Neuronal substrates of relapse to cocaine-seeking behavior: role of prefrontal cortex

Role of Glial Cells in Neuronal Function, Mood Disorders, and Drug Addiction

Mood disorders and substance use disorder (SUD) are of immense medical and social concern. Although significant progress on neuronal involvement in mood and reward circuitries has been achieved, it is only relatively recently that the role of glia in these disorders has attracted attention. Detailed understanding of the glial functions in these devastating diseases could offer novel interventions. Here, following a brief review of circuitries involved in mood regulation and reward perception, the specific contributions of neurotrophic factors, neuroinflammation, and gut microbiota to these diseases are highlighted. In this context, the role of specific glial cells (e.g., microglia, astroglia, oligodendrocytes, and synantocytes) on phenotypic manifestation of mood disorders or SUD are emphasized. In addition, use of this knowledge in the potential development of novel therapeutics is touched upon.

Sex differences in the neuroadaptations associated with incubated cocaine-craving: A focus on the dorsomedial prefrontal cortex

Introduction

Women have a shorter course from initial cocaine use to meeting the criteria for cocaine use disorder as compared to men. Preclinical findings similarly indicate that females develop key features of an addiction-like phenotype faster than males, including an enhanced motivation for cocaine and compulsive use, indicating that this phenomenon is biologically based. The goals of this study were to determine whether cocaine-craving, another key feature of addiction, also develops sooner during withdrawal in females than males and to determine whether there are sex differences in the molecular mechanisms associated with its development focusing on markers known to mediate cocaine-craving in males (i.e., dorsomedial prefrontal cortex, dmPFC, expression of brain-derived neurotrophic factor exon-IV, Bdnf-IV, and NMDA receptor subunits, Grin2a, Grin2b, and Grin1).

Methods

Cocaine-craving was assessed following extended-access cocaine self-administration and 2, 7, or 14 days of withdrawal using an extinction/cue-induced reinstatement procedure. Tissue was obtained from the dmPFC immediately after reinstatement testing and gene expression changes were analyzed using real-time qPCR.

Results

In males, cocaine-craving (total extinction and cue-induced reinstatement responding) progressively increased from early to later withdrawal time-points whereas in females, cocaine-craving was already elevated during early withdrawal (after 2 days) and did not further increase at later withdrawal time-points. Levels of cocaine-craving, however, were similar between the sexes. Gene expression changes differed markedly between the sexes such that males showed the expected relapse- and withdrawal-associated changes in Bdnf-IV, Grin2a, Grin2b, and Grin1 expression, but females only showed a modest increase Grin1 expression at the intermediate withdrawal timepoint.

Discussion

These findings indicate that cocaine-craving is similarly expressed in males and females although the time-course for its incubation appears to be accelerated in females; the molecular mechanisms also likely differ in females versus males.

Fos-expressing neuronal ensembles in rat infralimbic cortex encode initial and maintained oxycodone seeking in rats

Neuronal ensembles within the infralimbic cortex (IL) and their projections to the nucleus accumbens (NAc) mediate opiate seeking in well-trained rats. However, it is unclear how early this circuitry is recruited during oxycodone self-administration. Here, we used retrograde labelling (CTb) and immunohistochemistry to identify NAc-projecting neurons in the IL that were activated during initial oxycodone seeking. Next, we sought to determine the role of IL neuronal ensembles in initial oxycodone self-administration. We used the Daun02 procedure in male and female Fos-LacZ rats to chemogenetically inactivate IL Fos-expressing neurons at different time points in oxycodone self-administration training: immediately after meeting criteria for acquisition of behaviour and following nine daily sessions with increasing schedules of reinforcement (FR1, FR2 and FR3) in which rats demonstrated stable oxycodone intake under increasing effort to self-administer. We found that Daun02 infusions attenuated oxycodone seeking at both the initial learning and well-trained time points. These results suggest that IL neuronal ensembles are formed during initial learning of oxycodone self-administration and required for the maintenance and expression of oxycodone seeking.

Alterations in Excitatory and Inhibitory Synaptic Development Within the Mesolimbic Dopamine Pathway in a Mouse Model of Prenatal Drug Exposure

The rise in rates of opioid abuse in recent years in the United States has led to a dramatic increase in the incidence of neonatal abstinence syndrome (NAS). Despite improved understanding of NAS and its acute symptoms, there remains a paucity of information regarding the long-term effects of prenatal exposure to drugs of abuse on neurological development. The primary goal of this study was to investigate the effects of prenatal drug exposure on synaptic connectivity within brain regions associated with the mesolimbic dopamine pathway, the primary reward pathway associated with drug abuse and addiction, in a mouse model. Our secondary goal was to examine the role of the Ca+2 channel subunit α2δ-1, known to be involved in key developmental synaptogenic pathways, in mediating these effects. Pregnant mouse dams were treated orally with either the opioid drug buprenorphine (commonly used in medication-assisted treatment for substance use patients), gabapentin (neuropathic pain drug that binds to α2δ-1 and has been increasingly co-abused with opioids), a combination of both drugs, or vehicle daily from gestational day 6 until postnatal day 11. Confocal fluorescence immunohistochemistry (IHC) imaging of the brains of the resulting wild-type (WT) pups at postnatal day 21 revealed a number of significant alterations in excitatory and inhibitory synaptic populations within the anterior cingulate cortex (ACC), nucleus accumbens (NAC), and medial prefrontal cortex (PFC), particularly in the buprenorphine or combinatorial buprenorphine/gabapentin groups. Furthermore, we observed several drug- and region-specific differences in synaptic connectivity between WT and α2δ-1 haploinsufficient mice, indicating that critical α2δ-1-associated synaptogenic pathways are disrupted with early life drug exposure.

Novelty preferences and cocaine-associated cues influence regions associated with the salience network in juvenile female rats

Preferences for novel environments (novelty-seeking) is a risk factor for addiction, with little known about its underlying circuitry. Exposure to drug cues facilitates addiction maintenance, leading us to hypothesize that exposure to a novel environment activates a shared neural circuitry. Stimulation of the D1 receptor in the prelimbic cortex increases responsivity to drug-associated environments. Here, we use D1 receptor overexpression in the prelimbic cortex to probe brain responses to novelty-preferences (in a free-choice paradigm) and cocaine-associated odors following place conditioning. These same cocaine-conditioned odors were used to study neural circuitry with Blood Oxygen Level Dependent (BOLD) activity. D1 overexpressing females had deactivated BOLD signals related to novelty-preferences within the insula cortex and amygdala and activation in the frontal cortex and dopamine cell bodies. BOLD responses to cocaine cues were also sensitive to D1. Control females demonstrated a place preference for cocaine environments with no significant BOLD response, while D1 overexpressing females demonstrated a place aversion and weak BOLD responses to cocaine-conditioned odor cues within the insula cortex. For comparison, we provide data from an earlier study with juvenile males overexpressing D1 that show a strong preference for cocaine and elevated BOLD responses. The results support the use of a pharmacological manipulation (e.g., D1 overexpression) to probe the neural circuitry downstream from the prelimbic cortex.

Integration of value and action in medial prefrontal neural systems

The rodent medial prefrontal cortex (mPFC) plays a key role in regulating cognition, emotion, and behavior. mPFC neurons are activated in diverse experimental paradigms, raising the questions of whether there are specific task elements or dimensions encoded by mPFC neurons, and whether these encoded parameters are selective to neurons in particular mPFC subregions or networks. Here, we consider the role of mPFC neurons in processing appetitive and aversive cues, outcomes, and related behaviors. mPFC neurons are strongly activated in tasks probing value and outcome-associated actions, but these responses vary across experimental paradigms. Can we identify specific categories of responses (e.g., positive or negative value), or do mPFC neurons exhibit response properties that are too heterogeneous/complex to cluster into distinct conceptual groups? Based on a review of relevant studies, we consider what has been done and what needs to be further explored in order to address these questions.

Cocaine-Dependent Acquisition of Locomotor Sensitization and Conditioned Place Preference Requires D1 Dopaminergic Signaling through a Cyclic AMP, NCS-Rapgef2, ERK, and Egr-1/Zif268 Pathway

Elucidation of the mechanism of dopamine signaling to ERK that underlies plasticity in dopamine D1 receptor-expressing neurons leading to acquired cocaine preference is incomplete. NCS-Rapgef2 is a novel cAMP effector, expressed in neuronal and endocrine cells in adult mammals, that is required for D1 dopamine receptor-dependent ERK phosphorylation in mouse brain. In this report, we studied the effects of abrogating NCS-Rapgef2 expression on cAMP-dependent ERK→Egr-1/Zif268 signaling in cultured neuroendocrine cells; in D1 medium spiny neurons of NAc slices; and in either male or female mouse brain in a region-specific manner. NCS-Rapgef2 gene deletion in the NAc in adult mice, using adeno-associated virus-mediated expression of cre recombinase, eliminated cocaine-induced ERK phosphorylation and Egr-1/Zif268 upregulation in D1-medium spiny neurons and cocaine-induced behaviors, including locomotor sensitization and conditioned place preference. Abrogation of NCS-Rapgef2 gene expression in mPFC and BLA, by crossing mice bearing a floxed Rapgef2 allele with a cre mouse line driven by calcium/calmodulin-dependent kinase IIα promoter also eliminated cocaine-induced phospho-ERK activation and Egr-1/Zif268 induction, but without effect on the cocaine-induced behaviors. Our results indicate that NCS-Rapgef2 signaling to ERK in dopamine D1 receptor-expressing neurons in the NAc, but not in corticolimbic areas, contributes to cocaine-induced locomotor sensitization and conditioned place preference. Ablation of cocaine-dependent ERK activation by elimination of NCS-Rapgef2 occurred with no effect on phosphorylation of CREB in D1 dopaminoceptive neurons of NAc. This study reveals a new cAMP-dependent signaling pathway for cocaine-induced behavioral adaptations, mediated through NCS-Rapgef2/phospho-ERK activation, independently of PKA/CREB signaling.SIGNIFICANCE STATEMENT ERK phosphorylation in dopamine D1 receptor-expressing neurons exerts a pivotal role in psychostimulant-induced neuronal gene regulation and behavioral adaptation, including locomotor sensitization and drug preference in rodents. In this study, we examined the role of dopamine signaling through the D1 receptor via a novel pathway initiated through the cAMP-activated guanine nucleotide exchange factor NCS-Rapgef2 in mice. NCS-Rapgef2 in the NAc is required for activation of ERK and Egr-1/Zif268 in D1 dopaminoceptive neurons after acute cocaine administration, and subsequent enhanced locomotor response and drug seeking behavior after repeated cocaine administration. This novel component in dopamine signaling provides a potential new target for intervention in psychostimulant-shaped behaviors, and new understanding of how D1-medium spiny neurons encode the experience of psychomotor stimulant exposure.

Infralimbic Cortex Biases Preference Decision Making for Offspring over Competing Cocaine-Associated Stimuli in New Mother Rats

In the context of drug abuse, converging evidence suggests that cocaine use in new mothers is significantly reduced by the competing motivation related to child rearing. Given the demonstrated importance of the medial prefrontal cortex (mPFC) in decision-making processes, we investigated the contribution of distinct regions of the mPFC [anterior cingulate (Cg1), prelimbic (PrL), infralimbic (IL)] to decision making in new mother rats performing a concurrent pup/cocaine choice conditioned place preference (CPP) task. When given a choice, inactivation of IL cortex significantly biased decision making of mother rats toward cocaine-associated cues, highly contrasting the distribution of preferences by control groups. In contrast, inactivation of PrL cortex had the opposite effect, significantly increasing offspring bias in the decision making, such that none of the mothers chose the cocaine-associated alternative. Cg1 inactivation was without effect. Functional inactivation of these same mPFC subregions had no effect in a non-conflict CPP task in which context-induced cocaine or pup seeking were examined separately. Notably, inactivation of the IL cortex also interfered with maternal behavior. Taken together, we have identified a specific role of the IL cortex in the prioritization of offspring over drug competing alternatives, thus promoting resistance to drug use in new mothers.

PI3K activation within ventromedial prefrontal cortex regulates the expression of drug-seeking in two rodent species

Phosphatidylinositide 3-kinases (PI3Ks) are intracellular signal transducer enzymes that recruit protein kinase B (aka Akt) to the cell membrane, the subsequent activation of which regulates many cellular functions. PI3K/Akt activity is up-regulated within mesocorticolimbic structures in animal models of alcoholism, but less is known regarding PI3K/Akt activity in animal models of cocaine addiction. Given that prefrontal cortex (PFC) is grossly dysregulated in addiction, we studied how cocaine affects protein indices of PFC PI3K/Akt activity in rat and mouse models and examined the relevance of PI3K activity for cocaine-related learning. Immunoblotting of mouse medial PFC at 3 weeks withdrawal from a cocaine-sensitization regimen (seven injections of 30 mg/kg, intraperitoneal [IP]) revealed increased kinase activity, as did immunoblotting of tissue from the ventral PFC of rats with a history of long-access intravenous cocaine self-administration (0.25 mg/0.1 mL infusion; 10 days of 6 h/d cocaine access). Interestingly, increased Akt phosphorylation was observed in rat ventromedial PFC at both 3- and 30-day withdrawal only in animals re-exposed to cocaine-associated cues. A conditioned place-preference paradigm in mice and a cue-elicited drug-seeking test in rats were conducted to determine the functional relevance for elevated PI3K activity for addiction-related behavior. In both cases, an intra-PFC infusion of the PI3K inhibitor wortmannin (50μM) reduced drug-seeking behavior. Taken together, this cross-species, interdisciplinary, study provides convincing evidence that cocaine history produces an enduring increase in PI3K/Akt-dependent signaling within the more ventral aspect of the PFC that is relevant to behavioral reactivity to drug-associated cues/contexts. As such, PI3K inhibitors may well serve as an effective strategy for reducing drug cue reactivity and craving in cocaine addiction.

Cocaine Promotes Coincidence Detection and Lowers Induction Threshold during Hebbian Associative Synaptic Potentiation in Prefrontal Cortex

Addictive drugs usurp neural plasticity mechanisms that normally serve reward-related learning and memory, primarily by evoking changes in glutamatergic synaptic strength in the mesocorticolimbic dopamine circuitry. Here, we show that repeated cocaine exposure in vivo does not alter synaptic strength in the mouse prefrontal cortex during an early period of withdrawal, but instead modifies a Hebbian quantitative synaptic learning rule by broadening the temporal window and lowers the induction threshold for spike-timing-dependent LTP (t-LTP). After repeated, but not single, daily cocaine injections, t-LTP in layer V pyramidal neurons is induced at +30 ms, a normally ineffective timing interval for t-LTP induction in saline-exposed mice. This cocaine-induced, extended-timing t-LTP lasts for ∼1 week after terminating cocaine and is accompanied by an increased susceptibility to potentiation by fewer pre-post spike pairs, indicating a reduced t-LTP induction threshold. Basal synaptic strength and the maximal attainable t-LTP magnitude remain unchanged after cocaine exposure. We further show that the cocaine facilitation of t-LTP induction is caused by sensitized D1-cAMP/protein kinase A dopamine signaling in pyramidal neurons, which then pathologically recruits voltage-gated l-type Ca²⁺ channels that synergize with GluN2A-containing NMDA receptors to drive t-LTP at extended timing. Our results illustrate a mechanism by which cocaine, acting on a key neuromodulation pathway, modifies the coincidence detection window during Hebbian plasticity to facilitate associative synaptic potentiation in prefrontal excitatory circuits. By modifying rules that govern activity-dependent synaptic plasticity, addictive drugs can derail the experience-driven neural circuit remodeling process important for executive control of reward and addiction.

SIGNIFICANCE STATEMENT

It is believed that addictive drugs often render an addict's brain reward system hypersensitive, leaving the individual more susceptible to relapse. We found that repeated cocaine exposure alters a Hebbian associative synaptic learning rule that governs activity-dependent synaptic plasticity in the mouse prefrontal cortex, characterized by a broader temporal window and a lower threshold for spike-timing-dependent LTP (t-LTP), a cellular form of learning and memory. This rule change is caused by cocaine-exacerbated D1-cAMP/protein kinase A dopamine signaling in pyramidal neurons that in turn pathologically recruits l-type Ca²⁺ channels to facilitate coincidence detection during t-LTP induction. Our study provides novel insights on how cocaine, even with only brief exposure, may prime neural circuits for subsequent experience-dependent remodeling that may underlie certain addictive behavior.

The amphetamine-associated context exerts a stronger motivational effect in low-anxiety rats than in high-anxiety rats

This study used the conditioned place preference test to explore the effects of subchronic amphetamine administration on drug-associated cues in rats with different emotional reactivity. We also examined the changes in markers of dopaminergic activity in brain regions in response to the amphetamine-paired context, after a withdrawal period preceded by subchronic amphetamine treatment. We used low-anxiety (LR) and high-anxiety (HR) rats, which are known to exhibit distinct levels of susceptibility to amphetamine. Compared to HR rats, LR rats spent significantly more time in the amphetamine-paired compartment after the withdrawal period preceded by subchronic amphetamine treatment. Compared to HR control rats, LR control rats showed higher expression of the D1 receptor in the nucleus accumbens core (NAC core) and basolateral amygdala and higher expression of the D2 receptor in the NAC core. After the amphetamine treatment and withdrawal period, the LR rats showed higher D1 receptor expression in the NAC core, an increased level of homovanilic acid (HVA) in the prefrontal cortex, the NAC and the central amygdala than HR rats, as well as lower D2 receptor expression in the NAC core and the amygdala than LR control rats. These results indicate that the differences in the activity of the dopaminergic mesolimbic system in the HR and LR rats are maintained and even enhanced after a multi-day break in the use of the drug, indicating the occurrence of sensitisation. These findings show that the innate reactivity of the limbic dopaminergic innervations, dependent on the level of emotional reactivity, may significantly and chronically modify the development and maintenance of sensitisation to amphetamine.

Frontostriatal Circuit Dynamics Correlate with Cocaine Cue-Evoked Behavioral Arousal during Early Abstinence

It is thought that frontostriatal circuits play an important role in mediating conditioned behavioral responses to environmental stimuli that were previously encountered during drug administration. However, the neural correlates of conditioned responses to drug-associated cues are not well understood at the level of large populations of simultaneously recorded neurons, or at the level of local field potential (LFP) synchrony in the frontostriatal network.

It is thought that frontostriatal circuits play an important role in mediating conditioned behavioral responses to environmental stimuli that were previously encountered during drug administration. However, the neural correlates of conditioned responses to drug-associated cues are not well understood at the level of large populations of simultaneously recorded neurons, or at the level of local field potential (LFP) synchrony in the frontostriatal network. Here we introduce a behavioral assay of conditioned arousal to cocaine cues involving pupillometry in awake head-restrained mice. After just 24 h of drug abstinence, brief exposures to olfactory stimuli previously paired with cocaine injections led to a transient dilation of the pupil, which was greater than the dilation effect to neutral cues. In contrast, there was no cue-selective change in locomotion, as measured by the rotation of a circular treadmill. The behavioral assay was combined with simultaneous recordings from dozens of electrophysiologically identified units in the medial prefrontal cortex (mPFC) and ventral striatum (VS). We found significant relationships between cocaine cue-evoked pupil dilation and the proportion of inhibited principal cells in the mPFC and VS. Additionally, LFP coherence analysis revealed a significant correlation between pupillary response and synchrony in the 25–45 Hz frequency band. Together, these results show that pupil dilation is sensitive to drug-associated cues during acute stages of abstinence, and that individual animal differences in this behavioral arousal response can be explained by two complementary measures of frontostriatal network activity.

Cocaine Self-Administration Elevates GluN2B within dmPFC Mediating Heightened Cue-Elicited Operant Responding

Cue-elicited drug-craving correlates with hyperactivity within prefrontal cortex (PFC), which is theorized to result from dysregulated excitatory neurotransmission. The NMDA glutamate receptor is highly implicated in addiction-related neuroplasticity. As NMDA receptor function is regulated critically by its GluN2 subunits, herein, we assayed the relation between incubated cue-elicited cocaine-seeking following extended access to intravenous cocaine (6 h/d; 0.25 mg/infusion for 10 d) and the expression of GluN2A/B receptor subunits within PFC sub regions during early versus late withdrawal (respectively, 3 vs. 30 days). Cocaine-seeking rats exhibited elevated GluN2B expression within the dorsomedial aspect of the PFC (dmPFC); this effect was apparent at both 3 and 30 days withdrawal and occurred in cocaine-experienced rats, regardless of experiencing an extinction test or not. Thus, elevated dmPFC GluN2B expression appears to reflect a pharmacodynamic response to excessive cocaine intake that is independent of the duration of drug withdrawal or re-exposure to drug-taking context. The functional relevance of elevated dmPFC GluN2B expression for drug-seeking was assessed by the local infusion of the prototypical GluN2B-selective antagonist ifenprodil (1.0 µg/side). Ifenprodil did not alter cue-elicited responding in animals with a history of saline self-administration. In contrast, ifenprodil lowered cue-elicited cocaine-seeking, while potentiating cue-elicited sucrose-seeking. Thus, the effects of an intra-dmPFC ifenprodil infusion upon cue reactivity are reinforcer-specific, arguing in favor of targeting GluN2B-containing NMDA receptors as a pharmacological strategy for reducing behavioral reactivity to drug-associated cues with the potential benefit of heightening the reinforcing properties of cues associated with non-drug primary rewards.

Differential roles of medial prefrontal subregions in the regulation of drug seeking

The prefrontal cortex plays an important role in shaping cognition and behavior. Many studies have shown that medial prefrontal cortex (mPFC) plays a key role in seeking, extinction, and reinstatement of cocaine seeking in rodent models of relapse. Subregions of mPFC appear to play distinct roles in these behaviors, such that the prelimbic cortex (PL) is proposed to drive cocaine seeking and the infralimbic cortex (IL) is proposed to suppress cocaine seeking after extinction. This dichotomy of mPFC function may be a general attribute, as similar dorsal-ventral distinctions exist for expression vs. extinction of fear conditioning. However, other results indicate that the role of mPFC neurons in reward processing is more complex than a simple PL-seek vs. IL-extinguish dichotomy. Both PL and IL have been shown to drive and inhibit drug seeking (and other types of behaviors) depending on a range of factors including the behavioral context, the drug-history of the animal, and the type of drug investigated. This heterogeneity of findings may reflect multiple subcircuits within each of these PFC areas supporting unique functions. It may also reflect the fact that the mPFC plays a multifaceted role in shaping cognition and behavior, including those overlapping with cocaine seeking and extinction. Here we discuss research leading to the hypothesis that dorsal and ventral mPFC differentially control drug seeking and extinction. We also present recent results calling the absolute nature of a PL vs. IL dichotomy into question. Finally, we consider alternate functions for mPFC that correspond less to response execution and inhibition and instead incorporate the complex cognitive behavior for which the mPFC is broadly appreciated.

Incubation of cocaine-craving relates to glutamate over-flow within ventromedial prefrontal cortex

Craving elicited by drug-associated cues intensifies across protracted drug abstinence - a phenomenon termed "incubation of craving" - and drug-craving in human addicts correlates with frontal cortical hyperactivity. Herein, we employed a rat model of cue-elicited cocaine-craving to test the hypothesis that the time-dependent incubation of cue-elicited cocaine-craving is associated with adaptations in dopamine and glutamate neurotransmission within the ventromedial prefrontal cortex (vmPFC). Rats were trained to self-administer intravenous cocaine (6 h/day × 10 days) and underwent in vivo microdialysis procedures during 2 h-tests for cue-elicited cocaine-craving at either 3 or 30 days withdrawal. Controls rats were trained to either self-administer sucrose pellets or received no primary reinforcer. Cocaine-seeking rats exhibited a withdrawal-dependent increase and decrease, respectively, in cue-elicited glutamate and dopamine release. These patterns of neurochemical change were not observed in either control condition. Thus, cue-hypersensitivity of vmPFC glutamate terminals is a biochemical correlate of incubated cocaine-craving that may stem from dopamine dysregulation in this region.

Cocaine-conditioned odor cues without chronic exposure: Implications for the development of addiction vulnerability

Adolescents are highly vulnerable to addiction and are four times more likely to become addicted at first exposure than at any other age. The dopamine D1 receptor, which is typically overexpressed in the normal adolescent prefrontal cortex, is involved in drug cue responses and is associated with relapse in animal models. In human drug addicts, imaging methods have detected increased activation in response to drug cues in reward- and habit-associated brain regions. These same methods can be applied more quantitatively to rodent models. Here, changes in neuronal activation in response to cocaine-conditioned cues were observed using functional magnetic resonance imaging in juvenile rats that were made to over-express either D1 receptors or green fluorescent protein by viral-mediated transduction. Reduced activation was observed in the amygdala and dopamine cell body regions in the low cue-preferring/control juvenile rats in response to cocaine cues. In contrast, increased activation was observed in the dorsal striatum, nucleus accumbens, prefrontal cortex, and dopamine cell bodies in high cue-preferring/D1 juveniles. The increase in cue salience that is mediated by increased D1 receptor density, rather than excessive cocaine experience, appears to underlie the transition from aversion to reward in cue-induced neural response and may form the basis for habit-forming vulnerability.

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Increased D1 receptors in prefrontal cortex increase BOLD in addiction regions.

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Cocaine-associated cues activated the amygdala when cocaine was preferred.

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Cocaine cues deactivated the amygdala in the absence of cocaine preference.

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Genetic engineering can be used to isolate functional responses in neural circuitry.

Disruption of columnar and laminar cognitive processing in primate prefrontal cortex following cocaine exposure

Prefrontal cortical activity in primate brain plays a critical role in cognitive processes involving working memory and the executive control of behavior. Groups of prefrontal cortical neurons within specified cortical layers along cortical minicolumns differentially generate inter- and intra-laminar firing to process relevant information for goal oriented behavior. However, it is not yet understood how cocaine modulates such differential firing in prefrontal cortical layers. Rhesus macaque nonhuman primates (NHPs) were trained in a visual delayed match-to-sample (DMS) task while the activity of prefrontal cortical neurons (areas 46, 8 and 6) was recorded simultaneously with a custom multielectrode array in cell layers 2/3 and 5. Animals were reinforced with juice for correct responses. The first half of the recording session (control) was conducted following saline injection and in the second half of the same session cocaine was administered. Prefrontal neuron activity with respect to inter- and intra-laminar firing in layers 2/3 and 5 was assessed in the DMS task before and after the injection of cocaine. Results showed that firing rates of both pyramidal cells and interneurons increased on Match phase presentation and the Match Response (MR) in both control and cocaine halves of the session. Differential firing under cocaine vs. control in the Match phase was increased for interneurons but decreased for pyramidal cells. In addition, functional' interactions between prefrontal pyramidal cells in layer 2/3 and 5 decreased while intra-laminar cross-correlations in both layers increased. These neural recordings demonstrate that prefrontal neurons differentially encode and process information within and between cortical cell layers via cortical columns which is disrupted in a differential manner by cocaine: administration.

Chronic cocaine disrupts mesocortical learning mechanisms

The addictive power of drugs of abuse such as cocaine comes from their ability to hijack natural reward and plasticity mechanisms mediated by dopamine signaling in the brain. Reward learning involves burst firing of midbrain dopamine neurons in response to rewards and cues predictive of reward. The resulting release of dopamine in terminal regions is thought to act as a teaching signaling to areas such as the prefrontal cortex and striatum. In this review, we posit that a pool of extrasynaptic dopaminergic D1-like receptors activated in response to dopamine neuron burst firing serve to enable synaptic plasticity in the prefrontal cortex in response to rewards and their cues. We propose that disruptions in these mechanisms following chronic cocaine use contribute to addiction pathology, in part due to the unique architecture of the mesocortical pathway. By blocking dopamine reuptake in the cortex, cocaine elevates dopamine signaling at these extrasynaptic receptors, prolonging D1-receptor activation and the subsequent activation of intracellular signaling cascades, and thus inducing long-lasting maladaptive plasticity. These cellular adaptations may account for many of the changes in cortical function observed in drug addicts, including an enduring vulnerability to relapse. Therefore, understanding and targeting these neuroadaptations may provide cognitive benefits and help prevent relapse in human drug addicts.

Effect of fendiline on the maintenance and expression of methamphetamine-induced conditioned place preference in Sprague-Dawley rats

RATIONALE

Fendiline is a GABAB receptor-positive allosteric modulator and L-type Ca²⁺ channel blocker that is safe for human use. Based on these pharmacological properties, fendiline may be useful to disrupt associative memories that can drive relapse to drug use in drug-addicted individuals

OBJECTIVE

The current study evaluated the potential of fendiline to inhibit the maintenance and expression of learned associations between methamphetamine (meth) and an environmental context using conditioned place preference (CPP) in rats, to model for the associative learning that occurs during drug abuse by humans

METHODS

Following meth conditioning (1 mg/kg), fendiline (5 mg/kg) was administered at various post-conditioning times to ascertain if there was a temporal window during which fendiline would be effective.

RESULTS

Two once-daily injections of fendiline did not influence the maintenance of CPP regardless of the post-conditioning treatment time while 10 once-daily fendiline treatments inhibited CPP maintenance (p < 0.05). Fendiline administered immediately prior to the CPP test inhibited expression of meth-induced CPP in rats with a fendiline treatment history of 10 once-daily injections (p < 0.05) or those that received two injections that corresponded to the last 2 days of the 10-day treatment (p < 0.05). Fendiline did not produce preference or aversion on its own, nor did it alter motivated motor behavior.

CONCLUSION

Maintenance and expression of meth CPP is mitigated by repeated fendiline treatments when administered during the days that precede CPP testing. Reduction in the significance of meth-associated cues can reduce relapse; therefore, fendiline may be of value for addiction therapy in abstinent, meth-addicted humans.

Extinction of an instrumental response: a cognitive behavioral assay in Fmr1 knockout mice

Fragile X (FX) is the most common genetic cause of intellectual disability and autism. Previous studies have shown that partial inhibition of metabotropic glutamate receptor signaling is sufficient to correct behavioral phenotypes in a mouse model of FX, including audiogenic seizures, open-field hyperactivity and social behavior. These phenotypes model well the epilepsy (15%), hyperactivity (20%) and autism (30%) that are comorbid with FX in human patients. Identifying reliable and robust mouse phenotypes to model cognitive impairments is critical considering the 90% comorbidity of FX and intellectual disability. Recent work characterized a five-choice visuospatial discrimination assay testing cognitive flexibility, in which FX model mice show impairments associated with decreases in synaptic proteins in prefrontal cortex (PFC). In this study, we sought to determine whether instrumental extinction, another process requiring PFC, is altered in FX model mice, and whether downregulation of metabotropic glutamate receptor signaling pathways is sufficient to correct both visuospatial discrimination and extinction phenotypes. We report that instrumental extinction is consistently exaggerated in FX model mice. However, neither the extinction phenotype nor the visuospatial discrimination phenotype is corrected by approaches targeting metabotropic glutamate receptor signaling. This work describes a novel behavioral extinction assay to model impaired cognition in mouse models of neurodevelopmental disorders, provides evidence that extinction is exaggerated in the FX mouse model and suggests possible limitations of metabotropic glutamate receptor-based pharmacotherapy.

Reducing substance use during adolescence: a translational framework for prevention

RATIONALE

Most substance use is initiated during adolescence when substantial development of relevant brain circuitry is still rapidly maturing. Developmental differences in reward processing, behavioral flexibility, and self-regulation lead to changes in resilience or vulnerability to drugs of abuse depending on exposure to risk factors. Intervention and prevention approaches to reducing addiction in teens may be able to capitalize on malleable brain systems in a predictable manner.

OBJECTIVE

This review will highlight what is known about how factors that increase vulnerability to addiction, including developmental stage, exposure to early life adversity (ranging from abuse, neglect, and bullying), drug exposure, and genetic predisposition, impact the development of relevant systems.

RESULTS AND CONCLUSIONS

Appropriate, early intervention may restore the normal course of an abnormal trajectory and reduce the likelihood of developing a substance use disorder (SUD) later in life. A considerable amount is known about the functional neuroanatomy and/or pharmacology of risky behaviors based on clinical and preclinical studies, but relatively little has been directly translated to reduce their impact on addiction in high-risk children or teenagers. An opportunity exists to effectively intervene before adolescence when substance use is likely to emerge.

New technologies for examining the role of neuronal ensembles in drug addiction and fear

Correlational data suggest that learned associations are encoded within neuronal ensembles. However, it has been difficult to prove that neuronal ensembles mediate learned behaviours because traditional pharmacological and lesion methods, and even newer cell type-specific methods, affect both activated and non-activated neurons. In addition, previous studies on synaptic and molecular alterations induced by learning did not distinguish between behaviourally activated and non-activated neurons. Here, we describe three new approaches--Daun02 inactivation, FACS sorting of activated neurons and Fos-GFP transgenic rats--that have been used to selectively target and study activated neuronal ensembles in models of conditioned drug effects and relapse. We also describe two new tools--Fos-tTA transgenic mice and inactivation of CREB-overexpressing neurons--that have been used to study the role of neuronal ensembles in conditioned fear.

Developmental disruption of gamma-aminobutyric acid function in the medial prefrontal cortex by noncontingent cocaine exposure during early adolescence

BACKGROUND

Drug experimentation during adolescence is associated with increased risk of drug addiction relative to any other age group. To further understand the neurobiology underlying such liability, we investigate how early adolescent cocaine experience impacts medial prefrontal cortex (mPFC) network function in adulthood.

METHODS

A noncontingent administration paradigm was used to assess the impact of early adolescent cocaine treatment (rats; postnatal days [PD] 35-40) on the overall inhibitory regulation of mPFC activity in adulthood (PD 65-75) by means of histochemical and in vivo electrophysiological measures combined with pharmacologic manipulations.

RESULTS

Cocaine exposure during early adolescence yields a distinctive hypermetabolic prefrontal cortex state that was not observed in adult-treated rats (PD 75-80). Local field potential recordings revealed that early adolescent cocaine exposure is associated with an attenuation of mPFC gamma-aminobutyric acid (GABA)ergic inhibition evoked by ventral hippocampal stimulation at beta and gamma frequencies that endures throughout adulthood. Such cocaine-induced mPFC disinhibition was not observed in adult-exposed animals. Furthermore, the normal developmental upregulation of parvalbumin immunoreactivity observed in the mPFC from PD 35 to PD 65 is lacking following early adolescent cocaine treatment.

CONCLUSIONS

Our data indicate that repeated cocaine exposure during early adolescence can elicit a state of mPFC disinhibition resulting from a functional impairment of the local prefrontal GABAergic network that endures through adulthood. A lack of acquisition of prefrontal GABAergic function during adolescence could trigger long-term deficits in the mPFC that may increase the susceptibility for the onset of substance abuse and related psychiatric disorders.

Different populations of subthalamic neurons encode cocaine vs. sucrose reward and predict future error

The search for treatment of cocaine addiction raises the challenge to find a way to diminish motivation for the drug without decreasing it for natural rewards. Subthalamic nucleus (STN) inactivation decreases motivation for cocaine while increasing motivation for food, suggesting that STN can dissociate different rewards. Here, we investigated how rat STN neurons respond to cues predicting cocaine or sucrose and to reward delivery while rats are performing a discriminative stimuli task. We show that different neuronal populations of STN neurons encode cocaine and sucrose. In addition, we show that STN activity at the cue onset predicts future error. When changing the reward predicted unexpectedly, STN neurons show capacities of adaptation, suggesting a role in reward-prediction error. Furthermore, some STN neurons show a response to executive error (i.e., "oops neurons") that is specific to the missed reward. These results position the STN as a nexus where natural rewards and drugs of abuse are coded differentially and can influence the performance. Therefore, STN can be viewed as a structure where action could be taken for the treatment of cocaine addiction.

Columnar processing in primate pFC: evidence for executive control microcircuits

A common denominator for many cognitive disorders of human brain is the disruption of neural activity within pFC, whose structural basis is primarily interlaminar (columnar) microcircuits or "minicolumns." The importance of this brain region for executive decision-making has been well documented; however, because of technological constraints, the minicolumnar basis is not well understood. Here, via implementation of a unique conformal multielectrode recording array, the role of interlaminar pFC minicolumns in the executive control of task-related target selection is demonstrated in nonhuman primates performing a visuomotor DMS task. The results reveal target-specific, interlaminar correlated firing during the decision phase of the trial between multielectrode recording array-isolated minicolumnar pairs of neurons located in parallel in layers 2/3 and layer 5 of pFC. The functional significance of individual pFC minicolumns (separated by 40 μm) was shown by reduced correlated firing between cell pairs within single minicolumns on error trials with inappropriate target selection. To further demonstrate dependence on performance, a task-disrupting drug (cocaine) was administered in the middle of the session, which also reduced interlaminar firing in minicolumns that fired appropriately in the early (nondrug) portion of the session. The results provide a direct demonstration of task-specific, real-time columnar processing in pFC indicating the role of this type of microcircuit in executive control of decision-making in primate brain.

Context-dependent effects of a single administration of mirtazapine on the expression of methamphetamine-induced conditioned place preference

Re-exposure to cues repeatedly associated with methamphetamine (Meth) can trigger Meth-seeking and relapse in the abstinent abuser. Weakening the conditioned Meth-associated memory during cue re-exposure may provide a means for relapse-reduction pharmacotherapy. Accordingly, we sought to determine if the atypical antidepressant mirtazapine disrupted the persistence of Meth-induced conditioned place preference (CPP) when administered in conjunction with re-exposure to contextual conditioning cues, and if this effect was altered by Meth being present during cue re-exposure. First, we evaluated the effect of mirtazapine on the maintenance of Meth-induced CPP during re-exposure to either the saline- or Meth-paired chamber 12 days after conditioning. Meth-conditioned rats subsequently administered mirtazapine expressed CPP independent of re-exposure to the saline- or Meth-paired chamber; but the magnitude of CPP was significantly less for mirtazapine-treated rats re-exposed to the Meth-paired chamber. Next, we evaluated the effect of mirtazapine on a "reinforced re-exposure" to the Meth-paired context. Administration of mirtazapine vehicle and Meth, prior to re-exposure to the Meth-paired chamber did not disrupt the ability of rats to demonstrate CPP 15 days after conditioning; however, CPP was disrupted when rats were administered mirtazapine and Meth prior to re-exposure to the Meth-paired chamber. These results indicate that the capacity of mirtazapine to diminish Meth-induced CPP is promoted if mirtazapine treatment is coupled with Meth administration in the Meth-associated context and thus appears to be the consequence of disrupting processes necessary to reconsolidate CPP following activation of drug-associated memories.

Developmental trajectories during adolescence in males and females: a cross-species understanding of underlying brain changes

Adolescence is a transitional period between childhood and adulthood that encompasses vast changes within brain systems that parallel some, but not all, behavioral changes. Elevations in emotional reactivity and reward processing follow an inverted U shape in terms of onset and remission, with the peak occurring during adolescence. However, cognitive processing follows a more linear course of development. This review will focus on changes within key structures and will highlight the relationships between brain changes and behavior, with evidence spanning from functional magnetic resonance imaging (fMRI) in humans to molecular studies of receptor and signaling factors in animals. Adolescent changes in neuronal substrates will be used to understand how typical and atypical behaviors arise during adolescence. We draw upon clinical and preclinical studies to provide a neural framework for defining adolescence and its role in the transition to adulthood.

Glutamatergic plasticity in medial prefrontal cortex and ventral tegmental area following extended-access cocaine self-administration

Glutamate signaling in prefrontal cortex and ventral tegmental area plays an important role in the molecular and behavioral plasticity associated with addiction to drugs of abuse. The current study investigated the expression and postsynaptic density redistribution of glutamate receptors and synaptic scaffolding proteins in dorsomedial and ventromedial prefrontal cortex and ventral tegmental area after cocaine self-administration. After 14 days of extended-access (6h/day) cocaine self-administration, rats were exposed to one of three withdrawal regimen for 10 days. Animals either stayed in home cages (Home), returned to self-administration boxes with the levers withdrawn (Box), or underwent extinction training (Extinction). Extinction training was associated with significant glutamatergic plasticity. In dorsomedial prefrontal cortex of the Extinction group, there was an increase in postsynaptic density GluR1, PSD95, and actin proteins; while postsynaptic density mGluR5 protein decreased and there was no change in NMDAR1, Homer1b/c, or PICK1 proteins. These changes were not observed in ventromedial prefrontal cortex or ventral tegmental area. In ventral tegmental area, Extinction training reversed the decreased postsynaptic density NMDAR1 protein in the Home and Box withdrawal groups. These data suggest that extinction of drug seeking is associated with selective glutamatergic plasticity in prefrontal cortex and ventral tegmental area that include modulation of receptor trafficking to postsynaptic density.

Extinction of drug seeking

Drug seeking behavior can be reduced or inhibited via extinction. The brain mechanisms for extinction of drug seeking are poorly understood but are of significant interest because of their potential to identify novel approaches that promote abstinence from drug taking. Here we review recent literature on the neural mechanisms for extinction in drug self-administration paradigms. First, we consider the brain regions important for extinction of drug seeking. Functional inactivation studies have identified infralimbic prefrontal cortex, nucleus accumbens shell, as well as medial dorsal hypothalamus in the expression of extinction of drug seeking. These structures have been implicated in extinction expression across several reinforcers including cocaine, heroin, and alcohol. Second, we consider molecular studies which show that extinction training is associated with plasticity in glutamatergic signaling in both nucleus accumbens shell and core, and that this training may reverse or ameliorate the neuroadaptations produced by chronic drug exposure and spontaneous withdrawal. Finally, we consider the neural circuitry for extinction of drug seeking. Functional disconnection and neuroanatomical tracing studies show that extinction expression depends, at least in part, on cortico-striatal-hypothalamic and cortico-hypothalalmic-thalamic pathways. Moreover, they indicate that the expression of extinction and reinstatement of drug seeking may depend on parallel pathways that converge within lateral hypothalamus and paraventricular thalamus.

Elevated BDNF after cocaine withdrawal facilitates LTP in medial prefrontal cortex by suppressing GABA inhibition

Medial prefrontal cortex (mPFC) is known to be involved in relapse after cocaine withdrawal, but the underlying cellular mechanism remains largely unknown. Here, we report that after terminating repeated cocaine exposure in rats, a gradual increase in the expression of brain-derived neurotrophic factor (BDNF) in the mPFC facilitates activity-induced long-term potentiation (LTP) of excitatory synapses on layer V pyramidal neurons. This enhanced synaptic plasticity could be attributed to BDNF-induced suppression of GABAergic inhibition in the mPFC by reducing the surface expression of GABA(A) receptors. The BDNF effect was mediated by BDNF-TrkB-phosphatase 2A signaling pathway. Downregulating TrkB expression bilaterally in the mPFC reduced the locomotor hypersensitivity to cocaine 8 days after cocaine withdrawal. Thus, elevated BDNF expression after cocaine withdrawal sensitizes the excitatory synapses in the mPFC to undergo activity-induced persistent potentiation that may contribute to cue-induced drug craving and drug-seeking behavior.

Activators of G-protein signaling 3: a drug addiction molecular gateway

Drug addiction is marked by continued drug-seeking behavior despite deleterious consequences and a heightened propensity to relapse not withstanding long, drug-free periods. The enduring nature of addiction has been hypothesized to arise from perturbations in intracellular signaling, gene expression, and brain circuitry induced by substance abuse. Ameliorating some of these aberrations should abate behavioral and neurochemical markers associated with an 'addiction phenotype'. This review summarizes data showing that protein expression and signaling through the nonreceptor activator of G-protein signaling 3 (AGS3) are altered by commonly abused substances in rat and in in-vitro addiction models. AGS3 structure and function are unrelated to the more broadly studied regulator of G-protein signaling family. Thus, the unique role of AGS3 is the focus of this review. Intriguingly, AGS3 protein changes persist into drug abstinence. Accordingly, studies probing the role of AGS3 in the neurochemistry of drug-seeking behavior and relapse are studied in detail. To illuminate this study, AGS3 structure, cellular localization, and function are covered so that an idealized AGS3-targeted pharmacotherapy can be proposed.

Glial dysfunction in abstinent methamphetamine abusers

Persistent neurochemical abnormalities in frontal brain structures are believed to result from methamphetamine use. We developed a localized (13)C magnetic resonance spectroscopy (MRS) assay on a conventional MR scanner, to quantify selectively glial metabolic flux rate in frontal brain of normal subjects and a cohort of recovering abstinent methamphetamine abusers. Steady-state bicarbonate concentrations were similar, between 11 and 15 mmol/L in mixed gray-white matter of frontal brain of normal volunteers and recovering methamphetamine-abusing subjects (P>0.1). However, glial (13)C-bicarbonate production rate from [1-(13)C]acetate, equating with glial tricarboxylic acid (TCA) cycle rate, was significantly reduced in frontal brain of abstinent methamphetamine-addicted women (methamphetamine 0.04 micromol/g per min (N=5) versus controls 0.11 micromol/g per min (N=5), P=0.001). This is equivalent to 36% of the normal glial TCA cycle rate. Severe reduction in glial TCA cycle rate that normally comprises 10% of total cerebral metabolic rate may impact operation of the neuronal glial glutamate cycle and result in accumulation of frontal brain glutamate, as observed in these recovering methamphetamine abusers. Although these are the first studies to define directly an abnormality in glial metabolism in human methamphetamine abuse, sequential studies using analogous (13)C MRS methods may determine 'cause and effect' between glial failure and neuronal injury.

The infralimbic cortex regulates the consolidation of extinction after cocaine self-administration

The infralimbic cortex (IL) regulates the consolidation of extinction learning for fear conditioning. Whether the IL influences the consolidation of extinction learning for cocaine self-administration is unknown. To address this issue, male Sprague-Dawley rats underwent 2 wk of cocaine self-administration followed by extinction training. On the first 5 d of extinction, rats underwent brief (15- or 30-min) extinction sessions and received intra-IL microinjections immediately after each extinction session. On days 6-12 of extinction, rats underwent full-length (2-h) extinction sessions that were used to assess the retention of the extinction learning from the short sessions. IL inactivation via microinjections of the GABA agonists baclofen and muscimol (BM) immediately after the extinction sessions (days 1-5) impaired the retention of extinction learning. Control experiments demonstrated that this effect was not due to inactivation of the prelimbic cortex or due to effects of the drugs on the subsequent day's behavior. In contrast, post-training intra-IL microinjections of the allosteric AMPA receptor potentiator 4-[2-(phenylsulfonylamino)ethylthio]-2,6-difluorophenoxyacetamide (PEPA) enhanced retention of the extinction learning. As evidence suggests a role for the beta-adrenergic receptors in memory consolidation, other rats received microinjections of the beta(2)-adrenergic receptor agonist clenbuterol or antagonist ICI-118,551 (ICI). Post-training intra-IL administration of clenbuterol or pre-training administration of ICI enhanced or impaired, respectively, the retention of extinction learning. These data indicate that the IL, and specifically the glutamatergic and beta-adrenergic systems in the IL, regulates the consolidation of extinction of cocaine self-administration and that the IL can be manipulated to influence the retention of extinction.

Role of the prefrontal cortex and nucleus accumbens in reinstating methamphetamine seeking

Although the involvement of the medial prefrontal cortex projection to the nucleus accumbens in the reinstatement of cocaine seeking has been well studied, it is not known if this projection plays a similar role in the reinstatement of cue- and methamphetamine-induced drug seeking in animals extinguished from methamphetamine self-administration. Accordingly, following extinction from long-access methamphetamine self-administration, rats were bilaterally microinjected with either a combination of the GABA agonists baclofen/muscimol or vehicle (artificial cerebrospinal fluid) into the infralimbic or prelimbic subcompartments of the medial prefrontal cortex or into the shell or core subcompartments of the nucleus accumbens. Similar to cocaine seeking, inactivation of either the prelimbic cortex or accumbens core eliminated cue- and methamphetamine-induced reinstatement, and inactivation of neither the infralimbic cortex nor shell subcompartments inhibited methamphetamine-induced drug seeking. However, in contrast to previous reports with cocaine, cue-induced reinstatement of methamphetamine seeking was inhibited by inactivation of the infralimbic cortex. In conclusion, although a primary role in reinstated drug seeking by the prelimbic and the accumbens core is similar between cocaine and methamphetamine, the recruitment of the infralimbic cortex by conditioned cues differs between these two psychostimulant drugs.

Adaptations in medial prefrontal cortex function associated with amphetamine-induced behavioral sensitization

Neuroadaptations in the prefrontal cortex (PFC) are hypothesized to play an important role in the behavioral changes associated with repeated psychostimulant exposure, but there are few published studies that measure neuronal activity during the development and expression of sensitization. To address this, we recorded single neuron activity in the medial PFC (mPFC) of male rats that were exposed for 5 days to saline or amphetamine (AMPH; 1.0 mg/kg i.p.) and then given saline or AMPH challenges following a three-day withdrawal. We found that rats exposed to AMPH developed locomotor sensitization to the drug that emerged on the fifth treatment session and became statistically significant at AMPH challenge. This was associated with no change in baseline (i.e., pre-injection) activity of mPFC neurons across the treatment or challenge sessions. Following the first AMPH injection, mPFC neurons responded primarily with reductions in firing, with the overall pattern and magnitude of responses remaining largely similar following repeated treatment. The exception was in the minority of cells that respond to AMPH with increases in firing rate. In this population, the magnitude of excitations peaked during the fifth AMPH exposure and was still relatively elevated at the AMPH challenge. Furthermore, these units increased firing during a saline challenge that was given to assess associative conditioning. These results suggest that AMPH-induced adaptations in mPFC function are not as apparent as AMPH-induced adaptations in behavior. When mPFC adaptations do occur, they appear limited to the population of neurons that increase their firing in response to AMPH.

Plasticity of L-type Ca2+ channels after cocaine withdrawal

Increased reactivity of certain frontal cortical brain regions to cocaine re-exposure or drug-associated cues in cocaine-abstinent human addicts is linked to drug craving. Similarly, in rats tested after withdrawal from repeated cocaine exposure, cocaine or other strong excitatory stimuli produce greater activation of pyramidal neurons in the medial prefrontal cortex (mPFC). Our recent findings indicate that the increased mPFC neuronal activation depends primarily upon enhanced voltage-sensitive Ca(2+) influx, most likely through high-voltage activated (HVA) L-type Ca(2+) channels, but the mechanism underlying the enhanced Ca(2+) currents is unknown. In this study, we used a protein crosslinking assay to show that repeated cocaine injections, resulting in behavioral sensitization, increased total protein levels and cell surface expression of HVA-Ca(v)1.2 L-type channels in pyramidal neurons in deep layers of the mPFC. These changes in Ca(v)1.2 L-channels were time dependent and subtype specific (i.e., differed from those observed for Ca(v)1.3 L-channels). Furthermore, we found enhanced PKA activity in the mPFC of cocaine-sensitized rats that persisted for 21 days after withdrawal. PKA phosphorylation of L-channels increases their activity, so Ca(2+) currents after cocaine withdrawal could be enhanced as a result of both increased activity and number of HVA-Ca(v)1.2 L-channels on the cell surface. By increasing the suprafiring threshold excitability of mPFC pyramidal neurons, excessive upregulation of HVA L-channel activity and number may contribute to the cortical hyper-responsiveness that enhances vulnerability to cocaine craving and relapse. More generally, our results are the first to demonstrate that repeated cocaine exposure alters the membrane trafficking of a voltage-sensitive ion channel.

The encoding of cocaine vs. natural rewards in the striatum of nonhuman primates: categories with different activations

The behavioral and motivational changes that result from use of abused substances depend upon activation of neuronal populations in the reward centers of the brain, located primarily in the corpus striatum in primates. To gain insight into the cellular mechanisms through which abused drugs reinforce behavior in the primate brain, changes in firing of neurons in the ventral (VStr, nucleus accumbens) and dorsal (DStr, caudate-putamen) striatum to "natural" (juice) vs. drug (i.v. cocaine) rewards were examined in four rhesus monkeys performing a visual Go-Nogo decision task. Task-related striatal neurons increased firing to one or more of the specific events that occurred within a trial represented by (1) Target stimuli (Go trials) or (2) Nogotarget stimuli (Nogo trials), and (3) Reward delivery for correct performance. These three cell populations were further subdivided into categories that reflected firing exclusively on one or the other type of signaled reward (juice or cocaine) trial (20%-30% of all cells), or, a second subpopulation that fired on both (cocaine and juice) types of rewarded trial (50%). Results show that neurons in the primate striatum encoded cocaine-rewarded trials similar to juice-rewarded trials, except for (1) increased firing on cocaine-rewarded trials, (2) prolonged activation during delivery of i.v. cocaine infusion, and (3) differential firing in ventral (VStr cells) vs. dorsal (DStr cells) striatum cocaine-rewarded trials. Reciprocal activations of antithetic subpopulations of cells during different temporal intervals within the same trial suggest a functional interaction between processes that encode drug and natural rewards in the primate brain.

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

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

Dopaminergic receptor D5 mRNA expression is increased in circulating lymphocytes of Tourette syndrome patients

Tourette syndrome (TS) is a neuropsychiatric disorder in which dopaminergic dysfunction and immune system abnormalities seem to coexist. Using real-time PCR, we determined mRNA expression of dopamine receptors (DRs) D1-5 in peripheral blood lymphocytes (PBLs) from 15 TS patients and 15 sex- and age-matched healthy controls (HCs). DRD5 mRNA levels in cells from TS were higher than in cells from HCs. In TS patients with obsessive-compulsive disorder, DRD5 mRNA levels in PBLs showed a highly positive correlation with the severity of compulsive symptoms. DRD5 mRNA upregulation in PBLs from TS patients may represent a peripheral marker of dopaminergic dysfunction and supports the involvement of the immune system in TS.

Protracted 'anti-addictive' effects of adolescent phenylpropanolamine exposure in C57BL/6J mice

Exposure to the once highly prevalent over-the-counter (OTC) sympathomimetic phenylpropanolamine (PPA; +/--norephedrine) during pre-adolescence alters the developmental trajectory of catecholamine and amino acid neurotransmitter systems in the nucleus accumbens (NAC) that culminate in a 'pro-addictive' phenotype in adulthood. Thus, the present study sought to extend these earlier data by examining the long-term consequences of repeated PPA treatment during adolescence upon the behavioral and neurochemical responses to cocaine. For this, C57BL/6J mice were pre-treated with PPA (0-40 mg/kg) during postnatal days 35-44, and the capacity of cocaine (4 x 15 mg/kg) to elicit a conditioned place-preference, as well as behavioral and neurochemical sensitization within the NAC, were then assessed in adulthood. While adolescent PPA exposure did not influence spontaneous locomotor activity or the motor responses to either acute or repeated cocaine (4 x 15 mg/kg), PPA pre-exposure dose-dependently reduced the expression of a conditioned place-preference. As observed previously for juvenile PPA treatment, adolescent PPA administration blunted the dopamine and norepinephrine response to acute cocaine, prevented the development of catecholamine sensitization but did not influence cocaine-induced elevations in serotonin. However, unlike juvenile PPA treatment, adolescent PPA also prevented the development of glutamate sensitization within the NAC. These data provide evidence that adolescent exposure to a formerly prevalent OTC sympathomimetic produces protracted effects upon cocaine-induced changes in NAC glutamate transmission that may reduce vulnerability to cocaine addiction in later life and further the hypothesis that early exposure to sympathomimetic drugs may be an environmental factor contributing to the etiology of addiction.

Persistent alterations in mesolimbic gene expression with abstinence from cocaine self-administration

Cocaine-responsive gene expression changes have been described after either no drug abstinence or short periods of abstinence. Little data exist on the persistence of these changes after long-term abstinence. Previously, we reported that after discrete-trial cocaine self-administration and 10 days of forced abstinence, incubation of cocaine reinforcement was observable by a progressive ratio schedule. The present study used rat discrete-trial cocaine self-administration and long-term forced abstinence to examine extinction responding, mRNA abundance of known cocaine-responsive genes, and chromatin remodeling. At 30 and 100 days of abstinence, extinction responding increased compared to 3-day abstinent rats. Decreases in both medial prefrontal cortex (mPFC) and nucleus accumbens c-fos, Nr4a1, Arc, and EGR1 mRNA were observed, and in most cases persisted, for 100 days of abstinence. The signaling peptides CART and neuropeptide Y (NPY) transiently increased in the mPFC, but returned to baseline levels following 10 days of abstinence. To investigate a potential regulatory mechanism for these persistent mRNA changes, levels of histone H3 acetylation at promoters for genes with altered mRNA expression were examined. In the mPFC, histone H3 acetylation decreased after 1 and 10 days of abstinence at the promoter for EGR1. H3 acetylation increased for NPY after 1 day of abstinence and returned to control levels by 10 days of abstinence. Behaviorally, these results demonstrate incubation after discrete-trial cocaine self-administration and prolonged forced abstinence. This incubation is accompanied by changes in gene expression that persist long after cessation of drug administration and may be regulated by chromatin remodeling.

Delayed extinction and stronger reinstatement of cocaine conditioned place preference in adolescent rats, compared to adults

Adolescence is a transitional period during development that is associated with a greater likelihood of addiction to drugs than any other age. One possibility for this observation is that learned associations between the rewarding experience of drugs and drug-related cues may produce greater motivational salience, and thus are more difficult to extinguish. Using an unbiased place-conditioning paradigm with two doses of cocaine (10 mg/kg or 20 mg/kg), the authors show here that adolescents require 75 +/- 17% more extinction trials than adults to extinguish cocaine place-preferences. Furthermore, once extinguished, adolescents display a greater preference for a previously cocaine-paired environment upon drug-primed reinstatement compared with adults. These results suggest that adolescent vulnerability to addiction involves robust memories for drug-associated cues that are difficult to extinguish. Therefore, drug-addicted adolescents may have a higher risk of relapse than adults, leading to greater prevalence of addiction in this population.

Upregulation of Arc mRNA expression in the prefrontal cortex following cue-induced reinstatement of extinguished cocaine-seeking behavior

Cocaine-associated cues acquire incentive motivational effects that manifest as cue-elicited craving in humans and cocaine-seeking behavior in rats. Here we examine the hypothesis that neuronal processes associated with incentive motivational effects of cocaine cues involve increased expression of the plasticity-associated gene, Arc. Rats trained to self-administer cocaine subsequently underwent extinction training, during which cocaine-seeking behavior (i.e., responses without cocaine reinforcement) progressively decreased. Rats were then tested for cocaine-seeking behavior either with or without response-contingent presentations of light/tone cues that had been previously paired with cocaine infusions during self-administration training. Cues elicited reinstatement of cocaine-seeking behavior and were accompanied by increased Arc mRNA levels in the orbitofrontal, prelimbic, and anterior cingulate cortices, suggesting Arc involvement in conditioned plasticity associated with incentive motivational effects of cocaine cues. Additionally, rats with a history of cocaine self-administration and extinction exhibited upregulation of Arc expression in several limbic and cortical regions relative to saline-yoked controls regardless of cue exposure condition, suggesting persistent neuroadaptations involving Arc within these regions.

Anxiogenesis in adult rats treated chronically with cocaine during adolescence: effects of extended abstinence and 8-OH-DPAT treatment

Our laboratory has recently observed the development of an anxiogenic response after a short abstinence period ( approximately 10 days) in young adult rats treated repeatedly with cocaine during adolescence. The present study was conducted to determine if this effect persists into adulthood following extended durations of abstinence and whether it could be modulated with the 5-HT(1A) agonist (+/-)-8-hydroxy-2-(dipropylamino)tetralin hydrobromide (8-OH-DPAT). Accordingly, 30-day-old rats were injected with either 10mg/kg cocaine or saline for 8 consecutive days. Approximately 8 weeks after the final injection, anxiety levels in subjects were assessed with an elevated zero maze with a second assessment performed 4 weeks thereafter. Shortly prior to each test session, half the subjects in each of the two adolescent drug conditions received injections of 300 microg/kg 8-OH-DPAT while the other half received injections of the vehicle. Based on total time spent in the open areas of the maze, the results obtained at the first abstinent duration indicated that adolescent cocaine treatment did not induce an anxiogenic response. Assessment of maze behavior at the second abstinent duration was aided with a digital tracking and computerized scoring system (LimeLight, Actimetrics). Similar to the results obtained at the first abstinent duration, the amount of time spent in the open areas of the maze was unrelated to prior cocaine treatment. However, cocaine-treated rats did show evidence of an anxiogenic response at this abstinent duration based on more frequent entries into and out of the open and enclosed areas of the maze, more frequent and longer durations of exploratory bouts beyond the perimeter of the maze, excessive number of cautious protrusions into the open areas, and faster running velocities through the open areas. These results were not artifacts of generalized motor activation in that comparable total distance traveled scores were noted for all subjects. Treatment with 8-OH-DPAT shortly prior to elevated zero maze testing normalized these behaviors induced by adolescent cocaine. It is concluded that the anxiogenic response produced by cocaine exposure during adolescence persists 12 weeks beyond cessation of drug treatment and that this effect is reversible with 8-OH-DPAT. In addition, the results underscore the importance of taking multiple measures when assessing anxiety in experimental animals. Implications for the neurobiology of drug abuse and the role of serotonin are discussed.

Homers regulate drug-induced neuroplasticity: implications for addiction

Drug addiction is a chronic, relapsing disorder, characterized by an uncontrollable motivation to seek and use drugs. Converging clinical and preclinical observations implicate pathologies within the corticolimbic glutamate system in the genetic predisposition to, and the development of, an addicted phenotype. Such observations pose cellular factors regulating glutamate transmission as likely molecular candidates in the etiology of addiction. Members of the Homer family of proteins regulate signal transduction through, and the trafficking of, glutamate receptors, as well as maintain and regulate extracellular glutamate levels in corticolimbic brain regions. This review summarizes the existing data implicating the Homer family of protein in acute behavioral and neurochemical sensitivity to drugs of abuse, the development of drug-induced neuroplasticity, as well as other behavioral and cognitive pathologies associated with an addicted state.