Alteration of synaptic transmission in the hippocampal-mPFC pathway during extinction trials of context-dependent fear memory in juvenile rat stress models

The impact of early life maternal deprivation on the perineuronal nets in the prefrontal cortex and hippocampus of young adult rats

Early life stress negatively impacts brain development and affects structure and function of parvalbumin immunopositive (PV+) inhibitory neurons. Main regulators of PV+ interneurons activity and plasticity are perineuronal nets (PNNs), an extracellular matrix formation that enwraps PV+ interneurons mainly in the neocortex and hippocampus. To experimentally address the impact of early life stress on the PNNs and PV+ interneurons in the medial prefrontal cortex and dorsal hippocampus in rats, we employed a 24 h maternal deprivation protocol. We show that maternal deprivation in the medial prefrontal cortex of adult rats caused a decrease in density of overall PNNs and PNNs that enwrap PV+ interneurons in the rostral cingulate cortex. Furthermore, a staining intensity decrease of overall PNNs and PNN+/PV+ cells was found in the prelimbic cortex. Finally, a decrease in both intensity and density of overall PNNs and PNNs surrounding PV+ cells was observed in the infralimbic cortex, together with increase in the intensity of VGAT inhibitory puncta. Surprisingly, maternal deprivation did not cause any changes in the density of PV+ interneurons in the mPFC, neither had it affected PNNs and PV+ interneurons in the hippocampus. Taken together, our findings indicate that PNNs, specifically the ones enwrapping PV+ interneurons in the medial prefrontal cortex, are affected by early life stress.

Effects of a psychedelic 5-HT2A receptor agonist on anxiety-related behavior and fear processing in mice

Psychedelic-assisted psychotherapy gained considerable interest as a novel treatment strategy for fear-related mental disorders but the underlying mechanism remains poorly understood. The serotonin 2A (5-HT2A) receptor is a key target underlying the effects of psychedelics on emotional arousal but its role in fear processing remains controversial. Using the psychedelic 5-HT2A/5-HT2C receptor agonist 2,5-dimethoxy-4-iodoamphetamine (DOI) and 5-HT2A receptor knockout (KO) mice we investigated the effect of 5-HT2A receptor activation on emotional processing. We show that DOI administration did not impair performance in a spontaneous alternation task but reduced anxiety-like avoidance behavior in the elevated plus maze and elevated zero maze tasks. Moreover, we found that DOI did not block memory recall but diminished fear expression in a passive avoidance task. Likewise, DOI administration reduced fear expression in an auditory fear conditioning paradigm, while it did not affect retention of fear extinction when administered prior to extinction learning. The effect of DOI on fear expression was abolished in 5-HT2A receptor KO mice. Administration of DOI induced a significant increase of c-Fos expression in specific amygdalar nuclei. Moreover, local infusion of the 5-HT2A receptor antagonist M100907 into the amygdala reversed the effect of systemic administration of DOI on fear expression while local administration of DOI into the amygdala was sufficient to suppress fear expression. Our data demonstrate that activation of 5-HT2A receptors in the amygdala suppresses fear expression but provide no evidence for an effect on retention of fear extinction.

Longitudinal hippocampal circuit change differentiates persistence and remission of pediatric posttraumatic stress disorder

BACKGROUND

Previous studies have identified functional brain abnormalities in pediatric posttraumatic stress disorder (pPTSD) suggesting altered frontoparietal-subcortical function during emotion processing. However, little is known about how the brain functionally changes over time in recovery versus the persistence of pPTSD.

METHODS

This longitudinal study recruited 23 youth with PTSD and 28 typically developing (TD) youth (ages: 8.07-17.99). Within the PTSD group, nine remitted by the 1-year follow-up (Remit) while the remaining 14 persisted (PTSD). At each visit, youth completed an emotional processing task in which they viewed threat and neutral images during functional magnetic resonance imaging (fMRI). Voxelwise activation analyses using linear mixed-effects regression were conducted using a group (TD, Remit, PTSD) by time (baseline, follow-up) by valence (threat, neutral) design. Based on activation findings, a subsequent analysis of hippocampal functional connectivity was performed using a similar model.

RESULTS

PTSD youth showed significantly increasing hippocampal activation to threatening images compared to TD youth, while the Remit group showed more similar patterns to TD youth. Subsequent hippocampal functional connectivity analyses reveal the Remit group showed increasing functional connectivity between the hippocampus and visual cortex (V4) while viewing threat stimuli.

CONCLUSIONS

These findings represent one of the first preliminary reports of functional brain substrates of persistence and remission in pPTSD. Notably, increased hippocampal activation to threat and decreased connectivity in the hippocampal-V4 network over time may contribute to persistence in pPTSD. These findings suggest potential biomarkers that could be utilized to advance the treatment of pediatric PTSD.

In vivo blockade of 5HT3 receptors in the infralimbic medial prefrontal cortex enhances fear extinction in a rat model of PTSD

Objectives

Treatments that reverse deficits in fear extinction are promising for the management of post-traumatic stress disorder (PTSD). 5-Hydroxytryptamine type 3 (5-HT3) receptor is involved involved in the extinction of fear memories. The present work aims to investigate the role of 5HT3 receptors in the infralimbic part of the medial prefrontal cortex (IL-mPFC) in extinction of conditioned fear in the single prolonged stress (SPS) model of PTSD in rats.

Materials and Methods

The effect of SPS administration was evaluated on the freezing behavior in contextual and cued fear conditioning models. After the behavioral tests, levels of 5HT3 transcription in IL-mPFC were also measured in the same animals using the real-time RT-PCR method. To evaluate the possible role of local 5HT3 receptors on fear extinction, conditioned freezing was evaluated in another cohort of animals that received local microinjections of ondansetron (a 5HT3 antagonist) and ondansetron plus a 5HT3 agonist (SR 57227A) after extinction sessions.

Results

Our findings showed that exposure to SPS increased the freezing response in both contextual and cued fear models. We also found that SPS is associated with increased expression of 5HT3 receptors in the IL-mPFC region. Ondansetron enhanced the fear of extinction in these animals and the enhancement was blocked by the 5HT3 agonist, SR 57227A.

Conclusion

It seems that up-regulation of 5HT3 receptors in IL-mPFC is an important factor in the neurobiology of PTSD and blockade of these receptors could be considered a potential treatment for this condition.

Mini-review: Elucidating the psychological, physical, and sex-based interactions between HIV infection and stress

Stress is generally classified as any mental or emotional strain resulting from difficult circumstances, and can manifest in the form of depression, anxiety, post-traumatic stress disorder (PTSD), or other neurocognitive disorders. Neurocognitive disorders such as depression, anxiety, and PTSD are large contributors to disability worldwide, and continue to affect individuals and communities. Although these disorders affect men and women, women are disproportionately represented among those diagnosed with affective disorders, a result of both societal gender roles and physical differences. Furthermore, the incidence of these neurocognitive disorders is augmented among People Living with HIV (PLWH); the physical ramifications of stress increase the likelihood of HIV acquisition, pathogenesis, and treatment, as both stress and HIV infection are characterized by chronic inflammation, which creates a more opportunistic environment for HIV. Although the stress response is facilitated by the autonomic nervous system (ANS) and the hypothalamic pituitary adrenal (HPA) axis, when the response involves a psychological component, additional brain regions are engaged. The impact of chronic stress exposure and the origin of individual variation in stress responses and resilience are at least in part attributable to regions outside the primary stress circuity, including the amygdala, prefrontal cortex, and hippocampus. This review aims to elucidate the relationship between stress and HIV, how these interact with sex, and to understand the physical ramifications of these interactions.

Intra-amygdala metaplasticity modulation of fear extinction learning

The amygdala is a key brain region involved in emotional memory formation. It is also responsible for memory modulation in other brain areas. Under extreme conditions, amygdala modulation may lead to the generation of abnormal plasticity and trauma-related psychopathologies. However, the amygdala itself is a dynamic brain region, which is amenable to long-term plasticity and is affected by emotional experiences. These alterations may modify the way the amygdala modulates activity and plasticity in other related brain regions, which in turn may alter the animal's response to subsequent challenges in what could be termed as "Behavioral metaplasticity."Because of the reciprocal interactions between the amygdala and other emotion processing regions, such as the medial prefrontal cortex (mPFC) or the hippocampus, experience-induced intra-amygdala metaplasticity could lead to alterations in mPFC-dependent or hippocampus-dependent behaviors. While initiated by alterations within the basolateral amygdala (BLA), such alterations in other brain regions may come to be independent of BLA modulation, thus establishing what may be termed "Trans-regional metaplasticity." In this article, we review evidence supporting the notions of intra-BLA metaplasticity and how this may develop into "Trans-regional metaplasticity." Future research is needed to understand how such dynamic metaplastic alterations contribute to developing psychopathologies, and how this knowledge may be translated into promoting novel interventions in psychopathologies associated with fear, stress, and trauma.

Glucocorticoid-sensitive ventral hippocampal-orbitofrontal cortical connections support goal-directed action - Curt Richter Award Paper 2019

In an ever-changing and often ambiguous environment, organisms must use previously learned associations between antecedents and outcomes to predict future associations and make optimal choices. Chronic stress can impair one's ability to flexibly adjust behaviors when environmental contingencies change, particularly in cases of early-life stress. In mice, exposure to elevated levels of the primary stress hormone, corticosterone (CORT), during early adolescence is sufficient to impair response-outcome decision making later in life, biasing response strategies towards inflexible habits. Nevertheless, neurobiological mechanisms are still being defined. Here, we report that exposure to excess CORT in adolescence causes a loss of dendritic spines on excitatory pyramidal neurons in the lateral, but not medial, orbital prefrontal cortex (loPFC) of mice, and spine loss correlates with the severity of habit biases in adulthood. Excess CORT also reduces the presence of ventral hippocampal (vHC) axon terminals in the loPFC. To identify functional consequences, we inactivated vHC→loPFC projections in typical healthy mice during a period when mice must update response-outcome expectations to optimally acquire food reinforcers. Inactivation impaired the animals' subsequent ability to sustainably choose actions based on likely outcomes, causing them to defer to habit-based response strategies. Thus, vHC→loPFC projections are necessary for response-outcome expectancy updating and a target of excess glucocorticoids during early-life development. Their degradation is likely involved in long-term biases towards habit-based behaviors following glucocorticoid excess in adolescence.

Animal models of PTSD: a challenge to be met

Recent years have seen increased interest in psychopathologies related to trauma exposure. Specifically, there has been a growing awareness to posttraumatic stress disorder (PTSD) in part due to terrorism, climate change-associated natural disasters, the global refugee crisis, and increased violence in overpopulated urban areas. However, notwithstanding the increased awareness to the disorder, the increasing number of patients, and the devastating impact on the lives of patients and their families, the efficacy of available treatments remains limited and highly unsatisfactory. A major scientific effort is therefore devoted to unravel the neural mechanisms underlying PTSD with the aim of paving the way to developing novel or improved treatment approaches and drugs to treat PTSD. One of the major scientific tools used to gain insight into understanding physiological and neuronal mechanisms underlying diseases and for treatment development is the use of animal models of human diseases. While much progress has been made using these models in understanding mechanisms of conditioned fear and fear memory, the gained knowledge has not yet led to better treatment options for PTSD patients. This poor translational outcome has already led some scientists and pharmaceutical companies, who do not in general hold opinions against animal models, to propose that those models should be abandoned. Here, we critically examine aspects of animal models of PTSD that may have contributed to the relative lack of translatability, including the focus on the exposure to trauma, overlooking individual and sex differences, and the contribution of risk factors. Based on findings from recent years, we propose research-based modifications that we believe are required in order to overcome some of the shortcomings of previous practice. These modifications include the usage of animal models of PTSD which incorporate risk factors and of the behavioral profiling analysis of individuals in a sample. These modifications are aimed to address factors such as individual predisposition and resilience, thus taking into consideration the fact that only a fraction of individuals exposed to trauma develop PTSD. We suggest that with an appropriate shift of practice, animal models are not only a valuable tool to enhance our understanding of fear and memory processes, but could serve as effective platforms for understanding PTSD, for PTSD drug development and drug testing.

Adolescent conditioning affects rate of adult fear, safety and reward learning during discriminative conditioning

Fear and reward memories formed in adulthood are influenced by prior experiences. Experiences that occur during sensitive periods, such as adolescence, can have an especially high impact on later learning. Fear and reward memories form when aversive or appetitive events co-occur with initially neutral stimuli, that then gain negative or positive emotional load. Fear and reward seeking behaviours are influenced by safety cues, signalling the non-occurrence of a threat. It is unclear how adolescent fear or reward pre-conditioning influences later dynamics of these conditioned emotions, and conditioned safety. In this study, we presented male rats with adolescent fear or reward pre-conditioning, followed by discriminative conditioning in adulthood. In this discriminative task, rats are simultaneously conditioned to reward, fear and safety cues. We show that adolescent reward pre-conditioning did not affect the rate of adult reward conditioning, but instead accelerated adult safety conditioning. Adolescent fear pre-conditioning accelerated adult fear and reward seeking behaviours but delayed adult safety expression. Together, our results suggest that the dynamics of safety conditioning can be influenced by adolescent priming of different valences. Taking adolescent experiences into consideration can have implications on how we approach therapy options for later learned fear disorders where safety learning is compromised.

Neurobiological consequences of juvenile stress: A GABAergic perspective on risk and resilience

ALBRECHT, A., MÜLLER, I., ARDI, Z., ÇALIŞKAN, G., GRUBER, D., IVENS, S., SEGAL, M., BEHR, J., HEINEMANN, U., STORK, O., and RICHTER-LEVIN, G. Neurobiological consequences of juvenile stress: A GABAergic perspective on risk and resilience. NEUROSCI BIOBEHAV REV XXX-XXX, 2016.- Childhood adversity is among the most potent risk factors for developing mood and anxiety disorders later in life. Therefore, understanding how stress during childhood shapes and rewires the brain may optimize preventive and therapeutic strategies for these disorders. To this end, animal models of stress exposure in rodents during their post-weaning and pre-pubertal life phase have been developed. Such 'juvenile stress' has a long-lasting impact on mood and anxiety-like behavior and on stress coping in adulthood, accompanied by alterations of the GABAergic system within core regions for the stress processing such as the amygdala, prefrontal cortex and hippocampus. While many regionally diverse molecular and electrophysiological changes are observed, not all of them correlate with juvenile stress-induced behavioral disturbances. It rather seems that certain juvenile stress-induced alterations reflect the system's attempts to maintain homeostasis and thus promote stress resilience. Analysis tools such as individual behavioral profiling may allow the association of behavioral and neurobiological alterations more clearly and the dissection of alterations related to the pathology from those related to resilience.

Stress and Fear Extinction

Stress has a critical role in the development and expression of many psychiatric disorders, and is a defining feature of posttraumatic stress disorder (PTSD). Stress also limits the efficacy of behavioral therapies aimed at limiting pathological fear, such as exposure therapy. Here we examine emerging evidence that stress impairs recovery from trauma by impairing fear extinction, a form of learning thought to underlie the suppression of trauma-related fear memories. We describe the major structural and functional abnormalities in brain regions that are particularly vulnerable to stress, including the amygdala, prefrontal cortex, and hippocampus, which may underlie stress-induced impairments in extinction. We also discuss some of the stress-induced neurochemical and molecular alterations in these brain regions that are associated with extinction deficits, and the potential for targeting these changes to prevent or reverse impaired extinction. A better understanding of the neurobiological basis of stress effects on extinction promises to yield novel approaches to improving therapeutic outcomes for PTSD and other anxiety and trauma-related disorders.

Repeated fluvoxamine treatment recovers juvenile stress-induced morphological changes and depressive-like behavior in rats

Human studies have suggested that early life stress such as child abuse could enhance susceptibility to depressive disorders. Moreover, the abnormalities of the prefrontal cortex have been associated with depression. Although clinical studies have implied the negative effects of early life stress on brain development, the causality and the detailed morphogenetic changes has not been clearly elucidated. In the present study, we determined the effect of juvenile stress exposure on the presentation of depressive-like behavior and the neural mechanisms involved using a rodent model. Rat pups were exposed to footshock stress during postnatal days 21-25 followed by repeated oral administration of fluvoxamine (0 or 10mg/kg/d × 14 days), which is a selective serotonin reuptake inhibitor. At the postadolescent stage forced swim test assessment of depressive-like behavior and Golgi-Cox staining of medial prefrontal cortex pyramidal neurons followed by morphological analyses were carried out. Post-adolescent behavioral and morphological studies identified the presentation of increased depressive-like behaviors and reduced spine densities and dendritic lengths of layer II/III pyramidal neuron in the infralimbic cortex, but not in the prelimbic cortex of rats exposed to juvenile stress. Repeated fluvoxamine treatment recovered the increased depressive-like behavior and reduced spine densities/dendritic lengths observed in rats exposed to footshock stress. Cortical thicknesses in the infralimbic cortex and prelimbic cortex were also reduced by juvenile stress, but these reductions were not recovered by fluvoxamine treatment. The results demonstrate cortical sensitivities to stress exposures during the juvenile stage which mediate behavioral impairments, and provide a clue to find therapeutics for early life stress-induced emotional dysfunctions.

The metabotropic glutamate receptor, mGlu5, is required for extinction learning that occurs in the absence of a context change

The metabotropic glutamate (mGlu) receptors and, in particular, mGlu5 are crucially involved in multiple forms of synaptic plasticity that are believed to underlie explicit memory. MGlu5 is also required for information transfer through neuronal oscillations and for spatial memory. Furthermore, mGlu5 is involved in extinction of implicit forms of learning. This places this receptor in a unique position with regard to information encoding. Here, we explored the role of this receptor in context-dependent extinction learning under constant, or changed, contextual conditions. Animals were trained over 3 days to take a left turn under 25% reward probability in a T-maze with a distinct floor pattern (Context A). On Day 4, they experienced either a floor pattern change (Context B) or the same floor pattern (Context A) in the absence of reward. After acquisition of the task, the animals were returned to the maze once more on Day 5 (Context A, no reward). Treatment with the mGlu5 antagonist, 2-methyl-6-(phenylethynyl) pyridine, before maze exposure on Day 4 completely inhibited extinction learning in the AAA paradigm but had no effect in the ABA paradigm. A subsequent return to the original context (A, on Day 5) revealed successful extinction in the AAA paradigm, but impairment of extinction in the ABA paradigm. These data support that although extinction learning in a new context is unaffected by mGlu5 antagonism, extinction of the consolidated context is impaired. This suggests that mGlu5 is intrinsically involved in enabling learning that once-relevant information is no longer valid.

A hippocampal nicotinic acetylcholine alpha 7-containing receptor complex is linked to memory retrieval in the multiple-T-maze in C57BL/6j mice

The link between the cholinergic and serotonergic system in cognitive function is well-documented. There is, however, limited information on spatial memory and this formed the rationale to carry out a study with the aim to show a specific link between nicotinic and serotonergic receptor complexes rather than the corresponding subunits, to spatial memory retrieval in a land maze. A total of 46 mice were used and divided into two groups, trained and untrained (yoked) in the multiple-T-Maze (MTM) and following training during the first four days, probe trials for memory retrieval were performed on days 8, 16 and 30. Six hours following scarification, hippocampi were taken for the analysis of native receptor complex levels using blue-native gels followed by immunoblotting with specific antibodies. 5-HT1A-, 5-HT7-, nAChα4- and nACh-α7-containing receptor complexes were observed and were paralleling memory retrievals and receptor complex levels were shown to be significantly different between trained and yoked animals. Only levels of a nicotinic acetylcholine α7 receptor-containing complex at an apparent molecular weight of approximately 480kDa were shown to be linked to memory retrieval on day 8 but not to retrievals on days 16 and 30 when memory extinction has taken place. Correlation between nAChα4-, 5-HT1A- and 5-HT7-containing receptors and latencies on day 16 may point to a probable link in extinction mechanisms. A series of the abovementioned receptor complexes were correlating among each other probably indicating a serotonergic/cholinergic network paralleling spatial memory formation.

Early life stress affects the serotonergic system underlying emotional regulation

Traumatic events in early life are implicated in an increased risk of psychiatric diseases, such as depression and anxiety disorders. Serotonin is thought to play a central role in stress-induced psychiatric diseases. Serotonergic systems, including neural organization and receptor function, could dramatically change with each developmental stage. Here, we reviewed the persistent influence of early life stress on emotional regulation, focusing on the serotonergic system in rats. An aversive stimulus, foot shock (FS), during the early postnatal period (2-3 weeks after birth) produced behavioral, neuroanatomical and electrophysiological changes accompanied by serotonergic dysfunction, especially functional impairment of the serotonin (5-hydroxytryptamine; 5-HT)1A receptor in the cortico-limbic area. These findings suggest that normalization of the cortico-limbic serotonergic function has therapeutic potential for early stress-induced emotional disturbance.

Prepubertal stress and hippocampal function: sex-specific effects

The chances of developing psychiatric disorders in adulthood are increased when stress is experienced early in life. In particular, stress experienced in the childhood or 'prepubertal' phase is associated with the later development of disorders such as depression, anxiety, post-traumatic stress disorder, and psychosis. Relatively little is known about the biological basis of this effect, but one hypothesis is that prepubertal stress produces long-lasting changes in brain development, particularly in stress sensitive regions such as the hippocampus, leaving an individual vulnerable to disorders in adulthood. In this study, we used an animal model of prepubertal stress to investigate the hypothesis that prepubertal stress induces alterations in hippocampal function in adulthood. Male and female rats were exposed to a brief, variable prepubertal stress protocol (postnatal days 25-27), and their performance in two distinct hippocampal-dependent tasks (contextual fear and spatial navigation) was compared with controls in adulthood. Prepubertal stress significantly impaired contextual fear responses in males and enhanced performance in spatial navigation in females. These results demonstrate that exposure to a brief period of stress in the prepubertal phase alters hippocampal-dependent behaviors in adulthood in a sex-specific manner.

Adolescent traumatic stress experience results in less robust conditioned fear and post-extinction fear cue responses in adult rats

Early exposure to a traumatic event may produce lasting effects throughout the lifespan. Traumatic stress during adolescence may deliver a distinct developmental insult compared with more-often studied neonatal or juvenile traumatic stress paradigms. The present study describes the lasting effects of adolescent traumatic stress upon adulthood fear conditioning. Adolescent rats were exposed to a traumatic stressor (underwater trauma, UWT), then underwent fear conditioning during adulthood. Fear extinction was tested over five conditioned suppression extinction sessions three weeks later. The efficacies of two potential extinction-enhancing compounds, endocannabinoid reuptake inhibitor AM404 (10mg/kg) and M1 muscarinic positive allosteric modulator BQCA (10mg/kg), were also assessed. Finally, post-extinction fear responses were examined using a fear cue (light) as a prepulse stimulus. Rats traumatically stressed during adolescence showed blunted conditioned suppression on day 1 of extinction training, and AM404 reversed this effect. Post-extinction startle testing showed that fear conditioning eliminates prepulse inhibition to the light cue. Startle potentiation was observed only in rats without adolescent UWT exposure. AM404 and BQCA both ameliorated this startle potentiation, while BQCA increased startle in the UWT group. These results suggest that exposure to a traumatic stressor during adolescence alters developmental outcomes related to stress response and fear extinction compared to rats without adolescent traumatic stress exposure, blunting the adulthood fear response and reducing residual post-extinction fear expression. Efficacy of pharmacological interventions may also vary as a factor of developmental traumatic stress exposure.

A rat model of pre-puberty (juvenile) stress-induced predisposition to stress-related disorders: sex similarities and sex differences in effects and symptoms

OBJECTIVES

This study assessed the interactive effect of two risk factors: "Juvenile stress" and sex in the long-term consequences of "Juvenile stress" in male and female rats.

METHODS

Rats were exposed to "Juvenile stress" and to additional stress in adulthood. Measurements of anxiety and depressive-like behaviours were assessed in relation to each stress exposure and "Sex-specific" sets of criteria in order to characterize individual profiles of altered behaviours.

RESULTS

While both male and female rats were affected by exposure to "Juvenile stress", sex difference were evident in saccharine preference, coping with the stressful challenge of the two-way shuttle avoidance task, and on "Adult stress" induced changes in saccharine preference. "Profiling" altered behaviours revealed sex differences also in the prevalence of rats exhibiting different categories of "Affected" behaviours, indicating that female rats are more susceptible to the long-term effects of "Juvenile stress" and to the immediate effects of "Adulthood stress". Additionally, the prevalence of "Affected" animals among "Juvenile+ Adulthood stress" was similar, yet the profile of altered behaviours was significantly different.

CONCLUSIONS

The "Behavioural Profiling" approach presented here is of importance to understanding gender differences in the aetiology of predisposition to stress-related disorders, and of gender symptomatology differences in stress-related disorders.

Surgical incision-induced nociception causes cognitive impairment and reduction in synaptic NMDA receptor 2B in mice

Postoperative cognitive dysfunction (POCD) is associated with impairments in daily functioning, and increased morbidity and mortality. However, the causes and neuropathogenesis of POCD remain largely unknown. Uncontrolled pain often occurs postoperatively. We therefore set out to determine the effects of surgical incision-induced nociception on the cognitive function and its underlying mechanisms in 3- and 9-month-old mice. The mice had surgical incision in the hindpaw and then were tested for nociceptive threshold, learning, and memory. Brain levels of NMDA receptor and cyclin-dependent kinase 5 (CDK5) were also assessed. We found that surgical incision-induced nociception in mice led to a decreased freezing time in the tone test (which assesses the hippocampus-independent learning and memory function), but not the context test, of Fear Conditioning System at 3 and 7 d, but not 30 d post incision in 9-month-old, but not 3-month-old mice. Consistently, the surgical incision selectively decreased synaptic NMDA receptor 2B levels in the medial prefrontal cortex, and increased levels of tumor necrosis factor-α and CDK5 in the cortex, but not hippocampus, of the mice. Finally, eutectic mixture of local anesthetics and CDK5 inhibitor, roscovitine, attenuated the surgical incision-induced reduction in the synaptic NMDA receptor 2B levels and learning impairment. These results suggested that surgical incision-induced nociception reduced the synaptic NMDA receptor 2B level in the medial prefrontal cortex of mice, which might lead to hippocampus-independent learning impairment, contributing to POCD. These findings call for further investigation to determine the role of surgical incision-induced nociception in POCD.

Loss of Acid sensing ion channel-1a and bicarbonate administration attenuate the severity of traumatic brain injury

Traumatic brain injury (TBI) is a common cause of morbidity and mortality in people of all ages. Following the acute mechanical insult, TBI evolves over the ensuing minutes and days. Understanding the secondary factors that contribute to TBI might suggest therapeutic strategies to reduce the long-term consequences of brain trauma. To assess secondary factors that contribute to TBI, we studied a lateral fluid percussion injury (FPI) model in mice. Following FPI, the brain cortex became acidic, consistent with data from humans following brain trauma. Administering HCO₃⁻ after FPI prevented the acidosis and reduced the extent of neurodegeneration. Because acidosis can activate acid sensing ion channels (ASICs), we also studied ASIC1a^−/− mice and found reduced neurodegeneration after FPI. Both HCO₃⁻ administration and loss of ASIC1a also reduced functional deficits caused by FPI. These results suggest that FPI induces cerebral acidosis that activates ASIC channels and contributes to secondary injury in TBI. They also suggest a therapeutic strategy to attenuate the adverse consequences of TBI.

Effects of stressors in adolescence on learning and memory in rodent models

This article is part of a Special Issue "Puberty and Adolescence". Learning and memory is affected by a myriad of factors, including exposure to stressors and the corresponding rise in circulating glucocorticoids. Nevertheless, the effects of stressors depend on the sex, species, the type of stressor used, the duration of exposure, as well as the developmental time-point in which stressors are experienced. Effects of stress in adolescence, however, have received less attention than other developmental periods. In adolescence, the hypothalamic-pituitary-adrenal axis and brain regions involved in learning and memory, which also richly express corticosteroid receptors, are continuing to develop, and thus the effects of stress exposures would be expected to differ from those in adulthood. We conclude from a review of the available literature in animal models that hippocampal function is particularly sensitive to adolescent stressors, and the effects tend to be most evident several weeks after the exposure, suggesting stressors alter the developmental trajectory of the hippocampus.

Higher in vivo serotonin-1a binding in posttraumatic stress disorder: a PET study with [11C]WAY-100635

BACKGROUND

Brain serotonin-1A receptors (5-HT1A ) are implicated in anxiety. We compared regional brain 5-HT1A binding in medication-free participants with posttraumatic stress disorder (PTSD) and healthy volunteers using fully quantitative positron emission tomography (PET) methods.

METHODS

Twenty patients with DSM-IV PTSD (13 with comorbid major depressive disorder, [MDD]) and 49 healthy volunteers underwent PET imaging with 5-HT1A antagonist radioligand [C-11]WAY100635. Arterial blood sampling provided a metabolite-corrected input function and the concentration of free ligand in plasma (fP ) for estimation of regional binding potential, BPF ( = Bavailable / KD ). Linear mixed modeling compared BPF between groups across regions of interest (ROIs).

RESULTS

The PTSD group had higher 5-HT1A BPF across brain ROIs (P = .0006). Post hoc comparisons showed higher 5-HT1A BPF in PTSD in all cortical ROIs (26-33%), amygdala (34%), and brainstem raphe nuclei (43%), but not hippocampus. The subgroup of seven PTSD patients without comorbid MDD had higher 5-HT1A BPF compared with healthy volunteers (P = .03).

CONCLUSIONS

This is the first report of higher brainstem and forebrain 5-HT1A binding in vivo in PTSD. The finding is independent of MDD. PTSD and MDD have in common an upregulation of 5-HT1A binding including midbrain autoreceptors that would favor less firing and serotonin release. This abnormality may represent a common biomarker of these stress-associated brain disorders.

Facilitation of fear extinction by the 5-HT(1A) receptor agonist tandospirone: possible involvement of dopaminergic modulation

Fear extinction-based exposure treatment is an important component of psychotherapy for anxiety disorders such as posttraumatic stress disorder (PTSD). Recent studies have focused on pharmacological approaches combined with exposure therapy to augment extinction. In this study, we elucidated the therapeutic potential of the serotonin 1A (5-HT(1A) ) receptor agonist tandospirone compared with the effects of the N-methyl-D-aspartate partial agonist D-cycloserine (DCS), focusing on the possible involvement of dopaminergic mechanisms. We used a rat model of juvenile stress [aversive footshock (FS)] exposure during the third postnatal week (3wFS). The 3wFS group exhibited extinction deficit reflected in sustained fear-related behavior and synaptic dysfunction in the hippocampal CA1 field and medial prefrontal cortex (mPFC), which are responsible for extinction processes. Tandospirone administration (5 mg/kg, i.p.) before and after the extinction trials ameliorated both the behavioral deficit and synaptic dysfunction, i.e., synaptic efficacy in the CA1 field and mPFC associated with extinction training and retrieval, respectively, was potentiated in the tandospirone-treated 3wFS group. Extracellular dopamine release in the mPFC was increased by extinction retrieval in the non-FS control group. This facilitation was not observed in the 3wFS group; however, tandospirone treatment increased cortical dopamine levels after extinction retrieval. DCS (15 mg/kg, i.p.) also ameliorated the extinction deficit in the 3wFS group, but impaired extinction in the non-FS control group. These results suggest that tandospirone has therapeutic potential for enhancing synaptic efficacy associated with extinction processes by involving dopaminergic mechanisms. Pharmacological agents that target cortical dopaminergic systems may provide new insights into the development of therapeutic treatments of anxiety disorders, including PTSD.

Serotonergic modulation of conditioned fear

Conditioned fear plays a key role in anxiety disorders as well as depression and other neuropsychiatric conditions. Understanding how neuromodulators drive the associated learning and memory processes, including memory consolidation, retrieval/expression, and extinction (recall), is essential in the understanding of (individual differences in vulnerability to) these disorders and their treatment. The human and rodent studies I review here together reveal, amongst others, that acute selective serotonin reuptake inhibitor (SSRI) treatment facilitates fear conditioning, reduces contextual fear, and increases cued fear, chronic SSRI treatment reduces both contextual and cued fear, 5-HT1A receptors inhibit the acquisition and expression of contextual fear, 5-HT2A receptors facilitates the consolidation of cued and contextual fear, inactivation of 5-HT2C receptors facilitate the retrieval of cued fear memory, the 5-HT3 receptor mediates contextual fear, genetically induced increases in serotonin levels are associated with increased fear conditioning, impaired cued fear extinction, or impaired extinction recall, and that genetically induced 5-HT depletion increases fear conditioning and contextual fear. Several explanations are presented to reconcile seemingly paradoxical relationships between serotonin levels and conditioned fear.

Using the conditioned fear stress (CFS) animal model to understand the neurobiological mechanisms and pharmacological treatment of anxiety

SUMMARY

The mechanisms underlying the etiology and pathophysiology of anxiety disorders - the most prevalent class of mental disorders - remain unclear. Over the last 30 years investigators have used the animal model of conditioned fear stress (CFS) to investigate the brain structures and neurotransmitter systems involved in aversive emotional learning and memory. Recent studies have focused on the neuronal circuitry and cellular mechanisms of fearful emotional experiences. This review describes the CFS paradigm, discusses the neural circuit and neurotransmission underlying CFS, and explains the mechanism of action of pharmacological treatments of CFS. The focus of the review is on the molecular mechanisms of fear extinction, a phenomenon directly implicated in the clinical treatment of anxiety. Based on our assessment of previous work we will conclude by considering potential molecular targets for treating symptoms of anxiety and fear.

Photoperiod alters fear responses and basolateral amygdala neuronal spine density in white-footed mice (Peromyscus leucopus)

Photoperiodism is a biological phenomenon in which environmental day length is monitored to ascertain time of year to engage in seasonally appropriate adaptations. This trait is common among organisms living outside of the tropics. White-footed mice (Peromyscus leucopus) are small photoperiodic rodents which display a suite of adaptive responses to short day lengths, including reduced hippocampal volume, impairments in hippocampal-mediated memory, and enhanced hypothalamic-pituitary-adrenal axis reactivity. Because these photoperiodic changes in brain and behavior mirror some of the etiology of post-traumatic stress disorder (PTSD), we hypothesized that photoperiod may also alter fear memory and neuronal morphology within the hippocampus-basolateral amygdala-prefrontal cortex fear circuit. Ten weeks of exposure to short days increased fear memory in an auditory-cued fear conditioning test. Short days also increased dendritic spine density of the neurons of the basolateral amygdala, without affecting morphology of pyramidal neurons within the infralimbic region of the medial prefrontal cortex. Taken together, photoperiodic phenotypic changes in brain morphology and physiology induced by a single environmental factor, exposure to short day lengths, affect responses to fearful stimuli in white-footed mice. These results have potential implications for understanding seasonal changes in fear responsiveness, as well as for expanding translational animal models for studying gene-environment interactions underlying psychiatric diseases, such as PTSD.

Prenatal exposure to chronic mild stress increases corticosterone levels in the amniotic fluid and induces cognitive deficits in female offspring, improved by treatment with the antidepressant drug amitriptyline

Prenatal stress and associated in utero exposure to elevated levels of stress hormones can adversely affect the development of the central nervous system, thereby increasing the risk of mental illnesses in later life. Here, we examined the impact of prenatal exposure to chronic mild stress (CMS) on locomotion, anxiety-related behaviour, cognition and hippocampal serotonergic neurotransmission in juvenile and adult B6D2F2 mice, and whether antidepressant treatment in adulthood could reverse the observed behavioural disturbances. Pregnant B6D2F1 female mice were either subjected to CMS or left undisturbed until parturition. Three-week and 7-week-old male and female offspring were assessed in the open-field, novel object recognition and contextual fear conditioning tests. Hippocampal levels of serotonin and its major metabolite were then quantified using high-performance liquid chromatography. Some prenatally-stressed adult females were treated with amitriptyline (20mg/kg/day in drinking water) for 10 days, from the day prior to onset of behavioural testing. In a separate experiment, amniotic fluid was collected from stressed and non-stressed dams on gestational (G) days 13 and 18 to quantify levels of corticosterone. We found that prenatal CMS specifically impaired learning and memory performance in adult females. Amitriptyline elevated hippocampal serotonin levels and attenuated these cognitive deficits. Corticosterone levels in the amniotic fluid were increased by CMS on G13 but by G18, the levels in non-stressed dams reached those of stressed dams. These results suggest that female mice are particularly vulnerable to the adverse developmental effects of prenatal stress which can be improved by appropriate treatment strategies including antidepressants.

Learning-induced changes in mPFC-BLA connections after fear conditioning, extinction, and reinstatement of fear

The neural circuit linking the medial prefrontal cortex (mPFC) and the basolateral amygdala (BLA) has crucial roles in both the acquisition and the extinction of fear. However, the mechanism by which this circuit encodes fear and extinction remains unknown. In this study, we monitored changes in the magnitude of evoked field potentials (EFPs) in the mPFC-BLA and BLA-mPFC pathways following auditory fear conditioning and extinction, in freely moving rats. We report that extinction of fear is mediated by depression of the EFPs in the mPFC-BLA and by potentiation in the reciprocal pathway of BLA-mPFC. Interestingly, reinstatement of fear was associated with recovery of freezing and with reversal of the changes in EFPs that were observed following extinction in both pathways. The findings indicate that the mPFC-BLA circuit expresses differential changes following fear and extinction and point to dynamic and plastic changes underlying fear, extinction, and reinstatement. Manipulations targeting these different types of plasticity could constitute a therapeutic tool for the treatment of anxiety disorders.

Juvenile stress attenuates the dorsal hippocampal postsynaptic 5-HT1A receptor function in adult rats

RATIONALE

Traumatic events in early life are associated with an increased risk of psychiatric diseases in adulthood. 5-hydroxytryptamine (5-HT)(1A) receptors play a pivotal role in the 5-HTergic mechanisms associated with the etiology of stress-related disorders.

OBJECTIVE

The goal of the present study was to investigate whether juvenile stress influences emotional control via postsynaptic 5-HT(1A) receptor in the hippocampus and amygdala using contextual fear conditioning test in adult rats.

METHODS

The rats were subjected to aversive footshock (FS) during the third week of the postnatal period (3wFS group). During the postadolescent period (10-14 weeks postnatal), experiments were performed.

RESULTS

The systemic administration of the 5-HT(1A) receptor agonist R-(+)-8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) (0.2 mg/kg, i.p.) attenuated the freezing behavior in the non-FS group, but not in the 3wFS group. The bilateral local injection of 8-OH-DPAT (1 μg/side) into the amygdala decreased the freezing behavior in the non-FS group and the 3wFS group. However, the local injection of 8-OH-DPAT (1 μg/side) into the hippocampus decreased the freezing behavior in the non-FS group, but not in the 3wFS group. In a 5-HT(1A) receptor binding study, the Bmax of the 3wFS group decreased in the dorsal hippocampus, but not the amygdala in comparison with the non-FS group.

CONCLUSIONS

The juvenile stress attenuated the hippocampal postsynaptic 5-HT(1A) receptor function in context-dependent conditioned fear.

Pharmacological enhancement of memory and executive functioning in laboratory animals

Investigating how different pharmacological compounds may enhance learning, memory, and higher-order cognitive functions in laboratory animals is the first critical step toward the development of cognitive enhancers that may be used to ameliorate impairments in these functions in patients suffering from neuropsychiatric disorders. Rather than focus on one aspect of cognition, or class of drug, in this review we provide a broad overview of how distinct classes of pharmacological compounds may enhance different types of memory and executive functioning, particularly those mediated by the prefrontal cortex. These include recognition memory, attention, working memory, and different components of behavioral flexibility. A key emphasis is placed on comparing and contrasting the effects of certain drugs on different cognitive and mnemonic functions, highlighting methodological issues associated with this type of research, tasks used to investigate these functions, and avenues for future research. Viewed collectively, studies of the neuropharmacological basis of cognition in rodents and non-human primates have identified targets that will hopefully open new avenues for the treatment of cognitive disabilities in persons affected by mental disorders.

Mechanisms underlying ketamine-induced synaptic depression in rat hippocampus-medial prefrontal cortex pathway

The non-competitive N-methyl-D-aspartate NMDA receptor antagonist ketamine, a dissociative anesthetic capable of inducing analgesia, is known to have psychotomimetic actions, but the detailed mechanisms remain unclear because of its complex properties. The present study elucidated neural mechanisms of the effect of ketamine, at doses that exert psychotomimetic effects without anesthetic and analgesic effects, by evaluating cortical synaptic responses in vivo. Systemic administration (i.p.) of low (1 and 5 mg/kg), subanesthetic (25 mg/kg) and anesthetic (100 mg/kg) doses of ketamine dose-dependently decreased hippocampal stimulation-evoked potential in the medial prefrontal cortex (mPFC) in freely moving rats. The behavioral analysis assessed by prepulse inhibition (PPI) of acoustic startle response showed that ketamine (5 and 25 mg/kg, i.p.) produced PPI deficit. Thus, the psychotomimetic effects observed in ketamine-treated groups (5 and 25 mg/kg, i.p.) are associated with the induction of synaptic depression in the hippocampus-mPFC neural pathway. Based on these results, we further examined the underlying mechanisms of the ketamine-induced synaptic depression under anesthesia. Ketamine (5 and 25 mg/kg, i.p.) caused increases in dialysate dopamine in the mPFC in anesthetized rats. Moreover, the ketamine-induced decreases in the evoked potential, at the dose 5 mg/kg which has no anesthetic and analgesic effects, were indeed absent in dopamine-lesioned rats pretreated with 6-hydroxydopamine (6-OHDA; 150 μg/rat, i.c.v.). Ketamine (5 mg/kg, i.p.)-induced synaptic depression was blocked by pretreatment with dopamine D1 receptor antagonist SCH 23390 (10 μg/rat, i.c.v.) but not dopamine D2 receptor antagonist haloperidol (1.5 mg/kg, i.p.), suggesting that dopaminergic modulation mediated via D1 receptors are involved in the synaptic effects of ketamine. Furthermore, ketamine (5 mg/kg, i.p.)-induced synaptic depression was prevented also by GABAA receptor antagonist bicuculline (0.2 or 2 μg/rat, i.c.v.). These findings suggest that ketamine at the dose that exerts psychotomimetic symptoms depresses hippocampus-mPFC synaptic transmission through mechanisms involving dopaminergic modulation mediated via D1 receptors, which may lead to a net augmentation of synaptic inhibition mediated via GABAA receptors.

Adolescent development, hypothalamic-pituitary-adrenal function, and programming of adult learning and memory

Chronic exposure to stress is known to affect learning and memory in adults through the release of glucocorticoid hormones by the hypothalamic-pituitary-adrenal (HPA) axis. In adults, glucocorticoids alter synaptic structure and function in brain regions that express high levels of glucocorticoid receptors and that mediate goal-directed behaviour and learning and memory. In contrast to relatively transient effects of stress on cognitive function in adulthood, exposure to high levels of glucocorticoids in early life can produce enduring changes through substantial remodeling of the developing nervous system. Adolescence is another time of significant brain development and maturation of the HPA axis, thereby providing another opportunity for glucocorticoids to exert programming effects on neurocircuitry involved in learning and memory. These topics are reviewed, as is the emerging research evidence in rodent models highlighting that adolescence may be a period of increased vulnerability compared to adulthood in which exposure to high levels of glucocorticoids results in enduring changes in adult cognitive function.

Early stress exposure impairs synaptic potentiation in the rat medial prefrontal cortex underlying contextual fear extinction

Traumatic events during early life may affect the neural systems associated with memory function, including extinction, and lead to altered sensitivity to stress later in life. We recently reported that changes in prefrontal synaptic efficacy in response to extinction trials did not occur in adult rats exposed to early postnatal stress (i.e. footshock [FS] stress during postnatal day 21-25 [3W-FS group]). However, identifying neurocircuitry and neural mechanisms responsible for extinction retrieval after extinction training have not been precisely determined. The present study explored whether synaptic transmission in the hippocampal-medial prefrontal cortex (mPFC) neural pathway is altered by extinction retrieval on the day after extinction trials using electrophysiological approaches combined with behavioral analysis. We also elucidated the effects of early postnatal stress on the synaptic response in this neural circuit underlying extinction retrieval. Evoked potential in the mPFC was enhanced following extinction retrieval, accompanied by reduced freezing behavior. This synaptic facilitation (i.e. a long-term potentiation [LTP]-like response) did not occur; rather synaptic inhibition was observed in the 3W-FS group, accompanied by sustained freezing. The behavioral deficit and synaptic inhibition observed in the 3W-FS group were time-dependently ameliorated by the partial N-methyl-D-aspartate (NMDA) receptor agonist D-cycloserine (15 mg/kg, i.p.). These findings suggest that the LTP-like response in the hippocampal-mPFC pathway is associated with extinction retrieval of context-dependent fear memory. Early postnatal stress appears to induce neurodevelopmental dysfunction of this neural circuit and lead to impaired fear extinction later in life. The present data indicate that psychotherapy accompanied by pharmacological interventions that accelerate and strengthen extinction, such as d-cycloserine treatment, may have therapeutic potential for the treatment of anxiety disorders, including posttraumatic stress disorder.