Time-dependent changes in extinction behavior and stress-induced reinstatement of drug seeking following withdrawal from heroin in rats

Discrete cue-conditioned alcohol-seeking after protracted abstinence: pattern of neural activation and involvement of orexin₁ receptors

BACKGROUND AND PURPOSE

The enduring propensity for alcoholics to relapse even following years of abstinence presents a major hurdle for treatment. Here we report a model of relapse following protracted abstinence and investigate the pattern of neuronal activation following cue-induced reinstatement and administration of the orexin₁ receptor antagonist SB-334867 in inbred alcohol-preferring rats.

EXPERIMENTAL APPROACH

Rats were trained to self-administer alcohol under operant conditions and divided into two groups: immediate (reinstated immediately following extinction) and delayed (extinguished and then housed for 5 months before reinstatement). Prior to reinstatement, animals were treated with vehicle (immediate n= 11, delayed n= 11) or SB-334867 (20 mg·kg⁻¹ i.p.; immediate n= 6, delayed n= 11). Fos expression was compared between each group and to animals that underwent extinction only.

KEY RESULTS

SB-334867 significantly attenuated cue-induced reinstatement in both groups. Immediate reinstatement increased Fos expression in the nucleus accumbens (NAc), infra-limbic (IL), pre-limbic (PrL), orbitofrontal (OFC) and piriform cortices, the lateral and dorsomedial hypothalamus, central amygdala and basolateral amygdala (BLA), and the bed nucleus of the stria terminalis. Following delayed reinstatement, Fos expression was further elevated in cortical structures. Concurrent with preventing reinstatement, SB-334867 decreased Fos in NAc core, PrL and OFC following immediate reinstatement. Following protracted abstinence, SB-334867 treatment decreased reinstatement-induced Fos in the PrL, OFC and piriform cortices.

CONCLUSIONS AND IMPLICATIONS

Cue-induced alcohol seeking can be triggered following protracted abstinence in rats. The effects of SB-334867 on both behaviour and Fos expression suggest that the orexin system is implicated in cue-induced reinstatement, although some loci may shift following protracted abstinence.

Comparison of self-administration behavior and responsiveness to drug-paired cues in rats running an alley for intravenous heroin and cocaine

RATIONALE

Evidence suggests that responsiveness to a drug-paired cue is predicted by the reinforcing magnitude of the drug during prior self-administration. It remains unclear, however, if this principle holds true when comparisons are made across drug reinforcers.

OBJECTIVE

The current study was therefore devised to test the hypothesis that differences in the animals' responsiveness to a cocaine- or heroin-paired cue presented during extinction would reflect differences in the patterns of prior cocaine and heroin runway self-administration.

METHODS

Rats ran a straight alley for single intravenous injections of either heroin (0.1 mg/kg/inj) or cocaine (1.0 mg/kg/inj) each paired with a distinct olfactory cue. Animals experienced 15 trials with each drug reinforcer in a counterbalanced manner. Start latencies, run times, and retreat behaviors (a form of approach-avoidance conflict) provided behavioral indices of the subjects' motivation to seek the reinforcer on each trial. Responsiveness to each drug-paired cue was assessed after 7, 14, or 21 days of non-reinforced extinction trials. Other animals underwent conditioned place preference (CPP) testing to ensure that the two drug reinforcers were capable of producing drug-cue associations.

RESULTS

While both drugs produced comparable CPPs, heroin served as a stronger incentive stimulus in the runway as evidenced by faster start and run times and fewer retreats. In contrast, cocaine- but not heroin-paired cues produced increases in drug-seeking behavior during subsequent extinction trials.

CONCLUSIONS

The subjects' responsiveness to drug-paired cues during extinction was not predicted by differences in the motivation to seek heroin versus cocaine during prior drug self-administration.

Endogenous GDNF in ventral tegmental area and nucleus accumbens does not play a role in the incubation of heroin craving

Glial cell line-derived neurotrophic factor (GDNF) activity in ventral tegmental area (VTA) mediates the time-dependent increases in cue-induced cocaine-seeking after withdrawal (incubation of cocaine craving). Here, we studied the generality of these findings to incubation of heroin craving. Rats were trained to self-administer heroin for 10 days (6 hours/day; 0.075 mg/kg/infusion; infusions were paired with a tone-light cue) and tested for cue-induced heroin-seeking in extinction tests after 1, 11 or 30 withdrawal days. Cue-induced heroin seeking was higher after 11 or 30 days than after 1 day (incubation of heroin craving), and the time-dependent increases in extinction responding were associated with time-dependent changes in GDNF mRNA expression in VTA and nucleus accumbens. Additionally, acute accumbens (but not VTA) GDNF injections (12.5 µg/side) administered 1-3 hours after the last heroin self-administration training session enhanced the time-dependent increases in extinction responding after withdrawal. However, the time-dependent increases in extinction responding after withdrawal were not associated with changes in GDNF protein expression in VTA and accumbens. Additionally, interfering with endogenous GDNF function by chronic delivery of anti-GDNF monoclonal neutralizing antibodies (600 ng/side/day) into VTA or accumbens had no effect on the time-dependent increases in extinction responding. In summary, heroin self-administration and withdrawal regulate VTA and accumbens GDNF mRNA expression in a time-dependent manner, and exogenous GDNF administration into accumbens but not VTA potentiates cue-induced heroin seeking. However, based on the GDNF protein expression and the anti-GDNF monoclonal neutralizing antibodies manipulation data, we conclude that neither accumbens nor VTA endogenous GDNF mediates the incubation of heroin craving.

A transcriptomic analysis of type I-III neurons in the bed nucleus of the stria terminalis

The activity of neurons in the anterolateral cell group of the bed nucleus of the stria terminalis (BNST(ALG)) plays a critical role in anxiety- and stress-related behaviors. Histochemical studies have suggested that multiple distinct neuronal phenotypes exist in the BNST(ALG). Consistent with this observation, the physiological properties of BNST(ALG) neurons are also heterogeneous, and three distinct cell types can be defined (Types I-III) based primarily on their expression of four key membrane currents, namely I(h), I(A), I(T), and I(K(IR)). Significantly, all four channels are multimeric proteins and can comprise of more than one pore-forming α subunit. Hence, differential expression of α subunits may further diversify the neuronal population. However, nothing is known about the relative expression of these ion channel α subunits in BNST(ALG) neurons. We have addressed this lacuna by combining whole-cell patch-clamp recording together with single-cell reverse transcriptase polymerase chain reaction (scRT-PCR) to assess the mRNA transcript expression for each of the subunits for the four key ion channels in Type I-III neurons of the BNST(ALG.) Here, cytosolic mRNA from single neurons was probed for the expression of transcripts for each of the α subunits of I(h) (HCN1-HCN4), I(T) (Ca(v)3.1-Ca(v)3.3), I(A) (K(v)1.4, K(v)3.4, K(v)4.1-K(v) 4.3) and I(K(IR)) (Kir2.1-Kir2.4). An unbiased hierarchical cluster analysis followed by discriminant function analysis revealed that a positive correlation exists between the physiological and genetic phenotype of BNST(ALG) neurons. Thus, the analysis segregated BNST(ALG) neurons into 3 distinct groups, based on their α subunit mRNA expression profile, which positively correlated with our existing electrophysiological classification (Types I-III). Furthermore, analysis of mRNA transcript expression in Type I-Type III neurons suggested that, whereas Type I and III neurons appear to represent genetically homologous cell populations, Type II neurons may be further subdivided into three genetically distinct subgroups. These data not only validate our original classification scheme, but further refine the classification at the molecular level, and thus identifies novel targets for potential disruption and/or pharmacotherapeutic intervention in stress-related anxiety-like behaviors.

Incubation of cue-induced cigarette craving during abstinence in human smokers

BACKGROUND

Abstinent drug users remain at risk for relapse long after withdrawal subsides. Animal studies indicate that responses to drug-related cues not only persist but increase with abstinence, a phenomenon termed "incubation of drug craving." It is unknown whether cue-induced craving increases, decreases, or remains constant with abstinence in humans. We investigated effects of abstinence on cue-induced craving in cigarette smokers.

METHODS

Eighty-six non-treatment-seeking, adult smokers (≥10 cigarettes daily) were paid to abstain for 7 (Group 1), 14 (Group 2), or 35 (Groups 3 and 4) days. Abstinence was verified daily. Groups 1, 2, and 3 underwent a single cue session on the final abstinence day (7, 14, or 35). Group 4 viewed cues on Days 7, 14, and 35.

RESULTS

Between and within groups, smoking-cue-induced craving increased with abstinence on some measures. Cue-induced craving was greater in Group 3 (35-day) compared with Group 1 (7-day). Within Group 4, cue-induced craving was greater at 35 than 14 days. Cue-induced craving did not decrease with abstinence on any measure.

CONCLUSIONS

We present initial evidence of incubation of cue-induced craving in humans. The observation that cue-induced craving increases with abstinence, even as "background" craving and withdrawal symptoms subside, might have treatment implications.

Episodic withdrawal promotes psychomotor sensitization to morphine

The relative intermittency or continuity of drug delivery is a major determinant of addictive liability, and also influences the impact of drug exposure on brain function and behavior. Events that occur during the offset of drug action (ie, acute withdrawal) may have an important role in the consequences of intermittent drug exposure. We assessed whether recurrent episodes of acute withdrawal contribute to the development of psychomotor sensitization in rodents during daily morphine exposure. The acoustic startle reflex--a measure of anxiety induced by opiate withdrawal-was used to resolve and quantify discrete withdrawal episodes, and pharmacological interventions were used to manipulate withdrawal severity. Startle potentiation was observed during spontaneous withdrawal from a single morphine exposure, and individual differences in initial withdrawal severity positively predicted the subsequent development of sensitization. Manipulations that reduce or exacerbate withdrawal severity also produced parallel changes in the degree of sensitization. These results demonstrate that the episodic experience of withdrawal during daily drug exposure has a novel role in promoting the development of psychomotor sensitization--a prominent model of drug-induced neurobehavioral plasticity. Episodic withdrawal may have a pervasive role in many effects of intermittent drug exposure and contribute to the development of addiction.

Ventral tegmental afferents in stress-induced reinstatement: the role of cAMP response element-binding protein

The transcription factor cAMP response element-binding protein (CREB) is required for stress- but not drug-induced reinstatement of cocaine conditioned place preference. To reveal the neural circuitry associated with this CREB dependence, we injected a retrograde tracer into the ventral tegmental area (VTA) and identified afferents that were activated after stress or cocaine exposure in both naive and cocaine-conditioned mice. Neuronal activation, as assessed by Fos expression, was greatly reduced in the dorsal and ventral bed nucleus of the stria terminalis (BNST), lateral septum, and nucleus accumbens shell in mice lacking CREB (CREBαΔ mice) after a 6 min swim stress but not after cocaine exposure (20 mg/kg). Additionally, activation of VTA afferent neurons in the ventral BNST and the infralimbic cortex in CREBαΔ mice was blunted in response to stress. This pattern of neuronal activation persisted in mice that were conditioned to a cocaine place preference procedure before stress exposure. Furthermore, lidocaine inactivation (0.4 μl, 4%) studies demonstrated the necessity of BNST activation for swim-stress-induced reinstatement of cocaine-conditioned reward. Together, the present studies demonstrate that CREB is required for the activation of a unique circuit that converges on the dopamine reward pathway to elicit reinstatement of drug reward and points to the BNST as a key intersection between stress and reward circuits.

Evaluation of the relationship between anxiety during withdrawal and stress-induced reinstatement of cocaine seeking

The initial termination of cocaine consumption in human addicts is associated with heightened anxiety states and low levels of craving. Craving, however, tends to increase progressively over time, remains high for extended periods of time, and can be exacerbated by stressors, leading to relapse. Laboratory rats, likewise, exhibit heightened states of anxiety after withdrawal from drug, and follow a time course of cocaine seeking that parallels the time course of craving reported in humans. In addition, laboratory rats show heightened susceptibility to relapse when exposed to stressors after extended periods of withdrawal, and exhibit persistent and heightened expressions of stress-induced anxiety. The general objective of this paper is to consider the relationship between anxiety states after withdrawal from cocaine and stress-induced reinstatement of cocaine seeking in laboratory rats, and to identify the neural substrates involved. The focus of the review is on studies addressing the roles of corticotropin-releasing factor (CRF) and noradrenaline pathways of the extended amygdala circuitry, and their direct or indirect interactions with the mesocorticolimbic dopamine system, in anxiety after withdrawal from cocaine and stress-induced reinstatement of cocaine seeking. Furthermore, the effects of time after withdrawal from cocaine and amount of cocaine exposure during self-administration on the activity of CRF, noradrenaline, and dopamine pathways of the extended amygdala and mesocorticolimbic systems will be considered. The review will highlight how changing levels of activity within these systems may serve to alter the nature of the relationship between anxiety and stress-induced reinstatement of cocaine seeking at different times after withdrawal from cocaine.

Role of innate and drug-induced dysregulation of brain stress and arousal systems in addiction: Focus on corticotropin-releasing factor, nociceptin/orphanin FQ, and orexin/hypocretin

Stress-like symptoms are an integral part of acute and protracted drug withdrawal, and several lines of evidence have shown that dysregulation of brain stress systems, including the extrahypothalamic corticotropin-releasing factor (CRF) system, following long-term drug use is of major importance in maintaining drug and alcohol addiction. Recently, two other neuropeptide systems have attracted interest, the nociceptin/orphanin FQ (N/OFQ) and orexin/hypocretin (Orx/Hcrt) systems. N/OFQ participates in a wide range of physiological responses, and the hypothalamic Orx/Hcrt system helps regulate several physiological processes, including feeding, energy metabolism, and arousal. Moreover, these two systems have been suggested to participate in psychiatric disorders, including anxiety and drug addiction. Dysregulation of these systems by chronic drug exposure has been hypothesized to play a role in the maintenance of addiction and dependence. Recent evidence demonstrated that interactions between CRF-N/OFQ and CRF-Orx/Hcrt systems may be functionally relevant for the control of stress-related addictive behavior. The present review discusses recent findings that support the hypotheses of the participation and dysregulation of these systems in drug addiction and evaluates the current understanding of interactions among these stress-regulatory peptides.

AMPA receptor plasticity in the nucleus accumbens after repeated exposure to cocaine

This review focuses on cocaine-induced postsynaptic plasticity in the nucleus accumbens (NAc) involving changes in AMPA receptor (AMPAR) transmission. First, fundamental properties of AMPAR in the NAc are reviewed. Then, we provide a detailed and critical analysis of literature demonstrating alterations in AMPAR transmission in association with behavioral sensitization to cocaine and cocaine self-administration. We conclude that cocaine exposure leads to changes in AMPAR transmission that depend on many factors including whether exposure is contingent or non-contingent, the duration of withdrawal, and whether extinction training has occurred. The relationship between changes in AMPAR transmission and responding to cocaine or cocaine-paired cues can also be affected by these variables. However, after prolonged withdrawal in the absence of extinction training, our findings and others lead us to propose that AMPAR transmission is enhanced, resulting in stronger responding to drug-paired cues. Finally, many results indicate that the state of synaptic transmission in the NAc after cocaine exposure is associated with impairment of AMPAR-dependent plasticity. This may contribute to a broad range of addiction-related behavioral changes.

alpha2A-adrenergic receptors heterosynaptically regulate glutamatergic transmission in the bed nucleus of the stria terminalis

Stress is a major driving force in reinstatement of drug-seeking behavior. The bed nucleus of the stria terminalis (BNST) has been identified as a key brain region in this behavior, and receives a dense input of the stress-neurotransmitter norepinephrine through the ventral noradrenergic bundle. Activation of alpha(2)-adrenergic receptors (alpha(2)-ARs) in the BNST blocks stress-induced reinstatement of drug-seeking, indicating a potentially important role for these receptors. Currently, it is unclear how alpha(2)-AR agonists elicit this behavioral action, or through which alpha(2)-AR subtype. Activation of alpha(2)-ARs decreases glutamatergic transmission in the BNST, an effect which is nearly absent in the alpha(2A)-AR knockout mouse. Here, we take advantage of a knock-in mouse in which a hemagglutinin-tagged alpha(2A)-AR was inserted into the endogenous locus, along with the alpha(2A)-AR selective agonist guanfacine, to further study the role of the alpha(2A)-AR subtype in modulation of neurotransmission in the BNST. Using immunohistochemistry, we find that alpha(2A)-ARs are highly expressed in the BNST, and that this expression is more similar in distribution to the vesicular glutamate transporters than to either norepinephrine transporter or tyrosine hydroxylase positive terminals. Using whole cell patch-clamp recordings, we show that guanfacine causes a depression of evoked excitatory and, to a more limited extent, inhibitory fast synaptic transmission. In total, these data support a prominent heterosynaptic role for alpha(2A)-ARs in modulating fast synaptic transmission in the BNST.

Subcellular plasticity of the corticotropin-releasing factor receptor in dendrites of the mouse bed nucleus of the stria terminalis following chronic opiate exposure

Chronic opiate administration alters the expression levels of the stress-responsive peptide, corticotropin-releasing factor (CRF), in the bed nucleus of the stria terminalis (BNST). This brain region contains CRF receptors that drive drug-seeking behavior exacerbated by stress. We used electron microscopy to quantitatively compare immunolabeling of the corticotropin-releasing factor receptor (CRFr) and CRF in the anterolateral bed nucleus of the stria terminalis (BSTal) of mice injected with saline or morphine in escalating doses for 14 days. We also compared the results with those in non-injected control mice. The tissue was processed for CRFr immunogold and CRF immunoperoxidase labeling. The non-injected controls had a significantly lower plasmalemmal density of CRFr immunogold particles in dendrites compared with mice receiving saline, but not those receiving morphine, injections. Compared with saline, however, mice receiving chronic morphine showed a significantly lower plasmalemmal, and greater cytoplasmic, density of CRFr immunogold in dendrites. Within the cytoplasmic compartment of somata and dendrites of the BSTal, the proportion of CRFr gold particles associated with mitochondria was three times as great in mice receiving morphine compared with saline. This subcellular distribution is consistent with morphine,- and CRFr-associated modulation of intracellular calcium release or oxidative stress. The between-group changes occurred without effect on the total number of dendritic CRFr immunogold particles, suggesting that chronic morphine enhances internalization or decreases delivery of the CRFr to the plasma membrane, a trafficking effect that is also affected by the stress of daily injections. In contrast, saline and morphine treatment groups showed no significant differences in the total number of CRF-immunoreactive axon terminals, or the frequency with which these terminals contacted CRFr-containing dendrites. This suggests that morphine does not influence axonal availability of CRF in the BSTal. The results have important implications for drug-associated adaptations in brain stress systems that may contribute to the motivation to continue drug use during dependence.

Role of dopamine D(1)-family receptors in dorsolateral striatum in context-induced reinstatement of heroin seeking in rats

RATIONALE

In humans, exposure to environmental contexts previously associated with heroin intake can provoke relapse to drug use. In rats, exposure to heroin-associated contexts after extinction of drug-reinforced responding in different contexts reinstates heroin seeking. This effect is attenuated by blockade of D(1)-family receptors in lateral or medial accumbens shell, but not accumbens core.

OBJECTIVES

In this study, we further characterized the role of striatal D(1)-family receptors in context-induced reinstatement by assessing the effect of dorsolateral or dorsomedial injections of the D(1)-family receptor antagonist SCH 23390 on this reinstatement.

MATERIALS AND METHODS

Rats were trained to self-administer heroin (0.05-0.10 mg/kg per infusion) for 12 days; drug infusions were paired with a discrete tone-light cue. Subsequently, heroin-reinforced lever pressing was extinguished in the presence of the discrete cue in a nondrug context. During reinstatement tests under extinction conditions, the D(1)-family receptor antagonist SCH 23390 (0.3-1.0 microg per side) was injected into the dorsolateral or dorsomedial striatum prior to exposure to heroin self-administration context or the nondrug (extinction) context. We then used a disconnection procedure to examine whether D(1)-family receptors in the dorsolateral striatum and lateral accumbens shell jointly or independently support context-induced reinstatement.

RESULTS

Dorsolateral but not dorsomedial SCH 23390 injections attenuated context-induced reinstatement of heroin seeking. SCH 23390 injections into the dorsolateral striatum of one hemisphere and lateral accumbens shell of the other hemisphere were ineffective.

CONCLUSIONS

Results indicate that dorsolateral striatum D(1)-family dopamine receptors are critical for context-induced reinstatement of heroin seeking. Results also suggest that D(1)-receptor-mediated dopamine transmission in the dorsolateral striatum and lateral accumbens shell independently support this reinstatement.

The mGlu5 receptor antagonist 2-methyl-6-(phenylethynyl)pyridine (MPEP) supports intravenous self-administration and induces conditioned place preference in the rat

We recently reported that the mGlu5 receptor antagonist 2-methyl-6-(phenylethynyl)pyridine (MPEP) reduces intravenous self-administration of ketamine and, to a lesser extent, heroin in rats. We also found that MPEP potentiates conditioned place preference induced by these drugs, suggesting that the reduction of self-administration results from an MPEP-induced potentiation of the rewarding effect of the self-administered drug. The aim of the present study was to examine whether MPEP has intrinsic positive reinforcing and rewarding effects. In experiment 1, rats were trained to self-administer either ketamine [0.5 mg/kg/infusion, 2 h sessions, fixed-ratio (FR) 3] or heroin (0.05 mg/kg/infusion, 1 h sessions, FR 10), followed by a number of substitution sessions with MPEP (1 mg/kg/infusion) or saline. In experiment 2, drug-naïve rats were allowed to acquire intravenous self-administration of MPEP (1 mg/kg/infusion, 2 h sessions, FR 3) or saline. In experiment 3, rats were subjected to a single-trial unbiased conditioned place preference protocol with MPEP (0.3-10 mg/kg i.v., 20 min conditioning). It was found that (1) substitution with MPEP in rats which had learned to self-administer ketamine or heroin resulted in stable self-administration behavior, whereas substitution with saline resulted in a typical extinction profile, (2) drug-naïve rats learned to self-administer MPEP, but not saline, and self-administration remained stable for at least 7 sessions, and (3) MPEP induced dose-dependent place preference with a minimal effective dose of 3 mg/kg. These data clearly demonstrate that MPEP has (weak) positive reinforcing and rewarding effects when administered i.v.

Neurochemistry underlying relapse to opiate seeking behaviour

Relapse is a major clinical problem and remains a major challenge in the treatment of addictions. A goal of current research is to gain a greater understanding of the neurochemistry underlying relapse to opiate use. Factors which trigger relapse in humans such as stress, exposure to opiates and/or drug-associated cues, can also trigger opiate-seeking in animals. This review will overview preclinical studies relating to the neurochemistry of opiate-seeking with a focus on studies published from 2005 to present.

Forced abstinence model of relapse to study pharmacological treatments of substance use disorder

Understanding and preventing relapse to drug use is one of the most difficult challenges faced by clinicians and practitioners in the struggle to help people remain abstinent. In this paper, we review basic preclinical research on forced abstinence periods that identify the neural substrates involved and neural adaptations that occur after a drug-free period. Our attention focuses on forced abstinence after self-administration because of its promise for translational research in the development of candidate medications to reduce relapse. This model requires subjects (often rats) to initially acquire drug self-administration. However, rather than extinguishing behavior with daily drug-free sessions as in the reinstatement model of drug seeking, subjects are removed from the self-administration situation and do not receive any exposure to the drug. Notably, the integrity of the drug-taking behavior and the drug-associated cues in the drug-taking environment are preserved because they are not experienced in the absence of the drug. Research shows time dependent increases in drug-seeking following forced abstinence periods. More so, neural substrates and adaptations within the mesocorticolimbic system and the nigrostriatal system have been identified that contribute to increased drug seeking following abstinence. From a translational perspective, behavioral and pharmacological treatment of substance use disorder often starts during this initial abstinence period (either forced or voluntary). The forced abstinence model simulates some of the features of this treatment situation and thus allows for the study of potential treatments that alter relapse of drug-seeking behaviors along with the accompanying neurobiological changes.

Effects of training and withdrawal periods on heroin seeking induced by conditioned cue in an animal of model of relapse

RATIONALE

A high incidence of relapse can be triggered by exposure to conditioned cues previously associated with heroin. Extended access to drug and withdrawal are thought to affect the motivation for drug seeking.

OBJECTIVES

The present study evaluated how different periods of training to self-administer heroin and different periods of withdrawal affected drug seeking.

MATERIALS AND METHODS

Following 1 to 14 days of heroin self-administration, rats were left in the home environment for 1 or 14 days. Subsequently, rats were evaluated for extinction of nose poke during the first hour after being returned to the training apparatus. One hour later, a conditioned stimulus was presented to initiate cue-induced reinstatement.

RESULTS

Extending the training period from 1 to 14 days caused an escalation of reinstatement of drug seeking induced by conditioned cues. Increasing the withdrawal period from 1 to 14 days produced a similar increase in reinstatement of drug seeking induced by cues. Reinstatement of drug seeking induced by cues was augmented by pretreatment with naltrexone (1, 5 mg/kg) 24 h prior to reinstatement on day 1, but not at 14 days of withdrawal from heroin self-administration.

CONCLUSIONS

These experiments demonstrate that increasing the duration of either heroin self-administration or the withdrawal periods from heroin self-administration augments the reinstatement induced by cues that were associated previously with heroin reinforcement. Additionally, we provide one of the first demonstrations that opiate withdrawal induces heroin seeking, as assessed in the reinstatement model.

Protracted withdrawal from alcohol and drugs of abuse impairs long-term potentiation of intrinsic excitability in the juxtacapsular bed nucleus of the stria terminalis

The juxtacapsular bed nucleus of the stria terminalis (jcBNST) is activated in response to basolateral amygdala (BLA) inputs through the stria terminalis and projects back to the anterior BLA and to the central nucleus of the amygdala. Here we show a form of long-term potentiation of the intrinsic excitability (LTP-IE) of jcBNST neurons in response to high-frequency stimulation of the stria terminalis. This LTP-IE, which was characterized by a decrease in the firing threshold and increased temporal fidelity of firing, was impaired during protracted withdrawal from self-administration of alcohol, cocaine, and heroin. Such impairment was graded and was more pronounced in rats that self-administered amounts of the drugs sufficient to maintain dependence. Dysregulation of the corticotropin-releasing factor (CRF) system has been implicated in manifestation of protracted withdrawal from dependent drug use. Administration of the selective corticotropin-releasing factor receptor 1 (CRF(1)) antagonist R121919 [2,5-dimethyl-3-(6-dimethyl-4-methylpyridin-3-yl)-7-dipropylamino-pyrazolo[1,5-a]pyrimidine)], but not of the CRF(2) antagonist astressin(2)-B, normalized jcBNST LTP-IE in animals with a history of alcohol dependence; repeated, but not acute, administration of CRF itself produced a decreased jcBNST LTP-IE. Thus, changes in the intrinsic properties of jcBNST neurons mediated by chronic activation of the CRF system may contribute to the persistent emotional dysregulation associated with protracted withdrawal.

Desperately driven and no brakes: developmental stress exposure and subsequent risk for substance abuse

Adverse life events are associated with a wide range of psychopathology, including an increased risk for substance abuse. In this review, we focus on the inter-relationship between exposure to adversity and brain development, and relate this to enhanced windows of vulnerability. This review encompasses clinical and preclinical data, drawing evidence from epidemiological studies, morphometric and functional imaging studies, and molecular biology and genetics. The interaction of exposure during a sensitive period and maturational events produces a cascade that leads to the initiation of substance use at younger ages, and increases the likelihood of addiction by adolescence or early adulthood. A stress-incubation/corticolimbic dysfunction model is proposed based on the interplay of stress exposure, development stage, and neuromaturational events that may explain the seeking of specific classes of drugs later in life. Three main factors contribute to this age-based progression of increased drug use: (1) a sensitized stress response system; (2) sensitive periods of vulnerability; and (3) maturational processes during adolescence. Together, these factors may explain why exposure to early adversity increases risk to abuse substances during adolescence.

A differential role for the adenosine A2A receptor in opiate reinforcement vs opiate-seeking behavior

The adenosine A(2A) receptor is specifically enriched in the medium spiny neurons that make up the 'indirect' output pathway from the ventral striatum, a structure known to have a crucial, integrative role in processes such as reward, motivation, and drug-seeking behavior. In the present study we investigated the impact of adenosine A(2A) receptor deletion on behavioral responses to morphine in a number of reward-related paradigms. The acute, rewarding effects of morphine were evaluated using the conditioned place preference paradigm. Operant self-administration of morphine on both fixed and progressive ratio schedules as well as cue-induced drug-seeking was assessed. In addition, the acute locomotor response to morphine as well as sensitization to morphine was evaluated. Decreased morphine self-administration and breakpoint in A(2A) knockout mice was observed. These data support a decrease in motivation to consume the drug, perhaps reflecting diminished rewarding effects of morphine in A(2A) knockout mice. In support of this finding, a place preference to morphine was not observed in A(2A) knockout mice but was present in wild-type mice. In contrast, robust cue-induced morphine-seeking behavior was exhibited by both A(2A) knockout and wild-type mice after a period of withdrawal. The acute locomotor response to morphine in the A(2A) knockout was similar to wild-type mice, yet A(2A) knockout mice did not display tolerance to chronic morphine under the present paradigm. Both genotypes display locomotor sensitization to morphine, implying a lack of a role for the A(2A) receptor in the drug-induced plasticity necessary for the development or expression of sensitization. Collectively, these data suggest a differential role for adenosine A(2A) receptors in opiate reinforcement compared to opiate-seeking.

Central amygdala extracellular signal-regulated kinase signaling pathway is critical to incubation of opiate craving

Cue-induced drug-seeking in rodents progressively increases after withdrawal from operant self-administration of cocaine, heroin, methamphetamine, and alcohol, a phenomenon termed "incubation of drug craving." Here, we used the opiate drug morphine and explored whether incubation of drug craving also occurs in a pavlovian conditioned place preference (CPP) procedure in which rats learn to associate drug effects with a distinct environmental context. We also explored the role of amygdala extracellular signal-regulated kinase (ERK) and cAMP response element-binding protein (CREB) in this incubation. We found that the expression of morphine CPP progressively increases over the first 14 d after the last drug exposure in rats receiving four pairings of low-dose (1 or 3 mg/kg) but not high-dose (10 mg/kg) morphine with a distinct environment. The progressive increase in low-dose (3 mg/kg) morphine CPP was associated with increased ERK phosphorylation (a measure of ERK activity) and CREB (a downstream target of ERK) phosphorylation in central but not basolateral amygdala. Furthermore, inhibition of central but not basolateral amygdala ERK and CREB phosphorylation by U0126 [1,4-diamino-2,3-dicyano-1,4-bis(o-aminophenylmercapto)butadiene] decreased the enhanced (incubated) drug CPP after 14 d of withdrawal from morphine. Finally, stimulation of central amygdala ERK and CREB phosphorylation by NMDA enhanced drug CPP after 1 d of withdrawal from morphine, an effect reversed by U0126. These findings indicate that the rat's response to environmental cues previously paired with morphine progressively increases or incubates over the first 14 d of withdrawal from low but not high morphine doses. Additionally, this "incubation of morphine craving" is mediated by acute activation of central amygdala ERK pathway.

Review. Psychological and neural mechanisms of relapse

Relapse, the resumption of drug taking after periods of abstinence, remains the major problem for the treatment of addiction. Even when drugs are unavailable for long periods or when users are successful in curbing their drug use for extended periods, individuals remain vulnerable to events that precipitate relapse. Behavioural studies in humans and laboratory animals show that drug-related stimuli, drugs themselves and stressors are powerful events for the precipitation of relapse. Molecular, neurochemical and anatomical studies have identified lasting neural changes that arise from mere exposure to drugs and other enduring changes that arise from learning about the relationship between drug-related stimuli and drug effects. Chronic drug exposure increases sensitivity of some systems of the brain to the effects of drugs and stressful events. These changes, combined with those underlying conditioning and learning, perpetuate vulnerability to drug-related stimuli. Circuits of the brain involved are those of the mesocorticolimbic dopaminergic system and its glutamatergic connections, and the corticotropin-releasing factor and noradrenergic systems of the limbic brain. This paper reviews advances in our understanding of how these systems mediate the effects of events that precipitate relapse and of how lasting changes in these systems can perpetuate vulnerability to relapse.

CNS genes implicated in relapse

Drug abuse is a condition that impacts not only the individual drug user, but society as a whole. Although prevention of initial drug use is the most effective way to prevent addiction, avoiding relapse is a crucial component of drug addiction recovery. Recent studies suggest that there is a set of genes whose expression is robustly and stably altered following drug use and ensuing abstinence. Such stable changes in gene expression correlate with ultrastructural changes in brain as well as alterations in behavior. As persistent molecular changes, these genes may provide targets for the development of therapeutics. Developing a list of well-characterized candidate genes and examining the effect of manipulating these genes will contribute to the ultimate goal of developing effective treatments to prevent relapse to drug use.

Heroin self-administration: II. CNS gene expression following withdrawal and cue-induced drug-seeking behavior

In the accompanying paper, we described incubation of heroin-seeking behavior in rats following 14 days of abstinence. To gain an understanding of genomic changes that accompany this behavioral observation, we measured the expression of genes previously reported to respond to drugs of abuse. Specifically, after 1 or 14 days of abstinence, mRNA expression was measured for 11 genes in the medial prefrontal cortex (mPFC) and nucleus accumbens (NAc) immediately following a single 90 min extinction session. Additionally, the role of contingency was examined in control rats that received yoked, response-independent heroin administration. Gene expression was quantified by real-time quantitative PCR. Expression of five genes (Arc, EGR1, EGR2, Fos, and Homer1b/c) was changed in the mPFC. EGR1 and EGR2 expression was increased following the 90 min extinction session in a contingency-specific manner and this increase persisted through the 14 days of abstinence. Fos expression was also increased after 1 and 14 days of abstinence, but at 14 days this increase was response-independent (i.e., it occurred in both the rats with a history of heroin self-administration and in the yoked controls). Arc expression increased following the extinction session only in rats with a history of heroin self-administration and only when tested following 1, but not 14, days of abstinence. Homer 1 b/c decreased after 14 days of enforced abstinence in rats that received non-contingent heroin. Expression of only a single gene (EGR2) was increased in the NAc. These data demonstrate that behavioral incubation is coincident with altered levels of specific transcripts and that this response is contingently-specific. Moreover, EGR1 and EGR2 are specifically upregulated in self-administering rats following extinction and this finding persists through 14 days of abstinence, suggesting that these genes are particularly associated with the incubation phenomenon. These latter observations of persistent changes in gene expression following abstinence may reflect molecular correlates of relapse liability.

Heroin self-administration: I. Incubation of goal-directed behavior in rats

This study used heroin self-administration to investigate incubation of goal-directed heroin-seeking behavior following abstinence. Male Sprague-Dawley rats self-administered heroin on a fixed ratio 10 (FR10) schedule of reinforcement with licking of an empty spout serving as the operant behavior during 14 daily 3 h sessions. After this acquisition period, all rats received a 90 min extinction session following either 1 day or 14 days of home cage abstinence. When the extinction session occurred after only 1 day of home cage abstinence, rats with a history of heroin self-administration divided their responses equally between the previously "active" and "inactive" spouts. However, when the extinction session occurred following 14 days of home cage abstinence, the rats exhibited marked goal-directed heroin-seeking behavior by licking more on the previously "active" than "inactive" spout. These findings demonstrate that heroin-seeking behavior incubates over time, resulting in goal-directed heroin-seeking behavior in rats following 14 days but not 1 day of abstinence. Moreover, this facilitatory effect occurred in response to a different training schedule, lower total drug intake, and shorter periods of daily access than previously reported with heroin.

Noradrenergic transmission in the extended amygdala: role in increased drug-seeking and relapse during protracted drug abstinence

Studies reviewed here implicate the extended amygdala in the negative affective states and increased drug-seeking that occur during protracted abstinence from chronic drug exposure. Norepinephrine (NE) and corticotropin-releasing factor (CRF) signaling in the extended amygdala, including the bed nucleus of the stria terminalis, shell of the nucleus accumbens, and central nucleus of the amygdala, are generally involved in behavioral responses to environmental and internal stressors. Hyperactivity of stress response systems during addiction drives many negative components of drug abstinence. In particular, NE signaling from the nucleus tractus solitarius (NTS) to the extended amygdala, along with increased CRF transmission within the extended amygdala, are critical for the aversiveness of acute opiate withdrawal as well as stress-induced relapse of drug-seeking for opiates, cocaine, ethanol, and nicotine. NE and CRF transmission in the extended amygdala are also implicated in the increased anxiety that occurs during prolonged abstinence from chronic opiates, cocaine, ethanol, and cannabinoids. Many of these stress-associated behaviors are reversed by NE or CRF antagonists given systemically or locally within the extended amygdala. Finally, increased Fos activation in the extended amygdala and NTS is associated with the enhanced preference for drugs and decreased preference for natural rewards observed during protracted abstinence from opiates and cocaine, indicating that these areas are involved in the altered reward processing associated with addiction. Together, these findings suggest that involvement of the extended amygdala and its noradrenergic afferents in anxiety, stress-induced relapse, and altered reward processing reflects a common function for these circuits in stress modulation of drug-seeking.

Role of ventral medial prefrontal cortex in incubation of cocaine craving

Cue-induced drug-seeking in rodents progressively increases after withdrawal from cocaine, suggesting that cue-induced cocaine craving incubates over time. Here, we explored the role of the medial prefrontal cortex (mPFC, a brain area previously implicated in cue-induced cocaine seeking) in this incubation. We trained rats to self-administer cocaine for 10days (6h/day, infusions were paired with a tone-light cue), and then assessed after 1 or 30 withdrawal days the effect of exposure to cocaine cues on lever presses in extinction tests. We found that cue-induced cocaine-seeking in the extinction tests was higher after 30 withdrawal days than after 1day. The time-dependent increases in extinction responding were associated with large (ventral mPFC) or modest (dorsal mPFC) increases in ERK phosphorylation (a measure of ERK activity and an index of neuronal activation). After 30 withdrawal days, ventral but not dorsal injections of muscimol+baclofen (GABAa+GABAb receptor agonists that inhibit neuronal activity) decreased extinction responding. After 1 withdrawal day, ventral but not dorsal mPFC injections of bicuculline+saclofen (GABAa+GABAb receptor antagonists that increase neuronal activity) strongly increased extinction responding. Finally, muscimol+baclofen had minimal effect on extinction responding after 1day, and in cocaine-experienced rats, ventral mPFC injections of muscimol+baclofen or bicuculline+saclofen had no effect on lever presses for an oral sucrose solution. The present results indicate that ventral mPFC neuronal activity plays an important role in the incubation of cocaine craving.

Characterization of central and peripheral components of the hypothalamus-pituitary-adrenal axis in the inbred Roman rat strains

Several studies performed in outbred Roman high- and low-avoidance lines (RHA and RLA, respectively) have demonstrated that the more anxious line (RLA) is characterized by a higher hypothalamic-pituitary-adrenal (HPA) response to certain stressors than the less anxious one (RHA). However, inconsistent results have also been reported. Taking advantage of the generation of an inbred colony of RLA and RHA rats (RHA-I and RLA-I, respectively), we have characterized in the two strains not only resting and stress levels of peripheral HPA hormones but also central components of the HPA axis, including CRF gene expression in extra-hypothalamic areas. Whereas resting levels of ACTH and corticosterone did not differ between the strains, a greater response to a novel environment was found in RLA-I as compared to RHA-I rats. RLA-I rats showed enhanced CRF gene expression in the paraventricular nucleus (PVN) of the hypothalamus, with normal arginin-vasopressin gene expression in both parvocellular and magnocellular regions of the PVN. This enhanced CRF gene expression is not apparently related to altered negative corticosteroid feedback as similar levels of expression of brain glucorticoid and mineralocorticoid receptors were found in the two rat strains. CRF gene expression tended to be higher in the central amygdala and it was significantly higher in the dorsal region of the bed nucleus of stria terminalis (BNST) of RLA-I rats, while no differences appeared in the ventral region of BNST. Considering the involvement of CRF and the BNST in anxiety and stress-related behavioral alterations, the present data suggest that the CRF system may be a critical neurobiological substrate underlying differences between the two rat strains.

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.

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.

Addiction and arousal: the hypocretin connection

The hypocretins, also known as orexins, are two neuropeptides now commonly described as critical components to maintain and regulate the stability of arousal. Several lines of evidence have raised the hypothesis that hypocretin-producing neurons are part of the circuitries that mediate the hypothalamic response to acute stress. Intracerebral administration of hypocretin leads to a dose-related reinstatement of drug and food seeking behaviors. Furthermore, stress-induced reinstatement can be blocked with hypocretin receptor 1 antagonism. These results, together with recent data showing that hypocretin is critically involved in cocaine sensitization through the recruitment of NMDA receptors in the ventral tegmental area, strongly suggest that activation of hypocretin neurons play a critical role in the development of the addiction process. The activity of hypocretin neurons may affect addictive behavior by contributing to brain sensitization or by modulating the brain reward system. Hypocretinergic cells, in coordination with brain stress systems may lead to a vulnerable state that facilitates the resumption of drug seeking behavior. Hence, the hypocretinergic system is a new drug target that may be used to prevent relapse of drug seeking.

Differential effects of blockade of dopamine D1-family receptors in nucleus accumbens core or shell on reinstatement of heroin seeking induced by contextual and discrete cues

In humans, exposure to environmental contexts previously associated with heroin intake can provoke drug relapse, but the neuronal mechanisms mediating this relapse are unknown. Using a drug relapse model, we found previously that reexposing rats to heroin-associated contexts, after extinction of drug-reinforced responding in different contexts, reinstates heroin seeking. This effect is attenuated by inhibition of glutamate transmission in the ventral tegmental area and medial accumbens shell, components of the mesolimbic dopamine system. Here, we explored the role of dopamine of the accumbens in context-induced reinstatement by using the D1-family receptor antagonist SCH 23390 [R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrochloride]. Rats were trained to self-administer heroin for 12 d; drug infusions were paired with a discrete tone-light cue. Subsequently, the heroin-reinforced lever pressing was extinguished in the presence of the discrete cue in a context that differed from the drug self-administration context in terms of visual, auditory, tactile, and circadian cues. When tested in the original drug self-administration context, systemic and medial or lateral accumbens shell SCH 23390 injections attenuated context-induced reinstatement of heroin seeking, whereas accumbens core SCH 23390 injections were ineffective. In contrast, core but not lateral or medial shell SCH 23390 injections attenuated discrete-cue-induced reinstatement in a nondrug context after extinction of lever presses without this cue. Results indicate that activation of medial and lateral accumbens shell D1-family dopamine receptors mediate context-induced reinstatement of heroin seeking and provide the first demonstration for a role of lateral shell dopamine in conditioned drug effects. Results also demonstrate novel dissociable roles of accumbens core and shell in context- versus discrete-cue-induced reinstatement of heroin seeking.

Peptide YY3-36 decreases reinstatement of high-fat food seeking during dieting in a rat relapse model

A major problem in treating obesity is high rates of relapse to maladaptive food-taking habits during dieting. This relapse is often provoked by acute re-exposure to palatable food, food-associated cues, or stress. We used a reinstatement model, commonly used to study relapse to abused drugs, to explore the effect of peptide YY3-36 (PYY3-36) on reinstatement of high-fat (35%, 45 mg pellets) food seeking induced by acute exposure to the pellets (pellet priming), a cue previously associated with pellet delivery (pellet cue), or yohimbine (2 mg/kg, a pharmacological stressor). Rats were placed on a restricted diet (16 g of chow per day) and lever-pressed for the pellets for 9-12 sessions (6 h/d, every 48 h); pellet delivery was paired with a tone-light cue. They were then given 10-20 extinction sessions wherein lever presses were not reinforced with the pellets and subsequently tested for reinstatement of food seeking. Systemic PYY3-36 injections (100-200 microg/kg) decreased pellet priming- and pellet cue-induced reinstatement of food seeking but not yohimbine-induced reinstatement. Arcuate nucleus (Arc) injections of PYY3-36 (0.4 microg per side) decreased pellet priming-induced reinstatement. The attenuation of pellet priming-induced reinstatement by systemic PYY3-36 was reversed by systemic (2 mg/kg) but not Arc (0.5 microg per side) injections of the Y2 receptor antagonist BIIE0246. Arc PYY3-36 injections did not decrease pellet cue-induced reinstatement. Finally, systemic PYY3-36 injections had minimal effects on ongoing food self-administration or heroin priming- or heroin cue-induced reinstatement of heroin seeking. These data identify an effect of systemic PYY3-36 on relapse to food seeking that is independent of Y2 receptor activation in Arc and suggest that PYY3-36 should be considered for the treatment of relapse to maladaptive food-taking habits during dieting.

CRF-CRF1 system activation mediates withdrawal-induced increases in nicotine self-administration in nicotine-dependent rats

Nicotine, the main psychoactive ingredient of tobacco, induces negative emotional symptoms during abstinence that contribute to a profound craving for nicotine. However, the neurobiological mechanisms underlying how nicotine produces dependence remains poorly understood. We demonstrate one mechanism for both the anxiety-like symptoms of withdrawal and excessive nicotine intake observed after abstinence, through recruitment of the extrahypothalamic stress peptide corticotropin-releasing factor (CRF) system and activation of CRF(1) receptors. Overactivation of the CRF-CRF(1) system may contribute to nicotine dependence and may represent a prominent target for investigating the vulnerability to tobacco addiction.

A conflict rat model of cue-induced relapse to cocaine seeking

RATIONALE AND OBJECTIVE

Relapse to drug use in humans can be induced by exposure to drug-associated cues. The ability of drug cues to provoke 'relapse' has been studied in laboratory animals using a reinstatement model in which resumption of drug seeking is assessed after extinction of drug-reinforced responding. In this model, there are no adverse consequences of drug-seeking behavior. However, in humans, abstinence is often self-imposed, and relapse episodes likely involve making a choice between the desire for the drug and the negative consequences of pursuing it (a conflict situation). In this paper, we describe a conflict model of cue-induced relapse in rats that approximate the human condition.

MATERIALS AND METHODS

Rats were trained to lever press for cocaine; infusions were paired with a discrete light cue. An 'electric barrier' was then introduced by electrifying the floor area near the levers. Responding decreased over days with increasing shock intensities, until the rats did not approach the levers for 3 days. Subsequently, the effect of intermittent noncontingent light-cue presentations on resumption of lever responding (relapse) was assessed in extinction tests, with the electric barrier remaining activated; during testing, lever presses led to contingent light-cue presentations.

RESULTS

Noncontingent cue exposure led to resumption of lever presses during the relapse tests in 14 of the 24 rats. Surprisingly, 24 h later, 11 of the 24 rats resumed lever responding in a subsequent post-noncontingent cue test under similar extinction conditions. Large individual differences in responding were observed during both tests.

CONCLUSIONS

At its current stage of development, the conflict relapse model appears particularly suitable for studying individual differences in cue-induced relapse to cocaine seeking or factors that promote this relapse.

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

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

Systemic and central amygdala injections of the mGluR2/3 agonist LY379268 attenuate the expression of incubation of sucrose craving in rats

We previously reported that systemic or central amygdala injections of the mGluR(2/3) agonist LY379268 (which decreases glutamate release) prevented enhanced cue-induced cocaine seeking in extinction tests after prolonged withdrawal (incubation of cocaine craving). Here, we report that systemic and central amygdala injections of LY379268 also prevented the enhanced cue-induced sucrose seeking in extinction tests after prolonged sucrose-free period (incubation of sucrose craving). These findings suggest that central amygdala glutamate plays an important role in the incubation of reward craving after withdrawal from both drug and non-drug rewards.

Systemic and central amygdala injections of the mGluR(2/3) agonist LY379268 attenuate the expression of incubation of cocaine craving

BACKGROUND

We and others reported time-dependent increases in cue-induced cocaine seeking after withdrawal, suggesting that craving incubates over time. Recently, we found that central amygdala extracellular signal-regulated kinases (ERK) and glutamate are involved in this incubation. Here, we further explored the role of central amygdala glutamate in the incubation of cocaine craving by determining the effect of systemic or central amygdala injections of the mGluR2/3 agonist LY379268 (which decreases glutamate release) on cue-induced cocaine seeking during early and late withdrawal.

METHODS

Rats were trained to self-administer cocaine for 10 days (6 hours/day); infusions were paired with a tone-light cue. Cocaine seeking and craving after systemic or central amygdala injections of LY379268 were then assessed in extinction tests in the presence of the cocaine-associated cues during early (day 3) or late (day 21) withdrawal.

RESULTS

Systemic (1.5 or 3 mg/kg) or central amygdala (.5 or 1.0 microg/side) injections of LY379268 attenuated enhanced extinction responding on day 21 but had no effect on lower extinction responding on day 3.

CONCLUSIONS

Results confirm our previous findings on the role of central amygdala glutamate in the incubation of cocaine craving and together with previous reports suggest that mGluR(2/3) agonists should be considered in the treatment of drug relapse.

Toward a model of drug relapse: an assessment of the validity of the reinstatement procedure

BACKGROUND AND RATIONALE

The reinstatement model is widely used to study relapse to drug addiction. However, the model's validity is open to question.

OBJECTIVE

We assess the reinstatement model in terms of criterion and construct validity.

RESEARCH HIGHLIGHTS AND CONCLUSIONS

We find that the reinstatement model has adequate criterion validity in the broad sense of the term, as evidenced by the fact that reinstatement in laboratory animals is induced by conditions reported to provoke relapse in humans. The model's criterion validity in the narrower sense, as a medication screen, seems promising for relapse to heroin, nicotine, and alcohol. For relapse to cocaine, criterion validity has not yet been established primarily because clinical studies have examined medication's effects on reductions in cocaine intake rather than relapse during abstinence. The model's construct validity faces more substantial challenges and is yet to be established, but we argue that some of the criticisms of the model in this regard may have been overstated.

NR2B-containing NMDA receptor is required for morphine-but not stress-induced reinstatement

Glutamate receptors are known to be densely distributed in the forebrain rewarding circuits, and glutamatergic transmission is actively involved in the regulation of rewarding and reinstating effects of drugs of abuse. Here we investigated the possible involvement of the N-methyl-D-aspartate (NMDA) receptors in the reinstatement of extinguished morphine conditioned place preference (CPP) in rats. We found that previously extinguished morphine (3 mg/kg, i.p.) CPP was markedly reinstated by a priming injection of morphine (2 mg/kg, i.p.) or an acute environmental stressor (forced swim for 10 min), but not by the stress induced by a 24-h food deprivation. Parallel with this, protein levels of the NMDA receptor 2B subunit (NR2B) were elevated in the nucleus accumbens (NAc) and the hippocampus, but not the prefrontal cortex, of reinstated rats. Systemic administration of an NR2B selective antagonist ifenprodil (1, 3, 10 mg/kg, i.p.) attenuated the reinstatement induced by a priming morphine injection, although not by the forced swim. Ifenprodil (2.0 microg/rat) directly injected into the NAc shell or the CA1 region of the dorsal hippocampus produced a similar effect. These results indicate that the NR2B-containing NMDA receptors in the NAc and the dorsal hippocampus play a significant role in mediating the reinstatement of rewarding responses to morphine.

The anxiogenic drug yohimbine reinstates palatable food seeking in a rat relapse model: a role of CRF1 receptors

The major problem in treating excessive eating is high rates of relapse to maladaptive eating habits during diet treatments; this relapse is often induced by stress or anxiety states. Preclinical studies have not explored this clinical problem. Here, we adapted a reinstatement model (commonly used to study relapse to abused drugs) to examine the role of stress and anxiety in relapse to palatable food seeking during dieting. Rats were placed on restricted diet (75-80% of daily standard food) and for 12 intermittent training days (9 h/day, every other day) lever-pressed for palatable food pellets (25% fat, 48% carbohydrate) under a fixed ratio 1 (20-s timeout) reinforcement schedule. Subsequently, the rats were given 10 daily extinction sessions during which lever presses were not reinforced, and were then injected with yohimbine (an alpha-2 adrenoceptor antagonist that induces stress and anxiety in humans and non-humans) or given a single food pellet to assess reinstatement of food seeking. The rats rapidly learned to lever press for the palatable pellets and across the training days the ratio of timeout nonreinforced lever presses to reinforced lever presses progressively increased more than three-fold, suggesting the development of compulsive eating behavior. After extinction, yohimbine injections and pellet priming reliably reinstated food seeking. The corticotropin-releasing factor1 (CRF1) receptor antagonist antalarmin attenuated the reinstatement induced by yohimbine, but not pellet priming. Antalarmin also reversed yohimbine's anxiogenic effects in the social interaction test. These data suggest that CRF is involved in stress-induced relapse to palatable food seeking, and that CRF1 antagonists should be considered for the treatment of maladaptive eating habits.

The group II metabotropic glutamate receptor agonist, LY379268, inhibits both cocaine- and food-seeking behavior in rats

RATIONALE

Group II metabotropic glutamate receptor (mGluR2/3) agonists are proposed to serve as potential treatment for addiction.

OBJECTIVES

The present study examined the hypothesis that mGluR2/3 agonists exert inhibitory effects on cocaine-induced reinstatement of cocaine-seeking.

METHODS

Rats were trained to self-administer either cocaine or control reinforcer (food), then responding on the reinforcer-paired lever was extinguished. Reinstatement of responding was induced by a noncontingent presentation of the self-administered reinforcer (10 mg/kg cocaine, i.p. or 765 mg of food). In one experiment, rats were systemically pretreated with vehicle (Veh) or the mGluR2/3 agonist LY379268 (0.3, 1, or 3 mg/kg, i.p.) 30 min before the reinstatement test session. In a second experiment, Veh or LY379268 (0.05, 0.5, or 5 nmol/side) was microinjected into the nucleus accumbens core (NAc core) 5 min before the reinstatement test session. The effects of LY379268 on cocaine- and food-induced reinstatement on reward seeking were assessed.

RESULTS

Both systemic and intra-NAc core pretreatment with LY379268 inhibited both cocaine- and food-seeking behavior. However, the effect of LY379268 appeared somewhat more effective for cocaine-seeking than food-seeking.

CONCLUSIONS

These results support a potential therapeutic role for mGluR2/3 agonists on relapse of cocaine-seeking. However, doses that inhibited cocaine-seeking were only threefold lower than those inhibiting food-seeking, indicating possible unacceptable nonspecific effects. In addition, the NAc core is one site of action where the mGluR2/3 agonists elicit effects on reward-seeking behavior.

Effect of methamphetamine self-administration on tyrosine hydroxylase and dopamine transporter levels in mesolimbic and nigrostriatal dopamine pathways of the rat

RATIONALE AND OBJECTIVES

Many studies have examined the effect of experimenter-delivered methamphetamine on the mesolimbic and nigrostriatal dopamine pathways. In contrast, little is known about the effect of methamphetamine self-administration on these neuronal pathways. We studied the effect of methamphetamine self-administration on two key regulators of dopamine transmission, tyrosine hydroxylase (TH), and dopamine transporter (DAT), in components of the mesolimbic and nigrostriatal dopamine pathways.

METHODS

Rats self-administered methamphetamine (0.1 mg/kg per infusion, fixed-ratio-1 reinforcement schedule) or saline (control condition) for 9 h/day over 10 days. The brains of these rats were collected after 1 or 30 days of forced abstinence and the expression levels of TH and DAT were assayed by in situ, hybridization and western blot.

RESULTS

TH mRNA and protein levels were increased in the ventral tegmental area (VTA, the cell body region of the mesolimbic dopamine system) and the substantia nigra pars compacta (SNC, the cell body region of the nigrostriatal dopamine system) after 1 day, but not 30 days, of forced abstinence from methamphetamine. In contrast, methamphetamine self-administration had no effect on TH protein levels in dopaminergic terminals located in the nucleus accumbens and caudate-putamen. In addition, methamphetamine self-administration had no effect on DAT mRNA levels in the VTA.

CONCLUSIONS

Results suggest that extended daily access to self-administered methamphetamine results in a transient, short-lasting effect on mesolimbic and nigrostriatal dopamine neurons of the rat brain.

Social stress is as effective as physical stress in reinstating morphine-induced place preference in mice

RATIONALE

Relapse to drug-seeking in abstinent heroin addicts and reinstatement in experimental animals are observed when exposed to drug-associated stimuli or cues, the drug itself, and stressful events. It has been shown that footshock-induced stress increases the rewarding effects of opiates, delays extinction, and induces the reinstatement of drug-seeking. However, the effects of social stress on the reinstatement of opiate-seeking after extinction has not been studied.

OBJECTIVES

The role of physical (restraint and tail pinch) and social (social defeat) stressors on the reinstatement of morphine-induced conditioned place preference (CPP) was evaluated.

METHODS

Adult male OF1 mice were conditioned with 10, 20, or 40 mg/kg of morphine or saline. Only morphine-conditioned animals acquired CPP. All mice underwent extinction sessions until the CPP was extinguished. Then, the effects of physical or social stress on the reinstatement of CPP were evaluated. Morphine- and saline-conditioned animals were exposed to the respective stressor or control stress condition immediately or 15 min before reinstatement tests. In experiment 1, animals underwent restraint for 15 min. In experiment 2, animals were exposed to tail pinch or placed in a cage without any manipulation for 15 min. In experiment 3, animals performed an agonistic encounter with an isolated or anosmic mouse or were placed in a cage without any social contact or manipulation.

RESULTS

Restraint, tail pinch, and social defeat in an agonistic encounter with an isolated mouse produce the reinstatement of CPP in morphine-conditioned animals.

CONCLUSIONS

These data demonstrate that social stress is as effective as physical stress in reinstating morphine-seeking.

Stress and arousal: the corticotrophin-releasing factor/hypocretin circuitry

The hypocretins (also know as orexins) are two neuropeptides now commonly described as critical components for maintaining and regulating the stability of arousal. Several lines of evidence have raised the hypothesis that hypocretin-producing neurons are part of the circuitries that mediate the hypothalamic response to acute stress. New data indicate that the corticotrophin-releasing factor (CRF) peptidergic system directly innervates hypocretin-expressing neurons. CRF depolarizes hypocretin neurons, and this effect is blocked by a CRF-R1 antagonist. Furthermore, activation of hypocretinergic neurons by stress is impaired in CRF-R1 knockout mice. These data suggest that CRF-R1 receptor mediates the stress-induced activation of the hypocretinergic system. A significant amount of evidence also indicates that hypocretin cells connect reciprocally to the CRF system. We propose that upon stressor stimuli, CRF activates the hypocretin system, which relays these signals to brain stem nuclei involved in the modulation of arousal as well as to the extended amygdala, a structure involved in the negative motivational state that drives addiction.

Effects of environmental and pharmacological stressors on c-fos and corticotropin-releasing factor mRNA in rat brain: Relationship to the reinstatement of alcohol seeking

We have observed marked heterogeneity among different stressors in their ability to reinstate alcohol seeking in rats. Of the stressors we have tested, only the environmental stressor footshock and the pharmacological stressor yohimbine induce reinstatement. The reasons for such differences among stressors are not known. The purpose of the experiments presented here is to determine the neuroanatomical substrates that underlie these behavioral differences. To this end, we assessed whether stressors effective in inducing reinstatement of alcohol seeking activate a different set of neuronal pathways than do those that are ineffective, using the technique of in situ hybridization of the mRNAs for c-fos, a marker of neuronal activation, and corticotropin-releasing factor (CRF), a stress-related peptide we have shown to be critical to footshock-induced reinstatement of alcohol seeking. Exposure of rats to the environmental stressors footshock, restraint or social defeat, or the pharmacological stressors yohimbine or FG-7142 increased levels of the mRNAs for c-fos and CRF in the brain in a number of areas previously shown to be responsive to stressors. We found regionally specific effects of the stressors on c-fos and CRF mRNA in brain regions associated with the rewarding effects of alcohol and other abused drugs. The two stressors we have previously shown to be effective in inducing reinstatement of alcohol seeking, footshock and yohimbine, induced c-fos mRNA in the shell of the nucleus accumbens, and the basolateral and central amygdalar nuclei. These two stressors also induced CRF mRNA in the dorsal region of the bed nucleus of the stria terminalis. Taken together, these results provide evidence that activity in these regions may be involved in the reinstatement of alcohol seeking induced by these stressors. These results are also in keeping with the previously demonstrated role of CRF neurons in the dorsal bed nucleus of the stria terminalis in the reinstatement of alcohol seeking induced by stress.

Behavioral and neural mechanisms of compulsive drug seeking

Not the mere procurement and use of drugs, but the fact that patterns of seeking and taking become compulsive after prolonged drug use is a defining characteristic of drug addiction. Development of a therapy that targets the compulsive aspects of drug use and thus addresses addiction at its core would therefore be very desirable. In the present review, we will discuss animal studies that attempt to model loss of control over drug use. Furthermore, we will try to put these studies in a theoretical perspective, and discuss the hypothesized underlying neural and behavioral mechanisms.

The contribution of drug history and time since termination of drug taking to footshock stress-induced cocaine seeking in rats

RATIONALE

There is reason to think that footshock stress-induced reinstatement of cocaine may be affected by the history of drug use and time since termination of drug taking.

OBJECTIVES

Here, we assessed the contribution of daily access (hours per day) and duration (number of days) of cocaine self-administration to propensity to reinstate drug seeking following footshock stress at three time points following cocaine self-administration.

METHODS

Rats were trained to self-administer cocaine (0.5 mg kg(-1) infusion(-1)) on a fixed ratio 1 schedule in one of four training combinations of hours per day and number of days [2/7, 2/21, 12/7, and 12/21 (h/day)]. Rats were then tested for the first time under extinction conditions at either day 1, 10, or 60 after termination of cocaine availability. Once extinction criterion was met (<15 lever presses in 1 h), rats were then tested for stress-induced reinstatement after 15 min of intermittent, inescapable footshock (0.8 mA, 0.5 s/shock, mean off period of 40 s).

RESULTS

Rats that were given 12-h access to cocaine during training responded less in tests of extinction than those rats given 2-h access. Rats in all groups tested in extinction at days 10 and 60 showed higher responding than at day 1, suggesting an incubation of responding. In footshock stress-induced reinstatement tests, rats with greater exposure to cocaine showed a similar suppression of responding at day 1 and enhanced responding at day 60. As expected, rats that were given 12-h/21-day access to cocaine had the greatest intake of cocaine across the training phase with a slow escalation of hourly intake.

CONCLUSION

Greater access to cocaine results in suppression of cocaine seeking following footshock stress at early time points and a progressive increase over time.