Modulatory functions of neurotransmitters in the striatum: ACh/dopamine/NMDA interactions

Studies of Monoamine Neurotransmitters at Nanomolar Levels Using Carbon Material Electrodes: A Review

Neurotransmitters (NTs) with hydroxyl groups can now be identified electrochemically, utilizing a variety of electrodes and voltammetric techniques. In particular, in monoamine, the position of the hydroxyl groups might alter the sensing properties of a certain neurotransmitter. Numerous research studies using electrodes modified on their surfaces to better detect specific neurotransmitters when other interfering factors are present are reviewed to improve the precision of these measures. An investigation of the monoamine neurotransmitters at nanoscale using electrochemical methods is the primary goal of this review article. It will be used to determine which sort of electrode is ideal for this purpose. The use of carbon materials, such as graphite carbon fiber, carbon fiber micro-electrodes, glassy carbon, and 3D printed electrodes are only some of the electrodes with surface modifications that can be utilized for this purpose. Electrochemical methods for real-time detection and quantification of monoamine neurotransmitters in real samples at the nanomolar level are summarized in this paper.

Cholinergic Receptor Modulation as a Target for Preventing Dementia in Parkinson's Disease

Parkinson’s disease (PD) is a neurodegenerative condition characterized by the loss of dopaminergic neurons in the substantia nigra pars compacta (SNpc) in the midbrain resulting in progressive impairment in cognitive and motor abilities. The physiological and molecular mechanisms triggering dopaminergic neuronal loss are not entirely defined. PD occurrence is associated with various genetic and environmental factors causing inflammation and mitochondrial dysfunction in the brain, leading to oxidative stress, proteinopathy, and reduced viability of dopaminergic neurons. Oxidative stress affects the conformation and function of ions, proteins, and lipids, provoking mitochondrial DNA (mtDNA) mutation and dysfunction. The disruption of protein homeostasis induces the aggregation of alpha-synuclein (α-SYN) and parkin and a deficit in proteasome degradation. Also, oxidative stress affects dopamine release by activating ATP-sensitive potassium channels. The cholinergic system is essential in modulating the striatal cells regulating cognitive and motor functions. Several muscarinic acetylcholine receptors (mAChR) and nicotinic acetylcholine receptors (nAChRs) are expressed in the striatum. The nAChRs signaling reduces neuroinflammation and facilitates neuronal survival, neurotransmitter release, and synaptic plasticity. Since there is a deficit in the nAChRs in PD, inhibiting nAChRs loss in the striatum may help prevent dopaminergic neurons loss in the striatum and its pathological consequences. The nAChRs can also stimulate other brain cells supporting cognitive and motor functions. This review discusses the cholinergic system as a therapeutic target of cotinine to prevent cognitive symptoms and transition to dementia in PD.

What Is the Role of Thalamostriatal Circuits in Learning Vocal Sequences?

Basal ganglia (BG) circuits integrate sensory and motor-related information from the cortex, thalamus, and midbrain to guide learning and production of motor sequences. Birdsong, like speech, is comprised of precisely sequenced vocal elements. Learning song sequences during development relies on Area X, a vocalization related region in the medial striatum of the songbird BG. Area X receives inputs from cortical-like pallial song circuits and midbrain dopaminergic circuits and sends projections to the thalamus. It has recently been shown that thalamic circuits also send substantial projections back to Area X. Here, we outline a gated-reinforcement learning model for how Area X may use signals conveyed by thalamostriatal inputs to direct song learning. Integrating conceptual advances from recent mammalian and songbird literature, we hypothesize that thalamostriatal pathways convey signals linked to song syllable onsets and offsets and influence striatal circuit plasticity via regulation of cholinergic interneurons (ChIs). We suggest that syllable sequence associated vocal-motor information from the thalamus drive precisely timed pauses in ChIs activity in Area X. When integrated with concurrent corticostriatal and dopaminergic input, this circuit helps regulate plasticity on medium spiny neurons (MSNs) and the learning of syllable sequences. We discuss new approaches that can be applied to test core ideas of this model and how associated insights may provide a framework for understanding the function of BG circuits in learning motor sequences.

An acetylcholine-dopamine interaction in the nucleus accumbens and its involvement in ethanol's dopamine-releasing effect

Alcohol use disorder is a chronic, relapsing brain disorder causing substantial morbidity and mortality. Cholinergic interneurons (CIN) within the nucleus accumbens (nAc) have been suggested to exert a regulatory impact on dopamine (DA) neurotransmission locally, and defects in CIN have been implied in several psychiatric disorders. The aim of this study was to investigate the role of CIN in regulation of basal extracellular levels of DA and in modulation of nAc DA release following ethanol administration locally within the nAc of male Wistar rats. Using reversed in vivo microdialysis, the acetylcholinesterase inhibitor physostigmine was administered locally in the nAc followed by addition of either the muscarinic acetylcholine (ACh) receptor antagonist scopolamine or the nicotinic ACh receptor antagonist mecamylamine. Further, ethanol was locally perfused in the nAc following pretreatment with scopolamine and/or mecamylamine. Lastly, ethanol was administered locally into the nAc of animals with accumbal CIN‐ablation induced by anticholine acetyl transferase‐saporin. Physostigmine increased accumbal DA levels via activation of muscarinic ACh receptors. Neither scopolamine and/or mecamylamine nor CIN‐ablation altered basal DA levels, suggesting that extracellular DA levels are not tonically controlled by ACh in the nAc. In contrast, ethanol‐induced DA elevation was prevented following coadministration of scopolamine and mecamylamine and blunted in CIN‐ablated animals, suggesting involvement of CIN‐ACh in ethanol‐mediated DA signaling. The data presented in this study suggest that basal extracellular levels of DA within the nAc are not sustained by ACh, whereas accumbal CIN‐ACh is involved in mediating ethanol‐induced DA release.

Melatonin and Depression: A Translational Perspective From Animal Models to Clinical Studies

Daily rhythm of melatonin synchronizes the body to the light/dark environmental cycle. Several hypotheses have been raised to understand the intersections between melatonin and depression, in which changes in rest-activity and sleep patterns are prominent. This review describes key experimental and clinical evidence that link melatonin with the etiopathology and symptomatology of depressive states, its role in the follow up of therapeutic response to antidepressants, as well as the clinical evidence of melatonin as MDD treatment. Melatonin, as an internal temporal cue contributing to circadian organization and best studied in the context of circadian misalignment, is also implicated in neuroplasticity. The monoaminergic systems that underly MDD and melatonin production overlap. In addition, the urinary metabolite 6-sulfatoxymelatonin (aMT6) has been proposed as biomarker for antidepressant responders, by revealing whether the blockage of noradrenaline uptake has taken place within 24 h from the first antidepressant dose. Even though animal models show benefits from melatonin supplementation on depressive-like behavior, clinical evidence is inconsistent vis-à-vis prophylactic or therapeutic benefits of melatonin or melatonin agonists in depression. We argue that the study of melatonin in MDD or other psychiatric disorders must take into account the specificities of melatonin as an integrating molecule, inextricably linked to entrainment, metabolism, immunity, neurotransmission, and cell homeostasis.

Loss of nigral excitation of cholinergic interneurons contributes to parkinsonian motor impairments

Progressive loss of dopamine inputs in Parkinson's disease leads to imbalances in coordinated signaling of dopamine and acetylcholine (ACh) in the striatum, which is thought to contribute to parkinsonian motor symptoms. As reciprocal interactions between dopamine inputs and cholinergic interneurons (ChIs) control striatal dopamine and ACh transmission, we examined how partial dopamine depletion in an early-stage mouse model for Parkinson's disease alters nigral regulation of cholinergic activity. We found region-specific alterations in how remaining dopamine inputs regulate cholinergic excitability that differ between the dorsomedial (DMS) and dorsolateral (DLS) striatum. Specifically, we found that dopamine depletion downregulates metabotropic glutamate receptors (mGluR1) on DLS ChIs at synapses where dopamine inputs co-release glutamate, abolishing the ability of dopamine inputs to drive burst firing. This loss underlies parkinsonian motor impairments, as viral rescue of mGluR1 signaling in DLS ChIs was sufficient to restore circuit function and attenuate motor deficits in early-stage parkinsonian mice.

Neurobehavioral and neurochemical effects in rats offspring co-exposed to arsenic and fluoride during development

Arsenic (iAs) and fluoride (F) are ubiquitous in the environment. All over the world, in many countries, thousands of people are suffering from the toxic effects of arsenicals ad fluorides. These two elements are recognized worldwide as the most serious inorganic contaminants in drinking water. When two different types of toxicants are simultaneously going inside the human body they may function independently or can act as synergistic or antagonistic to one another. Although there have been reports in literature of individual toxicity of iAs and F, however, not much is known about the effects following the combined exposure to the toxicants above mentioned. In this work, we investigated the effect of the co-exposure to low levels of iAs/F through drinking water during pregnancy and lactation on central nervous system functionality in the exposed rats offspring. Wistar rats were exposed to one of these solutions: 0.05 mg/L iAs and 5 mg/L F (Concentration A) or 0.10 mg/L iAs and 10 mg/L F (Concentration B) from gestational day 0 up to post-gestational day 21. Sensory-motor reflexes a Functional Observational Battery and the locomotor activity in an open field were assessed in offspring. Additionally, the transaminases, acethylcholinesterase and catalase levels in the striatum were determined to elucidate the possible molecular mechanisms involved in locomotor and neurobehavioral disorders. The results showed that iAs/F exposition during development produces a delay reach the maturity of sensorimotor reflexes. A decrease in the nociceptive reflex response, and increase in the locomotor activity in adult rats offspring were observed. The increase in oxidative stress, the inhibition of transaminases enzymes and the inhibition of AChE in the striatum may partially regulate all the neurobehavioral disorders observed.

Tonically Active Neurons in the Monkey Dorsal Striatum Signal Outcome Feedback during Trial-and-error Search Behavior

An animal's choice behavior is shaped by the outcome feedback from selected actions in a trial-and-error approach. Tonically active neurons (TANs), presumed cholinergic interneurons in the striatum, are thought to be involved in the learning and performance of reward-directed behaviors, but it remains unclear how TANs are involved in shaping reward-directed choice behaviors based on the outcome feedback. To this end, we recorded activity of TANs from the dorsal striatum of two macaque monkeys (Macaca fuscata; 1 male, 1 female) while they performed a multi-step choice task to obtain multiple rewards. In this task, the monkeys first searched for a rewarding target from among three alternatives in a trial-and-error manner and then earned additional rewards by repeatedly choosing the rewarded target. We found that a considerable proportion of TANs selectively responded to either the reward or the no-reward outcome feedback during the trial-and-error search, but these feedback responses were not observed during repeat trials. Moreover, the feedback responses of TANs were similarly observed in any search trials, without distinctions regarding the predicted probability of rewards and the location of chosen targets. Unambiguously, TANs detected reward and no-reward feedback specifically when the monkeys performed trial-and-error searches, in which the monkeys were learning the value of the targets and adjusting their subsequent choice behavior based on the reward and no-reward feedback. These results suggest that striatal cholinergic interneurons signal outcome feedback specifically during search behavior, in circumstances where the choice outcomes cannot be predicted with certainty by the animals.

A free-choice high-fat diet modulates the effects of a sucrose bolus on the expression of genes involved in glucose handling in the hypothalamus and nucleus accumbens

The consumption of saturated fat and sucrose can have synergistic effects on the brain that do not occur when either nutrient is consumed by itself. In this study we hypothesize that saturated fat intake modulates glucose handling in the hypothalamus and nucleus accumbens, both brain areas highly involved in the control of food intake. To study this, male Wistar rats were given a free-choice high fat diet (fcHFD) or a control diet for two weeks. During the last seven days rats were given a daily bolus of either a 30% sucrose solution or water. Rats were sacrificed on day eight, 30 minutes after the onset of drinking. mRNA and protein levels of genes involved in glucose handling were assessed in the hypothalamus and nucleus accumbens. We found increased Glut3 and Glut4 mRNA in the hypothalamus of fcHFD-fed rats without an additional effect of the sucrose bolus. In the nucleus accumbens, the sucrose bolus increased Glut3 mRNA and decreased Glut4 mRNA independent of prior diet exposure. The ATP-sensitive potassium channel subunit Kir6.1 in the nucleus accumbens tended to be affected by the synergistic effects of a fcHFD and a sucrose bolus. These data suggest that acute glucose handling in the hypothalamus and nucleus accumbens may be affected by prior high fat exposure.

Effect of pterois volitans (lionfish) venom on cholinergic and dopaminergic systems

Pterois volitans venom induces muscular fibrillation, which results from nerve transmission caused by the presence of acetylcholine (ACh). It also has cardiovascular effects that are due to its actions on muscarinic and nicotinic cholinergic receptors. In this study, we characterized the effects of P. volitans venom on nicotinic acetylcholine receptors (nAChRs) and dopaminergic neurons. After exposure to P. volitans venom, acetylcholinesterase (AChE) mRNA levels and the expression of the α2 subunit of nAChR increased in zebrafish embryos (15-20 somites). In addition, the lionfish venom blocked zebrafish α2 nAChR subunit functional expression and the ACh-induced response of human neuronal α3β2 receptors. The latter receptor was blocked by a protein fraction named F2, which was isolated from P. volitans venom using reversed phase high performance liquid chromatography (RP-HPLC). This venom causes death in dopaminergic neurons, and affects the cholinergic system. The effect of these two systems may result in retarded embryonic development of zebrafish, since the two systems function in a related manner to control growth hormone secretion.

Striatal Dopamine D2-Muscarinic Acetylcholine M1 Receptor-Receptor Interaction in a Model of Movement Disorders

Parkinson’s disease (PD) is a neurodegenerative disorder characterized by motor control deficits, which is associated with the loss of striatal dopaminergic neurons from the substantia nigra. In parallel to dopaminergic denervation, there is an increase of acetylcholine within the striatum, resulting in a striatal dopaminergic–cholinergic neurotransmission imbalance. Currently, available PD pharmacotherapy (e.g., prodopaminergic drugs) does not reinstate the altered dopaminergic–cholinergic balance. In addition, it can eventually elicit cholinergic-related adverse effects. Here, we investigated the interplay between dopaminergic and cholinergic systems by assessing the physical and functional interaction of dopamine D₂ and muscarinic acetylcholine M₁ receptors (D₂R and M₁R, respectively), both expressed at striatopallidal medium spiny neurons. First, we provided evidence for the existence of D₂R–M₁R complexes via biochemical (i.e., co-immunoprecipitation) and biophysical (i.e., BRET¹ and NanoBiT^®) assays, performed in transiently transfected HEK293T cells. Subsequently, a D₂R–M₁R co-distribution in the mouse striatum was observed through double-immunofluorescence staining and AlphaLISA^® immunoassay. Finally, we evaluated the functional interplay between both receptors via behavioral studies, by implementing the classical acute reserpine pharmacological animal model of experimental parkinsonism. Reserpinized mice were administered with a D₂R-selective agonist (sumanirole) and/or an M₁R-selective antagonist (VU0255035), and alterations in PD-related behavioral tasks (i.e., locomotor activity) were evaluated. Importantly, VU0255035 (10 mg/kg) potentiated the antiparkinsonian-like effects (i.e., increased locomotor activity and decreased catalepsy) of an ineffective sumanirole dose (3 mg/kg). Altogether, our data suggest the existence of putative striatal D₂R/M₁R heteromers, which might be a relevant target to manage PD motor impairments with fewer adverse effects.

Bioelectronic sensor mimicking the human neuroendocrine system for the detection of hypothalamic-pituitary-adrenal axis hormones in human blood

In the neuroendocrine system, corticotropin-releasing hormone (CRH) and adrenocorticotropic hormone (ACTH) play important roles in the regulation of the hypothalamic-pituitary-adrenal (HPA) system. Disorders of the HPA system lead to physiological problems, such as Addison's disease and Cushing's syndrome. Therefore, detection of CRH and ACTH is essential for diagnosing disorders related to the HPA system. Herein, receptors of the HPA axis were used to construct a bioelectronic sensor system for the detection of CRH and ACTH. The CRH receptor, corticotropin-releasing hormone receptor 1 (CRHR1), and the ACTH receptor, melanocortin 2 receptor (MC2R), were produced using an Escherichia coli expression system, and were reconstituted using nanodisc (ND) technology. The receptor-embedded NDs were immobilized on a floating electrode of a carbon nanotube field-effect transistor (CNT-FET). The constructed sensors sensitively detected CRH and ACTH to a concentration of 1 fM with high selectivity in real time. Furthermore, the reliable detection of CRH and ACTH in human plasma by the developed sensors demonstrated their potential in clinical and practical applications. These results indicate that CRHR1 and MC2R-based bioelectronic sensors can be applied for rapid and efficient detection of CRH and ACTH.

STUDY OF RAPITALAM INFLUENCE ON OXOTREMORINE-INDUCED TREMOR

Parkinson’s disease is the second most common (after Alzheimer’s) neurodegenerative disease. All over the world, there is a search for new drugs aimed at the treatment of Parkinson’s disease. Till up to the present, there is no “ideal” medicine that can completely cure this disease and has minimal adverse side effects. Belgorod research institute of pharmacology of living systems is studying Rapitalam, a new drug for the treatment of tremulous Parkinson’s disease. This is an agonist of the mGluR4 group of metabotropic receptors.

The aim of the article is to study Rapitalam influence on the oxotremorine-induced tremor in rats.

Methods. The study comprised 60 rats (6 groups of 10 males), which were administered intragastrically with the studied substances for 10 days. All the animal groups except Control group 1, were administered with Rapitalam and the reference drug Levodopa. 30 minutes after Rapitalam and Levodopa, they were administered abdominally with the solution of Oxotremorine at the dose of 1.5 mg/kg. The animals of Control group 1, instead of Oxotremorine, were similarly administered with a solvent of 0.9% sodium chloride in the equivalent volume.

Results. In comparison with the reference group, Rapitalam at the dose of 3 mg/kg significantly reduced the severity of tremor 50 min. after its administration. The same effect took place 30 min after the administration of Oxotremorine at the dose of 10 mg/kg. At the dose of 3 and 10 mg/kg, Rapitalam also decreased the number of rats in the group (in %) with the signs of tremor 60 min. and 50 min. after the administration of Oxotremorine, respectively.

Conclusion. The study revealed that Rapitalam has a pronounced anti-tremor effect. Its administration at the studied doses reduced the symptoms of Oxotremorine-induced tremor in rats.

Coordination of rapid cholinergic and dopaminergic signaling in striatum during spontaneous movement

Interplay between dopaminergic and cholinergic neuromodulation in the striatum is crucial for movement control, with prominent models proposing pro-kinetic and anti-kinetic effects of dopamine and acetylcholine release, respectively. However, the natural, movement-related signals of striatum cholinergic neurons and their relationship to simultaneous variations in dopamine signaling are unknown. Here, functional optical recordings in mice were used to establish rapid cholinergic signals in dorsal striatum during spontaneous movements. Bursts across the cholinergic population occurred at transitions between movement states and were marked by widespread network synchronization which diminished during sustained locomotion. Simultaneous cholinergic and dopaminergic recordings revealed distinct but coordinated sub-second signals, suggesting a new model where cholinergic population synchrony signals rapid changes in movement states while dopamine signals the drive to enact or sustain those states.

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.

Cholinergic modulation of striatal microcircuits

The purpose of this review is to bridge the gap between earlier literature on striatal cholinergic interneurons and mechanisms of microcircuit interaction demonstrated with the use of newly available tools. It is well known that the main source of the high level of acetylcholine in the striatum, compared to other brain regions, is the cholinergic interneurons. These interneurons provide an extensive local innervation that suggests they may be a key modulator of striatal microcircuits. Supporting this idea requires the consideration of functional properties of these interneurons, their influence on medium spiny neurons, other interneurons, and interactions with other synaptic regulators. Here, we underline the effects of intrastriatal and extrastriatal afferents onto cholinergic interneurons and discuss the activation of pre‐ and postsynaptic muscarinic and nicotinic receptors that participate in the modulation of intrastriatal neuronal interactions. We further address recent findings about corelease of other transmitters in cholinergic interneurons and actions of these interneurons in striosome and matrix compartments. In addition, we summarize recent evidence on acetylcholine‐mediated striatal synaptic plasticity and propose roles for cholinergic interneurons in normal striatal physiology. A short examination of their role in neurological disorders such as Parkinson's, Huntington's, and Tourette's pathologies and dystonia is also included.

Striatal cholinergic interneurons regulate cognitive and affective dysfunction in partially dopamine-depleted mice

Early non-motor symptoms such as mood disorders and cognitive deficits are increasingly recognised in Parkinson's disease (PD). They may precede the characteristic motor symptomatology caused by dopamine (DA) neuronal loss in the substantia nigra pars compacta (SNc). It is well known that striatal cholinergic interneurons (ChIs) are emerging as key regulators of PD motor symptom, however, their involvement in the cognitive and affective alterations occurring in the premotor phase of PD is poorly understood. We used optogenetic photoinhibition of striatal ChIs in mice with mild nigrostriatal 6-hydroxydopamine (6-OHDA) lesions and assessed their role in anxiety-like behaviour in the elevated plus maze, social memory recognition of a congener and visuospatial object recognition. In transgenic mice specifically expressing halorhodopsin (eNpHR) in cholinergic neurons, striatal ChIs photoinhibition reduced the anxiety-like behaviour and reversed social and spatial short-term memory impairment induced by moderate DA depletion (e.g., 50% loss of tyrosine hydroxylase TH-positive neurons in the SNc). Systemic injection of telenzepine (0.3 mg/kg), a preferential M1 muscarinic cholinergic receptors antagonist, improved anxiety-like behaviour, social memory recognition but not spatial memory deficits. Our results suggest that dysfunction of the striatal cholinergic system may play a role in the short-term cognitive and emotional deficits of partially DA-depleted mice. Blocking cholinergic activity with M1 muscarinic receptor antagonists may represent a possible therapeutic target, although not exclusive, to modulate these early non-motor deficits.

Amino acid modulation of dopamine in the nucleus accumbens mediates sex differences in nicotine withdrawal

The aversive effect of nicotine withdrawal is greater in female versus male rats, and we postulate that this sex difference is mediated in the nucleus accumbens (NAc). Nicotine withdrawal induces decreases in NAc dopamine and increases in acetylcholine (ACh) levels in male rats. To our knowledge, these neurochemical markers of nicotine withdrawal have not been compared in female versus male rats. Given the role of amino acids in modulating NAc dopaminergic and cholinergic transmission, concomitant measures of gamma-aminobutyric acid (GABA) and glutamate levels were also compared across sex. Rats received continuous nicotine exposure for 14 days, and then NAc dialysate was collected during baseline and following administration of the nicotinic receptor antagonist mecamylamine to precipitate withdrawal. Chronic nicotine exposure was associated with larger increases in baseline dopamine, GABA and glutamate levels in the NAc of female versus male rats, whereas baseline ACh was only increased in male rats. During withdrawal, both sexes displayed equivalent increases in NAc ACh levels. As expected, male rats displayed decreases in dopamine, coupled with increases in GABA and decreases in glutamate levels, suggesting the possibility of increased inhibitory tone in the NAc during withdrawal. Relative to males, female rats displayed larger decreases in NAc dopamine and related increases in GABAergic transmission. As female rats also showed elevated glutamate levels that persist during withdrawal, it is suggested that sex differences may arise from increased glutamatergic drive of inhibitory tone in the NAc. The findings provide a potential mechanism whereby the aversive effects of nicotine withdrawal are enhanced in female rats.

Ethovision™ analysis of open field behaviour in rats following bilateral vestibular loss

Bilateral vestibular loss (BVL) causes a unique behavioural syndrome in rodents, with symptoms such as locomotor hyperactivity and changes in exploratory behaviour. Many of these symptoms appear to be indirect consequences of the loss of vestibular reflex function and are difficult to explain. Although such symptoms have been reported before, there have been few systematic studies of the effects of BVL using automated digital tracking systems in which many behavioural symptoms can be measured simultaneously with high precision. In this study, data were obtained from rats with BVL induced by intratympanic sodium arsanilate injections (n = 7) or sham injections (n = 8) and their behaviour in the open field was measured at 3 days and 23 days post-injection using Ethovision™ tracking software. BVL rats demonstrated reduced thigmotaxis, with more time spent in the central zones. Twenty-three days post-injection, BVL animals showed increased locomotor activity in the open field. The increase in activity was also reflected in the number of transitions between each zone of the field. In addition to increased activity, BVL animals showed increased whole body rotations following lesions. Using linear discriminant analysis (LDA) and random forest classification (RFC), we were able to show that the indirect behavioural effects of BVL, excluding direct measurement of vestibular reflex function, could correctly predict whether animals had received a BVL with a high degree of accuracy at both day 3 and day 23 post-BVL (83% and 100% for LDA, and 100% and 100% for RFC, respectively). RFC has been similarly successful in classifying other hyperactivity syndromes such as attention deficit hyperactivity disorder. These results suggest that BVL results in a unique behavioural signature that can identify vestibular loss in rats even without direct vestibular reflex measurements.

Inhibition of basal and amphetamine-stimulated extracellular signal-regulated kinase (ERK) phosphorylation in the rat forebrain by muscarinic acetylcholine M4 receptors

The mitogen-activated protein kinase (MAPK), especially its extracellular signal-regulated kinase (ERK) subfamily, is a group of kinases enriched in the mammalian brain. While ERK is central to cell signaling and neural activities, the regulation of ERK by transmitters is poorly understood. In this study, the role of acetylcholine in the regulation of ERK was investigated in adult rat striatum in vivo. We focused on muscarinic M1 and M4 receptors, two principal muscarinic acetylcholine (mACh) receptor subtypes in the striatum. A systemic injection of the M1-preferring antagonist telenzepine did not alter ERK phosphorylation in the two subdivisions of the striatum, the caudate putamen and nucleus accumbens. Similarly, telenzepine did not affect ERK phosphorylation in the medial prefrontal cortex (mPFC), hippocampus, and cerebellum. Moreover, telenzepine had no effect on the ERK phosphorylation induced by dopamine stimulation with the psychostimulant amphetamine. In contrast to telenzepine, the M4-preferring antagonist tropicamide consistently increased ERK phosphorylation in the striatum and mPFC. This increase was rapid and transient. Tropicamide and amphetamine when coadministered at subthreshold doses induced a significant increase in ERK phosphorylation. These results demonstrate that mACh receptors exert a subtype-specific modulation of ERK in striatal and mPFC neurons. While the M1 receptor antagonist has no effect on ERK phosphorylation, M4 receptors inhibit constitutive and dopamine-stimulated ERK phosphorylation in these dopamine-innervated brain regions.

Tyrosine biosynthesis, metabolism, and catabolism in plants

L-Tyrosine (Tyr) is an aromatic amino acid (AAA) required for protein synthesis in all organisms, but synthesized de novo only in plants and microorganisms. In plants, Tyr also serves as a precursor of numerous specialized metabolites that have diverse physiological roles as electron carriers, antioxidants, attractants, and defense compounds. Some of these Tyr-derived plant natural products are also used in human medicine and nutrition (e.g. morphine and vitamin E). While the Tyr biosynthesis and catabolic pathways have been extensively studied in microbes and animals, respectively, those of plants have received much less attention until recently. Accumulating evidence suggest that the Tyr biosynthetic pathways differ between microbes and plants and even within the plant kingdom, likely to support the production of lineage-specific plant specialized metabolites derived from Tyr. The interspecies variations of plant Tyr pathway enzymes can now be used to enhance the production of Tyr and Tyr-derived compounds in plants and other synthetic biology platforms.

Regulation of Phosphorylation of AMPA Glutamate Receptors by Muscarinic M4 Receptors in the Striatum In vivo

The acetylcholine muscarinic 4 (M4) receptor is a principal muscarinic receptor subtype present in the striatum. Notably, G_αi/o-coupled M4 receptors and G_αs/G_olf-coupled dopamine D1 receptors are coexpressed in striatonigral projection neurons and are thought to interact with each other to regulate neuronal excitability, although underlying molecular mechanisms are poorly understood. In this study, we investigated the role of M4 receptors in the regulation of phosphorylation of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors in the rat normal and dopamine-stimulated striatum in vivo. We found that a systemic injection of a M4 antagonist tropicamide increased AMPA receptor GluA1 subunit phosphorylation at a protein kinase A-dependent site (S845) in the striatum. The tropicamide-induced S845 phosphorylation was rapid, reversible, and dose-dependent and occurred in the two subdivisions of the striatum, i.e., the caudate putamen and nucleus accumbens. Coadministration of subthreshold doses of tropicamide and a D1 agonist SKF81297 induced a significant increase in S845 phosphorylation. Coadministered tropicamide and a dopamine psychostimulant amphetamine at their subthreshold doses also elevated S845 phosphorylation. Tropicamide alone or coinjected with SKF81297 or amphetamine had no effect on GluA1 phosphorylation at S831. Tropicamide did not affect GluA2 phosphorylation at S880. These results reveal a selective inhibitory linkage from M4 receptors to GluA1 in S845 phosphorylation in striatal neurons. Blockade of the M4-mediated inhibition significantly augments constitutive and dopamine-stimulated GluA1 S845 phosphorylation.

Food Addiction and Binge Eating: Lessons Learned from Animal Models

The feeding process is required for basic life, influenced by environment cues and tightly regulated according to demands of the internal milieu by regulatory brain circuits. Although eating behaviour cannot be considered "addictive" under normal circumstances, people can become "addicted" to this behaviour, similarly to how some people are addicted to drugs. The symptoms, cravings and causes of "eating addiction" are remarkably similar to those experienced by drug addicts, and both drug-seeking behaviour as eating addiction share the same neural pathways. However, while the drug addiction process has been highly characterised, eating addiction is a nascent field. In fact, there is still a great controversy over the concept of "food addiction". This review aims to summarize the most relevant animal models of "eating addictive behaviour", emphasising binge eating disorder, that could help us to understand the neurobiological mechanisms hidden under this behaviour, and to improve the psychotherapy and pharmacological treatment in patients suffering from these pathologies.

Task-dependent function of striatal cholinergic interneurons in behavioural flexibility

Flexible switching of behaviours depends on integrative functioning through the neural circuit connecting the prefrontal cortex and the dorsomedial striatum (DMS). Although cholinergic interneurons modulate striatal outputs by diverse synaptic mechanisms, the roles of cholinergic interneurons in the DMS appear to vary among different models used to validate behavioural flexibility. Here, we conducted immunotoxin-mediated cell targeting of DMS cholinergic interneurons and examined the functions of these interneurons in behavioural flexibility, with the learning conditions differing in trial spacing and discrimination type in a modified T-maze. Elimination of the DMS cholinergic cell group normally spared reversal learning in place discrimination with an intertrial interval (ITI) of 15 s, but it impaired the reversal performance in response discrimination with the same ITI. In contrast, DMS cholinergic elimination resulted in enhanced reversal performance in both place and response discrimination tasks with a 10-min ITI and accelerated the reversal of response discrimination with a 20-min ITI. Our previous study also showed an enhanced influence of cholinergic targeting on place reversal learning with a 20-min ITI, and the present results demonstrate that DMS cholinergic interneurons act to inhibit both place and response reversal performance with a relatively longer ITI, whereas their functions differ between types of reversal performance in the tasks with a shorter ITI. These findings suggest distinct roles of the DMS cholinergic cell group in behavioural flexibility dependent on the trial spacing and discrimination type constituting the learning tasks.

Regulation of synaptic acetylcholine concentrations by acetylcholine transport in rat striatal cholinergic transmission

In addition to hydrolysis by acetylcholine esterase (AChE), acetylcholine (ACh) is also directly taken up into brain tissues. In this study, we examined whether the uptake of ACh is involved in the regulation of synaptic ACh concentrations. Superfusion experiments with rat striatal segments pre-incubated with [³ H]choline were performed using an ultra-mini superfusion vessel, which was developed to minimize superfusate retention within the vessel. Hemicholinium-3 (HC-3) at concentrations less than 1 μM, selectively inhibited the uptake of [³ H]choline by the high affinity-choline transporter 1 and had no effect on basal and electrically evoked [³ H]efflux in superfusion experiments. In contrast, HC-3 at higher concentrations, as well as tetraethylammonium (>10 μM), which inhibited the uptake of both [³ H]choline and [³ H]ACh, increased basal [³ H]overflow and potentiated electrically evoked [³ H]efflux. These effects of HC-3 and tetraethylammonium were also observed under conditions where tissue AChE was irreversibly inactivated by diisopropylfluorophosphate. Specifically, the potentiation of evoked [³ H]efflux was significantly higher in AChE-inactivated preparations and was attenuated by atropine. On the other hand, striatal segments pre-incubated with [³ H]ACh failed to increase [³ H]overflow in response to electrical stimulation. These results show that synaptic ACh concentrations are significantly regulated by the postsynaptic uptake of ACh, as well as by AChE hydrolysis and modulation of ACh release mediated through presynaptic muscarinic ACh receptors. In addition, these data suggest that the recycling of ACh-derived choline may be minor in cholinergic terminals. This study reveals a new mechanism of cholinergic transmission in the central nervous system.

Involvement of Striatal Cholinergic Interneurons and M1 and M4 Muscarinic Receptors in Motor Symptoms of Parkinson's Disease

Over the last decade, striatal cholinergic interneurons (ChIs) have reemerged as key actors in the pathophysiology of basal-ganglia-related movement disorders. However, the mechanisms involved are still unclear. In this study, we address the role of ChI activity in the expression of parkinsonian-like motor deficits in a unilateral nigrostriatal 6-hydroxydopamine (6-OHDA) lesion model using optogenetic and pharmacological approaches. Dorsal striatal photoinhibition of ChIs in lesioned ChAT(cre/cre) mice expressing halorhodopsin in ChIs reduces akinesia, bradykinesia, and sensorimotor neglect. Muscarinic acetylcholine receptor (mAChR) blockade by scopolamine produces similar anti-parkinsonian effects. To decipher which of the mAChR subtypes provides these beneficial effects, systemic and intrastriatal administration of the selective M1 and M4 mAChR antagonists telenzepine and tropicamide, respectively, were tested in the same model of Parkinson's disease. The two compounds alleviate 6-OHDA lesion-induced motor deficits. Telenzepine produces its beneficial effects by blocking postsynaptic M1 mAChRs expressed on medium spiny neurons (MSNs) at the origin of the indirect striatopallidal and direct striatonigral pathways. The anti-parkinsonian effects of tropicamide were almost completely abolished in mutant lesioned mice that lack M4 mAChRs specifically in dopamine D1-receptor-expressing neurons, suggesting that postsynaptic M4 mAChRs expressed on direct MSNs mediate the antiakinetic action of tropicamide. The present results show that altered cholinergic transmission via M1 and M4 mAChRs of the dorsal striatum plays a pivotal role in the occurrence of motor symptoms in Parkinson's disease.

SIGNIFICANCE STATEMENT

The striatum, where dopaminergic and cholinergic systems interact, is the pivotal structure of basal ganglia involved in pathophysiological changes underlying Parkinson's disease. Here, using optogenetic and pharmacological approaches, we investigated the involvement of striatal cholinergic interneurons (ChIs) and muscarinic receptor subtypes (mAChRs) in the occurrence of a wide range of motor deficits such as akinesia, bradykinesia, motor coordination, and sensorimotor neglect after unilateral nigrostriatal 6-hydroxydopamine lesion in mice. Our results show that photoinhibition of ChIs in the dorsal striatum and pharmacological blockade of muscarinic receptors, specifically postsynaptic M1 and M4 mAChRs, alleviate lesion-induced motor deficits. The present study points to these receptor subtypes as potential targets for the symptomatic treatment of parkinsonian-like motor symptoms.

Impaired novelty acquisition and synaptic plasticity in congenital hyperammonemia caused by hepatic glutamine synthetase deficiency

Genetic defects in ammonia metabolism can produce irreversible damage of the developing CNS causing an impairment of cognitive and motor functions. We investigated alterations in behavior, synaptic plasticity and gene expression in the hippocampus and dorsal striatum of transgenic mice with systemic hyperammonemia resulting from conditional knockout of hepatic glutamine synthetase (LGS-ko). These mice showed reduced exploratory activity and delayed habituation to a novel environment. Field potential recordings from LGS-ko brain slices revealed significantly reduced magnitude of electrically-induced long-term potentiation (LTP) in both CA3-CA1 hippocampal and corticostriatal synaptic transmission. Corticostriatal but not hippocampal slices from LGS-ko brains demonstrated also significant alterations in long-lasting effects evoked by pharmacological activation of glutamate receptors. Real-time RT-PCR revealed distinct patterns of dysregulated gene expression in the hippocampus and striatum of LGS-ko mice: LGS-ko hippocampus showed significantly modified expression of mRNAs for mGluR1, GluN2B subunit of NMDAR, and A1 adenosine receptors while altered expression of mRNAs for D1 dopamine receptors, the M1 cholinoreceptor and the acetylcholine-synthetizing enzyme choline-acetyltransferase was observed in LGS-ko striatum. Thus, inborn systemic hyperammonemia resulted in significant deficits in novelty acquisition and disturbed synaptic plasticity in corticostriatal and hippocampal pathways involved in learning and goal-directed behavior.

Anticholinergic activity in the nervous system: Consequences for visuomotor function

Acetylcholine is present in the peripheral and central nervous system, where it is involved in a number of fundamental physiological and biochemical processes. In particular, interaction with muscarinic receptors can cause adverse effects such as dry mouth, drowsiness, mydriasis and cognitive dysfunction. Despite the knowledge that exists regarding these common side-effects, little is known about how anticholinergic medications influence central motor processes and fine motor control in healthy individuals. This paper reviews critical visuomotor processes that operate in healthy individuals, and how controlling these motor processes are influenced by medications that interfere with central cholinergic neurotransmission. An overview of receptor function and neurotransmitter interaction following the ingestion or administration of anticholinergics is provided, before exploring how visuomotor performance is affected by anticholinergic medications. In particular, this review will focus on the effects that anticholinergic medications have on fixation stability, saccadic eye movements, smooth pursuit eye movements, and general pupil dynamics.

Temporal Rewiring of Striatal Circuits Initiated by Nicotine

Drug addiction has been conceptualized as maladaptive recruitment of integrative circuits coursing through the striatum, facilitating drug-seeking and drug-taking behavior. The aim of this study was to define temporal neuroadaptations in striatal subregions initiated by 3 weeks of intermittent nicotine exposure followed by protracted abstinence. Enhanced rearing activity was assessed in motor activity boxes as a measurement of behavioral change induced by nicotine (0.36 mg/kg), whereas electrophysiological field potential recordings were performed to evaluate treatment effects on neuronal activity. Dopamine receptor mRNA expression was quantified by qPCR, and nicotine-induced dopamine release was measured in striatal subregions using in vivo microdialysis. Golgi staining was performed to assess nicotine-induced changes in spine density of medium spiny neurons. The data presented here show that a brief period of nicotine exposure followed by abstinence leads to temporal changes in synaptic efficacy, dopamine receptor expression, and spine density in a subregion-specific manner. Nicotine may thus initiate a reorganization of striatal circuits that continues to develop despite protracted abstinence. We also show that the response to nicotine is modulated in previously exposed rats even after 6 months of abstinence. The data presented here suggests that, even though not self-administered, nicotine may produce progressive neuronal alterations in brain regions associated with goal-directed and habitual performance, which might contribute to the development of compulsive drug seeking and the increased vulnerability to relapse, which are hallmarks of drug addiction.

RasGRP1 promotes amphetamine-induced motor behavior through a Rhes interaction network ("Rhesactome") in the striatum

The striatum of the brain coordinates motor function. Dopamine-related drugs may be therapeutic to patients with striatal neurodegeneration, such as Huntington's disease (HD) and Parkinson's disease (PD), but these drugs have unwanted side effects. In addition to stimulating the release of norepinephrine, amphetamines, which are used for narcolepsy and attention-deficit/hyperactivity disorder (ADHD), trigger dopamine release in the striatum. The guanosine triphosphatase Ras homolog enriched in the striatum (Rhes) inhibits dopaminergic signaling in the striatum, is implicated in HD and L-dopa-induced dyskinesia, and has a role in striatal motor control. We found that the guanine nucleotide exchange factor RasGRP1 inhibited Rhes-mediated control of striatal motor activity in mice. RasGRP1 stabilized Rhes, increasing its synaptic accumulation in the striatum. Whereas partially Rhes-deficient (Rhes+/-) mice had an enhanced locomotor response to amphetamine, this phenotype was attenuated by coincident depletion of RasGRP1. By proteomic analysis of striatal lysates from Rhes-heterozygous mice with wild-type or partial or complete knockout of Rasgrp1, we identified a diverse set of Rhes-interacting proteins, the "Rhesactome," and determined that RasGRP1 affected the composition of the amphetamine-induced Rhesactome, which included PDE2A (phosphodiesterase 2A; a protein associated with major depressive disorder), LRRC7 (leucine-rich repeat-containing 7; a protein associated with bipolar disorder and ADHD), and DLG2 (discs large homolog 2; a protein associated with chronic pain). Thus, this Rhes network provides insight into striatal effects of amphetamine and may aid the development of strategies to treat various neurological and psychological disorders associated with the striatal dysfunction.

Expanding the repertoire of L-DOPA's actions: A comprehensive review of its functional neurochemistry

Though a multi-facetted disorder, Parkinson's disease is prototypically characterized by neurodegeneration of nigrostriatal dopaminergic neurons of the substantia nigra pars compacta, leading to a severe disruption of motor function. Accordingly, L-DOPA, the metabolic precursor of dopamine (DA), is well-established as a treatment for the motor deficits of Parkinson's disease despite long-term complications such as dyskinesia and psychiatric side-effects. Paradoxically, however, despite the traditional assumption that L-DOPA is transformed in residual striatal dopaminergic neurons into DA, the mechanism of action of L-DOPA is neither simple nor entirely clear. Herein, focussing on its influence upon extracellular DA and other neuromodulators in intact animals and experimental models of Parkinson's disease, we highlight effects other than striatal generation of DA in the functional profile of L-DOPA. While not excluding a minor role for glial cells, L-DOPA is principally transformed into DA in neurons yet, interestingly, with a more important role for serotonergic than dopaminergic projections. Moreover, in addition to the striatum, L-DOPA evokes marked increases in extracellular DA in frontal cortex, nucleus accumbens, the subthalamic nucleus and additional extra-striatal regions. In considering its functional profile, it is also important to bear in mind the marked (probably indirect) influence of L-DOPA upon cholinergic, GABAergic and glutamatergic neurons in the basal ganglia and/or cortex, while anomalous serotonergic transmission is incriminated in the emergence of L-DOPA elicited dyskinesia and psychosis. Finally, L-DOPA may exert intrinsic receptor-mediated actions independently of DA neurotransmission and can be processed into bioactive metabolites. In conclusion, L-DOPA exerts a surprisingly complex pattern of neurochemical effects of much greater scope that mere striatal transformation into DA in spared dopaminergic neurons. Their further experimental and clinical clarification should help improve both L-DOPA-based and novel strategies for controlling the motor and other symptoms of Parkinson's disease.

Integrated regulation of AMPA glutamate receptor phosphorylation in the striatum by dopamine and acetylcholine

Dopamine (DA) and acetylcholine (ACh) signals converge onto protein kinase A (PKA) in medium spiny neurons of the striatum to control cellular and synaptic activities of these neurons, although underlying molecular mechanisms are less clear. Here we measured phosphorylation of the α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor (AMPAR) at a PKA site (S845) as an indicator of AMPAR responses in adult rat brains in vivo to explore how DA and ACh interact to modulate AMPARs. We found that subtype-selective activation of DA D1 receptors (D1Rs), D2 receptors (D2Rs), or muscarinic M4 receptors (M4Rs) induced specific patterns of GluA1 S845 responses in the striatum. These defined patterns support a local multitransmitter interaction model in which D2Rs inhibited an intrinsic inhibitory element mediated by M4Rs to enhance the D1R efficacy in modulating AMPARs. Consistent with this, selective enhancement of M4R activity by a positive allosteric modulator resumed the cholinergic inhibition of D1Rs. In addition, D1R and D2R coactivation recruited GluA1 and PKA preferentially to extrasynaptic sites. In sum, our in vivo data support an existence of a dynamic DA-ACh balance in the striatum which actively modulates GluA1 AMPAR phosphorylation and trafficking. This article is part of the Special Issue entitled 'Ionotropic glutamate receptors'.

Effects of dopamine D1-like and D2-like antagonists on cocaine discrimination in muscarinic receptor knockout mice

Muscarinic and dopamine brain systems interact intimately, and muscarinic receptor ligands, like dopamine ligands, can modulate the reinforcing and discriminative stimulus (S(D)) effects of cocaine. To enlighten the dopamine/muscarinic interactions as they pertain to the S(D) effects of cocaine, we evaluated whether muscarinic M1, M2 or M4 receptors are necessary for dopamine D1 and/or D2 antagonist mediated modulation of the S(D) effects of cocaine. Knockout mice lacking M1, M2, or M4 receptors, as well as control wild-type mice and outbred Swiss-Webster mice, were trained to discriminate 10mg/kg cocaine from saline in a food-reinforced drug discrimination procedure. Effects of pretreatments with the dopamine D1 antagonist SCH 23390 and the dopamine D2 antagonist eticlopride were evaluated. In intact mice, both SCH 23390 and eticlopride attenuated the cocaine discriminative stimulus effect, as expected. SCH 23390 similarly attenuated the cocaine discriminative stimulus effect in M1 knockout mice, but not in mice lacking M2 or M4 receptors. The effects of eticlopride were comparable in each knockout strain. These findings demonstrate differences in the way that D1 and D2 antagonists modulate the S(D) effects of cocaine, D1 modulation being at least partially dependent upon activity at the inhibitory M2/M4 muscarinic subtypes, while D2 modulation appeared independent of these systems.

[Clinical syndromes of neurotransmitter system dysfunction in severe brain injury]

AIM

To explore neurotransmitter system dysfunctions involved in maintaining of consciousness and motor functions in patients with severe traumatic brain injury (TBI) and to assess their severity and predictive value.

MATERIAL AND METHODS

Authors examined 100 patients (34 women and 66 men), aged 32.0 ± 13.0 years, with severe TBI. Eighty-eight patients (31 women and 57 men) were studied in the acute stage (1-15 days, mean 5.8 ± 3.7 days) and 70 patients (24 women and 46 men) in the subacute stage (18-70 days, mean 30.4 ± 12.7 days). Inclusion criteria were: severe TBI with depression of consciousness (≤ 7 scores on the Glasgow Coma Scale), admission to the hospital in acute and subacute stages. Outcome of TBI was evaluated using the Glasgow Outcome Scale.

RESULTS AND CONCLUSION

The following clinical syndromes of neurotransmitter system dysfunction were singled out: excess or insufficiency of glutamate, cholinergic deficit, excess or insufficiency of dopamine. Their transformation during disease was identified. Predictive value of neurotransmitter dysfunctions for TBI is emphasized.

Pathophysiology of L-dopa-induced motor and non-motor complications in Parkinson's disease

Involuntary movements, or dyskinesia, represent a debilitating complication of levodopa (L-dopa) therapy for Parkinson's disease (PD). L-dopa-induced dyskinesia (LID) are ultimately experienced by the vast majority of patients. In addition, psychiatric conditions often manifested as compulsive behaviours, are emerging as a serious problem in the management of L-dopa therapy. The present review attempts to provide an overview of our current understanding of dyskinesia and other L-dopa-induced dysfunctions, a field that dramatically evolved in the past twenty years. In view of the extensive literature on LID, there appeared a critical need to re-frame the concepts, to highlight the most suitable models, to review the central nervous system (CNS) circuitry that may be involved, and to propose a pathophysiological framework was timely and necessary. An updated review to clarify our understanding of LID and other L-dopa-related side effects was therefore timely and necessary. This review should help in the development of novel therapeutic strategies aimed at preventing the generation of dyskinetic symptoms.

Dopaminergic Regulation of Striatal Interneurons in Reward and Addiction: Focus on Alcohol

Corticobasal ganglia networks coursing through the striatum are key structures for reward-guided behaviors. The ventral striatum (nucleus accumbens (nAc)) and its reciprocal connection with the ventral tegmental area (VTA) represent a primary component of the reward system, but reward-guided learning also involves the dorsal striatum and dopaminergic inputs from the substantia nigra. The majority of neurons in the striatum (>90%) are GABAergic medium spiny neurons (MSNs), but both the input to and the output from these neurons are dynamically controlled by striatal interneurons. Dopamine is a key neurotransmitter in reward and reward-guided learning, and the physiological activity of GABAergic and cholinergic interneurons is regulated by dopaminergic transmission in a complex manner. Here we review the role of striatal interneurons in modulating striatal output during drug reward, with special emphasis on alcohol.

Regulation of synaptic MAPK/ERK phosphorylation in the rat striatum and medial prefrontal cortex by dopamine and muscarinic acetylcholine receptors

Dopamine and acetylcholine are two principal transmitters in the striatum and are usually balanced to modulate local neural activity and to maintain striatal homeostasis. This study investigates the role of dopamine and muscarinic acetylcholine receptors in the regulation of a central signaling protein, i.e., the mitogen-activated protein kinase (MAPK). We focus on the synaptic pool of MAPKs because of the fact that these kinases reside in peripheral synaptic structures in addition to their somatic locations. We show that a systemic injection of dopamine D1 receptor (D1R) agonist SKF81297 enhances phosphorylation of extracellular signal-regulated kinases (ERKs), a prototypic subclass of MAPKs, in the adult rat striatum. Similar results were observed in another dopamine-responsive region, the medial prefrontal cortex (mPFC). The dopamine D2 receptor agonist quinpirole had no such effects. Pretreatment with a positive allosteric modulator (PAM) of muscarinic acetylcholine M4 receptors (M4Rs), VU0152100, attenuated the D1R agonist-stimulated ERK phosphorylation in the two regions, whereas the PAM itself did not alter basal ERK phosphorylation. All drug treatments had no effect on phosphorylation of c-Jun N-terminal kinases (JNKs), another MAPK subclass, in the striatum and mPFC. These results demonstrate that dopamine and acetylcholine are integrated to control synaptic ERK but not JNK activation in striatal and mPFC neurons in vivo. Activation of M4Rs exerts an inhibitory effect on the D1R-mediated upregulation of synaptic ERK phosphorylation.

Parallel basal ganglia circuits for voluntary and automatic behaviour to reach rewards

The basal ganglia control body movements, value processing and decision-making. Many studies have shown that the inputs and outputs of each basal ganglia structure are topographically organized, which suggests that the basal ganglia consist of separate circuits that serve distinct functions. A notable example is the circuits that originate from the rostral (head) and caudal (tail) regions of the caudate nucleus, both of which target the superior colliculus. These two caudate regions encode the reward values of visual objects differently: flexible (short-term) values by the caudate head and stable (long-term) values by the caudate tail. These value signals in the caudate guide the orienting of gaze differently: voluntary saccades by the caudate head circuit and automatic saccades by the caudate tail circuit. Moreover, separate groups of dopamine neurons innervate the caudate head and tail and may selectively guide the flexible and stable learning/memory in the caudate regions. Studies focusing on manual handling of objects also suggest that rostrocaudally separated circuits in the basal ganglia control the action differently. These results suggest that the basal ganglia contain parallel circuits for two steps of goal-directed behaviour: finding valuable objects and manipulating the valuable objects. These parallel circuits may underlie voluntary behaviour and automatic skills, enabling animals (including humans) to adapt to both volatile and stable environments. This understanding of the functions and mechanisms of the basal ganglia parallel circuits may inform the differential diagnosis and treatment of basal ganglia disorders.

Cholinergic interneurons in the dorsal and ventral striatum: anatomical and functional considerations in normal and diseased conditions

Striatal cholinergic interneurons (ChIs) are central for the processing and reinforcement of reward-related behaviors that are negatively affected in states of altered dopamine transmission, such as in Parkinson's disease or drug addiction. Nevertheless, the development of therapeutic interventions directed at ChIs has been hampered by our limited knowledge of the diverse anatomical and functional characteristics of these neurons in the dorsal and ventral striatum, combined with the lack of pharmacological tools to modulate specific cholinergic receptor subtypes. This review highlights some of the key morphological, synaptic, and functional differences between ChIs of different striatal regions and across species. It also provides an overview of our current knowledge of the cellular localization and function of cholinergic receptor subtypes. The future use of high-resolution anatomical and functional tools to study the synaptic microcircuitry of brain networks, along with the development of specific cholinergic receptor drugs, should help further elucidate the role of striatal ChIs and permit efficient targeting of cholinergic systems in various brain disorders, including Parkinson's disease and addiction.

The effect of deficient muscarinic signaling on commonly reported biochemical effects in schizophrenia and convergence with genetic susceptibility loci in explaining symptom dimensions of psychosis

With the advent of DSM 5 criticism has generally centered on a lack of biological validity of the diagnostic criteria. Part of the problem in describing a nosology of psychosis is the tacit assumption of multiple genetic causes each with an incremental loading on the clinical picture that fails to differentiate a clear underlying pathophysiology of high impact. The aim of this paper is to consolidate a primary theory of deficient muscarinic signaling underlying key clinical features of schizophrenia and its regulation by several important genetic associations including neuregulin, DISC and dysbindin. Secondary reductions in markers for GABAergic function and changes in the levels of interneuron calcium binding proteins parvalbumin and calbindin can be attributed to dysfunctional muscarinic transduction. A parallel association exists for cytokine production. The convergent pathway hypothesis is likewise used to model dopaminergic and glutamatergic theories of schizophrenia. The negative symptom dimension is correlated with dysfunction of Akt and ERK transduction, a major point of convergence. The present paradigm predicts the importance of a recent finding of a deletion in a copy number variant of PLCB1 and its potential use if replicated, as one of the first testable biological markers differentiating schizophrenia from bipolar disorder and further subtyping of schizophrenia into deficit and non-deficit. Potential limitations of PLCB1 as a prospective marker are also discussed.

The potency and efficacy of anticholinergics to inhibit haloperidol-induced catalepsy in rats correlates with their rank order of affinities for the muscarinic receptor subtypes

Extrapyramidal syndromes (EPS) caused by antipsychotic therapy are currently treated with anticholinergics that lack selectivity for the five muscarinic receptor subtypes. Since these receptors are heterogeneously expressed among the different classes of striatal neurons and their afferents, it can be expected that their simultaneous blockade will cause distinct, sometimes opposed, effects within the striatal circuitry. In order to test the hypothesis that the differential blockade of the muscarinic receptor subtypes would influence their potency and efficacy to prevent EPS, here we tested four anticholinergics with varying order of affinities for the muscarinic receptor subtypes, and compared their dose-response curves to inhibit haloperidol-induced catalepsy in male rats. Drugs were applied into the lateral ventricle 15 min before haloperidol (2 mg/kg, s.c.). Catalepsy was measured in the bar test at 15 min intervals during 5 h. The preferential M1/M4 antagonist pirenzepine (3, 10, 30, 100, and 300 nmol) caused a dose-dependent inhibition of catalepsy intensity: ED50 = 5.6 nmol [95% CI, 3.9-8.1], and latency: ED50 = 5.6 nmol [95% CI, 3.7-8.6]. Pirenzepine had the steepest dose-response curve, producing maximal inhibition (84 ± 5%) at the dose of 10 nmol, while its effect tended to reverse at higher doses (62 ± 11%). The purported M1/M3 antagonist 4-DAMP (30, 100, and 300 nmol) also caused a dose-dependent inhibition of catalepsy intensity: ED50 = 29.5 nmol [95% CI, 7.0 to 123.0], and latency: ED50 = 28.5 nmol [95% CI, 2.2 to 362.0]. However, the curve for 4-DAMP had a less pronounced slope, reaching its maximal effect (63 ± 14%) at the dose of 300 nmol. The M2/M4 antagonist AF-DX 116 (10, 30, and 300 nmol) only caused a partial inhibition of catalepsy (30 ± 11%) at the dose of 30 nmol, but this changed to a non-significant increment (15 ± 10%) at the dose of 100 nmol. The alleged M4 antagonist tropicamide (30, 100, 300, and 600 nmol) produced a partial inhibition of catalepsy (36 ± 12%) at the dose of 300 nmol, but lacked effect at higher or lower doses. Concurrent treatment with pirenzepine (10 nmol) and tropicamide (300 nmol) produced an effect similar to that of tropicamide alone. The greater potency and efficacy of pirenzepine for catalepsy inhibition could be due to its higher affinity for M1 receptors and, to a lesser extent, for M4 receptors. It is suggested that selective M1 antagonists would be more effective than M2, M3 or M4 antagonists to prevent EPS caused by antipsychotic drugs.

Acute and chronic effects of the M1/M4-preferring muscarinic agonist xanomeline on cocaine vs. food choice in rats

RATIONALE

We previously showed that the M1/M4-preferring muscarinic agonist xanomeline can acutely attenuate or eliminate cocaine self-administration in mice.

OBJECTIVE

Medications used to treat addictions will arguably be administered in (sub)chronic or repeated regimens. Tests of acute effects often fail to predict chronic effects, highlighting the need for chronic testing of candidate medications.

METHODS

Rats were trained to lever press under a concurrent FR5 FR5 schedule of intravenous cocaine and food reinforcement. Once baseline behavior stabilized, the effects of 7 days once-daily injections of xanomeline were evaluated.

RESULTS

Xanomeline pretreatment dose-dependently (1.8-10 mg/kg/day) shifted the dose-effect curve for cocaine rightward (up to 5.6-fold increase in A 50), with reallocation of behavior to the food-reinforced lever. There was no indication of tolerance, rather effects grew over days. The suppression of cocaine choice appeared surmountable at high cocaine doses, and xanomeline treatment did not significantly decrease total-session cocaine or food intake.

CONCLUSIONS

In terms of xanomeline's potential for promoting abstinence from cocaine in humans, the findings were mixed. Xanomeline did produce reallocation of behavior from cocaine to food with a robust increase in food reinforcers earned at some cocaine/xanomeline dose combinations. However, effects appeared surmountable, and food-maintained behavior was also decreased at some xanomeline/cocaine dose combinations, suggesting clinical usefulness may be limited. These data nevertheless support the notion that chronic muscarinic receptor stimulation can reduce cocaine self-administration. Future studies should show whether ligands with higher selectivity for M1 or M1/M4 subtypes would be less limited by undesired effects and can achieve higher efficacy.

GABAergic synaptic transmission onto striatal cholinergic interneurons in dopamine D2 receptor knock-out mice

Whole-cell or cell-attached analysis was carried out in dopamine (DA) D2 receptor (D2R) knock-out (KO) mice to elucidate the function of this receptor in the regulation of GABAergic synaptic transmission onto striatal cholinergic interneurons as well as their spontaneous firing. In slice preparation obtained from wild-type mice, evoked GABAergic inhibitory postsynaptic currents (IPSCs) showed frequency-dependent suppression, and this suppression significantly decreased in the presence of a D2-like receptor antagonist or in D2R KO mice. Contribution of N-type calcium channel was also significantly reduced in the striatal cholinergic interneurons of the D2R KO mice compared with that in the wild-type mice. Spontaneous firing of striatal cholinergic interneurons was inhibited by 5- or 10-Hz stimulation, and the suppression was decreased in the presence of a D2-like receptor antagonist or in D2R KO mice. These findings substantiate the physiological role of D2R in the regulation of GABAergic synaptic transmission onto striatal cholinergic interneurons as well as their excitability, confirming the tight coupling between D2R and N-type calcium channels in the regulation of GABA release.

Galantamine reverses scopolamine-induced behavioral alterations in Dugesia tigrina

In planaria (Dugesia tigrina), scopolamine, a nonselective muscarinic receptor antagonist, induced distinct behaviors of attenuated motility and C-like hyperactivity. Planarian locomotor velocity (pLMV) displayed a dose-dependent negative correlation with scopolamine concentrations from 0.001 to 1.0 mM, and a further increase in scopolamine concentration to 2.25 mM did not further decrease pLMV. Planarian hyperactivity counts was dose-dependently increased following pretreatment with scopolamine concentrations from 0.001 to 0.5 mM and then decreased for scopolamine concentrations ≥ 1 mM. Planarian learning and memory investigated using classical Pavlovian conditioning experiments demonstrated that scopolamine (1 mM) negatively influenced associative learning indicated by a significant decrease in % positive behaviors from 86 % (control) to 14 % (1 mM scopolamine) and similarly altered memory retention, which is indicated by a decrease in % positive behaviors from 69 % (control) to 27 % (1 mM scopolamine). Galantamine demonstrated a complex behavior in planarian motility experiments since co-application of low concentrations of galantamine (0.001 and 0.01 mM) protected planaria against 1 mM scopolamine-induced motility impairments; however, pLMV was significantly decreased when planaria were tested in the presence of 0.1 mM galantamine alone. Effects of co-treatment of scopolamine and galantamine on memory retention in planaria via classical Pavlovian conditioning experiments showed that galantamine (0.01 mM) partially reversed scopolamine (1 mM)-induced memory deficits in planaria as the % positive behaviors increased from 27 to 63 %. The results demonstrate, for the first time in planaria, scopolamine's effects in causing learning and memory impairments and galantamine's ability in reversing scopolamine-induced memory impairments.