Distribution of VTA Glutamate and Dopamine Terminals, and their Significance in CA1 Neural Network Activity

Neuronal biomarkers as potential therapeutic targets for drug addiction related to sex differences in the brain: Opportunities for personalized treatment approaches

Biological sex disparities manifest at various stages of drug addiction, including craving, substance abuse, abstinence, and relapse. These discrepancies are underpinned by notable distinctions in neurobiological substrates, encompassing brain structures, functions, and neurotransmitter systems implicated in drug addiction. Neuronal biomarkers, such as neurotransmitters, signaling proteins, and genes may be associated with the diagnosis, prognosis, and treatment outcomes in both biological sexes afflicted by drug abuse. Sex differences in the neural reward system, mainly through dopaminergic transmission during drug abuse, can be attributed to modifications in neurotransmitter systems and signaling pathways. This results in distinct patterns of neural activation and responsiveness to addictive substances in males and females. Sex hormones, the estrus/menstrual cycle, and cerebral neurochemistry contribute to the progression of psychological and physiological dependence in both male and female individuals grappling with addiction. Moreover, the alteration of sex hormone balance and neurotransmitter release plays a pivotal role in substance use disorders, subsequently modulating cognitive functions pertinent to reward, including memory formation, decision-making, and locomotor activity. Comparative investigations reveal distinctions in brain region volume, gene expression, neuronal firing, and circuitry in substance use disorders affecting individuals of both biological sexes. This review examines prevalent substance use disorders to elucidate the impact of sex hormones as therapeutic biomarkers on the mesocorticolimbic neurotransmitter systems via diverse mechanisms within the addicted brain. We underscore the imperative necessity of considering these variations to gain a deeper comprehension of addiction mechanisms and potentially discern sex-specific neuronal biomarkers for tailored therapeutic interventions.

Distinct catecholaminergic pathways projecting to hippocampal CA1 transmit contrasting signals during navigation in familiar and novel environments

Neuromodulatory inputs to the hippocampus play pivotal roles in modulating synaptic plasticity, shaping neuronal activity, and influencing learning and memory. Recently it has been shown that the main sources of catecholamines to the hippocampus, ventral tegmental area (VTA) and locus coeruleus (LC), may have overlapping release of neurotransmitters and effects on the hippocampus. Therefore, to dissect the impacts of both VTA and LC circuits on hippocampal function, a thorough examination of how these pathways might differentially operate during behavior and learning is necessary. We therefore utilized 2-photon microscopy to functionally image the activity of VTA and LC axons within the CA1 region of the dorsal hippocampus in head-fixed male mice navigating linear paths within virtual reality (VR) environments. We found that within familiar environments some VTA axons and the vast majority of LC axons showed a correlation with the animals' running speed. However, as mice approached previously learned rewarded locations, a large majority of VTA axons exhibited a gradual ramping-up of activity, peaking at the reward location. In contrast, LC axons displayed a pre-movement signal predictive of the animal's transition from immobility to movement. Interestingly, a marked divergence emerged following a switch from the familiar to novel VR environments. Many LC axons showed large increases in activity that remained elevated for over a minute, while the previously observed VTA axon ramping-to-reward dynamics disappeared during the same period. In conclusion, these findings highlight distinct roles of VTA and LC catecholaminergic inputs in the dorsal CA1 hippocampal region. These inputs encode unique information, with reward information in VTA inputs and novelty and kinematic information in LC inputs, likely contributing to differential modulation of hippocampal activity during behavior and learning.

Liver kinase B-1 modulates the activity of dopamine neurons in the ventral tegmental area and regulates social memory formation

Social memory is the ability to discriminate between familiar and unknown conspecifics. It is an important component of social cognition and is therefore essential for the establishment of social relationships. Although the neural circuit mechanisms underlying social memory encoding have been well investigated, little focus has been placed on the regulatory mechanisms of social memory processing. The dopaminergic system, originating from the midbrain ventral tegmental area (VTA), is a key modulator of cognitive function. This study aimed to illustrate its role in modulating social memory and explore the possible molecular mechanisms. Here, we show that the activation of VTA dopamine (DA) neurons is required for the formation, but not the retrieval, of social memory. Inhibition of VTA DA neurons before social interaction, but not 24 h after social interaction, significantly impaired social discrimination the following day. In addition, we showed that the activation of VTA DA neurons was regulated by the serine/threonine protein kinase liver kinase B1 (Lkb1). Deletion of Lkb1 in VTA DA neurons reduced the frequency of burst firing of dopaminergic neurons. Furthermore, Lkb1 plays an important role in regulating social behaviors. Both genetic and virus-mediated deletions of Lkb1 in the VTA of adult mice impaired social memory and subsequently attenuated social familiarization. Altogether, our results provide direct evidence linking social memory formation to the activation of VTA DA neurons in mice and illustrate the crucial role of Lkb1 in regulating VTA DA neuron function.

Hippocampal synaptic failure is an early event in experimental parkinsonism with subtle cognitive deficit

Learning and memory mainly rely on correct synaptic function in the hippocampus and other brain regions. In Parkinson's disease, subtle cognitive deficits may even precede motor signs early in the disease. Hence, we set out to unravel the earliest hippocampal synaptic alterations associated with human α-synuclein overexpression prior to and soon after the appearance of cognitive deficits in a parkinsonism model. We bilaterally injected adeno-associated viral vectors encoding A53T-mutated human α-synuclein into the substantia nigra of rats, and evaluated them 1, 2, 4 and 16 weeks post-inoculation by immunohistochemistry and immunofluorescence to study degeneration and distribution of α-synuclein in the midbrain and hippocampus. The object location test was used to evaluate hippocampal-dependent memory. Sequential window acquisition of all theoretical mass spectrometry-based proteomics and fluorescence analysis of single-synapse long-term potentiation were used to study alterations to protein composition and plasticity in isolated hippocampal synapses. The effect of L-DOPA and pramipexole on long-term potentiation was also tested. Human α-synuclein was found within dopaminergic and glutamatergic neurons of the ventral tegmental area, and in dopaminergic, glutamatergic and GABAergic axon terminals in the hippocampus from 1 week post-inoculation, concomitant with mild dopaminergic degeneration in the ventral tegmental area. In the hippocampus, differential expression of proteins involved in synaptic vesicle cycling, neurotransmitter release and receptor trafficking, together with impaired long-term potentiation were the first events observed (1 week post-inoculation), preceding cognitive deficits (4 weeks post-inoculation). Later on, at 16 weeks post-inoculation, there was a deregulation of proteins involved in synaptic function, particularly those involved in the regulation of membrane potential, ion balance and receptor signalling. Hippocampal long-term potentiation was impaired before and soon after the onset of cognitive deficits, at 1 and 4 weeks post-inoculation, respectively. L-DOPA recovered hippocampal long-term potentiation more efficiently at 4 weeks post-inoculation than pramipexole, which partially rescued it at both time points. Overall, we found impaired synaptic plasticity and proteome dysregulation at hippocampal terminals to be the first events that contribute to the development of cognitive deficits in experimental parkinsonism. Our results not only point to dopaminergic but also to glutamatergic and GABAergic dysfunction, highlighting the relevance of the three neurotransmitter systems in the ventral tegmental area-hippocampus interaction from the earliest stages of parkinsonism. The proteins identified in the current work may constitute potential biomarkers of early synaptic damage in the hippocampus and hence, therapies targeting these could potentially restore early synaptic malfunction and consequently, cognitive deficits in Parkinson's disease.

Investigations of brain-wide functional and structural networks of dopaminergic and CamKIIα-positive neurons in VTA with DREADD-fMRI and neurotropic virus tracing technologies

BACKGROUND

The ventral tegmental area (VTA) contains heterogeneous cell populations. The dopaminergic neurons in VTA play a central role in reward and cognition, while CamKIIα-positive neurons, composed mainly of glutamatergic and some dopaminergic neurons, participate in the reward learning and locomotor activity behaviors. The differences in brain-wide functional and structural networks between these two neuronal subtypes were comparatively elucidated.

METHODS

In this study, we applied a method combining Designer Receptors Exclusively Activated by Designer Drugs (DREADD) and fMRI to assess the cell type-specific modulation of whole-brain neural networks. rAAV encoding the cre-dependent hM3D was injected into the right VTA of DAT-cre or CamKIIα-cre transgenic rats. The global brain activities elicited by DREADD stimulation were then detected using BOLD-fMRI. Furthermore, the cre-dependent antegrade transsynaptic viral tracer H129ΔTK-TT was applied to label the outputs of VTA neurons.

RESULTS

We found that DREADD stimulation of dopaminergic neurons induced significant BOLD signal changes in the VTA and several VTA-related regions including mPFC, Cg and Septum. More regions responded to selective activation of VTA CamKIIα-positive neurons, resulting in increased BOLD signals in VTA, Insula, mPFC, MC_R (Right), Cg, Septum, Hipp, TH_R, PtA_R, and ViC_R. Along with DREADD-BOLD analysis, further neuronal tracing identified multiple cortical (MC, mPFC) and subcortical (Hipp, TH) brain regions that are structurally and functionally connected by VTA dopaminergic and CamKIIα-positive neurons.

CONCLUSIONS

Our study dissects brain-wide structural and functional networks of two neuronal subtypes in VTA and advances our understanding of VTA functions.

VTA multifaceted modulation of CA1 local circuits

Excitatory pyramidal (PYR) cell activation of interneurons (INT) produces network oscillations that underlie cognitive processes in the CA1. Neural projections from the ventral tegmental area (VTA) to the hippocampus contribute to novelty detection by modulating CA1 PYR and INT activity. The role of the VTA in the VTA-hippocampus loop is mostly attributed to the dopamine neurons although the VTA glutamate-releasing terminals are dominant in the hippocampus. Because of the traditional focus on VTA dopamine circuits, how VTA glutamate inputs modulate PYR activation of INT in CA1 neuronal ensembles is poorly understood and has not been distinguished from the VTA dopamine inputs. By combining CA1 extracellular recording with VTA photostimulation in anesthetized mice, we compared the effects of VTA dopamine and glutamate input on CA1 PYR/INT connections. Stimulation of VTA glutamate neurons shortened PYR/INT connection time without altering the synchronization or connectivity strength. Conversely, activation of VTA dopamine inputs delayed CA1 PYR/INT connection time and increased the synchronization in putative pairs. Taken together, we conclude that VTA dopamine and glutamate projections produce tract-specific effects on CA1 PYR/INT connectivity and synchrony. As such, selective activation or co-activation of these systems will likely produce a range of modulatory effects on local CA1 circuits.

Effects of chronic delta-opioid receptor agonist on the excitability of hippocampal glutamate and brainstem monoamine neurons, anxiety, locomotion, and habituation in rats

BACKGROUND

Short-term treatment with non-peptide agonists of delta-opioid receptors, such as agonist SNC80, induced behavioral effects in rodents, which could be modulated via changes in central neurotransmission. The present experiments aimed at testing the hypothesis that chronic treatment with SNC80 induces anxiolytic effects associated with changes in hippocampal glutamate and brainstem monoamine pathways.

METHODS

Adult male Wistar rats were used in experiments. Rats were treated with SNC80 (3 mg/kg/day) for fourteen days. Neuronal excitability was assessed using extracellular in vivo single-unit electrophysiology. The behavioral parameters were examined using the elevated plus maze and open field tests.

RESULTS

Chronic SNC80 treatment increased the excitability of hippocampal glutamate and ventral tegmental area dopamine neurons and had no effect on the firing activity of dorsal raphe nucleus serotonin cells. Chronic SNC80 treatment induced anxiolytic effects, which were, however, confounded by increased locomotor activity clearly confirmed in an open field test. The ability to cope with stressful situations and habituation processes in a novel environment was not influenced by chronic treatment with SNC80.

CONCLUSION

Our study suggests that the psychoactive effects of SNC80 might be explained by its ability to stimulate hippocampal glutamate and mesolimbic dopamine transmission.

Selective Modulation of Hippocampal Theta Oscillations in Response to Morphine versus Natural Reward

Despite the overlapping neural circuits underlying natural and drug rewards, several studies have suggested different behavioral and neurochemical mechanisms in response to drug vs. natural rewards. The strong link between hippocampal theta oscillations (4-12 Hz) and reward-associated learning and memory has raised the hypothesis that this rhythm in hippocampal CA1 might be differently modulated by drug- and natural-conditioned place preference (CPP). Time-frequency analysis of recorded local field potentials (LFPs) from the CA1 of freely moving male rats previously exposed to a natural (in this case, food), drug (in this case, morphine), or saline (control) reward cue in the CPP paradigm showed that the hippocampal CA1 theta activity represents a different pattern for entrance to the rewarded compared to unrewarded compartment during the post-test session of morphine- and natural-CPP. Comparing LFP activity in the CA1 between the saline and morphine/natural groups showed that the maximum theta power occurred before entering the unrewarded compartment and after the entrance to the rewarded compartment in morphine and natural groups, respectively. In conclusion, our findings suggest that drug and natural rewards could differently affect the theta dynamic in the hippocampal CA1 region during reward-associated learning and contextual cueing in the CPP paradigm.

VTA-NAc glutaminergic projection involves in the regulation of pain and pain-related anxiety

Background

Besides the established role of dopamine neurons and projections in nociceptive stimuli, the involvement of ventral tegmental area (VTA) glutamatergic projections to nucleus accumbens (NAc) in pain remains unknown. In the present study, we aimed to examine the role of VTA glutamatergic projections to NAc in painful stimuli and its related behavioral changes.

Methods

Unilateral chronic constrictive injury (CCI) of sciatic nerve or intraplantar hind paw injections (i.pl.) of complete Freund's adjuvant (CFA) were used to develop pathological pain models in wild-type and VGluT2-Cre mice. The involvement of VTA glutamatergic neurons with projections to NAc in CCI-induced pain model was noted by c-Fos labeling and firing rate recordings. Pain response and pain-related behavior changes to the artificial manipulation of the VTA glutamatergic projections to NAc were observed by Hargreaves tests, von Frey tests, open field tests, elevated maze tests, and sucrose preference tests.

Results

Glutamatergic neurons in VTA had efferent inputs to shell area of the NAc. The CCI pain model significantly increased neuronal activity and firing rate in VTA glutamate neurons with projections to NAc. The photoinhibition of these glutamatergic projections relieved CCI-induced neuropathic pain and CFA-induced acute and chronic inflammatory pain. Moreover, pathological neuropathic pain-induced anxiety and less sucrose preference were also relieved by inhibiting the VTA glutamatergic projections to NAc.

Conclusion

Together, glutamatergic inputs from VTA to NAc contribute to chronic neuropathic and inflammatory pain and pain-related anxiety and depressive behaviors, providing a mechanism for developing novel therapeutic methods.

Reward expectation extinction restructures and degrades CA1 spatial maps through loss of a dopaminergic reward proximity signal

Hippocampal place cells support reward-related spatial memories by forming a cognitive map that over-represents reward locations. The strength of these memories is modulated by the extent of reward expectation during encoding. However, the circuit mechanisms underlying this modulation are unclear. Here we find that when reward expectation is extinguished in mice, they remain engaged with their environment, yet place cell over-representation of rewards vanishes, place field remapping throughout the environment increases, and place field trial-to-trial reliability decreases. Interestingly, Ventral Tegmental Area (VTA) dopaminergic axons in CA1 exhibit a ramping reward-proximity signal that depends on reward expectation and inhibiting VTA dopaminergic neurons largely replicates the effects of extinguishing reward expectation. We conclude that changing reward expectation restructures CA1 cognitive maps and determines map reliability by modulating the dopaminergic VTA-CA1 reward-proximity signal. Thus, internal states of high reward expectation enhance encoding of spatial memories by reinforcing hippocampal cognitive maps associated with reward.

Anatomy and Function of Ventral Tegmental Area Glutamate Neurons

The ventral tegmental area (VTA) is well known for regulating reward consumption, learning, memory, and addiction behaviors through mediating dopamine (DA) release in downstream regions. Other than DA neurons, the VTA is known to be heterogeneous and contains other types of neurons, including glutamate neurons. In contrast to the well-studied and established functions of DA neurons, the role of VTA glutamate neurons is understudied, presumably due to their relatively small quantity and a lack of effective means to study them. Yet, emerging studies have begun to reveal the importance of glutamate release from VTA neurons in regulating diverse behavioral repertoire through a complex intra-VTA and long-range neuronal network. In this review, we summarize the features of VTA glutamate neurons from three perspectives, namely, cellular properties, neural connections, and behavioral functions. Delineation of VTA glutamatergic pathways and their interactions with VTA DA neurons in regulating behaviors may reveal previously unappreciated functions of the VTA in other physiological processes.

Agomelatine attenuates alcohol craving and withdrawal symptoms by modulating the Notch1 signaling pathway in rats

AIM

Alcohol abuse is a significant causative factor of death worldwide. The Notch1 signaling pathway is involved in alcohol tolerance, withdrawal and dependence. Agomelatine is a known antidepressant acting as a melatonin receptor (MT1/2) agonist and a 5-hydroxytryptamine receptor-2C antagonist. However, its effects on alcohol cravings and alcohol withdrawal symptoms have not been investigated. In this study, we assessed the possibility of using agomelatine for the treatment of these symptoms in a rat model of alcoholism and the possible role of Notch1 signaling.

MAIN METHODS

We induced alcoholism in rats using a free-choice drinking model for 60 days. From day 61, free-choice was continued until day 82 for the craving model, whereas only water was offered in the withdrawal model. Meanwhile, the treated groups for both models received agomelatine (50 mg/kg/day) orally from day 61 to 82, followed by behavioral, histopathological and biochemical assessment.

KEY FINDINGS

Agomelatine treatment caused significant decrease in alcohol consumption with a positive effect on anxiety-like behavior in the open field, memory in the Morris water maze and immobility in the forced swim test. Moreover, agomelatine induced the expression of Notch1 pathway markers, including Notch1, NICD, CREB, CCNE-2, Hes-1, both total and phosphorylated ERK1/2, MMP9, Per2and RGS-2 in the hippocampal formation. By contrast, NMDAR expression was reduced. Furthermore, agomelatine normalized the serum levels of BDNF, cortisol, dopamine and glutamate which were disrupted by alcohol consumption.

SIGNIFICANCE

Based on these findings, agomelatine reversed alcohol cravings and withdrawal symptoms associated with alcohol dependence by modulating the Notch1 signaling pathway.

The late consolidation of an aversive memory is promoted by VTA dopamine release in the dorsal hippocampus

The hippocampus has been implicated in the processing and storage of aversive memories but the precise mechanisms by which these memories persist in time remain elusive. We have demonstrated that dopaminergic neurotransmission in the dorsal hippocampus regulates the long-term storage of both appetitive and aversive memories at a critical time point known as "late consolidation" (12 hr after the learning experience). This modulation appears to have opposite effects depending on the valence of the stimuli, with hippocampal dopamine release peaking immediately and 13-17 hr after a rewarding experience. Here, we determined the release pattern of hippocampal dopamine following an aversive experience, in order to better understand this opposite modulation process. We observed significant increases in dopamine levels at several times (6-8, 11-12, and 15 hr) after subjecting rats to a conditioned place aversion (CPA) task with the aversive agent lithium chloride (LiCl). Early pharmacological blockade of hippocampal DA receptors impaired CPA memory consolidation. In addition and consistent with previous findings showing that late post-training infusions of dopaminergic agents into the hippocampus modulate the long-term storage of aversive memories, we found that the photostimulation of dopaminergic VTA fibers in the dorsal hippocampus 11-12 hr after CPA training was enough to transform a short-lasting long-term memory into a long-lasting one. The fact that the persistence of an aversive memory can still be affected several hours after the learning experience opens new avenues to develop behavioral and pharmacological strategies for the treatment of a variety of mental disorders.

Entacapone Treatment Modulates Hippocampal Proteins Related to Synaptic Vehicle Trafficking

Entacapone, a reversible inhibitor of catechol-O-methyl transferase, is used for patients in Parkinson’s disease because it increases the bioavailability and effectiveness of levodopa. In the present study, we observed that entacapone increases novel object recognition and neuroblasts in the hippocampus. In the present study, two-dimensional electrophoresis (2-DE) and matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry were performed to compare the abundance profiles of proteins expressed in the hippocampus after entacapone treatment in mice. Results of 2-DE, MALDI-TOF mass spectrometry, and subsequent proteomic analysis revealed an altered protein expression profile in the hippocampus after entacapone treatment. Based on proteomic analysis, 556 spots were paired during the image analysis of 2-DE gels and 76 proteins were significantly changed more than two-fold among identified proteins. Proteomic analysis indicated that treatment with entacapone induced expressional changes in proteins involved in synaptic transmission, cellular processes, cellular signaling, the regulation of cytoskeletal structure, energy metabolism, and various subcellular enzymatic reactions. In particular, entacapone significantly increased proteins related to synaptic trafficking and plasticity, such as dynamin 1, synapsin I, and Munc18-1. Immunohistochemical staining showed the localization of the proteins, and western blot confirmed the significant increases in dynamin I (203.5% of control) in the hippocampus as well as synapsin I (254.0% of control) and Munc18-1 (167.1% of control) in the synaptic vesicle fraction of hippocampus after entacapone treatment. These results suggest that entacapone can enhance hippocampal synaptic trafficking and plasticity against various neurological diseases related to hippocampal dysfunction.