Amygdala Intercalated Cells: Gate Keepers and Conveyors of Internal State to the Circuits of Emotion

Abstract

Generating adaptive behavioral responses to emotionally salient stimuli requires evaluation of complex associations between multiple sensations, the surrounding context, and current internal state. Neural circuits within the amygdala parse this emotional information, undergo synaptic plasticity to reflect learned associations, and evoke appropriate responses through their projections to the brain regions orchestrating these behaviors. Information flow within the amygdala is regulated by the intercalated cells (ITCs), which are densely packed clusters of GABAergic neurons that encircle the basolateral amygdala (BLA) and provide contextually relevant feedforward inhibition of amygdala nuclei, including the central and BLA. Emerging studies have begun to delineate the unique contribution of each ITC cluster and establish ITCs as key loci of plasticity in emotional learning. In this review, we summarize the known connectivity and function of individual ITC clusters and explore how different neuromodulators conveying internal state act via ITC gates to shape emotionally motivated behavior. We propose that the behavioral state-dependent function of ITCs, their unique genetic profile, and rich expression of neuromodulator receptors make them potential therapeutic targets for disorders, such as anxiety, schizophrenia spectrum, and addiction.

Figure 1.

Rostral-caudal segregation of ITC clusters in the mouse amygdala. Confocal image rendition of ITC clusters in 300 μm brain slices from a Drd1a-Cre-Ai9_tdTomato mouse line labeling D1R neurons in red. Scale bar, 200 μm.

Figure 2.

ITC inputs and outputs. This scheme demonstrates the current understanding of ITC cluster connectivity by consolidating experiments, including optogenetics, 3D neuron reconstruction, and electrical stimulation over the past 20 years. The general theme is that ITCs provide a wide web of sensory context to different amygdala and extra-amygdala structures. Black arrows indicate inputs to different clusters. Each ITC cluster and corresponding output are represented in their unique colors. MD, medial dorsal thalamus; TeA, temporal association cortex; BA_IL, Som, somatosensory cortex; BA neurons projecting to mPFC_IL; BA_PL, BA neurons projecting to mPFC_PL; Stria, striatum/transition zone striatum.

Figure 3.

ITCs shift amygdala circuits to facilitate fear learning and fear extinction. A, Fear conditioning is associated with long-lasting dampening of sensory thalamic and cortical drive to ITC_dm neurons, which suppresses ITC_dm-mediated feedforward inhibition to promote fear-related plasticity in LA. This results in increased synaptic drive from LA to ITC_dm neurons, which facilitate fear learning. During fear memory retrieval, increased activity in ITC_dm drives inhibition of extinction-promoting neurons in the ITC_vm cluster and BA. ITC_dm neurons could also inhibit extinction-promoting (PKCδ) neurons in the CeL. This leads to disinhibition of the CeM and the corresponding activation of fear-promoting brainstem structures. ITC_dm also signals via a disynaptic mechanism through the BA to inhibit infralimbic mPFC inputs. B, During fear extinction, ITC_dm is inhibited by the ITC_vm, which assumes the role of modulating amygdala circuits. Consequently, ITC_vm inhibits the fear-promoting CeM and BA neurons projecting to the mPFC_PL. In concerted fashion, fear extinction enhances PL to IL drive that further activates the IL cortex. IL-mediated top-down control of the amygdala for extinction is speculated to occur through the BMA and ITCs. Ba_extinction, BA neurons that project to the infralimbic mPFC; BA_fear, BA neurons that project to the mPFC_PL; VlPAG, ventrolateral periaqueductal gray of the midbrain. Dashes indicate speculative networks. Gray lines/circles represent decreased function.

Figure 4.

Dopamine and opioid signaling hyperpolarizes ITCs. Dopamine acting via D1R hyperpolarizes ITCs by activating GIRK channels. This hyperpolarization is further regulated by organic cation 3-mediated dopamine clearance as well as GABA corelease. There appears to be differences in dopamine release and responses between clusters with the notable discovery that the ITC_dm neurons (A) receive dopaminergic inputs from VTA and SNc, whereas VTA is the only source of dopamine to ITC_vm neurons (B) during fear extinction. The consequence of unbalanced dopamine transmission to the clusters can promote shifts in ITC activity. This is shown where ITC_dm are disproportionately hyperpolarized during fear extinction promoting ITC_vm activity (D). C, Opioid signaling involves axo-axonic communication that directly modulates both presynaptic and postsynaptic activity of ITCs. Opioid release from dense core vesicles from ITCs provides axo-axonic communication to local glutamatergic neurons and promotes decreased glutamatergic drive onto ITCs. It also functions to induce ITC membrane hyperpolarization via a Gi-coupled K-inward rectifying channel. Opioid signaling was also shown to reduce GABA release from ITCs. Neprilysin is an enzyme that mediates opioid clearance from the synaptic cleft and regulates opioid signaling at the ITC interface. OCT3, Organic cation 3; DOR, δ-opioid receptor; MOR, μ-opioid receptor; RMP, resting membrane potential; GIRK, G-protein-coupled inward rectifying channels; Gi, G-protein inhibitory subunit; KIR, inward rectifying potassium channel.