Background and Goal of Study
Lines of evidence suggest that the allodynia and hypersensitivity, the hallmarks of chronic pain, involve changes in the activity of central nervous system. As the central amygdala (CeA) is a brain nucleus that shows robust plastic changes in various pain models and patients with chronic pain, we hypothesized the pain-induced alterations in the CeA activity would lie upstream to the allodynia and hypersensitivity. For this goal, we used a “latent formalin inflammation” model in which mechanical allodynia (Carrasquillo and Gereau, 2007) and synaptic potentiation in the CeA (Miyazawa et al., 2015) appear several hours after the termination of the conventionally observed initial phases of acute nocifensive behaviors. In this study we developed a model that shows ectopic mechanical allodynia in the hind paw in animals with orofacial formalin-induced inflammation and examined an involvement of the CeA in this “face-to-hind limb” ectopic allodynia.
Materials and Methods
We subcutaneously injected 5% formalin solution into the left upper lip of the adult male Wistar rats. After the acute nociceptive behaviors (1 h), we measured mechanical paw withdrawal threshold (PWT) as an index of the ectopic mechanical allodynia. The effects of the following manipulations on the PWT were evaluated: 1) pharmacological blockade of calcitonin gene-related peptide (CGRP) receptors by local microinjections of the antagonists into the CeA and 2) chemogenetic inhibition of the GABAergic neuronal activity with clozapine-N-oxide (CNO, i.p.) in rats expressing the Designer Receptors Exclusively Activated by Designer Drugs (DREADD) in the CeA.

Results
The PWT was significantly lowered after the orofacial formalin injection, indicating an establishment of the stable ectopic allodynia. Microinjection of a CGRP receptor antagonist in the right CeA, but not that in the left CeA, at ~5 h after formalin injection and a DREADD agonist-induced inhibition of the GABAergic neuronal activity in the CeA at ~5 and ~24 h ameliorated the ectopic allodynia transiently.

Conclusions  
These results indicate that the CeA is not a passive site for pain-induced emotional changes but also a site actively controlling the sensitivity of sensory system to aversive inputs, as expressed as a “brain sensitization” in response to persistent peripheral inflammation, which would strongly complicate the pain phenotypes in chronic pain.

Wide-spread sensitization in inflammatory pain model is regulated by neuronal activities in the amygdala