The effect of sazetidine-A and other nicotinic ligands on nicotine controlled goal-tracking in female and male rats.

Nicotine is the primary addictive component of tobacco products and its complex stimulus effects are readily discriminated by human and non-human animals. Previous research with rodents directly investigating the nature of the nicotine stimulus has been limited to males. The current study began to address this significant gap in the literature by training female and male rats to discriminate 0.4 mg/kg nicotine from saline in the discriminated goal-tracking task. In this task, access to sucrose was intermittently available on nicotine session. On interspersed saline session, sucrose was not available. Both sexes acquired the discrimination as evidenced by increased head entries into sucrose receptacle (goal-tracking) evoked by nicotine; the nicotine generalization curves were also similar between females and males. The pharmacological profile of the nicotine stimulus was assessed using substitution and targeted combination tests with the following ligands: sazetidine-A, PHA-543613, PNU-120596, bupropion, nornicotine, and cytisine. For females and males, nornicotine fully substituted for the nicotine stimulus, whereas sazetidine-A, bupropion, and cytisine all evoked partial substitution. Female and male rats responded in a similar manner to interaction tests where a combination of 1 mg/kg of sazetidine-A plus nicotine or nornicotine shifted the nicotine dose-effect curve to the left. The combination of sazetidine-A plus bupropion or cytisine failed to do so. These findings begin to fill a significant gap the in scientific literature by studying the nature of the nicotine stimulus and response to therapeutically interesting combinations using a model that includes both sexes.

Iptakalim attenuates self-administration and acquired goal-tracking behavior controlled by nicotine.

Iptakalim is an ATP-sensitive potassium channel opener, as well as an α4ß2-containing nicotinic acetylcholine receptor (nAChR) antagonist. Pretreatment with iptakalim diminishes nicotine-induced dopamine (DA) and glutamate release in the nucleus accumbens. This neuropharmacological profile suggests that iptakalim may be useful for treatment of nicotine dependence. Thus, we examined the effects of iptakalim in two preclinical models. First, the impact of iptakalim on the interoceptive stimulus effect of nicotine was evaluated by training rats in a discriminated goal-tracking task that included intermixed nicotine (0.4 mg/kg, SC) and saline sessions. Sucrose was intermittently presented in a response-independent manner only on nicotine sessions. On intervening test days, rats were pretreated with iptakalim (10, 30, 60 mg/kg, IP). Results revealed that iptakalim attenuated nicotine-evoked responding controlled by the nicotine stimulus in a dose-dependent manner. In a separate study, the impact of iptakalim on the reinforcing effects of nicotine was investigated by training rats to lever-press to self-administer nicotine (0.01 mg/kg/infusion) [Dosage error corrected]. Results revealed that pretreatment with iptakalim (1, 3, 6 mg/kg, IV) decreased nicotine intake (i.e., less active lever responding). Neither behavioral effect was due to a non-specific motor effect of iptakalim, nor to an ability of iptakalim to inhibit DA transporter (DAT) or serotonin transporter (SERT) function. Together, these finding support the notion that iptakalim may be an effective pharmacotherapy for increasing smoking cessation and a better understanding of its action could contribute to medication development.

An investigation of bupropion substitution for the interoceptive stimulus effects of nicotine.

Although the exact mechanism that makes bupropion hydrochloride (Zyban) effective as a smoking cessation aid has not been fully elucidated, studies have found that bupropion and nicotine share behavioural and neurophysiological properties suggesting that bupropion might serve as a substitute for nicotine. In fact, bupropion prompts nicotine-appropriate responding in operant and Pavlovian drug discrimination studies with rats. A majority of the literature examining this substitution pattern has been done with an operant paradigm. The present research extended this literature by further characterising the behavioural and neuropharmacological properties underlying the substitution for a nicotine conditioned stimulus (CS). Examination of the dose-effect function and temporal dynamics of this substitution pattern showed that bupropion (20 mg/kg) produced conditioned responding similar to nicotine (0.4 mg base/kg) (ED(50) = 9.9 mg/kg) at 15 and 30 min after injection and partially substituted 5 and 60 min post-injection. Bupropion produced a pattern of conditioned responding similar to nicotine during a 60-min extinction test. Additionally, it has been hypothesised that bupropion and nicotine have an overlapping dopaminergic mechanism. We tested the effects of bupropion pretreatment, the nicotine dose-effect function and the ability of dopamine antagonist to block the substitution of bupropion for nicotine. Pretreatment with doses of bupropion that did not substitute for the nicotine stimulus (5 and 10 mg/kg) did not affect nicotine-conditioned responding; pretreatment with 20 mg/kg attenuated nicotine-evoked responding. Pretreatment with the dopamine antagonists SCH-23390 and eticlopride blocked the substitution. Finally, S,S-hydroxybupropion, the major metabolite of bupropion in humans, did not substitute for the nicotine CS.

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.

Effects of chronic caffeine pre-exposure on conditioned and unconditioned psychomotor activity induced by nicotine and amphetamine in rats.

Three experiments examined the effects of chronic pre-exposure to caffeine on the subsequent conditioned and unconditioned locomotor activating effects of nicotine or amphetamine in rats. Rats were given daily intraperitoneal injections of caffeine anhydrous (0, 10 or 30 mg/kg base) for 30 days. Conditioning (environment-drug pairings) began after the last day of caffeine pre-exposure. Pre-exposure to 30 mg/kg of caffeine enhanced the acute and chronic locomotor effects of amphetamine (0.5 mg/kg). A similar enhancement of activity was not seen with the high (0.421 mg/kg base) or low dose (0.175 mg/kg) of nicotine. In a drug-free test, the distinct environment paired with amphetamine and the high dose of nicotine evoked increases in activity relative to controls. Caffeine pre-exposure did not affect expression of this conditioned hyperactivity. These effects of caffeine pre-exposure on amphetamine-induced activity could not be attributed to non-specific effects of caffeine.

Dopaminergic and cholinergic antagonism in a novel-object detection task with rats.

In a free-choice test, rats display a tendency to interact more with a novel object than a familiar object. In the present report, we assessed the role of the dopaminergic and cholinergic systems in the expression of this novelty detection. Rats were injected with a dopaminergic antagonist (sulpiride, U-99194A, clozapine, or L-745,870) or a cholinergic antagonist (mecamylamine or scopolamine) prior to the free-choice novel-object test. The dopamine antagonists did not block novel-object detection. In contrast, scopolamine, but not mecamylamine, reliably blocked the expression of novelty detection, indicating a role for muscarinic receptors.

Chronic caffeine exposure in rats blocks a subsequent nicotine-conditioned taste avoidance in a one-bottle, but not a two-bottle test.

Two experiments were conducted in order to investigate nicotine-conditioned taste avoidance (CTA) following chronic preexposure to caffeine. Rats were given daily intraperitoneal injections of caffeine anhydrous (0, 10, or 30 mg/kg) for 10 or 30 days. Training of the nicotine-CTA began after the last day of caffeine preexposure. On five separate occasions access to a saccharin solution was followed immediately by an injection of 1.2 mg/kg nicotine hydrogen tartrate salt or saline. Nicotine-CTA readily developed in saline-preexposed controls. That is, paired rats drank less saccharin solution than unpaired rats after repeated saccharin-nicotine pairings. A similar pattern of nicotine-CTA was found for rats preexposed to 30 mg/kg caffeine for 10 days. Following 10 days of preexposure to 10 mg/kg caffeine, however, CTA did not develop under standard testing conditions. Thirty days of caffeine preexposure did not affect the development of a nicotine-CTA even though the anorexic effects of caffeine were evident after exposure to 30 mg/kg for this duration. Thus, caffeine exposure appears to weaken acquisition or expression of the conditioned avoidance properties of nicotine. This effect is sensitive to the dose of caffeine and duration of preexposure. Importantly, the pattern of nicotine-CTA does not appear to be due to nonspecific effects of caffeine.

Individual differences in behavioral responses to novelty and amphetamine self-administration in male and female rats.

Previous work has shown that individual differences in locomotor activity in an inescapable novel environment can predict acquisition of amphetamine self-administration. The current study examined whether individual differences in approach to novelty in a free choice test could also predict amphetamine self-administration. Further, the current study examined whether individual differences in either free choice or inescapable novelty tests could predict responding for a nondrug reinforcer (sucrose) in the presence and absence of amphetamine. Male and female rats were first tested for their response to free choice novelty (playground maze and novelty-induced place preference tests) and inescapable novelty. They were then tested for acquisition of sucrose-reinforced responding, amphetamine-induced changes in maintenance of sucrose-reinforced responding, and amphetamine self-administration. Based on the inescapable novelty test, acquisition of sucrose-reinforced responding was more rapid in male high responders (HR) compared to low responders (LR). This effect in males did not generalize to females. None of the novelty tests predicted the ability of amphetamine to decrease sucrose-maintained responding. However, using the inescapable novelty test, both male and female HRs self-administered more amphetamine than LRs within the dose range tested (0.03-0.16 mg/kg/infusion). Neither the playground maze nor the novelty-induced place preference test predicted amphetamine self-administration. These results indicate that responses to free choice novelty and inescapable novelty predict different components of amphetamine-induced behavior.

Nicotine-conditioned locomotor activity in rats: dopaminergic and GABAergic influences on conditioned expression.

Little is known about the processes that mediate acquisition and expression of conditioned associations between contextual cues and psychomotor effects of nicotine. In four separate experiments using rats, an environment repeatedly paired with nicotine acquired the ability to elicit increases in activity even in the absence of drug. This conditioned effect was sensitive to nicotine dose. Rats that had 0.6 or 1.2 mg/kg nicotine, but not 0.3 mg/kg, paired with the environment were more active than an unpaired control group (Experiment 1). In Experiment 2, control groups eliminated accounts based on nonspecific effects of nicotine and inhibitory conditioning decreasing activity in the unpaired controls of Experiment 1. Pretreatment on the test day with 100 mg/kg of gamma vinyl-GABA (GVG), a compound that inhibits the enzyme required to breakdown GABA, partially blocked the expression of locomotor conditioning without impairing activity in controls (Experiment 3). In Experiment 4, pretreatment on the test day with the dopamine D(1) receptor antagonist SCH-23390 (0.03 mg/kg) blocked expression of nicotine-conditioned locomotor activity; the D(2)/D(3) receptor antagonist eticlopride did not. Thus, the dopamine D(1) receptor subtype appears to play a role in context-elicited increases in activity conditioned by nicotine; GABA may also modulate the expression of this conditioned effect.

Individual differences in rat locomotor activity are diminished by nicotine through stimulation of central nicotinic acetylcholine receptors.

An increasing body of research has focused on isolating factors that predict or alter individual differences in the behavioral and neural processes mediating the effects of abused drugs. Within this framework, the current report assessed individual differences and the locomotor effect of nicotine. Rats were screened for activity induced by a novel environment. Rats, which were more active to initial environment exposure, remained more active even after seven additional 30-min exposures to the same environment. Treatment with nicotine-di-D tartrate (1 mg/kg, sc) disrupted this effect. This nicotine disruption of individual differences occurred whether nicotine suppressed locomotor activity (initial administration) or stimulated locomotor activity (seventh and eighth administration). Mecamylamine (1 mg/kg), but not hexamethonium (10 mg/kg), completely blocked the suppressant and stimulant effects of nicotine. Further, mecamylamine restored the nicotine-induced disruption of individual differences; hexamethonium had no effect. This data pattern suggests that the disruptive effects of acute and chronic nicotine on individual differences were mediated by neural nicotinic acetylcholine (nACh) receptors.

Environmental familiarization in rats: differential effects of acute and chronic nicotine.

If an environment is familiar, rats will interact more with a novel object than if the environment is unfamiliar. In two experiments we used this behavioral tendency to assess the effects of nicotine on environmental familiarization (i.e., an elevated platform). As expected, rats given 2 min of exposure to the platform on 2 consecutive days (familiarization phase) interacted more with a novel object in a subsequent test than rats that had not experienced the platform until the test day. During the familiarization phase acute pretreatment with nicotine (0.6 and 1.8 mg/kg, subcutaneous) 10 min before platform exposure interfered with familiarization processes, as measured by object interaction on the drug-free test day. Behavioral measures of activity (e.g., turning and midline crosses) eliminated an account based on nicotine-induced motor impairment. Furthermore, this effect of acute nicotine on familiarization was not due to nonspecific effects of nicotine. Controls that received equivalent nicotine exposure temporally separated from platform exposure interacted more with the novel object than similarly treated controls that were unfamiliar with the platform on the test day. Interestingly, rats treated once daily with 0.6 mg/kg nicotine for 14 days before the familiarization phase (chronic condition) did not show a decrease in environmental familiarity. This dissociation extends a growing literature finding that the behavioral and neurobiological effects of nicotine differ, in part, after acute and chronic exposure. Indeed, acute nicotine (0. 2, 0.6, and 1.2 mg/kg) in the present report consistently decreased the amount of time spent with one paw on the edge of the platform; chronic nicotine did not affect this behavior.

Nicotine enhances acquisition of a T-maze visual discrimination: assessment of individual differences.

In the present report, rats' performance was assessed in five tasks designed to measure behavioral response to different novel stimuli under different experimental situations. Daily nicotine treatment (0, 0.3 or 1.0 mg / kg) began after the conclusion of the behavioral tasks and continued throughout the experiment. Training of a T-maze visual discrimination task commenced after 11 days of nicotine pretreatment. As a group, rats treated with the higher dose of nicotine (1.0 mg / kg) made fewer errors to acquire the initial T-maze discrimination than saline-treated controls. Activity induced by an inescapable novel environment (i.e. first behavioral screen) was positively correlated with the number of errors to acquire the initial discrimination in the T-maze for the two nicotine-treated groups (0.3 and 1.0 mg / kg). To examine this positive correlation further, a median split analysis was conducted on the novelty-induced activity for each group. Nicotine, especially the high dose (1.0 mg / kg), enhanced performance in rats that were less active in the inescapable novel environment. Nicotine treatment did not affect the performance of rats that were more active in that environment. After the initial visual discrimination was acquired, the reverse discrimination was trained. Nicotine treatment did not affect performance; the number of errors to acquire the reversal for nicotine- and saline-treated rats did not differ. Overall a nicotine-induced improvement in performance is demonstrated which can be predicted by a rat's reactivity to environmental novelty.

Conditioned place preference: what does it add to our preclinical understanding of drug reward?

RATIONALE: Among the various experimental protocols that have been used to measure drug reward in laboratory animals, conditioned place preference (CPP) has been one of the most popular. However, a number of controversial issues have surrounded the use of this experimental protocol. OBJECTIVE: The present review provides a theoretical overview of some critical issues relevant to CPP. The advantages and limitations of CPP are also covered. RESULTS: Based on modern and traditional theoretical formulations of Pavlovian conditioning, CPP appears to reflect a preference for a context due to the contiguous association between the context and a drug stimulus. Within this theoretical framework, it seems clear that CPP measures a learning process that is fundamentally distinct from drug self-administration. The main advantages of CPP are that it: (1) tests animals in a drug-free state; (2) is sensitive to both reward and aversion; (3) allows for simultaneous determination of CPP and locomotor activity; (4) is adaptable to a variety of species; (5) typically yields dose-effect curves that are monophasic rather than biphasic; and (6) has utility in probing the neural circuits involved in drug reward. The main limitations of CPP are that it: (1) is subject to interpretation based on the notion of novelty seeking; (2) is cumbersome for providing the graded dose-effect curves needed for answering some pharmacological questions; (3) is difficult to interpret when animals prefer one context prior to drug conditioning; and (4) lacks face validity as an experimental protocol of drug reward in humans. CONCLUSION: Despite some limitations, CPP provides unique information about the rewarding effect of contextual cues associated with a drug stimulus.

Conditioned increase in place preference by access to novel objects: antagonism by MK-801.

In three separate place conditioning experiments with rats, repeated access to novel objects in one of two distinct environments conditioned an increase in preference for the novelty-paired environment. A conditioned increase in preference was found whether novel objects were paired with a randomly chosen environment or with the less preferred of two environments (conditioned against a preference). This enhanced preference did not depend on the control group employed. Control groups exposed only to the place conditioning apparatus or to both the apparatus and the novel objects showed no systematic shift in place preference. Intravenous infusions of cocaine also produced an increase in preference using the procedures employed with novel objects. Pretreatment with the N-methyl-D-aspartate (NMDA) receptor antagonist MK-801 (0.03 mg kg(-1)) blocked acquisition of the enhanced place preference conditioned by access to novel objects without decreasing time spent with objects or inducing a place aversion in controls. Combined, these results show that access to novel objects can serve as an appetitive stimulus like drugs of abuse and that this novelty-induced appetitive process involves NMDA receptors. These place-conditioning procedures may provide a good model for determining the behavioral and neural process underlying the appetitive effects of novelty.

Dopamine antagonism in a novel-object recognition and a novel-object place conditioning preparation with rats.

Access to novel objects, similar to drugs of abuse, can enhance a place preference in rats. In the present experiments, the dopamine D1 receptor antagonist SCH-23390 blocked an increase in place preference conditioned by access to novel objects at doses that did not interfere with object interaction (0.01 and 0.03 mg/kg) or produce a place aversion in controls. However, eticlopride, a D2/D3 dopamine receptor antagonist, only blocked the conditioned increase in place preference at a dose (0.3 mg/kg) that impaired object interaction. In contrast, neither SCH-23390 nor eticlopride blocked preference for the novel object in an object recognition task at doses that did not interfere with object interaction. These experiments provide further evidence that the neural processes controlling learned associations between novel stimuli and the environment overlap with drugs of abuse.

Locomotion and conditioned place preference produced by acute intravenous amphetamine: role of dopamine receptors and individual differences in amphetamine self-administration.

Although previous studies have shown that dopamine (DA) antagonists block amphetamine reward, these studies have utilized animal models that involve repeated exposures to amphetamine. The present investigation examined the effect of DA antagonists on single-trial conditioned place preference (CPP) produced by acute intravenous (i.v.) amphetamine in rats. In the first experiment, rats were prepared with a jugular catheter and then received an acute i.v. injection of amphetamine (0.1-3 mg/kg) paired with one compartment of a CPP apparatus. Relative to sham controls (no i.v. catheter), amphetamine produced a dose-dependent increase in locomotor activity and CPP. Two further experiments demonstrated that both effects of amphetamine were completely blocked by pretreating rats with the D1 DA antagonist SCH-23390 (0.025 and 0.25 mg/kg) or the D2 DA antagonist eticlopride (0.2 and 2 mg/kg) on the conditioning trial. In a final experiment, single-trial amphetamine CPP did not predict subsequent self-administration of i.v. amphetamine (10-50 micrograms/infusion) using either a fixed ratio (FR) 1 or progressive ratio (PR) schedule of reinforcement. Thus, while sharing a similar DA receptor mechanism, the present results indicate that single-trial CPP and self-administration are dissociable effects of i.v. amphetamine.

Morphine-conditioned changes in locomotor activity: role of the conditioned stimulus.

When a multisensory environment was reliably paired with morphine (2 mg/kg) in rats, that environment, in a drug-free test, evoked a hyperactive conditioned response (CR). When an olfactory cue (banana odor) was the only stimulus element reliably paired with morphine, it also elicited a hyperactive CR. However, a gustatory cue (saccharin solution) evoked a hypoactive CR. This taste-elicited decrease in activity was dose dependent; morphine at 2 and 4 mg/kg conditioned hypoactivity, whereas a higher dose (8 mg/kg) did not. A robust conditioned saccharin aversion occurred only at the highest dose of morphine, suggesting disassociation between the hypoactive CR and taste aversion. A taste cue present during context conditioning also prevented either acquisition or expression of the hyperactive CR to the context. The modality of the conditioned stimulus is a critical determinant of the form of the CR in a morphine locomotor conditioning paradigm.

7-OH-DPAT has d-amphetamine-like discriminative stimulus properties.

Rats were trained on a d-amphetamine (1 mg/kg) vs. saline discrimination task using food-maintained responding (fixed ratio = 25). In extinction tests, drug-appropriate responding decreased as the dose of amphetamine was substituted for the training dose decreased. The dopamine D2/D3 receptor agonist (+/-)7-hydroxy-N,N-di-n-propyl-2-aminotetralin (7-OH-DPAT) substituted fully for the amphetamine discriminative stimulus at the higher doses examined (0.1, 0.3, 1.0 mg/kg). This substitution was accompanied by a substantial decrease in overall response rates. Eticlopride, a dopamine D2/D3 receptor antagonist, partially blocked 7-OH-DPAT substitution. Thus, at the higher doses, 7-OH-DPAT shared sufficient discriminative stimulus properties with the amphetamine to prompt full substitution. Eticlopride antagonism suggests a role for the D2/D3 dopamine receptor in this substitution.

Converging evidence for one-trial context fear conditioning with an immediate shock: importance of shock potency.

In a sample of 208 Holtzman-descended albino rats, we found evidence with 4 measures of conditioning (freezing, defecation, side crossing, and nose crossing) that a single 2-s, 1.0-mA immediate shock could condition fear to a context (Experiments 1, 2, and 4). When we reduced the shock intensity to 0.5 mA, we obtained a complete immediate-shock conditioning deficit according to all measures in Experiment 3 and to all but the defecation measure in Experiment 4. Results suggest two conclusions: (a) Differences in shock potency between laboratories may help explain discrepant findings about whether immediate shock supports contextual conditioning; (b) theories of contextual conditioning need a mechanism that permits that conditioning to result from immediate shock.