Food deprivation affects extinction and reinstatement of responding in rats

The interaction between sexual reward/ deprivation and the acquisition, extinction and reinstatement of morphine-seeking behavior

Natural rewards and abused drugs affect the function of the common brain's reward system. Interaction between social and drug rewards can change the vulnerability to development of drug addiction. Here, we investigate the effects of sexual experience and sex deprivation on the acquisition, maintenance, and drug prime-induced reinstatement of morphine-seeking behavior in male mice using conditioned place preference (CPP). CPP induced with morphine (3, 5, 7 mg/kg, s.c. for 3 days) lasted for 10 days after cessation of morphine treatment and priming dose of morphine (2 mg/kg, s.c.) reinstated the extinguished CPP. In the post-test phase, sexually experienced animals showed a lower preference for morphine compared to sex-deprived males. In the extinction phase, sex deprivation shortened maintenance time compared to control animals. The preference for morphine in sexually experienced animals did not diminish by the seventeenth extinction day. In both groups, the priming injection of morphine after the extinction period could reinstate the extinguished morphine-induced CPP. Together, these data showed the interaction between sex and drug reward and that sexual behavior -a natural rewarding stimulus- can prolong, whereas sex deprivation can block the maintenance of morphine-seeking behaviors. Sexual experience may induce functional and morphological alterations in brain reward areas particularly the mesolimbic system similar to repeated exposure to abused drugs which can affect morphine-seeking behaviors.

Compulsive Conditioned Sexual Responding of Male Japanese Quail in Extinction

Previous experiments showed that following acquisition of an association between a terry-cloth object conditioned stimulus (CS) and a live female unconditioned stimulus (US), male quail increased the frequency of their copulations with the inanimate CS during subsequent CS-alone (extinction) trials. The present experiment was conducted to identify the potential factors responsible for this unexpected increase in conditioned sexual behavior during extinction. A total of 57 naïve male quail were given pairings of a terry-cloth CS with a live female during acquisition. A total of 36 of these quail (the approach responders) showed only conditioned approach response to the CS object, whereas the remaining 21 quail (the consummatory responders) also displayed copulatory or consummatory responses to the CS. In the extinction phase, these two sets of quail were divided into two subgroups: one subgroup received a female in their home cages while the other did not. Consummatory responders that were not exposed to a female quail in the home cage showed a significant increase in conditioned consummatory responding as the extinction trials progressed (i.e., compulsive conditioned sexual responding), whereas the other subgroup showed no change. However, both subgroups showed resistance to extinction in both conditioned approach and consummatory behavior. These findings indicate that the increase in copulation with the terry-cloth CS during extinction is possibly caused by US deprivation. The findings also suggest that conditioned copulation with the terry-cloth CS may lead to partial drive satisfaction, which may contribute to persistence of the behavior. Implications of these findings for paraphilias and compulsive sexual behavior are discussed.

Female rats display higher methamphetamine-primed reinstatement and c-Fos immunoreactivity than male rats

Methamphetamine (meth) dependence is often characterized by persistent and chronic relapse (i.e., return to drug use). Previous work suggests females may be at greater risk to relapse. In this study, we extended this limited evidence and identified sex-dependent neural substrates related to meth-triggered reinstatement. Male and female Sprague-Dawley rats were implanted with indwelling jugular catheters. Half of the rats were then trained to self-administer meth (0.05 mg/kg/inf); the other half self-administered saline during 21 daily sessions (2 h). Rats were then given 12 extinction sessions. Twenty-four hours after the last extinction session, rats received reinstatement testing. Half of the rats received a meth-prime (0.3 mg/kg, IP) injection and the remaining rats received a saline injection. This design resulted in 4 separate groups for each sex, allowing for careful investigation of brain regions related to meth-triggered reinstatement. Brains were harvested following the reinstatement session and c-Fos immunoreactivity was measured in multiple brain regions. Meth triggered reinstatement in both sexes and this effect was more robust in females compared to males. Significant sex differences were detected. Females showed greater c-Fos immunoreactivity in the cingulate cortex area 1, lateral orbitofrontal cortex, prelimbic cortex, caudate-putamen, nucleus accumbens core and shell, and central nucleus of the amygdala following meth-primed reinstatement.

Evaluation of the abuse potential of AM281, a new synthetic cannabinoid CB1 receptor antagonist

AM281 (1-(2,4-dichlorophenyl)-5-(4-iodophenyl)-4-methyl-N-4-morpholinyl-1H-pyrazole-3-carboxamide) is a new synthetic cannabinoid CB1 receptor antagonist. Similar to other cannabinoid antagonists, AM281 has been suggested to have therapeutic indications. However, recent reports have suggested that cannabinoid CB1 receptor antagonists may share similar behavioral effects with other drugs of abuse such as cocaine and amphetamine. These reports cast doubts on the safety profile of AM281. Thus, in the present study we evaluated the abuse potential (rewarding and reinforcing effects) of AM281 through two of the most widely used animal models for assessing the abuse potential of drugs: the conditioned place preference (CPP) and self-administration (SA) tests. Experiments were performed in Sprague-Dawley rats in various dosages [CPP (0.1, 0.5 or 2.5mg/kg), SA (0.005, 0.025 or 0.1mg/kg/infusion)]. We also delved into the consequences of repeated drug exposure on the subsequent response to the drug. Thus, parallel experiments were carried out in rats pretreated with AM281 for 7 or 14 days. Our findings indicated that AM281, at any dose, did not induce CPP and SA in drug-naïve rats. Interestingly, significant CPP (0.5mg/kg of AM281), but not SA, was observed in 14 days pretreated rats. These observations suggest that AM281 per se has no or minimal rewarding and reinforcing properties, but alterations in neuronal functions and behavior due to repeated AM281 exposure may contribute in part to the abuse potential of this drug. In view of this finding, we advocate the careful use, monitoring, and dispensation of AM281.

Moving beyond energy homeostasis: new roles for glucagon-like peptide-1 in food and drug reward

Glucagon-like peptide-1 (GLP-1), a hormone and neuropeptide, is known to regulate energy homeostasis in part through an established central role in controlling food intake. Historically this central role has largely been attributed to GLP-1 receptor signaling in the brainstem and hypothalamus. However, emerging data indicate that GLP-1 also contributes to non-homeostatic regulation of food reward and motivated behaviors in brain reward centers, including the ventral tegmental area and nucleus accumbens. The hypothesis that GLP-1 signaling modulates reward circuitry has provided the impetus for studies demonstrating that GLP-1 attenuates reward for psychostimulants and alcohol. Here, we examine current evidence for GLP-1-mediated regulation of food and drug reward and use these findings to hypothesize mechanisms of action within brain reward centers.

Ghrelin and ghrelin receptor modulation of psychostimulant action

Ghrelin (GHR) is an orexigenic gut peptide that modulates multiple homeostatic functions including gastric emptying, anxiety, stress, memory, feeding, and reinforcement. GHR is known to bind and activate growth-hormone secretagogue receptors (termed GHR-Rs). Of interest to our laboratory has been the assessment of the impact of GHR modulation of the locomotor activation and reward/reinforcement properties of psychostimulants such as cocaine and nicotine. Systemic GHR infusions augment cocaine stimulated locomotion and conditioned place preference (CPP) in rats, as does food restriction (FR) which elevates plasma ghrelin levels. Ghrelin enhancement of psychostimulant function may occur owing to a direct action on mesolimbic dopamine function or may reflect an indirect action of ghrelin on glucocorticoid pathways. Genomic or pharmacological ablation of GHR-Rs attenuates the acute locomotor-enhancing effects of nicotine, cocaine, amphetamine and alcohol and blunts the CPP induced by food, alcohol, amphetamine and cocaine in mice. The stimulant nicotine can induce CPP and like amphetamine and cocaine, repeated administration of nicotine induces locomotor sensitization in rats. Inactivation of ghrelin circuit function in rats by injection of a ghrelin receptor antagonist (e.g., JMV 2959) diminishes the development of nicotine-induced locomotor sensitization. These results suggest a key permissive role for GHR-R activity for the induction of locomotor sensitization to nicotine. Our finding that GHR-R null rats exhibit diminished patterns of responding for intracranial self-stimulation complements an emerging literature implicating central GHR circuits in drug reward/reinforcement. Finally, antagonism of GHR-Rs may represent a smoking cessation modality that not only blocks nicotine-induced reward but that also may limit weight gain after smoking cessation.

Effect of food restriction on cocaine locomotor sensitization in Sprague-Dawley rats: role of kappa opioid receptors

RATIONALE

The interaction between repeated cocaine exposure and food restriction on sensitization to the stimulatory effects of cocaine has not been characterized.

OBJECTIVES

To compare cocaine sensitization in rats free fed and food restricted, and begin to explore the role of the stress-responsive dynorphin/kappa opioid system.

METHODS

Male rats were maintained for 10 days on two feeding conditions: free fed or food restricted (85 % of free fed weight). Test 1 of locomotor reactivity to cocaine (3, 9, or 15 mg/kg, IP) was followed by a sensitizing regimen of cocaine exposure (0 or 30 mg/kg/day × 5 days, IP), by a 10-day drug-free period, and by Test 2 of reactivity to the same cocaine dose. In a second experiment, rats received an injection of norbinaltorphimine (nor-BNI; 0, 5 or 20 mg/kg, SC) 10 days prior to each locomotion test, and plasma corticosterone (CORT) was assessed after Test 2.

RESULTS

On Test 1, it was found that food restriction enhanced locomotor responses to all doses of cocaine. On Test 2, it was found that free fed and food restricted animals displayed similar sensitized responses to cocaine. This, however, was not observed in nor-BNI-treated rats. Furthermore, 20 mg/kg nor-BNI reduced both the locomotor response to cocaine on Test 2 and the effect of cocaine and food restriction on CORT plasma levels.

CONCLUSIONS

These results indicate that the interaction between cocaine sensitization and food restriction is not synergistic, and that it involves activation of kappa-opioid receptors.

Exercise to reduce the escalation of cocaine self-administration in adolescent and adult rats

RATIONALE

Concurrent access to an exercise wheel decreases cocaine self-administration under short access (5 h/day for 5 days) conditions and suppresses cocaine-primed reinstatement in adult rats.

OBJECTIVE

The effect of exercise (wheel running) on the escalation of cocaine intake during long access (LgA, 6 h/day for 26 days) conditions was evaluated.

METHODS

Adolescent and adult female rats acquired wheel running, and behavior was allowed to stabilize for 3 days. They were then implanted with an iv catheter and allowed to self-administer cocaine (0.4 mg/kg, iv) during 6-h daily sessions for 16 days with concurrent access to either an unlocked or a locked running wheel. Subsequently, for ten additional sessions, wheel access conditions during cocaine self-administration sessions were reversed (i.e., locked wheels became unlocked and vice versa).

RESULTS

In the adolescents, concurrent access to the unlocked exercise wheel decreased responding for cocaine and attenuated escalation of cocaine intake irrespective of whether the locked or unlocked condition came first. However, cocaine intake increased when the wheel was subsequently locked for the adolescents that had initial access to an unlocked wheel. Concurrent wheel access either before or after the locked wheel access did not reduce cocaine intake in adults.

CONCLUSIONS

Wheel running reduced cocaine intake during LgA conditions in adolescent but not adult rats, and concurrent access to the running wheel was necessary. These results suggest that exercise prevents cocaine seeking and that this effect is more pronounced in adolescents than adults.

Differential involvement of prelimbic and infralimbic medial prefrontal cortex in discrete cue-induced reinstatement of 3,4-methylenedioxymethamphetamine (MDMA; ecstasy) seeking in rats

RATIONALE

The amphetamine derivative 3,4-methylenedioxymethamphetamine (MDMA; ecstasy) is a widely abused drug, particularly in adolescent and young adult populations. Although it was shown that MDMA-associated cues reinstate extinguished MDMA seeking in an animal relapse model, there is little information regarding the neural mechanisms underlying this behavior.

OBJECTIVES

Because the medial prefrontal cortex (mPFC) plays an important role in relapse to cocaine and methamphetamine seeking, we tested the effects of lidocaine inactivation of prelimbic (PL) and infralimbic (IL) subregions of mPFC on cue-induced relapse to MDMA seeking.

METHODS

Rats were trained to respond for MDMA infusions (0.50 mg/kg/infusion, i.v.) paired with a discrete cue in daily 2-h sessions. Responding was reinforced contingent on a modified fixed ratio 5 schedule of reinforcement. Cue-induced reinstatement tests were conducted after responding was extinguished in the absence of MDMA and the conditioned cues. Prior to reinstatement tests, rats received bilateral microinjections of either lidocaine (100 μg/0.5 μl/side) or physiological saline (0.5 μl/side) delivered to either PL or IL mPFC.

RESULTS

Microinjections of lidocaine into PL completely blocked reinstatement of MDMA-seeking behavior compared with saline microinjections into the same region. Lidocaine microinjections did not, however, have an effect on food-maintained responding, ruling out a nonspecific disruption of motor performance. Conversely, lidocaine inactivation of IL had no effect on reinstatement of MDMA seeking or food-maintained responding.

CONCLUSIONS

Our results provide direct support for PL activation in reinstatement of MDMA-seeking behavior. Moreover, akin to cocaine seeking, there appears to be differential involvement of PL and IL subregions in this behavior.

A limited role for ghrelin in heroin self-administration and food deprivation-induced reinstatement of heroin seeking in rats

Food deprivation (FD) or restriction augments the locomotor activating and reinforcing effects of drugs of abuse. It has been proposed that these effects might be mediated by FD-induced increase in plasma levels of ghrelin, a 28-amino acid orexigenic peptide demonstrated to functionally interact with the mesolimbic dopaminergic system. However, a role for ghrelin has been demonstrated only with psychostimulant drugs and alcohol associated behaviors. We therefore examined the role of ghrelin in ongoing heroin self-administration and FD-induced reinstatement of extinguished heroin seeking. As expected, infusions of ghrelin [0.0, 1.5 and 3.0 µg/rat, intracerebroventricular (i.c.v.)] produced increases in breakpoints on a progressive ratio schedule of heroin reinforcement. In contrast, central administration of a ghrelin receptor antagonist, [D-Lys-3]-GHRP-6 (0.0, or 20.0 µg/rat, i.c.v.) had no effect on ongoing heroin self-administration under a fixed-ratio 1 schedule, or on FD-induced reinstatement of heroin seeking. These results suggest that signals mediated through ghrelin receptors play a limited role in FD-induced augmentation of heroin reinforcement and reinstatement of extinguished heroin seeking.

A role for neuropeptide Y Y5 but not the Y1-receptor subtype in food deprivation-induced reinstatement of heroin seeking in the rat

RATIONAL AND OBJECTIVES

Neuropeptide Y (NPY), an orexigenic peptide that is released during periods of food restriction, has been shown to have a significant modulatory impact on drug-related behaviors. We have previously reported that both acute food deprivation (FD) and NPY injections can reinstate extinguished drug-seeking behavior, a proposed animal model of relapse to drug abuse. However, it is not clear whether the FD effect on drug seeking is dependent on NPY transmission. Here, we used the reinstatement model to assess the role of NPY Y1 and Y5-receptor-mediated transmission in FD-induced reinstatement of heroin seeking.

METHODS

Rats were trained to self-administer heroin for 10-12 days (0.1 mg/kg/infusion/intravenous). Animals then underwent extinction training followed by drug-seeking reinstatement tests under 21 h of FD and sated conditions.

RESULTS

Injections of a novel NPY Y5-receptor antagonist, Lu AA33810 (0.0, 1.0, or 30.0 mg/kg/IP), resulted in a significant attenuation of FD-induced reinstatement of extinguished heroin seeking. However, no significant effects on reinstatement were found for the Y1-receptor antagonist, BIBO 3304 (0.0, 5.0, or 10.0 nmol/intracerebroventricular).

CONCLUSIONS

These results suggest that while signals mediated through NPY Y1 receptors play a modest role in reinstatement, activation of Y5 receptors has a critical function in FD-induced reinstatement of heroin-seeking behavior.

Involvement of the dorsal subiculum and rostral basolateral amygdala in cocaine cue extinction learning in rats

Memory system circuitry may regulate how cues associated with cocaine are extinguished, and understanding neurosubstrates of extinction may lead to the development of improved treatment strategies for cocaine addiction. Sites within the hippocampus and amygdala were investigated for their role in regulating cocaine cue extinction learning. Initially, rats were trained to self-administer cocaine under a second-order reinforcement schedule (cocaine and cocaine cues present) followed by a 2-week abstinence period. Using lidocaine, rats next underwent bilateral inactivation of the dorsal subiculum (dSUB) or rostral basolateral amygdala (rBLA), asymmetric inactivation of the dSUB and rBLA, unilateral inactivation of the dSUB or rBLA, or ipsilateral inactivation of the dSUB and rBLA prior to cocaine cue extinction training sessions (only cocaine cues present) on two consecutive days. Relative to vehicle, bilateral and asymmetric lidocaine treatments in the dSUB and rBLA slowed cocaine cue extinction learning. Specifically, vehicle-treated rats exhibited a significantly larger difference in responding from Day 1 to Day 2 of extinction training than lidocaine-treated rats. In comparison, unilateral or ipsilateral lidocaine treatments in the dSUB and rBLA did not slow cocaine cue extinction learning. Rats treated with lidocaine and vehicle exhibited a similar difference in responding from Day 1 to Day 2 of extinction training. These results indicate that sites within the hippocampus and amygdala need to be functionally active simultaneously in at least one brain hemisphere for acquisition of cocaine cue extinction learning. These results further suggest that a serial circuit within each hemisphere mediates acquisition of cocaine cue extinction learning.

Medial dorsal hypothalamus mediates the inhibition of reward seeking after extinction

Extinction promotes abstinence from drug seeking. Extinction expression is an active process, dependent on infralimbic prefrontal cortex (ilPFC). However, the neurocircuitry mediating extinction expression is unknown. Here we studied the neural mechanisms for expression of extinction of alcoholic beer seeking in rats. We first examined the pattern of activation in prefrontal cortex projections to medial dorsal hypothalamus (MDH) (i.e., perifornical and dorsomedial nuclei) during extinction expression. Double labeling for retrograde tracer cholera toxin B subunit (CTb) and the neuronal activity marker c-Fos revealed significant recruitment of MDH projecting ilPFC neurons during extinction expression. We then studied the causal role of MDH in inhibiting alcoholic beer seeking during extinction expression. MDH infusion of the inhibitory neuropeptide cocaine- and amphetamine-regulated transcript prevented extinction expression, showing that MDH is necessary for extinction expression. Next we examined the pattern of activation in MDH projections to paraventricular thalamus (PVT) during extinction expression. Double labeling for CTb and c-Fos revealed significant recruitment of PVT projecting MDH neurons during extinction expression. We also showed, using triple-label immunofluorescence, that the majority of PVT projecting extinction neurons express prodynorphin, suggesting that actions at κ opioid receptors (KORs) in PVT may be critical for inhibiting alcoholic beer seeking. Consistent with this, infusions of a KOR agonist into PVT prevented reinstatement of alcoholic beer seeking showing that PVT KOR activation is sufficient to inhibit alcoholic beer seeking. Together, these findings identify a role for MDH and its ilPFC afferents and PVT efferents in inhibiting alcoholic beer seeking during extinction expression.

Accumbens shell-hypothalamus interactions mediate extinction of alcohol seeking

The nucleus accumbens shell (AcbSh) is required to inhibit drug seeking after extinction training. Conversely, the lateral hypothalamus (LH), which receives projections from AcbSh, mediates reinstatement of previously extinguished drug seeking. We hypothesized that reversible inactivation of AcbSh using GABA agonists (baclofen/muscimol) would reinstate extinguished alcohol seeking and increase neuronal activation in LH. Rats underwent self-administration training for 4% (v/v) alcoholic beer followed by extinction. AcbSh inactivation reinstated extinguished alcohol seeking when infusions were made after, but not before, extinction training. We then used immunohistochemical detection of c-Fos as a marker of neuronal activity, combined with immunohistochemical detection of the orexin and cocaine- and amphetamine-related transcript (CART) peptides, to study the profile and phenotype of neural activation during reinstatement produced by AcbSh inactivation. AcbSh inactivation increased c-Fos expression in hypothalamus, as well as in paraventricular thalamus and amygdala. Within hypothalamus, there was an increase in the number of orexin and CART cells expressing c-Fos. Finally, we hypothesized that concurrent inactivation of LH would prevent reinstatement produced by inactivation of AcbSh alone. Our results confirmed this. Together, these findings suggest that AcbSh mediates extinction of reward seeking by inhibiting hypothalamic neuropeptide neurons. Reversible inactivation of the AcbSh removes this influence, thereby releasing hypothalamus from AcbSh inhibition and enabling reinstatement of reward seeking. These ventral striatal-hypothalamic circuits for extinction overlap with those that mediate satiety, and we suggest that extinction training inhibits drug seeking because it co-opts neural circuits originally selected to produce satiety.

Reduction of extinction and reinstatement of cocaine seeking by wheel running in female rats

RATIONALE AND OBJECTIVES

Previous work has shown that wheel running reduced the maintenance of cocaine self-administration in rats. In the present study, the effect of wheel running on extinction and reinstatement of cocaine seeking was examined. Female rats were trained to run in a wheel during 6-h sessions, and they were then catheterized and placed in an operant conditioning chamber where they did not have access to the wheel but were allowed to self-administer iv cocaine. Subsequently, rats were divided into four groups and were tested on the extinction and reinstatement of cocaine seeking while they had varying access to a wheel in an adjoining compartment. The four groups were assigned to the following wheel access conditions: (1) wheel running during extinction and reinstatement (WER), (2) wheel running during extinction and a locked wheel during reinstatement (WE), (3) locked wheel during extinction and wheel running during reinstatement (WR), and (4) locked wheel during extinction and reinstatement (WL). WE and WR were retested later to examine the effect of one session of wheel access on cocaine-primed reinstatement.

RESULTS

There were no group differences in wheel revolutions, in rate of acquisition of cocaine self-administration, or in responding during maintenance when there was no wheel access. However, during extinction, WE and WER responded less than WR and WL. WR and WER had lower cocaine-primed reinstatement than WE and WL. One session of wheel exposure in WE also suppressed cocaine-primed reinstatement.

CONCLUSIONS

Wheel running immediately and effectively reduced cocaine-seeking behavior, but concurrent access to running was necessary. Thus, exercise is a useful and self-sustaining intervention to reduce cocaine-seeking behavior.

Endocrine links between food reward and caloric homeostasis

Both intrinsic and extrinsic (endocrine) inputs to the central nervous system (CNS) modulate motivation for feeding. Endocrine inputs such as insulin and leptin can have very rapid effects, but also the potential for chronic actions to decrease rewarding attributes of food. Future studies should elucidate the neural and cellular mechanisms which underlie these endocrine actions in the CNS.

A cocaine hydrolase engineered from human butyrylcholinesterase selectively blocks cocaine toxicity and reinstatement of drug seeking in rats

Successive rational mutations of human butyrylcholinesterase (BChE) followed by fusion to human serum albumin have yielded an efficient hydrolase that offers realistic options for therapy of cocaine overdose and abuse. This albumin-BChE prevented seizures in rats given a normally lethal cocaine injection (100 mg/kg, i.p.), lowered brain cocaine levels even when administered after the drug, and provided rescue after convulsions commenced. Moreover, it selectively blocked cocaine-induced reinstatement of drug seeking in rats that had previously self-administered cocaine. The enzyme treatment was well tolerated and may be worth exploring for clinical application in humans.

Mouse model of relapse to the abuse of drugs: procedural considerations and characterizations

To identify genetic risk factors involved in relapse to the abuse of drugs in humans, it is essential for researchers to develop a reliable mouse model of relapse by extending well-established extinction-reinstatement procedures in rats. Because of technical difficulties such as the relatively short duration of catheter patency in mice, few reports are available on the characterization of extinction-reinstatement behavior in wild-type and genetically engineered mutant mice. In this review, efforts are made to describe practical considerations during the establishment of extinction-reinstatement procedure in mice, including drug-primed, cue-induced, and stress-triggered reinstatement of previously extinguished drug-seeking behavior. Next, attention will be given to some characteristics of extinction-reinstatement behavior in mice. The present review might provide a new impetus in the exploration of genetic risk factors involved in relapse to drug dependence/addiction in humans using extinction-reinstatement procedures in widely available mutant mice.

Level of operant training rather than cocaine intake predicts level of reinstatement

RATIONALE

Extended cocaine self-administration has been shown to potentiate reinstatement. This increased vulnerability to relapse could be attributed not only to extended cocaine exposure but also to extended operant training.

OBJECTIVE

This study was aimed at determining the influence of different operant training histories on cocaine-induced reinstatement when cocaine intake is kept constant.

MATERIALS AND METHODS

Cocaine intake and operant training were dissociated by using experimental procedures generating different histories of operant training but almost identical histories of cocaine intake. Rats were first trained to self-administer cocaine at a classical unit dose (250 microg/inf, FR1), then in independent groups, the level of operant response was changed for the next 20 sessions by changing either the unit dose available (83, 250, or 750 microg/inf, Experiment 1) or the fixed ratio required (FR-1, FR-3, or FR-10, Experiment 2). Then, all rats were tested for reinstatement with different priming doses of cocaine (0, 5, 10, and 15 mg/kg; i.p.) at an early and late stage of an extinction period.

RESULTS

Level of responding during training predicts the level of reinstatement later on, independently of the amount of cocaine consumed. High FR requirement and low unit dose access led to higher level of reinstatement at early and late stage of the extinction period, respectively.

CONCLUSIONS

This study shows that the level of operant responding required to maintain optimal cocaine intake directly influences later levels of reinstatement. This finding suggests that environmental constrains that make drug-taking demanding and effortful may increase the vulnerability to relapse.

Stress and Rodent Models of Drug Addiction: Role of VTA-Accumbens-PFC-Amygdala Circuit

Stress can trigger, intensify, and prolong drug consumption, as well as reinstate previously extinguished drug-taking behavior by directly impacting a neural circuit often referred to as a reward pathways. Animal models of drug abuse have been used to understand these neural circuits mediating stress-induced drug intake and relapse through examination of cellular and subcellular molecular mechanisms. Several types of intermittent stressors have been shown to induce cross-sensitization to psychomotor stimulants, enhance conditioned place preference under most conditions, increase self-administration of cocaine and amphetamine and induce reinstatement of heroin and cocaine seeking via activation of the mesocorticolimbic dopamine system.

A comparison of the effects of different operant training experiences and dietary restriction on the reinstatement of cocaine-seeking in rats

Studies on the reinstatement of drug-seeking after withdrawal from chronic drug self-administration have varied in terms of the procedures by which animals are initially trained to self-administer the drug. The current study directly compared whether prior operant training for food pellet reinforcement and/or maintained dietary restriction significantly altered the reinstatement of extinguished cocaine-seeking in the presence of cocaine-paired cues, a priming injection of cocaine (10 mg/kg; i.p.), and the pharmacological stressor, yohimbine (1.25 or 2.5 mg/kg, i.p.). Male Long Evans rats were divided into four groups as follows: a) trained to lever press for food pellets prior to cocaine self-administration and then maintained on a restricted diet, b) trained to lever press for food pellets prior to cocaine self-administration and then placed on an ad libitum diet, c) untrained and maintained on a restricted diet, or d) untrained and placed on ad libitum feeding. All rats readily self-administered cocaine (0.2 mg/50 mul/infusion) and were subsequently extinguished in the absence of cocaine or previously cocaine-paired cues (light+tone). Following extinction, rats experienced cue-, cocaine-, and yohimbine-induced reinstatement testing. No significant differences were seen between groups for lever responding during the maintenance phase and during extinction. Likewise, reinstatement of cocaine-seeking did not vary across groups for cue-, cocaine-, or yohimbine-induced reinstatement. Under these specific parameters, operant training prior to cocaine self-administration and/or dietary restriction do not significantly alter reinstatement of cocaine-seeking. The results arguably support the approach of not using prior lever training with a non-drug reinforcer and to limit the use of dietary restriction only to the acquisition phase of drug self-administration.

Modeling the role of environment in addiction

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

Black agouti (ACI) rats show greater drug- and cue-induced reinstatement of methamphetamine-seeking behavior than Fischer 344 and Lewis rats

Fischer 344 (F344) and Lewis (LEW) rats differ in methamphetamine self-administration (SA) and methamphetamine-induced reinstatement of previously extinguished behavior. We sought to determine whether genetic background also influences methamphetamine reinforcement efficacy, conditioned reinstatement, and methamphetamine-primed reinstatement of responding in F344, LEW, and Black Agouti (ACI) rats. We implanted rats with jugular catheters and trained them to self-administer methamphetamine (0.06 mg/kg/infusion) under a progressive ratio (PR) schedule of reinforcement during daily 2-h SA sessions. A compound stimulus (light+tone; LT) was paired with each infusion. Dose-dependent intake was determined for each rat. Rats then entered the extinction phase of the experiment where responding resulted in no programmed consequences. Following extinction sessions, rats underwent conditioned reinstatement testing. For conditioned reinstatement, rats received response-contingent presentations of the LT and no methamphetamine. Last, methamphetamine-primed reinstatement test sessions where conducted where subjects received experimenter delivered infusions of methamphetamine (0.06, 0.12, or 0.24 mg/kg). The strains did not differ in PR responding across the doses tested. The ACI rats demonstrated the highest behavioral output during extinction training, conditioned- and methamphetamine-primed reinstatement of previously extinguished behavior compared to the other strains. These data suggest that genetic background differentially influences extinction, conditioned reinstatement and methamphetamine-primed reinstatement in rats.

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.

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.

Regulation of drug taking by sensitization and habituation

The authors argue that drug taking is an operant behavior that is reinforced by the drug itself. The effectiveness of a drug as a reinforcer is modulated by sensitization and habituation to the drug as it is consumed. According to this model, drug taking stops when habituation reduces the ability of the drug to reinforce its own consumption. Drug taking resumes when spontaneous recovery restores the effectiveness of the drug as a reinforcer. This parsimonious model provides a framework for understanding many findings in the drug literature, including acute and chronic tolerance, the effect of deprivation on consumption, the contextual specificity of tolerance, polydrug abuse, cross-sensitization between stress and drugs, behavioral sensitization, priming, and reinstatement. Although this model cannot explain all aspects of drug taking (e.g., the effect of cognitive manipulations), it has many implications for understanding and controlling human drug consumption and addiction.

Neonatal isolation stress potentiates cocaine seeking behavior in adult male and female rats

Little is known with regard to how sex and stress might interact as vulnerability factors in cocaine abuse. In this study, we compared the effects of neonatal isolation stress on cocaine self-administration under extended access conditions and on subsequent responding in a cue-induced reinstatement paradigm in adult male and female rats. Pups from each litter were subjected to either neonatal isolation (1 h/day) or brief daily handling from postnatal day 2 through 12. Adults rats were then trained to self-administer cocaine, and once they acquired lever responding for cocaine under a fixed ratio 1 schedule, they were given 24-h access to intravenous cocaine infusions (1.5 mg/kg) that were available in discrete trials (4, 10 min trials/h) for 7 consecutive days. At 10 days after the last discrete trial session, responding was assessed during six to eight 1-h extinction sessions that were followed by a 1-h cue-induced reinstatement session. Results revealed that females took more cocaine than did males over the 7-day discrete trial self-administration period and tended to respond at higher levels during the initial extinction sessions. Although intake did not differ between handled control rats and isolated rats under extended access conditions, stress effects were observed under subsequent extinction and cue-induced reinstatement testing conditions with isolated rats responding at higher levels during both phases. Notably, stress seemed to obscure sex differences in extinction responding such both isolated males and females responded at high levels. These findings demonstrate robust and enduring effects of neonatal isolation stress on cocaine seeking behavior in adult male and female rats.

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

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

Effect of environmental stressors on opiate and psychostimulant reinforcement, reinstatement and discrimination in rats: a review

Studies in humans suggest that exposure to life stressors is correlated with compulsive drug abuse and relapse to drugs during periods of abstinence. The behavioral and neurobiological mechanisms involved in the effect of stress on drug abuse, however, are not known. Here, we review data from studies using preclinical models in rats on the effect of environmental stressors on opiate and psychostimulant reinforcement, as measured by the intravenous drug self-administration and conditioned place preference procedures, on relapse to these drugs, as measured by the reinstatement procedure, and on the subjective effects of these drugs, as measured by the drug discrimination procedure. The results of the studies reviewed here suggest that while stressors are important modulators of the behavioral effects of opiate and psychostimulant drugs, the effect of stress on behavior in these animal models is stressor-specific, and to some degree, procedure- and drug-class-specific. The review of studies on the neurobiological mechanisms underlying stress-drug interactions in these animal models indicate that central noradrenaline and extrahypothalamic corticotropin-releasing factor mediate the effect of one form of stress (intermittent footshock) on reinstatement of opiate and psychostimulant seeking after prolonged drug-free periods. At present, however, little is known about the neuronal events that mediate the effect of environmental stressors on opiate and psychostimulant reinforcement or discrimination. The broader implications of the data reviewed here for future research and for the treatment of opiate and psychostimulant addiction are briefly discussed.

Effect of response-independent candy on responding maintained by candy using a novel model of commodity acquisition and consumption in nonhuman primates

Ingestive behavior consists of appetitive or foraging behavior, i.e., "acquisition," followed by consummatory behavior. Responding of six adult rhesus monkeys, living in three-chambered enclosures, was studied under an operant chain schedule that simulated commodity acquisition and commodity consumption. Responding during the initial acquisition component was reinforced by stimuli paired with that commodity, while responding during the following consumption component was reinforced with that commodity. Throughout the 10-h experimental day, monkeys experienced multiple candy (plain M & Ms) and fruit-drink (Kool-Aid) sessions in different end chambers. The effects of response-independent candy reinforcement, in the context of extinction, were examined when monkeys received a daily food ration of 8 or 20 chow. Response-independent candy increased responding during the acquisition components of candy sessions when monkeys received a daily food ration of 8 chow but not when the food ration was 20 chow. Furthermore, response-independent candy increased candy choice over fruit-drink during choice opportunities and increased the length of time spent in the candy chamber when the candy stimulus lights were illuminated under both food ration conditions, i.e., location preference. The present procedure, which combines operant and place preference measures of commodity acquisition, when used in combination with methods of studying reinstatement of responding, may prove useful in analyzing factors affecting relapse.

Effects of excitotoxic lesions of the basolateral amygdala on cocaine-seeking behavior and cocaine conditioned place preference in rats

Incentive motivation for cocaine, elicited by cocaine-associated stimuli, is thought to be involved in craving and relapse. To examine the role of the basolateral amygdala complex (BLC) in this phenomenon, we assessed the effects of post-training BLC lesions on extinction of cocaine-seeking behavior and cocaine-conditioned place preference (CPP) and the effects of pre-training BLC lesions on acquisition of cocaine-CPP. In Experiment 1, rats were first trained to self-administer cocaine and then received bilateral infusions of the excitotoxin, N-methyl-D-aspartic acid (NMDA, 0.12 M; 0.3 microl/side), or vehicle into the BLC. They were then tested repeatedly for extinction of cocaine-seeking behavior (i.e. nonreinforced responses in the presence of cocaine-paired stimuli). Subsequently, they were trained and tested for acquisition of cocaine-CPP (i.e. increased time spent in a previously cocaine-paired, relative to a saline-paired, environment). Locomotion and compartment entries were also measured. In Experiment 2, rats were first trained and tested for cocaine-CPP, and then received NMDA or vehicle infusions into the BLC. Subsequently, they were tested repeatedly for extinction of cocaine-CPP. Post-training BLC lesions retarded extinction of cocaine-seeking behavior and cocaine-CPP, whereas pre-training lesions disrupted acquisition of cocaine-CPP. These effects did not appear to be related to changes in general activity. We suggest that pre-training BLC lesions disrupted acquisition of cocaine-CPP by impairing assignment of incentive value to cocaine-paired stimuli, whereas post-training BLC lesions disrupted extinction of cocaine-conditioned behaviors by impairing the assessment of the current incentive value of cocaine-paired stimuli.

Evaluation of the reinforcing effects of the cannabinoid CB1 receptor antagonist, SR141716, in rhesus monkeys

The abuse liability of a selective cannabinoid CB1 receptor antagonist, SR141716 (N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxyamide hydrochloride), was evaluated in rhesus monkeys. Four rhesus monkeys with chronically indwelling venous catheters were initially trained to self-administer cocaine (30 microg/kg/injection) during daily 1-h sessions under a fixed ratio 50 (FR50) schedule of reinforcement. SR141716 was subsequently substituted for cocaine, and SR141716 dose was varied from 1 to 100 microg/kg/injection. Each dose of SR141716 was tested for four consecutive sessions and each unit dose was separated by at least three sessions of cocaine self-administration. Substitution of SR141716 for cocaine resulted in rapid extinction of lever pressing and none of the doses of SR141716 tested was self-administered above the vehicle levels. When the highest dose of SR141716 (100 microg/kg/injection) was evaluated, self-administration behavior was suppressed below vehicle levels suggesting that behaviorally active doses were evaluated. Since positive results in self-administration tests are generally predictive of abuse potential, the negative results with SR141716 suggest that this drug would likely have low abuse liability.

GABA modulation of cocaine self-administration

Recent experiments suggest that GABA compounds produce a clinically relevant modulation of cocaine reinforcement. This review summarizes the results of a number of studies that examined the effect of the GABAB agonist baclofen on cocaine self-administration using a variety of schedules of reinforcement. The results demonstrate that baclofen causes a dramatic reduction in cocaine self-administration which does not appear to be accounted for by a general disruption of behavior. However, the effect is dependent on the unit injection dose of cocaine and on the response requirements of the schedule. These results predict that in a clinical setting any potential therapeutic effect of baclofen will interact with the cost and availability of cocaine.

Nucleus accumbens cell firing during maintenance, extinction, and reinstatement of cocaine self-administration behavior in rats

Electrophysiological recording procedures were used to examine nucleus accumbens (Acb) cell firing in rats (N=13) during cocaine self-administration sessions consisting of three phases. In phase one (maintenance), each lever press resulted in an intravenous cocaine infusion (0.33 mg, 6 s) paired with a tone-houselight stimulus (20 s). Of 144 Acb cells recorded during maintenance, 39 neurons (27%) exhibited phasic firing relative to the cocaine-reinforced response [4-8]. Briefly, Acb neurons showed increases in firing rate within seconds preceding the reinforced response and/or changes (increases or decreases) in activity within seconds following response completion. In phase two (extinction), saline was substituted for cocaine in the task. Results indicated that cells displaying exclusively anticipatory discharges during maintenance exhibited similar phasic activity during extinction. However, neurons that displayed post-response activity during the maintenance phase typically showed significant attenuation of phasic firing rates during extinction. After 30 min of no responding, animals were 'primed' with an intravenous infusion of cocaine, and self-administration was reestablished during phase three (reinstatement). Results showed that pre-response discharge patterns remained relatively intact while post-response cells typically exhibited a partial recovery of phasic activity. Similar findings were observed during other extinction experiments in which the stimulus only was removed (CS extinction). These findings support the notion that specific factors operating within the self-administration context differentially control pre- versus post-response discharge patterns of Acb neurons.

The role of corticotropin-releasing factor and corticosterone in stress- and cocaine-induced relapse to cocaine seeking in rats

We have shown previously that footshock stress and priming injections of cocaine reinstate cocaine seeking in rats after prolonged drug-free periods (Erb et al., 1996). Here we examined the role of brain corticotropin-releasing factor (CRF) and the adrenal hormone corticosterone in stress- and cocaine-induced reinstatement of cocaine seeking in rats. The ability of footshock stress and priming injections of cocaine to induce relapse to cocaine seeking was studied after intracerebroventricular infusions of the CRF receptor antagonist D-Phe CRF12-41, after adrenalectomy, and after adrenalectomy with corticosterone replacement. Rats were allowed to self-administer cocaine (1.0 mg/kg/infusion, i.v) for 3 hr daily for 10-14 d and were then placed on an extinction schedule during which saline was substituted for cocaine. Tests for reinstatement were given after intermittent footshock (10 min; 0.5 mA) and after priming injections of saline and cocaine (20 mg/kg, i.p.). Footshock reinstated cocaine seeking in both intact animals and animals with corticosterone replacement but not in adrenalectomized animals. The CRF receptor antagonist D-Phe CRF12-41 blocked footshock-induced reinstatement at all doses tested in both intact animals and animals with corticosterone replacement. Reinstatement by priming injections of cocaine was only minimally attenuated by adrenalectomy and by pretreatment with D-Phe CRF12-41. These data suggest that brain CRF plays a critical role in stress-induced, but only a modulatory role in cocaine-induced, reinstatement of cocaine seeking. Furthermore, the data show that although reinstatement of cocaine seeking by footshock stress requires minimal, basal, levels of corticosterone, stress-induced increases in corticosterone do not play a role in this effect.

Sexually arousing events and relapse to heroin-seeking in sexually experienced male rats

We have shown previously, using a reinstatement procedure, that both priming injections of heroin and exposure to footshock stress reinstate heroin-taking behavior following prolonged drug-free periods. In the present study, we examined the effect of another highly arousing event, exposure to a sexually active female, on reinstatement of heroin-seeking. Male rats were first given sexual experience, being allowed to copulate on 4 occasions with sexually active females and were then trained to self-administer heroin (100 micrograms/kg per infusion, IV) for 4 3-h sessions/day for 5-6 days and 1 6-h session/day for an additional 6 days. Extinction sessions were then given for 4 days, 6-h/day, during which saline was substituted for heroin. On tests for reinstatement, males were presented with: 1. The wire-mesh side of an empty cage (baseline condition), 2. the sight, odor, and smell of a sexually inactive female, 3. the sight, odor, and smell of a female in heat, 4. a female in heat, and allowed to copulate, 5. intermittent footshock (15 min, 0.5 mA, 0.5 s on, mean off period of 40 s), or 6. a priming injection of heroin (0.25 mg/kg, SC). Reinstatement of heroin-taking behavior was observed after exposure to the priming injection of heroin and to footshock stress. Reinstatement of heroin-taking behavior was not induced by exposure to the females in any of the conditions. Thus, motivational arousal, as such, does not appear to be a sufficient stimulus for relapse to heroin-taking.

Acquisition of i.v. amphetamine and cocaine self-administration in rats as a function of dose

The effect of dose on the acquisition of i.v. amphetamine and cocaine self-administration was examined. Three unit doses of amphetamine (0.03, 0.06 and 0.12 mg/kg) and three unit doses of cocaine (0.05, 0.2 and 0.8 mg/kg) were tested in separate groups of ten (amphetamine) or 13 (cocaine) rats. Autoshaping methods were used to train rats to press a lever that resulted in drug infusion under a fixed-ratio (FR) 1 schedule. A daily 6-h autoshaping component non-contingently delivered 60 infusions according to a 60-s random time schedule with ten infusions delivered during the first half of each h. Each day autoshaping sessions were followed by a 6-h self-administration session. The criterion for acquisition was a 5-day period during which a daily mean of 100, 50 or 25 infusions for the three amphetamine doses and 400, 100 or 25 infusions were earned during the 6-h self-administration period for the three cocaine doses, respectively. As dose increased, more rats per group acquired drug self-administration and the mean number of days to meet the acquisition criterion decreased. The percentage of rats acquiring amphetamine self-administration increased with dose and ranged from 80 to 100%. Only one rat at the lowest cocaine dose met the acquisition criterion, but 100 percent of the rats at the two higher doses acquired. During the last 2 days of acquisition, mean infusions decreased and mean drug intake (mg/kg) increased as dose increased. On the last day of acquisition, the time course of infusions during the 6-h self-administration component was characterized by a steady rate of infusions per hour, and number of infusions was inversely related to dose. These findings indicate that the initial available dose of a drug is an important determinant of the rate and probability that successful acquisition will occur.