Neurocircuits for motivation

The evolving neurobiology of early-life stress

Because early-life stress is common and constitutes a strong risk factor for cognitive and mental health disorders, it has been the focus of a multitude of studies in humans and experimental models. Yet, we have an incomplete understanding of what is perceived as stressful by the developing brain, what aspects of stress influence brain maturation, what developmental ages are particularly vulnerable to stress, which molecules mediate the effects of stress on brain operations, and how transient stressful experiences can lead to enduring emotional and cognitive dysfunctions. Here, we discuss these themes, highlight the challenges and progress in resolving them, and propose new concepts and avenues for future research.

A neuroimmune circuit mediates cancer cachexia-associated apathy

Cachexia, a severe wasting syndrome associated with inflammatory conditions, often leads to multiorgan failure and death. Patients with cachexia experience extreme fatigue, apathy, and clinical depression, yet the biological mechanisms underlying these behavioral symptoms and their relationship to the disease remain unclear. In a mouse cancer model, cachexia specifically induced increased effort-sensitivity, apathy-like symptoms through a cytokine-sensing brainstem-to-basal ganglia circuit. This neural circuit detects elevated interleukin-6 (IL-6) at cachexia onset and translates inflammatory signals into decreased mesolimbic dopamine, thereby increasing effort sensitivity. We alleviated these apathy-like symptoms by targeting key circuit nodes: administering an anti-IL-6 antibody treatment, ablating cytokine sensing in the brainstem, and optogenetically or pharmacologically boosting mesolimbic dopamine. Our findings uncovered a central neural circuit that senses systemic inflammation and orchestrates behavioral changes, providing mechanistic insights into the connection between chronic inflammation and depressive symptoms.

Human social behavior and oxytocin: Molecular and neuronal mechanisms

Oxytocin (OT) is a hormone that is crucial for regulating various human social behaviors, such as maternal instinct, empathy, and trust. Its secretion in the brain is triggered by social stimuli. Recent research demonstrated impaired regulation of OT secretion and reduced social behaviors in patients with arginine vasopressin deficiency (central diabetes insipidus). OT interacts with other hormones to regulate human trust. Moreover, it has been shown to generate feelings of attachment and trust toward other and familiar consumer brands, thereby, potentially impacting personal consumption, which is a significant aspect of economic activity. This review provided insights into the molecular and neural mechanisms of OT in regulating human social behavior, including both social and economic activities.

Oxytocin signaling in the ventral tegmental area mediates social isolation-induced craving for social interaction

BACKGROUND

Social interaction is crucial for mental health across animal species. Social experiences, especially in early-life stages, strongly influence brain function and social behavior later in life. Acute social isolation (SI) increases motivation to seek social interaction, but little is known about its underlying neuronal and circuitry mechanisms. Here, we focus on oxytocin signaling in the ventral tegmental area (VTA), a vital node of the brain's reward network, as a potential mechanism for SI-induced craving for social interaction.

METHODS

Adolescent (4-week-old) or adult (14-week-old) male C57BL/6J mice underwent a 1-week SI. Free interaction, object exploration, three-chamber social approach, and habituation tests were used to assess social and non-social behavior changes. Viral vectors were used to decipher the underlying neural circuitry, and chemogenetic techniques were applied to modify neuronal activity.

RESULTS

We found that in male C57BL/6J mice, SI during adolescence, but not adulthood, leads to increased craving for social interaction and object exploration, accompanied by impaired social habituation, social novelty preference, and social recognition memory (SRM). SI-induced craving for social interaction and SRM deficit is still observed upon regrouping. Through cell-type-specific manipulations with designer receptors exclusively activated by designer drugs (DREADD), we show that oxytocin neurons in the paraventricular nucleus of the hypothalamus (PVN) are crucial for SI-induced social behavior changes. Chemogenetic activation of PVN oxytocin neurons recapitulates social behavior changes observed in SI mice, whereas chemogenetic inhibition of oxytocin neurons prevents social behavior changes caused by SI. Moreover, we found that dopaminergic neurons in the VTA mediate SI-induced craving for social interaction through their projections to the medial prefrontal cortex (mPFC), but not to the nucleus accumbens. Injection of a specific oxytocin receptor antagonist L368,899 into the VTA or chemical lesions of dopaminergic axon terminals in the mPFC with local application of 6-hydroxydopamine ameliorates SI-induced social behavior changes.

CONCLUSIONS

These findings suggest that adolescent SI has enduring effects on social behaviors in male mice through an oxytocinergic modulation of the VTA-to-mPFC dopaminergic circuit activity.

The dopaminergic effects of esketamine are mediated by a dual mechanism involving glutamate and opioid receptors

Esketamine represents a new class of drugs for treating mood disorders. Unlike traditional monoaminergic-based therapies, esketamine primarily targets N-methyl-D-aspartate receptors (NMDAR). However, esketamine is a complex drug with low affinity for NMDAR and can also bind to other targets, such as opioid receptors. Its precise mechanism of action for its antidepressant properties remains debated, as does its potential for misuse. A key component at the intersection of mood and reward processing is the dopaminergic system. In this study, we evaluated the effects of esketamine in locomotion, anxiety tests and operant responding and we used in vivo fiber photometry to explore the neurochemical effects of esketamine in the nucleus accumbens of mice. Our findings demonstrated multifaceted effects of esketamine on neurotransmitter dynamics. In freely behaving mice, esketamine increased locomotion and increased extracellular dopamine tone -by impairing dopamine clearance rather than promoting dopamine release- while decreasing glutamatergic activity. However, it decreased dopamine spontaneous release event frequency and impaired reward-evoked dopamine release, leading to a reduction in operant responding rates. These dopaminergic effects were partially, and conditionally, blocked by the opioid antagonist naloxone and required glutamatergic input. In summary, our study reveals a complex interaction between neurotransmitter systems, suggesting that the neurochemical effects of esketamine are both circuit- and state-dependent.

Cognitive Impairment in MASLD is associated with Amygdala-Related Connectivity Dysfunction in the Prefrontal and Sensory Cortex

BACKGROUND

Metabolic dysfunction-associated steatotic liver disease (MASLD) is a common metabolism-related multisystem clinical disorder, often accompanied by a high comorbidity of mild cognitive impairment (MCI). Increasing evidence suggests that the amygdala is crucial in cognitive processing during metabolic dysfunction. Nevertheless, the role of the amygdala in the neural mechanisms of MASLD with MCI (MCI_MASLD) remains unclear.

METHODS

A total of 74 MASLD patients (43 with MCI_MASLD and 31 without MCI [nonMCI_MASLD]) and 62 demographic-matched healthy controls (HC) were enrolled. All participants underwent resting-state functional magnetic resonance imaging scans and psychological scale assessments. Liver fat content and blood index measurements were performed on the patients. Using the bilateral amygdala as seeds, the seed-based functional connectivity (FC) maps were calculated and one-way analysis of covariance with post hoc tests was performed to investigate the difference among the three groups.

RESULTS

Compared to nonMCI_MASLD patients, MCI_MASLD patients demonstrated enhanced FC between the right amygdala and the medial prefrontal cortex (mPFC), while reduced FC between the left amygdala and the left supplementary motor area (SMA). Interestingly, the FC values of the mPFC were correlated with the Montreal Cognitive Assessment Scale (MoCA) scores and liver controlled attenuation parameters, and the FC values of the SMA were also correlated with the MoCA scores. Furthermore, the FC values between the bilateral amygdala and regions within the frontal-limbic-mesencephalic circuits were higher in MASLD patients when compared to HC.

CONCLUSIONS

Aberrant FC of the amygdala can provide potential neuroimaging markers for MCI in MASLD, which is associated with amygdala-related connectivity disturbances in areas related to cognition and sensory processing. Moreover, visceral fat accumulation may exacerbate brain dysfunction.

Molecular and functional mapping of the neuroendocrine hypothalamus: a new era begins

BACKGROUND

Recent advances in neuroscience tools for single-cell molecular profiling of brain neurons have revealed an enormous spectrum of neuronal subpopulations within the neuroendocrine hypothalamus, highlighting the remarkable molecular and cellular heterogeneity of this brain area.

RATIONALE

Neuronal diversity in the hypothalamus reflects the high functional plasticity of this brain area, where multiple neuronal populations flexibly integrate a variety of physiological outputs, including energy balance, stress and fertility, through crosstalk mechanisms with peripheral hormones. Intrinsic functional heterogeneity is also observed within classically 'defined' subpopulations of neuroendocrine neurons, including subtypes with distinct neurochemical signatures, spatial organisation and responsiveness to hormonal cues.

AIM

The aim of this review is to critically evaluate past and current research on the functional diversity of hypothalamic neuroendocrine neurons and their plasticity. It focuses on how this neuronal plasticity in this brain area relates to metabolic control, feeding regulation and interactions with stress and fertility-related neural circuits.

CONCLUSION

Our analysis provides an original framework for improving our understanding of the hypothalamic regulation of hormone function and the development of neuroendocrine diseases.

A unified theoretical framework underlying the regulation of motivated behavior

To orchestrate behaviors for survival, multiple psychological components have evolved. The current theories do not clearly distinguish the distinct components. In this article, we provide a unified theoretical framework. To optimize survival, there should be four components; (1) "need", an alarm based on a predicted deficiency. (2) "motivation", a direct behavior driver. (3) "pleasure", a teacher based on immediate outcomes. (4) "utility", a teacher based on final delayed outcomes. For behavior stability, need should be accumulated into motivation to drive behavior. Based on the immediate outcome of the behavior, the pleasure should teach whether to continue the current behavior. Based on the final delay outcome, the utility should teach whether to increase future behavior by reshaping the other three components. We provide several neural substrate candidates in the food context. The proposed theoretical framework, in combination with appropriate experiments, will unravel the neural components responsible for each theoretical component.

Tracing volitional recovery in post-stroke akinetic mutism using longitudinal microstructure imaging: Insights from a single case study

Lesions in the frontal-subcortical circuitry can lead to akinetic mutism (AM) characterized by diminished volition. However, the microstructural changes in the damaged network underlying its recovery remain unknown. Clinical examination and neuropsychological assessment were performed on a patient with post-stroke AM. Multimodal MRI scans were performed at baseline and follow-ups. We used diffusion MRI and biophysical models, specifically utilizing neurite orientation dispersion and density imaging for assessing gray matter microstructure, and fixel-based analysis for the evaluation of white matter. Longitudinal comparisons were performed between the patient and healthy controls. Pronounced recovery of volition was observed after dopamine agonist therapy combined with physical therapy. In addition to infarcts in the bilateral medial cortex, microstructure imaging detected reduced neurite density in extensive areas, specifically in temporal areas and subcortical nuclei, and decreased fiber density of white matter tracts (TFCE-corrected p < .05). Microstructural degeneration in the anterior cingulate cortex and cingulum was relatively persistent (Bonferroni-corrected p < .05). However, most tracts within the frontal-subcortical circuitry showed increased fiber density during the recovery stage. Microstructure of an extensive network may contribute to the disruption and recovery of volition. Fiber density within the frontal-subcortical circuitry could be a promising biomarker indicating volitional recovery.

Dorsal Raphe Serotonergic Neurons-Ventral Tegmental Area Neural Pathway Promotes Wake From Sleep

BACKGROUND

Dorsal raphe nucleus (DRN) serotonergic neurons projecting to the ventral tegmental area (VTA) neural circuit participate in regulating wake-related behaviors; however, the effect and mechanism of which in regulating sleep-wake are poorly understood.

METHODS

Fiber photometry was used to study DRN serotonergic afferent activity changes in the VTA during sleep-wake processes. Optogenetics and chemogenetics were took advantage to study the effects of DRN serotonergic afferents modulating VTA during sleep-wake. In vivo electrophysiology was employed to investigate how VTA neuronal firings were influenced by upregulation of DRN serotonergic afferents during sleep-wake.

RESULTS

We found that DRN serotonergic afferent activity in the VTA was higher during wake than during NREM and REM sleep. Chemogenetic activation of VTA-projecting DRN serotonergic neurons increased wake, and optogenetic activation of DRN serotonergic terminals in the VTA induced wake during NREM and REM sleep. Furthermore, we found that optogenetic activation of DRN serotonergic terminals in the VTA increased glutamatergic neuronal firing, decreased dopaminergic neuronal firing, but not influenced GABAergic neuronal firing during NREM sleep.

CONCLUSION

Our findings provide evidence in understanding the role of DRN serotonergic neurons-VTA neural pathway in regulating sleep-wake, in which dynamic VTA dopaminergic, glutamatergic, and GABAergic neuronal firing changes responded to the wake promoting effect of DRN serotonergic afferents.

Sequential Activation of Lateral Hypothalamic Neuronal Populations during Feeding and Their Assembly by Gamma Oscillations

Neural circuits supporting innate behaviors, such as feeding, exploration, and social interaction, intermingle in the lateral hypothalamus (LH). Although previous studies have shown that individual LH neurons change their firing relative to the baseline during one or more behaviors, the firing rate dynamics of LH populations within behavioral episodes and the coordination of behavior-related LH populations remain largely unknown. Here, using unsupervised graph-based clustering of LH neurons firing rate dynamics in freely behaving male mice, we identified distinct populations of cells whose activity corresponds to feeding, specific times during feeding bouts, or other innate behaviors-social interaction and novel object exploration. Feeding-related cells fired together with a higher probability during slow and fast gamma oscillations (30-60 and 60-90 Hz) than during nonrhythmic epochs. In contrast, the cofiring of neurons signaling other behaviors than feeding was overall similar between slow gamma and nonrhythmic epochs but increased during fast gamma oscillations. These results reveal a neural organization of ethological hierarchies in the LH and point to behavior-specific motivational systems, the dysfunction of which may contribute to mental disorders.

Modulation of cortical representations of sensory and contextual information underlies aversive associative learning

Cortical neurons encode both sensory and contextual information, yet it remains unclear how experiences modulate these cortical representations. Here, we demonstrate that trace eyeblink conditioning (TEC), an aversive associative-learning paradigm linking conditioned (CS) with unconditioned stimuli (US), finely tunes cortical coding at both population and single-neuron levels. Initially, we show that the primary somatosensory cortex (S1) is necessary for TEC acquisition, as evidenced by local muscimol administration. At the population level, TEC enhances activity in a small subset (∼20%) of CS- or US-responsive primary neurons (rPNs) while diminishing activity in non-rPNs, including locomotion-tuned or unresponsive PNs. Crucially, TEC learning modulates the encoding of sensory versus contextual information in single rPNs: CS-responsive neurons become less responsive, while US-responsive neurons gain responses to CS. Moreover, we find that the cholinergic pathway, via nicotinic receptors, underlies TEC-induced modulations. These findings suggest that experiences dynamically tune cortical representations through cholinergic pathways.

Distinct lateral hypothalamic CaMKIIα neuronal populations regulate wakefulness and locomotor activity

For nearly a century, evidence has accumulated indicating that the lateral hypothalamus (LH) contains neurons essential to sustain wakefulness. While lesion or inactivation of LH neurons produces a profound increase in sleep, stimulation of inhibitory LH neurons promotes wakefulness. To date, the primary wake-promoting cells that have been identified in the LH are the hypocretin/orexin (Hcrt) neurons, yet these neurons have little impact on total sleep or wake duration across the 24-h period. Recently, we and others have identified other LH populations that increase wakefulness. In the present study, we conducted microendoscopic calcium imaging in the LH concomitant with EEG and locomotor activity (LMA) recordings and found that a subset of LH neurons that express Ca2+/calmodulin-dependent protein kinase IIα (CaMKIIα) are preferentially active during wakefulness. Chemogenetic activation of these neurons induced sustained wakefulness and greatly increased LMA even in the absence of Hcrt signaling. Few LH CaMKIIα-expressing neurons are hypocretinergic or histaminergic while a small but significant proportion are GABAergic. Ablation of LH inhibitory neurons followed by activation of the remaining LH CaMKIIα neurons induced similar levels of wakefulness but blunted the LMA increase. Ablated animals showed no significant changes in sleep architecture but both spontaneous LMA and high theta (8 to 10 Hz) power during wakefulness were reduced. Together, these findings indicate the existence of two subpopulations of LH CaMKIIα neurons: an inhibitory population that promotes locomotion without affecting sleep architecture and an excitatory population that promotes prolonged wakefulness even in the absence of Hcrt signaling.