Midbrain Dopamine Controls Anxiety-like Behavior by Engaging Unique Interpeduncular Nucleus Microcircuitry.

BACKGROUND: Dopamine (DA) is hypothesized to modulate anxiety-like behavior, although the precise role of DA in anxiety behaviors and the complete anxiety network in the brain have yet to be elucidated. Recent data indicate that dopaminergic projections from the ventral tegmental area (VTA) innervate the interpeduncular nucleus (IPN), but how the IPN responds to DA and what role this circuit plays in anxiety-like behavior are unknown. METHODS: We expressed a genetically encoded G protein-coupled receptor activation-based DA sensor in mouse midbrain to detect DA in IPN slices using fluorescence imaging combined with pharmacology. Next, we selectively inhibited or activated VTA→IPN DAergic inputs via optogenetics during anxiety-like behavior. We used a biophysical approach to characterize DA effects on neural IPN circuits. Site-directed pharmacology was used to test if DA receptors in the IPN can regulate anxiety-like behavior. RESULTS: DA was detected in mouse IPN slices. Silencing/activating VTA→IPN DAergic inputs oppositely modulated anxiety-like behavior. Two neuronal populations in the ventral IPN (vIPN) responded to DA via D1 receptors (D1Rs). vIPN neurons were controlled by a small population of D1R neurons in the caudal IPN that directly respond to VTA DAergic terminal stimulation and innervate the vIPN. IPN infusion of a D1R agonist and antagonist bidirectionally controlled anxiety-like behavior. CONCLUSIONS: VTA DA engages D1R-expressing neurons in the caudal IPN that innervate vIPN, thereby amplifying the VTA DA signal to modulate anxiety-like behavior. These data identify a DAergic circuit that mediates anxiety-like behavior through unique IPN microcircuitry.

A circuit-based mechanism underlying familiarity signaling and the preference for novelty.

Novelty preference (NP) is an evolutionarily conserved, essential survival mechanism often dysregulated in neuropsychiatric disorders. NP is mediated by a motivational dopamine signal that increases in response to novel stimuli, thereby driving exploration. However, the mechanism by which once-novel stimuli transition to familiar stimuli is unknown. Here we describe a neuroanatomical substrate for familiarity signaling, the interpeduncular nucleus (IPN) of the midbrain, which is activated as novel stimuli become familiar with multiple exposures. In mice, optogenetic silencing of IPN neurons increases salience of and interaction with familiar stimuli without affecting novelty responses, whereas photoactivation of the same neurons reduces exploration of novel stimuli mimicking familiarity. Bidirectional control of NP by the IPN depends on familiarity signals and novelty signals arising from excitatory habenula and dopaminergic ventral tegmentum inputs, which activate and reduce IPN activity, respectively. These results demonstrate that familiarity signals through unique IPN circuitry that opposes novelty seeking to control NP.

Anxiety and Nicotine Dependence: Emerging Role of the Habenulo-Interpeduncular Axis.

While innovative modern neuroscience approaches have aided in discerning brain circuitry underlying negative emotional behaviors including fear and anxiety responses, how these circuits are recruited in normal and pathological conditions remains poorly understood. Recently, genetic tools that selectively manipulate single neuronal populations have uncovered an understudied circuit, the medial habenula (mHb)-interpeduncular (IPN) axis, that modulates basal negative emotional responses. Interestingly, the mHb-IPN pathway also represents an essential circuit that signals heightened anxiety induced by nicotine withdrawal. Insights into how this circuit interconnects with regions more classically associated with anxiety, and how chronic nicotine exposure induces neuroadaptations resulting in an anxiogenic state, may thereby provide novel strategies and molecular targets for therapies that facilitate smoking cessation, as well as for anxiety relief.

Increased CRF signalling in a ventral tegmental area-interpeduncular nucleus-medial habenula circuit induces anxiety during nicotine withdrawal.

Increased anxiety is a prominent withdrawal symptom in abstinent smokers, yet the neuroanatomical and molecular bases underlying it are unclear. Here we show that withdrawal-induced anxiety increases activity of neurons in the interpeduncular intermediate (IPI), a subregion of the interpeduncular nucleus (IPN). IPI activation during nicotine withdrawal was mediated by increased corticotropin releasing factor (CRF) receptor-1 expression and signalling, which modulated glutamatergic input from the medial habenula (MHb). Pharmacological blockade of IPN CRF1 receptors or optogenetic silencing of MHb input reduced IPI activation and alleviated withdrawal-induced anxiety; whereas IPN CRF infusion in mice increased anxiety. We identified a mesointerpeduncular circuit, consisting of ventral tegmental area (VTA) dopaminergic neurons projecting to the IPN, as a potential source of CRF. Knockdown of CRF synthesis in the VTA prevented IPI activation and anxiety during nicotine withdrawal. These data indicate that increased CRF receptor signalling within a VTA-IPN-MHb circuit triggers anxiety during nicotine withdrawal.

Modeling key pathological features of frontotemporal dementia with C9ORF72 repeat expansion in iPSC-derived human neurons.

The recently identified GGGGCC repeat expansion in the noncoding region of C9ORF72 is the most common pathogenic mutation in patients with frontotemporal dementia (FTD) or amyotrophic lateral sclerosis (ALS). We generated a human neuronal model and investigated the pathological phenotypes of human neurons containing GGGGCC repeat expansions. Skin biopsies were obtained from two subjects who had >1,000 GGGGCC repeats in C9ORF72 and their respective fibroblasts were used to generate multiple induced pluripotent stem cell (iPSC) lines. After extensive characterization, two iPSC lines from each subject were selected, differentiated into postmitotic neurons, and compared with control neurons to identify disease-relevant phenotypes. Expanded GGGGCC repeats exhibit instability during reprogramming and neuronal differentiation of iPSCs. RNA foci containing GGGGCC repeats were present in some iPSCs, iPSC-derived human neurons and primary fibroblasts. The percentage of cells with foci and the number of foci per cell appeared to be determined not simply by repeat length but also by other factors. These RNA foci do not seem to sequester several major RNA-binding proteins. Moreover, repeat-associated non-ATG (RAN) translation products were detected in human neurons with GGGGCC repeat expansions and these neurons showed significantly elevated p62 levels and increased sensitivity to cellular stress induced by autophagy inhibitors. Our findings demonstrate that key neuropathological features of FTD/ALS with GGGGCC repeat expansions can be recapitulated in iPSC-derived human neurons and also suggest that compromised autophagy function may represent a novel underlying pathogenic mechanism.