The intercalated paracapsular islands as a module for integration of signals regulating anxiety in the amygdala

The intercalated paracapsular (IPC) islands are clusters of dopamine-D1-and μ-opioid 1-receptor rich GABAergic neurons which surround the rostral half of the basolateral complex of the amygdala (BLA) giving rise to several subgroups which can be further subdivided. IPC cells are small-sized and have an axonal and dendritic pattern which differs according to the group they belong. Functionally, IPC neurons are endowed with unique properties that set them apart from other amygdaloid interneurons and allow them to participate in integrative functions. Consistent with this role IPC cells usually remain confined within the amygdala where they receive BLA and cortical inputs and interact synaptically with each other. They project into both the central (CeA) and medial (MeA) amygdaloid nuclei. Their main effect at the network level seems to control the trafficking of nerve impulses to the main input (BLA) and output (CeA) stations of the amygdala. Such a task seems to be accomplished by providing feedforward inhibition to BLA neurons from putative inputs of the medial prefrontal cortex (mPFC) and to CeA from both mPFC and BLA projections. Current experimental evidence will be discussed suggesting that through feedforward inhibitory effects on specific amygdaloid nuclei IPC neurons participate in the maintenance of basal anxiety as well as in the modulation of unconditioned and conditioned fear, and in the process of fear extinction.This article is part of a Special Issue entitled: Brain Integration.

► Structural and functional features of intercalated paracapsular masses are reviewed.
► Intercalated paracapsular islands module fear and anxiety.
► Intercalated paracapsular cells have a prominent role in fear extinction.
► GABAergic cells within intercalated paracapsular islands have unique properties.