Psychopharmacology (2010) 211:43–54 DOI 10.1007/s00213-010-1866-z
ORIGINAL INVESTIGATION
The effects of response operandum and prior food training on intravenous nicotine self-administration in rats Kelly J. Clemens & Stephanie Caillé & Martine Cador
Received: 21 January 2010 / Accepted: 10 April 2010 / Published online: 1 May 2010 # Springer-Verlag 2010
Abstract Rationale Nicotine intravenous self-administration (IVSA) in rats has been conducted using a variety of methodological procedures with equally variable results. Objectives Here, we addressed the importance of the type of response operandum and prior instrumental training with a natural reinforcer on nicotine IVSA and reinstatement. Methods Rats were tested for spontaneous acquisition of IVSA using either nose poke (NP) or lever press (LVR) operandum. A dose-response test was then conducted, followed by extinction and cue- and nicotine-induced reinstatement. Results The use of the NP operandum resulted in markedly higher levels of IVSA across acquisition and across doseresponse testing compared with the LVR group. Whereas both groups reinstated following a nicotine prime, only the LVR group demonstrated cue-induced reinstatement. As a positive control, the experiment was repeated with cocaine as the reinforcer: equivalent levels of IVSA were observed across all tests, irrespective of operandum. When rats selfadministering nicotine received instrumental training with a sucrose reinforcer prior to IVSA, a facilitated acquisition of IVSA was observed in both LVR and NP groups to a similar extent (the effect of operandum remained), but had This research and KJC were supported by funding from the Association pour la Recherche sur les Nicotianées (ARN), France. Electronic supplementary material The online version of this article (doi:10.1007/s00213-010-1866-z) contains supplementary material, which is available to authorized users. K. J. Clemens (*) : S. Caillé : M. Cador CNRS UMR 5227, Team “Neuropsychopharmacology of Addiction”, University of Bordeaux 2, Bordeaux, France e-mail:
[email protected]
little effect on responding thereafter. During reinstatement testing, both groups now displayed cue- and nicotineinduced reinstatement, but this was also evident in saline control animals that had never received nicotine. Conclusions These results suggest that, unlike cocaine, an increased physical response requirement can decrease nicotine intake. It also indicates that operandum and prior sucrose training may influence the role that visual cues play in nicotine dependence. Keywords Nicotine . Self-administration . Reinstatement . Cue . Operandum . Food training . Cocaine
Introduction More people use tobacco on a daily basis than any other addictive substance, including alcohol (SAMHSA 2009). Up to 50% of those people who experiment with smoking become addicted (Henningfield et al. 1998). Central to the ongoing problem of tobacco dependence is the difficulty smokers have in quitting; only 10–20% of smokers receiving treatment remain abstinent for more than 6 months (Hughes et al. 1999; West et al. 2001). The development of nicotine dependence, abstinence, and relapse can be modeled in the preclinical setting through nicotine intravenous self-administration (IVSA), extinction, and reinstatement. Intravenous self-administration is a reliable and widely used model of drug addiction, exhibiting high face, and predictive validity for a variety of addictive substances (Panlilio and Goldberg 2007). Increasing evidence suggests that nicotine addiction can also be accurately modeled using this paradigm. However, due to the relatively short history of nicotine IVSA, in part related to difficulties with establishing nicotine IVSA in the first instance, the procedure and
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parameters for obtaining nicotine IVSA vary considerably between laboratories. These differences are reflected in the wide range of levels of nicotine IVSA reported (from 8 to 20 infusions per hour) and a similar variability in the presence and extent of reinstatement following either cue or nicotine priming (Andreoli et al. 2003; Bespalov et al. 2005; Caggiula et al. 2001; Diergaarde et al. 2008a; Dravolina et al. 2007; Fattore et al. 2009; LeSage et al. 2004; Shoaib et al. 1997). To date, there have been few studies exploring the causes of such variability. Amongst nicotine IVSA reports, there are two major factors that vary between studies and may influence nicotine IVSA results: (1) the nature of the response operandum and (2) training of the instrumental response with a natural reinforcer prior to commencing IVSA. The use of a nose poke (NP) rather than lever pressing (LVR) operandum has been shown to facilitate the spontaneous acquisition of food (Caine et al. 1999), cocaine (Caine et al. 1999) and combined cocaine/heroin (David et al. 2001) IVSA in mice. Similarly, the spontaneous IVSA of the cannabinoid agonist WIN 55, 212-2 is more rapid, and occur in a greater proportion of rats when a NP, rather than LVR, operandum is used (Deiana et al. 2007). In these situations, the facilitated acquisition of IVSA does not lead to long-term effects of operandum on IVSA. Thus, the use of a NP operandum may be more effective in promoting initial IVSA of a drug that has an otherwise weak reinforcing efficacy, as has been suggested for nicotine (Manzardo et al. 2002; Stolerman and Jarvis 1995). However, the effect that operandum may have on other measures of reward-related behavior has not been addressed. Of particular interest is the reinstatement of reward-seeking behavior where subtle differences in motivation are integral to the expression of relapse. The second potentially contentious factor associated with nicotine IVSA that has yet to be examined experimentally is the use of instrumental training with a natural reinforcer prior to the onset of nicotine IVSA. This technique is employed in the majority of nicotine IVSA studies as a means of facilitating the acquisition of IVSA, and in some instances involves extensive training (Liu et al. 2007b; Shram et al. 2008). With respect to cocaine IVSA, prior food training acts only to facilitate acquisition and has no lasting effects on the sensitivity to reward, extinction, or reinstatement (Bongiovanni and See 2008); however, the impact of such training has not been specifically tested with nicotine. In this experiment, we sought to clarify the roles that the nature of the response operandum and prior instrumental training with a natural reinforcer may have on nicotine IVSA, and how this might translate to changes in susceptibility to cue- and drug-induced reinstatement. As a marked effect of operandum was observed during nicotine IVSA, an alternative reinforcer (cocaine) was used as a
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means of determining the specificity of this effect to nicotine. In addition, control groups receiving intravenous saline were used under each condition, thus accounting for any non-nicotine locomotor or response-related effects.
Materials and methods Subjects Male Sprague Dawley rats (175–200 g; Charles River Laboratories, France) were housed two to three animals per cage on a 12 h reverse light/dark cycle (lights off 09:00 h) with water ad libitum. During experimentation (nicotine sucrose training and IVSA), rats received a maintenance diet of 20 g rat chow per day immediately after the test session. Rats were anesthetized with a mixture of ketamine (100 mg/kg) and Xylazine (12 mg/kg) and implanted with a chronic indwelling catheter as described previously (Guillem et al. 2005). Catheter patency was verified with an infusion of the short-acting barbiturate Hypnomidate® (2 mg/ml, Janssen-Cilag), when necessary. All surgical and experimental procedures were performed in accordance with the European directive (86/ 609/EEC) and with the Centre National de la Recherche Scientifique approval. Drugs (−) Nicotine hydrogen tartrate (Sigma, St. Louis, MO, USA) and cocaine hydrochloride (Cooperative Pharmaceutique Française, France) were dissolved in sterile physiological saline (0.9%). Nicotine for subcutaneous injection was pH adjusted to 7.4. Nicotine doses are expressed as the base and cocaine doses as the salt. Apparatus Two identical sets of 12 standard operant conditioning chambers (30 cm height×40 cm length×36 cm depth, Imetronic, Pessac, France) were located in separate experimental rooms equipped with white noise generators. Experimental chambers were individually housed in sound attenuation boxes with inbuilt exhaust fans. External pumps were fitted with syringes that were connected with Tygon tubing to a fluid swivel and weighted pulley system inside the chamber. The swivel was attached to a spring connector (protecting Tygon tubing) and attached to the back-mount of the animal. The first 12 experimental chambers were equipped with two retractable levers (0.03 N force required for activation) on either side of the left wall (7 cm above the floor) counterbalanced as active or inactive across left and right. The second
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set of 12 chambers was equipped with two NP operanda, one in the center of each side wall (5 cm above the floor) as rats nose poking, but not lever pressing, exhibit a strong preference for the NP hole closest to the door (unpublished data). Illumination of a dim houselight signaled the start of each 2-h test session. Responses in the active hole/lever resulted in a 100-µL infusion of nicotine across 3 (LVR) or 4 (NP) s. This time difference was due to minor differences in the pumps used and is unlikely to be of consequence (Samaha et al. 2005; Sorge and Clarke 2009). Each infusion was accompanied by compound visual stimulus (VS) consisting of 3 s cue light above the active operandum and extinguishing of the houselight for the time-out period (20 s). Responses on the active NP or LVR operandum during the time out or the inactive operandum at any time were recorded, but were of no consequence. Experiment 1: the effect of operandum on the acquisition, dose response, extinction, and reinstatement of nicotine IVSA
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same as for IVSA except nicotine was not available. Following re-extinction, the second reinstatement test (VS plus nicotine) replicated the first but included a nicotine injection (0.3 mg/kg subcutaneous; s.c.) administered immediately prior to the test session (LeSage et al. 2004). Following a second period of re-extinction, the final reinstatement test involved injection of nicotine (0.3 mg/ kg) prior to an extinction test session (no VS). Positive control: cocaine A cocaine group (NP: n=6, LVR: n=6) was used as a positive control to determine if any group differences were specific to nicotine or were a general feature of stimulant drugs. Procedures were conducted as outlined above, except that rats self-administered 0.75 mg/kg/infusion of cocaine during training, 0.19, 0.38, 0.75, and 1.5 mg/kg/ infusion during dose-response testing and received 10 mg/ kg i.p. of cocaine for reinstatement testing. Negative control: saline
Acquisition Rats were randomly assigned to spontaneously acquire nicotine (30 µg/kg/infusion) IVSA using a NP (n=12) or LVR (n=12) operandum under a fixed ratio 1 (FR-1) schedule of reinforcement for 10 days, FR-2 for 3 days, and then FR-5 for 12 days. Three nicotine LVR rats failed to adequately transition to FR-5 and thus received 9 days of additional training (FR-2, FR-3, and FR-4). This extra training did not lead to any lasting changes in behavior (total nicotine intake or responding during reinstatement). Dose response Dose response testing was carried out Tuesday-Friday, with a return to the training dose each Monday. All rats received 7.5, 15, 30, or 60 µg/kg/infusion of nicotine in a random order for 2 days or until responding was stable. Data are represented as an average across the last 2 days at each dose. Extinction and reinstatement All nicotine rats were returned to the training dose of nicotine for a minimum of 5 days. Extinction commenced by removal of nicotine delivery and all associated VS. Extinction continued for a minimum of 10 days or until rats responded at a level less than 30% of the last 3 days of IVSA for two consecutive days or when less than 10 (LVR) or 20 (NP) responses were made on the active operandum (Paterson et al. 2005). Reinstatement testing commenced with cue (VS) reinstatement. The VS reinstatement procedure was exactly the
A third group (NP: n=6, LVR: n=6) received only saline during IVSA sessions to control for non-drug related behavior. The procedure was identical as that outlined above for nicotine except that rats only ever received IV infusions of saline at any time. Reinstatement testing was identical to that described for the nicotine group. Experiment 2: the effect of sucrose training on the acquisition, dose response, extinction and reinstatement of nicotine IVSA The procedure was exactly as described for experiment 1, with the exception that prior to surgery all rats were trained to lever press (n=6) or NP (n=6) for 0.15 mL of a 5% sucrose solution. Sucrose dippers were inserted into instrumental chambers between the two levers or on the left of the active NP hole. Rats received 2×dipper training (15 reinforcements, 30 min), 2×FR-1 (30 min or max 25 reinforcements) and then FR-5 (until criterion of 25 reinforcements in 30 min) sessions. The houselight was illuminated for the entire test session and no other cues were presented at any time. Rats were trained twice daily for 3–5 days before returning to full food ration prior to surgery. The experiment then continued to progress exactly as outlined in experiment 1. Positive control: cocaine A positive control group examining the effects of prior food training on cocaine IVSA was not included as this has already been well described (Bongiovanni and See 2008).
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Negative control: saline The procedure was identical as that outlined above for sucrose trained nicotine rats except that saline rats (NP: n=6, LVR: n=6) only ever received IV infusions of saline. Data analysis Throughout the experiment a total of 10 rats were excluded: seven due to a loss of catheter patency and three from extinction and reinstatement testing due to a failure to meet the extinction criteria (1 NP cocaine, 2 NP nicotine). All IVSA data were analyzed with repeated measures analysis of variance (ANOVA) with day as the withinsubjects factor and operandum (LVR or NP) as the between subjects factor. For dose-response data, two-way repeated measures ANOVA were used with dose as a within-subjects factor and operandum as a between subjects factor. For extinction data, Student’s t tests were used to compare the number of days required to reach the extinction criterion. For reinstatement data, test (extinction or test day) and drug source (active or inactive NP or LVR) were within-subjects factors and a Wilcoxon test for non-parametric paired data was used when appropriate. Post hoc analyses were carried out using the Fisher’s protected LSD test. A probability factor less than 0.05 was considered statistically significant.
Results Experiment 1: the effect of operandum on the acquisition, dose response, extinction and reinstatement of nicotine IVSA Nicotine Rats nose poking for nicotine initially responded much more than those lever pressing (F1,198 =15.41, p< 0.001; Fig. 1a), with the NP group correspondingly receiving more infusions across this time (F1,198 =15.66; p
