Loss of VGLUT3 Produces Circadian-Dependent Hyperdopaminergia and Ameliorates Motor Dysfunction and l-Dopa-Mediated Dyskinesias in a Model of Parkinson's Disease

Adaptation to photoperiod via dynamic neurotransmitter segregation

Changes in the amount of daylight (photoperiod) alter physiology and behaviour1,2. Adaptive responses to seasonal photoperiods are vital to all organisms-dysregulation associates with disease, including affective disorders3 and metabolic syndromes4. The circadian rhythm circuitry is implicated in such responses5,6, yet little is known about the precise cellular substrates that underlie phase synchronization to photoperiod change. Here we identify a brain circuit and system of axon branch-specific and reversible neurotransmitter deployment that are critical for behavioural and sleep adaptation to photoperiod. A type of neuron called mrEn1-Pet17 in the mouse brainstem median raphe nucleus segregates serotonin from VGLUT3 (also known as SLC17A8, a proxy for glutamate) to different axonal branches that innervate specific brain regions involved in circadian rhythm and sleep-wake timing8,9. This branch-specific neurotransmitter deployment did not distinguish between daylight and dark phase; however, it reorganized with change in photoperiod. Axonal boutons, but not cell soma, changed neurochemical phenotype upon a shift away from equinox light/dark conditions, and these changes were reversed upon return to equinox conditions. When we genetically disabled Vglut3 in mrEn1-Pet1 neurons, sleep-wake periods, voluntary activity and clock gene expression did not synchronize to the new photoperiod or were delayed. Combining intersectional rabies virus tracing and projection-specific neuronal silencing, we delineated a preoptic area-to-mrEn1Pet1 connection that was responsible for decoding the photoperiodic inputs, driving the neurotransmitter reorganization and promoting behavioural synchronization. Our results reveal a brain circuit and periodic, branch-specific neurotransmitter deployment that regulates organismal adaptation to photoperiod change.

The human VGLUT3-pT8I mutation elicits uneven striatal DA signaling, food or drug maladaptive consumption in male mice

Cholinergic striatal interneurons (ChIs) express the vesicular glutamate transporter 3 (VGLUT3) which allows them to regulate the striatal network with glutamate and acetylcholine (ACh). In addition, VGLUT3-dependent glutamate increases ACh vesicular stores through vesicular synergy. A missense polymorphism, VGLUT3-p.T8I, was identified in patients with substance use disorders (SUDs) and eating disorders (EDs). A mouse line was generated to understand the neurochemical and behavioral impact of the p.T8I variant. In VGLUT3T8I/T8I male mice, glutamate signaling was unchanged but vesicular synergy and ACh release were blunted. Mutant male mice exhibited a reduced DA release in the dorsomedial striatum but not in the dorsolateral striatum, facilitating habit formation and exacerbating maladaptive use of drug or food. Increasing ACh tone with donepezil reversed the self-starvation phenotype observed in VGLUT3T8I/T8I male mice. Our study suggests that unbalanced dopaminergic transmission in the dorsal striatum could be a common mechanism between SUDs and EDs.

The Role of Vesicular Glutamate Transporter Type 3 in Social Behavior, with a Focus on the Median Raphe Region

Social behavior is important for our well-being, and its dysfunctions impact several pathological conditions. Although the involvement of glutamate is undeniable, the relevance of vesicular glutamate transporter type 3 (VGluT3), a specific vesicular transporter, in the control of social behavior is not sufficiently explored. Since midbrain median raphe region (MRR) is implicated in social behavior and the nucleus contains high amount of VGluT3+ neurons, we compared the behavior of male VGluT3 knock-out (KO) and VGluT3-Cre mice, the latter after chemogenetic MRR-VGluT3 manipulation. Appropriate control groups were included. Behavioral test battery was used for social behavior (sociability, social discrimination, social interaction, resident intruder test) and possible confounding factors (open field, elevated plus maze, Y-maze tests). Neuronal activation was studied by c-Fos immunohistochemistry. Human relevance was confirmed by VGluT3 gene expression in relevant human brainstem areas. VGluT3 KO mice exhibited increased anxiety, social interest, but also aggressive behavior in anxiogenic environment and impaired social memory. For KO animals, social interaction induced lower cell activation in the anterior cingulate, infralimbic cortex, and medial septum. In turn, excitation of MRR-VGluT3+ neurons was anxiolytic. Inhibition increased social interest 24 h later but decreased mobility and social behavior in aggressive context. Chemogenetic activation increased the number of c-Fos+ neurons only in the MRR. We confirmed the increased anxiety-like behavior and impaired memory of VGluT3 KO strain and revealed increased, but inadequate, social behavior. MRR-VGluT3 neurons regulated mobility and social and anxiety-like behavior in a context-dependent manner. The presence of VGluT3 mRNA on corresponding human brain areas suggests clinical relevance.

Dopamine-mediated plasticity preserves excitatory connections to direct pathway striatal projection neurons and motor function in a mouse model of Parkinson's disease

The cardinal symptoms of Parkinson's disease (PD) such as bradykinesia and akinesia are debilitating, and treatment options remain inadequate. The loss of nigrostriatal dopamine neurons in PD produces motor symptoms by shifting the balance of striatal output from the direct (go) to indirect (no-go) pathway in large part through changes in the excitatory connections and intrinsic excitabilities of the striatal projection neurons (SPNs). Here, we report using two different experimental models that a transient increase in striatal dopamine and enhanced D1 receptor activation, during 6-OHDA dopamine depletion, prevent the loss of mature spines and dendritic arbors on direct pathway projection neurons (dSPNs) and normal motor behavior for up to 5 months. The primary motor cortex and midline thalamic nuclei provide the major excitatory connections to SPNs. Using ChR2-assisted circuit mapping to measure inputs from motor cortex M1 to dorsolateral dSPNs, we observed a dramatic reduction in both experimental model mice and controls following dopamine depletion. Changes in the intrinsic excitabilities of SPNs were also similar to controls following dopamine depletion. Future work will examine thalamic connections to dSPNs. The findings reported here reveal previously unappreciated plasticity mechanisms within the basal ganglia that can be leveraged to treat the motor symptoms of PD.

Metabolic Basis of Circadian Dysfunction in Parkinson's Disease

Parkinson's disease (PD) is one of the most common neurodegenerative disorders. The management of PD is a challenging aspect for general physicians and neurologists. It is characterized by the progressive loss of dopaminergic neurons. Impaired α-synuclein secretion and dopamine release may cause mitochondrial dysfunction and perturb energy metabolism, subsequently altering the activity and survival of dopaminergic neurons, thus perpetuating the neurodegenerative process in PD. While the etiology of PD remains multifactorial, emerging research indicates a crucial role of circadian dysfunction in its pathogenesis. Researchers have revealed that circadian dysfunction and sleep disorders are common among PD subjects and disruption of circadian rhythms can increase the risk of PD. Hence, understanding the findings of circadian biology from translational research in PD is important for reducing the risk of neurodegeneration and for improving the quality of life. In this review, we discuss the intricate relationship between circadian dysfunction in cellular metabolism and PD by summarizing the evidence from animal models and human studies. Understanding the metabolic basis of circadian dysfunction in PD may shed light on novel therapeutic approaches to restore circadian rhythm, preserve dopaminergic function, and ameliorate disease progression. Further investigation into the complex interplay between circadian rhythm and PD pathogenesis is essential for the development of targeted therapies and interventions to alleviate the burden of this debilitating neurodegenerative disorder.

A brain circuit and neuronal mechanism for decoding and adapting to change in daylength

Changes in daylight amount (photoperiod) drive pronounced alterations in physiology and behaviour1,2. Adaptive responses to seasonal photoperiods are vital to all organisms - dysregulation is associated with disease, from affective disorders3 to metabolic syndromes4. Circadian rhythm circuitry has been implicated5,6 yet little is known about the precise neural and cellular substrates that underlie phase synchronization to photoperiod change. Here we present a previously unknown brain circuit and novel system of axon branch-specific and reversible neurotransmitter deployment that together prove critical for behavioural and sleep adaptation to photoperiod change. We found that the recently defined neuron type called mrEn1-Pet17 located in the mouse brainstem Median Raphe Nucleus (MRN) segregates serotonin versus VGLUT3 (here proxy for the neurotransmitter glutamate) to different axonal branches innervating specific brain regions involved in circadian rhythm and sleep/wake timing8,9. We found that whether measured during the light or dark phase of the day this branch-specific neurotransmitter deployment in mrEn1-Pet1 neurons was indistinguishable; however, it strikingly reorganizes on photoperiod change. Specifically, axonal boutons but not cell soma show a shift in neurochemical phenotype upon change away from equinox light/dark conditions that reverses upon return to equinox. When we genetically disabled the deployment of VGLUT3 in mrEn1-Pet1 neurons, we found that sleep/wake periods and voluntary activity failed to synchronize to the new photoperiod or was significantly delayed. Combining intersectional rabies virus tracing and projection-specific neuronal silencing in vivo, we delineated a Preoptic Area-to-mrEn1Pet1 connection responsible for decoding the photoperiodic inputs, driving the neurochemical shift and promoting behavioural synchronization. Our results reveal a previously unrecognized brain circuit along with a novel form of periodic, branch-specific neurotransmitter deployment that together regulate organismal adaptation to photoperiod changes.

A mini-review of the role of vesicular glutamate transporters in Parkinson's disease

Parkinson's disease (PD) is a common neurodegenerative disease implicated in multiple interacting neurotransmitter pathways. Glutamate is the central excitatory neurotransmitter in the brain and plays critical influence in the control of neuronal activity. Impaired Glutamate homeostasis has been shown to be closely associated with PD. Glutamate is synthesized in the cytoplasm and stored in synaptic vesicles by vesicular glutamate transporters (VGLUTs). Following its exocytotic release, Glutamate activates Glutamate receptors (GluRs) and mediates excitatory neurotransmission. While Glutamate is quickly removed by excitatory amino acid transporters (EAATs) to maintain its relatively low extracellular concentration and prevent excitotoxicity. The involvement of GluRs and EAATs in the pathophysiology of PD has been widely studied, but little is known about the role of VGLUTs in the PD. In this review, we highlight the role of VGLUTs in neurotransmitter and synaptic communication, as well as the massive alterations in Glutamate transmission and VGLUTs levels in PD. Among them, adaptive changes in the expression level and function of VGLUTs may exert a crucial role in excitatory damage in PD, and VGLUTs are considered as novel potential therapeutic targets for PD.

Continuous cholinergic-dopaminergic updating in the nucleus accumbens underlies approaches to reward-predicting cues

The ability to learn Pavlovian associations from environmental cues predicting positive outcomes is critical for survival, motivating adaptive behaviours. This cued-motivated behaviour depends on the nucleus accumbens (NAc). NAc output activity mediated by spiny projecting neurons (SPNs) is regulated by dopamine, but also by cholinergic interneurons (CINs), which can release acetylcholine and glutamate via the activity of the vesicular acetylcholine transporter (VAChT) or the vesicular glutamate transporter (VGLUT3), respectively. Here we investigated behavioural and neurochemical changes in mice performing a touchscreen Pavlovian approach task by recording dopamine, acetylcholine, and calcium dynamics from D1- and D2-SPNs using fibre photometry in control, VAChT or VGLUT3 mutant mice to understand how these signals cooperate in the service of approach behaviours toward reward-predicting cues. We reveal that NAc acetylcholine-dopaminergic signalling is continuously updated to regulate striatal output underlying the acquisition of Pavlovian approach learning toward reward-predicting cues.

Chemogenetic activation of VGLUT3-expressing neurons decreases movement

Vesicular glutamate transporters (VGLUTs) are responsible for the storage of glutamate into secretory vesicles. The VGLUT3 isoform is mainly expressed in neurons that secrete other classical neurotransmitters, including the cholinergic interneurons in the striatum, and VGLUT3-expressing neurons often secrete two distinct neurotransmitters. VGLUT3 is discretely distributed throughout the brain and is found in subpopulations of spinal cord interneurons, in subset of neurons in the dorsal root ganglion, and in Merkel cells. Mice with a global loss of VGLUT3 are hyperactive and the modulation of specific VGLUT3-expressing circuits can lead to changes in movement. In this study, we tested the hypothesis that increased activity of VGLUT3-expressing neurons is associated with decreased movement. Using a mouse line expressing excitatory designer receptor exclusively activated by designer drugs (hM3Dq-DREADD) on VGLUT3-expressing neurons, we showed that activation of hM3Dq signalling acutely decreased locomotor activity. This decreased locomotion was likely not due to circuit changes mediated by glutamate nor acetylcholine released from VGLUT3-expressing neurons, as activation of hM3Dq signalling in mice that do not release glutamate or acetylcholine from VGLUT3-expressing neurons also decreased locomotor activity. This suggests that other neurotransmitters are likely driving this hypoactive phenotype. We used these mouse lines to compare the effects of DREADD agonists in vivo. We observed that clozapine-N-oxide (CNO), clozapine, compound 21 and perlapine show small differences in the speed at which they prompt behavioural responses but the four of them are selective DREADD ligands.

VGLUT3 Ablation Differentially Modulates Glutamate Receptor Densities in Mouse Brain

Type 3 vesicular glutamate transporter (VGLUT3) represents a unique modulator of glutamate release from both nonglutamatergic and glutamatergic varicosities within the brain. Despite its limited abundance, VGLUT3 is vital for the regulation of glutamate signaling and, therefore, modulates the activity of various brain microcircuits. However, little is known about how glutamate receptors are regulated by VGLUT3 across different brain regions. Here, we used VGLUT3 constitutive knock-out (VGLUT3^-/-) mice and explored how VGLUT3 deletion influences total and cell surface expression of different ionotropic and metabotropic glutamate receptors. VGLUT3 deletion upregulated the overall expression of metabotropic glutamate receptors mGluR5 and mGluR2/3 in the cerebral cortex. In contrast, no change in the total expression of ionotropic NMDAR glutamate receptors were observed in the cerebral cortex of VGLUT3^-/- mice. We noted significant reduction in cell surface levels of mGluR5, NMDAR2A, NMDAR2B, as well as reductions in dopaminergic D₁ receptors and muscarinic M1 acetylcholine receptors in the hippocampus of VGLUT3^-/- mice. Furthermore, mGluR2/3 total expression and mGluR5 cell surface levels were elevated in the striatum of VGLUT3^-/- mice. Last, AMPAR subunit GluA1 was significantly upregulated throughout cortical, hippocampal, and striatal brain regions of VGLUT3^-/- mice. Together, these findings complement and further support the evidence that VGLUT3 dynamically regulates glutamate receptor densities in several brain regions. These results suggest that VGLUT3 may play an intricate role in shaping glutamatergic signaling and plasticity in several brain areas.

Molecular Mechanisms and Therapeutic Strategies for Levodopa-Induced Dyskinesia in Parkinson's Disease: A Perspective Through Preclinical and Clinical Evidence

Parkinson's disease (PD) is the second leading neurodegenerative disease that is characterized by severe locomotor abnormalities. Levodopa (L-DOPA) treatment has been considered a mainstay for the management of PD; however, its prolonged treatment is often associated with abnormal involuntary movements and results in L-DOPA-induced dyskinesia (LID). Although LID is encountered after chronic administration of L-DOPA, the appearance of dyskinesia after weeks or months of the L-DOPA treatment has complicated our understanding of its pathogenesis. Pathophysiology of LID is mainly associated with alteration of direct and indirect pathways of the cortico-basal ganglia-thalamic loop, which regulates normal fine motor movements. Hypersensitivity of dopamine receptors has been involved in the development of LID; moreover, these symptoms are worsened by concurrent non-dopaminergic innervations including glutamatergic, serotonergic, and peptidergic neurotransmission. The present study is focused on discussing the recent updates in molecular mechanisms and therapeutic approaches for the effective management of LID in PD patients.

Functional dissociation of behavioral effects from acetylcholine and glutamate released from cholinergic striatal interneurons

In the striatum, cholinergic interneurons (CINs) have the ability to release both acetylcholine and glutamate, due to the expression of the vesicular acetylcholine transporter (VAChT) and the vesicular glutamate transporter 3 (VGLUT3). However, the relationship these neurotransmitters have in the regulation of behavior is not fully understood. Here we used reward‐based touchscreen tests in mice to assess the individual and combined contributions of acetylcholine/glutamate co‐transmission in behavior. We found that reduced levels of the VAChT from CINs negatively impacted dopamine signalling in response to reward, and disrupted complex responses in a sequential chain of events. In contrast, diminished VGLUT3 levels had somewhat opposite effects. When mutant mice were treated with haloperidol in a cue‐based task, the drug did not affect the performance of VAChT mutant mice, whereas VGLUT3 mutant mice were highly sensitive to haloperidol. In mice where both vesicular transporters were deleted from CINs, we observed altered reward‐evoked dopaminergic signalling and behavioral deficits that resemble, but were worse, than those in mice with specific loss of VAChT alone. These results demonstrate that the ability to secrete two different neurotransmitters allows CINs to exert complex modulation of a wide range of behaviors.

A New Player in the Hippocampus: A Review on VGLUT3+ Neurons and Their Role in the Regulation of Hippocampal Activity and Behaviour

Glutamate is the most abundant excitatory amino acid in the central nervous system. Neurons using glutamate as a neurotransmitter can be characterised by vesicular glutamate transporters (VGLUTs). Among the three subtypes, VGLUT3 is unique, co-localising with other "classical" neurotransmitters, such as the inhibitory GABA. Glutamate, manipulated by VGLUT3, can modulate the packaging as well as the release of other neurotransmitters and serve as a retrograde signal through its release from the somata and dendrites. Its contribution to sensory processes (including seeing, hearing, and mechanosensation) is well characterised. However, its involvement in learning and memory can only be assumed based on its prominent hippocampal presence. Although VGLUT3-expressing neurons are detectable in the hippocampus, most of the hippocampal VGLUT3 positivity can be found on nerve terminals, presumably coming from the median raphe. This hippocampal glutamatergic network plays a pivotal role in several important processes (e.g., learning and memory, emotions, epilepsy, cardiovascular regulation). Indirect information from anatomical studies and KO mice strains suggests the contribution of local VGLUT3-positive hippocampal neurons as well as afferentations in these events. However, further studies making use of more specific tools (e.g., Cre-mice, opto- and chemogenetics) are needed to confirm these assumptions.

Current Knowledge on the Background, Pathophysiology and Treatment of Levodopa-Induced Dyskinesia-Literature Review

Levodopa remains the primary drug for controlling motor symptoms in Parkinson’s disease through the whole course, but over time, complications develop in the form of dyskinesias, which gradually become more frequent and severe. These abnormal, involuntary, hyperkinetic movements are mainly characteristic of the ON phase and are triggered by excess exogenous levodopa. They may also occur during the OFF phase, or in both phases. Over the past 10 years, the issue of levodopa-induced dyskinesia has been the subject of research into both the substrate of this pathology and potential remedial strategies. The purpose of the present study was to review the results of recent research on the background and treatment of dyskinesia. To this end, databases were reviewed using a search strategy that included both relevant keywords related to the topic and appropriate filters to limit results to English language literature published since 2010. Based on the selected papers, the current state of knowledge on the morphological, functional, genetic and clinical features of levodopa-induced dyskinesia, as well as pharmacological, genetic treatment and other therapies such as deep brain stimulation, are described.

Dopaminergic co-transmission with sonic hedgehog inhibits abnormal involuntary movements in models of Parkinson's disease and L-Dopa induced dyskinesia

L-Dopa induced dyskinesia (LID) is a debilitating side effect of dopamine replacement therapy for Parkinson’s Disease. The mechanistic underpinnings of LID remain obscure. Here we report that diminished sonic hedgehog (Shh) signaling in the basal ganglia caused by the degeneration of midbrain dopamine neurons facilitates the formation and expression of LID. We find that the pharmacological activation of Smoothened, a downstream effector of Shh, attenuates LID in the neurotoxic 6-OHDA- and genetic aphakia mouse models of Parkinson’s Disease. Employing conditional genetic loss-of-function approaches, we show that reducing Shh secretion from dopamine neurons or Smoothened activity in cholinergic interneurons promotes LID. Conversely, the selective expression of constitutively active Smoothened in cholinergic interneurons is sufficient to render the sensitized aphakia model of Parkinson’s Disease resistant to LID. Furthermore, acute depletion of Shh from dopamine neurons through prolonged optogenetic stimulation in otherwise intact mice and in the absence of L-Dopa produces LID-like involuntary movements. These findings indicate that augmenting Shh signaling in the L-Dopa treated brain may be a promising therapeutic approach for mitigating the dyskinetic side effects of long-term treatment with L-Dopa.

Lauren Malave et al. examine the impact of sonic hedgehog signaling in the dorsal striatum in L-Dopa induced dyskinesia (LID) animal models. Their results suggest that increasing sonic hedgehog signaling can reduce the severity of LID and abnormal involuntary movements, suggesting future therapeutic approaches to mitigate dyskinetic comorbidities of long-term treatment with L-Dopa.

Perinatal interference with the serotonergic system affects VTA function in the adult via glutamate co-transmission

Serotonin and dopamine are associated with multiple psychiatric disorders. How they interact during development to affect subsequent behavior remains unknown. Knockout of the serotonin transporter or postnatal blockade with selective serotonin reuptake inhibitors (SSRIs) leads to novelty-induced exploration deficits in adulthood, potentially involving the dopamine system. Here, we show in the mouse that raphe nucleus serotonin neurons activate ventral tegmental area dopamine neurons via glutamate co-transmission and that this co-transmission is reduced in animals exposed postnatally to SSRIs. Blocking serotonin neuron glutamate co-transmission mimics this SSRI-induced hypolocomotion, while optogenetic activation of dopamine neurons reverses this hypolocomotor phenotype. Our data demonstrate that serotonin neurons modulate dopamine neuron activity via glutamate co-transmission and that this pathway is developmentally malleable, with high serotonin levels during early life reducing co-transmission, revealing the basis for the reduced novelty-induced exploration in adulthood due to postnatal SSRI exposure.

Glutamic Acid Transporters: Targets for Neuroprotective Therapies in Parkinson's Disease

Parkinson's disease (PD) is a common neurodegenerative disease in middle-aged and elderly individuals. At present, no effective drug has been developed to treat PD. Although a variety of drugs exist for the symptomatic treatment of PD, they all have strong side effects. Most studies on PD mainly focus on dopaminergic neurons. This review highlights the function of glutamic acid transporters (GLTs), including excitatory amino acid transporters (EAATs) and vesicular glutamate transporters (VGLUTs), during the development of PD. In addition, using bioinformatics, we compared the expression of different types of glutamate transporter genes in the cingulate gyrus of PD patients and healthy controls. More importantly, we suggest that the functional roles of glutamate transporters may prove beneficial in the treatment of PD. In summary, VGLUTs and EAATs may be potential targets in the treatment of PD. VGLUTs and EAATs can be used as clinical drug targets to achieve better efficacy. Through this review article, we hope to enable future researchers to improve the condition of PD patients.

Leveraging VGLUT3 Functions to Untangle Brain Dysfunctions

Vesicular glutamate transporters (VGLUTs) were long thought to be specific markers of glutamatergic excitatory transmission. The discovery, two decades ago, of the atypical VGLUT3 has thoroughly modified this oversimplified view. VGLUT3 is strategically expressed in discrete populations of glutamatergic, cholinergic, serotonergic, and even GABAergic neurons. Recent reports show the subtle, but critical, implications of VGLUT3-dependent glutamate co-transmission and its roles in the regulation of diverse brain functions and dysfunctions. Progress in the neuropharmacology of VGLUT3 could lead to decisive breakthroughs in the treatment of Parkinson's disease (PD), addiction, eating disorders, anxiety, presbycusis, or pain. This review summarizes recent findings on VGLUT3 and its vesicular underpinnings as well as on possible ways to target this atypical transporter for future therapeutic strategies.

Outer Hair Cell Glutamate Signaling through Type II Spiral Ganglion Afferents Activates Neurons in the Cochlear Nucleus in Response to Nondamaging Sounds

Cochlear outer hair cells (OHCs) are known to uniquely participate in auditory processing through their electromotility, and like inner hair cells, are also capable of releasing vesicular glutamate onto spiral ganglion (SG) neurons: in this case, onto the sparse Type II SG neurons. However, unlike glutamate signaling at the inner hair cell-Type I SG neuron synapse, which is robust across a wide spectrum of sound intensities, glutamate signaling at the OHC-Type II SG neuron synapse is weaker and has been hypothesized to occur only at intense, possibly damaging sound levels. Here, we tested the ability of the OHC-Type II SG pathway to signal to the brain in response to moderate, nondamaging sound (80 dB SPL) as well as to intense sound (115 dB SPL). First, we determined the VGluTs associated with OHC signaling and then confirmed the loss of glutamatergic synaptic transmission from OHCs to Type II SG neurons in KO mice using dendritic patch-clamp recordings. Next, we generated genetic mouse lines in which vesicular glutamate release occurs selectively from OHCs, and then assessed c-Fos expression in the cochlear nucleus in response to sound. From these analyses, we show, for the first time, that glutamatergic signaling at the OHC-Type II SG neuron synapse is capable of activating cochlear nucleus neurons, even at moderate sound levels.SIGNIFICANCE STATEMENT Evidence suggests that cochlear outer hair cells (OHCs) release glutamate onto Type II spiral ganglion neurons only when exposed to loud sound, and that Type II neurons are activated by tissue damage. Knowing whether moderate level sound, without tissue damage, activates this pathway has functional implications for this fundamental auditory pathway. We first determined that OHCs rely largely on VGluT3 for synaptic glutamate release. We then used a genetically modified mouse line in which OHCs, but not inner hair cells, release vesicular glutamate to demonstrate that moderate sound exposure activates cochlear nucleus neurons via the OHC-Type II spiral ganglion pathway. Together, these data indicate that glutamate signaling at the OHC-Type II afferent synapse participates in auditory function at moderate sound levels.

Two Players in the Field: Hierarchical Model of Interaction between the Dopamine and Acetylcholine Signaling Systems in the Striatum

Tight interactions exist between dopamine and acetylcholine signaling in the striatum. Dopaminergic neurons express muscarinic and nicotinic receptors, and cholinergic interneurons express dopamine receptors. All neurons in the striatum are pacemakers. An increase in dopamine release is activated by stopping acetylcholine release. The coordinated timing or synchrony of the direct and indirect pathways is critical for refined movements. Changes in neurotransmitter ratios are considered a prominent factor in Parkinson’s disease. In general, drugs increase striatal dopamine release, and others can potentiate both dopamine and acetylcholine release. Both neurotransmitters and their receptors show diurnal variations. Recently, it was observed that reward function is modulated by the circadian system, and behavioral changes (hyperactivity and hypoactivity during the light and dark phases, respectively) are present in an animal model of Parkinson’s disease. The striatum is one of the key structures responsible for increased locomotion in the active (dark) period in mice lacking M₄ muscarinic receptors. Thus, we propose here a hierarchical model of the interaction between dopamine and acetylcholine signaling systems in the striatum. The basis of this model is their functional morphology. The next highest mode of interaction between these two neurotransmitter systems is their interaction at the neurotransmitter/receptor/signaling level. Furthermore, these interactions contribute to locomotor activity regulation and reward behavior, and the topmost level of interaction represents their biological rhythmicity.

Striatal and nigral muscarinic type 1 and type 4 receptors modulate levodopa-induced dyskinesia and striato-nigral pathway activation in 6-hydroxydopamine hemilesioned rats

Acetylcholine muscarinic receptors (mAChRs) contribute to both the facilitation and inhibition of levodopa-induced dyskinesia operated by striatal cholinergic interneurons, although the receptor subtypes involved remain elusive. Cholinergic afferents from the midbrain also innervate the substantia nigra reticulata, although the role of nigral mAChRs in levodopa-induced dyskinesia is unknown. Here, we investigate whether striatal and nigral M1 and/or M4 mAChRs modulate dyskinesia and the underlying striato-nigral GABAergic pathway activation in 6-hydroxydopamine hemilesioned rats. Reverse microdialysis allowed to deliver the mAChR antagonists telenzepine (M1 subtype preferring), PD-102807 and tropicamide (M4 subtype preferring), as well as the selective M4 mAChR positive allosteric modulator VU0152100 in striatum or substantia nigra, while levodopa was administered systemically. Dyskinetic movements were monitored along with nigral GABA (and glutamate) and striatal glutamate dialysate levels, taken as neurochemical correlates of striato-nigral pathway and cortico-basal ganglia-thalamo-cortical loop activation. We observed that intrastriatal telenzepine, PD-102807 and tropicamide alleviated dyskinesia and inhibited nigral GABA and striatal glutamate release. This was partially replicated by intrastriatal VU0152100. The M2 subtype preferring antagonist AFDX-116, used to elevate striatal acetylcholine levels, blocked the behavioral and neurochemical effects of PD-102807. Intranigral VU0152100 prevented levodopa-induced dyskinesia and its neurochemical correlates whereas PD-102807 was ineffective. These results suggest that striatal, likely postsynaptic, M1 mAChRs facilitate dyskinesia and striato-nigral pathway activation in vivo. Conversely, striatal M4 mAChRs can both facilitate and inhibit dyskinesia, possibly depending on their localization. Potentiation of striatal and nigral M4 mAChR transmission leads to powerful multilevel inhibition of striato-nigral pathway and attenuation of dyskinesia.

Alterations in the intrinsic properties of striatal cholinergic interneurons after dopamine lesion and chronic L-DOPA

Changes in striatal cholinergic interneuron (ChI) activity are thought to contribute to Parkinson's disease pathophysiology and dyskinesia from chronic L-3,4-dihydroxyphenylalanine (L-DOPA) treatment, but the physiological basis of these changes is unknown. We find that dopamine lesion decreases the spontaneous firing rate of ChIs, whereas chronic treatment with L-DOPA of lesioned mice increases baseline ChI firing rates to levels beyond normal activity. The effect of dopamine loss on ChIs was due to decreased currents of both hyperpolarization-activated cyclic nucleotide-gated (HCN) and small conductance calcium-activated potassium (SK) channels. L-DOPA reinstatement of dopamine normalized HCN activity, but SK current remained depressed. Pharmacological blockade of HCN and SK activities mimicked changes in firing, confirming that these channels are responsible for the molecular adaptation of ChIs to dopamine loss and chronic L-DOPA treatment. These findings suggest that targeting ChIs with channel-specific modulators may provide therapeutic approaches for alleviating L-DOPA-induced dyskinesia in PD patients.

Molecular, Structural, Functional, and Pharmacological Sites for Vesicular Glutamate Transporter Regulation

Vesicular glutamate transporters (VGLUTs) control quantal size of glutamatergic transmission and have been the center of numerous studies over the past two decades. VGLUTs contain two independent transport modes that facilitate glutamate packaging into synaptic vesicles and phosphate (Pi) ion transport into the synaptic terminal. While a transmembrane proton electrical gradient established by a vacuolar-type ATPase powers vesicular glutamate transport, recent studies indicate that binding sites and flux properties for chloride, potassium, and protons within VGLUTs themselves regulate VGLUT activity as well. These intrinsic ionic binding and flux properties of VGLUTs can therefore be modulated by neurophysiological conditions to affect levels of glutamate available for release from synapses. Despite their extraordinary importance, specific and high-affinity pharmacological compounds that interact with these sites and regulate VGLUT function, distinguish between the various modes of transport, and the different isoforms themselves, are lacking. In this review, we provide an overview of the physiologic sites for VGLUT regulation that could modulate glutamate release in an over-active synapse or in a disease state.

Evidence for the spread of human-derived mutant huntingtin protein in mice and non-human primates

In recent years, substantial evidence has emerged to suggest that spreading of pathological proteins contributes to disease pathology in numerous neurodegenerative disorders. Work from our laboratory and others have shown that, despite its strictly genetic nature, Huntington's disease (HD) may be another condition in which this mechanism contributes to pathology. In this study, we set out to determine if the mutant huntingtin protein (mHTT) present in post-mortem brain tissue derived from HD patients can induce pathology in mice and/or non-human primates. For this, we performed three distinct sets of experiments where homogenates were injected into the brains of adult a) Wild-type (WT) and b) BACHD mice or c) non-human primates. Neuropathological assessments revealed that, while changes in the endogenous huntingtin were not apparent, mHTT could spread between cellular elements and brain structures. Furthermore, behavioural differences only occurred in the animal model of HD which already overexpressed mHTT. Taken together, our results indicate that mHTT derived from human brains has only a limited capacity to propagate between cells and does not depict prion-like characteristics. This contrasts with recent work demonstrating that other forms of mHTT - such as fibrils of a pathological polyQ length or fibroblasts and induced pluripotent stem cells derived from HD cases - can indeed disseminate disease throughout the brain in a prion-like fashion.

Optimizing Nervous System-Specific Gene Targeting with Cre Driver Lines: Prevalence of Germline Recombination and Influencing Factors

The Cre-loxP system is invaluable for spatial and temporal control of gene knockout, knockin, and reporter expression in the mouse nervous system. However, we report varying probabilities of unexpected germline recombination in distinct Cre driver lines designed for nervous system-specific recombination. Selective maternal or paternal germline recombination is showcased with sample Cre lines. Collated data reveal germline recombination in over half of 64 commonly used Cre driver lines, in most cases with a parental sex bias related to Cre expression in sperm or oocytes. Slight differences among Cre driver lines utilizing common transcriptional control elements affect germline recombination rates. Specific target loci demonstrated differential recombination; thus, reporters are not reliable proxies for another locus of interest. Similar principles apply to other recombinase systems and other genetically targeted organisms. We hereby draw attention to the prevalence of germline recombination and provide guidelines to inform future research for the neuroscience and broader molecular genetics communities.

Consequences of VGluT3 deficiency on learning and memory in mice

The aim of the present study was to test the hypothesis that vesicular glutamate transporter 3 (VGluT3) deficiency is associated with cognitive impairments. Male VGluT3 knockout (KO) and wild type (WT) mice were exposed to a behavioral test battery covering paradigms based on spontaneous exploratory behavior and reinforcement-based learning tests. Reversal learning was examined to test the cognitive flexibility. The VGluT3 KO mice clearly exhibited the ability to learn. The social recognition memory of KO mice was intact. The y-maze test revealed weaker working memory of VGluT3 KO mice. No significant learning impairments were noticed in operant conditioning or holeboard discrimination paradigm. In avoidance-based learning tests (Morris water maze and active avoidance), KO mice exhibited slightly slower learning process compared to WT mice, but not a complete learning impairment. In tests based on simple associations (operant conditioning, avoidance learning) an attenuation of cognitive flexibility was observed in KO mice. In conclusion, knocking out VGluT3 results in mild disturbances in working memory and learning flexibility. Apparently, this glutamate transporter is not a major player in learning and memory formation in general. Based on previous characteristics of VGluT3 KO mice we would have expected a stronger deficit. The observed hypolocomotion did not contribute to the mild cognitive disturbances herein reported, either.

Demonstration of prion-like properties of mutant huntingtin fibrils in both in vitro and in vivo paradigms

In recent years, evidence has accumulated to suggest that mutant huntingtin protein (mHTT) can spread into healthy tissue in a prion-like fashion. This theory, however, remains controversial. To fully address this concept and to understand the possible consequences of mHTT spreading to Huntington’s disease pathology, we investigated the effects of exogenous human fibrillar mHTT (Q48) and huntingtin (HTT) (Q25) N-terminal fragments in three cellular models and three distinct animal paradigms. For in vitro experiments, human neuronal cells [induced pluripotent stem cell-derived GABA neurons (iGABA) and (SH-SY5Y)] as well as human THP1-derived macrophages, were incubated with recombinant mHTT fibrils. Recombinant mHTT and HTT fibrils were taken up by all cell types, inducing cell morphology changes and death. Variations in HTT aggregation were further observed following incubation with fibrils in both THP1 and SH-SY5Y cells. For in vivo experiments, adult wild-type (WT) mice received a unilateral intracerebral cortical injection and R6/2 and WT pups were administered fibrils via bilateral intraventricular injections. In both protocols, the injection of Q48 fibrils resulted in cognitive deficits and increased anxiety-like behavior. Post-mortem analysis of adult WT mice indicated that most fibrils had been degraded/cleared from the brain by 14 months post-surgery. Despite the absence of fibrils at these later time points, a change in the staining pattern of endogenous HTT was detected. A similar change was revealed in post-mortem analysis of the R6/2 mice. These effects were specific to central administration of fibrils, as mice receiving intravenous injections were not characterized by behavioral changes. In fact, peripheral administration resulted in an immune response mounting against the fibrils. Together, the in vitro and in vivo data indicate that exogenously administered mHTT is capable of both causing and exacerbating disease pathology.

Loss of PINK1 causes age-dependent decrease of dopamine release and mitochondrial dysfunction

Mutations and deletions in PTEN-induced kinase 1 (PINK1) cause autosomal recessive Parkinson's disease (PD), the second most common neurodegenerative disorder. PINK1 is a nuclear-genome encoded Ser/Thr kinase in mitochondria. PINK1 deletion was reported to affect dopamine (DA) levels in the striatum and mitochondrial functions but with conflicting results. The role of PINK1 in mitochondrial function and in PD pathogenesis remains to be elucidated thoroughly. In this study, we measured DA release using fast-scan cyclic voltammetry in acute striatal slices from both PINK1 knockout (KO) and wild-type (WT) mice at different ages. We found that single pulse-evoked DA release in the dorsal striatum of PINK1 KO mice was decreased in an age-dependent manner. Furthermore, the decrease was because of less DA release instead of an alteration of DA transporter function or DA terminal degeneration. We also found that PINK1 KO striatal slices had significantly lower basal mitochondria respiration compared with that of WT controls, and this impairment was also age-dependent. These results suggest that the impaired DA release is most likely because of mitochondrial dysfunction and lower ATP production.

VGLUT3 gates psychomotor effects induced by amphetamine

Several subtypes of modulatory neurons co-express vesicular glutamate transporters (VGLUTs) in addition to their cognate vesicular transporters. These neurons are believed to establish new forms of neuronal communication. The atypical VGLUT3 is of particular interest since in the striatum this subtype is found in tonically active cholinergic interneurons (TANs) and in a subset of 5-HT fibers. The striatum plays a major role in psychomotor effects induced by amphetamine. Whether and how VGLUT3-operated glutamate/ACh or glutamate/5HT co-transmissions modulates psychostimulants-induced maladaptive behaviors is still unknown. Here, we investigate the involvement of VGLUT3 and glutamate co-transmission in amphetamine-induced psychomotor effects and stereotypies. Taking advantage of constitutive and cell-type specific VGLUT3-deficient mouse lines, we tackled the hypothesis that VGLUT3 could gate psychomotor effects (locomotor activity and stereotypies) induced by acute or chronic administration of amphetamine. Interestingly, VGLUT3-null mice demonstrated blunted amphetamine-induced stereotypies as well as reduced striatal ∆FosB expression. VGLUT3-positive varicosities within the striatum arise in part from 5HT neurons. We tested the involvement of VGLUT3 deletion in serotoninergic neurons in amphetamine-induced stereotypies. Mice lacking VGLUT3 specifically in 5HT fibers showed no alteration to amphetamine sensitivity. In contrast, specific deletion of VGLUT3 in cholinergic neurons partially phenocopied the effects observed in the constitutive knock-out mice. Our results show that constitutive deletion of VGLUT3 modulates acute and chronic locomotor effects induced by amphetamine. They point to the fact that the expression of VGLUT3 in multiple brain areas is pivotal in gating amphetamine-induced psychomotor adaptations. OPEN SCIENCE BADGES: This article has received a badge for *Open Materials* because it provided all relevant information to reproduce the study in the manuscript. The complete Open Science Disclosure form for this article can be found at the end of the article. More information about the Open Practices badges can be found at https://cos.io/our-services/open-science-badges/.

Targeting VGLUT2 in Mature Dopamine Neurons Decreases Mesoaccumbal Glutamatergic Transmission and Identifies a Role for Glutamate Co-release in Synaptic Plasticity by Increasing Baseline AMPA/NMDA Ratio

Expression of the Vglut2/Slc17a6 gene encoding the Vesicular glutamate transporter 2 (VGLUT2) in midbrain dopamine (DA) neurons enables these neurons to co-release glutamate in the nucleus accumbens (NAc), a feature of putative importance to drug addiction. For example, it has been shown that conditional deletion of Vglut2 gene expression within developing DA neurons in mice causes altered locomotor sensitization to addictive drugs, such as amphetamine and cocaine, in adulthood. Alterations in DA neurotransmission in the mesoaccumbal pathway has been proposed to contribute to these behavioral alterations but the underlying molecular mechanism remains largely elusive. Repeated exposure to cocaine is known to cause lasting adaptations of excitatory synaptic transmission onto medium spiny neurons (MSNs) in the NAc, but the putative contribution of VGLUT2-mediated glutamate co-release from the mesoaccumbal projection has never been investigated. In this study, we implemented a tamoxifen-inducible Cre-LoxP strategy to selectively probe VGLUT2 in mature DA neurons of adult mice. Optogenetics-coupled patch clamp analysis in the NAc demonstrated a significant reduction of glutamatergic neurotransmission, whilst behavioral analysis revealed a normal locomotor sensitization to amphetamine and cocaine. When investigating if the reduced level of glutamate co-release from DA neurons caused a detectable post-synaptic effect on MSNs, patch clamp analysis identified an enhanced baseline AMPA/NMDA ratio in DA receptor subtype 1 (DRD1)-expressing accumbal MSNs which occluded the effect of cocaine on synaptic transmission. We conclude that VGLUT2 in mature DA neurons actively contributes to glutamatergic neurotransmission in the NAc, a finding which for the first time highlights VGLUT2-mediated glutamate co-release in the complex mechanisms of synaptic plasticity in drug addiction.

Dual modulation on glial cells by tetrahydroxystilbene glucoside protects against dopamine neuronal loss

Background

Microglia-mediated neuroinflammation is recognized to mainly contribute to the pathogenesis of Parkinson’s disease (PD). Tetrahydroxystilbene glucoside (TSG) has been proved to be beneficial for health with a great number of pharmacological properties. We examined the effects of TSG against dopamine (DA) neuronal loss towards development of a PD treatment strategy.

Methods

Substantia nigral stereotaxic single injection of lipopolysaccharide (LPS)-induced rat DA neuronal damage was employed to investigate TSG-produced neuroprotection. In addition, primary rat midbrain neuron-glia co-cultures were performed to explore the underlying mechanisms.

Results

Daily intraperitoneal injection of TSG for seven consecutive days significantly attenuated LPS-induced loss of DA neurons in the substantia nigra. In addition, glia-dependent mechanisms were responsible for TSG-mediated neuroprotection. First, TSG ameliorated microglia-mediated neuroinflammation and the subsequent production of various pro-inflammatory and neurotoxic factors. Second, astroglial neurotrophic factor neutralization weakened TSG-mediated neuroprotection, showing that TSG was protective in part via increasing astroglia-derived neurotrophic factor secretion.

Conclusions

TSG protects DA neurons against LPS-induced neurotoxicity through dual modulation on glial cells by attenuating microglia-mediated neuroinflammation and enhancing astroglia-derived neurotrophic effects. These findings might open new alternative avenues for PD treatment.

A new outlook on cholinergic interneurons in Parkinson's disease and L-DOPA-induced dyskinesia

Traditionally, dopamine (DA) and acetylcholine (ACh) striatal systems were considered antagonistic and imbalances or aberrant signaling between these neurotransmitter systems could be detrimental to basal ganglia activity and pursuant motor function, such as in Parkinson's disease (PD) and L-DOPA-induced dyskinesia (LID). Herein, we discuss the involvement of cholinergic interneurons (ChIs) in striatally-mediated movement in a healthy, parkinsonian, and dyskinetic state. ChIs integrate numerous neurotransmitter signals using intrinsic glutamate, serotonin, and DA receptors and convey the appropriate transmission onto nearby muscarinic and nicotinic ACh receptors to produce movement. In PD, severe DA depletion causes abnormal rises in ChI activity which promote striatal signaling to attenuate normal movement. When treating PD with L-DOPA, hyperkinetic side effects, or LID, develop due to increased striatal DA; however, the role of ChIs and ACh transmission, until recently has been unclear. Fortunately, new technology and pharmacological agents have facilitated understanding of ChI function and ACh signaling in the context of LID, thus offering new opportunities to modify existing and discover future therapeutic strategies in movement disorders.

Enhanced innate fear and altered stress axis regulation in VGluT3 knockout mice

Glutamatergic neurons, characterized by vesicular glutamate transporters (VGluT1-3) provide the main excitation in the brain. Their disturbances have been linked to various brain disorders, which could be also modeled by the contextual fear test in rodents. We aimed to characterize the participation of VGluT3 in the development of contextual fear through its contribution to hypothalamic-pituitary-adrenocortical axis (HPA) regulation using knockout (KO) mice. Contextual fear conditioning was induced by foot shock and mice were examined 1 and 7 d later in the same environment comparing wild type with KO. Foot shock increased the immobility time without context specificity. Additionally, foot shock reduced open arm time in the elevated plus maze (EPM) test, and distance traveled in the open field (OF) test, representing the generalization of fear. Moreover, KO mice spent more time with freezing during the contextual fear test, less time in the open arm of the EPM, and traveled a smaller distance in the OF, with less entries into the central area. However, there was no foot shock and genotype interaction suggesting that VGluT3 does not influence the fear conditioning, rather determines anxiety-like characteristic of the mice. The resting hypothalamic CRH mRNA was higher in KO mice with reduced stressor-induced corticosterone elevations. Immunohistochemistry revealed the presence of VGluT3 positive fibers in the paraventricular nucleus of hypothalamus, but not on the hypophysis. As a summary, we confirmed the involvement of VGluT3 in innate fear, but not in the development of fear memory and generalization, with a significant contribution to HPA alterations. Highlights VGluT3 KO mice show innate fear without significant influence on fear memory and generalization. A putative background is the higher resting CRH mRNA level in their PVN and reduced stress-reactivity.

CaMKII inhibition ameliorated levodopa-induced dyskinesia by downregulating tyrosine hydroxylase activity in an experimental model of Parkinson's disease

Levodopa (L-dopa) remains the best treatment for Parkinson's disease (PD). However, long-term L-dopa treatment induces dyskinesia. The mechanism of L-dopa-induced dyskinesia (LID) is not fully understood. Enhanced activity of protein kinase A (PKA) and pulsatile dopamine (DA) stimulation plays an important role in LID. Tyrosine hydroxylase (TH) is the rate-limiting enzyme for DA synthesis. Decreased TH activity causes reduced pulsatile DA stimulation, which in turn reduces LID. Moreover, TH is a substrate of CaMKII. However, it is unknown whether inhibition of CaMKII reduces LID by downregulating the activity of TH. In this study, we found that CaMKII antagonist KN-93 reduced DA released in PC12 cells; in the meantime, KN-93 reduced phosphorylated levels of CaMKIIα and TH at Ser 40. Intrastriatal administration of KN-93 reduced LID without affecting the antiparkinsonian effect of L-dopa in PD mice. Mechanistically, KN-93 treatmentreduced phosphorylated CaMKIIα levels and subsequently downregulated phosphorylated TH at Ser 40 expression. Consequently, extracellular DA efflux was reduced andthe activation threshold of the PKA pathway was lowered. Moreover, KN-93 treatment reduced the expression of Arc and Penk, two immediate early genes, induced by chronic L-dopa. These data indicate that inhibition of CaMKIIα decreases LID at least partially by suppressing TH activity and subsequently reducing extracellular DA efflux and the activity of the PKA pathway, suggesting that CaMKIIα may be an alternative target for the treatment of LID.

Emerging preclinical interest concerning the role of circadian function in Parkinson's disease

The importance of circadian function in the aetiology, progression and treatment of Parkinson's disease is a topic of increasing interest to the scientific and clinical community. While clinical studies on this theme are relatively new and limited in number there are many preclinical studies which explore possible circadian involvement in Parkinson's disease and speculate as to the mechanism by which clinical benefit can be derived by manipulating the circadian system. The present review explores the sequelae of circadian related studies from a historical perspective and reveals mechanisms that may be involved in the aetiology and progression of the disease. A systematic review of these studies also sets the stage for understanding the basic neuroscientific approaches which have been applied and provides new direction from which circadian function can be explored.

Layer- and cell type-selective co-transmission by a basal forebrain cholinergic projection to the olfactory bulb

Cholinergic neurons in the basal forebrain project heavily to the main olfactory bulb, the first processing station in the olfactory pathway. The projections innervate multiple layers of the main olfactory bulb and strongly influence odor discrimination, detection, and learning. The precise underlying circuitry of this cholinergic input to the main olfactory bulb remains unclear, however. Here, we identify a specific basal forebrain cholinergic projection that innervates select neurons concentrated in the internal plexiform layer of the main olfactory bulb. Optogenetic activation of this projection elicits monosynaptic nicotinic and GABAergic currents in glomerular layer-projecting interneurons. Additionally, we show that the projection co-expresses markers for GABAergic neurotransmission. The data thus implicate neurotransmitter co-transmission in the basal forebrain regulation of this inhibitory olfactory microcircuit.

Cholinergic neurons innervate multiple layers in the main olfactory bulb but the precise circuitry of this input is not known. Here the authors show that VGLUT3⁺ cholinergic neurons selectively innervate deep short axon cells in specific layers and elicit robust monosynaptic GABAergic and nicotinic postsynaptic currents.

Contribution of Vesicular Glutamate Transporters to Stress Response and Related Psychopathologies: Studies in VGluT3 Knockout Mice

Maintenance of the homeostasis in a constantly changing environment is a fundamental process of life. Disturbances of the homeostatic balance is defined as stress response and is induced by wide variety of challenges called stressors. Being the main excitatory neurotransmitter of the central nervous system glutamate is important in the adaptation process of stress regulating both the catecholaminergic system and the hypothalamic-pituitary-adrenocortical axis. Data are accumulating about the role of different glutamatergic receptors at all levels of these axes, but little is known about the contribution of different vesicular glutamate transporters (VGluT1-3) characterizing the glutamatergic neurons. Here we summarize basic knowledge about VGluTs, their role in physiological regulation of stress adaptation, as well as their contribution to stress-related psychopathology. Most of our knowledge comes from the VGluT3 knockout mice, as VGluT1 and 2 knockouts are not viable. VGluT3 was discovered later than, and is not as widespread as the VGluT1 and 2. It may co-localize with other transmitters, and participate in retrograde signaling; as such its role might be unique. Previous reports using VGluT3 knockout mice showed enhanced anxiety and innate fear compared to wild type. Moreover, these knockout animals had enhanced resting corticotropin-releasing hormone mRNA levels in the hypothalamus and disturbed glucocorticoid stress responses. In conclusion, VGluT3 participates in stress adaptation regulation. The neuroendocrine changes observed in VGluT3 knockout mice may contribute to their anxious, fearful phenotype.

Cholinergic/glutamatergic co-transmission in striatal cholinergic interneurons: new mechanisms regulating striatal computation

It is well established that neurons secrete neuropeptides and ATP with classical neurotransmitters; however, certain neuronal populations are also capable of releasing two classical neurotransmitters by a process named co-transmission. Although there has been progress in our understanding of the molecular mechanism underlying co-transmission, the individual regulation of neurotransmitter secretion and the functional significance of this neuronal 'bilingualism' is still unknown. Striatal cholinergic interneurons (CINs) have been shown to secrete glutamate (Glu) in addition to acetylcholine (ACh) and are recognized for their role in the regulation of striatal circuits and behavior. Our review highlights the recent research into identifying mechanisms that regulate the secretion and function of Glu and ACh released by CINs and the roles these neurons play in regulating dopamine secretion and striatal activity. In particular, we focus on how the transporters for ACh (VAChT) and Glu (VGLUT3) influence the storage of neurotransmitters in CINs. We further discuss how these individual neurotransmitters regulate striatal computation and distinct aspects of behavior that are regulated by the striatum. We suggest that understanding the distinct and complementary functional roles of these two neurotransmitters may prove beneficial in the development of therapies for Parkinson's disease and addiction. Overall, understanding how Glu and ACh secreted by CINs impacts striatal activity may provide insight into how different populations of 'bilingual' neurons are able to develop sophisticated regulation of their targets by interacting with multiple receptors but also by regulating each other's vesicular storage. This is an article for the special issue XVth International Symposium on Cholinergic Mechanisms.

Circadian Dysregulation in Parkinson's Disease

Parkinson's disease (PD) is the second most common neurodegenerative disorder that affects over one million individuals in the US alone. PD is characterized by a plethora of motor and non-motor manifestations, resulting from a progressive degeneration of dopaminergic neurons and disbalance of several other neurotransmitters. A growing body of evidence points to significant alterations of the circadian system in PD. This is not surprising given the pivotal role that dopamine plays in circadian regulation as well as the role of circadian influences in dopamine metabolism. In this review we present basic and clinical investigations that examined the function of the circadian system in PD.