Pre-exposure to amphetamine but not nicotine sensitizes rats to the motor activating effect of cocaine

PI4KIIIβ-Mediated Phosphoinositides Metabolism Regulates Function of the VTA Dopaminergic Neurons and Depression-Like Behavior

Phosphoinositides, including phosphatidylinositol-4,5-bisphosphate (PIP2), play a crucial role in controlling key cellular functions such as membrane and vesicle trafficking, ion channel, and transporter activity. Phosphatidylinositol 4-kinases (PI4K) are essential enzymes in regulating the turnover of phosphoinositides. However, the functional role of PI4Ks and mediated phosphoinositide metabolism in the central nervous system has not been fully revealed. In this study, we demonstrated that PI4KIIIβ, one of the four members of PI4Ks, is an important regulator of VTA dopaminergic neuronal activity and related depression-like behavior of mice by controlling phosphoinositide turnover. Our findings provide new insights into possible mechanisms and potential drug targets for neuropsychiatric diseases, including depression. Both sexes were studied in basic behavior tests, but only male mice could be used in the social defeat depression model.

Repeated chemogenetic activation of dopaminergic neurons induces reversible changes in baseline and amphetamine-induced behaviors

RATIONALE

Repeated chemogenetic stimulation is often employed to study circuit function and behavior. Chronic or repeated agonist administration can result in homeostatic changes, but this has not been extensively studied with designer receptors exclusively activated by designer drugs (DREADDs).

OBJECTIVES

We sought to evaluate the impact of repeated DREADD activation of dopaminergic (DA) neurons on basal behavior, amphetamine response, and spike firing. We hypothesized that repeated DREADD activation would mimic compensatory effects that we observed with genetic manipulations of DA neurons.

METHODS

Excitatory hM3D(Gq) DREADDs were virally expressed in adult TH-Cre and WT mice. In a longitudinal design, clozapine N-oxide (CNO, 1.0 mg/kg) was administered repeatedly. We evaluated basal and CNO- or amphetamine (AMPH)-induced locomotion and stereotypy. DA neuronal activity was assessed using in vivo single-unit recordings.

RESULTS

Acute CNO administration increased locomotion, but basal locomotion decreased after repeated CNO exposure in TH-CrehM3Dq mice relative to littermate controls. Further, after repeated CNO administration, AMPH-induced hyperlocomotion and stereotypy were diminished in TH-CrehM3Dq mice relative to controls. Repeated CNO administration reduced DA neuronal firing in TH-CrehM3Dq mice relative to controls. A two-month CNO washout period rescued the decreases in basal locomotion and AMPH response.

CONCLUSIONS

We found that repeated DREADD activation of DA neurons evokes homeostatic changes that should be factored into the interpretation of chronic DREADD applications and their impact on circuit function and behavior. These effects are likely to also be seen in other neuronal systems and underscore the importance of studying neuroadaptive changes with chronic or repeated DREADD activation.

Ethanol pre-exposure differentially impacts the rewarding and aversive effects of α-pyrrolidinopentiophenone (α-PVP): Implications for drug use and abuse

RATIONALE

Exposure to a drug can subsequently impact its own reactivity as well as that of other drugs. Given that users of synthetic cathinones, i.e., "bath salts", typically have extensive and varied drug histories, an understanding of the effects of drug history on the behavioral and physiological consequences of synthetic cathiones may be important to their abuse liability.

OBJECTIVES

The goal of the current work was to assess the effects of an ethanol pre-exposure on the rewarding and aversive effects of α-PVP.

METHODS

Adult male Sprague Dawley rats were exposed to ethanol prior to combined conditioned taste avoidance/conditioned place preference training in which rats were injected with 1.5, 3 or 5 mg/kg of racemic α-PVP or vehicle. Following a 7-day washout period, rats were then tested for thermoregulatory effects of α-PVP using subcutaneous probes to measure body temperature changes over the course of 8 h. This was followed 10 days later by assessments for α-PVP-induced locomotor activity and stereotypies over a 1-h session.

RESULTS

α-PVP induced significant dose- and trial-dependent taste avoidance that was significantly attenuated by ethanol history and dose- and time-dependent increases in locomotor activity that were significantly increased by ethanol. α-PVP also induced place preferences and dose- and time-dependent increases in body temperature, but these measures were unaffected by ethanol history.

CONCLUSIONS

α-PVP's aversive effects (as measured by taste avoidance) were attenuated, while its rewarding effects (as indexed by place preference conditioning) were unaffected, by ethanol pre-exposure. Such a pattern may indicate increased α-PVP abuse liability, as changes in the balance of aversion and reward may impact overall drug effects and likelihood of drug intake. Future self-administration studies will be necessary to explore this possibility.

Intermittent access cocaine self-administration produces psychomotor sensitization: effects of withdrawal, sex and cross-sensitization

RATIONALE

With repeated administration, the psychomotor activating effects of drugs such as cocaine or amphetamine can change in very different ways-showing sensitization or tolerance-depending on whether they are administered more or less intermittently. This behavioral plasticity is thought to reflect, at least in part, changes in dopamine (DA) neurotransmission, and therefore, may provide insights into the development of substance use disorders. Indeed, the most widely used preclinical model of cocaine addiction, which involves Long Access (LgA) self-administration procedures, is reported to produce tolerance to cocaine's psychomotor activating effects and effects on DA activity. In contrast, Intermittent Access (IntA) cocaine self-administration is more effective than LgA in producing addiction-like behavior, but sensitizes DA neurotransmission. There is, however, very little information concerning the effects of IntA experience on the psychomotor activating effects of cocaine.

OBJECTIVE

The objective of this study was to determine whether IntA experience produces psychomotor sensitization with similar characteristics to that produced by the intermittent, noncontingent administration of cocaine.

RESULTS

IntA to cocaine did indeed produce psychomotor sensitization that (1) was greater after a long (30 days) vs. short (1 day) period of withdrawal, (2) was greater in females than males, and (3) resulted in cross-sensitization to another psychomotor stimulant drug, amphetamine.

CONCLUSION

The tolerance sometimes associated with LgA cocaine self-administration has been cited in support of the idea that, in addiction, drug-seeking and drug-taking is motivated to overcome a DA deficiency and associated anhedonia. In contrast, the neurobehavioral sensitization associated with IntA cocaine self-administration favors an incentive-sensitization view.

Spontaneous appetence for wheel-running: a model of dependency on physical activity in rat

According to human observations of a syndrome of physical activity dependence and its consequences, we tried to examine if running activity in a free activity paradigm, where rats had a free access to activity wheel, may present a valuable animal model for physical activity dependence and most generally to behavioral dependence. The pertinence of reactivity to novelty, a well-known pharmacological dependence predictor was also tested. Given the close linkage observed in human between physical activity and drugs use and abuse, the influence of free activity in activity wheels on reactivity to amphetamine injection and reactivity to novelty were also assessed. It appeared that (1) free access to wheel may be used as a valuable model for physical activity addiction, (2) two populations differing in activity amount also differed in dependence to wheel-running. (3) Reactivity to novelty did not appeared as a predictive factor for physical activity dependence (4) activity modified novelty reactivity and (5) subjects who exhibited a high appetence to wheel-running, presented a strong reactivity to amphetamine. These results propose a model of dependency on physical activity without any pharmacological intervention, and demonstrate the existence of individual differences in the development of this addiction. In addition, these data highlight the development of a likely vulnerability to pharmacological addiction after intense and sustained physical activity, as also described in man. This model could therefore prove pertinent for studying behavioral dependencies and the underlying neurobiological mechanisms. These results may influence the way psychiatrists view behavioral dependencies and phenomena such as doping in sport or addiction to sport itself.

Repeated MDMA administration increases MDMA-produced locomotor activity and facilitates the acquisition of MDMA self-administration: role of dopamine D2 receptor mechanisms

RATIONALE

Repeated exposure to ±3, 4-methylenedioxymethamphetamine (MDMA) produces sensitization to MDMA-produced hyperactivity, but the mechanisms underlying the development of this sensitized response or the relationship to the reinforcing effects of MDMA is unknown.

OBJECTIVES

This study determined the effect of a sensitizing regimen of MDMA exposure on the acquisition of MDMA self-administration and investigated the role of dopamine D₂ receptor mechanisms.

METHODS

Rats received the selective D₂ antagonist, eticlopride (0.0 or 0.3 mg/kg, i.p.) and MDMA (0.0 or 10.0 mg/kg, i.p.) during a five-day pretreatment regimen. Two days following the final session, the locomotor activating effects of MDMA (5 mg/kg, i.p.) and the latency to acquisition of MDMA self-administration were determined.

RESULTS

Pretreatment with MDMA enhanced the locomotor activating effects of MDMA and facilitated the acquisition of MDMA self-administration. Administration of eticlopride during MDMA pretreatment completely blocked the development of sensitization to MDMA-produced hyperactivity but failed to significantly alter the facilitated acquisition of MDMA self-administration. Pretreatment with eticlopride alone facilitated the acquisition of self-administration.

CONCLUSIONS

These data suggest that repeated MDMA exposure sensitized both the locomotor activating and reinforcing effects of MDMA. Activation of D₂ receptors during MDMA pretreatment appears critical for the development of sensitization to MDMA-produced hyperactivity. The role of D₂ receptor mechanisms in the development of sensitization to the reinforcing effects of MDMA is equivocal.

Nicotine produces long-term increases in cocaine reinforcement in adolescent but not adult rats

Studies have shown that many smokers begin using nicotine during adolescence, yet the influence of early nicotine use on the response to other drugs of abuse in adulthood is not fully understood. In the current study, nicotine was administered to adolescent and adult rats for seven days. Thirty days later, cocaine-induced locomotor activity and cocaine self-administration were examined when the rats pretreated as adolescents were adults. Rats exposed to nicotine during early adolescence were sensitized thirty days later to the locomotor-activating effects of cocaine and self-administered a greater number of cocaine infusions than adolescent rats pretreated with vehicle. As a result of this increased intake, the cocaine self-administration dose-response curve was shifted upward indicating an increase in cocaine reinforcement. Rats pretreated with nicotine as adults, however, did not show a difference in locomotor activity or cocaine self-administration thirty days later compared to adult rats pretreated with vehicle. These findings suggest that early exposure to nicotine has long-term consequences on cocaine use. These data further suggest that nicotine use may carry a greater risk during adolescence than adulthood and adolescents who smoke may be particularly vulnerable to stimulant use. This article is part of a Special Issue entitled SI: Adolescent plasticity.

Repeated pre-exposure to tobacco smoke potentiates subsequent locomotor responses to nicotine and tobacco smoke but not amphetamine in adult rats

These studies investigated if pre-exposure to tobacco smoke affects the locomotor response to tobacco smoke, nicotine, and amphetamine in adult rats. The rats were habituated to an open field for 3-4 days and then exposed to tobacco smoke for 2h/day for 13-14 days. The effect of exposure to tobacco smoke on locomotor activity was investigated after 1, 7, and 14 days of smoke exposure and after one 2-hour exposure session that followed a 3-week off period. The effects of tobacco smoke on the locomotor responses to nicotine (0.04 and 0.4 mg/kg, base) and amphetamine (0.1 and 0.5mg/kg) were investigated on day 14, one day after the last smoke exposure session. The locomotor response to tobacco smoke was increased after 7 and 14 days of smoke exposure and after one exposure session after the 3-week off-period. The acute administration of the high dose of nicotine (0.4 mg/kg) led to a brief period of hypoactivity that was followed by a period of hyperactivity. Pre-exposure to tobacco smoke attenuated the nicotine-induced hypoactivity and potentiated the nicotine-induced hyperactivity. The low dose of nicotine (0.04 mg/kg) did not affect locomotor activity in the control rats but increased the total distance traveled in the tobacco smoke exposed rats. Exposure to tobacco smoke did not affect the locomotor response to amphetamine. These findings indicate that exposure to tobacco smoke leads to tolerance to the depressant effects of nicotine and potentiates the stimulant effects of nicotine and tobacco smoke.

Decreased reinforcing effects of cocaine following 2 weeks of continuous D-amphetamine treatment in rats

RATIONALE

Recent studies have investigated D-amphetamine as a potential agonist medication for cocaine dependence. In rats, a 14-day continuous infusion of D: -amphetamine via osmotic mini-pump has been shown to decrease cocaine-reinforced responding under a progressive ratio (PR) schedule of reinforcement.

OBJECTIVES

This study was designed to assess the influences of the D-amphetamine treatment dose and self-administered cocaine dose on the magnitude of this effect.

MATERIALS AND METHODS

Experiment 1: rats were trained to self-administer 1.5 mg/kg/inj cocaine under a PR schedule, then implanted with D-amphetamine mini-pumps for 14 days (days 1-7, 5 mg/kg/day; days 8-14, 7.5 mg/kg/day). Breakpoints were evaluated throughout the treatment period and 14 days post-treatment. Experiment 2: rats were trained to self-administer cocaine under a PR schedule and initial dose-response curves were determined before implantation of D-amphetamine mini-pumps. During the 14-day D-amphetamine (5 mg/kg/day) treatment period, rats self-administered one of four cocaine doses (0.19, 0.38, 0.75, or 1.5 mg/kg/inj). A post-treatment PR dose-response curve and responding under a fixed ratio 1 (FR1) schedule were evaluated after mini-pump removal.

RESULTS

Experiment 1: breakpoints for 1.5 mg/kg/inj cocaine were unchanged by the increasing dose of D-amphetamine. Experiment 2: the PR dose-response curve was shifted downward after the treatment period in rats that had self-administered 0.19 and 0.38 mg/kg/inj cocaine. In contrast, rats in the 0.75 and 1.5 mg/kg/inj groups demonstrated increased rates of cocaine intake under an FR1 schedule after the treatment period.

CONCLUSIONS

These data suggest that continuous D-amphetamine treatment attenuates the reinforcing effects of cocaine.

The Relationship Between Physical Activity and Cocaine Intake in Mice

Human studies suggest the existence of an exercise dependency syndrome and a link between drug intake and intense physical activity. Our aim was to assess whether a link actually existed between running activity and cocaine intake in mice. Thirty male Swiss mice were used. Ten mice were used as controls, individually housed in cages without a wheel, and 20 mice were in cages with free access to a running wheel. Cocaine preference was estimated as the ratio (as percent) of cocaine solution intake over total fuid intake in the course of free oral access to cocaine solution versus water. High cocaine scores were only found with high wheel activity. The lowest activity scores were found with low cocaine preference. A group of “high runners” impervious to cocaine appetence and to the effects of exercise withdrawal were found, which may suggest that shared mechanisms could be involved in both dependence on sport and drug taking. Findings suggest that moderate activity seems to be associated with low cocaine preference, and cocaine intake could increase in cases of intense activity. The urge for physical activity (as seen with top-level professional athletes) may theoretically combine with different degrees of vulnerability to cocaine. The use of substances by those engaging in intense physical activity, for performance enhancement or recreational purposes, could potentially trigger a pattern of consumption and addiction. This pattern corresponds with the theory that there may be an addictive element in physical activity. Animal models could prove useful for identifying biological or behavioral predictors of such vulnerability and identifying persons either at risk or possessing resistance.

Disparate cocaine-induced locomotion as a predictor of choice behavior in rats trained in a delay-discounting task

Heightened impulsivity and differential sensitivity to a drug's behavioral effects are traits that, individually, have been associated with chronic drug use and dependence. Here, we used an animal model to test whether individual differences in cocaine-induced activity are predictive of impulsive choice behavior. Adult, male Sprague-Dawley rats were given cocaine (10mg/kg, i.p.) and classified into low or high cocaine responders (LCRs or HCRs, respectively) based on their locomotor response in an open-field arena. Rats were then trained in a delay-discounting task that offers a choice between immediately delivered, but smaller reinforcements, or larger reinforcements that are delivered after a delay. We also examined the effects of amphetamine (AMPH; 0.3-1.0mg/kg) and the 5-HT1A agonist 8-OH-DPAT (0.3-1.0mg/kg) on delay-discounting. Lastly, all rats were retested in the open-field to determine if phenotypes were stable. We observed baseline differences in choice behavior between the groups, with HCRs behaving more impulsively (i.e., choosing the small reinforcement) compared to LCRs. AMPH decreased choice of the large reinforcement in LCRs, but did not alter choice in HCRs. Impulsive choice was increased in both phenotypes following 8-OH-DPAT, with LCRs exhibiting changes across a wider range of delays. When cocaine-induced open-field behavior was retested, responses in LCRs were similar whereas HCRs showed evidence of tolerance. Our results suggest that differential sensitivity to cocaine-induced locomotion is predictive of impulsivity and the potential neurobiological differences in LCRs and HCRs may provide insight into mechanisms contributing to vulnerability for chronic drug use and/or dependence.

The role of NMDA receptor antagonists in nicotine tolerance, sensitization, and physical dependence: a preclinical review

Nicotine, the primary psychoactive component of tobacco products, produces diverse neurophysiological, motivational, and behavioral effects through several brain regions and neurochemical pathways. Various neurotransmitter systems have been explored to understand the mechanisms behind nicotine tolerance, dependence, and withdrawal. Recent evidence suggests that glutamate neurotransmission has an important role in this phenomenon. The aim of the present review is to discuss preclinical findings concerning the role of N-methyl-D-aspartate (NMDA) receptor neurotransmission in mediating the behavioral effects of nicotine, tolerance, sensitization, dependence, and withdrawal. Based on preclinical findings, it is hypothesized that NMDA receptors mediate the common adaptive processes that are involved in the development, maintenance, and expression of nicotine addiction. Modulation of glutamatergic neurotransmission with NMDA receptor antagonists may prove to be useful in alleviating the symptoms of nicotine abstinence and facilitate tobacco-smoking cessation.

Cross-sensitization of the reinforcing effects of cocaine and amphetamine in rats

RATIONALE

Cross-sensitization between cocaine and amphetamine has been demonstrated in different behavioral paradigms. There is a relative paucity of studies examining whether cross-sensitization occurs between amphetamine and cocaine when both are self-administered.

OBJECTIVE

The current study was designed to test whether animals sensitized to the reinforcing effects of cocaine would show cross-sensitization of the reinforcing effects of amphetamine, using a self-administration paradigm.

MATERIALS AND METHODS

Male, Sprague-Dawley rats were trained to self-administer cocaine and given limited or high exposure to cocaine under a fixed ratio (FR) 1 procedure. After the initial exposure to cocaine, animals self-administered cocaine (1.5 mg/kg per injection) under a progressive ratio (PR) procedure. Subsequently, breakpoints on a PR schedule and rates of intake on an FR schedule maintained by different doses of amphetamine were assessed.

RESULTS

Animals with high initial exposure to cocaine (40 injections of 1.5 mg/kg per injection per day for 5 days) showed stable breakpoints throughout testing. Animals given limited initial cocaine exposure (20 injections of 0.75 mg/kg per injection for 1 day) produced a gradual increase in breakpoints maintained by cocaine over time (i.e., sensitization of the reinforcing effects of cocaine). When subsequently tested with amphetamine, the dose-effect curve was shifted upward in the limited-exposure group relative to the high-exposure group, suggesting cross-sensitization of the reinforcing effects of amphetamine.

CONCLUSIONS

Sensitization of the reinforcing effects of cocaine resulted in cross-sensitization of the reinforcing effects of amphetamine. This phenomenon occurs even when both drugs are self-administered.

How to make a rat addicted to cocaine

Procedures have been developed which provide extremely stable patterns of cocaine self-administration in rats and these have been useful in lesion and drug pretreatment studies aimed at understanding the neurobiology of cocaine reinforcement. The issue now is whether studying the neurobiology of reinforcement is the same as studying the neurobiology of addiction. If the goal is to understand a progressive and deteriorating disorder, then the self-administration procedures should model specific aspects of the progressive stages of the addiction process. Here we review theoretical strategies for modeling the addiction process and present data from a series of experiments from our laboratory showing conditions which produce a progressive change in the motivation to self-administer cocaine in rats. This phenomenon is revealed by an escalation in breakpoints on a progressive ratio schedule. The effect, which is robust and persistent, depends on dose and speed of injection. Interestingly, high drug intake can retard the development of this effect, which we argue indicates that the addiction process has a developmental sequence. Finally, we suggest that specific parameters (dose, price and availability) can be used to examine the transition from recreational use to binge-like intake.

Centrally administered vasopressin cross-sensitizes rats to amphetamine and drinking hypertonic NaCl

Prior sodium restriction cross-sensitizes rats to the psychomotor effects of amphetamines and vice versa. Repeated central injections of vasopressin (VP) induce a psychomotor sensitization similar to amphetamine sensitization and repeated sodium deficiency. Thus brain VP signaling may be a common mechanism involved in mediating these two motivational systems. In experiment 1, we tested the hypothesis that rats previously sensitized to central VP would show enhanced psychomotor responses to amphetamine. Rats were administered saline, VP (50 ng), or amphetamine (1 mg/kg or 3 mg/kg) on days 1 and 2, and given saline or amphetamine on day 3. Amphetamine produced psychomotor arousal in all groups. However, amphetamine on day 3 elicited a significantly greater psychomotor response in rats that had prior injections of amphetamine or VP than in rats previously treated with saline. In experiment 2, the hypothesis that prior experience with central VP would cross-sensitize rats to drinking hypertonic sodium (NaCl) solutions was tested. Rats were administered VP (50 ng) or saline for 3 days. On the fourth day, nondeprived rats were given access to 0.3 M NaCl and water for 1 h. Control and saline-treated rats only drank 1 ml of 0.3 M NaCl, but rats previously exposed to central VP drank significantly more hypertonic saline (4 ml). These results show that prior experience with central VP cross-sensitizes rats to the psychomotor stimulant effects of amphetamine and the ingestion of concentrated NaCl solutions. This pattern of cross-sensitization links central VP signaling, amphetamine, and sodium deficiency, and therefore it may play a role in the cross-sensitization between sodium appetite and amphetamines.

Evidence for sugar addiction: behavioral and neurochemical effects of intermittent, excessive sugar intake

[Avena, N.M., Rada, P., Hoebel B.G., 2007. Evidence for sugar addiction: Behavioral and neurochemical effects of intermittent, excessive sugar intake. Neuroscience and Biobehavioral Reviews XX(X), XXX-XXX]. The experimental question is whether or not sugar can be a substance of abuse and lead to a natural form of addiction. "Food addiction" seems plausible because brain pathways that evolved to respond to natural rewards are also activated by addictive drugs. Sugar is noteworthy as a substance that releases opioids and dopamine and thus might be expected to have addictive potential. This review summarizes evidence of sugar dependence in an animal model. Four components of addiction are analyzed. "Bingeing," "withdrawal," "craving" and "cross-sensitization" are each given operational definitions and demonstrated behaviorally with sugar bingeing as the reinforcer. These behaviors are then related to neurochemical changes in the brain that also occur with addictive drugs. Neural adaptations include changes in dopamine and opioid receptor binding, enkephalin mRNA expression and dopamine and acetylcholine release in the nucleus accumbens. The evidence supports the hypothesis that under certain circumstances rats can become sugar dependent. This may translate to some human conditions as suggested by the literature on eating disorders and obesity.

Cocaine during adolescence enhances dopamine in response to a natural reinforcer

The use of cocaine during adolescent development could alter the normal growth of brain regions affected by cocaine, specifically the reward system, and impact the adult mesolimbic system. However, there is scant literature aimed at determining whether animals are more vulnerable to the adverse effects of drugs during adolescence. The present study investigated whether cocaine pretreatment in either adolescence or adulthood altered the dopaminergic response to a naturally reinforcing substance in adulthood. To evaluate the responsivity of the mesolimbic system after repeated cocaine, sucrose was offered during the dialysis procedure and dialysates were collected. Regardless of age all saline pretreated rats had significant increases in sucrose-induced extracellular dopamine (DA) levels in the nucleus accumbens septi (NAcc) as compared to baseline levels. Rats pretreated with cocaine as adults also had significant increases in DA levels after sucrose. Interestingly, sucrose intake significantly enhanced DA levels in cocaine pretreated adolescent rats as compared to all other conditions. The results from the present study show that in rats pretreated with cocaine during adolescence there is an enhanced response of the dopaminergic system in animals exposed to a naturally reinforcing substance. Therefore, cocaine exposure during adolescence results in long-term functional changes in the mesolimbic pathway. Future studies need to ascertain the underlying mechanisms and their potential role in cocaine addiction.

Rapid and persistent sensitization to the reinforcing effects of cocaine

The development of drug addiction involves a transition from recreational use to compulsive drug seeking and taking, and this progression can occur rapidly with cocaine use. These data highlight the importance of early drug exposure and the development of drug dependence; however, little experimental attention has been paid to this phenomenon in animal models of drug abuse. The present experiments demonstrate a progressive and rapid sensitization to the reinforcing strength of cocaine assessed using a progressive ratio (PR) schedule in rats. The first experiment found that rats show increased breakpoints over a 2-week period following acquisition. Subsequent experiments examined the role of total cocaine intake during the initial exposure period and found that low intakes (20 mg/kg/day x 5 days) resulted in sensitization, whereas relatively higher intake (60 or 100 mg/kg/day x 5 days) suppressed the development of sensitization. In contrast, this higher level of intake (60 mg/kg/day x 5 days) only transiently suppressed the expression of sensitization. Examination of breakpoints maintained by various doses of cocaine revealed an upward and leftward displacement of the cocaine dose-effect curve, relative to nonsensitized animals. These studies describe a form of sensitization that occurs rapidly to the reinforcing effects of cocaine, and provide a model to study the potential impact of initial experience on the development of drug dependence.

Contribution of dihydro-beta-erythroidine sensitive nicotinic acetylcholine receptors in the ventral tegmental area to cocaine-induced behavioral sensitization in rats

Nicotinic acetylcholine receptors (nAChRs) are known to play a role in several aspects of cocaine addiction. Recently, systemic administration of the nicotinic receptor antagonist mecamylamine was shown to block the induction of long-term locomotor sensitization to cocaine. Behavioral sensitization being a model of long-term neuroadaptations to chronic cocaine exposure, the goal of the current study was to identify the anatomical localization, as well as the nature, of the nicotinic receptors involved. Male Sprague-Dawley rats were stereotaxically implanted with bilateral cannula into either the ventral tegmental area (VTA) or the nucleus accumbens (Nacc). On each of the six consecutive days, rats were microinjected bilaterally with the selective nicotinic antagonists dihydro-beta-erythroidine (DHbetaE), methyllycaconitine (MLA) or saline, followed by an intra-peritoneal injection of cocaine (15 mg/kg, i.p.) or saline. Following a 2-week withdrawal period, rats received a final injection of cocaine in the absence of nicotinic antagonist to test for sensitization. When microinjected into the VTA, DHbetaE, but not MLA, prevented the induction of behavioral sensitization to cocaine. In contrast, behavioral sensitization was present in rats receiving DHbetaE microinjections into the Nacc. Neither antagonist, whether injected into the VTA or the Nacc had any significant effect on the acute locomotor response to cocaine. Given the subtype selectivity of the nicotinic antagonists employed, heteromeric beta2-containing (beta2*) nAChRs, but not homomeric alpha7* nAChRs, in the VTA may be involved in the neuroadaptive changes underlying cocaine sensitization.

Sensitization of the reinforcing effects of self-administered cocaine in rats: effects of dose and intravenous injection speed

Speed of drug onset is assumed to be an important determinant of the abuse liability of a drug. Studies in human and non-human primates suggest that the subjective and reinforcing effects of cocaine can be influenced by route of administration and/or speed of intravenous injection. Sensitization to the reinforcing effects of cocaine was studied in rats and the effects of various injection durations (i.e. speed of injection) on the development of sensitization was examined using a progressive ratio schedule. In addition, the effects of cocaine dose on sensitization and the effects of injection duration on the acute reinforcing effects of cocaine were examined. The initial study demonstrated that the development of sensitization (i.e. progressive increases in breakpoints) was dose-dependent. A robust sensitization of the reinforcing effects of cocaine was replicated in animals receiving cocaine at the highest rate (i.e. shortest duration; 5 s), but not in animals receiving the same dose over 25 or 50 s. Subsequent testing revealed that injection duration did not have profound effects on the acute reinforcing effects of cocaine (assessed by breakpoints or rate of responding on a fixed ratio schedule). These findings are similar to recent studies demonstrating that the development of sensitization, but not the acute responsivity, to cocaine's locomotor-activating effects are influenced by rate of intravenous injection. Taking these findings together, we hypothesize that the process of drug addiction involves both the acute reinforcing effects and the development of sensitization.

Sensitization of midbrain dopamine neuron reactivity and the self-administration of psychomotor stimulant drugs

Psychostimulant drugs like amphetamine are readily self-administered by humans and laboratory animals by virtue of their actions on dopamine (DA) neurons in the midbrain. Exposing animals to this drug either systemically or in the cell body region of these neurons in the ventral tegmental area leads to long-lasting alterations in dopaminergic function. These have most often been assessed as increased locomotor activity and enhanced DA overflow in the nucleus accumbens (NAcc) after re-exposure to the drug weeks to months later. Evidence is presented showing that manipulations that produce this sensitization of midbrain DA neuron reactivity enhance the pursuit and self-administration of psychostimulant drugs. Procedures known to prevent the induction of sensitization by amphetamine also prevent the facilitation of drug taking. Enhanced drug self-administration and primed reinstatement of drug seeking are also accompanied by enhanced NAcc DA reactivity. Finally, drugs that increase NAcc DA overflow acutely but fail to produce sensitization of this effect are not associated with the subsequent enhancement of self-administration. These results indicate a direct relationship between the sensitization of midbrain dopamine neuron reactivity and the excessive pursuit and self-administration of psychostimulant drugs. Understanding the neuronal events and adaptations that underlie the induction and expression of sensitization may thus help elucidate how drug abuse develops, how it is reinstated and ultimately how both may be prevented.

Chronic nicotine differentially alters cocaine-induced locomotor activity in adolescent vs. adult male and female rats

Tobacco use is prevalent in the adolescent population. It is a major concern because tobacco is highly addictive and has also been linked to illicit drug use. There is not much research, however, on the interaction between nicotine and other stimulant drugs in animal models of early adolescence. This study examined the effects of chronic nicotine alone and on cocaine-stimulated activity in male and female periadolescent rats compared to male and female adult rats. During the seven-day nicotine pretreatment period, nicotine increased locomotor activity in all groups compared to vehicle controls. Male and female adult rats and female periadolescent rats developed sensitization to the locomotor-activating effects of nicotine over the 7-day treatment period, while male periadolescent rats did not. All groups treated with nicotine, however, exhibited sensitization to nicotine-induced repetitive motion over the 7-day nicotine treatment period. On day 8, male periadolescent rats pretreated with nicotine were more markedly sensitized to the locomotor-activating effects of cocaine than male adult rats, while female rats pretreated with nicotine were not sensitized to cocaine. In contrast, male and female periadolescent rats, but not adult rats, had increased amounts of repetitive beam breaks induced by cocaine after nicotine pretreatment. Overall, it appears that cross-sensitization to cocaine is greater in periadolescent than in adult rats, and that males are more sensitized than females. Thus, it may be that nicotine use during adolescence carries a greater risk than during adulthood and that male adolescents may be particularly vulnerable to the risk of cocaine abuse after nicotine use. This information should be taken into account so as to help us better understand the development of drug addiction in adolescents compared to adults.

A diet promoting sugar dependency causes behavioral cross-sensitization to a low dose of amphetamine

Previous research in this laboratory has shown that a diet of intermittent excessive sugar consumption produces a state with neurochemical and behavioral similarities to drug dependency. The present study examined whether female rats on various regimens of sugar access would show behavioral cross-sensitization to a low dose of amphetamine. After a 30-min baseline measure of locomotor activity (day 0), animals were maintained on a cyclic diet of 12-h deprivation followed by 12-h access to 10% sucrose solution and chow pellets (12 h access starting 4 h after onset of the dark period) for 21 days. Locomotor activity was measured again for 30 min at the beginning of days 1 and 21 of sugar access. Beginning on day 22, all rats were maintained on ad libitum chow. Nine days later locomotor activity was measured in response to a single low dose of amphetamine (0.5 mg/kg). The animals that had experienced cyclic sucrose and chow were hyperactive in response to amphetamine compared with four control groups (ad libitum 10% sucrose and chow followed by amphetamine injection, cyclic chow followed by amphetamine injection, ad libitum chow with amphetamine, or cyclic 10% sucrose and chow with a saline injection). These results suggest that a diet comprised of alternating deprivation and access to a sugar solution and chow produces bingeing on sugar that leads to a long lasting state of increased sensitivity to amphetamine, possibly due to a lasting alteration in the dopamine system.

Previous exposure to VTA amphetamine enhances cocaine self-administration under a progressive ratio schedule in an NMDA, AMPA/kainate, and metabotropic glutamate receptor-dependent manner

Previous exposure to amphetamine (AMPH) in the ventral tegmental area (VTA) enhances cocaine self-administration in a D(1) dopamine receptor-dependent manner. The present study examined the contribution of VTA NMDA, AMPA/kainate, and metabotropic glutamate (mGlu) receptors to this effect. Rats in different groups received three intra-VTA injections, one every third day, of either saline (0.5 microl/side), AMPH (2.5 microg/0.5 microl/side), AMPH+CPP (NMDA receptor antagonist; 10 microM or 100 microM/0.5 microl/side), AMPH+CNQX (AMPA/kainate receptor antagonist; 0.3 mM or 1 mM/0.5 microl/side), AMPH+MCPG (mGlu receptor antagonist; 0.5 mM or 50 mM/0.5 microl/side), or the glutamate receptor antagonists alone. Starting 7-10 days after the last pre-exposure injection, rats were trained to self-administer cocaine (0.3 mg/kg/infusion) and then tested under a progressive ratio (PR) schedule of reinforcement for 6 consecutive days. As reported previously, VTA AMPH pre-exposed rats worked more and obtained more infusions of cocaine than saline pre-exposed animals. Coadministration of CPP, CNQX, or MCPG with AMPH during pre-exposure dose-dependently blocked this enhancement of cocaine self-administration. Rats pre-exposed to the glutamate receptor antagonists alone did not differ on the test days from the saline pre-exposed controls. These results indicate that, in a manner paralleling the induction of sensitization of the locomotor stimulating effects of AMPH, activation of NMDA, AMPA/kainate, and mGlu receptors during pre-exposure to AMPH in the VTA is necessary for the enhancement of cocaine self-administration to develop.

Evidence of cross-tolerance between behavioural effects of nicotine and cocaine in mice

RATIONALE

Studies have reported that chronic exposure to nicotine does not alter the effects of cocaine on locomotor activity, and vice versa. However, the apparent lack of effect of one drug on the behavioural response to the other may be due to an exclusive focus on locomotor activity as the target behaviour.

OBJECTIVE

To test whether repeated pretreatment with nicotine causes tolerance or sensitization to cocaine's effects on diverse behaviours: locomotion, rearing, grooming, and immobility. Similarly, the effects of repeated cocaine treatment on the acute response to nicotine were also tested.

METHODS

Mice were pretreated with 14 injections of nicotine (0.3 mg/kg), cocaine (5 mg/kg) or saline, the injections being given once daily, except for three breaks of two days each. Two days after the final pretreatment injection, mice were given a challenge injection of saline, cocaine (3 or 5 mg/kg) or nicotine (0.3 or 1 mg/kg), and observed in a large test cage for 40 min using a time-sampling procedure.

RESULTS

Repeated administration of either drug produced some tolerance to subsequent challenge with the same dose of the drug. Prior nicotine exposure significantly attenuated cocaine-induced decreases in grooming and increases in rearing, but did not significantly affect other behaviours. In contrast, prior cocaine exposure failed to alter nicotine's effects on any behaviour.

CONCLUSIONS

Cross-tolerance between nicotine and cocaine (but not vice-versa) can be demonstrated if several behaviours are observed; measures of locomotor activity are less sensitive to the effect. The asymmetrical pattern of cross-tolerance may be due to differential inhibition of dopamine uptake by the two drugs.

Smoking, stress, and negative affect: correlation, causation, and context across stages of smoking

This transdisciplinary review of the literature addresses the questions, Do stress and negative affect (NA) promote smoking? and Does smoking genuinely relieve stress and NA? Drawing on both human and animal literatures, the authors examine these questions across three developmental stages of smoking--initiation, maintenance, and relapse. Methodological and conceptual distinctions relating to within- and between-subjects levels of analyses are emphasized throughout the review. Potential mechanisms underlying links between stress and NA and smoking are also reviewed. Relative to direct-effect explanations, the authors argue that contextual mediator-moderator approaches hold greater potential for elucidating complex associations between NA and stress and smoking. The authors conclude with recommendations for research initiatives that draw on more sophisticated theories and methodologies.

Amphetamine-sensitized rats show sugar-induced hyperactivity (cross-sensitization) and sugar hyperphagia

The goal was to determine the locomotor and consummatory effects of sugar in amphetamine-sensitized rats. Following a 30-min locomotor activity baseline using a photocell cage, male rats were administered either 3.0 mg/kg amphetamine or saline i.p. daily for 6 days. On the final day of injections, locomotor activity was measured again to affirm amphetamine sensitization. Experiment 1: Seven days later, half of each group was offered 10% sucrose or water for 1 min in the home cages, followed by a 30-min locomotor activity test to determine whether or not the animals had become hyperactive in response to sugar. Results showed that amphetamine-sensitized animals were hyperactive following a taste of sugar, but not water. Experiment 2: All subjects were then given access to 10% sucrose for 1 h daily for five consecutive days. Results showed that the amphetamine-sensitized group consumed more sucrose across the 5-day measurement period. These results suggest that sugar may be acting on the same system as amphetamine to trigger hyperactivity, and that alterations in this system caused by repeated doses of amphetamine can instigate an appetite for sugar that persists for at least a week.

Pretreatment with methylphenidate sensitizes rats to the reinforcing effects of cocaine

Repeated administration of cocaine produces sensitization to its locomotor-activating effects and increases the rate at which cocaine self-administration behavior is acquired. Methylphenidate is administered clinically on a daily basis, predominantly to children and adolescents, for the treatment of attention-deficit hyperactivity disorder (ADHD). It has been demonstrated previously that pretreatment with methylphenidate administered to periadolescent rats decreased the latency to acquisition of cocaine self-administration. Since methylphenidate is often also administered to adults with ADHD, the present study was conducted to determine the effects of prior administration of methylphenidate (5 or 20 mg/kg/day for 9 days) to adult rats on the rate of acquisition for cocaine self-administration (0.25 mg/kg/infusion). The higher dose of methylphenidate significantly decreased the latency for acquisition of this behavior, suggesting that the rats were sensitized to the reinforcing effects of cocaine after treatment with methylphenidate. These findings add to the growing body of evidence suggesting cross-sensitization between the behavioral effects of psychostimulants. Further, insofar as self-administration is a reliable measure of abuse liability, these data suggest that a short-duration pretreatment with a high dose of methylphenidate to adults increases vulnerability to cocaine abuse.

Psychostimulant-induced behavioral sensitization depends on nicotinic receptor activation

Animal studies have shown that nicotine and psychostimulant drugs (amphetamine and cocaine) share the property of inducing long-lasting behavioral and neurochemical sensitization, which is thought to contribute to their addictive properties. Neuroplasticity subserving learning and memory mechanisms is considered to be involved in psychostimulant-induced sensitization and addiction behavior. Because nicotinic receptors in the brain play a role in the storage of drug-related information underlying reinforcement learning, we evaluated the possibility that activation of central nicotinic receptors may underlie psychostimulant-induced sensitization. Repeated exposure of rats to nicotine profoundly enhanced the psychomotor effects of nicotine and amphetamine 3 weeks after nicotine pretreatment. Moreover, the nicotinic receptor antagonist mecamylamine completely blocked the induction, but not the long-term expression, of behavioral sensitization to amphetamine in amphetamine-pretreated rats. Mecamylamine also prevented the development of cocaine-induced behavioral sensitization. Behavioral sensitization induced by nicotine, amphetamine, or cocaine was associated with an increase in the electrically evoked release of [(3)H]dopamine from nucleus accumbens slices. Coadministration of mecamylamine during pretreatment with nicotine, amphetamine, or cocaine prevented the development of this long-term hyperreactivity of nucleus accumbens dopamine neurons. Similarly, the high-affinity non-alpha7 subtype nicotinic receptor antagonist dihydro-beta-erythroidine prevented the development of amphetamine-induced behavioral and neurochemical sensitization. These data indicate that nicotinic receptor activation (by endogenously released acetylcholine) is a common denominator initiating neuroplasticity involved in the development of amphetamine, as well as cocaine-induced sensitization.

Effect of the kappa-opioid receptor agonist, U69593, on reinstatement of extinguished amphetamine self-administration behavior

Previous research has indicated that pretreatment with the kappa-opioid receptor agonist, U69593, decreased the ability of experimenter-administered cocaine to reinstate extinguished cocaine self-administration behavior. This effect was specific to cocaine-produced drug seeking since U69593 failed to attenuate the ability of experimenter-administered amphetamine to reinstate extinguished cocaine self-administration behavior. One possibility is that U69593 selectively attenuates the behavioral effects of the drug that was originally self-administered. In order to test this hypothesis, the present study examined the effect of U69593 (0.0 or 0.32 mg/kg) on the reinstatement of extinguished amphetamine self-administration behavior produced by experimenter-administered injections of cocaine and amphetamine. Following extinction of amphetamine self-administration (0.04 mg/kg/infusion) the ability of cocaine (0.0, 5.0, 10.0 or 20.0 mg/kg) or amphetamine (0.0, 0.3, 1.0 or 3.0 mg/kg) to reinstate extinguished self-administration behavior was measured. Both drugs reinstated extinguished responding and the reinstatement was attenuated by pretreatment with U69593. The data indicate that the ability of U69593 to decrease drug seeking is not restricted to subjects experienced with cocaine self-administration. Self-administration history does, however, determine the effect of U69593 on amphetamine-produced drug seeking.

A comparison of the behavioral effects of the repeated administration of PTT, 2beta-propanoyl-3beta-(4-tolyl)tropane and cocaine

In the present study the effects of the repeated administration of the novel tropane analog PTT (2beta-propanoyl-3beta(4-tolyl)tropane) on spontaneous locomotor activity were compared to those of cocaine. Previous reports describing the in vivo effects of PTT have focused solely on its acute effects following a single administration. In Experiment 1, PTT (1.0, 3.0 mg/kg), cocaine (30 mg/kg), or vehicle were administered intraperitoneally to male Sprague-Dawley rats daily for 10 consecutive days and locomotor activity was assessed. In Experiment 2, the locomotor effects of PTT (1.0 mg/kg) and cocaine (15 mg/kg) were assessed following 5 days of drug exposure to either PTT (1.0, 3.0 mg/kg) or cocaine (30 mg/kg) and 18 days of withdrawal. In both paradigms, PTT (1.0, 3.0 mg/kg) and cocaine (30 mg/kg) produced marked increases in locomotor activity acutely and an augmented response to drug challenge following repeated exposure, indicative of behavioral sensitization. These data indicate that, despite differences in the pharmacological profiles of PTT and cocaine, both drugs produce behavioral sensitization. These data are consistent with previous reports in the literature describing the effects of the repeated administration of other dopamine reuptake inhibitors and suggest that the development of behavioral sensitization is a uniform characteristic of this class of drugs.

The role of excitatory amino acids in behavioral sensitization to psychomotor stimulants

Behavioral sensitization refers to the progressive augmentation of behavioral responses to psychomotor stimulants that develops during their repeated administration and persists even after long periods of withdrawal. It provides an animal model for the intensification of drug craving believed to underlie addiction in humans. Mechanistic similarities between sensitization and other forms of neuronal plasticity were first suggested on the basis of the ability of N-methyl-D-aspartate (NMDA) receptor antagonists to prevent the development of sensitization [Karler, R., Calder, L. D., Chaudhry, I. A. and Turkanis, S. A. (1989) Blockade of "reverse tolerance" to cocaine and amphetamine by MK-801. Life Sci., 45, 599-606]. This article will review the large number of subsequent studies addressing: (1) the roles of NMDA, alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) and metabotropic glutamate receptors in the development and expression of behavioral sensitization, (2) excitatory amino acids (EAAs) and the role of conditioning in sensitization, (3) controversies regarding EAA involvement in behavioral sensitization based on studies with MK-801, (4) the effects of acute and repeated stimulant administration on EAA neurochemistry and EAA receptor expression, and (5) the neuroanatomy of EAA involvement in sensitization. To summarize, NMDA, AMPA metabotropic glutamate receptors all participate in the development of sensitization, while maintenance of the sensitized state involves alterations in neurochemical measures of EAA transmission as well as in the expression and sensitivity of AMPA and NMDA receptors. While behavioral sensitization likely involves complex neuronal circuits, with EAAs participating at several points within this circuitry, EAA projections originating in prefrontal cortex may play a particularly important role in the development of sensitization, perhaps via their regulatory effects on midbrain dopamine neurons. The review concludes by critically evaluating various hypotheses to account for EAA involvement in the development of behavioral sensitization, and considering the question of whether EAA receptors are involved in mediating the rewarding effects of psychomotor stimulants and sensitization of such rewarding effects.

Effects of acute and repeated administration of N-methyl-D-aspartate (NMDA) into the ventral tegmental area: locomotor activating effects of NMDA and cocaine

Repeated, intermittent administration of psychostimulants produces an enhancement of the subsequent behavioral effects of these drugs. This behavioral sensitization has been implicated in maintenance of and relapse to drug-taking. As a result, there has been great interest in elucidating the mechanisms underlying both the development and expression of sensitization. An accumulation of data from studies of stimulant-induced locomotor activity has implicated excitatory amino acids in the development of behavioral sensitization. In the present study, N-methyl-D-aspartate (NMDA) (0.6, 1.25 or 2.5 microg) infused bilaterally into the ventral tegmental area (VTA) produced dose-dependent locomotor activation. The locomotor activating effect of NMDA was increased following repeated NMDA administration (two exposures to intra-VTA NMDA), suggesting sensitization. However, repeated intra-VTA NMDA failed to sensitize rats to the locomotor activating effects of systemically administered cocaine (5.0, 10.0 or 20.0 mg/kg). These findings are consistent with the notion that repeated activation of NMDA receptors is sufficient for the development of behavioral sensitization to NMDA. Other neuroadaptations produced by repeated psychostimulant administration are required in order for the development of sensitization to the behavioral effects of those drugs.

Context-dependent cross-sensitization between cocaine and amphetamine

We investigated the effect of amphetamine pretreatment on the locomotor response to subsequent cocaine challenge. Rats were administered either 0.75 mg/kg amphetamine in a testing environment and saline in their home cage, saline in the testing environment and 0.75 mg/kg amphetamine in their home cage, or saline in both the testing environment and home cage. After 5 pairings of drug to environment, conditioning was tested by administration of a saline injection. Both amphetamine treated groups exhibited increased locomotion in response to saline injection. After the eighth session of the pairing regimen, all animals were administered 5 mg/kg cocaine in the test environment and their behavior measured for 30 min. Sensitization to cocaine was observed only in rats with previous amphetamine exposure in the testing environment. There was no difference between groups in whole brain, striatal, or plasma levels of cocaine. The data support the hypothesis that sensitization is independent of brain cocaine levels.

Effects of environmental conditioning on the development of nicotine sensitization: behavioral and neurochemical analysis

We have investigated the effects of environmental conditioning on the induction of nicotine sensitization of locomotion, stereotypy and nucleus accumbens dopamine release. Sprague-Dawley rats, some of which had been previously implanted with a microdialysis guide cannula over the nucleus accumbens, were sensitized with 5 days of repeated nicotine (0.6 mg/kg per day, SC) or saline injections (1 ml/kg per day). During nicotine treatment the drug administration was either paired with the microdialysis/activity monitor testing chamber (conditioned) (n = 6) or with the animal's home cage (unconditioned) (n = 6) and after 60 min the animal was returned to home cage and received a second injection of saline 15 min later. A third group received saline in the testing apparatus followed by nicotine in the home cage (pseudo-conditioned) (n = 6). In the guide cannulated animals, 2 mm microdialysis probes were inserted after completing day 5 of treatment and all animals were tested for their response to nicotine (0.6 mg/kg, SC) on day 6. Both locomotor activity and nucleus accumbens dopamine release showed a larger response subsequent to nicotine challenge in the nicotine versus saline pretreated animals in the conditioned group, but not in the unconditioned group. In the pseudo-conditioned group there was an increase in the stereotypy responses to nicotine, however the locomotor and dopamine release responses were not significantly enhanced. The results from the conditioned group were confirmed in animals which were tested for behavioral activation and dopamine release simultaneously (n = 5). These findings indicate that nicotine sensitization of locomotor activity and nucleus accumbens dopamine release (using a 5-day pretreatment protocol) is dependent on conditioning the animal to the testing environment during nicotine pretreatment.

Locomotor activation and dopamine release produced by nicotine and isoarecolone in rats

1. Isoarecolone was approximately 250 times less potent than nicotine as an inhibitor of [3H]-nicotine binding to rat brain membranes. Isoarecolone failed to inhibit the binding of the nicotinic ligand [125I]-alpha-bungarotoxin or of the muscarinic ligand [3H]-QNB. 2. Nicotine (0.01-30 microM) evoked the release of [3H]-dopamine from striatal and frontal cortex synaptosomes, with EC50 values of approximately 0.5 microM in each case. This release was largely mecamylamine-sensitive. 3. Isoarecolone (1-200 microM) evoked predominantly mecamylamine-sensitive dopamine release from both striatal and cortical synaptosomes, with a potency at least 20 times less than that of nicotine. The maximum effect of isoarecolone was less than that of nicotine, particularly in the frontal cortex preparation. 4. In control rats treated chronically with saline, neither nicotine nor isoarecolone had clear effects on locomotor activity at the doses tested. Chronic treatment with nicotine clearly sensitized rats to the locomotor activating effect of isoarecolone was seen at a dose about 40 times larger than that of nicotine. 5. The low potency and efficacy of isoarecolone in facilitating sensitized locomotor activity resembled its lower potency and efficacy, compared with nicotine, in evoking dopamine release in vitro. The agonist profile of the nicotinic receptor population mediating dopamine release may determine the pharmacological characteristics of consequent locomotor behaviour.

The role of dopamine in drug abuse viewed from the perspective of its role in motivation

Drugs of abuse share with conventional reinforcers the activation of specific neural pathways in the CNS that are the substrate of their motivational properties. Dopamine is recognized as the transmitter of one such neural pathway, being involved in at least three major aspects of motivation: modulation of motivational state, acquisition (incentive learning) and expression of incentive properties by motivational stimuli. Drugs of abuse of different pharmacological classes stimulate in the low dose range dopamine transmission particularly in the ventral striatum. Apart from psychostimulants, the evidence that stimulation of dopamine transmission by drugs of abuse provides the primary motivational stimulus for drug self-administration is either unconvincing or negative. However, stimulation of dopamine transmission is essential for the activational properties of drugs of abuse and might be instrumental for the acquisition of responding to drug-related incentive stimuli (incentive learning). Dopamine is involved in the induction and in the expression of behavioural sensitization by repeated exposure to various drugs of abuse. Sensitization to the dopamine-stimulant properties of specific drug classes leading to facilitation of incentive learning of drug-related stimuli might account for the strong control over behaviour exerted by these stimuli in the addiction state. Withdrawal from drugs of abuse results in a reduction in basal dopamine transmission in vivo and in reduced responding for conventional reinforcers. Although these changes are likely to be the expression of a state of dependence of the dopamine system their contribution to the motivational state of drug addiction is unclear.

Induction but not expression of behavioural sensitization to nicotine in the rat is dependent on glucocorticoids

Behavioural sensitization has been implicated in the development of addictive behaviour, and several studies suggest that corticosteroids may be involved in this phenomenon. In the present study, the effects of adrenalectomy and steroid replacement treatments on the behavioural sensitization observed after daily injections of nicotine (0.4 mg/kg s.c.) were investigated in the rat. Adrenalectomy completely prevented sensitization to the locomotor stimulating effect of nicotine after repeated injections but did not influence the acute locomotor activating effect of the drug or an already established sensitization to nicotine. In adrenalectomized animals receiving replacement treatment with corticosterone or dexamethasone, but not aldosterone, repeated administration of nicotine produced behavioural sensitization. Repeated dexamethasone treatment per se failed, however, to sensitize rats to nicotine. Post mortem neurochemical studies showed that repeated administration of nicotine significantly increased homovanillic acid (HVA) levels, as well as the dihydroxyphenylacetic acid (DOPAC)/dopamine quotient, in the limbic forebrain. Adrenalectomy per se significantly increased HVA levels and tended to elevate the DOPAC/dopamine quotient. When repeatedly treated with nicotine, adrenalectomized rats displayed a higher DOPAC/dopamine quotient, but no significant difference in HVA levels, compared to nicotine-treated sham-operated controls. In the striatum and the cortex, no significant effects of nicotine treatment or adrenalectomy were observed on any of the neurochemical measures. The present results suggest that glucocorticoid (type II) receptor activation is required for induction of sensitization to the locomotor stimulatory effect of nicotine, whereas corticosteroids are not required for the expression of the behavioural sensitization once established. Provided that HVA levels and the DOPAC/dopamine quotient relatively well reflect the presynaptic dopamine activating effect of nicotine, it may be suggested that corticosteroid-related mechanisms associated with behavioural sensitization to nicotine are post- rather than presynaptically located in relation to mesolimbic dopamine neurons.

Tolerance to the reinforcing effects of cocaine in a progressive ratio paradigm

This experiment used rats to test whether a regimen of chronic cocaine would produce tolerance to cocaine i.v. self-administration under a progressive ratio (PR) schedule of reinforcement. Under this PR schedule, an increasing number of responses was required to complete the ratio for each subsequent cocaine injection, and failure to complete the required ratio for the next injection within 1 h of the previous cocaine injection terminated the session. The number of injections taken in the session was termed the breaking point and used as the dependent variable. Rats were trained under this schedule until breaking point values were stable, after which cocaine dose-effect data were obtained: the breaking point increased as the dose of cocaine increased. Subsequently, rats were assigned to one of two groups for 7 days of chronic treatment: one group was infused with cocaine (18 mg/kg, given over 20 min once every 8 h) and the other group received 0.9% saline. Following termination of chronic treatment, cocaine dose-effect data were redetermined in both groups. Chronic cocaine treatment significantly decreased breaking point values across the entire dose-effect curve, although the effect was observed in only four of seven subjects. In contrast, chronic saline treatment produced no significant effect on the breaking point measures. Following a further 5 days of recovery from chronic treatment, cocaine dose-effect data were redetermined in both groups; these curves were essentially identical to those obtained before chronic treatments. These data support the hypothesis that tolerance occurs to the reinforcing effects of cocaine, as measured by a decrease in the breaking point, at least for a subset of animals.

Sensitization to cocaine's motor activating properties produced by electrical kindling of the medial prefrontal cortex but not of the hippocampus

A substantial body of evidence has accumulated that implicates NMDA systems in the neural changes that are associated with the development of both electrical kindling of limbic sites and sensitization to the behavioral effects of repeated stimulant exposure. This study sought to establish whether electrical kindling of the brain was a sufficient condition for inducing sensitization to cocaine's motor activating effects and, if so, whether the cross sensitization was a result of kindling of a specific locus. Rats received daily electrical stimulation of either the medial prefrontal cortex or the hippocampus. Other rats received the electrode implants and were handled daily but received no electrical stimulation. Stage 5 seizures developed in response to the stimulation in 32-35 days. Once this criterion of kindling was established and following a 14 day waiting period the effectiveness of cocaine (0.0, 5.0 or 10.0 mg/kg) in elevating horizontal motor activity was determined. For all 3 groups (sham controls, prefrontal cortical and hippocampal stimulated rats) cocaine produced a dose-dependent increase in horizontal activity. The sham controls and hippocampal rats did not differ in the magnitude of the cocaine-produced effect. However, rats that had received stimulation of the prefrontal cortex showed heightened levels of cocaine-induced activity that were particularly apparent in response to 10.0 mg/kg cocaine. These data suggest that kindling of the prefrontal cortex had sensitized rats to the behavioral effects of cocaine.(ABSTRACT TRUNCATED AT 250 WORDS)

Injections of 6-hydroxydopamine into the ventral tegmental area destroy mesolimbic dopamine neurons but spare the locomotor activating effects of nicotine in the rat

The locomotor response to nicotine was assessed four weeks following destruction of mesolimbic dopamine (DA) neurons in rats by infusion of 6-hydroxydopamine into the ventral tegmental area. Resulting depletions of nucleus accumbens (N.Acc.) DA of up to 100% of control concentrations did not block the acute locomotor response to nicotine (0.4 mg/kg, base, s.c.). Such depletions also did not prevent the progressive enhancement of nicotine's locomotor effects when injections were repeated daily for nine days. These results suggest that mesolimbic DA is not necessary for the elicitation of locomotor activation by nicotine.

Persistence of the ability of amphetamine preexposure to facilitate acquisition of cocaine self-administration

This study assessed the enduring effects of amphetamine preexposure on the subsequent reinforcing effects of cocaine. Rats received nine daily injections of either d-amphetamine SO4 (2.0 mg/kg, IP) or vehicle 45 days prior to testing of the acquisition of cocaine (0.25 mg/kg/infusion) self-administration. The latency to acquire reliable cocaine self-administration was shorter (day 3) in the amphetamine-preexposed rats than in the vehicle-preexposed rats (day 6). These data are comparable to those observed when testing was carried out 1 day following the treatment. In previous studies, shorter latencies to acquisition also occurred when the training dose of cocaine was increased, suggesting that the treatment had sensitized rats to cocaine's reinforcing properties. These effects of amphetamine exposure persist for 45 days following treatment, which suggest a long-lasting phenomenon.

Augmentation of the neurochemical effects of cocaine in the ventral striatum and medial prefrontal cortex following preexposure to amphetamine, but not nicotine: an in vivo microdialysis study

This study assessed the ability of cocaine to increase synaptic levels of dopamine (DA) in the ventral striatum (VS) and medial prefrontal cortex (mPFC) following repeated daily exposure to amphetamine or nicotine. In vivo microdialysis was used to assay DA levels in the awake freely moving male Sprague-Dawley rats. Three days following guide cannula implantation, subjects received 9 daily preexposure injections of amphetamine (1.0 mg/kg salt weight, IP), nicotine (0.6 mg/kg base weight, SC), or vehicle. Probes were then inserted and perfused with Ringer's solution overnight. Once stable baseline levels of DA were established, the subjects received a cocaine (15 mg/kg, IP) challenge injection and dialysate samples were collected every 20 minutes. Subjects preexposed to vehicle exhibited a 200% increase in DA levels in the VS and a 170% increase in the mPFC following the cocaine injection. DA recovery was significantly augmented in both regions in amphetamine-preexposed subjects with 450% and 285% increases above baseline levels observed in the VS and mPFC, respectively. In contrast, cocaine-induced DA levels in nicotine-preexposed subjects did not differ significantly from vehicle-preexposed controls in either brain region. Data are discussed in terms of alterations in the locomotor activating and reinforcing effects of cocaine following similar preexposure procedures.

Psychostimulant-induced activity is attenuated by two putative dopamine release inhibitors

Centrally administered amphetamine (AMPH), cathinone, (CATH), or cocaine (COC) have each been shown to produce elevated activity in rats and this effect is dose responsive. The question remains whether these psychostimulants share a common mechanism of action (i.e., do these psychostimulants act by releasing dopamine to increase activity levels?). Experiments were, therefore, conducted to measure the spontaneous activity of these three centrally administered psychostimulants in rats following pretreatment with two putative dopamine release inhibitors, viz., 5-(4-methyl-1 piperazinyl)imidazol(2,1-b) (1,3,5)-benzothiadiazepine maleate [CGS 10746B (CGS); 20 mg/kg)] and 4-(4-benzofurazanyl)-1,4-dihydro-2,6-dimethyl-3,5-pyridinedicar boxylic acid methyl 1-methyl-ethyl ester [isradipine (ISR); 2.5 mg/kg)]. Rats fitted with chronic indwelling ventricular cannulae received a single dose of ICV-administered CATH (32 micrograms), AMPH (16 micrograms), COC (100 micrograms), or vehicle. Selection of these ICV doses of stimulant drugs was based upon results obtained in preliminary studies that indicated similar elevations of activity. ICV administration of each of these drugs/doses was preceded (20 min) by peripherally administered CGS, ISR, or vehicle. Results show that ICV CATH (32 micrograms), AMPH (16 micrograms), COC (100 micrograms) equieffectively elevate activity (two- to threefold) and that, in each case, this increase was significantly attenuated by pretreatment with CGS or ISR.

Neurotransmitter regulation of dopamine neurons in the ventral tegmental area

Over the last 10 years there has been important progress towards understanding how neurotransmitters regulate dopaminergic output. Reasonable estimates can be made of the synaptic arrangement of afferents to dopamine and non-dopamine cells in the ventral tegmental area (VTA). These models are derived from correlative findings using a variety of techniques. In addition to improved lesioning and pathway-tracing techniques, the capacity to measure mRNA in situ allows the localization of transmitters and receptors to neurons and/or axon terminals in the VTA. The application of intracellular electrophysiology to VTA tissue slices has permitted great strides towards understanding the influence of transmitters on dopamine cell function, as well as towards elucidating relative synaptic organization. Finally, the advent of in vivo dialysis has verified the effects of transmitters on dopamine and gamma-aminobutyric acid transmission in the VTA. Although reasonable estimates can be made of a single transmitter's actions under largely pharmacological conditions, our knowledge of how transmitters work in concert in the VTA to regulate the functional state of dopamine cells is only just emerging. The fact that individual transmitters can have seemingly opposite effects on dopaminergic function demonstrates that the actions of neurotransmitters in the VTA are, to some extent, state-dependent. Thus, different transmitters perform similar functions or the same transmitter may perform opposing functions when environmental circumstances are altered. Understanding the dynamic range of a transmitter's action and how this couples in concert with other transmitters to modulate dopamine neurons in the VTA is essential to defining the role of dopamine cells in the etiology and maintenance of neuropsychiatric disorders. Further, it will permit a more rational exploration of drugs possessing utility in treating disorders involving dopamine transmission.

Development and expression of sensitization to cocaine's reinforcing properties: role of NMDA receptors

Acquisition of cocaine self-administration (0.125, 0.25 or 0.5 mg/kg/infusion) was assessed in rats that had received prior exposure to either saline or amphetamine (2.0 mg/kg). Acquisition of self-administration was dose-dependent, with the highest dose leading to the shortest latency to reliably discriminate between depression of a lever that resulted in drug delivery and an inactive lever. Latency to acquisition of the lever discrimination for rats that had received prior exposure to amphetamine was shorter than for the saline-pretreated counterparts in each cocaine dosage group. This suggests that repeated exposure to this drug prior to self-administration testing sensitized the rats to the reinforcing effects of cocaine. Co-administration of MK-801 (0.25 mg/kg, IP), a non-competitive NMDA antagonist, blocked the ability of chronic exposure to amphetamine to sensitize rats to cocaine. In experienced self-administering rats, acute pretreatment with MK-801 resulted in a loss of discriminative responding. The number of inactive lever responses was consistently higher than the number of active lever responses across all cocaine dosage groups. These data suggest that the NMDA receptor, possibly through interactions with dopamine systems, is critical for both the development and expression of sensitization to cocaine's reinforcing effects produced by intermittent preexposures to amphetamine.

The effects of acute and repeated nicotine treatment on nucleus accumbens dopamine and locomotor activity

1. The effects of acute and subchronic nicotine and (+)-amphetamine on the extracellular levels of dopamine and its metabolites, dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in nucleus accumbens (NAc) have been studied in conscious, freely-moving rats by use of in vivo microdialysis. 2. In rats which had been habituated to the test apparatus for approximately 80 min, the acute subcutaneous (s.c.) administration of nicotine (0.1 or 0.4 mg kg-1) caused a dose-dependent increase (P less than 0.01) in spontaneous activity and evoked significant increases (P less than 0.05) in the extracellular levels of DOPAC and HVA. 3. Measurements made 24 h after the last injection of nicotine showed that pretreatment with the higher doses tested (0.4 mg kg-1) resulted in increased basal levels of dopamine (P less than 0.01) and decreased basal levels of DOPAC (P less than 0.05) in the NAc dialysates. 4. Pretreatment with nicotine (0.1 or 0.4 mg kg-1 daily for 5 days) enhanced the effects of the drug on spontaneous locomotor activity and enhanced the effects of the drug on extracellular levels of dopamine to the extent that the response became significant (P less than 0.05). 5. If a dopamine uptake inhibitor, nomifensine, was added to the Ringer solution used to dialyse the probe, the s.c. administration of both acute and subchronic nicotine (0.4 mg kg-1) resulted in significant increases (P less than 0.05) in the dopamine concentration in the dialysate. Under these conditions, pretreatment with nicotine prior to the test day prolonged (P less than 0.05) the dopamine response to a challenge dose of nicotine.6. Subcutaneous injections of (+)-amphetamine (0.2 or 0.5 mg kg-') evoked dose-dependent increases in both spontaneous activity and the concentration of dopamine in NAc dialysates. These responses were unaffected by 5 days pretreatment with the drug.7. The results of this study support the conclusion that the enhanced locomotor response to nicotine observed in animals pretreated with the drug prior to the test day is associated with potentiation of its effects on dopamine secretion in the NAc.

Preexposure to amphetamine and nicotine predisposes rats to self-administer a low dose of cocaine

The acquisition of low-dose (0.25 mg/kg/infusion) intravenous cocaine self-administration was measured in rats that had received nine daily injections of amphetamine (1.0 mg/kg, IP), nicotine (0.6 mg/kg base weight, SC) or vehicle. For control rats, the acquisition of self-administration was gradual with the number of responses per 2 h daily test session increasing between days 3 and 9. By comparison, rats preexposed with amphetamine and nicotine demonstrated elevated response means during the early days of testing, suggesting more rapid acquisition. All groups eventually reached similar asymptotic levels of responding. The enhanced responding observed during the early days of testing in the rats preexposed with amphetamine and nicotine was due to an increased number of subjects that reliably self-administered cocaine. Thus, the rats preexposed with amphetamine and nicotine seemed predisposed to the reinforcing effects of cocaine. In contrast to the self-administration data, preexposure to nicotine failed to sensitize rats to the locomotor activating effects of cocaine. In fact, the same preexposure regimen appeared to produce tolerance to this effect of cocaine. These data give evidence that different mechanisms may mediate sensitization to the reinforcing and locomotor activating effects of cocaine.