Selective Knockout of the Vesicular Monoamine Transporter 2 (Vmat2) Gene in Calbindin2/Calretinin-Positive Neurons Results in Profound Changes in Behavior and Response to Drugs of Abuse

Neurotoxicology of dopamine: Victim or assailant?

Since the identification of dopamine as a neurotransmitter in the mid-20th century, investigators have examined the regulation of dopamine homeostasis at a basic biological level and in human disorders. Genetic animal models that manipulate the expression of proteins involved in dopamine homeostasis have provided key insight into the consequences of dysregulated dopamine. As a result, we have come to understand the potential of dopamine to act as an endogenous neurotoxin through the generation of reactive oxygen species and reactive metabolites that can damage cellular macromolecules. Endogenous factors, such as genetic variation and subcellular processes, and exogenous factors, such as environmental exposures, have been identified as contributors to the dysregulation of dopamine homeostasis. Given the variety of dysregulating factors that impact dopamine homeostasis and the potential for dopamine itself to contribute to further cellular dysfunction, dopamine can be viewed as both the victim and an assailant of neurotoxicity. Parkinson's disease has emerged as the exemplar case study of dopamine dysregulation due to the genetic and environmental factors known to contribute to disease risk, and due to the evidence of dysregulated dopamine as a pathologic and pathogenic feature of the disease. This review, inspired by the talk, "Dopamine in Durham: location, location, location" presented by Dr. Miller for the Jacob Hooisma Memorial Lecture at the International Neurotoxicology Association meeting in 2023, offers a primer on dopamine toxicity covering endogenous and exogenous factors that disrupt dopamine homeostasis and the actions of dopamine as an endogenous neurotoxin.

Transport and inhibition mechanism for VMAT2-mediated synaptic vesicle loading of monoamines

Monoamine neurotransmitters such as serotonin and dopamine are loaded by vesicular monoamine transporter 2 (VMAT2) into synaptic vesicles for storage and subsequent release in neurons. Impaired VMAT2 function underlies various neuropsychiatric diseases. VMAT2 inhibitors reserpine and tetrabenazine are used to treat hypertension, movement disorders associated with Huntington's Disease and Tardive Dyskinesia. Despite its physiological and pharmacological significance, the structural basis underlying VMAT2 substrate recognition and its inhibition by various inhibitors remains unknown. Here we present cryo-EM structures of human apo VMAT2 in addition to states bound to serotonin, tetrabenazine, and reserpine. These structures collectively capture three states, namely the lumen-facing, occluded, and cytosol-facing conformations. Notably, tetrabenazine induces a substantial rearrangement of TM2 and TM7, extending beyond the typical rocker-switch movement. These functionally dynamic snapshots, complemented by biochemical analysis, unveil the essential components responsible for ligand recognition, elucidate the proton-driven exchange cycle, and provide a framework to design improved pharmaceutics targeting VMAT2.

Vesicular monoamine transporter (VMAT) regional expression and roles in pathological conditions

Vesicular monoamine transporters (VMATs) are key regulators of neurotransmitter release responsible for controlling numerous physiological, cognitive, emotional, and behavioral functions. They represent important therapeutic targets for numerous pathological conditions. There are two isoforms of VMAT transporter proteins that function as secondary active transporters into the vesicle for storage and release via exocytosis: VMAT1 (SLC18A1) and VMAT2 (SLC18A2) which differ in their function, quantity, and regional expression. VMAT2 has gained considerable interest as a therapeutic target and diagnostic marker. Inhibitors of VMAT2 have been used as an effective therapy for a range of pathological conditions. Additionally, the functionality and phenotypic classification of classical and nonclassical catecholaminergic neurons are identified by the presence of VMAT2 in catecholaminergic neurons. Dysregulation of VMAT2 is also implicated in many neuropsychiatric diseases. Despite the complex role of VMAT2, many aspects of its function remain unclear. Therefore, our aim is to expand our knowledge of the role of VMAT with a special focus on VMAT2 in different systems and cellular pathways which may potentially facilitate development of novel, more specific therapeutic targets. The current review provides a summary demonstrating the mechanism of action of VMAT, its functional role, and its contribution to disease progression and utilization as therapeutic targets.

The Effects of Chronic Unpredictable Mild Stress and Semi-Pure Diets on the Brain, Gut and Adrenal Medulla in C57BL6 Mice

Chronic stress is known to perturb serotonergic regulation in the brain, leading to mood, learning and memory impairments and increasing the risk of developing mood disorders. The influence of the gut microbiota on serotonergic regulation in the brain has received increased attention recently, justifying the investigation of the role of diet on the gut and the brain in mood disorders. Here, using a 4-week chronic unpredictable mild stress (CUMS) model in mice, we aimed to investigate the effects of a high-fat high-glycaemic index (HFD) and high-fibre fruit & vegetable "superfood" (SUP) modifications of a semi-pure AIN93M diet on behaviour, serotonin synthesis and metabolism pathway regulation in the brain and the gut, as well as the gut microbiota and the peripheral adrenal medullary system. CUMS induced anxiety-like behaviour, dysregulated the tryptophan and serotonin metabolic pathways in the hippocampus, prefrontal cortex, and colon, and altered the composition of the gut microbiota. CUMS reduced the catecholamine synthetic capacity of the adrenal glands. Differential effects were found in these parameters in the HFD and SUP diet. Thus, dietary modifications may profoundly affect the multiple dynamic systems involved in mood disorders.

Increased sucrose consumption in mice gene-targeted for Vmat2 selectively in NeuroD6-positive neurons of the ventral tegmental area

Ventral tegmental area (VTA) dopamine (DA) neurons are implicated in reward processing, motivation, reward prediction error, and in substance use disorder. Recent studies have identified distinct neuronal subpopulations within the VTA that can be clustered based on their molecular identity, neurotransmitter profile, physiology, projections and behavioral role. One such subpopulation is characterized by expression of the NeuroD6 gene, and projects primarily to the nucleus accumbens medial shell. We recently showed that optogenetic stimulation of these neurons induces real-time place preference while their targeted deletion of the Vmat2 gene caused altered response to rewarding substances, including ethanol and psychostimulants. Based on these recent findings, we wanted to further investigate the involvement of the NeuroD6-positive VTA subpopulation in reward processing. Using the same NeuroD6^Cre+/wt ;Vmat2^flox/flox mice as in our prior study, we now addressed the ability of the mice to process sucrose reward. In order to assess appetitive behavior and motivation to obtain sucrose reward, we tested conditional knockout (cKO) and control littermate mice in an operant sucrose self-administration paradigm. We observed that cKO mice demonstrate higher response rates to the operant task and consume more sucrose rewards than control mice. However, their motivation to obtain sucrose is identical to that of control mice. Our results highlight previous observations that appetitive behavior and motivation to obtain rewards can be served by distinct neuronal circuits, and demonstrate that the NeuroD6 VTA subpopulation is involved in mediating the former, but not the latter. Together with previous studies on the NeuroD6 subpopulation, our findings pinpoint the importance of unraveling the molecular and functional role of VTA subpopulations in order to better understand normal behavior and psychiatric disease.

Cre-recombinase systems for induction of neuron-specific knockout models: a guide for biomedical researchers

Gene deletion has been a valuable tool for unraveling the mysteries of molecular biology. Early approaches included gene trapping and gene targetting to disrupt or delete a gene randomly or at a specific location, respectively. Using these technologies in mouse embryos led to the generation of mouse knockout models and many scientific discoveries. The efficacy and specificity of these approaches have significantly increased with the advent of new technology such as clustered regularly interspaced short palindromic repeats for targetted gene deletion. However, several limitations including unwanted off-target gene deletion have hindered their widespread use in the field. Cre-recombinase technology has provided additional capacity for cell-specific gene deletion. In this review, we provide a summary of currently available literature on the application of this system for targetted deletion of neuronal genes. This article has been constructed to provide some background information for the new trainees on the mechanism and to provide necessary information for the design, and application of the Cre-recombinase system through reviewing the most frequent promoters that are currently available for genetic manipulation of neurons. We additionally will provide a summary of the latest technological developments that can be used for targeting neurons. This may also serve as a general guide for the selection of appropriate models for biomedical research.

Midbrain Dopamine Neurons Defined by TrpV1 Modulate Psychomotor Behavior

Dopamine (DA) neurons of the ventral tegmental area (VTA) continue to gain attention as far more heterogeneous than previously realized. Within the medial aspect of the VTA, the unexpected presence of TrpV1 mRNA has been identified. TrpV1 encodes the Transient Receptor Potential cation channel subfamily V member 1, TRPV1, also known as the capsaicin receptor, well recognized for its role in heat and pain processing by peripheral neurons. In contrast, the brain distribution of TrpV1 has been debated. Here, we hypothesized that the TrpV1+ identity defines a distinct subpopulation of VTA DA neurons. To explore these brain TrpV1+ neurons, histological analyses and Cre-driven mouse genetics were employed. TrpV1 mRNA was most strongly detected at the perinatal stage forming a band of scattered neurons throughout the medial VTA, reaching into the posterior hypothalamus. Within the VTA, the majority of TrpV1 co-localized with both Tyrosine hydroxylase (Th) and Vesicular monoamine transporter 2 (Vmat2), confirming a DA phenotype. However, TrpV1 also co-localized substantially with Vesicular glutamate transporter 2 (Vglut2), representing the capacity for glutamate (GLU) release. These TrpV1+/Th+/Vglut2+/Vmat2+ neurons thus constitute a molecularly and anatomically distinct subpopulation of DA-GLU co-releasing neurons. To assess behavioral impact, a TrpV1Cre -driven strategy targeting the Vmat2 gene in mice was implemented. This manipulation was sufficient to alter psychomotor behavior induced by amphetamine. The acute effect of the drug was accentuated above control levels, suggesting super-sensitivity in the drug-na ve state resembling a "pre-sensitized" phenotype. However, no progressive increase with repeated injections was observed. This study identifies a distinct TrpV1+ VTA subpopulation as a critical modulatory component in responsiveness to amphetamine. Moreover, expression of the gene encoding TRPV1 in selected VTA neurons opens up for new possibilities in pharmacological intervention of this heterogeneous, but clinically important, brain area.