Co-transmission of dopamine and glutamate

Synaptic ensembles between raphe and D1R-containing accumbens shell neurons underlie postisolation sociability in males

Social animals expend considerable energy to maintain social bonds throughout their life. Male and female mice show sexually dimorphic behaviors, yet the underlying neural mechanisms of sociability and their dysregulation during social disconnection remain unknown. Dopaminergic neurons in dorsal raphe nucleus (DRN^TH) is known to contribute to a loneliness-like state and modulate sociability. We identified that activated subpopulations in DRN^TH and nucleus accumbens shell (NAc^sh) during 24 hours of social isolation underlie the increase in isolation-induced sociability in male but not in female mice. This effect was reversed by chemogenetically and optogenetically inhibiting the DRN^TH-NAc^sh circuit. Moreover, synaptic connectivity among the activated neuronal ensembles in this circuit was increased, primarily in D₁ receptor-expressing neurons in NAc^sh. The increase in synaptic density functionally correlated with elevated dopamine release into NAc^sh. Overall, specific synaptic ensembles in DRN^TH-NAc^sh mediate sex differences in isolation-induced sociability, indicating that sex-dependent circuit dynamics underlie the expression of sexually dimorphic behaviors.

Dopamine: The Neuromodulator of Long-Term Synaptic Plasticity, Reward and Movement Control

Dopamine (DA) is a key neurotransmitter involved in multiple physiological functions including motor control, modulation of affective and emotional states, reward mechanisms, reinforcement of behavior, and selected higher cognitive functions. Dysfunction in dopaminergic transmission is recognized as a core alteration in several devastating neurological and psychiatric disorders, including Parkinson's disease (PD), schizophrenia, bipolar disorder, attention deficit hyperactivity disorder (ADHD) and addiction. Here we will discuss the current insights on the role of DA in motor control and reward learning mechanisms and its involvement in the modulation of synaptic dynamics through different pathways. In particular, we will consider the role of DA as neuromodulator of two forms of synaptic plasticity, known as long-term potentiation (LTP) and long-term depression (LTD) in several cortical and subcortical areas. Finally, we will delineate how the effect of DA on dendritic spines places this molecule at the interface between the motor and the cognitive systems. Specifically, we will be focusing on PD, vascular dementia, and schizophrenia.

Changes in Striatal Medium Spiny Neuron Morphology Resulting from Dopamine Depletion Are Reversible

The classical motor symptoms of Parkinson’s disease (PD) are caused by degeneration of dopaminergic neurons in the substantia nigra, which is followed by secondary dendritic pruning and spine loss at striatal medium spiny neurons (MSN). We hypothesize that these morphological changes at MSN underlie at least in part long-term motor complications in PD patients. In order to define the potential benefits and limitations of dopamine substitution, we tested in a mouse model whether dendritic pruning and spine loss can be reversible when dopaminergic axon terminals regenerate. In order to induce degeneration of nigrostriatal dopaminergic neurons we used the toxicity of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) in C57BL/6J mice; 30 mg/kg MPTP was applied i.p. on five consecutive days. In order to assess the consequences of dopamine depletion, mice were analyzed 21 days after the last injection. In order to test reversibility of MSN changes we exploited the property of this model that striatal axon terminals regenerate by sprouting within 90 days and analyzed a second cohort 90 days after MPTP. Degeneration of dopaminergic neurons was confirmed by counting TH-positive neurons in the substantia nigra and by analyzing striatal catecholamines. Striatal catecholamine recovered 90 days after MPTP. MSN morphology was visualized by Golgi staining and quantified as total dendritic length, number of dendritic branch points, and density of dendritic spines. All morphological parameters of striatal MSN were reduced 21 days after MPTP. Statistical analysis indicated that dendritic pruning and the reduction of spine density represent two distinct responses to dopamine depletion. Ninety days after MPTP, all morphological changes recovered. Our findings demonstrate that morphological changes in striatal MSN resulting from dopamine depletion are reversible. They suggest that under optimal conditions, symptomatic dopaminergic therapy might be able to prevent maladaptive plasticity and long-term motor complications in PD patients.

Synaptic signalling in a network of dopamine neurons: what prevents proper intercellular crosstalk?

Human embryonic stem cell (hESC)-derived midbrain dopamine (DA) neurons stand out as a cell source for transplantation with their sustainability and consistency superior to the formerly used fetal tissues. However, multiple studies of DA neurons in culture failed to register action potential (AP) generation upon synaptic input. To test whether this is due to deficiency of NMDA receptor (NMDAR) coagonists released from astroglia, we studied the functional properties of neural receptors in hESC-derived DA neuronal cultures. We find that, apart from an insufficient amount of coagonists, lack of interneuronal crosstalk is caused by hypofunction of synaptic NMDARs due to their direct inhibition by synaptically released DA. This inhibitory tone is independent of DA receptors and affects the NMDAR coagonist binding site.

Neuromodulators and Long-Term Synaptic Plasticity in Learning and Memory: A Steered-Glutamatergic Perspective

The molecular pathways underlying the induction and maintenance of long-term synaptic plasticity have been extensively investigated revealing various mechanisms by which neurons control their synaptic strength. The dynamic nature of neuronal connections combined with plasticity-mediated long-lasting structural and functional alterations provide valuable insights into neuronal encoding processes as molecular substrates of not only learning and memory but potentially other sensory, motor and behavioural functions that reflect previous experience. However, one key element receiving little attention in the study of synaptic plasticity is the role of neuromodulators, which are known to orchestrate neuronal activity on brain-wide, network and synaptic scales. We aim to review current evidence on the mechanisms by which certain modulators, namely dopamine, acetylcholine, noradrenaline and serotonin, control synaptic plasticity induction through corresponding metabotropic receptors in a pathway-specific manner. Lastly, we propose that neuromodulators control plasticity outcomes through steering glutamatergic transmission, thereby gating its induction and maintenance.

Shared Behavioral and Neurocircuitry Disruptions in Drug Addiction, Obesity, and Binge Eating Disorder: Focus on Group I mGluRs in the Mesolimbic Dopamine Pathway

Accumulated data from clinical and preclinical studies suggest that, in drug addiction and states of overeating, such as obesity and binge eating disorder (BED), there is an imbalance in circuits that are critical for motivation, reward saliency, executive function, and self-control. Central to these pathologies and the extensive topic of this Review are the aberrations in dopamine (DA) and glutamate (Glu) within the mesolimbic pathway. Group I metabotropic glutamate receptors (mGlus) are highly expressed in the mesolimbic pathway and are poised in key positions to modulate disruptions in synaptic plasticity and neurotransmitter release observed in drug addiction, obesity, and BED. The use of allosteric modulators of group I mGlus has been studied in drug addiction, as they offer several advantages over traditional orthosteric agents. However, they have yet to be studied in obesity or BED. With the substantial overlap between the neurocircuitry involved in drug addiction and eating disorders, group I mGlus may also provide novel targets for obesity and BED.

Peptide-Mediated Neurotransmission Takes Center Stage

Today, we understand peptide transmitters to be signaling molecules that modulate neural activity. However, in 1982 little was known about neuropeptides and their role in neural communication. The influential 1982 paper by Jan and Jan reported definitive evidence that a presynaptically released neuropeptide evokes postsynaptic responses in an identified cholinergic synapse, thereby fueling a new era in neuroscience.

Tissue engineered nigrostriatal pathway for treatment of Parkinson's disease

The classic motor deficits of Parkinson's disease are caused by degeneration of dopaminergic neurons in the substantia nigra pars compacta, resulting in the loss of their long-distance axonal projections that modulate the striatum. Current treatments only minimize the symptoms of this disconnection as there is no approach capable of replacing the nigrostriatal pathway. We are applying microtissue engineering techniques to create living, implantable constructs that mimic the architecture and function of the nigrostriatal pathway. These constructs consist of dopaminergic neurons with long axonal tracts encased within hydrogel microcolumns. Microcolumns were seeded with dopaminergic neuronal aggregates, while lumen extracellular matrix, growth factors, and end targets were varied to optimize cytoarchitecture. We found a 10-fold increase in axonal outgrowth from aggregates versus dissociated neurons, resulting in remarkable axonal lengths of over 6 mm by 14 days and 9 mm by 28 days in vitro. Axonal extension was also dependent upon lumen extracellular matrix, but did not depend on growth factor enrichment or neuronal end target presence. Evoked dopamine release was measured via fast scan cyclic voltammetry and synapse formation with striatal neurons was observed in vitro. Constructs were microinjected to span the nigrostriatal pathway in rats, revealing survival of implanted neurons while maintaining their axonal projections within the microcolumn. Lastly, these constructs were generated with dopaminergic neurons differentiated from human embryonic stem cells. This strategy may improve Parkinson's disease treatment by simultaneously replacing lost dopaminergic neurons in the substantia nigra and reconstructing their long-projecting axonal tracts to the striatum.

Mimicking Neurotransmitter Release in Chemical Synapses via Hysteresis Engineering in MoS2 Transistors

Neurotransmitter release in chemical synapses is fundamental to diverse brain functions such as motor action, learning, cognition, emotion, perception, and consciousness. Moreover, improper functioning or abnormal release of neurotransmitter is associated with numerous neurological disorders such as epilepsy, sclerosis, schizophrenia, Alzheimer's disease, and Parkinson's disease. We have utilized hysteresis engineering in a back-gated MoS₂ field effect transistor (FET) in order to mimic such neurotransmitter release dynamics in chemical synapses. All three essential features, i.e., quantal, stochastic, and excitatory or inhibitory nature of neurotransmitter release, were accurately captured in our experimental demonstration. We also mimicked an important phenomenon called long-term potentiation (LTP), which forms the basis of human memory. Finally, we demonstrated how to engineer the LTP time by operating the MoS₂ FET in different regimes. Our findings could provide a critical component toward the design of next-generation smart and intelligent human-like machines and human-machine interfaces.

Increased nicotine response in iPSC-derived human neurons carrying the CHRNA5 N398 allele

Genetic variation in nicotinic receptor alpha 5 (CHRNA5) has been associated with increased risk of addiction-associated phenotypes in humans yet little is known the underlying neural basis. Induced pluripotent stem cells (iPSCs) were derived from donors homozygous for either the major (D398) or the minor (N398) allele of the nonsynonymous single nucleotide polymorphism (SNP), rs16969968, in CHRNA5. To understand the impact of these nicotinic receptor variants in humans, we differentiated these iPSCs to dopamine (DA) or glutamatergic neurons and then tested their functional properties and response to nicotine. Results show that N398 variant human DA neurons differentially express genes associated with ligand receptor interaction and synaptic function. While both variants exhibited physiological properties consistent with mature neuronal function, the N398 neuronal population responded more actively with an increased excitatory postsynaptic current response upon the application of nicotine in both DA and glutamatergic neurons. Glutamatergic N398 neurons responded to lower nicotine doses (0.1 μM) with greater frequency and amplitude but they also exhibited rapid desensitization, consistent with previous analyses of N398-associated nicotinic receptor function. This study offers a proof-of-principle for utilizing human neurons to study gene variants contribution to addiction.

Simultaneous analysis of vascular norepinephrine and ATP release using an integrated microfluidic system

BACKGROUND

Sympathetic nerves are known to release three neurotransmitters: norepinephrine, ATP, and neuropeptide Y that play a role in controlling vascular tone. This paper focuses on the co-release of norepinephrine and ATP from the mesenteric arterial sympathetic nerves of the rat.

NEW METHOD

In this paper, a quantification technique is described that allows simultaneous detection of norepinephrine and ATP in a near-real-time fashion from the isolated perfused mesenteric arterial bed of the rat. Simultaneous detection is enabled with 3-D printing technology, which is shown to help integrate the perfusate with different detection methods (norepinephrine by microchip-based amperometery and ATP by on-line chemiluminescence).

RESULTS

Stimulated levels relative to basal levels of norepinephrine and ATP were found to be 363nM and 125nM, respectively (n=6). The limit of detection for norepinephrine is 80nM using microchip-based amperometric detection. The LOD for on-line ATP detection using chemiluminescence is 35nM.

COMPARISON WITH EXISTING METHOD

In previous studies, the co-transmitters have been separated and detected with HPLC techniques. With HPLC, the samples from biological preparations have to be derivatized for ATP detection and require collection time before analysis. Thus real-time measurements are not made and the delay in analysis by HPLC can cause degradation.

CONCLUSIONS

In conclusion, the method described in the paper can be used to successfully detect norepinephrine and ATP simultaneously and in a near-real-time fashion.

Stimulation of glutamate receptors in the ventral tegmental area is necessary for serotonin-2 receptor-induced increases in mesocortical dopamine release

Modulation of dopamine (DA) released by serotonin-2 (5-HT2) receptors has been implicated in the mechanism of action of antipsychotic drugs. The mesocortical DA system has been implicated particularly in the cognitive deficits observed in schizophrenia. Agonism at 5-HT2A receptors in the prefrontal cortex (PFC) is associated with increases in cortical DA release. Evidence indicates that 5-HT2A receptors in the cortex regulate mesocortical DA release through stimulation of a "long-loop" feedback system from the PFC to the ventral tegmental area (VTA) and back. However, a causal role for VTA glutamate in the 5-HT2-induced increases in PFC DA has not been established. The present study does so by measuring 5-HT2 agonist-induced DA release in the cortex after infusions of glutamate antagonists into the VTA of the rat. Infusions of a combination of a N-methyl-d-aspartic acid (NMDA) (AP-5: 2-amino-5-phosphopentanoic acid) and an AMPA/kainate (CNQX: 6-cyano-7-nitroquinoxaline-2,3-dione) receptor antagonist into the VTA blocked the increases in cortical DA produced by administration of the 5-HT2 agonist DOI [(±)-2,5-dimethoxy-4-iodoamphetamine] (2.5mg/kg s.c.). These results demonstrate that stimulation of glutamate receptors in the VTA is necessary for 5-HT2 agonist-induced increases in cortical DA.

Local control of striatal dopamine release

The mesolimbic and nigrostriatal dopamine (DA) systems play a key role in the physiology of reward seeking, motivation and motor control. Importantly, they are also involved in the pathophysiology of Parkinson’s and Huntington’s disease, schizophrenia and addiction. Control of DA release in the striatum is tightly linked to firing of DA neurons in the ventral tegmental area (VTA) and the substantia nigra (SN). However, local influences in the striatum affect release by exerting their action directly on axon terminals. For example, endogenous glutamatergic and cholinergic activity is sufficient to trigger striatal DA release independently of cell body firing. Recent developments involving genetic manipulation, pharmacological selectivity or selective stimulation have allowed for better characterization of these phenomena. Such termino-terminal forms of control of DA release transform considerably our understanding of the mesolimbic and nigrostriatal systems, and have strong implications as potential mechanisms to modify impaired control of DA release in the diseased brain. Here, we review these and related mechanisms and their implications in the physiology of ascending DA systems.

Dual-transmitter neurons: functional implications of co-release and co-transmission

Co-transmission, the ability of a neuron to release multiple transmitters, has long been recognized in selected circuits. However, the release of multiple primary neurotransmitters from a single neuron is only beginning to be appreciated. Here we consider recent examples of co-transmission as well as co-release-the packaging of multiple neurotransmitters into a single vesicle. The properties associated with each mode of release greatly enhance the possible action of such neurons within circuits. The functional importance of dual- (or multi-) transmitter neurons extends beyond actions on postsynaptic receptors, due in part to differential spatial and temporal profiles of each neurotransmitter. Recent evidence also suggests that the dual-transmitter phenotype can be dynamically regulated during development and following injury or disease.

An update on the connections of the ventral mesencephalic dopaminergic complex

This review covers the intrinsic organization and afferent and efferent connections of the midbrain dopaminergic complex, comprising the substantia nigra, ventral tegmental area and retrorubral field, which house, respectively, the A9, A10 and A8 groups of nigrostriatal, mesolimbic and mesocortical dopaminergic neurons. In addition, A10dc (dorsal, caudal) and A10rv (rostroventral) extensions into, respectively, the ventrolateral periaqueductal gray and supramammillary nucleus are discussed. Associated intrinsic and extrinsic connections of the midbrain dopaminergic complex that utilize gamma-aminobutyric acid (GABA), glutamate and neuropeptides and various co-expressed combinations of these compounds are considered in conjunction with the dopamine-containing systems. A framework is provided for understanding the organization of massive afferent systems descending and ascending to the midbrain dopaminergic complex from the telencephalon and brainstem, respectively. Within the context of this framework, the basal ganglia direct and indirect output pathways are treated in some detail. Findings from rodent brain are briefly compared with those from primates, including humans. Recent literature is emphasized, including traditional experimental neuroanatomical and modern gene transfer and optogenetic studies. An attempt was made to provide sufficient background and cite a representative sampling of earlier primary papers and reviews so that people new to the field may find this to be a relatively comprehensive treatment of the subject.

Glutamatergic neurotransmission between the C1 neurons and the parasympathetic preganglionic neurons of the dorsal motor nucleus of the vagus

The C1 neurons are a nodal point for blood pressure control and other autonomic responses. Here we test whether these rostral ventrolateral medullary catecholaminergic (RVLM-CA) neurons use glutamate as a transmitter in the dorsal motor nucleus of the vagus (DMV). After injecting Cre-dependent adeno-associated virus (AAV2) DIO-Ef1α-channelrhodopsin2(ChR2)-mCherry (AAV2) into the RVLM of dopamine-β-hydroxylase Cre transgenic mice (DβH(Cre/0)), mCherry was detected exclusively in RVLM-CA neurons. Within the DMV >95% mCherry-immunoreactive(ir) axonal varicosities were tyrosine hydroxylase (TH)-ir and the same proportion were vesicular glutamate transporter 2 (VGLUT2)-ir. VGLUT2-mCherry colocalization was virtually absent when AAV2 was injected into the RVLM of DβH(Cre/0);VGLUT2(flox/flox) mice, into the caudal VLM (A1 noradrenergic neuron-rich region) of DβH(Cre/0) mice or into the raphe of ePet(Cre/0) mice. Following injection of AAV2 into RVLM of TH-Cre rats, phenylethanolamine N-methyl transferase and VGLUT2 immunoreactivities were highly colocalized in DMV within EYFP-positive or EYFP-negative axonal varicosities. Ultrastructurally, mCherry terminals from RVLM-CA neurons in DβH(Cre/0) mice made predominantly asymmetric synapses with choline acetyl-transferase-ir DMV neurons. Photostimulation of ChR2-positive axons in DβH(Cre/0) mouse brain slices produced EPSCs in 71% of tested DMV preganglionic neurons (PGNs) but no IPSCs. Photostimulation (20 Hz) activated PGNs up to 8 spikes/s (current-clamp). EPSCs were eliminated by tetrodotoxin, reinstated by 4-aminopyridine, and blocked by ionotropic glutamate receptor blockers. In conclusion, VGLUT2 is expressed by RVLM-CA (C1) neurons in rats and mice regardless of the presence of AAV2, the C1 neurons activate DMV parasympathetic PGNs monosynaptically and this connection uses glutamate as an ionotropic transmitter.

Region-specific expression of vesicular glutamate and GABA transporters under various ischaemic conditions in mouse forebrain and retina

There is accumulating evidence that glutamate and GABA release are key mechanisms of ischaemic events in the CNS. However, data on the expression of involved transporters for these mediators are inconsistent, potentially impeding further neuroprotective approaches. Here, we applied immunofluorescence labelling to characterise the expression pattern of vesicular glutamate (VGLUT) and GABA transporters (VGAT) after acute focal cerebral ischaemia and in two models of retinal ischaemia. Mice were subjected to filament-based focal cerebral ischaemia predominantly involving the middle cerebral artery territory, also leading to retinal ischaemia due to central retinal artery occlusion (CRAO). Alternatively, retinal ischaemia was induced by a transient increase of the intraocular pressure (HIOP). One day after ischaemia onset, diminished immunolabelling of neuronal nuclei and microtubule-associated protein 2-positive structures were found in the ipsilateral neocortex, subcortex and the retina, indicating neuronal degeneration. VGLUT1 expression did not change significantly in ischaemic tissues whereas VGLUT2 was down-regulated in specific areas of the brain. VGLUT3 expression was only slightly down-regulated in the ischaemia-affected neocortex, and was found to form clusters on fibrils of unknown origin in the ischaemic lateral hypothalamus. In contrast, retinae subjected to CRAO or HIOP displayed a rapid loss of VGLUT3-immunoreactivity. The expression of VGAT appears resistant to ischaemia as there was no significant alteration in all the regions analysed. In summary, these data indicate a region- and subtype-specific change of VGLUT expression in the ischaemia-affected CNS, whose consideration might help to generate specific neuroprotective strategies.