Effects of nicotine and footshock stress on dopamine release in the striatum and nucleus accumbens

E-cigarettes, nicotine, the lung and the brain: multi-level cascading pathophysiology

Tobacco smoking is highly addictive and causes respiratory disease, cardiovascular disease and multiple types of cancer. Electronic-cigarettes (e-cigarettes) are non-combustible tobacco alternatives that aerosolize nicotine and flavouring agents in a propylene glycol-vegetable glycerine vehicle. They were originally envisaged as a tobacco cessation aid, but whether or not they help people to quit tobacco use is controversial. In this review, we have compared and contrasted what is known regarding the effects of nicotine on the lungs vs. the effects of nicotine in the brain in the context of addiction. Critically, both combustible tobacco products and e-cigarettes contain nicotine, a highly addictive, plant-derived alkaloid that binds to nicotinic acetylcholine receptors (nAChRs). Nicotine's reinforcing properties are primarily mediated by activation of the brain's mesolimbic reward circuitry and release of the neurotransmitter dopamine that contribute to the development of addiction. Moreover, nicotine addiction drives repeated intake that results in chronic pulmonary exposure to either tobacco smoke or e-cigarettes despite negative respiratory symptoms. Beyond the brain, nAChRs are also highly expressed in peripheral neurons, epithelia and immune cells, where their activation may cause harmful effects. Thus, nicotine, a key ingredient of both conventional and electronic cigarettes, produces neurological effects that drive addiction and may damage the lungs in the process, producing a complex, multilevel pathological state. We conclude that vaping needs to be studied by multi-disciplinary teams that include pulmonary and neurophysiologists as well as behaviourists and addiction specialists to fully understand their impact on human physiology.

Alterations in the corticotropin-releasing hormone (CRH) neurocircuitry: Insights into post stroke functional impairments

Although it is well accepted that changes in the regulation of the hypothalamic-pituitary adrenal (HPA) axis may increase susceptibility to affective disorders in the general population, this link has been less examined in stroke patients. Yet, the bidirectional association between depression and cardiovascular disease is strong, and stress increases vulnerability to stroke. Corticotropin-releasing hormone (CRH) is the central stress hormone of the HPA axis pathway and acts by binding to CRH receptors (CRHR) 1 and 2, which are located in several stress-related brain regions. Evidence from clinical and animal studies suggests a role for CRH in the neurobiological basis of depression and ischemic brain injury. Given its importance in the regulation of the neuroendocrine, autonomic, and behavioral correlates of adaptation and maladaptation to stress, CRH is likely associated in the pathophysiology of post stroke emotional impairments. The goals of this review article are to examine the clinical and experimental data describing (1) that CRH regulates the molecular signaling brain circuit underlying anxiety- and depression-like behaviors, (2) the influence of CRH and other stress markers in the pathophysiology of post stroke emotional and cognitive impairments, and (3) context and site specific interactions of CRH and BDNF as a basis for the development of novel therapeutic targets. This review addresses how the production and release of the neuropeptide CRH within the various regions of the mesocorticolimbic system influences emotional and cognitive behaviors with a look into its role in psychiatric disorders post stroke.

Ventral tegmental area dopamine revisited: effects of acute and repeated stress

Aversive events rapidly and potently excite certain dopamine neurons in the ventral tegmental area (VTA), promoting phasic increases in the medial prefrontal cortex and nucleus accumbens. This is in apparent contradiction to a wealth of literature demonstrating that most VTA dopamine neurons are strongly activated by reward and reward-predictive cues while inhibited by aversive stimuli. How can these divergent processes both be mediated by VTA dopamine neurons? The answer may lie within the functional and anatomical heterogeneity of the VTA. We focus on VTA heterogeneity in anatomy, neurochemistry, electrophysiology, and afferent/efferent connectivity. Second, recent evidence for a critical role of VTA dopamine neurons in response to both acute and repeated stress will be discussed. Understanding which dopamine neurons are activated by stress, the neural mechanisms driving the activation, and where these neurons project will provide valuable insight into how stress can promote psychiatric disorders associated with the dopamine system, such as addiction and depression.

New mechanisms and perspectives in nicotine withdrawal

Diseases associated with tobacco use constitute a major health problem worldwide. Upon cessation of tobacco use, an unpleasant withdrawal syndrome occurs in dependent individuals. Avoidance of the negative state produced by nicotine withdrawal represents a motivational component that promotes continued tobacco use and relapse after smoking cessation. With the modest success rate of currently available smoking cessation therapies, understanding mechanisms involved in the nicotine withdrawal syndrome are crucial for developing successful treatments. Animal models provide a useful tool for examining neuroadaptative mechanisms and factors influencing nicotine withdrawal, including sex, age, and genetic factors. Such research has also identified an important role for nicotinic receptor subtypes in different aspects of the nicotine withdrawal syndrome (e.g., physical vs. affective signs). In addition to nicotinic receptors, the opioid and endocannabinoid systems, various signal transduction pathways, neurotransmitters, and neuropeptides have been implicated in the nicotine withdrawal syndrome. Animal studies have informed human studies of genetic variants and potential targets for smoking cessation therapies. Overall, the available literature indicates that the nicotine withdrawal syndrome is complex, and involves a range of neurobiological mechanisms. As research in nicotine withdrawal progresses, new pharmacological options for smokers attempting to quit can be identified, and treatments with fewer side effects that are better tailored to the unique characteristics of patients may become available. This article is part of the Special Issue entitled 'The Nicotinic Acetylcholine Receptor: From Molecular Biology to Cognition'.

Acupuncture Reduces Alcohol Withdrawal Syndrome and c-Fos Expression in Rat Brain

Acupuncture as a therapeutic intervention is widely practiced in the treatment of many functional disorders including alcohol abuse. In the present study, the effects of acupuncture on alcohol withdrawal syndrome (AWS) and Fos-like immunoreactivity (FLI) in the striatum and the nucleus accumbens (NAC) of rats were investigated. During 3 days of cessation following chronic administration of ethanol (3 g/kg, i.p. for 3 weeks), rats showed a significant increase in AWS, such as hypermotility, tail rigidity, grooming and tremor, and an increase in FLI in the dopamine terminal areas of the brain. Treatment with acupuncture at zusanli (ST36) or sanyinjiao (SP6) during the withdrawal period inhibited both AWS and FLI of rats undergoing ethanol injection. These results suggest that acupuncture may be useful in the treatment of alcoholism by modulating post-synaptic neural activation in the striatum and NAC.

Activating effects of chronic variable stress in rats with different exploratory activity: association with dopamine d(1) receptor function in nucleus accumbens

BACKGROUND/AIMS

Rats display persistent behavioural phenotypes of low (LE) versus high (HE) exploratory activity in the exploration box paradigm. LE rats that prefer passive coping strategies show differential dopaminergic activity in the striatum. The main hypothesis of this study was that chronic variable stress (CVS) would have a higher impact on LE rats.

METHODS

Animals were submitted to a CVS regimen lasting 32 days that was followed by a behavioural test battery. The functional states of their dopamine D(1) and D(2) receptors were measured in the striatum and nucleus accumbens (NAcc). Cerebral oxidative metabolism was assessed via cytochrome c oxidase histochemistry in 65 brain regions.

RESULTS

CVS decreased weight gain, to a higher extent in LE rats, and lowered the sucrose preference after the first week, but habituation to the anhedonic effect had developed by the end of the experiment. CVS did not change the behavioural phenotypes initially assigned. No effect of stress on D(2) receptor function was found. Chronically stressed animals exhibited higher levels of social interaction and D(1) receptor-mediated cAMP accumulation in the NAcc, but not in the striatum. CVS was associated with higher oxidative metabolism levels in the anteroventral thalamus, median raphe nuclei and central periaqueductal grey matter. These changes after stress did not depend upon the exploratory phenotype.

CONCLUSION

This study revealed changes in brain biochemistry after habituation to CVS that might be implicated in successful adaptation to chronic stress.

Potential programming of dopaminergic circuits by early life stress

Stress and high levels of glucocorticoids during pre- and early postnatal life seem to alter developmental programs that assure dopaminergic transmission in the mesolimbic, mesocortical, and nigrostriatal systems. The induced changes are likely to be determined by the ontogenetic state of development of these brain regions at the time of stress exposure and their stability is associated with increased lifetime susceptibility to psychiatric disorders, including drug addiction. This article is intended to serve as a starting point for future studies aimed at the attenuation or reversal of the effects of adverse early life events on dopamine-regulated behaviors.

Developmental pathology, dopamine, stress and schizophrenia

Psychological stress is a contributing factor for a wide variety of neuropsychiatric diseases including substance use disorders, anxiety, depression and schizophrenia. However, it has not been conclusively determined how stress augments the symptoms of these diseases. Here we review evidence that the ventral hippocampus may be a site of convergence whereby a number of seemingly discrete risk factors, including stress, may interact to precipitate psychosis in schizophrenia. Specifically, aberrant hippocampal activity has been demonstrated to underlie both the elevated dopamine neuron activity and associated behavioral hyperactivity to dopamine agonists in a verified animal model of schizophrenia. In addition, stress, psychostimulant drug use, prenatal infection and select genetic polymorphisms all appear to augment ventral hippocampal function that may therefore exaggerate or precipitate psychotic symptoms. Such information is critical for our understanding into the pathology of psychiatric disease with the ultimate aim being the development of more effective therapeutics.

Chronic stress, drug use, and vulnerability to addiction

Stress is a well-known risk factor in the development of addiction and in addiction relapse vulnerability. A series of population-based and epidemiological studies have identified specific stressors and individual-level variables that are predictive of substance use and abuse. Preclinical research also shows that stress exposure enhances drug self-administration and reinstates drug seeking in drug-experienced animals. The deleterious effects of early life stress, child maltreatment, and accumulated adversity on alterations in the corticotropin releasing factor and hypothalamic-pituitary-adrenal axis (CRF/HPA), the extrahypothalamic CRF, the autonomic arousal, and the central noradrenergic systems are also presented. The effects of these alterations on the corticostriatal-limbic motivational, learning, and adaptation systems that include mesolimbic dopamine, glutamate, and gamma-amino-butyric acid (GABA) pathways are discussed as the underlying pathophysiology associated with stress-related risk of addiction. The effects of regular and chronic drug use on alterations in these stress and motivational systems are also reviewed, with specific attention to the impact of these adaptations on stress regulation, impulse control, and perpetuation of compulsive drug seeking and relapse susceptibility. Finally, research gaps in furthering our understanding of the association between stress and addiction are presented, with the hope that addressing these unanswered questions will significantly influence new prevention and treatment strategies to address vulnerability to addiction.

In vivo neurochemical monitoring and the study of behaviour

In vivo neurochemical monitoring techniques measure changes in the extracellular compartment of selected brain regions. These changes reflect the release of chemical messengers and intermediates of brain energy metabolism resulting from the activity of neuronal assemblies. The two principal techniques used in neurochemical monitoring are microdialysis and voltammetry. The presence of glutamate in the extracellular compartment and its pharmacological characteristics suggest that it is released from astrocytes and acts as neuromodulator rather than a neurotransmitter. The changes in extracellular noradrenaline and dopamine reflect their role in the control of behaviour. Changes in glucose and oxygen, the latter a measure of local cerebral blood flow, reflect synaptic processing in the underlying neuronal networks rather than a measure of efferent output from the brain region. In vivo neurochemical monitoring provides information about the intermediate processing that intervenes between the application of the stimulus and the resulting behaviour but does not reflect the final efferent output that leads to behaviour.

Long-lasting, sex- and age-specific effects of social stressors on corticosterone responses to restraint and on locomotor responses to psychostimulants in rats

Many neural systems are undergoing marked development over adolescence, which may heighten an animal's vulnerability to stressors. One consequence may be altered sensitivity to drugs of abuse. We previously reported that social stressors in adolescence increased behavioral sensitization to nicotine in adulthood in female, but not male, rats. Here we examined whether social stressors in adolescence alter the functioning of the hypothalamic-pituitary-adrenal (HPA) axis by examining corticosterone release in response to restraint in adulthood. To further assess effects of social stressors on behavioral sensitivity to psychostimulants, we examined locomotor activity in response to nicotine and to amphetamine. In a second set of experiments, we investigated whether the same procedure of social stressors administered in adulthood produces effects similar to that observed when administered in adolescence. Rats underwent daily 1 h isolation followed by pairing with a new cage mate on either postnatal days 33-48 (pubertal stress: PS) or days 65-80 (adult stress: AS). Three weeks later rats tested for either: (a) corticosterone levels were measured in response to restraint, or (b) locomotor sensitization to nicotine (0.25 mg/kg; 5 days) followed by an amphetamine challenge (0.5 mg/kg) 24 h later. Effects of social stressors were evident only in females. PS females had increased locomotor activity to amphetamine compared to controls, and AS females had increased corticosterone release compared to controls. No effect of the social stressors was found in males at either age except for reduced weight gain during the stress procedure. Thus, females are more susceptible to the enduring effects of these moderate social stressors than are males. However, in terms of behavioral sensitivity to drugs of abuse, females may be more susceptible to stressors during adolescence than adulthood, although the reverse appears to be true for HPA function.

Stress during adolescence enhances locomotor sensitization to nicotine in adulthood in female, but not male, rats

A wide body of research has indicated that perinatal exposure to stressors alters the organism, notably by programming behavioral and neuroendocrine responses and sensitivity to drugs of abuse in adulthood. Recent evidence suggests that adolescence also may represent a sensitive period of brain development, and yet there has been little research on the long-lasting effects of stressors during this period. We investigated the effects of pubertal social stress (PS; daily 1-h isolation followed by pairing with a new cage mate on postnatal days 33-48) on locomotor sensitization to injections of nicotine and corticosterone response to restraint stress when the rats were adults (approximately 3 weeks after PS). There were no differences among the groups in locomotor activity to injections of saline. However, PS females had enhanced locomotor sensitization to repeated doses of nicotine compared to control (non-stressed; NS) females, whereas PS males and NS males did not differ. PS enhanced the corticosterone response to restraint in male rats previously sensitized to nicotine and decreased the corticosterone response in nonsensitized male rats. In contrast, PS females and NS females did not differ in plasma corticosterone levels in response to restraint stress, but NS females showed enhanced corticosterone release to restraint after sensitization to nicotine. Thus, during adolescence, social stressors can have long-lasting effects, and the effects appear to differ for males and females.

Sexual-incentive motivation and paced sexual behavior in female rats after treatment with drugs modifying dopaminergic neurotransmission

The effects of the dopamine receptor agonist apomorphine, the dopamine releaser amphetamine, and the dopamine receptor antagonist cis(Z)-flupenthixol on sexual-incentive motivation and on paced-mating behavior were studied in female rats. Apomorphine, in the doses of 0.125 and 0.5 mg/kg, showed a tendency to reduce incentive motivation. Ambulatory activity was inhibited, evidenced both by diminished distance moved and reduced velocity of movement. Amphetamine (0.25 and 1 mg/kg) and flupenthixol (0.25 and 0.5 mg/kg) failed to modify incentive motivation while stimulating and reducing ambulatory activity, respectively. In the mating test, apomorphine enhanced the latency to enter the male's half and reduced the number of proceptive behaviors. However, these effects were associated with the appearance of stereotyped sniffing. Amphetamine increased the propensity to escape from the male after a mount without having other effects. Flupenthixol augmented the duration of the lordosis posture. Neither amphetamine nor flupenthixol affected sniffing. These data show that facilitated dopaminergic neurotransmission stimulates neither paced female sexual behavior nor sexual-incentive motivation. Dopamine receptor blockade has slight consequences. It is concluded that dopamine is not a transmitter of major importance for unconditioned female sexual motivation and behavior.

Has dopamine a physiological role in the control of sexual behavior? A critical review of the evidence

The role of dopaminergic systems in the control of sexual behavior has been a subject of study for at least 40 years. Not surprisingly, reviews of the area have been published at variable intervals. However, the earlier reviews have been summaries of published research rather than a critical analysis of it. They have focused upon the conclusions presented in the original research papers rather than on evaluating the reliability and functional significance of the data reported to support these conclusions. During the last few years, important new knowledge concerning dopaminergic systems and their behavioral functions as well as the possible role of these systems in sexual behavior has been obtained. For the first time, it is now possible to integrate the data obtained in studies of sexual behavior into the wider context of general dopaminergic functions. To make this possible, we first present an analysis of the nature and organization of sexual behavior followed by a summary of current knowledge about the brain structures of crucial importance for this behavior. We then proceed with a description of the dopaminergic systems within or projecting to these structures. Whenever possible, we also try to include data on the electrophysiological actions of dopamine. Thereafter, we proceed with analyses of pharmacological data and release studies, both in males and in females. Consistently throughout this discussion, we make an effort to distinguish pharmacological effects on sexual behavior from a possible physiological role of dopamine. By pharmacological effects, we mean here drug-induced alterations in behavior that are not the result of the normal actions of synaptically released dopamine in the untreated animal. The conclusion of this endeavor is that pharmacological effects of dopaminergic drugs are variable in both males and females, independently of whether the drugs are administered systemically or intracerebrally. We conclude that the pharmacological data basically reinforce the notion that dopamine is important for motor functions and general arousal. These actions could, in fact, explain most of the effects seen on sexual behavior. Studies of dopamine release, in both males and females, have focused on the nucleus accumbens, a structure with at most a marginal importance for sexual behavior. Since accumbens dopamine release is associated with all kinds of events, aversive as well as appetitive, it can have no specific effect on sexual behavior but promotes arousal and activation of non-specific motor patterns. Preoptic and paraventricular nucleus release of dopamine may have some relationship to mechanisms of ejaculation or to the neuroendocrine consequences of sexual activity or they can be related to other autonomic processes associated with copulation. There is no compelling indication in existing experimental data that dopamine is of any particular importance for sexual motivation. There is experimental evidence showing that it is of no importance for sexual reward.

125I-iomazenil - benzodiazepine receptor binding and serum corticosterone level during psychological stress in a rat model

To test the hypothesis that benzodiazepine receptor density decreases in response to stress, we correlated (125)I-iomazenil ((125)I-IMZ) binding with serum corticosterone levels in a rat model. Wistar male rats were divided into four groups; control group (CON, 10 rats), no physical or psychological stress; and one-, three-, and five-day stress groups of 12 rats each (1-DAY, 3-DAY, and 5-DAY, respectively), receiving psychological stress for the given number of days. Psychological stress were given to rats with a communication box. The standardized uptake value (SUV) of (125)I-iomazenil of the 3-DAY and 5-DAY showed that (125)I-iomazenil-benzodiazepine receptor binding was significantly reduced in the cortices, accumbens nuclei, amygdala and caudate putamen (p<0.05). Serum corticosterone level ratio appeared to be slightly elevated in 3-DAY and 5-DAY, although this elevation was not significant. These data suggest that (125)I-IMZ is a useful radioligand to reflect received stress and its binding in the cortices, accumbens nuclei, amygdala and caudate putamen is strongly affected by psychological stress.

Smoking, stress, and negative affect: correlation, causation, and context across stages of smoking

This transdisciplinary review of the literature addresses the questions, Do stress and negative affect (NA) promote smoking? and Does smoking genuinely relieve stress and NA? Drawing on both human and animal literatures, the authors examine these questions across three developmental stages of smoking--initiation, maintenance, and relapse. Methodological and conceptual distinctions relating to within- and between-subjects levels of analyses are emphasized throughout the review. Potential mechanisms underlying links between stress and NA and smoking are also reviewed. Relative to direct-effect explanations, the authors argue that contextual mediator-moderator approaches hold greater potential for elucidating complex associations between NA and stress and smoking. The authors conclude with recommendations for research initiatives that draw on more sophisticated theories and methodologies.

Blockade of mesolimbic dopamine transmission dramatically increases sensitivity to the rewarding effects of nicotine in the ventral tegmental area

Nicotine produces rewarding and aversive motivational effects in humans and other animal species. Here, we report that the mammalian ventral tegmental area (VTA) represents a critical neural substrate for the mediation of both the rewarding and aversive properties of nicotine. We demonstrate that direct infusions of nicotine into the VTA can produce both rewarding and aversive motivational effects. While the rewarding effects of higher doses of nicotine were not attenuated by dopamine (DA) receptor blockade, blockade of mesolimbic DA signalling with either systemic or intra-nucleus accumbens (NAc) neuroleptic pretreatment potentiated the sensitivity to nicotine's rewarding properties over a three-order-of-magnitude dose range. Furthermore, the behavioural effects of lower doses of intra-VTA nicotine were reversed, switching the motivational valence of nicotine from aversive to rewarding. Our results suggest that blockade of mesolimbic DA signalling induced by neuroleptic medications may block selectively the aversive properties of nicotine, thus increasing the vulnerability to nicotine's rewarding and addictive properties by inducing a unique, drug-vulnerable phenotype.

Nicotine-induced fos expression in the pedunculopontine mesencephalic tegmentum in the rat

The aim of this study was to assess the effects of a single dose of nicotine (NIC, 0.3 or 1.0 mg/kg, s.c.), after survival times of 30, 60 or 120 min, on immediate early gene expression in the pedunculopontine mesencephalic tegmentum (PMT), using Fos-immunocytochemistry. Either doses of NIC strongly increased Fos-immunoreactivity in both the pedunculopontine tegmental nucleus (PPTg) and the laterodorsal tegmental nucleus (LDTg), as compared to the saline controls, at 30 min and 60 min. In comparison, the effects of NIC-induced Fos expression in the caudate-putamen (CP) were not as strong as the ones observed in the PPTg and LDTg. In fact, at 30 min the 0.3 mg/kg dose of NIC did not induce Fos-expression, unlike the PPTg and LDTg. The CP response was more noticeable in the mediodorsal than in the laterodorsal region. Double-labelling studies using Fos-immunoreactivity and NADPH-diaphorase histochemistry for cholinergic cells in the PPTg and LDTg revealed that, in general, cholinergic neurons had Fos negative nuclei, although double-labelled neurons were occasionally seen in the PPTg. In conclusion, systemically administered NIC activates the neuronal population of the PPTg and the LDTg possibly by directly targeting nicotinic receptors that may be located in non-cholinergic neurons. We postulate that activation of these non-cholinergic neurons modulates the activity of cholinergic cells in the PMT, which in turn may alter dopamine release in the mesolimbic system.

Previous exposure to footshock stress attenuates nicotine-induced serotonin release in rat striatum during the subsequent stress

We have analyzed the effects of chronic or repeated footshock stress on the release of serotonin (5-hydroxytryptamine: 5-HT) in the striatum of rats that received nicotine by using a microdialysis technique. Neither local infusion of nicotine alone nor stress application alone changed 5-HT release. Local infusion of 1 mM nicotine to the striatum, however, significantly increased 5-HT release in the striatum to 145.9 +/- 30.8 pg/dialysate during simultaneous stress application. These increases of extracellular 5-HT release induced by the combination of nicotine and stress application were also observed in rats that had received daily chronic footshock. However, the previously administered footshock induced the reduced release of 5-HT from the striatum to 33.5 +/- 8. 6 (repeated footshock) and 10.0 +/- 3.6 pg/dialysate (daily footshock) when footshock was given together with nicotine infusion. These results suggest that previous exposure to stress attenuated the nicotine-induced 5-HT release in the striatum during the subsequent stress.

Effect of acute nicotine administration on striatal dopamine output and metabolism in rats kept at different ambient temperatures

1. The effect of ambient temperature on the nicotine-induced (0.3, 0.5 or 0.8 mg kg(-1) s.c.) changes of the striatal concentrations of dopamine (DA) and its metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) was studied in freely-moving rats by in vivo microdialysis. 2. At the ambient temperature of 30 - 33 degrees C, but not at 20 - 23 degrees C, nicotine doses of 0.5 (P<0. 01) and 0.8 mg kg(-1) (P<0.05) significantly increased the extracellular DA concentration. The nicotine doses of 0.5 and 0.8 mg kg(-1) increased the DA metabolite levels similarly at both ambient temperatures studied (P Collapse

Key Words

• nicotine
• dopamine
• ambient temperature
• microdialysis
• striatum
• serum corticosterone

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MESH Headings

• Animals
• Corpus Striatum/metabolism
• Corticosterone/blood
• Dopamine/metabolism
• Male
• Microdialysis
• Nicotine/pharmacokinetics
• Nicotine/pharmacology
• Rats
• Rats, Wistar
• Receptors, Nicotinic/physiology
• Temperature

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Affiliation(s)

Tiina Seppä Division of Pharmacology and Toxicology, Department of Pharmacy, P.O. Box 56, FIN-00014 University of Helsinki, Finland
Minna Ruotsalainen Division of Pharmacology and Toxicology, Department of Pharmacy, P.O. Box 56, FIN-00014 University of Helsinki, Finland
Into Laakso Division of Pharmacognosy, Department of Pharmacy, P.O. Box 56, FIN-00014 University of Helsinki, Finland
Raimo Tuominen Division of Pharmacology and Toxicology, Department of Pharmacy, P.O. Box 56, FIN-00014 University of Helsinki, Finland
Liisa Ahtee Division of Pharmacology and Toxicology, Department of Pharmacy, P.O. Box 56, FIN-00014 University of Helsinki, Finland

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Differential effects of nicotine against stress-induced changes in dopaminergic system in rat striatum and hippocampus

A number of studies have shown an increase in nicotine self-administration among smokers when exposed to stress. Since it is well known that nicotine or stress alter the dopaminergic system, we examined the effect of chronic nicotine administration on the dopamine level and its metabolism in the striatum and the hippocampus during stressful conditions in rats. Nicotine (0.4 mg/kg, i.p. for 14 days) increased the dopamine level in the striatum (P<0. 05) and decreased it in the hippocampus (P<0.05) in comparison with the effect of saline. Three hours of water-immersion restraint stress sharply elevated the dopamine level (P<0.05) and reduced the 3-methoxytyramine level (P ranged from 0.05 to 0.001 depending on the area and time point) in both brain regions studied, while dihydroxyphenylacetic acid and homovanilic acid levels were not altered. Nicotine pretreatment attenuated some of these changes in a region- and time-dependent manner. However, stress induced a decrease in dopamine turnover in the hippocampus (P<0.05) but not in the striatum, and nicotine failed to prevent this effect. Stress-induced alterations gradually returned toward normal during the 48-h observation period, and in some cases this was facilitated by nicotine. Thus, we demonstrated differential, region- and time-dependent protective effects of chronic nicotine administration against stress-induced changes in dopamine levels and release in brain regions critically affected by stress.

Nicotine attenuates stress-induced changes in plasma amino acid concentrations and locomotor activity in rats

It is known that stressor stimuli (both systemic and processive) and nicotine activate central nervous system. Surprisingly, numerous studies have demonstrated an increase in nicotine self-administration among smokers when exposed to stress in order to reduce the stress-related tension. Therefore, in the present study, we decided to investigate the influence of nicotine on both behavioral (i.e., on locomotor activity) and metabolic (i.e., on the level of amino acids in the plasma) changes following water immersion restraint stress in rats. As expected, the stress produced evident decline in locomotor activity of the rats (p < 0.001) and in the levels of all plasma amino acids studied (p < 0.05). Nicotine alone also significantly reduced locomotor activity (p < 0.05) and the levels of some plasma amino acids. However, when administered to rats subjected to water immersion and restraint, nicotine attenuated both stress-induced decrease in locomotor activity (p < 0.05) and in some plasma amino acids. Thus, this study demonstrated that the mode of action of nicotine is strongly dependent on the level of initial brain activity, which provide new evidence for arousal-modulation model of nicotine action.

Enhancement of sensitization to nicotine-induced ambulatory stimulation by psychological stress in rats

1. The authors investigated the influence of psychological stress (PSY-stress) on sensitization to nicotine (0.5 mg/kg, s.c.)-induced ambulatory stimulation. 2. Rats were exposed to the emotional responses of animals which received foot-shock (FS), which they, themselves, did not receive. Ten daily exposures to PSY-stress for 20 min enhanced sensitization to the nicotine-induced ambulatory stimulation compared to that in non-stress rats. 3. However, the increased serum corticosterone levels following nicotine (0.5 mg/kg, s.c.) administered 24 hr after the tenth injection of nicotine in the behavioral study was almost the same in the rats exposed to PSY-stress as compared to non-stress rats. 4. These results suggest that PSY-stress may promote sensitization to nicotine-induced ambulatory stimulation and that the combined effect of PSY-stress and nicotine would facilitate the development of sensitization to nicotine.

Receptor systems participating in nicotine-specific effects

It is generally accepted that self-administration of drugs is prompted primarily by a reward system driven by an increase in extracellular dopamine in the nucleus accumbens. Recent findings that dopamine increase in the accumbens can be caused by many other factors, among them stress, suggest a more complex mechanism, and possibly differences in the reward system for different compounds. In the present paper we compare the effects of receptor-specific antagonists on the increase of dopamine induced by nicotine with that induced by cocaine in the nucleus accumbens in conscious rats. The compounds alone or together were injected intravenously, and dopamine level changes were measured via microdialysis. When administered together the effect of nicotine and cocaine on the level of dopamine in the accumbens was additive. Apparently there is some interaction between the two compounds, since nicotine had no effect after combined nicotine and cocaine administration. Perhaps the available dopamine pool was exhausted by the prior administration. The nicotinic antagonist mecamylamine, the muscarinic antagonist atropine, and the NMDA glutamate receptor antagonist MK-801 each blocked nicotine-induced dopamine release in the accumbens, indicating the participation of more than a single receptor system in the nicotine-induced effect. These three antagonists did not inhibit cocaine-induced dopamine increase in the accumbens, indicating the lack of a role of these receptors in the cocaine effect under our experimental conditions. SCH-23390, a dopamine D1 receptor antagonist, blocked both nicotine- and cocaine-induced effects, indicating the possible role of this receptor in these reward effects. The results indicate that there are differences in some of the receptors mediating the central effects of the two compounds examined, nicotine and cocaine, although each influences dopamine levels, and that the two compounds interact.