Spinal Mechanisms of Hyperalgesic Priming

The mechanisms that mediate the maintenance of chronic pain states are poorly understood, but elucidation of such could yield insight into how pain becomes chronic and how the process can potentially be reversed. This thesis investigated the role of ascending and descending spinal dorsal horn circuitry and interneurons in the plasticity that mediates a transition to pathological pain plasticity using hyperalgesic priming model. The results showed that, while dorsal horn neurokinin 1 receptor-positive neurons or descending serotonergic neurons mediated IL-6- and carrageenan-induced acute mechanical hypersensitivity, they were not required for PGE₂-induced mechanical hypersensitivity. In stark contrast, ablation of dopaminergic neurons did interrupt the IL-6- and carrageenan-induced mechanical hypersensitivity, but the subsequent PGE₂ injection failed to cause mechanical hypersensitivity - thereby reflecting that primed state plasticity is driven by differential mechanisms. In addition, the pharmacological antagonism of spinal dopamine D1/D5 receptors reversed priming and its agonism induced mechanical hypersensitivity exclusively in primed mice, which suggests dopaminergic control of pathological pain plasticity in a D1/D5-dependent manner. Moreover, in a primed state, changes to spinal dorsal horn GABA pharmacology were accompanied by upregulation of neuroligin 2 mRNA and protein expression. These findings 1) indicate a novel role for descending dopaminergic neurons in the maintenance of pathological pain plasticity, and 2) point to the inhibitory synaptic expression of neuroligin-2 as the spinal determinants of this type of pain plasticity