A role for the CRF-containing pathway from central nucleus of the amygdala to bed nucleus of the stria terminalis in the stress-induced reinstatement of cocaine seeking in rats

Differential involvement of the core and shell subregions of the nucleus accumbens in conditioned cue-induced reinstatement of cocaine seeking in rats

RATIONALE

The nucleus accumbens (NAC) is theorized to be a critical element of the neural circuitry that mediates relapse to cocaine seeking. Evidence suggests that the NAC is a functionally heterogeneous structure, and the core (NACc) and shell (NACs) regions of the NAC may play a differential role in stimulus-induced motivated behavior. Thus, determination of the involvement of NAC subregions in conditioned cue-induced reinstatement of cocaine seeking is warranted.

OBJECTIVES

The present study compared the effects of GABA agonist-induced inactivation of the NACc versus NACs on conditioned cue-induced reinstatement of cocaine seeking behavior.

METHODS

Rats were trained to lever press for cocaine infusions (0.20 mg/infusion, IV) paired with presentations of a light-tone stimulus complex. Responding was then allowed to extinguish prior to reinstatement testing. Reinstatement of cocaine seeking (i.e. responses on the previously cocaine-paired lever) was measured in the presence of response-contingent presentation of the light-tone stimulus complex following microinfusion of muscimol+baclofen (Mus+Bac, 0.1/1.0 mM, respectively, 0.3 microl/side) or vehicle into the NACc or NACs. The effects of these manipulations on locomotor activity were also examined.

RESULTS

Mus+Bac-induced inactivation of the NACc abolished, whereas inactivation of the NACs failed to alter, conditioned cue-induced reinstatement of operant responding relative to vehicle pretreatment. Time course analyses of the effects of these manipulations on locomotion versus operant responding confirmed that the effects of Mus+Bac on reinstatement were not due to suppression of general activity.

CONCLUSIONS

The functional integrity of the NACc, but not the NACs, is necessary for conditioned cue-induced reinstatement of cocaine seeking behavior.

Corticotropin-releasing factor in brain: a role in activation, arousal, and affect regulation

Organisms exposed to challenging stimuli that alter the status quo inside or outside of the body are required for survival purposes to generate appropriate coping responses that counteract departures from homeostasis. Identification of an executive control mechanism within the brain capable of coordinating the multitude of endocrine, physiological, and functional coping responses has high utility for understanding the response of the organism to stressor exposure under normal or pathological conditions. The corticotropin-releasing factor (CRF)/urocortin family of neuropeptides and receptors constitutes an affective regulatory system due to the integral role it plays in controlling neural substrates of arousal, emotionality, and aversive processes. In particular, available evidence from pharmacological intervention in multiple species and phenotyping of mutant mice shows that CRF/urocortin systems mediate motor and psychic activation, stimulus avoidance, and threat recognition responses to aversive stimulus exposure. It is suggested that affective regulation is exerted by CRF/urocortin systems within the brain based upon the sensitivity of local brain sites to CRF/urocortin ligand administration and the appearance of hypothalamo-pituitary-adrenocortical activation following stressor exposure. Moreover, these same stress neuropeptides may constitute a mechanism for learning to avoid noxious stimuli by facilitating the formation of so-called emotional memories. A conceptual framework is provided for extrapolation of animal model findings to humans and for viewing CRF/urocortin activation as a continuum measure linking normal and pathological states.

Extinction-induced neuroplasticity attenuates stress-induced cocaine seeking: a state-dependent learning hypothesis

Chronic drug use weakens excitatory neocortical input to the nucleus accumbens (NAc). We previously reported that extinction training, a form of inhibitory learning that progressively reduces cocaine-seeking behaviour when reward is withheld, reverses this deficit by up-regulating GluR1 and GluR2/3 subunits of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) glutamate receptors in the NAc. The level of GluR1 up-regulation is positively associated with a reduction in cocaine seeking, suggesting that extinction-induced up-regulation in AMPA receptors in the NAc opposes motivational influences that maintain cocaine seeking. This hypothesis is supported by the finding that over-expression of GluR1 and GluR2 in the NAc facilitates extinction of cocaine self-administration. Furthermore, a single extinction training session conducted during GluR1 and GluR2 over-expression strongly and selectively attenuates the ability of an environmental stressor to trigger relapse to cocaine seeking long after GluR1 and GluR2 over-expression declines. These results could suggest that excitatory input to the NAc promotes extinction learning, but only when memory is recalled under stressful situations. Recent studies indicate that both environmental stress and the frustrative stress of withholding reward during extinction of drug self-administration induce similar neurochemical events in the NAc. These neurochemical events could impose a "state-dependency" on extinction learning such that subsequent exposure to stress acts as a cue to enhance retrieval of extinction memory. Our results suggest that extinction-induced up-regulation in NAc AMPA receptors acts reciprocally to facilitate state-dependent extinction learning, as stressful situations evoke extinction memories that exert powerful inhibitory control over drug-seeking behaviour. These results may have important therapeutic implications for behaviour-based approaches aimed at treating drug addiction.

The anxiogenic drug yohimbine reinstates methamphetamine seeking in a rat model of drug relapse

BACKGROUND

Brain noradrenaline is involved in footshock stress-induced reinstatement of drug seeking in a rat relapse model. We studied whether yohimbine, an alpha-2 adrenoceptor antagonist that increases noradrenaline release and induces anxiety-like responses in human and nonhuman subjects, would reinstate methamphetamine seeking in rats.

METHODS

In experiment 1, the effect of yohimbine (1.25-2.5 mg/kg) on reinstatement was compared with that of intermittent footshock (5 min;.2-.6 mA) in rats that were trained to lever press for intravenous methamphetamine (9-11 days) and subsequently underwent 7 days of extinction training. In experiment 2, the effect of yohimbine on reinstatement of drug seeking was determined during early (1 day) and late (21 or 51 days) withdrawal periods. On the test days, rats were first given 3-hour extinction sessions and were then tested for reinstatement induced by yohimbine.

RESULTS

In experiment 1, both yohimbine and footshock stress reinstated methamphetamine seeking after extinction. In experiment 2, extinction responding was higher after 21 or 51 withdrawal days than after 1 withdrawal day. In contrast, no significant time-dependent changes in yohimbine-induced reinstatement were observed.

CONCLUSIONS

Results indicate that yohimbine is a potent stimulus for reinstatement of methamphetamine seeking in a rat relapse model.

Limbic and motor circuitry underlying footshock-induced reinstatement of cocaine-seeking behavior

The role of limbic, cortical, and striatal circuitry in a footshock reinstatement model of relapse to cocaine seeking was evaluated. Transient inhibition of the central extended amygdala [CEA; including the central nucleus of the amygdala (CN), ventral bed nucleus of the stria terminalis (BNSTv), and nucleus accumbens shell (NAshell)], ventral tegmental area (VTA), and motor circuitry [including the dorsal prefrontal cortex (PFCd), nucleus accumbens core (NAcore), and ventral pallidum (VP)] blocked the ability of footshock stress to reinstate lever pressing previously associated with cocaine delivery. However, inhibition of the basolateral amygdala, mediodorsal nucleus of the thalamus, or the ventral prefrontal cortex had no effect on drug-seeking behavior. These data suggest that footshock stress activates limbic circuitry of the CEA that, via the VTA, activates motor output circuitry responsible for producing lever press responding. Consistent with this notion, the D1/D2 dopamine receptor antagonist fluphenazine blocked footshock-induced reinstatement when infused into the PFCd. Further, inhibition of the NAshell blocked a footshock-induced increase in dopamine within the PFC and concomitantly blocked reinstatement responding. Also supporting the idea of a CEA-VTA-motor circuit in stress-induced reinstatement of cocaine seeking, inactivation of the PFCd was shown to block stress-induced glutamate release within the NAcore while concurrently inhibiting reinstatement responding. Taken together, these data suggest that footshock activates limbic circuitry in the CEA, which in turn activates a VTA dopamine projection to the PFCd. The rise in dopamine within the PFCd initiates reinstatement via a glutamatergic projection to the NAcore.

A circadian rhythm in the expression of PERIOD2 protein reveals a novel SCN-controlled oscillator in the oval nucleus of the bed nucleus of the stria terminalis

Circadian rhythms in mammals are regulated not only globally by the master clock in the suprachiasmatic nucleus (SCN), but also locally by widely distributed populations of clock cells in the brain and periphery that control tissue-specific rhythmic outputs. Here we show that the oval nucleus of the bed nucleus of the stria terminalis (BNST-OV) exhibits a robust circadian rhythm in expression of the Period2 (PER2) clock protein. PER2 expression is rhythmic in the BNST-OV in rats housed under a light/dark cycle or in constant darkness, in blind rats, and in mice, and is in perfect synchrony with the PER2 rhythm of the SCN. Constant light or bilateral SCN lesions abolish the rhythm of PER2 in the BNST-OV. Large abrupt shifts in the light schedule transiently uncouple the BNST-OV rhythm from that of the SCN. Re-entrainment of the PER2 rhythm is faster in the SCN than in the BNST-OV, and it is faster after a delay than an advance shift. Bilateral adrenalectomy blunts the PER2 rhythm in the BNST-OV. Thus, the BNST-OV contains circadian clock cells that normally oscillate in synchrony with the SCN, but these cells appear to require both input from the SCN and circulating glucocorticoids to maintain their circadian oscillation. Taken together with what is known about the functional organization of the connections of the BNST-OV with systems of the brain involved in stress and motivational processes, these findings place BNST-OV oscillators in a position to influence specific physiological and behavioral rhythms downstream from the SCN clock.

Influence of early postnatal rearing conditions on mesocorticolimbic dopamine and behavioural responses to psychostimulants and stressors in adult rats

While many experiment with drugs, relatively few individuals develop a true addiction. We hypothesized that, in rats, such individual differences in the actions of addictive drugs might be determined by postnatal rearing conditions. To test this idea, we investigated whether stimulant- and stress-induced activation of nucleus accumbens dopamine transmission and dopamine-dependent behaviours might differ among adults rats that had been either repeatedly subjected to prolonged maternal separation or a brief handling procedure or left undisturbed (non-handled) during the first 14 days of life. We found that, in comparison with their handled counterparts, maternally separated and non-handled animals are hyperactive when placed in a novel setting, display a dose-dependent higher sensitivity to cocaine-induced locomotor activity and respond to a mild stressor (tail-pinch) with significantly greater increases in nucleus accumbens dopamine levels. In addition, maternally separated animals were found to sensitize to the locomotor stimulant action of amphetamine when repeatedly stressed under conditions that failed to sensitize handled and non-handled animals. Finally, quantitative receptor autoradiography revealed a lower density of nucleus accumbens-core and striatal dopamine transporter sites in maternally separated animals. Interestingly, we also found greatly reduced D(3) dopamine receptor binding and mRNA levels in the nucleus accumbens-shell of handled animals. Together, these findings provide compelling evidence that disruptions in early postnatal rearing conditions can lead to profound and lasting changes in the responsiveness of mesocorticolimbic dopamine neurons to stress and psychostimulants, and suggest a neurobiological basis for individual differences in vulnerability to compulsive drug taking.

Patterns of functional activity associated with cocaine self-administration in the rat change over time

RATIONALE

Although imaging studies in human addicts have been valuable for identifying the neural substrates of the effects of abused drugs, few studies have used this approach in animal models where conditions can be carefully controlled.

OBJECTIVE

To define the substrates that mediate the effects of cocaine in a rodent model of cocaine self-administration using the 2-[(14)C]deoxyglucose method and to assess changes in these patterns over the course of drug exposure.

METHODS

Male Sprague-Dawley rats self-administered cocaine (0.75 mg/kg per injection; FR2; 21 injections/session) and control rats received saline infusions in the same pattern as the cocaine rats for 5 or 30 days. Metabolic mapping was applied immediately after the final session.

RESULTS

Following 5 days of self-administration, rates of glucose utilization were decreased in the nucleus accumbens, and increased in autonomic brainstem structures and in sensorimotor regions. After 30 days of cocaine exposure, self-administration reduced glucose utilization throughout the dorsal and ventral striatum, central nucleus of the amygdala, medial forebrain bundle, and infralimbic and prelimbic prefrontal cortices. In addition, at this time point glucose utilization was no longer elevated in any autonomic or sensorimotor brain regions.

CONCLUSIONS

These data demonstrate that the distribution of functional activity associated with self-administered cocaine undergoes considerable change over the course of drug exposure. While increases in metabolic rates were largely found in autonomic and sensorimotor structures after short-term cocaine access, decreases were prominent in mesocorticolimbic regions after prolonged exposure. These differences in the patterns of brain activity that develop with long-term cocaine self-administration may play a role in the transition to habitual drug seeking behavior.

N-methyl-D-aspartate receptors in the amygdala are necessary for the acquisition and expression of conditioned defeat

Here, we describe a biologically relevant model called conditioned defeat that is used to examine behavioral responses to social defeat in Syrian hamsters. In this model experimental animals that are normally aggressive experience social defeat and consequently display high levels of submissive/defensive behavior even in response to non-threatening conspecifics. N-methyl-D-aspartate (NMDA) receptors within the amygdala play an important role in conditioned fear; therefore, the purpose of this study was to examine whether NMDA receptors within the amygdala are necessary for the acquisition and expression of conditioned defeat. Specifically, the present study examined whether bilateral infusions of the NMDA receptor antagonist DL-2-amino-5-phosphonopentanoic acid (AP5; 0.625, 1.25, 2.5, 5.0, 10.0 microg) into the amygdala would block the acquisition of conditioned defeat. Subsequently, we examined whether bilateral infusions of AP5 (0.625, 1.25, 2.5, 5.0 microg) into the amygdala prior to testing would block the expression of conditioned defeat. Infusions of AP5 into the amygdala immediately before the initial social defeat significantly reduced submissive/defensive behavior when hamsters were tested the following day with a non-aggressive intruder. Similarly, infusions of AP5 into the amygdala immediately before exposure to a non-aggressive intruder significantly attenuated the display of submissive/defensive behavior. These data demonstrate that NMDA receptors are necessary for both the acquisition and expression of conditioned defeat. We believe that conditioned defeat is a unique and valuable animal model with which to investigate the neurobiology of fear-related changes in social behavior.

Effects of chronic cocaine exposure on corticotropin-releasing hormone binding protein in the central nucleus of the amygdala and bed nucleus of the stria terminalis

The neuropeptide, corticotropin-releasing hormone (CRH), has been shown to play a role in behavioral and neurobiological effects of drugs of abuse. An important modulator of CRH, the CRH binding protein (CRH-BP), has not, on the other hand, been assessed for its role in drug-associated effects. The primary objective of the present experiment was to assess whether prior, chronic exposure to cocaine modulates expression of CRH-BP, and to compare expression of the BP with that of the peptide itself. We assessed CRH-BP and CRH mRNA expression in two brain regions where CRH is known to affect responses to drugs of abuse; namely, the central nucleus of the amygdala (CeA) and bed nucleus of the stria terminalis (BNST). Male Long-Evans rats were given 14 daily injections of cocaine (30 mg/kg, i.p.) or saline. One, 3, 10, 28, or 42 days post-treatment, animals were killed and adjacent brain sections through the CeA and BNST were processed for CRH-BP and CRH by in situ hybridization. In the CeA, cocaine pre-exposure increased both CRH and CRH-BP mRNA expression 1 day post-treatment. In the dorsal BNST, cocaine pre-exposure elevated levels of CRH-BP, but not CRH, mRNA 3 days post-treatment. Taken together, the results suggest that withdrawal-induced changes in the expression of the CRH-BP, and CRH itself, are relatively short-lived and that a dysregulation in basal expression of either gene is not likely responsible for long-lasting behavioral effects noted with cocaine and other drugs of abuse.

Selective induction of c-Fos immunoreactivity in the prelimbic cortex during reinstatement of heroin seeking induced by acute food deprivation in rats

We previously reported that acute 1-day food deprivation reinstates heroin seeking in rats. The goal of the present study was to begin identifying brain sites potentially involved in this effect. For this purpose, we measured, by immunohistochemistry, the expression of c-Fos following a test for food deprivation-induced reinstatement. Groups of rats (n=9-10 per group) were trained to lever-press for heroin (0.05-0.1mg/kg/infusion) or saline for 10 days (9 h/day); each infusion was paired with a cue light. Rats were then given 10 days of extinction during which the heroin and saline syringes were removed. Next, a test for reinstatement was conducted after exposure to 0 (baseline) or 1-day food deprivation. During training, lever pressing for heroin increased over days, while responding for saline infusions paired with the cue light decreased over time. During extinction, responding on the heroin-paired lever decreased over time, while responding on the saline-paired lever remained low. In heroin-trained rats, food deprivation induced a large increase in responding on the lever associated with drug infusions. Surprisingly, food deprivation also modestly increased responding in the saline-trained rats. Food deprivation selectively increased c-Fos immunoreactivity (IR) in the prelimbic cortex of heroin-trained, but not saline-trained, rats (n=4 per condition). Food deprivation also increased c-Fos IR in both heroin- and saline-trained rats in the basolateral amygdala and the ventrolateral bed nucleus of stria terminalis (BNST), but had no effect on c-Fos expression in the dorsolateral BNST, cingulate cortex, nucleus accumbens, and central amygdala. These results raise the possibility that the prelimbic cortex is involved in food deprivation-induced reinstatement of heroin seeking.

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 blockade of corticotropin-releasing factor receptors in the median raphe nucleus on stress-induced c-fos mRNA in the rat brain

Using a rat relapse model, we have shown that infusion of a corticotropin-releasing factor (CRF) receptor antagonist into the median raphe nucleus (MRN) blocks footshock stress-induced reinstatement of alcohol seeking in rats. The goal of the present study was to begin identifying brain sites potentially involved in this effect. For this purpose, we measured levels of c-fos mRNA in discrete nuclei of the rat brain following exposure to intermittent footshock, which was preceded by intra-MRN infusions of a CRF receptor antagonist, d-Phe CRF (0 or 50 ng). Exposure to intermittent footshock increased the expression of c-fos mRNA in a number of brain regions previously shown to be responsive to stressful stimuli. Pretreatment with d-Phe CRF in the MRN selectively attenuated the increases in c-fos mRNA induced by footshock in the central nucleus of the amygdala (CeA). These findings are consistent with previous data on the important role for the CeA in stress-induced reinstatement of drug seeking. These results also suggest that inhibition of CeA activity may contribute to the blockade of alcohol-seeking induced by footshock that we have observed following injections of d-Phe into the MRN.

Chronic stress and obesity: a new view of "comfort food"

The effects of adrenal corticosteroids on subsequent adrenocorticotropin secretion are complex. Acutely (within hours), glucocorticoids (GCs) directly inhibit further activity in the hypothalamo-pituitary-adrenal axis, but the chronic actions (across days) of these steroids on brain are directly excitatory. Chronically high concentrations of GCs act in three ways that are functionally congruent. (i) GCs increase the expression of corticotropin-releasing factor (CRF) mRNA in the central nucleus of the amygdala, a critical node in the emotional brain. CRF enables recruitment of a chronic stress-response network. (ii) GCs increase the salience of pleasurable or compulsive activities (ingesting sucrose, fat, and drugs, or wheel-running). This motivates ingestion of "comfort food." (iii) GCs act systemically to increase abdominal fat depots. This allows an increased signal of abdominal energy stores to inhibit catecholamines in the brainstem and CRF expression in hypothalamic neurons regulating adrenocorticotropin. Chronic stress, together with high GC concentrations, usually decreases body weight gain in rats; by contrast, in stressed or depressed humans chronic stress induces either increased comfort food intake and body weight gain or decreased intake and body weight loss. Comfort food ingestion that produces abdominal obesity, decreases CRF mRNA in the hypothalamus of rats. Depressed people who overeat have decreased cerebrospinal CRF, catecholamine concentrations, and hypothalamo-pituitary-adrenal activity. We propose that people eat comfort food in an attempt to reduce the activity in the chronic stress-response network with its attendant anxiety. These mechanisms, determined in rats, may explain some of the epidemic of obesity occurring in our society.

Brain circuitry and the reinstatement of cocaine-seeking behavior

RATIONALE

Recent studies have attempted to identify the neuroanatomical substrates underlying primed reinstatement of drug-seeking behavior. Identification of neuronal substrates will provide a logical rationale for designing pharmacological interventions in treating drug relapse.

OBJECTIVE

The objective was to identify brain circuitry that is shared between cue-, drug- and stress-primed reinstatement, as well as identifying aspects of brain circuitry that are distinct for each stimulus modality. The resulting circuit offers theoretical interpretations for consideration in future studies.

RESULTS

Aspects of the circuitry mediating reinstatement can be identified with reasonable confidence. The role of the basolateral amygdala in cue-primed reinstatement, the role of the ventral tegmental area in drug-primed reinstatement and the role of adrenergic innervation of the extended amygdala in stress-primed reinstatement are well characterized. Also, all three modes for priming reinstatement may converge on the anterior cingulate cortex and have a final common output through the core of the nucleus accumbens. Lacunae in our understanding of the circuit were identified, especially with regard to how stress priming is conveyed from the extended amygdala to the shared anterior cingulate accumbens core circuit.

CONCLUSIONS

The proposed convergence of priming stimuli into the glutamatergic projection from anterior cingulate to the accumbens core combined with the changes in glutamate transmission and signaling that accompany repeated psychostimulant administration points to the potential value of pharmacological agents that manipulate glutamate release or postsynaptic glutamate receptor signaling and trafficking in treating primed relapse in addicts.

A role for the prefrontal cortex in stress- and cocaine-induced reinstatement of cocaine seeking in rats

RATIONALE AND OBJECTIVE

It is well established that stress induces reinstatement of drug seeking in an animal model of relapse. Here we studied the role of the medial prefrontal cortex (mPFC) and orbitofrontal cortex (OFC) in foot-shock stress-induced reinstatement of cocaine seeking.

METHODS

Groups of rats were trained to self-administer cocaine (0.5 mg/kg per infusion, i.v., 3 h/day for 9 days) and after ten drug-free days were exposed to extinction and reinstatement test sessions. Each 60 min of extinction was separated by a 30-min time-out period after which the lever and stimulus lights were reintroduced. Rats were given four 1-h extinction sessions on day 1 and then on subsequent days were given two to three 1-h extinction sessions that were followed by a 3-h test for reinstatement. Tests were run every 48 h. In one set of experiments, the effects of inactivation of the prelimbic (PL), infralimbic (IL) or OFC by tetrodotoxin (TTX, 5 ng/0.5 micro l per side) on reinstatement induced by foot shock (5 min, intermittent, 1 mA) or priming injections of cocaine (20 mg/kg, i.p.) were determined. In a second set, the effects of infusions of the D1-like and D2-like dopamine receptor antagonists (SCH 23390 and raclopride) were studied using the same methods.

RESULTS

TTX infusions into the PL cortex blocked both foot shock and cocaine-induced reinstatement. TTX into OFC attenuated foot-shock-induced, but not cocaine-induced reinstatement. Infusions into IL were ineffective. Infusions of SCH 22390 (0.25 micro g/0.5 micro l per side) into either PL or OFC blocked foot-shock-induced reinstatement, but infusions into PL had no effect on cocaine-induced reinstatement. Raclopride (5 micro g/0.5 micro l per side) had no effect on foot-shock-induced reinstatement in either PL or OFC or on cocaine-induced reinstatement when infused into PL. Neither TTX nor SCH23390 infusions into PL or OFC had any effect on lever pressing for sucrose.

CONCLUSIONS

These results suggest that the PL and OFC regions form part of the circuitry mediating the effects of foot shock stress on reinstatement of drug seeking and that the PL region may be a common pathway for cue, drug and foot-shock stress-induced reinstatement of drug seeking.

The reinstatement model of drug relapse: history, methodology and major findings

RATIONAL AND OBJECTIVES

The reinstatement model is currently used in many laboratories to investigate mechanisms underlying relapse to drug seeking. Here, we review briefly the history of the model and describe the different procedures that have been used to study the phenomenon of reinstatement of drug seeking. The results from studies using pharmacological and neuroanatomical techniques to determine the neuronal events that mediate reinstatement of heroin, cocaine and alcohol seeking by acute priming injections of drugs, drug-associated cues and environmental stressors are summarized. In addition, several issues are discussed, including (1) the concordance between the neuronal mechanisms involved in drug-induced reinstatement and those involved in drug reward and discrimination, (2) the role of drug withdrawal states and periods in reinstatement of drug seeking, (3) the role of neuronal adaptations induced by exposure to drugs in relapse, and (4) the degree to which the rat reinstatement model provides a suitable preclinical model of relapse to drug taking.

CONCLUSIONS

The data derived from studies using the reinstatement model suggest that the neuronal events that mediate drug-, cue- and stress-induced reinstatement of drug seeking are not identical, that the mechanisms underlying drug-induced reinstatement are to some degree different from those mediating drug discrimination or reward, and that the duration of the withdrawal period following cocaine and heroin self-administration has a profound effect on reinstatement induced by drug cues and stress. Finally, there appears to be a good correspondence between the events that induce reinstatement in laboratory animals and those that provoke relapse in humans.

A spoonful of sugar: feedback signals of energy stores and corticosterone regulate responses to chronic stress

To begin to understand the effects of chronic stress on food intake and energy stores, the effects of increased activity in the hypothalamo-pituitary-adrenal (HPA) axis and glucocorticoids (GCs) on the body and brain must first be understood. We propose two major systems that are both GC sensitive: a metabolic feedback that is inhibitory and a direct central GC drive. Under basal conditions, the metabolic feedback signal to brain is dominant, although infusion of GC into a lateral brain ventricle blocks the effects of the metabolic feedback. Chronic stress activates GC secretion and brain nuclear GC receptor occupancy, markedly changing the normal relationships between these two major corticosteroid-activated systems. The stressor-induced switch in the relative strengths of these signals determines subsequent brain regulation of stress responses (behavioral, neuroendocrine and autonomic outflows). The metabolic feedback effects of GCs are mimicked by voluntary sucrose ingestion in adrenalectomized rats, and experiments suggest that the metabolic feedback also inhibits the stressor-induced direct GC drive on brain. We speculate that the interaction between peripheral and central GC-sensitive signaling systems may be coupled through the inhibitory actions of endogenous opiatergic inputs on corticotropin-releasing factor (CRF) neurons.

Inhibition of the amygdala: key to pathological states?

The amygdala plays an important role in associating sensory stimuli with aversive or appetitive outcomes. Conditioning procedures potentiate inputs to the amygdala, which facilitate emotional responses via subcortical and cortical outputs. Powerful inhibitory circuits exist that control expression of conditioned responses in the amygdala, including inhibition from prefrontal cortex. Deficient inhibitory tone in the amygdala could lead to overexpression of conditioned responses, producing pathological states such as anxiety disorders and drug-seeking behavior. Support for this comes from several lines of animal research: (1) GABA antagonist-induced priming of anxiety states, (2) extinction of conditioned fear, (3) modulation of inhibitory avoidance memory, and (4) cue-induced reinstatement of drug seeking. Cue-induced craving in humans is associated with feelings of fear and autonomic arousal, suggesting a link between fear and addiction in the amygdala. Future therapies aimed at increasing inhibitory tone in the amygdala, either locally or via the prefrontal cortex, may prevent anxiety disorders and addiction relapse. Novel neuropeptides, which can either excite or inhibit specific components of anxiety responses, offer promise in this regard.

Role of the bed nucleus of the stria terminalis versus the amygdala in fear, stress, and anxiety

The bed nucleus of the stria terminalis is a limbic forebrain structure that receives heavy projections from, among other areas, the basolateral amygdala, and projects in turn to hypothalamic and brainstem target areas that mediate many of the autonomic and behavioral responses to aversive or threatening stimuli. Despite its strategic anatomical position, initial attempts to implicate the bed nucleus of the stria terminalis in conditioned fear were largely unsuccessful. Recent studies have shown, however, that the bed nucleus of the stria terminalis does participate in certain types of anxiety and stress responses. In this work, we review these findings and suggest from the emerging pattern of evidence that, although the bed nucleus of the stria terminalis may not be necessary for rapid-onset, short-duration behaviors which occur in response to specific threats, the bed nucleus of the stria terminalis may mediate slower-onset, longer-lasting responses that frequently accompany sustained threats, and that may persist even after threat termination.

Chronic cocaine self-administration upregulates the norepinephrine transporter and alters functional activity in the bed nucleus of the stria terminalis of the rhesus monkey

The bed nucleus of the stria terminalis (BNST) is in a key position to influence the integration of motivational and visceral functions, receiving inputs from limbic regions, including the amygdala, and sending projections to areas central to reward processing, including the ventral tegmental area and nucleus accumbens. The BNST also possesses a high density of noradrenergic fibers. The purpose of the present studies was to characterize the effects of cocaine self-administration on the regulation of norepinephrine transporter (NET) distribution and functional activity in the BNST of rhesus monkeys in the initial (5 d) or chronic (100 d) phases of cocaine self-administration. NET binding site densities in the BNST were assessed with quantitative in vitro receptor autoradiography using [(3)H]nisoxetine, and rates of local cerebral glucose utilization in the BNST were measured in the same monkeys using the 2-[(14)C]deoxyglucose method. Chronic exposure to cocaine self-administration resulted in significantly higher NET binding site densities (up to 52% relative to controls) throughout the BNST. Furthermore, cerebral metabolism was depressed significantly in a time-dependent manner with larger decreases after 100 d of cocaine self-administration. These data represent the first report of significant changes in the regulation of the NET resulting from cocaine exposure in primates. Furthermore, given the role of the BNST in cocaine withdrawal and stress-related reinstatement of self-administration, the changes reported here may provide a substrate for these phenomena.

The role of corticotropin-releasing factor in the median raphe nucleus in relapse to alcohol

Using an animal model of drug relapse, we found that intermittent footshock stress reinstates alcohol seeking, an effect attenuated by the 5-HT reuptake blocker fluoxetine and by corticotropin-releasing factor (CRF) receptor antagonists. Here we studied the role of the 5-HT cell body region of the median raphe nucleus (MRN) and CRF receptors in this site in reinstatement of alcohol seeking. Rats were given alcohol in a two-bottle choice procedure (water vs alcohol) for 25 d and were then trained for 1 hr/d to press a lever for alcohol (12% w/v) for 23-30 d. Subsequently, lever pressing for alcohol was extinguished by terminating drug delivery for 5-9 d. Tests for reinstatement of alcohol seeking were then performed under extinction conditions. Intra-MRN infusions of 8-OH-DPAT [8-hydroxy-2-(di-n-propylamino)tetralin] (a 5-HT1A agonist that decreases 5-HT cell firing and release) reinstated alcohol seeking. Reinstatement of alcohol seeking also was observed after intra-MRN infusions of low doses of CRF (3-10 ng), which mimicked the effect of ventricular infusions of higher doses of the peptide (300-1000 ng). Finally, intra-MRN infusions of the CRF receptor antagonist d-Phe CRF (50 ng) blocked the effect of intermittent footshock (10 min) on reinstatement. These data suggest that an interaction between CRF and 5-HT neurons within the MRN is involved in footshock stress-induced reinstatement of alcohol seeking.

Effects of lesions of various brain areas on drug priming or footshock-induced reactivation of extinguished conditioned place preference

We have previously shown with a model of morphine-induced conditioned place preference (CPP) that a brief exposure to footshock stress or a priming dose of morphine could reactivate drug-seeking behavior after a long drug-free period. The present study was designed to examine the possible role of certain brain areas in such a reactivation. After the rats were successfully trained with morphine (4 mg/kg, i.p.) through a CPP paradigm (10 sessions of daily pairing of morphine with one of the two compartments), different parts of nucleus accumbens (NAc), ventral tegmental area (VTA), and central (Ce) or lateral (La) nucleus of amygdala were lesioned with a DC current passing through the respective location. After a 9-day abstinence period, random intermittent footshock (DC square wave, 0.5 mA, 0.5 s width, off time 10-70 s) or drug priming (morphine 0.25 mg/kg, s.c.) reactivated the place preference in sham lesion rats. However, the effect of drug priming could be completely abolished by lesions placed either at VTA, or the majority or shell part, but not the core of NAc. On the other hand, the effect of footshock stressor could be eliminated by a lesion placed at Ce but not La. These results suggest that, while both drug priming and footshock stress are effective in reactivating drug-seeking behavior, they might work through different neurochemical mechanisms and anatomical pathways.

Blockade of stress-induced but not cocaine-induced reinstatement by infusion of noradrenergic antagonists into the bed nucleus of the stria terminalis or the central nucleus of the amygdala

Experiments in our laboratory have shown that central noradrenergic (NA) activation plays a major role in stress-induced reinstatement of drug seeking in rats. In the present experiments, we investigated the effects of blockade of beta-NA adrenoceptors in the bed nucleus of the stria terminalis (BNST) and in the region of the central nucleus of the amygdala (CeA) on footshock- and cocaine-induced reinstatement. Rats were trained to self-administer cocaine (0.5 mg/kg, i.v.) for 9 d and, after a 5-7 d drug-free period, were given extinction sessions followed by a test for footshock stress-induced (15 min of intermittent footshock, 0.8 mA) or cocaine-induced (20 mg/kg, i.p.) reinstatement. Before the test, different groups of rats were given bilateral infusions of one of four doses of a mixture of the beta(1)- and beta(2)-receptor antagonists betaxolol and ICI-118,551 (vehicle, 0.25, 0.5, and 1 nmol of each compound in 0.5 microliter) into either the BNST or CeA. We observed a dose-dependent reduction of stress-induced reinstatement after infusions into the BNST and a complete blockade of stress-induced reinstatement after infusions into the CeA at all doses tested. The same treatments did not block cocaine-induced reinstatement when given at either site. These data suggest that stress-induced NA activation in the BNST and in the region of the CeA is critical to relapse to drug seeking induced by stress but not to relapse induced by priming injections of cocaine, and we hypothesize that NA activity leads to activation of corticotropin-releasing factor neurons in these regions.