Abstract
Enk neurons in CeA modulate the activity of the amygdala projection neurons and it is very likely that changes of Enk signaling cause the heightened anxiety that accompanies chronic pain. We use chemogenetics and transgenic mice to investigate the effects of acute and continuous activation of the amygdala Enk neurons on persistent pain and anxiodepressive-like behavior in mice. Enk-cre mice were injected bilaterally into the CeA with cre-activated AAV-DREADD/Gq/mCherry, while neuropathic pain was induced by sciatic nerve constriction. A single injection of DREADD's ligand CNO decreased the anxiety-like behavior in both, uninjured mice and in mice with neuropathic pain and produced robust analgesia that lasted for 24 h. Furthermore, the activation of Enk neurons by the DREADD ligand led to increased c-Fos expression in PKC-δ interneurons of the CeA and in non-serotonergic neurons in the ventrolateral periaqueductal gray (vlPAG), a brain structure that is an essential part of the descending pain inhibitory system. Next, we added CNO to the drinking water of the experimental mice for 14 days in order to assess the effects of continuous activation of CeA Enk interneurons on anxiodepressive-like behavior, which is affected by chronic pain. The prolonged activation of the CeA Enk interneurons reduced neohypophagia in the novelty suppressed feeding test and increased ΔFosB (a marker for sustained neuronal activation) in the vlPAG of mice with chronic pain. All together, the results of our experiments point to an important role of the CeA Enk neurons in the control of both nociception and emotion. Activation of Enk neurons resulted in sustained analgesia accompanied by anxiolysis and antidepressant effects. Very likely, these effects of CeA Enk neurons are result of the activation of vlPAG, a brain region that is essential not only for descending inhibition of pain but it is also a core element in the resilience to stress.