Cue-induced Cocaine-seeking Behavior Research Articles

MGlu5 inhibition in the basolateral amygdala prevents estrous cycle-dependent changes in cue-induced cocaine seeking

Drug associated cues are a common relapse trigger for individuals recovering from cocaine use disorder. Sex and ovarian hormones influence patterns of cocaine use and relapse vulnerability, with studies indicating that females show increased cue-induced craving and relapse vulnerability compared to males. In a rodent model of cocaine craving and relapse vulnerability, cue-induced cocaine seeking behavior following weeks of withdrawal from extended-access cocaine self-administration is higher in females in the estrus stage of the reproductive (estrous) cycle (Estrus Females) compared to both Males and females in all other stages (Non-Estrus Females). However, the neuronal substrates and cellular mechanisms underlying these sex differences is not fully understood. One region that contributes to both sex differences in behavioral responding and cue-induced cocaine seeking is the basolateral amygdala (BLA), while one receptor known to play a critical role in mediating cocaine seeking behavior is metabotropic glutamate receptor 5 (mGlu5). Here we assessed the effects of BLA mGlu5 inhibition following prolonged withdrawal from cocaine self-administration on observed estrous cycle-dependent changes in cue-induced cocaine seeking behavior. We found that BLA microinjections of the mGlu5 antagonist MTEP selectively reduced the enhanced cue-induced cocaine seeking normally observed in Estrus Females while having no effect on cocaine seeking in Males and Non-Estrus Females. These findings identify a unique interaction between cocaine-exposure, estrous cycle fluctuations and BLA mGlu5-dependent transmission on cue-induced cocaine seeking behavior.

Enhancement of the GluN2B subunit of glutamatergic NMDA receptors in rat brain areas after cocaine abstinence.

Cocaine use disorder is associated with compulsive drug-seeking and drug-taking, whereas relapse may be induced by several factors, including stress, drug-related places, people, and cues. Recent observations strongly support the involvement of the N-methyl-D-aspartate (NMDA) receptors in cocaine use disorders and abstinence, whereas withdrawal in different environments may affect the intensification of relapse. The aim of this study was to examine the GluN2B subunit expression and its association with the postsynaptic density protein 95 (PSD95) in several brain structures in rats with a history of cocaine self-administration and housed either in an enriched environment or in an isolated condition. Furthermore, a selective antagonist of the GluN2B subunit-CP 101,606 (10 and 20 mg/kg) administered during exposure to cocaine or a drug-associated conditional stimulus (a cue) was used to evaluate seeking behavior in rats. In rats previously self-administering cocaine, we observed an increase in the GluN2B expression in the total homogenate from the dorsal hippocampus under both enriched environment and isolation. Cocaine abstinence under isolation conditions increased the GluN2B and GluN2B/PSD95 complex levels in the PSD fraction of the prelimbic cortex in rats previously self-administering cocaine. Administration of CP 101,606 attenuated cue-induced cocaine-seeking behavior only in isolation-housed rats. In summary, in this study we showed region-specific changes in both the expression of GluN2B subunit and NMDA receptor trafficking during cocaine abstinence under different housing conditions. Furthermore, we showed that the pharmacological blockade of the GluN2B subunit may be useful in attenuating cocaine-seeking behavior.

Accumbens D2-MSN hyperactivity drives antipsychotic-induced behavioral supersensitivity

Antipsychotic-induced dopamine supersensitivity, or behavioral supersensitivity, is a problematic consequence of long-term antipsychotic treatment characterized by the emergence of motor abnormalities, refractory symptoms, and rebound psychosis. The underlying mechanisms are unclear and no approaches exist to prevent or reverse these unwanted effects of antipsychotic treatment. Here we demonstrate that behavioral supersensitivity stems from long-lasting pre, post and perisynaptic plasticity, including insertion of Ca2+-permeable AMPA receptors and loss of D2 receptor-dependent inhibitory postsynaptic currents (IPSCs) in D2 receptor-expressing medium spiny neurons (D2-MSNs) in the nucleus accumbens core (NAcore). The resulting hyperexcitability, prominent in a subpopulation of D2-MSNs (21%), caused locomotor sensitization to cocaine and was associated with behavioral endophenotypes of antipsychotic treatment resistance and substance use disorder, including disrupted extinction learning and augmented cue-induced cocaine-seeking behavior. Chemogenetic restoration of IPSCs in D2-MSNs in the NAcore was sufficient to prevent antipsychotic-induced supersensitivity, pointing to an entirely novel therapeutic direction for overcoming this condition.

Dnmt3a2 in the Nucleus Accumbens Shell Mediates Cue-Induced Cocaine-Seeking Behavior

One of the main obstacles in treating drug addiction is the high rates of relapse even after prolonged periods of abstinence. Neuronal adaptions play a major role in drug craving and relapse ([Kalivas and Volkow, 2005][1]). Many of these adaptions involve changes in gene expression ([Nestler, 2001][

The Naturally Occurring Compound Garcinia Indica Selectively Impairs the Reconsolidation of a Cocaine-Associated Memory.

Sustained abstinence from cocaine use is frequently compromised by exposure to environmental stimuli that have previously been strongly associated with drug taking. Such cues trigger memories of the effects of the drug, leading to craving and potential relapse. Our work has demonstrated that manipulating cocaine-cue memories by destabilizing them through interfering with the reconsolidation process is one potential therapeutic tool by which to prolong abstinence. Here, we examine the use of the naturally occurring amnestic agent garcinol to manipulate an established cocaine-cue memory. Rats underwent 12 days of cocaine self-administration training during which time active lever presses resulted in an i.v. infusion of cocaine that was paired with a light/tone cue. Next rats underwent lever extinction for 8 days followed by light/tone reactivation and a test of cue-induced cocaine-seeking behavior. Systemic injection of garcinol 30 min after reactivation significantly impaired the reconsolidation of the cocaine-associated cue memory. Further testing revealed that garcinol had no effect on drug-induced cocaine-seeking, but was capable of blocking the initial conditioned reinforcing properties of the cue and prevents the acquisition of a new response. Additional experiments showed that the effects of garcinol are specific to reactivated memories only, temporally constrained, cue-specific, long-lasting, and persist following extended cocaine access. These data provide strong evidence that the naturally occurring compound, garcinol, may be a potentially useful tool to sustain abstinence from drug abuse.

Prevention of the incubation of cocaine seeking by aerobic exercise in female rats.

Recent research has demonstrated that aerobic exercise can attenuate craving for drugs of abuse and reduce escalation and reinstatement of drug-seeking behavior in animal models. The present study examined the effects of aerobic exercise on the development of the incubation of cocaine-seeking behavior or the progressive increase in cocaine seeking over a protracted withdrawal period from cocaine self-administration. Female rats were trained to self-administer cocaine (0.4mg/kg/inf) during daily 6-h sessions for 10days. Subsequently, access to cocaine and cocaine-paired cues was discontinued during a 3- or 30-day withdrawal period when rats had access to either a locked or unlocked running wheel. At the end of the withdrawal period, rats were reintroduced to the operant conditioning chamber and reexposed to cocaine-paired cues to examine cocaine-seeking behavior under extinction conditions. Rats with access to a locked running wheel during 30days of withdrawal had significantly greater cue-induced cocaine-seeking behavior than rats that had access to an unlocked running wheel for 30days. Further, there was robust incubation of cocaine seeking in rats with access to a locked running wheel as cocaine seeking was notably elevated at 30 vs. 3days of withdrawal. However, cocaine-seeking behavior did not differ between rats with access to an unlocked running wheel for 30 vs. 3days, indicating that incubation of cocaine seeking was suppressed following access to exercise for 30days. Aerobic exercise during extended withdrawal from cocaine self-administration decreased incubation of cue-induced cocaine-seeking behavior and may reduce vulnerability to relapse.

Anxiolytic effects of oxytocin in cue-induced cocaine seeking behavior in rats.

Oxytocin (OT) is a neuropeptide previously related to reward, learning, memory, and stress, events associated with cocaine addiction. OT has shown anxiolytic properties in different animal models of anxiety. Moreover, previous data have demonstrated an increase in mRNA OT levels within the nucleus accumbens (NAc) following acute and chronic cocaine exposure in rats. Therefore, OT might play a modulatory role in the rewarding properties of cocaine. The present set of experiments aims to examine the role of OT on environmentally elicited cocaine-seeking behavior and whether OT could reduce anxiety associated with this behavior. Separate groups of rats were trained in a cue-elicited cocaine-seeking behavior paradigm. Prior to the reinstatement phase, animals received microinfusions of artificial cerebrospinal fluid (aCSF), OT, OT agonist (TgOT), or OT antagonist (OTA) within the intracerebral ventricular intracerebroventricular (ICV) system. To test OT anxiolytic effects in reinstatement behavior, separate groups of animals were trained in a cue-elicited cocaine-seeking behavior protocol or in a cocaine-conditioning paradigm. At the end of each behavioral training, all animals were ICV pretreated with aCSF or OT, and then exposed to an elevated plus maze. Results showed that OT and TgOT pretreatment significantly reduced reinstatement of cocaine-seeking behavior. Most significantly, OT treatment reduced the anxiety triggered by cue-induced reinstatement conditions and cocaine-paired conditioned locomotion. The present study demonstrates for the first time that OT actions within the brain mediate the anxiety response triggered by cues previously paired with cocaine intake.

A novel mGluR5 antagonist, MFZ 10‐7, inhibits cocaine‐taking and cocaine‐seeking behavior in rats

Pre-clinical studies suggest that negative allosteric modulators (NAMs) of the metabotropic glutamate receptor subtype 5 (mGluR5), including 2-methyl-6-(phenylethynyl)pyridine (MPEP), 3-[(2-methyl-1,3-thiazol-4-yl)ethynyl]pyridine (MTEP) and fenobam are highly effective in attenuating drug-taking and drug-seeking behaviors. However, both MPEP and MTEP have no translational potential for use in humans because of their off-target effects and short half-lives. Here, we report that 3-fluoro-5-[(6-methylpyridin-2-yl)ethynyl]benzonitrile (MFZ 10-7), a novel mGluR5 NAM, is more potent and selective than MPEP, MTEP and fenobam in both in vitro binding and functional assays. Similar to MTEP, intraperitoneal administration of MFZ 10-7 inhibited intravenous cocaine self-administration, cocaine-induced reinstatement of drug-seeking behavior and cocaine-associated cue-induced cocaine-seeking behavior in rats. Although MFZ 10-7 and MTEP lowered the rate of oral sucrose self-administration, they did not alter total sucrose intake. Further, MFZ 10-7 appeared to be more potent than MTEP in inducing downward shifts in the cocaine dose-response curve, but less effective than MTEP in attenuating sucrose-induced reinstatement of sucrose-seeking behavior. MFZ 10-7 and MTEP had no effect on basal locomotor behavior. These findings not only provide additional evidence supporting an important role for mGluR5 in cocaine reward and addiction, but also introduce a new tool for both in vitro and in vivo investigations with which to further characterize this role.

β-Endorphin via the Delta Opioid Receptor is a Major Factor in the Incubation of Cocaine Craving

Cue-induced cocaine craving intensifies, or 'incubates', during the first few weeks of abstinence and persists over extended periods of time. One important factor implicated in cocaine addiction is the endogenous opioid β-endorphin. In the present study, we examined the possible involvement of β-endorphin in the incubation of cocaine craving. Rats were trained to self-administer cocaine (0.75 mg/kg, 10 days, 6 h/day), followed by either a 1-day or a 30-day period of forced abstinence. Subsequent testing for cue-induced cocaine-seeking behavior (without cocaine reinforcement) was performed. Rats exposed to the drug-associated cue on day 1 of forced abstinence demonstrated minimal cue-induced cocaine-seeking behavior concurrently with a significant increase in β-endorphin release in the nucleus accumbens (NAc). Conversely, exposure to the cue on day 30 increased cocaine seeking, while β-endorphin levels remained unchanged. Intra-NAc infusion of an anti-β-endorphin antibody (4 μg) on day 1 increased cue-induced cocaine seeking, whereas infusion of a synthetic β-endorphin peptide (100 ng) on day 30 significantly decreased cue response. Both intra-NAc infusions of the δ opioid receptor antagonist naltrindole (1 μg) on day 1 and naltrindole together with β-endorphin on day 30 increased cue-induced cocaine-seeking behavior. Intra-NAc infusion of the μ opioid receptor antagonist CTAP (30 ng and 3 μg) had no behavioral effect. Altogether, these results demonstrate a novel role for β-endorphin and the δ opioid receptor in the development of the incubation of cocaine craving.

Role of the Major Glutamate Transporter GLT1 in Nucleus Accumbens Core Versus Shell in Cue-Induced Cocaine-Seeking Behavior

Relapse to cocaine-seeking behavior requires an increase in nucleus accumbens (NAc) core glutamate transmission. Decreased expression of glutamate type I transporter (GLT1), which is responsible for >90% of glutamate clearance, occurs in the core of rats withdrawn from cocaine self-administration, while treatment with ceftriaxone, a β-lactam antibiotic previously shown to increase GLT1 expression and function in rodents, upregulates GLT1 and attenuates cue-induced cocaine reinstatement. Here, we tested the effects of increasing GLT1 expression on cue-induced cocaine seeking in rats exposed to either limited (2 h/d) or extended (6 h/d) cocaine access followed by short (2 d) or long (45 d) withdrawal periods. Treatment with ceftriaxone (200 mg/kg, i.p.) upregulated core GLT1 expression and attenuated cue-induced cocaine-seeking behavior only in rats exposed to long withdrawal periods, with a greater effect in the extended-access condition. Pearson's correlation revealed GLT1 expression in core to be inversely correlated with cue-induced cocaine-seeking behavior. To localize the effects of GLT1 upregulation within NAc, we tested the hypothesis that blockade of GLT1 in NAc core, but not shell, would reverse the ceftriaxone-mediated effect. Rats withdrawn from cocaine self-administration were treated with the same dose of ceftriaxone followed by intracore or intrashell infusions of one of two GLT1 blockers, dihydrokainic acid (500 μM) or DL-threo-β-benzyloxyaspartate (250 μM), or saline. Our results reveal that the ceftriaxone-mediated attenuation of cue-induced cocaine reinstatement is reversed by GLT1 blockade in core, but not shell, and further implicate core GLT1 as a potential therapeutic target for cocaine relapse.

Fenobam sulfate inhibits cocaine-taking and cocaine-seeking behavior in rats: implications for addiction treatment in humans

The metabotropic glutamate receptor subtype 5 (mGluR5) has been reported to be critically involved in drug reward and addiction. Because the mGluR5 negative allosteric modulators (NAMs) 2-methyl-6-(phenylethynyl)pyridine (MPEP) and 3-((2-methyl-1,3-thiazol-4-yl)ethynyl)pyridine (MTEP) significantly inhibit addictivelike behaviors of cocaine and other drugs of abuse in experimental animals, it has been suggested that mGluR5 NAMs may have translational potential for treatment of addiction in humans. However, neither MPEP nor MTEP have been evaluated in humans due to their off-target actions and rapid metabolism. Herein, we evaluate a potential candidate for translational addiction research: a new sulfate salt formulation of fenobam, a selective mGluR5 NAM that has been investigated in humans. In rats, fenobam sulfate had superior pharmacokinetics compared to the free base, with improved maximal plasma concentration (C max) and longer half life. Oral (p.o.) administration of fenobam sulfate (30 or 60 mg/kg) inhibited intravenous (i.v.) cocaine self-administration, cocaine-induced reinstatement of drug-seeking behavior, and cocaine-associated cue-induced cocaine-seeking behavior in rats. Fenobam sulfate also inhibited p.o. sucrose self-administration and sucrose-induced reinstatement of sucrose-seeking behavior, but had no effect on locomotion. This study provides additional support for the role of mGluR5 signaling in cocaine addiction and suggests that fenobam sulfate may have translational potential in medication development for the treatment of cocaine addiction in humans.

Deficits in Ventromedial Prefrontal Cortex Group 1 Metabotropic Glutamate Receptor Function Mediate Resistance to Extinction during Protracted Withdrawal from an Extensive History of Cocaine Self-Administration

Anomalies in prefrontal cortex (PFC) function are posited to underpin difficulties in learning to suppress drug-seeking behavior during abstinence. Because group 1 metabotropic glutamate receptors (mGluRs) regulate drug-related learning, we assayed the consequences of extended access to intravenous cocaine (6 h/d; 0.25 mg/infusion for 10 d) on the PFC expression of group 1 mGluRs and the relevance of observed changes for cocaine seeking. After protracted withdrawal, cocaine-experienced animals exhibited a time-dependent intensification of cue-induced cocaine-seeking behavior and an impaired extinction of this behavior. These behavioral phenomena were associated with a time-dependent reduction in mGluR1/5 expression within ventromedial PFC (vmPFC) of cocaine-experienced animals exposed to extinction testing but not in untested ones. Interestingly, pharmacological manipulations of vmPFC mGluR1/5 produced no immediate effects on cue-induced cocaine-seeking behavior but produced residual effects on a subsequent test for cocaine seeking. At 3 d withdrawal, cocaine-experienced rats infused intra-vmPFC with mGluR1/5 antagonists, either before or after an initial test for cocaine seeking, persisted in their cocaine seeking akin to cocaine-experienced rats in protracted withdrawal. Conversely, cocaine-experienced rats infused with an mGluR1/5 agonist before the initial test for cocaine-seeking at 30 d withdrawal exhibited a facilitation of extinction learning. These data indicate that cue-elicited deficits in vmPFC group 1 mGluR function mediate resistance to extinction during protracted withdrawal from a history of extensive cocaine self-administration and pose pharmacological stimulation of these receptors as a potential approach to facilitate learned suppression of drug-seeking behavior that may aid drug abstinence.

The mGluR2 Positive Allosteric Modulator BINA Decreases Cocaine Self-Administration and Cue-Induced Cocaine-Seeking and Counteracts Cocaine-Induced Enhancement of Brain Reward Function in Rats

Metabotropic glutamate receptor 2/3 (mGluR2/3) agonists were shown previously to nonselectively decrease both cocaine- and food-maintained responding in rats. mGluR2 positive allosteric modulators (PAMs) may represent improved therapeutic compounds because of their modulatory properties and higher selectivity for mGluR2. We analyzed the effects of the selective, brain penetrant, and systemically active mGluR2 PAM potassium 3'-([(2-cyclopentyl-6-7-dimethyl-1-oxo-2,3-dihydro-1H-inden-5-yl)oxy]methyl)biphenyl l-4-carboxylate (BINA) and the mGluR2/3 agonist LY379268 on intravenous cocaine self-administration and cocaine-seeking behavior in rats that had short (1 h, ShA) or long (6 h, LgA) access to cocaine. The effects of BINA on food responding and food-seeking behavior were also analyzed. Finally, we examined the effects of BINA on brain reward function and cocaine-induced reward enhancement using the intracranial self-stimulation procedure. BINA decreased cocaine self-administration in both ShA and LgA rats, with no effect on food self-administration. Alternatively, LY379268 nonselectively decreased both cocaine and food self-administration. BINA decreased cue-induced reinstatement of cocaine seeking with no effect on food seeking. The cocaine-induced enhancement of brain reward function was blocked by BINA, although the highest doses of BINA decreased brain reward function when administered alone, suggesting additive, rather than interactive, effects of BINA and cocaine. In conclusion, BINA attenuated the reinforcing and counteracted the reward-enhancing effects of cocaine and decreased cue-induced cocaine-seeking behavior, without affecting behaviors motivated by food reinforcement. The higher selectivity of BINA compared with an mGluR2/3 agonist for drug- vs food-motivated behaviors suggests a therapeutic role for mGluR2 PAMs for the treatment of cocaine addiction and possibly other drugs of abuse.

Effects of RGH-237 [N-{4-[4-(3-Aminocarbonyl-phenyl)-piperazin-1-yl]-butyl}-4-bromo-benzamide], an Orally Active, Selective Dopamine D3Receptor Partial Agonist in Animal Models of Cocaine Abuse

Dopamine D(3) receptor partial agonism has been suggested as a potential therapeutic intervention in cocaine addiction. RGH-237 [N-{4-[4-(3-aminocarbonyl-phenyl)-piperazin-1-yl]-butyl}-4-bromo-benzamide] was identified as a novel selective dopamine D(3) receptor partial agonist and used for testing this hypothesis in animal models. The compound showed nanomolar affinity to human (K(i) = 6.7 nM) and rat (K(i) = 1.6 nM) D(3) receptors with an intrinsic activity of approximately 50%. It possessed several hundredfold selectivity over the D(2) receptor. The molecule bound with moderate (100-250 nM) affinity to 5-hydroxytryptamine 1A (5-HT(1A)) and nonselectively labeled opiate receptors. RGH-237 proved to be practically inactive on more than 40 other targets, including monoaminergic, cholinergic, GABAergic, and glutamatergic receptors. In rats orally administered RGH-237 was well and rapidly absorbed yielding 41% oral bioavailability. At its pharmacologically active dose (10 mg/kg p.o.), the brain concentration of RGH-237 reached 110 ng/g. Its blood and brain levels were sustained for 3 h. RGH-237 at the oral dose of 10 mg/kg moderately but significantly inhibited the acquisition of cocaine-induced place preference, although by itself, it had no place-conditioning effect. The compound did not affect fixed ratio 1 cocaine self-administration. In a reinstatement paradigm of cocaine self-administration, the compound potently and dose-dependently blocked the cue-induced cocaine-seeking behavior of rats at 10 and 30 mg/kg oral doses. RGH-237 did not affect seeking activity for natural rewards, such as sucrose and water. It did not exert notable effect on spontaneous motor activity of rats. Our results demonstrate that selective D(3) partial agonists may be an effective therapeutic means in the treatment of cocaine abuse.

Dopamine D1 or D2 receptor antagonism within the basolateral amygdala differentially alters the acquisition of cocaine-cue associations necessary for cue-induced reinstatement of cocaine-seeking

The basolateral amygdala complex has been implicated in the formation and utilization of cocaine-cue associations in rat models of cue-induced reinstatement to cocaine-seeking behavior. We have previously demonstrated the importance of dopamine inputs to the basolateral amygdala complex in the reinstatement of cocaine-seeking behavior following chronic cocaine self-administration. Here we show that selective blockade of amygdalar dopamine D1 and D2 receptors during acquisition of cocaine-cue associations has distinctive effects on subsequent conditioned-cued cocaine-seeking behavior. Male, Sprague–Dawley rats were first trained to self-administer i.v. cocaine on a fixed ratio 1 schedule for 5 days. Subjects then received bilateral, intra-basolateral amygdala complex infusions of a dopamine D1 receptor antagonist (SCH23390, 0.25–2.0μg/side; experiment 1), a dopamine D2 receptor antagonist (raclopride, 0.625–5.0μg/side; experiment 2), or vehicle just prior to a single classical conditioning session, during which a light+tone cue was discretely paired with passive infusions of cocaine in the absence of lever responding. Following five additional days of cocaine self-administration and 7–10 days of extinction training, animals underwent multiple tests for cue-induced reinstatement. SCH23390 (2.0μg/side), administered at the time of cocaine-cue association only, produced an attenuation of reinstatement to cue-induced cocaine-seeking behavior. In contrast, low doses of raclopride potentiated, while a higher dose of raclopride attenuated cue-induced reinstatement. These results demonstrate unique contributions of D1 vs. D2 receptors in mediating dopamine inputs within the basolateral amygdala complex during the formation of cocaine-stimulus associations that are critical for cue-induced reinstatement.

Lidocaine Inactivation of Ventral Subiculum Attenuates Cocaine-Seeking Behavior in Rats

The role of the ventral subiculum in cocaine- or cue-induced cocaine-seeking behavior was investigated in rats tested on a between-session reinstatement model. Rats were trained to self-administer cocaine (0.25 mg/infusion, i.v.) in a lever-pressing operant task in a daily 2 hr session. Responding was reinforced contingent on a modified fixed-ratio 5 schedule. Reinstatement tests began after the lever-pressing behavior was extinguished in the absence of cocaine and conditioned cues (light and tone). Bilateral microinjections of lidocaine (100 microg) into the ventral subiculum decreased cocaine- or cue-induced reinstatement of cocaine-seeking behavior compared with saline microinjections into the same area in another group of rats. Lidocaine microinjections, however, had no effect on cocaine self-administration behavior or food-maintained or food-reinstated responding. Collectively, these results suggest that the ventral subiculum plays an important role in cocaine-seeking behavior. Considering the role of this structure in context learning, our data suggest that the full expression of cocaine- or cue-induced reinstatement may depend on the context in which the cocaine experience occurred.

Muscarinic receptor antagonism in the basolateral amygdala blocks acquisition of cocaine-stimulus association in a model of relapse to cocaine-seeking behavior in rats

Recent evidence has demonstrated a critical role for the basolateral amygdala complex in the reinstatement of extinguished drug-seeking behavior produced by drug-paired cues. In the current study, we utilized a model of the acquisition and expression of cocaine-stimulus associative pairing in order to study the role of cholinergic input to the basolateral amygdala in mediating conditioned-cued reinstatement. Male, Sprague–Dawley rats were first trained daily to self-administer i.v. cocaine on a fixed ratio 1 schedule of reinforcement. The muscarinic acetylcholine receptor antagonist, scopolamine, was directly infused into the basolateral amygdala prior to: a) classically conditioned pairing of a tone+light stimulus with cocaine infusions (acquisition), or b) testing of conditioned-cued reinstatement following a period of withdrawal from cocaine and extinction of cocaine-paired lever responding. Infusion of scopolamine just prior to the classical conditioning trial produced a dose-dependent disruption of cocaine-seeking behavior maintained by cocaine-paired cues during the reinstatement test. In contrast, infusion of scopolamine prior to the reinstatement test had no effect on conditioned-cued reinstatement of cocaine-seeking behavior. These results indicate a crucial role for cholinergic innervation of muscarinic acetylcholine receptors in the basolateral amygdala during the formation, but not the expression, of stimulus–reward associations that mediate cue-induced cocaine-seeking behavior.

Cocaine-seeking behavior in response to drug-associated stimuli in rats: involvement of D3 and D2 dopamine receptors.

Previous studies employed a second-order schedule paradigm maintained by cocaine reinforcement to show that BP897, a dopamine D(3) partial agonist, selectively modulated drug-seeking behavior. We investigated its effect on drug-seeking behavior induced by presentation of stimuli associated with and predictive of cocaine availability after a period of extinction and in the absence of any further cocaine. Male rats were trained to associate discriminative stimuli (S(D)) with the availability of intravenous (i.v.) 0.25 mg/0.1 ml/infusion cocaine (S(D+)) or no-reward (S(D-)) saline solution. Each infusion of cocaine or saline was followed by a response-cue signaling 20-s time-out (TO). After meeting the self-administration training criterion rats were placed on extinction conditions during which i.v. solutions and S(D)s were withheld. Every other 3 days on which rats met the extinction criterion, reinstatement tests were conducted, presenting the S(D+) or S(D-) noncontingently together with a contingent presentation of cocaine- or saline-cues signaling 20-s TO. Regardless of the order of presentation or the nature of the stimuli (auditory or visual), cocaine-associated but not saline-associated stimuli reinstated responding on the previously active lever. Presentation of cocaine-associated stimuli induced lasting drug-seeking behavior for at least eight test sessions. BP897 (1.0 mg/kg i.p.) significantly attenuated this behavior. Since it has been reported that BP897 can interact with a panel of different receptors with high affinity, we evaluated the effects of 7-OH-DPAT, an agonist to D(3) receptors, raclopride, a preferential antagonist to D(2) receptors, and WAY 100,635, an antagonist at 5-HT(1A) receptors, on drug-seeking behavior. 7-OH-DPAT (0.1-3.0 mg/kg i.p.) had biphasic effects on reinstatement induced by the cocaine-associated cues, low dosages reducing and high dosages increasing the impact of cocaine-associated stimuli on rats' behavior. Raclopride (0.1, 0.3 mg/kg s.c.) completely prevented drug-seeking behavior induced by the reintroduction of cocaine-associated stimuli. WAY 100,635 (0.1-1.0 mg/kg s.c.) had no effect on this behavior. These results, while confirming that the partial agonist at the D(3) receptors, BP897, might be a useful medication, also suggest a role of D(2) receptors in cue-induced cocaine-seeking behavior.

Enduring Resistance to Extinction of Cocaine-Seeking Behavior Induced by Drug-Related Cues

The conditioning of cocaine's pharmacological actions with environmental stimuli is thought to be a critical factor in long-lasting relapse risk associated with cocaine addiction. To study the significance of environmental stimuli in enduring vulnerability to relapse, the resistance to extinction of drug-seeking behavior elicited by a cocaine-related stimulus was examined. Male Wistar rats were trained to associate discriminative stimuli (SD) with the availability of intravenous cocaine (S+) vs. the availability of non-rewarding (S−) saline solution, and then placed on extinction conditions during which intravenous solutions and SD were withheld. The rats were then presented with the S+ or S− alone in 60-min reinstatement sessions conducted at 3-day intervals. To examine the long-term persistence of the motivating effects of the cocaine S+, a subgroup of rats was re-tested following an additional three months of abstinence during which time the rats remained confined to their home cages. Re-exposure to the cocaine S+ selectively elicited robust responding at the previously active lever. The efficacy and selectivity of this stimulus to elicit responding remained unaltered throughout a 34-day phase of repeated testing as well as following the additional extended abstinence period. In pharmacological tests, conducted in a separate group of rats, the dopamine (DA) D1 antagonist SCH 39166 (10 μg/kg), the D2/3 antagonist nafadotride (1 mg/kg), and the D2/3 agonist PD 128907 (0.3 mg/kg) suppressed the cue-induced response reinstatement while the D1 agonist SKF 81297 (1.0 mg/kg) produced a variable behavioral profile attenuating cue-induced responding in some rats while exacerbating this behavior in others. The results suggest that the motivating effects of cocaine-related stimuli are highly resistant to extinction. The undiminished efficacy of the cocaine S+ to induce drug-seeking behavior both with repeated testing and following long-term abstinence parallels the long-lasting nature of conditioned cue reactivity and cue-induced cocaine craving in humans, and confirms a significant role of learning factors in long-lasting vulnerability to relapse associated with cocaine addiction. Finally, the results support a role of DA neurotransmission in cue-induced cocaine-seeking behavior.