Molecularly defined circuits for cardiovascular and cardiopulmonary control.

The sympathetic and parasympathetic nervous systems regulate the activities of internal organs1, but the molecular and functional diversity of their constituent neurons and circuits remains largely unknown. Here we use retrograde neuronal tracing, single-cell RNA sequencing, optogenetics and physiological experiments to dissect the cardiac parasympathetic control circuit in mice. We show that cardiac-innervating neurons in the brainstem nucleus ambiguus (Amb) are comprised of two molecularly, anatomically and functionally distinct subtypes. The first, which we call ambiguus cardiovascular (ACV) neurons (approximately 35 neurons per Amb), define the classical cardiac parasympathetic circuit. They selectively innervate a subset of cardiac parasympathetic ganglion neurons and mediate the baroreceptor reflex, slowing heart rate and atrioventricular node conduction in response to increased blood pressure. The other, ambiguus cardiopulmonary (ACP) neurons (approximately 15 neurons per Amb) innervate cardiac ganglion neurons intermingled with and functionally indistinguishable from those innervated by ACV neurons. ACP neurons also innervate most or all lung parasympathetic ganglion neurons-clonal labelling shows that individual ACP neurons innervate both organs. ACP neurons mediate the dive reflex, the simultaneous bradycardia and bronchoconstriction that follows water immersion. Thus, parasympathetic control of the heart is organized into two parallel circuits, one that selectively controls cardiac function (ACV circuit) and another that coordinates cardiac and pulmonary function (ACP circuit). This new understanding of cardiac control has implications for treating cardiac and pulmonary diseases and for elucidating the control and coordination circuits of other organs.

Raphe GABAergic neurons mediate the acquisition of avoidance after social defeat.

Serotonin (5-HT) modulates neural responses to socioaffective cues and can bias approach or avoidance behavioral decisions, yet the cellular mechanisms underlying its contribution to the regulation of social experiences remain poorly understood. We hypothesized that GABAergic neurons in the dorsal raphe nucleus (DRN) may participate in socioaffective regulation by controlling serotonergic tone during social interaction. We tested this hypothesis using whole-cell recording techniques in genetically identified DRN GABA and 5-HT neurons in mice exposed to social defeat, a model that induces long-lasting avoidance behaviors in a subset of mice responsive to serotonergic antidepressants. Our results revealed that social defeat engaged DRN GABA neurons and drove GABAergic sensitization that strengthened inhibition of 5-HT neurons in mice that were susceptible, but not resilient to social defeat. Furthermore, optogenetic silencing of DRN GABA neurons disinhibited neighboring 5-HT neurons and prevented the acquisition of social avoidance in mice exposed to a social threat, but did not affect a previously acquired avoidance phenotype. We provide the first characterization of GABA neurons in the DRN that monosynaptically inhibit 5-HT neurons and reveal their key role in neuroplastic processes underlying the development of social avoidance.

HDAC6 regulates glucocorticoid receptor signaling in serotonin pathways with critical impact on stress resilience.

Genetic variations in certain components of the glucocorticoid receptor (GR) chaperone complex have been associated with the development of stress-related affective disorders and individual variability in therapeutic responses to antidepressants. Mechanisms that link GR chaperoning and stress susceptibility are not well understood. Here, we show that the effects of glucocorticoid hormones on socioaffective behaviors are critically regulated via reversible acetylation of Hsp90, a key component of the GR chaperone complex. We provide pharmacological and genetic evidence indicating that the cytoplasmic lysine deacetylase HDAC6 controls Hsp90 acetylation in the brain, and thereby modulates Hsp90-GR protein-protein interactions, as well as hormone- and stress-induced GR translocation, with a critical impact on GR downstream signaling and behavior. Pet1-Cre-driven deletion of HDAC6 in serotonin neurons, the densest HDAC6-expressing cell group in the mouse brain, dramatically reduced acute anxiogenic effects of the glucocorticoid hormone corticosterone in the open-field, elevated plus maze, and social interaction tests. Serotonin-selective depletion of HDAC6 also blocked the expression of social avoidance in mice exposed to chronic social defeat and concurrently prevented the electrophysiological and morphological changes induced, in serotonin neurons, by this murine model of traumatic stress. Together, these results identify HDAC6 inhibition as a potential new strategy for proresilience and antidepressant interventions through regulation of the Hsp90-GR heterocomplex and focal prevention of GR signaling in serotonin pathways. Our data thus uncover an alternate mechanism by which pan-HDAC inhibitors may regulate stress-related behaviors independently of their action on histones.

A brainstem circuit for phonation and volume control in mice.

Mammalian vocalizations are critical for communication and are produced through the process of phonation, in which expiratory muscles force air through the tensed vocal folds of the larynx, which vibrate to produce sound. Despite the importance of phonation, the motor circuits in the brain that control it remain poorly understood. In this study, we identified a subpopulation of ~160 neuropeptide precursor Nts (neurotensin)-expressing neurons in the mouse brainstem nucleus retroambiguus (RAm) that are robustly activated during both neonatal isolation cries and adult social vocalizations. The activity of these neurons is necessary and sufficient for vocalization and bidirectionally controls sound volume. RAm Nts neurons project to all brainstem and spinal cord motor centers involved in phonation and activate laryngeal and expiratory muscles essential for phonation and volume control. Thus, RAm Nts neurons form the core of a brain circuit for making sound and controlling its volume, which are two foundations of vocal communication.

Cellular mechanisms of deep brain stimulation: activity-dependent focal circuit reprogramming?

Deep brain stimulation (DBS) is a well-established treatment modality for movement disorders. As more behavioral disorders are becoming understood as specific disruptions in neural circuitry, the therapeutic realm of DBS is broadening to encompass a wider range of domains, including disorders of compulsion, affect, and memory, but current understanding of the cellular mechanisms of DBS remains limited. We review progress made during the last decade focusing in particular on how recent methods for targeted circuit manipulations, imaging and reconstruction are fostering preclinical and translational advances that improve our neurobiological understanding of DBS's action in psychiatric disorders.

Antidepressant-like effects of cortical deep brain stimulation coincide with pro-neuroplastic adaptations of serotonin systems.

BACKGROUND: Cortical deep brain stimulation (DBS) is a promising therapeutic option for treatment-refractory depression, but its mode of action remains enigmatic. Serotonin (5-HT) systems are engaged indirectly by ventromedial prefrontal cortex (vmPFC) DBS. Resulting neuroplastic changes in 5-HT systems could thus coincide with the long-term therapeutic activity of vmPFC DBS. METHODS: We tested this hypothesis by evaluating the antidepressant-like activity of vmPFC DBS in the chronic social defeat stress (CSDS) model of depression (n = 8-13 mice/group). Circuit-wide activation induced by vmPFC DBS was mapped with c-Fos immunolabeling. The effects of chronic vmPFC DBS on the physiology and morphology of genetically identified 5-HT cells from the dorsal raphe nucleus (DRN) were examined with whole-cell recording, somatodendritic three-dimensional reconstructions and morphometric analyses of presynaptic boutons along 5-HT axons. RESULTS: Acute DBS drove c-Fos expression locally in the vmPFC and in several distal monosynaptically connected regions, including the DRN. Chronic DBS reversed CSDS-induced social avoidance, restored the disrupted balance of excitatory/inhibitory inputs onto 5-HT neurons, and reversed 5-HT hypoexcitability observed after CSDS. Furthermore, vmPFC DBS reversed CSDS-induced arborization of 5-HT dendrites in the DRN and increased the size and density of 5-HT presynaptic terminals in the dentate gyrus and vmPFC. CONCLUSIONS: We validate a new preclinical paradigm to examine cellular mechanisms underlying the antidepressant-like activity of vmPFC DBS and identify dramatic circuit-mediated cellular adaptations that coincide with this treatment. These neuroplastic changes of 5-HT neurons might contribute to the progressive mood improvements reported in patients treated with chronic courses of cortical DBS.

Engineering Escherichia coli for light-activated cytolysis of mammalian cells.

By delivering payloads in response to specific exogenous stimuli, smart bacterial therapeutics have the potential to overcome many limitations of conventional therapies, including poor targeting specificity and dosage control in current cancer treatments. Although not yet explored as a trigger for bacterial drug delivery, light is an ideal induction mechanism because it offers fine spatiotemporal control and is easily and safely administered. Using recent advances in optogenetics, we have engineered two strains of Escherichia coli to secrete a potent mammalian cytotoxin in response to blue or red light. The tools in this study demonstrate the initial feasibility of light-activated bacterial therapeutics for applications such as tumor cytolysis, and their modular nature should enable simple substitution of other payloads of interest.