New insights into the distribution of BA-NAc neurons activated by emotionally salient stimuli
The NAc core/shell bregma level of AP +1.0 to +1.2 was used in order to facilitate comparison with previous studies that investigated BA projections to NAc at +1.0 to +1.5 . Retrograde tracer uptake by BA neuron somata was maximal at bregma -2.0 to -2.6 in terms of absolute number and density; this is similar to previous studies, which report a maximal number of BA-NAc neurons at -2.4 to -2.8 using percentage CTB+ neurons/large DAPI neurons and at -2.0 using density . Moving the NAc injection site to +1.6 resulted in a corresponding, topographic shift in the AP distribution of BA-NAc neuron somata. The overall absolute number/density of BA-NAc neurons activated by social stimuli in terms of c-Fos was highest at BA -1.6 to -2.0 and comprised a similar amount of neurons responsive to SR and SA; at BA -2.6 there was a high absolute number/density of BA-NAc neurons activated by SA specifically. Relative to total BA-NAc neurons, in int-BA some 15-17% were activated by SR or SA: this could reflect that 15-17% of int-BA-NAc neurons are dual-valent “social-stimulus” neurons, or that 30-35% of int-BA-neurons are monovalent “SR” or “SA neurons”, or some combination of these two extremes. Using single BA-NAc neuron electrophysiological recording to identify neurons responsive to reward (sucrose) or aversion (quinine), 16% were excited by reward, 3% by aversion and 3% by reward and aversion, and these neurons were distributed rather evenly along the AP axis of BA . We also assessed BA-NAc neuron activation by the physical aversive stimulus of TMT odor and the number of activated neurons was substantially lower compared with social stimuli. Whilst the medial-lateral distribution of BA-NAc neurons responsive to SR and SA was not quantified, anecdotally, in the int-BA they were quite evenly distributed on the ML axis in both cases, consistent with their presence in both the medial anterior/magnocellular and lateral posterior/parvocellular sub-regions . The current findings also indicate the high number of BA cells activated by SR and/or SA that do not project to intermediate NAc core/shell; these are likely to include glutamate neurons projecting to other AP NAc sub-regions, as well as central amygdala, hippocampus and prefrontal cortex, among others .