Conditioned Fear Stress Research Articles

Effects of PB190 and PB212, new σ receptor ligands, on glucocorticoid receptor-mediated gene transcription in LMCAT cells

The hyperactivity of the hypothalamic-pituitary-adrenocortical (HPA) axis is often observed in patients with major depression. It has even been implicated in the pathophysiology of this disease. Some antidepressant drugs (ADs) inhibit glucocorticoid receptor (GR) function under in vitro conditions. The σ1 receptor agonists reveal potential antidepressant activity in animals, moreover, igmesine is promising as an AD in humans. As already shown, σ receptors are involved in stress-induced responses (e.g., conditioned fear stress in mice). The aim of the present study was to find out whether the new selective σ receptor ligands, PB190 and PB212, are able to affect directly the endocrine system activity. To this end, we evaluated their influence on GR function in mouse fibroblast cells (L929), stably transfected with mouse mammary tumor virus-chloramphenicol acetyltransferase (MMTV-CAT) plasmid (LMCAT cells). Fluvoxamine, a selective serotonin reuptake inhibitor, recognized as a σ1 receptor agonist was used for comparison. The obtained results showed that both PB190 and PB212 (potential σ1 receptor agonist and antagonist, respectively) like fluvoxamine, decreased the corticosterone-induced CAT activity in a concentration-dependent manner. The significance of this fact remains ambiguous and requires further studies.

Pharmacological Interaction With the Sigma1 (σ1)-Receptor in the Acute Behavioral Effects of Antidepressants

Selective agonists of the sigma-1 (σ1) ligand–operated chaperone protein, like igmesine or PRE-084, are antidepressants in preclinical depression models. σ1-Protein activation may contribute to the antidepressant efficacy of drugs known to act as selective serotonin-reuptake inhibitors (SSRI) or noradrenaline reuptake inhibitors through direct or indirect involvement of the σ1-receptor in the drug effect. We here compared antidepressant effects in two behavioral procedures, the forced swimming test (FST) and conditioned fear stress (CFS). The involvement of the σ1-receptor was examined using a co-treatment with the σ1-antagonist BD1047 or using σ1-knockout (KO) mice. Igmesine but not PRE-084 decreased FST immobility. The SSRI fluoxetine and sertraline, but not fluvoxamine, and the tricyclic antidepressants imipramine, desipramine, and amitriptyline were also effective. Only the effect of igmesine was blocked by BD1047 or in σ1-KO mice. Igmesine, PRE-084, fluvoxamine, and sertraline decreased the CFS immobility in a BD1047- and σ1-KO–sensitive manner. Among tricyclics, only amitriptyline was effective and its effect was unaffected by BD1047 or in σ1-KO mice. The behavioral effects induced by mixed σ1-receptor/SSRI antidepressants, like fluvoxamine or sertraline, may therefore involve a non-selective action at both targets. Moreover, the CFS appears to more reliably uncover a σ1 pharmacological component in antidepressant screening.

Aripiprazole and haloperidol suppress excessive dopamine release in the amygdala in response to conditioned fear stress, but show contrasting effects on basal dopamine release in methamphetamine-sensitized rats

Although emotional dysfunction in patients with schizophrenia is thought to be associated with poorer outcomes in terms of overall quality of well-being, only a few basic studies have examined the biochemical effect of antipsychotics on emotional function. In this investigation, we examined differences in the effects of aripiprazole and haloperidol on the conditioned fear response in methamphetamine-sensitized and fear-conditioned rats in an in vivo microdialysis study. Aripiprazole is the first antipsychotic drug with an action involving partial dopamine D 2 receptor agonism, thus differing from haloperidol, a typical antipsychotic that shows selective dopamine D 2 receptor full antagonism. After exposure to a conditioned stimulus, methamphetamine-sensitized rats exhibited significantly higher dopamine release in the amygdala than unsensitized rats. We considered this hypersensitivity of dopamine release to be a biochemical marker of hypersensitivity and vulnerability to stress in psychosis. In the present study, we found that aripiprazole and haloperidol equally suppressed the marked increase in extracellular dopamine levels in fear-conditioned rats, whereas haloperidol increased and aripiprazole decreased tonic dopamine levels. In conclusion, the effect of an antipsychotic drug is likely to be involved in attenuation of the phasic increase in dopamine associated with the fear response, at least in the amygdala. In addition, the contrasting effects of haloperidol and aripiprazole on tonic dopamine levels in the amygdala are likely due to the difference in their actions (selective dopamine D 2 receptor full antagonist vs. partial agonist, respectively).

Anxiolytic-like profiles of histamine H3 receptor agonists in animal models of anxiety: a comparative study with antidepressants and benzodiazepine anxiolytic

Histamine H3 receptor functions as a presynaptic auto- and hetero-receptor on histaminergic and non-histaminergic neurons in the brain regulating the synaptic release of numerous neurotransmitters. Therefore, the ligands for this receptor have been proposed to be of therapeutic interest for the treatment of various neuropsychiatric disorders. At present, however, the psychopharmacological profiles of H3 ligands, particularly H3 agonists, have not been extensively studied. The present study investigated the anxiolytic-like profiles of H3-selective agonists in a variety of classical (benzodiazepine-sensitive) and atypical (antidepressant-effective) animal models of anxiety. Comparator drugs used were diazepam and both fluvoxamine and desipramine in the former and latter models, respectively. H3 agonist R-alpha-methylhistamine and immepip were inactive in rat elevated plus maze test and Vogel type conflict test where diazepam (5 mg/kg) produced significant anxiolytic-like effects. Meanwhile, these H3 agonists (10-30 mg/kg) significantly reduced isolation-induced vocalizations in guinea pig pups and isolation-induced aggressive behavior in mouse resident-intruder test. Moreover, in rat conditioned fear stress test, R-alpha-methylhistamine (30 mg/kg) and immepip (10 mg/kg) significantly decreased freezing time, which were completely reversed by concomitant treatment with H3 antagonist, thioperamide (10 mg/kg). In contrast to the limited efficacy obtained with desipramine (30 mg/kg), fluvoxamine (20-60 mg/kg) exhibited anxiolytic-like effects in all the latter three atypical models. These data suggest that the H3 agonists may have anxiolytic-like effects similar to those of selective serotonin reuptake inhibitors but not benzodiazepine anxiolytics and represent a novel strategy for the treatment of some anxiety disorders in which selective serotonin reuptake inhibitors are prescribed.

Anxiolytic-like profile of mirtazapine in rat conditioned fear stress model: Functional significance of 5-hydroxytryptamine 1A receptor and α1-adrenergic receptor

Mirtazapine is an antidepressant with a unique mechanism of action and has been categorized as a Noradrenergic and Specific Serotonergic Antidepressant (NaSSA). Although numerous clinical trials suggested the usefulness of mirtazapine for not only major depressive disorders but also a variety of anxiety disorders, efficacy studies in animal anxiety models have been rarely reported. The present study investigated a potential anxiolytic-like profile of mirtazapine in rat conditioned fear stress model. A 5-hydroxytryptamine (5-HT) 1A receptor partial agonist, buspirone (1–5 mg/kg) exhibited a significant reduction in freezing time, and its maximal effect was reversed by a selective 5-HT 1A antagonist, WAY-100635 (1 mg/kg). Mirtazapine (1–10 mg/kg) also reduced the freezing time in a dose-related fashion, a substantial proportion (approx. 50%) of which was likewise antagonized by WAY-100635 (1 mg/kg). Mianserin (1–30 mg/kg), a structural analogue for mirtazapine, was ineffective. Furthermore, co-administration of α1 adrenoceptor antagonist, prazosin (0.03 mg/kg) completely reversed mirtazapine (10 mg/kg)-induced reduction of freezing time. These findings represent the first demonstration that the anxiolytic-like action of mirtazapine involves activation of 5-HT 1A receptor and α1 adrenoceptor to different extents, and are compatible with one aspect of mirtazapine's pharmacological profile as NaSSA.

Effects of acute citalopram on the expression of conditioned freezing in naive versus chronic citalopram-treated rats

An acute challenge with selective serotonin (5-HT) reuptake inhibitors (SSRIs) reduces the conditioned freezing in rats, a model of anxiety. The increase in the 5-HT levels in the nerve terminal induced by SSRIs is closely related to its pharmacological effects. Clinically, SSRIs exert an anxiolytic effect after chronic treatment. The effects of repeated treatment with citalopram on conditioned freezing in rats were examined in the present study. Acute citalopram (10 mg/kg) reduced freezing at a short post-training interval (1 day) significantly. While the effect of citalopram (10 mg/kg) on freezing was diminished by prolonging the interval between conditioning and the exposure to conditioned fear stress, repeated citalopram (10 mg/kg) injection twice daily for 7 days restored the inhibitory effect of acute challenge of citalopram (10 mg/kg) on freezing. By prolonging the period between conditioning and exposure to conditioned fear stress, this model may have a more precise predictive validity of anxiety disorder as an animal model.

Pharmacokinetic interaction between tandospirone and fluvoxamine in the rat contextual conditioned fear stress model and its functional consequence: Involvement of cytochrome P450 3A4

In a previous study it was demonstrated that the anxiolytic action of tandospirone, a 5-hydroxytryptamine (5-HT)(1A) receptor agonist, is facilitated by cytochrome P450 (CYP) 3A4 inhibitors, such as ketoconazole and cimetidine. It is also known that fluvoxamine, a selective serotonin re-uptake inhibitor (SSRI), inhibits CYP3A4. The purpose of the present study was to clarify the pharmacokinetic interaction between tandospirone and fluvoxamine and to evaluate their combined effect in the rat anxiety model. The anxiolytic action of co-administration of tandospirone and fluvoxamine was examined using the rat contextual conditioned fear stress model. After testing the conditioned fear, plasma concentrations of tandospirone and its major metabolite 1-(2-pyrimidyl) piperazine were determined. One day after fear conditioning, both tandospirone (60 mg/kg, p.o.) and fluvoxamine (60 mg/kg, p.o.) significantly inhibited conditioned freezing and their combination effect was additive. In addition, plasma concentration of tandospirone was increased by fluvoxamine. There is a CYP3A4-related drug-drug interaction between tandospirone and fluvoxamine. Therefore, fluvoxamine may facilitate the anxiolytic effect of tandospirone via CYP3A4 inhibition.

Changes in amygdala neural activity that occur with the extinction of context-dependent conditioned fear stress

The purpose of the present study was to characterize functional changes in the amygdala that accompany the extinction of context-dependent conditioned fear stress in a rat, an animal model of anxiety. Specifically, the effect of extinction of conditioned fear-induced cyclic AMP responsive element-binding protein (CREB) phosphorylation in the amygdala was investigated using immunohistochemistry. Experiments demonstrated that CREB phosphorylation in the basal nucleus of the amygdala decreased with the extinction of context-dependent conditioned fear-induced freezing behavior. These data suggest that the basal nucleus of the amygdala plays an essential role in the expression of context-dependent conditioned fear. Further, this is the first study to demonstrate that CREB phosphorylation in the basal nucleus of the amygdala changes in parallel with the extinction of context-dependent conditioned fear.

Effect of co-administration of the selective 5-HT 1A receptor antagonist WAY 100,635 and selective 5-HT 1B/1D receptor antagonist GR 127,935 on anxiolytic effect of citalopram in conditioned fear stress in the rat

This study investigated the effect of co-administration of the selective 5-HT 1A receptor antagonist WAY 100,635 and selective 5-HT 1B/1D receptor antagonist GR 127,935 with a subactive dose of citalopram [selective serotonin (5-HT) reuptake inhibitor (SSRI)] on the expression of conditioned freezing, an index of fear. In the present study, acute administration of citalopram (s.c.) reduced freezing significantly at high doses (10, 30 and 100 mg/kg), while showing no significant effect at low doses (1 and 3 mg/kg). Co-administration of WAY 100,635 (0.15 mg/kg) with citalopram (3 mg/kg) reduced freezing markedly and significantly, as compared with either drug alone. However, the addition of GR 127,935 (4 mg/kg) did not potentiate the effects of citalopram (3 mg/kg) on freezing and did not enhance the effect of WAY 100,635 (0.15 mg/kg) with citalopram (3 mg/kg). Co-administration of WAY 100,635 (0.15 mg/kg) or GR 127,935 (4 mg/kg) gave no effect on high-dose citalopram (30 mg/kg)-induced inhibition of freezing behavior. These results suggest that co-administration of WAY 100,635 (0.15 mg/kg) strengthens the anxiolytic effect of citalopram (3 mg/kg) by facilitating central 5-HT neurotransmission. Since GR 127,935 (4 mg/kg) failed to accelerate the inhibition of freezing induced by citalopram (3 mg/kg) with WAY 100,635 (0.15 mg/kg) or citalopram (3 mg/kg) alone, it is suggested that blocking 5-HT 1A receptors is more effective in facilitating the anxiolytic effect of citalopram than blocking 5-HT 1B/1D receptors.

Inhibitory Effects of Ramosetron, a Potent and Selective 5-HT3–Receptor Antagonist, on Conditioned Fear Stress–Induced Abnormal Defecation and Normal Defecation in Rats: Comparative Studies With Antidiarrheal and Spasmolytic Agents

We examined the effect of ramosetron, a potent serotonin (5-HT)3-receptor antagonist for irritable bowel syndrome with diarrhea, on conditioned fear stress (CFS)-induced defecation and normal (non-stressed) defecation in rats and compared ramosetron with the antidiarrheal agent loperamide and the spasmolytic agents trimebutine and tiquizium. Ramosetron, loperamide, trimebutine, and tiquizium significantly inhibited CFS-induced defecation in a dose-dependent manner with ED50(95% confidence limit) values of 0.019 (0.01 –0.028), 9.4 (4.0 –22), 850 (520 –2,400), and 300 (190 –450) mg/kg, respectively. A significant effect of ramosetron on CFS-induced defecation appeared at 10 min after dosing and was sustained for 8 h. In contrast, loperamide, trimebutine, and tiquizium significantly inhibited CFS-induced defecation between 1 –8, 1 –4, and 1–8h after administration, respectively. High doses of ramosetron did not affect normal defecation, whereas loperamide, trimebutine, and tiquizium significantly inhibited this process. In conclusion, ramosetron has potent, rapid-onset, and long-lasting inhibitory effects on CFS-induced defecation in rats, but does not influence normal defecation. The present findings indicate that ramosetron will be a useful therapeutic agent for irritable bowel syndrome with diarrhea, showing greater efficacy and safety than other antidiarrheal and spasmolytic agents.

Fluoxetine protects hippocampal plasticity during conditioned fear stress and prevents fear learning potentiation

Contextual fear conditioning can produce both changes in hippocampal synaptic efficacy and potentiation of subsequent fear learning. In this study, we tested whether fluoxetine reverses these effects. In the first experiment, we examined alterations of baseline synaptic efficacy and induction of synaptic plasticity in the CA3 region of the hippocampus during re-exposure of rats, treated with fluoxetine (7 mg/kg) or vehicle, in a context where they previously received 15 eyelid shocks or no shock (controls). In the second experiment, fear learning potentiation was examined in rats that were initially submitted to conditioning (15 eyelid shocks) and extinction training and then re-exposed to a less intense stressor (three eyelid shocks). Conditioned fear stress decreased synaptic efficacy and blocked the induction of synaptic potentiation in the fimbria-CA3 pathway. Conditioned rats treated with fluoxetine were protected against these electrophysiological changes and did not differ from controls (i.e., no depression and normal induction of potentiation of synaptic efficacy). However, fluoxetine treatment did not suppress conditioned freezing. After fear extinction, exposure of rats to a subconditioning stressor provoked conditioning (fear learning potentiation) in rats treated with vehicle but not in those treated with fluoxetine. These findings indicate that fluoxetine treatment, which is ineffective on conditioned fear stress-induced freezing, may have beneficial effects on conditioned fear stress-induced disturbance of hippocampal plasticity. These data also suggest that restoration of hippocampal functioning may contribute to protection against exaggerated reactions to mild stressors reported in patients with post-traumatic stress disorder.

Synergistic effects of tandospirone and selective serotonin reuptake inhibitors on the contextual conditioned fear stress response in rats

The purpose of this study was to investigate the effects of co-administration of tandospirone (a 5-HT 1A receptor agonist) and individual selective serotonin reuptake inhibitors (SSRIs) on the contextual conditioned fear stress, using an anxiety model in rats. One day after fear conditioning, tandospirone (0.3–3 mg/kg, s.c.), paroxetine (5–20 mg/kg, i.p.), fluvoxamine (30–60 mg/kg, i.p.) and citalopram (3–30 mg/kg, i.p.) significantly inhibited the conditioned freezing in a dose-dependent manner, whereas, 14 days after fear conditioning, the anxiolytic effects of these drugs were weakened. Fourteen days after fear conditioning, co-administration of tandospirone (0.3 mg/kg, s.c.) with each SSRI [paroxetine (5 mg/kg, i.p.), fluvoxamine (30 mg/kg, i.p.) and citalopram (10 mg/kg, i.p.)], given at subeffective doses, markedly inhibited the conditioned freezing without affecting the locomotor activities and CYP3A4-related pharmacokinetic drug–drug interaction. These results elucidate the pharmacodynamic synergistic effects of tandospirone and SSRIs. Therefore, this augmentation therapy (SSRI + 5-HT 1A receptor agonist) may prove useful for some anxiety disorders.

Effects of cytochrome P450 (CYP) 3A4 inhibitors on the anxiolytic action of tandospirone in rat contextual conditioned fear

The azapirone derivatives, including tandospirone and buspirone, are anxiolytics with 5-HT 1A receptor agonistic action. Previous in vitro studies have suggested these azapirone derivatives are mainly metabolized by the cytochrome P450 (CYP) 3A4 isoform. The purpose of this study was to clarify the effects CYP3A4 inhibitors have on the anxiolytic action of tandospirone in a conditioned fear stress rat model. One day after fear conditioning, the orally administered tandospirone (30–100 mg/kg) significantly inhibited conditioned freezing in a dose-dependent manner. Co-administration of oral tandospirone and CYP3A4 inhibitors [ketoconazole (10 mg/kg, i.p.) and cimetidine (200 mg/kg, p.o.)] markedly inhibited conditioned freezing. Ketoconazole significantly increased the anxiolytic effect of buspirone similar to tandospirone. As with freezing behavior, the plasma concentrations of tandospirone and buspirone were increased by CYP3A4 inhibitors. This suggests the CYP3A4 isoform is involved in the metabolism of tandospirone, in vivo. Therefore, drugs with CYP3A4 inhibitory property may facilitate the anxiolytic effect of tandospirone when treating human anxiety disorders.

Pharmacological profile of ramosetron, a novel therapeutic agent for IBS

Ramosetron is a potent and selective serotonin (5-HT)(3) receptor antagonist that has been shown to affect abnormal colonic function and abdominal pain in animals. Ramosetron (0.3 to 100 microg/kg, p.o.) has been found to significantly suppress abnormal defecation induced by conditioned-fear stress (CFS), restraint stress, corticotropin releasing factor (CRF) and 5-HT in rats and mice, and these effects were more potent than those of alosetron, cilansetron or loperamide. On the other hand, ramosetron (3,000 microg/kg, p. o., once daily for 7 days) did not inhibit normal defecation in dogs while tiquizium significantly inhibited it. Ramosetron (3 to 100 microg/kg, p. o.) also significantly prevented CFS-induced acceleration of colonic transit and CRF-induced abnormal water transport in rats, respectively. Moreover, ramosetron (0.3 to 3 microg/kg, p. o.) significantly suppressed restraint stress-induced decrease in colonic pain threshold, an effect not observed with loperamide. These results indicate that ramosetron produce beneficial clinical effects on IBS symptoms.

An mGluR2/3 antagonist, MGS0039, exerts antidepressant and anxiolytic effects in behavioral models in rats

Abnormalities of glutamatergic neurotransmission have been reportedly observed in psychiatric disorders. Previously, we demonstrated that (1R, 2R, 3R, 5R, 6R)-2-Amino-3-(3,4-dichlorobenzyloxy)-6-fluorobicyclo[3.1.0]hexane-2,6-dicarboxylic acid (MGS0039) is a selective antagonist for group II metabotropic glutamate receptors (mGluR2/3), and that it exerted antidepressant effects in some animal behavioral tests. In the present study, we provide additional evidence that MGS0039 exhibits antidepressant and anxiolytic effects in experimental rodent models, which are predictive of clinical efficacy. The learned helplessness (LH) paradigm, which is a common model used to examine the depressive state, was used to assess antidepressant effects of MGS0039. Moreover, anxiolytic effects of MGS0039 were investigated in the conditioned fear stress (CFS) model, which represents emotional abnormality, including anxiety. Intraperitoneal administration of MGS0039 (10 mg/kg) to rats for 7 days elicited a significant reduction in escape failures in the LH paradigm. In addition, rats treated with MGS0039 (2 mg/kg) showed significantly attenuated freezing behavior in a CFS model, indicating the anxiolytic-like potential of MGS0039. These results suggest that the blockade of mGluR2/3 with MGS0039 may be effective in the treatment of depressive and anxiety disorders.

Target brain sites of the anxiolytic effect of citalopram, a selective serotonin reuptake inhibitor

The purpose of the present study was to elucidate the brain regions in which citalopram, a selective serotonin reuptake inhibitor (SSRI), exerts its anxiolytic effects in conditioned fear stress (CFS) in rats, an animal model of anxiety. The effect of citalopram on CFS-induced c-Fos expression was investigated using immunohistochemistry. Systemic administration of citalopram attenuated contextual CFS-induced c-Fos expression in the secondary motor cortex, primary somatosensory cortex, and basolateral nucleus of the amygdala. Among these regions the basolateral nucleus of the amygdala is a likely candidate region in which citalopram exerts its anxiolytic effect.

5-HT 1A receptor agonist affects fear conditioning through stimulations of the postsynaptic 5-HT 1A receptors in the hippocampus and amygdala

Evidence from preclinical and clinical studies has shown that 5-HT 1A receptor agonists have anxiolytic actions. The anxiolytic actions of 5-HT 1A receptor agonists have been tested by our previous studies using fear conditioning. However, little is known about the brain regions of anxiolytic actions of 5-HT 1A receptor agonists in this paradigm. In the present study, we investigated the effects of bilateral microinjections of flesinoxan, a selective 5-HT 1A receptor agonist, into the hippocampus, amygdala and medial prefrontal cortex on the expression of contextual conditioned freezing and the defecation induced by conditioned fear stress in rats. These results reveal that both intrahippocampal and intraamygdala injections of flesinoxan decreased the expression of conditioned freezing, while injections into the medial prefrontal cortex did not. In addition, intraamygdala injection of flesinoxan attenuated the increased defecation induced by conditioned fear, but injections into the hippocampus and medial prefrontal cortex failed. These results suggest that flesinoxan exerts its anxiolytic actions in the fear conditioning through stimulations of the postsynaptic 5-HT 1A receptors in the hippocampus and amygdala.

Lesions of the habenula produce stress- and dopamine-dependent alterations in prepulse inhibition and locomotion

The habenula complex modulates the activity of dopamine and serotonin systems in the brain. An important question remains whether there is a link between habenula dysfunction and monoamine-related disorders, such as schizophrenia. In this study, we describe an interaction between habenula lesions and stress that produces long-lasting effects on behavior. Mice received control lesions or bilateral electrolytic lesions of the habenula and were tested for fear-potentiated startle and freezing measures of conditioned fear. They were also tested for prepulse inhibition (PPI) and locomotor activity in the presence or absence of a dopaminergic agonist (apomorphine) or an atypical antipsychotic with mixed dopamine/serotonin antagonist properties (clozapine). There were no detectable effects of habenula lesions on fear conditioning and no effects on PPI in the absence of stress. However, following conditioned fear stress, habenula-lesioned animals showed decreased PPI which normalized with clozapine. Lesioned animals also showed diminished activity at baseline, with hyperlocomotion following apomorphine. These data support the hypothesis that the habenula may be normally involved in stress-dependent regulation of monoamine systems.

The pharmacokinetics and pharmacodynamics of tandospirone in rats exposed to conditioned fear stress

The 5-HT1, agonist tandospirone is generally thought to have a weak anxiolytic effect with a slow onset of action. Our recent clinical study suggested that a comparatively high dose of tandospirone has excellent anxiolytic efficacy and is without significant adverse effects. The present study was designed to clarify the relationship between the anxiolytic effect of tandospirone and its plasma and brain concentrations. The anxiolytic effect was estimated by determining the conditioned fear stress-induced freezing behavior in rats after tandospirone administration. Obvious correlations between anxiolytic effect and brain concentration of tandospirone were observed 0.5 and 4 h after tandospirone administration, while the anxiolytic effect was dependent on the plasma concentration of at 0.5 h but not 4 h after tandospirone administration. The plasma concentration was significantly correlated with the brain concentration. These findings suggest that the potency of the anxiolytic effect is dependent on both the plasma and brain concentration.

Amygdalic levels of dopamine and serotonin rise upon exposure to conditioned fear stress without elevation of glutamate

Conditioned fear is an artificial stress, induced by a stimulus, such as a tone, that does not elicit fear in nature. This fear response is acquired by experimental animals when tone is combined with an unconditioned stimulus, such as electrical foot shock. The amygdala is considered to be the area involved in acquisition, consolidation and recall of fear. A series of previous pharmacological studies showed antagonists of dopamine D 1 and D 2, glutamate N-methyl- d-asparatate and (±)-α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptors to prevent the acquisition of conditioned fear. However, little is known about the types of neurotransmitters released when conditioned fear is acquired and recalled. The present study was designed to continuously monitor changes in extracellular levels of glutamate, dopamine and serotonin in the amygdala, at the acquisition of conditioned fear on Day 1 and at fear recall in response to a tone as a conditioned stimulus on Day 2, using the in vivo microdialysis method. Glutamate was elevated only on Day 1, while dopamine and serotonin rose on both days. The periods of elevated dopamine and serotonin were longer on Day 1 than on Day 2. These results suggest that greater amounts of glutamate, dopamine and serotonin are necessary for acquisition than for recall of conditioned fear.