Controlling the Brain’s Emotional Switch: Biased Cholinergic Input within the Basolateral Amygdala

The basolateral amygdala (BLA), consisting of the basolateral (BL) and lateral (LA) nuclei, is an important emotional processing center in the brain, encoding salient stimuli with emotional value or valence. Previous studies indicate valence is assigned through the selective excitation of specific pyramidal neurons (PNs) in the BLA that project to appetitive or aversive behavioral circuits. Two distinct PN populations have been identified within the BL, segregated to the anterior (BLa) or posterior (BLp) subdivisions. Moreover, PNs in the BLa have been shown to be involved in encoding stimuli with negative valence, whereas BLp PNs have been shown to be involved in encoding stimuli with positive valence. The mechanisms underlying such coding, however, remain unclear. Previous studies have shown that the BLA receives dense cholinergic innervation from the basal forebrain. This suggests acetylcholine plays a role in modulating the excitation of BLA PNs and may mediate valence assignment. To better understand the anatomical basis for this modulation, the present study used confocal immunofluorescence and retrograde tract-tracing in C57BL/6J mice to examine the differential distribution of cholinergic innervation and signaling within the BLA. We employed vesicular acetylcholine transporter (VAChT) antibody to define the distribution of cholinergic axons by labeling cholinergic terminals. VAChT immunoreactivity (ir) was significantly higher in the BLa than in the LA or BLp. Linear regression of VAChT-ir across all subdivisions revealed a negative anterior-posterior gradient. To further characterize cholinergic signaling, M1 muscarinic acetylcholine receptors (M1Rs) were immunolabeled. Perikaryal M1R ir was significantly higher in the dorsal BLa than in the BLp or LA. Finally, M1R ir of PNs projecting to the prelimbic (PL) or infralimbic (IL) cortex was evaluated to determine if cholinergic modulation differed according to PN projection target. M1R ir proved to be determined by the location of the cell body within the BLA rather than the projection target. Altogether, these findings indicate cholinergic signaling is biased towards BLa PNs, irrespective of their valence-specific projection targets, to predominately mediate negative valence coding for aversive behavioral responses. Supported by the NIMH (R01MH104638 to DDM and AJM) and NIH (GM076277 to JMA).