Escalation of methamphetamine self-administration in adolescent and adult rats

Abstract

Background

Methamphetamine (METH) use has increased substantially in the last 10 years and poses a serious health concern, especially for young populations. Drug abuse primarily begins during adolescence, when uninhibited and excessive and drug intake is a common occurrence; thus, understanding the developmental patterns of addiction during this critical period is an essential step in its prevention. In the present study, the effect of age on the vulnerability to METH abuse was examined using a rat model of bingeing (i.e., escalation).

Methods

Adolescent and adult rats were compared during short (ShA, 2-h) and long-access (LgA, 6-h) to METH self-administration. On postnatal (PN) days 23 (adolescents) and 90 (adults), rats were implanted with i.v. catheters and trained to lever press for infusions of METH (0.05 mg/kg) during 2-h sessions. Once the rats reached a steady rate of METH self-administration, they were divided into ShA or LgA groups and allowed to self-administer METH for 15 additional days.

Results

Results indicated that adolescent rats earned significantly more infusions than adults under the LgA condition, but the age groups did not differ during ShA. Adolescents, but not adults, also significantly increased (i.e., escalated) METH self-administration across the 15 days of testing under the LgA condition. Further analysis indicated excessive responding during infusions in the LgA METH-exposed adolescents compared to the other groups, suggesting elevated impulsivity or motivation for drug.

Conclusion

These results demonstrate that adolescents are more vulnerable to the escalation of METH than adults during LgA.

1. Introduction

Methamphetamine (METH) use is a widespread problem (Rawson et al., 2007). The United Nations Office of Drug and Crime estimates that between 13.7 and 52.9 million people used an amphetamine-like substance at least once in 2010 (nearly half the North American users consumed METH), figures similar to or exceeding cocaine or heroin on a global scale. The growing popularity of METH is attributed to its potency and price. The street cost of METH has continually decreased over the past decade while the drug's purity and availability have increased, thus making first time use more accessible and more addictive (World Drug Report, 2010). Adolescents may be particularly vulnerable to METH abuse given the drug's growing popularity, availability, potency, and low price, and data from U.S. treatment programs indicate that adolescents comprise more than 20% of those admitted for METH abuse or dependence (Rawson et al., 2007; Gonzales et al., 2008).

Drug use generally starts and progresses more rapidly during adolescence. For example, the time from the initial use to the onset of dependence for most drugs of abuse is shorter in adolescents compared to adults (Clark et al., 1998), and once dependence develops, adolescents are more likely to engage in harmful and potentially lethal drug binges (Baumeister and Tossmann, 2005; Estroff et al., 1989; McCambridge and Strang, 2005). Even if abstinence does occur, adolescents are more resistant to treatment interventions and are at an increased risk for relapse compared to adults (Brown and D'Amico, 2001; Catalano et al., 1990; Chung et al., 2006; Dennis et al., 2004; Perepletchikova et al., 2008; Winters and Lee, 2008). These drug abuse patterns are enduring, as epidemiological evidence indicates that adolescence marks a period when individuals are most susceptible to developing lifelong drug addiction (Schramm-Sapyta et al., 2009; Spear, 2004, 2010; Spear and Varlinskaya, 2005, 2010; Doremus-Fitzwater et al., 2010).

Animal models have helped characterize underlying factors that influence drug abuse vulnerability in adolescents. This work has indicated that adolescents (vs. adults) are less sensitive to the aversive aspects of drugs of abuse such as negative withdrawal-related effects that may otherwise limit drug intake (Doremus et al., 2003; Infurna and Spear, 1979; Kota et al., 2007; O'Dell et al., 2007a,b; Schramm-Sapyta et al., 2006; Spear, 2010; Spear and Varlinskaya, 2005; Varlinskaya and Spear, 2004a,b; Doremus-Fitzwater et al., 2010). Furthermore, several studies have indicated that adolescents consume greater amounts of cocaine per kg of body weight (Anker and Carroll, 2010; Anker et al., 2011; Perry et al., 2007), are more resistant to extinction of responding previously reinforced with i.v. cocaine (Anker and Carroll, 2010; Anker et al., 2011), and are more vulnerable to the reinstatement of cocaine-seeking behavior than adult rats (Anker and Carroll, 2010). Thus, animal work supports findings from clinical studies indicating that adolescents have greater drug abuse vulnerability than adults.

The purpose of the present study was to examine METH self-administration using an animal model of drug bingeing (i.e., escalation). This pattern of drug intake was examined because evidence suggests that adolescent humans are particularly prone to engaging in dangerous levels of binge-like drug intake (Baumeister and Tossmann, 2005; Estroff et al., 1989; McCambridge and Strang, 2005), yet it remains unknown if this occurs with respect to METH.

2. Materials and methods

2.1. Subjects

Fourteen adult (postnatal (PN) day 90) and thirty adolescent (PN day 23) male Wistar rats served as subjects in the study. Adolescent rats were bred at the University of Minnesota from parents obtained from Harlan Sprague-Dawley, Inc., Indianapolis, IN, USA and housed in temperature- (24 °C) and humidity-controlled holding rooms under a constant light/dark cycle (12:12 h with room lights on at 6:00 a.m.)where they had ad libitum access to food and water. Adults were obtained from the same supplier and were housed under the same conditions. Adult male rats weighed 450–500g at the beginning of the study, and adolescent rats weighed 70–90g. Male rats are considered adults around PNday 60 (Spear, 2000); thus, adolescence was defined as PN days 21–60. On PN days 23–25 for adolescents and 90–100 for adults, rats were implanted with an intravenous catheter following the procedure outlined by Carroll and Boe (1982). During experimental sessions, adolescent and adult rats had free access to water and were given 20 g ground food (Purina Laboratory Chow) at the end of each session (3:00 p.m.). The experimental protocol (1008A87754) was approved by the University of Minnesota Institutional Animal Care and Use Committee (IACUC), and the experiment was conducted in accordance with the Principles of Laboratory Animal Care (National Research Council, 2003) in laboratory facilities accredited by the American Association for the Accreditation of Laboratory Animal Care.

2.2. METH self-administration

Operant conditioning chambers previously described in detail were used in the present study (Carroll et al., 2002). Three days following surgery, rats were trained to self-administer 0.05mg/kg i.v. METH under a fixed-ratio 1 (FR-1) schedule of reinforcement following methods reported by Anker et al. (2010). Briefly, training sessions began at 9:00 a.m. with the illumination of the house light and ended at 11:00 a.m. with its termination. During each self-administration session, a response on the left lever (active lever) delivered a single infusion of 0.05 mg/kg METH and activated the set of stimulus lights located above the lever for the duration of the infusion. Responses on the active lever that occurred during infusions were recorded but did not result in subsequent infusions. There was no timeout period following infusions. A response on the right lever (inactive lever) also illuminated the stimulus lights above that lever (to control for responding maintained by cues) but did not produce i.v. infusions. Acquisition of METH self-administration occurred once rats earned at least 35 unassisted infusions with a 2:1 active/inactive lever response ratio.

Due to the brief duration of adolescence in rats, and the goal of comparing adolescents and adults on escalation of intake, an abbreviated escalation procedure was used (15 days) as opposed to the standard 21-day procedure (e.g., Anker et al., 2010; Perry et al., 2007). Following acquisition, rats were allowed to maintain stable METH intake under experimental conditions identical to those described for training with the exception that self-administration took place during a short access 2-h session (9:00 a.m.–11:00 a.m.) or an extended access 6-h session (9:00 a.m.–3:00 p.m.) for 15 additional days.

2.3. Drugs

METH was supplied by National Institute of Drug Abuse (Research Triangle Institute, Research Triangle Park, NC, USA), dissolved in 0.9% NaCl at a concentration of 0.2 mg METH/1 ml saline, and refrigerated. Heparin (1/200 ml saline) was added to the METH solution to prevent catheter occlusion from thrombin build-up. The flow rate of each METH infusion was 0.025 ml/s, and the duration of pump activation (1 s/100 g of body weight) was adjusted to maintain rats at a 0.05 mg/kg METH dose throughout self-administration testing. Due to the rapid weight gain in the adolescent group, the infusion duration was adjusted approximately every three days for all rats to maintain the 0.05 mg/kg METH dose.

2.4. Data analysis

Infusions and active and inactive lever responses during the differential access condition served as the primary dependent measures. Responses and infusions were averaged across the first and last 3 days of the differential access condition for the ShA and LgA conditions and were compared with 2-factor repeated-measures ANOVAs (age × days). Responses per second during an infusion were also analyzed for the ShA and LgA groups during the differential access condition to examine age differences in cocaine seeking during brief periods of nonavailabilityto additional i.v. cocaine. After a significant main effect, post hoc tests were conducted using Fisher's LSD protected t-tests. All data analyses were conducted using GB Stat (Dynamic Microsystems, Inc., Silver Spring, MD, USA).

3. Results

Table 1 shows the mean food intake and weight at the start and the end of the differential access phase (days 1–3 vs. days 13–15) for adolescent and adult rats under the ShA and LgA conditions. There were no significant differences in food intake due to age or access condition. Examination of weights indicated that adolescent rats under the ShA and LgA condition significantly increased weight during the end of the procedure compared to the beginning (F_{1, 59} = 53.49, p < 0.01), while adult rats did not show a significant change. Average infusion duration during the study ranged from 1.12 to 2.29 s for adolescents and 3.45 to 4.06 s for adults.

Table 1.

Mean food intake and weight (g) in adolescent and adult rats under ShA(2h) and LgA(6h) to i.v. METH.

	ShA				LgA
	Adolescent		Adult		Adolescent		Adult
Days	1–3	13–15	1–3	13–15	1–3	13–15	1–3	13–15
Food intake ± SEM (g)	15.6 ± 0.9	17.1 ± 0.6	20.2 ± 0.1	20.0 ± 0.0	14.8 ± 0.6	16.7 ± 1.1	19.9 ±0.1	20.0 ±0.1
Weight ± SEM (g)	128.9 ± 5.4	194.7 ± 11.3a	374.2 ± 4.2	378.8 ± 6.5	146.1 ± 8.4	185.8 ± 5.9a	373.6 ± 8.3	373.4 ± 8.7

^aAdolescents (ShA and LgA) weighed significantly more at day 15 of the study compared today 1 (p < 0.01).

3.1. Differential access

METH infusions (Fig. 1) and active lever presses (Table 2) did not significantly differ between adolescent and adult rats, and there were no differences between the first and last 3 days of the ShA condition. However, during LgA, adolescents earned significantly more METH infusions (F_{1, 61} =19.00, p < 0.01) and pressed the active lever (F_{1, 61} = 19.66, p < 0.01) significantly more than adults. In addition, both adolescents and adults earned more infusions (F_{1, 61} =4.50, p < 0.05) and made more active lever responses (F_{1, 61} =6.56, p < 0.05) during the last 3 days compared to the first 3 days of the LgA condition. Adolescents also made more inactive lever responses under the ShA (F_{1, 25} = 8.34, p < 0.05) and LgA (F_{1, 61} = 6.98, p < 0.05) conditions during this time (Table 2). In addition, under the LgA (but not the ShA) condition, adolescent rats (vs. adult rats) made significantly more responses during infusions (t = 5.49, df = 25, p < 0.01; Fig. 2).

Fig. 1.

Mean (±SEM) METH infusions earned by adolescent and adult male rats during fifteen ShA (2 h) or LgA (6 h) sessions. Adolescents earned significantly more METH infusions than adults under LgA sessions (*p < 0.01). However, there were no significant age differences in the ShA groups. Adolescents and adults under the LgA condition escalated METH intake during the last 3 days compared to the first 3 days of testing (^†p < 0.01).

Table 2.

Mean responses on the active and inactive levers by adolescent and adult male rats during fifteen ShA (2 h) or LgA (6 h) sessions.

	ShA				LgA
	Adolescent		Adult		Adolescent		Adult
Days	1–3	13–15	1–3	13–15	1–3	13–15	1–3	13–15
Active lever± SEM	40.8 ± 7.2	41.9 ± 7.6	28.2 ± 6.7	42.3 ±11.0	104.1 ± 7.2a	135.4 ± 16.3a,b	59.4 ± 7.4	85.6 ± 12.9b
Inactive lever ± SEM	20.4 ± 7.2a	25.1±7.1a	4.2 ±1.4	4.9 ±1.6	54.6 ± 15.3a	67.1 ± 26.7a	8.6 ± 4.9	17.6 ± 13.5

^aAdolescents > adults (p < 0.05).

^bDays 13–15 >days 1–3 (p < 0.05).

Fig. 2.

Mean (±SEM) nonreinforced active lever presses per 1 s of infusion time by adolescent and adult rats with ShA or LgA to i.v. METH. Adolescents (vs. adults) made significantly more nonreinforced responses during LgA sessions (*p < 0.05), but both groups responded equally under ShA conditions.

An additional analysis was conducted to determine if the elevated METH intake in adolescents (vs. adults) during LgA sessions was a result of METH-seeking behavior and not due to general activity. Results confirmed that under the LgA condition adolescents responded more on the active lever (paired with METH infusion) compared to the inactive lever (t = 4.40, df = 39, p < 0.01). Thus, age differences were not due to indiscriminate responding in adolescents under the LgA condition.

4. Discussion

In the present study, adolescent rats earned significantly more METH infusions than adult rats under conditions of prolonged access (LgA) to i.v. METH. These results suggest that adolescence marks a period of increased vulnerability to the self-administration of METH. Similarly, in a recent study, Zakharova et al. (2009) demonstrated that the conditioned rewarding effects of METH were significantly greater in adolescent vs. adult rats under a conditioned place preference (CPP) procedure. Results during the LgA condition also support other work indicating enhanced sensitivity of adolescents vs. adults to cocaine-induced locomotor activity (Caster et al., 2005; Parylak et al., 2008; Snyder et al., 1998) and cocaine induced CPP (Badanich et al., 2006; Brenhouse and Andersen, 2008; Brenhouse et al., 2008; Zakharova et al., 2009). Adolescents have also been reported to exceed adults in self-administration of other drugs of abuse such as amphetamine (Shahbazi et al., 2008), cocaine (Anker and Carroll, 2010; Anker et al., 2011), nicotine (Chen et al., 2007; Levin et al., 2003, 2007), and alcohol (Fullgrabe et al., 2007; Vetter et al., 2007).

Escalation of METH self-administration occurred in adolescent and adult rats under the LgA condition which resembled the escalation of cocaine (Anker et al., 2010; Gipson et al., 2011; Perry et al., 2007) and heroin (McNamara et al., 2010; Vendruscolo et al., 2011) intake reported in previous studies.The pattern of escalating METH intake in the LgA groups resembled the binge-like patterns of drug use prevalent in adolescent humans (Baumeister and Tossmann, 2005; Estroff et al., 1989; McCambridge and Strang, 2005). The higher intake of METH by adolescent compared to adults under the LgA condition supports results from several animal studies demonstrating increased vulnerability in adolescent (vs. adult) rats during other critical phases of the drug abuse process such as acquisition (Anker and Carroll, 2010; Perry et al., 2007), extinction (Anker and Carroll, 2010; Anker et al., 2011), and reinstatement (Anker and Carroll, 2010). In contrast to the LgA condition, adolescent and adult rats earned a similar number of METH infusions during ShA. This is in line with several other findings demonstrating no age differences in cocaine self-administration under similar ShA conditions (2 h) (Frantz et al., 2007; Kantak et al., 2007; Kerstetter and Kantak, 2007; Li and Frantz, 2009). Furthermore, the lack of escalation in both the adolescent and adult ShA groups supports previous reports demonstrating that escalation of METH self-administration in rats is restricted to LgA conditions and does not occur in ShA conditions (Kitamura et al., 2006; Marusich et al., 2010; Schwendt et al., 2009).

Adolescents (vs. adults) under the LgA condition made significantly more active lever responses during brief periods of nonavailability to i.v. METH that occurred during infusions. Since the duration of METH infusions was based on the rat's weight, the adolescents' infusions were far shorter in duration than those of adults. Despite this, LgA adolescents responded more than LgA adults during these brief periods suggesting they were more persistent in seeking METH despite not being rewarded. Other studies support this preservative nonreinforced responding in adolescent rats. For example, adolescents were more likely than adults to respond for amphetamine (Shahbazi et al., 2008) and cocaine (Anker et al., 2011) infusions during periods of signaled nonavailability to drug, a possible indicator of an underlying deficit in response inhibition. Alternately, the nonreinforced responding in adolescents may be due to a general increase in behavior or to a difference in cue reactivity compared to adults.

Similar to a previous study (Anker and Carroll, 2010), adolescent rats responded significantly more on the inactive lever compared to adults during the LgA condition suggesting that increased METH seeking was a result of indiscriminate lever pressing rather than motivation to receive METH infusions. However, adolescents responded significantly more on the lever reinforced with i.v. infusions of METH compared to the inactive lever. Furthermore, indiscriminate active lever responding due to stereotypy was controlled by placing levers at a height the required rats to make a concerted effort to achieve a lever press (i.e., partially stand on haunches and exert pressure downward). Thus, despite increased responding over the 15 days in adolescent vs. adults during LgA, behavior was primarily directed toward drug reinforcement. Results from the differential access phase in the present study indicated that adolescent rats, compared to adults, were more sensitive to the reinforcing effects of METH, and they had a decreased ability to control responding as measured by responding that occurred during an infusion that did not yield additional reinforcement.

Findings from animal and human studies suggest that these age differences may be due to differential activation of brain regions that control reward-related behaviors. Several neurodevelopmental changes occur in the mesolimbic pathway and prefrontal cortex during adolescence (Crews et al., 2007). Dopamine release in the mesolimbic pathway in response to stimulant self-administration is involved in attributing salience to drug and nondrug rewards (Carelli and Deadwyler, 1994; Schultz et al., 1993). In both animals and humans, adolescence is characterized by elevated neuronal dopamine activity in the mesolimbic (reward) pathway of the brain (Chambers et al., 2003) suggesting that adolescents are hypersensitive to reward (Casey et al., 2008). In contrast, during adolescence, the frontal cortex involved in the inhibition of potentially harmful and dangerous behaviors are underdeveloped (Montague et al., 1999). Further, deficits in prefrontal cortex function are associated with decreased executive control and increased vulnerability to relapse in adults with METH dependence (Hester et al., 2010). Overactive mesolimbic and underactive prefrontal cortical function in adolescents may result in behaviors that are driven by rewards rather than executive decision making. This is supported by both human and animal studies that show amplified mesolimbic responses and diminished prefrontal cortical responses in anticipation of rewards (Casey et al., 2008). Thus, with respect to the current study, increased METH self-administration in adolescent rats may have been a result of underdeveloped frontal cortical and mesolimbic areas that guide reward-related behaviors.

The growing occurrence of METH abuse worldwide imposes a pressing need to understand its consequences and patterns of use. Our findings suggest that adolescents are more likely to engage in binge-like METH intake that can be exceedingly harmful and potentially lethal. While METH use is dangerous regardless of the user's age, the consequences adolescent users face are far more serious and detrimental than those faced by adults. Our findings reinforce the importance of METH abuse prevention at young ages before serious drug abuse can occur.