Nicotine enhancement of dopamine release by a calcium-dependent increase in the size of the readily releasable pool of synaptic vesicles

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.

Role of voltage-dependent calcium channels on the striatal in vivo dopamine release induced by the organophosphorus pesticide glyphosate

In the present study, we investigated the role of voltage-sensitive calcium channels (VSCCs) on the striatal dopamine release induced by the pesticide glyphosate (GLY) using selective VSCC inhibitors. The dopamine levels were measured by in vivo cerebral microdialysis coupled to HPLC-ED. Nicardipine (L-type VSCC antagonist) or ω-conotoxin MVIIC (non-selective P/Q-type antagonist) had no effect on dopamine release induced by 5 mM GLY. In contrast, flunarizine (T-type antagonist) or ω-conotoxin GVIA (neuronal N-type antagonist) significantly reduced GLY-stimulated dopamine release. These results suggest that GLY-induced dopamine release depends on extracellular calcium and its influx through the T- and N-type VSCCs. These findings were corroborated by molecular docking, which allowed us to establish a correlation between the effect of GLY on blocked VSCC with the observed dopamine release. We propose new molecular targets of GLY in the dorsal striatum, which could have important implications for the assessment of pesticide risks in non-target organisms.

Correlation of SV2C rs1423099 single nucleotide polymorphism with sporadic Parkinson's disease in Han population in Southern China

BACKGROUND

The synaptic vesicle glycoprotein 2 (SV2) has been implicated in synaptic function throughout the brain. Accumulating evidence investigated that SV2C contributed to dopamine release and the disrupted expression of SV2C was considered to be a unique feature of PD that may facilitate dopaminergic neuron dysfunction.

OBJECTIVE

This study aimed to examine the relationship between the SV2C rs1423099 single nucleotide polymorphism and sporadic Parkinson's disease (PD) in the Chinese Han population.

MATERIALS AND METHODS

This study enrolled 351 patients with sporadic PD and 240 normal controls in Chinese Han population. Peripheral blood DNA was extracted by DNA extraction kits and the rs1423099 genotype was analyzed by Agena MassARRAY DNA mass spectrometry. The differences in genotype and allele distribution frequencies between PD patients and control groups were compared using chi-squared tests or Fisher's exact tests.

RESULTS

No statistical difference was revealed in age and sex distribution between the cases and control groups, and the distribution of genotype and allele frequencies was consistent with the Hardy-Weinberg equilibrium test. In SV2C rs1423099 dominant model, the frequency of the CC/CT genotype was significantly higher in the PD group compared to the control group (OR = 4.065,95% CI: 2.801-10.870, p = 0.002). Nevertheless, in the recessive model, CC or CT/TT genotypes have no statistical difference in the two groups (p = 0.09). Additionally, in allelic analysis, the C allele was investigated to increase the risk of PD (OR = 1.346, 95% CI: 1.036-1.745, p = 0.026); Furthermore, subgroup analysis suggested that those carrying the C allele in the male subgroup were at a higher risk to afflicted with PD (OR = 1.637, 95% CI: 1.147-2.336, p = 0.006).

CONCLUSION

SV2C rs1423099 single nucleotide polymorphism was associated with sporadic Parkinson's disease in the Chinese Han population, particularly in males.

Axonal α7* nicotinic acetylcholine receptors modulate glutamatergic signaling and synaptic vesicle organization in ventral hippocampal projections

Modulation of the release of glutamate by activation of presynaptic nicotinic acetylcholine receptors (nAChRs) is one of the most prevalent mechanism of nicotinic facilitation of glutamatergic transmission in cortico-limbic circuits. By imaging gene chimeric co-cultures from mouse, we examined the role of α7* nAChRs mediated cholinergic modulation of glutamate release and synaptic vesicle organization in ventral hippocampal projections. We directly visualized exogenous and endogenous cholinergic facilitation of glutamate release in this specialized preparation of circuits in vitro. Disrupting α7* nAChRs mediated cholinergic signaling genetically or pharmacologically diminished cholinergic facilitation of glutamate release at presynaptic terminals. Alteration of α7* nAChRs mediated cholinergic signaling along glutamatergic axons also decreased functional synaptic vesicle clustering to presynaptic terminals. These findings suggest that presynaptic α7* nAChRs contribute to cholinergic modulation of glutamate release and synaptic vesicle organization.

Cocaine increases quantal norepinephrine secretion through NET-dependent PKC activation in locus coeruleus neurons

The norepinephrine neurons in locus coeruleus (LC-NE neurons) are essential for sleep arousal, pain sensation, and cocaine addiction. According to previous studies, cocaine increases NE overflow (the profile of extracellular NE level in response to stimulation) by blocking the NE reuptake. NE overflow is determined by NE release via exocytosis and reuptake through NE transporter (NET). However, whether cocaine directly affects vesicular NE release has not been directly tested. By recording quantal NE release from LC-NE neurons, we report that cocaine directly increases the frequency of quantal NE release through regulation of NET and downstream protein kinase C (PKC) signaling, and this facilitation of NE release modulates the activity of LC-NE neurons and cocaine-induced stimulant behavior. Thus, these findings expand the repertoire of mechanisms underlying the effects of cocaine on NE (pro-release and anti-reuptake), demonstrate NET as a release enhancer in LC-NE neurons, and provide potential sites for treatment of cocaine addiction.

Activation of β3-adrenoceptor increases the number of readily releasable glutamatergic vesicle via activating Ca2+/calmodulin/MLCK/myosin II pathway in the prefrontal cortex of juvenile rats

It is well known that β3-adrenoceptor (β3-AR) in many brain structures including prefrontal cortex (PFC) is involved in stress-related behavioral changes. SR58611A, a brain-penetrant β3-AR subtypes agonist, is revealed to exhibit anxiolytic- and antidepressant-like effects. Whereas activation of β3-AR exerts beneficial effects on cognitive function, the underlying cellular and molecular mechanisms have not been fully determined. In this study, whole cell patch-clamp recordings were employed to investigate the glutamatergic transmission of layer V/VI pyramidal cells in slices of the rat PFC. Our result demonstrated that SR58611A increased AMPA receptor-mediated excitatory postsynaptic currents (AMPAR-EPSCs) through activating pre-synaptic β3-AR. SR58611A enhanced the miniature EPSCs (mEPSCs) and reduced paired-pulse ratio (PPR) of AMPAR-EPSCs suggesting that SR58611A augments pre-synaptic glutamate release. SR58611A increased the number of readily releasable vesicle (N) and release probability (Pr) with no effects on the rate of recovery from vesicle depletion. Influx of Ca²⁺ through L-type Ca²⁺ channel contributed to SR58611A-mediated enhancement of glutamatergic transmission. We also found that calmodulin, myosin light chain kinase (MLCK) and myosin II were involved in SR58611A-mediated augmentation of glutamate release. Our current data suggest that SR58611A enhances glutamate release by the Ca²⁺/calmodulin/MLCK/myosin II pathway.

Endocannabinoid Modulation of Nucleus Accumbens Microcircuitry and Terminal Dopamine Release

The nucleus accumbens (NAc) is located in the ventromedial portion of the striatum and is vital to valence-based predictions and motivated action. The neural architecture of the NAc allows for complex interactions between various cell types that filter incoming and outgoing information. Dopamine (DA) input serves a crucial role in modulating NAc function, but the mechanisms that control terminal DA release and its effect on NAc neurons continues to be elucidated. The endocannabinoid (eCB) system has emerged as an important filter of neural circuitry within the NAc that locally shapes terminal DA release through various cell type- and site-specific actions. Here, we will discuss how eCB signaling modulates terminal DA release by shaping the activity patterns of NAc neurons and their afferent inputs. We then discuss recent technological advancements that are capable of dissecting how distinct cell types, their afferent projections, and local neuromodulators influence valence-based actions.

Relationships and Interactions between Ionotropic Glutamate Receptors and Nicotinic Receptors in the CNS

Although ionotropic glutamate receptors and nicotinic receptors for acetylcholine (ACh) have usually been studied separately, they are often co-localized and functionally inter-dependent. The objective of this review is to survey the evidence for interactions between the two receptor families and the mechanisms underlying them. These include the mutual regulation of subunit expression, which change the NMDA:AMPA response balance, and the existence of multi-functional receptor complexes which make it difficult to distinguish between individual receptor sites, especially in vivo. This is followed by analysis of the functional relationships between the receptors from work on transmitter release, cellular electrophysiology and aspects of behavior where these can contribute to understanding receptor interactions. It is clear that nicotinic receptors (nAChRs) on axonal terminals directly regulate the release of glutamate and other neurotransmitters, α7-nAChRs generally promoting release. Hence, α7-nAChR responses will be prevented not only by a nicotinic antagonist, but also by compounds blocking the indirectly activated glutamate receptors. This accounts for the apparent anticholinergic activity of some glutamate antagonists, including the endogenous antagonist kynurenic acid. The activation of presynaptic nAChRs is by the ambient levels of ACh released from pre-terminal synapses, varicosities and glial cells, acting as a 'volume neurotransmitter' on synaptic and extrasynaptic sites. In addition, ACh and glutamate are released as CNS co-transmitters, including 'cholinergic' synapses onto spinal Renshaw cells. It is concluded that ACh should be viewed primarily as a modulator of glutamatergic neurotransmission by regulating the release of glutamate presynaptically, and the location, subunit composition, subtype balance and sensitivity of glutamate receptors, and not primarily as a classical fast neurotransmitter. These conclusions and caveats should aid clarification of the sites of action of glutamate and nicotinic receptor ligands in the search for new centrally-acting drugs.

Spatial and temporal scales of dopamine transmission

Dopamine is a prototypical neuromodulator that controls circuit function through G protein-coupled receptor signalling. Neuromodulators are volume transmitters, with release followed by diffusion for widespread receptor activation on many target cells. Yet, we are only beginning to understand the specific organization of dopamine transmission in space and time. Although some roles of dopamine are mediated by slow and diffuse signalling, recent studies suggest that certain dopamine functions necessitate spatiotemporal precision. Here, we review the literature describing dopamine signalling in the striatum, including its release mechanisms and receptor organization. We then propose the domain-overlap model, in which release and receptors are arranged relative to one another in micrometre-scale structures. This architecture is different from both point-to-point synaptic transmission and the widespread organization that is often proposed for neuromodulation. It enables the activation of receptor subsets that are within micrometre-scale domains of release sites during baseline activity and broader receptor activation with domain overlap when firing is synchronized across dopamine neuron populations. This signalling structure, together with the properties of dopamine release, may explain how switches in firing modes support broad and dynamic roles for dopamine and may lead to distinct pathway modulation.

The effect of acute and chronic nicotine consumption on intra-cortical inhibition and facilitation: A transcranial magnetic stimulation study

OBJECTIVE

The aim of the present study was to explore the impact of acute and chronic nicotine consumption on measures of intracortical inhibition and facilitation.

METHODS

This study involved 50 chronic heavy cigarette smokers and 40 healthy subjects matched for age, sex and educational level, with no history of chronic nicotine intake. Intracortical inhibition and facilitation were assessed using transcranial magnetic stimulation (TMS) measures of motor threshold (MT), short- and long-interval intra-cortical inhibition (SICI, LICI), cortical silent period (CSP) and intra-cortical facilitation (ICF). Basal serum levels of cotinine were measured in the healthy group and at ½ and 2 h after smoking a single cigarette in the chronic smokers.

RESULTS

There was enhanced SICI and reduced ICF in smokers (independent of time after smoking) compared with non-smokers. The former suggests a chronic effect of increased nicotine levels on GABA-A neurotransmission whereas the latter suggests an additional effect on glutamatergic transmission. There were no significant differences between smokers and non-smokers in other TMS parameters. There was a significant negative correlation between cotinine levels at ½ h after smoking and SICI at 3 ms ISI (P < 0.001). There were no significant differences in any of the neurophysiological measures between smokers at ½ h versus 2 h after smoking a single cigarette.

CONCLUSION

Chronic nicotine consumption enhances SICI, and reduces ICF, supporting the hypothesis that nicotine acts as a neuromodulator of GABA-A and glutamate neurotransmission.

Nicotinic acetylcholine receptors regulate clustering, fusion and acidification of the rat brain synaptic vesicles

The brain nicotinic acetylcholine receptors (nAChRs) expressed in pre-synaptic nerve terminals regulate neurotransmitter release. However, there is no evidence for the expression of nAChRs in synaptic vesicles, which deliver neurotransmitter to synaptic cleft. The aim of this paper was to investigate the presence of nAChRs in synaptic vesicles purified from the rat brain and to study their possible involvement in vesicles life cycle. According to dynamic light scattering analysis, the antibody against extracellular domain (1-208) of α7 nAChR subunit inhibited synaptic vesicles clustering. Sandwich ELISA with nAChR subunit-specific antibodies demonstrated the presence of α4β2, α7 and α7β2nAChR subtypes in synaptic vesicles and showed that α7 and β2 nAChR subunits are co-localized with synaptic vesicle glycoprotein 2A (SV2A). Pre-incubation with either α7-selective agonist PNU282987 or nicotine did not affect synaptic vesicles clustering but delayed their Ca2+-dependent fusion with the plasma membranes. In contrast, nicotine but not PNU282987 stimulated acidification of isolated synaptic vesicles, indicating that α4β2 but not α7-containing nAChRs are involved in regulation of proton influx and neurotransmitter refilling. Treatment of rats with levetiracetam, a specific modulator of SV2A, increased the content of α7 nAChRs in synaptic vesicles accompanied by increased clustering but decreased Ca2+-dependent fusion. These data for the first time demonstrate the presence of nAChRs in synaptic vesicles and suggest an active involvement of cholinergic regulation in neurotransmitter release. Synaptic vesicles may be an additional target of nicotine inhaled upon smoking and of α7-specific drugs widely discussed as anti-inflammatory and pro-cognitive tools.

Direct dopamine terminal regulation by local striatal microcircuitry

Regulation of axonal dopamine release by local microcircuitry is at the hub of several biological processes that govern the timing and magnitude of signaling events in reward-related brain regions. An important characteristic of dopamine release from axon terminals in the striatum is that it is rapidly modulated by local regulatory mechanisms. These processes can occur via homosynaptic mechanisms-such as presynaptic dopamine autoreceptors and dopamine transporters - as well heterosynaptic mechanisms such as retrograde signaling from postsynaptic cholinergic and dynorphin systems, among others. Additionally, modulation of dopamine release via diffusible messengers, such as nitric oxide and hydrogen peroxide, allows for various metabolic factors to quickly and efficiently regulate dopamine release and subsequent signaling. Here we review how these mechanisms work in concert to influence the timing and magnitude of striatal dopamine signaling, independent of action potential activity at the level of dopaminergic cell bodies in the midbrain, thereby providing a parallel pathway by which dopamine can be modulated. Understanding the complexities of local regulation of dopamine signaling is required for building comprehensive frameworks of how activity throughout the dopamine system is integrated to drive signaling and control behavior.

Does kynurenic acid act on nicotinic receptors? An assessment of the evidence

As a major metabolite of kynurenine in the oxidative metabolism of tryptophan, kynurenic acid is of considerable biological and clinical importance as an endogenous antagonist of glutamate in the central nervous system. It is most active as an antagonist at receptors sensitive to N-methyl-D-aspartate (NMDA) which regulate neuronal excitability and plasticity, brain development and behaviour. It is also thought to play a causative role in hypo-glutamatergic conditions such as schizophrenia, and a protective role in several neurodegenerative disorders, notably Huntington's disease. An additional hypothesis, that kynurenic acid could block nicotinic receptors for acetylcholine in the central nervous system has been proposed as an alternative mechanism of action of kynurenate. However, the evidence for this alternative mechanism is highly controversial, partly because at least eight earlier studies concluded that kynurenic acid blocked NMDA receptors but not nicotinic receptors and five subsequent, independent studies designed to repeat the results have failed to do so. Many studies considered to support the alternative 'nicotinic' hypothesis have been based on the use of analogs of kynurenate such as 7-chloro-kynurenic acid, or putatively nicotinic modulators such as galantamine, but a detailed analysis of the pharmacology of these compounds suggests that the results have often been misinterpreted, especially since the pharmacology of galantamine itself has been disputed. This review examines the evidence in detail, with the conclusion that there is no confirmed, reliable evidence for an antagonist activity of kynurenic acid at nicotinic receptors. Therefore, since there is overwhelming evidence for kynurenate acting at ionotropic glutamate receptors, especially NMDAR glutamate and glycine sites, with some activity at GPR35 sites and Aryl Hydrocarbon Receptors, results with kynurenic acid should be interpreted only in terms of these confirmed sites of action.

Electrochemical Measurements of Acetylcholine-Stimulated Dopamine Release in Adult Drosophila melanogaster Brains

The fruit fly, Drosophila melanogaster, is a popular model organism for studying neurological processes and diseases due to the availability of sophisticated genetic tools. While endogenous neurotransmitter release has been characterized in Drosophila larvae, here, we measured endogenous dopamine release in isolated adult Drosophila brains for the first time. Dopamine was measured with fast-scan cyclic voltammetry (FSCV), and acetylcholine or nicotine were used as the stimulus, as both interact with nicotinic acetylcholine receptors (nAChRs) to evoke endogenous dopamine release. Stimulations with 10 pmol of acetylcholine elicited 0.26 ± 0.05 μM dopamine, while 70 fmol nicotine stimulations evoked 0.29 ± 0.03 μM in the central complex. Nicotine-stimulated dopamine release lasted much longer than acetylcholine-stimulated release. Dopamine release is reduced in the presence of nAChR antagonist α-bungarotoxin and the sodium channel blocker tetrodotoxin, indicating release is mediated by nAChRs and exocytosis. The identity of dopamine was confirmed by using 3-iodotyrosine, a dopamine synthesis inhibitor, and by confirming that release was not changed in octopamine synthesis mutant flies, Tdc2 RO54. Additionally, the half-decay time ( t50) in fumin (67 ± 15 s), dopamine transporter mutant flies, was larger than in wild-type flies (16 ± 3.7 s) further proving that acetylcholine stimulation evokes dopamine release. This study demonstrates that stimulation of nAChRs can be used to elicit endogenous dopamine release in adult fly brains, which will be a useful technique for future studies probing dopamine changes during aging or in neurodegenerative diseases.

Computational modeling and biomarker studies of pharmacological treatment of Alzheimer's disease (Review)

Alzheimer's disease (AD) is a complex and multifactorial disease. In order to understand the genetic influence in the progression of AD, and to identify novel pharmaceutical agents and their associated targets, the present study discusses computational modeling and biomarker evaluation approaches. Based on mechanistic signaling pathway approaches, various computational models, including biochemical and morphological models, are discussed to explore the strategies that may be used to target AD treatment. Different biomarkers are interpreted on the basis of morphological and functional features of amyloid β plaques and unstable microtubule‑associated tau protein, which is involved in neurodegeneration. Furthermore, imaging and cerebrospinal fluids are also considered to be key methods in the identification of novel markers for AD. In conclusion, the present study reviews various biochemical and morphological computational models and biomarkers to interpret novel targets and agonists for the treatment of AD. This review also highlights several therapeutic targets and their associated signaling pathways in AD, which may have potential to be used in the development of novel pharmacological agents for the treatment of patients with AD. Computational modeling approaches may aid the quest for the development of AD treatments with enhanced therapeutic efficacy and reduced toxicity.

Neuregulin 3 Signaling Mediates Nicotine-Dependent Synaptic Plasticity in the Orbitofrontal Cortex and Cognition

Neuregulin 3 (NRG3) and ErbB4 have been linked to nicotine addiction; however, the neuronal mechanisms and behavioral consequences of NRG3-ErbB4 sensitivity to nicotine remain elusive. Recent literature suggests that relapse to smoking is due to a lack of impulsive control, which is thought to be due to altered functioning within the orbitofrontal cortex (OFC). Therefore, we examined circuitry changes within this structure following nicotine application. We report that nicotine controls synaptic plasticity in the OFC through NRG3/ErbB4-dependent regulation of GABAergic inhibition. We observed that both nicotine and NRG3 facilitated the conversion of long-term potentiation into long-term depression at cortical layer 3/5 synapses. Induction of long-term depression by nicotine relied on nicotinic receptor activation and key regulators of NRG3 signaling: (1) release of intracellular calcium, (2) activation of the BACE1 beta-secretase, and (3) ErbB4 receptor activation. Nicotine-induced synaptic plasticity was also associated with accumulation of intracellular GABA and was completely blocked by GABAA/GABAB antagonists. To test whether these mechanisms underlie OFC-dependent behavior, we evaluated the effects of nicotine in the go/no-go task. Nicotine-impaired stimulus discrimination in this task was rescued by pharmacologic disruption of the NRG3 receptor, ErbB4. Altogether, our data indicate that nicotine-induced synaptic plasticity in the OFC and cognitive changes depend on NRG3-ErbB4 signaling. We propose that nicotine activation of this pathway may contribute to nicotine addiction, particularly in individuals with genetic variation in NRG3.

To Do or Not to Do: Dopamine, Affordability and the Economics of Opportunity

Five years ago, we introduced the thrift hypothesis of dopamine (DA), suggesting that the primary role of DA in adaptive behavior is regulating behavioral energy expenditure to match the prevailing economic conditions of the environment. Here we elaborate that hypothesis with several new ideas. First, we introduce the concept of affordability, suggesting that costs must necessarily be evaluated with respect to the availability of resources to the organism, which computes a value not only for the potential reward opportunity, but also the value of resources expended. Placing both costs and benefits within the context of the larger economy in which the animal is functioning requires consideration of the different timescales against which to compute resource availability, or average reward rate. Appropriate windows of computation for tracking resources requires corresponding neural substrates that operate on these different timescales. In discussing temporal patterns of DA signaling, we focus on a neglected form of DA plasticity and adaptation, changes in the physical substrate of the DA system itself, such as up- and down-regulation of receptors or release probability. We argue that changes in the DA substrate itself fundamentally alter its computational function, which we propose mediates adaptations to longer temporal horizons and economic conditions. In developing our hypothesis, we focus on DA D2 receptors (D2R), arguing that D2R implements a form of “cost control” in response to the environmental economy, serving as the “brain’s comptroller”. We propose that the balance between the direct and indirect pathway, regulated by relative expression of D1 and D2 DA receptors, implements affordability. Finally, as we review data, we discuss limitations in current approaches that impede fully investigating the proposed hypothesis and highlight alternative, more semi-naturalistic strategies more conducive to neuroeconomic investigations on the role of DA in adaptive behavior.

Synaptic vesicle 2C and its synaptic-related function

Synaptic vesicle 2 C (SV2C) is a novel isoform belonging to the synaptic vesicle 2 (SV2) protein superfamily; a family of proteins known to have roles in vesicle trafficking, exocytosis and neurotransmission. In humans, SV2C is expressed in evolutionarily older brain regions, and is a known receptor for botulinum neurotoxin/A (BoNT/A), controlling glucose-evoked granule recruitment and regulating dopamine release, thus serving as a potential target molecule in the treatment of psychosis. In addition, recent researches have shown that SV2C regulates hypertension and accelerates venous thromboembolism (VTE) and coagulation pathways and may play roles in several non-nervous system diseases. In terms of regulation, SV2C is positively regulated by both alendronate and statins. As SV2C may provide a potential novel therapeutic target for psychosis and other diseases, this article reviews the progress made thus far in understanding the structure, distribution, function and regulation of SV2C.

Rapid signalling in distinct dopaminergic axons during locomotion and reward

Dopaminergic projections from the midbrain to striatum are critical for motor control, as their degeneration in Parkinson’s disease results in profound movement deficits. Paradoxically, most recording methods report rapid phasic dopamine signaling (~100ms bursts) to unpredicted rewards, with little evidence for movement-related signaling. The leading model posits that phasic signaling in striatum targeting dopamine neurons drive reward-based learning, while slow variations in firing (tens of seconds to minutes) in these same neurons bias animals towards or away from movement. However, despite widespread acceptance of this model, current methods have provided little evidence to support or refute it. Here, using new optical recording methods, we report the discovery of rapid phasic signaling in striatum-targeting dopaminergic axons that was associated with, and capable of triggering, locomotion in mice. Axons expressing these signals were largely distinct from those signaling during unexpected rewards. These results suggest that dopaminergic neuromodulation can differentially impact motor control and reward learning with sub-second precision and suggest that both precise signal timing and neuronal subtype are important parameters to consider in the treatment of dopamine-related disorders.

Nicotinic modulation of glutamate receptor function at nerve terminal level: a fine-tuning of synaptic signals

This review focuses on a specific interaction occurring between the nicotinic cholinergic receptors (nAChRs) and the glutamatergic receptors (GluRs) at the nerve endings level. We have employed synaptosomes in superfusion and supplemented and integrated our findings with data obtained using techniques from molecular biology and immuno-cytochemistry, and the assessment of receptor trafficking. In particular, we characterize the following: (1) the direct and unequivocal localization of native α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and N-methyl-D-aspartate (NMDA) glutamatergic receptors on specific nerve terminals, (2) their pharmacological characterization and functional co-localization with nAChRs on the same nerve endings, and (3) the existence of synergistic or antagonistic interactions among them. Indeed, in the rat nucleus accumbens (NAc), the function of some AMPA and NMDA receptors present on the dopaminergic and glutamatergic nerve terminals can be regulated negatively or positively in response to a brief activation of nAChRs. This effect occurs rapidly and involves the trafficking of AMPA and NMDA receptors. The event takes place also at very low concentrations of nicotine and involves the activation of several nAChRs subtypes. This dynamic control by cholinergic nicotinic system of glutamatergic NMDA and AMPA receptors might therefore represent an important neuronal presynaptic adaptation associated with nicotine administration. The understanding of the role of these nicotine-induced functional changes might open new and interesting perspectives both in terms of explaining the mechanisms that underlie some of the effects of nicotine addiction and in the development of new drugs for smoking cessation.

Neurotensinergic augmentation of glutamate release at the perforant path-granule cell synapse in rat dentate gyrus: Roles of L-Type Ca²⁺ channels, calmodulin and myosin light-chain kinase

Neurotensin (NT) serves as a neuromodulator in the brain where it is involved in modulating a variety of physiological functions including nociception, temperature, blood pressure and cognition, and many neurological diseases such as Alzheimer's disease, schizophrenia and Parkinson's disease. Whereas there is compelling evidence demonstrating that NT facilitates cognitive processes, the underlying cellular and molecular mechanisms have not been fully determined. Because the dentate gyrus expresses high densities of NT and NT receptors, we examined the effects of NT on the synaptic transmission at the synapse formed between the perforant path (PP) and granule cells (GC) in the rats. Our results demonstrate that NT persistently increased the amplitude of the AMPA receptor-mediated EPSCs at the PP-GC synapse. NT-induced increases in AMPA EPSCs were mediated by presynaptic NTS1 receptors. NT reduced the coefficient of variation and paired-pulse ratio of AMPA EPSCs suggesting that NT facilitates presynaptic glutamate release. NT increased the release probability and the number of readily releasable vesicles with no effects on the rate of recovery from vesicle depletion. NT-mediated augmentation of glutamate release required the influx of Ca(2+) via L-type Ca(2+) channels and the functions of calmodulin and myosin light chain kinase. Our results provide a cellular and molecular mechanism to explain the roles of NT in the hippocampus.

Varenicline enhances dopamine release facilitation more than nicotine after long-term nicotine treatment and withdrawal

An important factor contributing to the high relapse rates among smokers is nicotine withdrawal symptoms. Multiple studies suggest that decreased dopamine release in nucleus accumbens plays a key role in withdrawal. However, recent reports showed that long-term nicotine exposure itself also decreases accumbal dopamine release, suggesting that additional mechanisms are involved in withdrawal. Here, we used real-time cyclic voltammetry in brain slices containing the nucleus accumbens to further elucidate the changes in dopamine release linked to nicotine withdrawal. Rats received vehicle or nicotine via the drinking water for 2-3 months. Studies assessing the expression of somatic signs in vehicle-treated, nicotine-treated, and 24-h nicotine withdrawn rats showed that nicotine withdrawal led to a significant increase in somatic signs. Subsequent voltammetry studies showed that long-term nicotine decreased single-pulse-stimulated dopamine release via an interaction at α6β2* receptors. Nicotine withdrawal led to a partial recovery in α6β2* receptor-mediated release. In addition, long-term nicotine treatment alone increased dopamine release paired-pulse ratios and this was partially reversed with nicotine removal. We then evaluated the effect of bath-applied nicotine and varenicline on dopamine release. Nicotine and varenicline both decreased single-pulse-stimulated release in vehicle-treated, nicotine-treated, and nicotine withdrawn rats. However, bath-applied varenicline increased paired-pulse ratios to a greater extent than nicotine during long-term nicotine treatment and after its withdrawal. Altogether these data suggest that nicotine withdrawal is associated with a partial restoration of dopamine release measures to control levels and that varenicline's differential modulation of dopamine release may contribute to its mechanism of action.

Illicit dopamine transients: reconciling actions of abused drugs

Phasic increases in brain dopamine are required for cue-directed reward seeking. Although compelling within the framework of appetitive behavior, the view that illicit drugs hijack reward circuits by hyperactivating these dopamine transients is inconsistent with established psychostimulant pharmacology. However, recent work reclassifying amphetamine (AMPH), cocaine, and other addictive dopamine-transporter inhibitors (DAT-Is) supports transient hyperactivation as a unifying hypothesis of abused drugs. We argue here that reclassification also identifies generating burst firing by dopamine neurons as a keystone action. Unlike natural rewards, which are processed by sensory systems, drugs act directly on the brain. Consequently, to mimic natural rewards and exploit reward circuits, dopamine transients must be elicited de novo. Of available drug targets, only burst firing achieves this essential outcome.

Presynaptic α7 nicotinic acetylcholine receptors enhance hippocampal mossy fiber glutamatergic transmission via PKA activation

Nicotinic acetylcholine receptors (nAChRs) are expressed widely in the CNS, and mediate both synaptic and perisynaptic activities of endogenous cholinergic inputs and pharmacological actions of exogenous compounds (e.g., nicotine and choline). Behavioral studies indicate that nicotine improves such cognitive functions as learning and memory. However, the mechanism of nicotine's action on cognitive function remains elusive. We performed patch-clamp recordings from hippocampal CA3 pyramidal neurons to determine the effect of nicotine on mossy fiber glutamatergic synaptic transmission. We found that nicotine in combination with NS1738, an α7 nAChR-positive allosteric modulator, strongly potentiated the amplitude of evoked EPSCs (eEPSCs), and reduced the EPSC paired-pulse ratio. The action of nicotine and NS1738 was mimicked by PNU-282987 (an α7 nAChR agonist), and was absent in α7 nAChR knock-out mice. These data indicate that activation of α7 nAChRs was both necessary and sufficient to enhance the amplitude of eEPSCs. BAPTA applied postsynaptically failed to block the action of nicotine and NS1738, suggesting again a presynaptic action of the α7 nAChRs. We also observed α7 nAChR-mediated calcium rises at mossy fiber giant terminals, indicating the presence of functional α7 nAChRs at presynaptic terminals. Furthermore, the addition of PNU-282987 enhanced action potential-dependent calcium transient at these terminals. Last, the potentiating effect of PNU-282987 on eEPSCs was abolished by inhibition of protein kinase A (PKA). Our findings indicate that activation of α7 nAChRs at presynaptic sites, via a mechanism involving PKA, plays a critical role in enhancing synaptic efficiency of hippocampal mossy fiber transmission.

Promise of pharmacogenomics for drug discovery, treatment and prevention of Parkinson's disease. A perspective

Parkinson's disease (PD) is a progressive neurodegenerative disorder characterized by a heterogeneous array of motor and non-motor features. Anti-PD drugs that are in use target only the motor symptoms, may lose efficacy over time, and can cause serious adverse effects such as dyskinesia and psychosis. There are currently no preventative or disease modifying treatments. All attempts to develop disease modifying drugs have failed. Pharmacogenomics (PGx) has the potential to change the way new drugs are developed and the way drugs are prescribed. By using genetic markers that correlate with, and can therefore predict drug response, clinical trials can be designed to be enriched with individuals who are most likely to benefit from the drug, maximizing drug's efficacy, minimizing its adverse effects, and boosting the odds of successful drug discovery. Clinical application of PGx will help physicians to quickly and accurately determine the right drugs and the right doses for individuals, avoiding the lengthy trial and error approaches and adverse effects. In combination with known protective factors such as nicotine and caffeine, PGx may enable development of personalized methods for PD prevention and, by extension, care.

A genetic basis for the variable effect of smoking/nicotine on Parkinson's disease

Prior studies have established an inverse association between cigarette smoking and the risk of developing Parkinson's disease (PD), and currently, the disease-modifying potential of the nicotine patch is being tested in clinical trials. To identify genes that interact with the effect of smoking/nicotine, we conducted genome-wide interaction studies in humans and in Drosophila. We identified SV2C, which encodes a synaptic-vesicle protein in PD-vulnerable substantia nigra (P=1 × 10(-7) for gene-smoking interaction on PD risk), and CG14691, which is predicted to encode a synaptic-vesicle protein in Drosophila (P=2 × 10(-11) for nicotine-paraquat interaction on gene expression). SV2C is biologically plausible because nicotine enhances the release of dopamine through synaptic vesicles, and PD is caused by the depletion of dopamine. Effect of smoking on PD varied by SV2C genotype from protective to neutral to harmful (P=5 × 10(-10)). Taken together, cross-validating evidence from humans and Drosophila suggests SV2C is involved in PD pathogenesis and it might be a useful marker for pharmacogenomics studies involving nicotine.

Cortical excitability in smoking and not smoking individuals with and without nicotine

RATIONAL

Activation of nicotinic acetylcholine receptors has a major neuromodulatory impact on central nervous system function. Beyond acute activation, chronic nicotine intake has long-lasting effects on cortical excitability in animal experiments, caused by receptor up- or down-regulation. Knowledge about the impact of nicotine on cortical excitability in humans, however, is limited.

OBJECTIVES

We therefore aimed to explore the effect of nicotine intake on cortical excitability in healthy human smokers and non-smokers.

METHODS

The primary motor cortex served as model, and cortical excitability was monitored via transcranial magnetic stimulation (TMS). Corticospinal excitability and intracortical excitability were recorded before and after application of nicotine patch in both groups. Corticospinal excitability was explored by motor threshold and input/output curve (I/O curve), and intracortical excitability was explored by means of paired-pulse TMS techniques (intracortical facilitation (ICF), short-latency intracortical inhibition (SICI), I-wave facilitation and short-latency afferent inhibition (SAI)).

RESULTS

The results show that smokers during nicotine withdrawal display increased corticospinal excitability with regard to the I/O curve (TMS intensity 150 % of resting motor threshold) compared to non-smokers and furthermore enhanced SAI and diminished ICF at the intracortical circuit level. After administration of nicotine, intracortical facilitation in smokers increased, while in non-smokers, inhibition (SICI, SAI) was enhanced.

CONCLUSION

Our results show that chronic nicotine consumption in smokers alters cortical excitability independent from acute nicotine consumption and that acute nicotine has different effects on motor cortical excitability in both groups.

Dopamine D₂ and acetylcholine α7 nicotinic receptors have subcellular distributions favoring mediation of convergent signaling in the mouse ventral tegmental area

Alpha7 nicotinic acetylcholine receptors (α7nAChRs) mediate nicotine-induced burst-firing of dopamine neurons in the ventral tegmental area (VTA), a limbic brain region critically involved in reward and in dopamine D2 receptor (D2R)-related cortical dysfunctions associated with psychosis. The known presence of α7nAChRs and Gi-coupled D2Rs in dopamine neurons of the VTA suggests that these receptors are targeted to at least some of the same neurons in this brain region. To test this hypothesis, we used electron microscopic immunolabeling of antisera against peptide sequences of α7nACh and D2 receptors in the mouse VTA. Dual D2R and α7nAChR labeling was seen in many of the same somata (co-localization over 97%) and dendrites (co-localization over 49%), where immunoreactivity for each of the receptors was localized to endomembranes as well as to non-synaptic or synaptic plasma membranes often near excitatory-type synapses. In comparison with somata and dendrites, many more small axons and axon terminals were separately labeled for each of the receptors. Thus, single-labeled axon terminals were predominant for both α7nAChR (57.9%) and D2R (89.0%). The majority of the immunolabeled axonal profiles contained D2R-immunoreactivity (81.6%) and formed either symmetric or asymmetric synapses consistent with involvement in the release of both inhibitory and excitatory transmitters. Of 160 D2R-labeled terminals, 81.2% were presynaptic to dendrites that expressed α7nAChR alone or together with the D2R. Numerous glial processes inclusive of those enveloping either excitatory- or inhibitory-type synapses also contained single labeling for D2R (n=152) and α7nAChR (n=561). These results suggest that classic antipsychotic drugs, all of which block the D2R, may facilitate α7nAChR-mediated burst-firing by elimination of D2R-dependent inhibition in neurons expressing both receptors as well as by indirect pre-synaptic and glial mechanisms.

Ectopic vesicular glutamate release at the optic nerve head and axon loss in mouse experimental glaucoma

Although clinical and experimental observations indicate that the optic nerve head (ONH) is a major site of axon degeneration in glaucoma, the mechanisms by which local retinal ganglion cell (RGC) axons are injured and damage spreads among axons remain poorly defined. Using a laser-induced ocular hypertension (LIOH) mouse model of glaucoma, we found that within 48 h of intraocular pressure elevation, RGC axon segments within the ONH exhibited ectopic accumulation and colocalization of multiple components of the glutamatergic presynaptic machinery including the vesicular glutamate transporter VGLUT2, several synaptic vesicle marker proteins, glutamate, the soluble N-ethylmaleimide-sensitive factor attachment protein receptor complex and active zone cytomatrix components, as well as ultrastructurally identified, synaptophysin-containing vesicles. Ectopic vesicle exocytosis and glutamate release were detected in acute preparations of the LIOH ONH. Immunolocalization and analysis using the ionotropic receptor channel-permeant cation agmatine indicated that ONH axon segments and glia expressed glutamate receptors, and these receptors were more active after LIOH compared with controls. Pharmacological antagonism of glutamate receptors and neuronal activity resulted in increased RGC axon sparing in vivo. Furthermore, in vivo RGC-specific genetic disruption of the vesicular glutamate transporter VGLUT2 or the obligatory NMDA receptor subunit NR1 promoted axon survival in experimental glaucoma. As the inhibition of ectopic glutamate vesicular release or glutamate receptivity can independently modify the severity of RGC axon loss, synaptic release mechanisms may provide useful therapeutic entry points into glaucomatous axon degeneration.

Neurotransmitter receptors and cognitive dysfunction in Alzheimer's disease and Parkinson's disease

Cognitive dysfunction is one of the most typical characteristics in various neurodegenerative diseases such as Alzheimer's disease and Parkinson's disease (advanced stage). Although several mechanisms like neuronal apoptosis and inflammatory responses have been recognized to be involved in the pathogenesis of cognitive dysfunction in these diseases, recent studies on neurodegeneration and cognitive dysfunction have demonstrated a significant impact of receptor modulation on cognitive changes. The pathological alterations in various receptors appear to contribute to cognitive impairment and/or deterioration with correlation to diversified mechanisms. This article recapitulates the present understandings and concepts underlying the modulation of different receptors in human beings and various experimental models of Alzheimer's disease and Parkinson's disease as well as a conceptual update on the underlying mechanisms. Specific roles of serotonin, adrenaline, acetylcholine, dopamine receptors, and N-methyl-D-aspartate receptors in Alzheimer's disease and Parkinson's disease will be interactively discussed. Complex mechanisms involved in their signaling pathways in the cognitive dysfunction associated with the neurodegenerative diseases will also be addressed. Substantial evidence has suggested that those receptors are crucial neuroregulators contributing to cognitive pathology and complicated correlations exist between those receptors and the expression of cognitive capacities. The pathological alterations in the receptors would, therefore, contribute to cognitive impairments and/or deterioration in Alzheimer's disease and Parkinson's disease. Future research may shed light on new clues for the treatment of cognitive dysfunction in neurodegenerative diseases by targeting specific alterations in these receptors and their signal transduction pathways in the frontal-striatal, fronto-striato-thalamic, and mesolimbic circuitries.

Effects of acute administration of nicotine, amphetamine, diazepam, morphine, and ethanol on risky decision-making in rats

RATIONALE

Most individuals can accurately assess the risks and rewards associated with choice alternatives and decide accordingly; however, drug users often display maladaptive decision-making, such that choices are biased toward excessively risky options.

OBJECTIVE

The purpose of this study was to investigate the effects of a range of drugs of abuse on risky decision-making.

METHODS

Male Long-Evans rats were trained in the Risky Decision-Making Task, in which they chose between two levers, one which produced a small, "safe" food reward and the other which produced a large, "risky" food reward. The large reward was accompanied by the risk of a mild footshock, the probability of which increased over the course of each test session (0%, 25%, 50%, 75%, and 100%).

RESULTS

Nicotine (0.6 mg/kg) and amphetamine (1.5 mg/kg) caused a significant decrease in choice of the large risky reward (decreased risk taking). Diazepam (1.0 mg/kg) caused a significant increase in choice of the large risky reward (increased risk taking), whereas morphine (3.0 mg/kg) caused only a trend toward increased choice of the large risky reward. Ethanol had no effect on choice behavior.

CONCLUSIONS

These results show that acute administration of drugs of abuse can modulate risk taking in a drug-specific manner, either increasing or decreasing preference for highly rewarding, but risky, options.

Acetylcholine α7 nicotinic and dopamine D2 receptors are targeted to many of the same postsynaptic dendrites and astrocytes in the rodent prefrontal cortex

The alpha-7 nicotinic acetylcholine receptor (α7nAChR) and the dopamine D(2) receptor (D(2) R) are both implicated in attentional processes and cognition, mediated in part through the prefrontal cortex (PFC). We examined the dual electron microscopic immunolabeling of α7nAChR and either D(2) R or the vesicular acetylcholine transporter (VAChT) in rodent PFC to assess convergent functional activation sites. Immunoreactivity (ir) for α7nAChR and/or D(2) R was seen in the same as well as separate neuronal and glial profiles. At least half of the dually labeled profiles were somata and dendrites, while most labeled axon terminals expressed only D(2) R-ir. The D(2) R-labeled terminals were without synaptic specializations or formed inhibitory or excitatory-type synapses with somatodendritic profiles, some of which expressed the α7nAChR and/or D(2) R. Astrocytic glial processes comprised the majority of nonsomatodendritic α7nAChR or α7nAChR and D(2) R-labeled profiles. Glial processes containing α7nAChR-ir were frequently located near VAChT-labeled terminals and also showed perisynaptic and perivascular associations. We conclude that in rodent PFC α7nACh and D(2) R activation can dually modulate (1) postsynaptic dendritic responses within the same or separate but synaptically linked neurons in which the D(2) R has the predominately presynaptic distribution, and (2) astrocytic signaling that may be crucial for synaptic transmission and functional hyperemia.

In vitro findings of alterations in intracellular calcium homeostasis in schizophrenia

The pathogenesis of schizophrenia involves several complex cellular mechanisms and is not well understood. Recent research has demonstrated an association between primary disturbances characteristic of the disease, including altered dopaminergic and glutamatergic neurotransmission, and impairments in neuronal calcium (Ca(2+)) homeostasis and signaling. Emerging Ca(2+) hypothesis links and unifies various cellular processes involved in the pathogenesis of schizophrenia and suggests a central role of dysregulation of Ca(2+) homeostasis in the etiology of the disease. This review explores the in vitro data on Ca(2+) homeostasis and signaling in schizophrenia. Major limitation in this research is the lack of schizophrenia markers and validated disease models. As indicated in this review, one way to overcome these limitations may be analyses of Ca(2+) signalosomes in peripheral cells from schizophrenia patients. Validation of animal models of schizophrenia may permit the application of advanced Ca(2+) imaging techniques in living animals.

Long-term treatment with aripiprazole on the waking and postprandial urges to smoke in Chinese heavy smokers

The central dopaminergic system plays a critical role in the reinforcing effects of nicotine, which are key determinants in the urge to smoke. Previous study has demonstrated that immediate administration of 10-mg aripiprazole significantly decreased various subjective responses to smoking. The present study investigated whether 2-week treatment with 10-mg aripiprazole could attenuate waking and postprandial urges to smoke in Chinese male and female heavy smokers. A randomized and placebo-controlled pilot clinical study was conducted to assess the effect of aripiprazole on various responses to smoking. The primary outcomes were subject's ratings on questionnaires of smoking urge, withdrawal syndromes, and cigarette evaluation. All participants were administered either placebo or 10-mg aripiprazole for 2 weeks. Throughout the experiment, participants were required to self-report (1) smoking urge and nicotine withdrawal symptoms before their first cigarette after awakening and after lunch and (2) subjective responses to the first cigarette smoked of the day and after lunch. Aripiprazole was associated with significantly decreased waking and postprandial urges to smoke. Aripiprazole failed to produce a significant effect on overall nicotine withdrawal symptoms after awakening and after lunch. However, waking, but not postprandial, withdrawal craving and syndromes were significantly reduced by aripiprazole. Aripiprazole had no effect on the overall subjective responses to the first cigarette of the day and after lunch. The attenuating effects of aripiprazole on waking and postprandial urges to smoke demonstrate the promising effect of aripiprazole in the treatment of nicotine dependence.

Why do patients with schizophrenia smoke?

PURPOSE OF REVIEW

Among the mentally ill, smoking prevalence is highest in patients with schizophrenia ( approximately 70-80%). This can impose a significant financial burden on patients, not to speak of increased smoking-related morbidity and mortality. Therefore, it is critical for clinicians to understand why patients with schizophrenia smoke in order to adapt treatment schemes. Understanding the reasons may also help to develop new drugs that target the nicotinic system in the brain as well as smoking cessation programs that are specifically designed for this particular patient population.

RECENT FINDINGS

So far, several reasons have been identified which are believed to explain tobacco consumption in patients with schizophrenia. Originally, it was widely believed that patients with schizophrenia smoke to increase hepatic clearance and to restore the dopamine blockade of certain antipsychotic drugs to diminish their side effects. However, more recently it became obvious that cigarette smoking may also be reinforcing for patients because it improves psychiatric symptoms, most notably negative and cognitive symptoms. The underlying molecular mechanisms of these nicotine effects are currently under intensive investigation.

SUMMARY

Heavy smoking in schizophrenia cannot simply be viewed as a 'bad habit'. Rather, self-medication of clinical symptoms and side effects of antipsychotic drugs appear to play a major role.

Cross-regulation between colocalized nicotinic acetylcholine and 5-HT3 serotonin receptors on presynaptic nerve terminals

AIM

Substantial colocalization of functionally independent alpha4 nicotinic acetylcholine receptors and 5-HT(3) serotonin receptors on presynaptic terminals has been observed in brain. The present study was aimed at addressing whether nicotinic acetylcholine receptors and 5-HT(3) serotonin receptors interact on the same presynaptic terminal, suggesting a convergence of cholinergic and serotonergic regulation.

METHODS

Ca(2+) responses in individual, isolated nerve endings purified from rat striatum were measured using confocal imaging.

RESULTS

Application of 500 nmol/L nicotine following sustained stimulation with the highly selective 5-HT(3) receptor agonist m-chlorophenylbiguanide at 100 nmol/L resulted in markedly reduced Ca(2+) responses (28% of control) in only those striatal nerve endings that originally responded to m-chlorophenylbiguanide. The cross-regulation developed over several minutes. Presynaptic nerve endings that had not responded to m-chlorophenylbiguanide, indicating that 5-HT(3) receptors were not present, displayed typical responses to nicotine. Application of m-chlorophenylbiguanide following sustained stimulation with nicotine resulted in partially attenuated Ca(2+) responses (49% of control). Application of m-chlorophenylbiguanide following sustained stimulation with m-chlorophenylbiguanide also resulted in a strong attenuation of Ca(2+) responses (12% of control), whereas nicotine-induced Ca(2+) responses following sustained stimulation with nicotine were not significantly different from control.

CONCLUSION

These results indicate that the presynaptic Ca(2+) increases evoked by either 5-HT(3) receptor or nicotinic acetylcholine receptor activation regulate subsequent responses to 5-HT(3) receptor activation, but that only 5-HT(3) receptors cross-regulate subsequent nicotinic acetylcholine receptor-mediated responses. The findings suggest a specific interaction between the two receptor systems in the same striatal nerve terminal, likely involving Ca(2+)-dependent intracellular pathways that regulate these signaling systems at one or more levels.

Nicotine enhances automatic temporal processing as measured by the mismatch negativity waveform

INTRODUCTION

Cholinergic agonists and, more specifically, nicotine, have been found to enhance a number of cognitive processes. The effect of nicotine on temporal processing is not known. The use of behavioral measures of temporal processing to measure its effect could be confounded by the general effects of nicotine on attention. Mismatch negativity (MMN) has been used as a physiological measure of automatic temporal processing to avoid this potential confound.

METHODS

A total of 20 subjects (11 nonsmokers and 9 smokers following 2 hr of abstinence) participated in a two-visit single-blind, placebo-controlled crossover study of the effect of nicotine on MMN indices in response to an interstimulus interval deviant.

RESULTS

Nicotine-enhanced MMN amplitudes from baseline recording to postdrug recording greater than did the placebo condition. This enhancement was seen in both nonsmokers and smokers. Nicotine had no significant effect on MMN latency or N100 amplitude or latency.

DISCUSSION

This is the first study to demonstrate a nicotine-related enhancement of MMN amplitude to an interstimulus interval duration deviant and confirms our hypothesis that nicotine enhances preattentive temporal processing. Nicotinic agonists may represent a potential therapeutic option for individuals with abnormalities in early sensory or temporal processing related to cholinergic system abnormalities. Methodologically, our paradigm of nicotine administration in abstinent smokers is important because it resulted in both minimal withdrawal symptoms and meaningful data that are not attributable solely to relief of withdrawal.

Molecular and cellular mechanisms of action of nicotine in the CNS

Nicotine achieves its psychopharmacological effects by interacting with nicotinic acetylcholine receptors (nAChRs) in the brain. There are numerous subtypes of nAChR that differ in their properties, including their sensitivity to nicotine, permeability to calcium and propensity to desensitise. The nAChRs are differentially localised to different brain regions and are found on presynaptic terminals as well as in somatodendritic regions of neurones. Through their permeability to cations, these ion channel proteins can influence both neuronal excitability and cell signalling mechanisms, and these various responses can contribute to the development or maintenance of dependence. However, many questions and uncertainties remain in our understanding of these events and their relevance to tobacco addiction. In this chapter, we briefly overview the fundamental characteristics of nAChRs that are germane to nicotine's effects and then consider the cellular responses to acute and chronic nicotine, with particular emphasis on dopamine systems because they have been the most widely studied in the context of nicotine dependence. Where appropriate, methodological aspects are critically reviewed.

Involvement of nicotinic receptors in methamphetamine- and MDMA-induced neurotoxicity: pharmacological implications

During the last years, we have focused on the study of the neurotoxic effects of 3,4-methylenedioxymethamphetamine (MDMA) and methamphetamine (METH) on the central nervous system (CNS) and their pharmacological prevention methods. In the process of this research, we have used a semipurified synaptosomal preparation from striatum of mice or rats as a reliable in vitro model to study reactive oxygen species (ROS) production by these amphetamine derivatives, which is well-correlated with their dopaminergic injury in in vivo models. Using this preparation, we have demonstrated that blockade of alpha7 nicotinic receptors with methyllycaconitine (MLA) prevents ROS production induced by MDMA and METH. Consequently, in vivo, MLA significantly prevents MDMA- and METH-induced neurotoxicity at dopaminergic level (mouse striatum), without affecting hyperthermia induced by these amphetamines. Additionally, when neuroprotection was assayed with memantine (MEM), a dual antagonist of NMDA and alpha7 receptors, an effective neuroprotection was obtained also ahead of serotonergic injury induced by MDMA in rats. MEM also prevents MDMA effect on serotonin transporter functionality and METH effect on dopamine transporter (DAT), suggesting that behavioral effects of these psychostimulants can also be modulated by MEM. Finally, we have demonstrated that MEM prevents the impaired memory function induced by MDMA, and also, using binding studies with radioligands, we have characterized the interaction of these substances with nicotinic receptors. Studies at molecular level showed that both MDMA and METH displaced competitively the binding of radioligands with homomeric alpha7 and heteromeric nicotinic acetylcholine receptors (nAChRs), indicating that they can directly interact with them. In all the cases, MDMA displayed higher affinity than METH and it was higher for heteromeric than for alpha7 subtype. Pre-incubation of differentiated PC12 cells with MDMA or METH induces nAChR upregulation in a concentration- and time-dependent manner, as many nicotinic ligands do, supporting their functional interaction with nAChRs. Such interaction expands the pharmacological profile of amphetamines and can account for some of their effects.

Glutamatergic contributions to nicotinic acetylcholine receptor agonist-evoked cholinergic transients in the prefrontal cortex

Because modulation of cortical cholinergic neurotransmission has been hypothesized to represent a necessary mechanism mediating the beneficial cognitive effects of nicotine and nicotinic acetylcholine receptor (nAChR) subtype-selective agonists, we used choline-sensitive microelectrodes for the real-time measurement of ACh release in vivo, to characterize cholinergic transients evoked by nicotine and the alpha4beta2*-selective nAChR partial agonist 2-methyl-3-(2-(S)-pyrrolindinylmethoxy)pyridine dihydrochloride (ABT-089), a clinically effective cognition enhancer. In terms of cholinergic signal amplitudes, ABT-089 was significantly more potent than nicotine in evoking ACh cholinergic transients. Moreover, cholinergic signals evoked by ABT-089 were characterized by faster signal rise time and decay rate. The nAChR antagonist mecamylamine attenuated the cholinergic signals evoked by either compound. Cholinergic signals evoked by ABT-089 were more efficaciously attenuated by the relatively beta2*-selective nAChR antagonist dihydro-beta-erythroidine. The alpha7 antagonist methyllycaconitine did not affect choline signal amplitudes but partly attenuated the relatively slow decay rate of nicotine-evoked cholinergic signals. Furthermore, the AMPA receptor antagonist DNQX as well as the NMDA receptor antagonist APV more potently attenuated cholinergic signals evoked by ABT-089. Using glutamate-sensitive microelectrodes to measure glutamatergic transients, ABT-089 was more potent than nicotine in evoking glutamate release. Glutamatergic signals were highly sensitive to tetrodotoxin-induced blockade of voltage-regulated sodium channels. Together, the present evidence indicates that compared with nicotine, ABT-089 evokes more potent and sharper cholinergic transients in prefrontal cortex. Glutamatergic mechanisms necessarily mediate the cholinergic effects of nAChR agonists in the prefrontal cortex.

Repeated exposure to methamphetamine causes long-lasting presynaptic corticostriatal depression that is renormalized with drug readministration

Addiction-associated behaviors such as drug craving and relapse are hypothesized to result from synaptic changes that persist long after withdrawal and are renormalized by drug reinstatement, although such chronic synaptic effects have not been identified. We report that exposure to the dopamine releaser methamphetamine for 10 days elicits a long-lasting (>4 month) depression at corticostriatal terminals that is reversed by methamphetamine readministration. Both methamphetamine-induced chronic presynaptic depression and the drug's selective renormalization in drug-experienced animals are independent of corresponding long-term changes in synaptic dopamine release but are due to alterations in D1 dopamine and cholinergic receptor systems. These mechanisms might provide a synaptic basis that underlies addiction and habit learning and their long-term maintenance.

Nonsynaptic Chemical Transmission Through Nicotinic Acetylcholine Receptors

This review attempts to organize the different aspects of nicotinic transmission in the context of nonsynaptic interactions. Nicotinic acetylcholine receptors (nAChRs) dominantly operate in the nonsynaptic mode in the central nervous system despite their ligand-gated ion-channel nature, which would otherwise be better suited for fast synaptic transmission. This fast form of nonsynaptic transmission, most likely unique to nAChRs, represents a new avenue in the communication platforms of the brain. Cholinergic messages received by nAChRs, arriving at a later phase following synaptic activation, can interfere with dendritic signal integration. Nicotinic transmission plays a role in both neural plasticity and cellular learning processes, as well as in long-term changes in basic activity through fast activation, desensitization of receptors, and fluctuations of the steady-state levels of ACh. ACh release can contribute to plastic changes via activation of nAChRs in neurons and therefore plays a role in learning and memory in different brain regions. Assuming that nAChRs in human subjects are ready to receive long-lasting messages from the extracellular space because of their predominantly nonsynaptic distribution, they offer an ideal target for drug therapy at low, nontoxic drug levels.

Facilitation of cortico-amygdala synapses by nicotine: activity-dependent modulation of glutamatergic transmission

The basolateral nucleus of the amygdala (BLA) receives cholinergic innervation from the basal forebrain and nicotine, via activation of neuronal nicotinic acetylcholine receptors (nAChRs), can improve performance in amygdala-based learning tasks. We tested the hypothesis that acute and prenatal nicotine exposure modulates cortico-amygdala synaptic transmission. We found that low-dose, single-trial exposures to nicotine can elicit lasting facilitation, the extent of which is dependent on the level of stimulation of the cortical inputs to the BLA. In addition, sustained facilitation is ablated by prenatal exposure to nicotine. This study examined synaptic transmission in 238 patch-clamp recordings from BLA neurons in acute slice from mouse brain. Pharmacological studies in wild-type and nAChR subunit knock-out mice reveal that activation of presynaptic alpha 7, containing (alpha 7*) and non-alpha 7* nAChRs, facilitates glutamatergic transmission in an activity-dependent manner. Without prior stimulation, application of nicotine elicits modest and transient facilitation of glutamatergic postsynaptic currents (PSCs) in about 40% of BLA neurons. With low-frequency stimulation of cortical inputs nicotine elicits robust facilitation of transmission at about 60% of cortico-BLA synapses and synaptic strength remains elevated at about 40% of these connections for >15 min after nicotine washout. Following paired-pulse stimulation nicotine elicits long-lasting facilitation of glutamatergic transmission at about 70% of cortico-BLA connections. Nicotine reduces the threshold for activation of long-term potentiation of cortico-BLA synapses evoked by patterned stimulation. Prenatal exposure to nicotine reduced subsequent modulatory responses to acute nicotine application.

Muscle and neuronal nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors (nAChRs) are integral membrane proteins and prototypic members of the ligand-gated ion-channel superfamily, which has precursors in the prokaryotic world. They are formed by the assembly of five transmembrane subunits, selected from a pool of 17 homologous polypeptides (alpha1-10, beta1-4, gamma, delta, and epsilon). There are many nAChR subtypes, each consisting of a specific combination of subunits, which mediate diverse physiological functions. They are widely expressed in the central nervous system, while, in the periphery, they mediate synaptic transmission at the neuromuscular junction and ganglia. nAChRs are also found in non-neuronal/nonmuscle cells (keratinocytes, epithelia, macrophages, etc.). Extensive research has determined the specific function of several nAChR subtypes. nAChRs are now important therapeutic targets for various diseases, including myasthenia gravis, Alzheimer's and Parkinson's diseases, and schizophrenia, as well as for the cessation of smoking. However, knowledge is still incomplete, largely because of a lack of high-resolution X-ray structures for these molecules. Nevertheless, electron microscopy studies on 2D crystals of nAChR from fish electric organs and the determination of the high-resolution X-ray structure of the acetylcholine binding protein (AChBP) from snails, a homolog of the extracellular domain of the nAChR, have been major steps forward and the data obtained have important implications for the design of subtype-specific drugs. Here, we review some of the latest advances in our understanding of nAChRs and their involvement in physiology and pathology.

Cholinergic drugs affect novel object recognition in rats: relation with hippocampal EEG?

This study examined the role of cognitively enhancing cholinergic drugs on both object memory and brain activity in rats, as well as the possible relation between the two measures. A group of twenty-four animals was used for assessing object recognition. In another group of eight rats, an electrode was implanted into the dorsal hippocampus to record an electroencephalogram (EEG) and auditory evoked potentials (AEP). In both groups, animals were treated with saline, 0.1 mg/kg scopolamine, 0.1 mg/kg methylscopolamine, 3 mg/kg donepezil, donepezil combined with scopolamine, 0.1 mg/kg nicotine, and nicotine combined with scopolamine. Scopolamine, but not methylscopolamine, impaired object recognition. Both donepezil and nicotine reversed this impairment. The N1 and N2 components of the AEP became closer to baseline after scopolamine, which was not reversed by donepezil or nicotine. Scopolamine increased the theta frequency in the EEG. When combined with donepezil, theta increased even more. Conversely, nicotine reversed the theta increment to control level. It is suggested that scopolamine caused a decrement in arousal in this study. Furthermore, the current results suggest a relation between EEG and object memory after cholinergic drug treatment. However, there was a clear dissociation between memory performance and EEG after combined treatment with drugs, which makes additional research where EEG and performance measures are co-registered imperative.

The allosteric potentiation of nicotinic acetylcholine receptors by galantamine ameliorates the cognitive dysfunction in beta amyloid25-35 i.c.v.-injected mice: involvement of dopaminergic systems

Galantamine, a drug for Alzheimer's disease, is a novel cholinergic agent with a dual mode of action, which inhibits acetylcholinesterase and allosterically modulates nicotinic acetylcholine receptors (nAChRs), as a result stimulates catecholamine neurotransmission. In the present study, we investigated whether galantamine exerts cognitive improving effects through the allosteric modulation of nAChR in the intracerebroventricular beta amyloid (Abeta)(25-35)-injected animal model of Alzheimer's disease. Galantamine (3 mg/kg p.o.) significantly increased the extracellular dopamine release in the hippocampus of saline- and Abeta(25-35)-injected mice. The effects of nicotine on the extracellular dopamine release were potentiated by galantamine, but antagonized by mecamylamine, a nAChR antagonist. Abeta(25-35)-injected mice, compared with saline-injected mice, could not discriminate between new and familiar objects in the novel object recognition test and exhibited less freezing response in the fear-conditioning tasks, suggesting Abeta(25-35) induced cognitive impairment. Galantamine improved the Abeta(25-35)-induced cognitive impairment in the novel object recognition and fear-conditioning tasks. These improving effects of galantamine were blocked by the treatment with mecamylamine, SCH-23390, a dopamine-D1 receptor antagonist, and sulpiride, a dopamine-D2 receptor antagonist, but not by scopolamine, a muscarinic acetylcholine receptor antagonist. This study provides the first in vivo evidence that galantamine augments dopaminergic neurotransmission within the hippocampus through the allosteric potentiation of nAChRs. The improving-effects of galantamine on the Abeta(25-35)-induced cognitive impairment may be mediated through the activation of, at least in part, dopaminergic systems, and the enhancement of dopamine release may be one of multiple mechanisms underlying the therapeutic benefit of galantamine.

Intracellular Ca2+ dynamics of hippocampal interneurons following nicotinic acetylcholine receptor activation

Ca2+ permeability of central nicotinic acetylcholine receptors (nAChRs), especially the alpha7 subunits, are exceptionally high and this important feature provide a special functional importance for these receptors at the system level. Although studies at the cellular level extensively characterized the molecular properties of Ca2+ influx following nAChR activation, much less is known about the time-related Ca2+ dynamics during nicotine administration in integration units of neurons. Such studies are of particular relevance to understanding in situ nonsynaptic actions of nicotine. Puff ejection of drugs produce a rapid drug delivery and elimination from the cell surface allowing the activation of extrasynaptic receptors within desensitization time-frame. In this report we provide evidence that rapid nicotine application is able to produce irregular Ca2+ transients in the dendrites of stratum radiatum interneurons in the hippocampal CA1 region. Potential components and mechanisms of nAChR-mediated Ca2+ influx are discussed in details to demonstrate the unique feature of activation of nAChRs involved in nonsynaptic function in interneurons as compared to other types of nicotinic activity.