A large forebrain circuit, including the thalamus, amygdala and frontal cortical regions,
is responsible for the establishment and extinction of fear-related memories. Understanding
interactions among these three regions is critical to deciphering the basic mechanisms
of fear. With the advancement of molecular and optogenetics techniques, the mouse
has become the main species used to study fear-related behaviours. However, the basic
connectivity pattern of the forebrain circuits involved in processing fear has not
been described in this species. In this study we mapped the connectivity between three
key nodes of the circuit, i.e. the basolateral nucleus of the amygdala (BLA), the
mediodorsal nucleus of the thalamus (MD) and the medial prefrontal cortex, which were
shown to have closed triangular connectivity in rats. In contrast to rat, we found
no evidence for this closed loop in mouse. There was no major input from the BLA to
the MD and little overlap between medial prefrontal regions connected with both the
BLA and MD. The common nodes in the frontal cortex, which displayed reciprocal connection
with both the BLA and MD were the agranular insular cortex and the border zone of
the cingulate and secondary motor cortex. In addition, the BLA can indirectly affect
the MD via the orbital cortex. We attribute the difference between our results and
earlier rat studies to methodological problems rather than to genuine species difference.
Our data demonstrate that the BLA and MD communicate via cortical sectors, the roles
in fear-related behaviour of which have not been extensively studied. In general,
our study provides the morphological framework for studies of murine fear-related
behaviours.
