Effects of naltrexone on cocaine- and sucrose-seeking behaviour in response to associated stimuli in rats

The non-selective opioid receptor antagonist naltrexone reduces cocaine-induced reinstatement of drug-seeking behaviour in abstinent rats. The current study sought to determine whether the opioid system is also involved in cocaine-seeking behaviour induced by cocaine-associated stimuli in abstinent rats. Adult male rats were trained to press a lever either to self-administer cocaine or to obtain sucrose pellets in the presence of distinctive discriminative and conditioned stimuli. After a period of extinction, re-exposure to cocaine-associated cues selectively elicited robust and enduring responding at the active lever; sucrose pellet-associated cues revived seeking behaviour less pronouncedly. Pretreatment with naltrexone (0.25, 1, 2.5 mg/kg s.c., 20 min before reinstatement tests) dose dependently prevented cue-induced cocaine-seeking behaviour, whereas (2.5 mg/kg s.c.) did not affect the degree of cue-induced sucrose-seeking behaviour. These results provide the first evidence that naltrexone influences cocaine seeking induced by conditioned stimuli in abstinent rats; this effect appears selective for cocaine reinstatement as opposed to a non-drug reinforcer.     

Estrogen receptor beta, but not alpha, mediates estrogen's effect on cocaine-induced reinstatement of extinguished cocaine-seeking behavior in ovariectomized female rats.

Preclinical and clinical studies indicate that females are more vulnerable to relapse than males, and the neurobiological effects of estrogen are thought to mediate, in part, the sex differences in cocaine-taking behavior. The goal of the present study was to investigate the involvement of estrogen receptor alpha (ERalpha) and beta (ERbeta) in estrogen-mediated increases in cocaine-induced reinstatement of extinguished cocaine-seeking behavior in ovariectomized (OVX) female rats. Rats were initially trained to self-administer cocaine (0.4 mg/kg/inf, i.v.) under a fixed-ratio 1 (FR 1) schedule of reinforcement during daily 2-h sessions. After a 10-day maintenance period, cocaine solutions were replaced with saline, and self-administration was extinguished over a 14-day period. OVX rats were then treated with either the mixed ERalpha/beta agonist estradiol benzoate (EB), the ERalpha-selective agonist, propyl-pyrazole-triol (PPT), the ERbeta-selective agonist, diarylpropionitrile (DPN), or a vehicle control (dimethyl sulfoxide, DMSO). Treatment lasted a total of 9 days, and during this time, rats were assessed for nonreinforced reinstatement of extinguished cocaine-seeking behavior after priming injections of saline or cocaine (5, 10, or 15 mg/kg, i.p.). OVX rats showed no differences in self-administration during maintenance or extinction. OVX rats treated with EB exhibited greater responding for cocaine during reinstatement compared to OVX+DMSO controls. Selective activation of ERbeta with DPN also increased cocaine-induced reinstatement responding, whereas selective activation of ERalpha with PPT did not affect cocaine-seeking behavior. These results indicate that estrogen influences the propensity for reinstatement of extinguished cocaine-seeking behavior, and that estrogen-mediated enhancement of cocaine-induced reinstatement responding involves the activation of ERbeta.     

Protracted time-dependent increases in cocaine-seeking behavior during cocaine withdrawal in female relative to male rats

Rationale Female rats display higher sensitivity to cocaine relative to males under a variety of conditions. Time-dependent increases in cocaine-seeking behavior (as measured by nonreinforced operant responses) during cocaine withdrawal have been reported in male, but not female, rats. Objectives The present study determines sex and estrous cycle influences on time-dependent changes in cocaine-seeking behavior. Materials and methods Male and female Sprague-Dawley rats were reinforced for “active lever” responses by a cocaine infusion (0.50 mg/kg/infusion, i.v., fixed ratio schedule of reinforcement, FR1) followed by a 20-s time-out when reinforcement was not delivered. Infusions were paired with a light + tone conditioned stimulus. Next, rats underwent cocaine withdrawal for 1, 14, 60, or 180 days before testing cocaine-seeking behavior. Each rat was tested for extinction of operant responding, conditioned-cued reinstatement, and cocaine-primed (10 mg/kg, i.p.) reinstatement. Results Both males and females displayed a time-dependent increase in cocaine-seeking behavior (active lever presses) under extinction of operant responding and conditioned-cued reinstatement conditions after 60 days of cocaine withdrawal. Moreover, cocaine-seeking behavior during extinction of operant responding in females, but not males, remained elevated at 180 days of cocaine withdrawal. Furthermore, females tested during estrus exhibited higher cocaine-seeking behavior under both extinction of operant responding and cocaine-primed reinstatement conditions relative to other rats independent of the duration of cocaine withdrawal. Conclusions The effects of reproductive cycle and withdrawal duration on cocaine-seeking behavior are additive and time-dependent increases in cocaine-seeking behavior are more enduring in females than in male rats.     

Potentiated reinstatement of cocaine-seeking behavior following D-amphetamine infusion into the basolateral amygdala.

Reinstatement of extinguished drug-seeking behavior following chronic drug self-administration has been demonstrated in rats in the presence of conditioned cues. This experimental model of cue-induced relapse can be used to assess the neural circuitry involved in relapse. We have previously shown that blockade of dopamine D1 receptors in the basolateral amygdala (BLA) abolishes conditioned cue-induced reinstatement of cocaine-seeking behavior. The present study tested the hypothesis that D-amphetamine-induced facilitation of monoamine neurotransmission in the BLA would potentiate conditioned cue-induced reinstatement of extinguished drug-seeking behavior. During daily self-administration sessions over 10 consecutive days, rats pressed a lever to receive cocaine infusions (0.2 mg/0.05 ml) paired with a light+tone compound stimulus. Following self-administration, rats underwent daily extinction sessions, during which no stimuli were presented. On the test days, rats received intra-BLA D-amphetamine (10 or 30 micro g/side) or vehicle infusions followed by extinction or conditioned cue-induced reinstatement testing. D-amphetamine infusions did not alter extinction responding relative to vehicle infusions. During reinstatement testing, conditioned cue presentation significantly increased responding over extinction levels, and intra-BLA D-amphetamine produced a dose-dependent increase in lever responding relative to vehicle infusions. These findings suggest that enhanced monoamine tone in the BLA potentiates the motivational effect and/or salience of cocaine-paired cues during reinstatement.     

NADPH oxidase 1 mediates upregulation of thromboxane A2 synthase in human vascular smooth muscle cells: inhibition with iloprost

We have previously reported that selective blockade of brain dopamine D(3) receptors by SB-277011A significantly attenuates cocaine self-administration and cocaine-induced reinstatement of drug-seeking behavior. In the present study, we investigated whether SB-277011A similarly inhibits methamphetamine self-administration and methamphetamine-induced reinstatement to drug-seeking behavior. Male Long-Evans rats were allowed to intravenously self-administer methamphetamine (0.05 mg/kg/infusion) under fixed-ratio 2 (FR2) or progressive-ratio (PR) reinforcement conditions, and some rats were tested for methamphetamine-induced reinstatement of drug-seeking behavior after extinction of self-administration. The effects of SB-277011A on each of these methamphetamine-supported behaviors were then tested. Acute intraperitoneal (i.p.) administration of SB-277011A failed to alter methamphetamine self-administration under FR2 reinforcement, but significantly lowered the break-point for methamphetamine self-administration under PR reinforcement. SB-277011A also significantly inhibited methamphetamine-triggered reinstatement of extinguished drug-seeking behavior. Overall, these data show that blockade of dopamine D(3) receptors by SB-277011A attenuates the rewarding and incentive motivational effects of methamphetamine in rats, supporting the development of selective dopamine D(3) antagonists for the treatment of methamphetamine addiction.     

Ketoconazole does not block cocaine discrimination or the cocaine-induced reinstatement of cocaine-seeking behavior.

Ketoconazole is an FDA-approved antifungal agent that also blocks the synthesis of adrenocorticosteroids and functions as a glucocorticoid receptor antagonist. It has been previously demonstrated that this drug blocks the stress-induced reinstatement of cocaine-seeking behavior and reduces low-dose cocaine self-administration in rats. In the present experiments, the effects of ketoconazole on the cocaine-induced reinstatement of extinguished cocaine-seeking behavior and on cocaine discrimination were investigated in male Wistar rats. In rats trained to self-administer cocaine (0.5 mg/kg/infusion) by pressing a lever under a fixed-ratio 4 schedule of reinforcement, cocaine (5-20 mg/kg, IP) dose dependently reinstated cocaine-seeking behavior following at least 10 days of extinction, during which responding on the cocaine lever resulted in no programmed consequences. Ketoconazole (50 mg/kg, IP) failed to block cocaine-induced reinstatement despite blocking cocaine-induced increases in plasma corticosterone. Ketoconazole (25 or 50 mg/kg) also failed to block cocaine discrimination in rats trained to discriminate 10 mg/kg cocaine from saline. In these rats, generalization to the training dose of cocaine was observed in the absence of increases in plasma corticosterone. The results of these experiments indicate that corticosterone may mediate the effects of stressors on cocaine-seeking behavior but not the direct effects of cocaine itself.     

Blockade of 5-HT2A receptors in the medial prefrontal cortex attenuates reinstatement of cue-elicited cocaine-seeking behavior in rats

Rationale

The action of serotonin (5-HT) at the 5-HT_2A receptor subtype is thought to be involved in cocaine-seeking behavior that is motivated by exposure to drug-associated cues and drug priming. 5-HT_2A receptors are densely clustered in the ventromedial prefrontal cortex (vmPFC), an area that plays a role in mediating cocaine-seeking behavior.

Objectives

This study examined the hypothesis that M100907, a 5-HT_2A receptor antagonist, infused directly in the vmPFC attenuates cue- and cocaine-primed reinstatement of cocaine-seeking behavior.

Methods

Rats trained to self-administer cocaine (0.75?mg/kg, i.v.) paired with light and tone cues underwent extinction training during which operant responses produced no consequences. Once behavior extinguished, rats were tested for reinstatement of responding elicited by either response-contingent presentations of the cocaine-paired light/tone cues or by cocaine-priming injections (10?mg/kg, i.p.) within 1?min after pretreatment with microinfusions of M100907 (0.1, 0.3, 1.0, or 1.5???g/0.2???l/side) into the vmPFC.

Results

Intra-vmPFC M100907 decreased cue-elicited reinstatement at the two highest doses (1.0 and 1.5???g) but produced only a slight decrease in cocaine-primed reinstatement that was not dose dependent. The decrease in cue reinstatement was not likely due to impaired ability to respond since intra-vmPFC M100907 infusions had minimal effect on cocaine self-administration and no effect on cue-elicited sucrose-seeking behavior, or spontaneous or cocaine-induced locomotion. M100907 infusions into the adjacent anterior cingulate cortex had no effect on cue reinstatement.

Conclusions

The results suggest that the blockade of 5-HT_2A receptors in the vmPFC selectively attenuates the incentive motivational effects of cocaine-paired cues.     

Effect of kappa-opioid receptor agonists U69593, U50488H, spiradoline and salvinorin A on cocaine-induced drug-seeking in rats

Our previous work indicated that pretreatment with the selective kappa-opioid receptor (KOPr) agonist, U69593, attenuated the ability of priming injections of cocaine to reinstate extinguished cocaine-seeking behavior. The present study expanded these initial tests to include other traditional KOPr agonists, U50488H, spiradoline (SPR), and salvinorin A (Sal A), an active constituent of the plant Salvia divinorum. Following acquisition and stabilization of cocaine self-administration, cocaine-produced drug-seeking was measured. This test was conducted in a single day and comprised an initial phase of self-administration, followed by a phase of extinguished responding. The final phase examined reinstatement of extinguished cocaine self-administration followed by a priming injection of cocaine (20.0 mg/kg, intraperitoneal (I.P.)) in combination with the various KOPr agonists. Cocaine-induced drug-seeking was attenuated by pretreatment with U69593 (0.3 mg/kg, subcutaneous (S.C.)), U50488H (30.0 mg/kg, I.P.), SPR (1.0, 3.0 mg/kg, I.P.) and Sal A (0.3, 1.0 mg/kg, I.P.). Sal A (0.3, 1.0 mg/kg, I.P.) had no effect on operant responding to obtain sucrose reinforcement or on cocaine-induced hyperactivity. These findings show that Sal A, like other traditional KOPr agonists attenuates cocaine-induced drug-seeking behavior.     

Perinatal lead exposure and relapse to drug-seeking behavior in the rat: a cocaine reinstatement study

Rationale Intravenous self-administration of cocaine at low doses is increased by chronic low-level exposure to lead during gestation and lactation (perinatal lead exposure). Insofar as drug potency is increased by early lead exposure, it must be considered that cocaine-seeking and relapse after periods of withdrawal similarly may be enhanced by perinatal lead exposure. Objectives Employing an animal model, the present study examined the effects of lead exposure during gestation and lactation on cocaine-induced reinstatement of drug-seeking, when animals were tested as adults. Methods Adult female rats were gavaged once daily with 0 or 16 mg lead for 30 days prior to breeding with non-exposed males. This exposure regimen continued until offspring were weaned at postnatal day (PND) 21. At PND 120, male offspring were trained to self-administer cocaine intravenously (IV) [0.50 mg/kg cocaine per infusion on a fixed-ratio schedule where two lever presses resulted in drug delivery (FR-2 schedule)]. After steady-state responding was established, cocaine reinstatement responding was assessed for each group within an extinction paradigm. During the initial 1 h of reinstatement testing, the previous baseline contingencies were in place, i.e. animals operated under an FR-2 schedule for an infusion of 0.50 mg/kg cocaine. During the 2 h, 3 h, and 4 h of testing saline infusions were substituted for cocaine infusions. After responding extinguished during hour 4, reinstatement of responding was tested by administering an intraperitoneal (IP) priming injection of 0.00, 5.00, 10.00, or 20.00 mg/kg cocaine. Following these injections, lever responding for saline infusions was monitored during hour 5. Results The number of saline infusions self-administered during hour 5 increased in a dose-dependent fashion for both controls (group 0-mg) and lead-exposed (group 16-mg) animals. However, lead-exposed animals self-administered significantly more saline infusions than controls at the 5.00 mg/kg and 10.00 mg/kg doses. This apparent metal-related increase in sensitivity to cocaine was evident with blood lead in metal-exposed test animals returning to control levels. However, brain lead levels remained elevated in lead-exposed test animals, relative to controls. Conclusions The results of this investigation suggest that low-level lead exposure during gestation and lactation increases sensitivity to the relapse phase of drug abuse. It is further apparent that this increased sensitivity to the reinstatement of drug-seeking behavior is long-lasting.     

Serotonin depletion attenuates cocaine seeking but enhances sucrose seeking and the effects of cocaine priming on reinstatement of cocaine seeking in rats

RATIONALE: Acute serotonin (5-HT) depletion by the tryptophan hydroxylase inhibitor, para-chlorophenylalanine, attenuates cocaine seeking in rats. OBJECTIVE: The present study examined the effects of chronic 5-HT depletion on cocaine- and sucrose seeking using the 5-HT-selective neurotoxin 5,7-dihydroxytryptamine (5,7-DHT). METHODS: Separate groups of rats were trained to lever press for cocaine infusions (0.33 mg/kg/0.1 ml, i.v.) or for sucrose pellets (45 mg Noyes) on a fixed ratio (FR) 1 schedule of reinforcement during daily 2-h sessions. Subsequently, animals received i.c.v. infusions of either vehicle or 5,7-DHT (150 microg/6 microl or 200 microg/20 microl). After a minimum of 10 days post-lesion, cocaine- and sucrose seeking were measured as lever presses in the absence of reinforcement (extinction). Some cocaine-trained animals were also assessed for the re-establishment of self-administration and reinstatement of extinguished cocaine seeking by i.v. cocaine priming injections and response-contingent presentations of cocaine-paired stimuli. RESULTS: 5-HT depletion by the 150 microg/6 microl dose of 5,7-DHT failed to alter cocaine- and sucrose seeking despite producing a 42-77% depletion of 5-HT in limbic terminal regions. The 200 microg/20 microl dose of 5,7-DHT attenuated cocaine seeking but enhanced sucrose seeking during extinction and produced a 55-85% depletion of 5-HT. In addition, cocaine-paired cues and cocaine priming reinstated cocaine-seeking behavior, and responding was enhanced in 5,7-DHT-treated animals relative to vehicle-treated controls at the 1 mg/kg/0.1 ml priming dose. However, re-establishment of cocaine self-administration was not altered by 5,7-DHT. CONCLUSION: The results suggest that 5-HT depletion may attenuate cocaine seeking but may enhance sucrose seeking when animals are tested during extinction. Furthermore, 5-HT depletion may enhance cocaine seeking produced by cocaine itself. Together these findings suggest that 5-HT depletion may have opposite effects on incentive motivation for cocaine during abstinence versus relapse.     

Different patterns of pharmacological reinstatement of cocaine-seeking behavior between Fischer 344 and Lewis rats

Rationale Fischer 344 (F344) and Lewis (LEW) rats differ in cocaine self-administration behaviors. Whether or not these inbred strains of rats differ in pharmacological reinstatement of cocaine-seeking behavior is unknown.Objectives The purpose of the present study was to determine if inbred strains of rats demonstrate differences in alpha-amino-3-hydroxy-5-methylisoxazole-4-proprionic acid (AMPA) and cocaine-induced reinstatement of previously extinguished cocaine-seeking behavior.Methods F344 and LEW rats received indwelling jugular catheters, bilateral guide cannula aimed at the nucleus accumbens core, and were trained to lever press for 0.5 mg/kg intravenous cocaine during 2-h self-administration sessions. Following 14 sessions, rats underwent extinction sessions, where previously reinforced lever pressing resulted in no programmed consequences. Just prior to beginning extinction session 7, rats received an intracranial infusion of saline. Lever pressing was not reinforced. During subsequent extinction sessions, rats received AMPA injections (0.2, 0.4, or 0.6 nM). Dosing order was determined by a within-subject Latin square design. At least two extinction sessions separated each AMPA session. Rats then underwent cocaine-induced reinstatement test sessions (lever pressing was not reinforced). Rats received passive intravenous cocaine (0.0, 0.5, 1.0, or 2.0 mg/kg) after being placed in the experimental chamber. At least two extinction sessions separated each cocaine-prime session, and subjects were tested at each dose according to a within-subjects Latin square design.Results LEW rats demonstrated blunted maximal responding to AMPA-induced reinstatement and heightened sensitivity to cocaine-induced reinstatement compared with F344 rats.Conclusions This study demonstrates that inbred strains differ in pharmacological reinstatement of cocaine-seeking behavior.     

Effects of progesterone on the reinstatement of cocaine-seeking behavior in female rats

Estradiol benzoate (EB) facilitates the acquisition and reinstatement of cocaine-seeking behavior when administered to ovariectomized (OVX) rats. In contrast, progesterone (P) decreases acquisition of cocaine self-administration, but the effects of P on the reinstatement of drug seeking are not known. The purpose of the present study was to compare the effects of EB and P on the reinstatement of cocaine-seeking behavior in female rats. Rats received either OVX or sham surgery (SH) and were trained to lever press for intravenous cocaine infusions (0.4 mg/kg) under a fixed ratio 1, 20-s time-out schedule during daily 2-hr sessions. After 14 days of stable responding, saline replaced cocaine, and a 21-day extinction period began. After extinction, rats were separated into 5 treatment groups (OVX+EB, OVX+EB+P, OVX+vehicle [VEH], SH+P, or SH+VEH), and VEH, EB, or EB+P was administered 30 min prior to each session for 5 days. After 3 days of hormone treatment, rats received a saline or cocaine (10 mg/kg) injection, and reinstatement of lever responding was assessed. Reinstatement responding in the OVX+EB group was greater relative to the OVX+EB+P, SH+P, and OVX+VEH groups, which had low levels of cocaine-primed responding. The SH+VEH and OVX+EB groups displayed similar high levels of cocaine-elicited reinstatement. The suppression of cocaine-induced reinstatement following P treatment suggests a role for P in the prevention of relapse to cocaine self-administration in female cocaine users.     

Effect of short- vs. long-term estrogen on reinstatement of cocaine-seeking behavior in female rats

Estrogen effects on cocaine-induced reinstatement of lever responding were examined in sham-operated, vehicle-treated (SH+VEH), ovariectomized (OVX+VEH), and OVX female Wistar rats with estrogen replacement (OVX+EB). The effect of long- (64+/-1.56 days) and short-term (9 days) EB treatment on reinstatement of cocaine-seeking behavior was compared in Experiment 1 and 2, respectively, in order to compare the effect of EB when it was present during the development vs. expression of reinstatement of cocaine-seeking behavior. Rats were trained to self-administer 0.4 mg/kg/inf cocaine. After the acquisition criteria were met, rats continued to respond for cocaine for 2 h/day for a 14-day maintenance period. Cocaine was then replaced with saline and the 21-day extinction period commenced. Subsequently, rats were tested for reinstatement of lever responding on the previously drug-paired lever after alternating daily injections of saline or cocaine. In both experiments, there were no differences between groups in self-administration behavior during training, maintenance, or extinction. In Experiment 1, SH+VEH and chronically treated OVX+EB rats had greater cocaine-induced reinstatement than OVX+VEH rats. In Experiment 2, short-term treated OVX+EB rats also showed enhanced cocaine-induced reinstatement compared to OVX+VEH rats. The results indicate that EB-mediated enhancement of cocaine-induced reinstatement is dependent on EB presence during the expression of reinstatement but not during the formation of stimulus-reward associations during the development of cocaine-reinforced behavior.     

Attenuation of cocaine-seeking by GABA B receptor agonists baclofen and CGP44532 but not the GABA reuptake inhibitor tiagabine in baboons

The current study evaluated the effects of drugs that increase GABA levels by activation of GABA(B) receptors (baclofen and CGP44532) or by inhibition of GABA reuptake (tiagabine) on the reinstatement of extinguished lever responding produced by priming doses of cocaine in baboons (i.e., cocaine-seeking). Cocaine self-injection was established and maintained under a fixed ratio (FR10) schedule of reinforcement during daily 2h sessions. Lever responding was extinguished by substituting vehicle (saline) for cocaine until the number of self-injections decreased to 10 or less per session for two consecutive sessions (defined as extinction). Once extinction occurred, priming doses of cocaine (0.1-3.2mg/kg, i.v.) were administered during extinction conditions. Administration of priming doses of cocaine significantly increased cocaine-seeking in a dose-dependent manner. Cocaine-seeking produced by priming doses of cocaine were attenuated by pretreatment with baclofen (N=5) or CGP44532 (N=5) but not tiagabine (N=3). The doses of baclofen (0.32 mg/kg), and CGP445532 (0.32 mg/kg) that reduced cocaine-seeking produced by cocaine priming doses did not reinstate cocaine-seeking and did not produce overt effects when administered alone. These data indicate that GABA(B) agonists may reduce relapse to cocaine taking.     

Stimulation of Medial Prefrontal Cortex Serotonin 2C (5-HT2C) Receptors Attenuates Cocaine-Seeking Behavior

Serotonin 2C receptor (5-HT_2CR) agonists administered systemically attenuate both cocaine-primed and cue-elicited reinstatement of extinguished cocaine-seeking behavior. To further elucidate the function of these receptors in addiction-like processes, this study examined the effects of microinfusing the 5-HT_2CR agonist MK212 (0, 10, 30, 100 ng/side/0.2 μl) into the medial prefrontal cortex (mPFC) on cocaine self-administration and reinstatement of extinguished cocaine-seeking behavior. Male Sprague–Dawley rats were trained to self-administer cocaine (0.75 mg/kg, i.v.) paired with light and tone cues. Once responding stabilized, rats received MK212 microinfusions before tests for maintenance of cocaine self-administration. Next, extinction training to reduce cocaine-seeking behavior, defined as responses performed without cocaine reinforcement available, occurred until low extinction baselines were achieved. Rats then received MK212 microinfusions before tests for reinstatement of extinguished cocaine-seeking behavior elicited by cocaine-priming injections (10 mg/kg, i.p.) or response-contingent presentations of the cocaine-associated cues; operant responses during cocaine-primed reinstatement tests produced no consequences. MK212 microinfusions into the prelimbic and infralimbic, but not anterior cingulate, regions of the mPFC dose-dependently attenuated both cocaine-primed and cue-elicited reinstatement of extinguished cocaine-seeking behavior, but did not reliably affect cocaine self-administration. A subsequent experiment showed that the effects of MK212 (100 ng/side/0.2 μl) on reinstatement of extinguished cocaine-seeking behavior were blocked by co-administration of the 5-HT_2CR antagonist SB242084 (200 ng/side/0.2 μl). MK212 administered alone into the mPFC as a drug prime produced no discernable effects on cocaine-seeking behavior. These findings suggest that stimulation of 5-HT_2CRs in the mPFC attenuates the incentive motivational effects produced by sampling cocaine or exposure to drug-paired cues.     

Factors that determine a propensity for cocaine-seeking behavior during abstinence in rats.

Individual differences in locomotor responses to novelty and psychostimulants, and sensitization following repeated drug exposure, predict increased sensitivity to the reinforcing effects of psychostimulants and are thought to underlie vulnerability to drug addiction. This study tested whether these factors determine another core feature of drug addiction, the propensity for drug-seeking behavior during abstinence in rats with prior cocaine-self-administration experience. Low and high response groups for each of these factors were determined in outbred rats by the median locomotor response to novelty and amphetamine prior to cocaine self-administration (pre-test), and to amphetamine during abstinence (post-test). Cocaine-seeking behavior during abstinence was measured by the level of drug-paired lever responding during extinction, and also during reinstatement induced by cocaine-associated cues, an amphetamine priming injection, and footshock stress. Animals with low and high locomotor responses to novelty and the amphetamine pre-test showed similar levels of cocaine-seeking behavior during extinction and reinstatement testing. Locomotor responses to amphetamine following cocaine self-administration (post-test) also failed to determine amphetamine's ability to reinstate cocaine-seeking behavior. Conversely, high levels of amphetamine-induced reinstatement were associated specifically with escalating cocaine intake during prior self-administration. These animals also developed locomotor sensitization to amphetamine following cocaine self-administration (post-test vs. pre-test), but the capacity to develop locomotor sensitization was not sufficient to determine a propensity for cocaine-seeking behavior. The findings suggest that the relationship between locomotor responses to novelty, amphetamine and behavioral sensitization a,nd the propensity for cocaine-seeking behavior during abstinence is complex, while the level of drug intake during prior self-administration is a primary determinant of this behavior.     

Administration of the D2 dopamine receptor antagonist sulpiride into the shell, but not the core, of the nucleus accumbens attenuates cocaine priming-induced reinstatement of drug seeking.

Enhanced dopamine transmission in the nucleus accumbens plays an important role in cocaine priming-induced reinstatement of drug-seeking behavior. However, the contribution of each dopamine receptor subtype to this behavior remains unclear. The present experiments were designed to assess the role of D2-like dopamine receptors in the nucleus accumbens core and shell subregions in cocaine priming-induced reinstatement of drug seeking. Rats were trained to lever press for cocaine using a fixed ratio (FR) 5 schedule of reinforcement. After approximately 18 days of cocaine self-administration, the animals underwent an extinction phase during which cocaine was replaced with saline. Daily extinction sessions were conducted until responding was less than 10% of the response rate maintained by cocaine self-administration. Following the extinction phase, priming-induced reinstatement of cocaine-seeking behavior was assessed. A range of doses of antagonists selective for D2- (sulpiride, 0.2 or 2.0 microg), D3- (U99194A, 3.9 or 7.8 microg), or D4- (L-750,667, 5.5 or 11 microg) dopamine receptors were microinjected into either the nucleus accumbens core, shell or lateral septum prior to a priming injection of cocaine (10 mg/kg, i.p.). Following administration into the shell, but not core or lateral septum, sulpiride dose-dependently attenuated reinstatement induced by a cocaine priming injection. In contrast, U99194A and L-750,667 failed to influence cocaine seeking at any of the doses tested in either accumbal subregion. Collectively, these findings indicate that activation of D2 dopamine receptors mediates cocaine priming-induced reinstatement of cocaine seeking in a region-specific manner within the nucleus accumbens.     

Contributions of prolonged contingent and noncontingent cocaine exposure to enhanced reinstatement of cocaine seeking in rats

Rationale Recent evidence suggests that prolonged cocaine self-administration produces escalation in drug-seeking behavior in rats analogous to the increased intake patterns observed in cocaine-dependent individuals. However, the contributions of prolonged access to cocaine taking vs the pharmacologic effects of the consequent increased cocaine exposure on escalation of drug-seeking behaviors have not been investigated.Objective The present study assessed the effects of these two factors on maintenance of cocaine self-administration and reinstatement of cocaine seeking.Methods Male, Sprague–Dawley rats were trained to self-administer cocaine (0.2 mg/i.v. infusion; FR1) for 1 h per day for 10 sessions followed by short access (1 h/day), contingent long access (6 h/day), or noncontingent long access (1 h contingent + 5 h of yoked cocaine infusions/day; i.e., short access + yoked) to cocaine for 14 daily sessions. All rats underwent extinction training and were subsequently tested for the ability of cocaine-paired cues or a cocaine-priming injection (7.5 mg/kg i.p.) to reinstate extinguished cocaine seeking.Results Rats in all groups maintained stable responding for cocaine reinforcement and subsequently showed significant reinstatement of cocaine-seeking behavior. Conditioned-cued reinstatement was enhanced after the contingent long access and short access + yoked cocaine exposure relative to short access cocaine exposure. Conversely, cocaine-primed reinstatement was enhanced after contingent long-access cocaine exposure relative to the other two conditions.Conclusions Enhanced drug seeking produced by prolonged daily cocaine self-administration is due to a combination of behavioral and pharmacological factors. Specifically, conditioned-cued reinstatement is enhanced by increased cocaine intake and cocaine-primed reinstatement is enhanced by increased cocaine taking.     

The role of the dorsomedial prefrontal cortex, basolateral amygdala, and dorsal hippocampus in contextual reinstatement of cocaine seeking in rats.

The present study tested the hypothesis that separate neural substrates mediate cocaine relapse elicited by drug-associated contextual stimuli vs explicit conditioned stimuli (CSs) and cocaine. Specifically, we investigated the involvement of the dorsal hippocampus (DH), basolateral amygdala (BLA), and dorsomedial prefrontal cortex (dmPFC) in contextual reinstatement of cocaine-seeking behavior and the involvement of the DH in explicit CS- and cocaine-induced reinstatement. Rats were trained to self-administer cocaine in a distinct context or in the presence of CSs paired explicitly with cocaine infusions. Responding of context-trained rats was then extinguished in the previously cocaine-paired or an alternate context, whereas responding of explicit CS-trained rats was extinguished in the absence of the CSs. Subsequently, the target brain regions or anatomical control regions were functionally inactivated using tetrodotoxin (0 or 5 ng/side), and cocaine-seeking behavior (ie, nonreinforced responses) was assessed in the cocaine-paired context, in the alternate context, in the presence of the explicit CSs, or following cocaine priming (10 mg/kg, i.p.). DH inactivation abolished contextual, but failed to alter explicit CS- or cocaine-induced, reinstatement of cocaine-seeking behavior. BLA or dmPFC inactivation also abolished contextual reinstatement. Conversely, inactivation of the control brain regions failed to alter contextual reinstatement. In conclusion, the DH, BLA, and dmPFC play critical roles in contextual reinstatement. Previous findings suggest that the BLA is critical for explicit CS-induced, but not cocaine-primed, reinstatement and the dmPFC is critical for both explicit CS-induced and cocaine-primed reinstatement. Thus, distinct but partially overlapping neural substrates mediate context-induced, explicit CS-induced, and cocaine-primed reinstatement of extinguished 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.