Acute tolerance to the locomotor stimulant effects of nicotine in the rat

Developmental alterations in locomotor and anxiety-like behavior as a function of D1 and D2 mRNA expression

The majority of smokers start smoking in adolescence, beginning a potentially lifelong struggle with nicotine use and abuse. In rodent models of the effects of nicotine, the drug has been shown to elicit both locomotor and anxiety-like behavioral effects. Research suggests that these behavioral effects may be due in part to dopamine (DA) receptors D1 and D2 in the mesolimbic system, specifically the nucleus accumbens (NAc). We examined early adolescent (P28), late adolescent (P45), and adult (P80) male Long-Evans rats in the elevated plus maze (EPM) under normal conditions and the open field (OF) post-nicotine in order to test locomotor and anxiety-like behavior. These behavioral findings were then correlated with expression of DA D1 and D2 mRNA levels as determined via in situ hybridization. Nicotine-induced locomotor behavior was found to be significantly different between age groups. After a single injection of nicotine, early adolescents exhibited increases in locomotor behavior, whereas both late adolescents and adults responded with decreases in locomotor behavior. In addition, it was found that among, early adolescents, open arm and center time in the EPM were negatively correlated with D2 mRNA expression. In contrast, among adults, distance traveled in the center and center time in the OF were negatively correlated with D2 mRNA expression. This study suggests that DA D2 receptors play a role in anxiety-like behavior and that the relationship between observed anxiety-like behaviors and D2 receptor expression changes through the lifespan.

Translating laboratory discovery to the clinic: from nicotine and mecamylamine to Tourette's, depression, and beyond

The early development of novel nicotinic drugs for Tourette's and depression was a very long journey in discovery, which began with basic behavioral neuroscience studies aimed at understanding how cholinergic and dopaminergic systems interact in the basal ganglia to control goal directed movement. These early rodent studies with nicotine and dopamine antagonists formed the basis for investigating a potentially improved treatment for children suffering from Tourette's syndrome (TS). Clinically, the research trajectory first focused on studies employing the use of nicotine gum to potentiate the therapeutic effect of the dopamine receptor antagonist, haloperidol, in patients with TS. These projects led to the discovery of a new use for a decades-old blood pressure medication, mecamylamine, a nicotine antagonist, which also appeared to provide symptomatic relief in some TS patients when used clinically and was found to reduce symptoms of mood instability and depression. This unexpected discovery led to a new hypothesis regarding the mechanism of action of antidepressants as well as a series of successful independent trials employing mecamylamine, and its active enantiomer, TC5214, as an augmenting agent in the treatment of major depression. This article is a chronological mini review of these basic and clinical translational studies on nicotinic therapeutics for Tourette's syndrome and depression over the past 25 years.

Nicotine sensitization of monkey striatal dopamine release

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

Tobacco/nicotine and endogenous brain opioids

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

Nicotine induced behavioral locomotor sensitization

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

Neurotransmitter system interactions revealed by drug-induced changes in motivated behavior

The present article reviews studies conducted either in collaboration with Jac Herberg, or in parallel with those studies that used consummatory behavior and responding for intracranial self-stimulation (ICSS) to investigate interactions between neurotransmitter systems. The studies reviewed include investigations of the role of dopamine in 8-OH-DPAT-induced feeding; the role of 5-HT3 receptors in the stimulant and depressant effects of nicotine on responding for ICSS; the interaction of D2 and 5-HT2 antagonists in sucrose consumption, and the differential contributions of alpha2-adrenoceptor and 5-HT2 antagonism to the rapid recovery of ICSS responding from depression produced by atypical neuroleptics. Further studies of the role of alpha2-adrenoceptor antagonism in the pattern of response decrements produced by neuroleptics on schedule-controlled responding for food confirm that the behavioral effects of monoamine interactions vary, depending on the specific receptor subtypes targeted and the behavioral paradigm employed. Consequently, the clinical relevance of findings will crucially depend on the choice of appropriate behavioral measures.

Ibogaine and the dopaminergic response to nicotine

There is increasing evidence that the rewarding effect of nicotine is mediated by the mesolimbic dopamine system. The first objective of this study was to examine the dopamine response to repeated i.v. infusions of nicotine. Using in vivo microdialysis in awake and freely moving male Sprague-Dawley rats, we demonstrated that i.v. nicotine infusions (0.16 mg/kg or 0.32 mg/kg per infusion) produced increases in extracellular dopamine levels that were dose- and infusion order-dependent. Acute tolerance was evidenced by the smaller dopamine response produced by a second infusion of nicotine, administered 1 h after the first one. Tolerance was reversible, since the dopamine response to a second infusion of nicotine was unchanged when the interval between the infusions was increased to 3 h. Ibogaine, an alkaloid found in Tabernanthe iboga, is claimed to decrease smoking and to have an anti-nicotinic action. The second objective of this study was to establish whether this claim has any neurochemical basis. Pretreatment with ibogaine (40 mg/kg, i.p.) 19 h prior to the first nicotine infusion (0.32 mg/kg per infusion) significantly attenuated the increase in extracellular dopamine levels induced by-the nicotine infusions, suggesting that ibogaine may decrease the rewarding effect of nicotine.

Nicotine for the treatment of Tourette's syndrome

Recent evidence has demonstrated that nicotine may obtund the symptoms of Tourette's syndrome (TS). TS is a neuropsychiatric disorder characterized by motor and vocal tics, obsessions and compulsions, and frequently with impulsivity, distractibility, and visual-motor deficits. While neuroleptics, such as haloperidol, are most effective for treatment of the motor and vocal tics of TS, these medications have many side effects. In this article, we review the evidence, consistent with findings in animals, that administration of nicotine (either 2 mg nicotine gum or 7 mg transdermal nicotine patch) potentiates the therapeutic properties of neuroleptics in treating TS patients and that a single patch may be effective for a variable number of days. These findings suggest that transdermal nicotine could serve as an effective adjunct to neuroleptic therapy for TS.

Locomotor activity after nicotine infusions into the fourth ventricle of rats

Microinjections of nicotine into the fourth ventricle of rats were reported previously to produce a characteristic prostration syndrome; similar microinjections have been investigated for effects on locomotor activity. It was confirmed that nicotine (4 micrograms) administered into the fourth ventricle of rats produced prostration which was also manifested on a second challenge with the drug. Increasing doses of nicotine produced increasing magnitudes of prostration and dose-related decreases in locomotor activity. In rats pretreated with nicotine (0.4 mg/kg SC) for 10 days, no tolerance was seen to either the prostration response or the locomotor depression. Mecamylamine (1.0 mg/kg SC) completely prevented the prostration response produced by 4 micrograms of nicotine, but the locomotor depression was still evident. The locomotor changes following intraventricular administration of nicotine appeared to be different from the locomotor depression seen following systemic administration because the posture of the animals was different and the latter effects showed tolerance with repeated exposures to nicotine and were fully blocked by mecamylamine. These findings suggested that the prostration response and the locomotor depression were mediated by different mechanisms.

Evidence that nicotine can acutely desensitize central nicotinic acetylcholinergic receptors

Current concepts concerning nicotine's CNS mechanism(s) of action suggest that this drug produces its effects via an interaction at nicotinic-cholinergic receptors (nAChRs) sensitive to acetylcholine. In vitro research further suggests that, following its initial agonist effect, this cholinergic drug may also induce a rapid desensitization of the nAChR similar to that of acetylcholine, resulting in termination of its pharmacological effect. Research described in this paper provides evidence of this secondary desensitization process in vivo by demonstrating nicotine's ability to induce acute tolerance in a Discriminative Stimulus (DS) paradigm. The ability of nicotine (400 micrograms/kg, SC) to elicit DS control of behavior in a two-lever operant procedure was significantly reduced via a challenge dose (800 micrograms/kg, SC) of nicotine administered 15-180 min before the training dose. Twenty-three of 52 rats demonstrated this phenomenon. The time to develop acute tolerance varied, providing additional evidence that these effects may be contingent upon individual rat variability. In addition, physostigmine was also observed to induce a similar desensitization in a random population of desensitizing rats. Lastly, there were no differences between desensitizers and non-desensitizers in relation to the ability of mecamylamine (1000 micrograms/kg, SC) to antagonize the DS, while in both populations of rats scopolamine (100 micrograms/kg, SC) failed to antagonize the DS.