Induction but not expression of behavioural sensitization to nicotine in the rat is dependent on glucocorticoids

The Role of Glucocorticoid and Nicotinic Acetylcholine Receptors in the Reward-Enhancing Effects of Nicotine in the ICSS Procedure in Male and Female Rats

Tobacco use disorder is a chronic disorder that affects more than one billion people worldwide and causes the death of millions each year. The rewarding properties of nicotine are critical for the initiation of smoking. Previous research has shown that the activation of glucocorticoid receptors (GRs) plays a role in nicotine self-administration in rats. However, the role of GRs in the acute rewarding effects of nicotine are unknown. In this study, we investigated the effects of the GR antagonist mifepristone and the nicotinic acetylcholine receptor (nAChR) antagonist mecamylamine on the reward-enhancing effects of nicotine using the intracranial self-stimulation (ICSS) procedure in adult male and female rats. The rats were prepared with ICSS electrodes in the medial forebrain bundle and then trained on the ICSS procedure. Nicotine lowered the brain reward thresholds and decreased response latencies similarly in male and female rats. These findings suggest that nicotine enhances the rewarding effects of ICSS and has stimulant properties. Treatment with the GR antagonist mifepristone did not affect the reward-enhancing effects of nicotine but increased response latencies, suggesting a sedative effect. Mecamylamine did not affect the brain reward thresholds or response latencies of the control rats, but prevented the nicotine-induced decrease in brain reward thresholds and reward latencies. These findings indicate that the rewarding effects of nicotine are mediated via the activation of nAChRs, and that the activation of GRs does not contribute to the acute rewarding effects of nicotine. These studies enhance our understanding of the neurobiological mechanisms underlying tobacco use disorder.

Role of aldosterone and mineralocorticoid receptor (MR) in addiction: A scoping review

Preclinical and human studies suggest a role of aldosterone and mineralocorticoid receptor (MR) in addiction. This scoping review aimed to summarize (1) the relationship between alcohol and other substance use disorders (ASUDs) and dysfunctions of the aldosterone and MR, and (2) how pharmacological manipulations of MR may affect ASUD-related outcomes. Our search in four databases (MEDLINE, Embase, Web of Science, and Cochrane Library) indicated that most studies focused on the relationship between aldosterone, MR, and alcohol (n = 30), with the rest focused on opioids (n = 5), nicotine (n = 9), and other addictive substances (n = 9). Despite some inconsistencies, the overall results suggest peripheral and central dysregulations of aldosterone and MR in several species and that these dysregulations depended on the pattern of drug exposure and genetic factors. We conclude that MR antagonism may be a promising target in ASUD, yet future studies are warranted.

Nicotine, adolescence, and stress: A review of how stress can modulate the negative consequences of adolescent nicotine abuse

In order to continue the decline of smoking prevalence, it is imperative to identify factors that contribute to the development of nicotine and tobacco addiction, such as adolescent initiation of nicotine use, adolescent stress, and their interaction. This review highlights the biological differences between adolescent and adults in nicotine use and resulting effects, and examines the enduring consequences of adolescent nicotine administration. A review of both clinical and preclinical literature indicates that adolescent, but not adult, nicotine administration leads to increased susceptibility for development of long-lasting impairments in learning and affect. Finally, the role stress plays in normal adolescent development, the deleterious effects stress has on learning and memory, and the negative consequences resulting from the interaction of stress and nicotine during adolescence is reviewed. The review concludes with ways in which future policies could benefit by addressing adolescent stress as a means of reducing adolescent nicotine abuse.

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

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

Effects of simultaneous exposure to stress and nicotine on nicotine-induced locomotor activation in adolescent and adult rats

Preclinical studies have shown that repeated stress experiences can result in an increase in the locomotor response to the subsequent administration of drugs of abuse, a phenomenon that has been termed behavioral cross-sensitization. Behavioral sensitization reflects neuroadaptive processes associated with drug addiction and drug-induced psychosis. Although cross-sensitization between stress- and drug-induced locomotor activity has been clearly demonstrated in adult rats, few studies have evaluated this phenomenon in adolescent rats. In the present study, we determined if the simultaneous exposure to stress and nicotine was capable of inducing behavioral sensitization to nicotine in adolescent and adult rats. To this end, adolescent (postnatal day (P) 28-37) and adult (P60-67) rats received nicotine (0.4 mg/kg, sc) or saline (0.9% NaCl, sc) and were immediately subjected to restraint stress for 2 h once a day for 7 days. The control group for stress was undisturbed following nicotine or saline injections. Three days after the last exposure to stress and nicotine, rats were challenged with a single dose of nicotine (0.4 mg/kg, sc) or saline and nicotine-induced locomotion was then recorded for 30 min. In adolescent rats, nicotine caused behavioral sensitization only in animals that were simultaneously exposed to stress, while in adult rats nicotine promoted sensitization independently of stress exposure. These findings demonstrate that adolescent rats are more vulnerable to the effects of stress on behavioral sensitization to nicotine than adult rats.

Nicotine sensitization of monkey striatal dopamine release

This study with monkeys was designed to answer two questions. 1) Does acute nicotine preferentially release more dopamine in the striatum than in the prefrontal cortex? 2) Do repeated doses of nicotine produce sensitization of striatal dopamine release? Microdialysis techniques were used to measure extracellular dopamine in both brain regions in two separate groups of conscious animals. The acute nicotine i.v. dose schedule was a bolus of 32 microg/kg plus an infusion of +/-0.8 microg/kg/min and a 100 microg/kg bolus plus an infusion of +/-2.53 microg/kg/min for 30 min to mimic human tobacco smoking arterial plasma nicotine concentrations. Acute nicotine given i.v. released more dopamine in the striatum than in the prefrontal cortex. In the second experiment, for convenience, daily nicotine was given i.m. and not i.v. bid in doses of 32 or 100 microg/kg for nine days. Dopamine release was measured after overnight nicotine abstinence using the i.v. dose schedule from the first experiment. Baseline dopamine release was significantly reduced (77.6% of control, P<0.05). With a lowered baseline, a greater facilitation of dopamine release was produced by nicotine compared to that obtained under control conditions when the baseline was higher. The impaired dopamine release with overnight nicotine abstinence was transiently enhanced in a dose dependent manner. These data regarding the striatum are consistent with previous findings in rodents of nicotine sensitization of dopamine release especially in nucleus accumbens following repeated administration.

Tobacco/nicotine and endogenous brain opioids

Smoking is a major public health problem with devastating health consequences. Although many cigarette smokers are able to quit, equal numbers of others cannot! Standard medications to assist in smoking cessation, such as nicotine replacement therapies and bupropion, are ineffective in many remaining smokers. Recent developments in the neurobiology of nicotine dependence have identified several neurotransmitter systems that may contribute to the process of smoking maintenance and relapse. These include: especially dopamine, but also norepinephrine, 5-hydroxytryptamine, acetylcholine, endogenous opioids, gamma-aminobutyric acid (GABA), glutamate, and endocannabinoids. The present review examines the limited contribution of the endogenous opioid system to the complex effects of nicotine/tobacco smoking.

Corticotropin releasing factor-1 receptor antagonist, CP-154,526, blocks the expression of ethanol-induced behavioral sensitization in DBA/2J mice

RATIONALE

Manipulation of glucocorticoid receptor signaling has been shown to alter the acquisition and expression of ethanol-induced locomotor sensitization in mice. It is unknown if other components of the hypothalamic-pituitary-adrenal (HPA)-axis modulate locomotor sensitization resulting from repeated ethanol administration. In the present investigation, we determined if pretreatment with an i.p. injection of CP-154,526, a selective corticotropin releasing factor (CRF) type-1 receptor antagonist, would block the acquisition and/or expression of ethanol-induced locomotor sensitization in male DBA/2J mice.

METHODS

To assess the role of the CRF1 receptor in the acquisition of behavioral sensitization, mice were pretreated with an i.p. injection of CP-154,526 30 min before each of 10 sensitizing i.p. injections of ethanol. To determine the role of the CRF1 receptor in modulating the expression of ethanol-induced sensitization, mice that had previously been sensitized to the locomotor stimulant effects of ethanol were pretreated with CP-154,526 30 min before an i.p. injection of ethanol on the test day. In a third study, ethanol-naïve mice were pretreated with CP-154,526 30 min before an initial i.p. injection of ethanol to determine the combined effects of the CRF1 receptor antagonist and ethanol on locomotor activity. Blood ethanol concentrations were assessed at the termination of sensitization studies.

RESULTS

Pretreatment with CP-154,526 blocked the expression of ethanol-induced locomotor sensitization in DBA/2J mice but did not prevent the acquisition of sensitization. The ability of CP-154,526 to block the expression of ethanol-induced locomotor sensitization was not attributable to alterations in blood ethanol levels or possible sedative effects produced by the combined administration of CP-154,526 and ethanol.

CONCLUSIONS

These data provide novel evidence that CRF1 receptor signaling modulates the expression of ethanol-induced locomotor sensitization, and add to a growing literature suggesting a role for neurochemicals and hormones associated with the HPA-axis in behavioral sensitization resulting from repeated exposure to drugs of abuse.

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.

Long maternal separation accelerates behavioural sensitization to ethanol in female, but not in male mice

Early life stress is associated with dysfunction of the hypothalamic-pituitary-adrenal (HPA) axis, and with aspects involved in drug abuse. In this study, we investigated the effects of brief (BMS) and long maternal separation (LMS) on the HPA axis response and behavioural sensitization to ethanol (EtOH) in male and female mice. From PND 2 to 14, pups were subjected to daily maternal separation for 15 min (BMS) or 180 min (LMS) or no separated, only handled during cage cleaning (animal facility rearing-AFR). As adults, animals were treated every other day with saline (SAL) or EtOH (2.2g/kg), i.p., for 10 days, and immediately after each administration, their locomotor response was evaluated for 15 min. Forty-eight hours after the 5th administration, all animals were challenged with saline, followed 48 h later, by an EtOH challenge. Corticosterone (CORT) plasma levels were determined 3 times: basal, after the 1st administration and after the EtOH challenge. LMS females showed higher CORT levels than BMS females at basal, but not in response to acute or chronic EtOH administration. The CORT response to EtOH was more robust in LMS and BMS male than AFR male mice. Repeated EtOH treatment induced behavioural sensitization in all groups of male mice. In females, LMS induced a faster sensitization, although BMS females also exhibited behavioural sensitization (4th day and 5th day of treatment, respectively). In conclusion, LMS and BMS produced gender-dependent effects. In females, LMS and BMS facilitated the development of behavioural sensitization, but in the LMS group this effect occurred faster, which may represent increased vulnerability to drug abuse. Moreover, LMS females showed higher basal CORT levels compared to BMS. In males, LMS and BMS increased the CORT response to EtOH but did not modify behavioural sensitization. Therefore, we postulate that LMS female mice exhibited a faster development of behavioural sensitization, but CORT levels were not involved with this effect.

Locomotor activity to nicotine and Fos immunoreactivity in the paraventricular nucleus of the hypothalamus in adolescent socially-stressed rats

We reported previously that social stressors in adolescence (SS: one-hour isolation and new cage partners daily for 16 days) increased locomotor activity to nicotine and to amphetamine in females, but not in males, when tested as adults. Here, we investigated whether effects of stressors in adolescence on locomotor responses to nicotine would be observed in both sexes if tested closer in time to the stressor exposure. We also tested whether social instability was necessary to alter nicotine's effects on locomotor activity by including a group that underwent daily isolation but was housed with the same partner (ISO). The locomotor-activating effects of nicotine were lower in SS rats compared to ISO and non-stressed control rats. In males, but not in females, there were effects of nicotine treatment and of stress condition on Fos immunoreactive (Fos-ir) cell counts in the paraventricular nucleus (PVN) of the hypothalamus: SS males had higher Fos-ir counts than did ISO and non-stressed control males, and higher Fos-ir counts in the PVN were found in repeated-nicotine groups than in acute-nicotine and saline groups. These results add to evidence that adolescents are uniquely vulnerable to stressors due to ongoing brain development, and also indicate that effects are sex- and stressor-specific.

Smoking as a complex but critical covariate in neurobiological studies of posttraumatic stress disorders: a review

As smoking rates in the general population continue to fall in response to new information and changing social values, the continued high rate of smoking among persons with psychiatric disorders has caught the attention of society at many levels: public health officials, medical and mental health care providers, and concerned family members alike. As a consequence, research studies aimed at quantifying the problem and understanding its cause have increased dramatically over the past several years. The following review first examines epidemiological studies that have revealed a bidirectional causal relationship between tobacco dependence and posttraumatic stress disorder (PTSD), one of several mental health disorders in which tobacco dependence remains prevalent and resistant to intervention. Second, we use a translational neuroscience perspective to discuss possible neurobiological mediators of the relationship between PTSD and tobacco dependence, hoping to spur further human and animal research that will elucidate pathogenetic mechanisms involved and inspire novel treatment interventions. Finally, to enable more effective clinical research in this area, we provide an overview of effective scientific methods for assessing and managing 'smoking status' as an experimental variable in clinical research studies of PTSD as well as other mental health disorders.

Liver-derived IGF-I regulates exploratory activity in old mice

Growth hormone (GH) replacement in hypopituitary patients improves well-being and initiative. Experimental studies indicate that these psychic effects may be reflected in enhanced locomotor activity in mice. It is unknown whether these phenomena are mediated directly by GH or by circulating IGF-I. IGF-I production in the liver was inactivated at 6-10 wk of age (LI-IGF-I-/- mice), resulting in an 80-85% reduction of circulating IGF-I, and, secondary to this, increased GH secretion. Using activity boxes on three different occasions during 1 wk, 6-mo-old LI-IGF-I-/- mice had similar activity levels, and 14-mo-old mice had a moderate but significant decrease in activity level, compared with control mice. At 20 mo of age, the LI-IGF-I-/- mice displayed a more prominent decrease in activity level with decreased horizontal activity throughout the test period, and at day 1, there were several signs of an altered habituation process with different time patterns of locomotor activity and horizontal activity compared with the control mice. At days 3 and 5, rearing activity was lower in the 20-mo-old LI-IGF-I-/- mice. Anxiety level was unaffected in all age groups, as measured using the Montgomery's elevated plus-maze. In conclusion, old LI-IGF-I-/- mice displayed a decrease in both horizontal and rearing (exploratory) activity level and an altered habituation process. These results indicate that liver-derived IGF-I mediates at least part of the effects of GH on exploratory activity in mice.

Differential behavioral and neuroendocrine effects of repeated nicotine in adolescent and adult rats

Despite the high prevalence of tobacco abuse among adolescents, the neurobiology of nicotine addiction has been studied mainly in adult animals. Repeated administration of this drug to adult rats induces behavioral sensitization. Nicotine activates the HPA axis in adult rats as measured by drug-induced increases in ACTH and corticosterone. Both behavioral sensitization and corticosterone are implicated in drug addiction. We examined the expression of behavioral sensitization induced by nicotine as well as the changes in corticosterone levels after repeated injections of nicotine in adolescent and adult animals. Adolescent and adult rats received subcutaneous (s.c.) injections of saline or 0.4 mg/kg of nicotine once daily for 7 days. Three days after the last injection animals were challenged with saline or nicotine (0.4 mg/kg; s.c.). Nicotine-induced locomotion was recorded in an activity cage. Trunk blood samples were collected in a subset of adolescent and adult rats and plasma corticosterone levels were determined by radioimmunoassay. Adult, but not adolescent, rats expressed behavioral sensitization. Pretreatment with nicotine abolished corticosterone-activating effect of this drug only in adult animals, indicating the development of tolerance at this age. Our results provide evidence that adolescent rats exposed to repeated nicotine display behavioral and neuroendocrine adaptations distinct from that observed in adult animals.

Repeated maternal separation: differences in cocaine-induced behavioral sensitization in adult male and female mice

RATIONALE

Repeated maternal separations profoundly alter the adult stress response, the development of the hypothalamic-pituitary-adrenal axis, and prominently, the GABAergic and monoaminergic systems. These neural changes are postulated to influence the vulnerability to drugs of abuse implicating glucocortocoids in the behavioral responses to psychomotor stimulants.

OBJECTIVE

To investigate whether repeated brief maternal separation stress increases behavioral sensitization to cocaine in adult male and female mice, and to assess any concurrent changes in hippocampal glucocorticoid receptors and accumbal dopamine transporters.

METHODS

Half of the litters were separated from the nest for 1 h/day from post-natal days 1 to 13. Starting on post-natal day 50, all mice were injected with either cocaine (10.0 mg/kg) or saline for 10 consecutive days. Locomotor activity was assessed in an open field on days 50, 54 and 59 via a tracking system. Approximately 10 and 40 days later, all mice were challenged with 7.5 mg/kg cocaine.

RESULTS

Repeated maternal separation increased the hyperlocomotor response to 10.0 mg/kg cocaine regardless of gender. During expression tests (days 69/71, 99), male, but not female, mice with a history of maternal separation exhibited significant sensitized hyperactivity in response to cocaine. Male mice that were maternally separated and had no history of cocaine sensitization, demonstrated cross-sensitization to 7.5 mg/kg cocaine. Immunohistochemical analysis revealed that the hippocampal CA1 glucocorticoid receptor and nucleus accumbens dopamine transporter proteins were expressed more in females than in males, regardless of maternal separation experience.

CONCLUSIONS

Repeated maternal separation is a stressor that can induce heightened sensitivity to low doses of cocaine, as expressed by hyperactivity. Furthermore, sex differences in glucocorticoid receptor and dopamine transporter expression may be responsible for the sexual dimorphic expression of behavioral sensitization to cocaine.

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.

Selective psychostimulant sensitization by food restriction: differential changes in accumbens shell and core dopamine

We have recently reported that behavioural sensitization to morphine, amphetamine, cocaine and nicotine is associated with an increased response of dialysate dopamine to the same drugs in the nucleus accumbens core and/or a reduced response in the shell. Prolonged exposure to stressful stimuli also induces behavioural sensitization to drugs of abuse. We therefore investigated the effect of different drugs of abuse on behaviour and on dopamine transmission in the nucleus accumbens shell and core of rats stressed by 1 week schedule of food restriction. Food-restricted rats (80% of their initial body weight) were implanted with microdialysis probes in the nucleus accumbens shell and core and challenged with cocaine (5 and 10 mg/kg i.p.), amphetamine (0.25 and 0.5 mg/kg s.c.), morphine (1 and 2 mg/kg s.c.), nicotine (0.2 and 0.4 mg/kg s.c.) and the changes in dialysate dopamine transmission were monitored together with the behaviour. Food restricted rats showed strong behavioural sensitization to cocaine and amphetamine but not to morphine or nicotine as compared to ad libitum fed controls. Behavioural sensitization to psychostimulants was associated with an increased response of dialysate dopamine in the core and with an unchanged or even reduced response in the shell. No significant differences were observed between controls and food-restricted animals in the ability of morphine and nicotine to stimulate dopamine transmission in the shell and core. The present results indicate that a sensitized dopamine response in the nucleus accumbens core is a general feature of the expression of behavioural sensitization.

The role of cortisol suppression on craving for and satisfaction from nicotine in high and low impulsive subjects

The rewarding properties of drug self-administration have been shown to depend on mesolimbic dopamine release but also on the availability of corticosterone, as shown in rats. Since this became particularly evident when tested in rat strains bred for high (HR) and low (LR) reactivity to novel stimuli, the role of cortisol and personality in drug craving was investigated in 60 male nicotine-deprived smokers divided according to questionnaire scores into high and low impulsives, a dimension considered to represent HR/LR rats. They either received the peripheral cortisol blocker metyrapone (MET) or the centrally blocking substance dexamethasone (DEX) or placebo (n=20 each) and were then allowed to smoke. MET more than DEX reduced craving for cigarettes during deprivation. This was only observed in low impulsives while high impulsives developed low craving unmodified by blockade of cortisol. Satisfaction from smoking was less achieved with DEX than with MET and not modified by impulsivity. Differences between MET and DEX in influencing craving during deprivation and satisfaction from smoking were discussed on the basis of different glucocorticoid receptors. The mineralocorticoid receptor (MR) seems to be the 'turn on switch' and the glucocorticoid receptor (GR) the 'turn off switch' in mediating the rewarding properties of dopamine in drug self-administration.

Nicotine-induced behavioral disinhibition and ethanol preference correlate after repeated nicotine treatment

This study investigated the effects of repeated daily nicotine (0.35 mg/kg; 15 days) treatment on behavioral inhibition and locomotor activity in the elevated plus-maze and on voluntary ethanol consumption. When challenged with nicotine before the test, rats pretreated with repeated nicotine spent more time on and made more entries onto the open arms of an elevated plus-maze than did vehicle-pretreated animals. The ethanol preference and intake, measured during 3 h after a nicotine injection, was also higher in the nicotine-pretreated animals. In ethanol consumption experiments, there was a positive correlation between the % time and % entries made onto open arms vs. the ethanol preference and intake. However, no correlation between the total number of entries made in the elevated plus-maze and the measures of ethanol consumption was observed. These findings suggest that the ability of repeated nicotine administration to increase ethanol consumption is related to development of a nicotine-induced reduction of inhibitory control rather than development of locomotor sensitization.

Nicotine induced behavioral locomotor sensitization

1. Nicotine behavioral sensitization of locomotor activity was investigated in adult female Sprague Dawley rats. Five different experiments were performed with nicotine in various doses of 0.1, 0.32, or 1.0 mg/kg i.p. These included: 1) effects of daily nicotine for 6 days, 2) effects of once per week nicotine for 3 weeks, 3) effects of MK-801 on nicotine-induced locomotor activity, 4) effects of dexamethasone on nicotine-induced locomotor activity, 5) induction of tolerance to nicotine-induced locomotor sensitization and lack of cross tolerance to caffeine. 2. Locomotor activity was measured with a photoelectric computerized system. The first dose of nicotine (0.32 mg/kg) induced marked locomotor depression. Once daily injection of 0.32 mg/kg of nicotine for 6 days produced tolerance to its depressant effects and sensitized the rats to its stimulant effects. Three once weekly doses of 0.32 mg/kg of nicotine also produced tolerance to its depressant effects and some locomotor stimulation. 3. Daily pretreatment for 5 days with a dose of 0.18 mg/kg of MK-801 i.p. partially antagonized the locomotor depressant and stimulant actions of nicotine. 4. Dexamethasone (1 mg/kg i.p.) daily pretreatment barely reduced nicotine locomotor depression and only very slightly enhanced locomotor stimulation. 5. Accumulating doses of 0.32 and 1.0 mg/kg b.i.d. of nicotine produced tolerance to its locomotor stimulant effects in rats previously sensitized to 0.32 mg/kg. There was no cross-tolerance to 32 mg/kg of caffeine citrate in previously sensitized animals tolerant to the stimulant effects of nicotine.

Genetic dissection of nicotine-related behaviour: a review of animal studies

Nicotine has a broad spectrum of behavioural effects. A considerable body of data has emerged indicating genetic factors regulate the behavioural effects of nicotine. Experimental genetic techniques have been invaluable in generating knowledge on the interrelationship of genetic factors and behavioural responsiveness to nicotine. Three different approaches have been invoked to explore the relationship of genetic factors to response to nicotine. Firstly, the classical genetic tool of inbred lines has been exploited to delineate genetic influences in the effects of nicotine. Secondly, the use of selectively bred lines has been profitably employed to reveal genetic differences in behavioural responses, such as cognition and exploration, to nicotine. These approaches have also provided useful information on the contribution of genetic factors influencing nicotinic receptors function. Finally the molecular genetic technique of gene targetting to create mice with null mutations of specific genes in the central nervous system, which is having a tremendous impact in drug addiction research, has also been employed to gain insight into the molecular and cellular basis of nicotine action. These techniques are proving to be invaluable in dissecting the role of different subunits of the nicotinic acetylcholine receptors on behaviour. This paper provides a survey of the animal studies that have used the above mentioned techniques to gain insight into genetic basis of the behavioural effects of nicotine.

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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Nicotine induces disinhibitory behavior in the rat after subchronic peripheral nicotinic acetylcholine receptor blockade

The present study investigated the effects of subchronic nicotine, mecamylamine and hexamethonium, alone or in combinations, on locomotor activity and behavioral inhibition. Rats were divided into groups and tested for locomotor activity after acute nicotine. The different groups received vehicle, nicotine, mecamylamine, mecamylamine+nicotine, hexamethonium (two different concentrations) and hexamethonium+nicotine injections once a day for 15 days after which they were tested for nicotine-induced locomotor activity again. Acutely, nicotine stimulated locomotor activity, and repeated daily nicotine or hexamethonium+nicotine administration sensitized the animals to this nicotine-induced locomotor stimulation (locomotor sensitization). Mecamylamine administered subchronically in combination with nicotine was able to block the induction to locomotor sensitization to nicotine. None of the nicotinic receptor antagonists induced locomotor sensitization to nicotine by themselves. In the elevated plus-maze, subchronic nicotine treatment demonstrated a nicotine-induced behavioral disinhibition, measured as an increase of time spent in and entries made into open arms. In contrast to the findings regarding locomotor sensitization, none of the antagonists counteracted the induction of this nicotine-induced behavioral disinhibition after subchronic co-treatment with nicotine. In addition, both antagonists by themselves produced a similar effect as subchronic nicotine, i.e. promoted the development of nicotine-induced disinhibitory behavior. It was concluded that the induction of locomotor sensitization to nicotine involves stimulation of central nicotinic acetylcholine receptors, whereas the development of nicotine-induced behavioral disinhibition involves blockade of peripheral nicotinic acetylcholine receptors, and that the latter, but not the former, phenomenon from a pharmacological point of view appears to be related to the increased ethanol consummatory behavior observed after subchronic nicotine administration.

A single exposure to amphetamine is sufficient to induce long-term behavioral, neuroendocrine, and neurochemical sensitization in rats

Repeated treatment with psychostimulant drugs causes long-lasting behavioral sensitization and associated neuroadaptations. Although sensitization induced by a single psychostimulant exposure has also been reported, information on the behavioral and neurochemical consequences of a single psychostimulant exposure is sparse. Therefore, to evaluate whether behavioral sensitization evoked by single and repeated psychostimulant pretreatment regimens represent the same neurobiological phenomenon, the time-dependent expression of behavioral, neurochemical, and neuroendocrine sensitization after a single exposure to amphetamine was investigated in rats. A single exposure to amphetamine (5 mg/kg, i.p.) caused context-independent sensitization of the locomotor effects of amphetamine, which intensified over time. Thus, sensitization to amphetamine was marginal at 3 d after treatment and more evident after 1 week, whereas 3 weeks after treatment, profound sensitization, as well as cross-sensitization, to cocaine was observed. Amphetamine pretreatment caused an increase in the electrically evoked release of [(3)H]dopamine from nucleus accumbens, caudate putamen, and medial prefrontal cortex slices and of [(14)C]acetylcholine from accumbens and caudate slices. The hyperreactivity of dopaminergic nerve terminals appeared to parallel the development of locomotor sensitization, i.e., whereas hyperreactivity of accumbens dopaminergic terminals increased between 3 d and 3 weeks after treatment, the hyperreactivity of medial prefrontal dopaminergic terminals decreased. Pre-exposure to amphetamine also sensitized the hypothalamus-pituitary-adrenal axis response to amphetamine at 1 and 3 weeks, but not at 3 d after treatment. Because these data closely resemble those reported previously for repeated amphetamine pretreatment, it is concluded that a single exposure to amphetamine is sufficient to induce long-term behavioral, neurochemical, and neuroendocrine sensitization in rats.

Stressor- or drug-induced sensitization of the corticosterone response is not critically involved in the long-term expression of behavioural sensitization to amphetamine

Repeated exposure to drugs of abuse induces long-lasting behavioural sensitization, which is thought to play a role in the persistence of drug-seeking behaviour. Recently, we showed that repeated exposure of rats to cocaine resulted in a long-lasting (weeks) sensitization of the hypothalamus-pituitary-adrenal axis, i.e. hypersecretion of adrenocorticotropic hormone and of the glucocorticoid corticosterone. Moreover, we found that the administration of a glucocorticoid receptor antagonist abolished the expression of psychostimulant-induced behavioural sensitization. In the present study we tested whether stressor- or drug-induced long-term hypersecretion of corticosterone is associated with the long-term expression of behavioural sensitization to psychostimulant drugs. To that end, groups of male Wistar rats were exposed once to interleukin-1beta or to footshocks, treatments that are known to induce long-term sensitization of the hypothalamus-pituitary-adrenal axis, or were treated with amphetamine or morphine, according to protocols known to induce long-lasting behavioural (locomotor) sensitization. Three weeks later, the groups and their controls were challenged with amphetamine or vehicle. Previous exposure to interleukin-1beta or footshocks enhanced adrenocorticotropic hormone and corticosterone responses, but did not affect the long-term locomotor sensitization to amphetamine. Prior amphetamine treatment enhanced the locomotor response and the adrenocorticotropic hormone and corticosterone responses to amphetamine. Prior morphine treatment resulted in long-term locomotor sensitization, whereas the adrenocorticotropic hormone and corticosterone responses to amphetamine were decreased. From these findings and the absence of within-group correlation between corticosterone and locomotor responses in interleukin-1beta and morphine-pretreated rats, we conclude that there is no correlation between sensitization of the corticosterone response and behavioural sensitization to amphetamine. Apparently, sensitization of the corticosterone response is not a prerequisite for the long-term expression of behavioural sensitization, which suggests that drug-induced long-term behavioural sensitization may involve corticosteroid receptor-dependent (central) mechanisms that occur independent of hypothalamus-pituitary-adrenal axis responsiveness.

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.

Effects of corticosterone on excitatory amino acid responses in dopamine-sensitive neurons in the ventral tegmental area

The ventral tegmental area is involved in reward processes and in drug dependence and sends dopaminergic projections to the nucleus accumbens and prefrontal cortex. Stress, and glucocorticoid hormones, are thought to play an important role in the development of drug dependence, but there has been little investigation of the effects of these hormones on ventral tegmental function. The present study examined the effects of corticosterone on single-unit recordings from dopamine-sensitive neurons in the ventral tegmental area in midbrain slices. At concentrations of 100 nM and above, corticosterone potentiated the responses to N-methyl-D-aspartate. This effect was not seen when the calcium concentration of the bathing medium was reduced to 0.1 mM. Responses to alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) and kainic acid were also considerably potentiated, at concentrations of corticosterone over 100 nM, while there was some evidence of decreases in these responses at the 100 nM concentration of this hormone. Aldosterone, at concentrations of 100 nM and above reduced the responses to N-methyl-D-aspartate, but had no effects at lower concentrations. RU38486, which acts as an antagonist at glucocorticoid (Type II) receptors, prevented the effects of corticosterone on responses to N-methyl-D-aspartate, with no effect on the spontaneous firing rate or on the effects of N-methyl-D-aspartate in the absence of corticosterone. The latter result, and the effects of aldosterone, suggest that the potentiation of responses to N-methyl-D-aspartate was mediated through Type II glucocorticoid receptors. This study suggests that potentiation of responses to excitatory amino acids by corticosterone may alter the function of ventral tegmental neurons during stress, and it is possible that this effect is involved in the development of drug dependence.

Sympatho-adrenal involvement in methamphetamine-induced hyperthermia through skeletal muscle hypermetabolism

We investigated the involvement of the sympatho-adrenal axis in the hyperthermia induced by methamphetamine by using a biotelemetric system. The intraperitoneal injection of methamphetamine (1 mg/kg) induced hyperthermia preceded by an increase in oxygen consumption in freely moving rats. The hyperthermic effect of methamphetamine was completely blocked by chemical sympathectomy with 6-hydroxydopamine (50 mg/kg, i.p.). Adrenalectomy, but not adrenal demedullation, prevented the hyperthermia. In adrenalectomized rats, dexamethasone supplementation (0.5 mg/kg, s.c.) restored the methamphetamine-induced hyperthermia. Furthermore, dantrolene (1 or 2 mg/kg, i.v.), which blocks Ca2+ release from the sarcoplasmic reticulum in skeletal muscle, attenuated the hyperthermia. These results suggest that methamphetamine stimulates norepinephrine release from sympathetic nerve terminals, which then enhances thermogenesis in skeletal muscle under the permissive action of glucocorticoids.

The role of corticosteroids in nicotine's physiological and behavioral effects

This paper reviews evidence indicating that adrenal corticosteroids modulate the responsiveness of mice and rats to nicotine. Adrenalectomy increases, and both acute and chronic corticosteroid administration decrease, some of the physiological and behavioral effects of nicotine. One function of adrenal steroids may be to regulate stress-induced changes in nicotine sensitivity. Another is to mediate the development of chronic tolerance when nicotine is given intermittently, and when the resulting tolerance has a learned component. A role of glucocorticoids in the development of tolerance to nicotine is suggested by the findings that a conditioned elevation of plasma corticosterone, which anticipates nicotine delivery, accompanies the development of chronic tolerance and that environmental cues evoke a conditioned corticosterone response, but only after they have become associated with nicotine delivery. The mechanisms by which adrenal steroids modulate nicotine sensitivity are not known, although recent in vitro evidence suggests that steroids can rapidly and reversibly reduce nicotinic receptor function. While most of the data are consistent with the hypothesis that corticosteroids reduce nicotine responsiveness, and thus promote a learned form of tolerance, there are new findings that corticosteroids increase the development of sensitization to the locomotor-activating effects of nicotine. These data suggest that formulations postulating a unidirectional effect of corticosteroids on nicotine's actions (e.g. decreased sensitivity) must be revised to take into account interacting variables such as the specific nicotine effect being studied and whether that effect normally exhibits tolerance or sensitization. Finally, research is presented which indicates that the corticosterone-elevating effects of nicotine, previously reported for experimenter-administered drug, are also produced when nicotine administration is contingent on an operant response, and at a dose which sustains the development of nicotine self-administration in rats. These findings highlight the feasibility of using self-administration models in future explorations of the relationship between adrenal steroids and nicotine function.

Lack of effect of nicotine or ethanol on the activity of 11beta-hydroxysteroid dehydrogenase type 2

Low birth weight in combination with a large placenta predicts human hypertension. The pathophysiological link remains unclear, but glucocorticoid excess impairs fetal growth and leads to offspring hypertension. A key controller of fetal glucocorticoid exposure and local tissue availability is 11beta-hydroxysteroid dehydrogenase type 2 (11beta-HSD2). The activity of placental 11beta-HSD2 correlates with fetal growth in animals and humans. Ethanol abuse and smoking are known to retard fetal growth which may relate to altered glucocorticoid action or dynamics. This study has examined whether nicotine or ethanol modulate glucocorticoid action in the placenta or fetus by inhibiting 11beta-HSD2, using clonal cell cultures, freshly isolated dually perfused intact human placentas and placentas from in vivo treated rats. No significant effect on the activity of 11beta-HSD2 by pathophysiologically relevant nicotine or ethanol concentrations was observed. The mechanism of action of nicotine and ethanol relevant to reduced fetal growth requires further study.

Behavioral sensitization to ethanol: genetics and the effects of stress

Some aspects of drug abuse syndromes may be influenced by sensitization to some drug effects. This enhancement of drug effect has been associated with prior drug exposure and with exposure to stressful stimuli. It has been postulated that sensitization to psychomotor stimulant drug effects influences sensitivity to drug reward. The drugs of abuse best characterized for sensitization phenomena include cocaine, amphetamine, and morphine. In general, ethanol's molecular mechanisms of action have been difficult to define relative to drugs with known receptor or transporter binding sites and, likewise, ethanol sensitization has been less thoroughly examined. Evidence supporting the existence of behavioral sensitization to ethanol, for genetic differences in the occurrence of ethanol sensitization, and for the influence of corticosterone on the development of ethanol sensitization is reviewed herein. There appear to be different genetic determinants of acute drug sensitivity and sensitization. Cross-sensitization between stress and ethanol suggest a potential role for hypothalamic-pituitary-adrenal (HPA) axis associated changes in ethanol sensitization, consistent with mechanisms likely contributing to sensitization to other abused drugs. Furthermore, glucocorticoid receptors appear to mediate both ethanol- and stress-induced sensitization to ethanol. A biological link between drug reward and drug sensitization involving HPA axis hormones may exist and, thus, study of the sensitization process may elucidate mechanisms relevant to drug abuse.

Voluntary ethanol intake in the rat: effects of nicotinic acetylcholine receptor blockade or subchronic nicotine treatment

It has been suggested that the mesolimbic dopamine activating and the reinforcing properties of ethanol involve activation of central nicotinic acetylcholine receptors. To test this hypothesis, the effects of two nicotinic receptor antagonists and of subchronic nicotine treatment on voluntary ethanol consumption (ethanol 6% v/v or water) were studied in ethanol low-, medium- or high-preferring Wistar rats. After systemic mecamylamine (2 mg/kg) but not hexamethonium (0 mg/kg) high- but not low-preferring rats decreased their ethanol intake but, however, not their ethanol preference. When subchronically exposed to nicotine (0.35 mg/kg, s.c. daily) medium-preferring rats markedly increased their ethanol intake and preference. This effect lasted for more than 1 week after interrupting nicotine administration. Ethanol intake levels did not correlate with locomotor activity scores after nicotine challenge (0.35 mg/kg, s.c.) or with exploratory locomotor activity. However, exploratory locomotor activity correlated with locomotor activity scores both after nicotine (0.35 mg/kg, s.c.) and ethanol (0.125 g/kg i.p.) challenge. Dopamine release, as indicated by accumulation of 3-methoxytyramine after monoamine oxidase inhibition, was increased in the limbic forebrain (including the nucleus accumbens, the olfactory tubercles, the amygdala and the septum) after acute nicotine (0.35 mg/kg s.c.) or ethanol (2.5 g/kg i.p.) in animals subchronically exposed to nicotine compared to subchronically vehicle-treated controls. The present results further implicate central nicotinic receptors in the molecular events mediating the reinforcing properties of ethanol, and suggest that subchronic nicotine enhances the responsiveness of mesolimbic dopamine neurons both to nicotine and to ethanol. Clinical implications are discussed.