Contributions of prelimbic cortex, dorsal and ventral hippocampus, and basolateral amygdala to fear return induced by elevated platform stress in rats.

Fear relapse is a major challenge in the treatment of stress-related mental disorders. Most investigations have focused on fear return induced by stimuli associated with the initial fear learning, while little attention has been paid to fear return evoked after exposure to an unconditioned stressor. This study explored the neural mechanisms of fear return induced by elevated platform (EP) stressor in Sprague-Dawley rats initially subjected to auditory fear conditioning. The contributions of the prelimbic cortex (PL), dorsal hippocampus (DH), ventral hippocampus (VH), and basolateral amygdala (BLA) were examined by targeted bilateral intracerebral injection of the GABAA agonist muscimol after elevated platform (EP) stressor. Muscimol-induced inactivation of PL or BLA significantly impaired the return of conditioning fear, while inactivation of the DH or VH had no effect. These results suggest that fear return induced by non-associative stressor may depend on the PL and BLA but not on the hippocampus.

Melanin-concentrating hormone in rat nucleus accumbens or lateral hypothalamus differentially impacts morphine and food seeking behaviors.

BACKGROUND: Identifying neural substrates that are differentially affected by drugs of abuse and natural rewards is key to finding a target for an efficacious treatment for substance abuse. Melanin-concentrating hormone is a polypeptide with an inhibitory effect on the mesolimbic dopamine system. Here we test the hypothesis that melanin-concentrating hormone in the lateral hypothalamus and nucleus accumbens shell is differentially involved in the regulation of morphine and food-rewarded behaviors. METHODS: Male Sprague-Dawley rats were trained with morphine (5.0 mg/kg, subcutaneously) or food pellets (standard chow, 10-14 g) to induce a conditioned place preference, immediately followed by extinction training. Melanin-concentrating hormone (1.0 µg/side) or saline was infused into the nucleus accumbens shell or lateral hypothalamus before the reinstatement primed by morphine or food, and locomotor activity was simultaneously monitored. As the comparison, melanin-concentrating hormone was also microinjected into the nucleus accumbens shell or lateral hypothalamus before the expression of food or morphine-induced conditioned place preference. RESULTS: Microinfusion of melanin-concentrating hormone into the nucleus accumbens shell (but not into the lateral hypothalamus) prevented the reinstatement of morphine conditioned place preference but had no effect on the reinstatement of food conditioned place preference. In contrast, microinfusion of melanin-concentrating hormone into the lateral hypothalamus (but not in the nucleus accumbens shell) inhibited the reinstatement of food conditioned place preference but had no effect on the reinstatement of morphine conditioned place preference. CONCLUSIONS: These results suggest a clear double dissociation of melanin-concentrating hormone in morphine/food rewarding behaviors and melanin-concentrating hormone in the nucleus accumbens shell. Melanin-concentrating hormone could be a potential target for therapeutic intervention for morphine abuse without affecting natural rewards.

The effect of traumatic-like stress exposure on alterations in the temporal social behavior of a rodent population.

Though the relationship between traumatic stress and social behavior, which has been explored for years, is dynamic and largely estimated between dyads, little is known about the causal effects of traumatic stress exposure on the time-dependent dynamic alterations in the social behaviors on a large-group level. We thus investigated the effect of a single prolonged stress (SPS) exposure, a classical animal model that recapitulates posttraumatic stress disorder (PTSD)-like symptoms in rodents, on the spatiotemporal, social behavior changes within a large group of cohabiting rats. One-half of thirty-two Sprague-Dawley rats were assigned to the experimental group and subjected to SPS treatment administered two weeks after baseline social behavior recording; the other half served as the controls. Each group of rats (n = 16) was housed in one of two large custom-made cylinders. We used an automatic tracking system to record the behavioral indices of social behavior of the rats before SPS exposure, on the SPS exposure day, during a 7-day-long quiescent period after SPS treatment, as well as during subsequent behavioral test days. In addition to SPS-induced PTSD-like behaviors, SPS induced a time-dependent, oscillating change in active/passive social behaviors that lasted for 3 weeks. SPS treatment decreased active social behaviors (especially affiliative behaviors) but increased passive social behaviors (e.g. huddling) immediately following stress exposure. Increased active social interactions were observed during the early phase after SPS treatment; while increased passive social behaviors were observed during the late phase after SPS treatment. These dynamic changes were repeatedly observed when the rats underwent subsequent stressful behavioral tests and challenges. SPS induced a long-term, time-dependent oscillating change in indices of the social behavior. These changes may serve as an adaptive mechanism, and their manifestations critically depended on the time course following the traumatic stress exposure.

The effects of acute stress on consummatory and motivational responses for sucrose in rats after long-term withdrawal from morphine.

BACKGROUND: Negative affective states, e.g., anhedonia, may be linked to the long-lasting motivational processes associated with relapse. Here, this study investigated whether, and how, anhedonic states are influenced by stressful events that contribute to craving and relapse. METHODS: All male rats were pretreated with a binge-like morphine paradigm for five days. After 12 to 16 days of withdrawal, rats were subjected to a one-hour free consumption test or three operant tasks with increasing cost/benefit ratio, i.e., fixed ratio 1 (FR1), progressive ratio (PR), and PR-punishment procedure of reinforcement, with sucrose solutions of three concentrations (4%, 15%, and 60%) as rewards. The consumption and operant responses under FR1 and PR procedures were measured following exposure to acute foot-shock stress (intermittent foot shock, 0.5 mA × 0.5 s × 10 min; mean intershock interval, 40 s), and the operant responses for 60% sucrose solution under PR-punishment procedure was measured following a forced-swim stress (5 minutes). RESULT: Foot-shock stress increased water consumption in a subpopulation of rats and decreased consumption of sucrose solutions, while it did not influence the operant responses for sucrose solutions under either FR1 or PR procedure. The forced-swim stress reduced operant responses for 60% sucrose solution under PR-punishment procedure, but did not influence responding for 60% sucrose solution under PR procedure. In addition, the forced-swim stress also elevated anxiety level of rats in an open area test. CONCLUSIONS: Acute stress induced hedonic but not motivational deficit for sucrose reward in protracted drug-abstinent animals. Additional negative emotional states besides anhedonia were evoked by acute stress.

A Conflict Model of Reward-seeking Behavior in Male Rats.

The present protocol describes a novel conflict task as a model of inhibitory control in rats. In this model, a natural rewarding stimulus (sexual stimulus) that represents a high-value reward, and the aversive stimuli (pins), are concurrently presented. The male rats have to climb or jump over the obstacle full of pins to approach and investigate the sexual partner. If the animal persists in their approaching behavior regardless of the aversive stimuli, it is considered as a maladaptive or risky reward-seeking behavior. The conflict task permits the evaluation of deficit in inhibitory control resulting from exposure to abused drug, such as morphine, or a stressful event. The main advantage of this model is that it provides a simple and quick way to discover the deficit in inhibitory control after exposure to opiate drugs or other stressful events. In addition to opiates, this behavioral model would also be useful for quickly discovering the inhibitory control deficits induced by other addictive drugs. However, the limitation is that the male rats' performance may be subject to exercising effects with repeated testing under this conflict task. In the future, one can hope that the individuals with the compulsive phenotype of reward-seeking behavior after exposure to opiates will be identified based on modifying this conflict model.

Acute stress worsens the deficits in appetitive behaviors for social and sexual stimuli displayed by rats after long-term withdrawal from morphine.

RATIONALE: Negative affective states, e.g., anhedonia, are suggested to be involved in the long-lasting motivational processes associated with relapse. Here, we investigated whether anhedonic behaviors could be elicited by an acute stress after protracted abstinence from morphine. OBJECTIVES: The behavioral responses to natural stimuli following exposure to an acute stress were examined after 14 days of withdrawal from morphine. Male rats were pretreated with either a binge-like morphine regimen or daily saline injections for 5 days. The motivation for two natural stimuli, i.e., a social stimulus (male rat) and a sexual stimulus (estrous female rat), was measured, following exposure to an acute stress (intermittent foot shock, 0.5 mA * 0.5 s * 10 min; mean inter-shock interval 40 s), under three conditions: free approach and effort- and conflict-based approaches. RESULTS: Foot-shock-induced stress did not influence free-approach behavior (sniffing time) towards the social or sexual stimulus. However, in the effort-based approach task, the stressed morphine-withdrawn rats demonstrated an attenuated motivation to climb over a partition to approach the social stimulus while the stressed saline-pretreated rats showed an increased motivation to approach the social stimulus. When an aversive stimulus (pins) was introduced in order to induce an approach-avoidance conflict, both drug-withdrawn and drug-naïve groups exhibited a bimodal distribution of approach behavior towards the sexual stimulus after the stress was introduced, i.e., the majority of rats had low risky appetitive behaviors but a minority of them showed rather highly "risky" approach behavior. CONCLUSIONS: The acute stress induces differential motivational deficits for social and sexual rewards in protracted drug-abstinent rats.

The consummatory and motivational behaviors for natural rewards following long-term withdrawal from morphine: no anhedonia but persistent maladaptive behaviors for high-value rewards.

RATIONALE: The negative affective state, e.g., anhedonia, emerges after abstinence from abused drugs may be linked to the motivational processes of drug craving and relapse. Although anhedonia diminishes over time with drug abstinence, it is not yet rather explicit whether anhedonia exists or not following protracted withdrawal. OBJECTIVES: The behavioral responses to natural rewards were examined after 2 to 3 weeks withdrawal from morphine. Male rats were pretreated with either a binge-like morphine paradigm or daily saline injection for 5 days. The consummatory and motivational behaviors for three natural rewards (sucrose solutions 4, 15, and 60%, social stimulus: male rat, and sexual stimulus: estrous female rat) were examined under varied testing conditions. RESULTS: The morphine-withdrawn rats significantly increased their intake of 15% sucrose solution during the 1-h consumption test and their operant responding for 15% sucrose solution under a progressive ratio (PR) schedule of reinforcement. When obtaining a reinforcer was associated with a 0.5 mA foot shock under a PR-punishment schedule, the morphine-withdrawn rats showed a higher performance for 60% sucrose solution. Meanwhile, the morphine-withdrawn rats displayed a higher motivation to sexual stimulus during the free-approach test and more approaching behaviors towards sexual stimulus in a conflict-based approach test (concurrent presence of reward and aversive stimulus). CONCLUSIONS: No anhedonia-like behavior but sensitized behaviors for natural rewards were found after long-term morphine withdrawal. Notably, the morphine-withdrawn rats displayed persistent motivated behaviors for high-value rewards (60% sucrose and sexual stimulus) in the conflict tests suggesting impairments in inhibitory control in morphine-treated rats.

Region-specific roles of the prelimbic cortex, the dorsal CA1, the ventral DG and ventral CA1 of the hippocampus in the fear return evoked by a sub-conditioning procedure in rats.

The return of learned fear is an important issue in anxiety disorder research since an analogous process may contribute to long-term fear maintenance or clinical relapse. A number of studies demonstrate that mPFC and hippocampus are important in the modulation of post-extinction re-expression of fear memory. However, the region-specific role of these structures in the fear return evoked by a sub-threshold conditioning (SC) is not known. In the present experiments, we first examined specific roles of the prelimbic cortex (PL), the dorsal hippocampus (DH, the dorsal CA1 area in particular), the ventral hippocampus (the ventral dentate gyrus (vDG) and the ventral CA1 area in particular) in this fear return process. Then we examined the role of connections between PL and vCA1 with this behavioral approach. Rats were subjected to five tone-shock pairings (1.0-mA shock) to induce conditioned fear (freezing), followed by three fear extinction sessions (25 tone-alone trials each session). After a post-test for extinction memory, some rats were retrained with the SC procedure to reinstate tone-evoked freezing. Rat groups were injected with low doses of the GABAA agonist muscimol to selectively inactivate PL, DH, vDG, or vCA1 120 min before the fear return test. A disconnection paradigm with ipsilateral or contralateral muscimol injection of the PL and the vCA1 was used to examine the role of this pathway in the fear return. We found that transient inactivation of these areas significantly impaired fear return (freezing): inactivation of the prelimbic cortex blocked SC-evoked fear return in particular but did not influence fear expression in general; inactivation of the DH area impaired fear return, but had no effect on the extinction retrieval process; both ventral DG and ventral CA1 are required for the return of extinguished fear whereas only ventral DG is required for the extinction retrieval. These findings suggest that PL, DH, vDG, and vCA1 all contribute to the fear return and connections between PL and vCA1 may be involved in the modulation of this process.

Post-training corticosterone inhibits the return of fear evoked by platform stress and a subthreshold conditioning procedure in Sprague-Dawley rats.

The return of fear is an important issue in anxiety disorder research. Each time a fear memory is reactivated, it may further strengthen overactivation of the fear circuit, which may contribute to long-term maintenance of the fear memory. Recent evidence indicates that glucocorticoids may help attenuate pathological fear, but its role in the return of fear is unclear. In the present study, systemic corticosterone (CORT; 25mg/kg) administration 1h after fear conditioning did not impair the consolidation process but significantly suppressed the return of fear evoked by a subthreshold conditioning (SC) procedure and elevated platform (EP) stress. Compared with the SC-induced return of fear, acute stress-induced return was state-dependent. In addition, post-training CORT treatment increased the adrenocorticotropic response after EP stress, which indicates that the drug-induced suppression of the return of fear may possibly derive from its regulation effect of the hypothalamic-pituitary-adrenal axis reactivity to stress. These results suggest that post-training CORT administration may help inhibit the return of fear evoked by EP or SC stress. The possible mechanisms involved in the high-dose CORT-induced suppression of the SC- and EP-induced return of fear are discussed.

Integration of animal behaviors under stresses with different time courses.

We used animal models of "forced swim stress" and "chronic unpredictable stress", and tried to reveal whether a passive coping style of high flotation behavior in forced swim stress predicts anhedonia behavior after chronic unpredictable stress, and whether the dopamine system regulates floating and anhedonia behaviors. Our results confirmed that depression-prone rats use "floating behavior" as a coping strategy in forced swim stress and more readily suffer from anhedonia during chronic unpredictable stress. Intraperitoneal injection or nucleus accumbens microinjection of the dopamine 2/3 receptor subtype agonist ropinirole reduced floating behaviors in depression-prone animals, but increased sucrose preference in rats showing anhedonia. These data indicate that floating behavior is a defensive mode that is preferred by susceptible individuals under conditions of acute stress. Simultaneously, these animals more readily experienced anhedonia under long-term stress; that is, they were more readily affected by depression. Our results suggest that dopamine 2/3 receptor subtypes in the nucleus accumbens play an important role in floating behaviors and anhedonia.

N-methyl-D-aspartate receptor-mediated glutamate transmission in nucleus accumbens plays a more important role than that in dorsal striatum in cognitive flexibility.

Cognitive flexibility is a critical ability for adapting to an ever-changing environment in humans and animals. Deficits in cognitive flexibility are observed in most schizophrenia patients. Previous studies reported that the medial prefrontal cortex-to-ventral striatum and orbital frontal cortex-to-dorsal striatum circuits play important roles in extra- and intra-dimensional strategy switching, respectively. However, the precise function of striatal subregions in flexible behaviors is still unclear. N-methyl-D-aspartate receptors (NMDARs) are major glutamate receptors in the striatum that receive glutamatergic projections from the frontal cortex. The membrane insertion of Ca(2+)-permeable α-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid receptors (AMPARs) depends on NMDAR activation and is required in learning and memory processes. In the present study, we measured set-shifting and reversal learning performance in operant chambers in rats and assessed the effects of blocking NMDARs and Ca(2+)-permeable AMPARs in striatal subregions on behavioral flexibility. The blockade of NMDARs in the nucleus accumbens (NAc) core by AP5 impaired set-shifting ability by causing a failure to modify prior learning. The suppression of NMDAR-mediated transmission in the NAc shell induced a deficit in set-shifting by disrupting the learning and maintenance of novel strategies. During reversal learning, infusions of AP5 into the NAc shell and core impaired the ability to learn and maintain new strategies. However, behavioral flexibility was not significantly affected by blocking NMDARs in the dorsal striatum. We also found that the blockade of Ca(2+)-permeable AMPARs by NASPM in any subregion of the striatum did not affect strategy switching. These findings suggest that NMDAR-mediated glutamate transmission in the NAc contributes more to cognitive execution compared with the dorsal striatum.

High-dose corticosterone after fear conditioning selectively suppresses fear renewal by reducing anxiety-like response.

Exposure therapy is widely used to treat anxiety disorders, including posttraumatic stress disorder (PTSD). However, preventing the return of fear is still a major challenge after this behavioral treatment. An increasing number of studies suggest that high-dose glucocorticoid treatment immediately after trauma can alleviate the symptoms of PTSD in humans. Unknown is whether high-dose glucocorticoid treatment following fear conditioning suppresses the return of fear. In the present study, a typical fear renewal paradigm (AAB) was used, in which the fear response to an auditory cue can be restored in a novel context (context B) when both training and extinction occur in the same context (context A). We trained rats for auditory fear conditioning and administered corticosterone (CORT; 5 and 25mg/kg, i.p.) or vehicle with different delays (1 and 24h). Forty-eight hours after drug injection, extinction was conducted with no drug in the training context, followed by a test of tone-induced freezing behavior in the same (AAA) or a shifted (AAB) context. Both immediate and delayed administration of high-dose CORT after fear conditioning reduced fear renewal. To examine the anxiolytic effect of CORT, independent rats were trained for cued or contextual fear conditioning, followed by an injection of CORT (5 and 25mg/kg, i.p.) or vehicle at a 1 or 24h delay. One week later, anxiety-like behavior was assessed in the elevated plus maze (EPM) before and after fear expression. We found that high-dose CORT decreased anxiety-like behavior without changing tone- or context-induced freezing. These findings indicate that a single high-dose CORT administration given after fear conditioning may selectively suppress fear renewal by reducing anxiety-like behavior and not by altering the consolidation, retrieval, or extinction of fear memory.

Complex motivated behaviors for natural rewards following a binge-like regimen of morphine administration: mixed phenotypes of anhedonia and craving after short-term withdrawal.

The anhedonia-like behaviors following about 1-week withdrawal from morphine were examined in the present study. Male rats were pretreated with either a binge-like morphine paradigm or daily saline injection for 5 days. Three types of natural reward were used, food reward (2.5, 4, 15, 30, 40, and 60% sucrose solutions), social reward (male rat) and sexual reward (estrous female rat). For each type of natural stimulus, consummatory behavior and motivational behaviors under varied testing conditions were investigated. The results showed that the morphine-treated rats significantly reduced their consumption of 2.5% sucrose solution during the 1-h consumption testing and their operant responding for 15, 30, and 40% sucrose solutions under a fixed ratio 1 (FR1) schedule. However, performance under a progressive ratio (PR) schedule increased in morphine-treated rats reinforced with 60% sucrose solution, but not in those reinforced with sucrose concentrations lower than 60%. Pretreatment with morphine significantly decreased the male rats' ejaculation frequency (EF) during the 1-h copulation testing, and impaired the maintenance of appetitive motivations to sexual and social stimuli under a free-approach condition. Moreover, the morphine-treated rats demonstrated a diminished motivation to approach social stimulus in the effort-based appetitive behavior test but showed a remarkable increase in motivation to approach sexual stimulus in the risky appetitive behavior test. These results demonstrated some complex motivated behaviors following about 1 week of morphine withdrawal: (1) The anhedonia-like behavior was consistently found in animals withdrawn from morphine. However, for a given reward, there was often a dissociation of the consummatory behaviors from the motivational behaviors, and whether the consummatory or the motivational anhedonia-like behaviors could be discovered heavily depended on the type and magnitude of the reward and the type of testing task; (2) These anhedonia-like behaviors coexisted with a craving for the high-incentive reward which was evidenced by the increased PR performance for the 60% sucrose solution and the heightened risky appetitive behavior for the sexual stimulus. The craving for the high-incentive reward alongside with the impaired inhibitory control in drug-withdrawn subjects might form one of psychological mechanisms underlying drug relapse after withdrawal.

Mineralocorticoid receptors in the ventral hippocampus are involved in extinction memory in rats.

Fear extinction decreases conditioned fear responses that normally occur when a conditioned stimulus (CS) is repeatedly presented in the absence of an aversive unconditioned stimulus (US), which is the behavioral basis of exposure therapy for posttraumatic stress disorder (PTSD). However, knowledge about the neurobiology of extinction is insufficient. The present study investigated changes in the protein expression of mineralocorticoid receptors (MRs) and glucocorticoid receptors (GRs) in the dorsal hippocampus (DH)and ventral hippocampus (VH), induced by extinction and return of conditioned fear responses to an auditory signal. Western blot analysis indicated that shock stress enhanced the expression of MRs in only the DH, whereas extinction selectively increased the expression of MRs in the VH. The infusion of MRs antagonist spironolactone in the VH indicated that MRs in the VH did not affect the retrieval of fear and extinction memories, but facilitated the formation of extinction memory. However, no changes in GRs in either the DH or VH were observed in each phase of auditory fear conditioning. These results suggest that MRs in the DH and VH have differential functions in the extinction of auditory fear conditioning. MRs in the DH appear to be related to only stressful experiences, whereas MRs in the VH are involved in extinction formation. The enhancement of MRs in the VH might be necessary to improve PTSD.

Altered phosphorylation of GluA1 in the striatum is associated with locomotor sensitization induced by exposure to increasing doses of morphine.

Escalation of drug consumption is involved in the transition from drug use to addiction. Our previous study demonstrated that neuronal activation in ventral tegmental area (VTA) and substantia nigra (SN) was associated with behavioral sensitization induced by increasing doses of morphine. Here we sought to characterize the molecular mechanism underlying this behavioral sensitization. We compared mitogen-activated protein kinase (MAPK) signaling following pretreatment with either increasing doses or fixed doses of morphine before and after behavioral sensitization. We found phospho-MAPK markedly increased in ventral striatum and decreased in dorsal striatum after either pretreatment group, but no further change after sensitization induced by 10mg/kg morphine challenge. Furthermore, we also evaluated the level of phospho-glutamate receptor 1 at serine 845 (pSer845-GluA1) and 831 (pSer831-GluA1) sites in ventral striatum and dorsal striatum. The results showed a significant increase in pSer845-GluA1/GluA1 ratio in ventral striatum but not dorsal striatum after pretreatment with increasing doses of morphine but not after fixed-dose or saline pretreatment. Importantly, pSer845-GluA1/GluA1 ratio was increased exclusively in dorsal striatum and not ventral striatum following acute morphine challenge specifically paired with increasing-dose pretreatment and not fixed-dose or saline. These findings indicate that behavioral sensitization-induced by chronic pretreatment with increasing doses of morphine might be more closely associated with the dynamic GluA1 activity in the striatum rather than the modulation of MAPK signaling. These findings also indicate that GluA1 phosphorylation may occur independent of MAPK activation.

Prefrontal high-frequency stimulation prevents sub-conditioning procedure-provoked, but not acute stress-provoked, reemergence of extinguished fear.

We have recently shown that post-extinction exposure of rats to a sub-conditioning procedure (SCP, i.e., retraining with a shock intensity that is too weak to induce by itself significant fear conditioning) or to acute stress provokes reemergence of extinguished fear. Furthermore, this SCP effect can be abolished by high-frequency stimulation (HFS) of the medial prefrontal cortex (mPFC), when applied following the SCP. The aim of the present study was to test whether HFS of the mPFC is effective in preventing both SCP-induced and acute stress-provoked fear reemergence. Rats implanted with stimulating electrodes in the mPFC were trained to acquire high levels of freezing to conditioned auditory cue. This fear response was then extinguished. Three weeks later, no spontaneous recovery was observed, but rats exposed to either the SCP or acute stress again exhibited high levels of freezing. HFS of the mPFC, applied before provoking fear reemergence, prevented the effects of SCP, but not acute stress. These data suggest that acute stress may have more impact on functions of the mPFC and/or associated structures than a situational reminder of fear conditioning.

Post-extinction fluoxetine treatment prevents stress-induced reemergence of extinguished fear.

RATIONALE: The post-extinction exposure of rats to a sub-conditioning procedure (SCP; i.e., retraining with a shock intensity that is too weak to induce by itself significant fear conditioning) has been reported to provoke the reemergence of extinguished fear. This phenomenon can be prevented by chronic fluoxetine treatment. OBJECTIVES: We sought to examine another potential inducer of fear reemergence, acute stress, in rats and determine whether fluoxetine prevents this phenomenon. METHODS: Because in previous studies fluoxetine was administered before extinction, we first analyzed its effect on the SCP-associated reemergence of auditory-cued conditioned fear in rats injected after extinction to avoid any interaction between fluoxetine and extinction learning. Next, we used the same protocol but replaced the SCP with acute stress. RESULTS: We found that the SCP and acute stress, which were carried out 3 weeks after fear extinction, similarly provoked the reemergence of extinguished fear in rats injected with vehicle during the 3-week period. In contrast, the animals treated with fluoxetine during this period behaved similarly to those not exposed to an inducer of fear reemergence. CONCLUSIONS: Our data establish acute stress as an inducer of fear reemergence. The results provide further support for the hypothesis that fluoxetine interfered with mechanisms that reactivated extinguished fear, even when administered after fear extinction.

Corticosterone combined with intramedial prefrontal cortex infusion of SCH 23390 impairs the strong fear response in high-fear-reactivity rats.

Accumulating evidence suggests that stress-dose corticosteroids impair fear memory in animals and humans. Corticosteroid treatment after critical illness is seen as a potential psychotropic medication by which to prevent posttraumatic stress disorder. However, individual difference in the responsiveness to stress (i.e., stress reactivity) is a factor that modulates the efficacy of corticosteroids. To understand the contribution of fear reactivity to the effect of post-stress corticosterone, male Sprague-Dawley rats were subjected to classical tone-cued fear conditioning and separated into high and low reactivity (HR and LR, respectively) responder groups based on their levels of freezing during conditioning. The HR rats showed significantly higher fear responses than the LR rats during conditioning as assessed by freezing behavior. At two intervals, 30 min and 48 hr later, the HR rats still displayed more pronounced conditioned responses to cued stimuli compared with the LR rats. Moreover, in contrast to the LR rats, the enhanced fear response in the HR rats was difficult to attenuate by post-training high-dose corticosterone. These results suggest that fear reactivity results in stronger fear memory, and that it is difficult to disrupt this strong fear memory in the HR phenotype using monotherapy. However, the strong fear memory in the HR rats was impaired by concurrent intramedial prefrontal cortex infusion of a high dose of the dopamine D1 receptor antagonist SCH 23390 and systemic administration of corticosterone. SCH 23390 and corticosterone alone did not decrease freezing levels in the HR rats. The fear impairment induced by SCH 23390 combined with corticosterone was not attributable to the effect of these drugs on locomotor activity. This effect was not found with administration of the D2 antagonist eticlopride combined with corticosterone. Our findings demonstrate that the conditioned fear memory in individuals with high stress reactivity is difficult to disrupt using monotherapy, but that combined pharmacotherapy may be useful for treating intervention-resistant fear.

Differential patterns of neuronal activation in rostral versus caudal ventral tegmental area involved in behavioral sensitization induced by an escalating-dose morphine administration paradigm.

The transition to addiction often involves a gradual process of escalated drug intake. The purpose of the present study was to characterize neuronal activation in the ventral tegmental area (VTA) and substantia nigra (SN) following chronic escalating-dose morphine exposure (days 1-7, 2 mg/kg/d; days 8-21, beginning at 10 mg/kg/d, increasing by 2 mg/kg/d), with steady-dose morphine (2 mg/kg/d, i.p., for 21 days) as the comparison. Using immunohistochemical double-staining for tyrosine hydroxylase (TH) and Fos, we found that the number of Fos(+)TH(+) neurons in the rostral VTA and number of Fos(+)TH(-) neurons in the lateral SNr were significantly increased in escalating-dose morphine-treated rats compared with steady-dose morphine-treated rats and acute morphine-treated rats. Meanwhile, this increase was associated with robust expression of behavioral sensitization after a challenge with 10 mg/kg morphine. The number of Fos(+)TH(+) neurons was significantly increased by acute morphine in the caudal VTA and SNc, but this number did not increase further with morphine pretreatment. These results demonstrate that behavioral sensitization was associated with elevated activation of dopaminergic neurons in the rostral VTA and nondopaminergic neurons in the lateral SNr, which could only be induced by chronic escalating-dose morphine rather than chronic steady-dose morphine pretreatment.

Roles of BDNF, dopamine D(3) receptors, and their interactions in the expression of morphine-induced context-specific locomotor sensitization.

Drug seeking, craving, and relapse can be triggered by environmental stimuli that acquire motivational salience through repeated associations with the drug's effects. Previous studies indicated that the dopamine D(3) receptor (Drd3) might be involved in the expression of drug-conditioned responses in rats, and brain-derived neurotrophic factor (BDNF) could modulate Drd3 expression in the nucleus accumbens (NAc). However, the involvement of neural regions with Drd3 activation and the underlying interaction between BDNF and Drd3 in the expression of behavioral responses controlled by a drug-associated environment have remained poorly understood. The present study used a conditioning procedure to assess the roles of BDNF, Drd3, and their interactions in the NAc in the expression of morphine-induced context-specific locomotor sensitization. We showed that the expression of locomotor sensitization in the morphine-paired environment was accompanied by significantly increased expression of Drd3 mRNA and BDNF mRNA and protein levels. Both sensitized locomotion in morphine-paired rats and enhanced Drd3 mRNA were suppressed by intra-NAc infusion of anti-tyrosine kinase receptor B (TrkB) IgG. Furthermore, intra-NAc infusion of the Drd3-selective antagonist SB-277011A significantly decreased the expression of context-specific locomotor sensitization and upregulated BDNF mRNA. Altogether, these results suggest that BDNF/TrkB signaling and activation of Drd3 in the NAc are required for the expression of morphine-induced context-specific locomotor sensitization.