Ventral tegmental area dopamine projections to the hippocampus trigger long-term potentiation and contextual learning.

In most models of neuronal plasticity and memory, dopamine is thought to promote the long-term maintenance of Long-Term Potentiation (LTP) underlying memory processes, but not the initiation of plasticity or new information storage. Here, we used optogenetic manipulation of midbrain dopamine neurons in male DAT::Cre mice, and discovered that stimulating the Schaffer collaterals - the glutamatergic axons connecting CA3 and CA1 regions - of the dorsal hippocampus concomitantly with midbrain dopamine terminals within a 200 millisecond time-window triggers LTP at glutamatergic synapses. Moreover, we showed that the stimulation of this dopaminergic pathway facilitates contextual learning in awake behaving mice, while its inhibition hinders it. Thus, activation of midbrain dopamine can operate as a teaching signal that triggers NeoHebbian LTP and promotes supervised learning.

Ventral pallidum GABA and glutamate neurons drive approach and avoidance through distinct modulation of VTA cell types.

The ventral pallidum (VP) contains GABA and glutamate neurons projecting to ventral tegmental area (VTA) whose stimulation drives approach and avoidance, respectively. Yet little is known about the mechanisms by which VP cell types shape VTA activity and drive behavior. Here, we found that both VP GABA and glutamate neurons were activated during approach to reward or by delivery of an aversive stimulus. Stimulation of VP GABA neurons inhibited VTA GABA, but activated dopamine and glutamate neurons. Remarkably, stimulation-evoked activation was behavior-contingent such that VTA recruitment was inhibited when evoked by the subject's own action. Conversely, VP glutamate neurons activated VTA GABA, as well as dopamine and glutamate neurons, despite driving aversion. However, VP glutamate neurons evoked dopamine in aversion-associated ventromedial nucleus accumbens (NAc), but reduced dopamine release in reward-associated dorsomedial NAc. These findings show how heterogeneous VP projections to VTA can be engaged to shape approach and avoidance behaviors.

Optogenetic activation of dorsal raphe serotonin neurons induces brain-wide activation.

Serotonin is a neuromodulator that affects multiple behavioral and cognitive functions. Nonetheless, how serotonin causes such a variety of effects via brain-wide projections and various receptors remains unclear. Here we measured brain-wide responses to optogenetic stimulation of serotonin neurons in the dorsal raphe nucleus (DRN) of the male mouse brain using functional MRI with an 11.7 T scanner and a cryoprobe. Transient activation of DRN serotonin neurons caused brain-wide activation, including the medial prefrontal cortex, the striatum, and the ventral tegmental area. The same stimulation under anesthesia with isoflurane decreased brain-wide activation, including the hippocampal complex. These brain-wide response patterns can be explained by DRN serotonergic projection topography and serotonin receptor expression profiles, with enhanced weights on 5-HT1 receptors. Together, these results provide insight into the DR serotonergic system, which is consistent with recent discoveries of its functions in adaptive behaviors.

Role of dopamine neurons in familiarity.

Dopamine neurons signal the salience of environmental stimuli and influence learning, although it is less clear if these neurons also determine the salience of memories. Ventral tegmental area (VTA) dopamine neurons increase their firing in the presence of new objects and reduce it upon repeated, inconsequential exposures, marking the shift from novelty to familiarity. This study investigates how dopamine neuron activity during repeated familiar object exposure affects an animal's preference for new objects in a subsequent novel object recognition (NOR) test. We hypothesize that a single familiarization session will not sufficiently lower dopamine activity, such that the memory of a familiar object remains salient, leading to equal exploration of familiar and novel objects and weaker NOR discrimination. In contrast, multiple familiarization sessions likely suppress dopamine activity more effectively, reducing the salience of the familiar object and enhancing subsequent novelty discrimination. Our experiments in mice indicated that multiple familiarization sessions reduce VTA dopamine neuron activation, as measured by c-Fos expression, and enhance novelty discrimination compared with a single familiarization session. Dopamine neurons that show responsiveness to novelty were primarily located in the paranigral nucleus of the VTA and expressed vesicular glutamate transporter 2 transcripts, marking them as dopamine-glutamate neurons. Chemogenetic inhibition of dopamine neurons during a single session paralleled the effects of multiple sessions, improving NOR. These findings suggest that a critical role of dopamine neurons during the transition from novelty to familiarity is to modulate the salience of an object's memory.

Stimulation of VTA dopamine inputs to LH upregulates orexin neuronal activity in a DRD2-dependent manner.

Dopamine and orexins (hypocretins) play important roles in regulating reward-seeking behaviors. It is known that hypothalamic orexinergic neurons project to dopamine neurons in the ventral tegmental area (VTA), where they can stimulate dopaminergic neuronal activity. Although there are reciprocal connections between dopaminergic and orexinergic systems, whether and how dopamine regulates the activity of orexin neurons is currently not known. Here we implemented an opto-Pavlovian task in which mice learn to associate a sensory cue with optogenetic dopamine neuron stimulation to investigate the relationship between dopamine release and orexin neuron activity in the lateral hypothalamus (LH). We found that dopamine release can be evoked in LH upon optogenetic stimulation of VTA dopamine neurons and is also naturally evoked by cue presentation after opto-Pavlovian learning. Furthermore, orexin neuron activity could also be upregulated by local stimulation of dopaminergic terminals in the LH in a way that is partially dependent on dopamine D2 receptors (DRD2). Our results reveal previously unknown orexinergic coding of reward expectation and unveil an orexin-regulatory axis mediated by local dopamine inputs in the LH.

Control of polymers' amorphous-crystalline transition enables miniaturization and multifunctional integration for hydrogel bioelectronics.

Soft bioelectronic devices exhibit motion-adaptive properties for neural interfaces to investigate complex neural circuits. Here, we develop a fabrication approach through the control of metamorphic polymers' amorphous-crystalline transition to miniaturize and integrate multiple components into hydrogel bioelectronics. We attain an about 80% diameter reduction in chemically cross-linked polyvinyl alcohol hydrogel fibers in a fully hydrated state. This strategy allows regulation of hydrogel properties, including refractive index (1.37-1.40 at 480 nm), light transmission (>96%), stretchability (139-169%), bending stiffness (4.6 ± 1.4 N/m), and elastic modulus (2.8-9.3 MPa). To exploit the applications, we apply step-index hydrogel optical probes in the mouse ventral tegmental area, coupled with fiber photometry recordings and social behavioral assays. Additionally, we fabricate carbon nanotubes-PVA hydrogel microelectrodes by incorporating conductive nanomaterials in hydrogel for spontaneous neural activities recording. We enable simultaneous optogenetic stimulation and electrophysiological recordings of light-triggered neural activities in Channelrhodopsin-2 transgenic mice.

Spatiotemporal Distributions of Acoustic Propagation in Skull During Ultrasound Neuromodulation.

There is widespread interest and concern about the evidence and hypothesis that the auditory system is involved in ultrasound neuromodulation. We have addressed this problem by performing acoustic shear wave simulations in mouse skull and behavioral experiments in deaf mice. The simulation results showed that shear waves propagating along the skull did not reach sufficient acoustic pressure in the auditory cortex to modulate neurons. Behavioral experiments were subsequently performed to awaken anesthetized mice with ultrasound targeting the motor cortex or ventral tegmental area (VTA). The experimental results showed that ultrasound stimulation (US) of the target areas significantly increased arousal scores even in deaf mice, whereas the loss of ultrasound gel abolished the effect. Immunofluorescence staining also showed that ultrasound can modulate neurons in the target area, whereas neurons in the auditory cortex required the involvement of the normal auditory system for activation. In summary, the shear waves propagating along the skull cannot reach the auditory cortex and induce neuronal activation. Ultrasound neuromodulation-induced arousal behavior needs direct action on functionally relevant stimulation targets in the absence of auditory system participation.

Neural inhibition as implemented by an actor-critic model involves the human dorsal striatum and ventral tegmental area.

Inhibition is implicated across virtually all human experiences. As a trade-off of being very efficient, this executive function is also prone to many errors. Rodent and computational studies show that midbrain regions play crucial roles during errors by sending dopaminergic learning signals to the basal ganglia for behavioural adjustment. However, the parallels between animal and human neural anatomy and function are not determined. We scanned human adults while they performed an fMRI inhibitory task requiring trial-and-error learning. Guided by an actor-critic model, our results implicate the dorsal striatum and the ventral tegmental area as the actor and the critic, respectively. Using a multilevel and dimensional approach, we also demonstrate a link between midbrain and striatum circuit activity, inhibitory performance, and self-reported autistic and obsessive-compulsive subclinical traits.

Ultrasound Stimulation Attenuates CRS-Induced Depressive Behavior by Modulating Dopamine Release in the Prefrontal Cortex.

Depression is one of the most serious mental disorders affecting modern human life and is often caused by chronic stress. Dopamine system dysfunction is proposed to contribute to the pathophysiology of chronic stress, especially the ventral tegmental area (VTA) which mainly consists of dopaminergic neurons. Focused ultrasound stimulation (FUS) is a promising neuromodulation modality and multiple studies have demonstrated effective ultrasonic activation of cortical, subcortical, and related networks. However, the effects of FUS on the dopamine system and the potential link to chronic stress-induced depressive behaviors are relatively unknown. Here, we measured the effects of FUS targeting VTA on the improvement of depression-like behavior and evaluated the dopamine concentration in the downstream region - medial prefrontal cortex (mPFC). We found that targeting VTA FUS treatment alleviated chronic restraint stress (CRS) -induced anhedonia and despair behavior. Using an in vivo photometry approach, we analyzed the dopamine signal of mPFC and revealed a significant increase following the FUS, positively associated with the improvement of anhedonia behavior. FUS also protected the dopaminergic neurons in VTA from the damage caused by CRS exposure. Thus, these results demonstrated that targeting VTA FUS treatment significantly rescued the depressive-like behavior and declined dopamine level of mPFC induced by CRS. These beneficial effects of FUS might be due to protection in the DA neuron of VTA. Our findings suggest that FUS treatment could serve as a new therapeutic strategy for the treatment of stress-related disorders.

Multimodal Interrogation of Ventral Pallidum Projections Reveals Projection-Specific Signatures and Effects on Cocaine Reward.

The ventral pallidum (VP) is a central hub in the reward circuitry with diverse projections that have different behavioral roles attributed mostly to the connectivity with the downstream target. However, different VP projections may represent, as in the striatum, separate neuronal populations that differ in more than just connectivity. In this study, we performed in mice of both sexes a multimodal dissection of four major projections of the VP-to the lateral hypothalamus (VP→LH), ventral tegmental area (VP→VTA), lateral habenula (VP→LHb), and mediodorsal thalamus (VP→MDT)-with physiological, anatomical, genetic, and behavioral tools. We also tested for physiological differences between VP neurons receiving input from nucleus accumbens medium spiny neurons (MSNs) that express either the D1 (D1-MSNs) or the D2 (D2-MSNs) dopamine receptor. We show that each VP projection (1) when inhibited during a cocaine conditioned place preference (CPP) test affects performance differently, (2) receives a different pattern of inputs using rabies retrograde labeling, (3) shows differentially expressed genes using RNA sequencing, and (4) has projection-specific characteristics in excitability and synaptic input characteristics using whole-cell patch clamp. VP→LH and VP→VTA projections have different effects on CPP and show low overlap in circuit tracing experiments, as VP→VTA neurons receive more striatal input, while VP→LH neurons receive more olfactory input. Additionally, VP→VTA neurons are less excitable, while VP→LH neurons are more excitable than the average VP neuron, a difference driven mainly by D2-MSN-responding neurons. Thus, VP→VTA and VP→LH neurons may represent largely distinct populations of VP neurons.

Dopamine regulates attitude toward risk.

Specific brain pathways can lower or raise the willingness of monkeys to take risks.

Balancing risk-return decisions by manipulating the mesofrontal circuits in primates.

Decision-making is always coupled with some level of risk, with more pathological forms of risk-taking decisions manifesting as gambling disorders. In macaque monkeys trained in a high risk-high return (HH) versus low risk-low return (LL) choice task, we found that the reversible pharmacological inactivation of ventral Brodmann area 6 (area 6V) impaired the risk dependency of decision-making. Selective optogenetic activation of the mesofrontal pathway from the ventral tegmental area (VTA) to the ventral aspect of 6V resulted in stronger preference for HH, whereas activation of the pathway from the VTA to the dorsal aspect of 6V led to LL preference. Finally, computational decoding captured the modulations of behavioral preference. Our results suggest that VTA inputs to area 6V determine the decision balance between HH and LL.

State and rate-of-change encoding in parallel mesoaccumbal dopamine pathways.

The nervous system uses fast- and slow-adapting sensory detectors in parallel to enable neuronal representations of external states and their temporal dynamics. It is unknown whether this dichotomy also applies to internal representations that have no direct correlation in the physical world. Here we find that two distinct dopamine (DA) neuron subtypes encode either a state or its rate-of-change. In mice performing a reward-seeking task, we found that the animal's behavioral state and rate-of-change were encoded by the sustained activity of DA neurons in medial ventral tegmental area (VTA) DA neurons and transient activity in lateral VTA DA neurons, respectively. The neural activity patterns of VTA DA cell bodies matched DA release patterns within anatomically defined mesoaccumbal pathways. Based on these results, we propose a model in which the DA system uses two parallel lines for proportional-differential encoding of a state variable and its temporal dynamics.

Glutamatergic Projection from the Ventral Tegmental Area to the Zona Incerta Regulates Fear Response.

Innate defensive behavior is important for animal survival. The Vglut2+ neurons in the ventral tegmental area (VTA) have been demonstrated to play important roles in innate defensive behaviors, but the neural circuit mechanism is still unclear. Here, we find that VTA - zona incerta (ZI) glutamatergic projection is involved in regulating innate fear responses. Combining calcium signal recording and chemogentics, we find that VTA-Vglut2+ neurons respond to foot shock stimulus. Inhibition of VTA-Vglut2+ neurons reduces foot shock-evoked freezing, while chemogentic activation of these neurons results in an enhanced fear response. Using viral tracing and immunofluorescence, we show that VTA - Vglut2+ neurons send direct excitatory outputs to the ZI. Moreover, we find that the activity of VTAVglut2 - ZI projection is pivotal in modulating fear response. Together, our study reveals a new VTA - ZI glutamatergic circuit in mediating innate fear response and provides a potential target for treating post-traumatic stress disorder.

A septal-ventral tegmental area circuit drives exploratory behavior.

To survive, animals need to balance their exploratory drive with their need for safety. Subcortical circuits play an important role in initiating and modulating movement based on external demands and the internal state of the animal; however, how motivation and onset of locomotion are regulated remain largely unresolved. Here, we show that a glutamatergic pathway from the medial septum and diagonal band of Broca (MSDB) to the ventral tegmental area (VTA) controls exploratory locomotor behavior in mice. Using a self-supervised machine learning approach, we found an overrepresentation of exploratory actions, such as sniffing, whisking, and rearing, when this projection is optogenetically activated. Mechanistically, this role relies on glutamatergic MSDB projections that monosynaptically target a subset of both glutamatergic and dopaminergic VTA neurons. Taken together, we identified a glutamatergic basal forebrain to midbrain circuit that initiates locomotor activity and contributes to the expression of exploration-associated behavior.

PI4KIIIß-Mediated Phosphoinositides Metabolism Regulates Function of the VTA Dopaminergic Neurons and Depression-Like Behavior.

Phosphoinositides, including phosphatidylinositol-4,5-bisphosphate (PIP2), play a crucial role in controlling key cellular functions such as membrane and vesicle trafficking, ion channel, and transporter activity. Phosphatidylinositol 4-kinases (PI4K) are essential enzymes in regulating the turnover of phosphoinositides. However, the functional role of PI4Ks and mediated phosphoinositide metabolism in the central nervous system has not been fully revealed. In this study, we demonstrated that PI4KIIIß, one of the four members of PI4Ks, is an important regulator of VTA dopaminergic neuronal activity and related depression-like behavior of mice by controlling phosphoinositide turnover. Our findings provide new insights into possible mechanisms and potential drug targets for neuropsychiatric diseases, including depression. Both sexes were studied in basic behavior tests, but only male mice could be used in the social defeat depression model.

Glutamate inputs send prediction error of reward, but not negative value of aversive stimuli, to dopamine neurons.

Midbrain dopamine neurons are thought to signal reward prediction errors (RPEs), but the mechanisms underlying RPE computation, particularly the contributions of different neurotransmitters, remain poorly understood. Here, we used a genetically encoded glutamate sensor to examine the pattern of glutamate inputs to dopamine neurons in mice. We found that glutamate inputs exhibit virtually all of the characteristics of RPE rather than conveying a specific component of RPE computation, such as reward or expectation. Notably, whereas glutamate inputs were transiently inhibited by reward omission, they were excited by aversive stimuli. Opioid analgesics altered dopamine negative responses to aversive stimuli into more positive responses, whereas excitatory responses of glutamate inputs remained unchanged. Our findings uncover previously unknown synaptic mechanisms underlying RPE computations; dopamine responses are shaped by both synergistic and competitive interactions between glutamatergic and GABAergic inputs to dopamine neurons depending on valences, with competitive interactions playing a role in responses to aversive stimuli.

Sensory Cues Potentiate VTA Dopamine Mediated Reinforcement.

Sensory cues are critical for shaping decisions and invigorating actions during reward seeking. Dopamine neurons in the ventral tegmental area (VTA) are central in this process, supporting associative learning in Pavlovian and instrumental settings. Studies of intracranial self-stimulation (ICSS) behavior, which show that animals will work hard to receive stimulation of dopamine neurons, support the notion that dopamine transmits a reward or value signal to support learning. Recent studies have begun to question this, however, emphasizing dopamine's value-free functions, leaving its contribution to behavioral reinforcement somewhat muddled. Here, we investigated the role of sensory stimuli in dopamine-mediated reinforcement, using an optogenetic ICSS paradigm in tyrosine hydroxylase (TH)-Cre rats. We find that while VTA dopamine neuron activation in the absence of explicit external cues is sufficient to maintain robust self-stimulation, the presence of cues dramatically potentiates ICSS behavior. Our results support a framework where dopamine can have some base value as a reinforcer, but the impact of this signal is modulated heavily by the sensory learning context.

A mesocortical glutamatergic pathway modulates neuropathic pain independent of dopamine co-release.

Dysfunction in the mesocortical pathway, connecting the ventral tegmental area (VTA) to the prefrontal cortex, has been implicated in chronic pain. While extensive research has focused on the role of dopamine, the contribution of glutamatergic signaling in pain modulation remains unknown. Using in vivo calcium imaging, we observe diminished VTA glutamatergic activity targeting the prelimbic cortex (PL) in a mouse model of neuropathic pain. Optogenetic activation of VTA glutamatergic terminals in the PL alleviates neuropathic pain, whereas inhibiting these terminals in naïve mice induces pain-like responses. Importantly, this pain-modulating effect is independent of dopamine co-release, as demonstrated by CRISPR/Cas9-mediated gene deletion. Furthermore, we show that VTA neurons primarily project to excitatory neurons in the PL, and their activation restores PL outputs to the anterior cingulate cortex, a key region involved in pain processing. These findings reveal a distinct mesocortical glutamatergic pathway that critically modulates neuropathic pain independent of dopamine signaling.

Ventral pallidum neurons projecting to the ventral tegmental area reinforce but do not invigorate reward-seeking behavior.

Reward-predictive cues acquire motivating and reinforcing properties that contribute to the escalation and relapse of drug use in addiction. The ventral pallidum (VP) and ventral tegmental area (VTA) are two key nodes in brain reward circuitry implicated in addiction and cue-driven behavior. In the current study, we use in vivo fiber photometry and optogenetics to record from and manipulate VP→VTA in rats performing a discriminative stimulus task to determine the role these neurons play in invigoration and reinforcement by reward cues. We find that VP→VTA neurons are active during reward consumption and that optogenetic stimulation of these neurons biases choice behavior and is reinforcing. Critically, we find no encoding of reward-seeking vigor, and optogenetic stimulation does not enhance the probability or vigor of reward seeking in response to cues. Our results suggest that VP→VTA activity is more important for reinforcement than for invigoration of reward seeking by cues.