Research Findings - Basic Behavioral Research
Enhanced Neuronal Activity in the Nucleus Accumbens Accompanies Relapse to Cocaine Self-Administration
When rats are trained to respond for i.v. drug administration, and subsequently withdrawn, they resume drug self-administration ("relapse") when returned to the environment where they previously received the drug. Researchers have identified a number of cellular changes that may contribute to relapse after a period of abstinence. For example, changes in the nucleus accumbens (NAc) are seen in gene expression, GABA concentration, and neuronal structure. Electrophysiological recording from the NAc of rats self-administering i.v. cocaine has revealed patterns of firing that are associated with anticipation of drug reward, and with response-reinforcement associations. Four well-defined types of firing patterns occur within seconds of drug reinforcement: Some cells increase their activity preceding the reinforced response and some increase or decrease immediately following the response. A fourth type exhibits a dual peak increase. Two other patterns have been identified that are long-duration cyclic discharges that span the inter-infusion interval of self-administration. Dr. Regina Carelli and her colleagues sought to determine whether these firing patterns show neuroadaptations that might be involved in relapse to drug seeking. She trained animals to self-administer cocaine and identified the six neuronal patterns described above on the basis of cell firing on the last self-administration session. After a one-month break, animals were tested in a one-hour self-administration session. Recordings from the NAc revealed significant increases in the number, percentage and strength of short-duration patterned discharges, and also in the long-term cyclic alterations associated with cocaine. The findings are significant in that the short discharges preceding drug administration are believed to encode cocaine-seeking behaviors, and the long-term firing patterns have been hypothesized to convert declining drug levels into increased motivation for more drug. The observation that these electrophysiological correlates are enhanced and associated with increased drug seeking during relapse suggests that these signals may be the output pathway for cellular, molecular and genetic changes seen during psychostimulant withdrawal. Hollander, J.A. and Carelli, R.M. Abstinence from Cocaine Self-administration Heightens Neural Encoding of Goal-directed Behaviors in the Accumbens. Neuropsychopharmacology, 30, pp. 1464-1474, 2005.
An Animal Model of Disruption of Maternal Behavior and Intergenerational
Pregnant women who use cocaine are more likely to abuse or neglect their children and to exhibit deficits in bonding and interacting with their infants. In addition, numerous studies have shown an intergenerational transfer of childhood maltreatment: daughters with a history of abuse or neglect are more likely to exhibit these same behaviors in rearing their own offspring. Dr. Josephine Johns and her colleagues have developed a rat model to investigate the causal factors involved in disruption of maternal behavior and intergenerational transfer. During gestation, they treated rat dams with cocaine, or with saline, or merely handled them as a control condition. Then, after the litters were culled to form comparable sized groups, the dams reared either their natural or cross-fostered litters such that some pups that were not themselves exposed to cocaine were reared by cocaine exposed dams and vice versa. Measures of maternal behavior were then collected on postpartum days 1, 5, 10, and 15. Subsequently, first generation daughters from these various litters were bred with no cocaine exposure, and their maternal behavior towards their natural litters was also studied. The authors found: 1) Dams treated with cocaine, regardless of whether they reared exposed or unexposed pups, showed disruption of the onset of maternal behavior, with a diminishing effect over the postpartum period; 2) Pups prenatally exposed to cocaine were treated differently by all dams, regardless of their own treatment condition. Overall, the dams had a tendency to spend less time caring for these pups, consistent with other animal studies suggesting that drug-exposed rat pups have attributes that may make them vulnerable to neglect; 3) Intergenerational deficits in maternal behavior were apparent in the first generation daughters. This effect largely resulted from prenatal exposure to cocaine, but it was also influenced by the treatment of the dams that had reared them. Johns, J.M., Elliott, D.L., Hofler, V.E., Joyner, P.W., McMurray, M.S., Jarrett, T.M., Haslup, A.M., Middleton, C.L., Elliott, J.C. and Walker, C.H. Cocaine Treatment and Prenatal Environment Interact to Disrupt Intergenerational Maternal Behavior in Rats. Behavioral Neuroscience, 119, pp. 1605-1618, 2005.
Personality Factors Influence the Effects of Acute Amphetamine
Individual differences in the drug-induced positive mood and other subjective effects of d-amphetamine have been linked to personality traits related to sensation seeking. The present study by Harriet de Wit and colleagues extended these associations to separate personality traits of reward sensitivity, physical fearlessness, and impulsivity. A total of 128 healthy volunteers were given oral doses of d-amphetamine (10 and 20 mg) or placebo in counterbalanced order. Their responses to the drug were measured using the Profile of Mood States, Addiction Research Center Inventory, and Drug Effects Questionnaire. Participants also completed the Multidimensional Personality Questionnaire Brief Form to assess personality traits related to reward sensitivity, physical fear (or harm avoidance), and impulsivity. De Wit reports three main findings from this study. First, compared to individuals with high trait physical fear, individuals with low trait physical fear reported greater positive activational responses to low-dose d-amphetamine, showing greater energy, intellectual activation, euphoria, vigor, arousal, elation, friendliness, positive mood, and less fatigue and sedation. Second, individuals with high trait reward sensitivity had marginally greater positive responses as well as drug liking and drug wanting after the 20 mg dose. Third, individuals with high mental imagery/_flexibility had greater positive drug responses to both doses, and these effects were more significant at the lower dose. These findings indicate that the subjective effects of amphetamine vary as a function of specific personality traits in healthy volunteers. White, T.L., Lott, D.C. and de Wit, H. Personality and the Subjective Effects of Acute Amphetamine in Healthy Volunteers. Neuropsychopharmacology, 31, pp. 1064-1074, May 2006.
Psychological Processes Underlying Risky Decisions
Poor self- control or disinhibition is a characteristic feature of young adults with drug addiction. NIDA researcher Julie Stout and her colleagues recently applied mathematical decision models with a simulated gambling task (SGT) to investigate the processes underlying decision making in 66 drug abusers and 58 control participants. The mathematical model enabled these investigators to separately examine the effects of wins and losses on the SGT. The investigators also assessed impulsivity, social deviance and harm avoidance. The results of the study showed that drug abusers differed from controls in SGT performance and in the processes underlying performance. The results showed also that individual differences, such as personality and drug abuse characteristics, are related to SGT performance. For the men, for example, drug group participants performed more poorly than did controls. Moreover, male drug abusers were more influenced by rewards than by punishments, consistent with other studies that have found drug abusers are hypersensitive to rewards and relatively insensitive to future consequences. In contrast to the findings from men, the performance of women was not straightforward. Control women performed at chance level, which is unusual for a control group on this task. By contrast, performance of the drug-abusing women fell below that of control men, but was slightly better than that of the drug-abusing men. Overall the findings suggest a difference between men and women in their approach to the SGT. Thus, generalizations from studies that have focused on male participants may not be germane to understanding cognitive aspects of drug abuse in women. Finally, individual differences in the decision processes used in performing the SGT task are related not only to drug abuse but also to personality factors. Stout, J.C., Rock, S.L., Campbell, M.C., Busemeyer, J.R. and Finn, P.R. Psychological Processes Underlying Risky Decisions in Drug Abusers. Psychology of Addictive Behaviors, 19, pp. 148-157, 2005.
Upregulation of Ionotropic Glutamate Receptor Subunits within Specific Mesocortico-limbic Regions during Chronic Nicotine Self-Administration
Little is known about the effects of chronic nicotine treatment on glutamate receptors. A model of chronic nicotine self-administration (SA) using rats, which emulates important aspects of nicotine intake by humans, has recently been employed by Burt M. Sharp and colleagues to determine whether glutamate receptor subunits are affected when animals freely take nicotine via the i.v. route of administration. In this study, two groups of rats could press a lever in their home cages to receive an infusion of either 0.03 mg/kg nicotine or saline. The investigators reported that animals working for nicotine responded twice as much than those working for saline. After 18 days of this SA procedure, ionotropic glutamate receptor subunit levels were determined in brain regions within the mesocorticolimbic system with Western blot techniques. In prefrontal cortex, the levels of NMDA receptor subunit 2A (NR2A) and NR2B were increased by 67% (p<0.04) and 83% (p<0.027), respectively, compared to saline controls. In the ventral tegmental area, glutamate receptor subunit 2/3 (GluR2/3) increased by 34% (p<0.011). By contrast, nicotine SA did not affect expression of these NMDA receptor subunits in the dorsal striatum or nucleus accumbens. These findings suggest that chronic nicotine SA selectively increases ionotropic glutamate receptor subunits in specific areas of the brain. Wang, F., Chen, H., Steketee, J.D. and Sharp, B.M. Upregulation of Ionotropic Glutamate Receptor Subunits within Specific Mesocorticolimbic Regions during Chronic Nicotine Self-Administration. Neuropsychopharmacology, advance online publication, 1 February 2006.
Potentiation of Cocaine-Primed Reinstatement of Drug Seeking in Female Rats During Estrus
Researchers at the Medical University of South Carolina previously reported that cue-induced reinstatement of cocaine-seeking was greater in male than in female rats at the highest (1.0 mg/kg) and lowest training dose (0.25 mg/kg), but did not differ at the intermediate training doses (0.4, 0.5, 0.6 mg/kg). At the lowest training dose, females in estrus failed to exhibit reinstatement. (Fuchs, R.A., Evans, A., Mehta, R.H., Case, J. M., and See, R.E. Influence of Sex and Estrous Cyclicity on Conditioned Cue-induced Reinstatement of Cocaine-seeking Behavior in Rats. Psychopharmacology, 179, pp.662-672, 2005.) In a follow up study, the researchers sought to determine whether sex and estrous cycle phase play a role in cocaine-primed reinstatement. Following the acquisition of cocaine self-administration (0.5 mg/kg per infusion) and subsequent extinction, reinstatement was assessed with a priming infusion of i.p. cocaine (0, 5.0 or 10.0 mg/kg) in males and in females tested in either the diestrus, proestrus, or estrus phase. Both males and females exhibited a dose-related increase in reinstatement. The effects, however, were opposite those previously observed under conditioned cue-reinstatement. At the 10.0 mg/kg priming dose, females in estrus were more susceptible to reinstatement than both males and non-estrus females (that did not differ). These sex differences in cocaine-induced versus conditioned cue-induced reinstatement could have implications for sex differences in relapse in humans, suggesting perhaps that the factors leading to relapse may differ in males and females. The neuroendocrine basis for the sex differences observed in the present study is unknown, although prior work has shown that dopamine transmission in the prefrontal cortex is associated with cocaine-primed reinstatement, whereas dopamine transmission in the basolateral amygdala is associated with conditioned cue-reinstatement. Further investigation into sex differences and hormonal factors involved in these brain regions and the relationship to reinstatement is warranted. Kippen, T.E., Fuchs, R.A., Mehta, R.H., Case, J.M., Parker, M.P., Bimonte-Nelson, H.A. and See, R.E. Potentiation of Cocaine-primed Reinstatement of Drug Seeking in Female Rats During Estrus. Psychopharmacology, 182, pp. 245-252, 2005.
Exogenous Estrogen Enhances Reinstatement of Cocaine-Seeking Behavior in Ovariectomized Female Rats
In recent studies from the Medical University of South Carolina, reinstatement of cocaine-seeking behavior was found to be affected by phase of the estrous cycle. In a separate investigation by researchers at the University of Minnesota, the role of estrogen as a modulator of reinstatement of cocaine-seeking behavior has been explicitly examined by comparing reinstatement in ovariectomized female rats administered estrogen replacement (OVX+EST), OVX females administered vehicle (OVX+VEH), and sham-operated females administered vehicle (SH+VEH). In the first of two experiments, in the OVX+EST females EST was administered long-term, during training of self-administration, maintenance, extinction and reinstatement. Results indicated that cocaine-primed reinstatement of responding was greater in the OVX+EST and the SH+VEH females than in the OVX+VEH females, indicating that estrogen mediates cocaine-primed reinstatement of responding. In order to distinguish between estrogen's effects on motivation versus learning, in the second experiment, EST was administered short-term, over 3 days prior to and during the reinstatement phase in OVX+EST. As in the first experiments, greater reinstatement occurred in the OVX+EST females compared to OVX+VEH controls. Comparison of OVX+EST rats in the two experiments (i.e., those receiving long versus short-term EST), however, revealed no differences in reinstatement, suggesting to the authors that estrogen's enhancing effects on reinstatement reflects a direct effect of estrogen's enhancement of motivation for cocaine as opposed to an effect of estrogen on associative learning. Larson, E.B., Roth, M.E., Anker, J.J. and Carroll, M.E. Effect of Short- vs. Long-term Estrogen on Reinstatement of Cocaine-seeking Behavior in Female Rats. Pharmacology, Biochemistry and Behavior, 82, pp. 98-108, 2005.
Nicotine Reward More Influenced by Dose Instructions in Female Than Male Smokers
Studies on smoking behavior have shown that females are more sensitive to the non-pharmacological cues associated with nicotine and less sensitive to nicotine dose compared to males. These findings suggest that different interventions may be effective in smoking cessation strategies for men and women. Ken Perkins and his colleagues investigated the effects of accurate or inaccurate instructions about nicotine content on smoking reward. The investigators report that accurate instructions increased smoking reward more in women than in men. In a follow-up study, these investigators enrolled 60 subjects who smoked more than 10 cigarettes per day for at least one year to assess the influence of dose instruction versus no instruction on male and female smokers. Subjects were abstinent overnight and were randomly assigned to one of four conditions on the study day. Subjects smoked either a normal cigarette with 0.6mg nicotine (nic) or a de-nicotinized brand with <0.05 mg. Half of each group was given instructions and half were not. After two puffs, they completed the Rose Sensory Questionnaire to assess smoking reward, and the Diener and Emmons Mood Scale. Craving was also measured and smoking behavior was quantified by number of puffs and latency to first puff during a 30-min ad lib smoking period. Data analysis revealed the following: For women, nic increased reward only in the presence of dose instructions, whereas for men, nic increased reward only in the absence of dose instructions. Nic increased positive affect overall. Craving decreased from baseline to post-puffs, but this decrease was greater for men than for women. Also, for men but not women, nic decreased craving in the presence, but not the absence, of instructions (opposite from the observation for smoking reward). In the 30-min ad lib smoking period, nic decreased the latency to the first puff for women, only in the presence of dose instruction; this effect was not seen in men. These observations add to a rapidly accumulating body of evidence that women's smoking behavior is guided by a different set of influences from those that are important for men. Perkins, K.A., Doyle, T., Ciccocioppo, M. Conklin, C., Sayette, M. and Caggiula, A. Sex Differences in the Influence of Nicotine Dose Instructions on the Reinforcing and Self-reported Rewarding Effects of Smoking. Psychopharmacology, 184, pp. 600-607, 2006.
Sibutramine Decreases the Appetitive and Consummatory Aspects of Feeding in Baboons
This study examined how sibutramine (0.06-4.0 mg/kg, i.m.), a clinically effective weight-loss medication that increases extracellular serotonin and norepinephrine levels, affected the appetitive and consummatory aspects of feeding of nonhuman primates (baboons) with task-dependent 24-hour access to food. Sibutramine effects were compared to those of dexfenfluramine (2.0-6.0 mg/kg, p.o.), which primarily increases extracellular serotonin levels. The baboons had to complete a two-response sequence to obtain food: responding on one lever during a 30-min appetitive phase was required before animals could start a consumption phase, where responding on a second lever led to food delivery. Responding during the appetitive phase resulted in presentations of food-related light stimuli only. Both males and females ate significantly less when treated with dexfenfluramine, but not sibutramine. Animals also ate fewer meals when treated with dexfenfluramine, but not with sibutramine. Although sibutramine had no effect on number of light presentations and did not alter performance during the appetitive phase, it did reduce consummatory behavior. Sibutramine increased the latency to the first meal of the session in females, but not males. By contrast, dexfenfluramine increased the latency to the first meal of the session, and decreased both appetitive and consummatory behavior in males and females. The presence of sibutramine's gender-specific effects suggests that sex may play a role in determining its effects on feeding behavior. The behavioral mechanism by which sibutramine decreases food intake appears distinct from anorectic drugs such as dexfenfluramine. That is, sibutramine has its anorectic effect by reducing consummatory behavior (eating), whereas dexfenfluramine reduces both consummatory and appetitive behavior (food seeking). Applied to drug abuse, this behavioral paradigm might be useful for understanding appetitive drug seeking and consummatory drug taking. Foltin, R.W. Effects of Sibutramine on the Appetitive and Consummatory Aspects of Feeding in Non-human Primates. Physiology and Behavior, 87, pp. 280-286, 2006.
Individual Differences in Anxiety and Stress Reactivity May Predict Nicotine Self-Administration
Previous research has revealed that anxiety, stress, and nicotine dependence interact in complex ways. Recently, NIDA grantee Dr. Richard De La Garza examined the relationship between anxiety and nicotine reinforcement by comparing self-administration behavior in Wistar-Kyoto (WK) rats, known to be anxiety prone and hyperresponsive to stress. These animals were compared to Wistar (W) controls in responding for a non-drug reinforcer (sucrose) and for self-administration of nicotine. Analysis revealed that W rats made significantly more responses and worked harder to obtain sucrose than did WK animals. In addition, over the course of 24 days of nicotine self-administration,W rats showed an increase in drug intake that significantly exceeded the stable intake of WK rats by Day 14. Taken together, these findings suggest a reduced motivation by WK rats for both non-drug and drug reinforcers. Moreover, further analysis showed that both groups had similar response rates for sucrose when low levels of effort were required, and that group differences were not seen until the response requirement increased to more than 23 lever presses for each sucrose delivery. To account for differences in responding for nicotine, the authors suggest that lower levels of responding for nicotine by WK rats might be attributable to drug-induced activation of endogenous stress systems, which would serve to reduce the reinforcing value of nicotine. De La Garza, R. Wistar Kyoto Rats Exhibit Reduced Sucrose Pellet Reinforcement Behavior and Intravenous Nicotine Self-administration. Pharmacology, Biochemistry and Behavior, 82, pp. 330-337, 2005.
Cocaine Impairs Social Communication in Baboons
Drugs of abuse have been observed to disrupt social behavior among non-human primates. Drugs like cocaine might produce these effects by altering the way in which nonhuman primates communicate. Recently, NIDA grantees Robert Hienz and Elise Weerts examined cocaine's effects on discriminations of socially important vocal communication - grunts - in baboons. Grunts are natural calls that are associated with differing motivational and/or social contexts, such as affiliation, dominance or aggression, and they also serve to identify the caller. Acoustically, they resemble human vowel sounds. Previously, these investigators reported that cocaine disrupted the baboons' perception of human vowel sounds. In the recent study they extended their research by testing digitized versions of baboon grunts that were recorded in the natural environment. Animals were trained to release a lever when they heard a different, or "target" grunt, inserted in a consecutive sequence of standard grunts. Cocaine was administered intramuscularly at doses between 0.032 and 0.56 mg/kg. Each dose was tested twice, with lower doses examined first to avoid the development of psychostimulant sensitization. Prior to drug administration, the four male baboons discriminated target vocalizations at the 80% correct level. Following cocaine, discrimination was significantly impaired, but interestingly, this effect was much greater than had previously been seen with human vowel sounds. Although these auditory stimuli are acoustically similar, the authors suggest that grunts are more affected because they are biologically and socially significant for the species. The detection of species-specific vocalizations may represent a more specialized perceptual process than the general process invoked by human vowel sounds. If similar drug effects occur with human cocaine use, it is possible that subtle social communications may be impaired and thus contribute to the addictive process. Hienz, R.D. and Weerts, E.M. Cocaine's Effects on the Perception of Socially Significant Vocalizations in Baboons. Pharmacology, Biochemistry and Behavior, 81, pp. 440-450, 2005.
Differential Responsivity to Non-Drug Reinforcers Predicts Vulnerability for Relapse in Female Rats
Previous studies have demonstrated that rats that freely explore novel environments, rats that have high activity levels, rats that are impulsive and rats that have a preference for highly palatable tastes readily acquire drug self-administration. Recently, Marilyn Carroll and colleagues examined the maintenance of i.v. cocaine intake and subsequent drug-primed relapse after an extended period of abstinence. In this study, female rats were allowed six hours per day access to running wheels for 21 days and then divided into groups that had high wheel running rates (H) or low rates (L) on the basis of a median split (mean revolutions/day). Patterns of wheel running over the 21 days increased in animals that were subsequently assigned to an H group, while steady rates were observed in the L group. Also, the investigators found that H rats took significantly more cocaine (0.4 mg/kg/infusion) over 14 days on a fixed ratio reinforcement schedule, than their low wheel running counterparts. Following self-administration tests, rats were withdrawn from cocaine by replacing saline in the infusion cannulae for 22 days. In response to a priming dose of cocaine on Day 23, both groups showed drug seeking by increasing their responses on the lever previously associated with drug, but the H group had a 7-fold increase over their prior response rates during extinction, as compared to a 3-fold increase by L animals. These differences cannot be accounted for by responses during extinction because both groups showed similar response rates during this phase. These findings support previous observations suggesting that inherent differences in an individual's behavior for non-drug reinforcers predict the propensity to engage in drug taking and drug seeking. Still to be determined is whether the increased vulnerability seen in maintenance and reinstatement reflects inherent differences in the sensitivity of motivational systems or more rapid neuroadaptations to cocaine exposure. However, as non-drug rewards and drugs of abuse activate similar neurobiological substrates, and H animals showed an escalating pattern of running wheel behavior, these animals may experience deprivation of hedonic effects produced by running. The authors suggest that they may show higher rates of drug intake and relapse in order to compensate for this loss. Larson, E.B. and Carroll, M.E. Wheel Running as a Predictor of Cocaine Self-administration and Reinstatement in Female Rats. Pharmacology Biochemistry and Behavior, 82, pp. 590-600, 2005.
Subthalamic Nucleus Lesions Enhance the Effects of Cocaine in Rats
The subthalamic nucleus (STN) is traditionally thought to be involved in motor control and has recently become a target for treatment of Parkinson's disease. Less attention has been paid to the role of STN in motivated behaviors, but recent reports suggest that it does have a role: in rats, bilateral lesions of the STN increase responding for food rewards, and high frequency stimulation of the STN has been reported to alter mood and motivation in patients treated for Parkinson's disease. Several years ago, Drs. Uslaner and Robinson noted that c-fos expression, a measure of neural activation, was robustly induced in STN in response to amphetamine or cocaine. They later found that this response increases after a sensitizing regimen of repeated drug administration. Now they have shown more directly that the STN plays an important role in motivational processes and the response to drugs of abuse. Rats received bilateral lesions of the STN and were tested for the psychomotor-activating effects of cocaine, the rate at which they acquired cocaine self-administration, and the motivation for cocaine as assessed with a progressive ratio schedule. STN lesions produced a dose-dependent increase in all of these measures. In addition, STN lesions enhanced the ability of cocaine to induce c-fos expression in the nucleus accumbens and caudate-putamen, structures known to be involved in mediating the psychomotor-activating and incentive motivational effects of drugs of abuse. These findings suggest that engagement of the STN normally serves to dampen psychomotor-activating and incentive motivational effects of drugs of abuse, and that the STN may serve as a novel target for therapeutic interventions aimed at treating drug dependence. Uslaner, J.M., Yang P. and Robinson, T.E. Subthalamic Nucleus Lesions Enhance the Psychomotor-activating, Incentive Motivational, and Neurobiological Effects of Cocaine. Journal of Neuroscience, 25, pp. 8407-8415, 2005.
Mechanisms of Orexin Action in Addiction-Related Neuroadaptations and Behaviors
The orexins (also known as hypocretins) are hypothalamic neuropeptides that were originally identified for their role in arousal and feeding. More recently, several lines of evidence have implicated orexin, and particularly a subset of the orexin neurons that project to the VTA as playing a role in drug addiction. Dr. Antonello Bonci's laboratory has recently investigated the nature of neuroadaptations produced by orexin A input to the VTA and how these are involved in behavioral sensitization to cocaine. The first set of experiments was carried out in brain slices, where orexin effects on synaptic function were explored in dopaminerigic neurons of the VTA. They found that orexin A input to the VTA: (1) potentiates NMDA receptor-mediated excitation in glutamatergic neurons; (2) increases the number of NMDA receptors, causing them to be inserted into the postsynaptic membrane, thereby increasing their availability; and (3) caused a longer-term potentiation of synaptic currents mediated by the AMPA glutamate receptors. They then investigated whether orexin signaling in the VTA was required for the long-term plasticity associated with the development of behavioral sensitization to cocaine. When an orexin A antagonist was given just before cocaine injections during a 5 day sensitization procedure, behavioral sensitization was blocked and the signature synaptic plasticity did not develop. Taken together, these results suggest that orexin produces a fast increase in NMDA receptor function, leading to a longer-term increase in AMPA receptor plasticity, which is associated with behavioral sensitization. In the context of results from a recent study in Dr. Gary Aston-Jones' laboratory, which showed that orexin injections into the VTA could reinstate drug seeking in drug-free rats, results from the current study indicate the cellular mechanisms that precipitate arousal-induced relapse to drug taking during abstinence. All of the recent studies showing a role for orexin in drug addiction indicate that this peptide may provide a target for addiction medications. Borgland, S.L., Taha, S.A., Sarti, F., Fields, H.L. and Bonci, A. Orexin A in the VTA is Critical for the Induction of Synaptic Plasticity and Behavioral Sensitization to Cocaine. Neuron, 49, pp. 589-601, 2006.
Nicotine Induces a Distinctive Gene Expression Profile in Adolescent Rat Forebrain
Smoking is typically initiated in adolescence, but comparatively few studies in animals have investigated whether the adolescent brain responds differently to nicotine than the adult brain. Differences in behavioral responses to nicotine have been observed in adolescent as compared to adult rats. Previously, these authors showed that adolescent rats fail to display long-term contextual cue conditioning, suggesting that neural plasticity related to memory formation may be different in the younger animals. In the current report, they examined the expression of a number of early response genes (arc, c-fos and NGFI-B) that have been implicated in synaptic plasticity and addiction, following acute nicotine in adolescent and adult rats. Baseline expression of arc and c-fos was higher in adolescent brains compared with adults. Following acute nicotine treatment, they found a higher induction of arc mRNA in the prefrontal cortex, and especially in orbitofrontal areas, of adolescents compared to adults. c-fos and NGFI-B were also upregulated by nicotine, but not in an age-related manner. They also measured the induction of these same genes in somatosensory cortex. As in the PFC, baseline levels were also higher in adolescents, but nicotine-induced expression was lower than in adults. Furthermore, in contrast to studies with other drugs of abuse, upregulation of these genes by nicotine was less pronounced in the ventral and dorsal striatum. These results indicate that in adolescence, the activity of specific early response genes is higher in brain regions critical for emotional regulation and decision making, which are known from other studies to be less mature at this age, and that nicotine affects arc in these areas more robustly in adolescents than in adults. Arc is a dendritically targeted gene that is important for synaptic plasticity involved in learning and memory, and it known to be upregulated by amphetamine and cocaine in various brain regions. These new results for nicotine suggest that adolescence may be a neurobiologically vulnerable period for nicotine exposure. Perhaps more importantly, the finding that nicotine has a more profound effect on cortical as compared to striatal regions in both age groups may indicate that addiction to nicotine, even more than to other drugs of abuse, depends on alterations of representations of reward value and other cognitive and attentional functions. Schochet, T.L., Kelley, A.E. and Landry, C.F. Differential Expression of arc mRNA and Other Plasticity-related Genes Induced by Nicotine in Adolescent Rat Forebrain. Neuroscience, 135, pp. 285-297, 2005.
The Cannabinoid System Affects Anxiety-Like Behavior Induced by Nicotine in Mice
The overlapping distribution of cannabinoid CB1 receptors and nicotinic acetyl choline receptors, along with other observations, suggests there may be functional interactions between these two systems. Dr. Rafael Maldonado and his colleagues have explored this interaction in a study on anxiety-like behaviors in mice using the elevated plus maze to study the anxiolytic or anxiogenic effects of various pharmacological treatments, relative to vehicle treatment. This apparatus has 4 arms - two that are enclosed and two that are open - and is suspended 30 cm above the ground. Less anxious animals spend more of their time on the maze and enter more often into the open arms than do more anxious animals. In a previous study, this group established that a single low dose of nicotine (0.05 mg/kg, s.c.) produced anxiolytic-like effects, whereas a high dose (0.8 mg/kg) was anxiogenic. In this study, they showed that pretreatment with the CB1 antagonist, rimonabant, dose-dependently abolished the anxiolytic effects of low-dose nicotine and increased the anxiogenic effects of the high dose. Treatment with _9-THC, a cannabinoid agonist, had no effect on the anxiolytic effects of nicotine, but attenuated the anxiogenic effects. They also tested a combination of subthreshold nicotine and _9-THC, which produced an anxiolytic-like response. The results of these studies indicate that the endogenous cannabinoid system regulates the effects of nicotine on anxiety-like behavior, and in particular that CB1 receptors must be active for nicotine to have an anxiolytic effect. The elucidation of this interaction between nicotine and the endogenous cannabinoid system provides support for the use of cannabinoid antagonists in the treatment of tobacco addiction. Balerio, G.N., Aso, E. and Maldonado, R. Role of the Cannabinoid System in the Effects Induced by Nicotine on Anxiety-like Behaviour in Mice. Psychopharmacology (Berlin), 184, pp. 504-513, 2006.
Morphine Conditioned Place Preference is Disrupted by Inhibition of Protein Synthesis During Memory Recall
A consensus is emerging that established memories of different sorts undergo a process called "reconsolidation" each time they are recalled. During recall and reconsolidation, memories can be disrupted by interfering events and pharmacological treatments, including inhibition of protein synthesis. Because craving and relapse are frequently evoked by the recall of memories associated with previous drug experience, the reconsolidation process offers an opportunity for disrupting these memories and potentially reducing relapse to drug seeking. In this study, Dr. Cristina Alberini and her colleagues used a rat model of drug seeking to test whether inhibiting protein synthesis after the reactivation of the memory trace can weaken the memory of a drug experience. They first established conditioned place preference for morphine (mCPP) over 4 days of treatment. One week later, the investigators attempted to disrupt memory reconsolidation in two different types of recall experiments. In the first, animals were tested in a contextual recall test; that is, they were tested for recall of the morphine-associated compartment. Immediately after this recall test, they were injected with either anisomycin or cycloheximide, two commonly used protein synthesis inhibitors. However, upon retesting the next day, the animals showed undisrupted mCPP. For the second experiment, the rats were given a new conditioning trial one week later, and then injected with inhibitor. In this case, when they were retested 24 hrs later for contextual recall, their memory was significantly impaired. With this single injection of inhibitor, recall was significantly recovered one week later, but with two injections, 5 hrs apart after the reconditioning trial, memory disruption persisted for at least 4 wks (the last time point tested). Thus, unlike with other types of memories that have been tested in similar paradigms, established mCPP did not become labile after contextual recall, but only after the concomitant re-experience of both the conditioning context and the drug. They then used focal injections of protein synthesis inhibitors and showed that an established mCPP was disrupted after the conditioning session if protein synthesis was blocked in the hippocampus, basolateral amygdala, or nucleus accumbens, but not in the ventral tegmental area. This study complements other recent studies that also showed disruption of drug-associated memories during recall. Although there are differences among the results of these studies that raise critical questions that can only be answered by further research, together they suggest the intriguing possibility that novel treatments with the goal of weakening memories for the drug experience might be developed to reduce the risk of relapse. Milekic, M.H., Brown, S.D., Castellini, C. and Alberini, C.M. Persistent Disruption of an Established Morphine Conditioned Place Preference. Journal of Neuroscience, 26, pp. 3010-3020, 2006.