Differential regulation of prelimbic and thalamic transmission to the basolateral amygdala by acetylcholine receptors

The amygdalar anterior basolateral nucleus (BLa) plays a vital role in emotional behaviors. This region receives dense cholinergic projections from basal forebrain which are critical in regulating neuronal activity and synaptic transmission. Cholinergic signaling in BLa is thought to occur through both a slow mode of volume transmission as well as a rapid, phasic mode. However, the relative effect of each mode of signaling in BLa is not understood. Here, we used electrophysiology and optogenetics in mouse brain slices to compare regulation of afferent input from cortex and thalamus to the BLa by these two modes of transmission. Phasic ACh release evoked by single pulse stimulation of cholinergic terminals had a biphasic effect on glutamatergic transmission at cortical input, producing rapid nicotinic receptor-mediated facilitation followed by slower muscarinic receptor (mAChR)-mediated depression. In contrast, tonic elevation of ACh through application of the cholinesterase inhibitor physostigmine suppressed glutamatergic transmission at cortical inputs through mAChRs only. This suppression was not observed at thalamic inputs to BLa. In agreement with this pathway-specificity, the mAChR agonist, muscarine more potently suppressed transmission at inputs from prelimbic cortex (PL) than thalamus. Muscarinic inhibition at PL input was dependent on presynaptic M4 mAChRs, while at thalamic input it depended upon M3 mAChR-mediated stimulation of retrograde endocannabinoid signaling. Muscarinic inhibition at both pathways was frequency-dependent, allowing only high frequency activity to pass. These findings demonstrate complex cholinergic regulation of afferent input to BLa that depends upon the mode of ACh release and is both pathway specific and frequency dependent. Significance statement Cholinergic modulation of the basolateral amygdala regulates formation of emotional memories, but the mechanisms underlying this regulation are not well understood. Here, we show, using mouse brain slices, that ACh differentially regulates afferent transmission to the BLa depending on the mode of cholinergic signaling. Rapid, phasic ACh produces a biphasic excitatory-inhibitory regulation of glutamatergic transmission mediated by nicotinic and muscarinic receptors, respectively. In contrast, slow, tonic ACh produces muscarinic inhibition only. Tonic regulation is pathway specific with cortical input regulated more strongly than thalamic input. This disparity is caused by differential regulation by M4 and M3 receptors at the two inputs. Specific targeting of these receptors may thus provide a therapeutic strategy to bias amygdalar processing and regulate emotional memory.