Effects of chronic oral nicotine treatment and its withdrawal on locomotor activity and brain monoamines in mice

Chronic oral nicotine administration and withdrawal regulate the expression of neuropeptide Y and its receptors in the mesocorticolimbic system

Neuropeptide Y (NPY) and its receptors are involved in the regulation of mood, stress, and anxiety. In parallel, NPY signaling may play a vital role in the negative affective state induced by drug withdrawal. This study examined the changes in the transcript levels of NPY, Y1, Y2, and Y5 receptors in the mesocorticolimbic system during chronic nicotine exposure and withdrawal. Rats were administered with nicotine (initial dose: 25 μg/ml, maintenance dose: 50 μg/ml, free base) in drinking water for 12 weeks. Control group received only tap water. In the final week of the study, some of the nicotine-treated animals continued to receive nicotine (0-W), whereas some were withdrawn for either 24 (24-W) or 48 (48-W) h. All animals were decapitated after the evaluation of somatic signs (frequency of gasps, eye blinks, ptosis, shakes, teeth chatter) and the duration of locomotor activity and immobility. mRNA levels of NPY, Y1, Y2, and Y5 receptors in the mesocorticolimbic system were measured by quantitative real-time PCR (qRT-PCR). Results showed that nicotine withdrawal increased overall somatic signs. Moreover, chronic nicotine treatment increased the duration of locomotor activity, whereas withdrawal increased the duration of immobility. qRT-PCR analysis revealed that chronic nicotine treatment increased NPY mRNA levels in the hippocampus. On the other hand, 24- and 48-h withdrawals increased NPY mRNA levels in the amygdala and medial prefrontal cortex (mPFC), Y1 and Y2 mRNA levels in the nucleus accumbens and mPFC, and Y5 mRNA levels in the mPFC. These findings suggest that nicotine withdrawal enhances NPY signaling in the mesocorticolimbic system, which could be an important mechanism involved in regulating the negative affective state triggered during nicotine withdrawal.

GABAB receptors blockage modulates somatic and aversive manifestations induced by nicotine withdrawal

There is substantial evidence that GABA_B agonist, baclofen, prevents somatic and motivational responses induced by nicotine withdrawal and may target drug cue vulnerabilities in humans. In this context, we explored different aspects associated with the possible mechanisms whereby the GABA_B receptors might influence nicotine withdrawal. Male mice received nicotine (2.5 mg/kg, s.c.) 4 times daily, for 7 consecutive days. Nicotine-treated mice received the nicotinic acetylcholine receptor antagonist, mecamylamine (MEC, 2 or 3.5 mg/kg, s.c.), to precipitate the withdrawal state. A second group of dependent mice received 2-hydroxysaclofen (GABA_B receptor antagonist, 1 mg/kg, s.c.) before MEC-precipitated abstinence. Somatic signs of nicotine withdrawal were measured for 30 min. Anxiogenic-like response associated to nicotine withdrawal was assessed by the elevated plus maze test. The dysphoric/aversive effect induced by nicotine withdrawal was evaluated using conditioned place aversion paradigm. Dopamine, serotonin and its metabolites concentrations were determined by HPLC in the striatum, cortex and hippocampus. Finally, α4β2 nicotinic acetylcholine receptor density was determined in several brain regions using autoradiography assays. The results showed that MEC-precipitated nicotine withdrawal induced somatic manifestations, anxiogenic-like response and dysphoric/aversive effect, and 2-hydroxysaclofen potentiated these behavioral responses. Additionally, 2-hydroxysaclofen was able to change striatal dopamine levels and α4β2 nicotinic acetylcholine receptor density, both altered by MEC-precipitated nicotine withdrawal. These findings provide important contributions to elucidate neurobiological mechanisms implicated in nicotine withdrawal. We suggest that GABA_B receptor activity is necessary to control alterations induced by nicotine withdrawal, which supports the idea of targeting GABA_B receptors to treat tobacco addiction in humans.

Effect of 4-Fluoro-N-(4-sulfamoylbenzyl) Benzene Sulfonamide on cognitive deficits and hippocampal plasticity during nicotine withdrawal in rats

Withdrawal from chronic nicotine has damaging effects on a variety of learning and memory tasks. Various Sulfonamides that act as carbonic anhydrase inhibitors have documented role in modulation of various cognitive, learning, and memory processing. We investigated the effects of 4-Fluoro-N-(4-sulfamoylbenzyl) Benzene Sulfonamide (4-FBS) on nicotine withdrawal impairments in rats using Morris water maze (MWM), Novel object recognition, Passive avoidance, and open field tasks. Also, Brain-derived neurotrophic factor (BDNF) profiling and in vivo field potential recording were assessed. Rats were exposed to saline or chronic nicotine 3.8 mg/kg subcutaneously for 14 days in four divided doses, spontaneous nicotine withdrawal was induced by quitting nicotine for 72 h (hrs). Animals received 4-FBS at 20, 40, and 60 mg/kg after 72 h of withdrawal in various behavioral and electrophysiological paradigms. Nicotine withdrawal causes a deficit in learning and long-term memory in the MWM task. No significant difference was found in novel object recognition tasks among all groups while in passive avoidance task nicotine withdrawal resulted in a deficit of hippocampus-dependent fear learning. Anxiety like behavior was observed during nicotine withdrawal. Plasma BDNF level was reduced during nicotine withdrawal as compared to the saline group reflecting mild cognitive impairment, stress, and depression. Withdrawal from chronic nicotine altered hippocampal plasticity, caused suppression of long-term potentiation (LTP) in the CA1 area of the hippocampus. Our results showed that 4-FBS at 40 and 60 mg/kg significantly prevented nicotine withdrawal-induced cognitive deficits in behavioral as well as electrophysiological studies. 4-FBS at 60 mg/kg upsurge nicotine withdrawal-induced decrease in plasma BDNF. We conclude that 4-FBS at 40 and 60 mg /kg effectively prevented chronic nicotine withdrawal-induced impairment in long term potentiation and cognitive performance.

The Role of Nicotinic Receptors in the Attenuation of Autism-Related Behaviors in a Murine BTBR T + tf/J Autistic Model

Nicotinic receptors are distributed throughout the central and peripheral nervous system. Postmortem studies have reported that some nicotinic receptor subtypes are altered in the brains of autistic people. Recent studies have demonstrated the importance of nicotinic acetylcholine receptors (nAChRs) in the autistic behavior of BTBR T + tf/J mouse model of autism. This study was undertaken to examine the behavioral effects of targeted nAChRs using pharmacological ligands, including nicotine and mecamylamine in BTBR T + tf/J and C57BL/6J mice in a panel of behavioral tests relating to autism. These behavioral tests included the three-chamber social interaction, self-grooming, marble burying, locomotor activity, and rotarod test. We examined the effect of various oral doses of nicotine (50, 100, and 400 mcg/mL; po) over a period of 2 weeks in BTBR T + tf/J mouse model. The results indicated that the chronic administration of nicotine modulated sociability and repetitive behavior in BTBR T + tf/J mice while no effects observed in C57BL/6J mice. Furthermore, the nonselective nAChR antagonist, mecamylamine, reversed nicotine effects on sociability and increased repetitive behaviors in BTBR T + tf/J mice. Overall, the findings indicate that the pharmacological modulation of nicotinic receptors is involved in modulating core behavioral phenotypes in the BTBR T + tf/J mouse model. LAY SUMMARY: The involvement of brain nicotinic neurotransmission system plays a crucial role in regulating autism-related behavioral features. In addition, the brain of the autistic-like mouse model has a low acetylcholine level. Here, we report that nicotine, at certain doses, improved sociability and reduced repetitive behaviors in a mouse model of autism, implicating the potential therapeutic values of a pharmacological intervention targeting nicotinic receptors for autism therapy. Autism Res 2020, 13: 1311-1334. © 2020 International Society for Autism Research, Wiley Periodicals, Inc.

Chronic Nicotine Exposure Alters Metabotropic Glutamate Receptor 5: Longitudinal PET Study and Behavioural Assessment in Rats

Using positron emission tomography (PET), a profound alteration of the metabotropic glutamate receptor 5 (mGluR5) was found in human smoking addiction and abstinence. As human PET data either reflect the impact of chronic nicotine exposure or a pre-existing vulnerability to nicotine addiction, we designed a preclinical, longitudinal study to investigate the effect of chronic nicotine exposure on mGluR5 with the novel radiotracer [18F]PSS232 using PET. Twelve male dark Agouti rats at the age of 6 weeks were assigned randomly to three groups. From day 0 to day 250 the groups received 0 mg/L, 4 mg/L, or 8 mg/L nicotine solution in the drinking water. From day 250 to 320 all groups received nicotine-free drinking water. PET scans with [18F]PSS232 were performed in all animals on days 0, 250, and 320. To assess locomotion, seven tests in square open field arenas were carried out 72 days after the last PET scan. During the first four tests, rats received 0 mg/L nicotine and for the last three tests 4 mg/L nicotine in the drinking water. After 250 days of nicotine consumption [18F]PSS232 binding was reduced in the striatum, hippocampus, thalamus, and midbrain. At day 320, after nicotine withdrawal, [18F]PSS232 binding increased. These effects were more pronounced in the 4 mg/L nicotine group. Chronic administration of nicotine through the drinking water reduced exploratory behaviour. This preliminary longitudinal PET study demonstrates that chronic nicotine administration alters behaviour and mGluR5 availability. Chronic nicotine administration leads to decreased [18F]PSS232 binding which normalizes after prolonged nicotine withdrawal.

Protective effects of forced exercise against nicotine-induced anxiety, depression and cognition impairment in rat

BACKGROUND

Nicotine is one of the psychostimulant agents displaying parasympathomimetic activity; the chronic neurochemical and behavioral effects of nicotine remain unclear. Exercise lowers stress and anxiety and can act as a non-pharmacologic neuroprotective agent. In this study, the protective effects of exercise in nicotine withdrawal syndrome-induced anxiety, depression, and cognition impairment were investigated.

METHODS

Seventy adult male rats were divided randomly into five groups. Group 1 served as negative control and received normal saline (0.2 mL/rat, i.p.) for 30 days, whereas group 2 (as positive control) received nicotine (6 mg/kg/day, s.c.) for the first 15 days. Groups 4, 5, and 6 were treated with nicotine (6 mg/kg/day, s.c.) for the first 15 days and then were treated with forced exercise, bupropion (20 mg/kg/day, i.p.), or a combination of the two for the following 15 days. Between day 25 and day 30, Morris water maze was used to evaluate spatial learning and memory. From days 31 to 35, the elevated plus maze (EPM), open field test (OFT), forced swim test (FST), and tail suspension test (TST) were used to investigate the level of anxiety and depression in the subjects.

RESULTS

Nicotine-dependent animals indicated a reflective depression and anxiety in a dose-dependent manner in FST, EPM, and TST, which were significantly different from the control group and also can significantly attenuate the motor activity and anxiety in OFT.

CONCLUSIONS

Forced exercise, bupropion, or their combination can attenuate nicotine cessation-induced anxiety, depression, and motor activity in the mentioned behavioral assay. We conclude that forced exercise can protect the brain against nicotine withdrawal-induced anxiety, depression, and cognitive alteration.

Performance of motor associated behavioural tests following chronic nicotine administration

Background

Nicotine has shown potential therapeutic value for neurodegenerative diseases though there are concerns that it may induce behavioural deficits.

Purpose

The present study sought to determine the effect of chronic nicotine administration on overall motor functions and coordination.

Methods

Forty adult female and male Wistar rats were randomly grouped into 4 groups. Treated groups were administered nicotine via subcutaneous injections at doses of 0.25, 2 and 4 mg/kg body weight for 28 days. Control groups received normal saline. All animals were monitored for the first few minutes after each injection for any observed immediate effect of drug administration. Motor associated behavioural tests performed include: open field test, string test for grip strength and limb impairment, movement initiation and step test.

Results

Nicotine induced muscular convulsions within the first 1-5 minutes following daily subcutaneous injections, throughout the period of administration. This was observed to be more severe in females. Nicotine did not produce major alterations in overall motor functions and coordination in both females and males.

Conclusion

The present study shows chronic nicotine treatment produces muscular convulsion but no major deficit in overall motor function and coordination and that any observed alterations may just be transient effects.

The effects of lobeline on nicotine withdrawal-induced depression-like behavior in mice

RATIONALE

Evidence suggests that neuronal nicotinic acetylcholine receptor (nAChR) ligand lobeline has antidepressant-like properties.

OBJECTIVES

The present study investigated the effects of lobeline on nicotine withdrawal-induced depression-like behavior.

METHODS

Adult C57BL/6J mice were exposed to nicotine (200 μg/ml) in drinking solution for 3 weeks. During withdrawal, depression-like behavior was measured by the forced swim test (FST). We also determined norepinephrine (NE) levels in the prefrontal cortex (PFC) and hippocampus during nicotine withdrawal. Furthermore, we determined the effects of repeated treatment with lobeline or a selective α4β2 nAChR ligand 3-(pyridine-3́-yl)-cytisine on brain-derived neurotrophic factor (BDNF) and phosphorylated cAMP-responsive element binding (p-CREB) protein expression in the hippocampus.

RESULTS

Withdrawal from chronic nicotine increased immobility time in the FST, a measure for depression-like behavior. Pretreatment with lobeline significantly decreased immobility time during nicotine withdrawal. In addition, pretreatment with lobeline attenuated nicotine withdrawal-induced increased NE levels in the PFC and hippocampus. Further, repeated treatment with lobeline or 3-(pyridine-3́-yl)-cytisine decreased immobility time in the FST and reduced withdrawal-induced increased BDNF and p-CREB expression in the hippocampus.

CONCLUSIONS

Taken together, our results indicate that lobeline attenuated nicotine withdrawal-induced depression-like behavior likely by targeting brain nAChRs, noradrenergic neurotransmission, and/or hippocampal BDNF. Thus, lobeline may have some potential to prevent smoking relapse by counteracting nicotine withdrawal-induced depression in humans.

Pharmacokinetic and pharmacodynamics studies of nicotine after oral administration in mice: effects of methoxsalen, a CYP2A5/6 inhibitor

INTRODUCTION

The use of novel oral nicotine delivery devices and compositions for human consumption and for animal research studies has been increasing in the last several years.

METHODS

Studies were undertaken to examine whether the systemic administration of methoxsalen, an inhibitor of human CYP2A6 and mouse CYP2A5, would modulate nicotine pharmacokinetics and pharmacological effects (antinociception in the tail-flick, and hot-plate tests, and hypothermia) in male ICR mouse after acute oral nicotine administration.

RESULTS

Administration of intra peritoneal (ip) methoxsalen significantly increased nicotine's Cmax, prolonged the plasma half-life (fourfold decrease) of nicotine, and increased its area under the curve (AUC) compared with ip vehicle treatment. Methoxsalen pretreatment prolonged the duration of nicotine-induced antinociception and hypothermia (15mg/kg, po) for periods up to 6- and 24-hr postnicotine administration, respectively. Additionally, methoxsalen potentiated nicotine-induced antinociception and hypothermia as evidenced by leftward shifts in nicotine's dose-response curve. Furthermore, this prolongation of nicotine's effects after methoxsalen was associated with a parallel prolongation of nicotine plasma levels in mice. These data strongly suggest that variation in the rates of nicotine metabolic inactivation substantially alter pharmacological effects of nicotine given orally.

CONCLUSION

We have shown that the pharmacological effects of inhibiting nicotine's metabolism after oral administration in mice are profound. Our results suggest that inhibiting nicotine metabolism can be used to dramatically enhance nicotine's bioavailability and its resulting pharmacology, which further supports this inhibitory approach for clinical development of an oral nicotine replacement therapy.

Mecamylamine-precipitated nicotine withdrawal syndrome and its prevention with baclofen: an autoradiographic study of α4β2 nicotinic acetylcholine receptors in mice

A previous study from our laboratory showed that baclofen (BAC, GABAB receptor agonist) was able to prevent the behavioral expression of nicotine (NIC) withdrawal syndrome. To further investigate the mechanisms underlying this effect, we conducted this study, with the aims of analyzing α4β2 nicotinic receptor density during NIC withdrawal and, in case we found any changes, of determining whether they could be prevented by pretreatment with BAC. Swiss Webster albino mice received NIC (2.5 mg/kg, s.c.) 4 times daily, for 7 days. On the 8th day, NIC-treated mice received the nicotinic antagonist mecamylamine (MEC; 2 mg/kg, i.p.) 1 h after the last dose of NIC. A second group of NIC-treated mice received BAC (2 mg/kg, i.p.) prior to MEC administration. Thirty minutes after MEC, mice were sacrificed and brain autoradiography with [(3)H]epibatidine was carried out at five different anatomical levels. Autoradiographic mapping showed a significant increase of α4β2 nicotinic receptor labeling during NIC withdrawal in the nucleus accumbens shell (AcbSh), medial habenular nucleus (HbM), thalamic nuclei, dorsal lateral geniculate (DLG) nucleus, fasciculus retroflexus (fr), ventral tegmental area, interpeduncular nucleus and superior colliculus. BAC pretreatment prevented the increased α4β2 nicotinic receptor binding sites in the AcbSh, MHb, thalamic nuclei, DLG nucleus and fr. The present results suggest a relationship between BAC's preventive effect of the expression of NIC withdrawal signs, and its ability to restore the changes in α4β2 nicotinic receptor labeling, evidenced in specific brain areas in NIC withdrawn animals.

Unusual effects of nicotine as a psychostimulant on ambulatory activity in mice

The present study examined the effect of nicotine, alone and in combination with various drugs that act on the CNS, on ambulatory activity, a behavioral index for locomotion, in ICR (CD-1) strain mice. Nicotine at 0.25–2 mg/kg acutely reduced ambulatory activity of ICR mice. The effect of nicotine was similar to that of haloperidol and fluphenazine but distinct from that of bupropion and methylphenidate. ICR mice developed tolerance against the inhibitory effect of nicotine on ambulatory activity when nicotine was repeatedly administered. This effect was also distinct from bupropion and methylphenidate as they produced augmentation of their ambulation-stimulating effects in ICR mice. Nicotine reduced the ambulation-stimulating effects of bupropion and methylphenidate as well as haloperidol and fluphenazine. Taken together, nicotine exhibited unusual effects as a psychostimulant on ambulatory activity in ICR mice.

Ability of baclofen to prevent somatic manifestations and neurochemical changes during nicotine withdrawal

BACKGROUND

Nicotine (NIC), the major active component of tobacco, is critical in the maintenance of the smoking habit. The aims of the present study were to analyze the behavioural and neurochemical variations during NIC withdrawal syndrome in mice, and whether they are prevented with baclofen (BAC, GABA(B) receptor agonist).

METHODS

Swiss-Webster albino mice received NIC (2.5 mg/kg, s.c.) 4 times daily, for 7 consecutive days. On day 8 (the day of the experiment), NIC-treated mice received the nicotine antagonist mecamylamine (MEC, 2 mg/kg, i.p.) 1h after the last dose of NIC. A second group of dependent mice received BAC (2mg/kg, i.p.) before MEC-precipitated abstinence. The somatic signs were measured for 30 min. Dopamine (DA), serotonin (5-hydroxytryptamine; 5-HT) and its metabolites concentrations were determined by HPLC in the striatum, cortex and hippocampus.

RESULTS

The global score was greater in the abstinent group compared to the control group. Moreover, the global score time course showed a higher increase at 10 min compared to the global score at 5 min or 30 min after MEC-precipitated NIC withdrawal. In addition, the global score was attenuated by BAC. The DA and dihydroxyphenyl acetic acid (DOPAC) cortical levels decreased in the abstinent group, while BAC reestablished these levels 10 min after NIC withdrawal. Furthermore, DA and 5-HT striatal levels decreased during NIC withdrawal, and BAC reverted this decrease.

CONCLUSION

In conclusion, the prevention of NIC withdrawal signs by BAC could be related to changes in dopaminergic and serotonergic activity.

Old mice lacking high-affinity nicotine receptors resist acoustic trauma

There is presently no clearly effective preventative medication against noise-induced hearing loss (NIHL). However, negative feedback systems that presumably evolved to modulate the sensitivity of the organ of Corti may incidentally confer protection. One feedback system implicated in protection from NIHL involves synaptic connections between the lateral olivocochlear efferent terminals and the afferent fibers of spiral ganglion neurons (SGNs). These connections operate via high-affinity nicotinic acetylcholine receptors containing the β2 subunit. We unexpectedly observed protection from NIHL in 9-month old knockout mice lacking the β2 subunit (β2(-/-)); however, the same protection was not observed in 2-month old β2(-/-) mice. This enigmatic observation led to the discovery that protection from acoustic trauma in older β2(-/-) mice is mainly mediated by an age-related increase of corticosterone, not disruption of efferent cholinergic transmission. Significant protection of inner hair cells after acoustic trauma in β2(-/-) mice was linked to the activation of glucocorticoid signaling pathways. However, significant loss of SGNs was observed in animals with chronically high systemic levels of corticosterone. These results suggested a "double-edge sword" nature of glucocorticoid signaling in neuronal protection, and a need for caution regarding when to apply synthetic glucocorticoid drugs to treat neural injury such as accompanies acoustic trauma.

Chronic oral nicotine increases brain [3H]epibatidine binding and responsiveness to antidepressant drugs, but not nicotine, in the mouse forced swim test

INTRODUCTION

Smoking rates among depressed individuals is higher than among healthy subjects, and nicotine alleviates depressive symptoms. Nicotine increases serotonergic and noradrenergic neuronal activity and facilitates serotonin and noradrenaline release. In mice, acute nicotine administration enhances the activity of antidepressants in the mouse forced swim (mFST) and tail suspension tests. Here, we investigated if this action of nicotine is also reflected in a chronic treatment regimen.

MATERIALS AND METHODS

After chronic treatment with nicotine in the drinking water, mice were challenged with nicotine, duloxetine, citalopram, and reboxetine in the mFST. Additionally, 8-OH-DPAT- and clonidine-induced hypothermia was tested in vehicle- and nicotine-pretreated mice, as a measure of 5-HT(1A) and alpha(2)-adrenoceptor function, respectively. Finally, the effects on the brain expression levels of high- and low-affinity nicotinic acetylcholine receptors (nAChRs) and the transporters for serotonin (SERT) and noradrenaline (NET) were assessed using [(3)H]epibatidine, [(3)H]alpha-bungarotoxin, [(3)H]citalopram, and [(3)H]nisoxetine binding, respectively.

RESULTS

In the mFST, nicotine-pretreated mice did not show altered response to the nicotine challenge, but increased responses to all three antidepressants tested were observed when compared to mice that had been administered drinking water without nicotine. There was no change in hypothermic responses to 8-OH-DPAT or clonidine. [(3)H]epibatidine binding was significantly increased in all brain regions investigated; whereas, [(3)H]alpha-bungarotoxin, [(3)H]citalopram, and [(3)H]nisoxetine binding were not altered, indicating that chronic oral nicotine increases the expression and/or affinity of high-affinity nAChRs, but not low-affinity nAChRs, SERT, or NET.

DISCUSSION

It is suggested that the increased sensitivity to antidepressants after chronic nicotine exposure involves increased high-affinity nAChR-mediated neurotransmission.

Brain activation by short-term nicotine exposure in anesthetized wild-type and beta2-nicotinic receptors knockout mice: a BOLD fMRI study

RATIONALE

The behavioral effects of nicotine and the role of the beta2-containing nicotinic receptors in these behaviors are well documented. However, the behaviors altered by nicotine rely on the functioning on multiple brain circuits where the high-affinity beta2-containing nicotinic receptors (beta2*nAChRs) are located.

OBJECTIVES

We intend to see which brain circuits are activated when nicotine is given in animals naïve for nicotine and whether the beta2*nAChRs are needed for its activation of the blood oxygen level dependent (BOLD) signal in all brain areas.

MATERIALS AND METHODS

We used functional magnetic resonance imaging (fMRI) to measure the brain activation evoked by nicotine (1 mg/kg delivered at a slow rate for 45 min) in anesthetized C57BL/6J mice and beta2 knockout (KO) mice.

RESULTS

Acute nicotine injection results in a significant increased activation in anterior frontal, motor, and somatosensory cortices and in the ventral tegmental area and the substantia nigra. Anesthetized mice receiving no nicotine injection exhibited a major decreased activation in all cortical and subcortical structures, likely due to prolonged anesthesia. At a global level, beta2 KO mice were not rescued from the globally declining BOLD signal. However, nicotine still activated regions of a meso-cortico-limbic circuit likely via alpha7 nicotinic receptors.

CONCLUSIONS

Acute nicotine exposure compensates for the drop in brain activation due to anesthesia through the meso-cortico-limbic network via the action of nicotine on beta2*nAChRs. The developed fMRI method is suitable for comparing responses in wild-type and mutant mice.

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.

Influence of Neuronal Nicotinic Receptors over Nicotine Addiction and Withdrawal

Cigarette smoking represents an enormous, global public health threat. Nearly five million premature deaths during a single year are attributable to smoking. Despite the resounding message of risks associated with smoking and numerous public health initiatives, cigarette smoking remains the most common preventable cause of disease in the United States. Fortunately, even in an adult smoker, smoking cessation can reverse many of the potential harmful effects. The symptoms associated with nicotine withdrawal represent the major obstacle to smoking cessation. This minireview examines the roles of various nicotinic receptors in the mechanisms of nicotine dependence, discusses the potential role of the habenula-interpeduncular nucleus axis in nicotine withdrawal, and highlights nicotinic receptors containing the β4 subunit as a potential pharmacological target for smoking cessation strategies.

Chronic nicotine exposure has dissociable behavioural effects on control and beta2-/- mice

Nicotine exerts beneficial effects on various neurological and psychiatric pathologies, yet its effects on cognitive performance remain unclear. Mice lacking the beta2 subunit of the nicotinic receptor (beta2-/-) show characteristic deficits in executive functions and are suggested as reliable animal models for some specific endophenotypes of human pathologies, notably ADHD. We use beta2-/- and their controls to investigate the consequences of chronic nicotine exposure on cognitive behaviour. We show that in control mice, this treatment elicits somewhat slight effects, particularly affecting nocturnal activity and self-grooming. By contrast, in beta2-/- mice, chronic nicotine treatment had restorative effects on exploratory behaviour in the open-field and affected rearing, but did not modify motor functions. We confirmed that beta2-/- mice exhibit impaired exploratory and social behaviour, and further demonstrated their nocturnal hyperactivity. These data support the proposal that beta2-/- mice represent a relevant model for cognitive disorders in humans and that nicotine administered chronically at low dose may relieve some of these.

Morphine-nicotine interaction in conditioned place preference in mice after chronic nicotine exposure

Previously we found that morphine's effects on locomotor activity and brain dopamine metabolism were enhanced in mice after cessation of 7-week oral nicotine treatment. In the present experiments we show that such chronic nicotine exposure cross-sensitizes NMRI mice to the reinforcing effect of morphine in the conditioned place preference paradigm. The nicotine-treated mice developed conditioned place preference after being conditioned twice with morphine 5 mg/kg s.c. whereas in control mice a higher dose (10 mg/kg) of morphine was required. Since the reinforcing effect of morphine is mediated via micro-opioid receptors we used [3H]DAMGO autoradiography to study whether the number (B(max)) or affinity (K(D)) of mu-opioid receptors in the mouse brain are affected following chronic nicotine exposure. However, no changes were found in the number or affinity of micro-opioid receptors in any of the brain areas studied. Neither did we find alterations in the functional activity of mu-opioid receptors studied by [35S]GTPgammaS-binding. In conclusion, chronic oral nicotine treatment augments the reinforcing effects of morphine in mice, and this cross-sensitization does not seem to be mediated by micro-opioid receptors.

Combined exposure to nicotine and ethanol in adolescent mice differentially affects anxiety levels during exposure, short-term, and long-term withdrawal

Smoking and consumption of alcoholic beverages are frequently associated during adolescence. This association could be explained by the cumulative behavioral effects of nicotine and ethanol, particularly those related to anxiety levels. However, despite epidemiological findings, there have been few animal studies of the basic neurobiology of the combined exposure in the adolescent brain. In the present work we assessed, through the use of the elevated plus maze, the short- and long-term anxiety effects of nicotine (NIC) and/or ethanol (ETOH) exposure during adolescence (from the 30th to the 45th postnatal day) in four groups of male and female C57BL/6 mice: (1) Concomitant NIC (nicotine free-base solution (50 microg/ml) in 2% saccharin to drink) and ETOH (ethanol solution (25%, 2 g/kg) i.p. injected every other day) exposure; (2) NIC exposure; (3) ETOH exposure; (4) Vehicle. C57BL/6 mice were selected, in spite of the fact that they present slower ethanol metabolism, because they readily consume nicotine in the concentration used in the present study. During exposure (45th postnatal day: PN45), our results indicated that ethanol was anxiolytic in adolescent mice and that nicotine reverted this effect. Short-term drug withdrawal (PN50) elicited sex-dependent effects: exposure to nicotine and/or ethanol was anxiogenic only for females. Although neither nicotine nor ethanol effects persisted up to 1 month postexposure (PN75), the coadministration elicited an anxiogenic response. In spite of the fact that generalizations based on the results from a single strain of mice are prone to shortcomings, our results suggest that the deficient response to the anxiolytic effects of ethanol in adolescents co-exposed to nicotine may drive higher ethanol consumption. Additionally, increased anxiety during long-term smoking and drinking withdrawal may facilitate relapse to drug use.

Uncoupling between noradrenergic and serotonergic neurons as a molecular basis of stable changes in behavior induced by repeated drugs of abuse

A challenge in drug dependence is to delineate long-term behavioral and neurochemical modifications induced by drugs of abuse. In rodents, drugs of abuse induce locomotor hyperactivity, and repeating injections enhance this response. This effect, called behavioral sensitization, persists many months after the last administration, thus mimicking long-term sensitivity to drugs observed in human addicts. Although addictive properties of drugs of abuse are generally considered to be mediated by an increased release of dopamine in the ventral striatum, recent pharmacological and genetic experiments indicate an implication of alpha1b-adrenergic receptors in behavioral and rewarding responses to psychostimulants and opiates. Later on, it was shown that not only noradrenergic but also serotonergic systems, through 5-HT(2A) receptors, were controlling behavioral effects of drugs of abuse. More recently, experiments performed in animals knockout for alpha1b-adrenergic or 5-HT(2A) receptors indicated that noradrenergic and serotonergic neurons, besides their activating effects, inhibit each other by means of the stimulation of alpha1b-adrenergic and 5-HT(2A) receptors and that this mutual inhibition vanishes in wild type mice with repeated injections of psychostimulants, opiates or alcohol. Uncoupling induced by repeated treatments with drugs of abuse installs a stable sensitization of noradrenergic and serotonergic neurons, thus explaining an increased reactivity of dopaminergic neurons and behavioral sensitization. We propose that noradrenergic/serotonergic uncoupling is a common stable neurochemical consequence of repeated drugs of abuse which may also occur during chronic stressful situations and facilitate the onset of mental illness. Drug consumption would facilitate an artificial re-coupling of these neurons, thus bringing a temporary relief.

Molecular mechanisms underlying the motivational effects of nicotine

In addition to the primary rewarding properties of nicotine and the alleviation of withdrawal symptoms, cues associated with smoking are critical contributors to maintenance of smoking behavior. Nicotine-paired cues are also critical for precipitating relapse after smoking cessation. An accumulation of evidence suggests that repeated exposure to tobacco, including the primary psychoactive ingredient, nicotine, changes brain neurochemistry in a way that promotes the control that cues associated with smoking or other rewards have over behavior. This chapter will consider the neurochemical mechanisms underlying these neuroadaptations. Targeting these molecular alterations may provide novel treatments for smoking cessation.

Translational research in medication development for nicotine dependence

A major obstacle to the development of medications for nicotine dependence is the lack of animal and human laboratory models with sufficient predictive clinical validity to support the translation of knowledge from laboratory studies to clinical research. This Review describes the animal and human laboratory paradigms commonly used to investigate the pathophysiology of nicotine dependence, and proposes how their predictive validity might be determined and improved, thereby enhancing the development of new medications.

Pleiotropic impact of constitutive fosB inactivation on nicotine-induced behavioral alterations and stress-related traits in mice

Multiple genes are thought to influence both susceptibility to nicotine dependence and its comorbid behavioral traits in humans. However, which specific genes contribute to this pleiotropic effect is poorly understood. Previous rodent studies have shown that many addictive substances and stressful stimuli increase the expression of the transcription factor FosB in limbic and associated regions and that the protein products of fosB contribute to certain behavioral effects of cocaine and morphine. However, the role of this gene in nicotine-regulated behaviors and dependence-related behavioral traits is unknown. We tested the hypothesis that a constitutive level of FosB affects nicotine-regulated behaviors and comorbid behavioral traits using constitutive fosB knockout (KO) mice. Following repeated or prolonged nicotine administration, but not a single acute administration, KO mice were impaired in conditioned place preference, oral nicotine intake and motor suppression. In wild-type mice, repeated nicotine injections, but not a single acute injection, increased the expression of FosB and its truncated variant DeltaFosB in the targets but not at the origins of the mesolimbic and nigrostriatal dopamine pathways; no detectable level of FosB/DeltaFosB was found in KO mice. In tasks designed to assess behavioral traits, KO mice exhibited more pronounced behavioral abnormalities when stress levels were high than when they were minimized. Our results suggest that the constitutive absence of fosB has a pleiotropic influence on the behavioral effects of repeated or prolonged nicotine administration and on stress-related behavioral traits in mice.

Monoamine oxidase A knockout mice exhibit impaired nicotine preference but normal responses to novel stimuli

Nicotine is thought to act on brain monoamine systems that normally mediate diverse motivational behaviors. How monoamine-related genes contribute to behavioral traits (e.g. responses to novel stimuli) comorbid with the susceptibility to nicotine addiction is still poorly understood. We examined the impact of constitutive monoamine oxidase A (MAOA) deficiency in mice on nicotine reward and responses to novel stimuli. Age-matched, male Maoa-knockout (KO) mice and wild-type (WT) littermates were tested for nicotine-induced conditioned place preference (CPP); voluntary oral nicotine preference/intake; spontaneous locomotor activity in a novel, inescapable open field; and novelty place preference. Nicotine preference in WT mice was reduced in Maoa-KO mice in the CPP and oral preference/intake tests. Control experiments showed that these phenotypes were not due to abnormalities in nicotine metabolism, fluid intake or response to taste. In contrast, Maoa-KO mice were normal in their behavioral response to a novel, inescapable open field and in their preference for a novel place. The observed phenotypes suggest that a constitutive deficiency of MAOA reduces the rewarding effects of nicotine without altering behavioral responses to novel stimuli in mice. Constitutive MAOA activity levels are likely to contribute to the vulnerability or resiliency to nicotine addiction by altering the rewarding effects of nicotine.

Monoamine oxidase inhibitors allow locomotor and rewarding responses to nicotine

Although nicotine is generally considered to be the main compound responsible for the addictive properties of tobacco, experimental data indicate that nicotine does not exhibit all the characteristics of other abused substances, such as psychostimulants and opiates. For example, nicotine is only a weak locomotor enhancer in rats and generally fails to induce a locomotor response in mice. This observation contradicts the general consensus that all drugs of abuse release dopamine in the nucleus accumbens, a subcortical structure, and thus increase locomotor activity in rodents. Because tobacco smoke contains monoamine oxidase inhibitors (MAOIs) and decreases MAO activity in smokers, we have combined MAOIs with nicotine to determine whether it is possible to obtain a locomotor response to nicotine in C57Bl6 mice. Among 15 individual or combined MAOIs, including harmane, norharmane, moclobemide, selegiline, pargyline, clorgyline, tranylcypromine and phenelzine, only irreversible inhibitors of both MAO-A and -B (tranylcypromine, phenelzine, and clorgyline+selegiline) allowed a locomotor response to nicotine. The locomotor stimulant interaction of tranylcypromine and nicotine was absent in beta2-nicotinic acetylcholine receptor subunit knockout mice. Finally, it was found that, whereas naïve rats did not readily self-administer nicotine (10 microg/kg/injection), a robust self-administration of nicotine occurred when animals were pretreated with tranylcypromine (3 mg/kg). Our data suggest that MAOIs contained in tobacco and tobacco smoke act in synergy with nicotine to enhance its rewarding effects.

Effects of chronic nicotine administration and its withdrawal on striatal FosB/DeltaFosB and c-Fos expression in rats and mice

DeltaFosB, a member of Fos family of transcription factors, is implicated in behavioural responses and synaptic plasticity induced by abused drugs. We studied the expressions of FosB/DeltaFosB and c-Fos immunohistochemically in two dopaminergic brain areas, nucleus accumbens (NAcc) and caudate-putamen (CPu). In mice neither 2- nor 7-week oral nicotine treatment induced expression of long-lived DeltaFosB isoforms although during the treatment in the NAcc FosB/DeltaFosB expression was increased as was c-Fos in the CPu. In rats given nicotine subcutaneously once daily for 5days FosB/DeltaFosB expression was elevated in the NAcc still after 24-h withdrawal suggesting accumulation of DeltaFosB but in the CPu neither FosB/DeltaFosB nor c-Fos expression was altered. Thus, in rats repeated nicotine administration seems mainly affect the NAcc paralleling with the evidence that nicotine stimulates preferentially mesolimbic dopamine system. Also, repeated nicotine induced behavioural sensitization in rats agreeing with suggested role of DeltaFosB in the development of psychomotor sensitization. However, in mice given nicotine via drinking fluid although striatal fosB and c-fos were activated by nicotine even after 7-week treatment no evidence of accumulation of long-lived DeltaFosB was found suggesting perhaps a species difference or more likely a role for the manner of administration.

Effect of quinpirole on striatal dopamine release and locomotor activity in nicotine-treated mice

The effect of chronic oral nicotine treatment which in its intermittent delivery resembles human smoking was studied on the sensitivity of dopamine autoreceptors in mice. On the 50th day of nicotine administration in the drinking water or after 23-25 h withdrawal quinpirole (D2/D3 agonist, 0.01-0.1 mg/kg s.c.) was given, and accumbal and dorsal striatal dopamine outflow, locomotor activity and body temperature were measured. Dorsal striatal extracellular dopamine concentration and locomotor activity were found to be elevated during nicotine administration. Chronic nicotine did not alter the effects of small, autoreceptor preferring doses of quinpirole on accumbal or dorsal striatal dopamine, locomotor activity or body temperature. However, quinpirole's locomotor activity reducing effect was slightly diminished in mice treated repeatedly with nicotine (0.4 mg/kg twice daily for 10 days s.c.). Thus, although repeated nicotine treatment for 5-14 days decreases dopamine autoreceptor sensitivity, after long-term oral nicotine treatment such a decrease is not seen. Thus, the changes occurring in the sensitivity of D2-like dopamine receptors probably play a minor role in regulating the dopaminergic transmission during long-term nicotine administration.

Effect of chronic nicotine treatment on localization of neuronal nicotinic acetylcholine receptors at cellular level

Chronic nicotine treatment increases the number of neuronal nicotinic acetylcholine receptors (nAChRs). Localization of nAChRs at a cellular level determines their functional role. However, changes in the localization of nAChRs caused by chronic nicotine treatment are not well known. In this study, we have examined the effects of chronic nicotine treatment on alpha7 and beta2 nAChR subunits in vitro in cell lines and in vivo in mouse striatum. In vitro, two different cell lines were used, SH-SY5Y cells endogenously expressing several nAChR subtypes and SH-EP1-halpha7 cells, transfected with the human alpha7 nAChR subunit gene. Effects of chronic nicotine treatment (10 microM, 3 days) were studied in vitro by using confocal and electron microscopy and calcium fluorometry. In vitro in SH-SY5Y cells, alpha7 and beta2 subunits formed groups, unlike alpha7 subunits in SH-EP1-halpha7 cells, which were partially localized on endoplastic reticulum. Chronic nicotine treatment did not change the localization of nAChRs in endosomes, but caused clustering of alpha7 subunits in SH-EP1-halpha7 cells. In vivo, nicotine was given to mice in their drinking water for 7 weeks. Results showed that alpha7 and beta2 subunits formed groups, and that chronic nicotine treatment increased the size of the clusters. As a conclusion, our data show that there are large intracellular pools of nAChR subunits, which are partially localized on endoplastic reticulum. Chronic nicotine treatment does not change endocytotic trafficking of nAChRs. Chronic nicotine treatment increased clustering of nAChRs, which could have a role in the release of dopamine (DA) evoked by nicotine.

Indirect modulation by alpha7 nicotinic acetylcholine receptors of noradrenaline release in rat hippocampal slices: interaction with glutamate and GABA systems and effect of nicotine withdrawal

Nicotinic acetylcholine receptors (nAChRs) can modulate transmitter release. Striatal [(3)H]dopamine ([(3)H]DA) release is regulated by presynaptic nAChR on dopaminergic terminals and alpha7 nAChR on neighboring glutamatergic afferents. Here, we explored the role of alpha7 nAChR in the modulation of [(3)H]noradrenaline ([(3)H]NA) release from rat hippocampal slices. The nicotinic agonist anatoxin-a (AnTx) evoked monophasic [(3)H]NA release (EC(50) = 1.2 microM) that was unaffected by alpha-conotoxin-MII or dihydro-beta-erythroidine, antagonists of alpha3/alpha6beta2* and beta2* nAChR, respectively. In contrast AnTx-evoked striatal [(3)H]DA release was biphasic (EC(50) = 138.9 nM; 7.1 microM) and blocked by these antagonists. At a high AnTx concentration (25 microM), alpha7 nAChR antagonists (methyllycaconitine, alpha-conotoxin-ImI) and glutamate receptor (GluR) antagonists [kynurenic acid, 6,7-dinitroquinoxaline-2,3-dione (DNQX)] partially inhibited [(3)H]NA release. The alpha7 nAChR-selective agonist choline evoked [(3)H]NA release (E(max) = 33% of that of AnTx) that was blocked by GluR antagonists, supporting a model in which alpha7 nAChRs trigger glutamate release that subsequently stimulates [(3)H]NA release. A GABAergic component was also revealed: choline-evoked [(3)H]NA release was partially blocked by the GABA(A) receptor antagonist bicuculline, and coapplication of bicuculline and DNQX fully abolished this response. These findings support alpha7 nAChR on GABAergic neurons that can promote GABA release which, in turn, leads to [(3)H]NA release, probably by disinhibition. To investigate the impact of long-term nicotine exposure on this model, rats were exposed for 14 days to nicotine (4 mg/kg/day) with or without 3 or 7 days of withdrawal. alpha7 nAChR responses were selectively and transiently up-regulated after 3 days of withdrawal. This functional up-regulation could contribute to the withdrawal effects of nicotine.

Nicotine physical dependence in the mouse: involvement of the alpha7 nicotinic receptor subtype

Although chronic nicotine produces dependence in mice, the role of specific nicotinic receptors has not been examined in knockout animals. The present study utilized alpha7 nicotinic receptor knockout mice to explore the role of this receptor subunit in nicotine dependence. Mice were chronically exposed to nicotine (0 or 200 microg/ml) in their drinking water and assayed for somatic withdrawal signs, hyperalgesia (tail-flick and hot-plate) and spontaneous activity following nicotine cessation. Nicotine withdrawal produced increased somatic signs in both strains and hyperalgesia in wild-type, but not in knockout animals. These results indicate that the alpha7 nicotinic receptor subunit may mediate some aspects of nicotine dependence.

Up-regulation of β2 and α7 subunit containing nicotinic acetylcholine receptors in mouse striatum at cellular level

Nicotine releases dopamine in the brain by activating neuronal nicotinic acetylcholine receptors (nAChRs). Chronic nicotine treatment increases the number of nAChRs, which represents plasticity of the brain. Together these phenomena have been suggested to have a role in the development of nicotine addiction. In the brain nAChRs can be localized synaptically, extrasynaptically or intracellularly. The purpose of these studies was to clarify the effects of chronic nicotine treatment on the localization of beta2 and alpha7 nAChR subunits in brain areas involved in nicotine addiction. Nicotine was administered orally in drinking water to male NMRI mice for 7 weeks. At the end of chronic nicotine treatment the localization of the nAChR subunits was studied in the dorsal striatum and in the ventral tegmental area (VTA) by using electron microscopy. In the brain areas studied beta2 and alpha7 subunits were localized presynaptically and postsynaptically in axon endings and in dendrites. In both areas the majority of the beta2 and alpha7 subunits were localized at extrasynaptic sites. In response to chronic nicotine treatment the beta2 and alpha7 nAChR subunit labelling was increased at synaptic and extrasynaptic sites as well as intracellularly. This suggests that the trafficking of nAChR subunits is increased as a result of chronic nicotine treatment and nAChRs in all parts of neurons could have functional roles in the formation of nicotine addiction.

Nicotine physical dependence and tolerance in the mouse following chronic oral administration

RATIONALE

Although nicotine dependence and tolerance develop in rats, few studies have examined these processes in the mouse. Establishing such mouse models would eventually allow for an examination of the role of specific nicotinic receptor subtypes in mediating these processes (i.e. through the use of receptor knockouts).

OBJECTIVES

The goals of the present study were to establish mouse models of nicotine dependence and tolerance.

METHODS

Mice were chronically exposed to nicotine (0-200 mug/ml) in their drinking solution and assayed for plasma nicotine and cotinine levels, withdrawal signs following nicotine cessation (spontaneous withdrawal) or nicotinic antagonist administration (precipitated withdrawal), or nicotine tolerance. Dependence assays included somatic sign observations (paw tremors, backing and head shakes), tail-flick, plantar stimulation, elevated plus-maze and spontaneous activity. Tolerance was assayed using tail-flick, hot-plate and body temperature tests.

RESULTS

Plasma nicotine and cotinine levels were elevated during oral nicotine exposure (15.85 ng/ml and 538.00 ng/ml, respectively) and quickly declined following nicotine cessation (<1 ng/ml and <2 ng/ml, respectively), providing evidence that the oral route was pharmacologically relevant. Nicotine withdrawal increased numbers of somatic signs (spontaneous and mecamylamine-precipitated withdrawal) and/or hyperalgesia (spontaneous withdrawal only). Chronic nicotine exposure also produced tolerance, as indicated by reduced responsivity to acute nicotine in assays of analgesia and hypothermia.

CONCLUSIONS

These results indicate that chronic oral nicotine produces dependence and tolerance in the mouse. Further, nicotine dependence may be mediated by multiple nicotinic receptor subtypes, since specific nicotinic receptor antagonists failed to precipitate withdrawal.

Beta2-subunit-containing nicotinic acetylcholine receptors are critical for dopamine-dependent locomotor activation following repeated nicotine administration

Activation of the mesolimbic dopamine system is a critical component underlying addictive behaviors, including smoking. It has been hypothesized that the initial effect of nicotine on the dopamine system is to activate high affinity nicotinic acetylcholine receptors (nAChRs) containing the beta2 subunit, but that these receptors rapidly desensitize and are not critical for ongoing dopaminergic activation. To clarify the role of beta2-subunit-containing (beta2*) nAChRs in activation of the dopamine system and subsequent locomotor activation by repeated nicotine administration, C57BL/6J (B6) mice were administered 200 microg/ml of nicotine in the drinking water and the onset of locomotor activation was measured. B6 mice showed an increase in locomotor activity in response to chronic nicotine which was blocked by oral administration of the dopamine receptor antagonist pimozide. Knockout mice lacking the beta2 subunit of the nAChR did not show locomotor activation in response to chronic nicotine exposure, suggesting that beta2* nAChRs are critical for ongoing activation of the dopamine system by chronic nicotine administration and the resulting locomotor activation in mice.

Lack of efficacy of a nicotine transdermal treatment on motor and cognitive deficits in Parkinson's disease

Studies assessing the efficacy of nicotine in Parkinson's disease (PD) have generated contradictory results. The controversy seems to stem from uncontrolled factors including the lack of objective measures, the practice effect in a test-retest design, and the absence of plasmatic dosage. This study aimed at further controlling these factors using transdermal nicotine in PD.

METHODS

Twenty-two nonsmoking PD patients received a transdermal nicotine treatment over 25 days in increasing titrated doses. Motor and cognitive assessments were carried out on days 11 and 25 (low-dose and high-dose assessments, respectively) and after a 14-day washout period.

RESULTS

Patients tolerated nicotine poorly. Thirteen (59%) withdrew, mostly because of acute side effects. In the remaining nine patients, nicotine neither improved nor worsened motor or cognitive functioning in comparison with 10 age, gender and education matched controls.

CONCLUSIONS

Transdermal nicotine is not effective in treating motor and cognitive deficits in PD. The results obtained with our objective measures confirm a recent double-blind, placebo-controlled study that used clinical measures. It is possible that nicotine lacks specificity in targeting critical nicotinic receptors that might be involved in PD pathophysiology. The low tolerability may be related to such a lack of specificity of nicotine, which would directly stimulate the autonomic nervous system.

Long-term nicotine exposure reduces Purkinje cell number in the adult rat cerebellar vermis

Nicotine affects functions of the central nervous system. A previous study showed that developing cerebellar Purkinje cells are targets for early postnatal nicotine exposure. In this study, we assessed the effects of long-term nicotine exposure on mature cerebellar Purkinje cells. This is particularly relevant since the majority of smokers are exposed to nicotine over a long period. Female adult Sprague-Dawley rats received three doses of nicotine (0.01%, 0.03%, or 0.06%) through their sole water source. After 8 weeks of nicotine exposure, the cerebellar vermis was removed and processed for stereological cell counting. The results showed that this long-term nicotine treatment did not change the cerebellum weight or the size (volume) of the cerebellar vermis. The long-term nicotine treatment regimen did result in a significant loss of mature Purkinje cells in the cerebellum, however, such a loss of Purkinje cells was not nicotine dose-related. These findings indicated that the mature adult cerebellum is susceptible to the damaging effects of nicotine in depleting Purkinje cells in the cerebellum.

Chronic nicotine and smoking exposure decreases GABA(B1) receptor expression in the rat hippocampus

Nicotine and smoking have long been proved to play an important role in cognition and memory in the hippocampus. This effect is closely related to the gamma-aminobutyric acid (GABA)ergic system. Previous research has focused on functional and pharmacological aspects of nicotine's modulation activity. In this study, the effects of nicotine and different doses of smoking on GABA(B1) expression in the rat hippocampus have been examined using in situ hybridization and RNase protection assay. GABA(B1) receptor mRNAs were intensely expressed in the CA1, CA2, CA3, and dentate gyrus areas of the hippocampus. Nicotine and smoking doses dependently decreased GABA(B1) receptor expression in the hippocampus. These results revealed new aspects of nicotine's modulation on GABA(B) receptor, and on learning and memory.

Mecamylamine decreases accumbal dopamine output in mice treated chronically with nicotine

The role of nicotinic acetylcholine receptor (nAChR) activation in accumbal dopamine (DA) release during chronic continuous nicotine treatment was studied by in vivo microdialysis in freely-moving mice. Nicotine was administered chronically to NMRI mice in their drinking water. On the 50th day of nicotine administration microdialysis samples were collected at 20 min intervals and their DA, 3,4-dihydroxyphenylacetic acid, homovanillic acid and 5-hydroxyindoleacetic acid contents were measured using a high-performance liquid chromatographic-electrochemical procedure. After collection of four baseline samples the nicotinic antagonist mecamylamine (2 mg/kg, s.c.) was given. The steady-state DA output was larger in the nicotine-treated mice than in the control mice. Mecamylamine reduced the DA output in the nicotine-treated but not in the control mice. Thus, after continuous 50-day administration nicotine still continues to activate nAChRs regulating accumbal DA release.

Pineal melatonin and brain transmitter monoamines in CBA mice during chronic oral nicotine administration

The effects of chronic oral nicotine administration on the pineal melatonin and brain transmitter monoamines were studied in male CBA mice, which possess a clear daily rhythm of melatonin secretion. On the 50th day of nicotine administration, pineal melatonin as well as cerebral dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), norepinephrine (NE), 3-methoxy-4-hydroxyphenylethyleneglycol (MHPG), serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) concentrations were determined at various times. The chronic nicotine treatment did not alter the timing of the pineal melatonin peak, which occurred at 10 h after the light offset. However, in mice drinking nicotine solution, the nocturnal pineal melatonin levels were lower than in control mice drinking tap water. The chronic nicotine treatment increased the striatal DA, DOPAC, HVA and 5-HIAA levels, the hypothalamic NE, MHPG and 5-HIAA and the cortical MHPG. Most prominent effects of nicotine were found at 8 h after the light offset, when the striatal levels of DA and HVA, hypothalamic NE and MHPG as well as cortical MHPG were significantly elevated in the nicotine-treated mice compared with the control mice. No direct correlation between nicotine's effects on brain transmitter monoamines and on pineal melatonin levels was apparent. The results suggest that chronic nicotine treatment slightly suppresses the melatonin production but does not alter the daily rhythm of pineal melatonin in mice maintained on a light-dark cycle. However, the results indicate that nicotinic receptors might be involved in the regulation of pineal function.

Effect of nicotine and nicotinic receptors on anxiety and depression

Nicotine has been shown to have effects on anxiety and depression in both human and animal studies. These studies suggest that nicotinic acetylcholine receptors (nAChRs) can modulate the function of pathways involved in stress response, anxiety and depression in the normal brain, and that smoking can result in alterations of anxiety level and mood. The effects of nicotine are complex however, and nicotine treatment can be either anxiolytic or anxiogenic depending on the anxiety model tested, the route of nicotine administration and the time course of administration. The paradoxical effects of nicotine on emotionality are likely due to the broad expression of nAChRs throughout the brain, the large number of nAChR subtypes that have been identified and the ability of nicotine treatment to both activate and desensitize nAChRs. Activation of nAChRs has been shown to modulate many systems associated with stress response including stress hormone pathways, monoaminergic transmission and release of classical neurotransmitters throughout the brain. Local administration studies in animals have identified brain areas that may be involved in the anxiogenic and anxiolytic actions of nicotine including the lateral septum, the dorsal raphe nuclei, the mesolimbic dopamine system and the hippocampus. The ensemble of studies to date suggest that under certain conditions nicotine can act as an anxiolytic and an antidepressant, but that following chronic use, adaptations to nicotine can occur resulting in increased anxiety and depression following withdrawal.

Nicotinic--serotonergic interactions in brain and behaviour

This review focuses on nicotinic--serotonergic interactions in the central nervous system (CNS). Nicotine increases 5-hydroxytryptamine (5-HT) release in the cortex, striatum, hippocampus, dorsal raphé nucleus (DRN), hypothalamus, and spinal cord. As yet, there is little firm evidence for nicotinic receptors on serotonergic terminals and thus nicotine's effects on 5-HT may not necessarily be directly mediated, but there is strong evidence that the 5-HT tone plays a permissive role in nicotine's effects. The effects in the cortex, hippocampus, and DRN involve stimulation of 5-HT(1A) receptors, and in the striatum, 5-HT(3) receptors. The 5-HT(1A) receptors in the DRN play a role in mediating the anxiolytic effects of nicotine and the 5-HT(1A) receptors in the dorsal hippocampus and lateral septum mediate its anxiogenic effects. The increased startle and anxiety during nicotine withdrawal is mediated by 5-HT(1A) and 5-HT(3) receptors. The locomotor stimulant effect of acute nicotine is mediated by 5-HT(1A) receptors and 5-HT(2) receptors may play a role in the expression of a sensitised response after chronic nicotine treatment. Unfortunately, the role of 5-HT(1A) receptors in mediating nicotine seeking has not yet been investigated and would seem an important area for future research. There is also evidence for nicotinic--serotonergic interactions in the acquisition of the water maze, passive avoidance, and impulsivity in the five-choice serial reaction task.