Genetic Disruption of System xc-Mediated Glutamate Release from Astrocytes Increases Negative-Outcome Behaviors While Preserving Basic Brain Function in Rat.

The importance of neuronal glutamate to synaptic transmission throughout the brain illustrates the immense therapeutic potential and safety risks of targeting this system. Astrocytes also release glutamate, the clinical relevance of which is unknown as the range of brain functions reliant on signaling from these cells hasn't been fully established. Here, we investigated system xc- (Sxc), which is a glutamate release mechanism with an in vivo rodent expression pattern that is restricted to astrocytes. As most animals do not express Sxc, we first compared the expression and sequence of the obligatory Sxc subunit xCT among major classes of vertebrate species. We found xCT to be ubiquitously expressed and under significant negative selective pressure. Hence, Sxc likely confers important advantages to vertebrate brain function that may promote biological fitness. Next, we assessed brain function in male genetically modified rats (MSxc) created to eliminate Sxc activity. Unlike other glutamatergic mechanisms, eliminating Sxc activity was not lethal and didn't alter growth patterns, telemetry measures of basic health, locomotor activity, or behaviors reliant on simple learning. However, MSxc rats exhibited deficits in tasks used to assess cognitive behavioral control. In a pavlovian conditioned approach, MSxc rats approached a food-predicted cue more frequently than WT rats, even when this response was punished. In attentional set shifting, MSxc rats displayed cognitive inflexibility because of an increased frequency of perseverative errors. MSxc rats also displayed heightened cocaine-primed drug seeking. Hence, a loss of Sxc-activity appears to weaken control over nonreinforced or negative-outcome behaviors without altering basic brain function.SIGNIFICANCE STATEMENT Glutamate is essential to synaptic activity throughout the brain, which illustrates immense therapeutic potential and risk. Notably, glutamatergic mechanisms are expressed by most types of brain cells. Hence, glutamate likely encodes multiple forms of intercellular signaling. Here, we hypothesized that the selective manipulation of astrocyte to neuron signaling would alter cognition without producing widespread brain impairments. First, we eliminated activity of the astrocytic glutamate release mechanism, Sxc, in rat. This impaired cognitive flexibility and increased expression of perseverative, maladaptive behaviors. Notably, eliminating Sxc activity did not alter metrics of health or noncognitive brain function. These data add to recent evidence that the brain expresses cognition-specific molecular mechanisms that could lead to highly precise, safe medications for impaired cognition.

Norepinephrine activates ß1 -adrenergic receptors at the inner nuclear membrane in astrocytes.

Norepinephrine exerts powerful influences on the metabolic, neuroprotective and immunoregulatory functions of astrocytes. Until recently, all effects of norepinephrine were believed to be mediated by receptors localized exclusively to the plasma membrane. However, recent studies in cardiomyocytes have identified adrenergic receptors localized to intracellular membranes, including Golgi and inner nuclear membranes, and have shown that norepinephrine can access these receptors via transporter-mediated uptake. We recently identified a high-capacity norepinephrine transporter, organic cation transporter 3 (OCT3), densely localized to outer nuclear membranes in astrocytes, suggesting that adrenergic signaling may also occur at the inner nuclear membrane in these cells. Here, we used immunofluorescence and western blot to show that ß1 -adrenergic receptors are localized to astrocyte inner nuclear membranes; that key adrenergic signaling partners are present in astrocyte nuclei; and that OCT3 and other catecholamine transporters are localized to astrocyte plasma and nuclear membranes. To test the functionality of nuclear membrane ß1 -adrenergic receptors, we monitored real-time protein kinase A (PKA) activity in astrocyte nuclei using a fluorescent biosensor. Treatment of astrocytes with norepinephrine induced rapid increases in PKA activity in the nuclear compartment. Pretreatment of astrocytes with inhibitors of catecholamine uptake blocked rapid norepinephrine-induced increases in nuclear PKA activity. These studies, the first to document functional adrenergic receptors at the nuclear membrane in any central nervous system cell, reveal a novel mechanism by which norepinephrine may directly influence nuclear processes. This mechanism may contribute to previously described neuroprotective, metabolic and immunoregulatory actions of norepinephrine.

Ventral hippocampal shock encoding modulates the expression of trace cued fear.

The hippocampus is crucial for associative fear learning when the anticipation of threat requires temporal or contextual binding of predictive stimuli as in trace and contextual fear conditioning. Compared with the dorsal hippocampus, far less is known about the contribution of the ventral hippocampus to fear learning. The ventral hippocampus, which is highly interconnected with defensive and emotional networks, has a prominent role in both innate and learned affective behaviors including anxiety, fear, and reward. Lesions or temporary inactivation of the ventral hippocampus impair both cued and contextual fear learning, but whether the ventral hippocampal role in learning is driven by affective processing, associative encoding, or both is not clear. Here, we used trace fear conditioning in mixed sex cohorts to assess the contribution of shock-encoding to the acquisition of cued and contextual fear memories. Trace conditioning requires subjects to associate an auditory conditional stimulus (CS) with a shock unconditional stimulus (UCS) that are separated in time by a 20-s trace interval. We first recorded neuronal activity in the ventral hippocampus during trace fear conditioning and found that ventral CA1 predominantly encoded the shock reinforcer. Potentiated firing to the CS was evident at testing, but no encoding of the trace interval was observed. We then tested the necessity of shock encoding for conditional fear acquisition by optogenetically silencing ventral hippocampal activity during the UCS on each trial of training. Contrary to our predictions, preventing hippocampal shock-evoked firing did not impair associative fear. Instead, it led to a more prolonged expression of CS freezing across test trials, an effect observed in males, but not females. Contextual fear learning was largely intact, although a subset of animals in each sex were differentially affected by shock-silencing. Taken together, the results show that shock encoding in the ventral hippocampus modulates the expression of learned fear in a sex-specific manner.

Ventral Hippocampal Input to the Prelimbic Cortex Dissociates the Context from the Cue Association in Trace Fear Memory.

The PFC, through its high degree of interconnectivity with cortical and subcortical brain areas, mediates cognitive and emotional processes in support of adaptive behaviors. This includes the formation of fear memories when the anticipation of threat demands learning about temporal or contextual cues, as in trace fear conditioning. In this variant of fear learning, the association of a cue and shock across an empty trace interval of several seconds requires sustained cue-elicited firing in the prelimbic cortex (PL). However, it is unknown how and when distinct PL afferents contribute to different associative components of memory. Among the prominent inputs to PL, the hippocampus shares with PL a role in both working memory and contextual processing. Here we tested the necessity of direct hippocampal input to the PL for the acquisition of trace-cued fear memory and the simultaneously acquired contextual fear association. Optogenetic silencing of ventral hippocampal (VH) terminals in the PL of adult male Long-Evans rats selectively during paired trials revealed that direct communication between the VH and PL during training is necessary for contextual fear memory, but not for trace-cued fear acquisition. The pattern of the contextual memory deficit and the disruption of local PL firing during optogenetic silencing of VH-PL suggest that the VH continuously updates the PL with the current contextual state of the animal, which, when disrupted during memory acquisition, is detrimental to the subsequent rapid retrieval of aversive contextual associations.SIGNIFICANCE STATEMENT Learning to anticipate threat from available contextual and discrete cues is crucial for survival. The prelimbic cortex is required for forming fear memories when temporal or contextual complexity is involved, as in trace fear conditioning. However, the respective contribution of distinct prelimbic afferents to the temporal and contextual components of memory is not known. We report that direct input from the ventral hippocampus enables the formation of the contextual, but not trace-cued, fear memory necessary for the subsequent rapid expression of a fear response. This finding dissociates the contextual and working-memory contributions of prelimbic cortex to the formation of a fear memory and demonstrates the crucial role for hippocampal input in contextual fear learning.

Prelimbic input to basolateral amygdala facilitates the acquisition of trace cued fear memory under weak training conditions.

The ability to predict the occurrence of an aversive outcome based on available cues requires associative learning and plastic changes in the amygdala. When the predictive cue and aversive shock outcome are separated in time as in trace fear conditioning, additional circuitry is needed, including the prelimbic (PL) area of the prefrontal cortex. We have previously shown that neuronal firing in the PL during the trace interval separating the cue and shock is required for trace cued fear memory formation, but whether this mnemonic signal is conveyed to the amygdala is unknown. Here we show in males that silencing PL activity during the trace interval reduces Arc protein in the basolateral amygdala (BLA) of trace-conditioned rats. Then, using pathway-specific optogenetic and chemogenetic silencing, we show a role for direct PL-BLA communication in trace cued fear learning under weak training conditions, but not standard training. These results suggest that PL input to the BLA may serve to promote cued learning when the cue-shock relationship is most ambiguous and that other trace fear circuitry can compensate for the loss of this connection with additional training. This also highlights the challenge to studying how emotional memories are formed and stored within a distributed network and suggests that the function of individual connections within such a network may best be determined using weak training conditions.

Estrous cycle stage gates sex differences in prefrontal muscarinic control of fear memory formation.

The association of a sensory cue and an aversive footshock that are separated in time, as in trace fear conditioning, requires persistent activity in prelimbic cortex during the cue-shock interval. The activation of muscarinic acetylcholine receptors has been shown to facilitate persistent firing of cortical cells in response to brief stimulation, and muscarinic antagonists in the prefrontal cortex impair working memory. It is unknown, however, if the acquisition of associative trace fear conditioning is dependent on muscarinic signaling in the prefrontal cortex. Here, we delivered the muscarinic receptor antagonist scopolamine to the prelimbic cortex of rats prior to trace fear conditioning and tested their memories of the cue and training context the following day. The effect of scopolamine on working memory performance was also tested using a spatial delayed non-match to sample task. Male and female subjects were included to examine potential sex differences in the modulation of memory formation, as we have previously observed for pituitary adenylate cyclase-activating polypeptide signaling in the prefrontal cortex (Kirry et al., 2018). We found that pre-training administration of intra-prelimbic scopolamine impaired the formation of cued and contextual fear memories in males, but not females at a dose that impairs spatial working memory in both sexes. Fear memory formation in females was impaired by a higher dose of scopolamine and this impairment was gated by estrous cycle stage: scopolamine failed to impair memory in rats in the diestrus or proestrus stages of the estrous cycle. These findings add to the growing body of evidence that the prefrontal cortex is sexually dimorphic in learning and memory and additionally suggest that males and females differentially engage prefrontal neuromodulatory systems in support of learning.

17ß-estradiol is necessary for extinction of cocaine seeking in female rats.

Human and preclinical models of addiction demonstrate that gonadal hormones modulate acquisition of drug seeking. Little is known, however, about the effects of these hormones on extinction of drug-seeking behavior. Here, we investigated how 17ß-estradiol (E2) affects expression and extinction of cocaine seeking in female rats. Using a conditioned place preference (CPP) paradigm, ovariectomized rats were maintained throughout conditioning with 2 d of E2 treatment followed by 2 d of vehicle treatment, or were injected with E2 daily. Hormone injections were paired or explicitly unpaired with place conditioning sessions. Expression of a cocaine CPP was of equal magnitude regardless of conditioning protocol, suggesting that E2 levels during conditioning did not affect subsequent CPP expression. During extinction, daily E2 administration initially enhanced expression of the cocaine CPP, but resulted in significantly faster extinction compared to controls. Whereas E2-treated rats were extinguished within 8 d, vehicle-treated rats maintained CPP expression for more than a month, indicative of perseveration. To determine whether E2 could rescue extinction in these rats, half were given daily E2 treatment and half were given vehicle. E2-treated rats showed rapid extinction, whereas vehicle-treated rats continued to perseverate. These data demonstrate for the first time that E2 is necessary for extinction of cocaine seeking in female rats, and that it promotes rapid extinction when administered daily. Clinically, these findings suggest that monitoring and maintaining optimal E2 levels during exposure therapy would facilitate therapeutic interventions for female cocaine addicts.

Pilot randomized controlled trial evaluating the effect of random nicotine delivery on cigarettes per day and smoke exposure.

BACKGROUND: Many smokers report attempting to quit each year, yet most relapse, in part due to exposure to smoking-related cues. It is hypothesized that extinction of the cue-drug association could be facilitated through random nicotine delivery (RND), thus making it easier for smokers to quit. The current study aimed to evaluate the effects of RND on smoking cessation-related outcomes including cigarettes per day (CPD) and exhaled carbon monoxide (CO). METHODS: Participants were current smokers (>9 CPD) interested in quitting. Novel trans-mucosal, orally dissolving nicotine films, developed by Bionex Pharmaceuticals, were used in the study. The pharmacokinetic profile of these films was assessed in single (Experiment 1) and multiple-dose (Experiment 2) administrations prior to the smoking cessation study (Experiment 3). In Experiment 3, participants were randomized 1:1:1 to recieve 4 nicotine films per day of either: placebo delivery (0 mg), steady-state delivery (2 mg), or random nicotine delivery (RND) (0 mg or 4 mg). After two weeks, participants were advised to quit (target quit date, TQD) and were followed up 4 weeks later to collect CPD and CO and to measure dependence (Penn State Cigarette Dependence Index; PSCDI) and craving (Questionnaire of Smoking Urges; QSU-Brief). Means and frequencies were used to describe the data and repeated measures ANOVA was used to determine differences between groups. RESULTS: The pharmacokinetic studies (Experiment 1 and 2) demonstrated that the films designed for this study delivered nicotine as expected, with the 4 mg film delivering a nicotine boost of approximately 12.4 ng/mL across both the single and the multiple dose administration studies. The films reduced craving for a cigarette and were well-tolerated, overall, and caused no changes in blood pressure or heart rate. Using these films in the cessation study (Experiment 3) (n = 45), there was a significant overall reduction in cigarettes smoked per day (CPD) and in exhaled CO, with no significant differences across groups (placebo, steady-state, RND). In addition, there were no group differences in dependence or craving. Adverse events included heartburn, hiccups, nausea, and to a lesser extent, vomiting and anxiety and there were no differences across groups. CONCLUSION: Overall, this pilot study found that RND via orally dissolving films was feasible and well tolerated by participants. However, RND participants did not experience a greater reduction in self-reported CPD and exhaled CO, compared with participants in the steady-state and placebo delivery groups. Future studies to evaluate optimal RND parameters with larger sample sizes are needed to fully understand the effect of RND on smoking cessation-related outcomes.

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.

Pituitary adenylate cyclase-activating polypeptide (PACAP) signaling in the prefrontal cortex modulates cued fear learning, but not spatial working memory, in female rats.

A genetic polymorphism within the gene encoding the pituitary adenylate cyclase- activating polypeptide (PACAP) receptor type I (PAC1R) has recently been associated with hyper-reactivity to threat-related cues in women, but not men, with post-traumatic stress disorder (PTSD). PACAP is a highly conserved peptide, whose role in mediating adaptive physiological stress responses is well established. Far less is understood about the contribution of PACAP signaling in emotional learning and memory, particularly the encoding of fear to discrete cues. Moreover, a neurobiological substrate that may account for the observed link between PAC1R and PTSD in women, but not men, has yet to be identified. Sex differences in PACAP signaling during emotional learning could provide novel targets for the treatment of PTSD. Here we investigated the contribution of PAC1R signaling within the prefrontal cortex to the acquisition of cued fear in female and male rats. We used a variant of fear conditioning called trace fear conditioning, which requires sustained attention to fear cues and depends on working-memory like neuronal activity within the prefrontal cortex. We found that cued fear learning, but not spatial working memory, was impaired by administration of a PAC1R antagonist directly into the prelimbic area of the prefrontal cortex. This effect was specific to females. We also found that levels of mRNA for the PAC1R receptor in the prelimbic cortex were greater in females compared with males, and were highest during and immediately following the proestrus stage of the estrous cycle. Together, these results demonstrate a sex-specific role of PAC1R signaling in learning about threat-related cues.

An intra-amygdala circuit specifically regulates social fear learning.

Adaptive social behavior requires transmission and reception of salient social information. Impairment of this reciprocity is a cardinal symptom of autism. The amygdala is a critical mediator of social behavior and is implicated in social symptoms of autism. Here we found that a specific amygdala circuit, from the lateral nucleus to the medial nucleus (LA-MeA), is required for using social cues to learn about environmental cues that signal imminent threats. Disruption of the LA-MeA circuit impaired valuation of these environmental cues and subsequent ability to use a cue to guide behavior. Rats with impaired social guidance of behavior due to knockout of Nrxn1, an analog of autism-associated gene NRXN, exhibited marked LA-MeA deficits. Chemogenetic activation of this circuit reversed these impaired social behaviors. These findings identify an amygdala circuit required to guide emotional responses to socially significant cues and identify an exploratory target for disorders associated with social impairments.

The role of dose and restriction state on morphine-, cocaine-, and LiCl-induced suppression of saccharin intake: A comprehensive analysis.

Rats avoid intake of a taste cue when paired with a drug of abuse or with the illness-inducing agent, lithium chloride (LiCl). Although progress has been made, it is difficult to compare the suppressive effects of abused agents and LiCl on intake of a gustatory conditioned stimulus (CS) because of the cross-laboratory use of different CSs, different unconditioned stimuli (USs), and different doses of the drugs, different conditioning regimens, and different restriction states. Here we have attempted to unify these variables by comparing the suppressive effects of a range of doses of morphine, cocaine, and LiCl on intake of a saccharin CS using a common regimen in non-restricted, food restricted, or water restricted male Sprague-Dawley rats. The results showed that, while the putatively aversive agent, LiCl, was effective in suppressing intake of the taste cue across nearly all doses, regardless of restriction state, the suppressive effects of both morphine and cocaine were greatly reduced when evaluated in either food or water restricted rats. Greater sensitivity to drug was revealed, at very low doses, when testing occurred in the absence of need (i.e., when the rats were non-restricted). Together, these results provide the first uniform and comprehensive analysis of the suppressive effects of morphine, cocaine, and LiCl as a function of dose and restriction state. In the present case, the suppressive effects of morphine and cocaine are found to differ from those of LiCl and, in some respects, from one another as well.

Blocking Infralimbic Basic Fibroblast Growth Factor (bFGF or FGF2) Facilitates Extinction of Drug Seeking After Cocaine Self-Administration.

Drug exposure results in structural and functional changes in brain regions that regulate reward and these changes may underlie the persistence of compulsive drug seeking and relapse. Neurotrophic factors, such as basic fibroblast growth factor (bFGF or FGF2), are necessary for neuronal survival, growth, and differentiation, and may contribute to these drug-induced changes. Following cocaine exposure, bFGF is increased in addiction-related brain regions, including the infralimbic medial prefrontal cortex (IL-mPFC). The IL-mPFC is necessary for extinction, but whether drug-induced overexpression of bFGF in this region affects extinction of drug seeking is unknown. Thus, we determined whether blocking bFGF in IL-mPFC would facilitate extinction following cocaine self-administration. Rats were trained to lever press for intravenous infusions of cocaine before extinction. Blocking bFGF in IL-mPFC before four extinction sessions resulted in facilitated extinction. In contrast, blocking bFGF alone was not sufficient to facilitate extinction, as blocking bFGF and returning rats to their home cage had no effect on subsequent extinction. Furthermore, bFGF protein expression increased in IL-mPFC following cocaine self-administration, an effect reversed by extinction. These results suggest that cocaine-induced overexpression of bFGF inhibits extinction, as blocking bFGF during extinction permits rapid extinction. Therefore, targeted reductions in bFGF during therapeutic interventions could enhance treatment outcomes for addiction.

Aversive stimuli drive drug seeking in a state of low dopamine tone.

BACKGROUND: Stressors negatively impact emotional state and drive drug seeking, in part, by modulating the activity of the mesolimbic dopamine system. Unfortunately, the rapid regulation of dopamine signaling by the aversive stimuli that cause drug seeking is not well characterized. In a series of experiments, we scrutinized the subsecond regulation of dopamine signaling by the aversive stimulus, quinine, and tested its ability to cause cocaine seeking. Additionally, we examined the midbrain regulation of both dopamine signaling and cocaine seeking by the stress-sensitive peptide, corticotropin releasing factor (CRF). METHODS: Combining fast-scan cyclic voltammetry with behavioral pharmacology, we examined the effect of intraoral quinine administration on nucleus accumbens dopamine signaling and hedonic expression in 21 male Sprague-Dawley rats. We tested the role of CRF in modulating aversion-induced changes in dopamine concentration and cocaine seeking by bilaterally infusing the CRF antagonist, CP-376395, into the ventral tegmental area (VTA). RESULTS: We found that quinine rapidly reduced dopamine signaling on two distinct time scales. We determined that CRF acted in the VTA to mediate this reduction on only one of these time scales. Further, we found that the reduction of dopamine tone and quinine-induced cocaine seeking were eliminated by blocking the actions of CRF in the VTA during the experience of the aversive stimulus. CONCLUSIONS: These data demonstrate that stress-induced drug seeking can occur in a terminal environment of low dopamine tone that is dependent on a CRF-induced decrease in midbrain dopamine activity.

Cocaine-induced suppression of saccharin intake: a model of drug-induced devaluation of natural rewards.

In Experiment 1, water-deprived Sprague-Dawley rats were given 5 min access to saccharin. This tube retracted, an empty tube advanced, and the rats were given 1 hr to lick the empty tube on a fixed-ratio 10 lick contingency to self-administer saline or cocaine (0.33 mg/infusion) via an intravenous catheter. The results showed that rats avoided intake of saccharin after saccharin-cocaine pairings and that greater avoidance of the gustatory cue was associated with greater cocaine self-administration. In Experiment 2, a similar dose-response function was obtained with either the empty tube or a lever as the operant. In Experiment 3, avoidance of the saccharin cue and the propensity to self-administer cocaine were maintained after at least 1 month of abstinence. As such, this paradigm may be useful as a model of cue-induced craving and drug-induced devaluation of natural rewards.

Alamar blue reagent interacts with cell-culture media giving different fluorescence over time: potential for false positives.

INTRODUCTION: The cell viability assay by alamar blue is based on the principle of reduction of the non-fluorescent reagent (resazurin) to a fluorescent compound (resarufin) by the intracellular reducing environment of living cells over time. In the present study, we have for the first time shown that even in the absence of cells, there occurs significant interaction between alamar blue and cell-culture media causing an increase in fluorescence. METHODS: We have used Opti-MEM, DMEM and 1:1 DMEM:Opti-MEM as three different media and determined the changes in their relative fluorescence units (RFUs) over time after the addition of 10% (v/v) alamar blue using two-way repeated measures analysis of variance (RM-ANOVA) followed by Tukey's post-hoc test. RESULTS: Our results show that upon the addition of alamar blue, there occurs a significant increase in RFUs in all the three media over time along with a significantly higher RFU for the Opti-MEM overall (p<0.05). We also show that the time-dependent change in RFU of 1:1 DMEM:Opti-MEM was more gradual compared to that of the other two media. DISCUSSION: These findings indicate that the reagent can itself interact with the media causing significantly different fluorescence over time in a manner independent from the effect of intracellular reducing environment of living cells on alamar blue. In addition our results indicate that fluorescence varies as a function of incubation time with the reagent. These findings signify the need for routine subtraction of the background fluorescence of media-only with alamar blue reagent during measurement of cell viability by this method in order to determine an accurate measurement of cell viability.

Bidirectional effects of inhibiting or potentiating NMDA receptors on extinction after cocaine self-administration in rats.

RATIONALE: Extinction of drug seeking is facilitated by NMDA receptor (NMDAr) agonists, but it remains unclear whether extinction is dependent on NMDAr activity. OBJECTIVES: We investigated the necessity of NMDArs for extinction of cocaine seeking and whether extinction altered NMDAr expression within extinction-related neuroanatomical loci. METHODS: Rats were trained to lever press for i.v. infusions of cocaine or sucrose reinforcement prior to extinction training or withdrawal. RESULTS: Administration of the NMDAr competitive antagonist CPP prior to four brief extinction sessions impaired subsequent extinction retention. In contrast, administration of the NMDAr coagonist D-serine after four brief extinction sessions attenuated lever pressing during subsequent extinction, indicative of facilitated consolidation of extinction. Furthermore, expression of the NMDAr subunits, GluN2A and GluN2B, was not altered in the ventromedial prefrontal cortex. However, both GluN2A and GluN2B subunit expression in the nucleus accumbens increased following cocaine self-administration, and this increased expression was relatively resistant to modulation by extinction. CONCLUSIONS: Our findings demonstrate that extinction of cocaine seeking is bidirectionally mediated by NMDArs and suggest that selective modulation of NMDAr activity could facilitate extinction-based therapies for treatment of cocaine abuse.

Neurobiological mechanisms that contribute to stress-related cocaine use.

The ability of stressful life events to trigger drug use is particularly problematic for the management of cocaine addiction due to the unpredictable and often uncontrollable nature of stress. For this reason, understanding the neurobiological processes that contribute to stress-related drug use is important for the development of new and more effective treatment strategies aimed at minimizing the role of stress in the addiction cycle. In this review we discuss the neurocircuitry that has been implicated in stress-induced drug use with an emphasis on corticotropin releasing factor actions in the ventral tegmental area (VTA) and an important pathway from the bed nucleus of the stria terminalis to the VTA that is regulated by norepinephrine via actions at beta adrenergic receptors. In addition to the neurobiological mechanisms that underlie stress-induced cocaine seeking, we review findings suggesting that the ability of stressful stimuli to trigger cocaine use emerges and intensifies in an intake-dependent manner with repeated cocaine self-administration. Further, we discuss evidence that the drug-induced neuroadaptations that are necessary for heightened susceptibility to stress-induced drug use are reliant on elevated levels of glucocorticoid hormones at the time of cocaine use. Finally, the potential ability of stress to function as a "stage setter" for drug use - increasing sensitivity to cocaine and drug-associated cues - under conditions where it does not directly trigger cocaine seeking is discussed. As our understanding of the mechanisms through which stress promotes drug use advances, the hope is that so too will the available tools for effectively managing addiction, particularly in cocaine addicts whose drug use is stress-driven. This article is part of a Special Issue entitled 'NIDA 40th Anniversary Issue'.

Yoked delivery of cocaine is aversive and protects against the motivation for drug in rats.

In Experiment 1, water-deprived rats had 5-min access to saccharin followed by active or yoked intravenous delivery of saline or cocaine (0.33 mg/infusion). Both cocaine groups avoided intake of the saccharin cue following saccharin-cocaine pairings; however, the rats in the yoked condition exhibited greater avoidance of the taste cue than did the actively administering rats. Experiment 2 evaluated subsequent self-administration behavior on fixed- and progressive-ratio schedules of reinforcement. The results showed that prior yoked exposure to cocaine reduced subsequent drug-taking behavior on a progressive-ratio but not on a fixed-ratio schedule. Finally, Experiment 3 used a choice test to determine the impact of yoked drug delivery on the relative preference for cocaine versus water. The results showed that rats with a history of self-administering cocaine preferred to perform operant behaviors on the side of the chamber previously paired with cocaine, whereas the rats with a history of yoked delivery of cocaine avoided this side. These data show that, in most rats, the unpredictable, uncontrollable delivery of cocaine protects against the subsequent motivation for cocaine through an aversive mechanism.