Stimulation of lateral parabrachial (LPB) to central amygdala (CeA) pituitary adenylate cyclase-activating polypeptide (PACAP) neurons induces anxiety-like behavior and mechanical allodynia

Abstract

Background: Anxiety disorders are the most prevalent psychiatric disorders, and they are highly comorbid with chronic pain conditions. The central nucleus of the amygdala (CeA) is known not only for its role in the regulation of anxiety but also as an important site for the negative affective dimension of pain. Pituitary adenylate cyclase activating polypeptide (PACAP), a neuropeptide whose terminals are abundant in the CeA, is strongly implicated in the stress response as well as in pain processing. Here, using Cre-dependent viral vectors, we explored in greater detail the role of the PACAP projection to the CeA that originates in the lateral parabrachial nucleus (LPB).

Methods: We first performed a circuit mapping experiment by injecting an anterograde Cre-dependent virus expressing a fluorescent reporter in the LPB of PACAP-Cre mice and observing their projections. Then, we used a chemogenetic approach (a Cre-dependent Designer Receptors Activated by Designer Drugs, DREADDs) to assess the effects of the direct stimulation of the PACAP LPB to CeA projection on general locomotor activity, anxiety-like behavior (using a defensive withdrawal test), and mechanical pain sensitivity (using the von Frey test).

Results: We found that the CeA, together with other areas, is one of the major downstream projection targets of PACAP neurons originating in the lateral parabrachial nucleus (LPB). In the DREADD experiment, we then found that the selective activation of this neuronal pathway is sufficient to increase both anxiety-like behavior and mechanical pain sensitivity in mice, without affecting general locomotor activity.

Conclusion: In conclusion, our data suggest that the dysregulation of this circuit may contribute to a variety of anxiety disorders and chronic pain states, and that PACAP may represent an important therapeutic target for the treatment of these conditions.

Keywords: Anxiety; Anxiogenic; Brainstem; CGRP; Extended amygdala; Fluorescence; Hyperalgesia; Neuropeptide; PAC1R; Pain; Stress; von Frey.