Research Findings - Basic Behavioral Research
Nicotinic Control of Axon Excitability Regulates Thalamocortical Transmission
Dr. Raju Metherate and his colleagues at the University of California, Irvine, have reported a new role for nicotinic acetylcholine receptors in regulating cortical information processing by an action on myelinated axons in a thalamocortical tract that carries sensory information from relay nuclei in the thalamus to the neocortex. Using both in vivo and in vitro preparations of the auditory thalamo-cortical pathway in adult mice, Dr. Metherate and his team showed that activation of nicotinic acetylcholine receptors in the initial portion of the path modulate transmission by means of regulating axonal excitability. Exogenously applied nicotine was found to increase the probability and synchrony of evoked action potential discharges in vitro, whereas blockade of the receptors in the pathway in vivo was accompanied by reduced sound-evoked cortical responses. Dr. Metherate proposes that this mechanism may be responsible for the facilitatory effect of nicotine on sound-evoked cortical potentials and the cholinergic regulation of sensory-cognitive function. Kawai, H., Lazar, R. and Metherate, R. Nicotinic Control of Axon Excitability Regulates Thalamocortical Transmission. Nature Neurosci., 10, pp. 1168-1175, 2007.
Repeated Cocaine in Rats Prior to Pregnancy Alters Subsequent Cortical Activation and Maternal Behavior
Drs. Marcelo Febo and Craig Ferris at the University of Massachusetts Medical Center administered cocaine to rats for 14 days and then withdrew treatment during breeding and pregnancy. On postpartum days 4-8 neural responses to suckling were measured with blood-oxygen-level-dependent (BOLD) MRI or microdialysis. Results showed that BOLD activation in the medial prefrontal cortex, septum, and auditory cortex was curtailed in these cocaine sensitized dams and the effect was long lasting. No differences were reported for other brain areas examined, including the nucleus accumbens and olfactory regions. Baseline, but not pup-stimulated, dopamine levels in the prefrontal cortex were lower in cocaine-sensitized dams. Cocaine-sensitized dams exhibited faster retrieval of pups, but no differences were seen in other maternal behaviors such as grouping, crouching or defending the nest. The authors proposed that this cocaine-associated change in pup retrieval behavior could be linked to hypo-responsivity of the medial prefrontal cortex. It is possible that cocaine's effect on neural activation associated with suckling reflects a cross-sensitization process, such as that previously observed by others when comparing drugs of abuse like cocaine with natural rewards. Febo, M. and Ferris, C.F. Development of Cocaine Sensitization Before Pregnancy Affects Subsequent Maternal Retrieval of Pups and Prefrontal Cortical Activity During Nursing. Neurosci, 148, pp. 400-412, 2007.
Chronic Nicotine Augments Hormonal Response to a Mild Stressor
NIDA grantee Dr. Burt Sharp has been investigating the effects of chronic nicotine self-administration (SA) on hypothalamo-pituitary-adrenal (HPA) responses to stress. In a recent study, he predicted that nicotine would act as a stressor, increasing plasma stress hormone levels and augmenting the increases produced by other stressors. Stress responsivity varies across rat strains, so Sprague-Dawley (SD) rats were comprehensively evaluated in this study and Lewis rats tested under limited conditions for comparison. Both strains were given the opportunity to self-administer (SA) nicotine or saline on a 23 hr/day schedule for 20 days. Blood samples were taken on the first and third day of access and analyzed for adrenocorticotropin (ACTH) and corticosterone (CORT). In both SD and Lewis rats, hormone levels increased on the first day of nicotine exposure, suggesting that acute drug activates a stress response, but normalized by the third day of nicotine SA. After 20 days of nicotine or saline SA, SD rats were exposed to one of three types of stress: mild or moderate foot shock stress, endotoxin [lipopolysaccharide; LPS], or immobilization. Foot shock was administered during a nicotine SA session, and lever-press behavior was assessed. Also, as all three types of stress have previously been observed to increase stress hormone levels, blood samples were taken for hormone level analysis. LPS and immobilization stressors were administered following a nicotine SA session, and only effects on plasma hormone levels were measured. For the Lewis rats, the only stressor tested was mild foot shock, which was also imposed during a nicotine SA session. In general, electric foot shock did not alter nicotine SA behavior regardless of strain or shock intensity. Chronic nicotine SA augmented the HPA hormonal response to mild foot shock in SD rats, and moderate foot shock-induced hormonal responses in this strain were not affected by chronic nicotine. Chronic nicotine also did not affect LPS- or immobilization-induced increases in ACTH or CORT. As predicted, nicotine activated the HPA axis in a similar manner to that produced by an imposed stressor -- initially increasing ACTH and CORT, with tolerance developing to this effect after several days of repeated administration. Although mild shock foot shock produced CORT increases in both SD and Lewis rats, their responses were not identical. Peak CORT responses were seen earlier in SD rats and nicotine's amplification of the CORT response in SD rats was greater than that seen in the Lewis strain. These findings indicate that type of stress, as well as its intensity, are important determinants of the stress-induced hormonal responses, and that nicotine augments the hormonal stress responses produced by these stressors. Taken together, the data suggest that nicotine may increase responses to a mild stressor in chronic cigarette smokers. Chen, H., Fu, Y., and Sharp, B.M. Chronic Nicotine Self-Administration Augments Hypothalamic-Pituitary-Adrenal Responses to Mild Acute Stress. Neuropsychopharm., pp. 1-10, 2007 (e-pub ahead of print).
Chronic Nicotine Increases _4 and _2-Containing- and Decreases _6-Containing-Nicotinic Acetylcholine Receptors
Nicotine regulates the expression of neuronal nicotinic acetylcholine receptors (nAChR). These receptors are typically heteromeric, containing both alpha- and beta-subtypes. Upregulation of _4_2 has been reported from human smokers, while nAChR receptors containing the _3_2 combination appear resistant to nicotine. Subunits of the _6 type in the striatum are involved in nicotine-stimulated dopamine release and levels of _6 are increased by long-term administration of nicotine. Dr. David Perry and colleagues have recently used three different approaches to assess the effects of chronic nicotine on _6 subunits and assess functional changes in striatal dopamine release that correspond to these changes: 1) autoradiographic binding of cone snail toxin _-conotoxin MII ([125I]_-CtxMII -- a radioligand predominantly selective for the _6 subtype) and [125I]A-85380 -- a radioligand selective for the _4_2 subtype; 2) immunoprecipi-tation with a battery of subunit-selective antibodies; and 3) determination of _6-CtxMII-sensitive dopamine release from striatal synaptosomes. Immunoprecipitation using antibodies for _2, _3, _4, _5, _6, _2, _3, and _4 subunits found that _4 and _2 receptor subunits represented about 80% of the labeled receptors in the striatum and the superior colliculus. Levels of both _2 and _4 subunits were negligible, with levels of _3, _5, _6 and _3 subunits falling in the 10-25% range. Receptor subunit levels were quantified with these techniques after chronic nicotine treatment (14 days), with 1-30 days of "recovery" time. [125I]A-85380 binding was increased in both regions, indicating a nicotine-induced increase in _4_2 subtype levels. In nicotine treated animals, there was a 28% reduction of ([125I]_-CtxMII binding in the striatum only. Immunoprecipitation of the _6 subunit also indicated significantly reduced levels, which was also selective for the striatum. Specifically, levels of _6-containing receptors decreased up to 3 days post-treatment, but recovered to near control levels by day 7, with complete recovery by 30 days. The decrease in _6-containing nACh receptors was accompanied by a decline of approximately 54% in nicotine-stimulated dopamine release from striatal synaptosomes. Thus, after chronic treatment with nicotine, _-CtxMII appears to be less effective in blocking nicotine-induced dopamine release in a population of striatal _6-containing AChRs, presumably due to down-regulation of the subunit. As the researchers did not observe a 100% decrease in the ability of _6-CtxMII to block nicotine stimulated dopamine release, it appears that there is a separate population of striatal _6-containing striatal receptors, that may be differentiated on the basis of subunit complement, and that this receptor constellation is not changed by chronic nicotine. Nicotine receptors in these regions also contain a _3 subunit, associated primarily with _4_2. Interestingly, while nicotine increased the number of receptors immunoprecipitated by _4 and _2 antibodies, and decreased the number detected by _6 antibodies, _3 levels showed no change. Conclusions from the study are as follows: The decrease in _6 levels in the striatum appears to be driven by a reduction in a population of AChRs that are _-CtxMII-sensitive. Furthermore, the researchers hypothesize that the presence of a _3 subunit modulates the regulatory effects of nicotine on _6_2_3-containing receptors, such that _-CtxMII-resistance (seen in the striatal _6 containing receptors that did not change after nicotine) is associated with the presence of this _3 subunit. They also postulate that _3 may allow more efficient assembly of the receptor subtype or stabilize assembly in some way, conferring resistance to chronic nicotine-induced down-regulation. Thus, chronic nicotine decreases binding to _6-containing receptors, but only to those that do not also contain a _3 subunit. This shift in the nicotinic receptor profile following chronic nicotine exposure may be relevant for understanding the development of nicotine dependence and identifying molecular targets in treatment. Perry, D.C., Mao, D., Gold, A.B., McIntosh, M., Pezzulo, J.C., and Kellar, K.J. Chronic Nicotine Differentially Regulates _6- and _3-Containing Nicotinic Cholinergic Receptors in Rat Brain. JPET 322, pp. 306-315, 2007.
Topiramate Increases Nicotine Reward but Reduces Nicotine Intake
Recently, NIDA grantee Malcolm Reid and colleagues have been testing the drug topiramate, which has both GABA agonist and AMPA glutamate antagonist activity, as a putative smoking cessation agent. Previous findings indicate that glutamatergic antagonists and GABAergic agonists reduce the acute rewarding and reinforcing effects of nicotine, however little is known about this drug's effects on subjective and physiological responses to nicotine withdrawal, cue-elicited craving, and the acute effects of smoked cigarettes. A total of 40 treatment-seeking smokers were enrolled in this study and were randomly assigned to either the topiramate treatment (75 mg/day) or a placebo control in a double-blind design. Baseline evaluations were conducted 30 minutes after a smoked cigarette and subjects were assessed with subjective rating scales such as the smoking urges-brief questionnaire (QSU), the withdrawal scale for tobacco (WST), and the cigarette evaluation questionnaire (CEQ). Following completion of a 9 day treatment schedule, subjects were re-evaluated at 3 hours of smoking abstinence. Post-treatment tests included skin temperature, blood pressure, heart rate and skin conductance, re-evaluation with the QSU and WST, and a choice procedure which asked patients to hypothetically choose between smoking a cigarette and receiving money. During cue exposure subjects were tested for craving with tactile, olfactory, visual, and audio smoking cues, or with neutral cues, and responses to the QSU, WST and choice test were again collected. In a post-treatment smoking test, subjects were asked to smoke a single cigarette. Cigarettes were smoked though a controlled- puff volume apparatus to measure number of puffs and volume per puff in to collect data on smoking topography. Before and after smoking, physiological measurements were again collected, and the QSU, WST, CEQ, and choice test were re-administered. After 3 hours of abstinence, subjects in the topiramate group reported greater QSU cigarette craving, WST craving, and WST withdrawal, suggesting that they experienced more withdrawal symptoms. There were no significant interactions between treatment condition and measures from the cue exposure test. The post-treatment cigarette reduced craving, withdrawal, and the amount of money chosen over a cigarette during the choice test, and medication did not alter these outcomes. Although topiramate-treated subjects reported increased smoking reward, they had lower puff volumes, total volume smoked, and plasma nicotine levels. These data suggest that topiramate augmented nicotine withdrawal following brief smoking abstinence, and enhanced the rewarding effects of nicotine when patients smoked. Given this observed enhancement of nicotine's rewarding effects, researchers question the utility of topiramate as putative aide in smoking cessation treatment. Reid, M.S., Palamar, J., Raghavan, S., and Flammino, F. Effects of Topiramate on Cue-induced Cigarette Craving and the Response to a Smoked Cigarette in Briefly Abstinent Smokers. Psychopharmacology, 192(1), pp. 147-158, 2007.
Social Reward in Juvenile Mice Reflects Genetic Variation
Vulnerability to drug abuse is different for adolescent humans and animals, compared to adults. We are beginning to understand some of the developmental factors that underlie this difference, including differences in the importance of social interactions. Dr. Garet Lahvis and his colleagues are developing a mouse model of social reward that can be used to investigate how social affiliative behavior and drug abuse vulnerability interact during different periods of development. In one study, they used a novel social conditioned place preference (SCPP) to examine social reward in juvenile mice and investigate its genetic basis by comparing different mouse strains. The SCPP procedure is similar to the conditioned place preference paradigm that is commonly used to study stimuli associated with drug rewards. In one type of environment with a particular type of bedding material, the mice were allowed to interact with their peers, while on alternate days, in the presence of different environmental stimuli, they experienced social isolation. Then on the test day, they were allowed to freely move between two compartments differentiated by the two types of bedding. The results showed that mice could be conditioned to prefer a stimulus previously associated with the availability of social interactions, and indicate that reward mediated by social contact is a fundamental aspect of juvenile mouse sociality. They also reveal that social proximity is rewarding for juvenile mice from three inbred strains (A/J, C57BL/6J and DBA/2J), but that mice from a fourth strain (BALB/cJ) were much less responsive to social contact (although the BALB mice can develop and demonstrate place conditioning for food). The SCPP phenotype was expressed early in development (postnatal day 25) and did not depend on the number of males or females in each conditioning group. They then conducted additional experiments to compare the strain that conditioned best (C57BL/6J) with the BALB mice that did not show SCPP. In these experiments, they were able to determine that SCPP responses result from an interaction between two distinct processes; that is, conditioning resulted from both facilitated approach toward environments associated with social salience and avoidance of environments associated with social isolation. A follow-up investigation examined the developmental trajectory of preference for social affiliation and behaviors that mediate it, such as ultrasonic communication, and this experiment also incorporated strain comparisons. Strain differences were also found in this study. Overall, these studies identify genetically prescribed processes that attribute value to conditions which predict social contact in juvenile mice - a form of social valuation that is not specifically related to reproductive behavior. This behavioral paradigm, and the identification of phenotypes that can be linked to a genetic substrate, can be a valuable tool for future research in the behavioral genetics of drug abuse vulnerability and development. Panksepp, J.B. and Lahvis, G.P. Social Reward Among Juvenile Mice. Genes, Brain and Behavior, 6, pp. 661-671, 2007. Panksepp, J.B., Jochman, K.A., Kim, J.U., Koy, J.J., Wilson, E.D., Chen, Q.L., Wilson, C.R., and Lahvis, G.P. Affiliative Behavior, Ultrasonic Communication and Social Reward Are Influenced by Genetic Variation in Adolescent Mice. PLoS ONE, 2, e351, 2007.
Enhanced Nicotinic Receptor Activity Links Individual Differences in Novelty Response to Drug Abuse Vulnerability
Animals that have high activity levels in a novel environment are more likely to self-administer abused drugs including nicotine, cocaine, amphetamine, and morphine. Other studies show that activation of nicotinic acetylcholine receptors (nAChRs) contributes to the rewarding effects of addictive drugs. Dr. Daniel McGehee and colleagues hypothesized that nAChR activity may contribute to differences seen in drug abuse vulnerability between these high- (HR) and low-novelty (LR) responding rats. They screened adult rats for their behavioral response to a novel environment and selected animals in the top and bottom quartiles to conduct electrophysio-logical studies of nAChR function in brain slices that included the mesoaccumbens dopamine neurons of the ventral tegmental area (VTA). They found a positive correlation between response to novelty and nAChR modulation of glutamatergic and GABAergic synaptic inputs to VTA dopamine neurons, and in the somatic nAChR responses of VTA neurons. It is also known that response to novelty and sensitivity to addictive drugs are positively correlated with hormonal responses to stress, as HR rats have higher corticosterone (CORT) levels after a stress challenge than their LR counterparts. Thus, the researchers also hypothesized that CORT could enhance nAChR responses. Consistent with this hypothesis, they found that 48 h of CORT treatment of dopaminergic cells in vitro enhanced nAChR responses. Furthermore, when LR animals (but not HR animals) were subjected to stress (repeated forced swimming in cold water for two days), nAChR responses were enhanced in the VTA. Both of these effects were inhibited by pretreatment with a glucocorticoid receptor antagonist, suggesting that a steroid hormone receptor-dependent process was involved. These findings suggest that differences in nAChR function within the mesoaccumbens dopamine system may contribute to individual differences in drug abuse vulnerability and that these differences can be linked to differences in stress hormone levels. Exactly how differences in nAChR function contribute to drug sensitivity and the predisposition to self-administer drugs requires additional study. Fagen, Z., Mitchum, R., Vezina, P., and McGehee, D.S. Enhanced Nicotinic Receptor Function and Drug Abuse Vulnerability. J. Neurosci., 27, pp. 8771-8778, 2007.
Antidepressant Treatment can Normalize Adult Behavioral Deficits Induced by Early-Life Exposure to Methylphenidate in an Animal Model
Animal models have been used to assess whether methylphenidate (MPH), a psychostimulant commonly prescribed for the treatment of attention-deficit/hyperactivity disorder, alters subsequent sensitivity to drugs of abuse or has other neurobiological or behavioral consequences in later life. Rats exposed to MPH during preadolescence show a decreased sensitivity to cocaine in adulthood, but they also show enhanced reactivity to stressful or anxiety provoking situations and a reduced sensitivity to natural and drug rewards. In the current study, Bolanos and his colleagues examined the ability of fluoxetine, a selective serotonin reuptake blocker, to normalize these depression-like symptoms. Male rats received MPH (2.0 mg/kg) or saline during preadolescence (postnatal day 20-35). As adults, they were tested for behavioral reactivity to emotion-eliciting stimuli and natural and drug rewards, with or without prior fluoxetine treatment. Sucrose preference was tested with a two-bottle test and morphine reward with conditioned place preference. Stress sensitivity and anxiety-like behavior were assessed with the forced swim test and the elevated-plus maze, respectively. Results show that MPH-treated rats were significantly less responsive to sucrose and morphine rewards and more sensitive to stress- and anxiety-eliciting situations. These MPH-induced behavioral deficits were reversed by fluoxetine. For all tests, a single, acute injection of fluoxetine given 24 hours before had little or no effect, whereas seven days of injections was effective. The results highlight the need for further research to understand the effects of stimulants on the developing nervous system and the potential enduring effects resulting from early-life exposure. Bolanos, C.A., Willey, M.D., Maffeo, M.L., Powers, K.D., Kinka, D.W., Grausam, K.B., and Henderson, R.P. Antidepressant Treatment Can Normalize Adult Behavioral Deficits Induced by Early-life Exposure to Methylphenidate. Biological Psychiatry, September 2007 (Epub ahead of print).
Rats Maintained on a High-Fat Diet Show Decreased Acquisition of Cocaine Self-Administration
Previous studies have shown that nutritional status can alter the rewarding effects of psychostimulants and the acquisition of drug self-administration. In general, these studies indicate that food deprivation enhances, while satiation diminishes, drug reward and self-administration. The present study by Wellman and his colleagues compared the rate of acquisition of cocaine self-administration between rats fed on a regular chow-pellet diet (about 10% fat) versus a high-fat diet (35.9% fat) for 45 days prior to cocaine self-administration testing. Rats maintained on the chow-pellet diet slowly acquired cocaine self-administration such that, at the end of the 25 day testing period, only 3 out of 7 of them had met criterion (on days 5, 7 and 15, respectively). In contrast, only 2 of the 8 rats that had ingested the high-fat diet acquired self-administration, and they did so on later testing days (16 and 24) than the chow fed rats. This effect was not a result of dietary-induced obesity as only 2 of the rats on the high fat diet gained significantly more weight than the chow-fed rats. The results suggest that prolonged exposure to a high-fat diet diminishes the efficacy of cocaine reinforcement and adds to the growing body of literature on nutritional modulation of drug self-administration. Wellman, P.J., Nation, J.R., and Davis, K.W. Impairment of Acquisition of Cocaine Self-administration in Rats Maintained on a High-fat Diet. Pharmacol. Biochem. Behav. 88, pp. 89-93, 2007.
Rat Conflict Model More Closely Approximates Human Relapse
Animal models of human relapse employ a reinstatement paradigm with three experimental phases: First, drug self-administration is established, and then responding for drug is extinguished with saline infusions instead of the drug. Next, either a stimulus cue previously paired with drug self administration, a 'priming' dose of the drug, or a stressor is introduced. All three manipulations reinstate responding for drug; in other words, produce drug-seeking behavior. This model has been criticized because human addicts rarely undergo forced extinction; rather, abstinence is self-imposed. Also, relapse to drug use is usually associated with negative consequences - that is, the return to drug taking is followed by adverse events. Recently, NIDA-funded investigator Dr. Abraham Zangen and Dr. Yavin Shaham from NIDA's IRP have proposed a conflict model of cue-induced relapse that may better mimic these aspects of human relapse. In their paradigm, rats are trained to press a lever for i.v. cocaine and when responding is stable an electrified grid is introduced. During this phase animals must cross the grid to gain access to the drug lever and eventually all animals develop self imposed abstinence. In phase three, a light cue previously paired with cocaine infusions is presented and animals are assessed for their willingness to cross the electrified grid and resume lever pressing - or to demonstrate drug seeking in the face of an adverse consequence (bar presses produce saline infusions in this phase). Surprisingly, in their study, 14 of 24 animals demonstrated drug seeking by crossing the electrified grid to press the lever, and some rats even crossed the barrier when placed in the operant chamber without the cue light illuminated. The most interesting finding was that there is great variability across animals, both in their willingness to cross the grid and in the number of operant responses made in the presence of the cue after crossing. Thus, this model may be valuable for investigating individual differences associated with the vulnerability to relapse under conditions that more closely approximate the human situation. Cooper, A., Barnea-Ygael, N., Levy, D., Shaham, Y., and Zangen, A. A Conflict Rat Model of Cue-induced Relapse to Cocaine Seeking. Psychopharmacology, 194, pp. 117-125, 2007.
Alternative Rewards Enhance Extinction in an Animal Model
When animals are trained to respond for natural or drug rewards in one environment (A), and extinction is carried out in an alternate environment (B), extinction does not reduce responding when animals are then placed back into environment A (an ABA procedure). The failure of extinction effects to transfer to the drug-taking environment, is reminiscent of extinction failure seen in therapeutic interventions with human addicts. When returning to the drug associated environment, the context may trigger relapse or it may serve as a setting that mediates the strength of drug-associated cues to elicit drug seeking. NIDA grantees Drs. David Kearns and Stanley Weiss conducted a study to determine if extinction transfer failure is due to the occasion setting properties of environment "A" (thus, a context renewal phenomenon), or due to learning a discrimination between A predicts 'drug' and B predicts 'no drug' (a simple operant discrimination). In their study, rats were trained in A to self-administer cocaine (coc) when a tone was present, but when the tone was off, responses were not reinforced with the drug. Then, in some the tone was presented alone in Context A until animals no longer responded to it. In others, the tone was presented alone in a new context - context B. All animals were tested for context renewal in the original environment (hence, AAA and ABA groups). As the researchers had predicted, ABA animals showed robust responding for coc when returned to A, whereas in AAA coc-seeking behavior had extinguished. Since animals in this study received discrimination training in phase one, they concluded that the "A" environment acts as an occasion setter to facilitate retrieval of the tone+drug association, triggering a reinstatement of drug seeking. Interestingly, additional animals were exposed to ABA conditions, but during extinction training in B they received food paired with the tone cue instead of coc. The tone+food pairing facilitated transfer of extinction effects to the A environment, whether food rewarded animals for not making an operant response when the tone was presented (that is, for not drug seeking in the presence of the cue), or if food was just delivered non-contingently with the tone presentation. Thus, it appears that the tone became associated with food reward and this new association interfered with the ability of the drug-associated cue to elicit coc seeking. In conclusion, pairing an alternative reward with drug associated cues during extinction may facilitate transfer of extinction effects back to the original drug-taking environment. Kearns, D.N. and Weiss, S.J. Contextual Renewal of Cocaine Seeking in Rats and its Attenuation by the Conditioned Effects of an Alternative Reinforcer. Drug and Alcohol Dep. 90, pp. 193-202, 2007.
Cortisol Response to Stress Predicts Progression in Youth Smoking
Dr. Harriet de Wit and colleagues recently conducted a laboratory investigation to assess individual vulnerability associated with the progression from occasional to habitual smoking in 18-24 year-old college students. In this study the researchers assessed 44 students for subjective and physiological responses to an amphetamine (or placebo) challenge, and to a Trier Social Stress Test (TSST). All subjects were light smokers who reported smoking an average of 10 cigarettes per week and none were nicotine dependent. The design followed a double-blind, placebo-controlled protocol, with 20mg of oral amphetamine used for the drug challenge. The POMS and the ARCI were used to collect self-report data; heart rate was measured, and saliva cortisol (cort) was sampled. Amphetamine induced the expected increases on scales such as Vigor, Elation, Euphoria, Friendliness and Arousal, and also increased HR and cort. The TSST increased reports of Vigor and Anxiety, and also increased HR and cort. Peak increases in cort levels after stress and amphetamine were positively correlated. Thirty-one students were followed up 6 months after the laboratory sessions and questioned about their frequency of smoking. Peak increases in cort after acute, laboratory stress were significantly positively related to smoking progression. That is, subjects who increased the amount they smoked in six months had an approximate thirteen-fold greater increase in peak cort response on the TSST than those that did not increase. While the authors argue that these results are from a small sample and should be accepted as preliminary, they suggest that sensitivity to stress and escalation of smoking may be linked to a common neurobiological predisposition. Alternatively, individuals with greater stress reactivity may experience greater positive mood effects from smoking. de Wit, H., Vicini, L., Childs, E., Sayla, M.A., and Terner, J. Does Stress Reactivity or Response to Amphetamine Predict Smoking Progression in Young Adults? A Preliminary Study. Pharmacol. Biochem. Behav. 86, pp. 312-319, 2007.
A Conditioned Inhibitor Signaling Unavailability of Food Reward Decreases Cocaine-Seeking
Environmental stimuli associated with drugs of abuse act as conditioned excitors to induce craving and trigger drug-seeking behavior. Conversely, a conditioned inhibitor that signals the absence of a reinforcer exerts behavioral control opposite to that produced by drug-paired cues, and instead suppresses behavior. Researchers at American University, in collaboration with scientists at NIDA's IRP, have previously observed that introducing a cue signaling the absence of cocaine, when cocaine is being self-administered in the presence of a drug-paired cue, suppresses cocaine (coc) responding by greater than 90%. However, a conditioned inhibitor can only reduce behavior as long as the conditioned excitor (drug-paired cue) retains its strength to activate behavior. If drugs are no longer available (e.g., during abstinence) then the inhibitor loses its effect. In a new study by these investigators, a cue signaling "no food" was found to dramatically suppress responding in animals trained to self administer coc. In a complex experimental design, rats were first trained to associate food with a click or a tone. They also received discrimination training so that when the stimulus cue was turned off, food reward was not available. Then all animals were trained to self-administer i.v. coc and the food+click group now learned to associate tone+coc, whereas the food+tone group had coc paired with the click. Again, discrimination training was conducted so that the absence of the reward-paired cue signaled unavailability of drug. Then both groups experienced a three component operant schedule in which animals had to respond appropriately during the click or tone to receive reward and a third component was an extinction component with no stimuli presented. Next all animals received training to establish a new stimulus as a conditioned inhibitor - a light was paired with the click that signaled food for one group of animals and coc for the other. Animals continued to respond appropriately for reward when the click or tone alone was present, but when the light+click compound stimulus was on, reinforcement was not available. In a subsequent test for conditioned inhibition, the inhibitor - the light - was now presented in compound stimulus with the tone and drug-seeking or food-seeking behavior was measured. During this test food or coc did not reinforce operant responses, but rats still made an average of 10-15 bar presses per minute (i.e., food- or drug-seeking responses). By contrast, when the uniquely paired light+tone combination was on, mean responses fell to less than one press per minute - representing a 94% and 87% response suppression for coc and food in the two groups (compared to tone alone). This result demonstrates that animals learned the light cue signaled "no food" - and thus it acquired properties of a conditioned inhibitor against the background of excitation induced by the food-associated click in cocaine-seeking animals. When then presented with the coc-associated tone, this inhibitor suppressed responding to receive drug. (A similar result was seen in food-seeking rats trained with food+tone). Cue-based extinction therapies have had limited success because of the strong control that drug related cues have to sustain the addict's behavior. These findings suggest that adding a conditioned inhibitor for alternate, non-drug rewards, to this context may increase treatment success. Weiss, S.J., Kearns, D.N., Christensen, C.J., Huntsberry, M.E., Schindler, C.W., and Panlilio, L.V. Reduction of Cocaine Seeking by a Food-based Inhibitor in Rats. Exper. Clin. Psychopharmacol. 15, pp. 359-367, 2007.
Drug Reinforcement is Enhanced by Social Stimuli
Many prior investigations have examined the interaction between social influences and the reinforcing properties of drugs of abuse. Reports from animal studies show that social isolation, or social subordination, is associated with an enhanced propensity to self-administer drugs such as cocaine or alcohol. In human subjects, subjective effects vary as a function of the social situation (alone or with peers) during testing. However, little is known about the effects of social stimuli, which are known to be inherently rewarding, on the reinforcing strength of drugs of abuse in the self-administration model. In the laboratory of Dr. Marilyn Carroll at the University of Minnesota, studies have been conducted to establish oral self-administration of PCP in rhesus monkeys. Dr. Carroll and colleagues have recently tested the effects of a nearby conspecific on PCP self administration in this model. Ten adult monkeys consumed 0.25 mg/ml PCP with water concurrently available, under fixed ratio (FR16) schedules. The test chamber could be fitted with a solid partition between two animal cages, or a grid that permitted access to a conspecific in the neighboring cage. After intake was stable with the solid partition in place, the grid wall was inserted for 14 days and when responding stabilized again, the solid partition was replaced. Salivary cortisol measures were taken during solid and grid partition phases. In the next phase of the experiment all animals were tested on progressive ratio (PR) schedules, (i.e., concurrent PRs for water and PCP), with increasing concentrations of the drug (0.125, 0.25, 0.5 and 1.0 mg/ml). Break points (BPs) were identified as the number of responses the animal was willing to make to receive drug at each of these concentrations. When observed under grid conditions monkeys vocalized (e.g., barking) and displayed interactive gesturing. Under these conditions, PCP intake was significantly greater than during the first, solid partition phase and remained significantly elevated even when the solid partition was replaced. Since water intake also increased under grid conditions, it is possible that the social stimuli non-specifically enhanced consummatory behavior. However, on the PR schedule, mean BPs under grid conditions were significantly higher for all but the highest dose of PCP, with no change in BPs for water, suggesting that these social stimuli facilitate drug intake. Furthermore, while presence of the neighboring monkey was a novel stimulus, it does not appear that elevated intake can be attributed to introduction of a stressor, since cortisol levels were no different during solid versus grid partition conditions. Newman, J.L., Perry J.L., and Carroll, M.E. Social Stimuli Enhance Phencyclidine (PCP) Self-administration in Rhesus Monkeys. Pharmacol. Biochem. Behav. 87, pp. 280-288, 2007.
Sex Differences in THC's Effects on Spatial Learning in Adult and Adolescent Rats
In 2006 Dr. Scott Swartzwelder and colleagues at Duke University reported effects of 2.5, 5.0, and 10.0mg/kg THC on learning in male adolescent and adult rats in the Morris water maze (Cha et al., 2006). The researchers found that THC disrupts both spatial and non-spatial learning more powerfully in adolescent rats than in adults at all dose levels. Dr. Swartzwelder and his colleagues have now published results from three studies that extend findings of this initial report. In the first study, effects of 5.0 mg/kg THC on spatial learning in the Morris Water maze were studied in male and female adolescent and adult rats over 5 daily test sessions. In males, spatial learning was impaired in adolescents on all test days, but was not impaired on any of the test days in adults. Among females, adolescents exhibited impairment on all test days, but adults were impaired only during the first two sessions. Thus, THC impairment was more potent in adolescents than adults and more potent in adult females than adult males. To assess the effects of chronic THC on subsequent learning, in a second experiment, separate groups of male and female adolescent and adults rats were treated with 5.0 mg//kg THC for 21 days and tested 28 days later in the Morris Water maze. Chronic THC treatment did not produce learning deficits in any of the groups. The third experiment was a THC dose-response study (2.5, 5.0, and 10.0mg/kg THC) using adolescent and adult female rats, conducted to parallel the initial experiment in males (Cha et al., 2006). Each dose was tested for 5 days. Consistent with the earlier outcome in males, THC produced a dose-response impairment in spatial learning. Although there was no overall age effect on spatial learning, there were differences in the dose-response function between adolescents and adults, suggesting a greater dose sensitivity in adolescents. Additionally, assessment of spatial memory indicated impairment in adolescents but not adults. These outcomes have important implications for future research on THC and other abused drugs. Animal model research in drug abuse has largely been conducted on adult males. The results of the present study join a growing body of drug abuse research showing that outcomes observed in adult males do not always generalize to females and to adolescents, highlighting the limitations of research conducted with only adult male subjects. Cha, Y.M., Jones, K.H., Kuhn, C.M., Wilson, W.A., and Swartzwelder, H.S. Sex Differences in the Effects of Delta(9)-tetrahydrocannabinol on Spatial Learning in Adolescent and Adult Rats. Behavioral Pharmacol. 18, pp. 563-569, 2007.
Sex Chromosome Complement Regulates Habit Formation
Gonadal hormones can cause sex differences via their brain organization effects during development and via their activational effects in puberty and adulthood. Other sex differences, however, are not due to gonadal hormones, but rather the direct action of the chromosome complement. Recent development of the 'four core genotype' mouse model permits dissociation of gonadal sex and chromosomal sex by comparison of males and females that are gonadally and chromosomally either congruent or incongruent. The two incongruent cases are (1) chromosomal XY males with deletion of the testis-determining Sry gene and are therefore gonadally female and (2) chromosomal XX females with insertion of the Sry gene and are therefore gonadally male. Dr. Jane Taylor and colleagues at Yale and UCLA have used this four core genotype mouse model to study the role of chromosomal sex versus gonadal sex on habit formation. In the first phase of their study, mice were trained to nose-poke in one of three apertures for a food pellet delivery. Following either moderate training (9 days) or extended training (15 days), food pellets were devalued in a conditioned taste aversion procedure in which food pellets were paired with injections of the emetic lithium chloride. The mice were then retested in the instrumental procedure. In past studies using this habit formation procedure, devaluation decreases instrumental performance prior to habit formation, but after habit formation, instrumental performance is unaffected by devaluation. Dr. Taylor and her colleagues found that XX mice that previously received moderate instrumental training, showed operant performance following devaluation that was insensitive to changes in reinforcer value, indicating formation of a habit. XY mice subjected to devaluation, however, exhibited poorer performance than those not subjected to devaluation of the reinforcer, indicating that XY mice had not yet acquired habit formation. This outcome was independent of gonadal phenotype and was independent of gonadectomy vs. sham-operated status, indicating that the effects did not depend on organizational or activational functions of gonadal hormones. Following extended instrumental training, XX mice exhibited better instrumental performance (more correct nose-pokes) than XY mice. After devaluation, XX and XY mice were equally insensitive to changes in reward value of the reinforcer, indicating that with extended training, all mice had developed an instrumental habit. The authors speculate that sex differences in habit formation, as determined by the sex chromosome complement, could have implications for studying and understanding sex differences in drug addiction. Quinn, J.J., Hitchcott, P.K., Umeda, E.A., Arnold, A.P. and Taylor, J.R. Sex Chromosome Complement Regulates Habit Formation. Nature Neurosci. 10, pp. 1398-1400, 2007.