Research Findings - Basic Behavioral Research
Stress System Mediation of the "Dark Side" of Compulsive Eating
Some types of obesity and eating disorders can be characterized as chronic relapsing disorders, similar to drug addiction. That is, periods of abstinence from "forbidden" highly palatable foods alternate with relapse to compulsive eating that continues despite negative consequences. Pietro Cottone and his colleagues investigated the role of negative reinforcement and the CRF system in this cycle, in a rat model. As opposed to positive reinforcement, negative reinforcement is the increased probability of a behavioral response produced by the removal of an aversive stimulus, e.g., the intake of palatable food to relieve a negative emotional state produced by abstinence. Rats were provided regular chow for 5 days, followed by a sugary diet for 2 days in a repeating cycle for 7 weeks (Chow/Palatable rats). Control rats had access to regular chow only. This regimen leads to undereating of chow and overeating of the preferred, highly palatable food. Rats then received a CRF1 receptor antagonist (R121919) in varying doses one hour before the switch from chow to palatable food or vice versa. The antagonist dose-dependently decreased palatable diet intake and increased chow intake in the Chow/Palatable rats, without affecting intake in chow controls. Thus, the CRF antagonist blunted the effect of diet cycling. The researchers then tested whether CRF1 receptors were involved in this negative emotional behavior that follows withdrawal from palatable food. After the 7-week feeding cycle, rats that are switched to chow show increased anxiety-like behavior, as measured by reduced time spent in the open arms of an elevated plus maze. However, treatment with the CRF1 antagonist normalized this behavior. In a final behavioral test, they found that the CRF1 antagonist reversed motivational deficits in responding for regular chow in a progressive ratio test exhibited by Chow/Palatable rats. Next, they measured CRF mRNA and peptide levels in the central nucleus of the amygdala. When Chow/Palatable rats were withdrawn from the palatable food, CRF mRNA expression increased five-fold and peptide levels were 70% higher. These measures were not significantly altered in other brain areas, including the hypothalamus, and they returned to baseline when rats regained access to the palatable food. Importantly, they did not observe changes in CRF after only one diet cycle, indicating that the CRF-CRF1 system is recruited progressively by diet history. These withdrawal-induced effects on CRF are similar to findings for drug and ethanol withdrawal. The authors propose that recruitment of anti-reward extrahypothalamic CRF-CRF1 systems during withdrawal from palatable food, analogous to abstinence from abused substances, promotes compulsive selection of palatable food, undereating of a more balanced diet, and a negative emotional state when intake of the palatable food is prevented. Cottone P, Sabino V, Roberto M, Bajo M, Pockros L, Frihauf JB, Fekete EM, Steardo L, Rice KC, Grigoriadis DE, Conti B, Koob GF, Zorrilla EP. CRF system recruitment mediates dark side of compulsive eating. Proc Natl Acad Sci USA. 2009 Nov 24; 106(47): 20016-20020.
Orexin A/Hypocretin-1 Selectively Promotes Motivation for High Salient Positive Rewards
A number of recent studies have indicated that the activation of orexin A/hypocretin-1 neurons in the hypothalamus, and activation of ox/hcrt-1 receptors in the VTA, are linked with drug reward and reinstatement to drug seeking behavior. The new report by Stephanie Borgland, Antonello Bonci, and colleagues shows that oxA/hcrt-1 signaling has a selective role in motivation for highly salient reinforcers. They trained different cohorts of rats to press a lever to obtain cocaine, regular food, or high-fat chocolate pellets and then tested them using a progress ratio procedure (PR) to assess the amount of effort the rats were willing to expend to obtain their respective reinforcers. Specifically, the PR procedure requires animals to press a lever an increasing number of times to obtain a reward and measures the "breakpoint" at which they are no longer willing to press. When ox/hcrt-1 receptor signaling was blocked, the breakpoints for the cocaine and high-fat pellet reinforcers decreased, while the breakpoint for regular food remained unchanged. To further test whether blocking this receptor affected motivation only for highly salient reinforcers, they placed high-fat pellets in one arm of a T maze and regular food in the other. Untreated rats were willing to climb over a significant inclined plane barrier to reach the high-fat pellets, but rats treated with an ox/hcrt-1 antagonist were more likely to take the easy path to the regular food. Next, they performed several electrophysiological studies on brain slices containing the VTA. Their previous studies had shown that oxA/hcrt-1 potentiates glutamate receptor responses in the VTA. In the new studies, they found that this oxA/hcrt-1-induced plasticity was significantly enhanced in rats with a history of cocaine or high-fat self-administration, but not in the rats trained with regular food. OxA/hcrt-1 mediated excitatory synaptic transmission was not, however, potentiated by arousing, aversive stimuli. This series of studies indicates that the oxA/hcrt-1 system may provide a unique opportunity to design novel therapies that selectively reduce excessive drive for highly salient positive reinforcers. Borgland SL, Chang SJ, Bowers MS, Thompson JL, Vittoz N, Floresco SB, Chou J, Chen BT, Bonci A. Orexin A/hypocretin-1 selectively promotes motivation for positive reinforcers. J Neurosci. 2009 Sep 9; 29(36): 11215-11225.
Sex Differences in the Effects of Allopregnanolone on Yohimbine-induced Reinstatement of Cocaine Seeking in Rats
Recent studies have shown that the gonadal hormones estrogen and progesterone play opposite roles in cocaine self-administration in rodent models. Dr. Marilyn Carroll and colleagues at the University of Minnesota, for example, found that whereas estrogen facilitated escalation of cocaine self-administration in ovariectomized female rats, progesterone prevented escalation (Larson et al., 2007). They also found that in females progesterone has an inhibitory effect on reinstatement of cocaine-primed responding, a commonly used model of relapse (Anker et al, 2007), whereas estrogen has previously been shown to play a facilitatory role. They subsequently showed that progesterone's suppression of cocaine-primed reinstatement was mediated by its metabolite allopregnanolone (ALLO), and that suppression by ALLO occurred only in females (Anker et al., 2009). More recently, they examined the effects of ALLO on stress-induced reinstatement of responding. Stress was produced by administration of the pharmacologic stressor yohimbine. They found that yohimbine-priming injections produced more reinstatement responding in females than males. Further, as previously found with cocaine-primed reinstatement, ALLO blocked yohimbine-induced reinstatement only in females. Collectively, these studies point to the potential clinical use of compounds that target progesterone, ALLO, or related compounds in the treatment of cocaine addiction. Higher doses may be required in males than in females. Anker JJ, Carroll ME. Sex differences in the effects of allopregnanolone on yohimbine-induced reinstatement of cocaine seeking in rats. Drug Alcohol Depend. 2010 Mar 1;107(2-3):264-7. Epub 2009 Dec 14.
The Medial Preoptic Area is Necessary for Motivated Choice of Pup- over Cocaine-associated Environments in Early Postpartum Rats
The motivation to seek cocaine during the postpartum period is significantly affected by the competing incentive of offspring, a stimulus unique to this life stage. The present study investigated the functional role of the medial preoptic area (mPOA), a critical site involved in maternal responsiveness, on processing incentive value of pup-associated cues, and directing response allocation for pup- over cocaine-associated environments. The study involved a concurrent pup/cocaine choice conditioned place preference (CPP) paradigm. Early postpartum females with bilateral guide cannulae near the mPOA or control sites were conditioned, from postpartum days (PPD) 4 to 7, to associate environments with pups or cocaine. CPP was tested on PPD8 following intra-mPOA infusions of either 2% bupivacaine to transiently inactivate the mPOA or saline vehicle. In separate experiments, the researchers examined effects of intra-mPOA infusions of bupivacaine on expression of conditioned responding induced by environments associated with either pups or cocaine. Bupivacaine infusion into the mPOA selectively blocked conditioned preferences for pup-associated environments, significantly contrasting the robust pup-CPP seen in non-surgical and intra-mPOA vehicle-treated females. Conversely, mPOA inactivation failed to alter cocaine-CPP in postpartum females. When given a choice between environments associated with pups or cocaine, transient functional inactivation of the mPOA altered choice behavior, biasing the preference of females toward cocaine-associated environments, such that almost all preferred cocaine- and none the pup-associated option. Anatomical specificity was revealed when inactivation of adjacent regions to the mPOA did not affect CPP responses for pups. These findings support a critical role for the mPOA in mediating pup-seeking behavior. They further suggest that competing properties of pups over alternative incentives, including drugs of abuse, rely on mPOA integrity to provide relevant pup-related information to the circuitry underlying the choice behavior between pups and alternative stimuli during the early postpartum period. Pereira M, Morrell JI. The medial preoptic area is necessary for motivated choice of pup- over cocaine-associated environments by early postpartum rats. Neurosci. 2010 Feb 12; [Epub ahead of print].
Stress-coping Profile of Opioid Dependent Individuals Entering Naltrexone Treatment: A Comparison with Healthy controls
Stress increases addiction vulnerability and risk of relapse to substance use. This study compared opioid dependent individuals entering naltrexone treatment with healthy controls on measures of stress, coping, and social support and examined the relative contribution of group membership, coping, and social support to stress within the sample. The results revealed that opioid dependent subjects reported greater stress, less use of adaptive coping, but comparable use of maladaptive/avoidant coping, as compared with their control counterparts. However, no differences were found between the two groups with respect to social support. Perceived stress was predicted by group membership, low social support, and greater use of maladaptive/avoidant coping, and the prediction by social support and maladaptive/avoidant coping did not differ by group. Findings suggest that novel treatment approaches that decrease maladaptive/avoidant coping and improve social support are important aspects of decreasing stress during early recovery from opioid addiction. Hyman SM, Hong KI, Chaplin TM, Dabre Z, Comegys AD, Simmerling A, Sinha R. A stress-coping profile of opioid dependent individuals entering naltrexone treatment: A comparison with healthy controls. Psychol Addict Behav. 2009; 23: 613-619.
"Emotional numbing" Associated with Heavy Smoking Among Veterans
There is a strong association between posttraumatic stress disorder (PTSD) and cigarette smoking, as individuals suffering from PTSD are approximately twice as likely to smoke as the general population. Although some research has examined the association between PTSD and smoking, there has been little investigation into the particular aspects of PTSD that may be responsible for this association. Therefore, NIDA grantee Dr. Jessica Cook and colleagues investigated overall PTSD severity, as well as symptom clusters, for their relationship to smoking status and heaviness of smoking in Iraq and Afganistan combat veterans. Smoking status was measured and subjects were classified as non-smokers, light (1-9 cigarettes per day; cpd), moderate (10-19 cpd) or heavy smokers (≥20 cpd). Most subjects were light or moderate smokers. The PTSD Checklist Military version was used to assess PTSD, and the Patient History Questionnaire was used to evaluate depression. Results indicate that the PTSD factors of reexperiencing, effortful avoidance, emotional numbing and hyperarousal were significantly positively correlated as symptom clusters (or "factors") of PTSD. The higher the PTSD symptom severity, the more likely that smoking was endorsed. Severity predicted heavy smoking, but not light or moderate smoking. Of the four PTSD factors, none were associated with light or moderate smoking and only emotional numbing was associated with heavy smoking. Like emotional numbing, depression also predicted heavy smoking. Because these two are also correlated with each other, both were included in a regression model. Neither depression nor emotional numbing predicted heavy smoking, suggesting that they may "cancel out" individual contributions toward heavy smoking. Therefore, the authors suggest that mechanisms maintaining smoking in both PTSD and depression are similar. One explanation for this association is that smoking elevates blunted positive emotions -- a symptom of both emotional numbing and depression. If this is the case, then perhaps early interventions aimed at treating PTSD (and/or depression) may decrease the likelihood of either initiating smoking, or escalating smoking. Cook J, Jakupcak M, Rosenheck R, Fontana A, McFall M. Influence of PTSD symptom clusters on smoking status among help-seeking Iraq and Afganistan veterans. Nic Tob Res. 2009; 10: 1189-1195.
Potential Pharmacotherapy for Methamphetamine Abuse and Attention Deficit Hyperactivity Disorder (ADHD) Differentially Affects Male and Female Periadolescent Rats
Lobeline is currently being tested in clinical trials for the treatment of methamphetamine abuse and ADHD. It is a nicotinic α4β2 receptor antagonist and is believed to disrupt vesicular monoamine transporters. Lobeline has been tested in animal models of substance use disorders and is neither rewarding nor reinforcing. However, it has not been tested in animal models of ADHD. It has also not been explicitly tested in comparisons between male and female animals. NIDA Grantee Steven Harrod and M. Lee Van Horn treated periadolescent male and female rats with one of five doses of lobeline (0, 1, 2, 5.6, or 10 mg/kg) for seven consecutive days, followed by a saline challenge, and examined drug effects on locomotor activity. Periadolescent rats were chosen in order to avoid the activational effects of gonadal hormones and locomotor activity was chosen because it has been previously shown to be sensitive to lobeline. As was seen in previous studies using adult rats, lobeline acutely reduced locomotor activity counts and distance traveled in a dose-dependent manner, with tolerance occurring over the seven lobeline sessions. Females developed tolerance more slowly than males at 5.6 mg/kg dose of lobeline. The saline challenge, administered one day after the seventh lobeline treatment, produced hyperactivity that was positively correlated with previous doses of lobeline. That is, the larger the dose of previous lobeline treatment, the greater the locomotor activity seen following the saline challenge. This suggests that cessation of chronic lobeline may have behavior consequences. In conclusion, the current results suggest that female patients taking lobeline may experience greater motor-depressant effects, although more animal behavior research, particularly with adult animals, is needed. Harrod SB, Van Horn ML. Sex differences in tolerance to the locomotor depressant effects of lobeline in periadolescent rats. Pharm Biochem Behav. 2009; 94: 296-304.
Alcohol Changes Men's, but not Women's, Smoking Behavior
Both epidemiological and laboratory studies have shown a strong positive relationship between alcohol drinking and cigarette smoking, and there is growing evidence of sex differences in these interactions. NIDA grantee, Dr. Andrea King, and colleagues further examined this interaction by exploring the role of nicotine. In this laboratory study, heavy social drinkers participated in a two-session, double-blind study that included a 15-min drinking period followed by 3 hours of post-drink assessment (measuring subjective effects and smoking behavior). The within-subjects factor was nicotinized versus denicotinized cigarettes (smoked through a topography device), and a between-subjects factor was alcohol (equivalent to 4-5 standard alcoholic drinks) versus placebo beverage. For both males and females, alcohol increased "urge to smoke", which was significantly correlated with subsequent smoking behavior, regardless of whether smoking nicotinized or denicotinized cigarettes. Sex differences emerged when looking at beverage type. In men, alcohol significantly increased smoking choice and three smoking topography measures (puff count, puff volume, puff duration). In women, there were no differences in whether they smoked, or in smoking topography, when comparing alcohol and placebo beverages. Alcohol selectively increased men's, but not women's, smoking behavior. The failure to find a preference for either nicotinized or denicotinized cigarettes was surprising and the authors postulate that non-nicotinic sensory factors significantly contributed to this finding. These findings suggest that the interactions among alcohol, cigarettes, and sex differences may be more complex than previously thought. King A, McNamara P, Conrad M, Cao D. Alcohol-induced increases in smoking behavior for nicotinized and denicotinized cigarettes in men and women. Psychopharmacol. 2009; 207:107-117.
Plasticity of Composite Stimulus Control of Motivated Behavior
Stimuli in the environment that are paired with positive reinforcers, such as food or drug, are capable of eliciting drug seeking behavior and act as triggers for continued drug taking and relapse. These cues also act as signals for reward availability and thus provide information that food or drug is available. Via the same associative processes, it is possible to learn that a discriminative cue (DS) signals reward unavailability. Thus, in a human drug abuse situation, stimuli in the home environment may provide discriminative cues that nicotine reward is available, whereas stimuli in the workplace signal that drug reward is unavailable. Research using animal models of operant behavior (performing responses to receive reward) have demonstrated that discriminative stimuli signaling reward availability (DS+) increase responding, whereas signals for extinction (no reward, DS-) reduce responding. Dr. Stanley Weiss of American University has demonstrated the ability of environmental stimuli to control responding for reward can be predicted by the sum of DS+ and DS- influences. Whereas animal models of drug abuse and relapse typically study the single discrete stimulus cues, this research on ‘composite stimuli' (bundled collections of reward-referenced cues) controlling behavior offers a more ecologically valid model of human addiction, where multiple environmental stimuli most likely influence complex behavior. It is also likely that composite stimuli controlling behavior include not only the presence of particular cues, but also the absence of other cues in the environment. Thus, the absence of a cue may also provide a signal that reward is available (DS+) or that reward is unavailable (DS-). The composite-stimulus control model accounts for combinations of these stimulus signals by conceptualizing cues in a ‘binary' manner - cues are either present (an "on state") or absent ("off state") and an organism can learn that either state signals reward (DS+) or extinction (DS-). Previously, Dr. Weiss demonstrated that DS- cues act as conditioned inhibitors, capable of reducing ongoing reinforced behavior when added to a composite set of cues. This suggests that the use of conditioned inhibitors may be a useful therapeutic approach in addiction treatment. However, as continued drug-taking in addiction is believed to be an automatic behavior, generated from circuits controlling habit learning, these over-trained responses may not be malleable. In a recent study, Dr. Weiss trained rats until they were under stimulus control by a composite that elicited responding for food reward, and a composite that signaled extinction - reducing responding to almost zero. Then, he retrained groups of rats with this history of different DS+ and DS- stimulus combinations (including the presence and absence of discrete cues), switching the stimuli in the composites that signaled extinction. He found that rats were able to learn a new stimulus combinations that signaled extinction, and the new composite became capable of reducing responding to very low rates as well. (For example, rats learning that absence of light + absence of tone signaled extinction, were switched to presence of light + presence of tone signaled extinction). This demonstration of reversibility in stimulus control of over-trained reward reinforced behavioral patterns suggests that stimulus-response associations contributing to drug taking may be malleable. Additional research with drug rewards should extend these findings. Weiss, SJ, Kearns, DN, Antoshina, M. Within-subject reversibility of discriminative function in the composite-stimulus control of behavior. J Exp Anal Behav. 2009; 92: 367-377.
N-methyl-D-aspartate (NMDA) Glycine-site Antagonists Produce Distinct Subjective Effects When Compared to Other NMDA Antagonists
N-methyl-D-aspartate antagonists have a variety of potential clinical uses, including the treatment of pain, depression and stroke. However, they also produce side effects that limit their clinical usefulness. NMDA glycine-site antagonists may be less likely to produce these effects than other NMDA antagonists. NIDA-grantee Dr. Robert Balster (Virginia Commonwealth University) and colleagues compared the discriminative stimulus effects of NMDA glycine-site drugs to those of channel blocking and competitive NMDA antagonists. Rats were trained in a drug discrimination paradigm with saline versus the NMDA channel blocker PCP (2 mg/kg i.p.) or the NMDA competitive antagonist NPC17742 (4 mg/kg i.p.) using a standard two-lever operant conditioning procedure under a FR32 (fixed ratio 32) operant schedule. Neither of the partial glycine-site agonists nor antagonists tested produced greater than 50% PCP-lever selection, although high doses that produced motor deficits were tested. Similarly, the partial agonists and antagonists tested did not consistently substitute for the NMDA competitive antagonist, NPC17742. Overall, these data suggest that NMDA glycine-site antagonists may be less likely to produce the undesirable psychotropic side effects seen in clinical trials with many other NMDA antagonists. Nicholson KL, Balster RL. The discriminative stimulus effects of N-methyl-D-aspartate glycine-site ligands in NMDA antagonist-trained rats. Psychopharmacol. 2009; 203:441-451.
Individual Differences in the Incentive Value of Cocaine-associated Cues
Stimuli associated with drugs can motivate drug-seeking and drug-taking behavior and over repeated pairing with drug, come to elicit approach behavior. In this study, the investigators asked whether individual differences in attention to cocaine-associated cues was correlated with the ability of these stimuli to motivate drug-related behavior. First, they trained rats in a Pavlovian procedure where a lighted lever was presented for 8 seconds and then a food pellet was delivered into a food hopper some distance from the lever. All of the rats learned lever signaled food arrival. However, when the lever appeared, some rats approached and engaged with the lever, moving to the food hopper only when the food came, whereas other rats went to the food hopper as soon as the lever appeared. Rats who exhibit the first pattern of behavior are called "sign trackers" (ST) and the others are called "goal trackers' (GT). They then compared rats with the strongest ST behavior and rats with the strongest GT behavior, on measures of drug self-administration. For the first experiment, they trained the rats to self-administer cocaine using two nose-poke ports. When a rat poked the active port, it received an infusion of cocaine and a light in the port was turned on. After initial training, they conducted a cue removal test for 2 days, in which everything was the same, except that the light was not turned on. Removal of the cue decreased self-administration in the STs, but not the GTs. Responding was restored to baseline with 4 more days of training (with the light present) and then the rats underwent 28 days of extinction training in which no cocaine was delivered, but the light was still turned on after a nose poke. The ST rats were more resistant to extinction: they continued to respond for the light alone much more than the GT rats. A second experiment with a new group of animals examined reinstatement behavior after 7 days of extinction training with no cocaine and no cue, and 30 days of abstinence with no testing. In this experiment also, ST rats responded more robustly than GT rats when the cue was again presented. These results indicate that for the ST animals that attributed the most incentive salience to a food cue, a cocaine cue spurred motivation to take drugs (they responded less when the cue was removed), and were more vulnerable to cue-induced reinstatement. For the GT animals, the cocaine cue had relatively less ability to motivate behavior. The study also suggests that it is possible to determine, using the sign-tracking vs. goal-tracking measure, which individuals will have the most difficulty resisting drug cues even before they have been exposed to drugs. This finding opens up many possibilities for neurobiological and other studies of drug-addiction vulnerability without the confound of drug exposure. Saunders BT, Robinson TE. A cocaine cue acts as an incentive stimulus in some but not others: implications for addiction. Biol Psychiat. 2009 Dec 31. [Epub ahead of print].
Dopaminergic Mechanisms Involved in Amphetamine-induced Impairment of Social Bonding
This study used the prairie vole to investigate drug-induced impairment of social bonding. The prairie vole is a socially monogamous rodent that forms pair bonds after mating. In the first experiment, Zuoxin Wang and his colleagues found that conditioned place preference (CPP) for an amphetamine-associated environment was mediated by D1-like DA receptors and that antagonism of D2-like DA receptors had no effect. Next, they showed that repeated AMPH exposure impaired the formation of mating-induced partner preferences in male voles. Partner preference after mating is a measure of social bonding. Importantly, the animals were tested a day after the final AMPH treatment, so that no drug was present during the behavioral test, and these drug-exposed animals were not impaired in mating behavior itself. Using in situ immunocytochemistry, they found that AMPH exposure increased the number of D1 receptors (D1Rs), but not D2Rs in the nucleus accumbens (NAcc). They had previously shown that, in male prairie voles, activation of D1Rs within the NAcc prevented formation of partner preferences, whereas activation of D2Rs in NAcc promotes this behavior. They tested therefore, the effect of blocking D1Rs in the NAcc during the AMPH exposure sessions and found that this D1R blockade did eliminate the impairment of partner preference formation. The finding indicates that distinct dopaminergic mechanisms in NAcc regulate AMPH- and partner-motivated behaviors. Their working model is that, under normal circumstances, a relatively low amount of DA is released in the NAcc during mating, which preferentially stimulates high-affinity D2Rs and promotes pair bonding. However, AMPH exposure releases a large amount of DA, which stimulates the lower affinity D1Rs and increases their number. After this treatment, DA released during mating is sufficient to stimulate D1Rs and block pair bonding. These studies, along with other research on drug impairment of maternal behavior and social play, are beginning to provide an understanding of the interactions between social behavior and drug use. Liu Y, Aragona BJ, Young KA, Dietz DM, Kabbaj M, Mazei-Robison M, Nestler EJ, Wang Z. Nucleus accumbens dopamine mediates amphetamine-induced impairment of social bonding in a monogamous rodent species. Proc Natl Acad Sci USA. 2009 Dec 29. [Epub ahead of print].
Environmental Enrichment Prevents Relapse in a Rat Model
Animal models have been used to study environmental stimulation effects on initiation, maintenance and relapse to drug-taking. Environmental stimulation is studied in experiments that rear some animals in "enriched" conditions (EC), consisting of social, physical and sensory stimulation, while control rats are raised in single-cage isolation. Previous studies have demonstrated that enriched conditions protect against the acquisition of i.v. drug self-administration; or, conversely, that isolation conditions (the typical laboratory housing condition) predispose animals to acquire this behavior. EC, imposed after conditioning, blocks the expression of cocaine-associated place preference and cue-elicited sucrose-seeking behavior. As drug-seeking can be triggered by exposure to drug paired cues, or by reintroducing the drug after drug-taking has extinguished, Dr. Janet Neisewander sought to determine if EC might also attenuate the ability of these experiences to produce relapse in the reinstatement paradigm. Using reinstatement, drug-seeking behavior (pressing a lever which previously delivered i.v. drug infusions) is measured in an attempt to mimic human relapse. In this study, rats were exposed to EC or isolation during a forced abstinence period, after previously acquiring steady i.v. self-administration of cocaine. EC conditions grouped five rats per cage, for 21 days, in a large plastic tub with running wheels and novel "toys". Extinction and reinstatement testing revealed that EC and isolated rats took equal amounts of drug prior to abstinence, but drug seeking was significantly reduced in the EC group during both extinction and cue-induced reinstatement. However, when given a 10 mg/kg i.p. priming injection of cocaine, both groups exhibited similar reinstatement of lever pressing behavior. A second experiment examined cocaine-seeking during an extinction phase in groups that were housed in isolation, in EC, or in pairs during abstinence. This study demonstrated that isolate rats had significantly higher rates of drug-seeking when returned to the drug-taking environment, compared to both pair-housed and EC groups. Thus, the ability of EC conditions to reduce drug-seeking in this model of relapse can be attributed to the EC complex of influences (e.g., not only social, but also sensory and motor). These findings are important in that they extend previous studies of EC in drug abuse to models of relapse and suggest that enrichment may be a part of the treatment strategy for relapse prevention. Thiel KJ, Sanabria F, Pentkowski NS, Neisewander JL. Anti-craving effects of environmental enrichment. Int J Neuropsychopharmacol. 2009; 12: 1151-1156.
Anxiolytic Effects of Nicotine Demonstrated with an Approach-avoidance Conflict Procedure
Smoking is generally believed to be driven by positive reinforcing or pleasurable subjective effects of nicotine. However, many reasons are cited for smoking, including relief from negative states such as stress. Thus, nicotine may also have anxiolytic effects that contribute to its continued use. Dr. Aaron Ettenberg developed an animal behavioral assay for separating a drug's positive subjective effects from those which are aversive. Using a runway procedure, he has demonstrated that cocaine induces both positive and aversive subjective effects. He has also used this paradigm to investigate drug interactions that may explain the concurrent use of different substances by human abusers. Lastly, by separating positive and aversive effects, the paradigm lends itself to study of treatment mechanisms of action (i.e., whether an addiction treatment attenuates hedonic impact, or perhaps increases unpleasant drug effects). In a recent study he investigated nicotine's putative anxiolytic properties by testing the drug's effect on retreat responses (indicating escape or avoidance of an unpleasant state) by measuring nicotine effects on approach-avoidance conflict in rats running down the alley to receive food reward that was paired with mild electric shock. During training, food-deprived animals learned to leave a start box and run down an alley to a goal box, where food was delivered. On day 11 of training, mild shock was delivered in the goal box, following the food, and animals continued to be tested every day until demonstrating at least three retreats per day. In the third phase of the study, rats were tested with a random order of five nicotine doses, administered i.v. by the experimenter, 10-min before runway testing. Separate groups of animals were tested to assess locomotor effects of representative nicotine doses and to collect a different measure of nicotine effects on anxiety using an open-field test. Results revealed anti-conflict effects (significant reductions in mean retreat frequencies) with the two highest doses tested: 0.06 and 0.075 mg/kg. While the nicotine doses tested did not elevate general locomotor activity, testing with 0.06mg/kg also showed that this dose significantly reduced entries into the center of an open field test box, providing additional evidence of nicotine's anxiolytic effects. Cohen A, Young RW, Velazquez MA, Groysman M, Noorbehesht K, Ben-Shahar OM, Ettenberg A. Anxiolytic effects of nicotine in a rodent test of approach-avoidance conflict. Psychopharmacol. 2009; 204: 541-549.
Extended Access to D-amphetamine Increases Impulsive Choice Behavior
Impulsivity has been associated with human drug abuse, but it is not known if highly impulsive individuals are more likely to become addicted, or if drugs of abuse increase impulsivity. Animal behavior studies of impulsivity are useful for studying the direct effects of drugs of abuse on this behavioral characteristic. While a number of different behavioral tests have been used to measure impulsive behavior, measures of impulsive choice, or the preference for immediate over delayed rewards, may best mimic the impulsivity seen on delay discounting tasks conducted with drug abusers. Studies in rat models have yielded inconsistent effects with oral psychostimulant administration, on delay discounting functions. However, as these earlier studies failed to employ a drug exposure regimen that mimics human patterns of compulsive drug abuse, Dr. Michael Bardo recently compared animals treated with extended access to d- amphetamine (amp), for 6 h/day, versus those that continue to self-administer i.v. drug for only 1 h/day. Before being allowed to self-administer amp, animals were first trained on a delay discounting (DD) task. Under this procedure animals could choose between responding on a lever that delivered one sucrose pellet immediately after a lever press or responding on another lever that rrsulted in the delivery of three sucrose pellets following a delay. The animals were given many choices for immediate and delayed pellets in a single session. Over the session, the delay to receiving the three food pellets varied depending on how frequently the animal chose this option. That is, if animals chose the three pellet option more over the course of the session, the delay for the three pellet choice increased. If they chose the three pellets less often, the delay decreased. This procedure resulted in a derived dependent variable "mean adjusted delay" or MAD score, was computed based on the proportion of delayed 3-pellet to choices to immediate 1-pellet choices. The Higher MAD score the less "impulsive" the animal is considered to be. After this phase of the experiment, i.v. catheters were implanted and rats were trained for i.v. self administration of (0.03 or 0.10 mg/kg infusion) amp for 1 h/day, while continuing DD trials each day prior to self administration. In the next phase, animals were split into two groups that either were allowed to self administer amp for 1 h/day and or 6 h/day extended access to amp. After 21 days, seven days of extinction with no drug were imposed, and DD testing was again conducted. Results show that prior to 6 h availability, animals had similar MAD scores to those assigned to the 1h/day self-administration group. However following access to amp, animals that self-administered amp for 6 h/day had significantly lower MAD scores (became more impulsive) over 21 days of self-administration and also had significantly lower MAD scores during the first three withdrawal sessions. When examining the pattern of drug intake over 21 days of access, the experimenter noted a typical escalation pattern in this 6h/day group- increasing drug intake over days - for the 0.03 dose only. Interestingly, both doses lowered the MMAD score in the DD procedure when drug was available for extended access periods over 21 days, whether or not a pattern of compulsive drug intake developed. Gipson CD, Bardo MT. Extended access to amphetamine self-administration increases impulsive choice in a delay discounting task in rats. Psychopharmacol. 2009; 207: 391-400.
Stress and Cue-induced Relapse Induce Additive Effects in Animal Reinstatement
Drug abusers relapse under conditions of stress and also may relapse when exposed to cues previously associated with drug use. Investigators have examined the neuroanatomical substrate for these relapse effects in animal studies, and identified unique but overlapping neural substrates for these two effects. As it is likely that human addicts are exposed to both stress and drug-related cues, simultaneously, in the natural environment, Dr. Ronald See sought to determine if exposures to both these factors would induce synergistic or additive effects. In his study, rats were trained to self administer i.v. cocaine to a criterion of 10 sessions with at least 10 drug infusions per session. Then they underwent extinction, during which time drug was not delivered for lever presses. Extinction sessions were followed by seven reinstatement tests, where rats were given either 15 min of intermittent foot shock followed by lever availability, or lever availability alone (without prior shock). On half of the sessions, pressing the active lever resulted in presentation of cues previously paired with i.v. cocaine infusion (cue only versus shock + cue conditions). Results revealed that all rats reached the self-administration criterion and decreased responding during extinction when drug and cues were not available. The ability of shock to enhance cue-induced relapse was evident by comparing the mean number of active lever responses during extinction, cue presentation alone, footshock alone, and the cue+shock condition: Cue presentation resulted in more than double the mean number of lever responses than seen during extinction; footshock also significantly increased responding over the mean seen during extinction. However, when shock at the two highest levels (0.5 and 0.75 mA) was delivered prior to cue presentation, mean active lever presses were approximately double that seen in response to cue alone. In conclusion, the findings reveal the ability of stress, in a dose-dependent manner, to enhance cue-induced relapse. Future studies will examine the neurobiological substrates that are involved in this additive effect. Buffalari DM, See RE. Footshock stress potentiates cue-induced cocaine-seeking in an animal model of relapse. Physiol Behav. 2009; 98:614-617.
Cannabinoid CB1 Receptors May Modulate Stress-induced Overeating
Exogenous and endogenous cannabinoid agonists increase food intake in humans and animals, and these effects are mediated through CB1 receptors. Conversely, CB1 antagonists, such as rimonabant, lower the motivation for food and may be useful for the treatment of obesity. Two hypotheses have been proposed to account for anorexigenic properties attributed to CB1 activation: Modification of the hedonic effects of food, or modification of the incentive motivation for food. As overeating may be related to stress, NIDA researchers recently investigated the effects of THC (a CB1 agonist) on stress-induced disruptions of operant responding for food in a mouse model. Food deprived mice were trained to respond for three kinds of food: Standard laboratory pellets, chocolate flavored pellets and high-fat pellets. Then footshock was introduced to reduce the rate of responding for food on the active lever, and animals increased their rate of response on the non-contingent lever. Rats responding for three different types of pellets were pretreated with either the CB1 agonist THC or an appropriate vehicle. The effects of associating footshock with food delivery resulted in similar rates of responses on the reinforced and the non-reinforced levers. At the end of training for food, mice responding for the chocolate pellets made significantly more responses than animals in the other two groups. Comparisons between THC treated and vehicle treated groups revealed that THC had no effect on operant responding for normal or high-fat pellets associated with the delivery of foot-shock. However, in mice responding to receive highly palatable chocolate flavored pellets + footshock, THC significantly improved the discrimination between active and inactive levers; thus, THC treatment returned response rates to the pattern seen before footshock was introduced. With THC treatment, mice responding for chocolate pellets made very few responses on the inactive lever. This effect may be due to the ability of CB1 activation to increase the incentive value of food, or to attenuate the stress induced by footshock. Pending further studies to separate these alternate explanations, CB1 receptors may be therapeutic targets for reducing stress-induced overeating. Barbano MF, Castane A, Martin-Garcia E, Maldonado R. Delta-9-tetrahydrocannabinol enhances food reinforcement in a mouse operant conflict test. Psychopharmacol. 2009; 205:475-487.