Nicotine and Behavioral Sensitization

Behavioral sensitization to psychostimulants and opioids: What is known in rodents and what still needs to be explored in humans?

Repeated exposure to substances of abuse results in an increase in some behavioral responses. This phenomenon, called behavioral sensitization (BS), is well described in preclinical models. However, its existence in humans is still a matter of debate. After a review of preclinical evidence of BS and its mechanisms in animal models, we reviewed the evidence supporting the existence of BS in humans, despite the limited research available in this regard. We focused our review on opioids and psychostimulants, since they share the ability to promote addictive behaviors. Further, they induce BS despite their distinct sedative and stimulant properties. Moreover, we proposed future research perspectives in this review to address the remaining unsolved questions, especially regarding BS in humans using a harm reduction approach.

Nonlinear relationship between CAN current and C a 2 + influx underpins synergistic action of muscarinic and NMDA receptors on bursts induction in midbrain dopaminergic neurons

Bursting of midbrain dopamine (DA) neurons is believed to represent an important reward signal that instructs and reinforces goal-directed behaviors. In DA neurons, many afferents, including cholinergic and glutamatergic inputs, induce bursting, and it is suggested that a synergy exists between these afferents in bursting induction. However, the underlying mechanisms of the role and the synergy of muscarinic receptors (mAChRs) and NMDA receptors (NMDARs) in bursting induction remain unclear. Present work bestowed analysis using a mathematical model of DA neurons to demonstrate the underlying mechanisms. Activation of mAChRs, leading to rapid translocation of TRPC channels to cell surface, recruited C a 2 + -activated nonspecific (CAN) current ( I CAN ), meanwhile NMDARs excitation triggered C a 2 + influx, which induced the positive feedback loop of C a 2 + and I CAN , respectively, yielded a robust ramping depolarization with a superimposed high-frequency spiking. In some DA cells, neither NMDARs nor mAChRs induced positive feedback loop unless they were activated simultaneously to induce bursting. Our experimental results verified those theoretical findings. These together unveil the underlying mechanisms of the role and synergy of mAChRs and NMDARs in bursting induction emerge from the nonlinear relationship between C a 2 + influx and I CAN . Given the diverse and complex nature of neural circuitry and the DA neuron heterogeneity, our work provides new insights to understand specific afferents, the synergy between those afferents, and the differences in intrinsic excitability to be integrated by the bursting to accurately characterize the dopamine signals in the valances of reward and reinforcement, and a broad spectrum of neuropsychiatric disorders.

The development of behavioral sensitization induced by a single morphine exposure in adult zebrafish (Danio rerio)

BACKGROUND

Accumulating evidence suggest that behavioral sensitization is involved in the process of drug addiction. Zebrafish are sensitive to a variety of addictive drugs and are thus suitable for the study of behavioral sensitization. However, in contrast to mature rodent models of behavioral sensitization, how this phenomenon manifests in aquatic organisms, especially zebrafish, is largely unknown. In this study, we developed a morphine-induced behavioral sensitization adult zebrafish model and performed a preliminary investigation of the underlying mechanisms.

METHODS

Behavioral sensitization was established in zebrafish by observing their behavior after treatment and challenge with morphine. The effect of morphine was evaluated by a behavioral locomotor test. Different doses of morphine and withdrawal times were used to evaluate the establishment of the behavioral sensitization model.

RESULTS

Hyperlocomotion was induced after administration of morphine in adult zebrafish. After withdrawing the drug for a period, challenge with low-dose morphine evoked behavioral sensitization in zebrafish acutely pre-treated with morphine. Low-dose morphine failed to induce behavioral sensitization in zebrafish if the withdrawal time was less than 5 days or more than 7 days. Morphine induced behavioral sensitization in zebrafish may involve dopaminergic, glutamatergic and opioid systems.

CONCLUSION

A single low-dose of morphine could induce behavioral sensitization in zebrafish acutely pre-treated with morphine, and this phenomenon was highly correlated with drug dose and withdrawal time. These findings suggest that zebrafish is a suitable model for the study of behavioral sensitization.

Anatabine, Nornicotine, and Anabasine Reduce Weight Gain and Body Fat through Decreases in Food Intake and Increases in Physical Activity

Obesity is a leading cause of preventable death in the United States. Currently approved pharmacotherapies for the treatment of obesity are associated with rebound weight gain, negative side effects, and the potential for abuse. There is a need for new treatments with fewer side effects. Minor tobacco alkaloids (MTAs) are potential candidates for novel obesity pharmacotherapies. These alkaloids are structurally related to nicotine, which can help reduce body weight, but without the same addictive potential. The purpose of the current study was to examine the effects of three MTAs (nornicotine, anatabine, and anabasine) and nicotine on weight gain, body composition, chow intake, and physical activity. We hypothesized that the MTAs and nicotine would reduce weight gain through reductions in chow intake and increases in physical activity. To test this, male Sprague Dawley rats were housed in metabolic phenotyping chambers. Following acclimation to these chambers and to (subcutaneous (sc)) injections of saline, animals received daily injections (sc) of nornicotine, anabasine, anatabine, or nicotine for one week. Compared to saline-injected animals that gained body weight and body fat during the treatment phase, injections of nornicotine and anatabine prevented additional weight gain, alongside reductions in body fat. Rats receiving anabasine and nicotine gained body weight at a slower rate relative to rats receiving saline injections, and body fat remained unchanged. All compounds reduced the intake of chow pellets. Nornicotine and nicotine produced consistent increases in physical activity 6 h post-injection, whereas anabasine’s and anatabine’s effects on physical activity were more transient. These results show that short-term, daily administration of nornicotine, anabasine, and anatabine has positive effects on weight loss, through reductions in body fat and food intake and increases in physical activity. Together, these findings suggest that MTAs are worthy of further investigations as anti-obesity pharmacotherapies.

Multiple nicotinic acetylcholine receptor subtypes regulate social or cognitive behaviors in mice repeatedly administered phencyclidine

Habitual smoking in patients with schizophrenia (SCZ) is considered to improve their own psychoses or to develop a vulnerability to psychological dependence on (-)-nicotine ([-]-NIC) by stimulating nicotinic acetylcholine receptors (nAChRs) in the central nervous system. In the present study, we investigated whether habitual smoking is due to get therapeutic effect or to psychological dependence and which nAChR subunits are associated with them using mice that were repeatedly administered phencyclidine (PCP: 10 mg/kg/day, s.c. for 14 days) as SCZ-like model mice. Mice that were repeatedly administered PCP showed impairments in social or cognitive behaviors; decreased expression of α7 and/or α4 nAChR subunits in the prefrontal cortex (PFC); and increased expression of α7, α4, and β2 nAChR subunits in the nucleus accumbens (NAc). These changes were attenuated by repeated administration of (-)-NIC. The attenuating effects on behavioral impairments were prevented by a selective α7 nAChR antagonist and a selective α4β2 nAChR antagonist. At non- or weak effective dose by themselves, co-administration of (-)-NIC (0.03 mg/kg) and risperidone (0.03 mg/kg) showed synergistic effects on behavioral impairments in PCP-administered mice. Repeated (-)-NIC administration did not affect the performance of conditioned place preference, while it showed behavioral sensitization to (-)-NIC in the PCP-administered mice. Repeated (-)-NIC administration did not affect the performance of conditioned place preference, while it showed behavioral sensitization to (-)-NIC and attenuating effect on haloperidol-induced catalepsy in the PCP-administered mice. Our findings suggest that habitual smoking in SCZ might be attributed to get therapeutic and reduce side effects mediated by α7 and α4β2 nAChR activation by (-)-NIC.

Modeling drug exposure in rodents using e-cigarettes and other electronic nicotine delivery systems

Smoking tobacco products is the leading cause of preventable death worldwide. Coordinated efforts have successfully reduced tobacco cigarette smoking in the United States; however, electronic cigarettes (e-cigarette) and other electronic nicotine delivery systems (ENDS) recently have replaced traditional cigarettes for many users. While the clinical risks associated with long-term ENDS use remain unclear, advancements in preclinical rodent models will enhance our understanding of their overall health effects. This review examines the peripheral and central effects of ENDS-mediated exposure to nicotine and other drugs of abuse in rodents and evaluates current techniques for implementing ENDS in preclinical research.

A comparison of implicit and explicit reward learning in low risk alcohol users versus people who binge drink and people with alcohol dependence

Chronic alcohol use leads to specific neurobiological alterations in the dopaminergic brain reward system, which probably are leading to a reward deficiency syndrome in alcohol dependence. The purpose of our study was to examine the effects of such hypothesized neurobiological alterations on the behavioral level, and more precisely on the implicit and explicit reward learning. Alcohol users were classified as dependent drinkers (using the DSM-IV criteria), binge drinkers (using criteria of the USA National Institute on Alcohol Abuse and Alcoholism) or low-risk drinkers (following recommendations of the Scientific board of trustees of the German Health Ministry). The final sample (n = 94) consisted of 36 low-risk alcohol users, 37 binge drinkers and 21 abstinent alcohol dependent patients. Participants were administered a probabilistic implicit reward learning task and an explicit reward- and punishment-based trial-and-error-learning task. Alcohol dependent patients showed a lower performance in implicit and explicit reward learning than low risk drinkers. Binge drinkers learned less than low-risk drinkers in the implicit learning task. The results support the assumption that binge drinking and alcohol dependence are related to a chronic reward deficit. Binge drinking accompanied by implicit reward learning deficits could increase the risk for the development of an alcohol dependence.

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Alcohol dependent patients were impaired in implicit and explicit reward learning.

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Alcohol dependence may lead to implicit and explicit reward learning deficits.

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Binge drinkers learned less than low-risk drinkers in the implicit learning task.

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Binge drinking is related to implicit reward learning deficits.

NMDA Receptor-Dependent Cholinergic Modulation of Mesolimbic Dopamine Cell Bodies: Neurochemical and Behavioral Studies

Substance abuse disorders are devastating, costly, and difficult to treat. Identifying the neurochemical mechanisms underlying reinforcement promises to provide critical information in the development of effective treatments. Several lines of evidence suggest that striatal dopamine (DA) release serves as a teaching signal in reinforcement learning, and that shifts in DA release from the primary reward to reward-predicting stimuli play a critical role in the self-administration of both natural and non-natural rewards. However, far less is known about the reinforcing effects of motivationally neutral sensory stimuli, or how these signals can facilitate self-administration behavior. Thus, we trained rats ( n = 7) to perform a visual stimulus-induced instrumental task, which involved lever pressing for activation of a stimulus light. We then microinfused vehicle (phosphate buffered saline), carbachol (acetylcholine receptor agonist), or carbachol in the presence of an N-methyl-d-aspartate (NMDA) receptor-specific drug (NMDA itself, or the antagonist, AP5) into the ventral tegmental area (VTA). This enabled us to directly evaluate how chemical modulation of dopamine cell bodies affects the instrumental behavior, as well as the nature of extracellular dopamine transients recorded in the nucleus accumbens shell (NAc shell) using fast-scan cyclic voltammetry (FSCV). Intra-VTA infusion of carbachol enhanced the magnitude and frequency of dopamine transients in the NAc shell and potentiated active lever responding without altering inactive lever responding, as compared to infusion of vehicle. Coinfusion of carbachol with AP5 abolished dopamine transients recorded in the NAc and attenuated active lever responding without altering inactive lever responding. Finally, coadministration of carbachol and NMDA into the VTA restored both lever pressing and dopaminergic signals recorded in the striatum. Together, these results suggest that acetylcholine and glutamate synergistically act at dopamine cells in the VTA to modulate VTA-NAc shell dopaminergic output, and this underlies motivation to lever press for a motivationally neutral visual stimulus.

Nicotine drug discrimination and nicotinic acetylcholine receptors in differentially reared rats

RATIONALE

Individuals vary in sensitivity to the behavioral effects of nicotine, resulting in differences in vulnerability to nicotine addiction. The role of rearing environment in determining individual sensitivity to nicotine is unclear. The neuropharmacological mechanisms mediating the effect of rearing environment on the behavioral actions of nicotine are also poorly understood.

OBJECTIVES

The contribution of rearing environment in determining the sensitivity to the interoceptive effects of nicotine was determined in rats reared in isolated conditions (IC) or enriched conditions (EC). The role of dopamine receptors and α4β2*-nicotinic acetylcholine (nACh) receptors in mediating the differential effect of IC and EC on the interoceptive action of nicotine was determined.

METHODS

The interoceptive action of nicotine was measured as the discriminative stimulus effect of nicotine. Mecamylamine- and eticlopride-inhibition of the nicotine stimulus were used to examine nACh and dopamine receptors, respectively. α4β2*-nACh receptor expression in the mesolimbic dopamine pathway was determined by quantitative autoradiography of [¹²⁵I]-epibatidine binding.

RESULTS

EC-reared rats are less sensitive than IC-reared rats to the discriminative stimulus effects of nicotine at all but maximally effective doses. Mecamylamine inhibited the nicotine stimulus threefold more potently in EC-reared rats (IC₅₀ = 0.25 mg/kg) compared to IC-reared rats (IC₅₀ = 0.75 mg/kg); eticlopride inhibition was not different. [¹²⁵I]-epibatidine binding in the ventral tegmental area of EC-reared rats was reduced (2.8 ± 0.3 fmol) compared to that of IC-reared rats (4.0 ± 0.4 fmol); there was no difference in the nucleus accumbens.

CONCLUSIONS

Rearing environment regulates the sensitivity to the interoceptive effects of nicotine and α4β2*-nACh receptor expression in the mesolimbic dopamine pathway.

Epigenetic mechanisms associated with addiction-related behavioural effects of nicotine and/or cocaine: implication of the endocannabinoid system

The addictive use of nicotine (NC) and cocaine (COC) continues to be a major public health problem, and their combined use has been reported, particularly during adolescence. In neural plasticity, commonly induced by NC and COC, as well as behavioural plasticity related to the use of these two drugs, the involvement of epigenetic mechanisms, in which the reversible regulation of gene expression occurs independently of the DNA sequence, has recently been reported. Furthermore, on the basis of intense interactions with the target neurotransmitter systems, the endocannabinoid (ECB) system has been considered pivotal for eliciting the effects of NC or COC. The combined use of marijuana with NC and/or COC has also been reported. This article presents the addiction-related behavioural effects of NC and/or COC, based on the common behavioural/neural plasticity and combined use of NC/COC, and reviews the interacting role of the ECB system. The epigenetic processes inseparable from the effects of NC and/or COC (i.e. DNA methylation, histone modifications and alterations in microRNAs) and the putative therapeutic involvement of the ECB system at the epigenetic level are also discussed.

Association study of DRD2 A2/A1, DRD3 Ser9Gly, DβH -1021C>T, OPRM1 A118G and GRIK1 rs2832407C>A polymorphisms with alcohol dependence

BACKGROUND

The reinforcing effects of alcohol are mediated through complex interactions between multiple neurochemical systems. Genes of dopaminergic (DRD2, DRD3 and DβH), opioid (OPRM1) and glutaminergic (GRIK1) systems mediate the dependent behavior via different mechanisms; however, they all target the serotonergic and dopaminergic pathways in the ventral tegmental area.

METHODS

In the present study, DRD2 A2/A1, DRD3 Ser9Gly, DβH -1021C>T, OPRM1 A118G and GRIK1 rs2832407C>A polymorphisms and their interactions were analyzed in 72 alcohol-dependent patients and 74 controls of Greek-Cypriot origin, using the PCR-RFLP method.

RESULTS

No differences were found in the genotype or allele distribution of DRD2 A2/A1, DRD3 Ser9Gly, DβH -1021C>T, OPRM1 A118G and GRIK1 rs2832407C>A between alcohol-dependent patients and controls. Additionally, we did not find any gene×gene interactions in association with alcohol dependence in the studied population.

CONCLUSIONS

Alcohol dependence is a complex interaction of genetic and environmental factors. In the present study, we have shown that DRD2 A2/A1, DRD3 Ser9Gly, DβH -1021C>T, OPRM1 A118G and GRIK1 rs2832407C>A are not associated with this dependent behavior alone or in interaction.

The effects of nicotine in the neonatal quinpirole rodent model of psychosis: Neural plasticity mechanisms and nicotinic receptor changes

Neonatal quinpirole (NQ) treatment to rats increases dopamine D2 receptor sensitivity persistent throughout the animal's lifetime. In Experiment 1, we analyzed the role of α7 and α4β2 nicotinic receptors (nAChRs) in nicotine behavioral sensitization and on the brain-derived neurotrophic factor (BDNF) response to nicotine in NQ- and neonatally saline (NS)-treated rats. In Experiment 2, we analyzed changes in α7 and α4β2 nAChR density in the nucleus accumbens (NAcc) and dorsal striatum in NQ and NS animals sensitized to nicotine. Male and female Sprague-Dawley rats were neonatally treated with quinpirole (1mg/kg) or saline from postnatal days (P)1-21. Animals were given ip injections of either saline or nicotine (0.5mg/kg free base) every second day from P33 to P49 and tested on behavioral sensitization. Before each injection, animals were ip administered the α7 nAChR antagonist methyllycaconitine (MLA; 2 or 4mg/kg) or the α4β2 nAChR antagonist dihydro beta erythroidine (DhβE; 1 or 3mg/kg). Results revealed NQ enhanced nicotine sensitization that was blocked by DhβE. MLA blocked the enhanced nicotine sensitization in NQ animals, but did not block nicotine sensitization. NQ enhanced the NAcc BDNF response to nicotine which was blocked by both antagonists. In Experiment 2, NQ enhanced nicotine sensitization and enhanced α4β2, but not α7, nAChR upregulation in the NAcc. These results suggest a relationship between accumbal BDNF and α4β2 nAChRs and their role in the behavioral response to nicotine in the NQ model which has relevance to schizophrenia, a behavioral disorder with high rates of tobacco smoking.

Nicotine-induced behavioral sensitization in an adult rat model of attention deficit/hyperactivity disorder (ADHD)

Attention deficit hyperactivity disorder (ADHD) is associated with increased risk of tobacco dependence. Nicotine, the main psychoactive component of tobacco, appears to be implicated in ADHD-related tobacco dependence. However, the behavioral responsiveness to nicotine of the prevalent animal model of ADHD, the spontaneously hypertensive rat (SHR), is currently underinvestigated. The present study examined the activational effects of acute and chronic nicotine on the behavior of adult male SHRs, relative to Wistar Kyoto (WKY) controls. Experiment 1 verified baseline strain differences in open-field locomotor activity. Experiment 2 tested for baseline strain differences in rotational behavior using a Rotorat apparatus. Adult SHR and WKY rats were then exposed to a 7-day regimen of 0.6mg/kg/d s.c. nicotine, or saline, prior to each assessment. A separate group of SHRs underwent similar training, but was pre-treated with mecamylamine, a cholinergic antagonist. Nicotine sensitization, context conditioning, and mecamylamine effects were then tested. Baseline strain differences were observed in open-field performance and in the number of full rotations in the Rotorat apparatus, but not in the number of 90° rotations or direction changes. In these latter measures, SHRs displayed weaker nicotine-induced rotational suppression than WKYs. Both strains expressed nicotine-induced sensitization of rotational activity, but evidence for strain differences in sensitization was ambiguous; context conditioning was not observed. Mecamylamine reversed the effects of nicotine on SHR performance. These findings are consistent with the hypothesis that a reduced aversion to nicotine (expressed in rats as robust locomotion) may facilitate smoking among adults with ADHD.

Diacylglycerol lipase disinhibits VTA dopamine neurons during chronic nicotine exposure

Chronic nicotine exposure (CNE) alters synaptic transmission in the ventral tegmental area (VTA) in a manner that enhances dopaminergic signaling and promotes nicotine use. The present experiments identify a correlation between enhanced production of the endogenous cannabinoid 2-arachidonoylglycerol (2-AG) and diminished release of the inhibitory neurotransmitter GABA in the VTA following CNE. To study the functional role of on-demand 2-AG signaling in GABAergic synapses, we used 1,2,3-triazole urea compounds to selectively inhibit 2-AG biosynthesis by diacylglycerol lipase (DAGL). The potency and selectivity of these inhibitors were established in rats in vitro (rat brain proteome), ex vivo (brain slices), and in vivo (intracerebroventricular administration) using activity-based protein profiling and targeted metabolomics analyses. Inhibition of DAGL (2-AG biosynthesis) rescues nicotine-induced VTA GABA signaling following CNE. Conversely, enhancement of 2-AG signaling in naïve rats by inhibiting 2-AG degradation recapitulates the loss of nicotine-induced GABA signaling evident following CNE. DAGL inhibition reduces nicotine self-administration without disrupting operant responding for a nondrug reinforcer or motor activity. Collectively, these findings provide a detailed characterization of selective inhibitors of rat brain DAGL and demonstrate that excessive 2-AG signaling contributes to a loss of inhibitory GABAergic constraint of VTA excitability following CNE.

Striatal NOS1 has dimorphic expression and activity under stress and nicotine sensitization

Nicotine exerts its addictive influence through the meso-cortico-limbic reward system, where the striatum is essential. Nicotine addiction involves different neurotransmitters, nitric oxide (NO) being especially important, since it triggers the release of the others by positive feedback. In the nervous system, NO is mainly produced by nitric oxide synthase 1 (NOS1). However, other subtypes of synthases can also synthesize NO, and little is known about the specific role of each isoform in the process of addiction. In parallel, NOS activity and nicotine addiction are also affected by stress and sexual dimorphism. To determine the specific role of this enzyme, we analyzed both NOS expression and NO synthesis in the striatum of wild-type and NOS1-knocked out (KO) mice of both sexes in situations of nicotine sensitization and stress. Our results demonstrated differences between the caudate-putamen (CP) and nucleus accumbens (NA). With respect to NOS1 expression, the CP is a dimorphic region (27.5% lower cell density in males), but with a stable production of NO, exclusively due to this isoform. Thus, the nitrergic system of CP may not be involved in stress or nicotine addiction. Conversely, the NA is much more variable and strongly involved in both situations: its NO synthesis displays dimorphic variations at both basal (68.5% reduction in females) and stress levels (65.9% reduction in males), which disappear when nicotine is infused. Thus, the KO animals showed an increase in NO production (21.7%) in the NA, probably by NOS3, in an attempt to compensate the lack of NOS1.

Cholinergic modulation of dopamine pathways through nicotinic acetylcholine receptors

Nicotine addiction is highly prevalent in current society and is often comorbid with other diseases. In the central nervous system, nicotine acts as an agonist for nicotinic acetylcholine receptors (nAChRs) and its effects depend on location and receptor composition. Although nicotinic receptors are found in most brain regions, many studies on addiction have focused on the mesolimbic system and its reported behavioral correlates such as reward processing and reinforcement learning. Profound modulatory cholinergic input from the pedunculopontine and laterodorsal tegmentum to dopaminergic midbrain nuclei as well as local cholinergic interneuron projections to dopamine neuron axons in the striatum may play a major role in the effects of nicotine. Moreover, an indirect mesocorticolimbic feedback loop involving the medial prefrontal cortex may be involved in behavioral characteristics of nicotine addiction. Therefore, this review will highlight current understanding of the effects of nicotine on the function of mesolimbic and mesocortical dopamine projections in the mesocorticolimbic circuit.

Gene-gene interaction of μ-opioid receptor and GluR5 kainate receptor subunit is associated with smoking behavior in a Greek population: presence of a dose allele effect

BACKGROUND

Components of nicotine reward system can potentially influence smoking behavior. The μ-opioid receptor (OPRM1) binds the endogenous opioid peptide β-endorphin and mediates the reinforcing effects of nicotine, while the GluR5 kainate receptor subunit (encoded by GRIK1 gene), a binding site for known mediators of glutamate neurotransmission, potentially affects the glutaminergic system that is also indirectly implicated in the reward system.

METHODS

In the present study, OPRM1 A118G and GRIK1 rs2832407C>A polymorphisms and their interactions were analyzed in 132 smoking initiators (SI) and 144 non-initiators (NI) of Greek origin, using the PCR-RFLP method.

RESULTS

No differences were found in the genotype or allele distribution of OPRM1 A118G and GRIK1 rs2832407C>A between SI and NI. However, we found a significant interaction of OPRM1 A118G and GRIK1 rs2832407C>A genotypes associated with smoking initiation in a model adjusted for age, sex, BMI and type 2 diabetes mellitus (odds ratio=1.341, 95% CI 1.024-1.755, p=0.033). A dose effect of OPRM1 and GRIK1 variant alleles was present. Increased number of variant alleles (from 0 to 4) was associated with smoking initiation in the same adjusted model (odds ratio=1.537, 95% CI 1.030-2.293, p=0.036).

CONCLUSIONS

Smoking phenotype is a complex interaction of genetic and environmental factors. In the present study, we have shown that gene-gene interaction of components of different systems associated with nicotine reinforcing effects, such as OPRM1 and GRIK1, rather than one gene polymorphism, is associated with smoking behavior.

Applications of the Neonatal Quinpirole Model to Psychosis and Convergence upon the Dopamine D2 Receptor

This mini review focuses on the importance of the dopamine D₂-like receptor family and its importance in psychosis. Past findings from this laboratory along with collaborators have been that neonatal quinpirole (a dopamine D₂-like receptor agonist) results in increases in dopamine D₂ receptor sensitivity that persists throughout the animal's lifetime. Findings from this model have been shown to have particular application and validity to schizophrenia, but may have broader implications toward other psychoses, which is reviewed in the present manuscript. In the present review, we also highlight other models of psychoses that have been centered on the subchronic administration of quinpirole to rats in order to model certain psychoses, which has uncovered some interesting and valid behavioral findings. This review highlights the importance of the combination of behavioral findings and neurobiological mechanisms focusing on neural plasticity in discovering underlying pathologies in these disorders that may lead to treatment discoveries, as well as the value of animal models across all psychoses.

Nicotinic, glutamatergic and dopaminergic synaptic transmission and plasticity in the mesocorticolimbic system: focus on nicotine effects

Cigarette smoking is currently the leading cause of preventable deaths and disability throughout the world, being responsible for about five million premature deaths/year. Unfortunately, fewer than 10% of tobacco users who try to stop smoking actually manage to do so. The main addictive agent delivered by cigarette smoke is nicotine, which induces psychostimulation and reward, and reduces stress and anxiety. The use of new technologies (including optogenetics) and the development of mouse models characterised by cell-specific deletions of receptor subtype genes or the expression of gain-of-function nAChR subunits has greatly increased our understanding of the molecular mechanisms and neural substrates of nicotine addiction first revealed by classic electrophysiological, neurochemical and behavioural approaches. It is now becoming clear that various aspects of nicotine dependence are mediated by close interactions of the glutamatergic, dopaminergic and γ-aminobutyric acidergic systems in the mesocorticolimbic system. This review is divided into two parts. The first provides an updated overview of the circuitry of the ventral tegmental area, ventral striatum and prefrontal cortex, the neurotransmitter receptor subtypes expressed in these areas, and their physiological role in the mesocorticolimbic system. The second will focus on the molecular, functional and behavioural mechanisms involved in the acute and chronic effects of nicotine on the mesocorticolimbic system.

Restraint stress attenuates nicotine's locomotor stimulant but not discriminative stimulus effects in rats

Stress enhances the locomotor stimulant and discriminative stimulus effects of several addictive drugs (e.g., morphine) in rodents, yet interactions between stress and nicotine's effects in these behavioral models have not been well established. To this end, the current studies examined the effects of restraint stress on nicotine-induced locomotor activity and nicotine discrimination in rats. We used a novel approach in which onset of stress and nicotine administration occurred concurrently (i.e., simultaneous exposure) to simulate effects of stress on ongoing tobacco use, as well as a more traditional approach in which a delay was imposed between stress and nicotine administration (i.e., sequential exposure). Simultaneous exposure to stress reduced the rate of locomotor sensitization induced by daily injections of nicotine (0.4 mg/kg, s.c.). A lower dose of nicotine (0.1mg/kg, s.c.) produced modest effects on activity that were generally unaffected by simultaneous exposure to stress. Sequential exposure to stress and nicotine (0.4 mg/kg, s.c.) slightly suppressed nicotine-induced activity but did not influence rate of locomotor sensitization. Neither simultaneous nor sequential exposure to stress influenced the discriminative stimulus effects of nicotine (0.01-0.2mg/kg, s.c.). These data show that restraint stress reduces nicotine's locomotor stimulant effects, particularly when onset of stress and nicotine exposure occurs simultaneously, but does not influence nicotine discrimination. These findings contrast with the ability of stress to enhance the effects of other drugs in these models. This study also suggests that studying the influence of simultaneous stress exposure on drug effects may be useful for understanding the role of stress in addiction.

Nicotine inhibits the proliferation by upregulation of nitric oxide and increased HDAC1 in mouse neural stem cells

Cigarette smoking (CS) is considered one of the major risk factors to cause neurodegenerative disorders. Nicotine is the main chemical in CS which is responsible for dysfunction of the brain as a neuroteratogen. Also, nicotine dependency is a real mental illness and disease. Recently, chronic nicotine exposure has been shown to cause oxidative/nitrosative stress leading to a deleterious condition to cellular death in different brain regions. However, little is known about the effects of nicotine on mouse neural stem cells (mNSCs). The aim of this study is to investigate the effects of nicotine on mNSCs and elucidate underlying mechanisms involved in expression of a diversity of genes regulated by nicotine. When mNSCs were isolated from the whole brain of embryonic day 16 mice treated with nicotine at vehicle, 100, 400, and 800 μM for 5 d, nicotine significantly decreased the number and size of neurospheres. In immunocytochemistry, nicotine-exposed mNSCs expressing nestin showed the shortened filaments and condensed nuclei. In RT-PCR, messenger RNA (mRNA) levels of proliferating cell nuclear antigen (PCNA) and sirtuin1 (SIRT1) were significantly decreased, while the production of nitric oxide and mRNA levels of cyclooxygenase2 (COX-2), tumor necrosis factor-alpha TNF-α, and histone deacetylase 1 (HDAC1) were increased in a dose-dependent manner. In addition, sodium butyrate and valproic acid, HDAC inhibitors, partially rescue proliferation of mNSCs via inhibition of HDAC1 expression and NO production. Taken together, these data demonstrate that prolonged exposure of nicotine decreased proliferation of mNSCs by increased NO and inflammatory cytokine through increased HDAC1. Furthermore, this study could help in the development of a therapy for nicotine-induced neurodegenerative disorder and drug abuse.

Critical role of peripheral sensory systems in mediating the neural effects of nicotine following its acute and repeated exposure

It is well established that the reinforcing properties of nicotine (NIC) depend on its action on nicotinic acetylcholine receptors expressed by brain neurons. However, when administered systemically, NIC first phasically activates nicotinic receptors located on the afferents of sensory nerves at the sites of drug administration before reaching the brain and directly interacting with central neurons. While this peripheral action of NIC has been known for years, it is usually neglected in any consideration of the drug's reinforcing properties and experience-dependent changes of its behavioral and physiological effects. The goal of this work was to review our recent behavioral, electrophysiological, and physiological data suggesting the critical importance of peripheral actions of NIC in mediating its neural effects following acute drug exposure and their involvement in alterations of NIC effects consistently occurring following repeated drug exposure. Because NIC, by acting peripherally, produces a rapid sensory signal to the central nervous system that is followed by slower, more prolonged direct drug actions in the brain, these two pharmacological actions interact in the central nervous system during repeated drug use with the development of Pavlovian conditioned association. This within-drug conditioning mechanism could explain the experience-dependent changes in the physiological, behavioral, and human psychoemotional effects of NIC, which, in drug-experienced individuals, always represent a combination of pharmacological and learning variables.

Exposure to nicotine enhances its subsequent self-administration: contribution of nicotine-associated contextual stimuli

Contextual stimuli present during nicotine exposure can come to act as conditioned stimuli and have been shown to play an important role in ongoing nicotine self-administration. In the present study, we characterized the effects of contextual stimuli previously paired with non-contingent nicotine exposure injections on subsequent nicotine self-administration. Rats were exposed to five injections of either saline or nicotine (0.4 mg/kg, i.p.) in either their home cage or a self-administration chamber with the levers retracted. Two weeks later, they were allowed to self-administer nicotine (30 μg/kg/infusion, IV) under fixed ratio (FR) schedules of reinforcement across 12 consecutive sessions. Lastly, responding under a progressive ratio (PR) schedule was assessed. Rats exposed to nicotine in the self-administration chamber subsequently increased their intake of nicotine across the FR test days, obtaining more infusions on average by days 7-12 compared to their saline exposed controls. This increase was not due to nicotine exposure alone as rats exposed to nicotine in the home cage did not show this effect. It was also not due to differences in the final ratio achieved between nicotine and saline exposed rats. Although rats exposed to nicotine in the self-administration chambers displayed reduced discrimination between the active and inactive levers during FR testing, they showed increased motivation to self-administer nicotine under the PR schedule. These results indicate that exposure to nicotine can enhance its subsequent self-administration and highlight the contribution of nicotine-associated contextual stimuli to the work output rats ultimately emit to obtain the drug.

Intravenous nicotine injection induces rapid, experience-dependent sensitization of glutamate release in the ventral tegmental area and nucleus accumbens

Although numerous data suggest that glutamate (GLU) is involved in mediating the neural effects of nicotine, direct data on nicotine-induced changes in GLU release are still lacking. Here, we used high-speed amperometry with enzyme-based GLU and enzyme-free GLU-null biosensors to examine changes in extracellular GLU levels in the ventral tegmental area (VTA) and nucleus accumbens shell (NAcc) induced by intravenous nicotine in a low, behaviorally active dose (30 μg/kg) in freely moving rats. Using this approach, we found that the initial nicotine injection in drug-naive conditions induces rapid, transient, and relatively small GLU release (~ 90 nM; latency ~ 15 s, duration ~ 60 s) that is correlative in the VTA and NAcc. Following subsequent nicotine injections within the same session, this phasic GLU release was supplemented by stronger tonic increases in GLU levels (100-300 nM) that paralleled increases in drug-induced locomotor activation. GLU responses induced by repeated nicotine injections were more phasic and stronger in the NAcc than in VTA. Therefore, GLU is phasically released within the brain's reinforcement circuit following intravenous nicotine administration. Robust enhancement of nicotine-induced GLU responses following repeated injections suggests this change as an important mediator of sensitized behavioral and neural effects of nicotine. By using high-speed amperometry with glutamate (GLU) biosensors, we show that i.v. nicotine at a low, behaviorally relevant dose induces rapid GLU release in the NAcc and VTA that is enhanced following repeated drug injections. This is the first study reporting second-scale fluctuations in extracellular GLU levels induced by nicotine in two critical structures of the motivation-reinforcement circuit and rapid sensitization of GLU responses coupled with locomotor sensitization.

Rapid sensitization of physiological, neuronal, and locomotor effects of nicotine: critical role of peripheral drug actions

Repeated exposure to nicotine and other psychostimulant drugs produces persistent increases in their psychomotor and physiological effects (sensitization), a phenomenon related to the drugs' reinforcing properties and abuse potential. Here we examined the role of peripheral actions of nicotine in nicotine-induced sensitization of centrally mediated physiological parameters (brain, muscle, and skin temperatures), cortical and VTA EEG, neck EMG activity, and locomotion in freely moving rats. Repeated injections of intravenous nicotine (30 μg/kg) induced sensitization of the drug's effects on all these measures. In contrast, repeated injections of the peripherally acting analog of nicotine, nicotine pyrrolidine methiodide (nicotine(PM), 30 μg/kg, i.v.) resulted in habituation (tolerance) of the same physiological, neuronal, and behavioral measures. However, after repeated nicotine exposure, acute nicotine(PM) injections induced nicotine-like physiological responses: powerful cortical and VTA EEG desynchronization, EMG activation, a large brain temperature increase, but weaker hyperlocomotion. Additionally, both the acute locomotor response to nicotine and nicotine-induced locomotor sensitization were attenuated by blockade of peripheral nicotinic receptors by hexamethonium (3 mg/kg, i.v.). These data suggest that the peripheral actions of nicotine, which precede its direct central actions, serve as a conditioned interoceptive cue capable of eliciting nicotine-like physiological and neural responses after repeated nicotine exposure. Thus, by providing a neural signal to the CNS that is repeatedly paired with the direct central effects of nicotine, the drug's peripheral actions play a critical role in the development of nicotine-induced physiological, neural, and behavioral sensitization.

Dual role of PPAR-γ in induction and expression of behavioral sensitization to cannabinoid receptor agonist WIN55,212-2

Behavioral sensitization (B.S.) is a pathophysiological animal model for stimulant-induced psychosis and addiction. Accumulated evidence indicates that inflammatory processes are involved in psychostimulants effects in the CNS. Cannabinoids like WIN55,212-2 act as potential activators of PPAR-γ and affects the inflammatory status of the CNS. The purpose of this study is to determine PPAR-γ role in induction and expression of B.S. and the coincident inflammatory responses developed by WIN55,212-2 (WIN). Using open-field test, locomotor activity was monitored in animals treated with intraperitoneal low-dose WIN single or repeated injections. Concurrent striatal COX-2 and TNF-α levels and PPAR-γ activity were determined by immunoblotting assay. Effects of concomitant chronic or acute PPAR-γ pharmacological inhibition (with GW9662) were then investigated on behavioral and biochemical variables. WIN enhanced locomotor activity and while administered chronically augmented cytosolic COX-2 and TNF-α and also PPAR-γ nuclear levels. GW9662 co-administration completely prevented the induction of sensitizing effects of chronic WIN and altered the inflammatory responses. However, the expression of B.S. was intensified with GW9662 as assessed by increased locomotion after WIN challenge following 48 h withdrawal. Neuroinflammation and locomotor excitability in animals received just a single-dose WIN were also escalated with GW9662. Our findings conclude that PPAR-γ could play different key roles during B.S. development by WIN. Although PPAR-γ is mostly known for neuroprotective and anti-inflammatory effects, our data indicate that it mediates the B.S. induction by chronic WIN. However, while the B.S. was induced, PPAR-γ could play a homeostatic role opposing the expressed B.S. escalation.

Implicit and explicit reward learning in chronic nicotine use

BACKGROUND

Chronic tobacco use is related to specific neurobiological alterations in the dopaminergic brain reward system that can be termed "reward deficiency syndrome" in dependent nicotine consumers. The close linkage of dopaminergic activity and reward learning led us to expect implicit and explicit reward learning deficits in dependent compared to non-smokers. Smokers who maintain a less regular, occasional use may also, to a lesser extent, show implicit reward learning deficits. The purpose of our study was to examine the behavioral effects of the neurobiological alterations on reward related learning. We also tested whether any deficits observed in an abstinent state are also present in a satiated state.

METHODS

In two studies, we examined implicit and explicit reward learning in smokers. Participants were administered a probabilistic implicit reward learning task, and an explicit reward- and punishment-based trial-and-error learning task. In Study 1, we compared dependent, occasional, and non-smokers, and in Study 2 satiated and abstinent smokers.

RESULTS

In Study 1, chronic and occasional smokers showed impairments in both, implicit and explicit reward learning tasks. In Study 2, satiated smokers did not perform better than abstinent smokers.

CONCLUSIONS

The results support the hypothesis of reward learning deficits. These deficits are not limited to explicit but extend to implicit reward learning and cannot be explained by tobacco withdrawal.

Nicotinic regulation of energy homeostasis

INTRODUCTION

The ability of nicotine, the primary psychoactive substance in tobacco smoke, to regulate appetite and body weight is one of the factors cited by smokers that prevents them from quitting and is the primary reason for smoking initiation in teenage girls. The regulation of feeding and metabolism by nicotine is complex, and recent studies have begun to identify nicotinic acetylcholine receptor (nAChR) subtypes and circuits or cell types involved in this regulation.

DISCUSSION

We will briefly describe the primary anatomical and functional features of the input, output, and central integration structures of the neuroendocrine systems that regulate energy homeostasis. Then, we will describe the nAChR subtypes expressed in these structures in mammals to identify the possible molecular targets for nicotine. Finally, we will review the effects of nicotine and its withdrawal on feeding and energy metabolism and attribute them to potential central and peripheral cellular targets.

Neuronal nicotinic acetylcholine receptors: neuroplastic changes underlying alcohol and nicotine addictions

Addictive drugs can activate systems involved in normal reward-related learning, creating long-lasting memories of the drug's reinforcing effects and the environmental cues surrounding the experience. These memories significantly contribute to the maintenance of compulsive drug use as well as cue-induced relapse which can occur even after long periods of abstinence. Synaptic plasticity is thought to be a prominent molecular mechanism underlying drug-induced learning and memories. Ethanol and nicotine are both widely abused drugs that share a common molecular target in the brain, the neuronal nicotinic acetylcholine receptors (nAChRs). The nAChRs are ligand-gated ion channels that are vastly distributed throughout the brain and play a key role in synaptic neurotransmission. In this review, we will delineate the role of nAChRs in the development of ethanol and nicotine addiction. We will characterize both ethanol and nicotine's effects on nAChR-mediated synaptic transmission and plasticity in several key brain areas that are important for addiction. Finally, we will discuss some of the behavioral outcomes of drug-induced synaptic plasticity in animal models. An understanding of the molecular and cellular changes that occur following administration of ethanol and nicotine will lead to better therapeutic strategies.

Glutamatergic and GABAergic metabolism in mouse brain under chronic nicotine exposure: implications for addiction

Background and Purpose

The effects of nicotine on cerebral metabolism and its influence on smoking behavior is poorly understood. An understanding of the chronic effects of nicotine on excitatory and inhibitory metabolic demand, and corresponding neurotransmission may provide clues for designing strategies for the optimal smoking cessation intervention. The objective of the current study was to investigate neuronal and astroglial metabolism in mice exposed to nicotine (0.5 and 2.0 mg/kg, sc) three times in a day for 4 weeks.

Experimental Approach/Principal Findings

Metabolic measurements were carried out by co-infusing [U-¹³C₆]glucose and [2-¹³C]acetate, and monitoring ¹³C labeling of amino acids in brain tissue extract using ¹H-[¹³C] and ¹³C-[¹H]-NMR spectroscopy. Concentration of ¹³C-labeled glutamate-C4 was increased significantly from glucose and acetate with chronic nicotine treatment indicating an increase in glucose oxidation by glutamatergic neurons in all brain regions and glutamate-glutamine neurotransmitter cycle in cortical and subcortical regions. However, chronic nicotine treatment led to increased labeling of GABA-C2 from glucose only in the cortical region. Further, increased labeling of glutamine-C4 from [2-¹³C]acetate is suggestive of increased astroglial activity in subcortical and cerebellum regions of brain with chronic nicotine treatment.

Conclusions and Significance

Chronic nicotine exposure enhanced excitatory activity in the majority of brain regions while inhibitory and astroglial functions were enhanced only in selected brain regions.

α6β2* and α4β2* nicotinic acetylcholine receptors as drug targets for Parkinson's disease

Parkinson's disease is a debilitating movement disorder characterized by a generalized dysfunction of the nervous system, with a particularly prominent decline in the nigrostriatal dopaminergic pathway. Although there is currently no cure, drugs targeting the dopaminergic system provide major symptomatic relief. As well, agents directed to other neurotransmitter systems are of therapeutic benefit. Such drugs may act by directly improving functional deficits in these other systems, or they may restore aberrant motor activity that arises as a result of a dopaminergic imbalance. Recent research attention has focused on a role for drugs targeting the nicotinic cholinergic systems. The rationale for such work stems from basic research findings that there is an extensive overlap in the organization and function of the nicotinic cholinergic and dopaminergic systems in the basal ganglia. In addition, nicotinic acetylcholine receptor (nAChR) drugs could have clinical potential for Parkinson's disease. Evidence for this proposition stems from studies with experimental animal models showing that nicotine protects against neurotoxin-induced nigrostriatal damage and improves motor complications associated with l-DOPA, the "gold standard" for Parkinson's disease treatment. Nicotine interacts with multiple central nervous system receptors to generate therapeutic responses but also produces side effects. It is important therefore to identify the nAChR subtypes most beneficial for treating Parkinson's disease. Here we review nAChRs with particular emphasis on the subtypes that contribute to basal ganglia function. Accumulating evidence suggests that drugs targeting α6β2* and α4β2* nAChR may prove useful in the management of Parkinson's disease.

Lack of association between dopamine receptor D4 variable numbers of tandem repeats gene polymorphism and smoking

Nicotine addiction, related to cigarette smoking, develops as a product of the complex interactions between social, environmental and genetic factors. Genes encoding the components of the dopaminergic system are thought to be associated with smoking. Literature data showed not only an association, but also a lack of association between variable number of tandem repeats (VNTR) polymorphism located in the third exon of dopamine D4 receptor (DRD4) gene and smoking. Repetitive sequence of DRD4 VNTR is 48 bp long and maximum 11 tandem copies were reported in humans. Presence of alleles with 6 and more repeats (i.e. long alleles) was associated with greater tendency to novelty seeking and addictive behaviors than the presence of 5 and less alleles (short alleles). The aim of this study was to determine the association between VNTR in DRD4 gene and present smoking status in ethnically homogenous Caucasian population from the Eastern European (Croatian) origin. Genotyping was done in 565 healthy subjects, 511 men and 54 women, respectively, who were subdivided into 176 smokers and 389 nonsmokers. Logistic regression analyses, adjusted for age and sex, revealed the lack of significant (p>0.05) effect of the 4/4, 4/7 and 7/7 genotypes, or carriers of the long and short allele, or all genotypes of the DRD4 VNTR on smoking status. The results of this study failed to confirm the hypothesis that long allele of the DRD4 VNTR is associated with smoking status in Caucasian subjects.

Varenicline is a potent partial agonist at α6β2* nicotinic acetylcholine receptors in rat and monkey striatum

Extensive evidence indicates that varenicline reduces nicotine craving and withdrawal symptoms by modulating dopaminergic function at α4β2* nicotinic acetylcholine receptors (nAChRs) (the asterisk indicates the possible presence of other nicotinic subunits in the receptor complex). More recent data suggest that α6β2* nAChRs also regulate dopamine release and mediate nicotine reinforcement. The present experiments were therefore done to test the effect of varenicline on α6β2* nAChRs and their function, because its interaction with this subtype is currently unclear. Receptor competition studies showed that varenicline inhibited α6β2* nAChR binding (K(i) = 0.12 nM) as potently as α4β2* nAChR binding (K(i) = 0.14 nM) in rat striatal sections and with ∼20-fold greater affinity than nicotine. Functionally, varenicline was more potent in stimulating α6β2* versus α4β2* nAChR-mediated [(3)H]dopamine release from rat striatal synaptosomes with EC(50) values of 0.007 and 0.086 μM, respectively. However, it acted as a partial agonist on α6β2* and α4β2* nAChR-mediated [(3)H]dopamine release with maximal efficacies of 49 and 24%, respectively, compared with nicotine. We also evaluated varenicline's action in striatum of monkeys, a useful animal model for comparison with humans. Varenicline again potently inhibited monkey striatal α6β2* (K(i) = 0.13 nM) and α4β2* (K(i) = 0.19 nM) nAChRs in competition studies. Functionally, it potently stimulated both α6β2* (EC(50) = 0.014 μM) and α4β2* (EC(50) = 0.029 μM) nAChR-mediated [(3)H]dopamine release from monkey striatal synaptosomes, again acting as a partial agonist relative to nicotine at both subtypes. These data suggest that the ability of varenicline to interact at α6β2* nAChRs may contribute to its efficacy as a smoking cessation aid.

How addictive drugs disrupt presynaptic dopamine neurotransmission

The fundamental principle that unites addictive drugs appears to be that each enhances synaptic dopamine by means that dissociate it from normal behavioral control, so that they act to reinforce their own acquisition. This occurs via the modulation of synaptic mechanisms that can be involved in learning, including enhanced excitation or disinhibition of dopamine neuron activity, blockade of dopamine reuptake, and altering the state of the presynaptic terminal to enhance evoked over basal transmission. Amphetamines offer an exception to such modulation in that they combine multiple effects to produce nonexocytic stimulation-independent release of neurotransmitter via reverse transport independent from normal presynaptic function. Questions about the molecular actions of addictive drugs, prominently including the actions of alcohol and solvents, remain unresolved, but their ability to co-opt normal presynaptic functions helps to explain why treatment for addiction has been challenging.

Brain nicotinic receptors as emerging targets for drug addiction: neurobiology to translational research

Drug addiction, a chronic relapsing disorder, is a serious public health problem around the world. A growing body of preclinical and clinical evidence suggests that mammalian brain nicotinic acetylcholine receptors (nAChRs), the heterogeneous family of ion channels, play a pivotal role in drug addiction, including nicotine and alcohol dependence. As a result, there is an increasing interest in developing nAChR-based therapies for the treatment of addictive disorders. The current review summarizes the important preclinical and clinical data, demonstrating the ability of nAChR ligands to modulate nicotine and alcohol-induced biobehavioral and neurochemical changes in laboratory animals and humans. Recent studies suggest that partial agonists and antagonists at nAChRs have therapeutic potential for the management of nicotine and alcohol dependence. The complexity of nAChRs and their regulation for the development of nAChR-based drug candidates as novel pharmacotherapy for other addictive disorders will also be discussed. Taken together, this review will provide new insights into nAChR-based compounds and offer innovative translational strategies for combating drug addictive disorders.

86Rb+ efflux mediated by alpha4beta2*-nicotinic acetylcholine receptors with high and low-sensitivity to stimulation by acetylcholine display similar agonist-induced desensitization

The nicotinic acetylcholine receptors (nAChR) assembled from alpha4 and beta2 subunits are the most densely expressed subtype in the brain. Concentration-effect curves for agonist activation of alpha4beta2*-nAChR are biphasic. This biphasic agonist sensitivity is ascribed to differences in subunit stoichiometry. The studies described here evaluated desensitization elicited by low concentrations of epibatidine, nicotine, cytisine or methylcarbachol of brain alpha4beta2-nAChR function measured with acetylcholine-stimulated (86)Rb(+) efflux from mouse thalamic synaptosomes. Each agonist elicited concentration-dependent desensitization. The agonists differed in potency. However, IC(50) values for each agonist for desensitization of (86)Rb(+) efflux both with high (EC(50) approximately 3 microM) and low (EC(50) approximately 150 microM) acetylcholine sensitivity were not significantly different. Concentrations required to elicit desensitization were higher that their respective K(D) values for receptor binding. Even though the two components of alpha4beta2*-nAChR-mediated (86)Rb(+) efflux from mouse brain differ markedly in EC(50) values for agonist activation, they are equally sensitive to desensitization by exposure to low agonist concentrations. Mice were also chronically treated with nicotine by continuous infusion of 0, 0.5 or 4.0mg/kg/h and desensitization induced by nicotine was evaluated. Consistent with previous results, chronic nicotine treatment increased the density of epibatidine binding sites. Acute exposure to nicotine also elicited concentration-dependent desensitization of both high-sensitivity and low-sensitivity acetylcholine-stimulated (86)Rb(+) efflux from cortical and thalamic synaptosomes. Although chronic nicotine treatment reduced maximal (86)Rb(+) efflux from thalamus, IC(50) values in both brain regions were unaffected by chronic nicotine treatment.

In vivo nicotine exposure in the zebra finch: a promising innovative animal model to use in neurodegenerative disorders related research

Nicotine improves cognitive enhancement and there are indications that neurodegenerative (age-related) cognitive disorders could be treated with nicotine-based drugs. The zebra finch is a well-recognized model to study cognitive functioning; hence this model could be used to study the effects of nicotine in neurodegenerative cognitive disorders. However, nicotine's in vivo physiological and behavioral effects have never been studied in the zebra finch. Here we present the first in vivo nicotine study in zebra finches. We evaluated the dose-response effects of nicotine on locomotor activity, song production, food intake and body weight. A liquid chromatography tandem mass spectrometry method was developed and validated for quantification of nicotine and cotinine in feces. The subcutaneous nicotine drug regiment (0.054-0.54mg/kg) induced physiologically significant values of nicotine and cotinine. The mid (0.18mg/kg) and high (0.54mg/kg) dose of nicotine promoted the development and expression of a sensitized response of song production and locomotor activity. Food intake and body weight were not affected following nicotine exposure. In conclusion, the zebra finch can be used as an innovative animal model not only in nicotine-related research studying cognitive functioning, but also in studies examining nicotine dependence and addictive mechanisms.