The effects of nicotine administration on 5-HT uptake and biosynthesis in rat brain

The influence of tobacco smoke and nicotine on antidepressant and memory-improving effects of venlafaxine

In experimental and clinical studies, central nicotinic systems have been shown to play an important role in cognitive function. Nicotinic acetylcholine receptors also mediate the reinforcing properties of nicotine (NIC) in tobacco products. A variety of studies have shown that acute treatment with NIC or nicotinic agonists can improve working memory function. Moreover, it is known that the monoaminergic system plays an important role in memory function. And there is evidence suggesting that prolonged use of NIC may exert antidepressant action via nicotinic receptors. The purpose of this study was to investigate the interactions between a novel antidepressant, venlafaxine (VEN), a blocker of noradrenaline and 5–hydroxytryptamine reuptake sites, and pure NIC in the context of antidepressant and memory function in tobacco smoke exposed and nonexposed rats. The animals were subjected to Porsolt's test for testing antidepressant activity and their memory function (spatial memory) was evaluated in the Morris Water Maze Test. In tobacco smoke non–exposed and exposed rats both single and chronic administration of VEN (20 mg/kg po) shortened immobility time. NIC (0.2 mg/kg sc) significantly reduced immobility time on the 1st, 7th and 14th test days in both non–exposed and exposed rats. Combined VEN–NIC treatment in tobacco smoke non–exposed rats reduced immobility too. This effect of the combination of drugs was significantly stronger as compared to the effects obtained after individual administration of VEN or NIC. In the group exposed to tobacco smoke, joint administration of VEN–NIC induced a significant reduction of immobility as compared to the control and NIC groups. In the Morris Water Maze Test single and chronic administration of VEN, lower values of escape latencies and lower numbers of crossed quadrants were noted in both exposed and non–exposed rats, which indicates improved performance. After administering NIC we could observe improvement of spatial memory in both the exposed and non–exposed group. A similar effect of improvement of spatial memory was observed after joint administration of VEN and NIC. The study results support the involvement of nicotinic systems in memory processes in rats. Memory improvement and antidepressant effects following joint administration of VEN and NIC may depend on nicotinic interactions with monoaminergic systems and VEN may represent a new therapeutic approach to smoking cessation.

Sex-influence of nicotine and nitric oxide on motor coordination and anxiety-related neurophysiological responses

RATIONALE

Nitric oxide (NO) is a messenger synthesized in both the neuronal and glial populations by nitric oxide synthase type 1 (NOS1). Nicotine regulates NO production in a sex-dependent manner, both molecules being involved in motor function.

OBJECTIVE

The present study evaluates sex differences in motor coordination, general movement, and anxiety-related responses resulting from both constant and continuous nicotine treatment and the genetic depletion of NOS1 activity.

METHODS

Male and female mice were analyzed with the open-field and the rotarod tests. To understand the role of NO, knockout mice for NOS1 (NOS1-/-) were analyzed. Nicotine was administered continuously at a dose of 24 mg/kg/day via osmotic mini-pumps over 14 days because the behavioral effects elicited are similar to those observed with discontinuous administration.

RESULTS

Data analyses revealed noteworthy sex differences derived from NOS1 depletion. Control NOS1-/- males exhibited an exacerbated anxiety-related response in relation to control NOS1-/- females and control wild-type (WT) males; these differences disappeared in the nicotine-administered NOS1-/- males. Additionally, nicotine administration differentially affected the horizontal movements of NOS1-/- females with respect to WT animals. NO depletion affected male but not female motor coordination improvement along the test days. However, the drug affected female motor coordination only at the end of the administration period.

CONCLUSIONS

We show for the first time that NO affects motor and anxiety behaviors in a sex-dependent manner. Moreover, the behavioral effects of constant nicotine administration are dimorphic and dependent on NO production.

Effect of gabapentin on cognitive processes in rats not exposed and exposed to tobacco smoke during fetal life

Cognitive deficits, including memory deficiencies, are currently deemed one of key symptoms of psychopathologic mental disorders or epilepsy. The impairment of neurocognitive processes could be due to the administered therapy, in particular combined therapy or therapy using antiepileptics of older type. Gabapentin (GBP) is one of new antiepileptics with normothymic properties. It is known that epileptic patients run a significant risk of developing depression and mood changes. Smoking may also have a negative effect on memory processes and efficacy of administered drugs. Note that smoking in pregnant women also leads to neurobehavioral changes in their children. The objective of our research was to evaluate the effect of GBP on memory functions and antidepressant effect in rats not exposed and exposed to tobacco smoke in fetal life. We were also intent on finding whether GBP has an anticonvulsant effect in contact and without contact with tobacco smoke, and whether it affects motor coordination in animals if administered in the dose of 25 mg/kg. Spatial memory of the animals was assessed in the Morris test and the antidepressant effect in the Porsolt test. The ED(50) value was determined in the Swinyard maximum electric shock test, and the effect on motor coordination was assessed in the chimney test. GBP administered in the dose of 25 mg/kg intraperitoneal (i.p.) significantly reduced the immobility time on days 1 and 7 of the test in animals exposed to tobacco smoke, and on days 7 and 14 of the test in rats not exposed to tobacco smoke. Upon single and multiple administration of GBP to animals not exposed to tobacco smoke, the spatial memory improved, whereas in animals exposed to tobacco smoke in fetal life tolerance for procognitive effect was observed on day 21 of the test. It has been found that in rats not exposed to tobacco smoke, ED(50) of GBP was 28.73 mg/kg, whereas in animals exposed to tobacco smoke in fetal life, ED(50) was 46.2 mg/kg. Upon 14 and 21 days of drug administration, motor coordination was impaired in both GBP receiving animal groups. In conclusion, GBP beside its anticonvulsant efficacy also improves memory processes and has antidepressant effect. We also proved that GBP may reverse cognitive deficits concerning working memory induced by prenatal exposure to tobacco smoke and may have antidepressant effect in rats exposed to tobacco smoke.

Chronic effect of nicotine on serotonin transporter mRNA in the raphe nucleus of rats: reversal by co-administration of bupropion

AIM

Epidemiologic studies suggest the existence of a biological link between nicotine withdrawal and depression. To investigate the neuronal mechanisms of the precipitation of depression during smoking cessation, an animal model of nicotine withdrawal was used, and the expression of serotonin transporter (5HTT), abnormality of which is implicated in the pathogenesis of depression, was investigated. The effect of co-administration of bupropion, which has been clinically shown to ameliorate nicotine withdrawal symptoms, was also investigated in this model.

METHODS

Male Wistar rats were implanted with a minipump s.c., which delivered nicotine at a rate of 6 mg/kg per day for 12 days (days 1-12). Rats given chronic nicotine were killed on day 13, or 2 days after the removal of minipump (withdrawal day 2). In a separate experiment, bupropion (15 or 30 mg/kg per day) was injected into the nicotine infused rats on days 2-12. The expression of mRNA for 5HTT in the dorsal raphe was determined on in situ hybridization.

RESULTS

Chronic nicotine infusion resulted in the reduction of 5HTT mRNA expression, which lasted through withdrawal day 2. Co-administration of bupropion, however, significantly antagonized this reduction.

CONCLUSIONS

Chronic nicotine infusion reduces the synthesis of 5HTT protein, which may consequently precipitate depression during nicotine withdrawal, but co-administration of bupropion may ameliorate withdrawal symptoms by counteracting nicotine's effect on 5HTT.

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.

Alpha4 containing nicotinic receptors are positioned to mediate postsynaptic effects on 5-HT neurons in the rat dorsal raphe nucleus

Nicotinic acetylcholine receptors containing the alpha4 and beta2 subunits constitute the most abundant high-affinity binding site of nicotine in the brain and are critical for the addictive qualities of nicotine. 5-HT neurotransmission is thought to be an important contributor to nicotine addiction. Therefore in this study it was examined how alpha4-containing receptors are positioned to modulate the function of 5-HT neurons using ultrastructural analysis of immunolabeling for the alpha4 receptor subunit in the dorsal raphe nucleus (DR), a primary source of forebrain 5-HT in the rat. Of 150 profiles labeled for the alpha4 subunit, 140 or 93% consisted of either soma or dendrites, these were often small-caliber (distal) dendrites <1.5 microm in diameter (63/150 or 42%). The majority (107/150 or 71%) of profiles containing labeling for alpha4 were dually labeled for the synthetic enzyme for 5-HT, tryptophan hydroxylase (TPH). Within dendrites immunogold labeling for alpha4 was present on the plasma membrane or near postsynaptic densities. However, labeling for alpha4 was commonly localized to the cytoplasmic compartment often associated with smooth endoplasmic reticulum, plausibly representing receptors in transit to or from the plasma membrane. Previous studies have suggested that nicotine presynaptically regulates activity onto 5-HT neurons, however alpha4 immunolabeling was detected in only 10 axons in the DR or 7% of profiles sampled. This finding suggest that alpha4 containing receptors are minor contributors to presynaptic regulation of synaptic activity onto 5-HT neurons, but rather alpha4 containing receptors are positioned to influence 5-HT neurons directly at postsynaptic sites.

Behavioural studies in humans: anxiety, stress and smoking

Numerous observers have reported that smokers smoke more under stressful conditions. The most frequent explanation is that nicotine reduces anxiety, an intervening variable identified as a negative reinforcer for smoking behaviour. The conditions under which anxiety reduction occurs in response to smoking, however, have not been well defined, nor are underlying mechanisms well understood. There are several possible explanations, including Schachter's theory based on stress-induced changes in urinary pH and the hypothesis of endogenous opioid involvement. The work of Collins and his associates in animals has shown that genetic variations in corticosteroid responsiveness to nicotine are associated with differences in sensitivity to nicotine. Research in our laboratory has extended to humans Collins' findings that sensitivity to nicotine is inversely related to corticosteroid activity. We also found that the combination of a psychological stressor and smoking produced additive effects on cortisol release in humans. These findings suggest a novel way of explaining the interaction between smoking and stress, in that increased nicotine intake in the context of stress may in part reflect behavioural compensation for diminished sensitivity to nicotine when corticosteroid activity is enhanced by the stressor.

Is the loudness dependence of auditory evoked potentials modulated by the selective serotonin reuptake inhibitor citalopram in healthy subjects?

The loudness dependence of auditory evoked potentials (LDAEP) has been discussed as a non-invasive in vivo marker of central serotonergic function. Evidence for this has been found in animal studies, but studies in humans provide less consistent results. In this study, the relationship between LDAEP and directly modulated central serotonergic activity in healthy subjects was investigated. In a single-blind cross-over design, the LDAEP of female participants (age: 24.0 +/- 2.3 years) was measured under two conditions: (1) infusion of 20 mg citalopram diluted in 250 ml 0.9% saline and (2) infusion of 250 ml 0.9% saline as placebo. LDAEP was measured at five different time points before, during and up to 60 min after drug/placebo administration and dipole source analysis was performed. The increase of the central serotonin activity in response to citalopram was not accompanied by a significant change of the LDAEP compared to the placebo condition. The result underlines that the acceptance of LDAEP as a marker of central serotonergic function still needs further discussion.

The effects of cholinergic and dopaminergic antagonists on nicotine-induced cerebral neurotransmitter changes

In a continuing study of nicotine-induced mechanisms in brain areas associated with cognitive processes, the effects of cholinergic and dopaminergic antagonists on nicotine-induced changes in dopamine, norepinephrine, and serotonin were examined. These effects were measured via in vivo microdialysis in the dorsal and ventral hippocampus and in the prefrontal and medial temporal cortex of conscious, freely moving, adult male rats. Nicotine (0.3 mg/kg, free base) was administered subcutaneously and the antagonists were infused locally via the microdialysis probe. Nicotine alone induced an increase of dopamine and its metabolites in all areas, an increase of norepinephrine in the cortex, and an increase of the norepinephrine metabolite 4-hydroxy-3-methoxy-phenylglycol in all areas. Serotonin was decreased in the hippocampus and increased in the cortex. Nicotine-induced dopamine increases were inhibited by nicotinic (mecamylamine 100 microM, methyllycaconitine 500 microM), muscarinic (atropine 100 microM), and dopaminergic D1 (SCH23390 100 microM) and D2 (eticlopride 100 microM) antagonists, in the hippocampal and cortical areas. In the hippocampal areas, these antagonists had less significant effect on norepinephrine and serotonin. However, in the cortical areas, all antagonists inhibited the nicotine-induced increase of serotonin to varying degrees; and some, primarily nicotinic and dopamine D1 antagonists, inhibited the induced increase of norepinephrine. In the hippocampal and cortical areas, the mechanisms of nicotine-induced dopamine increase seem to be similar, but the mechanisms seem to be different for noradrenergic and serotonergic systems, as shown by the fact that nicotine induces no change in norepinephrine and a decrease in serotonin in the hippocampus, while it induces an increase in both in the cortex. Nicotine-induced dopamine release seems to be mediated, in part locally, by nicotinic and muscarinic receptors on dopaminergic cells. In contrast, nicotine's effect on norepinephrine and serotonin is at least partially mediated by initial changes at other than local sites, and through different receptors. Thus, the effects of nicotine and the mechanisms involved differ for different neurotransmitters and in different brain areas.

Psychobiological Theories of Smoking and Smoking Motivation

Abstract. Theories of smoking have been developed about the conditions and causes of smoking as well as for explaining its maintenance. Moreover, factors of smoking motivation have been identified, which describe incentives to smoke and types of smoking behavior. The most frequently reported motives are psychosocial smoking, sensorimotor smoking, indulgent smoking, stimulation smoking, sedation smoking, dependent smoking, and automatic smoking. In the first phase after the start of smoking, psychosocial smoking is the dominating motive, which is best represented by theories of social psychology. Sensorimotor smoking may be best explained by theories of classical and operant conditioning. Indulgent smoking (= smoking for pleasure) may be explained by neurochemical theories and by the neurobiological theories of dependence emphasizing nicotine-induced activation of mesolimbic dopaminergic reinforcement. Stimulation smoking and sedation smoking are related to arousal models. Moreover, sedation smoking is also explained by aspects of reducing negative affect (as claimed by theories of affect regulation) and by biochemical theories emphasizing serotonergic mechanisms in modulating anxiety as well as by theories of dependence. Dependent smoking, which is mostly based on negative reinforcement (suppression of withdrawal symptoms) may be explained by neurochemical theories as well as by neurobiological theories of psychological and physical dependence explaining blunted dopaminergic and serotonergic responsivity as due to desensitization of respective receptors. Also automatic smoking may be explained by processes of habit learning and neurobiological theories of dependence. Finally, personality theories have been applied to all of these smoking motives.

Nicotine treatment produces persistent increases in amphetamine-stimulated locomotor activity in periadolescent male but not female or adult male rats

Nicotine is a popular addictive drug used among the adolescent population, and it has long been questioned whether nicotine use in adolescence may lead to the use of other psychostimulant drugs. It is not fully understood, however, how nicotine alters behavior and brain neurochemistry in the adolescent age cohort and how this may affect subsequent illicit drug use. In the current study, periadolescent and adult male and female rats were treated with nicotine for 7 days. One day or 30 days after this treatment, the effects of amphetamine on locomotor activity were studied. Sensitization to nicotine occurred in periadolescent female and adult male and female rats, but not in periadolescent male rats over the course of the 7-day treatment period. On day 8 (1 day after treatment with nicotine ended) and on day 37 (30 days after treatment with nicotine ended), nicotine-pretreated periadolescent male rats were sensitized to the locomotor-activating effects of amphetamine. The response to amphetamine of periadolescent female and adult male and female rats was unchanged at either time point after nicotine pretreatment. Thus, adolescent males are more sensitive than adults or females to the stimulant effects of amphetamine after exposure to nicotine, and this effect is long-lasting. These data suggest that nicotine use during adolescence may carry a greater risk than during adulthood and that male adolescent smokers may be particularly vulnerable to the risk of stimulant abuse.

N-methyl-D-aspartate antagonists as drug models of schizophrenia: a surprising link to tobacco smoking

The pros and cons of N-methyl-D-aspartate (NMDA) antagonists as drug models of schizophrenia are discussed in relation to the neuropathology of this complex mental spectrum of diseases. The role of acetylcholine, dopamine, gamma aminobutyric acid, glutamic acid, and serotonin emphasizes that multiple neurotransmitter system abnormalities are involved, even though current drug therapy involves primarily dopamine (D(2))/serotonin (5 HT(2)) antagonists. Only some of the fundamental aspects of schizophrenia are replicated by NMDA receptor antagonists of glutamic acid. Subchronic NMDA antagonism in an animal model results in decreased levels of dopamine in prefrontal cortex and increased D(1) receptor binding. The results of PET studies of schizophrenic patients imply decreased dopamine levels in their prefrontal cortex. Tobacco-smoking schizophrenic patients transiently normalize prepulse inhibition. Nicotine appears to be one factor that may help explain some of these phenomena.

Differences in platelet serotonin transporter sites between African-American tobacco smokers and non-smokers

RATIONALE

The serotonin transporter (5HTT) regulates the magnitude and duration of serotonergic neurotransmission. Although nicotine and other constituents of tobacco smoke may influence serotonin turnover among animals, few studies have examined whether smoking is associated with alteration in 5HTT in humans.

OBJECTIVE

We investigated whether tobacco smokers and non-smokers differed in platelet tritiated paroxetine binding, a measure of 5HTT sites, and whether severity of nicotine dependence (ND) was related to 5HTT measures.

METHODS

Tritiated paroxetine binding sites on platelets were assayed in 26 African-American smokers and 30 non-smokers. Severity of smoking was assessed using the Fagerstrom Test for Nicotine Dependence (FTND). Relationships between FTND scores and maximum number of transporter sites (B(max)) and affinity constant (K(d)) of paroxetine binding were determined.

RESULTS

B(max) values showed a significant negative correlation with FTND scores (rho=-0.28, P<0.01). Notably, smokers with higher ND had significantly lower B(max) compared to those with lower ND and non-smokers; the latter two groups did not differ in B(max) ( F=3.92, P<0.05). Smokers scored higher on impulsivity than non-smokers, however, behavioral variables did not influence the relationship of smoking with B(max). Age, gender and K(d) values were not associated with smoking or B(max).

CONCLUSIONS

Smoking, in particular higher nicotine dependence, appears to be correlated with decreased density of platelet 5HTT sites in African-Americans. The nature of the relationship and whether similar changes occur in the brain merit further investigation.

Involvement of serotonin in nicotine dependence: processes relevant to positive and negative regulation of drug intake

The neurobiological substrate of nicotine dependence has been the subject of extensive preclinical and clinical research. Many experimental reports have implicated the brain serotonin (5-HT) systems in processes relevant to nicotine dependence, but the specific role of this neurotransmitter system largely remains to be elucidated. This review will focus on the role of 5-HT in the acute and chronic effects of nicotine. In particular, the evidence for a role of 5-HT neurotransmission in brain processes thought to be involved in positive and negative control of nicotine use will be examined, and potential clinical implications discussed.

Neurobiology of the nicotine withdrawal syndrome

The aversive aspects of withdrawal from chronic nicotine exposure are thought to be an important motivational factor contributing to the maintenance of the tobacco habit in human smokers. Much emphasis has been placed on delineating the underlying neurobiological mechanisms mediating different components of the nicotine withdrawal syndrome. Recent studies have shown that both central and peripheral populations of nicotinic acetylcholine receptors (nAChRs) are involved in mediating somatic signs of nicotine withdrawal as measured by the rodent nicotine abstinence scale. However, only central populations of nAChRs are involved in mediating affective aspects of nicotine withdrawal, as measured by elevations in brain-stimulation reward thresholds and conditioned place aversion. Nicotine interacts with several neurotransmitter systems, including acetylcholine, dopamine, opioid peptides, serotonin, and glutamate systems. Evidence so far suggests that these neurotransmitters play a role in nicotine dependence and withdrawal processes. The available evidence also suggests that different underlying neurochemical deficits mediate somatic and affective components of nicotine withdrawal. The aim of the present review is to discuss preclinical findings concerning the neuroanatomical and neurochemical substrates involved in these different aspects of nicotine withdrawal.

Sex differences in [123I]beta-CIT SPECT measures of dopamine and serotonin transporter availability in healthy smokers and nonsmokers

Nicotine and other constituents of tobacco smoke elevate dopamine (DA) and serotonin (5-HT) levels in brain and may cause homeostatic adaptations in DA and 5-HT transporters. Since sex steroids alter DA and 5-HT transporter expression, the effects of smoking on DA and 5-HT transporter availability may differ between sexes. In the present study, DA and 5-HT transporter availabilities were quantitated using single photon emission computed tomography (SPECT) imaging approximately 22 h after bolus administration of [123I]beta-CIT, an analog of cocaine which labels DA and 5-HT transporters. Forty-two subjects including 21 pairs of age-, race-, and gender-matched healthy smokers and nonsmokers (12 female and 9 male pairs) were imaged. Regional uptake was assessed by the outcome measures, V3", which is the ratio of specific (i.e., ROI-cerebellar activity) to nondisplaceable (cerebellar) activity, and V3, the ratio of specific to free plasma parent. Overall, striatal and diencephalic [123I]beta-CIT uptake was not altered by smoking, whereas brainstem [123I]beta-CIT uptake was modestly higher (10%) in smokers vs. nonsmokers. When subgrouped by sex, regardless of smoking status, [123I]beta-CIT uptake was higher in the striatum (10%), diencephalon (15%), and brainstem (15%) in females vs. males. The sex*smoking interaction was not significant in the striatum, diencephalon, or brainstem, despite the observation of 20% higher brainstem [123I]beta-CIT uptake in male smokers vs. nonsmokers and less than a 5% difference between female smokers and nonsmokers. The results demonstrate higher DA and 5-HT transporter availability in females vs. males and no overall effect of smoking with the exception of a modest elevation in brainstem 5-HT transporters in male smokers. Although these findings are preliminary and need validation with a more selective 5-HT transporter radiotracer, the results suggest that brainstem 5-HT transporters may be regulated by smoking in a sex-specific manner.

Effects of 5-HT1A and 5-HT2 receptor agonists on the behavioral and neurochemical consequences of repeated nicotine treatment

This study investigated the effects of repeated daily (15 days) treatment with nicotine, alone or in combination with the 5-HT1A/7 receptor agonist (+/-)-8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) or the 5-HT2 receptor agonist (+/-)-2,5-dimethoxy-4-iodoamphetamine (DOI) on locomotor sensitization, mesolimbic dopamine neurochemistry and on behavioral inhibition in the rat. Acute nicotine elevated the extracellular dopamine levels in the nucleus accumbens and stimulated locomotor activity, effects that were sensitized after repeated nicotine treatment. Repeated nicotine administration also produced nicotine-induced behavioral disinhibition in the elevated plus-maze. Treatment with DOI counteracted the expression of the nicotine-induced locomotor and neurochemical sensitization, but had no effect on nicotine-induced behavioral disinhibition. Treatment with 8-OH-DPAT decreased the expression of nicotine-induced behavioral disinhibition, but had no effect on locomotor or neurochemical sensitization. Taken together, these findings suggest that the 5-HT1A and the 5-HT2 receptor subtypes are differentially involved in the effects of repeated nicotine on locomotor sensitization, behavioral inhibition and mesolimbic dopamine neurochemistry.

[Non-familial episodic ataxia possibly associated with the use of nicotine: case report]

The author reports a case of non-familial episodic ataxia responsive to acetazolamide, clinically similar to episodic ataxia type 2 (EA-2), in which nicotine is a possible factor in the origin of the ataxic episodes.

The effects of nicotine on neural pathways implicated in depression: a factor in nicotine addiction?

The prevalence of tobacco smoking varies considerably between different groups within the community, tobacco smoking being particularly prevalent in patients with depressive disorder. This review will focus on results, derived from animal studies, which suggest that, in addition to its primary reinforcing properties, nicotine also exerts effects in stressful environments, which may account for its enhanced addictive potential in depressed patients. It focuses on the evidence that depression sensitises patients to the adverse effects of stressful stimuli, and that this can be relieved by drugs that stimulate dopamine release in the forebrain. This mechanism, it is proposed, contributes to the increased craving to smoke in abstinent smokers exposed to such stimuli, because they become conditioned to use this property of nicotine to produce rapid alleviation of the adverse effects of the stress. The review also explores the possibility that chronic exposure to nicotine elicits changes in 5-HT formation and release in the hippocampus which are depressogenic. It is postulated that smokers are protected from the consequences of these changes, while they continue to smoke, by the antidepressant properties of nicotine. However, they contribute to the symptoms of depression experienced by many smokers when they first quit the habit.

Pharmacology of nicotine and its therapeutic use in smoking cessation and neurodegenerative disorders

During the last decade, nicotine has been used increasingly as an aid to smoking cessation and has been found to be a safe and efficacious treatment for the symptoms of nicotine withdrawal. This period has also seen significant advances in our understanding of the mechanisms underlying the psychopharmacological responses to nicotine, including, particularly, those that have been implicated in nicotine addiction. This paper reviews this decade of progress in the specific context of the therapeutic application of nicotine to the treatment of smoking cessation. Other putative future applications, particularly in the treatment of neurodegenerative disorders, are also reviewed.

Effects of systemic nicotine on serotonin release in rat brain

We used in vivo microdialysis to examine the acute effects of systemically administered nicotine (0.8-8.0 mg/kg, s.c.) on extracellular levels of serotonin (5-HT) in the frontal cortex of awake rats and animals anesthetized with chloralose/urethane. In anesthetized animals, 5-HT efflux was elevated during the initial 15 min after nicotine administration (2-8 mg/kg), but then returned to baseline values. All of the effective nicotine doses also lowered and then raised blood pressure in these animals. However, other drugs which raised (methoxamine, 0.07 mg/kg, i.v.) or lowered (mecamylamine, 5 mg/kg, i.p.) blood pressure without directly activating nicotinic receptors failed to alter 5-HT release. Moreover, pretreatment with a centrally active dose of mecamylamine, a known nicotinic antagonist, blocked the effects of nicotine (4 mg/kg) on 5-HT release. For studies on awake rats the perfusion fluid also contained fluoxetine, since basal 5-HT levels were barely detectable without this uptake blocker. In such animals, 1.6 mg/kg of nicotine significantly increased 5-HT release, an effect apparent in the initial 20 min after treatment and persisting for at least 2 h. These observations demonstrate that systemically administered nicotine increases frontocortical 5-HT release, that this effect is independent of the cardiovascular responses to the drug, and that it probably results from the activation of previously described nicotinic receptors on raphe neurons. The present findings are consistent with the hypothesis that the appetitive and mood disturbances associated with nicotine withdrawal may be mediated by diminished serotoninergic transmission.

The Psychopharmacology of 5-HT3Receptors

5-HT3 receptors have an exclusive neuronal location and evidence is presented of their involvement in behaviour. 5-HT3 receptor antagonists such as ondansetron, tropisetron and zacopride have provided the critical pharmacological tools to reveal a potent and efficacious ability to regulate disturbed behaviour. Thus the 5-HT3 receptor antagonists will restore to normal rodent and primate behaviour disturbed by increasing limbic dopamine function, aversive situations, cognitive impairments and drug abuse. The remarkable feature of their action is a failure to modify normal behaviour. This unique pharmacological signature has ensured a wide interest in the potential role of the 5-HT3 receptor antagonists in the treatment of schizophrenia, anxiety, age related memory impairment and the problems of withdrawal from drugs of abuse. The preclinical data and preliminary clinical observations are presented.

The influence of stress on psychopharmacological responses to nicotine

This essay considers the mechanisms which may mediate the apparent anxiolytic properties of nicotine and which are thought to be responsible for the 'calming' effect of tobacco smoke experienced by many smokers. It summarizes the evidence that, in many tests for anxiolytic activity, the effects of nicotine do not resemble those of established anxiolytic drugs such as diazepam and concludes that it is likely that neural systems other than those which mediate the responses to the benzodiazepines are responsible for the putative anxiolytic properties of nicotine. Circumstantial evidence which suggests that the increase in mesolimbic dopamine secretion evoked by nicotine may not only be rewarding per se but may also contribute to the ability of the drug to alleviate the effects of stress is presented. The essay also summarizes results which suggest that chronic nicotine evokes changes in the mesolimbic dopamine system which resemble those seen in animals treated chronically with antidepressant drugs and proposes that the mechanisms which mediate the ability of antidepressant drugs to alleviate the effects of stress may also mediate the apparent anxiolytic properties of nicotine. The possible consequences of this hypothesis for future research are considered.

Differential effects of prenatal and postnatal ACTH or nicotine exposure on 5-HT high affinity uptake in the neonatal rat brain

These studies were designed to examine the differential effects of prenatal or postnatal administration of ACTH 1-39 and nicotine, on 5-HT high affinity uptake in brainstem and hippocampal synaptosomes. ACTH was administered prenatally (to pregnant dams) and postnatally to the neonates. Postnatal administration of ACTH significantly increased high-affinity 5-HT uptake in the hippocampus and especially the brainstem at both 7 and 21 days after birth. Prenatal ACTH, on the other hand, transiently increased 5-HT uptake in only the brainstem at 7 days, a change that was reversed at 21 days. While the effects of postnatal nicotine administration were essentially the same as those of postnatal ACTH treatment, prenatal nicotine, unlike ACTH, did not alter 5-HT uptake in 7-day-old rats but did reduce uptake in both tissues at 21 days. The observation that postnatal nicotine mimics the effects of postnatal ACTH and that nicotine stimulates ACTH release, suggests that the postnatal effects of nicotine may be exerted through ACTH.

Ketotifen and its analogues reduce aversive responding in the rodent

The abilities of ketotifen and other 4-piperidylidene derivatives (HF200-184, HE36-953, SDZ209-321 and SDZ206-703) to inhibit aversive responding were compared in the mouse light/dark test box and in the rat social interaction test. Ketotifen and HF200-184 reduced aversive responding of the mouse to the brightly illuminated area of the test box and facilitated rat social interaction; HF200-184 was approximately 100 times more potent than ketotifen. The chronic administration and withdrawal from treatment with diazepam, ethanol, nicotine and cocaine in the mouse was associated with increased behavioural suppression which was prevented by the administration of ketotifen and HF200-184 during the period of withdrawal. HE36-953 also prevented the behavioural consequences of withdrawal from diazepam and cocaine. The relative potencies of ketotifen and its analogues to inhibit aversive responding did not correlate with their affinities for the 5-HT3 recognition site. It is concluded that compounds within the 4-piperidylidene series can reduce behavioural suppression in rodent models of anxiety and attenuate the behavioural consequences of withdrawing from treatment with drugs of abuse.

The psychopharmacology of 5-HT3 receptors

The review presents evidence that 5-HT3 receptors within the brain may contribute to the control of behavior. 5-HT3 receptor antagonists GR38032F, zacopride, ICS 205-930 and other agents are very potent in reducing mesolimbic dopamine hyperactivity caused by the injection of amphetamine or infusion of dopamine into the rat nucleus accumbens and amygdala, and the ventral striatum of the marmoset. Such actions are distinguished from those of neuroleptic agents by a failure to reduce normal levels of activity or to induce a rebound hyperactivity after discontinuation of treatment. Indeed, the 5-HT3 receptor antagonists can prevent the neuroleptic-induced rebound hyperactivity. Further evidence that 5-HT3 receptors moderate limbic dopamine function is shown by their ability to reduce both the behavioral hyperactivity and changes in limbic dopamine metabolism caused by DiMe-C7 injection into the ventral tegmental area. The 5-HT3 receptor antagonists also have an anxiolytic profile in the social interaction test in the rat, the light/dark exploration test in the mouse, the marmoset human threat test and behavioral observations in the cynomolgus monkey. They differ from the benzodiazepines by an absence of effect in the rat water lick conflict test and a withdrawal syndrome. Importantly, the 5-HT3 receptor antagonists are highly effective to prevent the behavioral syndrome following withdrawal from treatment with diazepam, nicotine, cocaine and alcohol. Intracerebral injection techniques in the mouse indicate that the dorsal raphe nucleus and amygdala may be important sites of 5-HT3 receptor antagonist action to inhibit aversive behavior. Studies with GR38032F indicate an additional effect in reducing alcohol consumption in the marmoset. The identification and distribution of 5-HT3 receptors in the brain using a number of 5-HT3 receptor ligands, [3H]65630, [3H]zacopride and [3H]ICS 205-930 correlates between studies, and the 5-HT3 recognition sites in cortical, limbic and other areas meet the criteria for 5-HT3 receptors to mediate the above behavioral effects. Thus the use of 5-HT3 receptor antagonists reveals an important role for 5-hydroxytryptamine in the control of disturbed behavior in the absence of effect on normal behavior. The profile of action of the 5-HT3 receptor antagonists has generated a major clinical interest in their potential use for schizophrenia, anxiety and in the control of drug abuse.

The actions of nicotine and cocaine in a mouse model of anxiety

The acute administration of nicotine (0.01-1.0 mg/kg IP) to the mouse increased the time spent and rearings and line crossings in the aversive brightly illuminated white area of a two compartment white/black test box, with a corresponding decrease in the black. This profile of change was maintained during twice daily administration (0.1 mg/kg IP) for 14 days. Eight to 96 hr following withdrawal of nicotine (14-day treatment), the behavioural profile was reversed to a preference for the black area: by 240 hr values had returned to control levels. In contrast to the effects of nicotine, an acute injection of cocaine (0.1-10 mg/kg IP) exacerbated the aversive response to the white area. However, similarly to nicotine, the administration of cocaine (1.0 mg/kg IP) twice daily for 14 days reduced the aversion to the white area and exacerbated the response following cocaine withdrawal. The effects of nicotine and cocaine to reduce and enhance responsiveness to the aversive properties of the white area are discussed in terms of an anxiolytic and anxiogenic response and the possibility of a serotonergic involvement.

Action of nicotine on accumbens dopamine and attenuation with repeated administration

The behavioral and physiological effects of repeated nicotine administration are complex; sedation and hypothermia are present early but become attenuated while locomotor activity increases. Maximal blood levels and behavioral changes occur within 10 min of s.c. injection. We examined the effects of 10 nicotine injections (0.8 mg/kg) in 14 days on the levels of brain amines following challenge with either saline or nicotine on the 15th day. Dopamine, DOPAC, HVA, 3-methoxytyramine, norepinephrine, 5-hydroxytyramine, and 5-HIAA were measured in the frontal cortex, olfactory tubercle, nucleus accumbens, caudate-putamen, substantia nigra and ventral tegmental area. Ten minutes after nicotine was given to rats that had previously received only saline the levels of dopamine and its metabolite DOPAC indicated an increase in dopamine turnover in the nucleus accumbens. Of the areas examined the accumbens was the most sensitive to nicotine, with few significant amine changes in other regions. Twenty-four hours after the last nicotine injection the levels of dopamine and its metabolites indicated a sustained decrease in dopamine turnover in the accumbens induced by repeated administration. Following repeated nicotine a nicotine challenge still induced an acute increase in dopamine turnover in the accumbens, but the response was less than in animals not previously given nicotine. The results confirm earlier studies indicating that the accumbens is a major site of nicotine action.

Evidence that tobacco smoking increases the density of (-)-[3H]nicotine binding sites in human brain

In a postmortem study of nicotinic receptors in human brain, cigarette smoking was found to be associated with increased (-)-[3H]nicotine binding to membranes prepared from gyrus rectus (Brodmann area 11) (p less than 0.001), hippocampal neocortex (Brodmann area 27), cerebellar cortex (p less than 0.01), hippocampal formation (Ammon's horn + subiculum), and the median raphe nuclei of the midbrain (p less than 0.05) but not the medulla oblongata. Analysis of the binding data suggested that the increased binding reflected an increase in the density of the receptors rather than a change in their affinity for (-)-nicotine. The effects of smoking were not influenced significantly by either the sex or age of the subject. It is concluded that smoking evokes an increase in high-affinity nicotine binding similar to that observed previously in animals treated chronically with nicotine and that the effect of smoking on these sites is probably caused by the nicotine present in the tobacco smoke.

Behavioural and adrenocortical responses to nicotine measured in rats with selective lesions of the 5-hydroxytryptaminergic fibres innervating the hippocampus

The effects of acute and subchronic (7) injections of nicotine (0.4 mg kg-1, s.c.) and of selective lesions of the 5-hydroxytryptaminergic (5-HTergic) pathways innervating the hippocampus on the spontaneous behaviour of rats in an elevated X-maze composed of two open and two enclosed runways have been examined. Subchronic, but not acute, nicotine increased total spontaneous activity. Neither acute nor subchronic nicotine altered the ratio of open:closed runway entries. Destruction of the 5-HTergic pathways innervating the hippocampus with 5,7-dihydroxytryptamine caused a reduction in the ratio of open:enclosed runway entries. Acute, but not subchronic, nicotine caused a significant increase in plasma corticosterone. The lesion had no effects on the plasma levels of this hormone. No significant interactions between the lesion and the responses to nicotine were observed. The data failed to provide any evidence that hippocampal 5-HTergic systems may be implicated in the effects of nicotine on the spontaneous behaviour of the rat.

Effects of nicotine on exploratory behavior in rats: correlation with regional brain monoamine levels

The results of a dose-response study of the effects of nicotine on exploratory behavior in male rats is reported. Nicotine at 0.5 mg/kg elevated locomotor activity without significantly changing other parameters of exploration. Low-dose nicotine (0.2 mg/kg) did not produce any effect on exploration measures, while high-dose nicotine (0.8 mg/kg) produced a state of ataxia in animals and decreased most exploration measures in general. Additional, nicotine at high doses seems to reduce the animal's state of fear/anxiety, while at low dose the drug seems to increase the animal's level of curiosity in a novel environment. Biochemically, nicotine has been found to accelerate dopamine synthesis and norepinephrine turnover, and to decrease serotonin turnover. More importantly, the amino acid precursors tyrosine and tryptophan were found to be the neurochemical measures most related to the behavioral changes produced by nicotine.

Reduction of nicotine-induced hyperactivity by pCPA

Spontaneous locomotion of female Wistar rats was measured in six to ten minute sessions in an automated tunnel maze consisting of a central arena and six radially symmetrical angled arms. Nicotine (0.2 mg/kg subcutaneous, 20-30 minutes pretest) increased total arm visit frequency, but intrasession habituation and number of repetitive arm visits in the first six choices were not affected. pCPA (300 mg/kg IP three days pretest) reduced arm-visit frequency in nicotine-, but not in saline-treated rats; it had no effect on intrasession habituation or number of repetitions in either treatment group. 5-HTP (50 mg/kg IP 90 minutes pretest) reduced arm entry frequency in saline-, nicotine-, and pCPA-treated groups. Possible reasons for this discrepancy are discussed.

Nicotine binding to brain tissue from drug-naive and nicotine-treated rats

Two nicotine binding sites with dissociation constants for nicotine of approximately 3 nM and 12 microM respectively have been found in homogenates of rat hippocampus, hypothalamus, parietal cortex and mesencephalon, the greatest density of high affinity binding sites being in parietal cortex (30.0 +/- 3.0 fmol (mg protein)-1), the lowest in hypothalamus (16.1 +/- 1.0 fmol (mg protein)-1). The density of the low affinity sites (approx. 20 pmol (mg protein)-1) did not show any regional variation. Neither site was present in homogenates of medulla oblongata. The accumulation of radioactivity following the subcutaneous administration of [3H]nicotine (0.4 mg kg-1) was rapid, the highest concentrations being found in the brain regions with the highest density of high affinity binding sites. Medulla oblongata did not accumulate radioactivity above the concentration found in plasma. The chronic administration of nicotine (0.4 mg kg-1 s.c. daily for 39 days) had no significant effects on [3H]nicotine binding to brain tissue or its accumulation into brain following subcutaneous administration. It is concluded that nicotine readily passes from plasma into brain tissue and is accumulated in the areas containing high affinity binding sites for the compound. It is also concluded that the biochemical and behavioural effects reported previously in response to the chronic daily administration of nicotine do not depend upon changes in its uptake or binding by brain tissue.