A limited and intermittent access to a high-fat diet modulates the effects of cocaine-induced reinstatement in the conditioned place preference in male and female mice

RATIONALE

Palatable food and drugs of abuse activate common neurobiological pathways and numerous studies suggest that fat consumption increases vulnerability to drug abuse. In addition, preclinical reports show that palatable food may relieve craving for drugs, showing that an ad libitum access to a high-fat diet (HFD) can reduce cocaine-induced reinstatement.

OBJECTIVE

The main aim of the present study was to evaluate the effect of a limited and intermittent exposure to HFD administered during the extinction and reinstatement processes of a cocaine-induced conditioned place preference (CPP).

METHODS

Male and female mice underwent the 10 mg/kg cocaine CPP. From post-conditioning onwards, animals were divided into four groups: SD (standard diet); HFD-MWF with 2-h access to the HFD on Mondays, Wednesdays, and Fridays; HFD-24h, with 1-h access every day; and HFD-Ext with 1-h access to the HFD before each extinction session.

RESULTS

Our results showed that all HFD administrations blocked reinstatement in males, while only the HFD-MWF was able to inhibit reinstatement in females. In addition, HFD-Ext males needed fewer sessions to extinguish the preference, which suggests that administration of fat before being exposed to the environmental cues is effective to extinguish drug-related memories. HFD did not affect Oprμ gene expression but increased CB1r gene expression in the striatum in HFD-Ext males.

CONCLUSIONS

These results support that palatable food could act as an alternative reward to cocaine, accelerating extinction and blocking reinstatement, these effects being sex specific.

Sodium butyrate enhances fear extinction and rescues hippocampal acetylcholinesterase activity in a rat model of posttraumatic stress disorder

It is believed that impaired extinction of fear memories is an underlying cause for the development of posttraumatic stress disorder (PTSD). Histone deacetylases (HDAC) are enzymes that modulate extinction by changing the chromatin structure and altering protein synthesis in the brain. Studies show that stress modifies both HDAC activity and cerebral cholinergic neurotransmission. The present work aims to evaluate the effect of sodium butyrate (NaBu), an HDAC inhibitor, on behavioral markers of extinction and biochemical changes in HDAC and acetylcholinesterase activity in the hippocampus. NaBu was administered for 7 days in a group of rats that were exposed to single prolonged stress (SPS), as a model for PTSD. Contextual fear conditioning was performed on the 8th day, and fear extinction was measured in the next 4 consecutive days. Other behavioral tests to measure anxiety, locomotor activity and working memory were performed for further interpretation of the results. Hippocampal acetylcholinesterase and HDAC activity were also measured through biochemical tests. Behavioral results showed that treatment with NaBu can reverse the SPS-induced extinction deficits. Biochemical data indicated that while SPS induced overactivity in hippocampal HDAC, it decreased acetylcholinesterase activity in the region. Both effects were reversed after NaBu treatment. It seems that at least part of extinction deficiency in SPS exposed rats is related to hypoacetylation of acetylcholinesterase in the hippocampus. Preemptive therapy with an HDAC inhibitor reverses this process and is worth further evaluation as a possible therapeutic approach in PTSD.

Fluoroethylnormemantine, A Novel Derivative of Memantine, Facilitates Extinction Learning Without Sensorimotor Deficits

BACKGROUND

Memantine, a noncompetitive N-methyl-D-aspartate receptor antagonist, has been approved for use in Alzheimer's disease, but an increasing number of studies have investigated its utility for neuropsychiatric disorders. Here, we characterized a novel compound, fluoroethylnormemtantine (FENM), which was derived from memantine with an extra Fluor in an optimized position for in vivo biomarker labeling. We sought to determine if FENM produced similar behavioral effects as memantine and/or if FENM has beneficial effects against fear, avoidance, and behavioral despair.

METHODS

We administered saline, FENM, or memantine prior to a number of behavioral assays, including paired-pulse inhibition, open field, light dark test, forced swim test, and cued fear conditioning in male Wistar rats.

RESULTS

Unlike memantine, FENM did not produce nonspecific side effects and did not alter sensorimotor gating or locomotion. FENM decreased immobility in the forced swim test. Moreover, FENM robustly facilitated fear extinction learning when administered prior to either cued fear conditioning training or tone reexposure.

CONCLUSIONS

These results suggest that FENM is a promising, novel compound that robustly reduces fear behavior and may be useful for further preclinical testing.

Effects of Combined Anti-Hypertensive and Statin Treatment on Memory, Fear Extinction, Adult Neurogenesis, and Angiogenesis in Adult and Middle-Aged Mice

Hyperlipidemia and hypertension are modifiable risk factors for cognitive decline. About 25% of adults over age 65 use both antihypertensives (AHTs) and statins to treat these conditions. Recent research in humans suggests that their combined use may delay or prevent dementia onset. However, it is not clear whether and how combination treatment may benefit brain function. To begin to address this question, we examined effects of atorvastatin, a 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitor, and Captopril, an angiotensin-converting enzyme inhibitor (ACEI), administration on memory function, anxiety-like behavior, adult hippocampal neurogenesis and angiogenesis in adult and middle-aged male C57Bl/6J mice. In adult mice (3-months-old) combination (combo) treatment, as well as administration of each compound individually, for six weeks, accelerated memory extinction in contextual fear conditioning. However, pattern separation in the touchscreen-based location discrimination test, a behavior linked to adult hippocampal neurogenesis, was unchanged. In addition, dentate gyrus (DG) neurogenesis and vascularization were unaffected. In middle-aged mice (10-months-old) combo treatment had no effect on spatial memory in the Morris water maze, but did reduce anxiety in the open field test. A potential underlying mechanism may be the modest increase in new hippocampal neurons (~20%) in the combo as compared to the control group. DG vascularization was not altered. Overall, our findings suggest that statin and anti-hypertensive treatment may serve as a potential pharmacotherapeutic approach for anxiety, in particular for post-traumatic stress disorder (PTSD) patients who have impairments in extinction of aversive memories.

Blockade of orexin receptors in the hippocampal dentate gyrus reduced the extinction latency of morphine-induced place preference in male rats

Relapse to drugs such as opioids is a major challenge in addiction therapy. It has been known that the orexinergic system has a significant role in mediating reward processing and addiction, as shown by the conditioned place preference (CPP). The dentate gyrus (DG) of the hippocampus receives orexinergic projections from the lateral hypothalamus that has been approved as a critical area arbitrating the maintenance of drug-seeking behavior following the extinction. The present study aimed to investigate the effects of intra-DG administration of the orexin-1 receptor (OX1R) and orexin-2 receptor (OX2R) antagonists on the extinction of morphine-induced CPP in male rats. Animals received different doses of SB334867 (as OX1R antagonist) or TCS OX2 29 (as OX2R antagonist) (0.5, 2.5, and 12.5 nM/0.5 μl DMSO 12 %) bilaterally into the DG during the extinction phase, after CPP had been induced by subcutaneous injection of morphine (5 mg/kg) during a 3-day conditioning phase. The conditioning scores were recorded during the test. The results demonstrated that intra-DG administration of the highest dose of OX1R antagonist (12.5 nM/0.5 μl DMSO 12 %) shortened the extinction latency of morphine-CPP compared to the DMSO group, while the OX2R antagonist did not significantly alter the latency. Findings imply that the blockade of OX1R, but not OX2R, within the DG facilitates the extinction of morphine-induced reward. In conclusion, the OX1R antagonist might be kept in mind as a convenient therapeutic factor in repressing drug-seeking behaviors in an optimum amount of treatment considering the low dose-treatments applied.

Stimulus-Based Extinction Generalization: Neural Correlates and Modulation by Cortisol

BACKGROUND

While healthy individuals and patients with anxiety disorders easily generalize fear responses, extinction learning is more stimulus specific. Treatments aiming to generalize extinction learning are urgently needed, since they comprise the potential to overcome stimulus specificity and reduce relapses, particularly in the face of stressful events.

METHODS

In the current 3-day functional magnetic resonance imaging fear conditioning paradigm, we aimed to create a generalized extinction memory trace in 60 healthy men and women by presenting multiple sizes of 1 conditioned stimulus during extinction training (CS+G; generalized), whereas the other conditioned stimulus was solely presented in its original size (CS+N; nongeneralized). Recall was tested on the third day after pharmacological administration of either the stress hormone cortisol or placebo.

RESULTS

After successful fear acquisition, prolonged activation of the amygdala and insula and deactivation of the ventromedial prefrontal cortex for CS+G compared with CS+N during extinction learning indicated sustained fear to the generalization stimuli. In line with our hypotheses, reduced amygdala activation was observed after extinction generalization on the third day in the contrast CS+G minus CS+N, possibly reflecting an attenuated return of fear. Cortisol administration before recall, however, blocked this effect.

CONCLUSIONS

Taken together, the findings show that extinction generalization was associated with decreased activation of the fear network during recall after prolonged activation of the fear network during extinction learning. However, the generalization of the extinction memory did not counteract the detrimental effects of stress hormones on recall. Thus, stimulus-based extinction generalization may not be sufficient to reduce relapses after stressful experiences.

IGF-1 facilitates extinction of conditioned fear

Insulin-like growth factor-1 (IGF-1) plays a key role in synaptic plasticity, spatial learning, and anxiety-like behavioral processes. While IGF-1 regulates neuronal firing and synaptic transmission in many areas of the central nervous system, its signaling and consequences on excitability, synaptic plasticity, and animal behavior dependent on the prefrontal cortex remain unexplored. Here, we show that IGF-1 induces a long-lasting depression of the medium and slow post-spike afterhyperpolarization (mAHP and sAHP), increasing the excitability of layer 5 pyramidal neurons of the rat infralimbic cortex. Besides, IGF-1 mediates a presynaptic long-term depression of both inhibitory and excitatory synaptic transmission in these neurons. The net effect of this IGF-1-mediated synaptic plasticity is a long-term potentiation of the postsynaptic potentials. Moreover, we demonstrate that IGF-1 favors the fear extinction memory. These results show novel functional consequences of IGF-1 signaling, revealing IGF-1 as a key element in the control of the fear extinction memory.

Role of prefrontal cortex projections to the nucleus accumbens core in mediating the effects of ceftriaxone on cue-induced cocaine seeking

Ceftriaxone is an antibiotic that reliably attenuates the reinstatement of cocaine seeking after extinction while preventing the nucleus accumbens (NA) core glutamate efflux that drives reinstatement. However, when rats undergo abstinence without extinction, ceftriaxone attenuates context-primed cocaine seeking but NA core glutamate efflux still increases. Here, we sought to determine if the same would occur when cocaine seeking is prompted by both context and discrete cues (cue-induced seeking) after cocaine abstinence. Male rats self-administered intravenous cocaine accompanied by drug-associated cues (light + tone) for 2 h/day for 14 days. Rats then experienced abstinence with daily handling but no extinction training for 2 weeks. Ceftriaxone (200 mg/kg IP) or vehicle was administered during the last 6 days of abstinence. During a cue-induced cocaine seeking test, microdialysis procedures were conducted. Rats were perfused at the end of the test for later Fos analysis. A separate cohort of rats was infused with the retrograde tracer cholera toxin B in the NA core and underwent the same self-administration and relapse procedures. Ceftriaxone increased baseline glutamate and attenuated both cue-induced cocaine seeking and NA core glutamate efflux during this test. Ceftriaxone reduced Fos expression in regions sending projections to the NA core (prefrontal cortex, basolateral amygdala, ventral tegmental area) and specifically reduced Fos in prelimbic cortex and not infralimbic cortex neurons projecting to the NA core. Thus, when cocaine seeking is induced by drug-associated cues, ceftriaxone is able to attenuate relapse by preventing NA core glutamate efflux, likely through reducing activity in prelimbic NA core-projecting neurons.

Effects of processed Aconiti tuber on the extinction and reinstatement of morphine-induced conditioned place preference in rats

AIM OF THE STUDY

To investigate the effect of processed Aconiti tuber (PAT) administered during or after the time of conditioned place preference (CPP) training on the extinction and reinstatement of morphine-priming CPP in rats. The dynorphin level in rats' nucleus accumbens (NAc) is detected as a target of the Dynorphin/Kappa Opioid Receptor (KOR) system for the possible mechanism.

MATERIALS AND METHODS

Eight groups of rats were subcutaneously (s.c.) injected with morphine (10mg/kg) (on days 2,4,6,8) or saline (1ml/kg) (on days 3,5,7,9) alternately for 8 days. Five groups, including groups (Mor + Water, Mor + PAT (1.0/3.0g/kg) (S) and Sal + PAT(1.0/3.0g/kg)), were orally given distilled water or PAT 1.0 or 3.0 g/kg daily on days 1-8 during CPP training while other three groups, including groups (Sal + Water and Mor + PAT (1.0/3.0g/kg)(P), were given distilled water or PAT daily from day 10 until CPP was extinct. Morphine 1mg/kg (s.c.) was used to reinstate the extinct CPP and the CPP scores were recorded. The dynorphin concentration in nucleus accumbens (NAc) was assayed by radioimmunoassay after the last CPP measurement.

RESULTS

1) The CPP extinction shortened in Mor + PAT (1.0/3.0 g/kg) (S) groups but extended in Mor + PAT (1.0/3.0 g/kg)(P) groups. 2) Morphine-priming CPP did not change either in Mor + PAT (1.0/3.0 g/kg) (S) or Mor + PAT (1.0/3.0 g/kg)(P) groups. 3) The dynorphin concentration in NAc increased either in Mor + PAT (1.0/3.0 g/kg)(S) or Mor + PAT (1.0/3.0 g/kg)(P) groups.

CONCLUSIONS

1) PAT shortened the extinction from morphine induced CPP when administrated before CPP acquisition, whereas it extended the extinction when administrated after CPP formation. 2) PAT administrated during or after CPP training did not affect morphine-priming reinstatement of morphine induced CPP. 3) Dynorphin/KOR system might be a target to regulate morphine-induced CPP extinction but not reinstatement.

Rat ultrasonic vocalizations as an index of memory

The emission of 50 kHz frequency-modulated ultrasonic vocalizations (FM USVs) in rats has been associated with positive affective states, while a decrease in FM USVs has been associated with anxiety-like states. We tested the hypothesis in male Sprague-Dawley rats that FM USVs would complement measures of aversive memories (decrease in FM USVs) in a conditioned fear task in which we examined extinction or reconsolidation disruption. In Experiment 1, rats were fear conditioned using low-level footshock followed by extinction while monitoring freezing and FM USVs. In Experiment 2, rats were fear conditioned, the alpha-1 antagonist prazosin was used to disrupt reconsolidation of memory, and freezing and FM USVs were measured. Rats fear conditioned with low-level shock showed minimal freezing that rapidly extinguished, despite a persistent decrease in FM USVs throughout extinction. Prazosin reduced freezing in a memory reactivation-dependent manner as expected, but the reduction in FM USVs after fear conditioning remained decreased, suggesting that an affective component of memory was not impacted by prazosin. These findings indicate that FM USVs may be used as an index of fear- or anxiety-like memory, and their measurement could benefit pre-clinical animal models for assessing reduction of aversive memories.

Effect of chemogenetic inhibition of lateral habenula neuronal activity on cocaine- and food-seeking behaviors in the rat

A major problem in the treatment of cocaine addiction is high rates of relapse. Relapse is often provoked by acute reexposure to cocaine-associated cues or to cocaine itself. The lateral habenula (LHb), an epithalamic nucleus, regulates midbrain dopaminergic systems that are known to be involved in cocaine taking and seeking behaviors. However, the role of this nucleus in cocaine self-administration and reinstatement of cocaine seeking has not been entirely parsed out. We used an operant self-administration and reinstatement procedure to explore the effect of Designer Receptors Exclusively Activated by Designer Drug (DREADD)-induced transient inhibition of LHb neurons on cocaine taking and seeking. Firstly, rats were injected with adeno-associated viral vectors expressing hM4 Di (a Gi/o -coupled DREADD) into the LHb, trained to self-administer cocaine (0.75 mg/kg/infusion), and the effect of clozapine-N-oxide (an inert ligand that activates DREADDs) was assessed on cocaine self-administration. Secondly, rats were injected with hM4 Di into the LHb, trained to self-administer cocaine; the operant response was extinguished, and cue- and cocaine priming-induced reinstatement was assessed. Thirdly, we tested the generality of the effect of inhibiting LHb neurons by assessing the effect of this manipulation on food-taking and seeking. hM4 Di -induced inhibition of LHb neurons increased cocaine but not food self-administration. In contrast, this manipulation decreased reinstatement of cocaine, but not food-seeking. Taken together, our data suggest that hM4 Di - induced LHb inhibition specifically mediates taking and seeking behaviors reinforced by cocaine but not by natural reinforcers. Further, our data indicate a dissociation in the role of LHb neurons on cocaine self-administration versus reinstatement of cocaine seeking.

Acute exercise enhances fear extinction through a mechanism involving central mTOR signaling

Impaired fear extinction, combined with the likelihood of fear relapse after exposure therapy, contributes to the persistence of many trauma-related disorders such as anxiety and post-traumatic stress disorder. Identifying mechanisms to aid fear extinction and reduce relapse could provide novel strategies for augmentation of exposure therapy. Exercise can enhance learning and memory and augment fear extinction of traumatic memories in humans and rodents. One factor that could contribute to enhanced fear extinction following exercise is the mammalian target of rapamycin (mTOR). mTOR is a translation regulator involved in synaptic plasticity and is sensitive to many exercise signals such as monoamines, growth factors, and cellular metabolism. Further, mTOR signaling is increased after chronic exercise in brain regions involved in learning and emotional behavior. Therefore, mTOR is a compelling potential facilitator of the memory-enhancing and overall beneficial effects of exercise on mental health.The goal of the current study is to test the hypothesis that mTOR signaling is necessary for the enhancement of fear extinction produced by acute, voluntary exercise. We observed that intracerebral-ventricular administration of the mTOR inhibitor rapamycin reduced immunoreactivity of phosphorylated S6, a downstream target of mTOR, in brain regions involved in fear extinction and eliminated the enhancement of fear extinction memory produced by acute exercise, without reducing voluntary exercise behavior or altering fear extinction in sedentary rats. These results suggest that mTOR signaling contributes to exercise-augmentation of fear extinction.

Neuropeptide Y as Alternative Pharmacotherapy for Antidepressant-Resistant Social Fear

In many social anxiety disorder (SAD) patients, the efficacy of antidepressant therapy is unsatisfactory. Here, we investigated whether mice deficient for the lysosomal glycoprotein acid sphingomyelinase (ASM−/−) represent an appropriate tool to study antidepressant-resistant social fear. We also investigated whether neuropeptide Y (NPY) reduces this antidepressant-resistant social fear in ASM−/− mice, given that NPY reduced social fear in a mouse model of SAD, namely social fear conditioning (SFC). We show that neither chronic paroxetine nor chronic amitriptyline administration via drinking water were successful in reducing SFC-induced social fear in ASM−/− mice, while the same treatment reduced social fear in ASM+/− mice and completely reversed social fear in ASM+/+ mice. This indicates that the antidepressants paroxetine and amitriptyline reduce social fear via the ASM-ceramide system and that ASM−/− mice represent an appropriate tool to study antidepressant-resistant social fear. The intracerebroventricular administration of NPY, on the other hand, reduced social fear in ASM−/− mice, suggesting that NPY might represent an alternative pharmacotherapy for antidepressant-resistant social fear. These results suggest that medication strategies aimed at increasing brain NPY concentrations might improve symptoms of social fear in SAD patients who fail to respond to antidepressant treatments.

Naltrexone attenuates methamphetamine-induced behavioral sensitization and conditioned place preference in mice

Methamphetamine addiction causes serious public health problems worldwide. However, there is no effective medication licensed for methamphetamine addiction. The endogenous opioid system is considered to be a common substrate in drug addiction due to its regulation of dopamine release. In recent clinical trials, (-)-naltrexone, an opioid receptors and Toll-like receptor 4 antagonist, has exhibited encouraging findings for treating methamphetamine addiction; however, the understanding of its pharmacological mechanisms remains insufficient. By using mice models of behavioral sensitization and conditioned place preference (CPP), the present study was performed to investigate the effects of naltrexone on the methamphetamine-associated properties of incentive salience and reward-related memory, the two crucial factors for the development of addictive process and relapse. We found that naltrexone reduced single methamphetamine-induced hyperlocomotion in mice. In the paradigm of methamphetamine-induced behavioral sensitization paired with contextual cues in mice, naltrexone suppressed the development and expression of locomotor sensitization, suggesting the decrease in incentive salience to methamphetamine and context. In the methamphetamine-induced CPP paradigm in mice, naltrexone attenuated both the expression and methamphetamine-priming reinstatement of CPP response, suggesting the impairment of either contextual cue- or drug-induced retrieval of methamphetamine-associated memory. After the establishment of methamphetamine-induced CPP in mice, naltrexone treatment during the extinction training produced conditioned place adverse response, suggesting that naltrexone facilitated negative affection-associated extinction learning. Taken together, these findings demonstrate that naltrexone could intervene in the properties of incentive salience and reward-related memory in methamphetamine addiction, which may contribute to its therapeutic effects on methamphetamine addicts in clinical studies.

Kaempferol Facilitated Extinction Learning in Contextual Fear Conditioned Rats via Inhibition of Fatty-Acid Amide Hydrolase

Background: Fear, stress, and anxiety-like behaviors originate from traumatic events in life. Stress response is managed by endocannabinoids in the body by limiting the uncontrolled retrieval of aversive memories. Pharmacotherapy-modulating endocannabinoids, especially anandamide, presents a promising tool for treating anxiety disorders. Here, we investigated the effect of kaempferol, a flavonoid, in the extinction of fear related memories and associated anxiety-like behavior. Methods: The ability of kaempferol to inhibit fatty-acid amide hydrolase (FAAH, the enzyme that catabolizes anandamide) was assessed in vitro using an enzyme-linked immunosorbent assay (ELISA) kit. For animal studies (in vivo), the extinction learning was evaluated using contextual fear conditioning (CFC, a behavioral paradigm based on ability to learn and remember aversive stimuli). Furthermore, an elevated plus-maze (EPM) model was used for measuring anxiety-like behavior, while serum corticosterone served as a biochemical indicator of anxiety. Lastly, the interaction of kaempferol with FAAH enzyme was also assessed in silico (computational study). Results: Our data showed that kaempferol inhibited the FAAH enzyme with an IC₅₀ value of 1 µM. In CFC, it reduced freezing behavior in rats. EPM data demonstrated anxiolytic activity as exhibited by enhanced number of entries and time spent in the open arm. No change in blood corticosterone levels was noted. Our computational study showed that Kaempferol interacted with the catalytic amino acids (SER241, PHE192, PHE381, and THR377) of FAAH enzyme Conclusion: Our study demonstrate that kaempferol facilitated the extinction of aversive memories along with a reduction of anxiety. The effect is mediated through the augmentation of endocannabinoids via the inhibition of FAAH enzyme.

Inhaled Lavandula angustifolia essential oil enhances extinction learning and inhibits memory updating in mice submitted to the contextual fear conditioning

ETHNOPHARMACOLOGICAL RELEVANCE

Lavender (Lavandula angustifolia) essential oil (EO) has a long history of use in emotional illness, including anxiety disorders. Cognitive mechanisms of learning and memory play a pivotal role in the etiology and maintenance of anxiety since exposure to cues related to aversive situations induces high arousal and anticipatory anxiety. Memory become labile after its reactivation and can be modulated by reconsolidation or extinction. Inhibition of memory reconsolidation or facilitation of memory extinction may be effective in preventing or minimizing the effect of contextual cues on anticipatory anxiety.

AIM OF THE STUDY

We investigated the effect of Lavandula angustifolia EO in the memory updating of conditioned contextual fear.

MATERIALS AND METHODS

Adult male C57Bl6 mice were submitted to fear conditioning. Two days after conditioning the mice underwent a reactivation session in a hybrid context and were then immediately exposed to vaporized water or essential oil at concentrations of 1%, 2.5% or 5% for 3 h. Two days later, the mice were tested in the original or an altered context and their freezing behavior was measured. In addition, mice were subjected to a fear memory recovery protocol followed by a reinstatement session.

RESULTS

In the contextual fear test, 1% essential oil, but not 2.5% or 5%, reduced the freezing behavior response, whereas after a reinstatement session, exposure to 1% essential oil increased the freezing behavior response.

CONCLUSIONS

These results suggest that Lavandula angustifolia essential oil enhances memory extinction and, consequently, inhibits memory updating.

Dissociable mesolimbic dopamine circuits control responding triggered by alcohol-predictive discrete cues and contexts

Context can influence reactions to environmental cues and this elemental process has implications for substance use disorder. Using an animal model, we show that an alcohol-associated context elevates entry into a fluid port triggered by a conditioned stimulus (CS) that predicted alcohol (CS-triggered alcohol-seeking). This effect persists across multiple sessions and, after it diminishes in extinction, the alcohol context retains the capacity to augment reinstatement. Systemically administered eticlopride and chemogenetic inhibition of ventral tegmental area (VTA) dopamine neurons reduce CS-triggered alcohol-seeking. Chemogenetically silencing VTA dopamine terminals in the nucleus accumbens (NAc) core reduces CS-triggered alcohol-seeking, irrespective of context, whereas silencing VTA dopamine terminals in the NAc shell selectively reduces the elevation of CS-triggered alcohol-seeking in an alcohol context. This dissociation reveals new roles for divergent mesolimbic dopamine circuits in the control of responding to a discrete cue for alcohol and in the amplification of this behaviour in an alcohol context.

Ceftriaxone and mGlu2/3 interactions in the nucleus accumbens core affect the reinstatement of cocaine-seeking in male and female rats

RATIONALE

The beta-lactam antibiotic ceftriaxone reliably attenuates the reinstatement of cocaine seeking. While the restoration of nucleus accumbens core (NA core) GLT-1 expression is necessary for ceftriaxone to attenuate reinstatement, AAV-mediated GLT-1 overexpression is not sufficient to attenuate reinstatement and does not prevent glutamate efflux during reinstatement.

AIMS

Here, we test the hypothesis that ceftriaxone attenuates reinstatement through interactions with glutamate autoreceptors mGlu2 and mGlu3 in the NA core.

METHODS

Male and female rats self-administered cocaine for 12 days followed by 2-3 weeks of extinction training. During the last 6-10 days of extinction, rats received ceftriaxone (200 mg/kg IP) or vehicle. In experiment 1, rats were killed, and NA core tissue was biotinylated for assessment of total and surface expression of mGlu2 and mGlu3 via western blotting. In experiment 2, we tested the hypothesis that mGlu2/3 signaling is necessary for ceftriaxone to attenuate cue- and cocaine-primed reinstatement by administering bilateral intra-NA core infusion of mGlu2/3 antagonist LY341495 or vehicle immediately prior to reinstatement testing.

RESULTS

mGlu2 expression was reduced by cocaine and restored by ceftriaxone. There were no effects of cocaine or ceftriaxone on mGlu3 expression. We observed no effects of estrus on expression of either protein. The antagonism of mGlu2/3 in the NA core during both cue- and cocaine-primed reinstatement tests prevented ceftriaxone from attenuating reinstatement.

CONCLUSIONS

These results indicate that ceftriaxone's effects depend on mGlu2/3 function and possibly mGlu2 receptor expression. Future work will test this hypothesis by manipulating mGlu2 expression in pathways that project to the NA core.

Ethanol-paired stimuli can increase reinforced ethanol responding

While ethanol-paired stimuli are frequently postulated to increase drinking motivation and thus increase ethanol responding and precipitate relapse, no study has demonstrated increases in ethanol-reinforced responding following presentation of an ethanol-paired stimulus that had not previously been part of a contingent relationship. Previous studies have shown that food-paired stimuli can increase food responding that is at low rates and increase food consumption in food-sated rats. In Experiment 1, we show that an ethanol-paired stimulus can increase ethanol responding that is at low levels late in the experimental session, presumably due to satiation. However, these increases may have resulted from either associative or non-associative mechanisms. In Experiment 2, we compared the effects of an ethanol-paired stimulus to those of the same stimulus in a Truly-Random-Control group. In a Truly-Random-Control, the stimulus and ethanol each are presented on independent random schedules, and thus any differences between the effects of the stimulus in the experimental and control groups is likely attributable to the association between the stimulus and ethanol. The stimulus increased ethanol-reinforced responding in both the experimental and control groups, but these increases were greater in the experimental than the control group. Thus, both stimulus-change and the pairing of the stimulus with ethanol may result in increases in ethanol-reinforced responding.

The effects of nicotinamide on reinstatement to cocaine seeking in male and female Sprague Dawley rats

RATIONALE

Interventions for psychostimulant use disorders are of significant need. Nicotinamide (NAM) is a small molecule that can oppose cellular adaptations observed following cocaine exposure in the rodent self-administration and reinstatement model of addiction. In addition, utility of NAM against symptoms of withdrawal and vulnerability to relapse to cocaine use has been suggested by case studies and anecdotal reports. However, the empirical effects of NAM on drug-seeking behaviors have not been examined.

OBJECTIVE

The objective of the current study was to investigate the effects of systemic NAM administration on reinstatement to cocaine seeking, using the rat self-administration/extinction/reinstatement model of cocaine addiction.

METHODS

Male and female Sprague Dawley rats were trained to self-administer i.v. cocaine or food pellets for 2 hrs per day for 12 days, followed by 14-17 days of extinction, during which i.p. NAM injections (0-120 mg/kg) were given 30 minutes prior to each extinction or reinstatement session. Rats were tested on cue-, cocaine-, or food-primed reinstatement, as well as locomotor activity.

RESULTS

Chronic NAM administered throughout extinction dose dependently attenuated cue-primed reinstatement in male rats, but not female rats. In contrast, acute NAM given once prior to reinstatement had no effect on reinstatement. Chronic NAM had no effect on locomotor activity or reinstatement to food seeking.

CONCLUSIONS

The specificity of NAM against cue-primed reinstatement indicates that NAM may influence responsiveness to drug-associated cues, specifically in males. Future studies will examine the mechanism(s) by which NAM may exert this effect.

Pharmacological Blockade of PPAR Isoforms Increases Conditioned Fear Responding in the Presence of Nociceptive Tone

Peroxisome proliferator-activated receptors (PPARs) are nuclear receptors with three isoforms (PPARα, PPARβ/δ, PPARγ) and can regulate pain, anxiety, and cognition. However, their role in conditioned fear and pain-fear interactions has not yet been investigated. Here, we investigated the effects of systemically administered PPAR antagonists on formalin-evoked nociceptive behaviour, fear-conditioned analgesia (FCA), and conditioned fear in the presence of nociceptive tone in rats. Twenty-three and a half hours following fear conditioning to context, male Sprague-Dawley rats received an intraplantar injection of formalin and intraperitoneal administration of vehicle, PPARα (GW6471), PPARβ/δ (GSK0660) or PPARγ (GW9662) antagonists, and 30 min later were re-exposed to the conditioning arena for 15 min. The PPAR antagonists did not alter nociceptive behaviour or fear-conditioned analgesia. The PPARα and PPARβ/δ antagonists prolonged context-induced freezing in the presence of nociceptive tone without affecting its initial expression. The PPARγ antagonist potentiated freezing over the entire trial. In conclusion, pharmacological blockade of PPARα and PPARβ/δ in the presence of formalin-evoked nociceptive tone, impaired short-term, within-trial fear-extinction in rats without affecting pain response, while blockade of PPARγ potentiated conditioned fear responding. These results suggest that endogenous signalling through these three PPAR isoforms may reduce the expression of conditioned fear in the presence of nociceptive tone.

Augmenting extinction learning with D-cycloserine reduces return of fear: a randomized, placebo-controlled fMRI study

D-cycloserine (DCS), a partial NMDA-receptor agonist, seems to be a promising enhancer for exposure therapy in anxiety disorders. It has been tested successfully in animal models of fear extinction, where DCS enhanced extinction learning. Applied in clinical studies, results of DCS-augmented exposure therapy remain ambiguous, calling for a deeper understanding of the underlying mechanisms of DCS and its exact effect on extinction learning and return of fear (ROF) in humans. In the present study, we investigated the effect of DCS-augmented extinction learning on behavioral, psychophysiological, and neural indices of ROF during a 24-h delayed recall test. Thirty-seven participants entered a randomized, placebo-controlled, double-blind, 3-day fear conditioning and delayed extinction fMRI design. One hour before extinction training, participants received an oral dose of 50 mg of DCS or a placebo. Behavioral arousal ratings revealed a generalized ROF during extinction recall in the placebo but not DCS group. Furthermore, participants receiving DCS compared to placebo showed attenuated differential BOLD responses in left posterior hippocampus and amygdala from extinction learning to extinction recall, due to increased hippocampal recruitment in placebo and trendwise decreased amygdala responding in DCS subjects. Our finding that DCS reduces ROF in arousal ratings and neural structures subserving defensive reactions support a role for NMDA receptors in extinction memory consolidation and encourage further translational research.

Changes in the elimination and resurgence of alcohol-maintained behavior in rats and the effects of naltrexone

Resurgence may be a mechanism of relapse in alcohol use disorder patients upon discharge from treatment as part of an abuse-treatment-relapse cycle. Adjunctive pharmacotherapies may be a means to facilitate behavioral treatments and block resurgence. Experiments were conducted using a model of alcohol self-administration to assess the repeatability of the elimination and resurgence of alcohol-maintained behavior and the effects of naltrexone. Experiments had three phases. In Phase 1, behavior was maintained by oral alcohol under a fixed-ratio schedule. In Phase 2, behavior was extinguished via condensed milk delivery under a differential-reinforcement-of-other-behavior (DRO) schedule. In Phase 3, the DRO schedule was eliminated. In Experiment 1, this 3-phase cycle was replicated 4 times. Across replications, response rates and dose of alcohol consumed did not differ in Phase 1, alcohol-maintained behavior was eliminated more rapidly in Phase 2, and the resurgence effect was generally stable in Phase 3. In Experiment 2, naltrexone was administered in Phase 2, Phase 3, or both Phases 2 and 3, to separate groups of rats. Naltrexone facilitated the elimination of alcohol-maintained behavior in Phase 2 and, the resurgence of alcohol-maintained behavior was reduced only for those rats that received naltrexone in both phases. Together, these experiments demonstrate that the resurgence of alcohol-maintained behavior is replicable within-subjects and, further, resurgence of alcohol-maintained behavior may be a useful model to evaluate pharmacological interventions to facilitate behavioral treatments and reduce the likelihood of relapse. Results with naltrexone support the use of medication-assisted therapy approaches to reduce relapse risk in patients. (PsycINFO Database Record (c) 2020 APA, all rights reserved).

Infralimbic cortex activity is required for the expression but not the acquisition of conditioned safety

The ability to discriminate between danger and safety is crucial for survival across species. Whereas danger signals predict the onset of a potentially threatening event, safety signals indicate the non-occurrence of an aversive event, thereby reducing fear and stress responses. While the neural basis of conditioned safety remains to be elucidated, fear extinction studies provide evidence that the infralimbic cortex (IL) modulates fear inhibition. In the current study, the IL was temporarily inactivated with local muscimol injections in male and female rats. The effect of IL inactivation on the acquisition and expression of conditioned safety was investigated utilizing the startle response. Temporary inactivation of the IL prior to conditioning did not affect the acquisition of conditioned safety, whereas IL inactivation during the expression test completely blocked the expression of conditioned safety in male and female rats. Inactivation of the neighboring prelimbic (PL) cortex during the expression test did not affect the expression of safety memory. Our findings suggest that the IL is a critical brain region for the expression of safety memory. Because patients suffering from anxiety disorders are often unable to make use of safety cues to inhibit fear, the present findings are of clinical relevance and could potentially contribute to therapy optimization of anxiety-related psychiatric disorders.

Operant Social Reward Decreases Incubation of Heroin Craving in Male and Female Rats

BACKGROUND

We recently reported that operant social choice-induced voluntary abstinence prevents incubation of methamphetamine craving. Here, we determined whether social choice-induced voluntary abstinence would prevent incubation of heroin craving. We also introduce a fully automatic social reward self-administration model that eliminates the intense workload and rat-human interaction of the original semiautomatic model.

METHODS

In experiment 1, we trained male and female rats for social self-administration (6 days) and then for heroin self-administration (12 days). Next, we assessed relapse to heroin seeking after 1 and 15 abstinence days. Between tests, the rats underwent either forced or social choice-induced abstinence. In experiment 2, we developed a fully automatic social self-administration procedure by introducing a screen between the self-administration chamber and the social-peer chamber; the screen allows physical contact but prevents rats from crossing chambers. Next, we compared incubation of craving in rats with a history of standard (no-screen) or automatic (screen) social self-administration and social choice-induced abstinence.

RESULTS

The time-dependent increase in heroin seeking after cessation of drug self-administration (incubation of craving) was lower after social choice-induced abstinence than after forced abstinence. There were no differences in social self-administration, social choice-induced abstinence, and incubation of craving in rats trained in the standard semiautomatic procedure versus the novel fully automatic procedure.

CONCLUSIONS

Our study demonstrates the protective effect of rewarding social interaction on heroin self-administration and incubation of heroin craving and introduces a fully automatic social self-administration and choice procedure to investigate the role of volitional social interaction in drug addiction and other psychiatric disorders.