Probes for SST

Major depressive disorder (MDD) patients display a common but often variable set of symptoms making successful, sustained treatment difficult to achieve. Separate depressive symptoms may be encoded by differential changes in distinct circuits in the brain, yet how discrete circuits underlie behavioral subsets of depression and how they adapt in response to stress has not been addressed. We identify two discrete circuits of parvalbumin-positive (PV) neurons in the ventral pallidum (VP) projecting to either the lateral habenula or ventral tegmental area contributing to depression. We find that these populations undergo different electrophysiological adaptations in response to social defeat stress, which are normalized by antidepressant treatment. Furthermore, manipulation of each population mediates either social withdrawal or behavioral despair, but not both. We propose that distinct components of the VP PV circuit can subserve related, yet separate depressive-like phenotypes in mice, which could ultimately provide a platform for symptom-specific treatments of depression.

The mammalian brain is composed of diverse, specialized cell populations. To systematically ascertain and learn from these cellular specializations, we used Drop-seq to profile RNA expression in 690,000 individual cells sampled from 9 regions of the adult mouse brain. We identified 565 transcriptionally distinct groups of cells using computational approaches developed to distinguish biological from technical signals. Cross-region analysis of these 565 cell populations revealed features of brain organization, including a gene-expression module for synthesizing axonal and presynaptic components, patterns in the co-deployment of voltage-gated ion channels, functional distinctions among the cells of the vasculature and specialization of glutamatergic neurons across cortical regions. Systematic neuronal classifications for two complex basal ganglia nuclei and the striatum revealed a rare population of spiny projection neurons. This adult mouse brain cell atlas, accessible through interactive online software (DropViz), serves as a reference for development, disease, and evolution.

Behavioral and molecular characterization of cell-type-specific populations governing fear learning and behavior is a promising avenue for the rational identification of potential therapeutics for fear-related disorders. Examining cell-type-specific changes in neuronal translation following fear learning allows for targeted pharmacological intervention during fear extinction learning, mirroring possible treatment strategies in humans. Here we identify the central amygdala (CeA) Drd2-expressing population as a novel fear-supporting neuronal population that is molecularly distinct from other, previously identified, fear-supporting CeA populations. Sequencing of actively translating transcripts of Drd2 neurons using translating ribosome affinity purification (TRAP) technology identifies mRNAs that are differentially regulated following fear learning. Differentially expressed transcripts with potentially targetable gene products include Npy5r, Rxrg, Adora2a, Sst5r, Fgf3, Erbb4, Fkbp14, Dlk1, and Ssh3. Direct pharmacological manipulation of NPY5R, RXR, and ADORA2A confirms the importance of this cellpopulation and these cell-type-specific receptors in fear behavior. Furthermore, these findings validate the use of functionally identified specific cell populations to predict novel pharmacological targets for the modulation of emotional learning.

Although neuropeptide Y (NPY) has potent anxiolytic actions within the basolateral amygdala (BLA), selective activation of BLA NPY Y2receptors (Y2R) acutely increases anxiety by an unknown mechanism. Using ex vivo male rat brain slice electrophysiology, we show that the selective Y2R agonist, [ahx5-24]NPY, reduced the frequency of GABAA-mediated miniature inhibitory post synaptic currents (mIPSC) in BLA principal neurons (PN). [ahx5-24]NPY also reduced tonic activation of GABAB receptors (GABABR), which increased PN excitability through inhibition of a tonic, inwardly-rectifying potassium current (KIR ). Surprisingly, Y2R-sensitive GABABR currents were action potential-independent, persisting after treatment with tetrodotoxin. Additionally, the Ca2+-dependent, slow afterhyperpolarizing K+ current (IsAHP ) was enhanced in roughly half of the Y2R-sensitive PNs, possibly from enhanced Ca2+ influx, permitted by reduced GABABR tone. In male and female mice expressing tdTomato in Y2R-expressing cells (tdT-Y2R mice), immunohistochemistry revealed that BLA somatostatin interneurons (SST IN) express Y2Rs, as do a significant subset of BLA PNs. In tdT-Y2R mice, [ahx5-24]NPY increased excitability and suppressed the KIR in nearly all BLA PNs independent of tdT-Y2R fluorescence, consistent with presynaptic Y2Rs on SST INs mediating the above effects. However, only tdT-Y2R-expressing PNs responded to [ahx5-24]NPY with an enhancement of the IsAHP Ultimately, increased PN excitability via acute Y2R activation likely correlates with enhanced BLA output, consistent with reported Y2R-mediated anxiogenesis. Furthermore, we demonstrate: 1) a novel mechanism whereby activity-independent GABA release can powerfully dampen BLA neuronal excitability via postsynaptic GABABRs, and 2) that this tonic inhibition can be interrupted by neuromodulation, here by NPY via Y2Rs.SIGNIFICANCE STATEMENTWithin the basolateral amygdala (BLA), neuropeptide Y (NPY) is potently anxiolytic. However, selective activation of NPY2-receptors (Y2R) increases anxiety by an unknown mechanism. We show that activation of BLA Y2Rs decreases tonic GABA release onto BLA principal neurons (PN), probably from Y2R-expressing somatostatin interneurons some of which co-express NPY. This increases PN excitability by reducing GABAB receptor (GABABR)-mediated activation of G-protein-coupled, inwardly-rectifying K+(GIRK) currents. Tonic, Y2R- sensitive GABABR currents unexpectedly persisted in the absence of action potential firing, revealing, to our knowledge, the first report of substantial, activity-independent GABABR activation. Ultimately, we provide a plausible explanation for Y2R-mediated anxiogenesis in vivo and describe a novel and modulatable means of damping neuronal excitability.