Stress rapidly dysregulates the glutamatergic synapse in the prefrontal cortex of cocaine-withdrawn adolescent rats

Temporal dynamics of BDNF signaling recruitment in the rat prefrontal cortex and hippocampus following a single infusion of a translational dose of ketamine

Despite several decades of investigations, the mechanisms underlying the rapid action of ketamine as antidepressant are still far from being completely understood. Several studies indicated Brain-Derived Neurotrophic Factor (BDNF) as critical for the fast antidepressant action of ketamine, due to its contribution in early and rapid synaptic adaptations. However, previous reports have been essentially based on ketamine dosing modes that differ from the clinical route of administration (slow intravenous infusion). In this report, we investigated the effects of a ketamine dosing mode in male Sprague-Dawley rats showed to be translational to the clinically effective mode in patients. We focused on the first 24 h after infusion to finely dissect potential differences in the contribution of BDNF signaling pathway in prefrontal cortex and hippocampus, two brain regions involved in the antidepressant effects of ketamine. Our data show that the slow ketamine infusion activates the BDNF-mTOR-S6 pathway in prefrontal cortex as early as 2 h and remains on until at least 6 h after the infusion. At the 12 h timepoint, this pathway is turned off in prefrontal cortex while it becomes activated in hippocampus. Interestingly, this pathway appears to be activated in both brain regions at 24 h through a BDNF-independent mechanism adding complexity to the early action of ketamine. We have captured previously unknown dynamics of the early effects of ketamine showing rapid activation/deactivation of BDNF and its downstream signaling in prefrontal cortex and hippocampus, following a precise temporal profile.

Recency memory is altered in cocaine-withdrawn adolescent rats: Implication of cortical mTOR signaling

In humans, cocaine abuse during adolescence poses a significant risk for developing cognitive deficits later in life. Among the regions responsible for cognitive processes, the medial prefrontal cortex (mPFC) modulates temporal order information via mechanisms involving the mammalian-target of rapamycin (mTOR)-mediated pathway and protein synthesis regulation. Accordingly, our goal was to study the effect of repeated cocaine exposure during both adolescence and adulthood on temporal memory by studying the mTOR pathway in the mPFC. Adolescent or adult rats underwent repeated cocaine injections for 15 days and, after two weeks of withdrawal, engaged in the temporal order object recognition (TOOR) test. We found that repeated cocaine exposure during adolescence impaired TOOR performance, while control or adult-treated animals showed no impairments. Moreover, activation of the mTOR-S6-eEF2 pathway following the TOOR test was diminished only in the adolescent cocaine-treated group. Notably, inhibition of the mTOR-mediated pathway by rapamycin injection impaired TOOR performance in naïve adolescent and adult animals, revealing this pathway to be a critical component in regulating recency memory. Our data indicate that withdrawal from cocaine exposure impairs recency memory via the dysregulation of protein translation mechanisms, but only when cocaine is administered during adolescence.

Transcriptional Profiling of Rat Prefrontal Cortex after Acute Inescapable Footshock Stress

Stress is a primary risk factor for psychiatric disorders such as Major Depressive Disorder (MDD) and Post Traumatic Stress Disorder (PTSD). The response to stress involves the regulation of transcriptional programs, which is supposed to play a role in coping with stress. To evaluate transcriptional processes implemented after exposure to unavoidable traumatic stress, we applied microarray expression analysis to the PFC of rats exposed to acute footshock (FS) stress that were sacrificed immediately after the 40 min session or 2 h or 24 h after. While no substantial changes were observed at the single gene level immediately after the stress session, gene set enrichment analysis showed alterations in neuronal pathways associated with glia development, glia-neuron networking, and synaptic function. Furthermore, we found alterations in the expression of gene sets regulated by specific transcription factors that could represent master regulators of the acute stress response. Of note, these pathways and transcriptional programs are activated during the early stress response (immediately after FS) and are already turned off after 2 h-while at 24 h, the transcriptional profile is largely unaffected. Overall, our analysis provided a transcriptional landscape of the early changes triggered by acute unavoidable FS stress in the PFC of rats, suggesting that the transcriptional wave is fast and mild, but probably enough to activate a cellular response to acute stress.

Long-lasting BDNF signaling alterations in the amygdala of adolescent female rats exposed to the activity-based anorexia model

Introduction: Anorexia nervosa (AN) is a severe psychiatric disorder characterized by a pathological fear of gaining weight, excessive physical exercise, and emotional instability. Since the amygdala is a key region for emotion processing and BDNF has been shown to play a critical role in this process, we hypothesized that alteration in the amygdalar BDNF system might underline vulnerability traits typical of AN patients. Methods: To this end, adolescent female rats have been exposed to the Activity-Based Anorexia (ABA) protocol, characterized by the combination of caloric restriction and intense physical exercise. Results: The induction of the anorexic phenotype caused hyperactivity and body weight loss in ABA animals. These changes were paralleled by amygdalar hyperactivation, as measured by the up-regulation of cfos mRNA levels. In the acute phase of the pathology, we observed reduced Bdnf exon IX, exon IV, and exon VI gene expression, while mBDNF protein levels were enhanced, an increase that was, instead, uncoupled from its downstream signaling as the phosphorylation of TrkB, Akt, and S6 in ABA rats were reduced. Despite the body weight recovery observed 7 days later, the BDNF-mediated signaling was still downregulated at this time point. Discussion: Our findings indicate that the BDNF system is downregulated in the amygdala of adolescent female rats under these experimental conditions, which mimic the anorexic phenotype in humans, pointing to such dysregulation as a potential contributor to the altered emotional processing observed in AN patients. In addition, since the modulation of BDNF levels is observed in other psychiatric conditions, the persistent AN-induced changes of the BDNF system in the amygdala might contribute to explaining the onset of comorbid psychiatric disorders that persist in patients even beyond recovery from AN.

The PI3-Kinase p110β Isoform Controls Severity of Cocaine-Induced Sequelae and Alters the Striatal Transcriptome

BACKGROUND

The PI3-kinase (PI3K) complex is a well-validated target for mitigating cocaine-elicited sequelae, but pan-PI3K inhibitors are not viable long-term treatment options. The PI3K complex is composed of p110 catalytic and regulatory subunits, which can be individually manipulated for therapeutic purposes. However, this possibility has largely not been explored in behavioral contexts.

METHODS

Here, we inhibited PI3K p110β in the medial prefrontal cortex (mPFC) of cocaine-exposed mice. Behavioral models for studying relapse, sensitization, and decision-making biases were paired with protein quantification, RNA sequencing, and cell type-specific chemogenetic manipulation and RNA quantification to determine whether and how inhibiting PI3K p110β confers resilience to cocaine.

RESULTS

Viral-mediated PI3K p110β silencing reduced cue-induced reinstatement of cocaine seeking by half, blocked locomotor sensitization, and restored mPFC synaptic marker content after exposure to cocaine. Cocaine blocked the ability of mice to select actions based on their consequences, and p110β inhibition restored this ability. Silencing dopamine D₂ receptor-expressing excitatory mPFC neurons mimicked cocaine, impairing goal-seeking behavior, and again, p110β inhibition restored goal-oriented action. We verified the presence of p110β in mPFC neurons projecting to the dorsal striatum and orbitofrontal cortex and found that inhibiting p110β in the mPFC altered the expression of functionally defined gene clusters within the dorsal striatum and not orbitofrontal cortex.

CONCLUSIONS

Subunit-selective PI3K silencing potently mitigates drug seeking, sensitization, and decision-making biases after exposure to cocaine. We suggest that inhibiting PI3K p110β provides neuroprotection against cocaine by triggering coordinated corticostriatal adaptations.

The effects of cocaine exposure in adolescence: Behavioural effects and neuroplastic mechanisms in experimental models

Drug addiction is a devastating disorder with a huge economic and social burden for modern society. Although an individual may slip into drug abuse throughout his/her life, adolescents are at higher risk, but, so far, only a few studies have attempted to elucidate the underlying cellular and molecular bases of such vulnerability. Indeed, preclinical evidence indicates that psychostimulants and adolescence interact and contribute to promoting a dysfunctional brain. In this review, we have focused our attention primarily on changes in neuroplasticity brought about by cocaine, taking into account that there is much less evidence from exposure to cocaine in adolescence, compared with that from adults. This review clearly shows that exposure to cocaine during adolescence, acute or chronic, as well as contingent or non‐contingent, confers a vulnerable endophenotype, primarily, by causing changes in neuroplasticity. Given the close relationship between drug abuse and psychiatric disorders, we also discuss the translational implications providing an interpretative framework for clinical studies involving addictive as well as affective or psychotic behaviours.

The role of the serotonin transporter in prefrontal cortex glutamatergic signaling following short- and long-access cocaine self-administration

Vulnerability to drug addiction relies on substantial individual differences. We previously demonstrated that serotonin transporter knockout (SERT^−/−) rats show increased cocaine intake and develop signs of compulsivity. However, the underlying neural mechanisms are not fully understood. Given the pivotal role of glutamate and prefrontal cortex in cocaine‐seeking behavior, we sought to investigate the expression of proteins implicated in glutamate neurotransmission in the prefrontal cortex of naïve and cocaine‐exposed rats lacking SERT. We focused on the infralimbic (ILc) and prelimbic (PLc) cortices, which are theorized to exert opposing effects on the control over subcortical brain areas. SERT^−/− rats, which compared to wild‐type (SERT^+/+) rats show increased ShA and LgA intake short‐access (ShA) and long‐access (LgA) cocaine intake, were sacrificed 24 h into withdrawal for ex vivo molecular analyses. In the ILc homogenate of SERT^−/− rats, we observed a sharp increase in glial glutamate transporter 1 (GLT‐1) after ShA, but not LgA, cocaine intake. This was paralleled by ShA‐induced increases in GluN1, GluN2A, and GluN2B NMDA receptor subunits and their scaffolding protein SAP102 in the ILc homogenate, but not postsynaptic density, of these knockout animals. In the PLc, we found no major changes in the homogenate; conversely, the expression of GluN1 and GluN2A NMDA receptor subunits was increased in the postsynaptic density under ShA conditions and reduced under LgA conditions. These results point to SERT as a critical regulator of glutamate homeostasis in a way that differs between the subregions investigated, the duration of cocaine exposure as well as the cellular compartment analyzed.

Bidirectional Control of Alcohol-drinking Behaviors Through Locus Coeruleus Optoactivation

The relationship between stress and alcohol-drinking behaviors has been intensively explored; however, neuronal substrates and neurotransmitter dynamics responsible for a causal link between these conditions are still unclear. Here, we optogenetically manipulated locus coeruleus (LC) norepinephrine (NE) activity by applying distinct stimulation protocols in order to explore how phasic and tonic NE release dynamics control alcohol-drinking behaviors. Our results clearly demonstrate contrasting behavioral consequences of LC-NE circuitry activation during low and high frequency stimulation. Specifically, applying tonic stimulation during a standard operant drinking session resulted in increased intake, while phasic stimulation decreased this measure. Furthermore, stimulation during extinction probe trials, when the lever press response was not reinforced, did not significantly alter alcohol-seeking behavior if a tonic pattern was applied. However, phasic stimulation substantially suppressed the number of lever presses, indicating decreased alcohol seeking under the same experimental condition. Given the well-established correlative link between stress and increased alcohol consumption, here we provide the first evidence that tonic LC-NE activity plays a causal role in stress-associated increases in drinking.

Anhedonic-like behavior and BDNF dysregulation following a single injection of cocaine during adolescence

We have previously demonstrated that a single exposure to cocaine during adolescence causes several behavioural and neurobiological changes, highlighting the unique vulnerability of this period of life. The purpose of our work was to investigate whether a single exposure to cocaine during brain development is sufficient to shape a negative emotional state in adolescent rats. A single injection of cocaine during adolescence followed by measurement of sucrose consumption, a measure of anhedonia, identifies two separate groups of rats, i.e. anhedonic (AN) and non anhedonic (NON-AN) rats. AN rats show reduced ability to synthesize, traffic and translate the neurotrophin BDNF at synaptic level, reduced activation of hippocampal BDNF signaling, reduced BDNF plasma levels and a steep rise of corticosterone secretion. Conversely, NON-AN rats exhibit reduced trafficking of BDNF while up-regulating hippocampal BDNF synthesis and stabilizing its downstream signaling with no changes of BDNF and corticosterone plasma levels. Adult rats exposed to cocaine showed no signs of anhedonia, an increase of BDNF both in hippocampus and plasma and decreased levels of corticosterone. In conclusion, our findings reveal a complex central and peripheral dysregulation of BDNF-related mechanisms that instead are preserved in NON-AN rats, suggesting that BDNF modulation dictates behavioural vulnerability vs. resiliency to cocaine-induced anhedonia, a profile uniquely restricted to adolescent rats.

Repeated cocaine exposure during adolescence impairs recognition memory in early adulthood: A role for BDNF signaling in the perirhinal cortex

The perirhinal cortex (PrhC) is critical for object recognition memory; however, information regarding the molecular mechanisms underlying this type of memory following repeated exposure to drugs of abuse during adolescence is unknown. To this end, adolescent or adult rats were exposed to cocaine from postnatal day (PND) 28 to PND 42 or PND 63 to PND 77, respectively. Two weeks later, rats were subjected to the cognitive test named Novel Object Recognition (NOR) test. We found that adolescent, but not adult, cocaine exposure caused a significant impairment in the NOR test, independently from changes in the stress response system. In adolescent saline-treated rats, NOR test up-regulated BDNF and its downstream signaling whereas a downregulation of the same pathway was observed in cocaine-treated rats together with a reduction of Arc/Arg3.1 and PSD95 expression, indicating reduced pro-cognitive structural adaptations in the PrhC. Of note, cocaine-treated adult rats correctly performed in the NOR test indicating intact recognition memory mechanisms, despite a significant cocaine-induced reduction of BDNF levels in the PrhC, suggesting that recognition memory is heavily dependent on BDNF during adolescence whereas during adulthood other mechanisms come into play.

Cocaine-induced Changes in the Expression of NMDA Receptor Subunits

Cocaine use disorder is manifested by repeated cycles of drug seeking and drug taking. Cocaine exposure causes synaptic transmission in the brain to exhibit persistent changes, which are poorly understood, while the pharmacotherapy of this disease has not been determined. Multiple potential mechanisms have been indicated to be involved in the etiology of co-caine use disorder. The glutamatergic system, especially N-methyl-D-aspartate (NMDA) receptors, may play a role in sever-al physiological processes (synaptic plasticity, learning and memory) and in the pathogenesis of cocaine use disorder. The composition of the NMDA receptor subunits changes after contingent and noncontingent cocaine administration and after drug abstinence in a region-specific and time-dependent manner, as well as depending on the different protocols used for co-caine administration. Changes in the expression of NMDA receptor subunits may underlie the transition from cocaine abuse to dependence, as well as the transition from cocaine dependence to cocaine withdrawal. In this paper, we summarize the cur-rent knowledge regarding neuroadaptations within NMDA receptor subunits and scaffolding proteins observed following voluntary and passive cocaine intake, as well as the effects of NMDA receptor antagonists on cocaine-induced behavioral changes during cocaine seeking and relapse.

Increased cocaine self-administration in rats lacking the serotonin transporter: a role for glutamatergic signaling in the habenula

Serotonin (5-HT) and the habenula (Hb) contribute to motivational and emotional states such as depression and drug abuse. The dorsal raphe nucleus, where 5-HT neurons originate, and the Hb are anatomically and reciprocally interconnected. Evidence exists that 5-HT influences Hb glutamatergic transmission. Using serotonin transporter knockout (SERT^-/- ) rats, which show depression-like behavior and increased cocaine intake, we investigated the effect of SERT reduction on expression of genes involved in glutamate neurotransmission under both baseline conditions as well as after short-access or long-access cocaine (ShA and LgA, respectively) intake. In cocaine-naïve animals, SERT removal led to reduced baseline Hb mRNA levels of critical determinants of glutamate transmission, such as SLC1A2, the main glutamate transporter and N-methyl-D-aspartate (Grin1, Grin2A and Grin2B) as well as α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (Gria1 and Gria2) receptor subunits, with no changes in the scaffolding protein Dlg4. In response to ShA and LgA cocaine intake, SLC1A2 and Grin1 mRNA levels decreased in SERT^+/+ rats to levels equal of those of SERT^-/- rats. Our data reveal that increased extracellular levels of 5-HT modulate glutamate neurotransmission in the Hb, serving as critical neurobiological substrate for vulnerability to cocaine addiction.

Short-term withdrawal from repeated exposure to cocaine during adolescence modulates dynorphin mRNA levels and BDNF signaling in the rat nucleus accumbens

BACKGROUND

Early-life stressful events affect the neurobiological maturation of cerebral circuitries including the endogenous opioid system and the effects elicited by adolescent cocaine exposure on this system have been poorly investigated. Here, we evaluated whether cocaine exposure during adolescence causes short- or long-term alterations in mRNAs codifying for selected elements belonging to the opioid system. Moreover, since brain-derived neurotrophic factor (BDNF) may undergo simultaneous alterations with the opioid peptide dynorphin, we also evaluated its signaling pathway as well.

METHODS

Adolescent male rats were exposed to cocaine (20 mg/kg/day) from post-natal day (PND) 28 to PND42, approximately corresponding to human adolescence. After short- (PND45) or long-term (PND90) abstinence, prodynorphin-κ-opioid receptor (pDYN-KOP) and pronociceptin-nociceptin receptor (pN/OFQ-NOP) gene expression were evaluated in the nucleus accumbens (NAc) and hippocampus (Hip) together with the analysis of BDNF signaling pathways.

RESULTS

In the NAc of PND45 rats, pDYN mRNA levels were up-regulated, an effect paralled by increased BDNF signaling. Differently from NAc, pDYN mRNA levels were down-regulated in the Hip of PND45 rats without significant changes of BDNF pathway. At variance from PND45 rats, we did not find any significant alteration of the investigated parameters either in NAc and Hip of PND90 rats.

CONCLUSIONS

Our results indicate that the short-term withdrawal from adolescent cocaine exposure is characterized by a parallel pDYN mRNA and BDNF signaling increase in the NAc. Given the depressive-like state experienced during short abstinence in humans, we hypothesize that such changes may contribute to promote the risk of cocaine abuse escalation and relapse.

Adolescent cocaine exposure enhanced negative affect following drug re-exposure in adult rats: Attenuation of c-Fos activation

BACKGROUND

The goal of the present study was to utilize the adolescent drug experience as an emerging vulnerability factor for developing psychiatric comorbidities in adulthood that could, in turn, help to elucidate and/or hypothesize possible mechanisms contributing to higher relapse rates.

OUTCOMES

The current results showed that adolescent cocaine exposure (15 mg/kg, intraperitoneally, seven days) during early-mid adolescence (postnatal days 33-39) enhanced negative affect in adulthood, by increasing behavioral despair following drug re-exposure and by increasing anhedonia. Thus, these behavioral data provided a good model to further ascertain the long-term cellular and molecular adaptations that might take place in the brain in response to adolescent cocaine exposure as well as the impact of drug re-exposure in adulthood. In this regard, the results showed that adolescent cocaine exposure did not modulate cell proliferation (Ki-67+ cells) or c-Fos protein activation in the dentate gyrus region of the hippocampus, but attenuated c-Fos activation in the dorsal striatum.

CONCLUSIONS

These results proved that a history of cocaine exposure during adolescence increased the vulnerability to induce negative affect (i.e. emergence of psychiatric comorbidity) in adulthood while it decreased neuronal activation in the dorsal striatum. Interestingly, these effects were only observed following cocaine re-exposure in adulthood, suggesting that avoiding drug contact in adulthood could prevent the long-term negative effects induced by adolescent cocaine.

A single cocaine administration alters dendritic spine morphology and impairs glutamate receptor synaptic retention in the medial prefrontal cortex of adolescent rats

The brain is still maturing during adolescence and interfering with such a vulnerable period may lead to structural and functional consequences. We investigated the effect of a single cocaine exposure on dendritic spine structure and glutamate dynamics in the medial prefrontal cortex (mPFC) of adolescent rats 7 days after a single cocaine administration. We found a reduced number of dendritic spines, suggesting that cocaine lowers the density of dendritic spines in the mPFC of adolescent rats. Since dendritic spines are postsynaptic glutamatergic protrusions, we investigated the main determinants of glutamate postsynaptic responsiveness. In the postsynaptic density, cocaine reduced the expression of the NMDA receptor subunits GluN1, GluN2A and GluN2B as well as of the AMPA GluA1 and GluA2 subunits. Cocaine also impaired their synaptic stability since the expression of the scaffolding proteins SAP102 and SAP97, critical for the anchoring of such receptors at the postsynaptic membrane, was reduced as well. The expression of PSD-95 and Arc/Arg3.1, which play structural and functional roles in glutamate neurons, was also similarly reduced. Such changes were not found in the whole homogenate, ruling out a translational effect of cocaine and implying, rather, an impaired synaptic retention at the active zones of the synapse. Notably, neither these critical glutamate determinants nor the density and morphology of the dendritic spines were altered in the mPFC of adult animals, suggesting that a single cocaine exposure selectively impairs the developmental trajectory of the glutamate synapse. These results indicate a dynamic impairment of mPFC glutamate homeostasis during a critical developmental window that persists for at least one week after a single cocaine administration. Our results identify dysfunctional glutamate synapse as a major contributor to the mechanisms that distinguish adolescent vs. adult outcomes of a single cocaine exposure.

Vesicular glutamate transporter 1 (VGLUT1)-mediated glutamate release and membrane GluA1 activation is involved in the rapid antidepressant-like effects of scopolamine in mice

Emerging data have identified certain drugs such as scopolamine as rapidly acting antidepressants for major depressive disorder (MDD) that increase glutamate release and induce neurotrophic factors through α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) activation in rodent models. However, little research has addressed the direct mechanisms of scopolamine on AMPAR activation or vesicular glutamate transporter 1 (VGLUT1)-mediated glutamate release in the prefrontal cortex (PFC) of mice. Herein, using a chronic unpredictable stress (CUS) paradigm, acute treatment with scopolamine rapidly reversed stress-induced depression-like behaviors in mice. Our results showed that CUS-induced depression-like behaviors, accompanied by a decrease in membrane AMPAR subunit 1 (GluA1), phosphorylated GluA1 Ser845 (pGluA1 Ser845), brain-derived neurotrophic factor (BDNF) and VGF (non-acronymic) and an increase in bicaudal C homolog 1 gene (BICC1) in the PFC of mice, and these biochemical and behavioral abnormalities were ameliorated by acute scopolamine treatments. However, pharmacological block of AMPAR by NBQX infusion into the PFC significantly abolished these effects of scopolamine. In addition, knock down of VGLUT1 by lentiviral-mediated RNA interference in the PFC of mice was sufficient to induce depression-like phenotype, to decrease extracellular glutamate accumulation and to cause similar molecular changes with CUS in mice. Remarkably, VGLUT1 knockdown alleviated the rapid antidepressant-like actions of scopolamine and the effects of scopolamine on membrane GluA1-mediated BDNF, VGF and BICC1 changes. Altogether, our findings suggest that VGLUT1-mediated glutamate release and membrane GluA1 activation may play a critical role in the rapid-acting antidepressant-like effects of scopolamine in mice.

Adolescent cocaine exposure induces prolonged synaptic modifications in medial prefrontal cortex of adult rats

Substance used during adolescent period increases the risk of psychiatric disorders in later life, but the underlying neural mechanisms remain unclear. We hypothesize that synaptic remodeling and changes of homeostasis in the medial prefrontal cortex (mPFC) following adolescent cocaine exposure may last for a long time, and these modifications may contribute to behavioral deficiencies in adulthood. To address this hypothesis, rats were exposed to cocaine hydrochloride from postnatal day 28 (P28) to P42. When reared to adulthood, rats were subjected to behavioral tests. On P75 and P76, cocaine-experienced rats exhibited increased locomotive and anxiety-like behaviors, as well as impaired non-selective attention. In the cocaine-experienced rats, both levels of synapse-related proteins (synapsin I and PSD-95) and density of synapse and dendrite spine in mPFC were significantly decreased when compared to controls. Unexpected, the expression of molecules related to oxidative stress, inflammation and apoptosis showed no significant changes in mPFC following adolescent cocaine exposure. These findings suggested that adolescent exposure to cocaine induce long-term modification on synapses in mPFC, which might contribute to long-term behavioral outcomes in adulthood.

Disruption of GluA2 phosphorylation potentiates stress responsivity

Cocaine addiction is characterized by persistent craving and addicts frequently relapse even after long periods of abstinence. Exposure to stress can precipitate relapse in humans and rodents. Stress and drug use can lead to common alterations in synaptic plasticity and these commonalities may contribute to the ability of stress to elicit relapse. These common changes in synaptic plasticity are mediated, in part, by alterations in the trafficking and stabilization of AMPA receptors. Exposure to both cocaine and stress can lead to alterations in protein kinase C-mediated phosphorylation of GluA2 AMPA subunits and thus alter the trafficking of GluA2-containing AMPARs. However, it is not clear what role AMPAR trafficking plays in the interactions between stress and cocaine. The current study utilized a mouse with a point mutation within the GluA2 subunit c-terminus resulting in a disruption of PKC-mediated GluA2 phosphorylation to examine stress responsivity. Although no differences were seen in the response to a forced swim stress in naïve mice, GluA2 K882A knock-in mice exhibited an increased stress response following cocaine self-administration. Furthermore, we demonstrated that disrupting GluA2 phosphorylation increases vulnerability to stress-induced reinstatement of both cocaine seeking and cocaine-conditioned reward. Finally, GluA2 K882A knock-in mice exhibit an increased vulnerability to social defeat as indicated by increased social avoidance. Taken together these results indicate that disrupting GluA2 phosphorylation leads to increased responsivity to acute stress following cocaine exposure and increased vulnerability to chronic stress. These results highlight the GluA2 phosphorylation site as a novel target for the stress-related disorders.

A single cocaine exposure disrupts actin dynamics in the cortico-accumbal pathway of adolescent rats: modulation by a second cocaine injection

RATIONALE AND OBJECTIVES

Among the changes caused by repeated exposure to drugs of abuse, structural rearrangements play a critical role, setting the stage for maladaptive responses to environmental challenges and sustaining drug-taking and drug-seeking behaviors. Given that adolescents are more vulnerable to drug abuse than adults and based on our recent data showing that a single exposure to cocaine during adolescence is sufficient to change the adolescent brain, we decided to investigate whether acute cocaine exposure may alter actin remodeling in reward-related brain regions.

METHODS

Accordingly, we decided to evaluate if F-actin/G-actin ratio was altered by a single injection of cocaine (20 mg/kg) at postnatal day 35. We also evaluated whether the first administration of cocaine influences such a ratio in response to a second injection (10 mg/kg) provided 24 h or 7 days later.

RESULTS

Within the medial prefrontal cortex, a single cocaine injection increases the F-actin/G-actin ratio. This effect lasts 1 week, and it is not affected by the second injection of cocaine, indicating a persistent effect of the first exposure. In the nucleus accumbens, cocaine reduces the F-actin/G-actin ratio 24 h later. Seven days later, instead, such a ratio is markedly increased: notably, the additional exposure to the psychostimulant normalizes the F-actin/G-actin ratio.

CONCLUSIONS

These results suggest that a single cocaine injection during adolescence causes possible changes in actin dynamics and influences the response to a second challenge of the psychostimulant, indicating that early cocaine priming might affect mechanisms regulating synaptic structural plasticity in specific brain regions.

Developmental Exposure to Cocaine Dynamically Dysregulates Cortical Arc/Arg3.1 Modulation in Response to a Challenge

During adolescence, the medial prefrontal cortex (mPFC) is still developing. We have previously shown that developmental cocaine exposure alters mPFC's ability to cope with challenging events. In this manuscript, we exposed rats developmentally treated with cocaine to a novelty task and analyzed the molecular changes of mPFC. Rats were exposed to cocaine from post-natal day (PND) 28 to PND 42 and sacrificed at PND 43, immediately after the novel object recognition (NOR) test. Cocaine-treated rats spent more time exploring the novel object than saline-treated counterparts, suggesting an increased response to novelty. The messenger RNA (mRNA) and protein levels of the immediate early gene Arc/Arg3.1 were reduced in both infralimbic (IL) and prelimbic (PL) cortices highlighting a baseline reduction of mPFC neuronal activity as a consequence of developmental exposure to cocaine. Intriguingly, significant molecular changes were observed in the IL, but not PL, cortex in response to the combination of cocaine exposure and test such as a marked upregulation of both Arc/Arg3.1 mRNA and protein levels only in cocaine-treated rats. As for proteins, such increase was observed only in the post-synaptic density and not in the whole homogenate, suggesting psychostimulant-induced changes in trafficking of Arc/Arg3.1 or an increased local translation. Notably, the same profile of Arc/Arg3.1 was observed for post-synaptic density (PSD)-95 leading to the possibility that Arc/Arg3.1 and PSD-95 bridge together to promote aberrant synaptic connectivity in IL cortex following repeated exposure to cocaine during brain development.

Dynamic Regulation of AMPAR Phosphorylation In Vivo Following Acute Behavioral Stress

The tuning of glutamatergic transmission is an essential mechanism for neuronal communication. α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) are ionotropic glutamate receptors that mediate fast synaptic transmission. The phosphorylation states of specific serine residues on the GluA1 and GluA2 AMPAR subunits are considered critical post-translational modifications that regulate AMPAR activity and subcellular trafficking. While behavioral stress, via stress hormones, exerts specific alterations on such glutamatergic processes, there have been conflicting data concerning the influence of stress on AMPAR phosphorylation in different brain regions, and the post-stress signaling mechanisms mediating these processes are not well delineated. Here, we examined the dynamics of phosphorylation at three AMPAR serine residues (ser831-GluA1, ser845-GluA1, and ser880-GluA2) in four brain regions [amygdala, medial prefrontal cortex (mPFC), dorsal hippocampus, and ventral hippocampus] of the rat during the hour following behavioral stress. We also tested the impact of post-stress corticosteroid receptor blockade on AMPAR phosphorylation. Both GluA1 subunit residues exhibited elevated phosphorylation after stress, yet post-stress administration of corticosteroid receptor antagonists curtailed these effects only at ser831-GluA1. In contrast, ser880-GluA2 displayed a time-dependent tendency for early decreased phosphorylation (that was selectively augmented by mifepristone treatment in the amygdala and mPFC of stressed animals) followed by increased phosphorylation later on. These findings show that the in vivo regulation of AMPAR phosphorylation after stress is a dynamic and subunit-specific process, and they provide support for the hypothesis that corticosteroid receptors have an ongoing role in the regulation of ser831-GluA1 phosphorylation during the post-stress interval.

The individual and combined effects of phenmetrazine and mgluR2/3 agonist LY379268 on the motivation to self-administer cocaine

BACKGROUND

The US Food and Drug Administration has not approved a treatment for cocaine addiction, possibly due in part to the fact that repeated cocaine use results in dysregulation of multiple neurotransmitter systems, including glutamate and dopamine, and an emergence of increased negative affective states and heightening motivation to take cocaine despite negative consequences. We used a combination therapy approach to assess whether modulation of both glutamate and dopamine transmission would reduce the motivation to self- administer cocaine compared to modulation of either system alone.

METHODS

The metabotropic glutamate 2/3 receptor agonist, LY379268, and the monoamine releaser, phenmetrazine, were used to assess their individual and combined ability to decrease the reinforcing efficacy of cocaine because they modulate glutamate and dopamine levels, respectively. Cocaine breakpoints and cocaine intake was assessed, using a progressive ratio schedule, at baseline in three groups based on dose of cocaine (0.19, 0.38, 0.75mg/kg/infusion), and following LY379268 (0.03 or 0.30mg/kg; i.p.), phenmetrazine (25mg/kg/day; osmotic minipump), and a combination of the two drugs.

RESULTS

LY379268 and phenmetrazine alone reduced breakpoints for all doses of cocaine. The combination of the two drugs showed a concerted effect in reducing breakpoints for all doses of cocaine, with the lowest dose of cocaine reduced by as much as 70%.

CONCLUSIONS

These data support combination therapy of dopamine and glutamate systems as an effective means to reduce the motivation to take cocaine since a combination of drugs can address neurobiological dysfunction in multiple neurotransmitter systems compared to therapies using single drugs.

Consequences of adolescent use of alcohol and other drugs: Studies using rodent models

Studies using animal models of adolescent exposure to alcohol, nicotine, cannabinoids, and the stimulants cocaine, 3,4-methylenedioxymethampethamine and methamphetamine have revealed a variety of persisting neural and behavioral consequences. Affected brain regions often include mesolimbic and prefrontal regions undergoing notable ontogenetic change during adolescence, although it is unclear whether this represents areas of specific vulnerability or particular scrutiny to date. Persisting alterations in forebrain systems critical for modulating reward, socioemotional processing and cognition have emerged, including apparent induction of a hyper-dopaminergic state with some drugs and/or attenuations in neurons expressing cholinergic markers. Disruptions in cognitive functions such as working memory, alterations in affect including increases in social anxiety, and mixed evidence for increases in later drug self-administration has also been reported. When consequences of adolescent and adult exposure were compared, adolescents were generally found to be more vulnerable to alcohol, nicotine, and cannabinoids, but generally not to stimulants. More work is needed to determine how adolescent drug exposure influences sculpting of the adolescent brain, and provide approaches to prevent/reverse these effects.

Short-term withdrawal from developmental exposure to cocaine activates the glucocorticoid receptor and alters spine dynamics

Although glucocorticoid receptors (GRs) contribute to the action of cocaine, their role following developmental exposure to the psychostimulant is still unknown. To address this issue, we exposed adolescent male rats to cocaine (20mg/kg/day) from post-natal day (PND) 28 to PND 42 and sacrificed them at PND 45 or 90. We studied the medial prefrontal cortex (mPFC), a brain region that is still developing during adolescence. In PND 45 rats we found enhanced GR transcription and translation as well as increased trafficking toward the nucleus of the receptor, with no alteration in plasma corticosterone levels. We also showed reduced expression of the GR co-chaperone FKBP51, that normally keeps the receptor in the cytoplasm, and increased expression of Src1, which cooperates in the activation of GR transcriptional activity, revealing that short withdrawal alters the finely tuned mechanisms regulating GR action. Since activation of GRs regulate dendritic spine morphology, we next investigated spine dynamics in cocaine-withdrawn rats. We found that PSD95, cofilin and F-actin, molecules regulating spine actin network, are reduced in the mPFC of PND 45 rats suggesting reduced spine density, confirmed by confocal imaging. Further, formation of filopodia, i.e. the inactive spines, is enhanced suggesting the formation of non-functional spines. Of note, no changes were found in molecules related to GR machinery or spine dynamics following long-term abstinence, i.e. in adult rats (PND 90). These findings demonstrate that short withdrawal promotes plastic changes in the developing brain via the dysregulation of the GR system and alterations in the spine network.

Hippocampal cell fate regulation by chronic cocaine during periods of adolescent vulnerability: Consequences of cocaine exposure during adolescence on behavioral despair in adulthood

Given that adolescence represents a critical moment for shaping adult behavior and may predispose to disease vulnerability later in life, the aim of this study was to find a vulnerable period during adolescence in which hippocampal cell fate regulation was altered by cocaine exposure, and to evaluate the long-term consequences of a cocaine experience during adolescence in affecting hippocampal plasticity and behavioral despair in adulthood. Study I: Male rats were treated with cocaine (15mg/kg, i.p.) or saline for 7 consecutive days during adolescence (early post-natal day (PND) 33-39, mid PND 40-46, late PND 47-53). Hippocampal plasticity (i.e., cell fate regulation, cell genesis) was evaluated 24h after the last treatment dose during the course of adolescence (PND 40, PND 47, PND 54). Study II: The consequences of cocaine exposure during adolescence (PND 33-39 or PND 33-46; 7 or 14days) were measured in adulthood at the behavioral (i.e., forced swim test, PND 62-63) and molecular (hippocampal cell markers, PND 64) levels. Chronic cocaine during early adolescence dysregulated FADD forms only in the hippocampus (HC), as compared to other brain regions, and during mid adolescence, impaired cell proliferation (Ki-67) and increased PARP-1 cleavage (a cell death maker) in the HC. Interestingly, chronic cocaine exposure during adolescence did not alter the time adult rats spent immobile in the forced swim test. These results suggest that this paradigm of chronic cocaine administration during adolescence did not contribute to the later manifestation of behavioral despair (i.e., one pro-depressive symptom) as measured by the forced swim test in adulthood.

A single exposure to cocaine during development elicits regionally-selective changes in basal basic Fibroblast Growth Factor (FGF-2) gene expression and alters the trophic response to a second injection

RATIONALE

During adolescence, the brain is maturing and more sensitive to drugs of abuse that can influence its developmental trajectory. Recently, attention has been focused on basic fibroblast growth factor (FGF-2) given that its administration early in life enhances the acquisition of cocaine self-administration and sensitization at adulthood (Turner et al. (Pharmacol Biochem Behav 92:100-4, 2009), Clinton et al. (Pharmacol Biochem Behav103:6-17, 2012)). Additionally, we found that abstinence from adolescent cocaine exposure long lastingly dysregulates FGF-2 transcription (Giannotti et al. (Psychopharmacology (Berl) 225:553-60, 2013 ).

OBJECTIVES

The objectives of the study are to evaluate if (1) a single injection of cocaine (20 mg/kg) at postnatal day 35 alters FGF-2 messenger RNA (mRNA) levels and (2) the first injection influences the trophic response to a second injection (10 mg/kg) provided 24 h or 7 days later.

RESULTS

We found regional differences in the FGF-2 expression pattern as either the first or the second injection of cocaine by themselves upregulated FGF-2 mRNA in the medial prefrontal cortex and nucleus accumbens while downregulating it in the hippocampus. The first injection influences the trophic response of the second. Of note, 24 h after the first injection, accumbal and hippocampal FGF-2 changes produced by cocaine in saline-pretreated rats were prevented in cocaine-pretreated rats. Conversely, in the medial prefrontal cortex and hippocampus 7 days after the first injection, the cocaine-induced FGF-2 changes were modified by the subsequent exposure to the psychostimulant.

CONCLUSIONS

These findings show that a single cocaine injection is sufficient to produce enduring changes in the adolescent brain and indicate that early cocaine priming alters the mechanisms regulating the trophic response in a brain region-specific fashion.

Orexin/hypocretin role in reward: implications for opioid and other addictions

Addiction is a devastating disorder that affects 15.3 million people worldwide. While prevalent, few effective treatments exist. Orexin receptors have been proposed as a potential target for anti-craving medications. Orexins, also known as hypocretins, are neuropeptides produced in neurons of the lateral and dorsomedial hypothalamus and perifornical area, which project widely throughout the brain. The absence of orexins in rodents and humans leads to narcolepsy. However, orexins also have an established role in reward seeking. This review will discuss some of the original studies describing the roles of the orexins in reward seeking as well as specific works that were presented at the 2013 International Narcotics Research Conference. Orexin signalling can promote drug-induced plasticity of glutamatergic synapses onto dopamine neurons of the ventral tegmental area (VTA), a brain region implicated in motivated behaviour. Additional evidence suggests that orexin signalling can also promote drug seeking by initiating an endocannabinoid-mediated synaptic depression of GABAergic inputs to the VTA, and thereby disinhibiting dopaminergic neurons. Orexin neurons co-express the inhibitory opioid peptide dynorphin. It has been proposed that orexin in the VTA may not mediate reward per se, but rather occludes the 'anti-reward' effects of dynorphin. Finally, orexin signalling in the prefrontal cortex and the central amygdala is implicated in reinstatement of reward seeking. This review will highlight recent work describing the role of orexin signalling in cellular processes underlying addiction-related behaviours and propose novel hypotheses for the mechanisms by which orexin signalling may impart drug seeking.

LINKED ARTICLES

This article is part of a themed section on Opioids: New Pathways to Functional Selectivity. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2015.172.issue-2.