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 .