Neural mechanism of acute stress regulation by trace aminergic signalling in the lateral habenula in male mice

Constitutive activity of the inhibitory G protein pathway mediated by non-visual opsin Opn7b reduces cFos activity in stress and fear circuits and modulates avoidance behavior

Constitutive activity of G protein-coupled receptors (GPCRs) plays an important role in brain function and disease including neurodegenerative and psychiatric disorders. The non-visual opsin Opn7b is a constitutively active G i/o coupled GPCR which has been used to synchronize neuronal networks. Here we show that expression of Opn7b in the bed nucleus of the stria terminalis and the ventral tegmental area, two interconnected brain areas involved in modulating fear and stress responses, reduces the number of cFos positive neurons and modulates avoidance behavior in mice. Thus, by constitutively activating the G i/o pathway Opn7b can be used as a tool to reduce cFos expression and to link cFos-expressing neurons to network- and pathway-specific behavior.

Lateral habenula induces cognitive and affective dysfunctions in mice with neuropathic pain via an indirect pathway to the ventral tegmental area

Neuropathic pain, which has become a major public health concern, is frequently accompanied by the deterioration of affective behavior and cognitive function. However, the brain circuitry underlying these changes is poorly understood. Therefore, we aimed to identify in a mouse model the converging circuit that influences the sensory, affective, and cognitive consequences of neuropathic pain. The lateral habenula (LHb) and ventral tegmental area (VTA) have been confirmed to play critical roles in the regulation of pain, cognition, and depression. Given the essential role of the LHb in depression and cognition, we attempted to clarify how neural circuitry involving the LHb integrates pain-related information. Our data confirmed that the VTA receives projections from the LHb, but our results suggest that inhibition of this direct pathway has no effect on the behavior of mice with chronic neuropathic pain. The rostromedial tegmental nucleus (RMTg), a GABAergic structure believed to underlie the transient inhibition of DAergic neurons in the VTA, received glutamatergic inputs from the LHb and projected strongly to the VTA. Furthermore, our data suggest that a projection from LHb glutamatergic neurons to RMTg GABAergic neurons in the VTA, constituting an indirect LHbGlu → RMTgGABA → VTADA pathway, participates in peripheral nerve injury-induced nociceptive hypersensitivity, depressive-like behavior, and cognitive dysfunction. Ex vivo extracellular recordings of LHb neurons showed that the proportion of burst-firing cells in the LHb was significantly increased in indirect projections rather than in direct projections. This may explain the functional discrepancies between direct and indirect projections of the LHb to the VTA. Collectively, our study identifies a pivotal role of the LHbGlu → RMTgGABA → VTADA pathway in processing pain. This pathway may offer new therapeutic targets to treat neuropathic pain and its associated depressive-like and cognitive impairments.

Trace amine-associated receptors as potential targets for the treatment of anxiety and depression

In the metabolic pathways associated with major biogenic amines, such as dopamine, noradrenaline, and serotonin, there exists a group of compounds known as trace amines. These trace amines share structural similarities with the major biogenic amines. Since the discovery of trace amine-associated receptors (TAARs) that are activated by trace amines, numerous studies have suggested that these receptors, particularly the TAAR1 subfamily, play a role in modulating the stress response and are involved in stress-related psychiatric disorders, including depression, bipolar disorder, and anxiety. Research indicates that TAAR1 regulates the release of neurotransmitters like dopamine and serotonin, which may be a potential mechanism underlying the involvement of trace amines and TAAR1 in response to stress. Several selective TAAR1 agonists have been evaluated in various animal models of depression and anxiety, showing that these compounds can be effective in alleviating depressive and anxiety-like behaviors. Additionally, TAAR5 has also been found to have an effect on anxiety; it is proposed that a TAAR5 antagonist might produce anxiolytic effects. Despite our limited understanding of the underlying mechanisms through which TAARs regulates stress-related disorders, current evidence strongly suggests that TAAR ligands could represent novel pharmacotherapy for treating psychiatric disorders such as depression, bipolar disorder, and anxiety disorders like post-traumatic stress disorder (PTSD). This offers hope for more effective and safer treatment options in the field of mental health.

Convergent state-control of endogenous opioid analgesia

Pain is a dynamic and nonlinear experience shaped by injury and contextual factors, including expectations of future pain or relief1. While μ opioid receptors are central to the analgesic effects of opioid drugs, the endogenous opioid neurocircuitry underlying pain and placebo analgesia remains poorly understood. The ventrolateral column of the posterior periaqueductal gray is a critical hub for nociception and endogenous analgesia mediated by opioid signaling2. However, significant gaps remain in understanding the cell-type identities, the sub-second neural dynamics involved in pain modulation, the role of endogenous peptide neuromodulators, and the contextual factors influencing these processes. Using spatial mapping with single-nuclei RNA sequencing of pain-active neurons projecting to distinct long-range brain targets, alongside cell type-specific and activity-dependent genetic tools for in vivo optical recordings and modulation of neural activity and opioid peptide release, we identified a functional dichotomy in the ventrolateral periaqueductal gray. Neurons expressing μ opioid receptors encode active nociceptive states, whereas enkephalin-releasing neurons drive pain relief during recovery from injury, in response to learned fear predictions, and during placebo analgesia. Finally, by leveraging the functional effects of placebo analgesia, we used direct optogenetic activation of vlPAG enkephalin neurons to drive opioid peptide release, resulting in a robust reduction in pain. These findings show that diverse need states converge on a shared midbrain circuit that releases endogenous opioids with high spatiotemporal precision to suppress nociceptive activity and promote analgesia.

Glutamatergic pathways from medial prefrontal cortex to paraventricular nucleus of thalamus contribute to the methamphetamine-induced conditioned place preference without affecting wakefulness

Methamphetamine (METH) is a commonly abused psychostimulant with a high addictive nature. The paraventricular nucleus of thalamus (PVT), a key nucleus for arousal, has attracted much attention in the reward process of substance use. However, at which stage dose the PVT encode the reward process? How to reduce the side-effects of modulating PVT on wakefulness during the treatment of substance use? These issues remain unclear. The goal of the current study is to explore the role of the PVT and the glutamatergic projections from medial prefrontal cortex (mPFC) to PVT in the reward process of METH. Methods: Here, the conditioned place preference (CPP) was used to assess the reward process of METH in male mice, combined with methods of c-Fos mapping, virus-based neural tracing, patch-clamp recording, EEG-EMG recordings, optogenetics and designer receptor exclusively activated by designer drugs (DREADDs). Results: The glutamatergic neurons in PVT (PVTGlu) were triggered during METH CPP-Test, rather than by METH CPP-Training. Suppressing either PVTGlu or glutamatergic projection from mPFC to PVT efficiently disrupted the acquisition of METH CPP in male mice, mainly mediated by the GluN2A subunit of NMDA receptor. Further, inhibition of PVTGlu affected the rhythm of EEG-EMG, whereas inhibition of glutamatergic projection from mPFC to PVT did not. Conclusion: PVTGlu is involved in the reward process of METH at the retrieval stage of METH-conditioned context, rather than at the stage of encoding association between METH and context. The glutamatergic projections from mPFC to PVT, especially the GluN2A molecule, may be a promising therapeutic target for reducing METH reward, as there are no significant side effects on wakefulness.

The disappointment centre of the brain gets exciting: a systematic review of habenula dysfunction in depression

BACKGROUND

The habenula is an epithalamic brain structure that acts as a neuroanatomical hub connecting the limbic forebrain to the major monoamine centres. Abnormal habenula activity is increasingly implicated in depression, with a surge in publications on this topic in the last 5 years. Direct activation of the habenula is sufficient to induce a depressive phenotype in rodents, suggesting a causative role in depression. However, the molecular basis of habenula dysfunction in depression remains elusive and it is unclear how the preclinical advancements translate to the clinical field.

METHODS

A systematic literature search was conducted following the PRISMA guidelines. The two search terms depress* and habenula* were applied across Scopus, Web of Science and PubMed databases. Studies eligible for inclusion must have examined the habenula in clinical cases of depression or preclinical models of depression and compared their measures to an appropriate control.

RESULTS

Preclinical studies (n = 63) measured markers of habenula activity (n = 16) and neuronal firing (n = 22), largely implicating habenula hyperactivity in depression. Neurotransmission was briefly explored (n = 15), suggesting imbalances within excitatory and inhibitory habenula signalling. Additional preclinical studies reported neuroconnectivity (n = 1), inflammatory (n = 3), genomic (n = 3) and circadian rhythm (n = 3) abnormalities. Seven preclinical studies (11%) included both males and females. From these, 5 studies (71%) reported a significant difference between the sexes in at least one habenula measure taken. Clinical studies (n = 24) reported abnormalities in habenula connectivity (n = 15), volume (n = 6) and molecular markers (n = 3). Clinical studies generally included male and female subjects (n = 16), however, few of these studies examined sex as a biological variable (n = 6).

CONCLUSIONS

Both preclinical and clinical evidence suggest the habenula is disrupted in depression. However, there are opportunities for sex-specific analyses across both areas. Preclinical evidence consistently suggests habenula hyperactivity as a primary driver for the development of depressive symptoms. Clinical studies support gross habenula abnormalities such as altered activation, connectivity, and volume, with emerging evidence of blood brain barrier dysfunction, however, progress is limited by a lack of detailed molecular analyses and limited imaging resolution.

Transcranial optogenetic brain modulator for precise bimodal neuromodulation in multiple brain regions

Transcranial brain stimulation is a promising technology for safe modulation of brain function without invasive procedures. Recent advances in transcranial optogenetic techniques with external light sources, using upconversion particles and highly sensitive opsins, have shown promise for precise neuromodulation with improved spatial resolution in deeper brain regions. However, these methods have not yet been used to selectively excite or inhibit specific neural populations in multiple brain regions. In this study, we created a wireless transcranial optogenetic brain modulator that combines highly sensitive opsins and upconversion particles and allows for precise bimodal neuromodulation of multiple brain regions without optical crosstalk. We demonstrate the feasibility of our approach in freely behaving mice. Furthermore, we demonstrate its usefulness in studies of complex behaviors and brain dysfunction by controlling extorting behavior in mice in food competition tests and alleviating the symptoms of Parkinson's disease. Our approach has potential applications in the study of neural circuits and development of treatments for various brain disorders.

The dopaminergic system mediates the lateral habenula-induced autonomic cardiovascular responses

The lateral habenula (LHb) has been implicated in stress coping and autonomic control. The LHb regulates the midbrain system of monoamine neurotransmitters such as dopamine, serotonin, and noradrenaline. However, how the LHb regulates autonomic cardiovascular control in stressful situations is unclear. In this study, we examined the participation of the midbrain dopaminergic system in the cardiovascular response elicited by activation of the LHb. We used urethane-anesthetized Wistar male rats. We performed electrical stimulation of the LHb to observe changes in heart rate and blood pressure. Stimulation of the LHb caused bradycardia and a pressor response. Application of a nonselective dopamine receptor antagonist attenuated both the heart rate and the blood pressure changes induced by the LHb. We also tested the effects of blockade of dopamine receptor subtypes in the LHb-induced cardiovascular responses. Application of selective dopamine D1/D5, D2/D3, or D4 receptor antagonists attenuated the LHb-induced pressor response but did not change the HR response. Furthermore, we examined the effect of inactivation of the ventral tegmental area (VTA) on the cardiovascular response induced by LHb stimulation. Inactivation of the VTA turned bradycardia into tachycardia caused by the LHb stimulation and attenuated the pressor response. Our results indicated that regulation of the dopaminergic system by the LHb mediates the generation of the autonomic cardiovascular response. Dopamine D1-like and D2-like receptors mediate the sympathoexcitation resulting from the activation of the LHb. The VTA is one of the dopaminergic origins related to the cardiovascular response originating from LHb activation.

The neural substrates of bruxism: current knowledge and clinical implications

Bruxism is a complex orofacial behavior that can occur during sleep or wakefulness, characterized by the involuntary grinding or clenching of teeth, involving repetitive activity of the jaw muscles. Its etiology is multifactorial, influenced by genetic, psychological, physiological, and lifestyle factors. While the mild bruxism may not necessitate treatment, severe bruxism can lead to significant consequences, including tooth damage, jaw pain, fatigue, and headaches. The bruxism has been associated with medical conditions, such as stress, anxiety, sleep disorders, and various neurological disorders; however, the exact pathophysiology remains elusive. Although the central nervous system is strongly implicated in the development of bruxism, specific neural substrates have not yet been conclusively established. Furthermore, there is evidence to suggest that individuals with bruxism may exhibit neural plasticity, resulting in the establishment of distinct neural circuitry that control the jaw movements. The application of various neurophysiological techniques in both clinical and pre-clinical studies provides valuable insights into the neural mechanisms underlying bruxism. This review aims to comprehensively examine the current literature on the neural pathways involved in bruxism, with the goal of improving the clinical approach and therapeutics for this condition. A deeper understanding of the neural circuitry controlling bruxism holds the potential to advance future treatment approaches and improve the management of patients with bruxism.

Mimicking opioid analgesia in cortical pain circuits

The anterior cingulate cortex plays a pivotal role in the cognitive and affective aspects of pain perception. Both endogenous and exogenous opioid signaling within the cingulate mitigate cortical nociception, reducing pain unpleasantness. However, the specific functional and molecular identities of cells mediating opioid analgesia in the cingulate remain elusive. Given the complexity of pain as a sensory and emotional experience, and the richness of ethological pain-related behaviors, we developed a standardized, deep-learning platform for deconstructing the behavior dynamics associated with the affective component of pain in mice-LUPE (Light aUtomated Pain Evaluator). LUPE removes human bias in behavior quantification and accelerated analysis from weeks to hours, which we leveraged to discover that morphine altered attentional and motivational pain behaviors akin to affective analgesia in humans. Through activity-dependent genetics and single-nuclei RNA sequencing, we identified specific ensembles of nociceptive cingulate neuron-types expressing mu-opioid receptors. Tuning receptor expression in these cells bidirectionally modulated morphine analgesia. Moreover, we employed a synthetic opioid receptor promoter-driven approach for cell-type specific optical and chemical genetic viral therapies to mimic morphine's pain-relieving effects in the cingulate, without reinforcement. This approach offers a novel strategy for precision pain management by targeting a key nociceptive cortical circuit with on-demand, non-addictive, and effective analgesia.

Input-output specific orchestration of aversive valence in lateral habenula during stress dynamics

Stress has been considered as a major risk factor for depressive disorders, triggering depression onset via inducing persistent dysfunctions in specialized brain regions and neural circuits. Among various regions across the brain, the lateral habenula (LHb) serves as a critical hub for processing aversive information during the dynamic process of stress accumulation, thus having been implicated in the pathogenesis of depression. LHb neurons integrate aversive valence conveyed by distinct upstream inputs, many of which selectively innervate the medial part (LHbM) or lateral part (LHbL) of LHb. LHb subregions also separately assign aversive valence via dissociable projections to the downstream targets in the midbrain which provides feedback loops. Despite these strides, the spatiotemporal dynamics of LHb-centric neural circuits remain elusive during the progression of depression-like state under stress. In this review, we attempt to describe a framework in which LHb orchestrates aversive valence via the input-output specific neuronal architecture. Notably, a physiological form of Hebbian plasticity in LHb under multiple stressors has been unveiled to incubate neuronal hyperactivity in an input-specific manner, which causally encodes chronic stress experience and drives depression onset. Collectively, the recent progress and future efforts in elucidating LHb circuits shed light on early interventions and circuit-specific antidepressant therapies.

The Formation and Function of the VTA Dopamine System

The midbrain dopamine system is a sophisticated hub that integrates diverse inputs to control multiple physiological functions, including locomotion, motivation, cognition, reward, as well as maternal and reproductive behaviors. Dopamine is a neurotransmitter that binds to G-protein-coupled receptors. Dopamine also works together with other neurotransmitters and various neuropeptides to maintain the balance of synaptic functions. The dysfunction of the dopamine system leads to several conditions, including Parkinson's disease, Huntington's disease, major depression, schizophrenia, and drug addiction. The ventral tegmental area (VTA) has been identified as an important relay nucleus that modulates homeostatic plasticity in the midbrain dopamine system. Due to the complexity of synaptic transmissions and input-output connections in the VTA, the structure and function of this crucial brain region are still not fully understood. In this review article, we mainly focus on the cell types, neurotransmitters, neuropeptides, ion channels, receptors, and neural circuits of the VTA dopamine system, with the hope of obtaining new insight into the formation and function of this vital brain region.

Association between RMTg Neuropeptide Genes and Negative Effect during Alcohol Withdrawal in Mice

Alcohol use disorders (AUDs) frequently co-occur with negative mood disorders, such as anxiety and depression, exacerbating relapse through dopaminergic dysfunction. Stress-related neuropeptides play a crucial role in AUD pathophysiology by modulating dopamine (DA) function. The rostromedial tegmental nucleus (RMTg), which inhibits midbrain dopamine neurons and signals aversion, has been shown to increase ethanol consumption and negative emotional states during abstinence. Despite some stress-related neuropeptides acting through the RMTg to affect addiction behaviors, their specific roles in alcohol-induced contexts remain underexplored. This study utilized an intermittent voluntary drinking model in mice to induce negative effect behavior 24 h into ethanol (EtOH) abstinence (post-EtOH). It examined changes in pro-stress (Pnoc, Oxt, Npy) and anti-stress (Crf, Pomc, Avp, Orx, Pdyn) neuropeptide-coding genes and analyzed their correlations with aversive behaviors. We observed that adult male C57BL/6J mice displayed evident anxiety, anhedonia, and depression-like symptoms at 24 h post-EtOH. The laser-capture microdissection technique, coupled with or without retrograde tracing, was used to harvest total ventral tegmental area (VTA)-projecting neurons or the intact RMTg area. The findings revealed that post-EtOH consistently reduced Pnoc and Orx levels while elevating Crf levels in these neuronal populations. Notably, RMTg Pnoc and Npy levels counteracted ethanol consumption and depression severity, while Crf levels were indicative of the mice's anxiety levels. Together, these results underscore the potential role of stress-related neuropeptides in the RMTg in regulating the negative emotions related to AUDs, offering novel insights for future research.

A nociceptive amygdala-striatal pathway for chronic pain aversion

The basolateral amygdala (BLA) is essential for assigning positive or negative valence to sensory stimuli. Noxious stimuli that cause pain are encoded by an ensemble of nociceptive BLA projection neurons (BLAnoci ensemble). However, the role of the BLAnoci ensemble in mediating behavior changes and the molecular signatures and downstream targets distinguishing this ensemble remain poorly understood. Here, we show that the same BLAnoci ensemble neurons are required for both acute and chronic neuropathic pain behavior. Using single nucleus RNA-sequencing, we characterized the effect of acute and chronic pain on the BLA and identified enrichment for genes with known functions in axonal and synaptic organization and pain perception. We thus examined the brain-wide targets of the BLAnoci ensemble and uncovered a previously undescribed nociceptive hotspot of the nucleus accumbens shell (NAcSh) that mirrors the stability and specificity of the BLAnoci ensemble and is recruited in chronic pain. Notably, BLAnoci ensemble axons transmit acute and neuropathic nociceptive information to the NAcSh, highlighting this nociceptive amygdala-striatal circuit as a unique pathway for affective-motivational responses across pain states.