Nicotinic acetylcholine receptors interact with dopamine in induction of striatal long-term depression

Evaluation of the potential role of glutamatergic, cholinergic, and nitrergic systems in the dopamine release induced by the pesticide glyphosate in rat striatum

Glyphosate (GLY) is a pesticide that severely alters nigrostriatal dopaminergic neurotransmission, inducing great increases in dopamine release from rat dorsal striatum. This GLY-induced striatal dopamine overflow occurs through mechanisms not yet fully understood, hence the interest in evaluating the role of other neurotransmitter systems in such effects. So, the main objective of this mechanistic study was to evaluate the possible mediation of the glutamatergic, cholinergic, and nitrergic systems in the GLY-induced in vivo dopamine release from rat dorsal striatum. The extracellular dopamine levels were measured by cerebral microdialysis and HPLC with electrochemical detection. Intrastriatal administration of GLY (5 mmol/L) significantly increased the dopamine release (1102%). Pretreatment with MK-801 (50 or 400 μmol/L), a non-competitive antagonist of NMDA receptors, significantly decreased the effect of GLY (by 70% and 74%, respectively), whereas AP-5 (400 μmol/L), a competitive antagonist of NMDA receptors, or CNQX (500 μmol/L), an AMPA/kainate receptor antagonist, had no significant effect. Administration of the nitric oxide synthase inhibitors, L-nitroarginine (L-NAME, 100 μmol/L) or 7-nitroindazole (7-NI, 100 μmol/L), also did not alter the effect of GLY on dopamine release. Finally, pretreatment of the animals with mecamylamine, an antagonist of nicotinic receptors, decreased the effect of GLY on dopamine release by 49%, whereas atropine, a muscarinic antagonist, had no significant effect. These results indicate that GLY-induced dopamine release largely depends on the activation of NMDA and nicotinic receptors in rat dorsal striatum. Future research is needed to determine the effects of this pesticide at environmentally relevant concentrations.

Astrocytic Regulation of Endocannabinoid-Dependent Synaptic Plasticity in the Dorsolateral Striatum

Astrocytes are pivotal for synaptic transmission and may also play a role in the induction and expression of synaptic plasticity, including endocannabinoid-mediated long-term depression (eCB-LTD). In the dorsolateral striatum (DLS), eCB signaling plays a major role in balancing excitation and inhibition and promoting habitual learning. The aim of this study was to outline the role of astrocytes in regulating eCB signaling in the DLS. To this end, we employed electrophysiological slice recordings combined with metabolic, chemogenetic and pharmacological approaches in an attempt to selectively suppress astrocyte function. High-frequency stimulation induced eCB-mediated LTD (HFS-LTD) in brain slices from both male and female rats. The metabolic uncoupler fluorocitrate (FC) reduced the probability of transmitter release and depressed synaptic output in a manner that was independent on cannabinoid 1 receptor (CB1R) activation. Fluorocitrate did not affect the LTD induced by the CB1R agonist WIN55,212-2, but enhanced CB1R-dependent HFS-LTD. Reduced neurotransmission and facilitated HFS-LTD were also observed during chemogenetic manipulation using Gi-coupled DREADDs targeting glial fibrillary acidic protein (GFAP)-expressing cells, during the pharmacological inhibition of connexins using carbenoxolone disodium, or during astrocytic glutamate uptake using TFB-TBOA. While pretreatment with the N-methyl-D-aspartate (NMDA) receptor antagonist 2-amino-5-phosphonopentanoic acid (APV) failed to prevent synaptic depression induced by FC, it blocked the facilitation of HFS-LTD. While the lack of tools to disentangle astrocytes from neurons is a major limitation of this study, our data collectively support a role for astrocytes in modulating basal neurotransmission and eCB-mediated synaptic plasticity.

Acetylcholine facilitates localized synaptic potentiation and location specific feature binding

Forebrain acetylcholine (ACh) signaling has been shown to drive attention and learning. Recent experimental evidence of spatially and temporally constrained cholinergic signaling has sparked interest to investigate how it facilitates stimulus-induced learning. We use biophysical excitatory-inhibitory (E-I) multi-module neural network models to show that external stimuli and ACh signaling can mediate spatially constrained synaptic potentiation patterns. The effects of ACh on neural excitability are simulated by varying the conductance of a muscarinic receptor-regulated hyperpolarizing slow K+ current (m-current). Each network module consists of an E-I network with local excitatory connectivity and global inhibitory connectivity. The modules are interconnected with plastic excitatory synaptic connections, that change via a spike-timing-dependent plasticity (STDP) rule. Our results indicate that spatially constrained ACh release influences the information flow represented by network dynamics resulting in selective reorganization of inter-module interactions. Moreover the information flow depends on the level of synchrony in the network. For highly synchronous networks, the more excitable module leads firing in the less excitable one resulting in strengthening of the outgoing connections from the former and weakening of its incoming synapses. For networks with more noisy firing patterns, activity in high ACh regions is prone to induce feedback firing of synchronous volleys and thus strengthening of the incoming synapses to the more excitable region and weakening of outgoing synapses. Overall, these results suggest that spatially and directionally specific plasticity patterns, as are presumed necessary for feature binding, can be mediated by spatially constrained ACh release.

Dopaminergic D2 receptor modulation of striatal cholinergic interneurons contributes to sequence learning

Learning action sequences is necessary for normal daily activities. Medium spiny neurons (MSNs) in the dorsal striatum (dStr) encode action sequences through changes in firing at the start and/or stop of action sequences or sustained changes in firing throughout the sequence. Acetylcholine (ACh), released from cholinergic interneurons (ChIs), regulates striatal function by modulating MSN and interneuron excitability, dopamine and glutamate release, and synaptic plasticity. Cholinergic neurons in dStr pause their tonic firing during the performance of learned action sequences. Activation of dopamine type-2 receptors (D2Rs) on ChIs is one mechanism of ChI pausing. In this study we show that deleting D2Rs from ChIs by crossing D2-floxed with ChAT-Cre mice (D2Flox-ChATCre), which inhibits dopamine-mediated ChI pausing and leads to deficits in an operant action sequence task and lower breakpoints in a progressive ratio task. These data suggest that D2Flox-ChATCre mice have reduced motivation to work for sucrose reward, but show no generalized motor skill deficits. D2Flox-ChATCre mice perform similarly to controls in a simple reversal learning task, indicating normal behavioral flexibility, a cognitive function associated with ChIs. In vivo electrophysiological recordings show that D2Flox-ChatCre mice have deficits in sequence encoding, with fewer dStr MSNs encoding entire action sequences compared to controls. Thus, ChI D2R deletion appears to impair a neural substrate of action chunking. Virally replacing D2Rs in dStr ChIs in adult mice improves action sequence learning, but not the lower breakpoints, further suggesting that D2Rs on ChIs in the dStr are critical for sequence learning, but not for driving the motivational aspects of the task.

Acute and chronic effects by nicotine on striatal neurotransmission and synaptic plasticity in the female rat brain

Introduction

Tobacco use is in part a gendered activity, yet neurobiological studies outlining the effect by nicotine on the female brain are scarce. The aim of this study was to outline acute and sub-chronic effects by nicotine on the female rat brain, with special emphasis on neurotransmission and synaptic plasticity in the dorsolateral striatum (DLS), a key brain region with respect to the formation of habits.

Methods

In vivo microdialysis and ex vivo electrophysiology were performed in nicotine naïve female Wistar rats, and following sub-chronic nicotine exposure (0.36 mg/kg free base, 15 injections). Locomotor behavior was assessed at the first and last drug-exposure.

Results

Acute exposure to nicotine ex vivo depresses excitatory neurotransmission by reducing the probability of transmitter release. Bath applied nicotine furthermore facilitated long-term synaptic depression induced by high frequency stimulation (HFS-LTD). The cannabinoid 1 receptor (CB1R) agonist WIN55,212-2 produced a robust synaptic depression of evoked potentials, and HFS-LTD was blocked by the CB1R antagonist AM251, suggesting that HFS-LTD in the female rat DLS is endocannabinoid mediated. Sub-chronic exposure to nicotine in vivo produced behavioral sensitization and electrophysiological recordings performed after 2-8 days abstinence revealed a sustained depression of evoked population spike amplitudes in the DLS, with no concomitant change in paired pulse ratio. Rats receiving sub-chronic nicotine exposure further demonstrated an increased neurophysiological responsiveness to nicotine with respect to both dopaminergic- and glutamatergic signaling. However, a tolerance towards the plasticity facilitating property of bath applied nicotine was developed during sub-chronic nicotine exposure in vivo. In addition, the dopamine D2 receptor agonist quinpirole selectively facilitate HFS-LTD in slices from nicotine naïve rats, suggesting that the tolerance may be associated with changes in dopaminergic signaling.

Conclusion

Nicotine produces acute and sustained effects on striatal neurotransmission and synaptic plasticity in the female rat brain, which may contribute to the establishment of persistent nicotine taking habits.

Local modulation by presynaptic receptors controls neuronal communication and behaviour

Central nervous system neurons communicate via fast synaptic transmission mediated by ligand-gated ion channel (LGIC) receptors and slower neuromodulation mediated by G protein-coupled receptors (GPCRs). These receptors influence many neuronal functions, including presynaptic neurotransmitter release. Presynaptic LGIC and GPCR activation by locally released neurotransmitters influences neuronal communication in ways that modify effects of somatic action potentials. Although much is known about presynaptic receptors and their mechanisms of action, less is known about when and where these receptor actions alter release, especially in vivo. This Review focuses on emerging evidence for important local presynaptic receptor actions and ideas for future studies in this area.

Effects of methamphetamine-induced neurotoxicity on striatal long-term potentiation

RATIONALE

Methamphetamine (METH) exposure is associated with damage to central monoamine systems, particularly dopamine signaling. Rodent models of such damage have revealed a decrease in the amplitude of phasic dopamine signals and significant striatal dysfunction, including changes in the molecular, system, and behavioral functions of the striatum. Dopamine signaling through D1 receptors promotes corticostriatal long-term potentiation (LTP), a critical substrate of these striatal functions.

OBJECTIVES

Therefore, the purpose of this study was to determine if METH-induced dopamine neurotoxicity would impair D1 receptor-dependent striatal LTP in mice.

METHODS

Mice were treated with a METH binge regimen (4 × 10 mg/kg d,l-methamphetamine, s.c.) that recapitulates all of the known METH-induced neurotoxic effects observed in humans, including dopamine toxicity. Three weeks later, acute brain slices containing either the dorsomedial striatum (DMS) or dorsolateral striatum (DLS) were prepared, and plasticity was assessed using white matter, high-frequency stimulation (HFS), and striatal extracellular electrophysiology.

RESULTS

Under these conditions, LTP was induced in brain slices containing the DMS from saline-pretreated mice, but not mice with METH-induced neurotoxicity. Furthermore, the LTP observed in DMS slices from saline-pretreated mice was blocked by the dopamine D1 receptor antagonist SCH23390, indicating that this LTP is dopamine D1 receptor-dependent. Finally, acute in vivo treatment of METH-pretreated mice with bupropion (50 mg/kg, i.p.) promoted LTP in DMS slices.

CONCLUSIONS

Together, these studies demonstrate that METH-induced neurotoxicity impairs dopamine D1 receptor-dependent LTP within the DMS and that the FDA-approved drug bupropion restores induction of striatal LTP in mice with METH-induced dopamine neurotoxicity.

Emotion Processing Dysfunction in Alzheimer's Disease: An Overview of Behavioral Findings, Systems Neural Correlates, and Underlying Neural Biology

We described behavioral studies to highlight emotional processing deficits in Alzheimer's disease (AD). The findings suggest prominent deficit in recognizing negative emotions, pronounced effect of positive emotion on enhancing memory, and a critical role of cognitive deficits in manifesting emotional processing dysfunction in AD. We reviewed imaging studies to highlight morphometric and functional markers of hippocampal circuit dysfunction in emotional processing deficits. Despite amygdala reactivity to emotional stimuli, hippocampal dysfunction conduces to deficits in emotional memory. Finally, the reviewed studies implicating major neurotransmitter systems in anxiety and depression in AD supported altered cholinergic and noradrenergic signaling in AD emotional disorders. Overall, the studies showed altered emotions early in the course of illness and suggest the need of multimodal imaging for further investigations. Particularly, longitudinal studies with multiple behavioral paradigms translatable between preclinical and clinical models would provide data to elucidate the time course and underlying neurobiology of emotion processing dysfunction in AD.

Corticostriatal Circuit Models of Cognitive Impairments Induced by Fetal Exposure to Alcohol

The term fetal alcohol spectrum disorder includes a group of diseases caused by fetal alcohol exposure (FAE). Patients with fetal alcohol spectrum disorder display heterogeneous socioemotional and cognitive deficits, particularly in the domain of executive function, that share symptoms with other neuropsychiatric disorders. Despite the availability of several preclinical models, the developmental brain defects causally linked to behavioral deficits induced by FAE remain poorly understood. Here, we first review the effects of FAE on corticostriatal development and its impact on both corticostriatal pathway function and cognitive abilities. We propose three non-mutually exclusive circuit models of corticostriatal dysfunctions to account for some of the FAE-induced cognitive deficits. One model posits that associative-sensorimotor imbalance causes hyper goal-directed behavior, and a second model implies that alteration of prefrontal-striatal behavioral suppression circuits results in loss of behavioral inhibition. A third model suggests that local striatal circuit deficits affect striatal neuronal ensemble function to impair action selection and performance. Finally, we discuss how preclinical approaches applied to these circuit models could offer potential rescue strategies for executive function deficits in patients with fetal alcohol spectrum disorder.

Effects of biperiden (cholinergic muscarinic m1/m4 receptor antagonist) on ethanol conditioned place preference in mice

BACKGROUND

Previous studies suggest that muscarinic cholinergic receptors might act upon the dopamine release in the mesolimbic system and alter drug-reinforcing values related to drug craving.

AIMS

We examined the effects of systemic biperiden administration, a muscarinic cholinergic (M1/M4) receptor antagonist, on ethanol (dose of 2 g/Kg) conditioned place preference (CPP), neuronal activation, dopamine and its metabolites levels in the nucleus accumbens.

METHODS

Thirty minutes before the ethanol-induced CPP test, mice received saline or biperiden at doses of 1.0, 5.0, or 10.0 mg/kg. The time spent in each compartment was recorded for 15 min. After the CPP protocol, animals were euthanized, and we investigated the activation of the nucleus accumbens by immunohistochemistry for Fos. We also quantified dopamine, homovanillic acid (HVA), and dihydroxyphenylacetic acid (DOPAC) levels in the nucleus accumbens by high-performance liquid chromatography (HPLC). Additionally, the rotarod was employed to evaluate the effects of biperiden on motor coordination.

RESULTS

Biperiden at different doses (1.0, 5.0, and 10.0 mg/kg) blocked the expression of ethanol-induced CPP. These biperiden doses increased the number of Fos-positive cells and the dopamine turnover in the nucleus accumbens. None of the doses affected the motor coordination evaluated by the rotarod.

CONCLUSIONS

Our results show that biperiden can modulate the effect of alcohol reward, and its mechanism of action may involve a change in dopamine and cholinergic mesolimbic neurotransmission.

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.

Bilateral responses of rat ventral striatum tonically active neurons to unilateral medial forebrain bundle stimulation

Unilateral medial forebrain bundle (MFB) stimulation is an extremely effective promoter of reinforcement learning irrespective of the conditioned cue's laterality. The effectiveness of unilateral MFB stimulation, which activates the mesolimbic pathway connecting the ventral tegmental area to the ventral striatum (vStr), is surprising considering that these fibers rarely cross to the contralateral hemisphere. Specifically, this type of biased fiber distribution entails the activation of brain structures that are primarily ipsilateral to the stimulated MFB, along with weak to negligible activation of the contralateral structures, thus impeding the formation of a cue-outcome association. To better understand the spread of activation of MFB stimulation across hemispheres, we studied whether unilateral MFB stimulation primarily activates the ipsilateral vStr or the vStr of both hemispheres. We simultaneously recorded neuronal activity in the vStr of both hemispheres in response to several sets of unilateral MFB stimulation in anesthetized and freely moving rats. Unilateral MFB stimulation evoked strong stimulus-dependent activation of vStr tonically active neurons (TANs), presumably the cholinergic interneurons, in both hemispheres. However, the TANs' activation patterns and responsiveness depended on whether the stimulus was delivered ipsilaterally or contralaterally to the recorded neuron. These findings indicate that unilateral MFB stimulation effectively activates the vStr in both hemispheres in a stimulus-dependent manner which may serve as neuronal substrate for the formation of cue-outcome associations during reinforcement learning.

The role of nicotinic cholinergic neurotransmission in delusional thinking

Delusions are a difficult-to-treat and intellectually fascinating aspect of many psychiatric illnesses. Although scientific progress on this complex topic has been challenging, some recent advances focus on dysfunction in neural circuits, specifically in those involving dopaminergic and glutamatergic neurotransmission. Here we review the role of cholinergic neurotransmission in delusions, with a focus on nicotinic receptors, which are known to play a part in some illnesses where these symptoms appear, including delirium, schizophrenia spectrum disorders, bipolar disorder, Parkinson, Huntington, and Alzheimer diseases. Beginning with what we know about the emergence of delusions in these illnesses, we advance a hypothesis of cholinergic disturbance in the dorsal striatum where nicotinic receptors are operative. Striosomes are proposed to play a central role in the formation of delusions. This hypothesis is consistent with our current knowledge about the mechanism of action of cholinergic drugs and with our abstract models of basic cognitive mechanisms at the molecular and circuit levels. We conclude by pointing out the need for further research both at the clinical and translational levels.

Synapse-specific expression of mu opioid receptor long-term depression in the dorsomedial striatum

The dorsal striatum is a brain region involved in action control, with dorsomedial striatum (DMS) mediating goal-directed actions and dorsolateral striatum (DLS) mediating habitual actions. Presynaptic long-term synaptic depression (LTD) plasticity at glutamatergic inputs to dorsal striatum mediates many dorsal striatum-dependent behaviors and disruption of LTD influences action control. Our previous work identified mu opioid receptors (MORs) as mediators of synapse-specific forms of synaptic depression at a number of different DLS synapses. We demonstrated that anterior insular cortex inputs are the sole inputs that express alcohol-sensitive MOR-mediated LTD (mOP-LTD) in DLS. Here, we explore mOP-LTD in DMS using mouse brain slice electrophysiology. We found that contrary to DLS, DMS mOP-LTD is induced by activation of MORs at inputs from both anterior cingulate and medial prefrontal cortices as well as at basolateral amygdala inputs and striatal cholinergic interneuron synapses on to DMS medium spiny neurons, suggesting that MOR synaptic plasticity in DMS is less synapse-specific than in DLS. Furthermore, only mOP-LTD at cortical inputs was sensitive to alcohol's deleterious effects. These results suggest that alcohol-induced neuroadaptations are differentially expressed in a synapse-specific manner and could be playing a role in alterations of goal-directed and habitual behaviors.

High-Frequency Activation of Nucleus Accumbens D1-MSNs Drives Excitatory Potentiation on D2-MSNs

Subtypes of nucleus accumbens medium spiny neurons (MSNs) promote dichotomous outcomes in motivated behaviors. However, recent reports indicate enhancing activity of either nucleus accumbens (NAc) core MSN subtype augments reward, suggesting coincident MSN activity may underlie this outcome. Here, we report a collateral excitation mechanism in which high-frequency, NAc core dopamine 1 (D1)-MSN activation causes long-lasting potentiation of excitatory transmission (LLP) on dopamine receptor 2 (D2)-MSNs. Our mechanistic investigation demonstrates that this form of plasticity requires release of the excitatory peptide substance P from D1-MSNs and robust cholinergic interneuron activation through neurokinin receptor stimulation. We also reveal that D2-MSN LLP requires muscarinic 1 receptor activation, intracellular calcium signaling, and GluR2-lacking AMPAR insertion. This study uncovers a mechanism for shaping NAc core activity through the transfer of excitatory information from D1-MSNs to D2-MSNs and may provide a means for altering goal-directed behavior through coordinated MSN activity.

Acute and chronic modulation of striatal endocannabinoid-mediated plasticity by nicotine

The endocannabinoid (eCB) system modulates several phenomena related to addictive behaviors, and drug‐induced changes in eCB signaling have been postulated to be important mediators of physiological and pathological reward‐related synaptic plasticity. Here, we studied eCB‐mediated long‐term depression (eCB‐LTD) in the dorsolateral striatum, a brain region critical for acquisition of habitual and automatic behavior. We report that nicotine differentially affects ex vivo eCB signaling depending on previous exposure in vivo. In the nicotine‐naïve brain, nicotine facilitates eCB‐signaling and LTD, whereas tolerance develops to this facilitating effect after subchronic exposure in vivo. In the end, a progressive impairment of eCB‐induced LTD is established after protracted withdrawal from nicotine. Endocannabinoid‐LTD is reinstated 6 months after the last drug injection, but a brief period of nicotine re‐exposure is sufficient to yet again impair eCB‐signaling. LTD induced by the cannabinoid 1 receptor agonist WIN55,212‐2 is not affected, suggesting that nicotine modulates eCB production or release. Nicotine‐induced facilitation of eCB‐LTD is occluded by the dopamine D2 receptor agonist quinpirole, and by the muscarinic acetylcholine receptor antagonist scopolamine. In addition, the same compounds restore eCB‐LTD during protracted withdrawal. Nicotine may thus modulate eCB‐signaling by affecting dopaminergic and cholinergic neurotransmission in a long‐lasting manner. Overall, the data presented here suggest that nicotine facilitates eCB‐LTD in the initial phase, which putatively could promote neurophysiological and behavioral adaptations to the drug. Protracted withdrawal, however, impairs eCB‐LTD, which may influence or affect the ability to maintain cessation.

Effects of stimulant drug use on the dopaminergic system: A systematic review and meta-analysis of in vivo neuroimaging studies

BACKGROUND

Stimulant drugs can cause persistent changes in the brain. Imaging studies show that these changes are most apparent in dopamine transporter (DAT) or receptor availability within the striatum.

METHODS

This work focuses on influences of stimulant use on dopaminergic function assessed using nuclear-medicine imaging (PET/SPECT). Included are 39 studies on 655 cocaine, amphetamine, methamphetamine or nicotine users, as well as 690 healthy controls. Metaanalyses were conducted separately for D2/D3 receptors and dopamine transporters of the entire striatum, its subregions caudate and putamen respectively.

RESULTS

Meta-analyses results regarding nicotine did not show significant effects between smokers and nonsmokers. In cocaine users there was a significant decrease in dopamine receptor availability in all regions. The striatal DAT availability was significantly increased in cocaine users. Methamphetamine users showed a significantly decreased dopamine receptor and transporter density in all regions. Significant results also indicate a lower transporter availability in all regions. Amphetamine users showed reduced DAT availability in the striatum, as well as in the sub regions.

CONCLUSION

This meta-analysis provides evidence that there are ongoing changes in the dopaminergic system associated with the use of stimulants. Especially the results of cocaine, methamphetamine and amphetamine use mainly showed a downregulation. In addition, this meta-analysis is the first to include nicotine. This subset of studies showed evidence for a decreased receptor and DAT availability but no significant results were found in the metaanalyses.

Disrupted reinforcement learning during post-error slowing in ADHD

ADHD is associated with altered dopamine regulated reinforcement learning on prediction errors. Despite evidence of categorically altered error processing in ADHD, neuroimaging advances have largely investigated models of normal reinforcement learning in greater detail. Further, although reinforcement leaning critically relies on ventral striatum exerting error magnitude related thresholding influences on substantia nigra (SN) and dorsal striatum, these thresholding influences have never been identified with neuroimaging. To identify such thresholding influences, we propose that error magnitude related activities must first be separated from opposite activities in overlapping neural regions during error detection. Here we separate error detection from magnitude related adjustment (post-error slowing) during inhibition errors in the stop signal task in typically developing (TD) and ADHD adolescents using fMRI. In TD, we predicted that: 1) deactivation of dorsal striatum on error detection interrupts ongoing processing, and should be proportional to right frontoparietal response phase activity that has been observed in the SST; 2) deactivation of ventral striatum on post-error slowing exerts thresholding influences on, and should be proportional to activity in dorsal striatum. In ADHD, we predicted that ventral striatum would instead correlate with heightened amygdala responses to errors. We found deactivation of dorsal striatum on error detection correlated with response-phase activity in both groups. In TD, post-error slowing deactivation of ventral striatum correlated with activation of dorsal striatum. In ADHD, ventral striatum correlated with heightened amygdala activity. Further, heightened activities in locus coeruleus (norepinephrine), raphe nucleus (serotonin) and medial septal nuclei (acetylcholine), which all compete for control of DA, and are altered in ADHD, exhibited altered correlations with SN. All correlations in TD were replicated in healthy adults. Results in TD are consistent with dopamine regulated reinforcement learning on post-error slowing. In ADHD, results are consistent with heightened activities in the amygdala and non-dopaminergic neurotransmitter nuclei preventing reinforcement learning.

Interaction of nicotinic acetylcholine receptors with dopamine receptors in synaptic plasticity of the mouse insular cortex

The insular cortex plays essential roles in nicotine addiction. However, much is still unknown about its cellular and synaptic mechanisms responsible for nicotine addiction. We have previously shown that in layer 5 pyramidal neurons of the mouse insular cortex, activation of the nicotinic acetylcholine receptors (nAChRs) suppresses synaptic potentiation through enhancing GABAergic synaptic transmission, although it enhances both glutamatergic and GABAergic synaptic transmission. In the present study, we examined whether dopamine receptors might contribute to the nicotine-induced inhibition of synaptic potentiation. The nicotine-induced inhibition of synaptic potentiation was decreased in the presence of a D1 dopamine receptor antagonist SCH23390 irrespective of the presence of a D2 dopamine receptor antagonist sulpiride, suggesting that D1 dopamine receptors are involved in nicotine-induced inhibition. We also investigated how dopamine receptors might contribute to the nAChR-induced enhancement of glutamatergic and GABAergic synaptic transmission. The nAChR-induced enhancement of GABAergic synaptic transmission was decreased in the presence of SCH23390 irrespective of the presence of sulpiride, whereas that of glutamatergic synaptic transmission was not altered in the presence of SCH23390 and sulpiride. These results suggest that D1 dopamine receptors are involved in the nAChR-induced enhancement of GABAergic synaptic transmission while dopamine receptors are not involved in that of glutamatergic synaptic transmission. These observations indicate that the interaction between nAChRs and D1 dopamine receptors plays critical roles in synaptic activities in layer 5 pyramidal neurons of the mouse insular cortex. These insular synaptic changes might be associated with nicotine addiction.

The novel hybrid agonist HyNDA-1 targets the D3R-nAChR heteromeric complex in dopaminergic neurons

In this paper, we designed, synthesized and tested a small set of three new derivatives potentially targeting the D3R-nAChR heteromer, a receptor complex recently identified and characterized as the molecular entity that, in dopaminergic neurons, mediates the neurotrophic effects of nicotine. By means of a partially rigidified spacer of variable length, we incorporated in the new compounds (1a-c) the pharmacophoric substructure of a known β2-subunit-containing nAChR agonist (A-84543) and that of the D2/D3R agonist drug ropinirole. All the compounds retained the ability to bind with high affinity both β2-subunit-containing nAChR and D3R. Compound 1a, renamed HyNDA-1, which is characterized by the shortest linker moiety, was the most interesting ligand. We found, in fact, that HyNDA-1 significantly modulated structural plasticity on both mice and human dopaminergic neurons, an effect strongly prevented by co-incubating this ligand with either nAChR or D3R antagonists. Moreover, the neurotrophic effects of HyNDA-1 were specifically lost by disrupting the complex with specific interfering peptides. Interestingly, by using the Bioluminescence Resonance Energy Transfer 2 (BRET²) assay in HEK-293 transfected cells, we also found that HyNDA-1 has the ability to increase the affinity of interaction between nAChR and D3R. Overall, our results indicate that the neurotrophic effects of HyNDA-1 are mediated by activation of the D3R-nAChR heteromeric complex specifically expressed on dopaminergic neurons.

Association and cis-mQTL analysis of variants in serotonergic genes associated with nicotine dependence in Chinese Han smokers

Variants in serotonergic genes are implicated in nicotine dependence (ND) in subjects of European and African origin, but their involvement with smoking in Asians is largely unknown. Moreover, mechanisms underlying the ND risk-associated single-nucleotide polymorphisms (SNPs) in these genes are rarely investigated. The Fagerström Test for Nicotine Dependence (FTND) score was used to assess ND in 2616 male Chinese Han smokers. Both association and interaction analysis were used to examine the association of variants in the serotonergic genes with FTND. Further, expression and methylation quantitative trait loci (cis-mQTL) analysis was employed to determine the association of individual SNPs with the extent of methylation of each CpG locus. Individual SNP-based association analysis revealed that rs1176744 in HTR3B was marginally associated with FTND (p = 0.042). Haplotype-based association analysis found that one major haplotype, T-T-A-G, formed by SNPs rs3758987-rs4938056-rs1176744-rs2276305, located in the 5' region of HTR3B, showed a significant association with FTND (p = 0.00025). Further, a significant genetic interactive effect affecting ND was detected among SNPs rs10160548 in HTR3A, and rs3758987, rs2276305, and rs1672717 in HTR3B (p = 0.0074). Finally, we found four CpG sites (CpG_4543549, CpG_4543464, CpG_4543682, and CpG_4546888) to be significantly associated with three cis-mQTL SNPs (i.e., rs3758987, rs4938056, and rs1176744) located in our detected haplotype within HTR3B. In sum, we showed SNP rs1176744 (Tyr129Ser) to be associated with ND. Together with the SNPs rs3758987 and rs4938056 in HTR3B, they formed a major haplotype, which had significant association with ND. We further showed these SNPs contribute to ND through four methylated sites in HTR3B. All these findings suggest that variants in the serotonergic system play an important role in ND in the Chinese Han population. More importantly, these findings demonstrated that the involvement of this system in ND is through gene-by-gene interaction and methylation.

Positive allosteric modulation of M1 and M4 muscarinic receptors as potential therapeutic treatments for schizophrenia

Current antipsychotic drugs provide symptomatic relief for positive symptoms of schizophrenia, but do not offer symptom management for negative and cognitive symptoms. In addition, many patients discontinue treatment due to adverse side effects. Therefore, there is a critical need to develop more effective and safe treatment options. Although the etiology of schizophrenia is unclear, considerable data from post-mortem, neuroimaging and neuropharmacology studies support a role of the muscarinic acetylcholine (mAChRs) in the pathophysiology of schizophrenia. Substantial evidence suggests that activation of mAChRs has the potential to treat all symptom domains of schizophrenia. Despite encouraging results in demonstrating efficacy, clinical trials of nonselective mAChR agonists were limited in their clinical utility due to dose-limiting peripheral side effects. Accordingly, efforts have been made to specifically target centrally located M1 and M4 mAChR subtypes devoid of adverse-effect liability. To circumvent this limitation, there have been tremendous advances in the discovery of ligands that bind at allosteric sites, binding sites distinct from the orthosteric site, which are structurally less conserved and thereby afford high levels of receptor subtype selectivity. The discovery of subtype-specific allosteric modulators has greatly advanced our understanding of the physiological role of various muscarinic receptor subtypes in schizophrenia and the potential utility of M1 and M4 mAChR subtypes as targets for the development of novel treatments for schizophrenia and related disorders. This article is part of the Special Issue entitled 'Neuropharmacology on Muscarinic Receptors'.

Alcohol exposure disrupts mu opioid receptor-mediated long-term depression at insular cortex inputs to dorsolateral striatum

Drugs of abuse, including alcohol, ablate the expression of specific forms of long-term synaptic depression (LTD) at glutamatergic synapses in dorsal striatum (DS), a brain region involved in goal-directed and habitual behaviors. This loss of LTD is associated with altered DS-dependent behavior. Given the role of the µ-opioid receptor (MOR) in behavioral responding for alcohol, we explored the impact of alcohol on various forms of MOR-mediated synaptic depression that we find are differentially expressed at specific DS synapses. Corticostriatal MOR-mediated LTD (mOP-LTD) in the dorsolateral striatum occurs exclusively at inputs from anterior insular cortex and is selectively disrupted by in vivo alcohol exposure. Alcohol has no effect on corticostriatal mOP-LTD in dorsomedial striatum, thalamostriatal MOR-mediated short-term depression, or mOP-LTD of cholinergic interneuron-driven glutamate release. Disrupted mOP-LTD at anterior insular cortex-dorsolateral striatum synapses may therefore be a key mechanism of alcohol-induced neuroadaptations involved in the development of alcohol use disorders.

Nicotine facilitates synaptic depression in layer V pyramidal neurons of the mouse insular cortex

The insular cortex is known to play a pivotal role in addiction to nicotine. Long-term depression (LTD) in the central nervous system is a major form of synaptic plasticity which is involved in learning and memory and in various pathological conditions such as nicotine addiction. Until now, effects of nicotine on LTD were mainly examined in the hippocampus and striatum, and there is no report showing the effects of nicotine on LTD in the insular cortex. In the present study, I show for the first time that nicotine facilitates LTD which is induced by combination of presynaptic stimulation with postsynaptic depolarization (paired training) in layer 5 pyramidal neurons of the mouse insular cortex using whole-cell patch-clamp recordings. The facilitatory effect of nicotine on LTD was blocked by GABA_A receptor antagonists, bicuculline and picrotoxin. Furthermore, blockade of β2-containing nicotinic acetylcholine receptors (nAChRs) prevented the effects of nicotine on LTD. Taken together, these results suggest that in layer 5 pyramidal neurons of the insular cortex, nicotine facilitates LTD through enhancement of GABAergic synaptic transmission, presumably mediated by activation of β2-containing nAChRs. These findings may provide the crucial synaptic basis for the insular cortical changes in nicotine addiction.

Extended nicotine self-administration increases sensitivity to nicotine, motivation to seek nicotine and the reinforcing properties of nicotine-paired cues

An array of pharmacological and environmental factors influence the development and maintenance of tobacco addiction. The nature of these influences likely changes across the course of an extended smoking history, during which time drug seeking can become involuntary and uncontrolled. The present study used an animal model to examine the factors that drive nicotine-seeking behavior after either brief (10 days) or extended (40 days) self-administration training. In Experiment 1, extended training increased rats' sensitivity to nicotine, indicated by a leftward shift in the dose-response curve, and their motivation to work for nicotine, indicated by an increase in the break point achieved under a progressive ratio schedule. In Experiment 2, extended training imbued the nicotine-paired cue with the capacity to maintain responding to the same high level as nicotine itself. However, Experiment 3 showed that the mechanisms involved in responding for nicotine or a nicotine-paired cue are dissociable, as treatment with the partial nicotine receptor agonist, varenicline, suppressed responding for nicotine but potentiated responding for the nicotine-paired cue. Hence, across extended nicotine self-administration, pharmacological and environmental influences over nicotine seeking increase such that nicotine seeking is controlled by multiple sources, and therefore highly resistant to change.

Metabolic, synaptic and behavioral impact of 5-week chronic deep brain stimulation in hemiparkinsonian rats

The long-term effects and action mechanisms of subthalamic nucleus (STN) high-frequency stimulation (HFS) for Parkinson's disease still remain poorly characterized, mainly due to the lack of experimental models relevant to clinical application. To address this issue, we performed a multilevel study in freely moving hemiparkinsonian rats undergoing 5-week chronic STN HFS, using a portable constant-current microstimulator. In vivo metabolic neuroimaging by (1) H-magnetic resonance spectroscopy (11.7 T) showed that STN HFS normalized the tissue levels of the neurotransmission-related metabolites glutamate, glutamine and GABA in both the striatum and substantia nigra reticulata (SNr), which were significantly increased in hemiparkinsonian rats, but further decreased nigral GABA levels below control values; taurine levels, which were not affected in hemiparkinsonian rats, were significantly reduced. Slice electrophysiological recordings revealed that STN HFS was, uniquely among antiparkinsonian treatments, able to restore both forms of corticostriatal synaptic plasticity, i.e. long-term depression and potentiation, which were impaired in hemiparkinsonian rats. Behavior analysis (staircase test) showed a progressive recovery of motor skill during the stimulation period. Altogether, these data show that chronic STN HFS efficiently counteracts metabolic and synaptic defects due to dopaminergic lesion in both the striatum and SNr. Comparison of chronic STN HFS with acute and subchronic treatment further suggests that the long-term benefits of this treatment rely both on the maintenance of acute effects and on delayed actions on the basal ganglia network. We studied the effects of chronic (5 weeks) continuous subthalamic nucleus (STN) high-frequency stimulation (HFS) in hemiparkinsonian rats. The levels of glutamate and GABA in the striatum () and substantia nigra reticulata (SNr) (), measured by in vivo proton magnetic resonance spectroscopy ((1) H-MRS), were increased by 6-hydroxydopamine (6-OHDA) lesion, which also disrupted corticostriatal synaptic plasticity () and impaired forepaw skill () in the staircase test. Five-week STN HFS normalized glutamate and GABA levels and restored both synaptic plasticity and motor function. A partial behavioral recovery was observed at 2-week STN HFS.

Activation of Phosphatidylinositol-Linked Dopamine Receptors Induces a Facilitation of Glutamate-Mediated Synaptic Transmission in the Lateral Entorhinal Cortex

The lateral entorhinal cortex receives strong inputs from midbrain dopamine neurons that can modulate its sensory and mnemonic function. We have previously demonstrated that 1 µM dopamine facilitates synaptic transmission in layer II entorhinal cortex cells via activation of D1-like receptors, increased cAMP-PKA activity, and a resulting enhancement of AMPA-receptor mediated currents. The present study assessed the contribution of phosphatidylinositol (PI)-linked D1 receptors to the dopaminergic facilitation of transmission in layer II of the rat entorhinal cortex, and the involvement of phospholipase C activity and release of calcium from internal stores. Whole-cell patch-clamp recordings of glutamate-mediated evoked excitatory postsynaptic currents were obtained from pyramidal and fan cells. Activation of D1-like receptors using SKF38393, SKF83959, or 1 µM dopamine induced a reversible facilitation of EPSCs which was abolished by loading cells with either the phospholipase C inhibitor U-73122 or the Ca2+ chelator BAPTA. Neither the L-type voltage-gated Ca2+ channel blocker nifedipine, nor the L/N-type channel blocker cilnidipine, blocked the facilitation of synaptic currents. However, the facilitation was blocked by blocking Ca2+ release from internal stores via inositol 1,4,5-trisphosphate (InsP3) receptors or ryanodine receptors. Follow-up studies demonstrated that inhibiting CaMKII activity with KN-93 failed to block the facilitation, but that application of the protein kinase C inhibitor PKC(19-36) completely blocked the dopamine-induced facilitation. Overall, in addition to our previous report indicating a role for the cAMP-PKA pathway in dopamine-induced facilitation of synaptic transmission, we demonstrate here that the dopaminergic facilitation of synaptic responses in layer II entorhinal neurons also relies on a signaling cascade dependent on PI-linked D1 receptors, PLC, release of Ca2+ from internal stores, and PKC activation which is likely dependent upon both DAG and enhanced intracellular Ca2+. These signaling pathways may collaborate to enhance sensory and mnemonic function in the entorhinal cortex during tonic release of dopamine.

Age-contingent influence over accumbal neurotransmission and the locomotor stimulatory response to acute and repeated administration of nicotine in Wistar rats

Nicotine addiction is one of the leading contributors to the global burden of disease, and early onset smokers report a more severe addiction with lower chance of cessation than those with a late onset. Preclinical research supports an age-dependent component to the rewarding and reinforcing properties of nicotine, and the aim of this study was to define behavioral adaptations and changes in accumbal neurotransmission that arise over 15 days of intermittent nicotine treatment (0.36 mg/kg/day) in rats of three different ages (5 weeks, 10 weeks, 36 weeks old). Repeated treatment increased the locomotor stimulatory response to nicotine in all age groups, but significantly faster in the two younger groups. In addition, nicotine decreased rearing activity in a way that sustained even after repeated administration in aged rats but not in the younger age groups. Electrophysiological field potential recordings revealed a decline in input/output function in the nucleus accumbens (NAc) of animals intermittently treated with nicotine starting at 5 weeks of age, but not in older animals. In drug naïve rats, acute administration of nicotine modulated both accumbal dopamine output and excitatory transmission in a partially age-dependent manner. Fifteen days of intermittent nicotine treatment did not alter the acute effect displayed by nicotine on dopamine levels or evoked field potentials. The data presented here show that both acute and repeated nicotine administration modulates accumbal neurotransmission and behavior in an age-contingent manner and that these age-dependent differences could reflect important neurobiological underpinnings associated with the increased vulnerability for nicotine-addiction in adolescents.

Dopamine-dependent corticostriatal synaptic filtering regulates sensorimotor behavior

Modulation of corticostriatal synaptic activity by dopamine is required for normal sensorimotor behaviors. After loss of nigrostriatal dopamine axons in Parkinson's disease, l-3,4-dihydroxyphenlalanine (l-DOPA) and dopamine D2-like receptor agonists are used as replacement therapy, although these drugs also trigger sensitized sensorimotor responses including dyskinesias and impulse control disorders. In mice, we lesioned dopamine projections to the left dorsal striatum and assayed unilateral sensorimotor deficits with the corridor test as well as presynaptic corticostriatal activity with the synaptic vesicle probe, FM1-43. Sham-lesioned mice acquired food equivalently on both sides, while D2 receptor activation filtered the less active corticostriatal terminals, a response that required coincident co-activation of mGlu-R5 metabotropic glutamate and CB1 endocannabinoid receptors. Lesioned mice did not acquire food from their right, but overused that side following treatment with l-DOPA. Synaptic filtering on the lesioned side was abolished by either l-DOPA or a D2 receptor agonist, but when combined with a CB1 receptor antagonist, l-DOPA or D2 agonists normalized both synaptic filtering and behavior. Thus, high-pass filtering of corticostriatal synapses by the coordinated activation of D2, mGlu-R5, and CB1 receptors is required for normal sensorimotor response to environmental cues.

Chronic methylphenidate exposure during adolescence reduces striatal synaptic responses to ethanol

Dopamine (DA) plays an important role in integrative functions contributing to adaptive behaviors. In support of this essential function, DA modulates synaptic plasticity in different brain areas, including the striatum. Many drugs used for cognitive enhancement are psychostimulants, such as methylphenidate (MPH), which enhance DA levels. MPH treatment is of interest during adolescence, a period of enhanced neurodevelopment during which the DA system is in a state of flux. Recent epidemiological studies report the co-abuse of MPH and ethanol in adolescents and young adults. Although repeated MPH treatment produces enduring changes that affect subsequent behavioral responses to other psychostimulants, few studies have investigated the interactions between MPH and ethanol. Here we addressed whether chronic therapeutic exposure to MPH during adolescence predisposed mice to an altered response to ethanol and whether this was accompanied by altered DA release and striatal plasticity. C57BL/6J mice were administered MPH (3-6 mg/kg/day) via the drinking water between post-natal days 30 and 60. Voltammetry experiments showed that sufficient brain MPH concentrations were achieved during adolescence in mice to increase the DA clearance in adulthood. The treatment also increased long-term depression and reduced the effects of ethanol on striatal synaptic responses. Although the injection of 0.4 or 2 g/kg ethanol dose-dependently decreased locomotion in control mice, only the higher dose decreased locomotion in MPH-treated mice. These results suggested that the administration of MPH during development promoted long-term effects on synaptic plasticity in forebrain regions targeted by DA. These changes in plasticity might, in turn, underlie alterations in behaviors controlled by these brain regions into adulthood.

Gene-environment interactions affect long-term depression (LTD) through changes in dopamine receptor affinity in Snap25 deficient mice

Genes and environmental conditions interact in the development of cognitive capacities and each plays an important role in neuropsychiatric disorders such as attention deficit/hyperactivity disorder (ADHD) and schizophrenia. Multiple studies have indicated that the gene for the SNARE protein SNAP-25 is a candidate susceptibility gene for ADHD, as well as schizophrenia, while maternal smoking is a candidate environmental risk factor for ADHD. We utilized mice heterozygous for a Snap25 null allele and deficient in SNAP-25 expression to model genetic effects in combination with prenatal exposure to nicotine to explore genetic and environmental interactions in synaptic plasticity and behavior. We show that SNAP-25 deficient mice exposed to prenatal nicotine exhibit hyperactivity and deficits in social interaction. Using a high frequency stimulus electrophysiological paradigm for long-term depression (LTD) induction, we examined the roles of dopaminergic D2 receptors (D2Rs) and cannabinoid CB1 receptors (CB1Rs), both critical for LTD induction in the striatum. We found that prenatal exposure to nicotine in Snap25 heterozygote null mice produced a deficit in the D2R-dependent induction of LTD, although CB1R regulation of plasticity was not impaired. We also show that prenatal nicotine exposure altered the affinity and/or receptor coupling of D2Rs, but not the number of these receptors in heterozygote null Snap25 mutants. These results refine the observations made in the coloboma mouse mutant, a proposed mouse model of ADHD, and illustrate how gene×environmental influences can interact to perturb neural functions that regulate behavior.

Selective activation of cholinergic interneurons enhances accumbal phasic dopamine release: setting the tone for reward processing

Dopamine plays a critical role in motor control, addiction, and reward-seeking behaviors, and its release dynamics have traditionally been linked to changes in midbrain dopamine neuron activity. Here, we report that selective endogenous cholinergic activation achieved via in vitro optogenetic stimulation of nucleus accumbens, a terminal field of dopaminergic neurons, elicits real-time dopamine release. This mechanism occurs via direct actions on dopamine terminals, does not require changes in neuron firing within the midbrain, and is dependent on glutamatergic receptor activity. More importantly, we demonstrate that in vivo selective activation of cholinergic interneurons is sufficient to elicit dopamine release in the nucleus accumbens. Therefore, the control of accumbal extracellular dopamine levels by endogenous cholinergic activity results from a complex convergence of neurotransmitter/neuromodulator systems that may ultimately synergize to drive motivated behavior.

Modulation of endocannabinoid-mediated long-lasting disinhibition of striatal output by cholinergic interneurons

The frequency and duration of glutamatergic inputs to the striatum are strong determinants of the net effect of retrograde endocannabinoid (eCB) signaling, and key factors in determining if long-term depression (LTD) has a net disinhibitory or inhibitory action in striatum. Low to moderate frequency stimulation in the dorsolateral striatum elevates eCB levels to an extent that primarily depresses transmitter release at inhibitory synapses, leading to a long-lasting disinhibition (DLL) of synaptic output. The aim of this study was to further characterize the basic features of endocannabinoid-mediated DLL of striatal output induced by moderate frequency stimulation (5 Hz, 60 s). DLL was inhibited in slices treated with the group 1 metabotropic glutamate receptor (mGluR) antagonists MPEP (40 μM) and CPCCOEt (40 μM), the dopamine D2 receptor antagonist sulpiride (5 μM), the L-type calcium channel blocker nifedipine (20 μM), the nicotinic receptor antagonist mecamylamine (10 μM), the muscarinic agonist oxotremorine sesquifumarate (10 μM), and strychnine (0.1 μM). Strychnine did not block DLL induced by WIN55,212-2 (250 nM), showing that glycine receptor-mediated modulation of eCB signaling occurs upstream from CB(1)R activation. Scopolamine (10 μM) restored DLL in strychnine-treated slices, suggesting that inhibition of glycine receptors on cholinergic interneurons could modulate eCB signaling by enhancing muscarinic receptor activation and reducing the opening of L-type calcium channels in response to depolarization. These data suggests that similar activation points are required for stimulation-induced DLL as for LTD at excitatory striatal synapses, and that cholinergic interneurons are key modulators of stimulation-induced eCB signaling in the striatum.

Nicotinergic impact on focal and non-focal neuroplasticity induced by non-invasive brain stimulation in non-smoking humans

Nicotine improves cognitive performance and modulates neuroplasticity in brain networks. The neurophysiological mechanisms underlying nicotine-induced behavioral changes have been sparsely studied, especially in humans. Global cholinergic activation focuses on plasticity in humans. However, the specific contribution of nicotinic receptors to these effects is unclear. Henceforth, we explored the impact of nicotine on non-focal neuroplasticity induced by transcranial direct current stimulation (tDCS) and focal, synapse-specific plasticity induced by paired associative stimulation (PAS) in healthy non-smoking individuals. Forty-eight subjects participated in the study. Each subject received placebo and nicotine patches combined with one of the stimulation protocols to the primary motor cortex in different sessions. Transcranial magnetic stimulation (TMS)-elicited motor-evoked potential (MEP) amplitudes were recorded as a measure of corticospinal excitability until the evening of the second day following the stimulation. Nicotine abolished or reduced both PAS- and tDCS-induced inhibitory neuroplasticity. Non-focal facilitatory plasticity was also abolished, whereas focal facilitatory plasticity was slightly prolonged by nicotine. Thus, nicotinergic influence on facilitatory, but not inhibitory plasticity mimics that of global cholinergic enhancement. Therefore, activating nicotinic receptors has clearly discernable effects from global cholinergic activation. These nicotine-generated plasticity alterations might be important for the effects of the drug on cognitive function.

Spike-timing dependent plasticity in striatal interneurons

Basal ganglia, an ensemble of interconnected subcortical nuclei, are involved in adaptive motor planning and procedural learning. Striatum, the primary input nucleus of basal ganglia, extracts the pertinent cortical and thalamic information from background noise in relation with the environmental stimuli and motivation. The striatum comprises different neuronal populations: the GABAergic striatal output neurons, three classes of GABAergic interneurons and the cholinergic cells. Striatal interneurons exert a powerful control of striatal output neuron excitability and therefore shape the cortico-basal ganglia information processing. Besides output neurons, striatal interneurons also receive directly cortical information and are able to adapt their behavior depending on the level of cortical and striatal activation. In this review, we focus on the corticostriatal long-term synaptic efficacy changes occurring in interneurons, and especially the spike-timing dependent plasticity (STDP), as a Hebbian synaptic learning rule. Combined with the striatal local interactions between interneurons and output neurons, we will consider the functional consequences of the interneuron plasticity on the striatal output. This article is part of a Special Issue entitled 'Synaptic Plasticity & Interneurons'.

Intermittent ethanol consumption depresses endocannabinoid-signaling in the dorsolateral striatum of rat

Recent research suggests that adaptations elicited by drugs of abuse share common features with traditional learning models, and that drugs of abuse cause long-term changes in behavior by altering synaptic function and plasticity. In this study, endocannabinoid (eCB) signaling in the dorsolateral striatum, a brain region vital for habit formation, was evaluated in acutely isolated brain slices from ethanol (EtOH)-consuming rats and control rats. EtOH-consuming rats had free access to a 20% EtOH solution for three 24 hour sessions a week during seven weeks and consumed an average of 3.4 g/kg per session. eCB-mediated long-lasting disinhibition (DLL) of population spike (PS) amplitude induced by moderate frequency stimulation was impaired in EtOH-consuming rats, and was not restored by the muscarinic receptor antagonist scopolamine (10 μM). The lack of DLL could be linked to a reduced GABA(A) receptor tone, since bicuculline-mediated disinhibition of striatal output was significantly reduced in slices from EtOH-consuming rats. However, eCB signaling induced by high frequency stimulation (HFS) was also impaired in slices from EtOH-consuming rats and isolated control rats. Activation of presynaptic cannabinoid 1 receptors (CB1R) with WIN55,212-2 (250 nM, 1 μM) significantly modulated PS amplitude in slices from age-matched control rats while slices from EtOH-consuming rats remained unaffected, indicating that eCB signaling is inhibited at a level that is downstream from CB1R activation. Intermittent alcohol intake for seven weeks might thus be sufficient to modulate a presynaptic mechanism that needs to be synergized with CB1R activation for induction of long-term depression (LTD). In conclusion, alcohol consumption inhibits striatal eCB signaling in a way that could be of importance for understanding the neurological underpinnings of addictive behavior.

Transient receptor potential vanilloid 1 channels control acetylcholine/2-arachidonoylglicerol coupling in the striatum

The neurotransmitter acetylcholine (Ach) controls both excitatory and inhibitory synaptic transmission in the striatum. Here, we investigated the involvement of the endocannabinoid system in Ach-mediated inhibition of striatal GABA transmission, and the potential role of transient receptor potential vanilloid 1 (TRPV1) channels in the control of Ach-endocannabinoid coupling. We found that inhibition of Ach degradation and direct pharmacological stimulation of muscarinic M1 receptors reduced striatal inhibitory postsynaptic currents (IPSCs) through the stimulation of 2-arachidonoylglicerol (2AG) synthesis and the activation of cannabinoid CB1 receptors. The effects of M1 receptor activation on IPSCs were occlusive with those of metabotropic glutamate receptor 5 stimulation, and were prevented in the presence of capsaicin, agonist of TRPV1 channels. Elevation of anandamide (AEA) tone with URB597, a blocker of fatty acid amide hydrolase, mimicked the effects of capsaicin, indicating that endogenous AEA acts as an endovanilloid substance in the control of M1-dependent 2AG-mediated synaptic effects in the striatum. Accordingly, both capsaicin and URB597 effects were absent in mice lacking TRPV1 channels. Pharmacological interventions targeting AEA metabolism and TRPV1 channels might be considered alternative therapeutic routes in disorders of striatal cholinergic or endocannabinoid neurotransmission.

Neurotransmitter roles in synaptic modulation, plasticity and learning in the dorsal striatum

The dorsal striatum is a large forebrain region involved in action initiation, timing, control, learning and memory. Learning and remembering skilled movement sequences requires the dorsal striatum, and striatal subregions participate in both goal-directed (action-outcome) and habitual (stimulus-response) learning. Modulation of synaptic transmission plays a large part in controlling input to as well as the output from striatal medium spiny projection neurons (MSNs). Synapses in this brain region are subject to short-term modulation, including allosteric alterations in ion channel function and prominent presynaptic inhibition. Two forms of long-term synaptic plasticity have also been observed in striatum, long-term potentiation (LTP) and long-term depression (LTD). LTP at glutamatergic synapses onto MSNs involves activation of NMDA-type glutamate receptors and D1 dopamine or A2A adenosine receptors. Expression of LTP appears to involve postsynaptic mechanisms. LTD at glutamatergic synapses involves retrograde endocannabinoid signaling stimulated by activation of metabotropic glutamate receptors (mGluRs) and D2 dopamine receptors. While postsynaptic mechanisms participate in LTD induction, maintained expression involves presynaptic mechanisms. A similar form of LTD has also been observed at GABAergic synapses onto MSNs. Studies have just begun to examine the roles of synaptic plasticity in striatal-based learning. Findings to date indicate that molecules implicated in induction of plasticity participate in these forms of learning. Neurotransmitter receptors involved in LTP induction are necessary for proper skill and goal-directed instrumental learning. Interestingly, receptors involved in LTP and LTD at glutamatergic synapses onto MSNs of the "indirect pathway" appear to have important roles in habit learning. More work is needed to reveal if and when synaptic plasticity occurs during learning and if so what molecules and cellular processes, both short- and long-term, contribute to this plasticity.

Nicotine normalizes intracellular subunit stoichiometry of nicotinic receptors carrying mutations linked to autosomal dominant nocturnal frontal lobe epilepsy

Autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE) is linked with high penetrance to several distinct nicotinic receptor (nAChR) mutations. We studied (alpha4)(3)(beta2)(2) versus (alpha4)(2)(beta2)(3) subunit stoichiometry for five channel-lining M2 domain mutations: S247F, S252L, 776ins3 in alpha4, V287L, and V287M in beta2. alpha4 and beta2 subunits were constructed with all possible combinations of mutant and wild-type (WT) M2 regions, of cyan and yellow fluorescent protein, and of fluorescent and nonfluorescent M3-M4 loops. Sixteen fluorescent subunit combinations were expressed in N2a cells. Förster resonance energy transfer (FRET) was analyzed by donor recovery after acceptor photobleaching and by pixel-by-pixel sensitized emission, with confirmation by fluorescence intensity ratios. Because FRET efficiency is much greater for adjacent than for nonadjacent subunits and the alpha4 and beta2 subunits occupy specific positions in nAChR pentamers, observed FRET efficiencies from (alpha4)(3)(beta2)(2) carrying fluorescent alpha4 subunits were significantly higher than for (alpha4)(2)(beta2)(3); the converse was found for fluorescent beta2 subunits. All tested ADNFLE mutants produced 10 to 20% increments in the percentage of intracellular (alpha4)(3)(beta2)(2) receptors compared with WT subunits. In contrast, 24- to 48-h nicotine (1 muM) exposure increased the proportion of (alpha4)(2)(beta2)(3) in WT receptors and also returned subunit stoichiometry to WT levels for alpha4S248F and beta2V287L nAChRs. These observations may be relevant to the decreased seizure frequency in patients with ADNFLE who use tobacco products or nicotine patches. Fluorescence-based investigations of nAChR subunit stoichiometry may provide efficient drug discovery methods for nicotine addiction or for other disorders that result from dysregulated nAChRs.

Double dissociation of pharmacologically induced deficits in visual recognition and visual discrimination learning

Monkeys trained in either one-trial recognition at 8- to 10-min delays or multi-trial discrimination habits with 24-h intertrial intervals received systemic cholinergic and dopaminergic antagonists, scopolamine and haloperidol, respectively, in separate sessions. Recognition memory was impaired markedly by scopolamine but not at all by haloperidol, whereas habit formation was impaired markedly by haloperidol but only minimally by scopolamine. These differential drug effects point to differences in synaptic modification induced by the two neuromodulators that parallel the contrasting properties of the two types of learning, namely, fast acquisition but weak retention of memories versus slow acquisition but durable retention of habits.

Dopamine D1 receptors and group I metabotropic glutamate receptors contribute to the induction of long-term potentiation in the nucleus accumbens

Long-term changes in the efficacy of glutamatergic synaptic transmission in the striatal complex are proposed to underlie motor learning and neuroadaptations leading to addiction. Dopamine and glutamate play key roles in the induction of long-term potentiation (LTP) and long-term depression (LTD) in the dorsal striatum, but their contribution to synaptic plasticity in the ventral striatum (nucleus accumbens, NAc) has been less extensively studied. We have examined the role of dopamine, glutamate and GABA in the induction of LTP in mouse brain slices containing the NAc. High-frequency stimulation of glutamatergic inputs elicited LTP of field excitatory postsynaptic potentials/population spikes (fEPSP/PSs) in the core region of the NAc. GABA did not seem to participate in LTP induction because LTP was not altered in the presence of either a GABA(A)- (bicuculline) or a GABA(B)- (CGP 55845) receptor antagonist. However, the dopamine D1 receptor antagonist SCH 23390, but not the dopamine D2 receptor antagonist sulpiride, impaired LTP. The dopamine reuptake blocker nomifensine also inhibited LTP induction. We found that group I metabotropic glutamate receptors (mGluRs) contribute to LTP induction because the mGluR1 antagonist LY 367385, or the mGluR5 antagonist MPEP, blocked LTP induction. Furthermore, the glutamate reuptake blocker DL-TBOA also impaired LTP. The present results demonstrate that dopamine and glutamate play critical roles in the mechanisms of induction of LTP in the NAc through the activation of dopamine D1 receptors and group I mGluRs. However, LTP is negatively regulated when endogenous levels of dopamine or glutamate are elevated.

Antimuscarinics for the treatment of overactive bladder: a review of central nervous system effects

Antimuscarinic drugs commonly used to treat overactive bladder are often associated with central nervous system (CNS) side effects including cognitive dysfunction, memory impairment, dizziness, fatigue, and headache. New agents show reduced CNS penetrance and better selectivity for the M3 muscarinic receptor. However, changes associated with aging may lead to alterations in blood-brain barrier permeability. Therefore, use of antimuscarinics in the elderly or in patients with Alzheimer's disease presents a significant challenge. This review highlights muscarinic receptor distribution and function in the CNS, provides a description and incidence of CNS side effects with therapy, offers information specific to currently available agents, and describes the use of antimuscarinics in special populations including children, the elderly, and patients with Alzheimer's disease.