Enkephalin knockout male mice are resistant to chronic mild stress

Highly sensitive in vivo detection of dynamic changes in enkephalins following acute stress

Enkephalins are opioid peptides that modulate analgesia, reward, and stress. In vivo detection of enkephalins remains difficult due to transient and low endogenous concentrations and inherent sequence similarity. To begin to address this we previously developed a system combining in vivo optogenetics with microdialysis and a highly sensitive mass spectrometry-based assay to measure opioid peptide release in freely moving rodents (Al-Hasani, 2018, eLife). Here we show improved detection resolution and stabilization of enkephalin detection, which allowed us to investigate enkephalin release during acute stress. We present an analytical method for real-time, simultaneous detection of Met- and Leu-Enkephalin (Met-Enk & Leu-Enk) in the mouse Nucleus Accumbens shell (NAcSh) after acute stress. We confirm that acute stress activates enkephalinergic neurons in the NAcSh using fiber photometry and that this leads to the release of Met- and Leu-Enk. We also demonstrate the dynamics of Met- and Leu-Enk release as well as how they correlate to one another in the ventral NAc shell, which was previously difficult due to the use of approaches that relied on mRNA transcript levels rather than post-translational products. This approach increases spatiotemporal resolution, optimizes the detection of Met-Enkephalin through methionine oxidation, and provides novel insight into the relationship between Met- and Leu-Enkephalin following stress.

Up-regulation of BDNF/TrkB signaling by δ opioid receptor agonist SNC80 modulates depressive-like behaviors in chronic restraint-stressed mice

Depressive disorder is a psychiatric disease characterized by its main symptoms of low mood and anhedonia. Due to its complex etiology, current clinical treatments for depressive disorder are limited. In this study, we assessed the role of the δ opioid receptor (δOR) system in the development of chronic-restraint-stressed (CRS)-induced depressive behaviors. We employed a 21-day CRS model and detected the c-fos activation and protein levels' changes in enkephalin (ENK)/δOR. It was found that the hippocampus and amygdala were involved in CRS-induced depression. The expression of pro-enkephalin (PENK), the precursors of the endogenous ligand for δOR, was significantly decreased in the hippocampus and amygdala following CRS. We then treated the mice with SNC80, a specific δOR agonist, to examine its anti-depressant effects in the tail suspension test (TST), forced swimming test (FST), and sucrose preference test (SPT). SNC80 administration significantly reversed depressive-like behaviors, and this antidepressant effect could be blocked by a TrkB inhibitor: ANA-12. Although ANA-12 treatment had no significant effect on the expression of ENK/δOR, it blocked the promoting effects of brain-derived neurotrophic factor (BDNF)/tyrosine kinase B(TrkB) signaling by SNC80 in the hippocampus and amygdala. Therefore, the present study demonstrates that SNC80 exerts anti-depressant effects by up-regulating the BDNF/TrkB signaling pathway in the hippocampus and amygdala in CRS-induced depression and provides evidence that δOR's agonists may be potential anti-depressant therapeutic agents.

Contribution of the Opioid System to the Antidepressant Effects of Fluoxetine

BACKGROUND

Selective serotonin reuptake inhibitors such as fluoxetine have a limited treatment efficacy. The mechanism by which some patients respond to fluoxetine while others do not remains poorly understood, limiting treatment effectiveness. We have found the opioid system to be involved in the responsiveness to fluoxetine treatment in a mouse model for anxiety- and depressive-like behavior.

METHODS

We analyzed gene expression changes in the dentate gyrus of mice chronically treated with corticosterone and fluoxetine. After identifying a subset of genes of interest, we studied their expression patterns in relation to treatment responsiveness. We further characterized their expression through in situ hybridization and the analysis of a single-cell RNA sequencing dataset. Finally, we behaviorally tested mu and delta opioid receptor knockout mice in the novelty suppressed feeding test and the forced swim test after chronic corticosterone and fluoxetine treatment.

RESULTS

Chronic fluoxetine treatment upregulates proenkephalin expression in the dentate gyrus, and this upregulation is associated with treatment responsiveness. The expression of several of the most significantly upregulated genes, including proenkephalin, is localized to an anatomically and transcriptionally specialized subgroup of mature granule cells in the dentate gyrus. We have also found that the delta opioid receptor contributes to some, but not all, of the behavioral effects of fluoxetine.

CONCLUSIONS

These data indicate that the opioid system is involved in the antidepressant effects of fluoxetine, and this effect may be mediated through the upregulation of proenkephalin in a subpopulation of mature granule cells.

Novel Pharmacological Approaches to the Treatment of Depression

Major depressive disorder is one of the most prevalent mental health disorders. Monoamine-based antidepressants were the first drugs developed to treat major depressive disorder. More recently, ketamine and other analogues were introduced as fast-acting antidepressants. Unfortunately, currently available therapeutics are inadequate; lack of efficacy, adverse effects, and risks leave patients with limited treatment options. Efforts are now focused on understanding the etiology of depression and identifying novel targets for pharmacological treatment. In this review, we discuss promising novel pharmacological targets for the treatment of major depressive disorder. Targeting receptors including N-methyl-D-aspartate receptors, peroxisome proliferator-activated receptors, G-protein-coupled receptor 39, metabotropic glutamate receptors, galanin and opioid receptors has potential antidepressant effects. Compounds targeting biological processes: inflammation, the hypothalamic-pituitary-adrenal axis, the cholesterol biosynthesis pathway, and gut microbiota have also shown therapeutic potential. Additionally, natural products including plants, herbs, and fatty acids improved depressive symptoms and behaviors. In this review, a brief history of clinically available antidepressants will be provided, with a primary focus on novel pharmaceutical approaches with promising antidepressant effects in preclinical and clinical studies.

Reported Benefits of Low-Dose Naltrexone Appear to Be Independent of the Endogenous Opioid System Involving Proopiomelanocortin Neurons and β-Endorphin

Naltrexone is an opioid receptor antagonist approved for the treatment of alcohol and opioid use disorders at doses of 50–150 mg/d. Naltrexone has also been prescribed at much lower doses (3–6 mg/d) for the off-label treatment of inflammation and pain. Currently, a compelling mechanistic explanation for the reported efficacy of low-dose naltrexone (LDN) is lacking and none of the proposed mechanisms can explain patient reports of improved mood and sense of well-being. Here, we examined the possibility that LDN might alter the activity of the endogenous opioid system involving proopiomelanocortin (POMC) neurons in the arcuate nucleus of the hypothalamus (ARH) in male and female mice. Known actions of POMC neurons could account for changes in pain perception and mood. However, using electrophysiologic, imaging and peptide measurement approaches, we found no evidence for such a mechanism. LDN did not change the sensitivity of opioid receptors regulating POMC neurons, the production of the β-endorphin precursor Pomc mRNA, nor the release of β-endorphin into plasma. Spontaneous postsynaptic currents (sPSCs) onto POMC neurons were slightly decreased after LDN treatment and GCaMP fluorescent signal, a proxy for intracellular calcium levels, was slightly increased. However, LDN treatment did not appear to change POMC neuron firing rate, resting membrane potential, nor action potential threshold. Therefore, LDN appears to have only slight effects on POMC neurons that do not translate to changes in intrinsic excitability or baseline electrical activity and mechanisms beyond POMC neurons and altered opioid receptor sensitivity should continue to be explored.

Analysis of Differentially Expressed Genes in the Dentate Gyrus and Anterior Cingulate Cortex in a Mouse Model of Depression

Major depressive disorder (MDD) is a prevalent, chronic, and relapse-prone psychiatric disease. However, the intermediate molecules resulting from stress and neurological impairment in different brain regions are still unclear. To clarify the pathological changes in the dentate gyrus (DG) and anterior cingulate cortex (ACC) regions of the MDD brain, which are the most closely related to the disease, we investigated the published microarray profile dataset GSE84183 to identify unpredictable chronic mild stress- (UCMS-) induced differentially expressed genes (DEGs) in the DG and ACC regions. Based on the DEG data, functional annotation, protein-protein interaction, and transcription factor (TF) analyses were performed. In this study, 1071 DEGs (679 upregulated and 392 downregulated) and 410 DEGs (222 upregulated and 188 downregulated) were identified in DG and ACC, respectively. The pathways and GO terms enriched by the DEGs in the DG, such as cell adhesion, proteolysis, ion transport, transmembrane transport, chemical synaptic transmission, immune system processes, response to lipopolysaccharide, and nervous system development, may reveal the molecular mechanism of MDD. However, the DEGs in the ACC involved metabolic processes, proteolysis, visual learning, DNA methylation, innate immune responses, cell migration, and circadian rhythm. Sixteen hub genes in the DG (Fn1, Col1a1, Anxa1, Penk, Ptgs2, Cdh1, Timp1, Vim, Rpl30, Rps21, Dntt, Ptk2b, Jun, Avp, Slit1, and Sema5a) were identified. Eight hub genes in the ACC (Prkcg, Grin1, Syngap1, Rrp9, Grwd1, Pik3r1, Hnrnpc, and Prpf40a) were identified. In addition, eleven TFs (Chd2, Zmiz1, Myb, Etv4, Rela, Tcf4, Tcf12, Chd1, Mef2a, Ubtf, and Mxi1) were predicted to regulate more than two of these hub genes. The expression levels of ten randomly selected hub genes that were specifically differentially expressed in the MDD-like animal model were verified in the corresponding regions in the human brain. These hub genes and TFs may be regarded as potential targets for future MDD treatment strategies, thus aiding in the development of new therapeutic approaches to MDD.

δ Opioid Receptor Inverse Agonists and their In Vivo Pharmacological Effects

The discovery of δ opioid receptor inverse agonist activity induced by ICI-174,864, which was previously reported as an δ opioid receptor antagonist, opened the door for the investigation of inverse agonism/constitutive activity of the receptors. Various peptidic or non-peptidic δ opioid receptor inverse agonists have since been developed. Compared with the reports dealing with in vitro inverse agonist activities of novel compounds or known compounds as antagonists, there have been almost no publications describing the in vivo pharmacological effects induced by a δ opioid receptor inverse agonist. After the observation of anorectic effects with the δ opioid receptor antagonism was discussed in the early 2000s, the short-term memory improving effects and antitussive effects have been very recently reported as possible pharmacological effects induced by a δ opioid receptor inverse agonist. In this review, we will survey the developed δ opioid receptor inverse agonists and summarize the possible in vivo pharmacological effects by δ opioid receptor inverse agonists. Moreover, we will discuss important issues involved in the investigation of the in vivo pharmacological effects produced by a δ opioid receptor inverse agonist.

Cell type-specific transcriptional programs in mouse prefrontal cortex during adolescence and addiction

Coordinated activity-induced transcriptional changes across multiple neuron subtypes of the prefrontal cortex (PFC) play a pivotal role in encoding and regulating major cognitive behaviors. Yet, the specific transcriptional programs in each neuron subtype remain unknown. Using single-cell RNA sequencing (scRNA-seq), here we comprehensively classify all unique cell subtypes in the PFC. We analyze transcriptional dynamics of each cell subtype under a naturally adaptive and an induced condition. Adaptive changes during adolescence (between P21 and P60), a highly dynamic phase of postnatal neuroplasticity, profoundly impacted transcription in each neuron subtype, including cell type-specific regulation of genes implicated in major neuropsychiatric disorders. On the other hand, an induced plasticity evoked by chronic cocaine addiction resulted in progressive transcriptional changes in multiple neuron subtypes and became most pronounced upon prolonged drug withdrawal. Our findings lay a foundation for understanding cell type-specific postnatal transcriptional dynamics under normal PFC function and in neuropsychiatric disease states.

Discovery of δ Opioid Receptor Full Inverse Agonists and Their Effects on Restraint Stress-Induced Cognitive Impairment in Mice

The cyclopropylmethyl group in classical δ opioid receptor (DOR) antagonist NTI, BNTX, and NTB was replaced with various electron-withdrawing groups to develop DOR inverse agonists. N-Benzyl NTB derivative SYK-657 was a potent DOR full inverse agonist and its potency was over 10-fold potent than that of a reference compound ICI-174,864. Intraperitoneal administration of SYK-657 induced the short-term memory improving effect in mice without abnormal behaviors.

Isolation stress impacts Met-enkephalin in the hypothalamo-pituitary-adrenocortical axis in growing Polish Mountain sheep: a possible role of the opioids in modulation of HPA axis

It was hypothesized that there is cross-talk between the classical constituents of the hypothalamo-pituitary-adrenocortical axis (HPA) and Met-enkephalin in the HPA axis. The study examined effects of isolation stress, sex, and age on concentrations of native Met-enkephalin and pro-enkephalin (PENK) gene expression in tissues of the HPA (hypothalamus, pituitary gland and adrenal cortex) in 3-, 6- and 9-month old female and male lambs. In addition, the effects of isolation stress on in vitro release Met-enkephalin from fragments of the hypothalamus or adrenal cortex were examined. Isolation stress was followed by decreases in the concentration of Met-enkephalin in both the pituitary gland and adrenal cortex. There were also increases in the hypothalamic concentration of Met-enkephalin together with increases in PENK gene expression in the HPA in 6- and 9-months old females and males. There were reductions in release of Met-enkephalin from hypothalamic and adrenocortical tissue in vitro after isolation stress. In the presence of naltrexone, there were increases in basal release in vitro of Met-enkephalin from hypothalamic tissue from control and stressed female lambs but a decrease in tissue from stressed male lambs. In a somewhat similar manner, the presence of naltrexone was associated with increases in the basal release of Met-enkephalin from adrenocortical tissue from control female lambs but a decrease with tissue from stressed female and both stressed and control male lambs. Lay summary   The present studies examine the impact of isolation stress on Met-enkephalin in growing female and male lambs. The results clearly showed the involvement of Met-enkephalin modulation of the psychological stress response in growing female and male lambs.

Nicotinic Cholinergic System in the Hypothalamus Modulates the Activity of the Hypothalamic Neuropeptides During the Stress Response

BACKGROUND

The hypothalamus harbors high levels of cholinergic neurons and axon terminals. Nicotinic acetylcholine receptors, which play an important role in cholinergic neurotransmission, are expressed abundantly in the hypothalamus. Accumulating evidence reveals a regulatory role for nicotine in the regulation of the stress responses. The present review will discuss the hypothalamic neuropeptides and their interaction with the nicotinic cholinergic system. The anatomical distribution of the cholinergic neurons, axon terminals and nicotinic receptors in discrete hypothalamic nuclei will be described. The effect of nicotinic cholinergic neurotransmission and nicotine exposure on hypothalamic-pituitaryadrenal (HPA) axis regulation at the hypothalamic level will be analyzed in view of the different neuropeptides involved.

METHODS

Published research related to nicotinic cholinergic regulation of the HPA axis activity at the hypothalamic level is reviewed.

RESULTS

The nicotinic cholinergic system is one of the major modulators of the HPA axis activity. There is substantial evidence supporting the regulation of hypothalamic neuropeptides by nicotinic acetylcholine receptors. However, most of the studies showing the nicotinic regulation of hypothalamic neuropeptides have employed systemic administration of nicotine. Additionally, we know little about the nicotinic receptor distribution on neuropeptide-synthesizing neurons in the hypothalamus and the physiological responses they trigger in these neurons.

CONCLUSION

Disturbed functioning of the HPA axis and hypothalamic neuropeptides results in pathologies such as depression, anxiety disorders and obesity, which are common and significant health problems. A better understanding of the nicotinic regulation of hypothalamic neuropeptides will aid in drug development and provide means to cope with these diseases. Considering that nicotine is also an abused substance, a better understanding of the role of the nicotinic cholinergic system on the HPA axis will aid in developing improved therapeutic strategies for smoking cessation.

Stress Impacts the Regulation Neuropeptides in the Rat Hippocampus and Prefrontal Cortex

Adverse life experiences increase the lifetime risk to several stress-related psychopathologies, such as anxiety or depressive-like symptoms following stress in adulthood. However, the neurochemical modulations triggered by stress have not been fully characterized. Neuropeptides play an important role as signaling molecules that contribute to physiological regulation and have been linked to neurological and psychiatric diseases. However, little is known about the influence of stress on neuropeptide regulation in the brain. Here, we have performed an exploratory study of how neuropeptide expression at adulthood is modulated by experiencing a period of multiple stressful experiences. We have targeted hippocampus and prefrontal cortex (PFC) brain areas, which have previously been shown to be modulated by stressors, employing a targeted liquid chromatography-mass spectrometry (LC-MS) based approach that permits broad peptide coverage with high sensitivity. We found that in the hippocampus, Met-enkephalin, Met-enkephalin-Arg-Phe, and Met-enkephalin-Arg-Gly-Leu were upregulated, while Leu-enkephalin and Little SAAS were downregulated after stress. In the PFC area, Met-enkephalin-Arg-Phe, Met-enkephalin-Arg-Gly-Leu, peptide PHI-27, somatostatin-28 (AA1-12), and Little SAAS were all downregulated. This systematic evaluation of neuropeptide alterations in the hippocampus and PFC suggests that stressors impact neuropeptides and that neuropeptide regulation is brain-area specific. These findings suggest several potential peptide candidates, which warrant further investigations in terms of correlation with depression-associated behaviors.

Faecal corticosterone metabolite assessment in socially housed male and female Wistar rats

Knowledge of animals' hormonal status is important for conservation studies in wild or semi-free-ranging conditions as well as for behavioural and clinical experiments conducted in laboratory research, mostly performed on rats and mice. Faecal sampling is a useful non-invasive method to obtain steroid hormone assessments. Nevertheless, in laboratory studies, unlike other contexts, faecal sampling is less utilised. One of the issues raised is the necessity to collect samples belonging to different animals, separately. Usually, researchers using faecal sampling solve this problem through the isolation of animals or taking the cage rather than single animal as unit of study. These solutions though, could lead to unreliable measurements, and cannot be applied in many studies. Our aim was to show the biological reliability of individual faecal corticosterone metabolite (FCM) assessments in socially housed male and female Wistar rats. We analytically validated the enzyme immunoassay kit used for FCM assessments. Then, we exposed the animals to two different stress stimuli that are known to activate the hypothalamus-pituitary-adrenal axis and the following release of corticosterone to biologically validate the EIA kit: environmental enrichment and predator odour. Individual faecal sampling from social animals was collected through short-time handling. The results demonstrated that both the stimuli increased FCM levels in male and female rats showing the reliability of EIA kit assessment and the applicability of our sampling method. We also found a diurnal rhythm in FCM levels. These results could help to increase the use of faecal hormone metabolite determinations in studies conducted on rats.

Enkephalins: Endogenous Analgesics with an Emerging Role in Stress Resilience

Psychological stress is a state of mental or emotional strain or tension that results from adverse or demanding circumstances. Chronic stress is well known to induce anxiety disorders and major depression; it is also considered a risk factor for Alzheimer's disease. Stress resilience is a positive outcome that is associated with preserved cognition and healthy aging. Resilience presents psychological and biological characteristics intrinsic to an individual conferring protection against the development of psychopathologies in the face of adversity. How can we promote or improve resilience to chronic stress? Numerous studies have proposed mechanisms that could trigger this desirable process. The roles of enkephalin transmission in the control of pain, physiological functions, like respiration, and affective disorders have been studied for more than 30 years. However, their role in the resilience to chronic stress has received much less attention. This review presents the evidence for an emerging involvement of enkephalin signaling through its two associated opioid receptors, μ opioid peptide receptor and δ opioid peptide receptor, in the natural adaptation to stressful lifestyles.

Involvement of the CXCL12 System in the Stimulatory Effects of Prenatal Exposure to High-Fat Diet on Hypothalamic Orexigenic Peptides and Behavior in Offspring

Exposure to a high fat diet (HFD) during gestation stimulates neurogenesis and expression of hypothalamic orexigenic neuropeptides that affect consummatory and emotional behaviors. With recent studies showing a HFD to increase inflammation, this report investigated the neuroinflammatory chemokine, CXCL12, and compared the effects of prenatal CXCL12 injection to those of prenatal HFD exposure, first, by testing whether the HFD affects circulating CXCL12 in the dam and the CXCL12 system in the offspring brain, and then by examining whether prenatal exposure to CXCL12 itself mimics the effects of a HFD on hypothalamic neuropeptides and emotional behaviors. Our results showed that prenatal exposure to a HFD significantly increased circulating levels of CXCL12 in the dam, and that daily injections of CXCL12 induced a similar increase in CXCL12 levels as the HFD. In addition, prenatal HFD exposure significantly increased the expression of CXCL12 and its receptors, CXCR4 and CXCR7, in the hypothalamic paraventricular nucleus (PVN) of the offspring. Finally, the results revealed strong similarities in the effects of prenatal HFD and CXCL12 administration, which both stimulated neurogenesis and enkephalin (ENK) expression in the PVN, while having inconsistent or no effect in other regions of the hypothalamus, and also increased anxiety as measured by several behavioral tests. These results focus attention specifically on the CXCL12 chemokine system in the PVN of the offspring as being possibly involved in the stimulatory effects of prenatal HFD exposure on ENK-expressing neurons in the PVN and their associated changes in emotional behavior.

The exercise-glucocorticoid paradox: How exercise is beneficial to cognition, mood, and the brain while increasing glucocorticoid levels

Exercise is known to have beneficial effects on cognition, mood, and the brain. However, exercise also activates the hypothalamic-pituitary-adrenal axis and increases levels of the glucocorticoid cortisol (CORT). CORT, also known as the "stress hormone," is considered a mediator between chronic stress and depression and to link various cognitive deficits. Here, we review the evidence that shows that while both chronic stress and exercise elevate basal CORT levels leading to increased secretion of CORT, the former is detrimental to cognition/memory, mood/stress coping, and brain plasticity, while the latter is beneficial. We propose three preliminary answers to the exercise-CORT paradox. Importantly, the elevated CORT, through glucocorticoid receptors, functions to elevate dopamine in the medial prefrontal cortex under chronic exercise but not chronic stress, and the medial prefrontal dopamine is essential for active coping. Future inquiries may provide further insights to promote our understanding of this paradox.

Endogenous opiates and behavior: 2014

This paper is the thirty-seventh consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2014 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (endogenous opioids and receptors), and the roles of these opioid peptides and receptors in pain and analgesia (pain and analgesia); stress and social status (human studies); tolerance and dependence (opioid mediation of other analgesic responses); learning and memory (stress and social status); eating and drinking (stress-induced analgesia); alcohol and drugs of abuse (emotional responses in opioid-mediated behaviors); sexual activity and hormones, pregnancy, development and endocrinology (opioid involvement in stress response regulation); mental illness and mood (tolerance and dependence); seizures and neurologic disorders (learning and memory); electrical-related activity and neurophysiology (opiates and conditioned place preferences (CPP)); general activity and locomotion (eating and drinking); gastrointestinal, renal and hepatic functions (alcohol and drugs of abuse); cardiovascular responses (opiates and ethanol); respiration and thermoregulation (opiates and THC); and immunological responses (opiates and stimulants). This paper is the thirty-seventh consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2014 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (endogenous opioids and receptors), and the roles of these opioid peptides and receptors in pain and analgesia (pain and analgesia); stress and social status (human studies); tolerance and dependence (opioid mediation of other analgesic responses); learning and memory (stress and social status); eating and drinking (stress-induced analgesia); alcohol and drugs of abuse (emotional responses in opioid-mediated behaviors); sexual activity and hormones, pregnancy, development and endocrinology (opioid involvement in stress response regulation); mental illness and mood (tolerance and dependence); seizures and neurologic disorders (learning and memory); electrical-related activity and neurophysiology (opiates and conditioned place preferences (CPP)); general activity and locomotion (eating and drinking); gastrointestinal, renal and hepatic functions (alcohol and drugs of abuse); cardiovascular responses (opiates and ethanol); respiration and thermoregulation (opiates and THC); and immunological responses (opiates and stimulants).

Opioid-dependent regulation of high and low fear responses in two inbred mouse strains

The molecular mechanisms underlying the susceptibility or resilience to trauma-related disorders remain incompletely understood. Opioids modulate emotional learning, but the roles of specific receptors are unclear. Here, we aimed to analyze the contribution of the opioid system to fear responses in two inbred mouse strains exhibiting distinct behavioral phenotypes. SWR/J and C57BL/6J mice were subjected to five consecutive electric footshocks (1mA each), and the contextual freezing time was measured. Stress-induced alterations in gene expression were analyzed in the amygdala and the hippocampus. In both strains, the fear response was modulated using pharmacological tools. SWR/J mice did not develop conditioned fear but exhibited increased transcriptional expression of Pdyn and Penk in the amygdala region. Blocking opioid receptors prior to the footshocks using naltrexone (2 mg/kg) or naltrindole (5 mg/kg) increased the freezing responses in these animals. The C57BL/6J strain displayed high conditioned fear, although no alteration in the mRNA abundance of genes encoding opioid precursors was observed. Double-injection of morphine (20 mg/kg) following stress and upon context re-exposure prevented the enhancement of freezing. Moreover, selective delta and kappa agonists caused a reduction in conditioned fear responses. To summarize, the increased expression of the Pdyn and Penk genes corresponded to reduced intensity of fear responses. Blockade of the endogenous opioid system restored freezing behavior in stress-resistant animals. The pharmacological stimulation of the kappa and delta opioid receptors in stress-susceptible individuals may alleviate fear. Thus, subtype-selective opioid receptor agonists may protect against the development of trauma-related disorders.