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NIDA Home > Publications > Director's Reports > September, 2006 Index    

Director's Report to the National Advisory Council on Drug Abuse - September, 2006



Research Findings - Basic Behavioral Research

An Experimental Paradigm for Reinstatement of Nicotine-Seeking Behavior with Drug-Associated Stimuli

In typical drug self-administration procedures, animals are first taught to respond for food on an active lever (while a second inactive lever serves as a control) until responding is stable, whereupon food is replaced by drug. To test for possible response bias in this procedure, Drs. Lui, Caggiula and colleagues, trained one group of rats to respond for nicotine on the same lever that had been active for food, and another group to respond for nicotine by responding on the formerly inactive lever. During the nicotine self-administration sessions, an illuminated light above the active lever was associated with nicotine delivery. Following acquisition of bar pressing for nicotine, the behavior was extinguished by substituting saline for nicotine and omitting the light following active lever presses. Following extinction, presentation of the light was sufficient to reinstate nicotine-seeking (i.e., lever pressing on the lever formerly associated with nicotine). Pretreatment with mecamylamine, a nicotinic antagonist attenuated the visual stimulus' effectiveness in producing reinstatement. Although there were initial differences in response rates depending upon which lever was used to train nicotine self-administration, no overall response biases were found. Since the observed differences were found in early stages of training for nicotine self-administration, this lever-switch paradigm may be useful for studying the addictive properties of nicotine in short duration experiments. Lui, X., Caggiula, A.R., Yee, S.K., Nobuta, H., Poland, R.E. and Pechnick, R.N. Reinstatement of Nicotine-Seeking Behavior by Drug-Associated Stimuli after Extinction in Rats. Psychopharmacology, 184, pp. 417-425, 2006.

Rats Given Prolonged Access to Nicotine Display Age, But Not Gender, Differences

In a recent study by NIDA grantee Dr. B. M. Sharp, adolescent rats were housed in operant chambers allowing 23 hr access to self-administered nicotine via lever-pressing on one of two available levers (one active, one inactive). During acquisition of self-administration behavior, males displayed greater variation in responding on the active lever, but there were no significant gender differences in acquisition. Thus, both genders rapidly learned to self-administer nicotine and there were no gender differences in sensitivity to dose. However, adult female rats acquired nicotine self-administration at a slower rate than adolescent females and the adult females self-administered significantly less drug than their adolescent counterparts. (Adult male rats were not tested in this phase of the study). The procedure used by Dr. Sharp and his colleagues differs from those previously employed to establish nicotine self-administration in that animals were not food restricted (which may affect growth, especially in animals that are continuing to develop). This experimental paradigm more closely mimics the conditions under which humans self-administer nicotine. Additionally, since the "adolescent" phase of development is short in rats, this model allows for rapid acquisition of the self-administration behavior for study. Chen, H., Matta, S.G. and Sharp, B.M. Acquisition of Nicotine Self-Administration in Adolescent Rats Given Prolonged Access to the Drug. Neuropsychopharmacology, (e-pub): 1-10, 2006.

Hedonic Response to an Initial Lapse Predicts Progression to Second Lapse (Relapse), but Aversive Ratings or High Dose NRT Do Not

In a NIDA supported investigation, Dr. Chad Gwaltney, asked smokers to use electronic diaries to record their smoking and subjective responses to smoking during baseline and test periods. Following baseline assessments (2 weeks prior to a previously set quit date), subjects were treated with either a high dose nicotine patch (35 mg), or a placebo patch. Initial abstinence was defined as 24 consecutive hours without smoking. Subjects continued to record observations in the electronic diaries for 6 weeks following the quit date. These data were used to assess (1) the pleasantness and satisfaction ("hedonic rating"), and aversive effects, of smoking lapse and relapse; (2) the relationship between the hedonic effects of first and second lapses, amount smoked, and likelihood of future lapses; and (3) the effect of a high dose nicotine patch (35 mg) on the hedonic effects of initial lapse. Dr. Gwaltney found that the likelihood of a second lapse was increased when a larger amount of the initial lapse cigarette was smoked, and if the subject reported greater 'liking' the cigarette. By contrast, aversive rating did not affect likelihood for a second lapse. Moreover, the high dose nicotine patch did not affect time to first or second lapse, and did not affect hedonic or aversive ratings. Programs examining the affective variables that affect lapse and relapse in smoking behavior have translational potential for interventions that may lead to smoking cessation. Shiffman, S., Ferguson, S.G. and Gwaltney, C.J. Immediate Hedonic Response to Smoking Lapses: Relationship To Smoking Relapse, and Effects of Nicotine Replacement Therapy. Psychopharmacology, 184, pp. 608-618, 2006.

Effects of Early Drug Exposure on the Transition to Addiction

The development of sensitization is believed to contribute to the emergence of addiction, and may underlie the rapid progression to drug dependence seen in some individuals. Animal models have been developed to study escalated, compulsive drug intake, and these excessive patterns of intake have been associated with sensitization. For example, using progressive ratio (PR) operant schedules, Dr. David Roberts and colleagues have demonstrated sensitization to the reinforcing strength of cocaine in animals allowed to self-administer in a binge pattern over two weeks. These animals show increased break points for self-administered cocaine, often making hundreds of responses over sessions to receive a single drug infusion. In the present study, the investigators assessed break points and determined dose-response curves for cocaine self-administration in animals with varied initial exposures to cocaine self-administration. Initially, all rats in these studies were trained to self-administer cocaine on a FR1 schedule over 1-5 days. Total intake during this training ranged from 60-150 mg/kg/day. As soon as animals acquired the self-administration behavior, they were switched to a PR schedule for 14 days. Other groups of animals received additional self-administration sessions for five more days before being switched to a PR schedule. Animals in these extended exposure groups self-administered 13, 40 or 67 injections of cocaine per day prior to PR testing; thus, 20, 60 or 100 mg/kg/day. Then, in a final study, rats with extensive initial cocaine exposures, (100 mg/kg/day x 5 days), were tested to determine a cocaine dose-response curve for self-administration following the 14 days of testing on a PR schedule. The investigators found that in animals with only one day of FR training, break points successively increased over 14 days of PR testing, suggesting a progressive increase in reinforcer strength of the drug. Then, to investigate a threshold for initial exposure in this sensitization process, animals undergoing previous FR self-administration to receive 20, 60 or 100 mg/kg/day were compared on 14 days of PR testing for cocaine. Only animals in the 20 mg/kg group showed this same progressive increase in break points and the sensitization effect was masked by high levels of drug intake in the other groups. In fact, a significant change in break point emerged by day four of testing in this low dose group, so low levels of cocaine exposure may be optimal for inducing a rapid, escalated pattern of drug intake. The authors argue that this low dose exposure leads to a sensitization of cocaine's reinforcing effects and contributes to the transition to addiction, whereas higher drug exposures produce a different outcome - for example, a deterioration of behavioral regulation as has been reported in earlier studies. Morgan, D., Liu, Y. and Roberts, D.C.S. Rapid and Persistent Sensitization to the Reinforcing Effects of Cocaine. Neuropsychopharmacology, 31, pp. 121-128, 2006.

Cocaine Self Administration, but Not Passive Cocaine Exposure, Selectively Abolishes Long-Term Depression in the Nucleus Accumbens Core

Dr. Antonello Bonci and his colleagues have discovered a mechanism that may underlie two phenomena that are important for understanding the neural basis of drug addiction. One of these is the differential roles of the core versus the shell of the nucleus accumbens (NAcc) in drug-related behaviors. The core is thought to regulate the control of behavior by salient stimuli, such as cue- and stress-induced reinstatement of drug seeking. The role of the shell is less well understood, but it appears to be involved in the primary reinforcing effects of drugs of abuse. A second subject of interest is the differential behavioral and neurobiological effects of drugs when they are obtained by self administration versus when they are passively received. In this study, the ability of cells in the NAcc to develop long-term depression (LTD) was investigated in four groups of animals: [1] rats trained to self administer cocaine ('cocaine rats') or [2] food ('food rats') for 14-19 days; [3] rats that received cocaine passively whenever their paired cocaine rat pressed the bar for cocaine ('yoked rats'); and [4] sham-naïve controls, which were exposed to the operant chamber, but not trained or exposed to cocaine. LTD was evaluated by whole-cell patch clamp recordings from brain slices. LTD is a form of synaptic plasticity, revealed as a reduction in synaptic currents, after a cell has been stimulated with a train of pulses at a low rate. This form of synaptic plasticity is, essentially, the opposite of long-term potentiation and is believed to be necessary for forgetting and new learning. One day after the training period, LTD was measured in brain slices from half of the rats in each group. NAcc neurons in sham-naïve animals showed robust LTD in both the core and shell, while LTD failed in both areas in cocaine rats. In the yoked and food rats, levels of LTD were similar to those in the sham-naïve animals. These results indicate that cocaine self administration blocks LTD, and that this loss cannot be attributed to either cocaine exposure alone (yoked rats) or operant training alone (food rats). The investigators then measured LTD in the other half of the rats after 21 days of abstinence. At this time, LTD could be induced in NAcc shell in all groups of rats, but it continued to be blocked in the NAcc core of the cocaine rats. These results indicate that voluntary cocaine self-administration selectively induces long-lasting neuroadaptations in the NAcc core. This loss of LTD may occlude further learning and thus contribute to the reduced behavioral flexibility often observed in drug addicts. It may also reflect ongoing memory consolidation of stimuli related to drug self-administration and thereby facilitate reactivity to relapse-inducing stimuli. Martin, M., Chen, B.T., Hopf, F.W., Bowers, M.S., and Bonci, A. Cocaine Self-Administration Selectively Abolishes LTD in the Core of the Nucleus Accumbens. Nature Neuroscience, 9, pp. 868-869, 2006.

Prior Cocaine Exposure Disrupts Extinction of Fear Conditioning

Exposure to drugs of abuse is known to disrupt, or in some cases, to enhance, various sorts of learning and memory. In this study Dr. Schoenbaum and his colleagues tested the effect of cocaine exposure on extinction of fear conditioning. Recent research in animal models of this type of learning and extinction indicate that fear conditioning is initially established by circuits in the amygdala, but its extinction requires new learning (i.e., learning not to respond to stimuli previously associated with shock) that involves the medial prefrontal cortex (mPFC) -- an area critical for the suppression of inappropriate responses. Because psychostimulant exposure has been shown to cause molecular and cellular changes in prefrontal cortex, it has been hypothesized that these changes might affect the operation of prefrontal-limbic circuits and disrupt their normal role in controlling behavior, thereby contributing to compulsive drug-seeking. The investigator reasoned that, if this hypothesis is correct, cocaine exposure should interfere with extinction learning. They found that rats that had been exposed to a sensitizing regimen of cocaine for two weeks showed similar rates of fear conditioning as saline controls when a shock was paired with a tone stimulus, but their extinction learning rate was slower. Extinction was tested in two different ways when the animals were exposed to the tone cue in the absence of shock - by a reduction in freezing behavior, and by reductions in the ability of the tone to suppress bar-pressing for sucrose pellets (conditioned suppression). In another part of the experiment, half the rats were exposed to an "inflation" procedure after fear conditioning. In this procedure, animals are given several shocks of greater magnitude than those used for fear conditioning, but un-paired with the tone stimulus. This treatment increased the freezing response to the tone, particularly at the beginning of the extinction trials, but inflation did not differ between the cocaine- and saline-treated rats. The authors argue that because inflation involves enhancement of associations in the amygdala, whereas extinction learning depends on mPFC, these results support the hypothesis that control processes in the mPFC are impaired by cocaine exposure. Dr. Schoenbaum's group and other laboratories have previously shown that drugs of abuse impair behaviors that depend on the orbitofrontal cortex, such as adjustments of behavior in response to devaluation of reinforcers and reversal learning. This new study, which implicates deficits in mPFC function after drug exposure, further highlights the importance of neural changes in frontal areas for understanding long-term consequences of drug abuse. Burke, K.A., Franz, T.M., Gugsa, N., Schoenbaum, G. Prior Cocaine Exposure Disrupts Extinction of Fear Conditioning. Learning and Memory, 13, pp. 416-421, 2006.

Sex Difference in Visual Cue Enhancement of Nicotine Reinforcement

Human laboratory studies have shown that sensitivity to nicotine is lower in females than in males but that sensitivity to nonpharmacological stimuli associated with cigarette use is higher in females than in males. The importance of stimuli associated with nicotine to the reinforcing effects of nicotine has been established by researchers at the University of Pittsburg. That research was conducted previously in male rats and the researchers have now extended this work to female rats and have observed sex differences in this effect. Following acquisition of i.v. self-administration of either 0.03, 0.06, or 0.15 mg/kg nicotine and stable responding, nicotine deliveries were subsequently paired with a visual cue. In the absence of the visual cue, nicotine alone functioned as a reinforcer in both males and females at the .06 and 0.15 mg/kg doses, but not the 0.03 mg/kg dose. Addition of the visual cue produced a robust enhancement of nicotine reinforcement (response rate, infusions earned, and nicotine intake) in both males and females at the lower doses, and this enhancement at the 0.06 mg/kg dose was greater in females than males. The authors suggest possible explanations for this greater enhancement in females, including a greater synergistic interaction between nicotine and the visual cue and sex differences in limbic responses to nicotine and nicotine cues. This sex difference in visual cue enhancement of nicotine reinforcement could have implications for the use of pharmacologic versus non-pharmacologic strategies for nicotine cessation in males versus females. Chaudhri, N., Cagguila, A.R., Donny, E.C., Booth, S., Gharib, M.A., Craven, L.A., Shannon, S.A., Alan, F.S., and Perkins, K.A. Sex Differences in the Contribution of Nicotine and Nonpharmacological Stimuli to Nicotine Self-Administration in Rats. Psychopharmacology, 180, pp. 258-266, 2005.

THC Produces More Impairment in Learning in Adolescent Versus Adult Male Rats

In rats, levels of CB1 receptors for THC, the major psychoactive constituent in marijuana, are maximal in early adolescence, and then decrease into adulthood, thus suggesting that THC would have greater effects on the adolescent than adult brain. Dr. Scott Swartzwelder and colleagues at Duke University compared the effects of an acute dose THC (2.5, 5.0, and 10.0mg/kg) in a water maze learning task in adolescent and adult male rats. The water maze task had both a spatial and a non-spatial variation. In the spatial task, the rats had to learn the fixed location of a platform submerged slightly below the water surface within the water tank. In the non-spatial variation of the task, the platform was raised above the surface of the water and its location within the water tank was changed from trial to trial. At all dose levels, THC disrupted both spatial and non-spatial learning more powerfully in adolescent animals than in adults as measured by the distance rats swam to reach the goal platform. In a study of the chronic effects of THC in adolescents and adults, both groups received 21 daily injections of 5.0 mg/kg THC and following the last injection were tested 28 days later at which time the adolescents had reached adult age. In this chronic THC study, THC had no effect on subsequent maze learning either in the adolescents or the adults. The authors caution, however, that the results from this chronic study do not rule out the possibility of persisting effects on maze learning which possibly could have been unmasked by various experimental challenges, and they further note that the findings may not extend to other learning tasks. Importantly, the authors conclude that the data from the acute study suggest that "adolescence is a period of heightened sensitivity to the neurobehavioral effects of THC." Cha, Y.M., White, A.M., Kuhn, C.M., Wilson, W.A., and Swartzwelder, H.S. Differential Effects of Delta9-THC on Learning in Adolescent and Adult Rats. Pharmacology, Biochemistry and Behavior, 83, pp. 448-455, 2006.

PET Imaging of Dopamine D2 Receptors Before, During and After Cocaine Self-Administration in Rhesus Monkeys

Research with both human and non-human primates has demonstrated an association between susceptibility to the reinforcing and positive subjective effects of psychostimulants and D2 receptor availability. Further, among cocaine users, D2 receptor availability has been found to be lower than that seen in age-matched non-cocaine users. Dr. Mike Nader and his colleagues at Wake Forest University School of Medicine recently sought to determine whether D2 receptor availability is a trait variable that confers vulnerability to cocaine, whether cocaine use alters D2 receptor availability, and whether D2 receptor availability changes during cocaine abstinence. Twelve experimentally naïve adult male rhesus monkeys underwent PET scans to determine baseline D2 receptor availability and were then exposed to a schedule of response-contingent food and cocaine availability. Data indicated that from weeks 4 to10 the rate of cocaine self-administration was negatively associated with baseline D2 receptor availability. Over 12 months of cocaine self-administration, the number of cocaine injections did not change, but D2 receptor availability declined approximately 22% and the decline was independent of the baseline levels. Following the one year of cocaine self-administration, five of the 12 monkeys were studied in abstinence. Within three months, three of the monkeys exhibited recovery of baseline D2 receptor availability, whereas after 12 months, the remaining two monkeys failed to exhibit recovery. These data suggest that D2 receptor availability is both a predisposing trait for cocaine abuse and a consequence of cocaine exposure and that there are individual differences in whether there is recovery of D2 receptor availability during abstinence. Nader, M.A., Morgan, D., Gage, H.D., Nader, S.H., Calhoun, T.L., Buchheimer, N., Ehrenkaufer, R., and Mach, R.H. PET Imaging of Dopamine D2 Receptors During Cocaine Self-Administration in Monkeys. Nature Neuroscience, 9, pp. 1050-1056, 2006.

Hyperthermic Effects of (+/-)3,4-Methylenedioxymethamphetamine, (+/-)3,4-Methylenedioxyamphetamine and Methamphetamine in Rhesus Monkeys

Severe hyperthermia leading to death has been reported in human recreational users of 3,4-methylene-dioxymethamphetamine (MDMA), 3,4-methylenedioxyamphetamine (MDA), and methamphetamine (METH). In rodents, the degree of hyperthermia produced by these compounds has also been associated with the extent of amphetamine-induced neurotoxicity. Dr. Michael Taffe and his associates at The Scripps Research Institute sought to systematically determine the thermoregulatory risks of recreational doses of these compounds in nonhuman primates. Six rhesus monkeys were given a range of doses of each drug via intramuscular injection and then monitored for body temperature and locomotor activity in their home cages via telemetry. All three compounds significantly raised body temperature in the animals, although in a manner unrelated to dose and with varying time courses. The effects of METH lasted hours longer than MDMA or MDA and disrupted nighttime circadian cooling for as long as 18 hours as well. In general, activity levels were not affected by the doses tested. These findings demonstrate that this paradigm can serve as a reliable primate model for studying the thermoregulatory effects of drugs of abuse, and found that three commonly identified constituents of ecstasy produce hyperthermia in monkeys under ambient temperatures. R.D. Crean, S.A. Davis, S.N. Von Huben, C.C. Lay, S.N. Katner, and M.A. Taffe. Effects of (+/-)3,4-Methylenedioxymethamphetamine, (+/-)3,4-Methylenedioxyamphetamine and Methamphetamine on Temperature and Activity in Rhesus Macaques. Neuroscience, advance online publication, July 27, 2006.

Cognitive Performance During Simulated Shift Work

Individuals who are required to work irregular or rotating shifts (e.g., healthcare workers and military personnel) frequently adjust their sleep-wake cycles and report sleep disruptions and increased sleepiness while working. These conditions may contribute to diminished performance and work-related accidents. One strategy used to offset shift change-related disruptions is the administration of psychostimulants. Psychostimulants such as the amphetamines, however, pose a risk of dependence. An alternative to psychostimulants is the alerting agent Modafinil. The purpose of this double-blind, within-participant study was to examine the effects of Modafinil on cognitive/psychomotor performance, mood, and measures of sleep during simulated shift work. In all, 11 participants completed this 23-day residential laboratory study. They received a single oral Modafinil dose (0, 200, 400 mg) 1 h after waking for three consecutive days under two shift conditions: day shift and night shift. Shifts alternated three times during the study, and shift conditions were separated by an 'off' day. When participants received placebo, cognitive performance and subjective ratings of mood were disrupted during the night shift, relative to the day shift. Objective and subjective measures of sleep were also disrupted, but to a lesser extent. Modafinil reversed disruptions in cognitive performance and mood during the night shift. While Modafinil produced few effects on sleep measures during the night shift, the largest dose produced several sleep alterations during the day shift. These data demonstrate that abrupt shift changes produce cognitive performance impairments and mood disruptions during night shift work. Therapeutic doses of Modafinil attenuated night-shift-associated disruptions, but the larger dose produced some sleep impairments when administered during day-shift work. Hart, C.L., Haney, M., Vosburg, S.K., Comer, S.D., Gunderson, E., and Foltin, R.W. Modafinil Attenuates Disruptions in Cognitive Performance During Simulated Night-Shift Work. Neuropsychopharmacology, 31, pp. 1526-1536, 2006.

Mechanism of the Immunomodulatory Effects of Opioids

It has been known for a long time that opiate abusers are at high risk for infectious diseases. Although the high incidence of infections among opioid users is due in part to increased exposure to germs from needle sharing or non-sterile iv drug use, opioids themselves contribute to increased infection by direct effects on the immune system. However, there is little known about the mechanism of the immunomodulatory effects of opioids. Whereas, there is evidence that opioids such as morphine modulate the immune system via activation of CNS _-opioid receptors, the neural pathways involved in these immunomodulatory effects have not been well-characterized. In a recent study, NIDA grantee Donald Lysle and colleagues showed that the dopaminergic system was critical for the induction of morphine-induced suppression of natural killer (NK) cell activity by demonstrating that the administration of the dopamine D1 receptor antagonist SCH-23390 into the nucleus accumbens shell, but not the core, dose dependently blocked the suppressive effect of morphine on splenic NK cell activity, while injection of the D2 receptor antagonist raclopride did not. In support of these findings, morphine-induced reductions of NK activity were also prevented in animals that received intra-accumbens microinfusions of the dopaminergic immunotoxin anti-DAT-saporin. Furthermore, administration of the D1 agonist SKF-38393 into the nucleus accumbens produced reductions in splenic NK activity comparable to morphine, suggesting a critical role for accumbens D1 receptors in the modulation of splenic NK activity. These findings demonstrate that dopaminergic inputs to the nucleus accumbens are critically involved in opioid-induced immunosuppression and suggest that acute increases in dopamine signaling may have adverse consequences on the immune system. Saurer, T.B., Carrigan, K.A., Ijames, S.G., and Lysle, D.T. Suppression of Natural Killer Cell Activity by Morphine is Mediated by the Nucleus Accumbens Shell. Journal of Neuroimmunology, 173, pp. 3-11, 2006. In a follow-up study, the role of the sympathetic transmitter neuropeptide Y (NPY) in mediating morphine-induced immune alterations was investigated. The results showed that administration of the selective NPY Y1 receptor antagonist BIBP3226 blocked morphine's effect on splenic NK activity. The results showed that microinjection of the D1 receptor agonist SKF-38393 into the nucleus accumbens shell induced a suppression of NK activity that was reversed by BIBP3226. Collectively, these findings demonstrate that NPY Y1 receptors mediate morphine's suppressive effect on NK activity and further suggest that opioid induced increases in nucleus accumbens D1 receptor activation inhibit splenic NK activity via increased NPY release from the sympathetic nervous system Saurer, T.B., Ijames, S.G., and Lysle, D.T.. Neuropeptide Y Y(1) Receptors Mediate Morphine-Induced Reductions of Natural Killer Cell Activity. Journal of Neuroimmunology, June 9, 2006 [Epub ahead of print].

Negative Consequences of Nicotine Withdrawal are Less in Adolescent Compared to Adult Rats

There is now general agreement that adolescence is a period marked by enhanced vulnerability to the effects of nicotine, and that nicotine addiction is more common among smokers who initially started smoking at a young age. A number of studies have indicated that the reinforcing effects of nicotine are greater in adolescent animals and contribute significantly to the rapid development of dependence in this group. Although nicotine addiction has been hypothesized to be influenced by both positive reinforcing effects of the drug and negative effects of withdrawal, little research has focused on the severity of the withdrawal symptoms in adolescents. Drs. Athina Markou and George Koob and colleagues at the Scripps Research Institute compared the motivational (changes in intracranial self-stimulation thresholds) and somatic (e.g. writhing, tremors, headshakes) signs of mecamylamine-precipitated nicotine withdrawal in adult and adolescent rats that had been chronically exposed to nicotine via implanted minipumps. They reported that adolescents displayed fewer symptoms of withdrawal than did adults, and that these differences did not appear to be due to differences in metabolism. The authors hypothesized that the positive effects of nicotine are enhanced and the negative effects of withdrawal are diminished in adolescence, which in turn could help explain the enhanced vulnerability to addiction seen during this developmental period. O'Dell, L.E., Bruijnzeel, A.W., Smith, R.T., Parsons, L.H., Merves, M.L., Goldberger, B.A., Richardson, H.A., Koob, G.F., and Markou, A. Diminished Nicotine Withdrawal in Adolescent Rats: Implications for Vulnerability to Addiction. Psychopharmacology 186, pp. 612-619, 2006.

Cue-Conditioned Withdrawal Responses Motivate Escalated Drug Taking in an Animal Model

A great deal of anecdotal evidence suggests that continued drug taking behavior is motivated, in part, by relief from affective, negative states in withdrawal. However, empirical evidence to support a role for withdrawal states in motivating drug consumption is lacking. In animal models using intracranial self-stimulation (ICSS) to quantify central motivational state, withdrawal from addictive drugs shifts thresholds for responding to higher values, indicating a suppression of brain reward systems. Since it is known that reward deficits seen in withdrawal can be conditioned to environmental cues paired with reward, investigators at The Scripps Research Institute sought to determine if withdrawal-associated cues can motivate continued drug taking behavior. Rats were trained on unlimited heroin self-administration (i.v.), available for nose-poke responses over 23 h per day to induce physical dependence, while control animals self-administered for only 1 h each day. All rats were also trained to response for ICSS and tested until their individual thresholds were stable. Then, thresholds were determined every day after heroin self-administration. The 1 h group showed decreased thresholds over 24 days of testing, presumably a direct pharmacological effect of the opiate on central reward systems. However, 23 h (dependent) rats had a steady rise in ICSS threshold over the 24 days, suggesting a progressive blunting of central reward circuits due to neuroadapative changes produced by this dependence-inducing regimen. In a second experiment, withdrawal was precipitated in these two groups with naloxone injections over 4 days of self-administration. Both 1 h and 23 h groups increased their heroin intake following each naloxone injection, presumably in an attempt to counteract the opioid receptor blockade. However, only in the 23 h group did investigators see evidence that this shifted threshold became conditioned to environmental cues present when the naloxone was administered. Thus, during a test on day five, saline was injected prior to heroin self-administration, rather than naloxone, and a significant increase in heroin intake was seen only in the 23 h animals. Lastly, when ICSS thresholds were examined over the four days of naloxone pre-session injections, naloxone reversed heroin's threshold lowering effects in the 1 h rats but this effect was not conditioned to cues in the environment. In 23 h rats, naloxone injections prior to heroin self-administration raised threshold above their already elevated level (produced by heroin alone), and this effect was conditioned to the test environment. These findings show that self-administered heroin, in a regimen previously demonstrated to induce dependence, induces persistent alterations in the sensitivity of central reward systems. The authors argue that their results also indicate that attenuated central reward sensitivity can drive increased heroin intake to counter this deficit, resulting in the escalation seen in addiction. Furthermore, the negative, affective state induced by precipitated withdrawal can be conditioned to the environment, under conditions of high drug intake and dependence, and the authors argue that this conditioned affect can also motivate greater heroin intake. Kenny, P.J., Chen, S.A., Kitamura, O., Markou, A. and Koob, G.F. Conditioned Withdrawal Drives Heroin Consumption and Decreases Reward Sensitivity. The Journal of Neuroscience, 26, pp. 5894-5900, 2006.


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