Environmental and pharmacological sensitization: effects of repeated administration of systemic or intra-nucleus accumbens cocaine

Exercise Pills for Drug Addiction: Forced Moderate Endurance Exercise Inhibits Methamphetamine-Induced Hyperactivity through the Striatal Glutamatergic Signaling Pathway in Male Sprague Dawley Rats

Physical exercise reduces the extent, duration, and frequency of drug use in drug addicts during the drug initiation phase, as well as during prolonged addiction, withdrawal, and recurrence. However, information about exercise-induced neurobiological changes is limited. This study aimed to investigate the effects of forced moderate endurance exercise training on methamphetamine (METH)-induced behavior and the associated neurobiological changes. Male Sprague Dawley rats were subjected to the administration of METH (1 mg/kg/day, i.p.) and/or forced moderate endurance exercise (treadmill running, 21 m/min, 60 min/day) for 2 weeks. Over the two weeks, endurance exercise training significantly reduced METH-induced hyperactivity. METH and/or exercise treatment increased striatal dopamine (DA) levels, decreased p(Thr308)-Akt expression, and increased p(Tyr216)-GSK-3β expression. However, the phosphorylation levels of Ser9-GSK-3β were significantly increased in the exercise group. METH administration significantly increased the expression of NMDAr1, CaMKK2, MAPKs, and PP1 in the striatum, and exercise treatment significantly decreased the expression of these molecules. Therefore, it is apparent that endurance exercise inhibited the METH-induced hyperactivity due to the decrease in GSK-3β activation by the regulation of the striatal glutamate signaling pathway.

Limonene Inhibits Methamphetamine-Induced Sensitizations via the Regulation of Dopamine Receptor Supersensitivity

Limonene is a cyclic terpene found in citrus essential oils and inhibits methamphetamine-induced locomotor activity. Drug dependence is a severe neuropsychiatric condition that depends in part on changes in neurotransmission and neuroadaptation, induced by exposure to recreational drugs such as morphine and methamphetamine. In this study, we investigated the effects of limonene on the psychological dependence induced by drug abuse. The development of sensitization, dopamine receptor supersensitivity, and conditioned place preferences in rats was measured following administration of limonene (10 or 20 mg/kg) and methamphetamine (1 mg/kg) for 4 days. Limonene inhibits methamphetamine-induced sensitization to locomotor activity. Expression of dopamine receptor supersensitivity induced by apomorphine, a dopamine receptor agonist, was significantly reduced in limonenepretreated rats. However, there was no significant difference in methamphetamine-induced conditioned place preferences between the limonene and control groups. These results suggest that limonene may ameliorate drug addiction-related behaviors by regulating postsynaptic dopamine receptor supersensitivity.

Unique Behavioral and Neurochemical Effects Induced by Repeated Adolescent Consumption of Caffeine-Mixed Alcohol in C57BL/6 Mice

The number of highly caffeinated products has increased dramatically in the past few years. Among these products, highly caffeinated energy drinks are the most heavily advertised and purchased, which has resulted in increased incidences of co-consumption of energy drinks with alcohol. Despite the growing number of adolescents and young adults reporting caffeine-mixed alcohol use, knowledge of the potential consequences associated with co-consumption has been limited to survey-based results and in-laboratory human behavioral testing. Here, we investigate the effect of repeated adolescent (post-natal days P35-61) exposure to caffeine-mixed alcohol in C57BL/6 mice on common drug-related behaviors such as locomotor sensitivity, drug reward and cross-sensitivity, and natural reward. To determine changes in neurological activity resulting from adolescent exposure, we monitored changes in expression of the transcription factor ΔFosB in the dopaminergic reward pathway as a sign of long-term increases in neuronal activity. Repeated adolescent exposure to caffeine-mixed alcohol exposure induced significant locomotor sensitization, desensitized cocaine conditioned place preference, decreased cocaine locomotor cross-sensitivity, and increased natural reward consumption. We also observed increased accumulation of ΔFosB in the nucleus accumbens following repeated adolescent caffeine-mixed alcohol exposure compared to alcohol or caffeine alone. Using our exposure model, we found that repeated exposure to caffeine-mixed alcohol during adolescence causes unique behavioral and neurochemical effects not observed in mice exposed to caffeine or alcohol alone. Based on similar findings for different substances of abuse, it is possible that repeated exposure to caffeine-mixed alcohol during adolescence could potentially alter or escalate future substance abuse as means to compensate for these behavioral and neurochemical alterations.

A Scutellaria baicalensis radix water extract inhibits morphine-induced conditioned place preference

CONTEXT

Scutellaria baicalensis Georgi (Lamiaceae) has been used as a traditional herbal preparation for the treatment of neuropsychiatric disorders in Asian countries for centuries.

OBJECTIVE

To evaluate the effects of S. baicalensis on morphine-induced drug dependence in rats.

MATERIALS AND METHODS

In order to evaluate the effect of S. baicalensis and baicalin on morphine-induced dependence-like behavior, a water extract of S. baicalensis [500 mg/kg, intraperitoneally (i.p.)] or baicalin (50 mg/kg, i.p., a flavonoid found in S. baicalensis) was administered prior to morphine injection [5 and 2.5 mg/kg, respectively, subcutaneously (s.c.)] to rats for 8 and 4 d, respectively. Morphine-induced conditioned place preference was assessed by measuring the time spent in a drug-paired chamber. The effect of S. baicalensis on dopamine receptor supersensitivity (locomotor activity) and dopamine agonist-induced climbing behavior due to a single apomorphine treatment (2 mg/kg, s.c.) was also measured.

RESULTS

At 50 mg/kg, a water extract of S. baicalensis decreased morphine (5 mg/kg)-induced conditioned place preference by 86% in rats. Apomorphine (2 mg/kg)-induced locomotor activity (dopamine receptor supersensitivity) in rats and climbing behavior in mice were attenuated after pretreatment with 500 mg/kg of S. baicalensis water extract by 41% and 56%, respectively. In addition, baicalin-reduced morphine-induced conditioned places preference by 86% in rats at 50 mg/kg.

DISCUSSION AND CONCLUSION

These results suggest that S. baicalensis can ameliorate drug addiction-related behavior through functional regulation of dopamine receptors.

L-tetrahydropalmatine inhibits methamphetamine-induced locomotor activity via regulation of 5-HT neuronal activity and dopamine D3 receptor expression

Methamphetamine (METH) is a psychomotor stimulant that produces hyperlocomotion in rodents. l-tetrahydropalmatine (l-THP) is an active ingredient found in Corydalis ternata which has been used as a traditional herbal preparation in Asian countries for centuries, however, the effect of l-THP on METH-induced phenotypes largely unknown. In this study, to evaluate the effect of l-THP on METH-induced psychotropic effects, rats were pretreated with l-THP (10 and 15 mg/kg) before acute METH injection, following which the total distance the rats moved in an hour was measured. To clarify a possible mechanism underlying the effect of l-THP on METH-induced behavioral changes, dopamine receptor mRNA expression levels in the striatum of the rats was measured following the locomotor activity study. In addition, the effect of l-THP (10 and 15 mg/kg) on serotonergic (5-HTergic) neuronal pathway activation was studied by measurement of 5-HT (80 μg/10μl/mouse)-induced head twitch response (HTR) in mice. l-THP administration significantly inhibited both hyperlocomotion in rats and HTR in mice. l-THP inhibited climbing behavior-induced by dopaminergic (DAergic) neuronal activation in mice. Furthermore, l-THP attenuated the decrease in dopamine D3 receptor mRNA expression levels in the striatum of the rats induced by METH. These results suggest that l-THP can ameliorate behavioral phenotype induced by METH through regulation of 5-HT neuronal activity and dopamine D3 receptor expression.

Limonene inhibits methamphetamine-induced locomotor activity via regulation of 5-HT neuronal function and dopamine release

Methamphetamine is a psychomotor stimulant that produces hyperlocomotion in rodents. Limonene (a cyclic terpene from citrus essential oils) has been reported to induce sedative effects. In this study, we demonstrated that limonene administration significantly inhibited serotonin (5-hydroxytryptamine, 5-HT)-induced head twitch response in mice. In rats, pretreatment with limonene decreased hyperlocomotion induced by methamphetamine injection. In addition, limonene reversed the increase in dopamine levels in the nucleus accumbens of rats given methamphetamine. These results suggest that limonene may inhibit stimulant-induced behavioral changes via regulating dopamine levels and 5-HT receptor function.

A silent synapse-based mechanism for cocaine-induced locomotor sensitization

Locomotor sensitization is a common and robust behavioral alteration in rodents whereby following exposure to abused drugs such as cocaine, the animal becomes significantly more hyperactive in response to an acute drug challenge. Here, we further analyzed the role of cocaine-induced silent synapses in the nucleus accumbens (NAc) shell and their contribution to the development of locomotor sensitization. Using a combination of viral vector-mediated genetic manipulations, biochemistry, and electrophysiology in a locomotor sensitization paradigm with repeated, daily, noncontingent cocaine (15 mg/kg) injections, we show that dominant-negative cAMP-element binding protein (CREB) prevents cocaine-induced generation of silent synapses of young (30 d old) rats, whereas constitutively active CREB is sufficient to increase the number of NR2B-containing NMDA receptors (NMDARs) at synapses and to generate silent synapses. We further show that occupancy of CREB at the NR2B promoter increases and is causally related to the increase in synaptic NR2B levels. Blockade of NR2B-containing NMDARs by administration of the NR2B-selective antagonist Ro256981 directly into the NAc, under conditions that inhibit cocaine-induced silent synapses, prevents the development of cocaine-elicited locomotor sensitization. Our data are consistent with a cellular cascade whereby cocaine-induced activation of CREB promotes CREB-dependent transcription of NR2B and synaptic incorporation of NR2B-containing NMDARs, which generates new silent synapses within the NAc. We propose that cocaine-induced activation of CREB and generation of new silent synapses may serve as key cellular events mediating cocaine-induced locomotor sensitization. These findings provide a novel cellular mechanism that may contribute to cocaine-induced behavioral alterations.

Alcohol-preferring (P) rats are more sensitive than Wistar rats to the reinforcing effects of cocaine self-administered directly into the nucleus accumbens shell

Wistar rats will self-administer cocaine directly into the nucleus accumbens shell (AcbSh), but not into the nucleus accumbens core. In human and animal literature, there is a genetic association between alcoholism and cocaine dependency. The current experiment examined whether selective breeding for high alcohol preference is also associated with greater sensitivity of the AcbSh to the reinforcing properties of cocaine. P and Wistar rats were given cocaine (0, 100, 200, 400, or 800 pmol/100 nl) to self-infuse into the AcbSh. Rats were given cocaine for the first 4 sessions (acquisition), artificial CSF for sessions 5 and 6 (extinction), and cocaine again in session 7 (reinstatement). During acquisition, P rats self-infused 200-800 pmol cocaine (59 infusions/session), whereas Wistar rats only reliably self-infused 800 pmol cocaine (38 infusions/session). Furthermore, P rats received a greater number of cocaine infusions in the 200, 400 and 800 pmol cocaine groups compared to respective Wistar groups during acquisition. Both P and Wistar rats reduced responding on the active lever when aCSF was substituted for cocaine, and reinstated responding in session 7 when cocaine was restored. However, P rats had significantly greater infusions during session 7 compared to session 4 at all concentrations of cocaine tested, whereas Wistar rats only displayed greater infusions during session 7 compared to session 4 at the 400 and 800 pmol cocaine concentrations. The present results suggest that, compared to Wistar rats, the AcbSh of P rats was more sensitive to the reinforcing effects of cocaine. The reinstatement data suggest that the AcbSh of P rats may have become sensitized to the reinforcing effects of cocaine. Overall, the findings from this study support a genetic association between high alcohol preference and greater sensitivity to the reinforcing effects of cocaine.

Parametric analysis of the development and expression of ethanol-induced behavioral sensitization in female Swiss mice: effects of dose, injection schedule, and test context

RATIONALE

Repeated administrations of ethanol induce a progressive and enduring increase in its locomotor stimulant effects, a phenomenon termed behavioral sensitization that has not been systematically characterized.

OBJECTIVE

The aim of the present studies was to characterize the development and expression of ethanol sensitization in female Swiss mice by examining (1) the doses of ethanol that induce behavioral sensitization, (2) the doses of acute ethanol challenges that are necessary to express behavioral sensitization, (3) the effects of the intervals between administrations, and (4) the context dependency of ethanol sensitization.

MATERIALS AND METHODS

Mice were i.p. injected for 8 days with various ethanol doses, and locomotion was recorded for 5 min. Two days after the last sensitization session, ethanol sensitization was tested in 30-min test sessions.

RESULTS

Mice repeatedly injected with 2.5 g/kg ethanol showed a progressive (200-300%) increase in locomotor activity. In response to a 2.5 g/kg ethanol challenge, the mice repeatedly treated with doses above 1.5 g/kg showed a significant sensitization. Following the induction of sensitization with the maximally effective sensitizing dose (2.5 g/kg), mice showed greater activation after challenges with 1.5, 2.0, 2.5, and 3.0 g/kg ethanol. The intervals (24, 48, or 96 h) between ethanol injections did not affect the induction or expression of sensitization. Finally, sensitization to 2.5 g/kg ethanol was expressed regardless of the context in which it was induced.

CONCLUSIONS

Female Swiss mice develop a robust context-independent sensitization after repeated ethanol injections at all doses above 1.5 g/kg, including highly sedative doses such as 4 g/kg.

Sleep rebound attenuates context-dependent behavioural sensitization induced by amphetamine

We have recently demonstrated that paradoxical sleep deprivation (PSD) potentiates the induction of amphetamine (AMPH)-induced behavioural sensitization by increasing its conditioned component. In the present study, the effects of sleep rebound (induced by 24 h recovery period from PSD) were studied on AMPH-induced behavioural sensitization. Sleep rebound attenuated the acute locomotor-stimulating effect of AMPH. AMPH-induced behavioural sensitization was context-specific and was also attenuated by sleep rebound. These results strengthen the notion that sleep conditions can influence AMPH-induced behavioural sensitization.

The endomorphin system and its evolving neurophysiological role

Endomorphin-1 (Tyr-Pro-Trp-Phe-NH2) and endomorphin-2 (Tyr-Pro-Phe-Phe-NH2) are two endogenous opioid peptides with high affinity and remarkable selectivity for the mu-opioid receptor. The neuroanatomical distribution of endomorphins reflects their potential endogenous role in many major physiological processes, which include perception of pain, responses related to stress, and complex functions such as reward, arousal, and vigilance, as well as autonomic, cognitive, neuroendocrine, and limbic homeostasis. In this review we discuss the biological effects of endomorphin-1 and endomorphin-2 in relation to their distribution in the central and peripheral nervous systems. We describe the relationship between these two mu-opioid receptor-selective peptides and endogenous neurohormones and neurotransmitters. We also evaluate the role of endomorphins from the physiological point of view and report selectively on the most important findings in their pharmacology.

Drug-induced home cage conspecifics' behavior can potentiate behavioral sensitization in mice

The effect of home cage conspecifics' behavior on locomotor sensitization to amphetamine (AMP) or ethanol (ETOH) were investigated. Female mice were repeatedly treated with saline or AMP (2.0 mg/kg for 13 days--Experiment 1) or saline or ETOH (1.8 g/kg for 21 days--Experiment 2) in home cages where all the animals had the same treatment (homogeneous home cages--HOM-HC) or in home cages where half of the animals were drug-treated and half of them were saline-treated (heterogeneous home cages--HET-HC). Behavioral sensitization was evaluated by the quantification of open-field locomotor activity after AMP or ETOH challenge injection, respectively. In both experiments, behavioral sensitization was potentiated in HOM-HC maintained animals. These results suggest that the behavioral sensitization phenomenon can be modified by home cage conspecifics' behavior.

Neurotensin receptor activation sensitizes to the locomotor stimulant effect of cocaine: A role for NMDA receptors

This study was aimed at determining whether repeated activation of neurotensin receptors sensitizes to cocaine-induced locomotor activity and whether this effect can be prevented by blockade of N-methyl-d-aspartate receptors. Independent groups of male rats were injected on four occasions, every other day (training phase), with vehicle or one of two doses (4 and 8 mg/kg) of the NMDA antagonist CPP [(+/-)-3-(2-carboxypiperazine-4-yl)-propanephosphonic)] followed by an intracerebroventricular injection of 18 nmol/10 microl of d-Tyr[(11)]neurotensin, or its vehicle. Ambulatory, non-ambulatory and vertical movements were measured for 2 h on every test day. One week after the last day of the training phase, locomotor responses to a single injection of cocaine (7.5 mg/kg, ip) were measured in all rats; a second cocaine challenge test was performed 3 weeks post-training. Results show that during the training phase d-Tyr[(11)]neurotensin produced an initial suppression of all locomotor responses followed by an augmentation of ambulatory and non-ambulatory activity compared to controls, effects that were only slightly altered by CPP. Cocaine produced higher ambulatory and non-ambulatory activity in animals pre-exposed to neurotensin than in the vehicle pre-exposed animals, a sensitization effect that was not prevented by CPP at 1 week post-training but that was blocked at 3 weeks at the high dose. When given alone, the low dose of CPP produced an effect very similar to that of neurotensin on cocaine sensitization. These results further confirm that neurotensin plays a role in sensitization to psychostimulant drugs and suggests that NMDA receptors are involved in the long-term effect of exposure to neurotensin.

Role of the ERK pathway in psychostimulant-induced locomotor sensitization

BACKGROUND

Repeated exposure to psychostimulants results in a progressive and long-lasting facilitation of the locomotor response that is thought to have implications for addiction. Psychostimulants and other drugs of abuse activate in specific brain areas extracellular signal-regulated kinase (ERK), an essential component of a signaling pathway involved in synaptic plasticity and long-term effects of drugs of abuse. Here we have investigated the role of ERK activation in the behavioral sensitization induced by repeated administration of psychostimulants in mice, using SL327, a brain-penetrating selective inhibitor of MAP-kinase/ERK kinase (MEK), the enzyme that selectively activates ERK.

RESULTS

A dose of SL327 (30 mg/kg) that reduced the number of activated ERK-positive neurons by 62 to 89% in various brain areas, had virtually no effect on the spontaneous locomotor activity or the acute hyperlocomotion induced by cocaine or D-amphetamine. Pre-treatment with SL327 (30 mg/kg) prior to each drug administration prevented the locomotor sensitization induced by repeated injections of D-amphetamine or cocaine. The SL327 pre-treatment abolished also conditioned locomotor response of mice placed in the context previously paired with cocaine or D-amphetamine. In contrast, SL327 did not alter the expression of sensitized response to D-amphetamine or cocaine.

CONCLUSION

Altogether these results show that ERK has a minor contribution to the acute locomotor effects of psychostimulants or to the expression of sensitized responses, whereas it is crucial for the acquisition of locomotor sensitization and psychostimulant-conditioned locomotor response. This study supports the important role of the ERK pathway in long-lasting behavioral alterations induced by drugs of abuse.

D3 dopamine receptor, behavioral sensitization, and psychosis

Behavioral sensitization is a progressive, enduring enhancement of behaviors that develops following repeated stimulant administration. It is mediated in part by dopaminergic pathways that also modulate a number of psychiatric conditions including the development of psychosis. We propose that down-regulation of D3 dopamine receptor function in critical brain regions contributes to sensitization. Rodent locomotion, a sensitizable behavior, is regulated by the opposing influence of dopamine receptor subtypes, with D3 stimulation opposing concurrent D1 and D2 receptor activation. The D3 dopamine receptor has a 70-fold greater affinity for dopamine than D1 or D2 dopamine receptors. This imbalance in ligand affinity dictates greater occupancy for D3 than D1 or D2 receptors at typical dopamine concentrations following stimulant drug administration, resulting in differences in the relative tolerance at D3 vs D1 and D2 receptors. Sensitization may therefore result in part from accommodation of the inhibitory D3 receptor 'brake' on D1/D2 mediated behaviors, leading to a progressive locomotion increase following repeated stimulant exposure. The requirement for differential tolerance at D3 vs D1 and D2 receptors may explain the observed development of sensitization following application of cocaine, but not amphetamine, directly into nucleus accumbens. If correct, the 'D3 Dopamine Receptor Hypothesis' suggests D3 antagonists could prevent sensitization, and may interrupt the development of psychosis when administered during the prodromal phase of psychotic illness. Additional study is needed to clarify the role of the D3 dopamine receptor in sensitization and psychosis.

Effects of repeated GBR 12909 administration on brain stimulation reward

Male rats were trained at three separate currents to bar press for intracranial self-stimulation. On days 1 and 15, all subjects were given 1-(2-bis(4-fluorophenyl)-methoxy)-ethyl-4-(3-phenylpropyl) piperazine, also known as GBR 12909 (10 mg/kg, i.p.), prior to test session. Between these days, the paired Chronic-before group was injected (every other day) with GBR 12909 prior to intracranial self-stimulation, while unpaired, Chronic-after group was given the drug just after the end of the session. A third group (Control) received saline injections (i.p.) 20 min following the session. Although GBR 12909 was found to be reward enhancing, neither sensitization nor tolerance developed to the rewarding and performance/motor effects regardless of the injection regimen. In addition, the rewarding effects of intracranial self-stimulation were found to be independent of both current and environment-specific pairing. The present data obtained for GBR 12909 agree with previous observations of the effects of repeated administration of drugs of abuse on intracranial self-stimulation.

Differential effects of endomorphin-1 and -2 on amphetamine sensitization: neurochemical and behavioral aspects

Mu-opioid receptors are known to modulate mesolimbic dopaminergic activity in the ventral tegmental area via disinhibition of GABA-containing neurons. Recently, two novel tetrapeptides, endomorphin-1 and endomorphin-2, were identified in the mammalian brain and reported to have high binding affinities toward mu-opioid receptors. To determine if endomorphins would modulate the development of amphetamine sensitization, we administered endomorphins locally into the rat brain followed by behavioral and neurochemical examinations. The results indicate that rats pretreated with endomorphin-1 or -2 (5 microg per side for 7 days) in the ventral tegmental area developed locomotor sensitization to the challenge injection of amphetamine (1 mg/kg). On the other hand, when endomorphins were given in the lateral ventricle (20 microg for 5 days) of amphetamine-sensitized rats (5 mg/kg x 14 days) during the withdrawal period (w5-w9), neither peptide had a modulatory effect on locomotor sensitization. Biochemical analyses revealed that treatment with endomorphins in the ventral tegmental area significantly increased the levels of glutamate in the medial prefrontal cortex and ventral and dorsal striatum to levels comparable to those observed in the amphetamine-sensitized rats. In the same animals, endomorphins also caused decreases in the levels of serotonin and its metabolite, 5-hydroxyindoleacetic acid, in the medial prefrontal cortex. Interestingly, although there was no behavioral significance, endomorphin-1 treatment in the lateral ventricle of control and amphetamine-sensitized rats during withdrawal resulted in decreases of GABA, aspartate, dopamine, and its metabolite 3,4-dihydroxyphenylacetic acid in the ventral striatum. We conclude that endomorphins, by stimulating the mu-opioid receptors in the ventral tegmental area, could sensitize the behavioral response to amphetamine. The results also demonstrate that there are differential responses between endomorphin-1 and -2 on behavioral amphetamine sensitization and the underlying neurochemical substrates.

Effects of cocaine context on NAcc dopamine and behavioral activity after repeated intravenous cocaine administration

In two conditioning experiments, identical procedures (previously shown to produce place preferences for a cocaine-paired environment) were used to assess dopaminergic and behavioral activity correlates of cocaine reward conditioning and sensitization. In these experiments, animals received repeated injections of intravenous cocaine (4.2 mg/kgx6) or saline (0.2 mlx6) on alternating days. One group in each of these experiments ('Cocaine Cues') occupied a consistent distinctive environment during cocaine treatments and testing sessions. For the other conditioned group ('Novel'), all procedures were the same, except that the last cocaine injection was administered while animals were occupying a novel environment. During day 1 and day 6 of the cocaine treatment, behavioral activity was assessed in experiment 1 and in vivo microdialysis procedures were conducted in experiment 2. Over the course of the conditioning sessions, cocaine-induced behavioral activity (locomotion and rearing) increased significantly in the Cocaine Cues group, but not in the Novel group. In addition, cocaine-induced increases in NAcc dopamine levels were significantly greater when cocaine-experienced animals were tested in a cocaine-paired environment compared to equally experienced and cocaine-naive animals tested in a novel environment. Context-dependent behavioral sensitization is a well-documented phenomenon. The observation of a corresponding enhancement of dopamine efflux in lieu of a lengthy withdrawal period is uncommon, but can be attributed to methodological differences across studies. The present study uniquely demonstrates concurrent context-dependent potentiation of behavioral and dopaminergic responses to cocaine occurring in conjunction with cocaine reward.

Sensitization of anorexia and locomotion induced by chronic administration of ephedrine in rats

Repeated daily administration of the sympathomimetic agent ephedrine (EPH) leads to an augmentation (sensitization) of locomotor activity in rats. The present experiments examined the impact of repeated administration of the (-)- and (+)-EPH enantiomers on feeding in rats to assess whether the anorexic activity of EPH exhibits tolerance or sensitization during chronic exposure and whether the time course of these effects follows that observed in studies of locomotion. Adult male rats were injected once daily for 12 days with either vehicle or 5, 10 or 20 mg/kg (-)-EPH or with 10 or 20 mg/kg (+)-EPH. Horizontal locomotion and diet consumption were assessed for 60 min in an activity chamber. Suppression of feeding and the induction of locomotion were augmented over the first four days of administration of either 10 mg/kg or 20 mg/kg of the (-)-EPH enantiomer. In contrast, repeated administration of 20 mg/kg (+)-EPH resulted in augmentation of appetite suppression but not locomotion. These results confirm and extend the phenomenon of locomotor and feeding sensitization for ephedrine, but suggest that these effects may differ for the two enantiomers of ephedrine.

Cocaine preexposure fails to sensitize the acquisition of cocaine-induced taste aversions

In two separate experiments, rats were given either an intraperitoneal (IP) injection of 10 mg/kg cocaine once a day for 10 consecutive days (Experiment 1) or a single IP injection of 40 mg/kg of cocaine (Experiment 2) prior to receiving repeated pairings of a novel saccharin solution with cocaine (32 mg/ kg; subcutaneous; SC). Although vehicle-preexposed subjects given saccharin-cocaine pairings readily acquired an aversion to the cocaine-associated saccharin solution, subjects preexposed to cocaine (whether 10 times or only once) displayed a retarded acquisition of the aversion. That is, cocaine preexposure attenuated the acquisition of cocaine-induced taste aversions. There was no difference in the degree of attenuation between the two preexposure conditions. Thus, under conditions that are effective in inducing sensitization within other behavioral preparations there was no evidence of sensitized cocaine-induced taste aversions. The results from the present investigation are similar to reports from this laboratory and others demonstrating that preexposure to cocaine, as with a range of other psychoactive drugs, results in weaker taste aversions. The basis for the attenuating effects of cocaine preexposure was discussed in terms of an adaptation to the aversive effects of cocaine.

Behavioural topography in the striatum: differential effects of quinpirole and D-amphetamine microinjections

Behavioural evidence has accumulated that supports the hypothesis that specific territories of the striatum contribute differentially to the control of motor behaviours. The present experiments compare the behavioural effects of microinjections of amphetamine (20 microg/0.5 microl) with those elicited by the D2-class dopamine receptor agonist quinpirole (3 microg/0.5 microl) following direct microinjection into three anatomically distinct sectors of the striatum: the nucleus accumbens, the ventrolateral striatum and the anterodorsal striatum. Our findings demonstrate that site-specific behavioural responses are induced by microinjections of amphetamine, but not of quinpirole, into the striatum. Our results suggest that widespread areas of the striatum are implicated in the induction of a syndrome of sedation, yawning and motor inhibition, observed readily following microinjections of quinpirole into the striatum. This evidence supports both homogeneity and segregation of function in the striatum at the behavioural level. Further, the results suggest that the elicitation of site-specific action sequences at the level of the striatum seems to require cooperative interactions between D1-class and D2-class dopamine receptors.

Repeated injection of GBR 12909, but not cocaine or WIN 35,065-2, into the ventral tegmental area induces behavioral sensitization

A role for the mesolimbic dopamine system in the development of behavioral sensitization to psychostimulants, such as cocaine and amphetamine, is well established. Previous reports have suggested that the ventral tegmental area (VTA) is involved in the initiation of, while the nucleus accumbens is in involved in the expression of behavioral sensitization. This hypothesis is supported in part, by studies which demonstrated that behavioral sensitization could be induced by repeated intra-VTA, but not intra-accumbal, administration of amphetamine. The present studies were designed to determine whether repeated intra-VTA cocaine would similarly induce behavioral sensitization. Rats receiving four daily injections of cocaine (1.5, 5 or 15 nmol/side) into the VTA did not show a sensitized behavioral response when challenged with cocaine (15 mg/kg, ip) 1 week later. In contrast to this, repeated injection of the specific dopamine reuptake inhibitor, GBR 12909 (15 nmol/side) produced behavioral sensitization to a challenge injection of cocaine. Repeated injections of the cocaine analog WIN 35,065-2 did not induce behavioral sensitization to cocaine, suggesting that the local anesthetic properties of cocaine were not responsible for the inability of intra-VTA cocaine to induce sensitization. In summary, the data suggest that sensitization to cocaine may involve mechanisms different from amphetamine.

Localization of genes mediating acute and sensitized locomotor responses to cocaine in BXD/Ty recombinant inbred mice

Sensitization to the psychostimulant effects of cocaine has received widespread attention because concomitant changes occur in neurochemical pathways that are part of the brain reward pathway. The current study was undertaken with the purpose of mapping genes determining sensitivity to the acute stimulant and sensitizing effects of cocaine. Sensitivity and sensitization to cocaine (5, 10, and 40 mg/kg) were measured in 25 BXD/Ty recombinant inbred (BXD RI) strains and the progenitor C57BL/6J (B6) and DBA/2J (D2) strains. Quantitative trait locus (QTL) mapping provisionally localized cocaine sensitivity genes to regions on all chromosomes except 6, 11, 17, and X; sensitization QTLs were localized to chromosomes 1-10, 13, 15, 18, 19, and X. Provisional QTLs for locomotion after saline injection in a novel setting were mapped to chromosomes 1, 3-6, 9, 12, 13, 18, and 19 and in a familiar setting to chromosomes 4-7, 9, 13, and 19. There were both common and unique QTL regions across the phenotypes. Evidence for a genetic association between magnitude of acute cocaine response and sensitization was obtained for only the 10 mg/kg dose. Some common QTL regions for cocaine, ethanol, and methamphetamine responses suggest the possibility that these drugs induce stimulant effects or sensitization through some common mechanisms. However, independent mechanisms were also indicated. Many candidate genes reside near the provisional QTLs mapped for cocaine responses, including genes coding a variety of neurotransmitter and hormone receptors. These data, once confirmed, should prove useful for directing investigations of acute and chronic cocaine effects down already suspected and novel avenues.

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.

A circuitry model of the expression of behavioral sensitization to amphetamine-like psychostimulants

Repeated exposure to psychostimulants such as cocaine and amphetamine produces behavioral sensitization, which is characterized by an augmented locomotor response to a subsequent psychostimulant challenge injection. Experimentation focused on the neural underpinnings of behavioral sensitization has progressed from a singular focus on dopamine transmission in the nucleus accumbens and striatum to the study of cellular and molecular mechanisms that occur throughout the neural circuitry in which the mesocorticolimbic dopamine projections are embedded. This research effort has yielded a conglomerate of data that has resisted simple interpretations, primarily because no single neuronal effect is likely to be responsible for the expression of behavioral sensitization. The present review examines the literature and critically evaluates the extent to which the neural consequences of repeated psychostimulant administration are associated with the expression of behavioral sensitization. The neural alterations found to contribute to the long-term expression of behavioral sensitization are centered in a collection of interconnected limbic nuclei, which are termed the 'motive' circuit. This neural circuit is used as a template to organize the relevant biochemical and molecular findings into a model of the expression of behavioral sensitization.

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.

Neonatal and preweanling rats are able to express short-term behavioral sensitization to cocaine

The present study assessed the ability of suckling rats to express short-term behavioral sensitization to cocaine prior to weaning. Rat pups, aged either 3, 5, 10, 12, 17 or 19 days at the beginning of the experiment, were placed in a chamber after daily injection with cocaine (7.5 or 15 mg/kg. i.p.) for either 2 or 4 consecutive days, and were tested for behavioral responsiveness to cocaine in the same chamber 24 h later (at either 7, 14 or 21 days of age). Such a short post-treatment interval was adopted, along with a consistent pairing of the testing context with the drug effect and a sensitive technique of behavioral measurement (video recording), in order to maximize the possibility of detecting any cocaine sensitization. Locomotion was sensitized at all ages, after both regimens in 14-day-old pups, but solely after 2 injections in 21- and 4 injections in 7-day-old pups. Sensitization was also expressed via behaviors specific to each age. Four cocaine injections augmented cocaine-induced uncoordinated movements of head, paws and body (horizontal activity) in 7-day-old pups, and mouth movements in 14-day-old pups. In 21-day-old pups, sensitization was dose- and regimen-dependently expressed via adult-like stereotyped head movements. In neonatal 7-day-old pups, cocaine sensitization was also visible as reductions in immobility (both injection regimens). Contrary to previous studies, these results indicate that, given the use of an appropriate methodology, short-term sensitization to the motoric effects of cocaine can be expressed by suckling rats prior to weaning, even after relatively short regimens of daily injections.

Long-term monitoring of extracellular dopamine concentration in the rat striatum by a repeated microdialysis procedure

This study examined a protocol for repeated measurement of the extracellular dopamine (DA) concentration in the rat striatum by microdialysis. Rats were implanted with a guide cannula in the striatum and the probe was inserted on each dialysing day, i.e. ten times over a 23 day period. During this period the animals were submitted to a control saline treatment. DA concentration was measured using the no-net-flux method. In these conditions, DA concentration remained remarkably constant over the 23 day period. The histological analysis using glial fibrillary acidic protein (GFAP), dopamine (DA) and tyrosine hydroxylase (TH) immunocytochemistry showed a moderate gliosis and a discrete increase of immunoreactivity of catecholaminergic fibres around the probe implantation site. This increase is probably related to a plasticity of the dopaminergic system in response to the lesion due to the probe implantations. This study shows that such a paradigm makes possible to measure the whole time course of the DA concentration in the rat striatum during chronic treatments with psychoactive drugs such cocaine or other compounds acting in the nigrostriatal system.

Systemic cocaine challenge after chronic cocaine treatment reveals sensitization of extracellular dopamine levels in nucleus accumbens but direct cocaine perfusion into nucleus accumbens does not: implications for the neural locus of cocaine sensitization

Rats were treated chronically with 20 mg/kg/day cocaine (by intraperitoneal injection) for 16 days, followed by 7 days of cocaine wash-out. On the next day, rats were challenged with an acute dose of cocaine administered by one of two routes (systemic or intracranial), and extra-cellular dopamine (DA) in the nucleus accumbens (Acb) was measured by in vivo microdialysis. Rats acutely challenged systemically with 20 mg/kg cocaine showed enhanced Acb extracellular DA levels (compared to control rats that had not previously received chronic cocaine). However, rats acutely challenged with intracranial cocaine by perfusion of 10(-5) M cocaine directly into the Acb did not. It is suggested that both the development and triggering of cocaine sensitization, as manifested by enhanced Acb DA content to subsequent acute cocaine challenge, may involve more than just neural mechanisms occurring locally within the Acb.

Modulation of drug-induced sensitization processes by endogenous opioid systems

Behavioural sensitization involves progressive increases in behavioural responses to repeated intermittent administration of drugs of abuse. Behavioural sensitization is observed to the locomotor stimulant, rewarding and discriminative effects of a drug. These are effects which seem to be essential in the initiation, expression and maintenance of a drug-seeking behaviour. Therefore the phenomenon of behavioural sensitization may have important implications for the understanding of addictive processes. Findings given in this review demonstrate the involvement of endogenous opioid systems in the initiation of sensitized responses on the neurochemical level, i.e., within the mesolimbic dopaminergic system, as well as on the behavioural level. Specifically, it is shown that behavioural sensitization to morphine and cocaine is modulated by endogenous kappa-opioid systems.

Evidence of a complete independence of the neurobiological substrates for the induction and expression of behavioral sensitization to amphetamine

The repeated administration of amphetamine in rats produces behavioral sensitization which is characterized either by a progressive enhancement of the locomotor activity induced by the drug or by an enduring behavioral hypersensitivity to the drug after the cessation of the treatment. Some authors have suggested that the action of amphetamine at the level of the nucleus accumbens is responsible for the expression of behavioral sensitization, whereas the action of amphetamine at the level of the dopamine cell bodies in the ventral tegmental area induces some changes responsible for the initiation of the phenomenon. The present study fully tested this hypothesis. In two separate experiments, the effects of different doses of amphetamine repeatedly administered in the ventral tegmental area or in the nucleus accumbens were tested on the later behavioral reactivity to the administration of amphetamine in the nucleus accumbens. Independent groups of rats received five repeated administrations (one injection every other day) of different doses of amphetamine either in the ventral tegmental area (0, 1, 2.5, 5 micrograms/0.5 microliters per side) or in the nucleus accumbens (0, 1, 3, 10 micrograms/l microliters per side). Two days following the last intracerebral amphetamine injection, each group received a phosphate buffer solution challenge directly into the nucleus accumbens followed two days later by an amphetamine challenge (1 microgram/l microliters per side) in the nucleus accumbens and two days later by a peripheral challenge with amphetamine (0.5 mg/kg, s.c.). Locomotor responses were recorded following each injection. Results showed that injections of amphetamine into the nucleus accumbens induced a dose-dependent increase in locomotor activity which remained identical with the repetition of the injections. No difference between the different intra-accumbens pretreated groups was observed following the diverse phosphate-buffered saline solution and amphetamine challenges. In contrast, intra-ventral tegmental area administration of amphetamine did not produce any modification of locomotor activity. However, whereas no difference between the differently pretreated groups was observed following phosphate-buffered saline administration into the nucleus accumbens, a potentiation of the locomotor response to a challenge dose of amphetamine into the nucleus accumbens was observed which was dependent on the dose of amphetamine pretreatment into the ventral tegmental area. Similar potentiation was observed following peripheral challenge with amphetamine. Finally, cross-sensitization was observed when a challenge dose of cocaine (10 micrograms/1 microliter per side) was injected into the nucleus accumbens, as well as when a peripheral challenge dose of morphine (2.5 mg/kg, s.c.) was administered to the ventral tegmental area-pretreated groups.(ABSTRACT TRUNCATED AT 400 WORDS)

Cocaine preexposure sensitizes conditioned fear in a potentiated acoustic startle paradigm

The consequences of chronic cocaine administration on fear-potentiated startle were evaluated in two experiments. Cocaine treatment (40 mg/kg) for 7 days prior to fear acquisition (light + shock pairings) had an attenuating influence on the ability of the conditioned stimulus (CS) to increase acoustic startle. When cocaine was administered in the context of the CS, following fear conditioning, a marked enhancement of potentiated startle was observed. In contrast, an extinction of the fear response was seen in saline and procaine animals repeatedly exposed to the nonreinforced CS. The results from control subjects injected with cocaine either in the shock chambers (contextual cues) or in their home cage environment, suggest that the systemic effects of this stimulant served to intensify the fear-eliciting properties acquired by the CS during fear conditioning. These findings demonstrate a cocaine sensitization of conditioned fear, and were related to the emotional and psychological disturbances associated with long-term cocaine use.

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)

Animals predisposed to develop amphetamine self-administration show higher susceptibility to develop contextual conditioning of both amphetamine-induced hyperlocomotion and sensitization

It has been shown that rats, like humans, display individual differences in the propensity to develop psychostimulant self-administration. Animals showing the highest locomotor reactivity to novelty (HRs: High Responders) are more prone to develop amphetamine self-administration than rats having a low locomotor response to novelty (LRs: Low Responders). The present study was designed to ascertain whether individual differences are also present in the conditioning of drug effects, a process involved in the maintenance of addiction. After pairing the drug effect with a particular set of environmental cues, only HRs showed conditioned hyperlocomotion and environment-specific sensitization to the effect of amphetamine. Unconditioned sensitization was, however, observed in LRs but not in HRs. The environment-specific sensitization disappeared on extinction of the conditioned hyperlocomotion in HRs, indicating that conditioning facilitates the expression of sensitization. In contrast, an inhibitory influence of conditioning on sensitization emerged from the analysis of the same results over all the experimental groups, without taking individual differences into account. In conclusion, our results show that: (i) locomotor reactivity to novelty predicts both vulnerability to develop self-administration and contextual conditioning of drug effects, which suggests that the two phenomena are two related features and that conditioning plays an important role not only in the maintenance of drug intake but also in its development; (ii) conditioned and unconditioned sensitization can be developed separately in different individuals which suggests that they are independent phenomena; (iii) analysis of individual differences is relevant to pharmacological studies, especially with respect to drugs of abuse.

A ketamine mixture anesthetic inhibits neuroendocrine and behavioral consequences of cocaine administration

Cocaine is known to affect different brain systems, particularly those associated with arousal, motor and motivational functions. In order to identify a possible neurochemical link among these systems, we investigated the effects of the non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist and dissociative anesthetic, ketamine (as a mixture with the sedatives acepromazine and xylazine) on the secretion of pituitary adrenocorticotropin hormone (ACTH) and on the development of behavioral sensitization induced by cocaine. Pretreatment with the ketamine anesthetic mixture (1.6 ml/kg; s.c.) completely blocked the stimulation of ACTH by cocaine (5 mg/kg, i.v.; administered 30 min after the ketamine mixture) without interfering with ACTH secretion induced by exogenous corticotropin-releasing factor (CRF; 5 micrograms/kg; i.v.) or interleukin-1 beta (IL-1 beta; 100 ng/kg; i.v.). Administration of the ketamine mixture prior to each of five repeated cocaine injections (15 mg/kg; i.p.) also completely reversed the behavioral sensitization observed in saline-treated control animals. Administration of the anesthetic mixture did not appear to impair the dopamine (DA) re-uptake blocking properties of cocaine in the nucleus accumbens since substantial increases in extracellular DA were observed in the presence of the ketamine mixture. In addition to the present results, no behavioral sensitization was also observed in rats anesthetized with a different general anesthetic (pentobarbital, 50 mg/kg) under similar conditions to that of the ketamine mixture. Taken together, these results are in accordance with the hypothesis that stimulation of excitatory amino acid receptor function may be just one of the mechanisms whereby cocaine exerts its effects on neuroendocrine and behavioral activating systems.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of cocaine microinjections into the nucleus accumbens and medial prefrontal cortex on schedule-induced behaviour: comparison with systemic cocaine administration

The effects of cocaine HCl infusions into either the nucleus accumbens (NACC) or medial prefrontal cortex (PFC) were compared on the performance of schedule-induced polydipsia (SIP) and related behaviours. Food-deprived rats were exposed to a fixed-time 60-s schedule of food delivery in daily 30-min sessions until stable levels of behaviour were obtained (14 days). Rats were then bilaterally infused with cocaine into either the NACC or PFC via chronically indwelling guide cannulae. Each subject received a sequence of five cocaine infusions (0, 12.5, 25, 50, 100 micrograms) according to a Latin Square design. For comparison, following these intracranial infusions each rat received a sequence of five IP injections of cocaine (0, 2.5, 5, 10, 20 mg/kg) also in a counterbalanced order. NACC and PFC infusions of cocaine and IP cocaine dose-dependently reduced SIP. Cocaine infusions into the NACC, but not the PFC, increased locomotor activity but the characteristic temporal profile of locomotor activity during SIP was retained. IP cocaine also increased locomotor activity in a dose-dependent manner, but the temporal profile of activity was flattened following 20 mg/kg cocaine. NACC and PFC infusions of cocaine had little effect on the total number of panel presses to gain access to the food pellets, but did slightly decrease the high rates of responding immediately prior to the pellet delivery. IP cocaine increased the total number of panel presses at the higher doses, mainly by increasing the low rates of responding. The effects of cocaine infusions into the PFC were behaviourally the most selective, as they reduced SIP without having substantial effects either on locomotor activity or panel pressing.(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in dopamine-dependent electrochemical signal in the nucleus accumbens associated with repeated cocaine injections in rats

High-speed chronoamperometry with Nafion-coated monoamine-sensitive electrodes was used to evaluate changes in the extracellular dopamine (DA) concentrations in the nucleus accumbens following each of a series of daily cocaine injections (15 mg/kg, i.p.; 5 days and challenge 3-4 days later) in freely moving rats. The first drug injection caused triphasic changes in the DA-dependent electrochemical signal. An initial transitory decrease in signal (equivalent of -45 nM DA) was followed by a slow increase (from 16 to 100 min; +155 nM) than a subsequent long-term and profound decrease (2-4 h; -100 to 200 nM). Subsequent daily injections did not cause an initial decrease; immediately after cocaine injection the signal increased rapidly to reach a peak of approximately 150-200 nM. The initial component of this signal increase (2-6 min) was more powerful with repeated daily injections, while the amplitude and duration of the signal increase were similar. Similar changes in locomotion were seen following repeated cocaine injections: movement activation occurred with shorter latency and reached a peak earlier, but total amount of counts was relatively stable. The gradual enhancement of the initial components of the electrochemical signal increase following repeated drug injections suggests that the mechanisms of conditioning contribute to the changes of mesolimbic DA accompanying development of cocaine sensitization.

Assessment of the relative contribution of peripheral and central components in cocaine place conditioning

A balanced place conditioning paradigm was used to assess the contribution of peripheral and central factors mediating place conditioning induced by cocaine HCl. The first experiment was conducted to examine changes in locomotor activity and extracellular dopamine (DA) concentrations in the nucleus accumbens (NACC) following intraperitoneal (IP) injections of cocaine HCl (15 mg/kg) or cocaine methiodide (19.6 mg/kg). IP cocaine HCl significantly increased locomotor activity and extracellular NACC DA, whereas IP cocaine methiodide failed to increase either locomotor activity or extracellular DA in the NACC. In the second experiment, IP cocaine HCl (15 mg/kg) induced a significant conditioned place preference; however, neither IP procaine HCl (25 or 50 mg/kg) nor IP cocaine methiodide (4.9, 9.8, or 19.6 mg/kg) induced preferences for the drug-paired compartment. In the third experiment, intracerebroventricular (ICV) infusions of cocaine HCl (25 micrograms/2 microliters) or cocaine methiodide (1 or 5 micrograms/2 microliters) induced significant place conditioning for the drug-paired compartment. These results suggest place conditioning induced by cocaine HCl is mediated centrally and that the local anaesthetic properties alone do not contribute to this effect to any significant degree.