The amygdala-ventral pallidum pathway contributes to a hypodopaminergic state in the ventral tegmental area during protracted abstinence from chronic cocaine

BACKGROUND AND PURPOSE

Incubation of craving, the progressive increase in drug seeking over the first weeks of abstinence, is associated with temporal changes during abstinence in the activity of several structures involved in drug-seeking behaviour. Decreases of dopamine (DA) release and DA neuronal activity (hypodopaminergic state) have been reported in the ventral tegmental area (VTA) during cocaine abstinence, but the mechanisms underlying these neuroadaptations are not well understood. We investigated the potential involvement of a VTA inhibiting circuit (basolateral amygdala [BLA]-ventral pallidum [VP] pathway) in the hypodopaminergic state associated with abstinence from chronic cocaine.

EXPERIMENTAL APPROACH

In a model of cocaine self-administration, we performed in vivo electrophysiological recordings of DA VTA neurons and BLA neurons from anaesthetised rats during early and protracted abstinence and evaluated the involvement of the BLA-VP pathway using a pharmacological approach.

KEY RESULTS

We found significant decreases in VTA DA population activity and significant increases in BLA activity after protracted but not after short-term abstinence from chronic cocaine. The decrease in VTA DA activity was restored by pharmacological inhibition of the activity of either the BLA or the VP, suggesting that these regions exert a negative influence on DA activity.

CONCLUSION AND IMPLICATIONS

Our study sheds new lights on neuroadaptations occurring during incubation of craving leading to relapse. In particular, we describe the involvement of the BLA-VP pathway in cocaine-induced decreases of DA activity in the VTA. This study adds important information about the specific brain network dysfunctions underlying hypodopaminergic activity during abstinence.

Pair housing does not alter incubation of craving, extinction, and reinstatement after heroin self-administration in female and male rats

Evidence suggests that single housing in rats acts as a chronic stressor, raising the possibilities that it contributes to measures of heroin craving and that pair housing ameliorates such measures. This study aimed to determine whether pair housing after heroin self-administration reduces the incubation of craving, extinction, and reinstatement of heroin seeking. Single-housed female and male Sprague-Dawley rats underwent daily 6-hr heroin self-administration, wherein active lever presses produced a heroin infusion paired with light/tone cues. One day after self-administration, rats underwent a baseline cued-seeking test wherein active lever presses only produced light/tone cues. Immediately following this cued-seeking test, rats were either pair-housed with weight- and sex-matched naïve rat or remained single-housed for the rest of the study. For 14 days, rats remained in their homecages, after which they underwent a cued-seeking test to assess the incubation of craving compared to their baseline test. Rats then underwent extinction sessions followed by cue-induced and heroin-primed reinstatements. The findings reveal that pair-housed rats did not differ from single-housed rats in terms of the incubation of craving, extinction, or reinstatement of heroin seeking. Additionally, the results did not reveal any evidence of sex-based differences in the study. The present work indicates that pair housing during the forced abstinence period does not alter measures of heroin craving/seeking. These findings suggest that the chronic stress of single housing specifically during forced abstinence does not contribute to the degree of such measures. (PsycInfo Database Record (c) 2023 APA, all rights reserved).

Increased heroin intake and relapse vulnerability in intermittent relative to continuous self-administration: Sex differences in rats

BACKGROUND AND PURPOSE

Studies using intermittent-access drug self-administration show increased motivation to take and seek cocaine and fentanyl, relative to continuous access. In this study, we examined the effects of intermittent- and continuous-access self-administration on heroin intake, patterns of self-administration and cue-induced heroin-seeking, after forced or voluntary abstinence, in male and female rats. We also modelled brain levels of heroin and its active metabolites.

EXPERIMENTAL APPROACH

Rats were trained to self-administer a palatable solution and then heroin (0.075 mg·kg^-1 per inf) either continuously (6 h·day^-1 ; 10 days) or intermittently (6 h·day^-1 ; 5-min access every 30-min; 10 days). Brain levels of heroin and its metabolites were modelled using a pharmacokinetic software. Next, heroin-seeking was assessed after 1 or 21 abstinence days. Between tests, rats underwent either forced or voluntary abstinence. The oestrous cycle was measured using a vaginal smear test.

KEY RESULTS

Intermittent access exacerbated heroin self-administration and was characterized by a burst-like intake, yielding higher brain peaks of heroin and 6-monoacetylmorphine concentrations. Moreover, intermittent access increased cue-induced heroin-seeking during early, but not late abstinence. Heroin-seeking was higher in females after intermittent, but not continuous access, and this effect was independent of the oestrous cycle.

CONCLUSIONS AND IMPLICATIONS

Intermittent heroin access in rats resembles critical features of heroin use disorder: a self-administration pattern characterized by repeated large doses of heroin and higher relapse vulnerability during early abstinence. This has significant implications for refining animal models of substance use disorder and for better understanding of the neuroadaptations responsible for this disorder.

LINKED ARTICLES

This article is part of a themed issue on Advances in Opioid Pharmacology at the Time of the Opioid Epidemic. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v180.7/issuetoc.

Persistent Neuroadaptations in the Nucleus Accumbens Core Accompany Incubation of Methamphetamine Craving in Male and Female Rats

Relapse is a major problem in treating methamphetamine use disorder. "Incubation of craving" during abstinence is a rat model for persistence of vulnerability to craving and relapse. While methamphetamine incubation has previously been demonstrated in male and female rats, it has not been demonstrated after withdrawal periods greater than 51 d and most mechanistic work used males. Here, we address both gaps. First, although methamphetamine intake was higher in males during self-administration training (6 h/d × 10 d), incubation was similar in males and females, with "incubated" craving persisting through withdrawal day (WD)100. Second, using whole-cell patch-clamp recordings in medium spiny neurons (MSNs) of the nucleus accumbens (NAc) core, we assessed synaptic levels of calcium-permeable AMPA receptors (CP-AMPARs), as their elevation is required for expression of incubation in males. In both sexes, compared with saline-self-administering controls, CP-AMPAR levels were significantly higher in methamphetamine rats across withdrawal, although this was less pronounced in WD100-135 rats than WD15-35 or WD40-75 methamphetamine rats. We also examined membrane properties and NMDA receptor (NMDAR) transmission. In saline controls, MSNs from males exhibited lower excitability than females. This difference was eliminated after incubation because of increased excitability of MSNs from males. NMDAR transmission did not differ between sexes and was not altered after incubation. In conclusion, incubation persists for longer than previously described and equally persistent CP-AMPAR plasticity in NAc core occurs in both sexes. Thus, abstinence-related synaptic plasticity in NAc is similar in males and females although other methamphetamine-related behaviors and neuroadaptations show differences.

Tramadol and Tapentadol Induce Conditioned Place Preference with a Differential Impact on Rewarding Memory and Incubation of Craving

Tramadol and tapentadol, synthetic opioids commonly prescribed for moderate-to-severe pain, have a unique pharmacology that optimizes their analgesia and safety. However, they are not devoid of risks, presenting addictive, abuse, and dependence potential. While tramadol-reinforcing properties have been documented by various studies with human and animal models, including conditioned place preference (CPP) assays, no similar studies have been performed with tapentadol. In the present study, we performed CPP assays by intraperitoneally administering Wistar rats with a tramadol/tapentadol therapeutic dose. Animal permanence and the number of entries in the CPP compartments were recorded in the preconditioning phase and then 1 (T1), 7 (T7), and 14 (T14) days after conditioning. Both opioids induced a change in place preference (T1), suggesting that they have short-term reinforcing properties. However, only tramadol was associated with place preference retention (T7 and T14), with an increase in the number of entries in the opioid-paired compartment (T1 and T7), showing that it causes rewarding memory and incubation of craving. The results indicate that at therapeutic doses: (1) both drugs cause short-term rewarding effects and (2) as opposed to tramadol, tapentadol does not cause CPP retention, despite its higher central nervous system activity and stricter scheduling.

GluA2-lacking AMPA receptors in the nucleus accumbens core and shell contribute to the incubation of oxycodone craving in male rats

One of the most challenging issues in the treatment of substance use disorder, including misuse of opioids such as oxycodone, is persistent vulnerability to relapse, often triggered by cues or contexts previously associated with drug use. In rats, cue-induced craving progressively intensifies ('incubates') during withdrawal from extended-access self-administration of several classes of misused drugs, including the psychostimulants cocaine and methamphetamine. For these psychostimulants, incubation is associated with strengthening of excitatory synapses in the nucleus accumbens (NAc) through incorporation of α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors that lack the GluA2 subunit and are therefore Ca2+ -permeable (CP-AMPARs). Once CP-AMPAR upregulation occurs, their stimulation is required for expression of incubation. It is not known if a similar mechanism contributes to incubation of oxycodone craving. Using male rats, we established that incubation occurs by withdrawal day (WD) 15 and persists through WD30. Then, using cell-surface biotinylation, we found that surface levels of the AMPAR subunit GluA1 but not GluA2 are elevated in NAc core and shell of oxycodone rats on WD15, although this wanes by WD30. Next, using intra-NAc injection of the selective CP-AMPAR antagonist Naspm before a seeking test, we demonstrate that CP-AMPAR blockade in either subregion decreases oxycodone seeking on WD15 or WD30 (after incubation), but not WD1, and has no effect in saline self-administering animals. The Naspm results suggest CP-AMPARs persist in synapses through WD30 even if total cell surface levels wane. These results suggest that a common neurobiological mechanism contributes to expression of incubation of craving for oxycodone and psychostimulants.

Acute high-intensity interval exercise attenuates incubation of craving for foods high in fat

OBJECTIVE

Food-seeking behaviors can be driven by food-associated cues, and palatable food seeking in response to food cues is a risk factor for obesity development. Cue-induced food seeking increases following a period of abstinence, a behavioral phenomenon known as "incubation of craving," which may contribute to an individual's difficulty abstaining from palatable foods. Pharmacological and environmental manipulations have been employed to try and reduce incubation of craving, albeit primarily in drug abuse paradigms. The goal of this study was to determine whether forced exercise can attenuate incubation of high-fat food craving.

METHODS

Male Sprague Dawley rats learned to self-administer high-fat pellets (60%) in combination with a compound cue (light + tone). The influence of high-intensity interval exercise on the time-dependent increase in cue-induced lever responding was investigated 30 days after the first cue test.

RESULTS

Rats exposed to exercise during abstinence did not express incubation of craving.

CONCLUSIONS

The results suggest that high-intensity exercise can prevent the establishment of incubation of craving for foods high in fat and may reduce cue-induced maladaptive food-seeking behaviors that contribute to overeating and obesity.

Profiling prefrontal cortex protein expression in rats exhibiting an incubation of cocaine craving following short-access self-administration procedures

INTRODUCTION

Incubation of drug-craving refers to a time-dependent increase in drug cue-elicited craving that occurs during protracted withdrawal. Historically, rat models of incubated cocaine craving employed extended-access (typically 6 h/day) intravenous drug self-administration (IV-SA) procedures, although incubated cocaine craving is reported to occur following shorter-access IV-SA paradigms. The notoriously low-throughput of extended-access IV-SA prompted us to determine whether two different short-access IV-SA procedures akin to those in the literature result in qualitatively similar changes in glutamate receptor expression and the activation of downstream signaling molecules within prefrontal cortex (PFC) subregions as those reported previously by our group under 6h-access conditions.

METHODS

For this, adult, male Sprague-Dawley rats were trained to intravenously self-administer cocaine for 2 h/day for 10 consecutive days (2-h model) or for 6 h on day 1 and 2 h/day for the remaining 9 days of training (Mixed model). A sham control group was also included that did not self-administer cocaine.

RESULTS

On withdrawal day 3 or 30, rats were subjected to a 2-h test of cue-reinforced responding in the absence of cocaine and a time-dependent increase in drug-seeking was observed under both IV-SA procedures. Immunoblotting of brain tissue collected immediately following the cue test session indicated elevated phospho-Akt1, phospho-CaMKII and Homer2a/b expression within the prelimbic subregion of the PFC of cocaine-incubated rats. However, we failed to detect incubation-related changes in Group 1 metabotropic glutamate receptor or ionotropic glutamate receptor subunit expression in either subregion.

DISCUSSION

These results highlight further a role for Akt1-related signaling within the prelimbic cortex in driving incubated cocaine craving, and provide novel evidence supporting a potential role also for CaMKII-dependent signaling through glutamate receptors in this behavioral phenomenon.

FACTORS CONTRIBUTING TO THE ESCALATION OF ALCOHOL CONSUMPTION

Understanding factors that contribute to the escalation of alcohol consumption is key to understanding how an individual transitions from non/social drinking to AUD and to providing better treatment. In this review, we discuss how the way ethanol is consumed as well as individual and environmental factors contribute to the escalation of ethanol consumption from intermittent low levels to consistently high levels. Moreover, we discuss how these factors are modelled in animals. It is clear a vast array of complex, interacting factors influence changes in alcohol consumption. Some of these factors act early in the acquisition of ethanol consumption and initial escalation, while others contribute to escalation of ethanol consumption at a later stage and are involved in the development of alcohol dependence. There is considerable need for more studies examining escalation associated with the formation of dependence and other hallmark features of AUD, especially studies examining mechanisms, as it is of considerable relevance to understanding and treating AUD.

Increased cocaine motivation in tree shrews is modulated by striatal dopamine D1 receptor-mediated upregulation of Cav 1.2

The progressively increased motivation for cocaine during abstinence is closely associated with the dysfunction of dopamine (DA) system. As DA receptors also dynamically regulate L-type calcium channels (LTCCs), in this study we examined how DA receptors (D1R or D2R) and LTCCs (Ca_v 1.2 or Ca_v 1.3) exert their influences on cocaine-seeking in a tree shrew (Tupaia belangeri chinensis) model. First, we demonstrated the 'incubation' effect by showing tree shrews exhibited a significantly higher seeking behaviour on withdrawal day (WD) 45 than on WD1. Then, we confirmed that longer abstinence period induced higher D1R expression in the nucleus accumbens (NAc). Next, we showed that LTCCs in the NAc participated in drug seeking. Moreover, Ca_v 1.2 expression in the NAc was increased on WD45, and disruption of the Ca_v 1.2 inhibited drug seeking. Finally, we found that D1R antagonist blocked the increase of Ca_v 1.2 on drug-seeking test. Collectively, these findings suggest that D1R-mediated upregulation of Ca_v 1.2 is involved in the incubation of cocaine craving.

Orbitofrontal cortex and dorsal striatum functional connectivity predicts incubation of opioid craving after voluntary abstinence

We recently introduced a rat model of incubation of opioid craving after voluntary abstinence induced by negative consequences of drug seeking. Here, we used resting-state functional MRI to determine whether longitudinal functional connectivity changes in orbitofrontal cortex (OFC) circuits predict incubation of opioid craving after voluntary abstinence. We trained rats to self-administer for 14 d either intravenous oxycodone or palatable food. After 3 d, we introduced an electric barrier for 12 d that caused cessation of reward self-administration. We tested the rats for oxycodone or food seeking under extinction conditions immediately after self-administration training (early abstinence) and after electric barrier exposure (late abstinence). We imaged their brains before self-administration and during early and late abstinence. We analyzed changes in OFC functional connectivity induced by reward self-administration and electric barrier-induced abstinence. Oxycodone seeking was greater during late than early abstinence (incubation of oxycodone craving). Oxycodone self-administration experience increased OFC functional connectivity with dorsal striatum and related circuits that was positively correlated with incubated oxycodone seeking. In contrast, electric barrier-induced abstinence decreased OFC functional connectivity with dorsal striatum and related circuits that was negatively correlated with incubated oxycodone seeking. Food seeking was greater during early than late abstinence (abatement of food craving). Food self-administration experience and electric barrier-induced abstinence decreased or maintained functional connectivity in these circuits that were not correlated with abated food seeking. Opposing functional connectivity changes in OFC with dorsal striatum and related circuits induced by opioid self-administration versus voluntary abstinence predicted individual differences in incubation of opioid craving.

Isolation housing elevates amphetamine seeking independent of nucleus accumbens glutamate receptor adaptations

Overdose death rates caused by psychostimulants have increased by 22.3% annually from 2008 to 2017. Cue-evoked drug craving progressively increases and contributes to perpetual relapse. Preclinical models have determined that glutamate receptor plasticity within the nucleus accumbens (NAc) drives amplified cue-evoked drug seeking after prolonged abstinence (>40 days). Isolated condition (IC) rearing increases cocaine and amphetamine (AMP) self-administration and cue-induced reinstatement. We tested the hypothesis that housing in the IC will augment AMP seeking after short and prolonged abstinence from AMP self-administration when compared with rats reared in the enrichment condition (EC). EC and IC male rats acquired stable AMP or SAL self-administration and were tested in a cue-induced AMP-seeking test after 1 and 40 days of abstinence. After the seeking test, the whole NAc was extracted and prepared for western blot analysis. Results indicate that IC rats had more active lever presses during a brief extinction interval and during the cue-induced seeking test. After 40 days of abstinence, IC rats had more active lever presses than EC rats during the cue-induced seeking test. Western blots indicated that the expression ratio between GluA1:mGlur5 was reduced only in IC-AMP-trained rats and the ratio between GluA1:mGlur1 was positively correlated with AMP seeking after prolonged abstinence in IC-AMP rats. These results indicate that IC housing engenders a vulnerable phenotype prone to persistent AMP seeking. The behavioural momentum of this vulnerable phenotype is further revealed when AMP-associated cues are presented following prolonged abstinence.

Sex differences in the effect of chronic delivery of the buprenorphine analogue BU08028 on heroin relapse and choice in a rat model of opioid maintenance

BACKGROUND AND PURPOSE

Maintenance treatment with opioid agonists (buprenorphine, methadone) decreases opioid use and relapse. We recently modelled maintenance treatment in rats and found that chronic delivery of buprenorphine or the μ opioid receptor partial agonist TRV130 decreased relapse to oxycodone seeking and taking. Here, we tested the buprenorphine analogue BU08028 on different heroin relapse-related measures and heroin versus food choice.

EXPERIMENTAL APPROACH

For relapse assessment, we trained male and female rats to self-administer heroin (6 h·day^-1 , 14 days) in Context A and then implanted osmotic minipumps containing BU08028 (0, 0.03 or 0.1 mg·kg^-1 ·d^-1 ). Effects of chronic BU08028 delivery were tested on (1) incubation of heroin-seeking in a non-drug Context B, (2) extinction responding reinforced by heroin-associated discrete cues in Context B, (3) reinstatement of heroin-seeking induced by re-exposure to Context A and (4) re-acquisition of heroin self-administration in Context A. For choice assessment, we tested the effect of chronic BU08028 delivery on heroin versus food choice.

KEY RESULTS

Chronic BU08028 delivery decreased incubation of heroin seeking. Unexpectedly, BU08028 increased re-acquisition of heroin self-administration selectively in females. Chronic BU08028 had minimal effects on context-induced reinstatement and heroin versus food choice in both sexes. Finally, exploratory post hoc analyses suggest that BU08028 decreased extinction responding selectively in males.

CONCLUSIONS AND IMPLICATIONS

Chronic BU08028 delivery had both beneficial and detrimental, sex-dependent, effects on different triggers of heroin relapse and minimal effects on heroin choice in both sexes. Results suggest that BU08028 would not be an effective opioid maintenance treatment in humans.

CaMKII Modulates Diacylglycerol Lipase-α Activity in the Rat Nucleus Accumbens after Incubation of Cocaine Craving

Relapse is a major challenge to the treatment of substance use disorders. A progressive increase in cue-induced drug craving, termed incubation of craving, is observed after withdrawal from multiple drugs of abuse in humans and rodents. Incubation of cocaine craving involves the strengthening of excitatory synapses onto nucleus accumbens (NAc) medium spiny neurons via postsynaptic accumulation of high-conductance Ca²⁺-permeable AMPA receptors. This enhances reactivity to drug-associated cues and is required for the expression of incubation. Additionally, incubation of cocaine craving is associated with loss of the synaptic depression normally triggered by stimulation of metabotropic glutamate receptor 5 (mGlu5), leading to endocannabinoid production, and expressed presynaptically via cannabinoid receptor 1 activation. Previous studies have found alterations in mGlu5 and Homer proteins associated with the loss of this synaptic depression. Here we conducted coimmunoprecipitation studies to investigate associations of diacylglycerol lipase-α (DGL), which catalyzes formation of the endocannabinoid 2-arachidonylglycerol (2-AG), with mGlu5 and Homer proteins. Although these interactions were unchanged in the NAc core at incubation-relevant withdrawal times, the association of DGL with total and phosphorylated Ca²⁺/calmodulin-dependent protein kinase IIα (CaMKIIα) and CaMKIIβ was increased. This would be predicted, based on other studies, to inhibit DGL activity and therefore 2-AG production. This was confirmed by measuring DGL enzymatic activity. However, the magnitude of DGL inhibition did not correlate with the magnitude of incubation of craving for individual rats. These results suggest that CaMKII contributes to the loss of mGlu5-dependent synaptic depression after incubation, but the functional significance of this loss remains unclear.

Incubation of alcohol craving as it naturally occurs in a developmentally diverse sample of dependent and nondependent drinkers

Longer periods of abstinence are shown to enhance response to alcohol cues among alcohol-dependent animals and humans, a phenomenon described as "incubation of craving." The present work examined the effects of days since last drink on general craving and alcohol-cued craving as it occurs in daily life and explored whether effects were influenced by age and dependence. Three samples were combined to include 266 drinkers ranging in age from 14 to 67 years recruited from the community; about half (59.4%) met criteria for alcohol dependence. Drinkers used handheld electronic devices to rate their subjective alcohol craving (assessed as "urge to drink") and situational context (e.g., presence of visible alcohol cues) at nondrinking times in daily life, with days since last alcohol use culled from timeline follow-back interviews and real-world reports. Drinkers at the lower end of the age range in this sample reported greater intensification of craving with more days of continuous abstinence than drinkers at the upper end of the age range. Age was not related to incubation of cue-elicited craving, in specific, however. For drinkers with dependence, craving when in the presence of visible alcohol cues intensified with more days of continuous abstinence, suggesting craving incubation. This study builds from important foundational work to demonstrate that incubation of cue-elicited craving occurs in dependent drinkers and applies regardless of age. Inasmuch as craving is a motivational drive that maintains alcohol use, understanding factors that influence craving in daily life holds promise for improving clinical care.

Role of orbitofrontal cortex in incubation of oxycodone craving in male rats

One of the main challenges in treating opioid-use disorders is relapse during abstinence, triggered by re-exposure to drug-associated cues. Previous studies have demonstrated that drug-seeking in rats progressively increases over time during withdrawal (incubation of drug craving). Here, we used male rats and examined neural mechanisms underlying incubation of craving to oxycodone, a commonly abused prescription opioid, and we focused on orbitofrontal cortex (OFC), a brain region previously implicated in incubation of heroin craving. We first used neuronal activity marker Fos and measured neuronal activation in OFC (ventral and lateral OFC) associated with day-1 and day-15 relapse tests. Next, we determined the effect of pharmacological reversible inactivation of OFC on incubated oxycodone seeking on withdrawal day 15. Finally, we determined the effect of reversible inactivation of OFC on nonincubated oxycodone seeking on withdrawal day 1. We found that lever presses during relapse tests were higher on withdrawal day 15 than on withdrawal day 1 (incubation of oxycodone craving). Incubation of oxycodone craving is accompanied with a time-dependent increase of Fos protein expression in both ventral and lateral OFC. Lastly, OFC inactivation decreased oxycodone seeking on withdrawal day 15 but had no effect on withdrawal day 1. Together with the previous heroin study, results here show that OFC plays a critical role in incubation of opioid craving.

Effects of Saccharin Consumption on Operant Responding for Sugar Reward and Incubation of Sugar Craving in Rats

Repeated experience with artificial sweeteners increases food consumption and body weight gain in rats. Saccharin consumption may reduce the conditioned satiety response to sweet-tasting food. Rats were trained to press a lever to obtain sucrose for five days. A compound cue (tone + light) was presented with every sucrose delivery. On the following day, each lever press produced only the compound cue (cue-reactivity test). Subjects were then provided with yogurt for three weeks in their home cages. The rats were divided into two groups. Rats in the saccharin group received yogurt sweetened with saccharin on some days and unsweetened yogurt on others. For the plain group, only unsweetened plain yogurt was provided. Subsequently, the cue-reactivity test was conducted again. On the following day, the rats underwent a consumption test in which each lever press was reinforced with sucrose. Chow consumption and body weight gain were larger in the saccharin group than in the plain group. Lever responses increased from the first to the second cue-reactivity tests (incubation of craving) in both groups. During the consumption test, lever responses were higher in the saccharin group than in the plain group, suggesting that the conditioned satiety response was impaired in the saccharin group.

Repeated restraint stress exposure during early withdrawal accelerates incubation of cue-induced cocaine craving

A major challenge for treating cocaine addiction is the propensity for abstinent users to relapse. Two important triggers for relapse are cues associated with prior drug use and stressful life events. To study their interaction in promoting relapse during abstinence, we used the incubation model of craving and relapse in which cue-induced drug seeking progressively intensifies ('incubates') during withdrawal from extended-access cocaine self-administration. We tested rats for cue-induced cocaine seeking on withdrawal day (WD) 1. Rats were then subjected to repeated restraint stress or control conditions (seven sessions held between WD6 and WD14). All rats were tested again for cue-induced cocaine seeking on WD15, 1 day after the last stress or control session. Although controls showed a time-dependent increase in cue-induced cocaine seeking (incubation), rats exposed to repeated stress in early withdrawal exhibited a more robust increase in seeking behavior between WD1 and WD15. In separate stressed and control rats, equivalent cocaine seeking was observed on WD48. These results indicate that repeated stress in early withdrawal accelerates incubation of cocaine craving, although craving plateaus at the same level were observed in controls. However, 1 month after the WD48 test, rats subjected to repeated stress in early withdrawal showed enhanced cue-induced cocaine seeking following acute (24 hours) food deprivation stress. Together, these data indicate that chronic stress exposure enhances the initial rate of incubation of craving during early withdrawal, resulting in increased vulnerability to cue-induced relapse during this period, and may lead to a persistent increase in vulnerability to the relapse-promoting effects of stress.

Prolonged withdrawal from cocaine self-administration affects prefrontal cortex- and basolateral amygdala-nucleus accumbens core circuits but not accumbens GABAergic local interneurons

Withdrawal from extended-access cocaine self-administration leads to progressive intensification ('incubation') of cocaine craving. After prolonged withdrawal (1-2 months), when craving is high, expression of incubation depends on strengthening of excitatory inputs to medium spiny neurons (MSN) of the nucleus accumbens (NAc). These excitatory inputs interact with the intra-NAc GABAergic 'microcircuit', composed of MSN axon collaterals and GABAergic interneurons. Here, we investigated whether the increased glutamatergic neurotransmission observed after prolonged withdrawal is accompanied by altered GABAergic neurotransmission, focusing on NAc core. Rats self-administered cocaine or saline (6 hours/day) and then underwent >40 days of withdrawal. First, we investigated parvalbumin positive (PV+) interneurons, GABAergic fast-spiking interneurons that regulate MSN activity. Immunohistochemical studies revealed no significant change in PV signal intensity or the number of PV+ cells in cocaine rats versus saline controls. We then screened PV and other interneuron markers using immunoblotting. We detected no changes in levels of PV, calretinin, calbindin or neuronal nitric oxide synthase. Because expression of these markers is activity dependent, our results suggest no marked changes in interneuron activity. Finally, we utilized local field potential recording, which can detect GABA-mediated alterations at the circuit level, to investigate potential changes in two circuits implicated in cocaine craving: prelimbic prefrontal cortex to NAc core and basolateral amygdala to NAc core. We detected differential adaptations in these circuits, some of which may involve GABA. Overall, our results suggest that alterations in GABA transmission may accompany incubation of cocaine craving, but they are circuit specific and less pronounced than alterations in glutamate transmission.

Role of Central Amygdala Neuronal Ensembles in Incubation of Nicotine Craving

The craving response to smoking-associated cues in humans or to intravenous nicotine-associated cues in adult rats progressively increases or incubates after withdrawal. Here, we further characterized incubation of nicotine craving in the rat model by determining whether this incubation is observed after adolescent-onset nicotine self-administration. We also used the neuronal activity marker Fos and the Daun02 chemogenetic inactivation procedure to identify cue-activated neuronal ensembles that mediate incubation of nicotine craving. We trained adolescent and adult male rats to self-administer nicotine (2 h/d for 12 d) and assessed cue-induced nicotine seeking in extinction tests (1 h) after 1, 7, 14, or 28 withdrawal days. In both adult and adolescent rats, nicotine seeking in the relapse tests followed an inverted U-shaped curve, with maximal responding on withdrawal day 14. Independent of the withdrawal day, nicotine seeking in the relapse tests was higher in adult than in adolescent rats. Analysis of Fos expression in different brain areas of adolescent and adult rats on withdrawal days 1 and 14 showed time-dependent increases in the number of Fos-positive neurons in central and basolateral amygdala, orbitofrontal cortex, ventral and dorsal medial prefrontal cortex, and nucleus accumbens core and shell. In adult Fos-lacZ transgenic rats, selective inactivation of nicotine-cue-activated Fos neurons in central amygdala, but not orbitofrontal cortex, decreased "incubated" nicotine seeking on withdrawal day 14. Our results demonstrate that incubation of nicotine craving occurs after adolescent-onset nicotine self-administration and that neuronal ensembles in central amygdala play a critical role in this incubation.

SIGNIFICANCE STATEMENT

The craving response to smoking-associated cues in humans or to intravenous nicotine-associated cues in adult rats progressively increases or incubates after withdrawal. It is currently unknown whether incubation of craving also occurs after adolescent-onset nicotine self-administration. The brain areas that mediate such incubation are also unknown. Here, we used a rat model of incubation of drug craving, the neuronal activity marker Fos, and the Daun02 chemogenetic inactivation method to demonstrate that incubation of nicotine craving is also observed after adolescent-onset nicotine self-administration and that neuronal ensembles in the central nucleus of the amygdala play a critical role in this incubation in adult rats.

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

BACKGROUND

Recent research has demonstrated that aerobic exercise can attenuate craving for drugs of abuse and reduce escalation and reinstatement of drug-seeking behavior in animal models. The present study examined the effects of aerobic exercise on the development of the incubation of cocaine-seeking behavior or the progressive increase in cocaine seeking over a protracted withdrawal period from cocaine self-administration.

METHODS

Female rats were trained to self-administer cocaine (0.4 mg/kg/inf) during daily 6-h sessions for 10 days. Subsequently, access to cocaine and cocaine-paired cues was discontinued during a 3- or 30-day withdrawal period when rats had access to either a locked or unlocked running wheel. At the end of the withdrawal period, rats were reintroduced to the operant conditioning chamber and reexposed to cocaine-paired cues to examine cocaine-seeking behavior under extinction conditions.

RESULTS

Rats with access to a locked running wheel during 30 days of withdrawal had significantly greater cue-induced cocaine-seeking behavior than rats that had access to an unlocked running wheel for 30 days. Further, there was robust incubation of cocaine seeking in rats with access to a locked running wheel as cocaine seeking was notably elevated at 30 vs. 3 days of withdrawal. However, cocaine-seeking behavior did not differ between rats with access to an unlocked running wheel for 30 vs. 3 days, indicating that incubation of cocaine seeking was suppressed following access to exercise for 30 days.

CONCLUSION

Aerobic exercise during extended withdrawal from cocaine self-administration decreased incubation of cue-induced cocaine-seeking behavior and may reduce vulnerability to relapse.

β-endorphin via the delta opioid receptor is a major factor in the incubation of cocaine craving

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

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.