Adrenocortical hormone regulation of nicotine sensitivity in mice

The impact of adolescent nicotine exposure on alcohol use during adulthood: The role of neuropeptides

Nicotine and alcohol abuse and co-dependence represent major public health crises. Indeed, previous research has shown that the prevalence of alcoholism is higher in smokers than in non-smokers. Adolescence is a susceptible period of life for the initiation of nicotine and alcohol use and the development of nicotine-alcohol codependence. However, there is a limited number of pharmacotherapeutic agents to treat addiction to nicotine or alcohol alone. Notably, there is no effective medication to treat this comorbid disorder. This chapter aims to review the early nicotine use and its impact on subsequent alcohol abuse during adolescence and adulthood as well as the role of neuropeptides in this comorbid disorder. The preclinical and clinical findings discussed in this chapter will advance our understanding of this comorbid disorder's neurobiology and lay a foundation for developing novel pharmacotherapies to treat nicotine and alcohol codependence.

FKBP5 variation is associated with the acute and chronic effects of nicotine

Stress and hormones released in response to stress influence the effects of nicotine and the severity of nicotine withdrawal. Here, we systematically examine the contribution of a stress response gene, FKBP5, to the acute and chronic behavioral effects of nicotine in smokers. Subjects were European- and African-American (EA and AA) heavy smokers who participated in an intravenous (IV) nicotine administration study (total n=169). FKBP5 rs3800373 genotype was analyzed for association to several outcomes, including nicotine withdrawal and the acute subjective, heart rate (HR), blood pressure and plasma cortisol responses to IV nicotine. Nicotine withdrawal was also examined in relation to rs3800373 allele frequencies in an independent cohort of EA and AA current smokers (n=3821). For a subset of laboratory subjects FKBP5 mRNA (n=48) expression was explored for an association to the same outcomes. The rs3800373 minor allele was associated with less severe nicotine withdrawal in laboratory subjects and the independent cohort of smokers. The rs3800373 minor allele was also associated with lower subjective ratings of negative drug effects in response to IV nicotine. Low FKBP5 mRNA expression was associated lower cortisol levels, lower subjective ratings of negative drug effects and a blunted HR response to nicotine. Stress hormone regulation via FKBP5 warrants further investigation as a potential contributor to the effects of nicotine withdrawal, which occurs commonly, and has an important role in the maintenance of smoking behavior and relapse following a quit attempt.

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.

Genetic matters: thirty years of progress using mouse models in nicotinic research

This research update summarizes thirty years of studies on genetic influences on responses to the acute or chronic administration of nicotine. Early studies established that various inbred mice are differentially sensitive to the effects of the drug. Classical genetic analyses confirmed that nicotine effects on locomotion, body temperature and seizures are heritable. A significant inverse correlation between the locomotor and hypothermic effects and the density of nicotine binding sites suggested that differential expression α4β2-neuronal nicotinic acetylcholine receptor (nAChR) mediated some of this genetic variability. Subsequent studies with α4 and β2 nAChR null (decreased sensitivity) and gain of function mutants (increased sensitivity) supports the role of the α4β2*nAChR subtype. However, null mutant mice still respond to nicotine, indicating that other nAChR subtypes also mediate these responses. Mice differing in initial sensitivity to nicotine also differ in tolerance development following chronic treatment: those mice that are initially more sensitive to nicotine develop tolerance at lower treatment doses than less sensitive mice, indicating that tolerance is an adaptive response to the effects of nicotine. In contrast, the sensitivity of mice to pre-pulse inhibition of acoustic startle response is correlated with the expression of α7-nAChR. While genetic variability in nAChR expression and function is an important factor contributing to differences in response to nicotine, the observations that altered activity of opioid, glutamate, and cannabinoid receptors among others also change nicotine sensitivity reinforces the proposal that the genetics of nicotine response is more complex than differences in nAChRs.

Acute restraint stress prevents nicotine-induced mesolimbic dopaminergic activation via a corticosterone-mediated mechanism: a microdialysis study in the rat

BACKGROUND

Stress affects the responsiveness to nicotine (NIC), by increasing drug use, facilitating relapse and reinstating NIC self administration even after prolonged abstinence. In turn, high corticosterone (CORT) blood levels induced by stress may alter the neurobiological properties of NIC by acting on the dopamine (DA) mesolimbic system.

METHODS

In this study, we evaluated the effect of exposure to acute restraint stress on NIC-induced stimulation of the mesolimbic DA system of the rat, by studying extracellular DA levels in the nucleus accumbens shell (NAccs) with microdialysis.

RESULTS

NIC intravenous administration (130 μg/kg) increased DA levels in the NAccs in control rats but not in subjects exposed to stress; this latter phenomenon was prevented by blockade of CORT effects with the inhibitor of corticosterone synthesis metirapone (100 mg/kg) or the glucorticoid receptor antagonist mifepristone (150 μmol/kg).

CONCLUSIONS

These observations show that exposure to acute stress inhibits the stimulatory response of the mesolimbic DA system to NIC and suggest that this effect is mediated by circulating CORT acting on its receptors. These results may bear relevance in explaining the role played by stressful stimuli in NIC-seeking and taking behavior.

Tobacco addiction and the dysregulation of brain stress systems

Tobacco is a highly addictive drug and is one of the most widely abused drugs in the world. The first part of this review explores the role of stressors and stress-associated psychiatric disorders in the initiation of smoking, the maintenance of smoking, and relapse after a period of abstinence. The reviewed studies indicate that stressors facilitate the initiation of smoking, decrease the motivation to quit, and increase the risk for relapse. Furthermore, people with depression or an anxiety disorder are more likely to smoke than people without these disorders. The second part of this review describes animal studies that investigated the role of brain stress systems in nicotine addiction. These studies indicate that corticotropin-releasing factor, Neuropeptide Y, the hypocretins, and norepinephrine play a pivotal role in nicotine addiction. In conclusion, the reviewed studies indicate that smoking briefly decreases subjective stress levels but also leads to a further dysregulation of brain stress systems. Drugs that decrease the activity of brain stress systems may diminish nicotine withdrawal and improve smoking cessation rates.

Locomotor and stress responses to nicotine differ in adolescent and adult rats

Since adolescence is a critical period for the initiation of tobacco use, we have systematically compared behavioral and endocrine responses to nicotine in Sprague-Dawley rats of both sexes at early adolescence (postnatal day (P) 28), mid- adolescence (P38) and adulthood (P90). Locomotion and center time in a novel open field were evaluated for 30min following intravenous injection of saline or nicotine (60microg/kg), followed by measurement of plasma corticosterone. Complex age and sex differences in behavioral and endocrine response were observed, which were dependent on the functional endpoint examined. Whereas there were age differences in nicotine effects on all functional measures, sex differences were largely restricted to adult stress-related corticosterone and center-time responses. Although significant drug effects were detected at P28 and P90, there was no effect of nicotine at P38 on any measure examined. In saline-treated males, but not females, there were significant positive correlations across age between initial ambulatory counts and both initial vertical counts and total center time. Nicotine treatment increased correlations in both sexes, and yielded a significant negative interaction between initial ambulatory counts and plasma corticosterone. The unique responses of adolescents to nicotine are consistent with an immature function of nicotinic acetylcholine receptors at this age.

Effects of acute psychosocial stress on cigarette craving and smoking

INTRODUCTION

Stress is thought to influence use of drugs, including cigarette smoking, but the mechanisms by which it does so are not clear. In this study, we investigated the effects of acute psychosocial stress on cigarette craving, the subjective effects of smoking, and smoking behavior in daily smokers.

METHODS

Healthy male and female smokers participated in two experimental sessions in which they were exposed to the Trier Social Stress Test or a nonstressful control task. For 2 hr after each task, they had repeated opportunities to either smoke or earn money. Physiological (heart rate, cortisol, and alpha-amylase) and subjective (anxiety and desire to smoke) measures were obtained before and after the tasks and after each smoking opportunity.

RESULTS

Stress significantly increased cigarette craving but it did not increase smoking. When individual differences in nicotine dependence were taken into account, stress influenced CO boost and pleasure from smoking the first cigarette.

DISCUSSION

Our results support previous evidence that acute psychosocial stress increases smoking desire.

Purine receptor antagonist modulates serology and affective behaviors in lupus-prone mice: evidence of autoimmune-induced pain?

Neurologic and psychiatric (NP) manifestations are severe complications of systemic lupus erythematosus (SLE). As commonly seen in patients, spontaneous disease onset in the MRL/MpJ-Fas(lpr)/J (MRL-lpr) mouse model of NP-SLE is accompanied by increased autoantibodies, pro-inflammatory cytokines and behavioral dysfunction which precede neuroinflammation and structural brain lesions. The role of purinergic receptors in the regulation of immunity and behavior remains largely unexplored in the field of neuropsychiatry. To examine the possibility that purinoception is involved in the development of affective behaviors, the P2X purinoceptor antagonist, suramin, was administered to lupus-prone mice from 5 to 14 weeks of age. In addition to food and water measures, novel object and sucrose preference tests were performed to assess neophobic anxiety- and anhedonic-like behaviors. Enzyme-linked immunosorbant assays for anti-nuclear antibodies (ANA) and pro-inflammatory cytokines were employed in immunopathological analyses. Changes in dendritic morphology in the hippocampal CA1 region were examined by a Golgi impregnation method. Suramin significantly lowered serum ANA and prevented behavioral deficits, but did not prevent neuronal atrophy in MRL-lpr animals. In a new batch of asymptomatic mice, systemic administration of corticosterone was found to induce aberrations in CA1 dendrites, comparable to the "stress" of chronic disease. The precise mechanism(s) through which purine receptor inhibition exerted beneficial effects is not known. The present data supports the hypothesis that activation of the peripheral immune system induces nociceptive-related behavioral symptomatology which is attenuated by the analgesic effects of suramin. Hypercortisolemia may also initiate neuronal damage, and metabolic perturbations may underlie neuro-immuno-endocrine imbalances in MRL-lpr mice.

Regulation of endocrine function by the nicotinic cholinergic receptor

One important neuroendocrine action of nicotine in the male rat is an increase in the secretion of corticosterone which is seen upon acute and acute intermittent exposure to nicotine. Tolerance develops to this action of nicotine upon chronic exposure, and in the withdrawal phase serum corticosterone levels are substantially reduced. In contrast, no significant increases of serum corticosterone levels were observed upon acute intermittent treatment with nicotine in the dioestrous rat. Available evidence indicates that corticosterone can modulate dopamine transmission in the basal ganglia via glucocorticoid receptors within the nucleus accumbens and neostriatum, and via glucocorticoid receptor immunoreactivity in nigrostriatal and mesolimbic dopamine pathways. Through concerted pre- and postsynaptic actions glucocorticoids may decrease dopamine transmission, especially that mediated by D2 receptors in these regions. In view of the hypothesis that the mesolimbic dopamine pathways mediate the euphoric effects of nicotine, the secretion of corticosterone induced by nicotine in the smoking male may substantially influence the mood elevating activity of nicotine. Thus, individual smoking habits may depend on the ability of nicotine to induce corticosterone secretion, which obviously would also vary with the degree of stress. The glucocorticoids may in a similar way influence the arousal action of nicotine because of the high number of glucocorticoid receptors present both in noradrenaline cell bodies of the locus ceruleus and within the entire cerebral cortex.

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.

Modulation of nicotine receptors by chronic exposure to nicotinic agonists and antagonists

Although numerous studies have demonstrated that chronic nicotine treatment often results in tolerance to this drug, the mechanisms that underlie this tolerance are not well defined. Recent evidence suggests that chronic nicotine treatment results in an up-regulation of brain nicotinic receptors, but the majority of these receptors may be desensitized or inactivated, thereby explaining tolerance. There is evidence that while all mouse strains show increased receptor numbers following chronic nicotine treatment, some mouse strains develop maximal changes in [3H] nicotine binding before any tolerance is detected. Other strains show a high correlation between increase in receptor number and tolerance. Studies with several other nicotinic agonists indicate that up-regulation of nicotine receptors can occur without changes in drug sensitivity. Similarly, chronic antagonists treatment can also elicit changes in receptors without affecting sensitivity to nicotine. Some of these discrepancies may be due to genetically influenced interactions between the adrenal steroid, corticosterone (CCS), and the nicotinic receptors. The addition of CCS in vitro inhibits binding to nicotinic receptors, and chronic CCS treatment results in decreases in the number of brain nicotinic receptors measured by [125I] bungarotoxin binding. Either of these biochemical measures may explain why altering CCS concentrations in vivo results in altered sensitivity to nicotine. It may be that both changes in the number of receptors and altered steroid interactions with the nicotinic receptors explain tolerance to nicotine.

Age, sex and early environment contribute to individual differences in nicotine/acetaldehyde-induced behavioral and endocrine responses in rats

Neonatal handling was used to evaluate the influence of early environment on responses to nicotine. Rats exposed as pups to daily short-term separation from the dam (H) were compared to non-handled (NH) controls. In experiment 1, prepubescent males and females, aged postnatal day (P) 30, were tested for the effect of nicotine/acetaldehyde (NicAc) on open field behavior and plasma corticosterone levels. NicAc induced increases in ambulatory activity and time spent in the center of the field in NH, but not H, males. Drug-induced increases in initial ambulatory activity, but not center time, were also seen in NH and H females. Handling, but not sex, contributed to group differences in plasma corticosterone levels. In experiment 2, NH and H rats were tested for acquisition of NicAc self-administration at three ages, P27-31, P34-38 and P90-94. Age and sex, but not handling, contributed to differences in performance of this task. Whereas males exhibited a decrease in responding with age, females did not. These findings demonstrate that neonatal handling may serve as an experimental model for individual differences in sensitivity to tobacco constituents. Furthermore, the current study indicates that stress reactivity, age and sex may play differential roles in initiating smoking behavior.

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.

Effect of nicotine and nicotinic receptors on anxiety and depression

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

Chronic corticosterone treatment alters sensory gating in C3H mice

Two methods of evaluating inhibitory sensory processing are prepulse inhibition of acoustic startle (PPI) and gating of auditory evoked potentials. Studies using both methods suggest nicotinic acetylcholinergic receptor modulation of gating, specifically the alpha-bungarotoxin (alpha-BTX) binding site (alpha7 receptor subtype). However, recent assessment of alpha7 null mutant mice failed to demonstrate any effect of the loss of this receptor in either gating paradigm. An alternate approach to assessing the effects of the alpha7 receptor is to reduce its numbers in mature inbred mice, thus, avoiding the twin problems of background and developmental compensation inherent in null mutant mouse studies. Numerous studies have shown that chronic corticosterone (CCS) treatment selectively reduces alpha-BTX binding sites. C3H mice were adrenalectomized and implanted with corticosterone or cholesterol (control) pellets. After 8 days, they were tested in one of the gating paradigms. PPI and auditory gating were significantly diminished in corticosterone-treated mice concomitant with a reduction in alpha-BTX binding in several brain regions. Cholesterol-treated mice had no change in either paradigm. Nicotine treatment (1 mg/kg) produced significant improvement in both paradigms in corticosterone-treated mice. These data agree with previous pharmacological studies suggesting modulation of gating occurs through a nicotinic receptor.

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.

Differential effects of response-contingent and response-independent nicotine in rats

Passive administration of nicotine activates the hypothalamic-pituitary-adrenocortical axis and sympathetic nervous system. However, little is known about the effects of self-administered nicotine. Drug-naive rats were trained to respond for food reinforcement and then tested in one, 1-h session in which they received response-contingent i.v. nicotine or response-independent i.v. nicotine or saline. Blood draws were taken immediately prior to the session, 15 min after the first infusion and immediately after the session. Both response-contingent and response-independent nicotine (RI/N) increased corticosterone within 15 min, however, corticosterone levels returned to baseline in animals receiving response-contingent nicotine (RC/N) by the end of the session while remaining elevated in those receiving RI/N. Furthermore, only RI/N increased plasma epinephrine and norepinephrine levels; RC/N produced no effect. These differences indicate that nicotine's acute effects are powerfully modified by the presence of a contingency relationship between drug administration and the animal's behavior and that this relationship develops very rapidly.

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.

Effects of nicotine on the hypothalamic-pituitary-axis (HPA) and immune function: introduction to the Sixth Nicotine Round Table Satellite, American Society of Addiction Medicine Nicotine Dependence Meeting, November 15, 1997

This meeting was the sixth consecutive Nicotine Round Table Satellite Meeting which was held in Washington, DC, on 15 November, 1996; previous meetings are presented in Table I. The overall objective of these meetings was to bring together scientists and clinicians as a means of developing a dialogue concerning the basic mechanisms of nicotine action and the effects of nicotine on the whole organism. The specific topic of this meeting was chosen because of the potent effects of nicotine on the hypothalamic-pituitary-axis (HPA), and newer concepts indicating that the immune system and the HPA are connected via a variety of neuroendocrine and neurotransmitter elements. Whereas the HPA appears to play a unique role in adapting to internal and external stressors, the immune system appears to act as a forward scout which provides information important to the HPA. This Satellite Meeting evaluated the effects of nicotine from three points of view: (1) the effects of nicotine on HPA function; (2) the effects of the HPA on the pharmacological effects of nicotine; and (3) the effects of nicotine on immune function. This specific presentation will provide an overview of the findings of the meeting and will discuss several of the overriding issues in this area of nicotine research.

Chronic corticosterone treatment elicits dose-dependent changes in mouse brain alpha-bungarotoxin binding

Previous studies have shown that adrenalectomy results in a small increase in hippocampal alpha-bungarotoxin binding, whereas seven days of chronic treatment with high doses of corticosterone results in decreases in alpha-bungarotoxin binding in several brain regions. The studies reported here examined the effects of different doses of corticosterone on brain alpha-bungarotoxin binding. C3H mice were adrenalectomized and treated with corticosterone-containing pellets (0.5-60%) for four days. Alpha-Bungarotoxin binding was measured in eight brain regions. Chronic treatment with corticosterone resulted in plasma corticosterone levels ranging from the low levels observed in an unstressed mouse during the daytime to levels significantly above those observed in mice during the night or as a result of stress. Adrenalectomy resulted in small increases in binding in hippocampus which was reversed by low dose corticosterone treatment. Chronic high-dose corticosterone treatment resulted in significant decreases in binding in four of the eight brain regions examined. Similar, but not identical, results were obtained in two other mouse strains (C57BL and DBA/2). These results argue that corticosterone levels play an important role in modulating the level of the brain nicotinic receptors that bind alpha-bungarotoxin with high affinity.

Corticosterone reversibly alters brain alpha-bungarotoxin binding and nicotine sensitivity

Previous studies have shown that chronic corticosterone (CCS) treatment via subcutaneous pellets elicits reduced sensitivity to many actions of nicotine in mice as well as decreased brain alpha-bungarotoxin (alpha-BTX) binding. We report here the time courses of altered sensitivity to nicotine, as measured by acoustic startle, Y-maze crossing and rearing activities, heart rate, and body temperature, and alpha-BTX binding during and after CCS treatment. CCS treatment resulted in rapid decreases in sensitivity to nicotine for four of the five responses that were measured, as well as rapid changes in alpha-BTX binding. Sensitivity to nicotine returned to control levels within 3 days following pellet removal, but alpha-BTX binding returned to control levels in most brain regions 9-11 days after pellet removal. Because the restoration of control sensitivity to nicotine occurred long before alpha-BTX binding returned to control levels, it seems likely that factors other than changes in alpha-BTX binding cause chronic CCS-induced changes in sensitivity to nicotine.

Attenuated cortisol response to psychological stress but not to CRH or ergometry in young habitual smokers

Salivary cortisol and heart rate responses to a) psychological stress (public speaking and mental arithmetic), b) human corticotropin-releasing hormone (hCRH), and c) bicycle ergometry until exhaustion were investigated in 10 smokers and 10 nonsmokers. Compared to d), an injection of physiological saline, psychological stress as well as hCRH resulted in significant elevations of salivary cortisol levels in the total group. Ergometry workload induced only moderately enhanced cortisol concentrations. Profound changes in heart rates were observed following bicycle ergometry [+83 beats per minure (bpm)] and after the psychological stress (+29 bpm). hCRH injection increased heart rate by 5 bpm while heart rates dropped after saline administration (-2 bpm). Smokers showed an attenuated cortisol response to the psychological stressor. Mean cortisol increases reached only one third in smokers compared to nonsmokers. Similarly, cortisol levels in smokers tended to be lower after hCRH injection; however, this difference was not statistically significant. Cortisol responses to ergometry did not differ between the two groups. Likewise, heart rates did not reveal different profiles in any of the three stimulations in smokers compared to nonsmokers.

Acute stress or corticosterone administration reduces responsiveness to nicotine: implications for a mechanism of conditioned tolerance

We have shown that conditioned tolerance develops to some of the behavioral and endocrine effects of nicotine in rats. Other investigators have suggested that tolerance to multiple nicotine injections in mice may be due, in part, to elevated plasma corticosterone (CORT) levels, since repeated nicotine injections are associated with elevated CORT, chronically elevated CORT reduces nicotine responsiveness and adrenalectomy disrupts nicotine tolerance. Three experiments tested the feasibility of this hypothesis, as a mechanism for conditioned nicotine tolerance in rats, by determining whether acute administration of CORT or manipulations that increase adrenocortical activity reduce nicotine responsiveness. In experiment 1, male rats were injected IP with CORT (1 mg/kg), vehicle (ETOH + distilled water) or no injection 10 min before nicotine (0.75 mg/kg, SC) and tested for nicotine-induced analgesia every other day for 10 days. A significant reduction in withdrawal latencies was obtained for CORT pretreated rats compared to animals given only nicotine. A similar reduction was produced by the vehicle pretreatment, which itself induced an elevation of endogenous CORT. Experiments 2 and 3 established that similar effects could be produced by doses of CORT as low as 0.125 mg/kg or by exposure to a novel environment which also elevated CORT levels. Results also suggest that a conditioned release of endogenous CORT was triggered by stimuli associated with nicotine delivery. These data are consistent with the hypothesis that a conditioned release of CORT could contribute to the development of tolerance to some of nicotine's effects.(ABSTRACT TRUNCATED AT 250 WORDS)

Adrenalectomy reverses chronic injection-induced tolerance to nicotine

A recent study from our laboratory has demonstrated that C57BL/6 male mice that are chronically injected with nicotine develop a profound tolerance to nicotine that is not associated with changes in brain nicotinic receptors. We have proposed that alterations in the secretion of corticosterone (CCS) may regulate tolerance development in chronically injected animals. In the present study we have directly tested this hypothesis. Female DBA/2 mice were injected three times each day for 12 days with saline or 2 mg/kg nicotine. Blood samples were collected at various time points during the course of treatment and plasma CCS levels were determined. The animals were divided into two groups following the last injection on day 12. The first group of animals was tested for nicotine-induced release of corticosterone on day 13 of the experiment and then sacrificed. The brains of these animals were subsequently used to measure nicotinic receptor binding. The second group of animals was adrenalectomized (ADX) or sham-operated on day 13 of the experiment and tested for nicotine sensitivity on day 14 of the experiment. Plasma CCS levels were significantly elevated in animals that were chronically injected with nicotine (versus saline controls) by the fourth day of the experiment. Chronic nicotine-injected animals were tolerant to nicotine-induced CCS release. Animals that were chronically injected with nicotine and sham-operated were tolerant to acute nicotine challenge; however, tolerance to nicotine was not detected in ADX animals. These data support the hypothesis that the capacity to release CCS may underscore the expression of tolerance to nicotine in chronically injected animals.

Tolerance to nicotine following chronic treatment by injections: a potential role for corticosterone

C57BL/6 male mice were injected intraperitoneally with nicotine (2.0 mg/kg) or saline three times each day (0800 h, 1300 h and 1800 h) for a period of 12 days and then tested for nicotine tolerance using a series of behavioral and physiological tests. For each of these tests, animals that received chronic nicotine treatment were significantly less sensitive to nicotine challenge than were animals that received chronic saline treatment, as indicated by shifts to the right of dose-response curves. Animals were retested for nicotine sensitivity 2 weeks following cessation of chronic nicotine injections. Tolerance to acute nicotine challenge persisted in nicotine-treated animals. Chronic nicotine treatment by injections did not alter the binding of L-[3H]-nicotine or alpha-[125I]-bungarotoxin in any of eight brain regions. Plasma corticosterone (CCS) levels were determined in animals prior to the initiation of the injection series (day 0), and on days 4, 8 and 12 of chronic treatment, immediately before the first injection of the day. CCS levels in nicotine-treated animals were elevated as compared to saline-injected controls by day 12 of treatment. Nicotine-treated animals also had elevated CCS levels 2 weeks after the last chronic injection. Nicotine-treated animals were, however, tolerant to nicotine-induced CCS release. Since previous studies from our laboratory have demonstrated that plasma CCS levels are inversely correlated with sensitivity to nicotine, it is possible that the tolerance to nicotine measured following chronic treatment by injections is due, at least in part, to the elevation in plasma CCS levels.

The neurobiology of tobacco addiction

The key property that makes nicotine addictive is an ability to support the drug-seeking behaviour that has been demonstrated in self-administration and place preference experiments. This reinforcing effect is complex, possibly involving subjective states of euphoria, cognitive enhancements, changed adaptation to stress, and relief from the nicotine withdrawal syndrome. The neural mechanisms, described here by Ian Stolerman and Mohammed Shoaib, include a primary action on central nicotinic acetylcholine receptors, associated with selective activation of the mesolimbic dopamine system that also mediates other sources of reinforcement. Structures such as the mesopontine tegmentum may also contribute to the reinforcing effect, whereas hippocampal and striatal regions seem to mediate other behavioural changes.

Research on stress and smoking: progress and problems

Despite evidence that smoking behaviour increases in the context of stress, there has yet to be a clear-cut demonstration that nicotine intake is similarly enhanced. Although nicotine intake has been shown to reduce reported anxiety in the context of stress, the controlling conditions (type of stressor, intensity, temporal relationships, etc.) need further exploration. Recent findings involving nicotine's effects on the hypophyseal-adrenal axis provide a new perspective on these issues, in that increased nicotine intake during exposure to a stressor may represent, at least in part, behavioral compensation for diminished sensitivity to nicotine brought about by nicotine-stimulated corticosteroid release. Corticosteroids may decrease central nervous system excitability in a way that could account for anxiety reduction; on the other hand, anxiety reduction may be an epiphenomenon with respect to the reinforcement of smoking behaviour. The integration of behavioural, physiological, and biochemical research exemplified by the above approach should lead to a better understanding of stress and smoking.

Acute and subchronic corticosterone treatment differentially modulates subcortical limbic and neostriatal nicotinic cholinergic receptors

The effects of acute and subchronic corticosterone treatment were analyzed on the competition by (-)nicotine ((-)nicotine hydrogen(+)-tartrate) on N-[3H]methylcarbamyl choline iodide ([ 3H]MCC) binding sites in membranes from the subcortical limbic forebrain and the neostriatum. Acute treatment with corticosterone (5 mg/kg, i.p., 2 h) increased the IC50 values of (-)nicotine by 230% in the subcortical limbic areas but not in the neostriatum. Subchronic corticosterone treatment (5 mg/kg, twice a day, 7 days) increased the IC50 values of (-)nicotine by 50% and slightly decreased the specific binding of [3H]MCC (5 nM) in the subcortical limbic area. In the neostriatum, subchronic treatment with corticosterone instead decreased by 50% the IC50 values of (-)nicotine and slightly increased the specific binding of [3H]MCC. The results indicate that corticosterone treatment selectively reduces the affinity of nicotinic cholinergic receptors within the subcortical limbic forebrain.

Cortisol response to a psychological stressor and/or nicotine

The effects of a psychological stressor and nicotine upon corticosteroid release were investigated using a full factorial, repeated-measures design in eight moderate smokers. Sessions involved the presentation of either competitive mental arithmetic or reading aloud and either smoking a usual cigarette or sham smoking. Self-reported anxiety increased after exposure to competitive mental arithmetic, confirming the stressfulness of the procedure. Cortisol levels increased significantly in response to psychological stress and showed a trend towards a significant elevation over time in response to nicotine self-administration. The two manipulations in combination produced additive effects upon plasma cortisol. These findings underscore the usefulness of the corticosteroid response as a marker of the impact of different procedures and suggest that it may provide an indicator for exploring the mechanisms by which nicotine-stress interactions are mediated. Systematic research that varies temporal and other parameters involving nicotine and various stressors will be needed to resolve inconsistencies in the literature on smoking and anxiety in the context of stress.

Chronic corticosterone administration modulates nicotine sensitivity and brain nicotinic receptor binding in C3H mice

Glucocorticoid regulation of nicotine sensitivity was investigated in adrenalectomized (ADX) and sham-operated C3H mice administered chronic corticosterone (CCS) replacement therapy. Hormone pellets (60% CCS or pure cholesterol) were implanted at the time of surgery and animals were tested for nicotine sensitivity in a battery of behavioral and physiological tests. ADX-induced increases in nicotine sensitivity were reversed by chronic CCS replacement. Sham-operated animals that received CCS supplementation were subsensitive to the effects of nicotine. In both ADX and sham-operated animals, chronic CCS administration induced a decrease in the number of CNS nicotinic cholinergic receptors labeled by alpha-[125I]-bungarotoxin. Binding was decreased by 30-60% depending on brain region; no changes in affinity (KD) were detected. The number of brain nicotinic sites labeled by [3H]-nicotine was unaltered following 1 week of chronic CCS administration. These data support the hypothesis that glucocorticoids modulate nicotine sensitivity in the C3H mouse. In animals chronically treated with CCS, nicotine tolerance may be due to CCS-induced changes in nicotinic cholinergic receptor binding or the presence of high CCS titers at the time of testing.

Strain differences in adrenalectomy-induced alterations in nicotine sensitivity in the mouse

Adult mice of four inbred strains (A, BUB, C57BL, DBA) and two selectively bred lines [Long-Sleep (LS) and Short-Sleep (SS)] were tested for differences in glucocorticoid regulation of nicotine sensitivity. One week following adrenalectomy (ADX), animals were tested for nicotine sensitivity in a battery of tests that included acoustic startle response, Y-maze activity (line crosses and rearings), heart rate and body temperature. Although each type of animal tested had increased nicotine sensitivity in at least one of the test battery measurements, there was clear evidence for a genetic influence on the scope of ADX-induced changes in sensitivity. LS animals had the largest increase in sensitivity with altered responses in four of five tests following ADX. The sensitivity of DBA animals was increased in two tests while for A, BUB, C57BL and SS animals, only one test was affected. ADX-induced alterations in nicotine sensitivity could not be explained on the basis of changes in nicotinic receptor number since changes were consistent across strains. The mechanism by which ADX causes increased nicotine sensitivity is not known. However, these data support the hypothesis that nicotine sensitivity is modulated by adrenal glucocorticoid secretion and also suggest that this phenomenon is under strict genetic control.

Dexamethasone attenuation of the cortisol response to nicotine in smokers

The effect of corticosteroids upon the cortisol response to nicotine from smoking was investigated in five heavy smokers. Corticosteroid activity was manipulated by administering dexamethasone, a synthetic glucocorticoid (1 mg orally, 14 h before), in a double-blind, placebo-controlled procedure. Testing took place in the middle of the day and involved the smoking of two high-nicotine (2.87 mg) research cigarettes over a 15-min period. The dexamethasone condition was characterized by a pronounced suppression of baseline plasma cortisol, as expected, and by a significant dampening of the cortisol response to nicotine, indicating diminished sensitivity to nicotine under conditions of enhanced corticosteroid activity.

Neuroregulatory effects of nicotine

The impact of nicotine on the central nervous system is, in an important sense, neuroregulatory, with cascading effects on physiological and biochemical function as well as on behavioral activity. Accordingly, the neurotransmitter and neuroendocrine effects of nicotine constitute a critical part of its biological action, which includes reinforcing as well as pathophysiological consequences. This review focuses on nicotine's effects on cholinergic and non-cholinergic nicotine receptors and on the responses of catecholamines, monoamines, hypophyseal hormones, and cortisol. The contribution of critical variables, such as timing and duration of neuroregulator release and the patterns that make up the total response, is still largely unknown, particularly with regard to the effects of environmental context, history of nicotine use, and mode of administration. The evidence suggests that by altering the bioavailability of the above-listed neuroregulators, nicotine serves as a pharmacological "coping response", providing immediate though temporary improvement in affect or performance in response to environmental demands. Much of what is known to date is based on studies involving the administration of agonists and antagonists under different environmental conditions. Newer technological approaches such as autoradiography and positron emission tomography show potential for determining the neuroregulatory patterns involved and specifying nicotine's locus of action relevant to its behavioral and physiological effects.