Medial prefrontal cortical innervation of the intercalated nuclear region of the amygdala

The projections of the infralimbic area (IL) of the medial prefrontal cortex to the intercalated nuclei (ICNs) of the amygdala are thought to form a critical component of the forebrain circuitry for fear extinction. Despite the importance of these projections, there have been no focussed anatomical studies that have investigated the extent of IL inputs to different portions of the ICN complex. The present investigation used anterograde tract tracing in the rat to study the projections of the ventromedial PFC, including the IL, to the ICNs and surrounding amygdalar regions. Immunohistochemistry for the μ-opioid receptor (MOR) was used to identify the ICNs. At rostral levels of the amygdala there was a very dense projection to a far lateral portion of the capsular subdivision of the central nucleus (CLC) located between the main and medial ICNs, but only very light projections to these ICNs and the lateral ICNs. This distinct portion of the CLC receiving strong IL inputs was termed the capsular infralimbic target zone (CITZ), and was MOR-negative. Likewise, at more caudal levels of the amygdala, IL projections to the medial, lateral, and dorsal ICNs were light to moderate compared with projections to adjacent portions of the basolateral amygdala and amygdalostriatal transitional area. These findings suggest that the putative role of the IL-to-ICN connection in fear inhibition may be mediated by light to moderate projections from the IL to the medial ICN, and that the CITZ may be an equally important amygdalar target for this function.

▶The infralimbic cortex (IL) has projections to the intercalated nuclei (ICN) of the amygdala. ▶The strongest projection to the ICN region is to a zone located between the main and medial ICNs. ▶This zone is termed the “capsular infralimbic target zone” (CITZ). ▶The CITZ is distinguished from the ICNs because of its lack of μ-opioid receptor immunoreactivity. ▶The IL projection to the ICNs is much lighter than to the CITZ and neighboring basolateral amygdala.