Nicotine injections as the conditioned stimulus in discrimination learning

Summary Rats were trained to respond for water rewards on different bars in a Skinner box depending on whether they had previously been injected with nicotine or with saline. No other drug tested could consistently elicit responses on the nicotine correct bar. Pre-treatment with mecamylamine abolished the rats' ability to distinguish between nicotine and saline but pretreatment with chlorisondamine did not.     

Nicotine as a discriminative stimulus in rats depleted of norepinephrine or 5-hydroxytryptamine

Rats were trained to make a specific behavioral response in a T-maze apparatus conditional upon whether they were injected with 0.4 mg/kg nicotine or saline. Depletion of brain levels of 5-hydroxytryptamine by orally administered para-chlorophenylalanine had no significant effect on the rats' ability to discriminate nicotine. However, both insoluble alpha-methyl-p-tyrosine and its ester, at intraperitoneal doses of 90 and 135 mg/kg, respectively, significantly decreased discrimination of nicotine at 180 and 270 min post-administration. At these doses and times, saline discrimination was not altered. The experimental evidence indicates that nicotine's CNS cueing effect is mediated by norepinephrine, and this is discussed in light of the Burn and Rand hypothesis. It is proposed that, nicotine may act on a specific nicotine-sensitive cholinergic receptor in the CNS, which causes release of norepinephrine which, in turn, produces the interoceptive cueing effect that enables the rats to make the appropriate behavioral response.Supported by grants from the A. M. A. Education and Research Foundation.Supported by grants from the A. M. A. Education and Research Foundation.     

Diprenorphine as a stimulus in drug discrimination learning.

Using the conditioned taste aversion baseline of drug discrimination learning, animals were trained to discriminate diprenorphine from distilled water. In subsequent generalization tests, the opiate antagonists naltrexone and naloxone and the mixed opiate agonist/antagonist nalorphine substituted for the diprenorphine stimulus in a dose-dependent manner, while the opiate agonist morphine and the nonopiate pentobarbital failed to substitute even at the highest doses tested. That a range of opiate antagonists substituted for the diprenorphine stimulus (and an opiate agonist and a nonopiate failed to substitute) suggest that diprenorphine's antagonist properties may mediate the discrimination, presumably by blocking endogenous opiate activity. The ability of these drugs to substitute for the diprenorphine stimulus may also be a function of this receptor activity. The differences in the specific generalization patterns reported in the present assessment and those of earlier reports were discussed.     

Drug trace discrimination with nicotine and morphine in rats

In typical drug discrimination experiments, subjects are exposed to psychoactive substances both prior to and during training sessions. The present experiments aimed to determine whether pre-session effects of drugs could serve as discriminative stimuli. Rats were trained in a two-lever discrimination procedure with food reinforcers presented on a tandem variable interval-fixed ratio (VI-FR) schedule. Injections of nicotine (0.6mg/kg 20 min pre-session) or saline were followed by administration of the nicotine antagonist mecamylamine (1.0 mg/kg 10 min pre-session) to block effects of nicotine during training sessions. Similarly, the action of morphine (10 mg/kg 30 min pre-session) was terminated by administering naloxone (0.1 mg/kg 10 min pre-session). These drug discriminations were acquired slowly to an accuracy of only 70-75% (n=10-12). Extinction tests confirmed stimulus control by nicotine in the presence of mecamylamine and by morphine in the presence of naloxone. The antagonists attenuated the response-rate reducing effects of the training doses of their respective agonists. The results are interpreted in terms of stimulus control by pre-session effects of the training drugs, but other explanations are considered. Stimulus control by pre-session drug states may be weak due to the time elapsed between termination of drug effects and training (trace conditioning).     

Nicotine does not interact with the discriminative stimulus effects of clozapine in rats

The purpose of the present study was to assess the potential interaction between clozapine, an atypical antipsychotic recently approved for clinical use in the United States and nicotine. Male Sprague-Dawley rats were trained to discriminate nicotine or clozapine in a standard two-lever operant drug discrimination procedure. Rats were tested for generalization to nicotine and clozapine, and the interactions produced by combining various doses of nicotine and clozapine were evaluated. Results suggest that nicotine does not interact with the discriminative stimulus effects of clozapine. Thus, patients who smoke while receiving clozapine therapy may not experience the decreased antipsychotic effectiveness which is common in patients who smoke while receiving treatment with typical antipsychotic compounds. © 1992 Wiley-Liss, Inc.     

Rolipram forms a potent discriminative stimulus in drug discrimination experiments in rats

Long Evans rats were trained to discriminate 0.2 mg/kg IP (±)-rolipram from vehicle injection in a food-motivated two-lever operant task. Eight out of nine rats acquired the discrimination after an average of 91 sessions (min 65, max 137). The ED₅₀ of (±)-rolipram was 0.06 mg/kg IP. Generalization tests with (–)- and (+)-rolipram showed that the (–)-isomer was 8 times more active than (+)-rolipram with an ED₅₀ of 0.06 and 0.4 mg/kg IP respectively. The phosphodiesterase inhibitor RO 20-1724 partially (83%) generalized to (±)-rolipram in doses of 0.6 and 1.0 mg/kg IP. IBMX 5 mg/kg IP showed 63% generalization. Tests with imipramine and the (+)- and (–)-isomer of the noradrenaline uptake inhibitor oxaprotiline suggest that NA-uptake inhibiting drugs do not form an interoceptive cue which is (±)-rolipram-like. dbcAMP 12.5 mg/kg SC and 100 mg/kg SC dbcGMP did not generalize to the training drug. The nature of the discriminative stimulus produced by this dose of (±)-rolipram in rats remains to be elucidated. Offprint requests to: R. Ortman     

Nicotine discrimination and self-administration in humans as a function of smoking status

Nicotine’s discriminative stimulus effects may be critical to understanding reinforcement of tobacco smoking. It is not known whether regular nicotine exposure produces tolerance or sensitivity to these effects. In this study, male and female smokers (n = 11) and never-smokers (n = 10) were trained to discriminate 20 μg/kg nicotine by nasal spray from placebo (0) on day 1. On day 2, both groups were tested on generalization of this discrimination across intermittent presentations of 0, 3, 6, 12, and 20 μg/kg nicotine in random order. Quantitative and quantal behavioral discrimination tasks, used in previous research, were employed. On day 3, subjects were instructed to self-administer sprays from the 20 μg/kg nicotine versus 0 bottles in a concurrent-choice procedure. All but one subject (female smoker) learned reliably to discriminate 20 μg/kg nicotine from placebo (≥ 80% correct) on day 1. Nicotine-appropriate responding on day 2 was attenuated in smokers versus never-smokers at 20 μg/kg on the quantitative task and at 12 μg/kg on the quantal task, suggesting tolerance. There was no difference in responding at other doses. Smokers also showed attenuated responses on the subjective measure of “head rush”, which was associated with discrimination responding in both groups. Nicotine self-administration was significantly greater in smokers versus never-smokers, who self-administered nicotine below chance levels, and was inversely related to discrimination behavior in never-smokers but unrelated in smokers. Women smokers showed less change in nicotine-appropriate responding across generalization doses, reported less confidence in discriminating training doses during acquisition on day 1, and tended to self-administer less nicotine on day 3. These results indicate that smokers may become tolerant to the discriminative stimulus effects of nicotine, perhaps promoting increased use. Received: 1 October 1996/Final version: 28 January 1997     

Discriminative stimulus effects of ethanol in rats using a three-choice ethanol-midazolam-water discrimination

Three-choice discrimination procedures are used to characterize how similar the discriminative stimulus effects of two drugs are in relation to each other. This procedure has suggested similarities between ethanol and ligands that positively modulate the gamma-aminobutyric acid type A (GABAA) receptor complex. As an extension to these studies, male Long-Evans rats were trained to discriminate midazolam (3.0 mg/kg, i.p.) from ethanol (1.0 g/kg, i.g.) from water (2.3 ml, i.g.) in a three-lever, food reinforced task. Substitution tests were conducted following administration of GABAA-positive modulators, noncompetitive N-methyl-D-aspartate (NMDA) antagonists, 5-HT1B agonists and isopropanol. Among the GABAA-positive modulators, diazepam was the only drug that completely substituted for midazolam; both pentobarbital and the neurosteroid allopregnanolone showed partial midazolam substitution. The NMDA antagonist dizocilpine substituted for ethanol, while phencyclidine showed no substitution for either ethanol or midazolam. The serotonin agonists tested also showed no substitution for either ethanol or midazolam. Isopropanol was the only other drug that completely substituted for ethanol. These data extend previous findings from an ethanol-pentobarbital-water discrimination and further define training conditions that result in a conditional basis for the ethanol discrimination where only those drugs with pharmacological heterogeneous effects similar to ethanol produce a full ethanol-like effect.     

Testosterone as a discriminative stimulus in male rats

Testosterone and other anabolic-androgenic steroids (AAS) are reinforcing in animals, as determined by conditioned place preference or self-administration. Most drugs of abuse produce subjective effects on mood and perception that initiate and maintain drug taking. Whether AAS have similar effects is not known. Food-restricted male Sprague-Dawley rats (n = 9) were tested for their ability to discriminate an injection of testosterone from the β-cyclodextrin vehicle using a standard two-lever operant paradigm. In drug discrimination, animals use the subjective effects of drug or vehicle to select the appropriate lever to obtain food pellets under an FR10 schedule of reinforcement. All rats demonstrated vigorous responding for food (1415.1 ± 76.1 responses/20 min) with 94.9% of responses on the active lever. For the first 30 days, rats received 1 mg/kg testosterone sc 30 min before testing. On Day 14, one rat achieved the discrimination criteria of 9/10 consecutive days with > 90% responses on the active lever and ≤ 5 responses on the inactive lever before the first reinforcement. Subsequently, rats were tested with testosterone at different doses (2, 7.5, 15 mg/kg at 30 min before testing) and times (2 mg/kg at 30 or 60 min before testing), each for 20 days. One additional rat demonstrated successful discrimination at Day 54 with 2 mg/kg testosterone 60 min before testing. The remaining 7 rats failed to discriminate testosterone within 110 days. When analyzed according to less-stringent standards, 4 additional rats met criteria for testosterone discrimination. However, continued performance was not stable. Thus, testosterone was unable to consistently support drug discrimination. We conclude that testosterone does not produce rapid interoceptive effects (NIH DA12843 to RIW).     

Discriminative stimulus effects of midazolam and abecarnil in rats treated chronically with diazepam or abecarnil

Abecarnil (ABC) is a beta-carboline that acts as an agonist at benzodiazepine (BZD) receptors. It possesses anxiolytic and anticonvulsant properties, but produces little sedation and is without muscle relaxant effects. To explain this unusual profile of activity, two hypotheses have been advanced: either 1) ABC acts as a partial agonist or 2) ABC acts as a full agonist, but only at a sub-population of BZD receptors. The present experiment used cross-tolerance profiles between BZDs and ABC to differentiate these hypotheses based upon predictions of receptor theory: tolerance produced to a full agonist should confer even greater cross-tolerance to a partial agonist. Rats were trained in a three-choice drug discrimination procedure to detect the benzodiazepine, midazolam (MDZ, 1.0 mg/kg) from pentylenetetrazole (PTZ, 20 mg/kg) from saline. Tested acutely, MDZ and ABC substituted for MDZ with similar potencies. Following chronic treatment with the BZD-agonist diazepam (DZP; 20 mg/kg per 8 h for 7 days), both the MDZ and ABC dose-effect curves were significantly shifted to the right, and both drugs showed a comparable three-fold decrease in potency. The chronic administration of ABC (4.0 mg/kg per 8 h for 7 days) produced a different spectrum of results. No significant shift occurred in the MDZ dose-effect curve, but there was a significant seven-fold shift to the right of the ABC dose-effect curve. Throughout all tests, PTZ-lever responding rarely occurred and did not account for more than 20% of lever selections for any individual test. These data support the hypothesis that ABC acts as a full agonist at a sub-population of BZD receptors, which mediate its substitution for MDZ.     

Nicotine enhances stimulus detection performance of middle- and old-aged rats: a longitudinal study

The effects of nicotine on sustained attention were tested in F344xBN male rats when they were chronologically middle and old aged. The rats (n = 11) were trained in a two-choice, stimulus detection task in which a press of one of two levers was reinforced with food, with the correct lever indicated by the position of a briefly illuminated light. They were tested when they were 24-25 and 34-35 months of age (i.e., at 60-68% and 85-95%, respectively of their expected median life span) after saline or 0.1-0.5 mg/kg doses of nicotine (SC). A significant dose-related improvement in percent correct choices and decrease in choice response times was found at both ages, and there was no significant main effect of age or an age by dose interaction. These results support the position that nicotine can enhance attentional processes in rats throughout their life span. Nicotine and other nicotinic agonists may have efficacy in the treatment of disorders such as Alzheimer's disease.     

Nicotine as a conditioned stimulus: impact of attention-deficit/hyperactivity disorder medications

People diagnosed with attention-deficit/hyperactivity disorder (ADHD) are at an increased risk to start smoking and have greater difficulty quitting. Nicotine, one of the principal addictive components of tobacco smoke, functioned as a conditioned stimulus (CS) for intermittent sucrose delivery in a Pavlovian drug discrimination task with rats. This study compared the ability of commonly prescribed ADHD medications (i.e., methylphenidate, atomoxetine, and bupropion) and additional dopamine reuptake inhibitors (i.e., cocaine and GBR 12909) to substitute for the CS effects of nicotine. Atomoxetine was also used to antagonize these CS effects. Rats acquired the discrimination as evidenced by increased dipper entries in nicotine (0.2 mg base/kg) sessions as compared with saline sessions. Nicotine generalization was dose dependent. Bupropion (10 and 20 mg/kg), methylphenidate (10 mg/kg), and cocaine (5 and 10 mg/kg) partially substituted for the 0.2 mg/kg nicotine CS. Atomoxetine did not substitute for the nicotine CS; however, atomoxetine (1 to 10 mg/kg) partially blocked nicotine's CS effects. These results suggest that atomoxetine, bupropion, and/or methylphenidate may be effective treatments for people diagnosed with ADHD and addicted to nicotine.     

Mianserin as a discriminative stimulus in rats: asymmetrical cross-generalization with scopolamine

The present study was conducted to determine if the tetracyclic antidepressant mianserin could be established as a discrminative stimulus in rats. One group of rats was trained to discriminate mianserin (4.0 mg/kg, IP) from saline in a two-lever drug discrimination procedure, and a second group of rats was trained to discriminate the muscarinic cholinergic antagonist scopolamine (0.25 mg/kg, IP) from saline. Generalization testing with the training drugs yielded an ED₅₀ of 0.502 mg/kg for the mianserin-trained rats and an ED₅₀ of 0.048 mg/kg for the scopolamine-trained rats. Asymmetrical cross-generalization between mianserin and scopolamine was observed, because scopolamine produced mianserin-appropriate responding, but mianserin did not produce scopolamine-appropriate responding. This study is the first demonstration that rats can be trained to discriminate mianserin from saline and that antagonism of muscarinic cholinergic receptors is sufficient to produce mianserin-appropriate responding.     

Self-administered ethanol as a discriminative stimulus in rats.

The neurochemical and behavioral effects produced by drugs can differ based on whether self-administered or experimenter-administered. In addition, self-administered drugs, particularly those taken orally or by inhalation, have peripheral stimulus effects that are not present following experimenter administration. One drug with highly prominent peripheral stimulus effects when taken orally is ethanol. The purpose of the present experiment was to examine whether orally self-administered (SA) ethanol would serve as a discriminative stimulus and to determine if the peripheral effects of ethanol play a major role in the discriminative stimulus of orally SA ethanol. Twelve Long-Evans rats were trained to orally self-administer 750 mg/kg of 10% (w/v) ethanol and then discriminate that dose of ethanol from SA water. Six of twelve rats were successfully trained to discriminate oral SA ethanol from water. Intraperitoneal experimenter-administered and orally SA ethanol doses of 100-1320 mg/kg were tested for substitution. SA and i.p. ethanol doses of 750, 1000, and 1320 mg/kg fully substituted for the SA training dose. SA doses of 100, 320 and 560 mg/kg partially substituted for the SA ethanol training dose, whereas the 100 and 320 mg/kg i.p. ethanol doses did not substitute for SA ethanol. The ED(50) values for SA and i.p. ethanol were not significantly different from one another. The results indicate that SA ethanol can serve as a discriminative stimulus in rats and that i.p. ethanol can substitute for SA ethanol. In addition, the results also show that the discriminative stimulus effects of SA ethanol are primarily mediated by CNS drug effects.     

Nicotine does not improve discrimination of brain stimulation reward by rats

Rats were trained to shuttle between two selected ("ON") arms of a Y maze, to obtain electrical stimulation of the medial forebrain bundle. Each shuttle response was rewarded with a brief pulse train. Repetitive entries into the same "ON" arm were not rewarded, nor were entries made into the third ("OFF") arm. Every 67s, stimulation was made available from a different pair of arms. Test sessions lasted for 80 min, beginning immediately after SC injection. Undrugged subjects responded faster, and with a greater proportion of rewarded responses, the higher the stimulation current. In non-tolerant rats, nicotine (0-0.4 mg/kg) depressed responding and induced ataxia shortly after injection; from 40 min, nicotine increased low rates of responding but decreased high rates. All these effects were dose-dependent. Mecamylamine (2.0 mg/kg) prevented the initial depressant action. With repeated daily injections of nicotine (0.4 mg/kg), a marked stimulant action emerged which replaced the initial depressant action, and this was dose-dependent. However, responding was increased by nicotine even when brain stimulation was not available ("time-out"). In contrast, an additional "rate-free" index based on discrimination showed that nicotine did not augment the rewarding properties of the brain stimulation.     

Nicotine and bupropion share a similar discriminative stimulus effect

Bupropion is a weakly potent central nervous system (CNS) stimulant that is marketed both as an antidepressant and as an anti-smoking aid. The mechanism(s) by which it produces its effects is not well understood. In the present study, the effect of bupropion was examined in rats trained to discriminate the stimulus effect of 0.60 mg/kg of (-)-nicotine from saline in a two-lever drug discrimination task. In tests of stimulus generalization (substitution), the nicotine (ED(50)=0.17 mg/kg) stimulus completely generalized to bupropion (ED(50)=5.50 mg/kg). In addition, interaction studies were conducted that evaluated the effect of 3.0 mg/kg of bupropion, a dose that when given alone produced saline-appropriate responding, in combination with various doses of nicotine. This application resulted in an enhancement of the potency of nicotine (ED(50)=0.05 mg/kg), as indicated by a leftward shift of the nicotine dose-effect function. In tests of stimulus antagonism, various doses of bupropion were administered prior to the training dose of nicotine and were found to be ineffective as antagonists of the nicotine stimulus. In contrast, the nicotinic acetylcholine receptor (nicotine receptor) antagonist mecamylamine (AD(50)=0.40 mg/kg) completely blocked the stimulus effect of nicotine. Mecamylamine did not attenuate the stimulus generalization of bupropion. The results demonstrated that bupropion can produce a nicotine-like response in nicotine-trained animals, but it does so via a mechanism of action that is unlike that of nicotine. It is speculated that bupropion may be somewhat effective as an anti-smoking treatment in people who are motivated to quit smoking because low doses of bupropion produce a nicotine-like effect(s) that serve as a suitable substitute for nicotine.     

alpha-Ethyltryptamine (alpha-ET) as a discriminative stimulus in rats

alpha-Ethyltryptamine (etryptamine, alpha-ET) is a drug of abuse that first appeared on the clandestine market in the mid-1980s. Its pharmacological actions are poorly understood. In this investigation, it is reported for the first time that alpha-ET serves as a training drug in drug discrimination studies. Male Sprague-Dawley rats were trained to discriminate (30-min pretreatment time) 2.5 mg/kg of alpha-ET (ED(50)=1.3 mg/kg) from saline vehicle using a standard two-lever operant paradigm and a VI-15s schedule of reinforcement for appetitive reward. Once established, the alpha-ET stimulus was shown to have an onset to action of 30 min and a duration of effect of at least 4 h. In tests of stimulus generalization (substitution), the alpha-ET stimulus generalized to S(-)alpha-ET (ED(50)=1.6 mg/kg) and R(+)alpha-ET (ED(50)=1.3 mg/kg). Tests of stimulus generalization were also conducted with prototypical phenylisopropylamines: (+)amphetamine, 1-(2,5-dimethoxy-4-methylphenyl)-2-aminopropane (DOM), and N-methyl-1-(4-methoxyphenyl)-2-aminopropane (PMMA). The alpha-ET stimulus generalized to DOM (ED(50)=0.4 mg/kg) and PMMA (ED(50)=0.7 mg/kg), but only partially generalized (ca. 40% maximal drug-appropriate responding) to (+)amphetamine. The results suggest that alpha-ET produces a complex stimulus.     

Nicotine discrimination in men and women.

Nicotine is the primary compound that maintains tobacco smoking behavior, and nicotine reinforcement may be related to its discriminative stimulus effects. Nicotine in novel form, isolated from tobacco smoke, is often reinforcing in men but not in women, and clinical trials with nicotine replacement via gum or patch have often shown less efficacy in women vs. men trying to quit smoking. We hypothesize that this sex difference in nicotine reinforcement or clinical efficacy may be related to reduced intensity of nicotine's discriminative stimulus effects in women. Using formal drug discrimination procedures, we have found in several studies that discrimination responding across nasal spray nicotine doses tends to be flatter for women than men (i.e., sex x dose interaction), suggesting reduced sensitivity to changes in dose. Results from the field of psychophysiology, involving detection of physiological changes, are generally consistent with our findings, and suggest that the environmental context accompanying physiological change is important in understanding this sex difference. The implications of this sex difference for smoking cessation treatment and future research directions are presented.     

Nicotine serves as a feature-positive modulator of Pavlovian appetitive conditioning in rats

The present experiments examined whether a nicotine state could set the occasion for a pairing between visual cues and a rewarding outcome in rats. Following nicotine administration, presentation of a conditional stimulus (CS; light-on) was followed by brief access to a sucrose solution. When saline was administered, the same CS was presented but was not followed by any consequence. In Experiment 1, two groups assessed whether rats could acquire this Pavlovian feature-positive discrimination via different training procedures. An anticipatory food-seeking conditioned response (CR) developed during the CS on nicotine sessions but not on saline sessions in both groups. In Experiment 2, centrally acting antagonists of nicotinic acetylcholine and opiate receptors (mecamylamine and naloxone, respectively) dose-dependently blocked nicotine's control of the CR, whereas the peripherally acting nicotinic antagonist hexamethonium had no effect. Increasing or decreasing the interval between nicotine administration and testing also attenuated the CR. These results are consistent with the hypothesis that nicotine can occasion appetitive Pavlovian relations via its action at central nervous system cholinergic receptors.     

Lack of evidence for a role of endorphinergic mechanisms in mediating a discriminative stimulus produced by diazepam in rats

Male Sprague-Dawley rats were trained in a two-lever food-reinforced procedure to discriminate between the effect of saline and diazepam (2.5 mg/kg). After acquisition of this discrimination, the ability of morphine to generalize, and naloxone to antagonize the diazepam discriminative stimulus was tested. The rats did not generalized the effect of morphine, and naloxone did not antagonize the diazepam discriminative stimulus whether it was given prior or subsequent to diazepam. These data suggest a lack of involvement of endorphins in mediating the discriminative stimulus property of diazepam.