Contextual Fear Paradigm Research Articles

Phosphodiesterase-2 inhibitor reverses post-traumatic stress induced fear memory deficits and behavioral changes via cAMP/cGMP pathway

Phosphodiesterase 2 is one of the phosphodiesterase (PDEs) family members that regulate cyclic nucleotide (namely cAMP and cGMP) concentrations. The present study determined whether PDE2 inhibition could rescue post-traumatic stress disorder (PTSD)-like symptoms. Mice were subjected to single prolonged stress (SPS) and treated with selective PDE2 inhibitor Bay 60–7550 (0.3, 1, or 3 mg/kg, i.p.). The behavioral tests such as forced swimming, sucrose preference test, open field, elevated plus maze, and contextual fear paradigm were conducted to determine the effects of Bay 60–7550 on SPS-induced depression- and anxiety-like behavior and fear memory deficits. The results suggested that Bay 60–7550 reversed SPS-induced depression- and anxiety-like behavior and fear memory deficits. Moreover, Bay 60–7550 prevented SPS-induced changes in the adrenal gland index, synaptic proteins synaptophysin and PSD95 expression, PKA, PKG, pCREB, and BDNF levels in the hippocampus and amygdala. These effects were completely prevented by PKG inhibitor KT5823. While PKA inhibitor H89 also prevented Bay 60-7550-induced pCREB and BDNF expression, but only partially prevented the effects on PSD95 expression in the hippocampus. These findings suggest that Bay 60–7550 protects mice against PTSD-like stress induced traumatic injury by activation of cGMP- or cAMP-related neuroprotective molecules, such as synaptic proteins, pCREB and BDNF.

Lycopene ameliorates PTSD-like behaviors in mice and rebalances the neuroinflammatory response and oxidative stress in the brain

Posttraumatic stress disorder (PTSD) is a mental disorder that can translate into severe economic problems. Lycopene is an aliphatic hydrocarbon carotenoid extracted from plants, including papayas, tomatoes, and water melons. Previous studies have shown that lycopene can produce antidepressant-like effects in rodent models of depression. However, little is known about its anti-PTSD-like effect. This was addressed in the present study by using the single prolonged stress (SPS) protocol to induce PTSD-like behavioral deficits in mice. Our results showed that 12 days of lycopene treatment at the dose of 10 and 20 mg/kg, but not at 5 mg/kg, ameliorated the PTSD-like phenotype induced by SPS, including the increase in freezing time in contextual fear paradigm, the decrease in time and entries in open arms in elevated plus maze test, and the decrease in distance and time in the central area of the open field test, without affecting the mouse locomotor activity. Mechanistic studies revealed that lycopene treatment (20 mg/kg, 12 days) could suppress the SPS-induced increase in levels of interleukin-6 (IL-6), IL-1β, tumor necrosis factor-α (TNF-α), and nitrite in the hippocampus and prefrontal cortex in mice, as well as the increased markers that indicate high levels of oxido-nitrosative stress in the hippocampus and prefrontal cortex in SPS mice. Lycopene treatment (20 mg/kg, 12 days) also suppressed the SPS-induced decrease in brain derived neurotrophic factor (BDNF) levels in the hippocampus and prefrontal cortex in mice. Overall, the anti-PTSD-like effect of lycopene may be associated with its anti-neuroinflammation and anti-oxidative stress activities.

Male mice placed on a ketogenic diet from postnatal day (P) 21 through adulthood have reduced growth, are hypoactive, show increased freezing in a conditioned fear paradigm, and have spatial learning deficits

The ketogenic diet (KD) is a non-pharmacological treatment for specific types of epilepsy. In addition, it has been shown to be effective in mitigating other neurologic disorders. The KD is also effective in reducing body mass, leading to an increase in use by the general population for weight loss. As the popularity of the clinical and general use of the KD has increased, it is important to develop adequate mouse models to better understand the effects of the KD in both normal and diseased states. Many times, the best outcome for disorders treatable with the KD would be achieved by commencing treatment in early life. Few studies have evaluated the cognitive effect of starting the KD in early life. To better understand these effects, male C57BL6/J mice were placed on a KD from postnatal day (P) 21 through young adulthood (~P90). KD-fed mice had increased blood ketone levels, reduced blood glucose, and reduced weight gain versus mice fed a control diet (CD). The weight loss in the KD-fed mice was not accompanied by a change in body fat percentage, suggesting that there was a loss of lean mass. Behavioral testing began on P60 while the mice were still on the diet. KD-fed mice were hypoactive with CD-fed mice. In the Morris water maze, KD-fed mice showed decreased path efficiency, suggesting a spatial learning deficits. No differences were observed in spatial memory or in novel object recognition memory. In a contextual and conditioned fear paradigm, the KD-fed mice had an increase in freezing behavior. These data suggest that early-life exposure to a KD leads to impaired body composition and long-term cognitive changes.

The anxiolytic-like effects of puerarin on an animal model of PTSD

Post traumatic stress disorder (PTSD) is a mental illness that affected numerous people. The anti-PTSD-like effects of puerarin is unknown, although the antidepressant- and anxiolytic- like effects of puerarin have been reported. The PTSD behavioral deficits in rats were induced by single prolonged stress (SPS), mainly including the reduced time/entries in the open arms and the elevated time/entries in the closed arms in elevated plus maze test, increased freezing duration in contextual fear paradigm and lowered time/entries in the central zone in open field test. However, the behavioral deficits were attenuated by puerarin (50 and 100 mg/kg) without affecting the locomotor activity. For the evaluation of mechanism, the decreased levels of progesterone, allopregnanolone, and the increased levels of corticosterone, corticotropin releasing hormone, and adrenocorticotropic hormone in the brain or serum were induced by SPS, which is blocked by puerarin. In summary, the anti-PTSD-like effects of puerarin were associated with biosynthesis of neurosteroids and normalized levels of stress hormones in HPA axis.

The anxiolytic-like effects of ginsenoside Rg2 on an animal model of PTSD

Post traumatic stress disorder (PTSD) is one of the mental illness. The antidepressant-like properties of ginsenoside Rg2 (GRg2) have been shown, while little is known about its anti-PTSD-like effects. In the present study, the PTSD-associated behavioral deficits in rats were induced following exposure to single prolonged stress (SPS). The results showed that the decreased time and entries in the open arms in elevated plus maze test (EPMT) and increased freezing duration in contextual fear paradigm (CFP) were reversed by GRg2 (10 and 20 mg/kg) without affecting the locomotor activity. In addition, GRg2 (10 and 20 mg/kg) could block the decreased levels of progesterone, allopregnanolone, serotonin (5-HT), 5-Hydroxyindoleacetic acid (5-HIAA), corticotropin releasing hormone (CRH), corticosterone (Cort) and adrenocorticotropic hormone (ACTH) in the brain or serum. In summary, GRg2 alleviated the PTSD-associated behavioral deficits with biosynthesis of neurosteroids, normalization of serotonergic system and HPA axis dysfunction.

Anxiolytic-like effects of paeoniflorin in an animal model of post traumatic stress disorder.

Post-traumatic stress disorder (PTSD) is the serious psychiatric disorder. Paeoniflorin (PF) produces the antidepressant-like properties. However, few studies are concerned about its anti-PTSD-like effects and mechanisms. To investigate these, the single prolonged stress (SPS) model was utilized. PTSD-like behavioral deficits in rats after exposure to SPS were improved by PF (10 and 20mg/kg, i.p.), evidenced by blocking increased freezing time in contextual fear paradigm (CFP) and increased time and entries in open arms in elevated plus maze (EPM) test without affecting the locomotor activity in open field (OF) test. We also found that increased levels of corticosterone (Cort), corticotropin releasing hormone (CRH) and adrenocorticotropic hormone (ACTH) after exposure to SPS were reversed by PF (10 and 20mg/kg, i.p.) in serum, respectively. Moreover, the decreased levels of serotonin (5-HT) and 5-Hydroxyindoleacetic acid (5-HIAA) in prefrontal cortex and hippocampus were reversed by PF (10 and 20mg/kg, i.p.), respectively. In summary, the anti-PTSD-like activities of PF were associated with the modulation of HPA axis and 5-HT system activation.

Trans-Resveratrol ameliorates anxiety-like behaviors and fear memory deficits in a rat model of post-traumatic stress disorder

trans-Resveratrol, a natural polyphenol enriched in grape seed and skin, has been extensively investigated for its antioxidant, anti-inflammatory and anti-psychiatric properties. The present study examined the effects of trans-resveratrol on ameliorating anxiety-like behaviors and fear memory deficits induced by time-dependent sensitization (TDS) procedure, which is a classical animal model for mimicking posttraumatic stress disorder (PTSD). The results suggested that trans-resveratrol at doses of 10, 20 and 40 mg/kg (via gavage, i.g.) reversed TDS-induced decreases in the percentage of time spent in the center of arena, the open arm entries and time spent in the open arms in the open field and elevated plus maze tests. It also decreased the percentage of freezing time in the contextual fear paradigm that was increased in TDS treated rats. Further study suggested that TDS-induced abnormality in the limbic hypothalamus-pituitary-adrenal gland (L-HPA) axis was reversed by trans-resveratrol, i.e. it reversed increased adrenal gland index and corticotropin-releasing factor (CRF) levels, and rescued the differential expression of glucocorticoid receptor (GR) in the hypothalamus, hippocampus and amygdala. Neurobiological studies suggested that trans-resveratrol increased phosphorylation of cAMP response element binding protein (pCREB) and brain derived neurotrophic factor (BDNF) levels, which were decreased in rats subjected to TDS. These results provide compelling evidence that trans-resveratrol protects neurons against PTSD-like stress insults by regulation of L-HPA axis function and activation of downstream neuroprotective molecules, such as pCREB and BDNF expression.

(+)-Borneol suppresses conditioned fear recall and anxiety-like behaviors in mice

Fear- and anxiety-related psychiatric disorders have been one of the major chronic diseases afflicting patients for decades, and new compounds for treating such disorders remain to be developed. (+)-Borneol, a bicyclic monoterpene found in several species of Artemisia and Dipterocarpaceae, is widely used for anxiety, pain and anesthesia in Chinese medicine. Meanwhile, it can potentiate GABA (γ-aminobutyric acid) activity directly in recombinant GABAA receptors. The present study was to investigate the effects of (+)-Borneol on both contextual and cued fear recall. Interestingly, microinjection of (+)-Borneol into the dorsal hippocampus inhibited 24 h and 7 d contextual fear, whereas its infusion into ventral hippocampus only reduced 24 h cued fear responses. Moreover, microinjection of (+)-Borneol into dorsal but not ventral hippocampus suppressed anxiety-like behaviors in the open field test, light/dark exploration and the elevated plus maze test. As selective GABAA receptor antagonist bicuculline reversed the effect of (+)-Borneol on contextual fear paradigm and the drug potentiated GABA-evoked currents in acute hippocampus slices, modulation of the GABAergic neurotransmission may explain the effects of (+)-Borneol. Our findings suggest that (+)-Borneol can serve as a new therapeutic in fear- and anxiety-related disorders.

Anti-PTSD-like effects of albiflorin extracted from Radix paeoniae Alba

Ethnopharmacology relevancePost-traumatic stress disorder (PTSD) is a severe psychiatric disorder that is characterized by symptoms of re-experiencing, avoidance and hyperarousal, as well as social and professional dysfunction at least one month after the exposure to a traumatic event. Biosynthesis of allopregnanolone has been suggested as one of the important contributors to PTSD. Albiflorin (AF) extracted from Radix paeoniae Alba had been shown to be effective in the therapy of depression. However, few studies were concerned about the anti-PTSD-like effects of AF. Aim of the studyThe current study aimed to evaluate the anti-PTSD-like effects of AF in an animal model and its possible mechanism. Materials and methodsTo evaluate this, the single prolonged stress (SPS) model was used in the present study. The SPS rats were administered by AF (at doses of 3.5, 7 and 14.0mg/kg, i.g.) after induction of SPS from days 2–13. After the exposure to SPS, behavioral assessments were conducted, including contextual fear paradigm (CFP), elevated plus-maze test (EPMT), open-field test (OFT). The rats were decapitated at the end of the behavioral tests and levels of allopregnanolone in prefrontal cortex, hippocampus and amygdala were measured by enzyme linked immunosorbent assay (ELISA). ResultsIt had been shown that behavioral deficits of SPS rats were reversed by AF (7.0 and 14.0mg/kg, i.g.), which attenuated the PTSD-like associated contextual freezing behavior in CFP and improved PTSD-like associated anxiogenic behavior in EPMT without affecting locomotor activity in OFT. Moreover, decreased levels of allopregnanolone in prefrontal cortex, hippocampus, and amygdala were reversed by AF (7.0 and 14.0mg/kg, i.g.), respectively. ConclusionIn summary, the present study indicated that AF exerted the anti-PTSD-like effects, which maybe associated with allopregnanolone biosynthesis in the brain.

The inulin-type oligosaccharides extract from morinda officinalis, a traditional Chinese herb, ameliorated behavioral deficits in an animal model of post-traumatic stress disorder.

Post-traumatic stress disorder (PTSD) is a severe psychiatric condition. The allopregnanolone biosynthesis has been implicated as one of the possible contributors to PTSD. Inulin-type oligosaccharides of morinda officinalis (IOMO) had been shown to be effective in the therapy of depression. However, few studies concern the anti-PTSD-like effects of IOMO. To evaluate this, the single prolonged stress (SPS) model was used in the present study. It had been shown that the behavioral deficits of SPS-treated rats were reversed by IOMO (25.0 and 50.0mg/kg, i.p.), which reversed the increased freezing time in contextual fear paradigm (CFP) and the decreased time and entries in open arms in the elevated plus maze (EPM) test without affecting the locomotor activity in the open field (OF) test. In addition, the decreased allopregnanolone in the prefrontal cortex, hippocampus, and amygdala was reversed by IOMO (25.0 and 50.0mg/kg, i.p.), respectively. In summary, the present study indicated that the IOMO exert anti-PTSD-like behaviors, which maybe associated with the brain allopregnanolone biosynthesis.

Free and Easy Wanderer Plus (FEWP) improves behavioral deficits in an animal model of post-traumatic stress disorder by stimulating allopregnanolone biosynthesis

Post-traumatic stress disorder (PTSD) is a severe psychiatric condition defined as a “trauma and stress-related disorder”. Dampened allopregnanolone biosynthesis has been implicated as a possible contributor to PTSD aetiology. Free and Easy Wanderer Plus (FEWP) is a traditional Chinese medicine previously shown to be effective in PTSD treatment. However, little is known about the role of allopregnanolone in the anti-PTSD effects of FEWP. To evaluate this, the single prolonged stress (SPS) model was used in the present study. SPS-induced rats were administered FEWP (at doses of 2.5, 5.0 and 10.0mg/kg, p.o.) after induction of SPS from days 2 through 15. After exposure to SPS, behavioral assessments were determined, including the open-field test, the contextual fear paradigm, and the elevated plus-maze test. The experimental model rats were decapitated at the end of the behavioral tests and the level of allopregnanolone in the prefrontal cortex, hippocampus and amygdala was measured by enzyme linked immunosorbent assay (ELISA). The behavioral deficits of the SPS-induced rats were significantly reversed by FEWP (at doses of 5.0 and 10.0mg/kg, p.o.). The level of allopregnanolone was increased by administration of FEWP. In summary, this study indicated that the anti-PTSD effects of FEWP were associated with allopregnanolone biosynthesis.

Tracking explicit and implicit long-lasting traces of fearful memories in humans

Recent accounts of Posttraumatic Stress Disorder (PTSD) suggest that the encoding of an episode within a fearful context generates different implicit and explicit memory representations. Whilst implicit memory traces include the associated emotional states, explicit traces include a recoding into an abstract or gist-based structural context of the episode. Theoretically, the long-term preservation of implicit memory traces may facilitate the often untreatable memory intrusions in PTSD. Here, we tracked in two experiments how implicit and explicit memory traces for fearful episodes dissociate and evolve over time. Subjects (N=86) were presented with semantically-related word-lists in a contextual fear paradigm and tested for explicit memories either immediately (i.e., 30min) or after a delay (i.e., 1 or 2weeks) with a verbal recognition task. Skin Conductance Response (SCR) was used to assess implicit memory responses.Subjects showed high memory accuracy for words when tested immediately after encoding. At test, SCR was higher during the presentation of verbatim but not gist-based words encoded in a fearful context, and remained unchanged after 2weeks, despite subjects being unaware of words’ encoding context. We found no clear evidence of accurate explicit memory traces for the fearful or neutral contexts of words presented during encoding, either 30min or 2weeks afterwards. These findings indicate that the implicit, but not the explicit, memory trace of a fearful context of an episode can be detected at long-term through SCR and is dissociated from the gist-based memory. They may have implicationstowards the understanding of how the processing of fearful memoriescould lead to PTSD.

Midazolam Ameliorates the Behavior Deficits of a Rat Posttraumatic Stress Disorder Model through Dual 18 kDa Translocator Protein and Central Benzodiazepine Receptor and Neurosteroidogenesis

Post-traumatic stress disorder (PTSD) is a debilitating anxiety disorder that may develop after an individual has experienced or witnessed a severe traumatic event. It has been shown that the 18 kDa translocator protein (TSPO) may be correlated with PTSD and that the TSPO ligand improved the behavioral deficits in a mouse model of PTSD. Midazolam, a ligand for TSPO and central benzodiazepine receptor (CBR), induces anxiolytic- and anti-depressant-like effects in animal models. The present study aimed to determine whether midazolam ameliorates PTSD behavior in rats as assessed by the single prolonged stress (SPS) model. The SPS rats received daily Sertraline (Ser) (15 mg/kg, p.o.) and midazolam (0.125, 0.25, 0.5, and 1 mg/kg, p.o.) during the exposure to SPS and behavioral assessments, which included the open field (OF) test, the contextual fear paradigm (CFP), and the elevated plus-maze (EPM). The results showed that, like Ser (15 mg/kg, p.o.), midazolam (0.25 and 0.5 mg/kg, p.o.) significantly reversed the behavioral deficiencies of the SPS rats, including PTSD-associated freezing and anxiety-like behavior but not the effects on spontaneous locomotor activity. In addition, the anti-PTSD effects of midazolam (0.5 mg/kg, p.o.) were antagonized by the TSPO antagonist PK11195 (3 mg/kg, i.p.), the CBR antagonist flumazenil (15 mg/kg, p.o.) and the inhibitor of steroidogenic enzymes finasteride (30 mg/kg, p.o.), which by themselves had no effect on PTSD-associated freezing and anxiety-like behavior. In summary, this study demonstrated that midazolam improves the behavioral deficits in the SPS model through dual TSPO and CBR and neurosteroidogenesis.

Perinatal exposure to methoxychlor enhances adult cognitive responses and hippocampal neurogenesis in mice.

During perinatal life, sex steroids, such as estradiol, have marked effects on the development and function of the nervous system. Environmental estrogens or xenoestrogens are man-made chemicals, which animal and human population encounter in the environment and which are able to disrupt the functioning of the endocrine system. Scientific interest in the effects of exposure to xenoestrogens has focused more on fertility and reproductive behaviors, while the effects on cognitive behaviors have received less attention. Therefore, the present study explored whether the organochlorine insecticide Methoxychlor (MXC), with known xenoestrogens properties, administered during the perinatal period (from gestational day 11 to postnatal day 8) to pregnant-lactating females, at an environmentally relevant dose (20 µg/kg (body weight)/day), would also affect learning and memory functions depending on the hippocampus of male and female offspring mice in adulthood. When tested in adulthood, MXC perinatal exposure led to an increase in anxiety-like behavior and in short-term spatial working memory in both sexes. Emotional learning was also assessed using a contextual fear paradigm and MXC treated male and female mice showed an enhanced freezing behavior compared to controls. These results were correlated with an increased survival of adult generated cells in the adult hippocampus. In conclusion, our results show that perinatal exposure to an environmentally relevant dose of MXC has an organizational effect on hippocampus-dependent memory and emotional behaviors.

Dopamine D2 receptors modulate the expression of contextual conditioned fear

Although dopaminergic systems are more commonly associated with the reinforcing effects of various stimuli, numerous reports have demonstrated a relationship between changes in dopaminergic transmission and aversive situations. In the present study, we examined the involvement of D1-like and D2-like receptors in the expression of conditioned freezing using the context as the conditioned stimulus. Intraperitoneal injections of the D1 agonist SKF38393 or the D1 antagonist SCH23390 did not change the conditioned freezing in rats subjected to the contextual fear paradigm. In contrast, intraperitoneal injections of the D2 agonist quinpirole and the D2 antagonist sulpiride caused a significant dose-dependent reduction in the expression of contextual conditioned freezing. As these data may reflect that the systemic manipulations acted on dopaminergic receptors in different brain areas, the effects of administration of quinpirole and sulpiride into the ventral tegmental area (VTA) and the basolateral amygdala complex (BLA) on the expression of contextual conditioned freezing were also evaluated. Intra-VTA quinpirole and intra-BLA sulpiride injections reduced the conditioned freezing response; intra-VTA sulpiride and intra-BLA quinpirole injections had no significant effects. These data suggest that D2-like receptors, but not D1-like receptors, play an important role in the expression of contextual conditioned freezing. Quinpirole may act at D2 presynaptic receptors located in the VTA, decreasing dopamine levels in the terminal fields of the mesolimbic pathway. The effects of sulpiride, in contrast, appear to be triggered by an action on postsynaptic dopaminergic receptors located in the BLA. However, it cannot be totally excluded that the injected solutions did not also affect neighboring amygdalar regions. Together with previous findings, the present data suggest the need to consider dopaminergic mechanisms in the mesolimbic circuit as novel targets for the pharmacological treatment of fear-related disorders, especially post-traumatic stress disorder.

Interfering with post-learning hippocampal activity does not affect long-term consolidation of a context fear memory outside the hippocampus

There are still uncertainties about the role of the hippocampus (HPC) in memory consolidation. One theory, systems consolidation, states that the HPC is required for the initial storage of certain memories that subsequently become established in non-HPC networks through a lengthy process, involving an interaction with the HPC. A similar process may underlie the ability of multiple, distributed learning episodes of contextual fear conditioning to create a HPC-independent context fear memory, in a memory task that does not undergo systems consolidation with the mere passage of time [5]. The current study examined whether post-learning HPC activity is necessary to establish these HPC-independent context memories through distributed learning episodes. Rats received either three or six context conditioning sessions over the course of three days. The HPC-dependence of context memories was assessed using multisite, temporary inactivation of the HPC using ropivacaine during retention testing. We established that six conditioning sessions, but not three, created a memory that could be retrieved while the HPC was inactive. To directly test our hypothesis, HPC was inactivated after half of the six context-shock pairings in an independent group of rats. The rats were then tested for retention of context fear in the absence of HPC activity. Post-learning inactivation of the HPC did not affect the establishment of a HPC-independent context memory. These results favor the idea that at least one memory system outside the HPC can acquire context memories independently.

Previous Stress Attenuates the Susceptibility to Midazolam's Disruptive Effect on Fear Memory Reconsolidation: Influence of Pre-Reactivation D-Cycloserine Administration

It is well known that, under certain boundary conditions, the retrieval of a stable consolidated memory results into a labile one. During this unstable phase, memory can be vulnerable to interference by a number of pharmacological agents, including benzodiazepines. One of the goals of this study was to evaluate the vulnerability to midazolam (MDZ) after reactivation of recent and remote contextual fear memories in animals that experienced a stressful situation before learning. Animals were subjected to a restraint session and trained in a contextual fear paradigm the following day; consolidated memories were reactivated at different times after learning and different MDZ doses (1.5, 3.0 mg/kg) were administered to rats after reactivation. Our results show that MDZ did not affect memory reconsolidation in older-than-one-day memories of stressed animals, even after the administration of a higher MDZ dose and a longer reactivation session (5 min). In contrast, MDZ was effective in blocking reconsolidation at all memory ages in unstressed animals. In addition, the current research investigated whether activating NMDA sites before reactivation promotes the destabilization of resistant memories such as those of stressed animals. We tested the influence of pre-reactivation D-cycloserine (DCS), a partial NMDA agonist, on MDZ's effect on fear memory reconsolidation in stressed animals. Our findings indicate that DCS before reactivation promotes retrieval-induced lability in resistant memory traces, as MDZ-induced memory impairment in stressed rats became evident with pre-reactivation DCS but not after pre-reactivation sterile isotonic saline.

Extracellular serotonin level in the basolateral nucleus of the amygdala and dorsal periaqueductal gray under unconditioned and conditioned fear states: An in vivo microdialysis study

Serotonin (5-HT) plays a key role in the neural circuitry mediating unconditioned and conditioned fear responses related to panic and generalized anxiety disorders. The basolateral nucleus of the amygdala (BLA) and the dorsal periaqueductal gray (dPAG) appear to be mainly involved in these conditions. The aim of this study was to measure the extracellular level of 5-HT and its metabolite 5-hydroxyindolacetic acid (5-HIAA) in the BLA and dPAG during unconditioned and conditioned fear states using in vivo microdialysis procedure. Thus, for the unconditioned fear test, animals were chemically stimulated in the dPAG with semicarbazide, an inhibitor of the γ-aminobutyric acid-synthesizing enzyme glutamic acid decarboxylase. For the conditioned fear test, animals were subjected to a contextual conditioned fear paradigm using electrical footshock as the unconditioned stimulus. The results show that the 5-HT and 5-HIAA level in the BLA and dPAG did not change during unconditioned fear, whereas 5-HT concentration, but not 5-HIAA concentration, increased in these brain areas during conditioned fear. The present study showed that the 5-HT system was activated during conditioned fear, whereas it remained unchanged during unconditioned fear, supporting the hypothesis that 5-HT has distinct roles in conditioned and unconditioned fear (dual role of 5-HT in anxiety disorders).

Disruptive Effect of Midazolam on Fear Memory Reconsolidation: Decisive Influence of Reactivation Time Span and Memory Age

Benzodiazepine (BDZ) administered shortly after retrieval disrupts the reconsolidation of fear memory. In this research, we explored the way in which different factors that limit the emergence of such process may affect BDZ's disruptive effect on fear memory reconsolidation. Animals were conditioned in a contextual fear paradigm; the consolidated memory was reactivated by exposure to the associated context for different periods of time that were followed by midazolam (MDZ) administration. We also studied MDZ amnesic effect after reactivating fear memories of several ages. We finally analyzed the effectiveness of different MDZ doses in preventing the reconsolidation of different age fear memories. The memory trace was disrupted following MDZ when the reactivation session lasted 3-5 min but it was not after a briefer 1-min reactivation period. Over a 10-min reactivation session, all animals gradually reduced their fear response, which indicates the emergence of the extinction process. When tested, MDZ rats exhibited a robust fear, suggesting that MDZ impaired the consolidation of extinction. In a 3-min reactivation session, MDZ (1-1.5 mg/kg) prevented the reconsolidation of recently acquired memories. A 21-day-old fear memory was only vulnerable to MDZ at a 1.5 mg/kg dose with a reactivation session of 5 and not 3 min, whereas a 36-day-old memory was only disrupted with a higher MDZ dose (3 mg/kg) regardless of the reactivation trial's duration. This study demonstrated MDZ's interference on fear-memory reconsolidation within a relatively short reactivation period in recently acquired memories. Over longer reexposure, MDZ disrupts the consolidation of extinction. A longer duration of the reexposure session, as well as higher MDZ doses, is required to prevent the reconsolidation process of remote fear memories.

Serotonergic mechanisms in the basolateral amygdala differentially regulate the conditioned and unconditioned fear organized in the periaqueductal gray

The amygdala is an important filter for unconditioned and conditioned aversive information. The amygdala synthesizes the stimuli input from the environment and then signals the degree of threat that they represent to the dorsal periaqueductal gray (dPAG), which would be in charge of selecting, organizing and executing the appropriate defense reaction. In this study, we examined the influence of fluoxetine microinjections (1.75 and 3.5 nmol/0.2 μL) into the lateral (LaA) and basolateral (BLA) amygdaloid nuclei on the freezing and escape responses induced by electrical stimulation of the dPAG. Freezing behavior was also measured after the interruption of the electrical stimulation of the dPAG. On the following day, these rats were also submitted to a contextual fear paradigm to examine whether these microinjections would affect the conditioned freezing to contextual cues previously associated with foot shocks. Fluoxetine injections into both amygdaloid nuclei did not change the freezing and escape thresholds, but disrupted the dPAG-post-stimulation freezing. Moreover, the conditioned freezing was enhanced by fluoxetine. Whereas 5-HT mechanisms in the amygdala facilitate the acquisition of conditioned fear they inhibit the dPAG-post-stimulation freezing. However, the unconditioned fear triggered by activation of the dPAG is produced downstream of the amygdala. These findings have important implications for the understanding of the neurochemical substrates that underlie panic and generalized anxiety disorders.