Keywords
basal amygdala; nucleus accumbens; social aversion and social reward; c-Fos; calcium activity
Introduction
Accurate detection of and adaptive responding to emotionally salient stimuli are vital brain functions; this applies to both rewarding stimuli that elicit appetitive and consummatory behaviors and aversive stimuli that elicit defensive avoidance and escape behaviors. Pathologies of emotional stimulus processing constitute major symptoms in various neuropsychiatric disorders, including major depressive disorder, schizophrenia, and social anxiety disorder. The amygdala is a major region in the neural circuitries of both aversion and reward processing . The lateral amygdala (LA) and basal/basolateral amygdala (BA) are cortex-like, comprising primarily pyramidal glutamatergic neurons . Glutamate neurons in LA receive and integrate inputs from thalamic and cortical sensory neurons encoding innate and conditioned stimuli , and are the major afferents onto BA glutamate neurons . The function(s) of BA glutamate neurons is less well understood: they project to several sub-cortical and cortical regions, including nucleus accumbens (NAc) core and shell, which are also major regions of reward and aversion processing .
In mice, a small number of studies of amygdala processing of stimulus valence (reward and aversion) have included the amygdala-to-NAc neural pathway. In a study of LA/BA-NAc neurons, mice acquired operant behavior for optogenetic self-stimulation of LA/BA-NAc somata, consistent with reward encoding . LA/BA-NAc neuronal density was highest in the anteroposterior (AP) intermediate LA/BA , where the BA comprises both medial anterior/magnocellular and lateral posterior/parvocellular sub-regions . Using retrograde tracing and in vivo single-cell electrophysiological recording to measure responsiveness of LA/BA-NAc neurons to sucrose reward (S) or quinine aversion (Q), 16% were excited by S, 3% by Q and 3% by S and Q (16% were inhibited by S, 13% by Q and 5% by S and Q) . Using putative marker genes for neurons responsive, in terms of immediate-early gene Fos expression, to either reward (female) or aversion (foot shock), 70% of BA-NAc neurons expressed the putative reward-neuron gene Ppp1r1b (protein phosphatase 1, regulatory inhibitor subunit 1b) and were posterior/parvocellular, and 30% expressed the putative aversion-neuron gene Rspo2 (R-spondin 2) and were intermediate/magnocellular. Optogenetic activation of Rspo2 + BA-NAc neurons resulted in contextual aversion conditioning . AnteriorRspo2 + BA-NAc neurons are reported to also express Fezf2 (FEZ family zinc finger 2), and to respond to aversion (air puff, tail shock) and not to reward (sucrose), as assessed using calcium-sensor fibre photometry .
One feature of BA-NAc neuron research to-date, as exemplified by the above studies, has been the focus on physical emotional stimuli. In male mice, estrous female was applied as a social reward stimulus (SR) in a single study , and aggressive-dominant male as social aversion stimulus (SA) remains to be applied. Given the particularly high emotional salience of social stimuli , and the common application of aggressive-dominant male as a chronic stressor , a study directly comparing engagement of BA-NAc neurons by estrous-receptive female and aggressive-dominant male would be informative. The iterative aims of the current study were to: (1) Establish the AP distribution of retrogradely-labelled BA-NAc (core and shell) neuronal somata activated in terms of c-Fos by SR and/or SA. (2) Using targeted recombination in active populations (TRAP) in an AP subregion with a relatively high density of SR and SA BA-NAc neurons, determine whether they are monovalent or dual-valent. (3) Having identified the engagement of BA-NAc neurons by SR (or SA) using ex vivo methods, apply bulk calcium-sensor fibre photometry to provide in vivo evidence for engagement of BA-NAc neurons during either distal or proximal interactions with SR. Aim 1 demonstrated that AP-intermediate BA was relatively rich in BA-NAc neurons responsive to SR and/or SA, and AP-posterior BA was also rich in BA-NAc neurons responsive to SA. Aim 2 demonstrated that substantial and equable proportions of intermediate BA-NAc neurons were monovalent with respect to SR and SA. Aim 3 demonstrated that intermediate BA-NAc neurons were engaged during distal and proximal interactions with SR. These findings further understanding of the anatomical organization and valence specificity of BA-NAc neurons engaged in the processing of social stimuli of high emotional significance; they are relevant to both adaptive socio-emotional processing and the pathologies thereof that are common in stress-related neuropsychiatric disorders.