation (SNL) mouse model of neuropathic pain leads to alterations in the functional connectivity of spinal cord circuits including lamina II excitatory interneurons. Here we show that SNL enhances excitation and decreases inhibition to lamina II excitatory interneurons along with their increased glutamate-evoked excitability. The enhanced excitatory postsynaptic input and connectivity evoked by SNL eventually return to normal levels concurrently with the resolution of the neuropathic pain states. The physiological pattern highly correlates with mouse pain behaviors following SNL, supporting a neurophysiological mechanism of central sensitization and neuropathic pain that is functionally localized to the spinal dorsal horn. Together, these data support that SNL induces functional changes in synaptic input and connectivity to lamina II excitatory interneurons that code for pain perception, and thus provide new insights into the mechanism and locus of pain hypersensitivity.

Background and Purpose: Nerve injury induces concurrent up‐regulation of the voltage‐gated calcium channel subunit Ca_vα₂δ₁and the extracellular matrix protein thrombospondin‐4 (TSP4) in dorsal root ganglia and dorsal spinal cord, leading to the development of a neuropathic pain state. Interactions of these proteins promote aberrant excitatory synaptogenesis that contributes to neuropathic pain state development through unknown mechanisms. We investigated the contributions of Ca_vα₂δ₁subunits and TSP4 to synaptogenesis, and the pathways involvedin vitro, and whether treatment with gabapentin could block this process and pain developmentin vivo.Experimental Approach: A co‐culture system of sensory and spinal cord neurons was used to study the contribution from each protein to synaptogenesis and the pathway(s) involved. Anti‐synaptogenic actions of gabapentin were studied in TSP4‐injected mice.Key Results: Only presynaptic, but not postsynaptic, Ca_vα₂δ₁subunits interacted with TSP4 to initiate excitatory synaptogenesis through a pathway modulated by T‐type calcium channels. Ca_vα₂δ₁/TSP4 interactions were not required for maintenance of already formed synapses.In vivo, early, but not delayed, treatment with low‐dose gabapentin blocked this pathway and the development of the pain state.Conclusions and Implications: Ca_vα₂δ₁/TSP4 interactions were critical for the initiation, but not for the maintenance, of abnormal synapse formation between sensory and spinal cord neurons. This process was blocked by early, but was not reversed by delayed, treatment with gabapentin. Early intervention with gabapentin may prevent the development of injury‐induced chronic pain, resulting from Ca_vα₂δ₁/TSP4‐initiated abnormal synapse formation.Linked Articles: This article is part of a themed section on Recent Advances in Targeting Ion Channels to Treat Chronic Pain. To view the other articles in this section visithttp://onlinelibrary.wiley.com/doi/10.1111/bph.v175.12/issuetoc

Voltage‐gated calcium channels (VGCCs) play important roles in physiological functions including the modulation of neurotransmitter release, neuronal network activities, intracellular signalling pathways and gene expression. Some pathological conditions, including nerve injuries, can cause the dysregulation of VGCCs and their subunits. This in turn can lead to a functional maladaptation of VGCCs and their subunits, which can contribute to the development of disorders such as pain sensations. This review has summarized recent findings related to maladaptive changes in the dysregulated VGCC α₂δ₁subunit (Ca_vα₂δ₁) with a focus on exploring the mechanisms underlying the contribution of Ca_vα₂δ₁to pain signal transduction. At least under neuropathic pain conditions, the dysregulated Ca_vα₂δ₁can modulate VGCC functions as well as other plasticity changes. The latter includes abnormal excitatory synaptogenesis resulting from its interactions with injury‐induced extracellular matrix glycoprotein molecule thrombospondins, which is independent of the VGCC functions. Blocking Ca_vα₂δ₁with gabapentinoids can reverse neuropathic pain significantly with relatively mild side effects, but only in a small population of neuropathic pain patients due to reasons yet to be explored. There are emerging data suggesting that early preventive treatment with gabapentinoids can prevent aberrant excitatory synapse formation and the development of chronic pain. If these findings are confirmed clinically, this could be an attractive approach for neuropathic pain management.Linked Articles: This article is part of a themed section on Recent Advances in Targeting Ion Channels to Treat Chronic Pain. To view the other articles in this section visithttp://onlinelibrary.wiley.com/doi/10.1111/bph.v175.12/issuetoc

Trigeminal nerves collecting sensory information from the orofacial area synapse on second‐order neurons in the dorsal horn of subnucleus caudalis and cervical C1/C2 spinal cord (Vc/C2, or trigeminocervical complex), which is critical for sensory information processing. Injury to the trigeminal nerves may cause maladaptive changes in synaptic connectivity that plays an important role in chronic pain development. Here we examined whether injury to the infraorbital nerve, a branch of the trigeminal nerves, led to synaptic ultrastructural changes when the injured animals have developed neuropathic pain states. Transmission electron microscopy was used to examine synaptic profiles in Vc/C2 at 3 weeks postinjury, corresponding to the time of peak behavioral hypersensitivity following chronic constriction injury to the infraorbital nerve (CCI‐ION). Using established criteria, synaptic profiles were classified as associated with excitatory (R‐), inhibitory (F‐), and primary afferent (C‐) terminals. Each type was counted within the superficial dorsal horn of the Vc/C2 and the means from each rat were compared between sham and injured animals; synaptic contact length was also measured. The overall analysis indicates that rats with orofacial pain states had increased numbers and decreased mean synaptic length of R‐profiles within the Vc/C2 superficial dorsal horn (lamina I) 3 weeks post‐CCI‐ION. Increases in the number of excitatory synapses in the superficial dorsal horn of Vc/C2 could lead to enhanced activation of nociceptive pathways, contributing to the development of orofacial pain states. J. Comp. Neurol. 524:309–322, 2016.

Background: Up‐regulation of voltage‐gated calcium channel α₂δ₁ subunit post spinal nerve ligation (SNL) injury or in α₂δ₁‐overexpressing transgenic (Tg) mice correlates with tactile allodynia, a pain state mediated mainly by Aβ sensory fibres forming synaptic connections with deep dorsal horn (DDH) neurons. It is not clear, however, whether dysregulated α₂δ₁ alters DDH synaptic neurotransmission that underlies tactile allodynia development post nerve injury.Methods: Tactile allodynia was tested in the SNL and α₂δ₁ Tg models. Miniature excitatory/inhibitory post‐synaptic currents were recorded in DDH neurons from these animal models using whole‐cell patch clamp slice recording techniques.Results: There was a significant increase in the frequency, but not amplitude, of miniature excitatory post‐synaptic currents (mEPSC) in DDH neurons that correlated with tactile allodynia in SNL and α₂δ₁ Tg mice. Gabapentin, an α₂δ₁ ligand that is known to block tactile allodynia in these models, also normalized mEPSC frequency dose‐dependently in DDH neurons from SNL and α₂δ₁ Tg mice. In contrast, neither frequency nor amplitude of miniature inhibitory post‐synaptic currents was altered in DDH neurons from SNL and α₂δ₁ Tg mice.Conclusion: Our data suggest that α₂δ₁ dysregulation is highly likely contributing to tactile allodynia through a pre‐synaptic mechanism involving facilitation of excitatory synaptic neurotransmission in DDH of spinal cord.