Research Findings - Basic Behavioral Research

Persistent Synaptic Potentiation Is Produced By the Self-Administration of Cocaine, But Not Natural Rewards

Relapse to drug-seeking and drug-taking behavior is a characteristic feature of addiction. A popular hypothesis suggests that drug addiction results from maladaptive co-option of the brain's natural reward-related learning and memory mechanism. In this study, Dr. Antonello Bonci and his colleagues examined whether long-term potentiation (LTP), a synaptic mechanism associated with learning and memory, could be elicited at excitatory synapses in ventral tegmental area (VTA) dopamine neurons following natural reward or cocaine self-administration training. Potentiation was assessed in electrophysiological experiments by measuring both the ratio of AMPA to NMDA glutamate receptors (indicating a postsynaptic change) and the frequency of miniature excitatory postsynaptic currents (mEPSCs, indicative of a presynaptic change). In all cases where LTP was found, both measures were elevated. LTP was induced by self-administration training for both natural reward (food or sucrose) and cocaine. However, the LTP produced by cocaine self-administration lasted for at least three months while that produced by natural rewards had dissipated by 21 days after training ceased. Interestingly, at 21 days, animals trained to respond for food, sucrose, or cocaine all continued to show high levels of reward seeking, as measured in progressive ratio tests. Thus, persistent LTP in VTA is not necessary for continued reward seeking per se, but in the case of cocaine, its persistence suggests that it may be involved in long-term vulnerability for relapse. In this regard, another important result from the study was that the LTP produced by cocaine self-administration was resistant to extinction. Rats trained to self-administer cocaine for 14 days followed by three weeks of daily two-hour extinction training, during which cocaine was not delivered to reinforce responding. One day after the last extinction session, half of the rats were sacrificed for electrophysiological experiments and the other half underwent a single cue-induced reinstatement session and were then sacrificed for electrophysiology. Both groups showed elevated AMPA:NMDA ratios and mEPSC frequencies similar to those of rats that had been abstinent from cocaine for 21 days after self-administration without extinction training. These data suggest that even when drug-seeking behaviors are extinguished, the enhancement in glutamate function induced by voluntary cocaine self-administration remains potentiated, and thus may be important in the resumption of the previously extinguished behavior, or relapse. The persistent potentiation of excitatory synapses in VTA dopamine neurons may represent a fundamental cellular phenomenon driving pathological drug-seeking behavior. Chen, B.T, Bowers, M.S., Martin, M., Hopf, F.W., Guillory, A.M., Carelli, R.M., Chou, J.K., and Bonci, A. Cocaine but Not Natural Reward Self-Administration nor Passive Cocaine Infusion Produces Persistent LTP in the VTA. Neuron, 59, pp. 288-297, 2008.

Novel Reactivity and Behavioral Inhibition: Inflexibility As A Vulnerability Phenotype?

Recently there has been great interest in using animal models to determine behavioral phenotypes that confer vulnerability to acquire or sustain drug abuse behavior, (e.g., drug self-administration). It has long been known that individual animals differ in basal activity levels and response to novel stimuli and environments. Animals with greater responses are those who more readily acquire drug self-administration. A number of other behavioral tests assay impulsivity and individual tasks may assess different components of impulsivity. While the link between addiction and impulsivity is clear, it is unknown whether impulsive behavior is antecedent to, or a consequence of, chronic exposure to drugs of abuse. Some studies suggest that animals high on impulsivity assessments are also more vulnerable to acquire self-administration, but other authors claim that impulsivity is more related to features of addiction such as compulsive drug taking and continued drug seeking in the face of adverse consequences. Thus there is an interest in determining if high novelty reactivity and impulsive behavior represent overlapping vulnerability phenotypes in animal models of drug abuse. Drs. Stoffel and Cunningham have been studying high and low-novel reactive rats (HR, LR) and animals that demonstrate impulsive behavior on an operant schedule called differential reinforcement of low-rate (DRL) responding. On this schedule, impulsive animals have difficulty withholding lever press responses for a specific interval in order to earn a reinforcer. On this task, animals can be separated into high disinhibition and low disinhibition phenotypes (HD and LD). In this study, the researchers found that (1) HR and HD animals were indistinguishable in their response patterns under the DRL schedule, indicating an overlap of phenotype, and (2) Both HR and HD rats showed less variability in responding on the DRL task, suggesting that a more rigid response typography is a common characteristic of this phenotype. This is an interesting observation, as chronic cocaine treatment has been associated with a narrowing of cognitive and behavioral flexibility. The finding from this animal model suggests that behavioral inflexibility may be a component of the vulnerability phenotype and deserves further investigation. Stoffel, E.C. and Cunningham, K.A. The Relationship Between the Locomotor Response to a Novel Environment and Behavioral Disinhibition in Rats. Drug and Alc. Dep., 92, pp. 69-78, 2008.

Activation of Circadian Genes With Repeated Binge-Like Cocaine Self-Administration

Drugs of abuse disrupt homeostatic processes such as sleep and eating, and psychostimulants activate genes involved in the regulation of circadian rhythms. Circadian genes may also play a role in modulating cocaine reward, as sensitization in drosophila depends on the activation of genes such as Period 1, Period 2 and Clock. Additionally, circadian associated genes located outside of the suprachiasmatic nucleus are found in regions such as the dorsal striatum, known to be important for drug-related learning and the development of habitual behaviors in addiction. For example, in mice, Erg1, a circadian associated gene, is required for long-lasting associations between cocaine and the drug-related environment. Drs. Wendy Lynch and colleagues at Yale University have been studying circadian related genes in a rat model of i.v. cocaine self-administration to identify changes in neural pathways, produced by repeated drug administration, which may be responsible for homeostatic dysregulation and involved in neuroadaptations seen in addiction. Using a continuous access paradigm with discrete trial drug availability, they mimicked high levels of cyclical intake seen in human abusers. After seven days of binge administration, gene expression was assayed using a custom microarray chip focused on growth factor signaling molecules, transcription factors and known classes of circadian or timekeeper genes. Profiles were compared to animals self-administering saline only. Genes found to be differentially upregulated in the striatum included transcription factors such as zinc finger genes, Fosl1 and CREB; growth factors such as Chgb; enzymes/kinases such as Camklg, Fdft1; receptors/signal transduction proteins (e.g., 5-HT2C, NMDA2A); and neurotransmitter signaling genes such as Park2. Twenty-seven genes with a known circadian function were also upregulated and additional validation using quantitative real-time PCR was performed for the self-administration group. They also assayed additional known circadian genes that were not represented on the gene array, using real-time PCR quantification of mRNA levels, to reveal differential regulation of Clock, Per2 and Cry1 in cocaine self-administering animals. In order to identify neural networks associated with self-administration, the investigators used functional network mapping software, GeneGo (MetaCore). Each of the cocaine-regulated genes were examined for their involvement in the circadian systems, focusing not only on the primary circadian genes (Arntl/Bmal1, Clock, Per1, Per2, Cry1, Cry2), but also genes that have been shown previously to be regulated by or associated with the circadian system. Thirty regulatory networks of significance were identified, suggesting inhibitory and activation interactions between genes important for circadian rhythms and related biological processes. The authors discuss common mechanisms by which cocaine-activated and circadian systems may be regulated (e.g., through D1-cAMP signaling pathways or NMDA regulation of Per1 gene expression, changes ERK/MAPK signaling pathways). Most importantly, results from this study suggest candidates for future investigation to manipulate these signaling pathways with specific inhibitors or activators and examine the consequence for behavior. Lynch, W.J., Girgenti, M.J., Breslin, F.J., Newton, S.S., and Taylor, J.R. Gene Profiling the Response to Repeated Cocaine Self-administration in Dorsal Striatum: A Focus on Circadian Genes. Brain Research, 1213, pp. 166-177, 2008.

Two Classes of Neurons in the Rostral Ventromedial Medulla (RVM) Independently Modulate Pain

Studies have identified two classes of neurons (based on electrophysiological characterization) in the RVM, a region importantly involved with pain modulation. These cells are termed "on-cells" and "off-cells" and fire during pain and analgesia, respectively. In the current study, NIDA-grantee Dr. Mary Heinricher and colleagues (Oregon Health & Science University) examined if the activation of on-cells directly inhibits off-cells, as is generally believed to be the case. They recorded activity of on- and off-cells during heat-evoked paw or tail withdrawal in lightly anesthetized rats and precisely measured the onsets of the off-cell pause and the on-cell burst. Contrary to what would be expected if on-cells were inhibitory interneurons, off-cells typically ceased firing before on-cells began firing (mean 481 ms lag). This suggests that on-cell activity does not directly inhibit off-cell activity. These data help us better understand the brainstem circuitry involved with pain and analgesia and suggest that the on- and off-cells work more independently than previously thought in modulating pain. Clearly, D.R., Neubert, M.J., and Heinricher, M.M, Neuroscience, 151, pp. 564-571, 2008.

Behavioral and Electrophysiological Indices of Negative Affect Predict Cocaine Self-Administration

The hypothesis that drug-associated cues can induce a negative affective state that would then induce drug taking was put forward more than 30 years ago. But the current study, a collaboration between Dr. Sue Grigson's and Dr. Regina Carelli's laboratories, is the first test of this hypothesis and its neurobiological basis in an animal model. In this study rats were given intraoral infusions of an orange or grape flavored saccharin solution. The rats were conditioned to associate one of the flavors with the opportunity to self-administer cocaine and, on alternate days, to associate the other flavor with saline self-administration. Flavor conditioning sessions consisted of thirty 3.5s infusions over a 30 min session, followed by drug availability during self-administration five minutes later. Initially, the rats made stereotyped orofacial responses (licking and sideways tongue protrusions), which are associated with palatable taste stimuli, for both flavors However, as one of the flavors came to predict cocaine, they began to show selective aversive taste reactions to that flavor (gaping), identical to responses rats make for unpalatable tastes such as quinine. Analysis of the aversive taste reactivity measures and measures of drug seeking behavior showed that stronger aversive reactions to the cocaine-associated taste predicted shorter latency to the first cocaine infusion and more "drug loading" (number of cocaine infusions early in the session). Previous studies of neurophysiological responses in the nucleus accumbens (NAc) have shown that stimuli with positive hedonic values generally suppress NAc activity, while those with negative values generally excite NAc neurons. Consistent with this characterization, presentation of either flavor solution in naive rats inhibited about 75% of the neurons recorded and excited only about 25%. However, after conditioning, responses to the cocaine-associated flavor were reversed (about 60% excitatory), while those to the saline-associated flavor remained the same (i.e., mostly inhibitory). These neurophysiological data demonstrate a shift in patterned NAc neuronal firing that corresponds with the learned shift in affect and the increased cocaine-seeking behavior. This study provides strong support for the hypothesis that drug-associated cues can trigger an aversive affective state that drives drug-seeking behavior, and it provides a novel behavioral model to further investigate the neural mechanisms by which cues can exert control over drug self-administration. Wheeler, R.A., Twining, R.C., Jones, J.L., Slater, J.M., Grigson, P.S. and Carelli, R.M. Behavioral and Electrophysiological Indices of Negative Affect Predict Cocaine Self-administration. Neuron, 57, pp. 774-785, 2008.

The Medial Prefrontal Cortex Mediates Differential Effects of Uncontrollable vs. Controllable Stress on Responses to Drugs of Abuse

A single session of uncontrollable (inescapable tailshock, IS), but not controllable (escapable tailshock, ES), stress enhances the conditioned place preference (CPP) response to morphine, even when the stressor and drug administration are separated temporally and spatially. Dr. Steven Maier and his colleagues have been systematically investigating the neural circuit that underlies this differential vulnerability to drug reward produced by controllable vs. uncontrollable stress. From previous studies, they knew that activation of serotonergic neurons in the dorsal raphe (DRN) during IS was necessary to potentiate dopamine efflux in the nucleus accumbens and enhance morphine CPP and also that activation of the ventral medial prefrontal cortex (mPFCv) can inhibit DRN activity. Thus, in this study, they investigated whether the mPFCv was a critical mediator of the effects of stressor controllability on morphine-CPP. In one experiment, they inactivated mPFCv just prior to stress exposure by microinjections of muscimol. This inactivation had no effect on the ability of IS to potentiate morphine CPP, but now ES also potentiated the behavior. In the second, complementary, experiment they microinjected picrotoxin to activate mPFCv. In this case, neither IS nor ES effectively enhanced morphine-CPP. Together, these results demonstrate that activation of mPFCv during stress is both necessary and sufficient to block stress-induced potentiation of morphine-CPP, regardless of the type of stressor. The results are consistent with a variety of other studies that implicate PFC hypoactivity in promoting addiction-related behaviors. The authors speculate that mPFCv may be responsible both for detecting stressor controllability and for dampening the stress-activated neural activity that increases drug reward and other deleterious behaviors. Rozeske, R.R., Der-Avakian, A., Bland, S.T., Beckley, J.T., Watkins, L.R., and Maier, S.F. The Medial Prefrontal Cortex Regulates the Differential Expression of Morphine-conditioned Place Preference Following a Single Exposure to Controllable or Uncontrollable Stress. Neuropsychopharmacology, [E-pub ahead of print], 2008.

Methylphenidate Disrupts Social Play Behavior in Adolescent Rats

Dr. Louk Vanderschuren and his colleagues at the University Medical Center Utrecht, Uterech, The Netherlands, report that low doses of methylphenidate, abolish social play behavior (i.e., pouncing and pinning which often involve boxing/wrestling and chasing) without altering general social interest (i.e, social exploration via sniffing, including anogenital) or locomotor behavior during social interactions in rats. This effect of methylphenidate did not depend upon the baseline level of social play and was not secondary to changes in locomotion. Furthermore, the play-suppressant effect of methylphenidate was not subject to tolerance or sensitization. Methylphenidate blocked both the initiation to play and the responsivity to play initiation. The effect of methylphenidate was mimicked by the noradrenaline reuptake inhibitor atomoxetine, which is also used for the treatment of ADHD, and was blocked by an alpha-2 adrenoceptor antagonist. In addition, combined administration of sub-effective doses of methylphenidate and atomoxetine suppressed social play. However, blockade of alpha-1 adrenoceptors, beta-adrenoceptors or dopamine receptors did not alter the effect of methylphenidate. These data show that methylphenidate selectively blocks the most vigorous part of the behavioral repertoire of adolescent rats, through a noradrenergic mechanism. The authors suggest that the effect of methylphenidate on social play is a reflection of its therapeutic effect in ADHD, i.e., improved behavioral inhibition. However, given the importance of social play for child development, these findings may also indicate a possible adverse side-effect of methylphenidate. Vanderschuren, L.J.M.J., Trezza, V., Griffioen-Roose, S., Schiepers, O.J.G., Van Leeuwen, N., De Vries, T.J., and Schoffelmeer, A.N.M. Methylphenidate Disrupts Social Play Behavior in Adolescent Rats. Neuropsychopharmacology, 2008 May 21 [E-pub ahead of print]. 10.1038/npp.2008.10. PMID: 18496520

The Relative Abuse Potential of Oral Oxycodone, Hydrocodone and Hydromorphone in Healthy Non-Dependent Prescription Opioid Abusers

Epidemiological studies and law enforcement data suggest that the abuse, medical consequences and diversion of prescription opioids represent a growing public health problem in the United States. Various opioid drugs are prescribed for pain relief, such as hydrocodone (brands include Vicodin, Anexsia, Lorcet, Norco), oxycodone (brands include Oxycontin, Oxycet, Percocet, Percodan), hydromorphone (brands include Dilaudid, Hydrostat IR, Palladone), and morphine. It is commonly believed that these drugs differ in their relative risks for abuse with some having greater abuse potential than others; indeed, physicians are inclined to prescribe more frequently those drugs presumed to have lower abuse potential in order to reduce the risk of misuse. Hydrocodone, for example, is the most commonly prescribed opioid in the United States, accounting for approximately 47% of the total number of opioid prescriptions, and is also less stringently regulated under the Federal Controlled Substances Act. Despite the precipitous rise of abuse of prescription opioids in the United States over the last decade, there have been few controlled comparisons of the abuse liability of the most commonly used opioids. Dr. Sharon Walsh and her colleagues at the University of Kentucky conducted an outpatient study employing a double-blind, randomized, within-subject, placebo-controlled design to examine the relative abuse potential and potency of oral oxycodone (10, 20 & 40 mg), hydrocodone (15, 30 & 45 mg), hydromorphone (10, 17.5 & 25 mg) and placebo. Nine healthy adult volunteers with sporadic prescription opioid abuse participated in 11 experimental sessions (6.5 hr in duration) conducted in a hospital setting. All three opioids produced a typical mu opioid agonist profile of subjective (increased ratings of liking, good effects, high and opiate symptoms), observer-rated, and physiological effects (miosis, modest respiratory depression, exophoria and decrements in visual threshold discrimination) that were generally dose-related. The overall profile of action did not differentiate these three drugs suggesting that they have similar abuse liability characteristics. Relative potency assays revealed that oxycodone was roughly equipotent to or slightly more potent than hydrocodone. Hydromorphone was only modestly more potent (less than two-fold) than either hydrocodone or oxycodone, which is inconsistent with prior estimates arising from analgesic studies. Estimates from prior studies based on the strength of these drugs to provide pain relief (rather than on their abuse-related, subjective features) have indicated that oxycodone and hydrocodone are approximately equally powerful analgesics and that hydromorphone is 4-7 more powerful than either. The present study suggests that the abuse liability profile and relative potency of these three commonly used opioids do not differ substantially from one another and that other factors may account for differential rates of abuse such as availability and/or formulation. These data also suggest that analgesic potency may not accurately reflect relative differences in the abuse liability of prescription opioids. Therefore, physicians who prescribe opioids should choose the drug which provides the best pain relief and side effect profile for a given patient while recognizing that these drugs may possess comparable risk for abuse. Walsh, S.L., Nuzzo, P.A., Lofwall, M.R., and Holtman, J.R., Jr. The Relative Abuse Liability of Oral Oxycodone, Hydrocodone and Hydromorphone Assessed in Prescription Opioid Abusers. Drug and Alcohol Dependence, 2008 Jul 5. [E-pub ahead of print]

Menstrual Cycle Modulates Striatal Dopamine in Drug-Naive Cynomolgus Monkeys

Several studies have reported that the subjective effects of abused stimulants in women vary with the menstrual cycle. Given the role of the brain dopamine system in abused drugs, these menstrual cycle effects may be mediated by interactions between gonadal hormones and dopamine. Drs. Paul Czoty and Michael Nader and colleagues at Wake Forest University School of Medicine conducted the first non-human primate study to examine whether basal measures of DA D2 receptor availability vary with the menstrual cycle phase. They tested seven drug-naive, individually-housed female cynomolgus monkeys using the D2-like receptor ligand [18F]fluoroclebopride (FCP). Menstrual cycle phase was determined by serum progesterone levels on the day of the PET scan. In both the caudate nucleus and the putamen, D2 receptor availability was found to be lower during the follicular phase than in the luteal phase. Previous studies examining menstrual cycle effects on D2 receptor availability in female human subjects have reported mixed results, including effects consistent with the present results, opposite the present results, and a report of no effects. Drs. Czoty, Nader and colleagues note that there are several potential explanations for these discrepancies, including their use of monkeys with no drug history of any kind (other than veterinary care with ketamine) and their use of a state-of-the-art primate microPET camera with high resolution. Whether the lower D2 receptor availability observed in the follicular phase this study reflects lower D2 receptor densities or higher levels of extracellular dopamine in the follicular phase is unclear; however, the authors note that rodent studies have shown that estrogen, which is higher in the follicular phase than luteal, has been shown to increase dopamine release. Several studies have shown that D2 receptor availability is inversely related to vulnerability to the abuse-related effects of cocaine. The present data, therefore, suggest that such effects in females would be greater in the follicular phase, which is an outcome that has been observed in several studies examining the subjective effects of stimulants in humans. Also, NIDA grantee Dr. Nancy Mello of the Harvard Medical School reported in 2007 that a low dose of cocaine produced a considerably greater progressive ratio breakpoint during the follicular phase than in the luteal phase in cynomolgus monkeys (Mello, N.K., Knudson, I.M., and Mendelson, J.H.. Sex and Menstrual Cycle Effects on Progressive Ratio Measures of Cocaine Self-Administration in Cynomolgus Monkeys. Neuropsychopharmacology, 32, pp. 1956-1966, 2007). Although only females were studied in the present study, data from a prior published study by these researchers examining D2 receptor availability in individually-housed male monkeys permitted an examination of sex differences. Results from that study indicated that D2 receptor availability in both the caudate nucleus and the putamen was lower than seen in females in either the luteal or follicular phase. This research highlights the need for models of the neurobiology of addiction to incorporate sex differences and the interactions of gonadal hormones and neurotransmitter systems underlying addiction. Czoty, P.W., Riddick, N.V., Gage, H.D., Sandridge, M., Nader, S.H., Garg, S., Bounds, M., Garg, P.K., and Nader, M.A. Effect of Menstrual Cycle Phase on Dopamine D2 Receptor Availability in Female Cynomolgus Monkeys. Neuropsychopharmacology. 2008 Feb 6. [E-pub ahead of print]

Developmental Differences In Cocaine-Induced Behavioral Sensitization In The Mouse

Although it is fairly well established that adolescent animals differ from adults in their behavioral and neurochemical sensitivity to drugs of abuse, less is known about the long-term neurobehavioral consequences of adolescent drug exposure. With rats, some investigators have found that adolescent cocaine exposure results in differential sensitivity to cocaine during adulthood, while others have failed to find this effect. The current experiment by Dr. M. Foster Olive sought to examine whether adolescent mice exposed to cocaine would be differentially sensitive to the behavioral and neurochemical effects of cocaine compared to mice similarly treated as adults. Initially, systemically administered cocaine produced increased locomotor activity in all mice, whereas saline had no effect. However, after 9 days of cocaine treatment, adolescent mice displayed higher levels of locomotor activity compared to comparably treated adults. Following 10 days of abstinence, a cocaine challenge produced higher levels of locomotor activity in adolescent mice with this cocaine history than in the adult group with similar prior drug exposure. Changes in central transmitter systems involved in motivational circuitry were assessed with in vivo microdialysis for dopamine and glutamate levels in the nucleus accumbens following the cocaine challenge. Compared to the mice previously treated with saline, mice with prior nine-day cocaine treatment tended to show larger increases in dopamine dialysate, and adult mice displayed greater peak dopamine dialysate levels as compared with the adolescents. There were no differences in levels of extracellular glutamate. The current findings reveal that although adolescent mice develop greater behavioral sensitization to the locomotor-stimulating effects of cocaine, this difference cannot be explained by age-related differences in limbic dopamine stimulation. Thus, the neurochemical mechanisms that account for greater locomotor sensitization in adolescent mice are yet to be determined. Camarini R, Griffin, W.C., Yanke, A.B., dos Santos, B.R. and Olive, M.R. Effects of Adolescent Exposure to Cocaine on Locomotor Activity and Extracellular Dopamine and Glutamate Levels in Nucleus Accumbens of DBA/2J mice. Brain Res. 1193, pp. 34-42, 2008.

Distal Cues in the Environment Produce Cue Reactivity in Abstinent Smokers

Smoking research has primarily focused on proximal cues (cue directly linked to smoking behavior) alone, or proximal cues in the presence of distal cues. Dr. Cynthia Conklin and colleagues investigated differences in smoking cue reactivity produced by proximal cues versus those produced by distal cues (indirectly linked, e.g., environmental cues). These cues were presented as pictorial stimuli, and featured either proximal or distal cues, but not both. The distal (environmental) cues included three smoking contexts and three non-smoking contexts. The proximal cues included three smoking cues (e.g., cigarette in ashtray, lighter, pack of cigarettes), and three nonsmoking cues. The six environmental cues were combined with the six proximal cues in a counter-balanced 12-trial cue reactivity paradigm. Adult smokers abstained from smoking for 6 hours prior to cue reactivity testing. During the session, heart rate and skin conductance were monitored, and following each pictorial stimuli presentation, subjective ratings were taken for craving, vividness, relevance, negative affect, positive affect, excited and calm. Both types of smoking cues elicited craving, with proximal cues eliciting higher craving ratings than distal cues. Smokers rated smoking cues as more vivid and more relevant than nonsmoking cues, and proximal cues were more vivid and more relevant than distal cues. Similar results were found with negative affect. Positive affect, on the other hand, was elicited by distal cues more than proximal cues. Although there was no main effect for excitement when comparing smoking and nonsmoking cues, there was a pairwise interaction that indicated that proximal cues elicited higher ratings on excitement as compared to distal cues. Results from this study indicate that it is possible to develop smoking environmental (distal) cues, using pictorial stimuli that successfully produce cue reactivity in abstinent smokers. In fact, smokers in this study were robustly reactive to the environmental cues and, to a lesser extent, proximal cues that are more often manipulated in craving and intervention research. Conklin, C.A., Robin, N., Perkins, K.A., Salkeld, R.P. and McClernon, F.J. Proximal Versus Distal Cues to Smoke: The Effects of Environments on Smokers' Cue-Reactivity. Exper. Clin. Psychopharm. 16(3), pp. 207-214, 2008.

Chronic Nicotine Restricts Weight Gain and Alters Expression of Two Neuropeptides that are Markers for Activation of the Hypothalamic-Pituitary Axis (HPA)

Long-time NIDA grantee, Dr. Burt Sharp, is investigating possible mechanisms responsible for stress-induced effects of chronic nicotine. Dr. Sharp has examined corticotropin-releasing factor (CRF) and arginine vasopressin (AV) as indicators of a stress reactivity. He found that rats self-administering (SA) intravenous nicotine during a 23 hour access period ate less than free fed saline SA control animals. This restricted weight gain was not simply due to lower calorie intake, as pair-fed controls gained more weight (free fed > pair-fed> nicotine SA). During nicotine SA, antibody probes of CRF and AV were used to visualize mRNA expression. While no differences were found between free-fed and pair-fed rats, animals in the chronic nicotine group had decreased CRF levels in the paraventricular nucleus, and significant increases in AV mRNA. When nicotine SA was extinguished (e.g., nicotine no longer available as a consequence of lever pressing), there was a reversal of the mRNA findings, with no signal intensity differences between nicotine SA and saline SA rats. Since chronic nicotine enhances the HPA response to mild footshock stress (mFSS), an experiment was conducted to examine the responsiveness of a subset of PVN neurons under mFSS as compared to mFSS + nicotine. Rats were trained to lever press to SA nicotine for 20 sessions (one session per day), then mFSS was introduced. PVN+ AV neurons responded to mFSS with increased mRNA expression and chronic nicotine augmented this increase. Although mFSS increased mRNA expression of CRF, chronic nicotine did not promote further increases. However, when neurons expressed mRNA for both AV and CRF, the mFSS-induced increases were further augmented by chronic nicotine. These data suggest that chronic nicotine produces a stress-like behavioral phenotype, (e.g., lower weight gain not due solely to reduced calorie intake), and augments stress-induced mRNA increases indexed by two markers of HPA axis activation. Yu, G., Chen, H., Zhao, W., Matta, S.G. and Sharp, B.M. Nicotine Self-Administration Differentially Regulates Hypothalamic Corticotropin-Releasing Factor and Arginine Vasopression mRNAs and Facilitates Stress-Induced Neuronal Activation. J. Neurosci. 28(1), pp. 2773-2782, 2008.

Blocking the Toll-like Receptor 4 (TLR4) Eliminates Neuropathic Pain in Rats

NIDA-grantee Dr. Linda Watkins and colleagues (University of Colorado, Boulder) have been looking at the involvement of spinal microglia in pain and opioid analgesia. The present studies demonstrate that the TLR4 is critical for maintaining neuropathic pain following sciatic nerve chronic constriction injury (CCI) in rats. Established neuropathic pain was reversed by an intrathecally delivered TLR4 receptor antagonist derived from lipopolysaccharide. Additionally, (+)-naltrexone, (+)-naloxone, and (-)-naloxone, which were also shown to be TLR4 antagonists, completely reversed neuropathic pain with chronic infusion. Immunohistochemical analyses of spinal cord tissue following chronic infusion revealed suppression of CCI-induced microglial activation by (+)-naloxone and (-)-naloxone. Together, these data support the conclusion that neuron-to-glia signaling through the TLR4 is important for maintaining neuropathic pain. The finding with (+)-naloxone is of potential clinical relevance since (+)-naloxone is an antagonist that is inactive at neuronal mu-opioid receptors that produce analgesia. Thus, drugs like (+)-naloxone may be useful clinically to suppress glial activation and pain while not interfering with mu-opioid agonist's ability to also suppress pain. Hutchinson, M.R., Zhang, Y., Brown, K., Coats, B.D., Shridhar, M., Sholar, P.W., Patel, S.J., Crysdale, N.Y., Harrison, J.A., Maier S.F., Rice, K.C., and Watkins, L.R., European Journal of Neuroscience, pp. 1-10, 2008.

Endocannabinoid Signaling Controls Anxiety in Hamsters

Available evidence suggests that endocannabinoid receptors and endogenous cannabinoids in the limbic system of the brain modulate emotional responding. NIDA investigators sought to investigate the existence of an endocannabinoid signaling system in the Syrian hamster using neuroanatomical, biochemical and pharmacological approaches. Using in vitro receptor binding and quantitative autoradiography they found a heterogeneous distribution of cannabinoid binding sites in hamster brain similar to that observed in rat. Dense binding was detected in basal ganglia and other motor structures, limbic structures including the hippocampus and cingulate cortex. Lower levels of binding were detected in the amygdala and brainstem. They also found that FAAH, the predominant enzyme that controls deactivation of the endogenous cannabinoid anandamide in other rodents plays a similar role in hamsters. They reported also that WIN55,212-2 induced CB1-mediated motor ataxia. Blockade of CB1 with rimonabant induced anxiogenic-like behavior in the elevated plus maze and stimulation of CB1 blocked anxiety in the plus maze. Finally, they found that neither unconditioned nor conditioned social defeat in the hamster is mediated by CB1 receptor activation. This research provides the first evidence that the hamster brain contain functional cannabinoid CB1 receptors, as well as FAAH, similar to that observed in non-hibernating rodent species. Moreover, this system serves naturally to modulate anxiety-like behavior in the hamster. Finally, the data suggest that cannabinoids and benzodiazepines differentially modulate anxiety. That is, although the benzodiazepine, diazepam, reduced anxiety-like behavior in the elevated plus maze and in the social defeat models, CB1 activation had no effect on social defeat. It did, however, induce anxiolytic effects in the plus maze. Moise, A.M., Eisenstein, S.A., Astarita, G., Piomelli, D., and Hohmann, A.G. An Endocannabinoid Signaling System Modulates Anxiety-like Behavior in Male Syrian Hamsters. Psychopharmacology, 2008, on-line publication.

Stimulus-Reward Learning about Cocaine in Adolescents vs Adults

This study investigated the effects of cocaine self-administration on stimulus-reward learning in adult male rats depending upon whether the drug experience occurred during adolescence or adulthood. In addition, to assess the effects of contingent vs noncontingent (or passive) cocaine exposure, a triadic design (i.e., continent cocaine, noncontingent cocaine, saline) was used with both age groups. Following drug exposure, all rats were returned to their home cages for an 18 day drug free period. Then they were assessed in a conditioned cue preference apparatus where they learned to associate particular cues with a food reward or lack thereof. Results indicated first that cocaine self-administration behavior was similar in adolescent and adult rats. However, despite similarities in the amount of cocaine consumed in adult and adolescent rats, subsequent learning was affected differentially after the drug-free period. Rats contingently self-administering cocaine or passively exposed to cocaine during adulthood showed learning deficits in the conditioned cue preference task, indicating a deficit in the stimulus-reward learning function of the amygdala reward system. Rats exposed to contingent or noncontingent cocaine during adolescence, on the other hand, showed strong conditioned cue preference. This indicates that the stimulus-reward learning function of the amygdala memory system was intact after adolescent cocaine exposure. Under passive saline control conditions, the strength of conditioning was similar in rats for whom drug-onset began either in adolescence or adulthood. These findings support a view that similar intakes of cocaine can have different consequences when drug use begins during adolescence vs. adulthood. The authors further suggest that adolescent rats are not as vulnerable to cocaine-induced devaluation of natural rewards (i.e., the comparison of food reward relative to cocaine) compared to adult rats and that this reduced sensitivity protects them from later impairment in stimulus-reward learning. Kerstetter, K.A., and Kantak, K.M. Differential Effects of Self-administered Cocaine in Adolescent and Adult Rats on Stimulus-reward Learning. Psychopharmacology, 194, pp. 403-311, 2007.

Neural Substrates Involved in Context Reinstatement of Cocaine Responding

Environmental cues associated with drugs of abuse contribute to the development, maintenance and relapse to addiction, but the neurobiological mechanisms responsible for these effects have yet to be fully elucidated. In the present report, Dr. Katherine Kantak and her colleagues investigated the role of the ventral hippocampus in regulating context and discrete cue-induced reinstatement of cocaine-seeking behavior. Animals were trained to self-administer cocaine in the presence of distinct contextual or discrete cues. Some animals had extensive exposure (21 days) to the drug associated contexts, whereas other had short-term exposure (3 days). To test the role of the hippocampus, half the animals in each training group were conditioned after infusion of lidocaine to inactivate the ventral hippocampus and half were trained following saline infusions into the same structure. Results showed that inactivation of the ventral hippocampus by lidocaine during conditioning blocked context-induced reinstatement in rats with short- but not long-term exposure to the contextual cues. These results suggest that hippocampal processing of drug-related information is still in progress after 3 pairings but not after 21 context-drug pairings. In contrast to the results for contextual cue-induced reinstatement, lidocaine did not affect reinstatement by discrete cues, even though rats had only 3 days of exposure, suggesting that either the ventral hippocampus isn't involved in processing of discrete cues, or that processing of these cues is completed after three days. Overall the findings suggest that the ventral hippocampus is involved in associative learning early in the development of cocaine addiction. Once the cocaine-paired cues are no longer novel, however, other brain areas, such as prefrontal cortex, may be more relevant for processing the contextual cues. Arkins, A. L., Mashhoon, Y., and Kantak, K.M. Hippocampal Regulation of Contextual Cue-induced Reinstatement of Cocaine-seeking Behavior. Pharmacology, Biochemistry and Behavior, 90, pp. 981-491, 2008.