Psychopharmacology (1999) 147:125–134
© Springer-Verlag 1999
O R I G I N A L I N V E S T I G AT I O N
Paul Vezina · Peter J. Pierre · Daniel S. Lorrain
The effect of previous exposure to amphetamine on drug-induced locomotion and self-administration of a low dose of the drug Received: 23 November 1998 / Final version: 19 June 1999
Abstract Rationale and objectives: In order to assess directly the relationship between locomotor activity and drug self-administration, the present experiment simultaneously measured these two behaviors in rats with different histories of pre-exposure to amphetamine either following or in the absence of priming injections of the drug. Methods: Different groups of rats were exposed to ten daily injections of either saline (1.0 ml/kg, i.p.) or amphetamine (1.5 mg/kg, i.p.) and, in each of 13 daily sessions starting 10 days later, were given the opportunity to lever press for a low dose of amphetamine (10 µg/kg per i.v. infusion) in a two-lever (active versus inactive) continuous reinforcement task. Animals were administered a priming injection of amphetamine (1.0 mg/kg, i.p.) immediately before testing on the first 8 days, a saline injection (1.0 ml/kg, i.p.) on the next 3 days and amphetamine on the final 2 days of testing. Results: Consistent with previous reports, prior exposure to amphetamine led to an enhanced locomotor response to the priming injection of amphetamine on the first day of testing. Little pressing for drug was observed on this day. Following priming injections on the subsequent test days, evidence for enhanced locomotion by amphetamine-pre-exposed rats diminished and both groups showed comparable and progressive increases in active versus inactive lever pressing. When priming injections were not made, however, only animals previously exposed to amphetamine maintained lever pressing for the drug. Under these conditions, these animals emitted more active lever presses and time-out responses and exhibited higher levels of locomotor activation in proximity to the active drug administering lever than did salinepre-exposed rats. Conclusions: These results are consistent with the view that previous exposure to amphetamine produces a long-lasting enhancement in the behavioral activation animals will direct toward stimuli asP. Vezina (✉) · P.J. Pierre · D.S. Lorrain Department of Psychiatry, The University of Chicago, 5841 S. Maryland Ave. MC3077, Chicago, IL 60637, USA e-mail:
[email protected], Tel.: +1-773-7022890, Fax: +1-773-7020857
sociated with the drug. This enhancement was displayed initially as a sensitized locomotor response to amphetamine on the first day of testing and was subsequently observed on those test days when no priming injections were given when animals continued to self-administer a low dose of amphetamine under a simple schedule of reinforcement. The implications of these findings for our understanding of the excessive expression of drug-directed behaviors are discussed. Key words Amphetamine · Pre-exposure · Sensitization · Drug self-administration · Locomotor activity · Predisposition
Introduction Repeated intermittent exposure to psychomotor stimulant drugs such as amphetamine has long been known to lead to sensitized behavioral responding to subsequent administrations of these drugs. Most often, this sensitization has been demonstrated as an enhanced locomotor response to a challenge drug injection (for review, see Kalivas and Stewart 1991). Attention is being given increasingly, however, to whether previous exposure to these drugs affects other behaviors and, in particular, to drug taking when pre-exposed individuals are subsequently exposed to drugs of abuse (for reviews, see Robinson and Berridge 1993; Piazza and Le Moal 1996; Schenk and Partridge 1997). The idea that psychomotor stimulant drugs not only produce locomotor effects but also support self-administration behaviors via their actions on a common neurobiological substrate, the mesoaccumbens dopamine (DA) system, has figured strongly in recent influential theories of drug taking. Wise and Bozarth (1987) argued, for example, that a wide range of addictive substances have in common the ability to elicit approach behaviors by virtue of their actions on this system. More recently, Robinson and Berridge (1993) proposed that the ability of addictive drugs to increase the incentive salience of drug relat-
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ed stimuli and to elicit craving stems from their ability to produce locomotor sensitization and enhance mesoaccumbens DA neuron reactivity. There is considerable evidence to support these views in the case of psychomotor stimulant drugs. It is generally agreed, for example, that the mesoaccumbens DA system is essential both for the self-administration of these drugs (Roberts et al. 1980; Pettit et al. 1984; Woolverton and Virus 1989; Caine and Koob 1994) and for their locomotor effects (Clarke et al. 1988; McCreary and Marsden 1993; Meyer et al. 1993; Vezina 1996). And, in a manner paralleling sensitization of the locomotor and nucleus accumbens DA responses to amphetamine and cocaine, previous exposure to these drugs has been shown to enhance the acquisition and expression of conditioned place preferences (Lett 1989; Shippenberg and Heidbreder 1995) and to produce a long-lasting enhancement of the predisposition to selfadminister them (Woolverton et al. 1984; Piazza et al. 1989, 1991; Horger et al. 1990, 1992; Valadez and Schenk 1994; Pierre and Vezina 1997; Lorrain et al. 1999; Mendrek et al. 1998). Importantly, manipulations known to block the induction of locomotor and dopaminergic sensitization by amphetamine, such as giving a D1 DA receptor antagonist just before each pre-exposure injection, have also been shown to block the facilitation of drug self-administration normally seen following previous drug exposure (Pierre and Vezina 1998). Such findings suggest that an enhanced predisposition to selfadminister amphetamine may represent another manifestation of the long-lasting changes in neuronal functioning that underlie the development and expression of sensitization to the drug as measured by enhanced locomotor responding and increased levels of extracellular DA in the nucleus accumbens. There are, however, a number of findings that appear not to support this general proposition. For example, previous exposure to nicotine has been found to facilitate the self-administration but not to enhance the locomotor effects of cocaine (Schenk et al. 1991; Horger et al. 1992). In another study, repeated exposure to amphetamine was reported to produce a slight but progressive enhancement of the locomotion produced by this drug but no change in its ability to facilitate intracranial selfstimulation (Wise and Munn 1993). It has also been argued that amphetamine conditioned locomotion and place preference can be dissociated (Swerdlow and Koob 1984; Brown and Fibiger 1993). Finally, in a different but relevant type of experiment, it was found that isolation-rearing enhanced the locomotor response to cocaine but did not facilitate the self-administration of a high dose of the drug (Phillips et al. 1994). Thus, even though procedural and neuropharmacological considerations may provide alternative interpretations in most of these latter cases (see Vezina and Stewart 1987a, 1987b; Carr et al. 1988; Borowski and Kokkinidis 1992; Vezina et al. 1992, 1994; Lorrain et al. 1998; Mendrek et al. 1998), it is clear from the above that the relationship between stimulant drug-induced locomotion and stimulant drug self-administration remains poorly understood.
Noting that in all of the above studies, rats’ locomotor and self-administration behaviors were assessed in separate experiments or experimental sessions, the following study investigated the relationship between these two stimulant drug-related behaviors by assessing them simultaneously. Rats with no previous lever-press training were exposed to a regimen of amphetamine injections known to enhance both of these behaviors and, starting 10 days later, their locomotor activity and self-administration of a low dose of the drug were assessed daily over the course of 13 test days. Because so-called “priming” injections are often used to facilitate responding during the training period in drug self-administration experiments and because such injections also permit testing for sensitization of locomotion, animals in the present experiment were tested following a systemic injection of amphetamine on the first 8 days, following a systemic saline injection on the next 3 days and again following an amphetamine injection on the final 2 days of testing.
Materials and methods Subjects Forty-eight male Sprague-Dawley rats (Harlan Sprague-Dawley, Madison, Wis.) weighing 250–300 g at the beginning of the experiment were used. They were individually housed and maintained with freely available food and water in a reverse cycle room (12 h light/12 h dark) for the duration of the experiment. Animals were handled daily for 2–3 days prior to testing and were always tested during the dark period of the light cycle. Apparatus Drug pre-exposure and testing for sensitization Twelve test chambers were used. Each chamber (21.5×41.5×27 cm, inner dimensions) was constructed of opaque plastic (rear and two side walls) and a Plexiglas front-hinged door with a ceiling and floor consisting of evenly spaced stainless-steel rods. Each chamber was enclosed in a plastic box that shielded animals from extraneous disturbances. White noise was supplied in each box by a ventilating fan. The test chambers were used to simultaneously measure locomotor activity and lever pressing. In each chamber, a retractable lever (5 cm above the floor and 4 cm from the rear wall; MED Associates Inc., St. Albans, Vt.) and a stimulus light (13.5 cm above the lever and 10 cm from the rear wall) were positioned on each of the side walls. One of the levers was designated an active lever (presses resulted in drug delivery) and the other an inactive lever (presses were counted but without consequence; see below). Both were retracted during the pre-exposure phase and introduced into the chambers for the test phase. Two infrared photo beams placed 3.5 cm above the cage floor and perpendicular to the long axis of each chamber were used to record locomotor activity. One photo beam was positioned on each side of the chamber, 11 cm from the same-sided lever and 26 cm from the lever located on the other side. Separate interruptions of the two photo beams were recorded. These were counted separately to provide estimates of locomotion on the active lever and inactive lever sides of the chamber, summed to provide an estimate of overall horizontal locomotor activity throughout the chamber and counted sequentially to measure crosses from one side to the other. Photo-beam interruptions with a duration of less than 0.5 s were not recorded in order to filter out rapid and repetitive movements in one place.
127 The test chambers were equipped with a liquid swivel system to allow for the self-administration of drug upon depression of the active lever. The liquid swivel system was comprised of a steelspring tether, a liquid swivel, and an infusion pump (Razel Scientific Inc., Model A.E). The tether was connected to the animal by screwing its captive collar onto the threaded portion of a customdesigned L-shaped Plastics-One cannula (20 gauge) secured to the animal’s skull. Separate interruptions of photo beams, lever presses, and drug infusions were detected, recorded, and controlled via an electrical interface by a computer using locally developed software. Acute locomotor responding to i.v. amphetamine Eight test chambers were used to assess the acute locomotor effects of different concentrations of amphetamine following noncontingent i.v. administration by the experimenter. In each chamber (28.5×36×34 cm, inner dimensions), one infrared photo beam was positioned 3.5 cm above the floor and 12 cm from each of the side walls. These chambers did not possess drug self-administration levers. All remaining details were as described above. Procedures Twenty-two rats were used to test the effect of previous exposure to amphetamine on drug-induced locomotion and self-administration of a low dose of the drug. This experiment consisted of three phases: pre-exposure, surgery, and testing for sensitization. A separate group of 26 rats was used to assess the acute locomotor response to different concentrations of amphetamine following noncontingent i.v. administration. This experiment consisted of two phases: surgery and locomotor testing. Pre-exposure In this phase, animals were administered ten injections of amphetamine (1.5 mg/kg, i.p.; amphetamine pre-exposed) or saline (1.0 ml/kg, i.p.; saline pre-exposed). Injections were made once a day for 5 days and, following 2 days without injection (intervening weekend), for another five consecutive days. Each day, immediately after their respective injections, rats were placed in the chambers and their locomotor activity was recorded for 2 h to verify the drug injections. The response levers were not present during this phase. Surgery During the 9 days between the pre-exposure and test phases, animals were surgically implanted with intravenous catheters into their right external jugular vein. The catheters were made of silastic tubing (Dow Corning, Inc.) on which two small bulbs of silicon were fashioned to allow securing them in the vein. Surgical procedures were conducted under sterile conditions and performed with the animal under deep anesthesia (ketamine HCl, 100 mg/kg, i.p. and xylazine HCl, 6 mg/kg, i.p.) according to an approved IACUC protocol and as described by Pierre and Vezina (1997). Testing for sensitization Testing began 10 days following the rats’ last pre-exposure injection and consisted of 13 2-h test sessions conducted daily less two intervening weekends. On each of these test days, rats were connected to the self-administration tether, placed in the chamber, and lever pressing (the number of active versus inactive lever presses) as well as locomotor activity were measured simultaneously over the course of the 2-h session. On test days 1–8, rats were administered a challenge systemic injection of amphetamine (1.0 mg/kg,
i.p.) immediately before being placed in the chamber. On test days 9–11, they were injected with saline (1.0 ml/kg, i.p.) and on test day 12 and day 13, they were again injected with amphetamine (1.0 mg/kg, i.p.) before being placed in the chamber. For all animals, presses on the active lever delivered an infusion of amphetamine through the catheter (10 µg/kg per infusion) under a schedule of continuous reinforcement. The amphetamine was injected in volumes of 0.10–0.13 ml/infusion at a rate of 1.6 ml/min. For 10 s immediately following depression of the active lever, the stimulus light located above the lever was lit. During the 10-s time-out period, presses on the active lever were recorded but did not lead to further infusions. Presses on the inactive lever were also recorded but were without consequence. Lever designation did not vary across test days. Rats were returned to their individual home cages after each session. Prior to and immediately following each testing session, approximately 0.15 ml of a flushing solution (containing 30 IU/ml heparin, 250 mg/ml ampicillin and 8333 IU/ml streptokinase diluted in 0.9% saline solution) was infused into the animals’ catheters. After the final testing session, a lethal dose of sodium pentobarbital was infused into animals’ catheters to determine their terminal patency. Rats that did not lose muscle tone within 5–10 s and expire readily were judged to have non-patent catheters and were excluded from the experiment. Three rats (one from the amphetamine- and two from the saline-pre-exposed groups) were thus excluded. A fourth rat (from the amphetamine-pre-exposed group) was excluded because, unlike all other animals, it failed to emit a lever press (active or inactive) on more than 1 day of testing. Data from the remaining 18 rats (9 in each of the pre-exposure groups) are presented below. Acute locomotor responding to i.v. amphetamine Testing began 5–10 days after animals were surgically prepared with intravenous catheters as described above and consisted of one test session. On the test day, rats were connected to the drug administration tether, placed in the chamber and allowed to locomote freely for 1 h. A non-contingent infusion of amphetamine (0, 20, 60, 120, 180, or 360 µg/kg, i.v.) was then administered by the experimenter, and the animals’ locomotor responses were measured for another hour. After testing, catheter patency was assessed as described above. All animals were judged to have patent catheters. Drugs The amphetamine (D-amphetamine sulfate; Research Biomedicals International, Natick, Mass.) was dissolved in saline. Doses refer to the weight of the salt. The sodium pentobarbital solution (Nembutal, 50 mg/ml) was obtained from Abbott Laboratories (North Chicago, Ill.). Data analysis Data were analyzed using analyses of variance (ANOVAs) followed by post-hoc Scheffé comparisons made according to Kirk (1968). In order to confirm injection integrity, the locomotor activity data obtained during the pre-exposure phase were analyzed with one-between, one-within ANOVA with pre-exposure condition (amphetamine/saline) as the between factor and days (10) as the within factor. The drug self-administration and locomotor activity data obtained in the testing phase were analyzed with onebetween, one-within ANOVA with pre-exposure condition (amphetamine/saline) as the between factor and time (twelve 10-min bins) or days (8 for test days 1–8, 3 for test days 9–11 and 2 for test days 12–13) as the within factor. The locomotor activity data obtained during the pre-exposure and testing phases were also analyzed using two-way within ANOVA with days (10 for pre-exposure and 8, 3 or 2 for testing) and side of chamber (active lever side/inactive lever side) as the two factors. Similarly, the drug
128 self-administration data obtained during the testing phase were also analyzed using two-way-within ANOVA with days (8, 3 or 2) and lever (active/inactive) as the two factors. The number of lever presses emitted by the two groups on day 1 of testing were also compared with unpaired t-tests. Finally, the locomotor activity data obtained following acute non-contingent i.v. infusions of amphetamine were analyzed with one-way between ANOVA with dose as the between factor.
Results Pre-exposure As expected, amphetamine produced significant increases in horizontal locomotion throughout the 10 days of pre-exposure (F1,16=175.2 and 103.7, P
