Discriminative Stimulus Effects Of Nicotine Research Articles

Evaluation of novel epibatidine analogs in the rat nicotine drug discrimination assay and in the rat chronic constriction injury neuropathic pain model.

Nicotine is the primary psychoactive component responsible for maintaining tobacco dependence in humans. Chronic pain is often a consequence of tobacco-related pathologies, and the development of a dual therapeutic that could treat chronic pain and tobacco dependence would be advantageous. Epibatidine reliably substitutes for nicotine in the drug discrimination assay, and is a potent analgesic, but has a side-effect profile that limits its therapeutic potential. Thus, considerable efforts to produce epibatidine derivatives are underway. Here we tested three epibatidine derivatives, 2'-fluoro-3'-(4-nitrophenyl)deschloroepibatidine (RTI-7527-102; i.e., RTI-102), 2'-fluorodeschloroepibatidine (RTI-7527-36; i.e., RTI-36), and 3'-(3″-dimethylaminophenyl)-epibatidine (RTI-7527-76; i.e., RTI-76) in both the rat nicotine drug discrimination assay as well as in the rat chronic constriction injury (CCI) of the sciatic nerve neuropathic pain model. Male and female Sprague-Dawley rats were trained on a fixed-ratio 10 schedule to discriminate nicotine (0.32mg/kg base) from vehicle. All compounds dose-dependently substituted for nicotine, without significant decreases in response rates. In the discrimination assay the rank order potency was RTI-36 > nicotine > RTI-102 > RTI-76. Evidence suggests the α4β2* subtype is particularly important to nicotine-related abuse potential. Thus, here we utilized the antagonist dihydro-β-erythroidine (DHβE) to examine relative β2 subunit contribution. DHβE (3.2mg/kg, s.c.) antagonized the discriminative stimulus effects of nicotine. However, relative to antagonism of nicotine, DHβE produced less antagonism of RTI-102 and RTI-76 and greater antagonism of RTI-36. It is likely that at nicotinic receptor subunits RTI-102, RTI-76 and RTI-36 possess differing activity. To confirm that the full discriminative stimulus of these compounds was due to nAChR activity beyond the β2 subunit, we examined these compounds in the presence of the non-selective nicotinic receptor antagonist mecamylamine. Mecamylamine (0.56mg/kg, s.c.) pretreatment abolished nicotine-paired lever responding for all compounds. In a separate cohort, male and female Sprague-Dawley rats underwent CCI surgery and tested for CCI-induced mechanical allodynia via the von Frey assay. Each compound produced CCI-induced mechanical allodynia reversal. RTI-36 displayed higher potency than either RTI-102 or RTI-76. These novel epibatidine analogs may prove to be useful tools in the fight against nicotine dependence as well as novel neuropathic pain analgesics.

Acute nicotine reinforcement requires ability to discriminate the stimulus effects of nicotine.

This review of research on behavioral discrimination of nicotine and how it informs public health policy for reducing risk of tobacco dependence is adapted from Kenneth A. Perkins's American Psychological Association Division 28 (Psychopharmacology and Substance Abuse) 2020 Med Associates Brady/Schuster Award Lecture. The author's initial programmatic clinical research on nicotine is introduced, especially efforts to develop and validate a novel method of acute nicotine dosing. After the public health rationale for characterizing the discriminative stimulus effects of nicotine in humans are described, details from two separate programs of research on nicotine discrimination in humans are presented. The first, conducted with nicotine dosing by nasal spray, documented that humans could discriminate nicotine administered rapidly, examined nicotine's neuropharmacological specificity, identified discrimination threshold dose in smokers and nonsmokers, and explored other conditions that might alter ability to discriminate its effects. The second, more recent program focused on threshold doses for discrimination of nicotine by cigarette smoking, a program that was very difficult to do until the past decade, and how nicotine's self-reported "reward" and preference via choice behavior relate to its discriminability. Differences due to menthol and degree of tobacco dependence were also examined. For each of these two programs, the main findings of selected studies are noted, followed by very recent work on nicotine discrimination and choice that informs Food and Drug Administration's efforts to formulate public policy to improve health and reduce the nearly half million American deaths per year due to persistent tobacco use. (PsycInfo Database Record (c) 2022 APA, all rights reserved).

Menthol blunts the interoceptive discriminative stimulus effects of nicotine in female but not male rats.

Menthol is a widely used tobacco constituent that has shown to enhance nicotine's reinforcing effects. To determine whether injected menthol also alters nicotine's stimulus effects, we used a drug discrimination task. A total of 57 adult Sprague-Dawley rats (28M, 29F) received 20 positive and 20 negative days (intermixed) of discrimination training. On positive days, rats received a group-specific menthol and nicotine injection (VEH + 0.1 NIC, 1 M + 0.1 NIC, 5 M + 0.1 NIC, VEH + 0.4 NIC, 1 M + 0.4 NIC, 5 M + 0.4 NIC; mg/kg) before eight 15-s cue light presentations (conditioned stimulus (CS)), each followed by 4-s sucrose access. On negative days, all rats were injected with vehicle and saline before eight non-reinforced CS presentations. Next, rats underwent generalization testing with 30 dose combinations of menthol and nicotine. The change in drug-mediated anticipatory goal tracking during the CS was calculated as a difference score (CS minus pre-CS responding). All groups readily acquired drug discrimination. However, difference scores for the 5M + 0.1 NIC group were lower for females. Additionally, females had lower scores for 0.05, 0.1, and 0.4 mg/kg nicotine generalization tests. The lowest nicotine dose discriminable from saline was 0.05 mg/kg for females but 0.025 mg/kg for males. Co-administration with 5 or 10 mg/kg menthol weakened discrimination performance between 0.1 and 0.4 mg/kg and between 0.1 and 0.05 mg/kg nicotine for 0.1 mg/kg nicotine training groups. Female rats that were trained with 0.1 mg/kg nicotine were more sensitive to menthol's modulatory effects on nicotine's stimulus effects. This highlights the importance of taking sex and training dose into account when evaluating the interoceptive stimulus effects of nicotine and menthol.

Evaluation of Novel Epibatidine Analogs in Rats Discriminating Nicotine

Tobacco use is a persistent public health issue. One of every three cancer deaths is linked to tobacco use, including nearly 90% of all deaths related to lung cancer. Nicotine is the primary psychoactive component in tobacco, and nicotine has been identified as the primary compound responsible for maintaining tobacco dependence in humans. Drug discrimination is a pharmacologically selective bioassay, utilized in the past for elucidating the receptor pharmacology of nicotine. If a test compound shares discriminative stimulus effects with nicotine, then it might serve as an effective substitution pharmacotherapy. Epibatidine reliably substitutes for nicotine in the drug discrimination assay, but has a side‐effect profile that limits its therapeutic potential. Thus, considerable efforts are underway to produce derivatives of epibatidine. Here we tested three epibatidine derivatives, 2′‐fluoro‐3′‐(4‐nitrophenyl)deschloroepibatidine (RTI‐7527‐102; i.e., RTI‐102), 2′‐fluorodeschloroepibatidine (RTI‐7527‐36; i.e., RTI‐36), and 3′‐(3″‐dimethylaminophenyl)‐epibatidine (RTI‐7527‐76; i.e., RTI‐76). A total of 4 male and 4 female Sprague‐Dawley rats were trained on a fixed‐ratio 10 schedule to discriminate nicotine (0.32 mg/kg base) from vehicle. All compounds dose‐dependently substituted for nicotine, without significant decreases in response rates. The rank‐order potency in the discrimination assay was RTI‐36 > nicotine > RTI‐102 > RTI‐76. There is abundant evidence that the α4β2* subtype is of particular importance to the abuse potential of nicotine. Thus, we examined the relative contribution of the β2 subunit with the antagonist by dihydro‐β‐erythroidine (DHβE). DHβE antagonized the discriminative stimulus effects of nicotine. However, relative to antagonism of nicotine, DHβE produced less antagonism of RTI‐102, RTI‐36 and RTI‐76. Thus, it is likely that RTI‐102, RTI‐36 and RTI‐76 possess differing activity at nicotinic receptor subunits.Support or Funding InformationThis work was supported by the National Institutes of Health National Institute on Drug Abuse [DA25267] and [DA48353].

Discriminative stimulus effects of mecamylamine and nicotine in rhesus monkeys: Central and peripheral mechanisms

Mecamylamine is a non-competitive nicotinic acetylcholine receptor (nAChR) antagonist that has been prescribed for hypertension and as an off-label smoking cessation aid. Here, we examined pharmacological mechanisms underlying the interoceptive effects (i.e., discriminative stimulus effects) of mecamylamine (5.6 mg/kg s.c.) and compared the effects of nAChR antagonists in this discrimination assay to their capacity to block a nicotine discriminative stimulus (1.78 mg/kg s.c.) in rhesus monkeys. Central (pempidine) and peripherally restricted nAChR antagonists (pentolinium and chlorisondamine) dose-dependently substituted for the mecamylamine discriminative stimulus in the following rank order potency (pentolinium > pempidine > chlorisondamine > mecamylamine). In contrast, at equi-effective doses based on substitution for mecamylamine, only mecamylamine antagonized the discriminative stimulus effects of nicotine, i.e., pentolinium, chlorisondamine, and pempidine did not. NMDA receptor antagonists produced dose-dependent substitution for mecamylamine with the following rank order potency (MK-801 > phencyclidine > ketamine). In contrast, behaviorally active doses of smoking cessation aids including nAChR agonists (nicotine, varenicline, and cytisine), the smoking cessation aid and antidepressant bupropion, and the benzodiazepine midazolam did not substitute for the discriminative stimulus effects of mecamylamine. These data suggest that peripheral nAChRs and NMDA receptors may contribute to the interoceptive stimulus effects produced by mecamylamine. Based on the current results, the therapeutic use of mecamylamine (i.e., for smoking or to alleviate green tobacco sickness) should be weighed against the potential for mecamylamine to produce interoceptive effects that overlap with another class of abused drugs (i.e., NMDA receptor agonists).

Rapid nicotine tolerance and cross-tolerance to varenicline in rhesus monkeys: Drug discrimination.

Acute tolerance to effects of nicotine plays an important role in nicotine dependence, but the mechanism underlying these effects is unclear. Drug discrimination was used in the current study to examine the impact of nicotine pretreatment on sensitivity to the discriminative stimulus effects of nicotine and the FDA-approved smoking cessation pharmacotherapy varenicline. Rhesus monkeys (n = 4) discriminated 0.032 mg/kg nicotine base iv from saline under an FR5 schedule of stimulus-shock termination. Both nicotine and varenicline increased drug-appropriate responding; ED50 values (95% confidence limits) were 0.0087 [0.0025, 0.030] and 0.028 [0.0096, 0.082] mg/kg, respectively. Additional pretreatment injections of the training dose of nicotine (0.032 mg/kg, iv) produced tolerance to its discriminative stimulus effects and the magnitude of this effect was related to the number of pretreatment injections administered. Two pretreatment injections of the training dose of nicotine (0.032 mg/kg, iv) produced a 5.4-fold rightward shift in the nicotine dose-response function and a sevenfold rightward shift in the varenicline dose-response function. The duration of tolerance under these conditions was less than 60 min. These results demonstrate that tolerance to the discriminative stimulus effects of nicotine can be produced by acute nicotine exposure. Acute cross-tolerance from nicotine to varenicline is consistent with similar actions at nAChRs, and suggests that conditions resulting in acute nicotine tolerance could impact sensitivity to other nAChR agonists. (PsycINFO Database Record (c) 2018 APA, all rights reserved).

The discriminative stimulus effects of nicotine & ethanol with two distinct olfactory contexts in male and female rats

Odor cues and interoceptive cues can combine in promoting drug seeking behavior. Drug discrimination methodology was combined with odor-context conditioning in 8 male and 8 female rats. One drug (nicotine or EtOH) plus odorant (peppermint or anise) compound functioned as setting the occasion for sessions of food-reinforced nose poke responses (i.e., the SD) that were maintained on a variable interval 30 s schedule (VI-30), whereas the opposite drug (EtOH or nicotine) plus odorant (anise or peppermint) compound predicted intermixed sessions of non-reinforcement of nose poking (i.e., the SΔ). During brief non-reinforcement tests conducted with each condition there was significantly greater responding under the SD drug plus odor compound compared to the SΔ drug plus odor compound. Discriminative control was evident and there was a sex by stimulus role interaction with greater SΔ responding in females. The odor contexts and drug contexts alone also sustained strong stimulus control but to a lesser extent compared to the full drug-odor compounds. These data suggest configural learning among drug and odor cues.

Nicotine drug discrimination and nicotinic acetylcholine receptors in differentially reared rats.

Individuals vary in sensitivity to the behavioral effects of nicotine, resulting in differences in vulnerability to nicotine addiction. The role of rearing environment in determining individual sensitivity to nicotine is unclear. The neuropharmacological mechanisms mediating the effect of rearing environment on the behavioral actions of nicotine are also poorly understood. The contribution of rearing environment in determining the sensitivity to the interoceptive effects of nicotine was determined in rats reared in isolated conditions (IC) or enriched conditions (EC). The role of dopamine receptors and α4β2*-nicotinic acetylcholine (nACh) receptors in mediating the differential effect of IC and EC on the interoceptive action of nicotine was determined. The interoceptive action of nicotine was measured as the discriminative stimulus effect of nicotine. Mecamylamine- and eticlopride-inhibition of the nicotine stimulus were used to examine nACh and dopamine receptors, respectively. α4β2*-nACh receptor expression in the mesolimbic dopamine pathway was determined by quantitative autoradiography of [125I]-epibatidine binding. EC-reared rats are less sensitive than IC-reared rats to the discriminative stimulus effects of nicotine at all but maximally effective doses. Mecamylamine inhibited the nicotine stimulus threefold more potently in EC-reared rats (IC50 = 0.25mg/kg) compared to IC-reared rats (IC50 = 0.75mg/kg); eticlopride inhibition was not different. [125I]-epibatidine binding in the ventral tegmental area of EC-reared rats was reduced (2.8 ± 0.3fmol) compared to that of IC-reared rats (4.0 ± 0.4fmol); there was no difference in the nucleus accumbens. Rearing environment regulates the sensitivity to the interoceptive effects of nicotine and α4β2*-nACh receptor expression in the mesolimbic dopamine pathway.

The contribution of α4β2 and non-α4β2 nicotinic acetylcholine receptors to the discriminative stimulus effects of nicotine and varenicline in mice.

The extent to which non-α4β2 versus α4β2* nAChRs contribute to the behavioral effects of varenicline and other nAChR agonists is unclear. The purpose of this study was to characterize the discriminative stimulus effects of varenicline and nicotine using various nAChR agonists and antagonists to elucidate possible non-α4β2 nAChR mechanisms. Separate groups of male C57BL/6J mice were trained to discriminate varenicline (3.2mg/kg) or nicotine (1mg/kg). Test drugs included mecamylamine; the nAChR agonists epibatidine, nicotine, cytisine, varenicline, and RTI-102; the β2-containing nAChR antagonist dihydro-β-erythroidine (DHβE); the α7 nAChR agonist PNU-282987; the α7 antagonist methyllycaconitine (MLA); the α3β4 antagonist 18-methoxycoronaridine (18-MC); and the non-nAChR drugs midazolam and cocaine. In nicotine-trained mice, maximum nicotine-appropriate responding was 95% nicotine, 94% epibatidine, 63% varenicline, 58% cytisine, and less than 50% for RTI-102, PNU-282987, midazolam, and cocaine. In varenicline-trained mice, maximum varenicline-appropriate responding was 90% varenicline, 86% epibatidine, 74% cytisine, 80% RTI-102, 50% cocaine, and 50% or less for nicotine, PNU-282987, and midazolam. Drugs were studied to doses that abolished operant responding. Mecamylamine antagonized the discriminative stimulus effects, but not the rate-decreasing effects, of nicotine and varenicline. DHβE antagonized the discriminative stimulus and rate-decreasing effects of nicotine but not varenicline in either the nicotine or varenicline discrimination assays. The discriminative stimulus, but not the rate-decreasing, effects of epibatidine were antagonized by DHβE regardless of the training drug. These results suggest that α4β2* nAChRs differentially mediate the discriminative stimulus effects of nicotine and varenicline, and suggest that varenicline has substantial non-α4β2 nAChR activity.

Effects of nicotine in combination with drugs described as positive allosteric nicotinic acetylcholine receptor modulators in vitro: discriminative stimulus and hypothermic effects in mice

Some drugs that are positive allosteric nAChR modulators in vitro, desformylflustrabromine (dFBr), PNU-120596 and LY 2087101, have not been fully characterized in vivo. These drugs were examined for their capacity to share or modify the hypothermic and discriminative stimulus effects of nicotine (1mg/kg s.c.) in male C57Bl/6J mice. Nicotine, dFBr, and PNU-120596 produced significant hypothermia, whereas LY 2087101 (up to 100mg/kg) did not. Nicotine dose-dependently increased nicotine-appropriate responding and decreased response rate; the respective ED50 values were 0.56mg/kg and 0.91mg/kg. The modulators produced no more than 38% nicotine-appropriate responding up to doses that disrupted operant responding. Rank order potency was the same for hypothermia and rate-decreasing effects: nicotine>dFBr>PNU-120596=LY 2087101. Mecamylamine and the α4β2 nAChR antagonist dihydro-β-erythroidine, but not the α7 antagonist methyllycaconitine, antagonized the hypothermic effects of nicotine. In contrast, mecamylamine did not antagonize the hypothermic effects of the modulators. The combined discriminative stimulus effects of DFBr and nicotine were synergistic, whereas the combined hypothermic effects of nicotine with either dFBr or PNU-120596 were infra-additive. PNU-120596 did not modify the nicotine discriminative stimulus, and LY 2087101 did not significantly modify either effect of nicotine. Positive modulation of nicotine at nAChRs by PNU-120596 and LY 2087101 in vitro does not appear to confer enhancement of the nAChR-mediated hypothermic or discriminative stimulus effects of nicotine. However, dFBr appears to be a positive allosteric modulator of some behavioral effects of nicotine at doses of dFBr smaller than the doses producing unwanted effects (e.g. hypothermia) through non-nAChR mechanisms.

Acute Tolerance to the Discriminative Stimulus Effects of Nicotine in Monkeys

Tolerance to the effects of nicotine, a major psychoactive component of tobacco, contributes to escalation in the frequency and quantity of tobacco use. However, mechanisms of tolerance to the effects of nicotine are not well understood. Here, drug discrimination was used to examine the rapid decrease in sensitivity to nicotine (i.e., acute tolerance), including its underlying receptor pharmacology and its relationship to nicotine pharmacokinetics. Rhesus monkeys (n=6) discriminated 0.032 mg/kg of nicotine base administered i.v. under an FR5 schedule of stimulus‐shock termination. Nicotine dose‐dependently increased drug‐lever responding; the ED50 value (95% confidence limits) of nicotine to produce discriminative stimulus effects was 0.012 (0.0077–0.020) mg/kg. The β2‐selective nicotinic acetylcholine receptor (nAChR) antagonist dihydro‐β‐erythroidine produced a 4.8‐fold rightward shift in the nicotine dose‐response function. Methyllycaconitine, an α7‐selective nAChR antagonist, up to 10 mg/kg did not antagonize the nicotine discriminative stimulus. The duration of the nicotine discriminative stimulus was less than 30 min; however, concentrations of nicotine measured in saliva were maximal (45–65 ng/ml) 10–30 min post‐injection. Pretreatment with nicotine (0.032 mg/kg i.v.) 30 min before re‐determination of sensitivity to the effects of nicotine resulted in a 3‐fold rightward shift in the dose‐response function. Firstly, these data suggest that the i.v. nicotine discriminative stimulus is mediated by β2‐containing, not α7‐containing, nAChRs. Secondly, decreased sensitivity to the discriminative stimulus effects of nicotine when preceded by an additional dose of nicotine 30 min earlier provides evidence of acute tolerance. That the peak concentrations of nicotine in saliva persist beyond the offset of its discriminative stimulus effects is also consistent with acute pharmacodynamic tolerance. Collectively, these results demonstrate that tolerance to the discriminative stimulus effects of nicotine can be produced by acute exposure to nicotine and suggest that the mechanism underlying acute tolerance may be related to rapid desensitization of β2‐containing nAChRs.Support or Funding InformationSupported by United States Public Health Service Grant DA 25267.

Adolescent alcohol exposure decreased sensitivity to nicotine in adult Wistar rats.

Many adolescents engage in heavy alcohol use. Limited research in humans indicates that adolescent alcohol use predicts adult tobacco use. The present study investigated whether adolescent intermittent ethanol (AIE) exposure alters nicotine sensitivity in adulthood. Adolescent male Wistar rats (postnatal day 28-53) were exposed to AIE exposure that consisted of 5 g/kg of 25 percent ethanol three times per day in a 2 days on/2 days off regimen. Control rats received water with the same exposure regimen. In adulthood, separate groups of rats were tested for nicotine intravenous self-administration (IVSA), drug discrimination and conditioned taste aversion (CTA). The dose-response function for nicotine IVSA under a fixed-ratio schedule of reinforcement was similar in AIE-exposed and control rats. However, AIE-exposed rats self-administered less nicotine at the lowest dose, suggesting that low-dose nicotine was less reinforcing in AIE-exposed, compared with control rats. AIE-exposed rats self-administered less nicotine under a progressive-ratio schedule, suggesting decreased motivation for nicotine after AIE exposure. The discriminative stimulus effects of nicotine were diminished in AIE-exposed rats compared with control rats. No group differences in nicotine CTA were observed, suggesting that AIE exposure had no effect on the aversive properties of nicotine. Altogether, these results demonstrate that AIE exposure decreases sensitivity to the reinforcing, motivational and discriminative properties of nicotine while leaving the aversive properties of nicotine unaltered in adult rats. These findings suggest that drinking during adolescence may result in decreased sensitivity to nicotine in adult humans, which may in turn contribute to the higher rates of tobacco smoking.

Characterization of a Nicotine Discriminative Stimulus in Rhesus Monkeys

In general, the discriminative stimulus effects of drugs are examined following s.c. or i.m. administration; this may not be predictive of the effects of drugs that are abused by other means. Nicotine is primarily inhaled in the form of cigarette smoke. Here, two drug discrimination assays were used to compare the effects of nicotine administered i.v. or s.c. in rhesus monkeys responding under an FR5 schedule of stimulus shock termination. In monkeys discriminating 1.78 mg/kg nicotine s.c. (n=5), the ED50 of nicotine was 0.41 mg/kg. In monkeys discriminating 0.032 mg/kg nicotine i.v. (n=4), the ED50 of nicotine was 0.0067 mg/kg. In 3 of these monkeys, a dose of 3.2 mg/kg nicotine s.c. was required to substitute for the i.v. discriminative stimulus; up to 5.6 mg/kg nicotine did not substitute in the fourth monkey. The nicotinic acetylcholine receptor (nAChR) antagonist mecamylamine antagonized the discriminative stimulus effects of nicotine in both discriminations; 1 mg/kg mecamylamine produced rightward shifts of the nicotine dose‐effect function of 3.2‐fold in the s.c. discrimination and a minimum 15‐fold in the i.v. discrimination. A dose of 3.2 mg/kg of the β2 subtype‐selective nAChR antagonist dihydro‐β‐erythroidine (DHβE) did not significantly antagonize the discriminative stimulus effects of nicotine in the s.c. discrimination. However, when combined with the training dose (0.032 mg/kg) of nicotine in the i.v. discrimination, DHβE dose‐dependently decreased discriminative stimulus effects (ED50=0.17 mg/kg). These results indicate there is a critical difference in the involvement of the β2 subtype of nAChR between the two discriminations that cannot be explained by the difference in training dose. Supported by USPHS Grant DA25267.

The Discriminative Stimulus Effects of Nicotine, Epibatidine, and Varenicline in Mice: Involvement of β2 Containing Nicotinic Acetylcholine Receptor Subtypes

Nicotine is a high efficacy β2 containing nicotinic acetylcholine receptor (β2 nAChR) agonist, whereas varenicline has low efficacy at β2 nAChRs. To better understand the receptor pharmacology of varenicline, male C57BL/6J mice (n=8) were trained to discriminate varenicline (3.2 mg/kg s.c.) under an FR10 schedule of food presentation. Separate groups of mice (n=8 per group) were trained to discriminate nicotine (1 mg/kg base weight s.c.) or the β2 nAChR agonist epibatidine (0.0032 mg/kg s.c.). Mice trained with nicotine and epibatidine satisfied the test criteria after a mean of 87 and 89 sessions, respectively, whereas varenicline‐trained mice required significantly more (i.e., 113) sessions. Varenicline was less effective than nicotine and epibatidine in maintaining stimulus control as evidenced by the number of training sessions required between tests. The nicotine and epibatidine dose‐effect functions did not vary as a function of training drug; the functions were steep and characterized by low inter‐subject variability. In contrast, the varenicline dose‐effect functions were shallow and highly variable among mice. Varenicline produced a maximum 63% and 61% drug‐appropriate responding in the nicotine and epibatidine discriminations, respectively. The nAChR antagonist mecamylamine (3.2 mg/kg) antagonized the effects of nicotine, epibatidine, and varenicline. In contrast, the β2 antagonist DHβE (3.2 mg/kg) antagonized the effects of nicotine and epibatidine, but not varenicline. These results suggest that the discriminative stimulus effects of varenicline are not as robust as those of nicotine and epibatidine and are not mediated by β2 nAChRs.*Supported by USPHS grant DA25267

Agonist and antagonist effects of cytisine in vivo

Varenicline, the most successful smoking cessation aid, is a selective partial agonists at α4β2* nicotinic receptors. Its efficacy is likely to be shared by other drugs with similar receptor action, including cytisine. The present study aimed to characterize behavioral effects of cytisine compared with nicotine using locomotor activity tests, intracranial self-stimulation of ventral tegmental area (discrete-trial threshold current intensity titration procedure), drug discrimination (0.6 mg/kg nicotine from vehicle), physical dependence (osmotic minipumps delivering 6 mg/kg/day of nicotine) and intravenous nicotine self-administration (0.01 mg/kg per infusion) in adult Wistar rats. Cytisine (1–3 mg/kg) partially substituted for nicotine and at the highest dose tended to antagonize nicotine's discriminative stimulus effects. Nicotine (0.05–0.4 mg/kg), but not cytisine (0.3–3 mg/kg), lowered ICSS thresholds and cytisine dose-dependently reversed effects of nicotine. Nicotine (0.15–0.6 mg/kg), but not cytisine (0.3–3 mg/kg), stimulated locomotor activity and cytisine (3 mg/kg) fully reversed these effects of nicotine. Acute pretreatment with nicotine (0.15–0.6 mg/kg), but not cytisine (0.3–3 mg/kg), reinstated extinguished nicotine self-administration. Continuous infusion of nicotine induced physical dependence, as indicated by reduced rates of food-reinforced responding induced by a challenge dose of mecamylamine. At the highest tested dose (3 mg/kg), cytisine tended to reduce response rates irrespective of whether the rats were continuously exposed to nicotine or saline. Cytisine behaves like a weak partial agonist, mimicking effects of nicotine to a limited degree. Although cytisine reversed several effects of nicotine, it seemed to have a reduced potential to produce withdrawal signs in nicotine-dependent subjects.

Sensation within the skin.

This Viewpoint emphasizes interoceptive discriminative stimulus modulation of voluntary operant behavior (a la B. F. Skinner), rather than elicitation of Pavlovian conditioned reflexes. In doing so, I will restate how the operant drug discrimination paradigm may not only elucidate smoking and other addictive behaviors but also anxiety (Troisi, J. R., II. (2003) Spontaneous recovery during, but not following, extinction of the discriminative stimulus effects of nicotine in rats: Reinstatement of stimulus control. Psychol. Rec. 53, 579-592).

OR07-3 * SWEET TASTE ENHANCED SOME OF THE STIMULUS EFFECTS OF NICOTINE IN RODENTS

To evaluate non-pharmacological factors influencing smoking behavior, the effects of intake of sweet solutions on reinforcing and discriminative-stimulus effects of nicotine were examined in rodents. Methods. Effects of saccharin or glucose solutions, on (1) discriminative-stimulus effects were evaluated in male SD rats trained to discriminate 0.5 mg/kg nicotine, sc, from saline, and (2) reinforcing effects were evaluated by intravenous self-administration of nicotine (0.03 mg/kg/inj) under a progressive-ratio (PR) schedule in male SD rats, and by conditioned place preference (CPP) after 0.56 mg/kg nicotine in male C57/BL mice. Sweet solutions or water were freely available via drinking bottles for 5 min prior to the tests. Results. ED50 values for the discriminative-stimulus effects of nicotine after water (0.13 mg/kg) were decreased after 2% saccharin (0.09 mg/kg) while 0.2% saccharin and 3% glucose did not produce appreciable changes. However, the highest ratio values attained in the PR test were not significantly changed after 2% saccharin. In the CPP paradigm nicotine at 0.56 mg/kg did not produce significant preference, however after exposure to 1% saccharin a significant preference to the nicotine-paired compartment was induced. Conclusion. The present data suggest that sweet tastes can enhance the stimulus effects of nicotine under some circumstances.

Restraint stress attenuates nicotine's locomotor stimulant but not discriminative stimulus effects in rats

Stress enhances the locomotor stimulant and discriminative stimulus effects of several addictive drugs (e.g., morphine) in rodents, yet interactions between stress and nicotine's effects in these behavioral models have not been well established. To this end, the current studies examined the effects of restraint stress on nicotine-induced locomotor activity and nicotine discrimination in rats. We used a novel approach in which onset of stress and nicotine administration occurred concurrently (i.e., simultaneous exposure) to simulate effects of stress on ongoing tobacco use, as well as a more traditional approach in which a delay was imposed between stress and nicotine administration (i.e., sequential exposure). Simultaneous exposure to stress reduced the rate of locomotor sensitization induced by daily injections of nicotine (0.4mg/kg, s.c.). A lower dose of nicotine (0.1mg/kg, s.c.) produced modest effects on activity that were generally unaffected by simultaneous exposure to stress. Sequential exposure to stress and nicotine (0.4mg/kg, s.c.) slightly suppressed nicotine-induced activity but did not influence rate of locomotor sensitization. Neither simultaneous nor sequential exposure to stress influenced the discriminative stimulus effects of nicotine (0.01–0.2mg/kg, s.c.). These data show that restraint stress reduces nicotine's locomotor stimulant effects, particularly when onset of stress and nicotine exposure occurs simultaneously, but does not influence nicotine discrimination. These findings contrast with the ability of stress to enhance the effects of other drugs in these models. This study also suggests that studying the influence of simultaneous stress exposure on drug effects may be useful for understanding the role of stress in addiction.

Effects of the nicotinic acetylcholine receptor antagonist mecamylamine on the discriminative stimulus effects of cocaine in male rhesus monkeys.

Preclinical drug discrimination procedures have been useful in understanding the pharmacological mechanisms of the subjective-like effects of abused drugs. Converging lines of evidence from neurochemical and behavioral studies implicate a potential role of nicotinic acetylcholine (nACh) receptors in the abuse-related effects of cocaine. The aim of the present study was to determine the effects of the nACh receptor antagonist mecamylamine on the discriminative stimulus effects of cocaine in nonhuman primates. The effects of mecamylamine on the cocaine-like discriminative stimulus effects of nicotine were also examined. Male rhesus monkeys (n = 5) were trained to discriminate 0.32 mg/kg, IM cocaine from saline in a 2-key, food-reinforced discrimination procedure. Initially, potency and time course of cocaine-like discriminative stimulus effects were determined for nicotine and mecamylamine alone. Test sessions were then conducted examining the effects of mecamylamine on cocaine or the cocaine-like discriminative stimulus effects of nicotine. Curiously, mecamylamine produced partial cocaine-like discriminative stimulus effects. Mecamylamine did not significantly alter the discriminative stimulus effects of cocaine up to doses that significantly decreased rates of operant responding. Mecamylamine and nicotine combinations were not different than saline. These results confirm previous nonhuman primate studies of partial substitution with nicotine and extend these findings with mecamylamine. Furthermore, these results extend previous results in rats suggesting cocaine may have nACh receptor antagonist properties.

Nicotine-like behavioral effects of the minor tobacco alkaloids nornicotine, anabasine, and anatabine in male rodents.

Tobacco use is associated with lethal diseases in an estimated 440,000 persons in the United States each year (Centers for Disease Control and Prevention, 2005). Successful smoking quit-rates are estimated at 5%-8%, even though a quarter of those attempts included use of smoking-cessation aids (Messer et al., 2008; Henningfield et al., 2009). Current projections are that 16% of the U.S. population-35 million people-will still smoke in 2025, thus more effective smoking-cessation aids are urgently needed (Pollock et al., 2009). The minor tobacco alkaloids may be promising candidates, but further research is necessary (Hoffman & Evans, 2013). Accordingly, we systematically evaluated the minor tobacco alkaloids nornicotine, anabasine, and anatabine using assays of behavioral tolerability, nicotine withdrawal, nicotine discrimination, and nicotine self-administration in male rodents. At doses that were well tolerated, all 3 minor alkaloids dose-dependently engendered robust substitution for a nicotine discriminative stimulus in mice (0.32 mg/kg, IP), and anabasine attenuated nicotine withdrawal. When the ED50 dose of each alkaloid was administered in combination with nicotine, the discriminative stimulus effects of nicotine were not enhanced by any of the alkaloids, and anatabine blunted nicotine's effects. In drug self-administration studies, only nornicotine was self-administered by rats that self-administered nicotine intravenously; anabasine and anatabine had no reinforcing effects. Moreover, prior administration of each of the minor tobacco alkaloids dose-dependently decreased nicotine self-administration. Collectively these results suggest that the minor tobacco alkaloids may substitute for the subjective effects of nicotine and attenuate withdrawal and craving without the abuse liability of nicotine.