Neuron vasculature interactions in pain

Neuropathic spontaneous pain, a main symptom of neuropathic pain, is often a particularly intractable clinical problem. While much has been done to understand the sensory sensitization to evoked pain, the mechanisms underlying neuropathic spontaneous pain are less understood and believed to be different from evoked pain

mechanisms. Our collaborator Dr. Xinzhong Dong’s lab in Johns Hopkins University created Pirt-GCaMP6 mice, mice in which a genetically encoded Ca²+ indicator GCaMP6 is specifically expressed in >95% of dorsal root ganglion (DRG) neurons under the control of the Pirt promoter. With these mice, we were able to examine DRG

neuronal activity in vivo, still in its original environment and spatial arrangement. We found that after nerve injury, spatially clustered neurons with sporadic spontaneous activity formed within the DRG (Zheng et al., Neuron, 2022). Mice that displayed increased spontaneous pain behavior after nerve injury were found to highly correlate

with presence of these ectopic ‘cluster’ neurons. We further discovered that cluster firing of DRG neurons and neuropathic spontaneous pain could be triggered by sympathetic activation - effects blocked by both α- and β- adrenergic antagonists. However, the main target of sympathetic activation is not clear since adrenoceptors are

expressed in sensory neurons as well as the blood vessels in the DRGs. Results from our preliminary studies show that cluster firing and spontaneous pain behaviors are enhanced by local or systemic administration of vasoconstrictors (e.g., phenylephrine or angiotensin II), and that evoked and spontaneous cluster firing as well

as spontaneous pain are decreased by blocking the Piezo2 mechanoreceptors in the DRG pharmacologically or genetically. We thus hypothesize that neuropathic spontaneous pain and cluster firing of sensory neurons after peripheral nerve injury are triggered by the mechanical movement (i.e., vasoconstriction and/or vasodilation,

blood vessel displacement) of the blood vessels within the DRGs. The somata or axons of all neurons in the clusters are physically associated with branches of blood vessels supplying the DRG. We further hypothesize that Piezo2 receptors expressed in the majority of hyperexcitable DRG neurons mediate such effects. The

above-mentioned findings that sympathetically evoked cluster firing and spontaneous pain can be blocked by both α- and β-adrenergic antagonists support our hypothesis since blocking α- and β-adrenoceptors would block vasoconstriction and vasodilation, respectively, and both would result in stabilization of blood vessel movement.

In this application, using established mouse and rat neuropathic pain models, we propose to test the hypothesis in 3 Specific Aims (SA). SA1 will characterize the role of local vascular movement in triggering neuropathic spontaneous pain. SA2 will determine how mechanoreceptors in sensory neurons contribute to vascular

movement-evoked spontaneous pain. SA3 is to identify additional key risk factors for vascular movement-evoked spontaneous pain. The proposed experiments will investigate a novel mechanism for neuropathic spontaneous pain and provide a mechanistic basis for developing better therapeutics for this often-intractable clinical problem.