Multiple Nicotinic Acetylcholine Receptor Subtypes in the Mouse Amygdala Regulate Affective Behaviors and Response to Social Stress

Abstract

Electrophysiological and neurochemical studies implicate cholinergic signaling in the basolateral amygdala (BLA) in behaviors related to stress. Both animal studies and human clinical trials suggest that drugs that alter nicotinic acetylcholine receptor (nAChR) activity can affect behaviors related to mood and anxiety. Clinical studies also suggest that abnormalities in cholinergic signaling are associated with major depressive disorder, whereas pre-clinical studies have implicated both β2 subunit-containing (β2*) and α7 nAChRs in the effects of nicotine in models of anxiety- and depression-like behaviors. We therefore investigated whether nAChR signaling in the amygdala contributes to stress-mediated behaviors in mice. Local infusion of the non-competitive non-selective nAChR antagonist mecamylamine or viral-mediated downregulation of the β2 or α7 nAChR subunit in the amygdala all induced robust anxiolytic- and antidepressant-like effects in several mouse behavioral models. Further, whereas α7 nAChR subunit knockdown was somewhat more effective at decreasing anxiety-like behavior, only β2 subunit knockdown decreased resilience to social defeat stress and c-fos immunoreactivity in the BLA. In contrast, α7, but not β2, subunit knockdown effectively reversed the effect of increased ACh signaling in a mouse model of depression. These results suggest that signaling through β2* nAChRs is essential for baseline excitability of the BLA, and a decrease in signaling through β2 nAChRs alters anxiety- and depression-like behaviors even in unstressed animals. In contrast, stimulation of α7 nAChRs by acetylcholine may mediate the increased depression-like behaviors observed during the hypercholinergic state observed in depressed individuals.

Figure 1

Mecamylamine infusion into the amygdala has an antidepressant-like effect. (a) Placement of all on-target infusions into the amygdala. The yellow triangle represents an example of an off-target infusion. (b) Time spent immobile in the tail suspension test following acute infusion of mecamylamine. Data are expressed as means ±SEM. **p<0.01.

Figure 2

Effects of β2 and α7 nAChR subunit knockdown in the amygdala in tests of anxiety- and depression-like phenotypes. Time spent in the dark side of the light/dark box following (a) β2 or (b) α7 nAChR subunit knockdown in the amygdala. Time spent immobile in the tail suspension test following (c) β2 or (d) α7 nAChR subunit knockdown in the amygdala. Time spent immobile in the forced swim test following (e) β2 or (f) α7 nAChR subunit knockdown in the amygdala. Time spent interacting with a CD1 mouse after 4 days of social defeat following (g) β2 or (h) α7 nAChR subunit knockdown in the amygdala. Data are expressed as % means ±SEM normalized to each control group. *p<0.05; ***p<0.001.

Figure 3

Immunohistochemistry for GFP and c-fos in the amygdala after β2 or α7 knockdown. (a) Representative photomicrograph of GFP expression in the BLA following viral infusion. (b) Representative photomicrograph of c-fos immunohistochemistry in the amygdala following viral infusion. EC, external capsule. (c, d) Total number of c-fos immunoreactive cells in the amygdala following (c) β2 or (d) α7 nAChR subunit knockdown. Data are expressed as means ±SEM. *p<0.06; ***p<0.001.

Figure 4

α7 and β2* nAChRs in the amygdala mediate the antidepressant-like effects of a nicotinic antagonist. Effect of acute mecamylamine injection on (a) the time spent immobile in the tail suspension test and (b) time spent interacting with a CD1 mouse in the social defeat test following α7 or β2 nAChR subunit knockdown in the amygdala. Data are expressed as means ±SEM. *p<0.05; **p<0.01; ***p<0.001.

Figure 5

Knockdown of α7 or β2* nAChRs in the amygdala can oppose the effects of systemic physostigmine injection. (a) Time spent immobile in the tail suspension test. (b) Time spent interacting with a CD1 mouse in the forced swim test. Data are expressed as means ±SEM. *p<0.05; ***p<0.001; #p<0.007.