Operant responding for conditioned and unconditioned reinforcers in rats is differentially enhanced by the primary reinforcing and reinforcement-enhancing effects of nicotine

Rationale Nicotine self-administration in rats is modest when response-contingent nicotine infusions are delivered alone (primary reinforcement) but robust when nicotine infusions are combined with a mildly reinforcing non-pharmacological stimulus. Furthermore, response-independent (non-contingent) nicotine administration also elevates responding for that same non-pharmacological stimulus, suggesting that in addition to primary reinforcement, nicotine can enhance the incentive value of other reinforcers.Objectives In this study, we tested the hypothesis that the reinforcement-enhancing effects of non-contingent nicotine are more dependent on the reinforcing strength of the non-pharmacological stimulus than are the effects of contingent nicotine.Materials and methods A weakly reinforcing light-tone stimulus was established as a conditioned reinforcer by repeated pairings with sucrose for some rats, or by delivery in an explicitly unpaired design with sucrose to other rats. Subsequently, both groups lever pressed for the stimulus with contingent nicotine, non-contingent nicotine (0.06 mg kg⁻¹ per infusion, freebase), or non-contingent saline, according to fixed ratio and progressive ratio reinforcement schedules.Results Compared to sucrose-unpaired training, repeated association with sucrose established the light-tone stimulus as a robust conditioned reinforcer. Contingent and non-contingent nicotine equally elevated responding for this conditioned stimulus. Conversely, for the less reinforcing (sucrose-unpaired) stimulus contingent nicotine more effectively elevated behavior compared to non-contingent nicotine.Conclusions The reinforcement-enhancing effect of nicotine increases behavior controlled by both conditioned and unconditioned reinforcers; however, for less salient stimuli associative processes derived from the primary reinforcing effects of contingent nicotine may also be important. These data suggest that nicotine present in tobacco may differentially modulate stimulus-driven behavior in smokers.     

Effects of acute and chronic nicotine on responses maintained by primary and conditioned reinforcers in rats

There is growing recognition that nonnicotine factors, such as the sensory stimuli associated with smoking, can play a critical role in the maintenance of cigarette smoking. However, little is known about the effects of nicotine on responding maintained by these stimuli, which are assumed to be conditioned reinforcers. The authors used an animal model to examine the acute and chronic effects of nicotine on responses maintained by food and conditioned reinforcers (i.e., lights) and responses in the absence of programmed consequences (i.e., extinction). During the acute phase, 4 male rats received 5 doses of subcutaneous nicotine. One dose of nicotine was then administered for a minimum of 60 days. Food-maintained and extinction responses did not significantly increase during the acute phase; however, food-maintained responses did increase during the chronic phase. Relative to vehicle, intermediate doses increased responses maintained by conditioned reinforcers during both phases. The results suggest that nicotine enhances responding maintained by conditioned reinforcers and possibly by food.     

Operant responding for a visual reinforcer in rats is enhanced by noncontingent nicotine: implications for nicotine self-administration and reinforcement

Rationale Current conceptualizations of drug reinforcement assume that drug-taking behavior is a consequence of the contingent, temporal relationship between the behavior and drug reward. However, stimulant drugs also potentiate the rewarding effects of other reinforcers when administered noncontingently. Objectives These studies were designed to determine whether noncontingent nicotine enhances the reinforcing properties of a nonpharmacological reinforcer and whether this direct effect facilitates operant behavior within the context of a nicotine self-administration procedure. Methods Rats self-administered nicotine or food, or received noncontingent nicotine, saline, or food either with or without a response-contingent, unconditioned reinforcing visual stimulus (VS). Results Noncontingent nicotine, whether delivered as discrete injections based on a pattern of self-administered nicotine or as a continuous infusion, increased response rates maintained by the VS. There were no significant differences in responding by animals that received contingent compared with noncontingent nicotine when a VS was available. This increase was not observed in the absence of the VS or as a consequence of noncontingent food delivery. Operant behavior was equally attenuated and reinstated by the removal and subsequent replacement of contingent and noncontingent nicotine. Nicotine supported self-administration in the absence of response-contingent, nicotine-paired stimuli; however, response rates were drastically reduced compared with nicotine self-administration with the VS. Conclusions Nicotine influences operant behavior in two ways: by acting as a primary reinforcer when it is contingent upon behavior, and by directly potentiating the reinforcing properties of other stimuli through a nonassociative mechanism. Nicotine self-administration and smoking may be largely dependent upon this later action. This work was supported by National Institute on Drug Abuse research grants, DA-10464 and DA-12655. "Principles of laboratory animal care" (NIH No. 85-23, revised 1985) were followed throughout all experiments. This research was approved by the University of Pittsburgh Institutional Animal Care and Use Committee, Assurance Number A3187-01     

Age-dependent effects of nicotine on locomotor activity and conditioned place preference in rats

Rationale Most adult smokers start smoking during their adolescence. This adolescent initiation may be due to multiple factors, but little evidence is available regarding whether their brains are differentially sensitive to the addictive effects of nicotine during adolescence.Objective To test the hypothesis that adolescents are more sensitive than adults to nicotines rewarding actions.Methods An unbiased, counterbalanced, place-conditioning procedure was used to examine drug-induced reward and locomotor activity. Early adolescent (postnatal day 28), late adolescent (P38) and adult (P90) rats received either saline or nicotine (0.125, 0.25 or 0.5 mg/kg, s.c.) and were tested for place conditioning.Results During early adolescence, a single nicotine injection (0.5 mg/kg) induced significant conditioned place preference (CPP). In contrast, during late adolescence or adulthood, nicotine did not induce CPP after either one or four conditioning trials. Initial locomotor responses to acute nicotine administration during the first conditioning trial also differed with age, with no effect at P28, but substantial inhibitory responses at all doses studied (0.125–0.5 mg/kg) at later ages. Although not differing in their initial locomotor response to nicotine, there was a significantly greater tolerance/sensitization during the second and subsequent drug exposures in late adolescents than in adults.Conclusions These findings provide evidence that adolescent brain is differentially sensitive to both the acute and repeated effects of nicotine relative to adult brain. Furthermore, there are significant differences in nicotine sensitivity between early and late phases of adolescence.     

Self-administered and noncontingent nicotine enhance reinforced operant responding in rats: impact of nicotine dose and reinforcement schedule

Rationale Nicotine infusions that are self-administered (contingent) or response-independent (noncontingent) increase lever pressing for a reinforcing nonpharmacological stimulus in rats, suggesting that in addition to primary reinforcement, nicotine self-administration may result from nicotine enhancing the reinforcement derived from nonnicotine stimuli. Objectives Based on our previous research, in this study, we tested the hypothesis that contingent and noncontingent nicotine would equally elevate responding for a moderately reinforcing visual stimulus, across a range of nicotine doses on both fixed ratio and progressive ratio reinforcement schedules. Materials and methods The rats lever pressed for a visual stimulus with contingent nicotine, noncontingent nicotine, or contingent saline. Separate groups responded for saline or nicotine without the visual stimulus. Three doses of nicotine (0.01, 0.03, and 0.09 mg/kg per infusion, free base) were tested in a between-groups design. After responding on an escalating fixed ratio reinforcement schedule, the rats were tested on a progressive ratio schedule. Results Compared to responding for the visual stimulus with saline, both contingent and noncontingent nicotine equally elevated lever pressing for the stimulus at each dose on fixed and progressive ratio schedules. In the absence of the stimulus, only the highest nicotine dose sustained self-administration. Conclusions The ability of noncontingent nicotine to elevate responding for a moderately reinforcing visual stimulus occurs across a range of doses, and both self-administered and noncontingent nicotine equally increase motivation to obtain the stimulus, as reflected by performance on a progressive ratio schedule. In the absence of a contingent stimulus, primary reinforcement from nicotine only supports self-administration at high nicotine doses in rats.     

Toward a nonhuman model of contingency management: effects of reinforcing abstinence from nicotine self-administration in rats with an alternative nondrug reinforcer

Rationale  Reinforcing abstinence from drug use with alternative nondrug reinforcers (e.g., contingency management) is one of the most effective interventions for drug abuse. While nonhuman studies have also shown that access to alternative nondrug reinforcers reduces drug self-administration, this effect has not been examined in nicotine self-administration models. Moreover, abstinence contingencies per se under free-operant conditions have not been examined. Objective  The objective of this experiment was to begin development of a model of contingency management interventions by employing a differential-reinforcement-of-alternative-behavior (DRA) schedule of alternative nondrug reinforcement in rats self-administering nicotine. Methods  Two groups of rats were trained to self-administer nicotine under a multiple schedule of nicotine and sucrose delivery. The DRA-group was then exposed to an interlocking FR3 nicotine DRA t-sec sucrose schedule. Under this schedule, nicotine continued to be available under the FR schedule while a sucrose pellet was made available contingent upon every pause in self-administration responding (DRA interval) of 40, 80, or 160 s. The FT-group was exposed to noncontingent delivery of sucrose under fixed time (FT) schedules at an average rate equal to that obtained under the DRA schedule in the DRA-group. Results  The DRA schedule significantly reduced NSA by 73, 69, and 59% at the DRA 40, 80, and 160 s intervals, respectively, compared to baseline, while noncontingent sucrose had no significant effect. The effect of the DRA schedule was apparent throughout the NSA sessions. Conclusions  The present assay approximates the abstinence contingencies arranged in contingency management interventions for drug abuse and provides a preliminary nonhuman model of such interventions.     

Effects of changing dosage and urinary pH in rats self-administering nicotine on a food delivery schedule

The effects of different available dosage and of acidic and alkaline urinary pH have been investigated on the rates of self-administration of nicotine by rats on an FT60 food delivery schedule. Different groups of rats initially received one of 3 doses of nicotine (0.05, 0.1 and 0.25 mg/kg/infusion) contingent upon bar-pressing. The self-administration rates during an initial 6-day period of the 3 groups of rats were significantly different from each other, 26.1 ± 3.2 SEM), 15.4 ± 1.5 and 9.5 ± 0.9 at doses of 0.05, 0.1 and 0.25 mg/kg/infusion, respectively. However, once the rates of responding were established during the initial period, no significant changes occured when the doses were changed in all 3 groups after each subsequent 6-day period. These rates of self-administration decreased when saline replaced the available nicotine solution after Day 18. The urinary pH of groups of rats was maintained alkaline (pH9.0), acidic (pH 5.9) or normal (pH 6.7) by allowing them to drink sodium bicarbonate solution, ammonium chloride solution or water, respectively. The self-administration rates during the initial periods of these 3 groups of rats were also significantly different from each other 4.7 ± 0.66, 17.0 ± 0.76 and 9.4 ± 1.11, respectively). In contrast, however, when the rates of responding were established at normal urinary pH during the initial period when water was available, no significant changes occurred when urinary pH was subsequently changed in either an acidic or alkaline direction. The results suggest that the bar-pressing rates are dependent on the amount of nicotine available or present in plasma during the acquisition phase. Nevertheless, once the rate of bar-pressing is established on a food delivery schedule, it seems that the schedule exerts too powerful an effect on behavior for subsequent changes in nicotine levels to modify responding over the period of these experiments.     

The effects of pimozide and of reward omission on fixed-interval behavior of rats maintained by food and electrical brain stimulation

Pimozide (0.125 to 2.0 mg/kg) was administered to rats whose behavior was maintained by a fixed-interval schedule in which the reward was either food (Experiment 1) or electrical stimulation of the brain (Experiment 2). The effects of the drug were compared with the effects of withholding reward (i.e., extinction) in both experiments. Reward omission and administration of pimozide both resulted in decreases in overall rates of responding and increases in the time taken by the subjects to complete a specified number of fixed-intervals. The typical patterning of responding during the sessions of reward omission was also characteristic of the effects of pimozide with food reward but not with brain stimulation reward. The duration of trains of brain stimulation which was under the control of the subjects in Experiment 2, was not altered by administration of pimozide. The differences between the effects of pimozide on behavior maintained by intermittent food reward or by intermittent brain stimulation reward limits a global interpretation of the effects of neuroleptics.     

The effects of nicotine on locomotor behavior in non-tolerant rats: a multivariate assessment

Previous studies assessing the effects of nicotine on the locomotion of non-tolerant rats have yielded mixed results. High doses of nicotine have been reported to decrease both rearing and locomotor behavior. Low doses of nicotine have either decreased or had no effect on rearing, and have been reported to increase, decrease or have no effect on ambulation. The present study utilized ten indices of locomotor behavior collected simultaneously, which allowed for a more fine-grained analysis of locomotor behavior than has been possible previously. The results indicated that nicotine decreased measures of vertical movement (rearing) in a dose-related fashion. Measures of ambulation revealed a more complex pattern: the number of movements increased in a dose-dependent fashion, while average speed of movement exhibited a dose-related decrease. The present results may help to explain the disparate results reported by earlier investigators.     

Joint effects of medial septal lesions and amylobarbitone injections on resistance to extinction in the rat

32 male rats, of which half had sustained small electrolytic lesions in the medial septal area and half had received sham operations, were trained on continuous reinforcement to run an alley for water reward and then given four days of extinction testing. Half of both the lesioned and sham-operated groups were given sodium amylobarbitone on Days 1 and 2 of extinction and the other half on Days 3 and 4, saline being administered on non-drug days. The drug, unusually, decreased resistance to extinction. This effect was probably due to the subjects having taken part in a previous experiment in which they had received, without drugs, training and extinction under the same conditions as in this experiment. In the goal section of the alley, the drug effect was greater in lesioned than shamoperated rats. The lesions retarded extinction, and this effect was reduced by the drug. A model for the neural loci at which amylobarbitone acts to affect resistance to extinction, based on these and other results, is proposed.     

The effects of fenfluramine and fluoxetine on the acquisition of a conditioned avoidance response in rats

This study tested a behavior-suppressing punishment system and how its activity may be altered by agents known to interrupt or enhance serotonergic (5-HT) transmission. Holtzman male albino rats were tested for shuttle box avoidance acquisition and intertrial responding either 1 or 8 h following daily injections of fenfluramine (FEN) or fluoxetine (FXT). When the drug-test interval was 1 h, a time when both drugs are presumably potentiating 5-HT activity, avoidance acquisition and intertrial responding were impaired. When testing occurred 8 h after drug treatment, a time when 5-HT levels are unaltered by FXT and are maximally reduced by FEN, these drugs had no effect on avoidance acquisition, but FEN produced an increase in intertrial responses whereas FXT did not. These results support the proposal of an inhibitory 5-HT system. Furthermore, these data demonstrate that FEN is capable of exerting a biphasic action on intertrial responding and suggest that the time interval between drug administration and behavioural testing is a crucial variable when investigating FEN.     

Acute and chronic effects of nornicotine on locomotor activity in rats: altered response to nicotine

Rationale: Nicotine, a tobacco alkaloid, is known to be important in the acquisition and maintenance of tobacco smoking. Nornicotine, an active nicotine metabolite, stimulates nicotinic receptors and may produce psychomotor effects similar to nicotine. Objective: The present study determined the effects of acute and repeated administration of nornicotine on locomotor activity and compared its effects with those of nicotine. Methods: R(+)-Nornicotine (0.3–10 mg/kg), S(–)-nornicotine (0.3–10 mg/kg), S(–)-nicotine (0.1–1 mg/kg) or saline was administered s.c. to rats acutely or repeatedly (eight injections at 48-h intervals). Activity was recorded for 50 min immediately after each injection. Results: S(–)-Nicotine produced transient hypoactivity, followed by dose-related hyperactivity. Repeated S(–)-nicotine administration resulted in tolerance to the hypoactivity and sensitization to the hyperactivity. Subsequent testing following a saline injection revealed evidence of conditioned hyperactivity. Acute administration of 0.3 mg/kg or 1 mg/kg R(+)- or S(–)-nornicotine produced no effect. Transient hypoactivity was observed at 3 mg/kg and 10 mg/kg R(+)-nornicotine and at 10 mg/kg S(–)-nornicotine. However, rebound hyperactivity was not observed following acute administration of either nornicotine enantiomer, suggesting that nornicotine-induced psychomotor effects differ qualitatively from those of S(–)-nicotine. Repeated R(+)-nornicotine resulted in tolerance to the transient hypoactivity, however hyperactivity was not observed. Repeated S(–)-nornicotine resulted in tolerance to the hypoactivity and the appearance of hyperactivity. Repeated administration of either nornicotine enantiomer resulted in a dose-dependent alteration in response to a 1 mg/kg S(–)-nicotine challenge, suggesting some commonalities in the mechanism of action. Conclusion: Nornicotine likely contributes to the neuropharmacological effects of nicotine and tobacco use. Received: 11 January 1999 / Final version: 25 March 1999     

Investigation of endocannabinoid modulation of conditioned responding evoked by a nicotine CS and the Pavlovian stimulus effects of CP 55,940 in adult male rats

Rationale  The cannabinoid CB₁ receptor antagonist/inverse agonist rimonabant (SR 141716) has been shown to block reinforcing and rewarding effects of nicotine. Research has not investigated whether the cannabinoid system is involved in the interoceptive stimulus effects of nicotine functioning as a conditional stimulus (CS). Objective  We examined the effects of rimonabant and the CB_1/2 receptor agonist, CP 55,940, on responding evoked by a nicotine CS in rats. Additionally, we determined whether CP 55,940 functioned as a CS or a Pavlovian positive drug feature Materials and methods  Pavlovian discrimination training involved intermixed nicotine (0.2 mg base/kg) and saline sessions with intermittent access to water only on nicotine. Antagonism tests with rimonabant (0.1-3 mg/kg) and substitution tests with CP 55,940 (0.003–0.1 mg/kg) followed. An effective dose of CP 55,940 was tested against the nicotine generalization curve. A separate group received CS training with CP 55,940 (0.01 mg/kg). Two other groups were trained using CP 55,940 (0.01 or 0.03 mg/kg) as a positive drug feature in which a brief light CS signaled access to water only on CP 55,940 sessions Results  Rimonabant blocked nicotine-evoked responding. CP 55,940 partially substituted for nicotine and enhanced responding to lower nicotine doses. Overall, CP 55,940 did not acquire control of conditioned responding in either Pavlovian drug discrimination task Conclusions  The cannabinoid system was involved in the CS effects of nicotine. This finding is counter to the operant drug discrimination research with nicotine as a discriminative stimulus, warranting further research into this possible dissociation.     

The psychopharmacology of impulsive behaviour in rats VIII: effects of amphetamine,methylphenidate, and other drugs on responding maintained by a fixed consecutive number avoidance schedule

Rationale Impulsive behaviour is a component of psychiatric disorders such as bipolar disorder, attention deficit hyperactivity disorder (ADHD), or personality disorders. Most experimental studies on impulsive behaviour punish impulsive choices by loss or delay of reward. In the present study, impulsive behaviour was punished by an explicitly aversive stimulus, using a novel fixed consecutive number (FCN) schedule of electric shock avoidance.Objectives This study was conducted to demonstrate stable performance using the FCN avoidance procedure, and examine the effects of drugs previously shown to affect impulsive behaviour using a conventional FCN schedule.Methods First, rats were trained in the appetitive FCN procedure. Pressing the right lever in an operant conditioning chamber after having pressed the left lever at least six times delivered a food pellet (FCN6). Responses on the right lever before completing this ratio resulted in a time out and restarted the ratio. The rats were then switched to FCN avoidance. Responses on the right lever made before completion of the ratio also resulted in food delivery, but were accompanied by an electric shock.Results Chlordiazepoxide (10.0 mg/kg), ethanol (1.0 g/kg), and haloperidol (0.1 mg/kg) increased impulsive behaviour by reducing the number of left lever responses made before the right lever was pressed. Imipramine (1.0–10.0 mg/kg) and desipramine (1.0–10.0 mg/kg) had no effect on impulsive choice. Amphetamine (0.4 and 0.8 mg/kg) and methylphenidate (6.0 mg/kg) significantly increased the mean chain length, and the proportion of very long chains, indicative of reduced impulsivity, although this did not improve efficiency.Conclusions The increases in impulsivity produced by chlordiazepoxide, ethanol, and haloperidol were similar to those under appetitive FCN schedules. In contrast, amphetamine and methylphenidate, by reducing impulsivity in the FCN avoidance, induced effects opposite to those observed in an appetitive FCN procedure. These results suggest that the therapeutic actions of stimulants, to reduce impulsive behaviour in ADHD, may arise in part by increasing the control of behaviour by aversive stimuli.     

Acquisition, maintenance and reinstatement of intravenous cocaine self-administration under a second-order schedule of reinforcement in rats: effects of conditioned cues and continuous access to cocaine

 Second order schedules of IV cocaine reinforcement in rats provide a reliable method for evaluating the effects of conditioned stimuli on cocaine-seeking behaviour, and for measuring the motivational aspects of cocaine reinforcement. In the procedure established here, each infusion of cocaine (0.25 mg/infusion) was initially made contingent on a lever press and was paired with a 20-s light conditioned stimulus (CS). When rats acquired stable rates of cocaine self-administration, the response requirement for cocaine was increased progressively to a second-order schedule of the type FI15 min(FR10:S), whereby the IV cocaine infusion was self-administered following the completion of the first FR10 responses (and CS presentation) after a 15-min fixed interval (FI) had elapsed. Evaluation of the animals’ responding during the first, drug-free interval of each daily session provided a measure of cocaine-seeking behaviour, independent of other pharmacological effects of the self-administered drug. Thus, a dose-response study (dose range: 0.083, 0.25 and 0.50 mg/infusion) revealed that responding under this schedule during the initial, drug-free interval changed monotonically with dose, whereas an inverse relationship between cocaine dose and response level tended to appear during the rest of the session, after rats had self-administered the drug. Responding under this schedule was also shown to occur under the control of the CS, which had acquired conditioned reinforcing properties. Thus, a decrease in responding and an increase in the latency to initiate responding followed the omission of the CS for 3 consecutive days. In addition, extinction of cocaine-seeking behaviour was slower when contingent CS presentations occurred compared to extinction when the CS was not present. Furthermore, the reinstatement of responding for cocaine, which followed a brief period of non-contingent CS presentations, was retarded when this conditioned reinforcer had been extinguished together with cocaine. Finally, cocaine-seeking behaviour decreased markedly for the first 6 h that followed a 12-h period of continuous access to cocaine, when compared to responding 6 h after a 90-min session of limited access to the drug. Responding subsequently increased to baseline levels within 72 h. These results emphasise the utility of second-order schedules for studying drug-seeking behaviour and the importance of drug-associated cues in maintaining such responding for cocaine. Received: 4 December 1997 / Final version: 28 April 1998     

The effects of compounds related to γ-aminobutyrate and benzodiazepine receptors on behavioural responses to anxiogenic stimuli in the rat: Extinction and successive discrimination

In a first set of experiments rats were trained to run in a straight alley for food reward on a continuous reinforcement schedule and the running response was then extinguished. On the last 2 days of training and daily throughout extinction different groups of animals were injected IP with saline, 5 mg/kg chlordiazepoxide, 0.75 mg/kg picrotoxin, chlordiazepoxide + picrotoxin, chlordiazepoxide +1.5 mg/kg bicuculline, 0.00125 or 0.25 mg/kg muscimol, 1 mg/kg baclofen, chlordiazepoxide + baclofen, or 0.00125 mg/kg muscimol + baclofen. Chlordiazepoxide increased resistance to extinction, a well-known anxiolytic effect. This effect was blocked by both picrotoxin and bicuculline. Picrotoxin on its own reduced resistance to extinction (an anxiogenic-like effect). Whether given alone or in combination with other drugs, muscimol and baclofen had no effect.In a second set of experiments rats were trained in a successive operant discrimination (signalled by a flashing or steady light) between components in which sucrose reward was available on a variable-interval schedule for barpressing and components in which no reward was given. Chlordiazepoxide at 10 mg/kg increased responding in both rewarded and nonrewarded components, but more in the latter than could be accounted for by change in the former. This effect is as expected with an anxiolytic drug. It was not altered by administration of bicuculline at 1.5 or 1.75 mg/kg; at 2 mg/kg bicuculline acted synergistically with chlordiazepoxide. Picrotoxin (1 and 1.5 mg/kg) also acted synergistically with chloridazepoxide, enhancing the latter's rate-increasing effects, but only during rewarded components. Neither muscimol (0.00125 and 0.25 mg/kg) nor baclofen (0.01 mg/kg) affected response rates, whether given alone or in combination. However, baclofen in a dose of 1 mg/kg, provided it was given to rats also injected with muscimol (0.00125 or 0.25 mg/kg) at other times, significantly reduced responding during nonrewarded components (an apparently anxiogenic effect).The results of the two sets of experiments are discussed in relation to the hypothesis that anxiolytic drugs affect behaviour by increasing GABAergic inhibition. Offprint requests to: C. Buckland     

The effects of a single exposure to uncontrollable stress on the subsequent conditioned place preference responses to oxycodone,cocaine, and ethanol in rats

Rationale Acute stress has been shown to facilitate the rewarding effects of a number of commonly abused drugs, although the stressor typically must be administered either immediately before or during drug administration and often in the same environment. We have previously reported that a single session of an uncontrollable (inescapable tailshock, IS), but not controllable (escapable tailshock, ES), stressor can enhance the conditioned place preference (CPP) response to morphine, even when stressor and drug administration are separated temporally and spatially. However, this persistent, trans-situational enhancement did not occur to amphetamine CPP. Objectives The following experiments were conducted to determine whether the long-term effects of IS on drug reward are specific to opioids. Materials and methods Adult, male Sprague–Dawley rats were exposed to a single session of IS or remained in their home cages (HC). Twenty-four hours later, using an unbiased procedure, CPP conditioning was conducted with either oxycodone (0, 2, or 5 mg/kg, sc), cocaine (0, 1, 5, or 10 mg/kg, ip), or ethanol (0.3, 1, or 2 g/kg, ip). Another group of rats were exposed to IS, ES, or HC treatment and conditioned with oxycodone (5 mg/kg, sc) 24 h later. Results IS enhanced the subsequent CPP response to oxycodone, but not cocaine or ethanol. This enhancement was dependent on the controllability of the stressor, as ES did not affect oxycodone CPP. Conclusions These results indicate that the long-term, trans-situational enhancing effect of uncontrollable stress on drug reward is specific to opioids. This work was supported by NIH: DA13159, DA015642, and DA16004.     

The pharmacology of impulsive behaviour in rats VII: the effects of serotonergic agonists and antagonists on responding under a discrimination task using unreliable visual stimuli

Rationale: The serotonergic systems have been implicated in the pathological impulsive behaviour on the basis of both clinical and preclinical data. However, impulsivity is probably made up of several independent factors, and the involvement of the diverse regulatory mechanisms of the serotonergic systems has not been widely studied. Objective: The influence of a range of serotonergic agents on impulsivity was examined using a procedure designed to test the dimension of impulsivity termed ”reflection-impulsivity” in rats. Methods: An operant procedure was used in which the need to wait before responding was made explicit by using a signal which increased in predictive value the longer the subject waited before responding. First, the rats learned that a light signal indicated the availability of a food reinforcer if one of two levers was pressed. In the test procedure, on each trial, when the light was turned on it was only 50% likely to indicate the ”correct” lever. After a brief interval it was turned off and on again, this time with a slightly higher probability (>50%) of indicating the correct lever. Over a period of a few seconds the probability that the light indicated the correct lever increased to almost 100%. Thus a quick response to the light would result in many errors, whereas a slow response could always result in food delivery. Once trained the rats were treated with a series of drugs: citalopram, (selective serotonin reuptake inhibitor), p-chloramphetamine (PCA, serotonin releaser), 8-OH-DPAT (5-HT_1A agonist), RU24969 (primarily a 5-HT_1B receptor agonist), DOI, (5-HT₂ agonist), WAY-100,635 (5-HT_1A antagonist), ritanserin (5-HT₂ antagonist), and MDL-72222, (5-HT₃ antagonist). Results: Of the test compounds, PCA, DOI and 8-OH-DPAT increased reaction times, whereas ritanserin reduced them. Citalopram and WAY-100,635 had no significant effects, RU-24969 appeared to disrupt responding, and MDL-72222 reduced premature responses and the number of short reaction times. Conclusions: Since agonists at the 5-HT_1A and 5-HT₂ receptors both reduced impulsivity in this procedure, these data suggest that serotonin may promote ”reflection” in this procedure via stimulation of these receptor subtypes. Received: 25 February 1999 / Final version: 4 June 1999     

The intra- and inter-laboratory reproducibility and predictivity of the KeratinoSens assay to predict skin sensitizers in vitro: Results of a ring-study in five laboratories

Due to regulatory constraints and ethical considerations, research on alternatives to animal testing to predict the skin sensitization potential of novel chemicals has gained a high priority. Accordingly, different in vitro, in silico and in chemico approaches have been described in the scientific literature to achieve this goal. To replace regulatory approved animal tests, these alternatives need to be transferable to other labs, their within and between laboratory reproducibility must be assured, and their predictivity should be high. The KeratinoSens assay is a cell-based reporter gene assay to screen substances with a full dose-response assessment. It is based on a stable transgenic keratinocyte cell line. The induction of a luciferase gene under the control of the antioxidant response element (ARE) derived from the human AKR1C2 gene is determined. Here we report on the results of a ring-study with five laboratories performing the KeratinoSens assay on a set of 28 test substances. The assay was found to be easily transferable to all laboratories. Overall both the qualitative (sensitizer/non-sensitizer categorization) and the quantitative (concentration for significant gene induction) results were reproducible between laboratories. A detailed analysis of the transferability, the within- and between laboratory reproducibility and the predictivity is presented.