Nicotine tolerance: an analysis of the time course of its development and loss in the rat

Nicotine produces chronic behavioral sensitization with changes in accumbal neurotransmission and increased sensitivity to re-exposure

Tobacco use is often associated with long-term addiction as well as high risk of relapse following cessation. This is suggestive of persistent neural adaptations, but little is known about the long-lasting effects of nicotine on neural circuits. In order to investigate the long-term effects of nicotine exposure, Wistar rats were treated for 3 weeks with nicotine (0.36 mg/kg), and the duration of behavioral and neurophysiological adaptations was evaluated 7 months later. We found that increased drug-induced locomotion persisted 7 months after the initial behavioral sensitization. In vitro analysis of synaptic activity in the core and shell of the nucleus accumbens (nAc) revealed a decrease in input/output function in both regions of nicotine-treated rats as compared to vehicle-treated control rats. In addition, administration of the dopamine D2 receptor agonist quinpirole (5 μM) significantly increased evoked population spike amplitude in the nAc shell of nicotine-treated rats as compared to vehicle-treated control rats. To test whether nicotine exposure creates long-lasting malleable circuits, animals were re-exposed to nicotine 7 months after the initial exposure. This treatment revealed an increased sensitivity to nicotine among animals previously exposed to nicotine, with higher nicotine-induced locomotion responses than observed initially. In vitro electrophysiological recordings in re-exposed rats detected an increased sensitivity to dopamine D2 receptor activation. These results suggest that nicotine produces persistent neural adaptations that make the system sensitive and receptive to future nicotine re-exposure.

Comparison of effects produced by nicotine and the α4β2-selective agonist 5-I-A-85380 on intracranial self-stimulation in rats

Intracranial self-stimulation (ICSS) is one type of preclinical procedure for research on pharmacological mechanisms that mediate abuse potential of drugs acting at various targets, including nicotinic acetylcholine receptors (nAChRs). This study compared effects of the nonselective nAChR agonist nicotine (0.032-1.0 mg/kg) and the α4β2-selective nAChR agonist 5-I-A-85380 (0.01-1.0 mg/kg) on ICSS in male Sprague-Dawley rats. Rats were implanted with electrodes targeting the medial forebrain bundle at the level of the lateral hypothalamus and trained to respond under a fixed-ratio 1 schedule for a range of brain stimulation frequencies (158-56 Hz). A broad range of 5-I-A-85380 doses produced an abuse-related increase (or "facilitation") of low ICSS rates maintained by low brain-stimulation frequencies, and this effect was blocked by both the nonselective nAChR antagonist mecamylamine and the selective α4β2 antagonist dihyrdo-β-erythroidine (DHβE). Conversely, nicotine produced weaker ICSS facilitation across a narrower range of doses, and higher nicotine doses decreased high rates of ICSS maintained by high brain-stimulation frequencies. The rate-decreasing effects of a high nicotine dose were blocked by mecamylamine but not DHβE. Chronic nicotine treatment produced selective tolerance to rate-decreasing effects of nicotine but did not alter ICSS rate-increasing effects of nicotine. These results suggest that α4β2 receptors are sufficient to mediate abuse-related rate-increasing effects of nAChR agonists in this ICSS procedure. Conversely, nicotine effects at non-α4β2 nAChRs appear to oppose and limit abuse-related effects mediated by α4β2 receptors, although tolerance can develop to these non-α4β2 effects. Selective α4β2 agonists may have higher abuse potential than nicotine.

Common effects of fat, ethanol, and nicotine on enkephalin in discrete areas of the brain

Fat, ethanol, and nicotine share a number of properties, including their ability to reinforce behavior and produce overconsumption. To test whether these substances act similarly on the same neuronal populations in specific brain areas mediating these behaviors, we administered the substances short-term, using the same methods and within the same experiment, and measured their effects, in areas of the hypothalamus (HYPO), amygdala (AMYG), and nucleus accumbens (NAc), on mRNA levels of the opioid peptide, enkephalin (ENK), using in situ hybridization and on c-Fos immunoreactivity (ir) to indicate neuronal activity, using immunofluorescence histochemistry. In addition, we examined for comparison another reinforcing substance, sucrose, and also took measurements of stress-related behaviors and circulating corticosterone (CORT) and triglycerides (TG), to determine if they contribute to these substances' behavioral and physiological effects. Adult Sprague-Dawley rats were gavaged three times daily over 5 days with 3.5 mL of water, Intralipid (20% v/v), ethanol (12% v/v), nicotine (0.01% w/v) or sucrose (22% w/v) (approximately 7 kcal/dose), and tail vein blood was collected for measurements of circulating CORT and TG. On day five, animals were sacrificed, brains removed, and the HYPO, AMYG, and NAc processed for single- or double-labeling of ENK mRNA and c-Fos-ir. Fat, ethanol, and nicotine, but not sucrose, increased the single- and double-labeling of ENK and c-Fos-ir in precisely the same brain areas, the middle parvocellular but not lateral area of the paraventricular nucleus, central but not basolateral nucleus of the AMYG, and core but not shell of the NAc. While having little effect on stress-related behaviors or CORT levels, fat, ethanol, and nicotine all increased circulating levels of TG. These findings suggest that the overconsumption of these three substances and their potential for abuse are mediated by the same populations of ENK-expressing neurons in specific nuclei of the hypothalamus and limbic system.

Multiple nicotine training doses in mice as a basis for differentiating the effects of smoking cessation aids

RATIONALE

Receptor mechanisms underlying the behavioral effects of clinically used nicotinic acetylcholine receptor agonists have not been fully established.

OBJECTIVE

Drug discrimination was used to compare receptor mechanisms underlying the effects of smoking cessation aids.

METHODS

Separate groups of male C57BL/6J mice discriminated 0.56, 1, or 1.78 mg/kg of nicotine base. Nicotine, varenicline, and cytisine were administered alone, in combination with each other, and in combination with mecamylamine and dihydro-β-erythroidine (DHβE). Midazolam and morphine were tested to examine sensitivity to non-nicotinics.

RESULTS

The ED50 value of nicotine to produce discriminative stimulus effects systematically increased as training dose increased. Varenicline and cytisine did not fully substitute for nicotine and, as compared with nicotine, their ED50 values varied less systematically as a function of nicotine training dose. Morphine did not substitute for nicotine, whereas midazolam substituted for the low and not the higher training doses of nicotine. As training dose increased, the dose of mecamylamine needed to produce a significant rightward shift in the nicotine dose-effect function also increased. DHβE antagonized nicotine in animals discriminating the smallest dose of nicotine. Varenicline did not antagonize the effects of nicotine, whereas cytisine produced a modest though significant antagonism of nicotine.

CONCLUSIONS

These results suggest that differences in pharmacologic mechanism between nicotine, varenicline, and cytisine include not only differences in efficacy at a common subtype of nicotinic acetylcholine receptor, but also differential affinity and/or efficacy at multiple receptor subtypes.

Animal models of nicotine exposure: relevance to second-hand smoking, electronic cigarette use, and compulsive smoking

Much evidence indicates that individuals use tobacco primarily to experience the psychopharmacological properties of nicotine and that a large proportion of smokers eventually become dependent on nicotine. In humans, nicotine acutely produces positive reinforcing effects, including mild euphoria, whereas a nicotine abstinence syndrome with both somatic and affective components is observed after chronic nicotine exposure. Animal models of nicotine self-administration and chronic exposure to nicotine have been critical in unveiling the neurobiological substrates that mediate the acute reinforcing effects of nicotine and emergence of a withdrawal syndrome during abstinence. However, important aspects of the transition from nicotine abuse to nicotine dependence, such as the emergence of increased motivation and compulsive nicotine intake following repeated exposure to the drug, have only recently begun to be modeled in animals. Thus, the neurobiological mechanisms that are involved in these important aspects of nicotine addiction remain largely unknown. In this review, we describe the different animal models available to date and discuss recent advances in animal models of nicotine exposure and nicotine dependence. This review demonstrates that novel animal models of nicotine vapor exposure and escalation of nicotine intake provide a unique opportunity to investigate the neurobiological effects of second-hand nicotine exposure, electronic cigarette use, and the mechanisms that underlie the transition from nicotine use to compulsive nicotine intake.

Intermittent nicotine exposure upregulates nAChRs in VTA dopamine neurons and sensitises locomotor responding to the drug

Dopaminergic projections from the ventral tegmental area (VTA) to the nucleus accumbens (NAcc) mediate the behavioral and motivational effects of many drugs of abuse, including nicotine. Repeated intermittent administration of these drugs, a pattern often associated with initial drug exposure, sensitises the reactivity of dopamine (DA) neurons in this pathway, enhances the locomotor behaviors the drugs emit, and promotes their pursuit and self-administration. Here we show that activation of nicotinic acetylcholine receptors (nAChRs) in the VTA, but not the NAcc, is essential for the induction of locomotor sensitisation by nicotine. Repeated intermittent nicotine exposure (4 × 0.4 mg/kg, base, i.p., administered over 7 days), a regimen leading to long-lasting locomotor sensitisation, also produced upregulation of nAChRs in the VTA, but not the NAcc, in the hours following the last exposure injection. Functional nAChR upregulation was observed selectively in DA but not GABA neurons in the VTA. These effects were followed by long-term potentiation of excitatory inputs to these cells and increased nicotine-evoked DA overflow in the NAcc. Withdrawal symptoms were not observed following this exposure regimen. Thus, intermittent activation and upregulation by nicotine of nAChRs in DA neurons in the VTA may contribute to the development of behavioral sensitisation and increased liability for nicotine addiction.

Sazetidine-A, a selective α4β2 nicotinic acetylcholine receptor ligand: effects on dizocilpine and scopolamine-induced attentional impairments in female Sprague-Dawley rats

BACKGROUND

Neuronal nicotinic receptor systems have been shown to play key roles in cognition. Nicotine and nicotinic analogs improve attention and nicotinic antagonists impair it. This study was conducted to investigate the role of α4β2 nicotinic receptors in sustained attention using a novel selective α4β2 nicotinic receptor ligand, sazetidine-A.

METHODS

Female rats were trained to perform the signal detection task to a stable baseline of accuracy. The rats were injected with saline, sazetidine-A (0.01, 0.03, and 0.1 mg/kg), dizocilpine (0.05 mg/kg), or their combination; or, in another experiment, the rats were injected with the same doses of sazetidine-A, scopolamine (0.02 mg/kg), or their combination.

RESULTS

Percent hit and percent correct rejection showed that dizocilpine caused significant (p < 0.025) impairments in performance, which were significantly reversed by each of the sazetidine-A doses. Response omissions were significantly (p < 0.05) increased by dizocilpine, and this was also significantly reversed by each of the sazetidine-A doses. None of the sazetidine-A doses had significant effects on hit, correct rejection, or response omissions when given alone. Scopolamine also caused significant (p < 0.0005) impairments in percent hit and percent correct rejection and increased response omissions, which were significantly attenuated by all the sazetidine-A doses for percent hit and response omissions and by the highest dose of sazetidine-A for percent correct rejection. Both scopolamine and dizocilpine significantly (p < 0.0005) increased response latency, an effect which was significantly attenuated by sazetidine-A coadministration.

CONCLUSIONS

These studies imply an important role for α4β2 nicotinic receptors in improving sustained attention under conditions that disrupt it. Very low doses of sazetidine-A or drugs with a similar profile may provide therapeutic benefit for reversing attentional impairment in patients suffering from mental disorders and/or cognitive impairment.

Short- and long-lasting consequences of in vivo nicotine treatment on hippocampal excitability

The potential for relapse following cessation of drug use can last for years, implying the induction of stable changes in neural circuitry. In hippocampal slices from rats treated with nicotine for 1 week, withdrawal from nicotine in vivo produces an increase in CA1 pyramidal cell excitability that persists up to 9 months. Immediately upon drug cessation, the enhanced excitability depends on input from regions upstream of CA1, while the long-term excitability change (> 4 weeks) is expressed as an increase in the intrinsic excitability of CA1 neurons. Re-exposure to nicotine in vitro restores hippocampal function to control levels via activation of high-affinity nicotinic acetylcholine receptors after 1 d of withdrawal, but not at times >4 weeks. Thus, nicotine in vivo first induces homeostatic adaptations followed by other more robust neural changes. These mechanisms may contribute to hippocampal localized cue-motivated reinstatement of drug-seeking and/or cognitive deficits observed during withdrawal.

Neurobiological mechanisms involved in nicotine dependence and reward: participation of the endogenous opioid system

Nicotine is the primary component of tobacco that maintains the smoking habit and develops addiction. The adaptive changes of nicotinic acetylcholine receptors produced by repeated exposure to nicotine play a crucial role in the establishment of dependence. However, other neurochemical systems also participate in the addictive effects of nicotine including glutamate, cannabinoids, GABA and opioids. This review will cover the involvement of these neurotransmitters in nicotine addictive properties, with a special emphasis on the endogenous opioid system. Thus, endogenous enkephalins and beta-endorphins acting on mu-opioid receptors are involved in nicotine-rewarding effects, whereas opioid peptides derived from prodynorphin participate in nicotine aversive responses. An up-regulation of mu-opioid receptors has been reported after chronic nicotine treatment that could counteract the development of nicotine tolerance, whereas the down-regulation induced on kappa-opioid receptors seems to facilitate nicotine tolerance. Endogenous enkephalins acting on mu-opioid receptors also play a role in the development of physical dependence to nicotine. In agreement with these actions of the endogenous opioid system, the opioid antagonist naltrexone has shown to be effective for smoking cessation in certain sub-populations of smokers.

Cellular events in nicotine addiction

In the 25 years since the observation that chronic exposure to nicotine could regulate the number and function of high affinity nicotine binding sites in the brain there has been a major effort to link alterations in nicotinic acetylcholine receptors (nAChRs) to nicotine-induced behaviors that drive the addiction to tobacco products. Here we review the proposed roles of various nAChR subtypes in the addiction process, with emphasis on how they are regulated by nicotine and the implications for understanding the cellular neurobiology of addiction to this drug.

Nicotine psychopharmacology: policy and regulatory

Powerful nerve agent, poison, addictive drug, or wonder medicine of the future? Nicotine has had a long and storied history in pharmacology, physiology, public health and, more recently, in regulatory policy initiatives in the United States and internationally. Psychopharmacology research on nicotine and tobacco came to particular prominence in the latter third of the twentieth century with exploration addressing the role of nicotine in tobacco use, the potential categorization of nicotine as an addictive drug, the pharmacological basis for treatment of tobacco addiction, and the perspective of policy developers seeking to reduce the toll of tobacco use. In fact, the 2005 ratification of the World Health Organization's first global health treaty, the Framework Convention on Tobacco Control, provides further impetus for extending the science foundation for tobacco disease control and policy efforts. Implementation of the treaty's provisions will control tobacco use and reduce the 500 million premature deaths projected to occur in the first half of the twenty-first century from tobacco use. Psychopharmacological research on nicotine and tobacco was important in the rationale and development of the treaty. The public health relevance of psychopharmacology research continues to grow with the realization of the potential of nicotine and related drugs to treat or prevent a diverse range of disorders (e.g., Alzheimer's disease, attention deficit hyperactivity disorder, and pain). Although comprehensive review of the research and implications is beyond the scope of this article, the more modest goal of providing insight into the theoretical, clinical, and policy importance of key psychopharmacology research laboratories over the past few decades is attempted.

Cognitive deficits in schizophrenia: focus on neuronal nicotinic acetylcholine receptors and smoking

Patients with schizophrenia present with deficits in specific areas of cognition. These are quantifiable by neuropsychological testing and can be clinically observable as negative signs. Concomitantly, they self-administer nicotine in the form of cigarette smoking. Nicotine dependence is more prevalent in this patient population when compared to other psychiatric conditions or to non-mentally ill people. The target for nicotine is the neuronal nicotinic acetylcholine receptor (nAChR). There is ample evidence that these receptors are involved in normal cognitive operations within the brain. This review describes neuronal nAChR structure and function, focusing on both cholinergic agonist-induced nAChR desensitization and nAChR up-regulation. The several mechanisms proposed for the nAChR up-regulation are examined in detail. Desensitization and up-regulation of nAChRs may be relevant to the physiopathology of schizophrenia. The participation of several subtypes of neuronal nAChRs in the cognitive processing of non-mentally ill persons and schizophrenic patients is reviewed. The role of smoking is then examined as a possible cognitive remediator in this psychiatric condition. Finally, pharmacological strategies focused on neuronal nAChRs are discussed as possible therapeutic avenues that may ameliorate the cognitive deficits of schizophrenia.

Exposure to nicotine and sensitization of nicotine-induced behaviors

Evidence for an important link between sensitization of midbrain dopamine (DA) neuron reactivity and enhanced self-administration of amphetamine and cocaine has been reported. To the extent that exposure to nicotine also sensitizes nucleus accumbens DA reactivity, it is likely that it will also impact subsequent drug taking. It is thus necessary to gain an understanding of the long-term effects of exposure to nicotine on nicotinic acetylcholine receptors (nAChRs), neuronal excitability and behavior. A review of the literature is presented in which different regimens of nicotine exposure are assessed for their effects on upregulation of nAChRs, induction of LTP in interconnected midbrain nuclei and development of long-lasting locomotor and DA sensitization. Exposure to nicotine upregulates nAChRs and nAChR currents and produces LTP of excitatory inputs to midbrain DA neurons. These effects appear in the hours to days following exposure. Exposure to nicotine also leads to long-lasting sensitization of nicotine's nucleus accumbens DA and locomotor activating effects. These effects appear days to weeks after drug exposure. A model is proposed in which nicotine exposure regimens that produce transient nAChR upregulation and LTP consequently produce long-lasting sensitization of midbrain DA neuron reactivity and nicotine-induced behaviors. These neuroadaptations are proposed to constitute critical components of the mechanisms underlying the initiation, maintenance and escalation of drug use.

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.

Pre- and post-nicotine circadian activity rhythms can be differentiated by a paired environmental cue

Previous studies have shown that addictive drugs presented daily at fixed times produce circadian (oscillator-driven) anticipatory and evoked activity rhythms in rats. Other studies have shown that environmental cues paired with addictive drugs produce tolerance to drug effects and elicit craving behavior when presented without the drug. The present study tested these circadian entrainment and paired-cue conditioning effects together. This study compared the ability of daily nicotine and saline injections at different fixed times to entrain pre-injection (anticipatory) and post-injection (evoked) circadian activity rhythms in two groups of female Sprague-Dawley rats. One group (Paired) had an environmental cue (a tone) paired with the effects of the nicotine injection, and the second group (Unpaired) had the tone paired with the effects of the saline injection. The rats were housed singly for 56 days in chambers with attached wheels under constant dim light and rate-limited food access. During three separate injection phases, nicotine and saline were administered daily at different fixed times, and the tone was presented at the second injection time. Three multi-day test phases examined circadian activity (a) without injections or tone, (b) with the tone alone at normal and novel times, and (c) with the tone absent and with injections occurring at normal and at novel times. The results showed that nicotine entrained both pre- and post-injection circadian oscillators, and the nicotine-paired tone interfered with pre-injection anticipatory activity.

Atomoxetine reverses nicotine withdrawal-associated deficits in contextual fear conditioning

Recent evidence suggests that the cognitive symptoms of nicotine withdrawal and the cognitive symptoms of attention deficit hyperactivity disorder (ADHD) may share neural correlates. Thus, therapeutics that ameliorate ADHD symptoms may also ameliorate nicotine-withdrawal symptoms. The present research tested this hypothesis in an animal model of nicotine withdrawal-associated cognitive deficits using atomoxetine, a norepinephrine reuptake inhibitor that is approved by the FDA to treat the symptoms of ADHD. C57BL/6 mice were prepared with osmotic minipumps that administered 6.3 mg/kg/day of nicotine or saline, and the minipumps were removed after 12 days of continuous treatment. Twenty-four hours later, mice were trained in delay fear conditioning using two paired presentations of an auditory conditioned stimulus (CS) with a footshock unconditioned stimulus. Testing for freezing in response to the training context and for freezing in response to the CS occurred the next day. Nicotine-withdrawn mice and their saline-treated counterparts received either saline or atomoxetine before training and the context test. Consistent with previous research, the results indicate that mice withdrawn from chronic nicotine demonstrated lower levels of contextual fear conditioning than mice that were not withdrawn from chronic nicotine. Atomoxetine dose-dependently reversed the deficit, suggesting that nicotine withdrawal may be associated with changes in noradrenergic function, acetylcholinergic function, and/or with changes in cell signaling cascades that are activated by both nicotine and norepinephrine. These data suggest that atomoxetine may be efficacious for treating nicotine withdrawal-associated cognitive deficits that promote relapse in abstinent smokers.

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.

Nicotine psychopharmacology research contributions to United States and global tobacco regulation: a look back and a look forward

Nicotine has a long and storied history in physiology and pharmacology. Historically, it has been used as a tool to explore the nervous system, studied for its role in tobacco use, and more recently examined for its diverse potential medicinal uses. Psychopharmacology research has been pivotal in the science foundation for nicotine and tobacco product regulation.

Tolerance to nicotine in mice lacking alpha7 nicotinic receptors

RATIONALE

Previous studies have suggested that a knockout of the gene coding for alpha7 nicotinic receptor subunits influences the behaviour of undrugged mice but not the acute effect of nicotine on locomotor activity.

OBJECTIVES

The present studies extend these observations to nicotine tolerance assessed by means of schedule-controlled behaviour.

METHODS

Groups of alpha7-/- and alpha7+/+ mice were trained to press levers under an FR20 schedule of food reinforcement. The acute response rate-depressant effects of nicotine were determined in both genotypes and the mice were then subdivided into groups treated daily with nicotine (1.2 mg/kg/day) or saline. After 39 days of exposure to this regimen, the dose-response curves were re-determined.

RESULTS

Knockout of the alpha7 gene had no consistent effect on the lever-pressing behaviour of undrugged mice and did not influence the acute, dose-related, response rate-depressant effect of nicotine (0.2-1.2 mg/kg). When dose-response curves for nicotine (0.4-2.0 mg/kg) were re-determined after daily dosing with the drug, both wild-type and knockout mice developed similar tolerance to nicotine, as shown by approximately 2.5-fold shifts to the right of the dose-response curves.

CONCLUSIONS

Nicotinic receptors containing the alpha7 subunit do not play a significant role in the regulation of the lever-pressing behaviour studied or in the acute behavioural depressant effect of nicotine and the development of tolerance to that effect. Such results contrast with previous reports suggesting profound impairments in sensitivity to nicotine in nicotinic receptor beta2-/- mice.

Binding and functional activity of nicotinic cholinergic receptors in selected rat brain regions are increased following long-term but not short-term nicotine treatment

Chronic nicotine exposure up-regulates neuronal nicotinic receptors, but the functional consequences for these receptors is less well understood. Following 2 weeks of nicotine or saline treatment by osmotic minipump, the functional activity of nicotinic receptors was measured by concentration-response curves for epibatidine-stimulated (86)Rb efflux. Nicotine-treated animals had a significantly higher maximal efflux in cerebral cortex and superior colliculus, but not in thalamus or interpeduncular nucleus plus medial habenula. This increase was confirmed in a separate experiment with stimulation by single concentrations of epibatidine (cortex, superior colliculus) or nicotine (cortex only). Chronic nicotine did not alter (86)Rb efflux stimulated by cytisine, an alpha3beta4-selective agonist, or by potassium chloride, in any region. Short-term (16 h) nicotine exposure caused no changes in either (86)Rb efflux or receptor binding measured with [(3)H]epibatidine. Binding was significantly increased after 2 weeks nicotine exposure in cortex, superior colliculus and thalamus, but not in interpeduncular nucleus plus medial habenula. The increases in epibatidine-stimulated (86)Rb efflux in the four regions tested was linearly correlated with the increases in [(3)H]epibatidine binding in these regions (R(2) = 0.91), suggesting that rat brain receptors up-regulated by chronic nicotine are active. These results have important consequences for understanding nicotinic receptor neurobiology in smokers and users of nicotine replacement therapy.

Mecamylamine but not the alpha7 receptor antagonist alpha-bungarotoxin blocks sensitization to the locomotor stimulant effects of nicotine

The involvement of alpha7 receptors in the locomotor stimulant effects of nicotine has been examined by determining the ability of intracerebroventricular (i.c.v.) administration of the alpha7 receptor antagonist alpha-bungarotoxin (alpha-bgt) to modify sensitization to the locomotor activating effects of chronic nicotine. Intracerebroventricular administration of alpha-bgt (0.02 - 8 nmoles) produced a dose dependent increase in convulsive behaviour. At doses less than 1.0 nmole, minimal convulsive behaviour occurred but larger doses evoked convulsions in all rats which displayed a more rapid onset time as the dose increased. The binding distribution of alpha7 receptors 20 min and 3 h following an i.c.v. administration of [(125)I]-alpha-bgt (0.02 nmoles) revealed clear binding in the hippocampus, cingulate cortex and hypothalamus which was more intense after 3 h. Rats chronically treated with nicotine (0.4 mg kg(-1)) and exposed to the locomotor activity apparatus daily acquired an increase in locomotor activity relative to the control group after 3 days of treatment which reached a maximum after 7 days of treatment and was maintained for the 2 week treatment period. Pre-treatment with mecamylamine (1 mg kg(-1)) prevented the expression of the locomotor stimulant effects of nicotine but pre-treatment with i.c.v. alpha-bgt (0.02 nmoles) did not affect nicotine-induced changes in locomotor activity. The results of this study support the conclusion that nicotinic receptors of the alpha4beta2 subtype rather than the alpha7 subtype are important in mediating the expression of the locomotor stimulant effects of nicotine.

Persistent and delayed behavioral changes after nicotine treatment in adolescent rats

Despite the increasing use of tobacco by adolescents, few animal studies have addressed the neurobehavioral consequences of nicotine exposure during this period. We administered nicotine to adolescent rats via continuous infusion on postnatal days (PN) 30 through 47.5, using a dosage regimen that maintains plasma levels similar to those found in smokers or in users of the transdermal nicotine patch. Behavior in a novel open field and learning a passive avoidance task were assessed during nicotine treatment and for 2 weeks post-treatment. On PN44, during nicotine exposure, female rats showed decreased grooming, an effect not seen in males; this effect is opposite to the effects of nicotine in adult rats. Two weeks after cessation of nicotine administration, females showed deficits in locomotor activity and rearing, whereas males again were unaffected; the behavioral deficits appeared at the same age at which gender-selective brain cell damage emerges. In contrast, nicotine exposure enhanced passive avoidance, with the effect intensifying and persisting throughout the post-treatment period. These results reinforce the concept that developmental vulnerability to nicotine extends into adolescence, with patterns of drug effects different from those in earlier or later periods. The correlation of neurochemical with behavioral effects strengthens the connection between adolescent nicotine exposure and persistent functional changes that may influence drug habituation, learning and memory.

Chronic nicotine administration in the drinking water affects the striatal dopamine in mice

Although tobacco contains a large variety of substances, its addictive properties are most probably due to the reinforcing actions of nicotine that motivates continued tobacco use. Animals and humans self-administer nicotine, a response that appears to involve the mesolimbic dopamine system and to be common to other abused drugs. The present article reviews animal models to administer nicotine chronically. We also describe a new animal model in which nicotine is given to mice in drinking water as their sole source of fluid. This treatment produced nicotine plasma concentrations comparable to or above those found in smokers. We found that mice withdrawn from nicotine were tolerant to the effects of nicotine challenge on striatal dopamine metabolism as well as on body temperature and locomotor activity. Furthermore, 3H-nicotine binding in the cortex and midbrain was significantly increased in mice withdrawn from nicotine. The last part of the article will focus on the effects of this chronic nicotine treatment on striatal dopamine. Dopamine and its metabolites and locomotor activity were increased in the forenoon in mice still drinking nicotine solutions. We also report recent data in which chronic nicotine administration in the drinking water enhanced the effect of dopamine receptor agonist, quinpirole, on striatal metabolism. The animal model described appears to be a relevant method for studying the mechanisms that are thought to be involved in nicotine dependence.

Sex differences in nicotine effects and self-administration: review of human and animal evidence

Although both the human and animal literatures are notable for the general lack of attention paid to possible sex differences in drug self-administration behavior, evidence is accumulating to suggest that males and females may differ in factors that maintain tobacco smoking or nicotine self-administration. Self-administration of nicotine per se may be less robust in women, and women are less sensitive than men to some effects of nicotine that may be reinforcing. Compared to men, smoking behavior of women may be influenced more by non-nicotine stimuli associated with smoking, suggesting greater conditioned reinforcement of smoking in women. Moreover, nicotine replacement, the current standard treatment for smoking cessation, is sometimes less effective in women, further suggesting the need for greater consideration of non-nicotine factors that may maintain women's smoking. Very recent research on rats also indicates sex differences in nicotine self-administration. However, these differences are complex and suggest that nicotine-seeking behavior is composed of several components, including hedonic, incentive-motivational, and conditioning effects; males and females may differ in one or more of these components. Menstrual or estrous cycle phase effects on the maintenance of nicotine self-administration are not particularly apparent in humans or animals, although cycle phase may influence other stages of dependence (e.g., withdrawal symptoms during cessation). Future research should evaluate further the consistency of results across human and non-human species, identify the conditions and procedures under which sex differences are observed, and elucidate the specific components of reinforcement that may differ between males and females. Studies also should examine the possible generalizability of these sex differences to other drugs of abuse. Identification of specific factors responsible for these sex differences may lead to improved interventions for smoking cessation and other substance abuse in women.

Effects of chronic nicotine administration on locomotion depend on rat sex and housing condition

Nicotine's behavioral actions in the human smoker by self-report depend, in part, on the individual's gender and environment. The purpose of the present experiment was to determine whether effects of nicotine on unconditioned behaviors of rats also depend on sex and environmental conditions. Long-Evans rats (96 males and 96 females) living in individual or grouped housing were administered saline or 12 mg/kg/day nicotine via osmotic minipump for 14 days. Horizontal activity (a measure of overall activity and arousal), vertical activity (a measure of exploratory behavior), and center time (a possible index of anxiety) were measured on Day 10 of drug administration and on Day 2 of nicotine cessation. Group housing decreased horizontal and vertical activity and center time, with effects occurring sooner in females. Nicotine's effects depended on housing and sex. For males, nicotine altered indices of arousal and exploration, increasing these variables for group-housed males but decreasing them for individually housed males. For females, nicotine altered possible indices of anxiety, reducing anxiety for group-housed females. In cessation, housing effects continued in females and appeared more robustly in males. Results indicate that nicotine's chronic effects depend on subjects' sex and living environment.

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.

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.

Sex difference in up-regulation of nicotinic acetylcholine receptors in rat brain

This study tested for sex differences in the effects of chronic nicotine administration and withdrawal on nicotinic acetylcholine receptor binding in brain. Rats received nicotine (0.6 mg/kg, s.c.) or saline once daily for 15 days, and were sacrificed 1 or 20 days after termination of treatment. Saturation studies of nAChR binding were performed using [3H]cytisine as the radioligand in whole brain minus cerebellum taken from animals in the chronic treatment groups and from naive rats. Male but not female rats that received chronic nicotine had higher receptor densities than corresponding control groups; up-regulation of nAChR was not seen 20 days after withdrawal. Furthermore, in groups that showed no up-regulation (controls and rats withdrawn for 20 days), nAChR densities were higher in female rats than males. The findings underscore the importance of sex differences in pharmacological responses as well as in basal neurochemical parameters.

Contribution of CNS nicotine metabolites to the neuropharmacological effects of nicotine and tobacco smoking

Nicotine, the principal alkaloid in tobacco products, is generally accepted to be the active pharmacological agent responsible for CNS effects resulting from tobacco use. Arguments are presented in this commentary which take issue with this popular dogma, by providing evidence that nicotine metabolites may also be responsible for the CNS effects commonly attributed to nicotine. CNS effects attributed to nicotine include reinforcing effects, mood elevation, arousal, locomotor stimulant effects, and learning and memory enhancement. The reinforcing and locomotor stimulant effects of nicotine have been suggested to be the result of activation of CNS dopaminergic systems, and nicotine-induced modulation of dopaminergic neurotransmission has been studied in detail. Nicotine acts at a family of nicotinic receptor subtypes composed of multiple subunits; however, the exact composition of the subunits in native nicotinic receptors and the functional significance of the receptor subtype diversity are currently unknown. This nicotinic subtype diversity increases the complexity of the potential mechanisms of action of nicotine and its metabolites. Although peripheral metabolism of nicotine has been studied extensively, metabolism in the CNS has not been investigated to any great extent. Recently, studies from our laboratory have demonstrated that several nicotine metabolites are present in the CNS after acute nicotine administration. Moreover, nicotine metabolites are pharmacologically active in neurochemical and behavioral assays. Thus, CNS effects resulting from nicotine exposure may not be due solely to nicotine, but may result, at least in part, from the actions of nicotine metabolites.

A proposed test battery and constellations of specific behavioral paradigms to investigate the behavioral phenotypes of transgenic and knockout mice

Behavioral phenotyping of transgenic and knockout mice requires rigorous, formal analyses. Well-characterized paradigms can be chosen from the established behavioral neuroscience literature. This review describes (1) a series of neurological and neuropsychological tests which are effectively used as a first screen for behavioral abnormalities in mutant mice, and (2) a series of specific behavioral paradigms, clustered by category. Included are multiple paradigms for each category, including learning and memory, feeding, analgesia, aggression, anxiety, depression, schizophrenia, and drug abuse models. Examples are given from the experiences of the authors, in applying these experimental designs to transgenic and knockout mice. Extensive references for each behavioral paradigm are provided, to allow new investigators to access the relevant literature on behavioral methodology.

Tolerance to nicotine's effects on striatal dopamine metabolism in nicotine-withdrawn mice

After 7-week chronic administration of nicotine to mice in their drinking water, nicotine was withdrawn for 24 h. Acute nicotine challenge (1 mg/kg s.c., 60 min) elevated the striatal concentrations of dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) and decreased the concentration of 3-methoxytyramine significantly less in the mice withdrawn for 24 h from nicotine than in the control mice which had been drinking tap water under identical conditions for 7 weeks. Neither withdrawal nor the acute nicotine challenge altered the striatal dopamine concentration. No alterations were found in the density or affinity of the specific binding of [3H]SCH 23390 or [3H]spiperone to striatal membrane homogenates during nicotine treatment or after its withdrawal. Thus, our results show that tolerance to the acute effects of nicotine on striatal dopamine metabolism can be induced by administering nicotine to mice in the drinking water. However, neither chronic nicotine treatment nor its withdrawal seem to affect dopamine D1 and D2 receptors in the striatum.

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

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

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.

Individual variability in responses to nicotine

Individual variability in acute responses to nicotine, which may be defined as variable magnitude of effects following controlled dosing, is generally attributed to stable characteristics of tobacco users such as genetic/constitutional factors or to chronic behavioral factors (e.g., long-term use of other drugs). Often overlooked, however, is that such variability may also be due to the transient influence of the situational factors in which people consume nicotine, such as acute stress or physical activity. Results of selected studies from the author's laboratory provide examples of each of these sources of variability in nicotine responding on subjective, behavioral, and physiological measures. All studies used a nasal spray method of nicotine dosing or controlled smoking (paced puffing) to control acute nicotine exposure, an essential methodological feature of any research on individual differences in acute responses to nicotine. As an example of genetic/constitutional factors, gender differences in nicotine responding have begun to receive some attention, with few differences emerging. However, females may be more responsive than males to nonnicotine stimuli associated with smoking (e.g., sight and taste of smoke). In terms of chronic behavioral factors, long-term use of nicotine produces attenuation of most subjective and some behavioral effects of nicotine, reflecting chronic tolerance, and the possibility that chronic use of other drugs may alter responses to nicotine (i.e., cross-tolerance or cross-sensitization) deserves greater study. Of particular emphasis in this review is the modulating influence of acute situational factors on nicotine responding. Human studies have shown that magnitude of nicotine's subjective effects may depend on the predrug subjective state, level of physical activity vs. rest, and concurrent acute intake of other drugs, among other situational factors. Proper consideration of these situational factors may reveal the greatest source of individual variability in responding to nicotine and clarify the impact of more stable genetic/constitutional or chronic environmental factors.

Characteristics of tail-tremor induced by nicotine in rats

To characterize the tail-tremor and locomotor hyperactivity induced by repeated nicotine administration, the effects of nicotinic, alpha-adrenergic and dopaminergic blockers were investigated in rats. Daily administration of nicotine (0.5 mg/kg, s.c.) induced tail-tremor from the 4th day, which became more marked in intensity by subsequent administration. Locomotor hyperactivity was also induced by nicotine, which was enhanced by daily administration. The tail-tremor and locomotor hyperactivity induced by repeated nicotine administration were inhibited by mecamylamine (0.1-1 mg/kg, i.p.) but not by hexamethonium (0.5 and 1 mg/kg, i.p.). Clonidine (0.02 and 0.04 mg/kg, i.p.) and prazosin (0.5 and 1 mg/kg, i.p.) reduced tail-tremor more markedly than hyperactivity. However, haloperidol (0.05-0.2 mg/kg, i.p.) and chlorpromazine (1-5 mg/kg, i.p.) reduced hyperactivity more markedly than tail-tremor. These results suggest that nicotine-induced tail-tremor and hyperactivity are due to an increased susceptibility of central nicotinic receptors of nicotine followed by catecholaminergic mechanisms, and that tail-tremor may be more associated with the noradrenergic system than the dopaminergic system.

An analysis of response to nicotine infusion using an automated radiotelemetry system

Previous studies from our laboratory have demonstrated that chronic nicotine infusion evokes tolerance to nicotine injected IP several hours after withdrawal from chronic infusion. This method may introduce problems related to withdrawal reactions and to stress associated with handling of the animals. The studies reported here measured tolerance to nicotine in mice using an automated radiotelemetry system. DBA/2 mice were infused intravenously with saline for 4 days followed by infusion of a 4 mg/kg per h dose of nicotine for 7 days. After the nicotine treatment, the mice were infused with saline for 7 days. The nicotine was infused continuously or in four 1 mg/kg pulses, two 2 mg/kg pulses or one 4 mg/kg pulse each hour. Home cage activity and body temperature were measured throughout the treatment periods using a radiotelemetry system. Nicotine infusion produced an abrupt decrease in body temperature and activity, but this effect was totally reversed within 12 h in the continuously infused and four infusions/h treatment groups. Mice that received one or two infusions/h also showed a rapid response to nicotine that was reversed as treatment proceeded, but nicotine continued to produce a measurable effect for several days. After nicotine withdrawal, temperature and activity returned to predrug infusion values in all of the groups except those infused once per hour. This group showed depressed activity for a minimum of 3 days after nicotine treatment stopped. Thus, the kinetics of nicotine administration affected the intensity of response during continued treatment as well as activity after cessation of chronic treatment.

Absence of appreciable tolerance and morphine cross-tolerance in rats with adrenal medullary transplants in the spinal cord

Adrenal medullary transplants in the spinal subarachnoid space may be a means of achieving sustained local delivery of pain-reducing neuroactive substances on a continually renewable basis. However, a potential limitation of this approach is tolerance development to agents released from the transplanted cells. In particular, since adrenal medullary chromaffin cells release opioid peptides, reduced antinociceptive efficacy of opioids is possible. To determine this, alterations in the dose-effectiveness of morphine were assessed in animals with adrenal medullary transplants. Results indicated that, not only was there no apparent tolerance, but that adrenal medullary transplants could potentiate the analgesic efficacy of morphine. An additional goal of these studies was to determine whether chronic or intermittent nicotine could produce increased antinociception, since stimulation of cell surface nicotinic receptors increases release of neuroactive substances from chromaffin cells. This was assessed using subcutaneously implanted nicotine pellets or repeated systemic administration of nicotine. Findings indicated that exposure to nicotine results in an acute tolerance, or tachyphylaxis, to nicotine which is rapidly reversed following cessation of nicotinic stimulation. Together, these results suggest that adrenal medullary transplants may provide a constant source of opioid peptides, augmentable by intermittent nicotinic stimulation, without the development of appreciable tolerance to these pain-reducing neuroactive substances.

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)

Tolerance to the convulsions induced by daily nicotine treatment in rats

Development of tolerance to the nicotine-induced convulsions in rats was examined. Acute intraperitoneal (i.p.) administration of nicotine (2.5, 3.75 and 5 mg/kg) produced convulsions in a dose-dependent manner. Mecamylamine (1 mg/kg, i.p.) antagonized the convulsions, but hexamethonium (5 mg/kg, i.p.) did not modify them. Daily nicotine administration (2.5, 3.75 and 5 mg/kg, i.p.) once a day for 6 days developed tolerance to the convulsions induced by nicotine. After the daily administrations of nicotine for 6 days, the effects of a challenge administration of nicotine (2 mg/kg) on the nicotine-induced convulsions were tested on the 7th-day. Further tolerances were also developed by the 7th-day challenge administration. After the 7th-day test, nicotine levels of the brain and blood 15 min after the challenge injection were measured. With nicotine (5 mg/kg once a day)-treatment, nicotine levels of all the brain regions were increased. In contrast, a similar challenge injection had no effect on blood nicotine level. These results indicate that the development of tolerance to the nicotine-induced convulsions is produced relatively earlier and day by day by daily administrations to rats, which is closely related with the increase in brain nicotine level.

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.

Nicotine-induced sensitization to ambulatory stimulant effect produced by daily administration into the ventral tegmental area and the nucleus accumbens in rats

Bilateral injections of nicotine (30 micrograms/side) into the ventral tegmental area (VTA) and the nucleus accumbens (NACC) increased the ambulatory activity in rats. Moreover, daily injections of nicotine (10, 20 and 30 micrograms/side) into the VTA and the NACC for 6 successive days produced sensitization to the ambulatory stimulant effect of nicotine. Sensitization produced by daily injections of nicotine (20 micrograms/side) into both the sites was maintained for withdrawal periods of 10 days. Mecamylamine (2 mg/kg, i.p.), SCH23390 (0.05 mg/kg, i.p.) and spiperone (0.1 mg/kg, i.p.) antagonized nicotine-induced sensitization to the ambulatory stimulant nicotine-induced sensitization to the ambulatory stimulant effect produced by daily injections into the VTA. These results suggest that nicotine-induced sensitization to the ambulatory stimulant effect involves the stimulation of the mesolimbic dopaminergic pathway through the nicotinic acetylcholine receptor (nAChR) in the VTA and the NACC.

Kinetics of drug action in disease states. XXXVIII: Effect of body temperature on the convulsant activity of pentylenetetrazol in rats

The purpose of this investigation was to determine the effect of body temperature on the pharmacodynamics (convulsant activity) of pentylenetetrazol (PTZ). Rats received an iv infusion of PTZ until the onset of maximal seizures, at which time samples of cerebrospinal fluid (CSF), brain, and blood (for serum) were obtained for subsequent determination of PTZ concentrations by HPLC. The PTZ infusion caused a decrease in body temperature of approximately 4 degrees C within 20 min and onset of seizures in approximately 40 min. Compared with animals whose temperature was maintained in the normal range by heating pads, the hypothermic rats required significantly larger doses and higher serum, brain, and CSF concentrations of PTZ to produce seizures. Other rats received an injection of brewer's yeast to produce fever. Then, PTZ was infused 6, 12, or 24 h later when body temperature was elevated by an average of 1.3, 1.1, or 0.4 degrees C, respectively. Compared with control rats, whose temperature was maintained in the normal range by heating pads, moderate hyperthermia had no significant effect on the dose and concentrations of PTZ required to produce maximum seizures. Pentylenetetrazol exemplifies a drug that can produce hypothermia which, in turn, reduces the sensitivity of rats to its pharmacologic action. Unlike the central nervous system (CNS) depressants phenobarbital and ethanol, whose pharmacologic activity in rats is enhanced at elevated body temperature, the activity of the CNS stimulant PTZ is apparently not altered by fever.

Effects of nicotine withdrawal on the local cerebral glucose utilization in conscious rats

Chronic infusion of nicotine has been shown to result (1) in an increase in nicotine binding sites in the brain and (2) in a distinct pattern of increases in local cerebral metabolic rate for glucose (CMRglc). The present study addresses two questions: (1) whether a one-day withdrawal of nicotine after a two-week exposure is long enough to restore local CMRglc to the preinfusion values and (2) whether an acute nicotine infusion after one day's withdrawal would influence local CMRglc. Chronic infusion of L-nicotine (12.5 micrograms/kg/min) was performed by osmotic minipumps. Local CMRglc was measured using the quantitative 2-deoxyglucose method in conscious rats. The following results were obtained: (1) a one-day withdrawal of nicotine after a two-week chronic infusion restores local CMRglc to a pattern which is close to the control pattern obtained without any nicotine infusion, and (2) an acute infusion of nicotine after a one-day withdrawal of a chronic nicotine infusion induces distinct increases in local CMRglc of several brain structures; these are essentially identical with structures which are activated during an acute nicotine infusion in otherwise untreated rats (no chronic infusion). The data indicate: (1) The main effects of chronic nicotine infusion on local CMRglc have disappeared after one day of nicotine withdrawal. (2) An acute load of nicotine in such nicotine-withdrawal rats has effects on local CMRglc which resemble those found in previously untreated rats during an acute nicotine infusion. (3) In contrast to most binding studies which have shown persisting increases in nicotine binding sites after one day's withdrawal of chronic nicotine, local CMRglc is restored to control values and can be again activated by an acute nicotine infusion.

Dissociation of the apparent relationship between nicotine tolerance and up-regulation of nicotinic receptors

Saline or nicotine (0.8 mg/kg/hr) was administered chronically to rats via subcutaneous infusion for 7 days. After chronic treatment, the effects of a challenge dose of nicotine, injected subcutaneously, on open-field activity and body temperature were measured. Nicotine responses were measured 2 hours after infusion had been stopped (day 0) or 1, 2, 4, 8, or 21 days after cessation of treatment. Brain nicotinic receptors were measured using nicotine binding or bungarotoxin binding in six brain regions. Tolerance to nicotine's effects on open-field activity and body temperature were observed; this tolerance was lost before the fourth day of withdrawal. Both nicotine and BTX binding were increased after chronic nicotine treatment. The return of the receptors to control values was dependent on brain region, but in general receptors were still increased at 4 days after withdrawal. The half-lives for receptors returning to normal were greater for nicotine binding compared to BTX binding. These results suggest that a relatively short-lived tolerance results from chronic nicotine infusion and that alterations in nicotinic receptors can be dissociated from behavioral tolerance.

Desensitization of central cholinergic mechanisms and neuroadaptation to nicotine

This review focuses on neuroadaptation to nicotine. The first part of the paper delineates some possible general mechanisms subserving neuroadaptation to commonly abused drugs. The postulated role of the mesocorticolimbic neuroanatomical pathway and drug-receptor desensitization mechanisms in the establishment of tolerance to, dependence on, and withdrawal from psychoactive drugs are discussed. The second part of the review deals with the pharmacological effects of nicotine at both pre- and postsynaptic locations within the central nervous system, and the still-perplexing upregulation of brain nicotine-binding sites seen after chronic nicotine administration. A special emphasis has been put on desensitization of presynaptic cholinergic mechanisms, and postsynaptic neuronal nicotinic-receptor function and its modulation by endogenous substances. A comparison with the inactivation process occurring at peripheral nicotinic receptors is also included. Finally, a hypothesis on the possible connections between desensitization of central cholinergic mechanisms and neuroadaptation to nicotine is advanced. A brief comment on the necessity of fully understanding the effects of nicotine on the developing nervous system closes this work.

Chronic ethanol or nicotine treatment results in partial cross-tolerance between these agents

Female DBA/2Ibg mice were treated chronically (21 days) with ethanol- or dextrin-containing liquid diets or infused chronically with nicotine (8 mg/kg/h) or saline for 10 days. The responses of these animals to challenge doses of ethanol (2.5 g/kg) or nicotine (1 or 2 mg/kg) were measured using a test battery consisting of respiration rate, acoustic startle response, Y-maze crosses and rears, heart rate and body temperature. Chronic ethanol-treated animals were tolerant to the effects elicited by a challenge dose of ethanol on four of the six measures and were cross-tolerant to nicotine's effects on the acoustic startle test. Chronic nicotine-treated animals were tolerant to nicotine's effects on five of the six measures and cross-tolerant to ethanol's effects on heart rate and body temperature. Thus, partial cross-tolerance between ethanol and nicotine exists. Chronic nicotine treatment resulted in significant increases in L-[3H]-nicotine binding in six of seven brain regions and in alpha-[125I]-bungarotoxin binding in three of seven brain regions. Chronic ethanol treatment failed to alter the binding of either ligand. Therefore, the cross-tolerance that develops between ethanol and nicotine is not totally dependent on alterations in the number of brain nicotinic receptors.