The influence of environment on the induction of sensitization to the psychomotor activating effects of intravenous cocaine in rats is dose-dependent

Opposite environmental gating of the experienced utility ('liking') and decision utility ('wanting') of heroin versus cocaine in animals and humans: implications for computational neuroscience

BACKGROUND

In this paper, we reviewed translational studies concerned with environmental influences on the rewarding effects of heroin versus cocaine in rats and humans with substance use disorder. These studies show that both experienced utility ('liking') and decision utility ('wanting') of heroin and cocaine shift in opposite directions as a function of the setting in which these drugs were used. Briefly, rats and humans prefer using heroin at home but cocaine outside the home. These findings appear to challenge prevailing theories of drug reward, which focus on the notion of shared substrate of action for drug of abuse, and in particular on their shared ability to facilitate dopaminergic transmission.

AIMS

Thus, in the second part of the paper, we verified whether our findings could be accounted for by available computational models of reward. To account for our findings, a model must include a component that could mediate the substance-specific influence of setting on drug reward RESULTS: It appears of the extant models that none is fully compatible with the results of our studies.

CONCLUSIONS

We hope that this paper will serve as stimulus to design computational models more attuned to the complex mechanisms responsible for the rewarding effects of drugs in real-world contexts.

Heroin versus cocaine: opposite choice as a function of context but not of drug history in the rat

RATIONALE

Previous studies have shown that rats trained to self-administer heroin and cocaine exhibit opposite preferences, as a function of setting, when tested in a choice paradigm. Rats tested at home prefer heroin to cocaine, whereas rats tested outside the home prefer cocaine to heroin. Here, we investigated whether drug history would influence subsequent drug preference in distinct settings. Based on a theoretical model of drug-setting interaction, we predicted that regardless of drug history rats would prefer heroin at home and cocaine outside the home.

METHODS

Rats with double-lumen catheters were first trained to self-administer either heroin (25 μg/kg) or cocaine (400 μg/kg) for 12 consecutive sessions. Twenty-six rats were housed in the self-administration chambers (thus, they were tested at home), whereas 30 rats lived in distinct home cages and were transferred to self-administration chambers only for the self-administration session (thus, they were tested outside the home). The rats were then allowed to choose repeatedly between heroin and cocaine within the same session for seven sessions.

RESULTS

Regardless of the training drug, the rats tested outside the home preferred cocaine to heroin, whereas the rats tested at home preferred heroin to cocaine. There was no correlation between drug preference and drug intake during the training phase.

CONCLUSION

Drug preferences were powerfully influenced by the setting but, quite surprisingly, not by drug history. This suggests that, under certain conditions, associative learning processes and drug-induced neuroplastic adaptations play a minor role in shaping individual preferences for one drug or the other.

Exploring the role of locomotor sensitization in the circadian food entrainment pathway

Food entrainment is the internal mechanism whereby the phase and period of circadian clock genes comes under the control of daily scheduled food availability. Food entrainment allows the body to efficiently realign the internal timing of behavioral and physiological functions such that they anticipate food intake. Food entrainment can occur with or without caloric restriction, as seen with daily schedules of restricted feeding (RF) or restricted treat (RT) that restrict food or treat intake to a single feeding time. However, the extent of clock gene control is more pronounced with caloric restriction, highlighting the role of energy balance in regulating clock genes. Recent studies have implicated dopamine (DA) to be involved in food entrainment and caloric restriction is known to affect dopaminergic pathways to enhance locomotor activity. Since food entrainment results in the development of a distinct behavioral component, called food anticipatory activity (FAA), we examined the role of locomotor sensitization (LS) in food entrainment by 1) observing whether amphetamine (AMPH) sensitization results in enhanced locomotor output of FAA and 2) measuring LS of circadian and non-circadian feeding paradigms to an acute injection of AMPH (AMPH cross-sensitization). Unexpectedly, AMPH sensitization did not show enhancement of FAA. On the contrary, LS did develop with sufficient exposure to RF. LS was present after 2 weeks of RF, but not after 1, 3 or 7 days into RF. When food was returned and rats regain their original body weight at 10-15 days post-RF, LS remained present. LS did not develop to RT, nor to feedings of a non-circadian schedule, e.g. variable restricted feeding (VRF) or variable RT (VRT). Further, when RF was timed to the dark period, LS was observed only when tested at night; RF timed to the light period resulted in LS that was present during day and night. Taken together our results show that LS develops with food entrainment to RF, an effect that is dependent on the chronicity and circadian phase of RF but independent of body weight. Given that LS involves reorganization of DA-regulated motor circuitry, our work provides indirect support for the role of DA in the food entrainment pathway of RF. The findings also suggest differences in neuronal pathways involved in LS from AMPH sensitization and LS from RF.

Behavioral sensitization of the reinforcing value of food: What food and drugs have in common

Sensitization is a basic property of the nervous system whereby repeated exposure to a stimulus results in an increase in responding to that stimulus. This increase in responding contributes to difficulty with treatment of drug abuse, as stimuli associated with substance use become signals or triggers for drug craving and relapse. Our work over the past decade has applied the theoretical framework of incentive sensitization to overeating. We have shown, in several studies, that lean adults do not commonly demonstrate behavioral sensitization after repeated exposure to snack food, but a subset of obese adults reliably does. This review will discuss this change in behavioral response to repeated consumption of snack food in obese individuals and apply the theoretical framework of incentive sensitization to drugs of abuse to high fat/high sugar snack foods. We will also show data that suggest that behavioral sensitization to repeated administration of snack food is predictive of weight gain, which may enhance its utility as a diagnostic tool for identifying at-risk individuals for obesity. Finally, we will discuss the future directions of this line of research, including studying the phenomenon in children and adolescents and determining if similar principles can be used to increase motivation to eat healthier food. A combination of reductions in unhealthy food intake and increases and healthy food intake is necessary to reduce obesity rates and improve health.

Drugs, Environment, Brain, and Behavior

The effects of psychoactive drugs are not just a function of their pharmacological actions, but are due to complex interactions among pharmacological, psychological, and environmental factors. We discuss here how drug-environment interactions determine the likelihood that addictive drugs produce a persistent form of neurobehavioral plasticity (sensitization) thought to be involved in the pathophysiology of addiction and relapse.

Ultrasonic vocalization in rats self-administering heroin and cocaine in different settings: evidence of substance-specific interactions between drug and setting

RATIONALE

Clinical and preclinical evidence indicates that the setting of drug use affects drug reward in a substance-specific manner. Heroin and cocaine co-abusers, for example, indicated distinct settings for the two drugs: heroin being used preferentially at home and cocaine preferentially outside the home. Similar results were obtained in rats that were given the opportunity to self-administer intravenously both heroin and cocaine.

OBJECTIVES

The goal of the present study was to investigate the possibility that the positive affective state induced by cocaine is enhanced when the drug is taken at home relative to a non-home environment, and vice versa for heroin.

METHODS

To test this hypothesis, we trained male rats to self-administer both heroin and cocaine on alternate days and simultaneously recorded the emission of ultrasonic vocalizations (USVs), as it has been reported that rats emit 50-kHz USVs when exposed to rewarding stimuli, suggesting that these USVs reflect positive affective states.

RESULTS

We found that Non-Resident rats emitted more 50-kHz USVs when they self-administered cocaine than when self-administered heroin whereas Resident rats emitted more 50-kHz USVs when self-administering heroin than when self-administering cocaine. Differences in USVs in Non-Resident rats were more pronounced during the first self-administration (SA) session, when the SA chambers were completely novel to them. In contrast, the differences in USVs in Resident rats were more pronounced during the last SA sessions.

CONCLUSION

These findings indicate that the setting of drug taking exerts a substance-specific influence on the ability of drugs to induce positive affective states.

Impact of repeated intravenous cocaine administration on incentive motivation depends on mode of drug delivery

The incentive sensitization theory of addiction posits that repeated exposure to drugs of abuse, like cocaine, can lead to long-term adaptations in the neural circuits that support motivated behavior, providing an account of pathological drug-seeking behavior. Although pre-clinical findings provide strong support for this theory, much remains unknown about the conditions that support incentive sensitization. The current study examined whether the mode of cocaine administration is an important factor governing that drug's long-term impact on behavior. Separate groups of rats were allowed either to self-administer intravenous cocaine or were given an equivalent number and distribution of unsignaled cocaine or saline infusions. During the subsequent test of incentive motivation (Pavlovian-to-instrumental transfer), we found that rats with a history of cocaine self-administration showed strong cue-evoked food seeking, in contrast to rats given unsignaled cocaine or saline. This finding indicates that the manner in which cocaine is administered can determine its lasting behavioral effects, suggesting that subjective experiences during drug use play a critical role in the addiction process. Our findings may therefore have important implications for the study and treatment of compulsive drug seeking.

Region-specific effects of isoflurane anesthesia on Fos immunoreactivity in response to intravenous cocaine challenge in rats with a history of repeated cocaine administration

We have previously shown that acute intravenous (i.v.) administration of cocaine increases Fos immunoreactivity in rats under isoflurane anesthesia. Given that Fos expression is a marker of neural activation, the results suggested that isoflurane is appropriate for imaging cocaine effects under anesthesia. However, most imaging research in this area utilizes subjects with a history of repeated cocaine exposure and this drug history may interact with anesthetic use differently from acute cocaine exposure. Thus, this study further examined Fos expression under isoflurane in rats with a history of repeated i.v. cocaine administration. Rats received daily injections of either saline or cocaine (2mg/kg, i.v.) across 7 consecutive days, followed by 5 days of no drug exposure. On the test day, rats were either nonanesthetized or anesthetized under isoflurane and were given an acute challenge of cocaine (2mg/kg, i.v.). Additional saline-exposed controls received a saline challenge. Ninety min after the drug challenge, the rats were perfused under isoflurane anesthesia and their brains were processed for Fos protein immunohistochemistry. We found that challenge injections of cocaine following a regimen of repeated cocaine exposure resulted in Fos expression in the prefrontal cortex and striatum roughly equivalent to that found in rats who had received the cocaine challenge after a history of vehicle injections. Additionally, isoflurane anesthesia resulted in a heterogeneous attenuation of cocaine-induced Fos expression, with the most robust effect in the orbital cortex but no effect in the nucleus accumbens core (NAcC). These results indicate that cocaine-induced Fos is preserved in the NAcC under isoflurane, suggesting that isoflurane can be used in imaging studies involving cocaine effects in this region.

Neural substrates of amphetamine-induced behavioral sensitization: unconditioned (zero context) and conditioned (switch versus same context) components in c-fos overexpression

The neural substrates of the unconditioned and conditioned components of amphetamine (AMPH)-induced behavioral sensitization remain unknown. The present study examines the brain activation of rats in response to an AMPH challenge with augmented locomotion in groups receiving chronic AMPH under chloral hydrate anesthetization (i.e., the 'zero context') or when tested in the 'same context' as a chronic treatment, or when tested in a 'different context'. The neural activations of the three groups reveal fairly consistent patterns: (a) The substantia nigra is activated in the same context condition and the pure AMPH effect (i.e., the zero context with the unconditioned component), but not in the switch context condition. (b) The ventral pallidum showed Fos expression in the switch context and the same context, but not in the zero context condition. (c) The other nuclei, including the medial prefrontal cortex, nucleus accumbens, caudate putamen, medial thalamus, hippocampus, amygdala, and ventral tegmental area, are activated in all contextual conditions and the pure AMPH effect (the zero context). The context exerts definable effects on the mesocorticolimbic dopamine system on AMPH-induced behavioral sensitization. (d) The ventral pallidum and the substantia nigra activations dissociate the unconditioned component from the conditioned component in behavioral sensitization. Further studies are needed to determine how these two nuclei mediate the effect in terms of primary and conditioned rewards.

Importance of associative learning processes for one-trial behavioral sensitization of preweanling rats

During adulthood, associative learning is necessary for the expression of one-trial behavioral sensitization; however, it is uncertain whether the same associative processes are operative during the preweanling period. Two strategies were used to assess the importance of associative learning for one-trial behavioral sensitization of preweanling rats. In the initial experiments, we varied both the sequence and time interval between presentation of the conditioned stimulus (CS, novel environment) and unconditioned stimulus (US, cocaine). In the final experiment, we determined whether electroconvulsive shock-induced retrograde amnesia would disrupt one-trial behavioral sensitization. Results showed that robust-sensitized responding was apparent regardless of the sequence in which cocaine and the novel environment (the presumptive CS) were presented. Varying the time between CS and US presentation (0, 3, or 6 h) was also without effect. Results from experiment 3 showed that single or multiple electroconvulsive shock treatments did not alter the expression of the sensitized response. Therefore, these data indicated that one-trial behavioral sensitization of preweanling rats was exclusively mediated by nonassociative mechanisms and that associative processes did not modulate sensitized responding. These findings are in contrast to what is observed during adulthood, as adult rats exhibit one-trial behavioral sensitization only when associative processes are operative.

One-trial cocaine-induced behavioral sensitization in preweanling rats: role of contextual stimuli

Using a one-trial procedure, preweanling rats exhibit robust sensitization regardless of whether drug pretreatment and testing occur in the same or different environments. The purpose of the present study was to determine whether one-trial context-specific and context-independent sensitization of preweanling rats could be dissociated by varying the pretreatment dose of cocaine, by varying the pretreatment drug, or by minimizing interoceptive cues. In Experiments 1a and 1b, rats were pretreated with a broad dose range of cocaine (0-40 mg/kg) before placement in a novel activity chamber or the home cage. In Experiment 2, rats were pretreated with a locomotor-enhancing drug (e.g., methylphenidate, U50,488, or MK-801) before placement in a novel activity or anesthesia chamber. In Experiment 3, rats were anesthetized with isoflurane before cocaine administration to minimize the effects of interoceptive and injection cues. In all experiments, rats were challenged with cocaine on the test day (24 hr later), with locomotion being measured in activity chambers. Results showed that (a) the pretreatment dose of cocaine (10-40 mg/kg) did not differentially affect context-specific and context-independent sensitization; (b) cross-sensitization between methylphenidate and cocaine was observed in the context-specific condition, but not when using a context-independent procedure; and (c) sensitization was evident if injection and interoceptive cues were minimized. One possibility is that associative processes do not modulate the one-trial sensitization of preweanling rats. Alternatively, "unitization" may cause preweanling rats to treat the different environments as equivalent, thus permitting robust sensitization even when drug pretreatment and testing occur in different environments.

Apomorphine-induced context-specific behavioural sensitization is prevented by the D1 antagonist SCH-23390 but potentiated and uncoupled from contextual cues by the D2 antagonist sulpiride

RATIONALE

In the study of behavioural sensitization induced by dopamine agonists, D1 and D2 receptors have a critical, but a puzzling role.

OBJECTIVE

The objective of this study is to examine the effects of the D1 antagonist SCH-23390 and the D2 antagonist sulpiride given repeatedly alone or in combination with apomorphine upon apomorphine conditioning and sensitization.

METHODS

Apomorphine-induced (2.0 mg/kg) conditioning and sensitization were assessed following five paired/unpaired treatments. Sulpiride (10, 30 and 100 mg/kg) and SCH-23390 (0.01, 0.02 and 0.05 mg/kg) were administered alone or in combination with apomorphine. In experiment 1, the effect of 5 days of sulpiride and SCH-23390 treatments given alone were assessed on apomorphine reactivity. In experiment 2, sulpiride and SCH-23390 were co-administered with apomorphine for 5 days and subsequently, conditioning and sensitization tests were performed. In experiment 3, following five apomorphine treatment sessions, sulpiride and SCH-23390 were administered prior to the conditioning and sensitization tests.

RESULTS

SCH-23390 and sulpiride induced hyper-reactivity to apomorphine. SCH-23390 when given after the induction of apomorphine sensitization, blocked the expression of apomorphine sensitization. When given in combination with apomorphine, SCH-23390 blocked the apomorphine conditioning and sensitization, whereas low-dose sulpiride permitted conditioning and enhanced apomorphine sensitization and high-dose sulpiride blocked conditioning but permitted apomorphine sensitization. Both sulpiride doses transformed apomorphine sensitization from context-specific to context-independent sensitization.

CONCLUSION

The SCH-23390 findings are supportive of a critical role for D1 receptors in apomorphine effects whereas the sulpiride effects diminish the importance of conditioning and dopamine autoreceptor subsensitivity mechanisms in the mediation of apomorphine sensitization.

Importance of environmental context for one- and three-trial cocaine-induced behavioral sensitization in preweanling rats

RATIONALE

Preweanling rats, unlike adults, exhibit context-independent behavioral sensitization after a single pretreatment injection of cocaine.

OBJECTIVE

The purpose of this study was to examine environmental factors modulating one- and three-trial sensitization in preweanling rats.

METHODS

For preweanling rats, drug pretreatments occurred on postnatal day (PD) 17-PD 19 (experiment 1) or PD 19 (experiment 2). One set of rats was injected with cocaine (30 mg/kg) and placed in anesthesia ("small"), operant conditioning ("large"), or activity chambers for 30 min. Rats were returned to the home cage and injected with saline. Additional groups of rats were injected with saline and placed in small, large, or activity chambers for 30 min and then injected with cocaine after being returned to the home cage. Control groups were injected with saline at both time points. In separate experiments, rats were pretreated with cocaine or saline and restricted to the home cage. On PD 20, all rats were injected with cocaine (20 mg/kg) and placed in activity chambers where locomotor activity was assessed for 60 min. For comparison purposes, sensitization was also assessed in adult rats.

RESULTS

Adult male and female rats exhibited only context-dependent sensitization, whereas preweanling rats showed context-independent sensitization in a variety of conditions (e.g., when pretreated with cocaine in various novel chambers or the home cage).

CONCLUSIONS

These results suggest that nonassociative mechanisms underlying behavioral sensitization are functionally mature in preweanling rats, but associative processes modulating the strength of the sensitized response do not function in an adult-like manner during the preweanling period.

Persistence of one-trial cocaine-induced behavioral sensitization in young rats: regional differences in Fos immunoreactivity

RATIONALE

Unlike adult rats, young rats exhibit context-dependent and context-independent behavioral sensitization when assessed after a single pretreatment injection of cocaine.

OBJECTIVE

The purpose of this study was to determine whether: (1) the context-dependent and context-independent sensitization of young rats can be dissociated based on the persistence of the sensitized response and (2) the expression of behavioral sensitization is associated with region-specific increases in Fos immunoreactivity (Fos-IR).

MATERIALS AND METHODS

On postnatal day (PD) 19, rats were injected with either saline or cocaine (30 mg/kg) in a novel test chamber or the home cage. After 1, 3, 5, 7, 14, or 61 abstinence days, rats were challenged with 20 mg/kg cocaine and locomotor activity was measured for 60 min. In a separate experiment, rats pretreated on PD 19 were challenged with cocaine (10-30 mg/kg) on PD 80.

RESULTS

The sensitized responding of young rats persisted for the same length of time (5 days) regardless of whether cocaine pretreatment occurred in a novel environment or the home cage. Behavioral sensitization did not reemerge in adulthood. When assessed after three abstinence days (i.e., on PD 22), acute treatment with cocaine increased Fos-IR in various brain regions, but sensitized responding was associated with elevated Fos expression in only the caudate-putamen (CP) and prefrontal cortex (PFC).

CONCLUSIONS

Persistence of the sensitized response cannot be used to dissociate the one-trial context-dependent and context-independent sensitization of young rats. Fos data indicate that the CP and PFC may be involved in the mediation of short-term behavioral sensitization on PD 22.

Reduced sensitivity to the locomotor-stimulant effects of cocaine is associated with increased sensitivity to its discriminative stimulus properties

Outbred Long-Evans rats exhibit wide variation in their locomotor response to cocaine. Here, we investigated the relationship between these individual differences and interoceptive effects of cocaine in low cocaine responder (LCR) and high cocaine responder (HCR) phenotypes. Rats were trained to discriminate cocaine (10.0 mg/kg, intraperitoneally) from saline by repeated pairings of injections with one of two response levers. In subsequent tests for stimulus generalization to other cocaine doses (1.25-15.0 mg/kg), LCRs exhibited partial-to-full generalization at 1.85 and 2.5 mg/kg cocaine, respectively, whereas HCRs did not. When the selective 5-HT reuptake inhibitor fluoxetine (5.0 mg/kg) was coadministered with saline or different cocaine doses, we observed similar upward shifts in dose-response in both phenotypes. In contrast, coadministration of the 5-HT2A/2C agonist (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI; 0.3 mg/kg) led to partial substitution of DOI for cocaine and enhancement of the stimulus properties of 1.25 mg/kg cocaine in LCRs only. Finally, a retest of cocaine-induced locomotion after discrimination testing revealed marked behavioral sensitization in LCRs and modest changes in behavior in HCRs. Taken together, these results suggest that initial sensitivity to the locomotor-stimulant effects of cocaine is inversely related to its interoceptive properties and that differences in 5-HT systems may contribute to the phenotypic differences observed.

Prominent activation of brainstem and pallidal afferents of the ventral tegmental area by cocaine

Blockade of monoamine transporters by cocaine should not necessarily lead to certain observed consequences of cocaine administration, including increased firing of ventral mesencephalic dopamine (DA) neurons and accompanying impulse-stimulated release of DA in the forebrain and cortex. Accordingly, we hypothesize that the dopaminergic-activating effect of cocaine requires stimulation of the dopaminergic neurons by afferents of the ventral tegmental area (VTA). We sought to determine if afferents of the VTA are activated following cocaine administration. Rats were injected in the VTA with retrogradely transported Fluoro-Gold and, after 1 week, were allowed to self-administer cocaine or saline via jugular catheters for 2 h on 6 consecutive days. Other rats received a similar amount of investigator-administered cocaine through jugular catheters. Afterward, the rats were killed and the brains processed immunohistochemically for retrogradely transported tracer and Fos, the protein product of the neuronal activation-associated immediate early gene, c-fos. Forebrain neurons exhibiting both Fos and tracer immunoreactivity were enriched in both cocaine groups relative to the controls only in the globus pallidus and ventral pallidum, which, together, represented a minor part of total forebrain retrogradely labeled neurons. In contrast, both modes of cocaine administration strongly increased double-labeling relative to the controls in the brainstem, specifically in the caudal ventromedial mesencephalon and rostromedial pontine tegmentum. It is concluded that a previously unappreciated activation of pallidal and brainstem afferents may contribute to the modulation of dopaminergic neuronal activity following cocaine administration.

Decrease of D2 receptor binding but increase in D2-stimulated G-protein activation, dopamine transporter binding and behavioural sensitization in brains of mice treated with a chronic escalating dose 'binge' cocaine administration paradigm

Understanding the neurobiology of the transition from initial drug use to excessive drug use has been a challenge in drug addiction. We examined the effect of chronic 'binge' escalating dose cocaine administration, which mimics human compulsive drug use, on behavioural responses and the dopaminergic system of mice and compared it with a chronic steady dose (3 x 15 mg/kg/day) 'binge' cocaine administration paradigm. Male C57BL/6J mice were injected with saline or cocaine in an escalating dose paradigm for 14 days. Locomotor and stereotypy activity were measured and quantitative autoradiographic mapping of D(1) and D(2) receptors, dopamine transporters and D(2)-stimulated [(35)S]GTPgammaS binding was performed in the brains of mice treated with this escalating and steady dose paradigm. An initial sensitization to the locomotor effects of cocaine followed by a dose-dependent increase in the duration of the locomotor effect of cocaine was observed in the escalating but not the steady dose paradigm. Sensitization to the stereotypy effect of cocaine and an increase in cocaine-induced stereotypy score was observed from 3 x 20 to 3 x 25 mg/kg/day cocaine. There was a significant decrease in D(2) receptor density, but an increase in D(2)-stimulated G-protein activity and dopamine transporter density in the striatum of cocaine-treated mice, which was not observed in our steady dose paradigm. Our results document that chronic 'binge' escalating dose cocaine treatment triggers profound behavioural and neurochemical changes in the dopaminergic system, which might underlie the transition from drug use to compulsive drug use associated with addiction, which is a process of escalation.

The role of acetylcholine in cocaine addiction

Central nervous system cholinergic neurons arise from several discrete sources, project to multiple brain regions, and exert specific effects on reward, learning, and memory. These processes are critical for the development and persistence of addictive disorders. Although other neurotransmitters, including dopamine, glutamate, and serotonin, have been the primary focus of drug research to date, a growing preclinical literature reveals a critical role of acetylcholine (ACh) in the experience and progression of drug use. This review will present and integrate the findings regarding the role of ACh in drug dependence, with a primary focus on cocaine and the muscarinic ACh system. Mesostriatal ACh appears to mediate reinforcement through its effect on reward, satiation, and aversion, and chronic cocaine administration produces neuroadaptive changes in the striatum. ACh is further involved in the acquisition of conditional associations that underlie cocaine self-administration and context-dependent sensitization, the acquisition of associations in conditioned learning, and drug procurement through its effects on arousal and attention. Long-term cocaine use may induce neuronal alterations in the brain that affect the ACh system and impair executive function, possibly contributing to the disruptions in decision making that characterize this population. These primarily preclinical studies suggest that ACh exerts a myriad of effects on the addictive process and that persistent changes to the ACh system following chronic drug use may exacerbate the risk of relapse during recovery. Ultimately, ACh modulation may be a potential target for pharmacological treatment interventions in cocaine-addicted subjects. However, the complicated neurocircuitry of the cholinergic system, the multiple ACh receptor subtypes, the confluence of excitatory and inhibitory ACh inputs, and the unique properties of the striatal cholinergic interneurons suggest that a precise target of cholinergic manipulation will be required to impact substance use in the clinical population.

Opposite environmental regulation of heroin and amphetamine self-administration in the rat

RATIONALE

The circumstances of drug taking are thought to play a role in drug abuse but the evidence of it is anecdotal. Previous studies have shown that the intravenous self-administration of cocaine is facilitated in rats non-residing in the test chambers relative to rats that live in the test chambers at all times. We investigated here whether environmental context could exert its modulatory influence on heroin and amphetamine self-administration as well.

MATERIALS AND METHODS

Independent groups of rats were given the possibility to self-administer different doses of heroin or amphetamine (12.5, 25.0, or 50.0 microg/kg). Some animals were housed in the self-administration chambers (resident groups) whereas other rats were transported to the self-administration chambers only for the test sessions (non-resident groups).

RESULTS

Amphetamine-reinforcing effects were more pronounced in non-resident rats than in resident rats, as previously reported for cocaine. Quite unexpectedly, the opposite was found for heroin. Because of this surprising dissociation, some of the rats trained to self-administer amphetamine were later given the opportunity to self-administer heroin. Also in this case, resident rats took more heroin than non-resident rats.

CONCLUSIONS

These findings suggest an unforeseen dissociation between opioid and psychostimulant reward and demonstrate that even in the laboratory rat some contexts are associated with the propensity to self-administer more opioid than psychostimulant drugs and vice versa, thus indicating that drug taking is influenced not only by economical or cultural factors but also can be modulated at a much more basic level by the setting in which drugs are experienced.

Modulatory effect of environmental context and drug history on heroin-induced psychomotor activity and fos protein expression in the rat brain

The goal of the present study was to investigate the role of environmental context and drug history in modulating the effects of heroin on locomotor activity and Fos protein expression in the neocortex and striatal complex of the rat. It was found that (1) repeated i.p. administrations of a relatively low dose of heroin (1 mg/kg, i.p.) induced psychomotor sensitization only when the treatment was administered in a relatively 'novel' environment (ie, a unique test environment distinct from the home cage) but not when the same treatment was administered in the home cage; (2) environmental novelty facilitated heroin-induced Fos expression in the caudate, particularly in its most caudal regions; (3) environmental context also modulated heroin-induced Fos expression in the nucleus accumbens and in the neocortex; (4) repeated exposures to heroin dramatically altered its effects on Fos expression in the caudate and in the neocortex; and (5) Fos protein levels in the postero-dorsal caudate, in the shell of the nucleus accumbens, and in the barrel field cortex predicted most of the variance in heroin-induced activity scores, as shown by multiple regression analysis. The present report demonstrates that environment and drug history powerfully interact in shaping the neurobehavioral response to heroin, as previously shown for amphetamine and cocaine. Thus, a full understanding of the mechanisms responsible for the neurobehavioral adaptations produced by addictive drugs will also require taking into due consideration the environment in which drugs are experienced.

Modeling the role of environment in addiction

The aim of this review is to provide an overview of the main types of animal models used to investigate the modulatory role of environment on drug addiction. The environment can alter the responsiveness to addictive drugs in at least three major ways. First, adverse life experiences can make an individual more vulnerable to develop drug addiction or to relapse into drug seeking. Second, neutral environmental cues can acquire, through Pavlovian conditioning, the ability to trigger drug seeking even after long periods of abstinence. Third, the environment immediately surrounding drug taking can alter the behavioral, subjective, and rewarding effects of a given drug, thus influencing the propensity to use the same drug again. We have focused in particular on the results obtained using an animal model we have developed to study the latter type of drug-environment interaction.

Protracted 'pro-addictive' phenotype produced in mice by pre-adolescent phenylpropanolamine

For decades, the sympathomimetic phenylpropanolamine (PPA; +/- -norepinephrine) was an active ingredient found in popular children's over-the-counter (OTC) cold, cough, and allergy medications. To examine the possibility that pre-adolescent PPA exposure may induce neuroadaptations that influence behavioral and neurochemical responding to cocaine, C57BL/6J mice were pretreated with PPA (0-40 mg/kg) during postnatal days 21-31. The behavioral and neurochemical responses to acute and repeated cocaine (4 x 15 mg/kg) were then assessed in adulthood when the mice were 10 weeks of age. Whereas pre-adolescent PPA exposure did not influence the acute locomotor response to 15 mg/kg cocaine, PPA pre-exposure dose-dependently enhanced the expression of cocaine-induced place conditioning, reduced the expression of locomotor sensitization, but did not influence cocaine-induced stereotypy. Pre-adolescent PPA exposure completely prevented the capacity of cocaine to elevate extracellular levels of catecholamines in the nucleus accumbens, but facilitated the development of cocaine-induced glutamate sensitization. Neither acute nor repeated cocaine altered extracellular GABA levels in the accumbens of control mice; however, 15 mg/kg cocaine lowered GABA levels by approximately 40% in PPA pretreated mice and this effect showed tolerance with repeated cocaine administration. These data provide the first evidence that early exposure to an OTC compound produces protracted effects upon cocaine-induced changes in nucleus accumbens neurotransmission that may contribute to a 'pro-addictive' phenotype in adulthood.

Cocaine-induced behavioral sensitization in preweanling and adult rats: effects of a single drug-environment pairing

RATIONALE

Adult rats typically exhibit more robust behavioral sensitization than do preweanling rats. A possible explanation for this age-dependent difference is that environmental context may have relatively less impact on the psychostimulant-induced behaviors of preweanling rats.

OBJECTIVE

The purpose of this study was to assess the importance of environmental context for the development of cocaine-induced sensitization in preweanling and adult rats.

MATERIALS AND METHODS

On postnatal day (PD) 19 or PD 79, rats in the context-dependent condition were injected with 30 mg/kg cocaine immediately before being placed in a novel test chamber for 30 min. The same rats were then injected with saline 30 min after being returned to the home cage. Rats in the context-independent condition were injected with saline before being placed in the novel chamber and cocaine in the home age. Control rats were injected with saline at both time points. One day later, adult and preweanling rats were challenged with saline or 10 mg/kg cocaine (experiment 1), or preweanling rats were challenged with 5, 20, or 30 mg/kg cocaine (experiment 2). After being injected, rats were placed in the test chamber, and behavior was measured for 60 min.

RESULTS

Adult rats showed context-dependent locomotor sensitization and conditioned activity, with females exhibiting more locomotor activity than males. Preweanling rats did not exhibit conditioned activity, but they showed robust context-dependent and context-independent sensitization when challenged with 10-30 mg/kg cocaine.

CONCLUSIONS

Context did not influence the expression of behavioral sensitization in preweanling rats, suggesting that deficits in associative or memory processes may be responsible for age-dependent differences in behavioral sensitization and conditioned activity.

Deletion of the 5-HT(3A)-receptor subunit blunts the induction of cocaine sensitization

Serotonin (5-HT) receptors are classified into seven groups (5-HT(1-7)), comprising at least 14 structurally and pharmacologically distinct receptor subtypes. Pharmacological antagonism of ionotropic 5-HT(3) receptors has been shown to modulate both behavioral and neurochemical aspects of the induction of sensitization to cocaine. It is not known, however, if specific molecular subunits of the 5-HT(3) receptor influence the development of cocaine sensitization. To address this question, we studied the effects of acute and chronic intermittent cocaine administration in mice with a targeted deletion of the gene for the 5-HT(3A)-receptor subunit (5-HT(3A)-/-). 5-HT(3A) (-/-) mice showed blunted induction of cocaine-induced locomotor sensitization as compared with wild-type littermate controls. 5-HT(3A) (-/-) mice did not differ from wild-type littermate controls on measures of basal motor activity or response to acute cocaine treatment. Enhanced locomotor response to saline injection following cocaine sensitization was observed equally in 5-HT(3A) (-/-) and wild-type mice suggesting similar conditioned effects associated with chronic cocaine treatment. These data show a role for the 5-HT(3A)-receptor subunit in the induction of behavioral sensitization to cocaine and suggest that the 5-HT(3A) molecular subunit modulates neurobehavioral adaptations to cocaine, which may underlie aspects of addiction.

Environmental modulation of cocaine self-administration in the rat

RATIONALE

Previous studies have shown that environmental context can powerfully modulate the induction of psychomotor sensitization to cocaine in the rat. Rats that receive repeated administrations of cocaine in association with environmental novelty exhibit greater psychomotor sensitization than animals that receive the same treatments in their home cages.

OBJECTIVES

The goal of the present study was to investigate whether environmental context can exert its modulatory influence also on cocaine self-administration.

MATERIALS AND METHODS

Independent groups of rats with intravenous catheters were given the possibility to self-administer different doses of cocaine (0.0, 0.2, 0.4, and 0.8 mg/kg per infusion) under two environmental conditions. Some animals were housed in the self-administration cages (home groups), whereas other rats were transported to the self-administration cages only for the test sessions (novelty groups).

RESULTS

Environmental "novelty" facilitated the acquisition of cocaine self-administration at the doses of 0.2 and 0.4 mg/kg per infusion. When rats were given access to a higher dose of cocaine (0.8 mg/kg per infusion), there were no significant group differences in drug taking. Environmental context had no effect on the self-administration of the vehicle. Thus, it appears that environmental "novelty" produced a shift to the left in the dose-effect curve for cocaine self-administration. Furthermore, "novelty" enhanced the motivation of the rats to work for cocaine, as indicated by the results of a progressive ratio procedure.

CONCLUSIONS

The present findings demonstrate for the first time that the environment surrounding drug taking can alter both the intake of and motivation for cocaine.

Blockade of morphine-induced behavioral sensitization by a combination of amisulpride and RB101, comparison with classical opioid maintenance treatments

BACKGROUND AND PURPOSE

Maintenance treatments with methadone or buprenorphine are more or less efficient procedures for helping heroin addicts to stop or reduce drug abuse. Another approach to treat opiate dependence could be to target the endogenous opioid system by enhancing the effects of enkephalins by protecting them from enzymic degradation by the dual peptidase inhibitor RB101.

EXPERIMENTAL APPROACH

As chronic treatment with the dopamine D2 antagonist amisulpride facilitates RB101-induced behavioral effects, we chose in this study to treat mice previously sensitized to the hyperlocomotor effects induced by morphine with a combination of amisulpride and RB101.

KEY RESULTS

Expression of morphine-induced locomotor sensitization was abolished after combined treatment with amisulpride (20 mg x kg(-1), i.p.) and RB101 (80 mg x kg(-1), i.p.), whereas these drugs were not effective when used alone. We then compared these results with the effects of amisulpride combined with buprenorphine (0.1 mg x kg(-1), i.p.) or methadone (2.5 mg x kg(-1), i.p.) upon morphine-induced behavioral sensitization. Whereas the combination of amisulpride and buprenorphine partially blocked the expression of morphine sensitization, amisulpride+methadone was not effective in this paradigm.

CONCLUSIONS AND IMPLICATIONS

The combination of amisulpride+RB101 appears to be very efficient in blocking the expression of morphine-induced behavioral sensitization. This could reflect a reinstatement of a balance between the function of the dopamine and opioid systems and could represent a new approach in maintenance treatments for opiate addiction.

The neural substrates of amphetamine conditioned place preference: implications for the formation of conditioned stimulus-reward associations

Associations formed between conditioned stimuli and drug reward are major contributors in human drug addiction. To better understand the brain changes that accompany this process, we used immunohistochemistry for c-Fos (a neuronal activity marker), synaptophysin (a marker for synaptogenesis) and tyrosine kinase B receptor (a neurotrophic factor receptor that mediates synaptic plasticity) to investigate the neural substrates of amphetamine-induced conditioned place preference in rats. Conditioned place preference was induced by both 1.0 mg/kg and 0.3 mg/kg doses of amphetamine. Furthermore, amphetamine conditioning increased the density of c-Fos-immunoreactive cells and these cells were fully colocalized with the tyrosine kinase B receptor in the dentate gyrus, CA1 field and basolateral amygdala. Amphetamine conditioning increased the density of synaptophysin-immunoreactive varicosities in all brain regions studied, except the nucleus accumbens shell and dorsolateral striatum. The degree of conditioned place preference was highly correlated with c-Fos-immunoreactive cell density in the basolateral amygdala and with the density of synaptophysin-immunoreactive varicosities in all mesolimbic regions studied. The latter correlation was particularly impressive for the ventral pallidum and basolateral amygdala. The formation of conditioned stimulus-amphetamine reward associations is accompanied by tyrosine kinase B receptor expression in the basolateral amygdala and dentate gyrus, CA1 and CA3 fields of the hippocampus. These data therefore suggest that the formation of conditioned stimulus-reward associations requires, at least in part, activation of amygdalar-hippocampal circuits.

Cocaine‐induced locomotor activity and Fos expression in nucleus accumbens are sensitized for 6 months after repeated cocaine administration outside the home cage

Induction of the immediate early gene protein product Fos has been used extensively to assess neural activation in the striatum after repeated cocaine administration to rats in their home cages but rarely after repeated administration outside the home cage, which produces more robust locomotor sensitization. In the present study, we found cocaine-induced Fos expression in nucleus accumbens, but not caudate-putamen, was enhanced 1 and 6 months after repeated drug administration in locomotor activity chambers. Double-labelling of Fos protein and enkephalin mRNA indicated that Fos expression in nucleus accumbens was enhanced in enkephalin-positive, but not enkephalin-negative, medium spiny neurons. In contrast, cocaine-induced Fos expression was absent altogether in nucleus accumbens and unaltered in caudate-putamen 1 month after repeated cocaine administration in the home cage. As cocaine-induced locomotor activity was also enhanced 1 and 6 months after repeated cocaine administration in locomotor activity chambers, we wanted to confirm that neuronal activity in nucleus accumbens mediates cocaine-induced locomotor activity using our particular treatment regimen. Bilateral infusions of the GABA agonists baclofen and muscimol (1 microg/side) into nucleus accumbens of sensitized rats blocked cocaine-induced Fos expression and locomotor activity. Thus, while neuronal activity in both D1- and D2-type neurons in nucleus accumbens can mediate acute cocaine-induced locomotor activity, the enhanced activation of enkephalinergic D2-type neurons suggests that these latter neurons mediate the enhancement of cocaine-induced locomotor activity for up to 6 months after repeated drug administration outside the home cage.

Cocaine self-administration in rats with histories of cocaine exposure and discrimination

RATIONALE

Interrelationships between the discriminative stimulus and reinforcing properties of psychoactive drugs and the way in which they may interact to control drug intake are unclear. Studies have shown that drug history can influence the expression of drug-produced behavioral effects.

OBJECTIVE

The present study examined the acquisition and maintenance of intravenous cocaine self-administration in rats with a history of drug discrimination.

METHODS

Two groups of male hooded rats (n=12 each) were successfully trained in a single-lever food-reinforced procedure to discriminate cocaine (10 mg/kg) from saline. Control groups (n=12 each) received drug injections and/or saline injections only and lever-pressed for food reinforcers with no discrimination training. Subsequently, all subjects were implanted with chronic jugular catheters and allowed to nose-poke for infusions of cocaine (0.2 mg/kg per infusion).

RESULTS

Initial rates of responding were similar for all groups. Acquisition of self-administration on a FR-10 schedule of drug delivery was significantly faster for cocaine-exposed rats in comparison to all other groups (P<0.02). There were no differences between groups in the breaking points of cocaine and saline on a progressive ratio schedule of self-administration. Dose-response functions were obtained by two methods and were similar for all groups.

CONCLUSION

These results are consistent with earlier studies demonstrating weakly sensitized primary reinforcing properties of cocaine in preexposed rats. Previous learning to discriminate cocaine impaired this sensitization.

Importance of D(1) receptors for associative components of amphetamine-induced behavioral sensitization and conditioned activity: a study using D(1) receptor knockout mice

RATIONALE

Repeated exposure to psychostimulant drugs results in conditioned activity and behavioral sensitization. Nonassociative cellular changes are necessary for behavioral sensitization, while associative processes appear to modify the sensitized response.

OBJECTIVE

The purpose of the present study was to determine whether the absence of the D(1) receptor would disrupt associative processes modulating sensitization and conditioned activity.

METHODS

Wild-type and D(1) receptor knockout mice (i.e., D(1)-deficient mice) were injected with amphetamine (AMPH; 8 mg/kg, IP) before being placed in a previously novel test chamber (AMPH-Test group) or before being returned to the home cage (AMPH-Home group). Separate groups of mice were injected with saline (SAL) at the same time points. Distance traveled was measured 60 min each day, with the preexposure phase lasting 1 or 7 days. Sensitization was subsequently assessed after an injection of AMPH (1 mg/kg, IP), while conditioned activity was assessed after an injection of SAL.

RESULTS

After a 1-day preexposure phase, wild-type and D(1)-deficient mice exhibited similar patterns of sensitization and conditioned activity. After a 7-day preexposure phase, (1) D(1)-deficient mice exhibited more robust context-specific sensitization than wild-type mice, (2) only D(1)-deficient mice showed context-independent sensitization, and (3) only D(1)-deficient mice showed conditioned activity.

CONCLUSIONS

Repeatedly treating D(1)-deficient mice with AMPH appears to cause a general increase in responsivity. The reason for this hyper-responsivity is uncertain, but it is possible that cues from the testing environment were unable to inhibit responding (i.e., associative processes were disrupted). Alternatively, compensatory mechanisms (e.g., increases in D(2)-like receptors) may affect processes underlying sensitization and conditioned activity.

Heightened cocaine-induced locomotor activity in adolescent compared to adult female rats

Initiation and experimentation with illicit drugs often occurs in adolescence. Evidence suggests that adolescent rats are more sensitive to some of the effects of drugs of abuse than adult rats. The present study investigated whether adolescent and adult female Sprague Dawley rats differ in cocaine-induced locomotor activity. Animals were placed in the test environment for 30 minutes, and then administered an intraperitoneal (IP) injection of either cocaine (20mg/kg) or saline (0.9%). Both adult and adolescent animals showed significant increases in locomotor activity as a result of cocaine administration compared to saline controls. Interestingly, cocaine induced significantly more locomotor activity in the adolescent females compared to the adults, demonstrating that cocaine acts differently in developing animals.

Behavioural effects of acute and repeated cocaine treatments: a comparative study in sensitisation-prone RHA rats and their sensitisation-resistant RLA counterparts

RATIONALE

Dopamine (DA) transmission is critically involved in the motor effects of psychostimulants and opiates, as well as in the augmentation of these effects resulting from repeated drug administration-a process termed behavioural sensitisation. The latter is known to play a central role in the development and maintenance of drug addiction as well as in the high frequency of relapse observed in drug addicts following detoxification. The selective breeding of Roman high- (RHA) and low-avoidance (RLA) rats for extreme performances in the acquisition of avoidant behaviour has generated two phenotypes that differ in the functional properties of the mesocortical and mesolimbic DA systems and in their behavioural and neurochemical responses to the acute administration of psychostimulants and opiates. More recently, we showed that repeated morphine or amphetamine injections induce behavioural sensitisation in RHA, but not RLA, rats.

OBJECTIVE

To further characterize the differences in the susceptibility to develop psychostimulant sensitisation between the Roman lines, we evaluated the behavioural effects of acute cocaine (5 and 10 mg kg(-1), i.p.) 1 day before and 8 days after repeated administration of saline (2 ml kg(-1), i.p.) or cocaine (10 mg kg(-1), i.p. for 14 consecutive days).

RESULTS

We show that repeated cocaine administration elicits augmented behavioural responses to both challenge doses of the same drug only in RHA rats.

CONCLUSIONS

The Roman lines represent a useful model to investigate how, and to what extent, the genetic make-up influences the neural substrates of individual vulnerability to addiction.

The neural consequences of repeated cocaine exposure revealed by functional MRI in awake rats

The use of functional magnetic resonance imaging (fMRI) in animal models of cocaine addiction is an invaluable tool for investigating the neuroadaptations that lead to this psychiatric disorder. We used blood-oxygen-level-dependent (BOLD) MRI in awake rats to identify the neuronal circuits affected by repeated cocaine administration. Rats were given an injection of cocaine (15 mg/kg, i.p.) or its vehicle for 7 days, abstained from injections for 1 week, and challenged with an intracerebroventricular cocaine injection during functional imaging. Acute cocaine produced robust positive BOLD responses across well-known monoamine-enriched brain regions, such as the prefrontal cortex, nucleus accumbens, dorsal striatum, sensory cortex, hippocampus, thalamus, and midbrain areas. However, repeated cocaine administration resulted in lower BOLD responses in the prefrontal cortex, agranular insular cortex, nucleus accumbens, ventral pallidum, and dorsomedial thalamus, among other brain regions. Reductions in BOLD intensity were not associated with variations in cerebrovascular reactivity between drug naive rats and those repeatedly exposed to cocaine. Therefore, the lower metabolic activation in response to cocaine could reflect a reduced neuronal and/or synaptic activity upon repeated administration.

Estrogen influences cocaine-induced blood oxygen level-dependent signal changes in female rats

We investigated the effect of estrogen on cocaine-induced brain activity using blood oxygen level-dependent (BOLD) magnetic resonance imaging. Ovariectomized (Ovx) rats without estrogen and Ovx rats with estrogen (Ovx+E) were given a single saline or cocaine injection (15 mg/kg, i.p.) for 5 d. After 7 d of withdrawal from injections, rats were challenged with cocaine during functional imaging. Acute cocaine administration produced positive BOLD activation in the prefrontal cortex, nucleus accumbens, striatum, ventral tegmental area, and hippocampus, among other brain regions. Positive BOLD signal changes were lower in Ovx+E than in Ovx rats. With repeated cocaine administration, Ovx+E rats showed enhanced BOLD signal changes in the nucleus accumbens, ventral tegmental area, and hippocampus compared with acutely treated animals. Our results indicate that estrogen influences the effects of acute and repeated cocaine administration on BOLD signal changes. The data suggest that in females with estrogen, cocaine-induced neuronal activity is enhanced after repeated cocaine administration. It is possible that the actions of estrogen within the aforementioned brain regions potentiate the behavioral response to cocaine observed in female rats.

Sex differences in the escalation of intravenous cocaine intake following long- or short-access to cocaine self-administration

Preclinical data have indicated that extended access to cocaine self-administration (e.g., 6-12 h/day) facilitates an escalation in daily cocaine intake that is not seen when rats are given shorter (e.g., 1-2 h/day) access to cocaine for self-administration. Data from studies with rats have shown that females self-administer more cocaine than males during all phases of drug abuse (e.g., acquisition, maintenance, and reinstatement). The purpose of this study was to examine potential differences between males and females in the escalation of intravenous cocaine intake following a differential access (e.g., 1 vs. 6 h) period of cocaine self-administration. Four groups of rats were compared: (1) long-access (LgA; 6 h) females; (2) LgA males; (3) short-access (ShA; 1 h) females; and (4) ShA males. Animals were given LgA or ShA to intravenous cocaine (0.5 mg/kg/infusion) self-administration under an Fr 1 schedule for 21 days. Subsequently, access conditions were made equal (3 h) across groups, and dose-response curves for cocaine were compared. Results revealed that the LgA groups' dose-response curves were significantly elevated above those of ShA groups. Additionally, the dose-response curve of LgA female rats was significantly elevated above that of LgA male rats. These results suggest that female rats are more sensitive than male rats to factors that contribute to the escalation of cocaine intake (e.g., extended access conditions).

The induction of behavioural sensitization is associated with cocaine-induced structural plasticity in the core (but not shell) of the nucleus accumbens

Repeated exposure to cocaine increases the density of dendritic spines on medium spiny neurons in the nucleus accumbens (Acb) and pyramidal cells in the medial prefrontal cortex (mPFC). To determine if this is associated with the development of psychomotor sensitization, rats were given daily i.p. injections of 15 mg/kg of cocaine (or saline) for 8 days, either in their home cage (which failed to induce significant psychomotor sensitization) or in a distinct and relatively novel test cage (which induced robust psychomotor sensitization). Their brains were obtained 2 weeks after the last injection and processed for Golgi-Cox staining. In the Acb core (AcbC) cocaine treatment increased spine density only in the group that developed psychomotor sensitization (i.e. in the Novel but not Home group), and there was a significant positive correlation between the degree of psychomotor sensitization and spine density. In the Acb shell (AcbS) cocaine increased spine density to the same extent in both groups; i.e. independent of psychomotor sensitization. In the mPFC cocaine increased spine density in both groups, but to a significantly greater extent in the Novel group. Furthermore, when rats were treated at Home with a higher dose of cocaine (30 mg/kg), cocaine now induced psychomotor sensitization in this context, and also increased spine density in the AcbC. Thus, the context in which cocaine is experienced influences its ability to reorganize patterns of synaptic connectivity in the Acb and mPFC, and the induction of psychomotor sensitization is associated with structural plasticity in the AcbC and mPFC, but not the AcbS.

Structural plasticity associated with exposure to drugs of abuse

Persistent changes in behavior and psychological function that occur as a function of experience, such those associated with learning and memory, are thought to be due to the reorganization of synaptic connections (structural plasticity) in relevant brain circuits. Some of the most compelling examples of experience-dependent changes in behavior and psychological function, changes that can last a lifetime, are those that accrue with the development of addictions. However, until recently, there has been almost no research on whether potentially addictive drugs produce forms of structural plasticity similar to those associated with other forms of experience-dependent plasticity. In this paper we summarize evidence that, indeed, exposure to amphetamine, cocaine, nicotine or morphine produces persistent changes in the structure of dendrites and dendritic spines on cells in brain regions involved in incentive motivation and reward (such as the nucleus accumbens), and judgment and the inhibitory control of behavior (such as the prefrontal cortex). It is suggested that structural plasticity associated with exposure to drugs of abuse reflects a reorganization of patterns of synaptic connectivity in these neural systems, a reorganization that alters their operation, thus contributing to some of the persistent sequela associated with drug use--including addiction.

State-dependent action of cocaine on brain temperature and movement activity: implications for movement sensitization

Because neural activity is highly energy consuming and heat producing, brain temperature offers a reliable, real-time measure of an animal's activity state and its changes induced by environmental and drug challenges. Therefore, it allows evaluation of the activity state of an animal preceding drug administration and its relation to subsequent drug-induced neural effects. This approach was used to explore the state dependency of cocaine's effects. Brain and body temperatures, as well as locomotion were measured simultaneously in rats during repeated, daily administration of cocaine (15 mg/kg i.p., daily for 5 days) under different experimental conditions. The drug was administered via (a). a chronically implanted catheter in quiet resting conditions, (b). an injection made under quiet rest or (c). an injection under activated conditions associated with placement in the cage. Although brain temperature and movement increased after cocaine administration in each condition, cocaine's action (evaluated as cocaine-saline difference for both parameters) was situational. Catheter-administered cocaine induced the strongest movement activation and robust, monophasic temperature increase, which remained relatively stable following each subsequent drug infusion. Cocaine injected during quiet and, especially, activated conditions, induced a weaker locomotor activation, while the temperature response (evaluated as drug-saline difference) had a biphasic pattern. Cocaine initially inhibited the temperature increases seen in saline-treated animals (0-20 min) and then induced a more prolonged hyperthermia, which was about twofold weaker than that seen after catheter-administered drug. Although movement activation gradually increased following repeated treatment in activated conditions, the magnitude of this sensitized motor response barely reached the levels induced by the initial cocaine administration via catheter. These data suggest that both the acute effects of cocaine in the brain and their change following repeated drug administration are dependent upon the ongoing neural activity state of the animal. Cocaine's interaction with this activity state is a crucial factor determining the behavioral effects of this drug, including state-dependent motor sensitization.

Drug-induced neurobehavioral plasticity: the role of environmental context

Repeated administrations of addictive drugs produce long-lasting changes in brain and behavior. However, drug-induced neurobehavioral plasticity is not a mere function of the neuropharmacological actions of drugs, but the result of complex drug-environment interactions. In the present review we summarize results obtained in a series of studies using an animal model of drug-environment interaction, showing that environmental context and past drug history interact to modulate the effects of amphetamine, cocaine and morphine on behavior, gene expression and structural plasticity. These findings may help shed some light on the conditions necessary for addictive drugs to enduringly alter brain and behavior.

Repeated i.v. cocaine exposure produces long-lasting behavioral sensitization in pregnant adults, but behavioral tolerance in their offspring

Repeated exposure to cocaine during sensitive periods of forebrain development produces specific, long-lasting changes in the structure and function of maturing neural circuits. Similar regimens of drug exposure in adult animals with mature, homeostatically regulated nervous systems produce neuroadaptations that appear to be quite different in nature and magnitude. We studied the ability of cocaine to induce behavioral sensitization and/or tolerance following repeated administration of i.v. cocaine (3 mg/kg, twice daily) to pregnant rabbits during the period of peak differentiation within the rabbit cerebral cortex (embryonic day [E] 16-E25). Offspring and the adult mothers were behaviorally tested following acute administration of amphetamine 2 months after the litters were born. The offspring, having received cocaine during the prenatal sensitive period, showed profound behavioral tolerance to the amphetamine challenge. In contrast, the mothers of these offspring, who received cocaine at the same dose and duration, and experienced the same period of withdrawal, exhibited robust behavioral sensitization. These data indicate that specific adaptive changes in neural signaling and/or circuitry that occur in response to repeated exposure to psychostimulants are highly dependent upon the maturational state of the brain during which the exposure occurs.

The magnitude and the extinction duration of the cocaine-induced conditioned locomotion-activated response are related to the number of cocaine injections paired with the testing context in C57BL/6J mice

Behavioural activation repeatedly induced by a stimulant in rodents can persist in the absence of the drug if the animals are tested in the context where the drug was previously given, a phenomenon often explained in terms of Pavlovian conditioning. The aim of this study was to verify whether the amplitude of the putative CR (the drug-like activity) increases with the number of the US-CS associations (the number of drug-context pairings), one of the most representative rules of Pavlovian conditioning. The effect of the number of trials on the speed of extinction was also considered. C57BL/6J mice received 3, 6 or 12 once-daily injections of either saline or 12 mg/kg (-)-cocaine hydrochloride (s.c.) in the same test context, a photocell activity-box in which they were tested for 60 min after every injection. Other groups received the same treatments outside of the test context (being placed in a novel cage tub after each injection). Twenty-four hours after the last treatment session, all mice were challenged with saline in the test context (test for conditioned activity), extinction sessions taking place on the three subsequent days. Sensitisation to the locomotor-activating effect of cocaine developed only amongst the animals injected 6 or 12 times, the magnitude of the last sensitised response being comparable for these two injections regimen. On saline challenge, only the animals that had received 6 or 12 cocaine injections showed significant conditioned activity (CR), with the greatest response occurring following 12 injections. The 6-trial group reached the level of non-significance after fewer extinction sessions than the 12-trial group; however, the rates of extinction did not differ (comparable regression coefficients and quasi-parallel curves). These results suggest that the amplitude of the CR (cocaine-like stimulation after saline), and perhaps less convincingly the duration of extinction, are functions of the number of the US-CS (cocaine-context) pairings, supporting the Pavlovian nature of post-sensitisation placebo drug-like effects.

Environmental context and drug history modulate amphetamine-induced c-fos mRNA expression in the basal ganglia, central extended amygdala, and associated limbic forebrain

The context in which amphetamine is administered modulates its ability to induce both behavioral sensitization and immediate early gene expression. When given in a novel test environment amphetamine produces greater levels of c-fos and arc mRNA expression in many brain regions relative to when it is given in the home cage. The purpose of the current study was to determine if environment and drug history interact to influence amphetamine-induced c-fos mRNA expression. Rats with a unilateral 6-hydroxydopamine lesion were treated for 7 days with saline or 0.5 mg/kg of d-amphetamine (i.v.) in a distinct and relatively novel test environment (Novel), or in their home cage (Home). Following a 10-12-day withdrawal period, a challenge injection of either saline or 0.5 mg/kg d-amphetamine was administered. In situ hybridization histochemistry was used to examine c-fos mRNA expression in several regions of the basal ganglia, the central extended amygdala, and limbic forebrain. In most brain regions amphetamine given in the Novel environment produced greater c-fos mRNA expression than when given it was given at Home, and drug history had no effect on amphetamine-induced c-fos mRNA expression. However, within the subthalamic nucleus, substantia nigra reticulata, and central nucleus of the amygdala prior experience with amphetamine in the Novel but not Home environment enhanced the effect of an amphetamine challenge injection on c-fos mRNA expression. In contrast, there was a decrease in c-fos mRNA expression in amphetamine-pretreated animals, regardless of environmental context, in the ventral portion of the far caudal striatum. Reexposure to an environment previously paired with amphetamine produced a conditioned increase in c-fos mRNA expression in portions of the caudate-putamen, the subthalamic nucleus, the nucleus accumbens shell and a conditioned decrease in c-fos mRNA expression in the central nucleus of the amygdala. We conclude that environmental context and drug history interact to alter the basal ganglia and central extended amygdala circuitry engaged by subsequent exposure to amphetamine, or exposure to an environment previously paired with amphetamine.

Manipulations during the second, but not the first, week of life increase susceptibility to cocaine self-administration in female rats

We compared the effects of manipulations during week 1 vs week 2 of life on the propensity to self-administer cocaine. Pups received daily subcutaneous saline injections, were handled briefly, or remained undisturbed during their respective treatment periods. Animals handled during the second week of life exhibited increased locomotor response to novelty when tested on postnatal day (PND) 48, compared to all other groups. Rats were implanted with jugular catheters on PND 70 and then given the opportunity to self-administer (0.125 mg/kg/infusion) cocaine for 5 consecutive days (1 h sessions). The dose was then raised to 0.25 mg/kg/infusion for 5 days and to 0.5 mg/kg/infusion for the final 5 days of testing. Only animals manipulated during the second week of life acquired drug-taking behavior. These effects were both stimulus- and gender-specific. Females handled during the second week of life acquired cocaine self-administration (SA) at the lowest dose, and females injected during the second week of life acquired at the intermediate dose. Males injected during the second week of life showed a similar, but more variable, drug-taking pattern. There were no group differences in serum corticosterone response to novelty, although relative to undisturbed animals and those manipulated in the first week of life, female animals manipulated during the second week of life had lower basal expression of hippocampal glucocorticoid receptor mRNA in adulthood. We conclude that the second week of life in the rodent is a sensitive period during which manipulations result in a more vulnerable phenotype for the acquisition of cocaine SA.

H3 receptor blockade by thioperamide enhances cognition in rats without inducing locomotor sensitization

RATIONALE

Attention deficit hyperactivity disorder (ADHD) is currently treated with psychomotor stimulants, including methylphenidate and amphetamine. Several adverse effects are associated with these drugs, however, such as agitation and abuse. H(3) receptor antagonists are under clinical investigation for ADHD.

OBJECTIVES

To investigate the potential of thioperamide, a prototypical H(3) receptor antagonist, to enhance learning and attention while inducing no effects on locomotor stimulation and sensitization, or alterations in ACTH levels.

METHODS

Thioperamide (1, 3, 10, 30 mg/kg) was administered prior to testing in a multi-trial, inhibitory avoidance response in rat pups (five trials separated by 1 min) to evaluate attention/cognition. Locomotor sensitization and cross-sensitization was assessed following administration of methylphenidate (3 mg/kg), cocaine (10 mg/kg), or thioperamide (1, 3, 10 mg/kg).

RESULTS

Thioperamide significantly enhanced performance of the five-trial inhibitory avoidance response with efficacy similar to that previously reported for methylphenidate. Administration of amphetamine, methylphenidate and cocaine produced significant locomotor sensitization, however. In contrast, thioperamide did not induce locomotor stimulation or sensitization, nor did it cross-sensitize to the stimulant effects of amphetamine or cocaine. The repeated administration of methylphenidate significantly elevated ACTH levels, while thioperamide did not affect this neuroendocrine endpoint.

CONCLUSIONS

H(3) receptor blockade may offer a safer alternative to psychomotor stimulants for the treatment of ADHD.