Research Findings - Basic Behavioral Research

Low Dose Nicotine Pretreatment Increases Cocaine Reward in Adolescent but not Adult Rats

Prior animal behavioral studies have shown that compared to adult rats, adolescent rats exhibit greater sensitivity to nicotine's stimulant and rewarding effects and lesser sensitivity to nicotine's negative effects. In the present study, Dr. Frances Leslie and colleagues at the University of California at Irvine sought to determine if nicotine pretreatment differentially modulates cocaine self-administration in adolescent and adult rats. Male and female rats, aged postnatal day P28 and P86, were treated twice daily with nicotine (0.03 ug/kg/, i.v.) or saline daily for 4 days. At P32 and P90, the rats were tested for 5 consecutive days for acquisition of cocaine (0.2 or 0.5 mg/kg) self-administration. In the first hour of acquisition, there was more cocaine self-administration by the adolescent nicotine-pretreatment group than by adolescent saline-pretreated animals or by adult nicotine and saline groups. Nicotine pretreatment also significantly enhanced cocaine self-administration in the adolescent, but not adult rats, across the first four of five sessions of cocaine self-administration. In a separate similar experiment with adolescent rats, nicotine pretreatment failed to enhance acquisition of sucrose operant responding, suggesting that nicotine's enhancement of cocaine self-administration does not merely reflect a generalized increase in appetitive responding. The authors conclude these findings suggest "that brief treatment with low doses of nicotine in early adolescence, but not adulthood, may cross-sensitize the brain to cocaine reward" (p.70). McQuown, S.C., Belluzzi, J.D., and Leslie, F.M. Low Dose Nicotine Treatment during Early Adolescence Increases Subsequent Cocaine Reward. Neurotoxicology and Teratology, 29, pp. 66-73, 2007.

Sex Differences in Adolescent-onset vs Adult-onset Nicotine Self-administration

Dr. Ed Levin and colleagues at Duke University previously reported higher levels of nicotine self-administration in adolescent female rats than in adult female rats. This enhancement of nicotine self-administration in adolescence was still observed when animals were tested as adults (Levin et al. Psychopharmacology, 169, pp. 141-149, 2003). To complement this previous study, Dr. Levin and his colleagues have now conducted a parallel study in males. They found male adolescent rats, like female adolescent rats, self-administered more nicotine on a per kilogram basis than adults. Additionally, they found two important sex differences. First, males were more vulnerable to this adolescent enhancement effect than females: compared to adult onset nicotine self-administration, the adolescent enhancement was two-fold in females, but three-fold in males. Second, unlike the female rats that persisted in higher rates of self-administration when they became adults, male rats exhibit adult-like levels of nicotine self-administration after the adolescent period. Thus, male rats appear to be more vulnerable than females to the adolescent enhancement effect, but only females exhibited a persistence of this enhancement into adulthood. Levin, E.D., Lawrence, S.S., Petro, A., Horton, K., Rezvani, A.H., Seidler, F.J., and Slotkin, T.A. Adolescent vs. Adult-onset Nicotine Self-administration in Male rats: Duration of Effect and Differential Nicotinic Receptor Correlates. Neurotoxicology and Teratology, 29, pp. 458-465, 2007.

An Improved Transdermal Drug Delivery Patch using Voltgage-gated Carbon Nanotube Membranes

Dr. Bruce Hinds and his colleagues at the University of Kentucky have developed a novel membrane patch that can be opened or closed by applying a low voltage to provide an improved well-controlled method of transdermal drug delivery. The patch is made of a polymer film through which an array of aligned hollow-core carbon nanotubes passes. These permeable membranes are now being tested with drugs such as nicotine and fentanyl with the goal of providing a patient-controlled, noninvasive flexible dosing regimen. Majumber, M., Zhan, X., Andrews, R., and Hinds B.J. Voltage Gated Carbon Nanotube Membranes. Langmuir 23, pp. 8624-8631, 2007.

Antidepressant-like Properties of Cytisine, a Partial Agonist of Nicotinic Acetylcholine Receptors

Anecdotal and self-reports from smokers suggest that nicotine via cigarette smoking can help regulate mood. Indeed, published reports have indicated that depressed subjects have an increased incidence of smoking and that smoking cessation is often associated with increased signs of depression. Dr. Marina Picciotto and her colleagues at Yale University School of Medicine have studied the role that nicotine may play in modulating neuronal systems that control mood. Specifically, they examined the effects of cytisine, a partial agonist of alpha4-beta2 acetylcholine receptors and a full agonist at alpha3-beta4 receptors, using a number of mouse models of depression and c-fos expression. Cytisine exhibited antidepressant-like activity in the tail suspension, forced swim, and novelty-suppressed feeding tests, all shown by others to be responsive to acute administration of antidepressants in mice. These effects did not appear to involve motivation to acquire food, differences in metabolism, or levels of activity. Effects of cytisine on c-fos activity were examined to determine which particular brain regions might be involved. Interestingly, regions involved with dopaminergic and cholinergic neurotransmission did not appear to be involved, while C-fos activity in the amygdala was markedly reduced. These findings provide new information on the role of cholinergic systems involved in mood regulation, suggesting that cytisine and other such agents acting on the alpha4-beta2 neuronal acetylcholine receptors may be potential target compounds for new antidepressant drugs. Mineur, Y.S., Somenzi, O., and Picciotto, M.R. Cytisine, A Partial Agonist of High-affinity Nicotinic Acetylcholine Receptors, has Antidepressant-like Properties in Male C57BL/6J Mice. Neuropharmacology 52, pp. 1256-1262, 2007.

Effects of Uncontrollable Stress on Responses to Drugs of Abuse

Acute stress has been shown to facilitate the rewarding effects of a number of commonly abused drugs when the stressor is administered either immediately before or during drug administration and often in the same environment. Dr. Steven Maier and his colleagues have previously reported that a single session of uncontrollable (inescapable tailshock, IS), but not controllable (escapable tailshock, ES), stress can enhance the conditioned place preference (CPP) response to morphine, even when stressor and drug administration are separated temporally and spatially. However, this persistent, trans-situational enhancement was not seen with amphetamine CPP. In two new studies, they have further explored the ability of IS to enhance drug reward. The first study asked whether the long-term effect of IS is specific to opioids. Rats were exposed to a single session of IS or left in their home cage (HC). Twenty-four hours later, CPP was conducted with oxycodone, cocaine, or ethanol. IS enhanced the subsequent CPP response to oxycodone, but not to cocaine or ethanol. A complementary experiment tested whether enhancement of oxycodone CPP, as in the previous experiments with morphine, was dependent on the "escapability" of the stressor. Rats were exposed to IS, ES, or HC treatment and conditioned with oxycodone 24 h later. The results showed that enhancement was dependent on the stressor, as ES did not affect oxycodone CPP. Collectively, these findings indicate that the long-term, trans-situational enhancing effect of uncontrollable stress on drug reward is specific to opioids. In a second study, they asked whether IS might have a long-lasting effect on responses to drugs other than opioids if drug exposure were immediately preceded by a mild stressor. First, they measured the locomotor response to cocaine 48 h after a single session of IS. Then, this procedure was repeated, except that half of the rats received two footshocks immediately before cocaine administration. Finally, they manipulated the escapability of the initial stressor such that rats received either ES or IS 48 h prior to footshock + cocaine. IS did not affect the subsequent locomotor response to cocaine, but did enhance this response when cocaine administration was immediately preceded by two footshocks, while footshocks alone (in the absence of IC 48 hr earlier) were without effect. This sensitizing effect was dependent on the "escapability" of the initial stressor, as ES did not alter the locomotor response to footshock + cocaine. These results indicate that acute exposure to IS, but not ES, can sensitize the locomotor response to cocaine 48 h later, but only when cocaine administration is immediately preceded by a brief stressor. This research reveals that uncontrollable stress can have long-lasting effects on magnitude of drug reward, but that this effect varies and depends on the drug class. Der-Avakian, A., Rozeske, R.R., Bland, S.T., Watkins, L.R., and Maier, S.F. The Effects of a Single Session of Inescapable Tailshock on the Subsequent Locomotor Response to Brief Footshock and Cocaine Administration in Rats. Psychopharmacology (Berl). 191, pp. 899-907, 2007; Der-Avakian, A., Bland, S.T., Rozeske, R.R., Tamblyn, J.P., Hutchinson, M.R., Watkins, L.R., and Maier, S.F. The Effects of a Single Exposure to Uncontrollable Stress on the Subsequent Conditioned Place Preference Responses to Oxycodone, Cocaine, and Ethanol in Rats. Psychopharmacology (Berl), 191. pp. 909-917, 2007.

Exposure to Marijuana Smoke Impairs Memory Retrieval in Mice

Studies have shown that delta-9-tetrahydrocannabinol (_9-THC) and other cannabinoids disrupt performance in a wide range of animal models of learning and memory. However, few studies have investigated the effects of smoked marijuana in these paradigms, although marijuana smoke contains over 60 cannabinoid components in addition to _9-THC, and several hundred non-cannabinoid chemicals. In addition, cannabinoids are generally administered before acquisition, and retention is evaluated soon afterwards; thus it is difficult to distinguish between processes related to acquisition and retrieval. In the present study, Dr. Aron Lichtman and his colleagues investigated specific effects of marijuana smoke and injected _9-THC on acquisition versus memory retrieval in mice using a repeated acquisition Morris water-maze task. The smoke exposure system was recently developed in Dr. Lichtman's laboratory and has been used in two other recent studies. For the memory tests, in order to distinguish between acquisition and retrieval, subjects were administered _9-THC or exposed to marijuana smoke either 30 min before acquisition or 30 min before the retention test. Inhalation of marijuana smoke or injected _9-THC impaired the ability of the mice to learn the location of the hidden platform and also to recall the platform location once learning had already taken place. In contrast, when the platform was visible, neither drug impaired performance, indicating that the drugs had not impaired other abilities necessary to perform the task. They also found that the CB1 receptor antagonist rimonabant prevented the memory disrupting effects of both _9-THC and marijuana. This is the first study demonstrating that marijuana smoke impairs memory retrieval through a CB1 receptor mechanism, independent of its effects on sensorimotor performance, motivation, or initial acquisition. Niyuhire, F., Varvel, S.A., Martin, B.R., and Lichtman, A.H. Exposure to Marijuana Smoke Impairs Memory Retrieval in Mice. Journal of Pharmacology and Experimental Therapeutics, June 22, 2007. Epub ahead of print.

Single Gene Affects Nicotine Dependence and Stress-Related Behaviors

NIDA grantee Dr. Noboru Hiroi and colleagues investigated the role of fosB -- a Fos family transcription factor -- as a pleiotrophic factor. That is, their investigation approached the study of fosB as a single genetic factor that affects many phenotypes related to both nicotine-induced behavioral traits and stress-related traits. Genetically modified mice (wild-type and fosB knock-out) were compared using a conditioned place preference procedure. Although animals showed no initial difference in preference for the two sides of the chamber, environments paired with various doses of nicotine were differentially preferred and/or avoided. Specifically, the low dose of nicotine (0.2 mg/kg) was preferred by wild-type (WT) but not knockout (KO) mice. At the moderate dose of nicotine (0.6 mg/kg), WT type mice showed neither a preference nor aversion, however the KO mice showed a conditioned aversion. At the high dose of nicotine (0.8 mg/kg), both WT and KO showed a conditioned place aversion. These data suggest that the KO mice may be less sensitive to the rewarding properties of nicotine, but more sensitive to its negative properties. When WT and KO mice were given access to oral nicotine using a 2-bottle choice paradigm, the WT had greater nicotine intake (a finding that was not due to reduced fluid intake by the KO mice). This finding appears to be nicotine-specific, as there were no significant differences in the levels of saccharin preference or quinine aversion using the same choice paradigm. Paralleling findings from the CPP study, these choice data also suggest that KO mice have reduced sensitivity to the rewarding properties of nicotine. In addition to differences in nicotine reward and aversion, there were also differences in nicotine-suppressed motor activity. Although WT and KO mice displayed similar levels of nicotine-suppressed motor activity following acute, centrally administered nicotine to the nucleus accumbens, the KO mice lost motor suppression more quickly than WT mice. When administered into the nucleus accumbens and caudate putamen, repeated nicotine exposure did not stimulate fosB or delta fosB in the KO mice, but did induce them in WT mice. Testing with behavioral measures of stress (inescapable open field), revealed that KO mice had heightened responses. This was true following either saline or nicotine, indicating that enhanced stress responsivity was not just a selective sensitivity to the drug nicotine. In addition, a series of control experiments revealed that the differences found by Hiroi and colleagues could not be attributed merely to differences in nicotine metabolism, or differences between groups on novelty preference. Collectively, these findings suggest that fosB represents a single genetic factor that affects both nicotine-induced behavior and behavioral sensitivity to environmental stress. Zhu, H., Lee, M., Agatsuma, S., and Hiroi, N. Pleiotropic Impact of Constitutive fosB Inactivation on Nicotine-induced Behavioral Alterations and Stress-related Traits in Mice. Human Mol Genetics, 16(7), pp. 820-836, 2007.

Adolescent Rats Are Less Sensitive to the Aversive Effects of THC

Recent studies show that, compared to their adult counterparts, adolescent animals are more sensitive to the rewarding properties of many drugs of abuse and less sensitive to aversive properties. These differences may account for a propensity to initiate drug use during adolescence and greater risk to escalate and develop dependence. Given the high frequency of marijuana abuse in adolescents, researchers at Duke University Medical Center have been investigating developmental vulnerabilities to THC's behavioral effects. In a 2007 report from Schramm-Sapyta et al., evidence is provided to suggest that adolescent rats (28 days of age) are less sensitive to anxiogenic or aversive effects of 0.5 to 2.5 mg/kg THC than adults tested at 64-66 days of age. The researchers employed two tests of anxiety - the elevated plus maze (EPM) and the light-dark task (LDT). Adolescents were also less sensitive to THC's locomotor reducing effects on these two tasks. Two tests of the aversive properties of THC - conditioned taste aversion (CTA) and conditioned place aversion (CPA)- were employed. While both age groups demonstrated CTA, adults were more sensitive to this effect, showing a greater reduction in THC-paired saccharin than adolescents. On CPA, adults showed an aversion at 5.0 mg kg, and adolescents did not develop an aversion at any dose. The researchers also measured ACTH and corticosterone levels to examine differences in THC-induced HPA activation that might account for these behavioral differences. Adults had a higher ACTH response after THC, suggesting that the drug may induce a greater stress response. In conclusion, the authors report less THC-induced aversive and anxiety related effects in adolescents, that cannot be attributed to differential locomotor effects, since THC doses that produced these effects were associated with the least locomotor changes in these paradigms. Baseline observations corroborate previous studies to suggest that adolescents take more behavioral risks. It is unclear if maturation of central CB1 receptor systems might underlie these observations. Whatever the mechanism, less aversive effects of the drug THC at younger ages may contribute to this drug's popularity during adolescence. Schramm-Sapyta, NL., Cha, Y.M., Chaudhry, S., Wilson, W.A., Swartzwelder, H.S., and Kuhn, C.M. Differential Anxiogenic, Aversive, and Locomotor Effects of THC in Adolescent and Adult Rats. Psychopharmacology, 191, pp. 867-877, 2007.

Self-Control, Symptomatology and Substance Use in 9 yr Old Children

NIDA researcher, Rick Gibbons and colleagues, tested a theoretical model of how self-control constructs are related to psychological symptomatology and variables that predispose to involvement versus noninvolvement in substance use: willingness to use, affiliation with peers who use, and efficacy for resisting use. Data were obtained from a sample of 332 children, mean age = 9.3 years, who were interviewed in their homes. Overall, self-control constructs were significantly related to symptomatology or well-being. Moreover, the results showed significant pathways from symptomatology measures to predisposing factors plus a direct effect from poor self-control to lower resistance efficacy. The study was conducted with a diverse sample of children, and the results were obtained with control for relevant demographic characteristics, including ethnicity and parental education. The results may have implications for preventive interventions. The results suggest that self-control training components can be included in prevention programs focused on adolescent problem behaviors. The results for predisposing factors suggest that prevention programs for younger children can target perceptions of substance users and resistance efficacy, as there is evidence that both processes can be altered through family- or school-based interventions. Furthermore, prevention research may be designed to examine the implications of more distal factors (e.g., neighborhood disadvantage, racial discrimination), as well as more proximal factors such as peer affiliations. Finally, the present results suggest that cognitive and motivational processes are more salient for young boys, whereas social processes are more salient for young girls. These gender differences need to be replicated, but results of this type need exploration because of their implications for targeted prevention programs. Wills, T.A., Ainette, M.G., Mendoza, D., Gibbons, F.X., and Brody, G.H. Self-Control, Symptomatology, and Substance Use Precursors: Test of a Theoretical Model in a Community Sample of 9-Year-Old Children. Psychology of Addictive Behaviors, 21, pp. 205-215, 2007.

Protection Against Early Onset Of Substance Use And Sexual Behavior Among African Americans

NIDA researcher, Rick Gibbons and his colleagues, tested a theoretical model relevant for averting early onset of substance use and sexual behavior among rural African American youth. The model posits that substance use and early sexual behavior are influenced by a number of variables including the type of parenting a child experiences, self-control, ethnic esteem, academic and social competence, peer behavior, etc. A community sample of 670 African American youth, mean age of 11.2 years, were interviewed in their homes. The main outcome measures were use of cigarettes and alcohol and incidence of sexual behavior. Other variables included measures of parenting (parent-child interaction), racial socialization, self control, ethnic pride and self-esteem, academic and social competence, attitudes towards deviance, substance use willingness, substance use, sexual behavior, etc. Data were collected by self report and from caregivers and teachers where relevant. Prevalence data indicated that rates of substance use and sexual behavior were low but not zero. For alcohol, 14% of the participants indicated that they had ever drunk alcohol. For smoking, 6% of the participants indicated that they had ever smoked cigarettes. For sexual behavior, 3% of the sample indicated they had ever had sex. The data indicated generally low levels of willingness for substance use and high levels of resistance efficacy, consistent with previous data from similar populations. In comparison to data for substance use, the level of actual sexual behavior was lower, but the level of willingness was higher. Structural equation modeling indicated parenting was related to self-control and self-esteem, and racial socialization was related to ethnic pride. Self-control and self-esteem variables were related to levels of deviance-prone attitudes and to perceptions of those who engage in, or abstain from, substance use and sexual behavior. The proximal factors (behavioral willingness, resistance efficacy, and peer behavior) had substantial relations to substance use and sexual behavior. Thus, in this population, self-esteem and self-control are related to parenting approaches and have pathways to attitudes and social perceptions that are significant factors for predisposing to, or protecting against, early involvement in substance use and sexual behavior. Wills, T.A., Murry, V.M., Brody, G.H., Gibbons, F.X., Gerrard, M., Walker, C., and Ainette, M.G. Ethnic Pride and Self-Control Related to Protective and Risk Factors: Test of the Theoretical Model for the Strong African American Families Program. Health Psychology, 26, pp. 50-59, 2007.