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

Pavlovian Conditioning of Immunological and Neuroendocrine Functions

The phenomenon of behaviorally conditioned immunological and neuroendocrine functions has been investigated for the past 100 yr. The observation that associative learning processes can modify peripheral immune functions was first reported and investigated by Ivan Petrovic Pavlov and his co-workers. Their work later fell into oblivion, also because so little was known about the immune system’s function and even less about the underlying mechanisms of how learning, a central nervous system activity, could affect peripheral immune responses. With the employment of a taste-avoidance paradigm in rats, this phenomenon was rediscovered 45 yr ago as one of the most fascinating examples of the reciprocal functional interaction between behavior, the brain, and peripheral immune functions, and it established psychoneuroimmunology as a new research field. Relying on growing knowledge about efferent and afferent communication pathways between the brain, neuroendocrine system, primary and secondary immune organs, and immunocompetent cells, experimental animal studies demonstrate that cellular and humoral immune and neuroendocrine functions can be modulated via associative learning protocols. These (from the classical perspective) learned immune responses are clinically relevant, since they affect the development and progression of immune-related diseases and, more importantly, are also inducible in humans. The increased knowledge about the neuropsychological machinery steering learning and memory processes together with recent insight into the mechanisms mediating placebo responses provide fascinating perspectives to exploit these learned immune and neuroendocrine responses as supportive therapies, the aim being to reduce the amount of medication required, diminishing unwanted drug side effects while maximizing the therapeutic effect for the patient’s benefit.

HPA Axis Interactions with Behavioral Systems

Perhaps the most salient behaviors that individuals engage in involve the avoidance of aversive experiences and the pursuit of pleasurable experiences. Engagement in these behaviors is regulated to a significant extent by an individual's hormonal milieu. For example, glucocorticoid hormones are produced by the hypothalamic-pituitary-adrenocortical (HPA) axis, and influence most aspects of behavior. In turn, many behaviors can influence HPA axis activity. These bidirectional interactions not only coordinate an individual's physiological and behavioral states to each other, but can also tune them to environmental conditions thereby optimizing survival. The present review details the influence of the HPA axis on many types of behavior, including appetitively-motivated behaviors (e.g., food intake and drug use), aversively-motivated behaviors (e.g., anxiety-related and depressive-like) and cognitive behaviors (e.g., learning and memory). Conversely, the manuscript also describes how engaging in various behaviors influences HPA axis activity. Our current understanding of the neuronal and/or hormonal mechanisms that underlie these interactions is also summarized. © 2016 American Physiological Society. Compr Physiol 6:1897-1934, 2016.

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.

Potential substrates for nicotine and alcohol interactions: a focus on the mesocorticolimbic dopamine system

Epidemiological studies consistently find correlations between nicotine and alcohol use, yet the neural mechanisms underlying their interaction remain largely unknown. Nicotine and alcohol (i.e., ethanol) share many common molecular and cellular targets that provide potential substrates for nicotine-alcohol interactions. These targets for interaction often converge upon the mesocorticolimbic dopamine system, where the link to drug self-administration and reinforcement is well documented. Both nicotine and alcohol activate the mesocorticolimbic dopamine system, producing downstream dopamine signals that promote the drug reinforcement process. While nicotine primarily acts via nicotinic acetylcholine receptors, alcohol acts upon a wider range of receptors and molecular substrates. The complex pharmacological profile of these two drugs generates overlapping responses that ultimately intersect within the mesocorticolimbic dopamine system to promote drug use. Here we will examine overlapping targets between nicotine and alcohol and provide evidence for their interaction. Based on the existing literature, we will also propose some potential targets that have yet to be directly tested. Mechanistic studies that examine nicotine-alcohol interactions would ultimately improve our understanding of the factors that contribute to the associations between nicotine and alcohol use.

Individual phenotype predicts nicotine-haloperidol interaction in catalepsy: possible implication for the therapeutic efficacy of nicotine in Tourette's syndrome

In individuals with Tourette's syndrome, the therapeutic efficacy of haloperidol can be augmented by nicotine. In laboratory rats, the dopamine antagonist haloperidol produces catalepsy and nicotine can potentiate it, although this effect is variable and not always observed. Our aim was to understand this variability. In rats, the locomotor response to a novel environment predicts the magnitude of the locomotor response to nicotine. Since the psychostimulant effect of nicotine might counter catalepsy, we hypothesized that rats with a high locomotor response to novelty would show reduced vulnerability to nicotine potentiation of haloperidol catalepsy. First, we administered haloperidol (0, 0.1 or 0.3mg/kg, ip) and found stronger catalepsy in rats with low reactivity to novelty. Second, we administered haloperidol (0.3mg/kg) or haloperidol plus nicotine (0.1mg/kg, ip) and found that nicotine indeed potentiated haloperidol catalepsy but only in rats with low reactivity to novelty. Nicotine did not induce catalepsy on its own. Thus, previously reported inconsistencies in the catalepsy potentiating effect of nicotine may have been due to differential vulnerability to its stimulant actions. As previously observed, the potentiation of haloperidol catalepsy was greatest 4h after injection. Given the short half-life of nicotine, the mechanism(s) underlying the delayed expression of its pro-cataleptic capacity remains obscure.

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.

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.

Stress-induced attenuation of acoustic startle in low-saccharin-consuming rats

Exposure to stress can lead to either increased stress vulnerability or enhanced resiliency. Laboratory rats are a key tool in the exploration of basic biobehavioral processes underlying individual differences in the effect of stress on subsequent stressors' impact. The Occidental low (LoS) and high (HiS) saccharin-consuming rats, which differ in emotional reactivity, are useful in this effort. In the present study, footshock affected acoustic startle amplitude 4 h later among LoS but not HiS rats. Surprisingly, shock attenuated startle rather than sensitizing it, a finding not previously reported for male rats exposed to shock. Attenuation was blocked by administering the anxiolytic drug alprazolam prior to stress, implicating anxiety in the effect. Preliminary tests provided no evidence of mediation by adenosine or corticosterone. This novel result encourages further study of the stressor and dispositional variables that modulate the timecourse of effects of stress on startle and identification of its mechanisms.

The effects of non-genomic glucocorticoid mechanisms on bodily functions and the central neural system. A critical evaluation of findings

Mounting evidence suggests that--beyond the well-known genomic effects--glucocorticoids affect cell function via non-genomic mechanisms. Such mechanisms operate in many major systems and organs including the cardiovascular, immune, endocrine and nervous systems, smooth and skeletal muscles, liver, and fat cells. Non-genomic effects are exerted by direct actions on membrane lipids (affecting membrane fluidity), membrane proteins (e.g. ion channels and neurotransmitter receptors), and cytoplasmic proteins (e.g. MAPKs, phospholipases, protein kinases, etc.). These actions are mediated by the glucocorticoids per se or by the proteins dissociated from the liganded glucocorticoid receptor complex. The MR and GR also activate non-genomic mechanisms in certain cases. Some effects of glucocorticoids are shared by a variety of steroids, whereas others are more selective. Moreover, "ultra-selective" effects-mediated by certain glucocorticoids only-were also shown. Disparate findings suggest that non-genomic mechanisms also show "demand-specificity", i.e. require the coincidence of two or more processes. Some of the non-genomic mechanisms activated by glucocorticoids are therapeutically relevant; moreover, the "non-genomic specificity" of certain glucocorticoids raises the possibility of therapeutic applications. Despite the large body of evidence, however, the non-genomic mechanisms of glucocorticoids are still poorly understood. Criteria for differentiating genomic and non-genomic mechanisms are often loosely applied; interactions between various mechanisms are unknown, and non-genomic mechanism-specific pharmacological (potentially therapeutic) agents are lacking. Nevertheless, the discovery of non-genomic mechanisms is a major breakthrough in stress research, and further insights into these mechanisms may open novel approaches for the therapy of various diseases.

Wistar Kyoto and Wistar rats differ in the affective and locomotor effects of nicotine

Anhedonia is a characteristic of clinical depression and has been associated with dysfunction of the mesolimbic dopaminergic system, a system also involved in mediating nicotine reward. To further examine the relationship between anhedonia, clinical depression and nicotine reward, the present experiment determined if Wistar Kyoto (WKY) rats, an animal model of clinical depression, differed from Wistar rats in nicotine conditioned place preference (CPP). Strain differences in nicotine-induced changes in locomotor activity also were determined simultaneously. To determine if strain differences were specific to reward-based learning, nicotine or lithium chloride (LiCl) conditioned taste avoidance (CTA) experiments were conducted. Rats received vehicle or nicotine (0.4 or 0.8 mg/kg) during a multi-trial, biased CPP training procedure or received vehicle, nicotine (0.2, 0.4 or 0.8 mg/kg) or lithium chloride (LiCl; 0.0375, 0.075 or 0.15 M) during a multi-trial CTA training procedure. Whereas both nicotine doses (0.4 and 0.8 mg/kg) initially induced hypoactivity, only the moderate nicotine dose (0.4 mg/kg) induced hyperactivity with repeated administration and produced a CPP in Wistar rats. Both nicotine doses failed to alter locomotor activity or produce a CPP in WKY rats. WKY rats also acquired a LiCl CTA more slowly and less robustly compared to Wistar rats. In contrast, nicotine dose-dependently produced a CTA in both strains and WKY rats were more sensitive to the avoidance effects of nicotine compared to Wistar rats. Collectively, these results suggest that WKY rats show deficits in nicotine reward and specific aversive drug stimuli compared to Wistar rats.

Acquisition and extinction of conditioned nicotine analgesic tolerance

These studies demonstrated the acquisition and extinction of conditioned tolerance to the analgesic effect of nicotine in rats. In Experiment 1, distinctive environmental cues were either paired or unpaired with nicotine. Following acquisition, the paired group was more tolerant to nicotine than the unpaired and saline groups. Conditioned tolerance was extinguished in the paired group after placebo sessions in the distinctive environment. Experiment 2 examined whether the distinctive environment functioned as a CS or as an occasion setter for injection cues. After acquisition, exposure to the distinctive environment, with or without placebo injections, resulted in extinction. This demonstrates that the distinctive environment served as a CS, not as an occasion setter for injection cues.

Interdose interval effects on the development of contextual tolerance to nicotine's analgesic effects in rats (Rattus norvegicus)

Learning models of associative and nonassociative drug tolerance predict that the development of contextual tolerance to drug effects is disrupted when the drug is delivered at short interdose intervals (IDIs). The authors examined the impact of 1 long IDI and 2 short IDIs in the development of contextual nicotine tolerance. Associative tolerance was investigated by giving rats (Rattus norvegicus) 10 subcutaneous injections of nicotine at either long (72-hr) IDIs or short (6-hr and 4.5-hr) IDIs. The delivery of nicotine was either explicitly paired or explicitly unpaired with a distinctive context. A 3rd group of rats was exposed to the experimental procedures but received only saline. Associative tolerance to nicotine's analgesic effects was defined as a shift to the right of the dose-response curve (DRC) of rats in the explicitly paired condition with respect to the DRC of rats in the explicitly unpaired condition. Analgesia was assessed with the tail-flick and hot-plate devices. In the tail-flick assessment, associative tolerance was evident in the 72-hr and the 6-hr IDI conditions only. In the hot-plate assessment, associative tolerance was present in the 72-hr IDI condition only. The findings suggest that contextual tolerance to nicotine's analgesic effects are positively related to IDI length and are more readily demonstrated with the tail-flick method than with the hot-plate method. Overall, the results supported the thesis that nicotine tolerances that develop to different IDIs are qualitatively different and may be mediated by different psychological and physiological mechanisms.

Wistar Kyoto rats exhibit reduced sucrose pellet reinforcement behavior and intravenous nicotine self-administration

A phenotype of heightened anxiety-like behavior is hypothesized to be associated with altered reinforcement behavior. To test this hypothesis, we studied patterns of sucrose pellet intake and intravenous nicotine self-administration in animals that exhibit anxiety-like behavior at baseline, Wistar Kyoto (WKY) rats, as compared to normal controls (Wistar rats). WKY rats exhibited significantly reduced sucrose pellet self-administration behavior as assessed by both fixed and progressive ratio schedules of reinforcement and exhibited significantly reduced self-administration of intravenous nicotine. On the basis of previously published findings, we hypothesize that altered mesolimbic dopamine responses, as well as heightened HPA axis functioning, may account for reduced nicotine self-administration and sucrose pellet reinforcement responding in WKY rats. These studies highlight the role of heightened anxiety-like behavior, resulting from the genetic background of the animal, in altering behavioral responses to reinforcing stimuli.

Associative and behavioral tolerance to the analgesic effects of nicotine in rats: tail-flick and paw-lick assays

RATIONALE

Theories of drug tolerance differentiate between associative and behavioral (instrumental) drug tolerance. However, there is little research comparing these two forms of drug tolerance beyond alcohol and morphine.

OBJECTIVE

We examined the time course development of associative and behavioral tolerance to the analgesic effects of nicotine.

METHODS AND RESULTS

Associative tolerance was investigated by giving independent groups of rats one, five, 15, ten or 20 administrations of nicotine either explicitly paired or unpaired with a distinctive context. Associative tolerance, assessed in the tail flick, developed more rapidly and reached greater magnitude when nicotine and distinctive context were explicitly paired than when they were unpaired. This effect was evidenced after the fifth conditioning session and was maintained through the tenth, 15th, and 20th sessions. Contextual tolerance, assessed in the hot plate, was first evident after ten sessions. However, this effect disappeared safter 15 and 20 sessions. A second study examined the acquisition of behavioral tolerance to the disruptive effects of nicotine on the hot-plate response. Animals that practiced the test response while drugged developed greater tolerance than animals receiving as much nicotine and hot-plate practice but with these two conditions explicitly unpaired. This effect was evident in two different environments but did not generalize to the tail-flick test.

CONCLUSIONS

The findings suggest that contextual tolerance to drug effects is test specific, with tail-flick responses depending on cue-associative tolerance processes and hot-plate responses requiring procedures that allow the animal to practice the test response while drugged.

Plasma corticosterone in the rat in response to nicotine and saline injections in a context previously paired or unpaired with nicotine

RATIONALE

Following repeated injections with nicotine paired with a distinctive environment, some studies have reported that the distinctive context becomes a conditioned stimulus (CS) capable of eliciting conditioned corticosterone (CORT) release. Conversely, other studies have found that exposure to the CS results in conditioned attenuation of nicotine-induced CORT release.

OBJECTIVE

The present study was designed to examine whether these sets of separate findings could be replicated in animals exposed to the same experimental procedures within the same study.

METHODS

CORT assessments were conducted in a distinctive context after independent groups of animals were injected with either saline or nicotine (1 mg/kg, salt) after five or ten nicotine injections either explicitly paired or unpaired with a distinctive context. The design also included groups of nicotine-naïve rats exposed to the experimental procedures and assessed for CORT levels following either nicotine or saline injections during their first, and after their fifth and tenth context exposures.

RESULTS

CORT levels were higher after nicotine than after saline, and higher in the paired than in the unpaired condition. Exposure to the context without nicotine produced conditioned CORT release and exposure to the context did not attenuate nicotine-induced CORT release.

CONCLUSIONS

The results supported the notion that a CS associated with nicotine effects elicit a conditioned response (CR) in the form of CORT release. Future research will be needed to examine whether conditioned CORT release can explain the context-dependent attenuations of nicotine-induced CORT.

Nicotine dependence: development, mechanisms, individual differences and links to possible neurophysiological correlates

There is now little doubt that the majority of people who smoke tobacco do so to experience the psychopharmacological properties of the nicotine present in the smoke and that a significant proportion of habitual tobacco users become addicted to the drug nicotine. In the US some 80% and in Europe (Germany) 39% of smokers have been classified as dependent according to the diagnostic guidelines of the American Psychiatric Association. As a result, direct nicotine replacement is used increasingly by many people who want to stop smoking. The objectives of this review are to outline the mechanisms involved in the development and maintenance of nicotine dependence and to link behavioural observations to possible neurophysiologic correlates.

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 sucrose intake enhances nicotine-induced antinociception in female but not male Long-Evans rats

Previous work has demonstrated that intake of palatable foods can alter the behavioral actions of opioid drugs. To investigate whether intake of palatable fare only affects opioid-induced behaviors or more generally influences drug-induced responses, this study examined the effects of chronic intake of a palatable sucrose solution on nicotine-induced antinociception. Eight male and eight female Long-Evans rats were provided with ground chow and water (control group), while eight males and eight females were provided with chow, water and a 32% sucrose solution (sucrose group). After 3 weeks of exposure to the dietary conditions, all rats were tested for nicotine-induced antinociception using the tail flick test. Nicotine, administered using a cumulative dose regime (0.03, 0.1, 0.3 and 1.0 mg/kg sc), led to dose-dependent increases in tail flick latencies in male and female rats. Females in the sucrose group displayed significantly greater antinociceptive responses to nicotine than those in the control group. Similar results were obtained when females were retested after an additional 2 weeks. Comparison of males and females, revealed that sucrose enhanced nicotine's antinociceptive action in female but not in male rats. While previous research suggested that sweet tasting substances might affect drug action by acting on the endogenous opioid system, the present results indicate that sucrose intake could also alter the cholinergic system and possibly other systems involved in nicotine antinociception.

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.

Operant ethanol self-administration after nicotine treatment and withdrawal

The effects of chronic intermittent administration of nicotine (NIC) and withdrawal on operant ethanol (EtOH) self-administration were tested in Long-Evans rats (n = 8). EtOH self-administration (10% v/v, Fixed Ratio 4 reinforcement schedule) was induced by the sucrose-substitution procedure. Then the animals were divided into two groups of four rats matched on EtOH self-administration and the locomotor activity following an injection of NIC (0.35 mg/kg, SC) or saline was measured. The groups then received 9 days of injection of either NIC (0.35 mg/kg) or saline and then motor activity was retested using the initial NIC dose. This was followed by 17 days of NIC injections (0.6 mg/kg) or saline injections. A final locomotor test using the higher NIC dose was then conducted. The initial acute administration of NIC had no effect on motor activity compared to saline (measured by the number of horizontal movements). However, after the repeated treatment, the group of animals injected chronically and acutely with NIC showed motor activation in comparison with the animals injected chronically with saline and injected acutely with NIC only on the days of activity testing. At the end of the chronic NIC treatment, operant EtOH self-administration was not changed. However, 6 days after the NIC injections were concluded, a change in the pattern of responding for EtOH was observed in the NIC group, showing a decrease in the mean rate of responding during the first half of the operant self-administration session. When both groups were again tested for locomotor activity at the end of the operant self-administration experiment, the increased motor activity in the NIC group was still observed. The results suggest that alterations in the nicotinic system may affect EtOH self-administration, but this appears to be only modulatory, even with a significant change in locomotor response to NIC following chronic NIC administration.

Influence of sex and female hormones on nicotine-induced changes in locomotor activity in rats

The acute and chronic effects of nicotine (0.4 mg/kg s.c.) on locomotor activity in photocell cages have been compared in male, female, and ovariectomized hooded rats. In Experiment 1, female rats displayed higher locomotion than males (n = 12); acutely, nicotine-reduced locomotion, and this effect was slightly larger in females than males. Daily administration of nicotine for 21 days produced a similar, gradual increase in activity in both sexes. Tests then confirmed greater activity in females than males and as a function of previous chronic exposure to nicotine (n = 6); there was an activating effect of nicotine challenge but no interaction of nicotine effects with sex. In Experiment 2, ovariectomized rats were primed with 17-beta-estradiol (50 microg/kg s.c.) and progesterone (2.5 mg/kg s.c.) or vehicle only. Acute administration of nicotine reduced activity in both groups similarly (n = 12). After nicotine daily for 21 days, there was increased activity as a function of both chronic nicotine and hormonal priming, and challenge with nicotine increased activity (n = 6). The effects of these challenges with nicotine were also slightly greater, as a function of previous nicotine exposure and priming. As a whole, these experiments showed robust effects of acute and chronic nicotine administration, sex, and hormonal priming; neither sex nor gonadal hormones had marked influences on changes in locomotor activity produced by nicotine.

Effects of chronic nicotine treatment and withdrawal on hypothalamic proopiomelanocortin gene expression and neuroendocrine regulation

Considerable evidence suggest that some responses to smoking and nicotine are mediated by forebrain beta-endorphinergic opioid mechanisms. It has also been demonstrated that nicotine stimulates rat tuberoinfundibular dopaminergic activity. Since we have proposed that interactions between mediobasohypothalamic (MBH) dopaminergic and beta-endorphinergic mechanisms have a key role in neuroendocrine integration, we investigated the effects of chronic nicotine treatment and withdrawal on: (1) MBH concentrations of proopiomelanocortin (POMC, precursor for beta-endorphin biosynthesis) mRNA; (2) MBH concentrations of tyrosine hydroxylase (TH, rate limiting enzyme in catecholamine biosynthesis) mRNA; (3) corresponding serum prolacin, corticosterone, luteinizing hormone (LH), and testosterone concentrations. POMC and TH mRNA levels were measured by RNase protection/solution hybridization assay; serum hormone levels were measured by radioimmunoassay. Adult male rats received subcutaneous injections of either nicotine or saline during the dark period of each day on an increasing frequency (1-3 injections/day) and dosage (0.4-0.5 mg nicotine/kg body weight) schedule over 4 weeks. The rats were sacrificed after 4 weeks treatment and at 1, 3, 7, 14 and 21 days withdrawal. Chronic daily nicotine administration induced significant changes in serum corticosterone, serum prolactin, MBH TH mRNA, and MBH POMC mRNA concentrations that tended to persist through day 3 of withdrawal; serum prolactin and MBH POMC mRNA concentrations were suppressed whereas serum corticosterone and MBH TH mRNA concentrations were stimulated. None of the parameters were significantly different from control levels following 7 or more days of withdrawal from nicotine, except for a significant decrease of MBH POMC mRNA concentrations on day 21. Chronic daily nicotine or withdrawal did not significantly alter serum LH or testosterone concentrations. These results suggest that chronic nicotine inhibited POMC gene expression and thus, probably, biosynthesis of beta-endorphin and other opiomelanocortins. We hypothesize that suppression of forebrain beta-endorphin synthesis in response to long-term nicotine exposure produces a chronically opioid deficient condition which may play an important role in maintaining nicotine self-administration and in mediating some changes during the nicotine withdrawal syndrome.

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.

Altered behavioral sensitivity of Ca(2+)-modulating drugs after chronic nicotine administration in mice

Numerous studies have demonstrated that tolerance develops to the physiological and behavioral effects of nicotine in animals after chronic administration of the drug. However, the mechanisms underlying tolerance to nicotine are not well known. There are several lines of evidence which support a role for Ca2+ in nicotine's acute pharmacological effects. The objective of the study was to determine whether Ca2+ plays a role in the development of tolerance to nicotine by investigating the behavioral activity of several Ca(2+)-modulating drugs after systemic (BAY K 8644: (+/-)-1,4-dihydro-2,6-dimethyl-5-nitro-4-[2-(trifluromethyl)-pheny l] -3-pyridine carboxylic acid methylester) and intrathecal administration (BAY K 8644, Ca2+ and thapsigargin) in nicotine-tolerant mice. The ability of BAY K 8644 to induce motor impairment and hypomotility after i.p. injection was decreased in nicotine-tolerant mice. In addition, tolerance to Ca2+, thapsigargin, and BAY K 8644-induced antinociception after i.t. injection also developed in nicotine-tolerant mice. ED50 values for BAY K 8644 and thapsigargin increased from 3.7 to 12 micrograms/mouse and 0.83 to 19.7 micrograms/mouse, respectively. The greatest tolerance developed to the effects of thapsigargin with an ED50 value that increased from 0.83 to 20 micrograms. Furthermore, chronic nicotine injections did not alter [3H]nitrendipine binding in the brain. These results suggest the involvement of Ca(2+)-dependent mechanisms in nicotine tolerance in mice.

Effect of prior nicotine treatment on drug induced changes in serum LH concentrations in rats

The effect of daily injections of nicotine on drug induced changes in LH secretion was investigated in male rats. Daily administration of nicotine for 7 days resulted in decreased basal serum LH concentrations. Nicotine treatment blocked naloxone induced LH release and reduced LHRH induced increases in serum LH. Clonidine induced increases in serum LH were not altered by nicotine treatment and haloperidol treatment did not alter nicotine induced decreases in serum LH. In an acute study nicotine blocked LH secretion induced by the long acting opioid antagonist naltrexone. Collectively these results indicate that opioidergic neurons are involved in the reduction in serum LH that occurs following nicotine. They also indicate that chronic nicotine treatment can reduce the pituitary gland response to LHRH.

Different methods of assessing nicotine-induced antinociception may engage different neural mechanisms

Two methods were used to assess nicotine-induced antinociception: tail withdrawal from a hot water bath and hind paw withdrawal from a hotplate. Nicotine doses which produced 75-80% maximum response were 0.75 mg/kg (free base) for tail withdrawal and 0.35 mg/kg for paw withdrawal. The peripheral blocker chlorisondamine (0.1 mg/kg, SC) and the central antagonist, mecamylamine (1 mg/kg, SC) were each effective in blocking nicotine-induced increases in tail withdrawal latencies, suggesting that this effect of nicotine depends on either the action of nicotine at peripheral receptors or the functional integrity of those receptors. In contrast, nicotine-induced increases in paw withdrawal latencies were blocked by mecamylamine but not by chlorisondamine, even at other agonist/antagonist dose combinations. The results indicate that these two effects of nicotine involve at least partially separate pathways and may reflect a different mix of the antinociceptive and motor depressing effects of nicotine.

Nicotine self-administration in rats

Considering the importance of self-administration models in determining mechanisms of drug maintained behavior, we attempted to replicate the findings of nicotine self-administration by Corrigall and Coen. Male, Sprague-Dawley rats, trained on food reinforcement, acquired relatively high and stable rates of self-administration of IV nicotine bitartrate (0.03 mg/kg, free base). Extinction and reacquisition followed substituting saline and then nicotine, respectively. Responses, infusions and intake decreased at 0.003 mg/kg, while intake increased at 0.06 mg/kg. This model of nicotine self-administration provides a reliable alternative to experimenter-administration models for examining the effects of nicotine.

Brain transcription factor gene expression, neurotransmitter levels, and novelty response behaviors: alterations during rat amphetamine withdrawal and following chronic injection stress

Transcription factors are known to act as gene expression regulators, possibly linking extracellular stimuli to long-term modifications at the neuronal level. Such modifications may potentially underlie chronic psychostimulant- and stress-induced behavioral alterations. This study illustrates how a 2 week, twice daily 7.5 mg/kg d-amphetamine or saline regimen alters rat brain regional expression of transcription factor genes, including c-fos, fos-B, jun-B, c-jun, and zif 268, and seeks potential correlations between those changes and alterations in neurotransmitter levels and behavioral novelty responses. Amphetamine withdrawal-induced decreases in transcription factor mRNA levels, assessed using Northern blot analysis, appear most prominent in prefrontal cortex, begin approximately 12 h after the last injection, and largely recover to control levels by 54 h. Prefrontal cortical and striatal dopamine content, assessed using HPLC, decrease and recover over a similar time course. Behavioral "stereotypy time" manifest by animals exposed to a novel environment, a measure sensitive to psychostimulant withdrawal, also decreases beginning 12 h after the last injection, is still significantly reduced at 54 h, and recovers at 72 h. Chronic saline injections are followed by a consistent decrease in transcription factor gene expression, observed 6 h after the last injection, followed by a "rebound" increase at 12 h. These changes are accompanied by dramatic, mostly biphasic alterations in prefrontal cortical biogenic amines and by a short-lived increase in striatal dopamine turnover. At the same time, rats display much longer-lasting decreases in locomotor responses when exposed to a novel environment, with recovery occurring only 54 h after the last injection. The delayed recovery of behavioral responses to novelty is consistent with potential involvement of changes in transcription factor-mediated gene expression in neurochemical mechanisms underlying psychostimulant withdrawal and chronic injection stress-induced behavioral alterations.