Chemogenetic synaptic silencing of neural circuits localizes a hypothalamus→midbrain pathway for feeding behavior

Does the lateral hypothalamus govern the transition between appetitive and consummatory feeding?

Feeding is a cyclic behaviour that includes appetitive, consummatory and termination phases. Identifying the neural circuits controlling these phases and triggering specific transitions between phases would be a key advance in understanding feeding behaviour. The lateral hypothalamus (LH) has long been recognized for its central role in feeding. We review evidence suggesting that the LH acts as a regulator of the appetitive-consummatory transition using a switchboard-like circuit architecture. Within the LH, several neuronal subpopulations can be defined based on molecular markers, and - although these subpopulations are functionally diverse - they contribute to appetitive and consummatory behaviours to varying extents. We summarise the current evidence on whether these subpopulations have functional identities and speculate on the role of the LH as a controller of behavioural transitions.

Astrocytes release ATP/ADP and glutamate in flashes via vesicular exocytosis

Astrocytes regulate brain functions through gliotransmitters like ATP/ADP and glutamate, but their release patterns and mechanisms remain controversial. Here, we visualized ATP/ADP and glutamate response following astrocyte activation and investigated their mechanisms in vivo. Employing cOpn5-mediated optogenetic stimulation, genetically encoded fluorescent sensors, and two-photon imaging, we observed ATP/ADP released as temporally prolonged and spatially extended flashes that later converted to adenosine. This release occurs via Ca2+ and VNUT-dependent vesicular exocytosis. Additionally, astrocytes also release glutamate in flashes through TeNT-sensitive exocytosis, independent of ATP/ADP release. ATP/ADP released by astrocytes triggers further ATP/ADP release from microglia through P2Y12- and VNUT-dependent mechanisms. VNUT in astrocytes and microglia also contributes to ATP/ADP release under LPS-induced brain inflammation. These findings establish Ca2+-dependent vesicular exocytosis as a key mode of action, reveal intricate astrocyte-microglia interactions, and suggest a role for gliotransmission in brain inflammation. Furthermore, the methodologies may provide valuable tools for deciphering glial physiology and pathophysiology.

Somatostatin neurons detect stimulus-reward contingencies to reduce neocortical inhibition during learning

Learning involves the association of discrete events in the world to infer causality, likely through a cascade of changes at input- and target-specific synapses. Transient or sustained disinhibition may initiate cortical circuit plasticity important for association learning, but the cellular networks involved have not been well defined. Using recordings in acute brain slices, we show that whisker-dependent sensory association learning drives a durable, target-specific reduction in inhibition from somatostatin (SST)-expressing GABAergic neurons onto pyramidal (Pyr) neurons in superficial but not deep layers of mouse somatosensory cortex. Critically, SST output was not altered when stimuli and rewards were unpaired, indicating that these neurons are sensitive to stimulus-reward contingency. Depression of SST output onto Pyr neurons could be phenocopied by chemogenetic suppression of SST activity outside of the training context. Thus, neocortical SST neuron output can undergo long-lasting modifications to selectively disinhibit superficial layers of sensory neocortex during learning.

Spatial localization of hippocampal replay requires dopamine signaling

Sequenced reactivations of hippocampal neurons called replays, concomitant with sharp-wave ripples in the local field potential, are critical for the consolidation of episodic memory, but whether replays depend on the brain's reward or novelty signals is unknown. Here, we combined chemogenetic silencing of dopamine neurons in ventral tegmental area (VTA) and simultaneous electrophysiological recordings in dorsal hippocampal CA1, in freely behaving male rats experiencing changes to reward magnitude and environmental novelty. Surprisingly, VTA silencing did not prevent ripple increases where reward was increased, but caused dramatic, aberrant ripple increases where reward was unchanged. These increases were associated with increased reverse-ordered replays. On familiar tracks this effect disappeared, and ripples tracked reward prediction error (RPE), indicating that non-VTA reward signals were sufficient to direct replay. Our results reveal a novel dependence of hippocampal replay on dopamine, and a role for a VTA-independent RPE signal that is reliable only in familiar environments.

Brainstem neuropeptidergic neurons link a neurohumoral axis to satiation

Hunger is evolutionarily hardwired to ensure that an animal has sufficient energy to survive and reproduce. Just as important as knowing when to start eating is knowing when to stop eating. Here, using spatially resolved single-cell phenotyping, we characterize a population of neuropeptidergic neurons in the brainstem's dorsal raphe nucleus (DRN) and describe how they regulate satiation. These neurons track food from sensory presentation through ingestion, integrate these signals with slower-acting humoral cues, and express cholecystokinin (CCK). These CCK neurons bidirectionally regulate meal size, driving a sustained meal termination signal with a built-in delay. They are also well positioned to sense and respond to ingestion: they express a host of metabolic signaling factors and are integrated into an extended network known to regulate feeding. Together, this work demonstrates how DRN CCK neurons regulate satiation and identifies a likely conserved cellular mechanism that transforms diverse neurohumoral signals into a key behavioral output.

Hippocampal Inhibitory Interneuron-Specific DREADDs Treatment Alters mTORC1-4E-BP Signaling and Impairs Memory Formation

Control of protein synthesis via the mechanistic target of rapamycin complex 1 (mTORC1) is essential for learning and memory. However, the cell-type-specific and spatiotemporal regulation of this pathway during memory formation is not well understood. In this study, we expressed artificial human muscarinic M3 [hM3D(Gq)] or M4 [hM4D(Gi)] designer receptors exclusively activated by designer drugs (DREADDs) in hippocampal CA1 excitatory or inhibitory neurons of adult mice. We studied the impact of clozapine-N-oxide (CNO), a synthetic DREADDs agonist, on the mTORC1 pathway and long-term memory. hM3D(Gq) and hM4D(Gi) activate or inactivate, respectively, mTORC1 signaling in hippocampal interneurons, as indicated by the phosphorylation of its targets, eukaryotic initiation factor 4E-binding proteins (4E-BP1/2) and ribosomal protein S6 (S6). Activation of either hM3D(Gq) or hM4D(Gi) in mice immediately after training in memory tasks impaired long-term memory formation in inhibitory, but not in excitatory neurons. The findings underscore the importance of activity-dependent mTORC1-4E-BP1/2 signaling in hippocampal inhibitory interneurons for memory formation.

Cationic peptides cause memory loss through endophilin-mediated endocytosis

The zeta inhibitory peptide (ZIP) interferes with memory maintenance and long-term potentiation (LTP)1 when administered to mice. However, mice lacking its putative target, protein kinase PKMζ, exhibit normal learning and memory as well as LTP2,3, making the mechanism of ZIP unclear. Here we show that ZIP disrupts LTP by removing surface AMPA receptors through its cationic charge alone. This effect requires endophilin-A2-mediated endocytosis and is fully blocked by drugs suppressing macropinocytosis. ZIP and other cationic peptides remove newly inserted AMPA receptor nanoclusters at potentiated synapses, providing a mechanism by which these peptides erase memories without altering basal synaptic function. When delivered in vivo, cationic peptides can modulate memories on local and brain-wide scales, and these mechanisms can be leveraged to prevent memory loss in a model of traumatic brain injury. Our findings uncover a previously unknown synaptic mechanism by which memories are maintained or lost.

GlyT2-Positive Interneurons Regulate Timing and Variability of Information Transfer in a Cerebellar-Behavioral Loop

GlyT2-positive interneurons, Golgi and Lugaro cells, reside in the input layer of the cerebellar cortex in a key position to influence information processing. Here, we examine the contribution of GlyT2-positive interneurons to network dynamics in Crus 1 of mouse lateral cerebellar cortex during free whisking. We recorded neuronal population activity using Neuropixels probes before and after chemogenetic downregulation of GlyT2-positive interneurons in male and female mice. Under resting conditions, cerebellar population activity reliably encoded whisker movements. Reductions in the activity of GlyT2-positive cells produced mild increases in neural activity which did not significantly impair these sensorimotor representations. However, reduced Golgi and Lugaro cell inhibition did increase the temporal alignment of local population network activity at the initiation of movement. These network alterations had variable impacts on behavior, producing both increases and decreases in whisking velocity. Our results suggest that inhibition mediated by GlyT2-positive interneurons primarily governs the temporal patterning of population activity, which in turn is required to support downstream cerebellar dynamics and behavioral coordination.

Projections from thalamic nucleus reuniens to hippocampal CA1 area participate in context fear extinction by affecting extinction-induced molecular remodeling of excitatory synapses

The ability to extinguish contextual fear in a changing environment is crucial for animal survival. Recent data support the role of the thalamic nucleus reuniens (RE) and its projections to the dorsal hippocampal CA1 area (RE→dCA1) in this process. However, it remains poorly understood how RE impacts dCA1 neurons during contextual fear extinction (CFE). Here, we reveal that the RE→dCA1 pathway contributes to the extinction of contextual fear by affecting CFE-induced molecular remodeling of excitatory synapses. Anatomical tracing and chemogenetic manipulation in mice demonstrate that RE neurons form synapses and regulate synaptic transmission in the stratum oriens (SO) and lacunosum-moleculare (SLM) of the dCA1 area, but not in the stratum radiatum (SR). We also observe CFE-specific structural changes of excitatory synapses and expression of the synaptic scaffold protein, PSD-95, in both strata innervated by RE, but not in SR. Interestingly, only the changes in SLM are specific for the dendrites innervated by RE. To further support the role of the RE→dCA1 projection in CFE, we demonstrate that brief chemogenetic inhibition of the RE→dCA1 pathway during a CFE session persistently impairs the formation of CFE memory and CFE-induced changes of PSD-95 levels in SLM. Thus, our data indicate that RE participates in CFE by regulating CFE-induced molecular remodeling of dCA1 synapses.

A septo-hypothalamic-medullary circuit directs stress-induced analgesia

Stress is a potent modulator of pain. Specifically, acute stress due to physical restraint induces stress-induced analgesia (SIA). However, where and how acute stress and pain pathways interface in the brain are poorly understood. Here, we describe how the dorsal lateral septum (dLS), a forebrain limbic nucleus, facilitates SIA through its downstream targets in the lateral hypothalamic area (LHA) of mice. Taking advantage of transsynaptic viral-genetic, optogenetic, and chemogenetic techniques, we show that the dLS→LHA circuitry is sufficient to drive analgesia and is required for SIA. Furthermore, our results reveal that the dLS→LHA pathway is opioid-dependent and modulates pain through the pro-nociceptive neurons in the rostral ventromedial medulla (RVM). Remarkably, we found that the inhibitory dLS neurons are recruited specifically when the mice struggle to escape under restraint and, in turn, inhibit excitatory LHA neurons. As a result, the RVM neurons downstream of LHA are disengaged, thus suppressing nociception. Together, we delineate a poly-synaptic pathway that can transform escape behavior in mice under restraint to acute stress into analgesia.

Neuroglia in eating disorders (obesity, Prader-Willi syndrome and anorexia nervosa)

The hypothalamus is widely recognized as one of the most extensively studied brain regions involved in the central regulation of energy homeostasis. Within the hypothalamus, peptidergic neurons play a crucial role in monitoring peripheral concentrations of metabolites and hormones, and they finely adjust the sensing of these factors, leading to the activation of either anorexigenic (appetite-suppressing) or orexigenic (appetite-stimulating) pathways. While cortical innervation of the hypothalamus does influence these processes, it is generally considered of secondary importance. Eating-related disorders, such as obesity and anorexia nervosa, are strongly associated with imbalances in energy intake and expenditure. The phenotypes of these disorders can be attributed to dysfunctions in the hypothalamus. Traditionally, it has been believed that hypothalamic dysfunction in these disorders primarily stems from defects in neural pathways. However, recent evidence challenges this perception, highlighting the active participation of neuroglial cells in shaping both physiologic and behavioral characteristics. This review aims to provide an overview of the latest insights into glial biology in three specific eating disorders: obesity, Prader-Willi syndrome, and anorexia. In these disorders, neural dysfunction coincides with glial malfunction, suggesting that neuroglia actively contribute to the development and progression of various neurologic disorders. These findings underscore the importance of glial cells and open up potential new avenues for therapeutic interventions.

Modulation of stress-related behaviour by preproglucagon neurons and hypothalamic projections to the nucleus of the solitary tract

Stress-induced behaviours are driven by complex neural circuits and some neuronal populations concurrently modulate diverse behavioural and physiological responses to stress. Glucagon-like peptide-1 (GLP-1)-producing preproglucagon (PPG) neurons within the lower brainstem caudal nucleus of the solitary tract (cNTS) are particularly sensitive to stressful stimuli and are implicated in multiple physiological and behavioural responses to interoceptive and psychogenic threats. However, the afferent inputs driving stress-induced activation of PPG neurons are largely unknown, and the role of PPG neurons in anxiety-like behaviour is controversial. Through chemogenetic manipulations we reveal that cNTS PPG neurons have the ability to moderately increase anxiety-like behaviours in mice in a sex-dependent manner. Using an intersectional approach, we show that input from the paraventricular nucleus of the hypothalamus (PVN) drives activation of both the cNTS as a whole and PPG neurons in particular in response to acute restraint stress, but that while this input is rich in corticotropin-releasing hormone (CRH), PPG neurons do not express significant levels of receptors for CRH and are not activated following lateral ventricle delivery of CRH. Finally, we demonstrate that cNTS-projecting PVN neurons are necessary for the ability of restraint stress to suppress food intake in male mice. Our findings reveal sex differences in behavioural responses to PPG neural activation and highlight a hypothalamic-brainstem pathway in stress-induced hypophagia.

The multi-stage plasticity in the aggression circuit underlying the winner effect

Winning increases the readiness to attack and the probability of winning, a widespread phenomenon known as the "winner effect." Here, we reveal a transition from target-specific to generalized aggression enhancement over 10 days of winning in male mice. This behavioral change is supported by three causally linked plasticity events in the ventrolateral part of the ventromedial hypothalamus (VMHvl), a critical node for aggression. Over 10 days of winning, VMHvl cells experience monotonic potentiation of long-range excitatory inputs, transient local connectivity strengthening, and a delayed excitability increase. Optogenetically coactivating the posterior amygdala (PA) terminals and VMHvl cells potentiates the PA-VMHvl pathway and triggers the same cascade of plasticity events observed during repeated winning. Optogenetically blocking PA-VMHvl synaptic potentiation eliminates all winning-induced plasticity. These results reveal the complex Hebbian synaptic and excitability plasticity in the aggression circuit during winning, ultimately leading to increased "aggressiveness" in repeated winners.

Striatal lateral inhibition regulates action selection in a mouse model of levodopa-induced dyskinesia

Striatal medium spiny neurons (MSNs) integrate multiple external inputs to shape motor output. In addition, MSNs form local inhibitory synaptic connections with one another. The function of striatal lateral inhibition is unknown, but one possibility is in selecting an intended action while suppressing alternatives. Action selection is disrupted in several movement disorders, including levodopa-induced dyskinesia (LID), a complication of Parkinson's disease (PD) therapy characterized by involuntary movements. Here, we identify chronic changes in the strength of striatal lateral inhibitory synapses in a mouse model of PD/LID. These synapses are also modulated by acute dopamine signaling. Chemogenetic suppression of lateral inhibition originating from dopamine D2 receptor-expressing MSNs lowers the threshold to develop involuntary movements in vivo, supporting a role in motor control. By examining the role of lateral inhibition in basal ganglia function and dysfunction, we expand the framework surrounding the role of striatal microcircuitry in action selection.

Molecular and circuit determinants in the globus pallidus mediating control of cocaine-induced behavioral plasticity

The globus pallidus externus (GPe) is a central component of the basal ganglia circuit that acts as a gatekeeper of cocaine-induced behavioral plasticity. However, the molecular and circuit mechanisms underlying this function are unknown. Here, we show that GPe parvalbumin-positive (GPePV) cells mediate cocaine responses by selectively modulating ventral tegmental area dopamine (VTADA) cells projecting to the dorsomedial striatum (DMS). Interestingly, GPePV cell activity in cocaine-naive mice is correlated with behavioral responses following cocaine, effectively predicting cocaine sensitivity. Expression of the voltage-gated potassium channels KCNQ3 and KCNQ5 that control intrinsic cellular excitability following cocaine was downregulated, contributing to the elevation in GPePV cell excitability. Acutely activating channels containing KCNQ3 and/or KCNQ5 using the small molecule carnosic acid, a key psychoactive component of Salvia rosmarinus (rosemary) extract, reduced GPePV cell excitability and impaired cocaine reward, sensitization, and volitional cocaine intake, indicating its therapeutic potential to counteract psychostimulant use disorder.

Chemogenetic silencing reveals presynaptic Gi/o protein-mediated inhibition of developing hippocampal synchrony in vivo

Recent advances in understanding how neuronal activity shapes developing brain circuits increasingly rely on Gi/o-dependent inhibitory chemogenetic tools (Gi-DREADDs). However, their mechanisms of action and efficacy in neurons with immature Gi/o signaling are elusive. Here, we express the Gi-DREADD hM4Di in glutamatergic telencephalic neurons and analyze its impact on CA1 pyramidal neurons in neonatal mice. Using acousto-optic two-photon Ca2+ imaging, we report that activation of hM4Di leads to a complete arrest of spontaneous synchrony in CA1 in vitro. We demonstrate that hM4Di does not cause somatic hyperpolarization or shunting but rather mediates presynaptic silencing of glutamatergic neurotransmission. In vivo, inhibition through hM4Di potently suppresses early sharp waves (eSPWs) and discontinuous oscillatory network activity in CA1 of head-fixed mice before eye opening. Our findings provide insights into the role of Gi/o signaling in synchronized activity in the neonatal hippocampus and bear relevance for applying chemogenetic silencing at early developmental stages.

Paraventricular hypothalamic RUVBL2 neurons suppress appetite by enhancing excitatory synaptic transmission in distinct neurocircuits

The paraventricular hypothalamus (PVH) is crucial for food intake control, yet the presynaptic mechanisms underlying PVH neurons remain unclear. Here, we show that RUVBL2 in the PVH is significantly reduced during energy deficit, and knockout (KO) of PVH RUVBL2 results in hyperphagic obesity in mice. RUVBL2-expressing neurons in the PVH (PVHRUVBL2) exert the anorexigenic effect by projecting to the arcuate hypothalamus, the dorsomedial hypothalamus, and the parabrachial complex. We further demonstrate that PVHRUVBL2 neurons form the synaptic connections with POMC and AgRP neurons in the ARC. PVH RUVBL2 KO impairs the excitatory synaptic transmission by reducing presynaptic boutons and synaptic vesicles near active zone. Finally, RUVBL2 overexpression in the PVH suppresses food intake and protects against diet induced obesity. Together, this study demonstrates an essential role for PVH RUVBL2 in food intake control, and suggests that modulation of synaptic plasticity could be an effective way to curb appetite and obesity.

Treating Alzheimer's disease with brain stimulation: From preclinical models to non-invasive stimulation in humans

Alzheimer's disease (AD) is a severe and progressive neurodegenerative condition that exerts detrimental effects on brain function. As of now, there is no effective treatment for AD patients. This review explores two distinct avenues of research. The first revolves around the use of animal studies and preclinical models to gain insights into AD's underlying mechanisms and potential treatment strategies. Specifically, it delves into the effectiveness of interventions such as Optogenetics and Chemogenetics, shedding light on their implications for understanding pathophysiological mechanisms and potential therapeutic applications. The second avenue focuses on non-invasive brain stimulation (NiBS) techniques in the context of AD. Evidence suggests that NiBS can successfully modulate cognitive functions associated with various neurological and neuropsychiatric disorders, including AD, as demonstrated by promising findings. Here, we critically assessed recent findings in AD research belonging to these lines of research and discuss their potential impact on the clinical horizon of AD treatment. These multifaceted approaches offer hope for advancing our comprehension of AD pathology and developing novel therapeutic interventions.

Modulation of human induced neural stem cell-derived dopaminergic neurons by DREADD reveals therapeutic effects on a mouse model of Parkinson's disease

BACKGROUND

Stem cell-based therapy is a promising strategy for treating Parkinson's disease (PD) characterized by the loss of dopaminergic neurons. Recently, induced neural stem cell-derived dopaminergic precursor cells (iNSC-DAPs) have been emerged as a promising candidate for PD cell therapy because of a lower tumor-formation ability. Designer receptors exclusively activated by designer drugs (DREADDs) are useful tools for examining functional synaptic connections with host neurons.

METHODS

DREADD knock-in human iNSCs to express excitatory hM3Dq and inhibitory hM4Di receptors were engineered by CRISPR. The knock-in iNSCs were differentiated into midbrain dopaminergic precursor cells (DAPs) and transplanted into PD mice. The various behavior test such as the Apomorphine-induced rotation test, Cylinder test, Rotarod test, and Open field test were assessed at 4, 8, or 12 weeks post-transplantation with or without the administration of CNO. Electrophysiology were performed to assess the integrated condition and modulatory function to host neurons.

RESULTS

DREADD expressing iNSCs were constructed with normal neural stem cells characteristics, proliferation ability, and differentiation potential into dopaminergic neuorns. DAPs derived from DREADD expressing iNSC showed matched function upon administration of clozapine N-oxide (CNO) in vitro. The results of electrophysiology and behavioral tests of transplanted PD mouse models revealed that the grafts established synaptic connections with downstream host neurons and exhibited excitatory or inhibitory modulation in response to CNO in vivo.

CONCLUSION

iNSC-DAPs are a promising candidate for cell replacement therapy for Parkinson's disease. Remote DREADD-dependent activation of iNSC-DAP neurons significantly enhanced the beneficial effects on transplanted mice with Parkinson's disease.

Targeted inhibition of kisspeptin neurons reverses hyperandrogenemia and abnormal hyperactive LH secretion in a preclinical mouse model of polycystic ovary syndrome

STUDY QUESTION

Do hyperactive kisspeptin neurons contribute to abnormally high LH secretion and downstream hyperandrogenemia in polycystic ovary syndrome (PCOS)-like conditions and can inhibition of kisspeptin neurons rescue such endocrine impairments?

SUMMARY ANSWER

Targeted inhibition of endogenous kisspeptin neuron activity in a mouse model of PCOS reduced the abnormally hyperactive LH pulse secretion and hyperandrogenemia to healthy control levels.

WHAT IS KNOWN ALREADY

PCOS is a reproductive disorder characterized by hyperandrogenemia, anovulation, and/or polycystic ovaries, along with a hallmark feature of abnormal LH hyper-pulsatility, but the mechanisms underlying the endocrine impairments remain unclear. A chronic letrozole (LET; aromatase inhibitor) mouse model recapitulates PCOS phenotypes, including polycystic ovaries, anovulation, high testosterone, and hyperactive LH pulses. LET PCOS-like females also have increased hypothalamic kisspeptin neuronal activation which may drive their hyperactive LH secretion and hyperandrogenemia, but this has not been tested.

STUDY DESIGN, SIZE, DURATION

Transgenic KissCRE+/hM4Di female mice or littermates Cre- controls were treated with placebo, or chronic LET (50 µg/day) to induce a PCOS-like phenotype, followed by acute (once) or chronic (2 weeks) clozapine-N-oxide (CNO) exposure to chemogenetically inhibit kisspeptin cells (n = 6 to 10 mice/group).

PARTICIPANTS/MATERIALS, SETTING, METHODS

Key endocrine measures, including in vivo LH pulse secretion patterns and circulating testosterone levels, were assessed before and after selective kisspeptin neuron inhibition and compared between PCOS groups and healthy controls. Alterations in body weights were measured and pituitary and ovarian gene expression was determined by qRT-PCR.

MAIN RESULTS AND THE ROLE OF CHANCE

Acute targeted inhibition of kisspeptin neurons in PCOS mice successfully lowered the abnormally hyperactive LH pulse secretion (P < 0.05). Likewise, chronic selective suppression of kisspeptin neuron activity reversed the previously high LH and testosterone levels (P < 0.05) down to healthy control levels and rescued reproductive gene expression (P < 0. 05).

LARGE SCALE DATA

N/A.

LIMITATIONS, REASONS FOR CAUTION

Ovarian morphology was not assessed in this study. Additionally, mouse models can offer mechanistic insights into neuroendocrine processes in PCOS-like conditions but may not perfectly mirror PCOS in women.

WIDER IMPLICATIONS OF THE FINDINGS

These data support the hypothesis that overactive kisspeptin neurons can drive neuroendocrine PCOS-like impairments, and this may occur in PCOS women. Our findings complement recent clinical investigations using NKB receptor antagonists to lower LH in PCOS women and suggest that pharmacological dose-dependent modulation of kisspeptin neuron activity may be a valuable future therapeutic target to clinically treat hyperandrogenism and lower elevated LH in PCOS women.

STUDY FUNDING/COMPETING INTEREST(S)

This research was supported by NIH grants R01 HD111650, R01 HD090161, R01 HD100580, P50 HD012303, R01 AG078185, and NIH R24 HD102061, and a pilot project award from the British Society for Neuroendocrinology. There are no competing interests.

Chemogenetic Tools and their Use in Studies of Neuropsychiatric Disorders

Chemogenetics is a newly developed set of tools that allow for selective manipulation of cell activity. They consist of a receptor mutated irresponsive to endogenous ligands and a synthetic ligand that does not interact with the wild-type receptors. Many different types of these receptors and their respective ligands for inhibiting or excitating neuronal subpopulations were designed in the past few decades. It has been mainly the G-protein coupled receptors (GPCRs) selectively responding to clozapine-N-oxide (CNO), namely Designer Receptors Exclusively Activated by Designer Drugs (DREADDs), that have been employed in research. Chemogenetics offers great possibilities since the activity of the receptors is reversible, inducible on demand by the ligand, and non-invasive. Also, specific groups or types of neurons can be selectively manipulated thanks to the delivery by viral vectors. The effect of the chemogenetic receptors on neurons lasts longer, and even chronic activation can be achieved. That can be useful for behavioral testing. The great advantage of chemogenetic tools is especially apparent in research on brain diseases since they can manipulate whole neuronal circuits and connections between different brain areas. Many psychiatric or other brain diseases revolve around the dysfunction of specific brain networks. Therefore, chemogenetics presents a powerful tool for investigating the underlying mechanisms causing the disease and revealing the link between the circuit dysfunction and the behavioral or cognitive symptoms observed in patients. It could also contribute to the development of more effective treatments.

Distinct roles of monkey OFC-subcortical pathways in adaptive behavior

Primates must adapt to changing environments by optimizing their behavior to make beneficial choices. At the core of adaptive behavior is the orbitofrontal cortex (OFC) of the brain, which updates choice value through direct experience or knowledge-based inference. Here, we identify distinct neural circuitry underlying these two separate abilities. We designed two behavioral tasks in which two male macaque monkeys updated the values of certain items, either by directly experiencing changes in stimulus-reward associations, or by inferring the value of unexperienced items based on the task's rules. Chemogenetic silencing of bilateral OFC combined with mathematical model-fitting analysis revealed that monkey OFC is involved in updating item value based on both experience and inference. In vivo imaging of chemogenetic receptors by positron emission tomography allowed us to map projections from the OFC to the rostromedial caudate nucleus (rmCD) and the medial part of the mediodorsal thalamus (MDm). Chemogenetic silencing of the OFC-rmCD pathway impaired experience-based value updating, while silencing the OFC-MDm pathway impaired inference-based value updating. Our results thus demonstrate dissociable contributions of distinct OFC projections to different behavioral strategies, and provide new insights into the neural basis of value-based adaptive decision-making in primates.

Mapping GABAergic projections that mediate feeding

Neuroscience offers important insights into the pathogenesis and treatment of obesity by investigating neural circuits underpinning appetite and feeding. Gamma-aminobutyric acid (GABA), one of the most abundant neurotransmitters in the brain, and its associated receptors represent an array of pharmacologically targetable mediators of appetite signalling. Targeting the GABAergic system is therefore an increasingly investigated approach to obesity treatment. However, the many GABAergic projections that control feeding have yet to be collectively analysed. This review provides a comprehensive analysis of the relationship between GABAergic signalling and appetite by examining both foundational studies and the results of newly emerging chemogenetic/optogenetic experiments. A current snapshot of these efforts to map GABAergic projections influencing appetite is provided, and potential avenues for further investigation are provided.

Chemogenetic inactivation of the nucleus reuniens and its projections to the orbital cortex produce deficits on discrete measures of behavioral flexibility in the attentional set-shifting task

The nucleus reuniens (RE) of the ventral midline thalamus is a critical node in the communication between the orbitomedial prefrontal cortex (OFC) and the hippocampus (HF). While RE has been shown to directly participate in memory-associated functions through its connections with the medial prefrontal cortex and HF, less is known regarding the role of RE in executive functioning. Here, we examined the involvement of RE and its projections to the orbital cortex (ORB) in attention and behavioral flexibility in male rats using the attentional set shifting task (AST). Rats expressing the hM4Di DREADD receptor in RE were implanted with indwelling cannulas in either RE or the ventromedial ORB to pharmacologically inhibit RE or its projections to the ORB with intracranial infusions of clozapine-N-oxide hydrochloride (CNO). Chemogenetic-induced suppression of RE resulted in impairments in reversal learning and set-shifting. This supports a vital role for RE in behavioral flexibility - or the ability to adapt behavior to changing reward or rule contingencies. Interestingly, CNO suppression of RE projections to the ventromedial ORB produced impairments in rule abstraction - or dissociable effects elicited with direct RE suppression. In summary, the present findings indicate that RE, mediated in part by actions on the ORB, serves a critical role in the flexible use of rules to drive goal directed behavior. The cognitive deficits of various neurological disorders with impaired communication between the HF and OFC, may be partly attributed to alterations of RE -- as an established intermediary between these cortical structures.

Chemogenetic Signaling in Space and Time: Considerations for Designing Neuroscience Experiments Using DREADDs

The use of designer receptors exclusively activated by designer drugs (DREADDs) has led to significant advances in our understanding of the neural circuits that govern behavior. By allowing selective control over cellular activity and signaling, DREADDs have become an integral tool for defining the pathways and cellular phenotypes that regulate sleep, pain, motor activity, goal-directed behaviors, and a variety of other processes. In this review, we provide a brief overview of DREADDs and discuss notable discoveries in the neurosciences with an emphasis on circuit mechanisms. We then highlight methodological approaches to achieve pathway specific activation of DREADDs. Finally, we discuss spatial and temporal constraints of DREADDs signaling and how these features can be incorporated into experimental designs to precisely dissect circuits of interest.

Molecular and circuit determinants in the globus pallidus mediating control of cocaine-induced behavioral plasticity

The globus pallidus externus (GPe) is a central component of the basal ganglia circuit, receiving strong input from the indirect pathway and regulating a variety of functions, including locomotor output and habit formation. We recently showed that it also acts as a gatekeeper of cocaine-induced behavioral plasticity, as inhibition of parvalbumin-positive cells in the GPe (GPe PV ) prevents the development of cocaine-induced reward and sensitization. However, the molecular and circuit mechanisms underlying this function are unknown. Here we show that GPe PV cells control cocaine reward and sensitization by inhibiting GABAergic neurons in the substantia nigra pars reticulata (SNr GABA ), and ultimately, selectively modulating the activity of ventral tegmental area dopamine (VTA DA ) cells projecting to the lateral shell of the nucleus accumbens (NAcLat). A major input to GPe PV cells is the indirect pathway of the dorsomedial striatum (DMS D 2 ), which receives DAergic innervation from collaterals of VTA DA →NAcLat cells, making this a closed-loop circuit. Cocaine likely facilitates reward and sensitization not directly through actions in the GPe, but rather in the upstream DMS, where the cocaine-induced elevation of DA triggers a depression in DMS D 2 cell activity. This cocaine-induced elevation in DA levels can be blocked by inhibition of GPe PV cells, closing the loop. Interestingly, the level of GPe PV cell activity prior to cocaine administration is correlated with the extent of reward and sensitization that animals experience in response to future administration of cocaine, indicating that GPe PV cell activity is a key predictor of future behavioral responses to cocaine. Single nucleus RNA-sequencing of GPe cells indicated that genes encoding voltage-gated potassium channels KCNQ3 and KCNQ5 that control intrinsic cellular excitability are downregulated in GPe PV cells following a single cocaine exposure, contributing to the elevation in GPe PV cell excitability. Acutely activating channels containing KCNQ3 and/or KCNQ5 using the small molecule carnosic acid, a key psychoactive component of Salvia rosmarinus (rosemary) extract, reduced GPe PV cell excitability and also impaired cocaine reward, sensitization, and volitional cocaine intake, indicating its potential as a therapeutic to counteract psychostimulant use disorder. Our findings illuminate the molecular and circuit mechanisms by which the GPe orchestrates brain-wide changes in response to cocaine that are required for reward, sensitization, and self-administration behaviors.

Inputs to the locus coeruleus from the periaqueductal gray and rostroventral medulla shape opioid-mediated descending pain modulation

The supraspinal descending pain modulatory system (DPMS) shapes pain perception via monoaminergic modulation of sensory information in the spinal cord. However, the role and synaptic mechanisms of descending noradrenergic signaling remain unclear. Here, we establish that noradrenergic neurons of the locus coeruleus (LC) are essential for supraspinal opioid antinociception. While much previous work has emphasized the role of descending serotonergic pathways, we find that opioid antinociception is primarily driven by excitatory output from the ventrolateral periaqueductal gray (vlPAG) to the LC. Furthermore, we identify a previously unknown opioid-sensitive inhibitory input from the rostroventromedial medulla (RVM), the suppression of which disinhibits LC neurons to drive spinal noradrenergic antinociception. We describe pain-related activity throughout this circuit and report the presence of prominent bifurcating outputs from the vlPAG to the LC and the RVM. Our findings substantially revise current models of the DPMS and establish a supraspinal antinociceptive pathway that may contribute to multiple forms of descending pain modulation.

Tonic noradrenergic input to neurons in the dorsal raphe nucleus mediates food intake in male mice

The dorsal raphe nucleus (DRN) is essential for the control of food intake. Efferent projections from the DRN extend to several forebrain regions that are involved in the control of food intake. However, the neurotransmitters released in the DRN related to the control of food intake are not known. We have previously demonstrated that a tonic α1 action on DRN neurons contributes to satiety in the fed rats. In this study we investigated the participation of norepinephrine (NE) signaling in the DRN in the satiety response. Intra-DRN administration of NE causes an increase in the 2-hour food intake of sated mice, an effect that was blocked by previous administration of yohimbine, an α2 antagonist. Similarly, Intra-DRN administration of clonidine, an α2 agonist, increases food intake in sated mice. This result indicates that in the satiated mice exogenous NE acts on α2 receptors to increase food intake. Furthermore, administration of phenylephrine, an α1 agonist, decreases food intake in fasted mice and prazosin, an α1 antagonist, increases food intake in the sated mice. Taken together these results indicate that, in a satiated condition, a tonic α1 adrenergic action on the DRN neurons inhibits food intake and that exogenous NE administered to the DRN acts on α2 adrenergic receptors to increase food intake. These data reinforce the intricate neuronal functioning of the DRN and its effects on feeding.

Functionally refined encoding of threat memory by distinct populations of basal forebrain cholinergic projection neurons

Neurons of the basal forebrain nucleus basalis and posterior substantia innominata (NBM/SIp) comprise the major source of cholinergic input to the basolateral amygdala (BLA). Using a genetically encoded acetylcholine (ACh) sensor in mice, we demonstrate that BLA-projecting cholinergic neurons can 'learn' the association between a naive tone and a foot shock (training) and release ACh in the BLA in response to the conditioned tone 24 hr later (recall). In the NBM/SIp cholinergic neurons express the immediate early gene, Fos following both training and memory recall. Cholinergic neurons that express Fos following memory recall display increased intrinsic excitability. Chemogenetic silencing of these learning-activated cholinergic neurons prevents expression of the defensive behavior to the tone. In contrast, we show that NBM/SIp cholinergic neurons are not activated by an innately threatening stimulus (predator odor). Instead, VP/SIa cholinergic neurons are activated and contribute to defensive behaviors in response to predator odor, an innately threatening stimulus. Taken together, we find that distinct populations of cholinergic neurons are recruited to signal distinct aversive stimuli, demonstrating functionally refined organization of specific types of memory within the cholinergic basal forebrain of mice.

The secondary somatosensory cortex gates mechanical and heat sensitivity

The cerebral cortex is vital for the processing and perception of sensory stimuli. In the somatosensory axis, information is received primarily by two distinct regions, the primary (S1) and secondary (S2) somatosensory cortices. Top-down circuits stemming from S1 can modulate mechanical and cooling but not heat stimuli such that circuit inhibition causes blunted perception. This suggests that responsiveness to particular somatosensory stimuli occurs in a modality specific fashion and we sought to determine additional cortical substrates. In this work, we identify in a mouse model that inhibition of S2 output increases mechanical and heat, but not cooling sensitivity, in contrast to S1. Combining 2-photon anatomical reconstruction with chemogenetic inhibition of specific S2 circuits, we discover that S2 projections to the secondary motor cortex (M2) govern mechanical and heat sensitivity without affecting motor performance or anxiety. Taken together, we show that S2 is an essential cortical structure that governs mechanical and heat sensitivity.

Functionally refined encoding of threat memory by distinct populations of basal forebrain cholinergic projection neurons

Neurons of the basal forebrain nucleus basalis and posterior substantia innominata (NBM/SIp) comprise the major source of cholinergic input to the basolateral amygdala (BLA). Using a genetically-encoded acetylcholine (ACh) sensor in mice, we demonstrate that BLA-projecting cholinergic neurons can "learn" the association between a naïve tone and a foot shock (training) and release ACh in the BLA in response to the conditioned tone 24h later (recall). In the NBM/SIp cholinergic neurons express the immediate early gene, Fos following both training and memory recall. Cholinergic neurons that express Fos following memory recall display increased intrinsic excitability. Chemogenetic silencing of these learning-activated cholinergic neurons prevents expression of the defensive behavior to the tone. In contrast, we show that NBM/SIp cholinergic neurons are not activated by an innately threatening stimulus (predator odor). Instead, VP/SIa cholinergic neurons are activated and contribute to defensive behaviors in response to predator odor, an innately threatening stimulus. Taken together, we find that distinct populations of cholinergic neurons are recruited to signal distinct aversive stimuli, demonstrating functionally refined organization of specific types of memory within the cholinergic basal forebrain of mice.

Microglia regulate sleep through calcium-dependent modulation of norepinephrine transmission

Sleep interacts reciprocally with immune system activity, but its specific relationship with microglia-the resident immune cells in the brain-remains poorly understood. Here, we show in mice that microglia can regulate sleep through a mechanism involving Gi-coupled GPCRs, intracellular Ca2+ signaling and suppression of norepinephrine transmission. Chemogenetic activation of microglia Gi signaling strongly promoted sleep, whereas pharmacological blockade of Gi-coupled P2Y12 receptors decreased sleep. Two-photon imaging in the cortex showed that P2Y12-Gi activation elevated microglia intracellular Ca2+, and blockade of this Ca2+ elevation largely abolished the Gi-induced sleep increase. Microglia Ca2+ level also increased at natural wake-to-sleep transitions, caused partly by reduced norepinephrine levels. Furthermore, imaging of norepinephrine with its biosensor in the cortex showed that microglia P2Y12-Gi activation significantly reduced norepinephrine levels, partly by increasing the adenosine concentration. These findings indicate that microglia can regulate sleep through reciprocal interactions with norepinephrine transmission.

A hippocampo-cortical pathway detects changes in the validity of an action as a predictor of reward

Much research has been dedicated to understanding the psychological and neural bases of goal-directed action, yet the relationship between context and goal-directed action is not well understood. Here, we used excitotoxic lesions, chemogenetics, and circuit-specific manipulations to demonstrate the role of the ventral hippocampus (vHPC) in contextual learning that supports sensitivity to action-outcome contingencies, a hallmark of goal-directed action. We found that chemogenetic inhibition of the ventral, but not dorsal, hippocampus attenuated sensitivity to instrumental contingency degradation. We then tested the hypothesis that this deficit was due to an inability to discern the relative validity of the action compared with the context as a predictor of reward. Using latent inhibition and Pavlovian context conditioning, we confirm that degradation of action-outcome contingencies relies on intact context-outcome learning and show that this learning is dependent on vHPC. Finally, we show that chemogenetic inhibition of vHPC terminals in the medial prefrontal cortex also impairs both instrumental contingency degradation and context-outcome learning. These results implicate a hippocampo-cortical pathway in adapting to changes in instrumental contingencies and indicate that the psychological basis of this deficit is an inability to learn the predictive value of the context. Our findings contribute to a broader understanding of the neural bases of goal-directed action and its contextual regulation.

Local and Remote Chemogenetic Suppression of Hippocampal Seizures in Rats

BACKGROUND

Innovative treatments of refractory epilepsy are widely desired, for which chemogenetic technology can provide region- and cell-type-specific modulation with relative noninvasiveness.

OBJECTIVES

We aimed to explore the specific applications of chemogenetics for locally and remotely networks controlling hippocampal seizures.

METHODS

A virus coding for a modified human Gi-coupled M4 muscarinic receptor (hM4Di) on pyramidal cells was injected into either the right hippocampal CA3 or the bilateral anterior nucleus of the thalamus (ANT) in rats. After one month, seizures were induced by 4-aminopyridine (4-AP) injection into the right CA3. Simultaneously, clozapine-N-oxide (CNO) (2.5 mg/kg) or clozapine (0.1 mg/kg), the specific ligands acting on hM4Di, were injected intraperitoneally. We also set up hM4Di control and clozapine control groups to eliminate the influence of viral transfection and the ligand alone on the experimental results.

RESULTS

For both local and remote controls, the mean seizure duration was significantly reduced upon ligand application in the experimental groups. Seizure frequency, on the other hand, only showed a significant decrease in local control, with a lower frequency in the clozapine group than in the CNO group. Both the effects of CNO and clozapine were time-dependent, and clozapine was faster than CNO in local seizure control.

CONCLUSION

This study shows the potency of chemogenetics to attenuate hippocampal seizures locally or remotely by activating the transfected hM4Di receptor with CNO or clozapine. ANT is suggested as a potentially safe chemogenetic application target in the epileptic network for focal hippocampal seizures.

Modern Methods for Unraveling Cell- and Circuit-Level Mechanisms of Neurophysiological Biomarkers in Psychiatry

Methods for studying the mammalian brain in vivo have advanced dramatically in the past two decades. State-of-the-art optical and electrophysiological techniques allow direct recordings of the functional dynamics of thousands of neurons across distributed brain circuits with single-cell resolution. With transgenic tools, specific neuron types, pathways, and/or neurotransmitters can be targeted in user-determined brain areas for precise measurement and manipulation. In this chapter, we catalog these advancements. We emphasize that the impact of this methodological revolution on neuropsychiatry remains uncertain. This stems from the fact that these tools remain mostly limited to research in mice. And while translational paradigms are needed, recapitulations of human psychiatric disease states (e.g., schizophrenia) in animal models are inherently challenging to validate and may have limited utility in heterogeneous disease populations. Here we focus on an alternative strategy aimed at the study of neurophysiological biomarkers-the subject of this volume-translated to animal models, where precision neuroscience tools can be applied to provide molecular, cellular, and circuit-level insights and novel therapeutic targets. We summarize several examples of this approach throughout the chapter and emphasize the importance of careful experimental design and choice of dependent measures.

The Diverse Network of Brain Histamine in Feeding: Dissect its Functions in a Circuit-Specific Way

Feeding is an intrinsic and important behavior regulated by complex molecular, cellular and circuit-level mechanisms, one of which is the brain histaminergic network. In the past decades, many studies have provided a foundation of knowledge about the relationship between feeding and histamine receptors, which are deemed to have therapeutic potential but are not successful in treating feeding- related diseases. Indeed, the histaminergic circuits underlying feeding are poorly understood and characterized. This review describes current knowledge of histamine in feeding at the receptor level. Further, we provide insight into putative histamine-involved feeding circuits based on the classic feeding circuits. Understanding the histaminergic network in a circuit-specific way may be therapeutically relevant for increasing the drug specificity and precise treatment in feeding-related diseases.

Imaging-based chemogenetics for dissecting neural circuits in nonhuman primates

Nonhuman primates, particularly macaque and marmoset monkeys, serve as invaluable models for studying complex brain functions and behavior. However, the lack of suitable genetic neuromodulation tools has constrained research at the network level. This review examines the application of a chemogenetic technology, specifically, designer receptors exclusively activated by designer drugs (DREADDs), to nonhuman primates. DREADDs offer a means of reversibly controlling neuronal activity within a specific cell type or neural pathway, effectively targeting multiple brain regions simultaneously. The combination of DREADDs with imaging techniques, such as positron emission tomography and magnetic resonance imaging, has significantly enhanced nonhuman primate research, facilitating the precise visualization and manipulation of specific brain circuits and enabling the detailed monitoring of changes in network activity, which can then be correlated with altered behavior. This review outlines these technological advances and considers their potential for enhancing our understanding of primate brain circuit function and developing novel therapeutic approaches for treating brain diseases.

Distinct roles of monkey OFC-subcortical pathways in adaptive behavior

To be the most successful, primates must adapt to changing environments and optimize their behavior by making the most beneficial choices. At the core of adaptive behavior is the orbitofrontal cortex (OFC) of the brain, which updates choice value through direct experience or knowledge-based inference. Here, we identify distinct neural circuitry underlying these two separate abilities. We designed two behavioral tasks in which macaque monkeys updated the values of certain items, either by directly experiencing changes in stimulus-reward associations, or by inferring the value of unexperienced items based on the task's rules. Chemogenetic silencing of bilateral OFC combined with mathematical model-fitting analysis revealed that monkey OFC is involved in updating item value based on both experience and inference. In vivo imaging of chemogenetic receptors by positron emission tomography allowed us to map projections from the OFC to the rostromedial caudate nucleus (rmCD) and the medial part of the mediodorsal thalamus (MDm). Chemogenetic silencing of the OFC-rmCD pathway impaired experience-based value updating, while silencing the OFC-MDm pathway impaired inference-based value updating. Our results thus demonstrate a dissociable contribution of distinct OFC projections to different behavioral strategies, and provide new insights into the neural basis of value-based adaptive decision-making in primates.

Advanced neurobiological tools to interrogate metabolism

Engineered neurobiological tools for the manipulation of cellular activity, such as chemogenetics and optogenetics, have become a cornerstone of modern neuroscience research. These tools are invaluable for the interrogation of the central control of metabolism as they provide a direct means to establish a causal relationship between brain activity and biological processes at the cellular, tissue and organismal levels. The utility of these methods has grown substantially due to advances in cellular-targeting strategies, alongside improvements in the resolution and potency of such tools. Furthermore, the potential to recapitulate endogenous cellular signalling has been enriched by insights into the molecular signatures and activity dynamics of discrete brain cell types. However, each modulatory tool has a specific set of advantages and limitations; therefore, tool selection and suitability are of paramount importance to optimally interrogate the cellular and circuit-based underpinnings of metabolic outcomes within the organism. Here, we describe the key principles and uses of engineered neurobiological tools. We also highlight inspiring applications and outline critical considerations to be made when using these tools within the field of metabolism research. We contend that the appropriate application of these biotechnological advances will enable the delineation of the central circuitry regulating systemic metabolism with unprecedented potential.

A non-canonical striatopallidal Go pathway that supports motor control

In the classical model of the basal ganglia, direct pathway striatal projection neurons (dSPNs) send projections to the substantia nigra (SNr) and entopeduncular nucleus to regulate motor function. Recent studies have re-established that dSPNs also possess axon collaterals within the globus pallidus (GPe) (bridging collaterals), yet the significance of these collaterals for behavior is unknown. Here we use in vivo optical and chemogenetic tools combined with deep learning approaches in mice to dissect the roles of dSPN GPe collaterals in motor function. We find that dSPNs projecting to the SNr send synchronous motor-related information to the GPe via axon collaterals. Inhibition of native activity in dSPN GPe terminals impairs motor activity and function via regulation of Npas1 neurons. We propose a model by which dSPN GPe axon collaterals (striatopallidal Go pathway) act in concert with the canonical terminals in the SNr to support motor control by inhibiting Npas1 neurons.

Inputs to the locus coeruleus from the periaqueductal gray and rostroventral medulla shape opioid-mediated descending pain modulation

The supraspinal descending pain modulatory system (DPMS) shapes pain perception via monoaminergic modulation of sensory information in the spinal cord. However, the role and synaptic mechanisms of descending noradrenergic signaling remain unclear. Here, we establish that noradrenergic neurons of the locus coeruleus (LC) are essential for supraspinal opioid antinociception. Unexpectedly, given prior emphasis on descending serotonergic pathways, we find that opioid antinociception is primarily driven by excitatory output from the ventrolateral periaqueductal gray (vlPAG) to the LC. Furthermore, we identify a previously unknown opioid-sensitive inhibitory input from the rostroventromedial medulla (RVM), the suppression of which disinhibits LC neurons to drive spinal noradrenergic antinociception. We also report the presence of prominent bifurcating outputs from the vlPAG to the LC and the RVM. Our findings significantly revise current models of the DPMS and establish a novel supraspinal antinociceptive pathway that may contribute to multiple forms of descending pain modulation.

Paraventricular nucleus projections to the nucleus tractus solitarii are essential for full expression of hypoxia-induced peripheral chemoreflex responses

The hypothalamic paraventricular nucleus (PVN) is essential to peripheral chemoreflex neurocircuitry, but the specific efferent pathways utilized are not well defined. The PVN sends dense projections to the nucleus tractus solitarii (nTS), which exhibits neuronal activation following a hypoxic challenge. We hypothesized that nTS-projecting PVN (PVN-nTS) neurons contribute to hypoxia-induced nTS neuronal activation and cardiorespiratory responses. To selectively target PVN-nTS neurons, rats underwent bilateral nTS nanoinjection of retrogradely transported adeno-associated virus (AAV) driving Cre recombinase expression. We then nanoinjected into PVN AAVs driving Cre-dependent expression of Gq or Gi designer receptors exclusively activated by designer drugs (DREADDs) to test the degree that selective activation or inhibition, respectively, of the PVN-nTS pathway affects the hypoxic ventilatory response (HVR) of conscious rats. We used immunohistochemistry for Fos and extracellular recordings to examine how DREADD activation influences PVN-nTS neuronal activation by hypoxia. Pathway activation enhanced the HVR at moderate hypoxic intensities and increased PVN and nTS Fos immunoreactivity in normoxia and hypoxia. In contrast, PVN-nTS inhibition reduced both the HVR and PVN and nTS neuronal activation following hypoxia. To further confirm selective pathway effects on central cardiorespiratory output, rats underwent hypoxia before and after bilateral nTS nanoinjections of C21 to activate or inhibit PVN-nTS terminals. PVN terminal activation within the nTS enhanced tachycardic, sympathetic and phrenic (PhrNA) nerve activity responses to hypoxia whereas inhibition attenuated hypoxia-induced increases in nTS neuronal action potential discharge and PhrNA. The results demonstrate the PVN-nTS pathway enhances nTS neuronal activation and is necessary for full cardiorespiratory responses to hypoxia. KEY POINTS: The hypothalamic paraventricular nucleus (PVN) contributes to peripheral chemoreflex cardiorespiratory responses, but specific PVN efferent pathways are not known. The nucleus tractus solitarii (nTS) is the first integration site of the peripheral chemoreflex, and the nTS receives dense projections from the PVN. Selective GqDREADD activation of the PVN-nTS pathway was shown to enhance ventilatory responses to hypoxia and activation (Fos immunoreactivity (IR)) of nTS neurons in conscious rats, augmenting the sympathetic and phrenic nerve activity (SSNA and PhrNA) responses to hypoxia in anaesthetized rats. Selective GiDREADD inhibition of PVN-nTS neurons attenuates ventilatory responses, nTS neuronal Fos-IR, action potential discharge and PhrNA responses to hypoxia. These results demonstrate that a projection from the PVN to the nTS is critical for full chemoreflex responses to hypoxia.

A head-to-head comparison of two DREADD agonists for suppressing operant behavior in rats via VTA dopamine neuron inhibition

RATIONALE

Designer receptors exclusively activated by designer drugs (DREADDs) are a tool for "remote control" of defined neuronal populations during behavior. These receptors are inert unless bound by an experimenter-administered designer drug, commonly clozapine-n-oxide (CNO). However, questions have emerged about the suitability of CNO as a systemically administered DREADD agonist.

OBJECTIVES

Second-generation agonists such as JHU37160 (J60) have been developed, which may have more favorable properties than CNO. Here we sought to directly compare effects of CNO (0, 1, 5, & 10 mg/kg, i.p.) and J60 (0, 0.03, 0.3, & 3 mg/kg, i.p.) on operant food pursuit.

METHODS

Male and female TH:Cre + rats and their wildtype (WT) littermates received cre-dependent hM4Di-mCherry vector injections into ventral tegmental area (VTA), causing inhibitory DREADD expression in VTA dopamine neurons of TH:Cre + rats. All rats were trained to stably lever press for palatable food on a fixed ratio 10 schedule, and doses of both agonists were tested on separate days in counterbalanced order.

RESULTS

All three CNO doses reduced operant rewards earned in rats with DREADDs, and no CNO dose had behavioral effects in WT controls. The highest J60 dose tested significantly reduced responding in DREADD rats, but this dose also increased responding in WTs, indicating non-specific effects. The magnitude of CNO and J60 effects in TH:Cre + rats were correlated and were present in both sexes.

CONCLUSIONS

Findings demonstrate the usefulness of directly comparing DREADD agonists when optimizing behavioral chemogenetics, and highlight the importance of proper controls, regardless of the DREADD agonist employed.

Activation of Basal Forebrain Astrocytes Induces Wakefulness without Compensatory Changes in Sleep Drive

Mammalian sleep is regulated by a homeostatic process that increases sleep drive and intensity as a function of prior wake time. Sleep homeostasis has traditionally been thought to be a product of neurons, but recent findings demonstrate that this process is also modulated by glial astrocytes. The precise role of astrocytes in the accumulation and discharge of sleep drive is unknown. We investigated this question by selectively activating basal forebrain (BF) astrocytes using designer receptors exclusively activated by designer drugs (DREADDs) in male and female mice. DREADD activation of the Gq-protein-coupled pathway in BF astrocytes produced long and continuous periods of wakefulness that paradoxically did not cause the expected homeostatic response to sleep loss (e.g., increases in sleep time or intensity). Further investigations showed that this was not because of indirect effects of the ligand that activated DREADDs. These findings suggest that the need for sleep is not only driven by wakefulness per se, but also by specific neuronal-glial circuits that are differentially activated in wakefulness.SIGNIFICANCE STATEMENT Sleep drive is controlled by a homeostatic process that increases sleep duration and intensity based on prior time spent awake. Non-neuronal brain cells (e.g., glial astrocytes) influence this homeostatic process, but their precise role is unclear. We used a genetic technique to activate astrocytes in the basal forebrain (BF) of mice, a brain region important for sleep and wake expression and sleep homeostasis. Astroglial activation induced prolonged wakefulness without the expected homeostatic increase in sleep drive (i.e., sleep duration and intensity). These findings indicate that our need to sleep is also driven by non-neuronal cells, and not only by time spent awake.

Olfactory-driven beta band entrainment of limbic circuitry during neonatal development

Cognitive processing relies on the functional refinement of the limbic circuitry during the first two weeks of life. During this developmental period, when the auditory, somatosensory and visual systems are still largely immature, the sense of olfaction acts as 'door to the world', providing an important source of environmental inputs. However, it is unknown whether early olfactory processing shapes the activity in the limbic circuitry during neonatal development. Here, we address this question by combining simultaneous in vivo recordings from the olfactory bulb (OB), lateral entorhinal cortex (LEC), hippocampus (HP) and prefrontal cortex (PFC) with olfactory stimulation as well as opto- and chemogenetic manipulations of mitral/tufted cells in the OB of non-anaesthetized neonatal mice of both sexes. We show that the neonatal OB synchronizes the limbic circuity in the beta frequency range. Moreover, it drives neuronal and network activity in LEC, as well as subsequently, HP and PFC via long-range projections from mitral cells to HP-projecting LEC neurons. Thus, OB activity shapes the communication within limbic circuits during neonatal development. KEY POINTS: During early postnatal development, oscillatory activity in the olfactory bulb synchronizes the limbic circuit. Olfactory stimulation boosts firing and beta synchronization along the olfactory bulb-lateral entorhinal cortex-hippocampal-prefrontal pathway. Mitral cells drive neuronal and network activity in the lateral entorhinal cortex (LEC), as well as subsequently, the hippocampus (HP) and the prefrontal cortex (PFC) via long-range projections from mitral cells to HP-projecting LEC neurons. Inhibition of vesicle release on LEC targeting mitral cell axons reveals direct involvement of LEC in the olfactory bulb-driven oscillatory entrainment of the limbic circuitry.

Subcortico-amygdala pathway processes innate and learned threats

Behavioral flexibility and timely reactions to salient stimuli are essential for survival. The subcortical thalamic-basolateral amygdala (BLA) pathway serves as a shortcut for salient stimuli ensuring rapid processing. Here, we show that BLA neuronal and thalamic axonal activity in mice mirror the defensive behavior evoked by an innate visual threat as well as an auditory learned threat. Importantly, perturbing this pathway compromises defensive responses to both forms of threats, in that animals fail to switch from exploratory to defensive behavior. Despite the shared pathway between the two forms of threat processing, we observed noticeable differences. Blocking β-adrenergic receptors impairs the defensive response to the innate but not the learned threats. This reduced defensive response, surprisingly, is reflected in the suppression of the activity exclusively in the BLA as the thalamic input response remains intact. Our side-by-side examination highlights the similarities and differences between innate and learned threat-processing, thus providing new fundamental insights.

Two Ascending Thermosensory Pathways from the Lateral Parabrachial Nucleus That Mediate Behavioral and Autonomous Thermoregulation

Thermoregulatory behavior in homeothermic animals is an innate behavior to defend body core temperature from environmental thermal challenges in coordination with autonomous thermoregulatory responses. In contrast to the progress in understanding the central mechanisms of autonomous thermoregulation, those of behavioral thermoregulation remain poorly understood. We have previously shown that the lateral parabrachial nucleus (LPB) mediates cutaneous thermosensory afferent signaling for thermoregulation. To understand the thermosensory neural network for behavioral thermoregulation, in the present study, we investigated the roles of ascending thermosensory pathways from the LPB in avoidance behavior from innocuous heat and cold in male rats. Neuronal tracing revealed two segregated groups of LPB neurons projecting to the median preoptic nucleus (MnPO), a thermoregulatory center (LPB→MnPO neurons), and those projecting to the central amygdaloid nucleus (CeA), a limbic emotion center (LPB→CeA neurons). While LPB→MnPO neurons include separate subgroups activated by heat or cold exposure of rats, LPB→CeA neurons were only activated by cold exposure. By selectively inhibiting LPB→MnPO or LPB→CeA neurons using tetanus toxin light chain or chemogenetic or optogenetic techniques, we found that LPB→MnPO transmission mediates heat avoidance, whereas LPB→CeA transmission contributes to cold avoidance. In vivo electrophysiological experiments showed that skin cooling-evoked thermogenesis in brown adipose tissue requires not only LPB→MnPO neurons but also LPB→CeA neurons, providing a novel insight into the central mechanism of autonomous thermoregulation. Our findings reveal an important framework of central thermosensory afferent pathways to coordinate behavioral and autonomous thermoregulation and to generate the emotions of thermal comfort and discomfort that drive thermoregulatory behavior.SIGNIFICANCE STATEMENT Coordination of behavioral and autonomous thermoregulation is important for maintaining thermal homeostasis in homeothermic animals. However, the central mechanism of thermoregulatory behaviors remains poorly understood. We have previously shown that the lateral parabrachial nucleus (LPB) mediates ascending thermosensory signaling that drives thermoregulatory behavior. In this study, we found that one pathway from the LPB to the median preoptic nucleus mediates heat avoidance, whereas the other pathway from the LPB to the central amygdaloid nucleus is required for cold avoidance. Surprisingly, both pathways are required for skin cooling-evoked thermogenesis in brown adipose tissue, an autonomous thermoregulatory response. This study provides a central thermosensory network that coordinates behavioral and autonomous thermoregulation and generates thermal comfort and discomfort that drive thermoregulatory behavior.

The Secondary Somatosensory Cortex Gates Mechanical and Thermal Sensitivity

The cerebral cortex is vital for the perception and processing of sensory stimuli. In the somatosensory axis, information is received by two distinct regions, the primary (S1) and secondary (S2) somatosensory cortices. Top-down circuits stemming from S1 can modulate mechanical and cooling but not heat stimuli such that circuit inhibition causes blunted mechanical and cooling perception. Using optogenetics and chemogenetics, we find that in contrast to S1, an inhibition of S2 output increases mechanical and heat, but not cooling sensitivity. Combining 2-photon anatomical reconstruction with chemogenetic inhibition of specific S2 circuits, we discover that S2 projections to the secondary motor cortex (M2) govern mechanical and thermal sensitivity without affecting motor or cognitive function. This suggests that while S2, like S1, encodes specific sensory information, that S2 operates through quite distinct neural substrates to modulate responsiveness to particular somatosensory stimuli and that somatosensory cortical encoding occurs in a largely parallel fashion.

Divergent projections of the prelimbic cortex mediate autism- and anxiety-like behaviors

The comorbidity of autism spectrum disorder and anxiety is common, but the underlying circuitry is poorly understood. Here, Tmem74-/- mice showed autism- and anxiety-like behaviors along with increased excitability of pyramidal neurons (PNs) in the prelimbic cortex (PL), which were reversed by Tmem74 re-expression and chemogenetic inhibition in PNs of the PL. To determine the underlying circuitry, we performed conditional deletion of Tmem74 in the PNs of PL of mice, and we found that alterations in the PL projections to fast-spiking interneurons (FSIs) in the dorsal striatum (dSTR) (PLPNs-dSTRFSIs) mediated the hyperexcitability of FSIs and autism-like behaviors and that alterations in the PL projections to the PNs of the basolateral amygdaloid nucleus (BLA) (PLPNs-BLAPNs) mediated the hyperexcitability of PNs and anxiety-like behaviors. However, the two populations of PNs in the PL had different spatial locations, optogenetic manipulations revealed that alterations in the activity in the PL-dSTR or PL-BLA circuits led to autism- or anxiety-like behaviors, respectively. Collectively, these findings highlight that the hyperactivity of the two populations of PNs in the PL mediates autism and anxiety comorbidity through the PL-dSTR and PL-BLA circuits, which may lead to the development of new therapeutics for the autism and anxiety comorbidity.