Distinct neuronal populations in the basolateral and central amygdala are activated with acute pain, conditioned fear, and fear-conditioned analgesia

Fear-conditioned analgesia (FCA) is modulated by brain areas involved in the descending inhibitory pain pathway such as the basolateral (BLA) and central amygdala (CEA). The BLA contains Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) and parvalbumin (PV) neurons. CEA neurons are primarily inhibitory (GABAergic) that comprise enkephalin (ENK) interneurons and corticotropin-releasing factor (CRF) – neurons that project to the periaqueductal grey. The purpose of our experiment was to determine the pattern of activation of CaMKII/PV and ENK/CRF neurons following the expression of acute pain, conditioned fear, and FCA. A significant reduction was observed in nociceptive behaviors in mice re-exposed to a contextually-aversive environment. Using NeuN and cFos as markers for activated neurons, CaMKII, PV, ENK, or CRF were used to identify neuronal subtypes. We find that mice expressing conditioned fear displayed an increase in c-Fos/CaMKII co-localization in the lateral amygdala and BLA compared to controls. Additionally a significant increase in cFos/CRF co-localization was observed in mice expressing FCA. These results show that amygdala processing of conditioned contextual aversive, nociceptive, and FCA behaviors involve different neuronal phenotypes and neural circuits between, within, and from various amygdala nuclei. This information will be important in developing novel therapies for treating pain and emotive disorders in humans.