Glutamate Cotransmission in Cholinergic, GABAergic and Monoamine Systems: Contrasts and Commonalities

Aging disrupts the coordination between mRNA and protein expression in mouse and human midbrain

Age-related dopamine (DA) neuron loss is a primary feature of Parkinson's disease. However, it remains unclear whether similar biological processes occur during healthy aging, albeit to a lesser degree. We therefore determined whether midbrain DA neurons degenerate during aging in mice and humans. In mice, we identified no changes in midbrain neuron numbers throughout aging. Despite this, we found age-related decreases in midbrain mRNA expression of tyrosine hydroxylase (Th), the rate limiting enzyme of DA synthesis. Among midbrain glutamatergic cells, we similarly identified age-related declines in vesicular glutamate transporter 2 (Vglut2) mRNA expression. In co-transmitting Th +/Vglut2 + neurons, Th and Vglut2 transcripts decreased with aging. Importantly, striatal Th and Vglut2 protein expression remained unchanged. In translating our findings to humans, we found no midbrain neurodegeneration during aging and identified age-related decreases in TH and VGLUT2 mRNA expression similar to mouse. Unlike mice, we discovered diminished density of striatal TH+ dopaminergic terminals in aged human subjects. However, TH and VGLUT2 protein expression were unchanged in the remaining striatal boutons. Finally, in contrast to Th and Vglut2 mRNA, expression of most ribosomal genes in Th + neurons was either maintained or even upregulated during aging. This suggests a homeostatic mechanism where age-related declines in transcriptional efficiency are overcome by ongoing ribosomal translation. Overall, we demonstrate species-conserved transcriptional effects of aging in midbrain dopaminergic and glutamatergic neurons that are not accompanied by marked cell death or lower striatal protein expression. This opens the door to novel therapeutic approaches to maintain neurotransmission and bolster neuronal resilience.

Characterizing enteric neurons in dopamine transporter (DAT)-Cre reporter mice reveals dopaminergic subtypes with dual-transmitter content

The enteric nervous system (ENS) comprises a complex network of neurons whereby a subset appears to be dopaminergic although the characteristics, roles, and implications in disease are less understood. Most investigations relating to enteric dopamine (DA) neurons rely on immunoreactivity to tyrosine hydroxylase (TH)-the rate-limiting enzyme in the production of DA. However, TH immunoreactivity is likely to provide an incomplete picture. This study herein provides a comprehensive characterization of DA neurons in the gut using a reporter mouse line, expressing a fluorescent protein (tdTomato) under control of the DA transporter (DAT) promoter. Our findings confirm a unique localization of DA neurons in the gut and unveil the discrete subtypes of DA neurons in this organ, which we characterized using both immunofluorescence and single-cell transcriptomics, as well as validated using in situ hybridization. We observed distinct subtypes of DAT-tdTomato neurons expressing co-transmitters and modulators across both plexuses; some of them likely co-releasing acetylcholine, while others were positive for a slew of canonical DAergic markers (TH, VMAT2 and GIRK2). Interestingly, we uncovered a seemingly novel population of DA neurons unique to the ENS which was ChAT/DAT-tdTomato-immunoreactive and expressed Grp, Calcb, and Sst. Given the clear heterogeneity of DAergic gut neurons, further investigation is warranted to define their functional signatures and decipher their implication in disease.

Evidence for a Role of 5-HT-glutamate Co-releasing Neurons in Acute Stress Mechanisms

A major subpopulation of midbrain 5-hydroxytryptamine (5-HT) neurons expresses the vesicular glutamate transporter 3 (VGLUT3) and co-releases 5-HT and glutamate, but the function of this co-release is unclear. Given the strong links between 5-HT and uncontrollable stress, we used a combination of c-Fos immunohistochemistry and conditional gene knockout mice to test the hypothesis that glutamate co-releasing 5-HT neurons are activated by stress and involved in stress coping. Acute, uncontrollable swim stress increased c-Fos immunoreactivity in neurons co-expressing VGLUT3 and the 5-HT marker tryptophan hydroxylase 2 (TPH2) in the dorsal raphe nucleus (DRN). This effect was localized in the ventral DRN subregion and prevented by the antidepressant fluoxetine. In contrast, a more controllable stressor, acute social defeat, had no effect on c-Fos immunoreactivity in VGLUT3-TPH2 co-expressing neurons in the DRN. To test whether activation of glutamate co-releasing 5-HT neurons was causally linked to stress coping, mice with a specific deletion of VGLUT3 in 5-HT neurons were exposed to acute swim stress. Compared to wildtype controls, the mutant mice showed increased climbing behavior, a measure of active coping. Wildtype mice also showed increased climbing when administered fluoxetine, revealing an interesting parallel between the behavioral effects of genetic loss of VGLUT3 in 5-HT neurons and 5-HT reuptake inhibition. We conclude that 5-HT-glutamate co-releasing neurons are recruited by exposure to uncontrollable stress. Furthermore, natural variation in the balance of 5-HT and glutamate co-released at the 5-HT synapse may impact stress susceptibility.

Architecture of the subthalamic nucleus

The subthalamic nucleus (STN) is a major neuromodulation target for the alleviation of neurological and neuropsychiatric symptoms using deep brain stimulation (DBS). STN-DBS is today applied as treatment in Parkinson´s disease, dystonia, essential tremor, and obsessive-compulsive disorder (OCD). STN-DBS also shows promise as a treatment for refractory Tourette syndrome. However, the internal organization of the STN has remained elusive and challenges researchers and clinicians: How can this small brain structure engage in the multitude of functions that renders it a key hub for therapeutic intervention of a variety of brain disorders ranging from motor to affective to cognitive? Based on recent gene expression studies of the STN, a comprehensive view of the anatomical and cellular organization, including revelations of spatio-molecular heterogeneity, is now possible to outline. In this review, we focus attention to the neurobiological architecture of the STN with specific emphasis on molecular patterns discovered within this complex brain area. Studies from human, non-human primate, and rodent brains now reveal anatomically defined distribution of specific molecular markers. Together their spatial patterns indicate a heterogeneous molecular architecture within the STN. Considering the translational capacity of targeting the STN in severe brain disorders, the addition of molecular profiling of the STN will allow for advancement in precision of clinical STN-based interventions.

Drug repositioning targeting glutaminase reveals drug candidates for the treatment of Alzheimer's disease patients

BACKGROUND

Despite numerous clinical trials and decades of endeavour, there is still no effective cure for Alzheimer's disease. Computational drug repositioning approaches may be employed for the development of new treatment strategies for Alzheimer's patients since an extensive amount of omics data has been generated during pre-clinical and clinical studies. However, targeting the most critical pathophysiological mechanisms and determining drugs with proper pharmacodynamics and good efficacy are equally crucial in drug repurposing and often imbalanced in Alzheimer's studies.

METHODS

Here, we investigated central co-expressed genes upregulated in Alzheimer's disease to determine a proper therapeutic target. We backed our reasoning by checking the target gene's estimated non-essentiality for survival in multiple human tissues. We screened transcriptome profiles of various human cell lines perturbed by drug induction (for 6798 compounds) and gene knockout using data available in the Connectivity Map database. Then, we applied a profile-based drug repositioning approach to discover drugs targeting the target gene based on the correlations between these transcriptome profiles. We evaluated the bioavailability, functional enrichment profiles and drug-protein interactions of these repurposed agents and evidenced their cellular viability and efficacy in glial cell culture by experimental assays and Western blotting. Finally, we evaluated their pharmacokinetics to anticipate to which degree their efficacy can be improved.

RESULTS

We identified glutaminase as a promising drug target. Glutaminase overexpression may fuel the glutamate excitotoxicity in neurons, leading to mitochondrial dysfunction and other neurodegeneration hallmark processes. The computational drug repurposing revealed eight drugs: mitoxantrone, bortezomib, parbendazole, crizotinib, withaferin-a, SA-25547 and two unstudied compounds. We demonstrated that the proposed drugs could effectively suppress glutaminase and reduce glutamate production in the diseased brain through multiple neurodegeneration-associated mechanisms, including cytoskeleton and proteostasis. We also estimated the human blood-brain barrier permeability of parbendazole and SA-25547 using the SwissADME tool.

CONCLUSIONS

This study method effectively identified an Alzheimer's disease marker and compounds targeting the marker and interconnected biological processes by use of multiple computational approaches. Our results highlight the importance of synaptic glutamate signalling in Alzheimer's disease progression. We suggest repurposable drugs (like parbendazole) with well-evidenced activities that we linked to glutamate synthesis hereby and novel molecules (SA-25547) with estimated mechanisms for the treatment of Alzheimer's patients.

Absence of VGLUT3 Expression Leads to Impaired Fear Memory in Mice

Fear is an emotional mechanism that helps to cope with potential hazards. However, when fear is generalized, it becomes maladaptive and represents a core symptom of posttraumatic stress disorder (PTSD). Converging lines of research show that dysfunction of glutamatergic neurotransmission is a cardinal feature of trauma and stress related disorders such as PTSD. However, the involvement of glutamatergic co-transmission in fear is less well understood. Glutamate is accumulated into synaptic vesicles by vesicular glutamate transporters (VGLUTs). The atypical subtype, VGLUT3, is responsible for the co-transmission of glutamate with acetylcholine, serotonin, or GABA. To understand the involvement of VGLUT3-dependent co-transmission in aversive memories, we used a Pavlovian fear conditioning paradigm in VGLUT3^-/- mice. Our results revealed a higher contextual fear memory in these mice, despite a facilitation of extinction. In addition, the absence of VGLUT3 leads to fear generalization, probably because of a pattern separation deficit. Our study suggests that the VGLUT3 network plays a crucial role in regulating emotional memories. Hence, VGLUT3 is a key player in the processing of aversive memories and therefore a potential therapeutic target in stress-related disorders.

Unveiling behavioral and molecular neuroadaptations related to the antidepressant action of cannabidiol in the unpredictable chronic mild stress model

Introduction: This study aims to further characterize cannabidiol's pharmacological and molecular profile as an antidepressant. Methods: Effects of cannabidiol (CBD), alone or combined with sertraline (STR), were evaluated in male CD1 mice (n = 48) exposed to an unpredictable chronic mild stress (UCMS) procedure. Once the model was established (4 weeks), mice received CBD (20 mg·kg-1, i.p.), STR (10 mg·kg-1, p.o.) or its combination for 28 days. The efficacy of CBD was evaluated using the light-dark box (LDB), elevated plus maze (EPM), tail suspension (TS), sucrose consumption (SC) and novel object recognition (NOR) tests. Gene expression changes in the serotonin transporter, 5-HT1A and 5-HT2A receptors, BDNF, VGlut1 and PPARdelta, were evaluated in the dorsal raphe, hippocampus (Hipp) and amygdala by real-time PCR. Besides, BDNF, NeuN and caspase-3 immunoreactivity were assessed in the Hipp. Results: CBD exerted anxiolytic and antidepressant-like effects at 4 and 7 days of treatment in the LDB and TS tests, respectively. In contrast, STR required 14 days of treatment to show efficacy. CBD improved cognitive impairment and anhedonia more significantly than STR. CBD plus STR showed a similar effect than CBD in the LBD, TST and EPM. However, a worse outcome was observed in the NOR and SI tests. CBD modulates all molecular disturbances induced by UCMS, whereas STR and the combination could not restore 5-HT1A, BDNF and PPARdelta in the Hipp. Discussion: These results pointed out CBD as a potential new antidepressant with faster action and efficiency than STR. Particular attention should be given to the combination of CBD with current SSRI since it appears to produce a negative impact on treatment.

Delayed effects of neonatal immune activation on brain neurochemistry and hypothalamic-pituitary-adrenal axis functioning

During the postnatal period, the brain is highly sensitive to stress and inflammation, which are hazardous to normal growth and development. There is increasing evidence that inflammatory processes in the early postnatal period increase the risk of psychopathologies and cognitive impairment later in life. On the other hand, there are few studies on the ability of infectious agents to cause long-term neuroinflammation, leading to changes in the hypothalamic-pituitary-adrenal axis functioning and an imbalance in the neurotransmitter system. In this review, we examine short- and long-term effects of neonatal-induced inflammation in rodents on glutamatergic, GABAergic and monoaminergic systems and on hypothalamic-pituitary-adrenal axis activity.

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.

Chemogenetic activation of noradrenergic A5 neurons increases blood pressure and visceral sympathetic activity in adult rats

In mammals, the pontine noradrenergic system influences nearly every aspect of central nervous system function. A subpopulation of pontine noradrenergic neurons, called A5, are thought to be important in the cardiovascular response to physical stressors, yet their function is poorly defined. We hypothesized that activation of A5 neurons drives a sympathetically mediated increase in blood pressure (BP). To test this hypothesis, we conducted a comprehensive assessment of the cardiovascular effects of chemogenetic stimulation of A5 neurons in male and female adult rats using intersectional genetic and anatomical targeting approaches. Chemogenetic stimulation of A5 neurons in freely behaving rats elevated BP by 15 mmHg and increased cardiac baroreflex sensitivity with a negligible effect on resting HR. Importantly, A5 stimulation had no detectable effect on locomotor activity, metabolic rate, or respiration. Under anesthesia, stimulation of A5 neurons produced a marked elevation in visceral sympathetic nerve activity (SNA) and no change in skeletal muscle SNA, showing that A5 neurons preferentially stimulate visceral SNA. Interestingly, projection mapping indicates that A5 neurons target sympathetic preganglionic neurons throughout the spinal cord and parasympathetic preganglionic neurons throughout in the brainstem, as well as the nucleus of the solitary tract, and ventrolateral medulla. Moreover, in situ hybridization and immunohistochemistry indicate that a subpopulation of A5 neurons coreleases glutamate and monoamines. Collectively, this study suggests A5 neurons are a central modulator of autonomic function with a potentially important role in sympathetically driven redistribution of blood flow from the visceral circulation to critical organs and skeletal muscle.

Nanoscopic distribution of VAChT and VGLUT3 in striatal cholinergic varicosities suggests colocalization and segregation of the two transporters in synaptic vesicles

Striatal cholinergic interneurons (CINs) use acetylcholine (ACh) and glutamate (Glut) to regulate the striatal network since they express vesicular transporters for ACh (VAChT) and Glut (VGLUT3). However, whether ACh and Glut are released simultaneously and/or independently from cholinergic varicosities is an open question. The answer to that question requires the multichannel detection of vesicular transporters at the level of single synaptic vesicle (SV). Here, we used super-resolution STimulated Emission Depletion microscopy (STED) to characterize and quantify the distribution of VAChT and VGLUT3 in CINs SVs. Nearest-neighbor distances analysis between VAChT and VGLUT3-immunofluorescent spots revealed that 34% of CINs SVs contain both VAChT and VGLUT3. In addition, 40% of SVs expressed only VAChT while 26% of SVs contain only VGLUT3. These results suggest that SVs from CINs have the potential to store simultaneously or independently ACh and/or Glut. Overall, these morphological findings support the notion that CINs varicosities can signal with either ACh or Glut or both with an unexpected level of complexity.

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.

Excitatory selective LTP of supramammillary glutamatergic/GABAergic cotransmission potentiates dentate granule cell firing

It is now established that many neurons can release multiple transmitters. Recent studies revealed that fast-acting neurotransmitters, glutamate and GABA, are coreleased from the same presynaptic terminals in some adult brain regions. The dentate gyrus (DG) granule cells (GCs) are innervated by the hypothalamic supramammillary nucleus (SuM) afferents that corelease glutamate and GABA. However, how these functionally opposing neurotransmitters contribute to DG information processing remains unclear. We show that glutamatergic, but not GABAergic, cotransmission exhibits long-term potentiation (LTP) at SuM-GC synapses. By the excitatory selective LTP, the excitation/inhibition balance of SuM inputs increases, and GC firing is enhanced. This study provides evidence that glutamatergic/GABAergic cotransmission balance is rapidly changed in an activity-dependent manner, and such plasticity may modulate DG activity.

Emerging evidence indicates that the functionally opposing neurotransmitters, glutamate and GABA, are coreleased from the same presynaptic terminals in some adult brain regions. The supramammillary nucleus (SuM) is one region that coreleases glutamate and GABA in the dentate gyrus (DG) through its afferents. Although the SuM-DG pathway has been implicated in various brain functions, little is known about the functional roles of the peculiar features of glutamate/GABA corelease. Here, we show that depolarization of granule cells (GCs) triggers postsynaptic long-term potentiation (LTP) of glutamatergic, but not GABAergic, cotransmission at SuM-GC synapses. Moreover, the burst activity of perforant-path inputs heterosynaptically induces LTP at excitatory SuM-GC synapses. This non-Hebbian LTP requires postsynaptic Ca²⁺ influx, Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) activity, and exocytosis of AMPA receptors. Glutamatergic transmission-selective expression of LTP increases the excitatory drive such that SuM inputs become sufficient to discharge GCs. Our results highlight a form of LTP, which dynamically and rapidly changes the glutamatergic/GABAergic cotransmission balance and contributes to DG network activity.

Noncanonical genomic imprinting in the monoamine system determines naturalistic foraging and brain-adrenal axis functions

Noncanonical genomic imprinting can cause biased expression of one parental allele in a tissue; however, the functional relevance of such biases is unclear. To investigate ethological roles for noncanonical imprinting in dopa decarboxylase (Ddc) and tyrosine hydroxylase (Th), we use machine learning to decompose naturalistic foraging in maternal and paternal allele mutant heterozygous mice. We uncover distinct roles for the maternal versus paternal alleles on foraging, where maternal alleles affect sons while daughters are under paternal allelic control. Each parental allele controls specific action sequences reflecting decisions in naive or familiar contexts. The maternal Ddc allele is preferentially expressed in subsets of hypothalamic GABAergic neurons, while the paternal allele predominates in subsets of adrenal cells. Each Ddc allele affects distinct molecular and endocrine components of the brain-adrenal axis. Thus, monoaminergic noncanonical imprinting has ethological roles in foraging and endocrine functions and operates by affecting discrete subsets of cells.

Colocalization of different neurotransmitter transporters on synaptic vesicles is sparse except for VGLUT1 and ZnT3

Vesicular transporters (VTs) define the type of neurotransmitter that synaptic vesicles (SVs) store and release. While certain mammalian neurons release multiple transmitters, it is not clear whether the release occurs from the same or distinct vesicle pools at the synapse. Using quantitative single-vesicle imaging, we show that a vast majority of SVs in the rodent brain contain only one type of VT, indicating specificity for a single neurotransmitter. Interestingly, SVs containing dual transporters are highly diverse (27 types) but small in proportion (2% of all SVs), excluding the largest pool that carries VGLUT1 and ZnT3 (34%). Using VGLUT1-ZnT3 SVs, we demonstrate that the transporter colocalization influences the SV content and synaptic quantal size. Thus, the presence of diverse transporters on the same vesicle is bona fide, and depending on the VT types, this may act to regulate neurotransmitter type, content, and release in space and time.

Hippocampal acetylcholine modulates stress-related behaviors independent of specific cholinergic inputs

Acetylcholine (ACh) levels are elevated in actively depressed subjects. Conversely, antagonism of either nicotinic or muscarinic ACh receptors can have antidepressant effects in humans and decrease stress-relevant behaviors in rodents. Consistent with a role for ACh in mediating maladaptive responses to stress, brain ACh levels increase in response to stressful challenges, whereas systemically blocking acetylcholinesterase (AChE, the primary ACh degradative enzyme) elicits depression-like symptoms in human subjects, and selectively blocking AChE in the hippocampus increases relevant behaviors in rodents. We used an ACh sensor to characterize stress-evoked ACh release, then used chemogenetic, optogenetic and pharmacological approaches to determine whether cholinergic inputs from the medial septum/diagonal bands of Broca (MSDBB) or ChAT-positive neurons intrinsic to the hippocampus mediate stress-relevant behaviors in mice. Chemogenetic inhibition or activation of MSDBB cholinergic neurons did not result in significant behavioral effects, while inhibition attenuated the behavioral effects of physostigmine. In contrast, optogenetic stimulation of septohippocampal terminals or selective chemogenetic activation of ChAT-positive inputs to hippocampus increased stress-related behaviors. Finally, stimulation of sparse ChAT-positive hippocampal neurons increased stress-related behaviors in one ChAT-Cre line, which were attenuated by local infusion of cholinergic antagonists. These studies suggest that ACh signaling results in maladaptive behavioral responses to stress if the balance of signaling is shifted toward increased hippocampal engagement.

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.

The Effect of the Low Glutamate Diet on the Reduction of Psychiatric Symptoms in Veterans With Gulf War Illness: A Pilot Randomized-Controlled Trial

The objective of this pilot study was to examine the effects of the low glutamate diet on anxiety, post-traumatic stress disorder (PTSD), and depression in veterans with Gulf War Illness (GWI). The low glutamate diet removes dietary excitotoxins and increases consumption of micronutrients which are protective against glutamatergic excitotoxicity. This study was registered at ClinicalTrials.gov (NCT#03342482). Forty veterans with GWI completed psychiatric questionnaires at baseline and after 1-month following the low glutamate diet. Participants were then randomized into a double-blind, placebo-controlled crossover challenge with monosodium glutamate (MSG; a dietary excitotoxin) vs. placebo over three consecutive days per week, with assessments on day three. Data were analyzed across the full sample and with participants categorized by baseline symptom severity. Pre-post-dietary intervention change scores were analyzed with Wilcoxon signed-rank tests and paired sample t-tests across the full sample, and changes across symptom severity categories were analyzed using ANOVA. Crossover challenge results were analyzed with linear mixed modeling accounting for challenge material (MSG v. placebo), sequence (MSG/placebo v. placebo/MSG), period (challenge week 1 v. week 2), pre-diet baseline symptom severity category (minimal/mild, moderate, or severe), and the challenge material^*symptom severity category interaction. A random effect of ID (sequence) was also included. All three measures showed significant improvement after 1 month on the diet, with significant differences between baseline severity categories. Individuals with severe psychological symptoms at baseline showed the most improvement after 1 month on the diet, while those with minimal/mild symptoms showed little to no change. Modeling results from the challenge period demonstrated a significant worsening of anxiety from MSG in only the most severe group, with no significant effects of MSG challenge on depression nor PTSD symptoms. These results suggest that the low glutamate diet may be an effective treatment for depression, anxiety, and PTSD, but that either (a) glutamate is only a direct cause of symptoms in anxiety, or (b) underlying nutrient intake may prevent negative psychiatric effects from glutamate exposure. Future, larger scale clinical trials are needed to confirm these findings and to further explore the potential influence of increased micronutrient intake on the improvements observed across anxiety, PTSD, and depression.

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.

Neuronal pericellular baskets: neurotransmitter convergence and regulation of network excitability

A pericellular basket is a presynaptic configuration of numerous axonal boutons outlining a target neuron soma and its proximal dendrites. Recent studies show neurochemical diversity of pericellular baskets and suggest that neurotransmitter usage together with the dense, soma-proximal boutons may permit strong input effects on different timescales. Here we review the development, distribution, neurochemical phenotypes, and possible functions of pericellular baskets. As an example, we highlight pericellular baskets formed by projections of certain Pet1/Fev neurons of the serotonergic raphe nuclei. We propose that pericellular baskets represent convergence sites of competition or facilitation between neurotransmitter systems on downstream circuitry, especially in limbic brain regions, where pericellular baskets are widespread. Study of these baskets may enhance our understanding of monoamine regulation of memory, social behavior, and brain oscillations.

Bidirectional role of dopamine in learning and memory-active forgetting

Dopaminergic neurons projecting from the Substantia Nigra to the Striatum play a critical role in motor functions while dopaminergic neurons originating in the Ventral Tegmental Area (VTA) and projecting to the Nucleus Accumbens, Hippocampus and other cortical structures regulate rewarding learning. While VTA mainly consists of dopaminergic neurons, excitatory (glutamate) and inhibitory (GABA) VTA-neurons have also been described: these neurons may also modulate and contribute to shape the final dopaminergic response, which is critical for memory formation. However, given the large amount of information that is handled daily by our brain, it is essential that irrelevant information be deleted. Recently, apart from the well-established role of dopamine (DA) in learning, it has been shown that DA plays a critical role in the intrinsic active forgetting mechanisms that control storage information, contributing to the deletion of a consolidated memory. These new insights may be instrumental to identify therapies for those disorders that involve memory alterations.

Dopaminergic neurons establish a distinctive axonal arbor with a majority of non-synaptic terminals

Chemical neurotransmission typically occurs through synapses. Previous ultrastructural examinations of monoamine neuron axon terminals often failed to identify a pre- and postsynaptic coupling, leading to the concept of "volume" transmission. Whether this results from intrinsic properties of these neurons remains undefined. We find that dopaminergic neurons in vitro establish a distinctive axonal arbor compared to glutamatergic or GABAergic neurons in both size and propensity of terminals to avoid direct contact with target neurons. While most dopaminergic varicosities are active and contain exocytosis proteins like synaptotagmin 1, only ~20% of these are synaptic. The active zone protein bassoon was found to be enriched in dopaminergic terminals that are in proximity to a target cell. Finally, we found that the proteins neurexin-1α^SS4- and neuroligin-1^A+B play a critical role in the formation of synapses by dopamine (DA) neurons. Our findings suggest that DA neurons are endowed with a distinctive developmental connectivity program.

Change in the neurochemical signature and morphological development of the parvocellular isthmic projection to the avian tectum

Neurons can change their classical neurotransmitters during ontogeny, sometimes going through stages of dual release. Here, we explored the development of the neurotransmitter identity of neurons of the avian nucleus isthmi parvocellularis (Ipc), whose axon terminals are retinotopically arranged in the optic tectum (TeO) and exert a focal gating effect upon the ascending transmission of retinal inputs. Although cholinergic and glutamatergic markers are both found in Ipc neurons and terminals of adult pigeons and chicks, the mRNA expression of the vesicular acetylcholine transporter, VAChT, is weak or absent. To explore how the Ipc neurotransmitter identity is established during ontogeny, we analyzed the expression of mRNAs coding for cholinergic (ChAT, VAChT, and CHT) and glutamatergic (VGluT2 and VGluT3) markers in chick embryos at different developmental stages. We found that between E12 and E18, Ipc neurons expressed all cholinergic mRNAs and also VGluT2 mRNA; however, from E16 through posthatch stages, VAChT mRNA expression was specifically diminished. Our ex vivo deposits of tracer crystals and intracellular filling experiments revealed that Ipc axons exhibit a mature paintbrush morphology late in development, experiencing marked morphological transformations during the period of presumptive dual vesicular transmitter release. Additionally, although ChAT protein immunoassays increasingly label the growing Ipc axon, this labeling was consistently restricted to sparse portions of the terminal branches. Combined, these results suggest that the synthesis of glutamate and acetylcholine, and their vesicular release, is complexly linked to the developmental processes of branching, growing and remodeling of these unique axons.

The Long-Term Effects of Neonatal Inflammatory Pain on Cognitive Function and Stress Hormones Depend on the Heterogeneity of the Adolescent Period of Development in Male and Female Rats

Exposure to stress at an early age programs the HPA axis which can lead to cognitive deficits in adults. However, it is not known whether these deficits emerge in adulthood or are expressed earlier in life. The aims of the study were to investigate (1) the immediate effects of early injury-induced stress in one-day-old (P1) and repeated stress on at P1 and P2 rat pups on plasma corticosterone levels; and (2) examine the subsequent long-term effects of this early stress on spatial learning and memory, and stress reactivity in early P26-34 and late P45-53 adolescent male and female rats. Intra-plantar injection of formalin induced prolonged and elevated levels of corticosterone in pups and impaired spatial learning and short- and long-term memory in late adolescent males and long-term memory in early adolescent females. There were sex differences in late adolescence in both learning and short-term memory. Performance on the long-term memory task was better than that on the short-term memory task for all early adolescent male and female control and stressed animals. Short-term memory was better in the late age control rats of both sexes and for formalin treated females as compared with the early age rats. These results are consistent with an impaired function of structures involved in memory (the hippocampus, amygdala, prefrontal cortex) after newborn pain. However, activation of the HPA axis by neonatal pain did not directly correlate with spatial learning and memory outcomes and the consequences of neonatal pain remain are likely multi-determined.

Vesicular glutamate transporter modulates sex differences in dopamine neuron vulnerability to age-related neurodegeneration

Age is the greatest risk factor for Parkinson's disease (PD) which causes progressive loss of dopamine (DA) neurons, with males at greater risk than females. Intriguingly, some DA neurons are more resilient to degeneration than others. Increasing evidence suggests that vesicular glutamate transporter (VGLUT) expression in DA neurons plays a role in this selective vulnerability. We investigated the role of DA neuron VGLUT in sex- and age-related differences in DA neuron vulnerability using the genetically tractable Drosophila model. We found sex differences in age-related DA neurodegeneration and its associated locomotor behavior, where males exhibit significantly greater decreases in both DA neuron number and locomotion during aging compared with females. We discovered that dynamic changes in DA neuron VGLUT expression mediate these age- and sex-related differences, as a potential compensatory mechanism for diminished DA neurotransmission during aging. Importantly, female Drosophila possess higher levels of VGLUT expression in DA neurons compared with males, and this finding is conserved across flies, rodents, and humans. Moreover, we showed that diminishing VGLUT expression in DA neurons eliminates females' greater resilience to DA neuron loss across aging. This offers a new mechanism for sex differences in selective DA neuron vulnerability to age-related DA neurodegeneration. Finally, in mice, we showed that the ability of DA neurons to achieve optimal control over VGLUT expression is essential for DA neuron survival. These findings lay the groundwork for the manipulation of DA neuron VGLUT expression as a novel therapeutic strategy to boost DA neuron resilience to age- and PD-related neurodegeneration.

Long-Range GABAergic Inhibition Modulates Spatiotemporal Dynamics of the Output Neurons in the Olfactory Bulb

Local interneurons of the olfactory bulb (OB) are densely innervated by long-range GABAergic neurons from the basal forebrain (BF), suggesting that this top-down inhibition regulates early processing in the olfactory system. However, how GABAergic inputs modulate the OB output neurons, the mitral/tufted cells, is unknown. Here, in male and female mice acute brain slices, we show that optogenetic activation of BF GABAergic inputs produced distinct local circuit effects that can influence the activity of mitral/tufted cells in the spatiotemporal domains. Activation of the GABAergic axons produced a fast disinhibition of mitral/tufted cells consistent with a rapid and synchronous release of GABA onto local interneurons in the glomerular and inframitral circuits of the OB, which also reduced the spike precision of mitral/tufted cells in response to simulated stimuli. In addition, BF GABAergic inhibition modulated local oscillations in a layer-specific manner. The intensity of locally evoked θ oscillations was decreased on activation of top-down inhibition in the glomerular circuit, while evoked γ oscillations were reduced by inhibition of granule cells. Furthermore, BF GABAergic input reduced dendrodendritic inhibition in mitral/tufted cells. Together, these results suggest that long-range GABAergic neurons from the BF are well suited to influence temporal and spatial aspects of processing by OB circuits.SIGNIFICANCE STATEMENT Disruption of GABAergic inhibition from the basal forebrain (BF) to the olfactory bulb (OB) impairs the discrimination of similar odors, yet how this centrifugal inhibition influences neuronal circuits in the OB remains unclear. Here, we show that the BF GABAergic neurons exclusively target local inhibitory neurons in the OB, having a functional disinhibitory effect on the output neurons, the mitral cells. Phasic inhibition by BF GABAergic neurons reduces spike precision of mitral cells and lowers the intensity of oscillatory activity in the OB, while directly modulating the extent of dendrodendritic inhibition. These circuit-level effects of this centrifugal inhibition can influence the temporal and spatial dynamics of odor coding in the OB.

Structural basis for noradrenergic regulation of neural circuits in the mouse olfactory bulb

Olfactory input is processed in the glomerulus of the main olfactory bulb (OB) and relayed to higher centers in the brain by projection neurons. Conversely, centrifugal inputs from other brain regions project to the OB. We have previously analyzed centrifugal inputs into the OB from several brain regions using single-neuron labeling. In this study, we analyzed the centrifugal noradrenergic (NA) fibers derived from the locus coeruleus (LC), because their projection pathways and synaptic connections in the OB have not been clarified in detail. We analyzed the NA centrifugal projections by single-neuron labeling and immunoelectron microscopy. Individual NA neurons labeled by viral infection were three-dimensionally traced using Neurolucida software to visualize the projection pathway from the LC to the OB. Also, centrifugal NA fibers were visualized using an antibody for noradrenaline transporter (NET). NET immunoreactive (-ir) fibers contained many varicosities and synaptic vesicles. Furthermore, electron tomography demonstrated that NET-ir fibers formed asymmetrical synapses of varied morphology. Although these synapses were present at varicosities, the density of synapses was relatively low throughout the OB. The maximal density of synapses was found in the external plexiform layer; about 17% of all observed varicosities contained synapses. These results strongly suggest that NA-containing fibers in the OB release NA from both varicosities and synapses to influence the activities of OB neurons. The present study provides a morphological basis for olfactory modulation by centrifugal NA fibers derived from the LC.

Cholinergic dysfunction in the dorsal striatum promotes habit formation and maladaptive eating

Dysregulation of habit formation has been recently proposed as pivotal to eating disorders. Here, we report that a subset of patients suffering from restrictive anorexia nervosa have enhanced habit formation compared with healthy controls. Habit formation is modulated by striatal cholinergic interneurons. These interneurons express vesicular transporters for acetylcholine (VAChT) and glutamate (VGLUT3) and use acetylcholine/glutamate cotransmission to regulate striatal functions. Using mice with genetically silenced VAChT (VAChT conditional KO, VAChTcKO) or VGLUT3 (VGLUT3cKO), we investigated the roles that acetylcholine and glutamate released by cholinergic interneurons play in habit formation and maladaptive eating. Silencing glutamate favored goal-directed behaviors and had no impact on eating behavior. In contrast, VAChTcKO mice were more prone to habits and maladaptive eating. Specific deletion of VAChT in the dorsomedial striatum of adult mice was sufficient to phenocopy maladaptive eating behaviors of VAChTcKO mice. Interestingly, VAChTcKO mice had reduced dopamine release in the dorsomedial striatum but not in the dorsolateral striatum. The dysfunctional eating behavior of VAChTcKO mice was alleviated by donepezil and by l-DOPA, confirming an acetylcholine/dopamine deficit. Our study reveals that loss of acetylcholine leads to a dopamine imbalance in striatal compartments, thereby promoting habits and vulnerability to maladaptive eating in mice.

Single-Cell Transcriptomic Reveals Dual and Multi-Transmitter Use in Neurons Across Metazoans

Neurotransmitter expression is widely used as a criterion for classifying neurons. It was initially thought that neurons express a single type of neurotransmitter, a phenomenon commonly recognized as Dale's principle: “one neuron, one transmitter.” Consequently, the expression of a single neurotransmitter should determine stable and distinguishable neuronal characteristics. However, this notion has been largely challenged and increasing evidence accumulates supporting a different scenario: “one neuron, multiple neurotransmitters.” Single-cell transcriptomics provides an additional path to address coexpression of neurotransmitters, by investigating the expression of genes involved in the biosynthesis and transmission of fast-acting neuromodulators. Here, we study neuronal phenotypes based on the expression of neurotransmitters, at single-cell resolution, across different animal species representing distinct clades of the tree of life. We take advantage of several existing scRNAseq datasets and analyze them in light of neurotransmitter plasticity. Our results show that while most neurons appear to predominantly express a single type of neurotransmitter, a substantial number of neurons simultaneously expresses a combination of them, across all animal species analyzed.

Absence of metabotropic glutamate receptor homolog(s) accelerates acetylcholine neurotransmission in Caenorhabditis elegans

Glutamate (Glu) and Acetylcholine (ACh), are excitatory neurotransmitters, acting through ionotropic (iR) and metabotropic receptors (mR). Importantly, both neurotransmitters and their signalling are impaired in the prevalent neurodegenerative disease-Alzheimer disease (AD). Glu and its signalling cascade's influence on ACh-neurotransmission (NT) are sparsely understood. The mGluRs coupled to G-protein signalling acting through PI3K cascade (GrpI) or inhibition of adenylate cyclase-cAMP cascade (GrpII and GrpIII) brings about long-lasting structural/functional changes. These complexities are challenging to decipher. Here, we report that human/mouse mGluRs when compared with their Caenorhabditis elegans homologs, MGL-1-3 showed overall of homology of ∼31-39 %. Phylogeneitc analysis revealed homology of MGL-2 to GrpI, MGL-3 with Grp1 &II and GRM6 of GrpIII and MGL-1, a low homology that falls between GrpI & GrpII. Then, alteration of ACh-NT in C. elegans loss-of-function mutants of mgl-1, mgl-2, mgl-3, PI3K (age-1) and iGluR (NMDA)(nmr-1) was estimated by well-established acute aldicarb (Ald), that increases ACh at synapse, and levamisole (Lev) (postsynaptic activation of levamisole sensitive iAChR) induced time-dependent paralysis assays. Surprisingly, all of them were hypersensitive to Ald and Lev compared to wildtype (in percentage), namely, mgl-1 -17, 54; mgl-2 - 7.2, 24; mgl-3 -52, 64; age-1 - 27, 32; nmr-1- 24, 48; respectively. Of the three, mgl-3 contributes to maximal overall acceleration of ACh-NT. Adenylate cyclase, acy-1 gain-of-function mutant showed less hypersensitivity, Ald - 7% and Lev- 25 %. Together, Glu receptors and signalling cascades are altering ACh-NT permanently, thus establishing the interplay between them thereby provide potential drug targets to be considered for AD.

Sex differences in the transcription of glutamate transporters in major depression and suicide

BACKGROUND

Accumulating evidence indicates that the glutamate system contributes to the pathophysiology of major depressive disorder (MDD) and suicide. We previously reported higher mRNA expression of glutamate receptors in the dorsolateral prefrontal cortex (DLPFC) of females with MDD.

METHODS

In the current study, we measured the expression of mRNAs encoding glutamate transporters in the DLPFC of MDD subjects who died by suicide (MDD-S, n = 51), MDD non-suicide subjects (MDD-NS, n = 28), and individuals who did not have a history of neurological illness (CTRL, n = 32).

RESULTS

Females but not males with MDD showed higher expression of EAATs and VGLUTs relative to CTRLs. VGLUT expression was significantly higher in the female MDD-S group, relative to the other groups. EAAT expression was lower in the male violent suicides.

LIMITATIONS

This study has limitations common to most human studies, including medication history and demographic differences between the diagnostic groups. We mitigated the effects of confounders by including them as covariates in our analyses.

CONCLUSIONS

We report sex differences in the expression of glutamate transporter genes in the DLPFC in MDD. Increased neuronal glutamate transporter expression may increase synaptic glutamate, leading to neuronal and glial loss in the DLPFC in MDD. These deficits may lower DLPFC activity, impair problem solving and impair executive function in depression, perhaps increasing vulnerability to suicidal behavior. These data add to accumulating support for the hypothesis that glutamatergic transmission is dysregulated in MDD and suicide. Glutamate transporters may be novel targets for the development of rapidly acting antidepressant therapies.

Vesicular neurotransmitter transporters in Drosophila melanogaster

Drosophila melanogaster express vesicular transporters for the storage of neurotransmitters acetylcholine, biogenic amines, GABA, and glutamate. The large array of powerful molecular-genetic tools available in Drosophila enhances the use of this model organism for studying transporter function and regulation.

Heterogeneous expression of dopaminergic markers and Vglut2 in mouse mesodiencephalic dopaminergic nuclei A8-A13

Co-transmission of glutamate by brain dopaminergic (DA) neurons was recently proposed as a potential factor influencing cell survival in models of Parkinson's disease. Intriguingly, brain DA nuclei are differentially affected in Parkinson's disease. Whether this is associated with different patterns of co-expression of the glutamatergic phenotype along the rostrocaudal brain axis is unknown in mammals. We hypothesized that, as in zebrafish, the glutamatergic phenotype is present preferentially in the caudal mesodiencephalic DA nuclei. Here, we used in mice a cell fate mapping strategy based on reporter protein expression (ZsGreen) consecutive to previous expression of the vesicular glutamate transporter 2 (Vglut2) gene, coupled with immunofluorescence experiments against tyrosine hydroxylase (TH) or dopamine transporter (DAT). We found three expression patterns in DA cells, organized along the rostrocaudal brain axis. The first pattern (TH-positive, DAT-positive, ZsGreen-positive) was found in A8-A10. The second pattern (TH-positive, DAT-negative, ZsGreen-positive) was found in A11. The third pattern (TH-positive, DAT-negative, ZsGreen-negative) was found in A12-A13. These patterns should help to refine the establishment of the homology of DA nuclei between vertebrate species. Our results also uncover that Vglut2 is expressed at some point during cell lifetime in DA nuclei known to degenerate in Parkinson's disease and largely absent from those that are preserved, suggesting that co-expression of the glutamatergic phenotype in DA cells influences their survival in Parkinson's disease.

Roles of dopamine and glutamate co-release in the nucleus accumbens in mediating the actions of drugs of abuse

Projections of ventral tegmental area dopamine (DA) neurons to the medial shell of the nucleus accumbens have been increasingly implicated as integral to the behavioral and physiological changes involved in the development of substance use disorders (SUDs). Recently, many of these nucleus accumbens-projecting DA neurons were found to also release the neurotransmitter glutamate. This glutamate co-release from DA neurons is critical in mediating the effect of drugs of abuse on addiction-related behaviors. Potential mechanisms underlying the role(s) of dopamine/glutamate co-release in contributing to SUDs are unclear. Nevertheless, an important clue may relate to glutamate's ability to potentiate loading of DA into synaptic vesicles within terminals in the nucleus accumbens in response to drug-induced elevations in neuronal activity, enabling a more robust release of DA after stimulation. Here, we summarize how drugs of abuse, particularly cocaine, opioids, and alcohol, alter DA release in the nucleus accumbens medial shell, examine the potential role of DA/glutamate co-release in mediating these effects, and discuss future directions for further investigating these mechanisms.

Dynamic neurotransmitter specific transcription factor expression profiles during Drosophila development

The remarkable diversity of neurons in the nervous system is generated during development, when properties such as cell morphology, receptor profiles and neurotransmitter identities are specified. In order to gain a greater understanding of neurotransmitter specification we profiled the transcription state of cholinergic, GABAergic and glutamatergic neurons in vivo at three developmental time points. We identified 86 differentially expressed transcription factors that are uniquely enriched, or uniquely depleted, in a specific neurotransmitter type. Some transcription factors show a similar profile across development, others only show enrichment or depletion at specific developmental stages. Profiling of Acj6 (cholinergic enriched) and Ets65A (cholinergic depleted) binding sites in vivo reveals that they both directly bind the ChAT locus, in addition to a wide spectrum of other key neuronal differentiation genes. We also show that cholinergic enriched transcription factors are expressed in mostly non-overlapping populations in the adult brain, implying the absence of combinatorial regulation of neurotransmitter fate in this context. Furthermore, our data underlines that, similar to Caenorhabditis elegans, there are no simple transcription factor codes for neurotransmitter type specification.

This article has an associated First Person interview with the first author of the paper.

Summary: Transcriptome profiling of cholinergic, GABAergic and glutamatergic neurons in Drosophila identified multiple transcription factors as potential regulators of neurotransmitter fate.

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.

Paradoxical network excitation by glutamate release from VGluT3+ GABAergic interneurons

In violation of Dale's principle several neuronal subtypes utilize more than one classical neurotransmitter. Molecular identification of vesicular glutamate transporter three and cholecystokinin expressing cortical interneurons (CCK+VGluT3+INTs) has prompted speculation of GABA/glutamate corelease from these cells for almost two decades despite a lack of direct evidence. We unequivocally demonstrate CCK+VGluT3+INT-mediated GABA/glutamate cotransmission onto principal cells in adult mice using paired recording and optogenetic approaches. Although under normal conditions, GABAergic inhibition dominates CCK+VGluT3+INT signaling, glutamatergic signaling becomes predominant when glutamate decarboxylase (GAD) function is compromised. CCK+VGluT3+INTs exhibit surprising anatomical diversity comprising subsets of all known dendrite targeting CCK+ interneurons in addition to the expected basket cells, and their extensive circuit innervation profoundly dampens circuit excitability under normal conditions. However, in contexts where the glutamatergic phenotype of CCK+VGluT3+INTs is amplified, they promote paradoxical network hyperexcitability which may be relevant to disorders involving GAD dysfunction such as schizophrenia or vitamin B6 deficiency.

Neurochemical organization of the ventral striatum's olfactory tubercle

The ventral striatum is a collection of brain structures, including the nucleus accumbens, ventral pallidum and the olfactory tubercle (OT). While much attention has been devoted to the nucleus accumbens, a comprehensive understanding of the ventral striatum and its contributions to neurological diseases requires an appreciation for the complex neurochemical makeup of the ventral striatum's other components. This review summarizes the rich neurochemical composition of the OT, including the neurotransmitters, neuromodulators and hormones present. We also address the receptors and transporters involved in each system as well as their putative functional roles. Finally, we end with briefly reviewing select literature regarding neurochemical changes in the OT in the context of neurological disorders, specifically neurodegenerative disorders. By overviewing the vast literature on the neurochemical composition of the OT, this review will serve to aid future research into the neurobiology of the ventral striatum.

Octopamine neuron dependent aggression requires dVGLUT from dual-transmitting neurons

Neuromodulators such as monoamines are often expressed in neurons that also release at least one fast-acting neurotransmitter. The release of a combination of transmitters provides both "classical" and "modulatory" signals that could produce diverse and/or complementary effects in associated circuits. Here, we establish that the majority of Drosophila octopamine (OA) neurons are also glutamatergic and identify the individual contributions of each neurotransmitter on sex-specific behaviors. Males without OA display low levels of aggression and high levels of inter-male courtship. Males deficient for dVGLUT solely in OA-glutamate neurons (OGNs) also exhibit a reduction in aggression, but without a concurrent increase in inter-male courtship. Within OGNs, a portion of VMAT and dVGLUT puncta differ in localization suggesting spatial differences in OA signaling. Our findings establish a previously undetermined role for dVGLUT in OA neurons and suggests that glutamate uncouples aggression from OA-dependent courtship-related behavior. These results indicate that dual neurotransmission can increase the efficacy of individual neurotransmitters while maintaining unique functions within a multi-functional social behavior neuronal network.

Neurocircuitry of Reward and Addiction: Potential Impact of Dopamine-Glutamate Co-release as Future Target in Substance Use Disorder

Dopamine-glutamate co-release is a unique property of midbrain neurons primarily located in the ventral tegmental area (VTA). Dopamine neurons of the VTA are important for behavioral regulation in response to rewarding substances, including natural rewards and addictive drugs. The impact of glutamate co-release on behaviors regulated by VTA dopamine neurons has been challenging to probe due to lack of selective methodology. However, several studies implementing conditional knockout and optogenetics technologies in transgenic mice have during the past decade pointed towards a role for glutamate co-release in multiple physiological and behavioral processes of importance to substance use and abuse. In this review, we discuss these studies to highlight findings that may be critical when considering mechanisms of importance for prevention and treatment of substance abuse.

Glutamate at the Vertebrate Neuromuscular Junction: From Modulation to Neurotransmission

Although acetylcholine is the major neurotransmitter operating at the skeletal neuromuscular junction of many invertebrates and of vertebrates, glutamate participates in modulating cholinergic transmission and plastic changes in the last. Presynaptic terminals of neuromuscular junctions contain and release glutamate that contribute to the regulation of synaptic neurotransmission through its interaction with pre- and post-synaptic receptors activating downstream signaling pathways that tune synaptic efficacy and plasticity. During vertebrate development, the chemical nature of the neurotransmitter at the vertebrate neuromuscular junction can be experimentally shifted from acetylcholine to other mediators (including glutamate) through the modulation of calcium dynamics in motoneurons and, when the neurotransmitter changes, the muscle fiber expresses and assembles new receptors to match the nature of the new mediator. Finally, in adult rodents, by diverting descending spinal glutamatergic axons to a denervated muscle, a functional reinnervation can be achieved with the formation of new neuromuscular junctions that use glutamate as neurotransmitter and express ionotropic glutamate receptors and other markers of central glutamatergic synapses. Here, we summarize the past and recent experimental evidences in support of a role of glutamate as a mediator at the synapse between the motor nerve ending and the skeletal muscle fiber, focusing on the molecules and signaling pathways that are present and activated by glutamate at the vertebrate neuromuscular junction.

Differential Expression of VGLUT2 in Mouse Mesopontine Cholinergic Neurons

Vesicular glutamate transporters (VGLUTs) mediate the synaptic uptake of glutamate from the cytosol into synaptic vesicles and are considered unambiguous neurochemical markers of glutamate neurons. However, many neurons not classically thought of as glutamatergic also express a VGLUT and co-release glutamate. Using a genetic fate-mapping strategy we found that most cholinergic neurons in the mouse mesopontine tegmentum express VGLUT2 at some point during development, including the pedunculopontine tegmental nucleus (PPTg), laterodorsal tegmental nucleus, and parabigeminal nucleus (PBG), but not the oculomotor nucleus. In contrast, very few of these cholinergic neurons displayed evidence of vesicular GABA transporter expression. Using multiplex fluorescent in situ hybridization, we determined that only PBG cholinergic neurons are also predominantly positive for VGLUT2 mRNA in the adult, with only small numbers of PPTg cholinergic neurons overlapping with VGLUT2 mRNA. Using Cre-dependent viral vectors we confirm these in situ hybridization data, and demonstrate projection patterns of cholinergic and glutamatergic populations. These results demonstrate that most mesopontine cholinergic neurons may transiently express VGLUT2, but that a large majority of PBG neurons retain VGLUT2 expression throughout adulthood, and support a growing body of literature indicating that distinct cholinergic populations have differing potential for GABA or glutamate co-release.

Translating striatal activity from brain slice to whole animal neurophysiology: A guide for neuroscience research integrating diverse levels of analysis

An important goal of this review is highlighting research in neuroscience as examples of multilevel functional and anatomical analyses addressing basic science issues and applying results to the understanding of diverse disorders. The research of Dr. Michael Levine, a leader in neuroscience, exemplifies this approach by uncovering fundamental properties of basal ganglia function and translating these findings to clinical applications. The review focuses on neurophysiological research connecting results from in vitro and in vivo recordings. A second goal is to utilize these research connections to produce novel, accurate descriptions for corticostriatal processing involved in varied, complex functions. Medium spiny neurons in striatum act as integrators combining input with baseline activity creating motivational "events." Basic research on corticostriatal synapses is described and links developed to issues with clinical relevance such as inhibitory gating, self-injurious behavior, and relative reward valuation. Work is highlighted on dopamine-glutamate interactions. Individual medium spiny neurons express both D₁ and D₂ receptors and encode information in a bivalent manner depending upon the mix of receptors involved. Current work on neurophysiology of reward processing has taken advantage of these basic approaches at the cellular and molecular levels. Future directions in studying physiology of reward processing and action sequencing could profit by incorporating the divergent ways dopamine modulates incoming neurochemical signals. Primary investigators leading research teams should mirror Mike Levine's efforts in "climbing the mountain" of scientific inquiry by performing analyses at different levels of inquiry, integrating the findings, and building comprehensive answers to problems unsolvable without this bold approach.

Components of the GABAergic signaling in the peripheral cholinergic synapses of vertebrates: a review

Gamma-aminobutyric acid (GABA) is the main inhibitory neurotransmitter in the mammalian central nervous system. Since the 1970s, many studies have focused on the role of GABA in the mammalian peripheral nervous system, and particularly in the cholinergic synapses. In this review, we present current findings for the cholinergic neurons of vegetative ganglia as well as for the neurons innervating smooth and striated muscles. Synaptic contacts formed by these neurons contain GABA and the enzyme, glutamic acid decarboxylase, which catalyzes the synthesis of GABA from glutamate. Newly formed GABA is released in the cholinergic synapses and mostly all the peripheral cholinergic synaptic contacts contain iono- and metabotropic GABA receptors. Although the underlying molecular mechanism of the release is not well understood, still, it is speculated that GABA is released by a vesicular and/or non-vesicular way via reversal of the GABA transporter. We also review the signaling role of GABA in the peripheral cholinergic synapses by modulating acetylcholine release, but its exact physiological function remains to be elucidated.

The NeuroD6 Subtype of VTA Neurons Contributes to Psychostimulant Sensitization and Behavioral Reinforcement

Reward-related behavior is complex and its dysfunction correlated with neuropsychiatric illness. Dopamine (DA) neurons of the ventral tegmental area (VTA) have long been associated with different aspects of reward function, but it remains to be disentangled how distinct VTA DA neurons contribute to the full range of behaviors ascribed to the VTA. Here, a recently identified subtype of VTA neurons molecularly defined by NeuroD6 (NEX1M) was addressed. Among all VTA DA neurons, less than 15% were identified as positive for NeuroD6. In addition to dopaminergic markers, sparse NeuroD6 neurons expressed the vesicular glutamate transporter 2 (Vglut2) gene. To achieve manipulation of NeuroD6 VTA neurons, NeuroD6(NEX)-Cre-driven mouse genetics and optogenetics were implemented. First, expression of vesicular monoamine transporter 2 (VMAT2) was ablated to disrupt dopaminergic function in NeuroD6 VTA neurons. Comparing Vmat2^{lox/lox;NEX-Cre} conditional knock-out (cKO) mice with littermate controls, it was evident that baseline locomotion, preference for sugar and ethanol, and place preference upon amphetamine-induced and cocaine-induced conditioning were similar between genotypes. However, locomotion upon repeated psychostimulant administration was significantly elevated above control levels in cKO mice. Second, optogenetic activation of NEX-Cre VTA neurons was shown to induce DA release and glutamatergic postsynaptic currents within the nucleus accumbens. Third, optogenetic stimulation of NEX-Cre VTA neurons in vivo induced significant place preference behavior, while stimulation of VTA neurons defined by Calretinin failed to cause a similar response. The results show that NeuroD6 VTA neurons exert distinct regulation over specific aspects of reward-related behavior, findings that contribute to the current understanding of VTA neurocircuitry.

Contribution of cholinergic interneurons to striatal pathophysiology in Parkinson's disease

Parkinson's disease (PD) is a neurodegenerative disorder caused by the loss of nigral dopaminergic neurons innervating the striatum, the main input structure of the basal ganglia. This creates an imbalance between dopaminergic inputs and cholinergic interneurons (ChIs) within the striatum. The efficacy of anticholinergic drugs, one of the earliest therapy for PD before the discovery of L-3,4-dihydroxyphenylalanine (L-DOPA) suggests an increased cholinergic tone in this disease. The dopamine (DA)-acetylcholine (ACh) balance hypothesis is now revisited with the use of novel cutting-edge techniques (optogenetics, pharmacogenetics, new electrophysiological recordings). This review will provide the background of the specific contribution of ChIs to striatal microcircuit organization in physiological and pathological conditions. The second goal of this review is to delve into the respective contributions of nicotinic and muscarinic receptor cholinergic subunits to the control of striatal afferent and efferent neuronal systems. Special attention will be given to the role played by muscarinic acetylcholine receptors (mAChRs) in the regulation of striatal network which may have important implications in the development of novel therapeutic strategies for motor and cognitive impairment in PD.