Whole-brain mapping of direct inputs to midbrain dopamine neurons

DRN-SNc serotonergic circuit drives stress-induced motor deficits and Parkinson's disease vulnerability

Stress is a recognized risk factor for Parkinson's disease (PD), but the mechanisms by which stress exacerbates PD symptoms through the serotonergic system are not fully understood. This study investigates the role of serotonergic (5-HT) neurons in the dorsal raphe nucleus (DRN) in mediating stress-induced motor deficits and PD progression. Acute and chronic stress were induced in mice using an elevated platform (EP) and combined with MPTP administration to model early-stage PD. Acute EP stress caused transient motor deficits and significant activation of DRN5-HT neurons projecting to substantia nigra compacta (SNc) dopaminergic (DA) neurons. Manipulating the DRN-SNc pathway with optogenetics and chemogenetics confirmed its critical role in stress-induced motor deficits. Activation of the SNc 5-HT2C receptor with an agonist replicated these deficits, while receptor inhibition prevented them, underscoring its importance. Chronic EP stress worsened MPTP-induced deficits and caused significant SNcDA neurons loss, suggesting it accelerates PD progression. Prolonged chemogenetic inhibition of the DRN-SNc circuit mitigated chronic stress effects in MPTP-treated mice. These findings highlight the crucial role of the DRN-SNc serotonergic circuit and 5-HT2C receptors in stress-related motor deficits, suggesting potential targets for therapies aimed at treating both stress-related motor disorders and Parkinson's disease.

Glycine-gated extrasynaptic NMDARs activated during glutamate spillover drive burst firing in nigral dopamine neurons

Burst firing in substantia nigra pars compacta dopamine neurons is a critical biomarker temporally associated to movement initiation. This phasic change is generated by the tonic activation of NMDARs but the respective role of synaptic versus extrasynaptic NMDARs in the ignition of a burst and what is their level of activation remains unknown. Using ex vivo electrophysiological recordings from adolescent rats, we demonstrate that extrasynaptic NMDARs are the primary driver of burst firing. This pool of receptors is recruited during intense synaptic activity via spillover of glutamate and require the binding of NMDAR co-agonist glycine for full activation. Basal synaptic transmission activating only synaptic NMDARs with the support of D-serine is insufficient to generate a burst. Notably, both synaptic and extrasynaptic NMDARs share the same subunit composition but are regulated by distinct co-agonists. Location of NMDARs and regionalization of co-agonists but not NMDAR subunit composition underly burst generation and may serve as a guideline in understanding the physiological role of dopamine in signaling movement.

A transgenic mouse line for rabies virus-mediated trans-synaptic tracing in the postnatal developing brain

Neural circuits are composed of numerous neurons that perform diverse functions. Understanding the mechanisms of neural processing requires elucidating the connections among individual neurons. Rabies virus (RV)-mediated trans-synaptic tracing enables the visualization of direct presynaptic neurons of a defined neural population, facilitating the precise mapping of neural circuits across various brain regions. This method relies on RV mutants that require the expression of the TVA receptor and rabies glycoprotein to infect and spread to presynaptic neurons. Traditionally, adeno-associated virus (AAV) has been used to express these proteins. However, because AAV requires several weeks to achieve sufficient gene expression, it is challenging to use this approach for studying neural connections during postnatal development. To address this limitation, we generated a transgenic mouse line, termed Ai162-nCTG, which expresses nuclear-localized mCherry, the TVA receptor, and rabies glycoprotein in a Cre-dependent manner. As a proof-of-principle, we crossed the Ai162-nCTG line with the vasopressin-Cre line. In the paraventricular hypothalamic nucleus, where a major cluster of vasopressin neurons exists, mCherry expression was highly specific to vasopressin neurons, although not all vasopressin neurons co-expressed mCherry. We injected RV into the paraventricular hypothalamic nucleus and compared the labeling patterns with those of the conventional AAV-based approach. Although both methods labeled input cells in similar brain regions, the AAV-based approach was superior in terms of labeling efficiency. We also demonstrated that the Ai162-nCTG-based method enables rabies virus-mediated trans-synaptic tracing in mice at postnatal day 7 and 30. The distribution of presynaptic neurons was largely similar in the juvenile and adult stages, suggesting that paraventricular vasopressin neurons do not significantly change their presynaptic inputs during post-weaning development. Taken together, these findings suggest that the Ai162-nCTG line can be used for rabies virus-mediated trans-synaptic tracing when AAV administration is challenging. We also acknowledge and discuss the technical constraints associated with this mouse line.

Electrophysiological characterisation of intranigral-grafted hiPSC-derived dopaminergic neurons in a mouse model of Parkinson's disease

BACKGROUND

Parkinson's disease (PD) is a complex neurological disorder characterized by the progressive degeneration of midbrain dopaminergic (mDA) neurons in the substantia nigra (SN). This degeneration disrupts the basal ganglia loops, leading to both motor and non-motor dysfunctions. Cell therapy for PD aims to replace lost mDA neurons to restore the DA neurotransmission in the denervated forebrain targets. In clinical trials for PD, mDA neurons are implanted into the target area, the striatum, and not in the SN where they are normally located. This ectopic localisation of cells may affect the functionality of transplanted neurons due to the absence of appropriate host afferent regulation. We recently demonstrated that human induced pluripotent stem cells (hiPSCs) derived mDA progenitors grafted into the substantia nigra pars compacta (SNpc) in a mouse model of PD, differentiated into mature mDA neurons, restored the degenerated nigrostriatal pathway, and induced motor recovery. The objective of the present study was to evaluate the long-term functionality of these intranigral-grafted mDA neurons by assessing their electrophysiological properties.

METHODS

We performed intranigral transplantation of hiPSC-derived mDA progenitors in a 6-hydroxydopamine RAG2-KO mouse model of PD. We recorded in vivo unit extracellular activity of grafted mDA neurons in anesthetised mice from 9 to 12 months post-transplantation. Their electrophysiological properties, including firing rates, patterns and spike characteristics, were analysed and compared with those of native nigral dopaminergic neurons from control mice.

RESULTS

We demonstrated that these grafted mDA neurons exhibited functional characteristics similar to those of native nigral dopaminergic neurons, such as large bi- or triphasic spike waveforms, low firing rates, pacemaker-like properties, and two single-spike firing patterns. Although grafted mDA neurons also displayed low discharge frequencies below 10 Hz, their mean frequency was significantly lower than that of nigral mDA neurons, with a differential pattern distribution.

CONCLUSIONS

Our findings indicate that grafted mDA neurons exhibit dopaminergic-like functional properties, including intrinsic membrane potential oscillations leading to regular firing patterns. Additionally, they demonstrated irregular and burst firing patterns, suggesting they receive modulatory inputs. However, grafted mDA neurons displayed distinct properties, potentially related to their human origin or the incomplete maturation one year after transplantation.

Projections from ventral hippocampus to nucleus accumbens' cholinergic neurons are altered in depression

The cholinergic interneurons (ChIs) of the nucleus accumbens (NAc) have a critical role in the activity of this region, specifically in the context of major depressive disorder. To understand the circuitry regulating this behavior, we sought to determine the areas that directly project to these interneurons by utilizing the monosynaptic cell-specific tracing technique. Mapping showed monosynaptic projections that are exclusive to NAc ChIs. To determine if some of these projections are altered in a depression mouse model, we used mice that do not express the calcium-binding protein p11 specifically in ChIs (ChAT-p11 cKO) and display a depressive-like phenotype. Our data demonstrated that while the overall projection areas remain similar between wild type and ChAT-p11 cKO mice, the number of projections from the ventral hippocampus (vHIP) is significantly reduced in the ChAT-p11 cKO mice. Furthermore, using optogenetics and electrophysiology we showed that glutamatergic projections from vHIP to NAc ChIs are severely altered in mutant mice. These results show that specific alterations in the circuitry of the accumbal ChIs could play an important role in the regulation of depressive-like behavior, reward-seeking behavior in addictions, or psychiatric symptoms in neurodegenerative diseases.

Volume electron microscopy for genetically and molecularly defined neural circuits

The brain networks responsible for adaptive behavioral changes are based on the physical connections between neurons. Light and electron microscopy have long been used to study neural projections and the physical connections between neurons. Volume electron microscopy has recently expanded its scale of analysis due to methodological advances, resulting in complete wiring maps of neurites in a large volume of brain tissues and even entire nervous systems in a growing number of species. However, structural approaches frequently suffer from inherent limitations in which elements in images are identified solely by morphological criteria. Recently, an increasing number of tools and technologies have been developed to characterize cells and cellular components in the context of molecules and gene expression. These advancements include newly developed probes for visualization in electron microscopic images as well as correlative integration methods for the same elements across multiple microscopic modalities. Such approaches advance our understanding of interactions between specific neurons and circuits and may help to elucidate novel aspects of the basal ganglia network involving dopamine neurons. These advancements are expected to reveal mechanisms for processing adaptive changes in specific neural circuits that modulate brain functions.

Multidimensional mechanisms of anxiety and depression in Parkinson's disease: Integrating neuroimaging, neurocircuits, and molecular pathways

Anxiety and depression are common non-motor symptoms of Parkinson's disease (PD) that significantly affect patients' quality of life. In recent years, our understanding of PD has advanced through multifaceted studies on the pathological mechanisms associated with anxiety and depression in PD. These classic psychiatric symptoms involve complex pathophysiology, with both distinct features and connections to the mechanisms underlying the aetiology of PD. Furthermore, the co-occurrence of anxiety and depression in PD blurs the boundaries between them. Therefore, a comprehensive summary of the pathogenic mechanisms associated with anxiety and depression will aid in better addressing the emergence of these classic psychiatric symptoms in PD. This article integrates neuroanatomical, neural projection, neurotransmitter, neuroinflammatory, brain-gut axis, neurotrophic, hypothalamic-pituitary-adrenal axis, and genetic perspectives to provide a comprehensive description of the core pathological alterations underlying anxiety and depression in PD, aiming to provide an up-to-date perspective and broader therapeutic prospects for PD patients suffering from anxiety or depression.

Repeated activation of preoptic area recipient neurons in posterior paraventricular nucleus mediates chronic heat-induced negative emotional valence and hyperarousal states

Mental and behavioral disorders are associated with extended period of hot weather as found in heatwaves, but the underlying neural circuit mechanism remains poorly known. The posterior paraventricular thalamus (pPVT) is a hub for emotional processing and receives inputs from the hypothalamic preoptic area (POA), the well-recognized thermoregulation center. The present study was designed to explore whether chronic heat exposure leads to aberrant activities in POA recipient pPVT neurons and subsequent changes in emotional states. By devising an air heating paradigm mimicking the condition of heatwaves and utilizing emotion-related behavioral tests, viral tract tracing, in vivo calcium recordings, optogenetic manipulations, and electrophysiological recordings, we found that chronic heat exposure for 3 weeks led to negative emotional valence and hyperarousal states in mice. The pPVT neurons receive monosynaptic excitatory and inhibitory innervations from the POA. These neurons exhibited a persistent increase in neural activity following chronic heat exposure, which was essential for chronic heat-induced emotional changes. Notably, these neurons were also prone to display stronger neuronal activities associated with anxiety responses to stressful situations. Furthermore, we observed saturated neuroplasticity in the POA-pPVT excitatory pathway after chronic heat exposure that occluded further potentiation. Taken together, long-term aberration in the POA to pPVT pathway offers a neurobiological mechanism of emotional and behavioral changes seen in extended periods of hot weather like heatwaves.

Identification of a stress-sensitive endogenous opioid-containing neuronal population in the paranigral ventral tegmental area

Nociceptin/orphanin FQ (N/OFQ), an endogenous opioid neuropeptide, and its G-protein coupled receptor NOPR have been implicated in motivation, feeding behaviors, and aversion. Stress-induced dysfunction in these states is central to the development of numerous psychiatric disorders, and the N/OFQ-NOPR system's role in reward- and stress-related responses has driven broad interest in NOPR as a therapeutic target for anxiety and depression. However, the impact of stress on N/OFQ signaling in the context of its influence on discrete midbrain reward circuitry remains unknown. To this end, we focused on a possible candidate population of N/OFQ neurons in the paranigral ventral tegmental area (pnVTA PNOC ) that have been shown to act locally on NOPR-containing VTA dopamine neurons to suppress motivation. Here we report and characterize pnVTA PNOC sensitivity to stress exposure and identify a functional excitatory and inhibitory afferent input to this subpopulation from the lateral hypothalamus (LH). Our results indicate that pnVTA PNOC neurons become recruited during exposure to a range of acute stressor types, whereas the GABAergic input from the LH to this population is suppressed by predator odor stress, providing a mechanism for disinhibition of these neurons. These findings suggest that this N/OFQ population in the pnVTA could act as a critical bridge between stress and motivation through interactions with upstream hypothalamic circuitry.

Anatomical mapping of whole-brain monosynaptic inputs to the orbitofrontal cortex

The orbitofrontal cortex (ORB) exhibits a complex structure and diverse functional roles, including emotion regulation, decision-making, and reward processing. Structurally, it comprises three distinct regions: the medial part (ORBm), the ventrolateral part (ORBvl), and the lateral part (ORBl), each with unique functional attributes, such as ORBm's involvement in reward processing, ORBvl's regulation of depression-like behavior, and ORBl's response to aversive stimuli. Dysregulation of the ORB has been implicated in various psychiatric disorders. However, the neurocircuitry underlying the functions and dysfunctions of the ORB remains poorly understood. This study employed recombinant adeno-associated viruses (rAAV) and rabies viruses with glycoprotein deletion (RV-ΔG) to retrogradely trace monosynaptic inputs to three ORB subregions in male C57BL/6J mice. Inputs were quantified across the whole brain using fluorescence imaging and statistical analysis. Results revealed distinct input patterns for each ORB subregion, with significant contributions from the isocortex and thalamus. The ORBm received prominent inputs from the prelimbic area, agranular insular area, and hippocampal field CA1, while the ORBvl received substantial intra-ORB inputs. The ORBl exhibited strong inputs from the somatomotor and somatosensory areas. Thalamic inputs, particularly from the mediodorsal nucleus and submedial nucleus of the thalamus, were widespread across all ORB subregions. These findings provide novel insights into the functional connectivity of ORB subregions and their roles in neural circuit mechanisms underlying behavior and psychiatric disorders.

The Futile Creatine Cycle powers UCP1-independent thermogenesis in classical BAT

Classical brown adipose tissue (BAT) is traditionally viewed as relying exclusively on uncoupling protein 1 (UCP1) for thermogenesis via inducible proton leak. However, the physiological significance of UCP1-independent mechanisms linking substrate oxidation to ATP turnover in classical BAT has remained unclear. Here, we identify the Futile Creatine Cycle (FCC), a mitochondrial-localized energy-wasting pathway involving creatine phosphorylation by creatine kinase b (CKB) and phosphocreatine hydrolysis by tissue-nonspecific alkaline phosphatase (TNAP), as a key UCP1-independent thermogenic mechanism in classical BAT. Reintroducing mitochondrial-targeted CKB exclusively into interscapular brown adipocytes in vivo restores thermogenesis and cold tolerance in mice lacking native UCP1 and CKB, in a TNAP-dependent manner. Furthermore, mice with inducible adipocyte-specific co-deletion of TNAP and UCP1 exhibit severe cold-intolerance. These findings challenge the view that BAT thermogenesis depends solely on UCP1 because of insufficient ATP synthase activity and establishes the FCC as a physiologically relevant thermogenic pathway in classical BAT.

Superior Colliculus Controls the Activity of the Substantia Nigra Pars Compacta and Ventral Tegmental Area in an Asymmetrical Manner

Dopaminergic (DA) neurons of the substantia nigra pars compacta (SNc) and ventral tegmental area (VTA) play a crucial role in controlling animals' orienting and approach behaviors toward relevant environmental stimuli. The ventral midbrain receives sensory input from the superior colliculus (SC), a tectal region that processes information from contralateral receptive fields of various modalities. Given the significant influence of dopamine release imbalance in the left and right striatum on animals' movement direction, our study aimed to investigate the lateralization of the connection between the lateral SC and the midbrain DA system in male rats. We explored the circuit's anatomy using transsynaptic viral tract-tracing and its physiology using in vivo single-unit and ex vivo multi-electrode array recordings of SNc and VTA neuronal activity combined with optogenetic stimulation of either the ipsilateral or contralateral SC or its terminals. During the experiments, DA neurons were identified optogenetically (in vivo recordings) or pharmacologically (ex vivo recordings). Anatomical findings revealed a bilateral innervation pattern of the lateral SC to the ventral midbrain, with a significantly stronger ipsilateral connection, particularly evident in the SNc, involving both DA and non-DA neurons. This anatomical asymmetry was also expressed during in vivo and ex vivo recordings, which showed a predominance of ipsilateral connections, especially within the SNc. Ex vivo recordings also confirmed that this lateralized pathway is direct. The described features of the SC→VTA/SNc neuronal circuit, particularly its anatomical and physiological asymmetry, suggest its involvement in orienting and approach behaviors guided by the direction of incoming sensory stimuli.

Dopamine in the tail of the striatum facilitates avoidance in threat-reward conflicts

Responding appropriately to potential threats before they materialize is critical to avoiding disastrous outcomes. Here we examine how threat-coping behavior is regulated by the tail of the striatum (TS) and its dopamine input. Mice were presented with a potential threat (a moving object) while pursuing rewards. Initially, the mice failed to obtain rewards but gradually improved in later trials. We found that dopamine in TS promoted avoidance of the threat, even at the expense of reward acquisition. Furthermore, the activity of dopamine D1 receptor-expressing neurons promoted threat avoidance and prediction. In contrast, D2 neurons suppressed threat avoidance and facilitated overcoming the potential threat. Dopamine axon activation in TS not only potentiated the responses of dopamine D1 receptor-expressing neurons to novel sensory stimuli but also boosted them acutely. These results demonstrate that an opponent interaction of D1 and D2 neurons in the TS, modulated by dopamine, dynamically regulates avoidance and overcoming potential threats.

Dorsal column pathway is involved in tactile reward-induced affective 50-kHz ultrasonic vocalizations in rats

Rhythmic stroking induces positive emotions in rats via the mesolimbic dopamine system. However, the ascending pathways underlying the affective 50-kHz ultrasonic vocalizations (USVs) induced by somatosensory stimulation remain unknown. The dorsal column consists of ascending spinal tracts that convey innocuous tactile information from the spinal cord to the brain. Here, we investigated whether the somatosensory signals transmitted through the dorsal column pathway contribute to the induction of positive 50-kHz USVs during rhythmic stroking. The 50-kHz USVs, behavior, approach latency, and mechanical tactile thresholds of animals with dorsal column lesions (DCL) at the upper thoracic level were compared with those in sham-operated animals. The DCL significantly reduced the number of 50-kHz USVs, harmonics, and split calls during rhythmic stroking, and the number of hedonic frequency-modulated calls (trill, complex, and step up calls) after rhythmic stroking. The DCL significantly increased the approach latency compared to presurgical controls. Sham-operated rats demonstrated a significant increase in the number of 50-kHz USVs and shortened approach latency compared with presurgical control values. Our results suggest that the somatosensory input conveyed by the dorsal column triggers the affective 50-kHz USVs during rhythmic stroking and approach behaviors. These findings contribute to revealing the neural circuits underlying somatosensory-emotional integration.

Leptin activates dopamine and GABA neurons in the substantia nigra via a local pars compacta-pars reticulata circuit

Adipose-derived leptin contributes to energy homeostasis by balancing food intake and motor output, but how leptin acts in brain motor centers remains poorly understood. We investigated the influence of leptin on neuronal activity in two basal ganglia nuclei involved in motor control: the substantia nigra pars compacta (SNc) and pars reticulata (SNr). Using a mouse reporter line to identify cells expressing leptin receptors (LepRs), we found that in both sexes, a majority of SNc dopamine neurons express a high level of LepR. Whole-cell recording in ex vivo midbrain slices from male wild-type mice showed that leptin activates SNc dopamine neurons directly and increases somatodendritic dopamine release. Although LepR expression in SNr GABA output neurons was low, leptin also activated these cells. Additional experiments showed that the influence of leptin on SNr neurons is indirect and involves D1 dopamine receptors and TRPC3 channels. Administration of leptin to male mice increased locomotor activity, consistent with activation of dopamine neurons in the SNc coupled to previously reported amplification of axonal dopamine release by leptin in striatal slices. These findings indicate that in addition to managing energy homeostasis through its actions as a satiety hormone, leptin also promotes axonal and somatodendritic dopamine release that can influence motor output.Significance statement Dopamine neurons regulate motivated behaviors, but how they are influenced by metabolic hormones, like leptin, is incompletely understood. We show here that leptin increases the activity of substantia nigra (SN) pars compacta dopamine neurons directly, and that this enhances somatodendritic dopamine release. Leptin also increases the activity of GABAergic neurons in the SN pars reticulata, but does so indirectly via D1 dopamine receptors activated by locally released dopamine. Consistent with increased nigral dopamine neuron activity and previous evidence showing that leptin amplifies striatal dopamine release, systemic leptin increases locomotor behavior. This increase in motor activity complements the well-established inhibitory effect of leptin on food intake and adds an additional dimension to the regulation of energy balance by this hormone.

Molecularly distinct striatonigral neuron subtypes differentially regulate locomotion

Striatonigral neurons, traditionally known for promoting locomotion, comprise diverse subtypes with distinct transcriptomic profiles. However, their specific contributions to locomotor regulation remain incompletely understood. Using the genetic markers Kremen1 and Calb1, we demonstrate in mouse models that Kremen1+ and Calb1+ striatonigral neurons exerted opposing effects on locomotion. Kremen1+ neurons displayed delayed activation at locomotion onset but exhibited increasing activity during locomotion offset. In contrast, Calb1+ neurons showed early activation at locomotion onset and decreasing activity during locomotion offset. Optogenetic activation of Kremen1+ neurons suppressed ongoing locomotion, whereas activation of Calb1+ neurons promoted locomotion. Activation of Kremen1+ neurons induced a greater reduction in dopamine release than Calb1+ neurons, followed by a post-stimulation rebound. Conversely, activation of Calb1+ neurons triggered an initial increase in dopamine release. Furthermore, genetic knockdown of GABA-B receptor Gabbr1 in Aldh1a1+ nigrostriatal dopaminergic neurons (DANs) reduced DAN inhibition and completely abolished the locomotion-suppressing effect of Kremen1+ neurons. Together, these findings reveal a cell type-specific mechanism within striatonigral neuron subtypes: Calb1+ neurons promote locomotion, while Kremen1+ neurons terminate ongoing movement by inhibiting Aldh1a1+ DAN activity via GABBR1 receptors.

Cross-species dissection of the modular role of the ventral tegmental area in depressive disorders

Depressive disorders, including major depressive disorder (MDD), represent one of the most prevalent set of disorders worldwide. MDD is characterized by a range of cognitive, behavioral, and neurobiological changes that contribute to the vast array of symptom profiles that make this disorder particularly difficult to treat. A multitude of established evidence suggests a role for the dopamine system, stemming in part from the ventral tegmental area (VTA), in mediating symptoms and behavioral changes that underlie depression. Developments in cutting-edge technologies in pre-clinical models of depressive phenotypes, such as retrograde tracing, electrophysiological recordings, immunohistochemistry, and molecular profiling, have allowed a deeper characterization of singular VTA neuron molecular, physiological, and projection properties. These developments have highlighted that the VTA is not a homogenous cell population but instead comprises vast cellular diversity that underscores its modular role across various functions related to reward processing, aversion, salience processing, learning and motivation. In this review, we begin by introducing the various cell types and brain regions that comprise the VTA circuitry. Then, we introduce the role of the VTA in reward processing as it compares to aversion processing. Next, we characterize distinct neural pathways within the VTA circuitry to understand the effects of chronic social and non-social stress and tie together how these neurobiological changes manifest into specific behavioral phenotypes. Finally, we relate these preclinical findings to clinical findings to parse the heterogeneity of depressive phenotypes and explain the efficacy of recent novel pharmacological interventions that may target the VTA in MDD.

VTA dopaminergic neurons involved in chronic spared nerve injury pain-induced depressive-like behavior

Affective disorders, such as depression, are commonly associated with the development of chronic pain, but the underlying mechanisms still remain unclear. The dopaminergic system, located in the midbrain, is considered one of the regions where algesia and emotional processing overlap. This suggests a structural basis hypothesis for the comorbidity of chronic pain and depression, highlighting the interplay between nociceptive and affective processing. But there are more and more evidences show that somatic and head/facial pain involve different neuronal overlap. In previous study, the research show that VTA dopaminergic system involved in pIONT surgery induced depressive-like behaviors in mice. But there still no evidence shows if chronic somatic pain will induce depressive-like behaviors and which neuronal circle pathway is underly. In this study, we assessed depressive-like behaviors and performed artificial interference of VTA (ventral tegmental area) dopaminergic neurons in a mouse model of chronic peripheral neuropathic pain induced by the spared nerve injury (SNI) model. After a 4-week duration of hyperalgesia and allodynia resulting from SNI surgery, social withdraw and other depressive-like behaviors were observed in the SNI group. Furthermore, the dopaminergic cells' excitability in VTA were significantly increased in SNI mice. The excitability alteration was improved play a key role in the development and modulation of the chronic peripheral neuropathic pain-induced depressive-like behaviors. It has been shown pain and affections have structural and functional circuits to interact with each other, therefore the neuroplastic changes and functional role of VTA dopaminergic neurons within these circuits may serve as potential targets for understanding and therapeutically addressing the development of depressive-like symptoms accompanied by prolonged pain syndromes in humans.

Segregated input to thalamic areas that project differently to core and shell auditory cortical fields

Perception of the environment is multimodal in nature, with sensory systems intricately interconnected. The ability to integrate multimodal sensations while preserving the distinct characteristics of each sensory modality is crucial, and the underlying mechanisms of the organization that facilitate this process require further elucidation. In the auditory system, although the concept of core and shell pathways is well established, the brain-wide input/output relationships of thalamic regions projecting to auditory-responsive cortical areas remain insufficiently studied, particularly in relation to non-auditory structures. In this study, we utilized functional imaging and viral tracing techniques to map the brain-wide connections of core and shell pathways. We identified three distinct shell pathways, in addition to a core pathway, each exhibiting unique associations with non-auditory structures involved in behavior, emotion, and other functions. This architecture suggests that these pathways contribute differentially to various aspects of multimodal sensory integration.

A brain-wide map of descending inputs onto spinal V1 interneurons

Motor output results from the coordinated activity of neural circuits distributed across multiple brain regions that convey information to the spinal cord via descending motor pathways. Yet the organizational logic through which supraspinal systems target discrete components of spinal motor circuits remains unclear. Here, using viral transsynaptic tracing along with serial two-photon tomography, we have generated a whole-brain map of monosynaptic inputs to spinal V1 interneurons, a major inhibitory population involved in motor control. We identified 26 distinct brain structures that directly innervate V1 interneurons, spanning medullary and pontine regions in the hindbrain as well as cortical, midbrain, cerebellar, and neuromodulatory systems. Moreover, we identified broad but biased input from supraspinal systems onto V1Foxp2 and V1Pou6f2 neuronal subsets. Collectively, these studies reveal elements of biased connectivity and convergence in descending inputs to molecularly distinct interneuron subsets and provide an anatomical foundation for understanding how supraspinal systems influence spinal motor circuits.

Neuronal stress-coping mechanisms in postpartum females

Animals exhibit a wide range of stress responses aimed at restoring homeostasis and promoting adaptation. In response to stress, they employ coping mechanisms to maintain physiological balance. Dysregulated stress-coping strategies have been associated with mental disorders, including depression, anxiety, and post-traumatic stress disorder. Understanding the neuronal mechanisms that regulate stress-coping is critical for elucidating normal physiological responses and addressing the pathological processes underlying these disorders. Stress responses are influenced by sex and life stage, with notable variability in the prevalence and severity of mental disorders based on these factors. Stress-coping mechanisms are pivotal in determining the vulnerability or resilience of an individual to stress. Thus, identifying differences in stress-coping strategies between sexes and across life stages is essential for advancing prevention and treatment strategies for stress-related mental disorders. This review explores the neuronal mechanisms underlying stress responses, emphasizing the distinct stress-coping strategies utilized by postpartum females. Highlighting these differences underscores the need for targeted prevention and treatment approaches that consider sex- and life stage-specific variations in stress-coping mechanisms.

Interactions of sex and stress in modulation of ventral tegmental area dopaminergic activity

Dopaminergic (DA) neurons of the ventral tegmental area (VTA) have long been studied for their role in reward prediction and goal-directed behaviors. However, appreciation is growing for a complementary role of VTA DA neurons in responding to aversive stimuli and as critical substrates for behavioral sequelae of stressful experiences. As is the case across neuroscience, the majority of our knowledge about VTA DA neurons comes from studies in male subjects. Recent years have seen an increase in inclusion of female subjects and exploration of sex differences. There is now an emerging body of literature showing that although there are minimal basal structural and functional sex differences in VTA DA neurons, experience-dependent changes in these neurons can differ significantly between males and females. Here, we discuss potential implications of sex differences in VTA function and review recent data on sex differences and similarities of DA neurons at baseline and following stress.

How Dopamine Enables Learning from Aversion

Dopamine is heavily studied for its role in reward learning, but it is becoming increasingly appreciated that dopamine can also enable learning from aversion. Dopamine neurons modulate their firing and neurotransmitter release patterns in response to aversive outcomes. However, there is considerable heterogeneity in the timing and directionality of the modulation. Open questions remain as to the factors that determine this heterogeneity and how varying patterns of responses to aversion in different dopamine-receptive brain regions contribute to value learning, decision-making, and avoidance. Here, we review recent progress in this area and highlight important future directions.

Expanding the brain's terrain for reward

A previously unknown region in the brainstem controls dopamine activity.

Dissection of the long-range circuit of the mouse intermediate retrosplenial cortex

The retrosplenial cortex (RSP) is a complex brain region with multiple interconnected subregions that plays crucial roles in various cognitive functions, including memory, spatial navigation, and emotion. Understanding the afferent and efferent connectivity of the RSP is essential for comprehending the underlying mechanisms of its functions. Here, via viral tracing and fluorescence micro-optical sectioning tomography (fMOST), we systematically investigated the anatomical organisation of the upstream and downstream circuits of glutamatergic and GABAergic neurons in the dorsal and ventral RSP. The cortical connections of the RSP show laminar organisation in which the input neurons are distributed more in the deeper layers of the upstream cortex. Although different types of neurons have similar upstream circuits, GABAergic neurons show bidirectional connections with the hippocampus, whereas glutamatergic neurons only show unidirectional connections. Moreover, GABAergic neurons receive more inputs from the primary sensory cortex than from the prefrontal cortex and association cortex. The dorsal and ventral subregions have preferred circuits such that the dorsal RSP exhibits spatially topological connections with the dorsal visual cortex and lateral thalamus. The systematic study on long-range connections across RSP subregions and cell types may provide useful information for future revealing of RSP working mechanisms.

The parasubthalamic nucleus: A novel eating center in the brain

Eating behavior stands as a fundamental determinant of animal survival and growth, intricately regulated by an amalgamation of internal and external stimuli. Coordinated movements of facial muscles and the mandible orchestrate prey capture and food processing, propelled by the allure of taste and rewarding food properties. Conversely, satiation, pain, aversion, negative emotion or perceived threats can precipitate the cessation or avoidance of eating activities. In recent years, the parasubthalamic nucleus (PSTN), located in the lateral hypothalamic area, has emerged as a focal point in feeding research. PSTN neurons assume pivotal roles within multiple feeding circuits, bridging central feeding centers with peripheral organs. They intricately modulate regulation of oral sensorimotor functions, hedonic feeding, appetite motivation and the processing of satiation and aversive signals, thereby orchestrating the initiation or termination of feeding behaviors. This review delves into the distinctive neuronal subpopulations within the PSTN and their associated neural networks, aiming to refine our comprehension of the neural underpinnings of feeding while also seeking to unearth more efficacious therapeutic avenues for feeding and eating disorders.

Dopaminergic signaling to ventral striatum neurons initiates sniffing behavior

Sniffing is a motivated behavior displayed by nearly all terrestrial vertebrates. While sniffing is associated with acquiring and processing odors, sniffing is also intertwined with affective and motivated states. The systems which influence the display of sniffing are unclear. Here, we report that dopamine release into the ventral striatum in mice is coupled with bouts of sniffing and that stimulation of dopaminergic terminals in these regions drives increases in respiratory rate to initiate sniffing whereas inhibition of these terminals reduces respiratory rate. Both the firing of individual neurons and the activity of post-synaptic D1 and D2 dopamine receptor-expressing neurons are coupled with sniffing and local antagonism of D1 and D2 receptors squelches sniffing. Together, these results support a model whereby sniffing can be initiated by dopamine's actions upon ventral striatum neurons. The nature of sniffing being integral to both olfaction and motivated behaviors implicates this circuit in a wide array of functions.

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.

A Vulnerable Subtype of Dopaminergic Neurons Drives Early Motor Deficits in Parkinson's Disease

In Parkinson's disease, dopaminergic neurons (DANs) in the midbrain gradually degenerate, with ventral substantia nigra pars compacta (SNc) DANs exhibiting greater vulnerability. However, it remains unclear whether specific molecular subtypes of ventral SNc DANs are more susceptible to degeneration in PD, and if they contribute to the early motor symptoms associated with the disease. We identified a subtype of Sox6+ DANs, Anxa1+, which are selectively lost earlier than other DANs, and with a time course that aligns with the development of motor symptoms in MitoPark mice. We generated a knock-in Cre mouse line for Anxa1+ DANs and showed differential anatomical inputs and outputs of this population. Further, we found that the inhibition of transmitter release in Anxa1+ neurons led to bradykinesia and tremor. This study uncovers the existence of a selectively vulnerable subtype of DANs that is sufficient to drive early motor symptoms in Parkinson's disease.

Melanin-concentrating hormone attenuates the hedonic feeding induced by orexin-A in the ventral tegmental area of high-fat diet male mice

Objective

The ventral tegmental area (VTA), a pivotal hub in the brain's reward circuitry, receives inputs from the lateral hypothalamic area (LHA). However, it remains unclear whether melanin-concentrating hormone (MCH) and orexin-A (OX-A) neurons in the LHA exert individual or cooperative influence on palatable food consumption in the VTA. This study aims to investigate the modulatory role of MCH and OX-A in hedonic feeding within the VTA of high-fat diet (HFD) mice.

Methods

Male mice were subjected to an 8-week high-fat diet. To visualize the projections from the LHA to VTA, we employed fluorescent gold retrograde tracing combined with immunofluorescence staining. Immunofluorescence staining or enzyme-linked immunosorbent assay was used to detect the activity of the VTA neurons, expression of OX-A or MCH in the LHA, as well as the activity of their receptors (OXR1 and MCHR1) in the VTA following a sucrose preference test. Single-unit extracellular electrical discharge recordings were conducted to assess the effects of OX-A and MCH on VTA neurons in HFD mice. Additionally, chemogenetic inhibition of MCH neurons and immunofluorescence staining were utilized to observe the regulatory roles of MCH in changes of hedonic feeding induced by OX-A in HFD mice.

Results

Sucrose intake resulted in lower activation of VTA neurons in the HFD mice compared to CON mice, while OX-Aergic and MCHergic neurons project from the LHA to the VTA. Although sucrose intake increased the expression of OX-A and MCH in HFD mice, it led to diminished activation of OXR1-positive and MCHR1-positive VTA neurons compared to CON mice. Extracellular single-unit recording revealed that MCH significantly suppressed the firing rate of OX-A-responsive neurons in the VTA. MCH attenuated the hedonic feeding response induced by OX-A in HFD mice, and administration of MCHR1 antagonist (SNAP94847) significantly potentiated the effect of OX-A. Chemogenetic inhibition of MCH neurons improved the activity of OXR1-expressing neurons, which could be reversed by pretreatment with an OXR1 antagonist (SB334867). Furthermore, chemogenetic inhibition of MCH enhanced hedonic feeding behavior, which was counteracted by SB334867 treatment in HFD mice.

Conclusion

Melanin-concentrating hormone could attenuate the hedonic feeding behavior induced by orexin-A in the VTA of HFD mice.

Transcriptomic cell-type specificity of local cortical circuits

Complex neocortical functions rely on networks of diverse excitatory and inhibitory neurons. While local connectivity rules between major neuronal subclasses have been established, the specificity of connections at the level of transcriptomic subtypes remains unclear. We introduce single transcriptome assisted rabies tracing (START), a method combining monosynaptic rabies tracing and single-nuclei RNA sequencing to identify transcriptomic cell types, providing inputs to defined neuron populations. We employ START to transcriptomically characterize inhibitory neurons providing monosynaptic input to 5 different layer-specific excitatory cortical neuron populations in mouse primary visual cortex (V1). At the subclass level, we observe results consistent with findings from prior studies that resolve neuronal subclasses using antibody staining, transgenic mouse lines, and morphological reconstruction. With improved neuronal subtype granularity achieved with START, we demonstrate transcriptomic subtype specificity of inhibitory inputs to various excitatory neuron subclasses. These results establish local connectivity rules at the resolution of transcriptomic inhibitory cell types.

Alcohol, flexible behavior, and the prefrontal cortex: Functional changes underlying impaired cognitive flexibility

Cognitive flexibility enables individuals to alter their behavior in response to changing environmental demands, facilitating optimal behavior in a dynamic world. The inability to do this, called behavioral inflexibility, is a pervasive behavioral phenotype in alcohol use disorder (AUD), driven by disruptions in cognitive flexibility. Research has repeatedly shown that behavioral inflexibility not only results from alcohol exposure across species but can itself be predictive of future drinking. Like many high-level executive functions, flexible behavior requires healthy functioning of the prefrontal cortex (PFC). The scope of this review addresses two primary themes: first, we outline tasks that have been used to investigate flexibility in the context of AUD or AUD models. We characterize these based on the task features and underlying cognitive processes that differentiate them from one another. We highlight the neural basis of flexibility measures, focusing on the PFC, and how acute or chronic alcohol in humans and non-human animal models impacts flexibility. Second, we consolidate findings on the molecular, physiological and functional changes in the PFC elicited by alcohol, that may contribute to cognitive flexibility deficits seen in AUD. Collectively, this approach identifies several key avenues for future research that will facilitate effective treatments to promote flexible behavior in the context of AUD, to reduce the risk of alcohol related harm, and to improve outcomes following AUD. This article is part of the Special Issue on "PFC circuit function in psychiatric disease and relevant models".

Developmental and Adult Striatal Patterning of Nociceptin Ligand Marks Striosomal Population With Direct Dopamine Projections

Circuit influences on the midbrain dopamine system are crucial to adaptive behavior and cognition. Recent developments in the study of neuropeptide systems have enabled high-resolution investigations of the intersection of neuromodulatory signals with basal ganglia circuitry, identifying the nociceptin/orphanin FQ (N/OFQ) endogenous opioid peptide system as a prospective regulator of striatal dopamine signaling. Using a prepronociceptin-Cre reporter mouse line, we characterized highly selective striosomal patterning of Pnoc mRNA expression in mouse dorsal striatum, reflecting the early developmental expression of Pnoc. In the ventral striatum, Pnoc expression in the nucleus accumbens core was grouped in clusters akin to the distribution found in striosomes. We found that PnoctdTomato reporter cells largely comprise a population of dopamine receptor D1 (Drd1) expressing medium spiny projection neurons localized in dorsal striosomes, known to be unique among striatal projection neurons for their direct innervation of midbrain dopamine neurons. These findings provide a new understanding of the intersection of the N/OFQ system among basal ganglia circuits with particular implications for developmental regulation or wiring of striato-nigral circuits.

Pre-existing visual responses in a projection-defined dopamine population explain individual learning trajectories

A key challenge of learning a new task is that the environment is high dimensional-there are many different sensory features and possible actions, with typically only a small reward-relevant subset. Although animals can learn to perform complex tasks that involve arbitrary associations between stimuli, actions, and rewards,1,2,3,4,5,6 a consistent and striking result across varied experimental paradigms is that in initially acquiring such tasks, large differences between individuals are apparent in the learning process.7,8,9,10,11,12 What neural mechanisms contribute to initial task acquisition, and why do some individuals learn a new task much more quickly than others? To address these questions, we recorded longitudinally from dopaminergic (DA) axon terminals in mice learning a visual decision-making task.7 Across striatum, DA responses tracked idiosyncratic and side-specific learning trajectories, consistent with widespread reward prediction error coding across DA terminals. However, even before any rewards were delivered, contralateral-side-specific visual responses were present in DA terminals, primarily in the dorsomedial striatum (DMS). These pre-existing responses predicted the extent of learning for contralateral stimuli. Moreover, activation of these terminals improved contralateral performance. Thus, the initial conditions of a projection-specific and feature-specific DA signal help explain individual learning trajectories. More broadly, this work suggests that functional heterogeneity across DA projections may serve to bias target regions toward learning about different subsets of task features, providing a potential mechanism to address the dimensionality of the initial task learning problem.

Basal ganglia: Uniting circuit logic between matrix and striosome

A new study has identified a novel direct-indirect circuit architecture connecting the striosome compartment of the striatum with midbrain dopamine neurons. This circuit has the potential to integrate limbic and sensorimotor functions and to exert substantial control over biological reinforcement leaning.

Molecular diversity and migration of GABAergic neurons in the developing ventral midbrain

Dopaminergic neurons in the ventral midbrain (mDA) are surrounded by GABAergic neurons. The full extent of GABAergic neuron subtypes occupying this region and the mechanisms that underlie their development and function are largely unknown. Therefore, we performed single-cell RNA sequencing (scRNA-seq) of fluorescence-activated cell sorting (FACS)-isolated GABAergic neurons in the developing mouse ventral midbrain. Several distinct GABAergic neuron subtypes were identified based on transcriptomic profiles and spatially assigned to the ventral midbrain using in situ hybridization and immunohistochemistry for specific markers. A subset of GABAergic clusters that co-expressed mDA markers was studied in more detail and showed distinctive molecular, functional, and wiring properties. Finally, migration of different GABAergic neuron subtypes required netrin-1 from different cellular sources acting via distinct receptor mechanisms. Overall, our work provides insight into the heterogeneity and spatial organization of GABAergic neurons in the developing ventral midbrain and begins to dissect the mechanisms that underlie their development.

Monosynaptic Inputs to Ventral Tegmental Area Glutamate and GABA Co-transmitting Neurons

A unique population of ventral tegmental area (VTA) neurons co-transmits glutamate and GABA. However, the circuit inputs to VTA VGluT2+VGaT+ neurons are unknown, limiting our understanding of their functional capabilities. By coupling monosynaptic rabies tracing with intersectional genetic targeting in male and female mice, we found that VTA VGluT2+VGaT+ neurons received diverse brainwide inputs. The largest numbers of monosynaptic inputs to VTA VGluT2+VGaT+ neurons were from superior colliculus (SC), lateral hypothalamus (LH), midbrain reticular nucleus, and periaqueductal gray, whereas the densest inputs relative to brain region volume were from the dorsal raphe nucleus, lateral habenula, and VTA. Based on these and prior data, we hypothesized that LH and SC inputs were from glutamatergic neurons. Optical activation of glutamatergic LH neurons activated VTA VGluT2+VGaT+ neurons regardless of stimulation frequency and resulted in flee-like ambulatory behavior. In contrast, optical activation of glutamatergic SC neurons activated VTA VGluT2+VGaT+ neurons for a brief period of time at high frequency and resulted in head rotation and arrested ambulatory behavior (freezing). Stimulation of glutamatergic LH neurons, but not glutamatergic SC neurons, was associated with VTA VGluT2+VGaT+ footshock-induced activity and inhibition of LH glutamatergic neurons disrupted VTA VGluT2+VGaT+ tailshock-induced activity. We interpret these results such that inputs to VTA VGluT2+VGaT+ neurons may integrate diverse signals related to the detection and processing of motivationally salient outcomes.

A brainstem circuit amplifies aversion

Dynamic gain control of aversive signals enables adaptive behavioral responses. Although the role of amygdalar circuits in aversive processing is well established, the neural pathway for amplifying aversion remains elusive. Here, we show that the brainstem circuit linking the interpeduncular nucleus (IPN) with the nucleus incertus (NI) amplifies aversion and promotes avoidant behaviors. IPN GABA neurons are activated by aversive stimuli and their predicting cues, with their response intensity closely tracking aversive values. Activating these neurons does not trigger aversive behavior on its own but rather amplifies responses to aversive stimuli, whereas their ablation or inhibition suppresses such responses. Detailed circuit dissection revealed anatomically distinct subgroups within the IPN GABA neuron population, highlighting the NI-projecting subgroup as the modulator of aversiveness related to fear and opioid withdrawal. These findings unveil the IPN-NI circuit as an aversion amplifier and suggest potential targets for interventions against affective disorders and opioid relapse.

Corticonigral projections recruit substantia nigra pars lateralis dopaminergic neurons for auditory threat memories

Dopaminergic neurons (DANs) in the lateral substantia nigra project to the tail of striatum (TS), which is involved in threat conditioning. Auditory cortex also contributes to threatening behaviors, but whether it directly interacts with midbrain DANs and how these interactions might influence threat conditioning remain unclear. Here, functional mapping revealed robust excitatory input from auditory and temporal association cortexes to substantia nigra pars lateralis (SNL) DANs, but not to pars compacta (SNc) DANs. SNL DANs exhibited unique firing patterns, with irregular pacemaking and higher maximal firing, reflecting different channel complements than SNc DANs. Behaviorally, inhibiting cortex to SNL projections impaired memory retrieval during auditory threat conditioning. Thus, we demonstrate robust corticonigral projections to SNL DANs, contrasting with previous observations of sparse cortical input to substantia nigra DANs. These findings distinguish SNL DANs from other nigral populations, highlighting their role in threatening behaviors and expanding knowledge of cortex to midbrain interactions.

Rikkunshito improves anorexia through ghrelin- and orexin-dependent activation of the brain hypothalamus and mesolimbic dopaminergic pathway in rats

BACKGROUND

Rikkunshito (RKT), a traditional Japanese medicine, can relieve epigastric discomfort and anorexia in patients with functional dyspepsia. RKT enhances the orexigenic hormone, ghrelin. Ghrelin regulates food motivation by stimulating the appetite control center in the hypothalamus and the brain mesolimbic dopaminergic pathway (MDPW). However, the effect of RKT on MDPW remains unclear. Here, we aimed to investigate the central neural mechanisms underlying the orexigenic effects of RKT, focusing on the MDPW.

METHODS

We examined the effects of RKT on food intake and neuronal c-Fos expression in restraint stress- and cholecystokinin octapeptide-induced anorexia in male rats.

KEY RESULTS

RKT treatment significantly restored stress- and cholecystokinin octapeptide-induced decreased food intake. RKT increased c-Fos expression in the ventral tegmental area (VTA), especially in tyrosine hydroxylase-immunoreactive neurons, and nucleus accumbens (NAc). The effects of RKT were suppressed by the ghrelin receptor antagonist [D-Lys3]-GHRP-6. RKT increased the number of c-Fos/orexin-double-positive neurons in the lateral hypothalamus (LH), which project to the VTA. The orexin receptor antagonist, SB334867, suppressed RKT-induced increase in food intake and c-Fos expression in the LH, VTA, and NAc. RKT increased c-Fos expression in the arcuate nucleus and nucleus of the solitary tract of the medulla, which was inhibited by [D-Lys3]-GHRP-6.

CONCLUSIONS & INFERENCES

RKT may restore appetite in subjects with anorexia through ghrelin- and orexin-dependent activation of neurons regulating the brain appetite control network, including the hypothalamus and MDPW.

The putative role of the habenula in animal migration

BACKGROUND

When an addicted animal seeks a specific substance, it is based on the perception of internal and external cues that strongly motivate to pursue the acquisition of that compound. In essence, a similar process acts out when an animal leaves its present area to begin its circannual migration. This review article examines the existence of scientific evidence for possible relatedness of migration and addiction by influencing Dorsal Diencephalic Conduction System (DDCS) including the habenula.

METHODS

For this review especially the databases of Pubmed and Embase were frequently and non-systematically searched.

RESULTS

The mechanisms of bird migration have been thoroughly investigated. Especially the mechanism of the circannual biorhythm and its associated endocrine regulation has been well elucidated. A typical behavior called "Zugunruhe" marks the moment of leaving in migratory birds. The role of magnetoreception in navigation has also been clarified in recent years. However, how bird migration is regulated at the neuronal level in the forebrain is not well understood. Among mammals, marine mammals are most similar to birds. They use terrestrial magnetic field when navigating and often bridge long distances between breeding and foraging areas. Population migration is further often seen among the large hoofed mammals in different parts of the world. Importantly, learning processes and social interactions with conspecifics play a major role in these ungulates. Considering the evolutionary development of the forebrain in vertebrates, it can be postulated that the DDCS plays a central role in regulating the readiness and intensity of essential (emotional) behaviors. There is manifold evidence that this DDCS plays an important role in relapse to abuse after prolonged periods of abstinence from addictive behavior. It is also possible that the DDCS plays a role in navigation.

CONCLUSIONS

The role of the DDCS in the neurobiological regulation of bird migration has hardly been investigated. The involvement of this system in relapse to addiction in mammals might suggest to change this. It is recommended that particularly during "Zugunruhe" the role of neuronal regulation via the DDCS will be further investigated.

The External Globus Pallidus as the Hub of the Auditory Cortico-Basal Ganglia Loop

The cortico-basal ganglia loop has traditionally been conceptualized as consisting of three distinct information networks: motor, limbic, and associative. However, this three-loop concept is insufficient to comprehensively explain the diverse functions of the cortico-basal ganglia system, as emerging evidence suggests its involvement in sensory processing, including the auditory systems. In the present study, we demonstrate the auditory cortico-basal ganglia loop by using transgenic mice and viral-assisted labelings. The caudal part of the external globus pallidus (GPe) emerged as a major output nucleus of the auditory cortico-basal ganglia loop with the cortico-striato-pallidal projections as its input pathway and pallido-cortical and pallido-thalamo-cortical projections as its output pathway. GABAergic neurons in the caudal GPe dominantly innervated the nonlemniscal auditory pathway. They also projected to various regions, including the substantia nigra pars lateralis, cuneiform nucleus, and periaqueductal gray. Considering the functions associated with these GPe-projecting regions, auditory cortico-basal ganglia circuits may play a pivotal role in eliciting defensive behaviors against acoustic stimuli.

Modulation of Hypothalamic Dopamine Neuron Activity by Interaction Between Caloric State and Amphetamine in Zebrafish Larvae

Dopamine (DA) signaling is evoked by both food and drugs that humans come to abuse. Moreover, physiological state (e.g., hunger versus satiety) can modulate the response. However, there is great heterogeneity among DA neurons. Limited studies have been performed that could resolve the interaction between physiological state and drug responsivity across groups of DA neurons. Here, we measured the activity of neurons in transgenic Tg (th2:GCaMP7s) zebrafish larva that expresses a calcium indicator (GCaMP7s) in A11 (posterior tuberculum) and a part of A14 (caudal hypothalamus and intermediate hypothalamus) DA populations located in the hypothalamus of the larval zebrafish. Fish were recorded in one of two physiological states: ad-libitum fed (AL) and food deprived (FD) and before and after acute exposure to different doses of the stimulant drug amphetamine (0, 0.7, and 1.5 μM). We quantified fluorescence change, activity duration, peak rise/fall time, and latency in the calcium spikes of the DA neurons. Our results show that baseline DA neuron activity amplitude, spike duration, and correlation between inter- and intra-DA neurons were higher in the FD than in the AL state. Dose-dependent AMPH treatment further increased the intensity of these parameters in the neuron spikes but only in the FD state. The DA activity correlation relatively increased in AL state post-AMPH treatment. Given that hunger increases drug reactivity and the probability of relapse to drug seeking, the results support populations of DA neurons as potential critical mediators of the interaction between physiological state and drug reinforcement.

Reward Bases: A simple mechanism for adaptive acquisition of multiple reward types

Animals can adapt their preferences for different types of reward according to physiological state, such as hunger or thirst. To explain this ability, we employ a simple multi-objective reinforcement learning model that learns multiple values according to different reward dimensions such as food or water. We show that by weighting these learned values according to the current needs, behaviour may be flexibly adapted to present preferences. This model predicts that individual dopamine neurons should encode the errors associated with some reward dimensions more than with others. To provide a preliminary test of this prediction, we reanalysed a small dataset obtained from a single primate in an experiment which to our knowledge is the only published study where the responses of dopamine neurons to stimuli predicting distinct types of rewards were recorded. We observed that in addition to subjective economic value, dopamine neurons encode a gradient of reward dimensions; some neurons respond most to stimuli predicting food rewards while the others respond more to stimuli predicting fluids. We also proposed a possible implementation of the model in the basal ganglia network, and demonstrated how the striatal system can learn values in multiple dimensions, even when dopamine neurons encode mixtures of prediction error from different dimensions. Additionally, the model reproduces the instant generalisation to new physiological states seen in dopamine responses and in behaviour. Our results demonstrate how a simple neural circuit can flexibly guide behaviour according to animals' needs.

A pontine-medullary loop crucial for REM sleep and its deficit in Parkinson's disease

Identifying the properties of the rapid eye movement (REM) sleep circuitry and its relation to diseases has been challenging due to the neuronal heterogeneity of the brainstem. Here, we show in mice that neurons in the pontine sublaterodorsal tegmentum (SubLDT) that express corticotropin-releasing hormone-binding protein (Crhbp+ neurons) and project to the medulla promote REM sleep. Within the medullary area receiving projections from Crhbp+ neurons, neurons expressing nitric oxide synthase 1 (Nos1+ neurons) project to the SubLDT and promote REM sleep, suggesting a positively interacting loop between the pons and the medulla operating as a core REM sleep circuit. Nos1+ neurons also project to areas that control wide forebrain activity. Ablating Crhbp+ neurons reduces sleep and impairs REM sleep atonia. In Parkinson's disease patients with REM sleep behavior disorders, CRHBP-immunoreactive neurons are largely reduced and contain pathologic α-synuclein, providing insight into the mechanisms underlying the sleep deficits characterizing this disease.

Dopamine and Norepinephrine Differentially Mediate the Exploration-Exploitation Tradeoff

Dopamine (DA) and norepinephrine (NE) have been repeatedly implicated in neuropsychiatric vulnerability, in part via their roles in mediating the decision-making processes. Although two neuromodulators share a synthesis pathway and are coactivated under states of arousal, they engage in distinct circuits and modulatory roles. However, the specific role of each neuromodulator in decision-making, in particular the exploration-exploitation tradeoff, remains unclear. Revealing how each neuromodulator contributes to exploration-exploitation tradeoff is important in guiding mechanistic hypotheses emerging from computational psychiatric approaches. To understand the differences and overlaps of the roles of these two catecholamine systems in regulating exploration, a direct comparison using the same dynamic decision-making task is needed. Here, we ran male and female mice in a restless two-armed bandit task, which encourages both exploration and exploitation. We systemically administered a nonselective DA antagonist (flupenthixol), a nonselective DA agonist (apomorphine), a NE beta-receptor antagonist (propranolol), and a NE beta-receptor agonist (isoproterenol) and examined changes in exploration within subjects across sessions. We found a bidirectional modulatory effect of dopamine on exploration. Increasing dopamine activity decreased exploration and decreasing dopamine activity increased exploration. The modulatory effect of beta-noradrenergic receptor activity on exploration was mediated by sex. Reinforcement learning model parameters suggested that dopamine modulation affected exploration via decision noise and norepinephrine modulation affected exploration via sensitivity to outcome. Together, these findings suggested that the mechanisms that govern the exploration-exploitation transition are sensitive to changes in both catecholamine functions and revealed differential roles for NE and DA in mediating exploration.

Transformation of valence signaling in a mouse striatopallidal circuit

The ways in which sensory stimuli acquire motivational valence through association with other stimuli is one of the simplest forms of learning. Although we have identified many brain nuclei that play various roles in reward processing, a significant gap remains in understanding how valence encoding transforms through the layers of sensory processing. To address this gap, we carried out a comparative investigation of the mouse anteromedial olfactory tubercle (OT), and the ventral pallidum (VP) - 2 connected nuclei of the basal ganglia which have both been implicated in reward processing. First, using anterograde and retrograde tracing, we show that both D1 and D2 neurons of the anteromedial OT project primarily to the VP and minimally elsewhere. Using two-photon calcium imaging, we then investigated how the identity of the odor and reward contingency of the odor are differently encoded by neurons in either structure during a classical conditioning paradigm. We find that VP neurons robustly encode reward contingency, but not identity, in low-dimensional space. In contrast, the OT neurons primarily encode odor identity in high-dimensional space. Although D1 OT neurons showed larger responses to rewarded odors than other odors, consistent with prior findings, we interpret this as identity encoding with enhanced contrast. Finally, using a novel conditioning paradigm that decouples reward contingency and licking vigor, we show that both features are encoded by non-overlapping VP neurons. These results provide a novel framework for the striatopallidal circuit in which a high-dimensional encoding of stimulus identity is collapsed onto a low-dimensional encoding of motivational valence.

The Cerebellum-Ventral Tegmental Area Microcircuit and Its Implications for Autism Spectrum Disorder: A Narrative Review

The cerebellum has long been implicated in the etiopathogenesis of autism spectrum disorder (ASD), and emerging evidence suggests a significant contribution by reciprocal neural circuits between the cerebellum and ventral tegmental area (VTA) in symptom expression. This review provides a concise overview of morphological and functional alterations in the cerebellum and VTA associated with ASD symptoms, primarily focusing on human studies while also integrating mechanistic insights from animal models. We propose that cerebello-VTA circuit dysfunctional is a major contributor to ASD symptoms and that these circuits are promising targets for drugs and therapeutic brain stimulation methods.

Decoding the brain: From neural representations to mechanistic models

A central principle in neuroscience is that neurons within the brain act in concert to produce perception, cognition, and adaptive behavior. Neurons are organized into specialized brain areas, dedicated to different functions to varying extents, and their function relies on distributed circuits to continuously encode relevant environmental and body-state features, enabling other areas to decode (interpret) these representations for computing meaningful decisions and executing precise movements. Thus, the distributed brain can be thought of as a series of computations that act to encode and decode information. In this perspective, we detail important concepts of neural encoding and decoding and highlight the mathematical tools used to measure them, including deep learning methods. We provide case studies where decoding concepts enable foundational and translational science in motor, visual, and language processing.

Expectancy-related changes in firing of dopamine neurons depend on hippocampus

The orbitofrontal cortex (OFC) and hippocampus (HC) both contribute to the cognitive maps that support flexible behavior. Previously, we used the dopamine neurons to measure the functional role of OFC. We recorded midbrain dopamine neurons as rats performed an odor-based choice task, in which expected rewards were manipulated across blocks. We found that ipsilateral OFC lesions degraded dopaminergic prediction errors, consistent with reduced resolution of the task states. Here we have repeated this experiment in male rats with ipsilateral HC lesions. The results show HC also shapes the task states, however unlike OFC, which provides information local to the trial, the HC is necessary for estimating upper-level hidden states that distinguish blocks. The results contrast the roles of the OFC and HC in cognitive mapping and suggest that the dopamine neurons access rich information from distributed regions regarding the environment's structure, potentially enabling this teaching signal to support complex behaviors.