Convergence and interaction of hippocampal and amygdalar projections within the prefrontal cortex in the rat

Late-adolescent onset of prefrontal endocannabinoid control of hippocampal and amygdalar inputs and its impact on trace-fear conditioning behavior

Prefrontal cortex (PFC) maturation during adolescence is characterized by structural and functional changes, which involve the remodeling of GABA and glutamatergic synapses, as well as changes in the endocannabinoid system. Yet, the way PFC endocannabinoid signaling interacts with local GABA and glutamatergic function to impact its processing of afferent transmission during the adolescent transition to adulthood remains unknown. Here we combined PFC local field potential recordings with local manipulations of 2-AG and anandamide levels to assess how PFC endocannabinoid signaling is recruited to modulate ventral hippocampal and basolateral amygdalar inputs in vivo in adolescent and adult male rats. We found that the PFC endocannabinoid signaling does not fully emerge until late-adolescence/young adulthood. Once present, both 2-AG and anandamide can be recruited in the PFC to limit the impact of hippocampal drive through a CB1R-mediated mechanism whereas basolateral amygdalar inputs are only inhibited by 2-AG. Similarly, the behavioral effects of increasing 2-AG and anandamide in the PFC do not emerge until late-adolescence/young adulthood. Using a trace fear conditioning paradigm, we found that elevating PFC 2-AG levels preferentially reduced freezing behavior during acquisition without affecting its extinction. In contrast, increasing anandamide levels in the PFC selectively disrupted the extinction of trace fear memory without affecting its acquisition. Collectively, these results indicate a protracted recruitment of PFC endocannabinoid signaling, which becomes online in late adolescence/young adulthood as revealed by its impact on hippocampal and amygdalar-evoked local field potential responses and trace fear memory behavior.

A "double-edged" role for type-5 metabotropic glutamate receptors in pain disclosed by light-sensitive drugs

Knowing the site of drug action is important to optimize effectiveness and address any side effects. We used light-sensitive drugs to identify the brain region-specific role of mGlu5 metabotropic glutamate receptors in the control of pain. Optical activation of systemic JF-NP-26, a caged, normally inactive, negative allosteric modulator (NAM) of mGlu5 receptors, in cingulate, prelimbic and infralimbic cortices and thalamus inhibited neuropathic pain hypersensitivity. Systemic treatment of alloswitch-1, an intrinsically active mGlu5 receptor NAM, caused analgesia, and the effect was reversed by light-induced drug inactivation in in the prelimbic and infralimbic cortices, and thalamus. This demonstrates that mGlu5 receptor blockade in the medial prefrontal cortex and thalamus is both sufficient and necessary for the analgesic activity of mGlu5 receptor antagonists. Surprisingly, when light was delivered in the basolateral amygdala, local activation of systemic JF-NP-26 reduced pain thresholds, whereas inactivation of alloswitch-1 enhanced analgesia. Electrophysiological analysis showed that alloswitch-1 increased excitatory synaptic responses in prelimbic pyramidal neurons evoked by stimulation of BLA input, and decreased feedforward inhibition of amygdala output neurons by BLA. Both effects were reversed by optical silencing and reinstated by optical reactivation of alloswitch-1. These findings demonstrate for the first time that the action of mGlu5 receptors in the pain neuraxis is not homogenous, and suggest that blockade of mGlu5 receptors in the BLA may limit the overall analgesic activity of mGlu5 receptor antagonists. This could explain the suboptimal effect of mGlu5 NAMs on pain in human studies and validate photopharmacology as an important tool to determine ideal target sites for systemic drugs.

Dynorphin / kappa-opioid receptor regulation of excitation-inhibition balance toggles afferent control of prefrontal cortical circuits in a pathway-specific manner

The medial prefrontal cortex (mPFC) controls behavior via connections with limbic excitatory afferents that engage various inhibitory motifs to shape mPFC circuit function. The dynorphin (Dyn) / kappa-opioid receptor (KOR) system is highly enriched in the mPFC, and its dysregulation is implicated in neuropsychiatric disorders. However, it is unclear how the Dyn / KOR system modulates excitatory and inhibitory circuits that are integral for mPFC information processing and behavioral control. Here, we provide a circuit-based framework wherein mPFC Dyn / KOR signaling regulates excitation-inhibition balance by toggling which afferents drive mPFC neurons. Dyn / KOR regulation of afferent inputs is pathway-specific. Dyn acting on presynaptic KORs inhibits glutamate release from afferent inputs to the mPFC, including the basolateral amygdala (BLA), paraventricular nucleus of the thalamus, and contralateral cortex. The majority of excitatory synapses to mPFC neurons, including those from the ventral hippocampus (VH), do not express presynaptic KOR, rendering them insensitive to Dyn / KOR modulation. Dyn / KOR signaling also suppresses afferent-driven recruitment of specific inhibitory sub-networks, providing a basis for Dyn to disinhibit mPFC circuits. Specifically, Dyn / KOR signaling preferentially suppresses SST interneuron- relative to PV interneuron-mediated inhibition. Selective KOR action on afferents or within mPFC microcircuits gates how distinct limbic inputs drive spiking in mPFC neurons. Presynaptic Dyn / KOR signaling decreases KOR-positive input-driven (e.g. BLA) spiking of mPFC neurons. In contrast, KOR-negative input recruitment of mPFC neurons is enhanced by Dyn / KOR signaling via suppression of mPFC inhibitory microcircuits. Thus, by acting on distinct circuit elements, Dyn / KOR signaling shifts KOR-positive and negative afferent control of mPFC circuits, providing mechanistic insights into the role of neuropeptides in shaping mPFC function. Together, these findings highlight the utility of targeting the mPFC Dyn / KOR system as a means to treat neuropsychiatric disorders characterized by dysregulation in mPFC integration of long-range afferents with local inhibitory microcircuits.

Endocannabinoid signaling and epigenetics modifications in the neurobiology of stress-related disorders

Stress exposure is associated with psychiatric conditions, such as depression, anxiety, and post-traumatic stress disorder (PTSD). It is also a vulnerability factor to developing or reinstating substance use disorder. Stress causes several changes in the neuro-immune-endocrine axis, potentially resulting in prolonged dysfunction and diseases. Changes in several transmitters, including serotonin, dopamine, glutamate, gamma-aminobutyric acid (GABA), glucocorticoids, and cytokines, are associated with psychiatric disorders or behavioral alterations in preclinical studies. Complex and interacting mechanisms make it very difficult to understand the physiopathology of psychiatry conditions; therefore, studying regulatory mechanisms that impact these alterations is a good approach. In the last decades, the impact of stress on biology through epigenetic markers, which directly impact gene expression, is under intense investigation; these mechanisms are associated with behavioral alterations in animal models after stress or drug exposure, for example. The endocannabinoid (eCB) system modulates stress response, reward circuits, and other physiological functions, including hypothalamus-pituitary-adrenal axis activation and immune response. eCBs, for example, act retrogradely at presynaptic neurons, limiting the release of neurotransmitters, a mechanism implicated in the antidepressant and anxiolytic effects after stress. Epigenetic mechanisms can impact the expression of eCB system molecules, which in turn can regulate epigenetic mechanisms. This review will present evidence of how the eCB system and epigenetic mechanisms interact and the consequences of this interaction in modulating behavioral changes after stress exposure in preclinical studies or psychiatric conditions. Moreover, evidence that correlates the involvement of the eCB system and epigenetic mechanisms in drug abuse contexts will be discussed.

Hippocampal sharp wave ripples underlie stress susceptibility in male mice

The ventral hippocampus (vHC) is a core brain region for emotional memory. Here, we examined how the vHC regulates stress susceptibility from the level of gene expression to neuronal population dynamics in male mice. Transcriptome analysis of samples from stress-naïve mice revealed that intrinsic calbindin (Calb1) expression in the vHC is associated with susceptibility to social defeat stress. Mice with Calb1 gene knockdown in the vHC exhibited increased stress resilience and failed to show the increase in the poststress ventral hippocampal sharp wave ripple (SWR) rate. Poststress vHC SWRs triggered synchronous reactivation of stress memory-encoding neuronal ensembles and facilitated information transfer to the amygdala. Suppression of poststress vHC SWRs by real-time feedback stimulation or walking prevented social behavior deficits. Taken together, our results demonstrate that internal reactivation of memories of negative stressful episodes supported by ventral hippocampal SWRs serves as a crucial neurophysiological substrate for determining stress susceptibility.

Episodic Memory formation: A review of complex Hippocampus input pathways

Memories of everyday experiences involve the encoding of a rich and dynamic representation of present objects and their contextual features. Traditionally, the resulting mnemonic trace is referred to as Episodic Memory, i.e. the "what", "where" and "when" of a lived episode. The journey for such memory trace encoding begins with the perceptual data of an experienced episode handled in sensory brain regions. The information is then streamed to cortical areas located in the ventral Medio Temporal Lobe, which produces multi-modal representations concerning either the objects (in the Perirhinal cortex) or the spatial and contextual features (in the parahippocampal region) of the episode. Then, this high-level data is gated through the Entorhinal Cortex and forwarded to the Hippocampal Formation, where all the pieces get bound together. Eventually, the resulting encoded neural pattern is relayed back to the Neocortex for a stable consolidation. This review will detail these different stages and provide a systematic overview of the major cortical streams toward the Hippocampus relevant for Episodic Memory encoding.

Memory-related neurophysiological mechanisms in the hippocampus underlying stress susceptibility

Stress-induced psychiatric symptoms, such as increased anxiety, decreased sociality, and depression, differ considerably across individuals. The cognitive model of depression proposes that biased negative memory is a crucial determinant in the development of mental stress-induced disorders. Accumulating evidence from both clinical and animal studies has demonstrated that such biased memory processing could be triggered by the hippocampus, a region well known to be involved in declarative memories. This review mainly describes how memory-related neurophysiological mechanisms in the hippocampus and their interactions with other related brain regions are involved in the regulation of stress susceptibility and discusses potential interventions to prevent and treat stress-related psychiatric symptoms. Further neurophysiological insights based on memory mechanisms are expected to devise personalized prevention and therapy to confer stress resilience.

Acute restraint stress redirects prefrontal cortex circuit function through mGlu5 receptor plasticity on somatostatin-expressing interneurons

Inhibitory interneurons orchestrate prefrontal cortex (PFC) activity, but we have a limited understanding of the molecular and experience-dependent mechanisms that regulate synaptic plasticity across PFC microcircuits. We discovered that mGlu5 receptor activation facilitates long-term potentiation at synapses from the basolateral amygdala (BLA) onto somatostatin-expressing interneurons (SST-INs) in mice. This plasticity appeared to be recruited during acute restraint stress, which induced intracellular calcium mobilization within SST-INs and rapidly potentiated postsynaptic strength onto SST-INs. Restraint stress and mGlu5 receptor activation each augmented BLA recruitment of SST-IN phasic feedforward inhibition, shunting information from other excitatory inputs, including the mediodorsal thalamus. Finally, studies using cell-type-specific mGlu5 receptor knockout mice revealed that mGlu5 receptor function in SST-expressing cells is necessary for restraint stress-induced changes to PFC physiology and related behaviors. These findings provide new insights into interneuron-specific synaptic plasticity mechanisms and suggest that SST-IN microcircuits may be promising targets for treating stress-induced psychiatric diseases.

Parsing neural circuits of fear learning and extinction across basic and clinical neuroscience: Towards better translation

Over the past decades, studies of fear learning and extinction have advanced our understanding of the neurobiology of threat and safety learning. Animal studies can provide mechanistic/causal insights into human brain regions and their functional connectivity involved in fear learning and extinction. Findings in humans, conversely, may further enrich our understanding of neural circuits in animals by providing macroscopic insights at the level of brain-wide networks. Nevertheless, there is still much room for improvement in translation between basic and clinical research on fear learning and extinction. Through the lens of neural circuits, in this article, we aim to review the current knowledge of fear learning and extinction in both animals and humans, and to propose strategies to fill in the current knowledge gap for the purpose of enhancing clinical benefits.

The effect of stress and reward on encoding future fear memories

Prior experience changes the way we learn about our environment. Stress predisposes individuals to developing psychological disorders, just as positive experiences protect from this eventuality (Kirkpatrick & Heller, 2014; Koenigs & Grafman, 2009; Pechtel & Pizzagalli, 2011). Yet current models of how the brain processes information often do not consider a role for prior experience. The considerable literature that examines how stress impacts the brain is an exception to this. This research demonstrates that stress can bias the interpretation of ambiguous events towards being aversive in nature, owed to changes in amygdala physiology (Holmes et al., 2013; Perusini et al., 2016; Rau et al., 2005; Shors et al., 1992). This is thought to be an important model for how people develop anxiety disorders, like post-traumatic stress disorder (PTSD; Rau et al., 2005). However, more recent evidence suggests that experience with reward learning can also change the neural circuits that are involved in learning about fear (Sharpe et al., 2021). Specifically, the lateral hypothalamus, a region typically restricted to modulating feeding and reward behavior, can be recruited to encode fear memories after experience with reward learning. This review discusses the literature on how stress and reward change the way we acquire and encode memories for aversive events, offering a testable model of how these regions may interact to promote either adaptive or maladaptive fear memories.

Differential Rearrangement of Excitatory Inputs to the Medial Prefrontal Cortex in Chronic Pain Models

Chronic pain patients suffer a disrupted quality of life not only from the experience of pain itself, but also from comorbid symptoms such as depression, anxiety, cognitive impairment, and sleep disturbances. The heterogeneity of these symptoms support the idea of a major involvement of the cerebral cortex in the chronic pain condition. Accordingly, abundant evidence shows that in chronic pain the activity of the medial prefrontal cortex (mPFC), a brain region that is critical for executive function and working memory, is severely impaired. Excitability of the mPFC depends on the integrated effects of intrinsic excitability and excitatory and inhibitory inputs. The main extracortical sources of excitatory input to the mPFC originate in the thalamus, hippocampus, and amygdala, which allow the mPFC to integrate multiple information streams necessary for cognitive control of pain including sensory information, context, and emotional salience. Recent techniques, such as optogenetic methods of circuit dissection, have made it possible to tease apart the contributions of individual circuit components. Here we review the synaptic properties of these main glutamatergic inputs to the rodent mPFC, how each is altered in animal models of chronic pain, and how these alterations contribute to pain-associated mPFC deactivation. By understanding the contributions of these individual circuit components, we strive to understand the broad spectrum of chronic pain and comorbid pathologies, how they are generated, and how they might be alleviated.

Maturation of Corticolimbic Functional Connectivity During Sensitive Periods of Brain Development

The maturation of key corticolimbic structures and the prefrontal cortex during sensitive periods of brain development from early life through adolescence is crucial for the acquisition of a variety of cognitive and affective processes associated with adult behavior. In this chapter, we first review how key cellular and circuit level changes during adolescence dictate the development of the prefrontal cortex and its capacity to integrate contextual and emotional information from the ventral hippocampus and the amygdala. We further discuss how afferent transmission from ventral hippocampal and amygdala inputs displays unique age-dependent trajectories that directly impact prefrontal functional maturation through adolescence. We conclude by proposing that time-sensitive strengthening of specific corticolimbic synapses is a critical contributing factor for the protracted maturation of cognitive and emotional regulation by the prefrontal cortex.

Effects of Acute Stress on the Oscillatory Activity of the Hippocampus-Amygdala-Prefrontal Cortex Network

Displaying a stress response to threatening stimuli is essential for survival. These reactions must be adjusted to be adaptive. Otherwise, even mental illnesses may develop. Describing the physiological stress response may contribute to distinguishing the abnormal responses that accompany the pathology, which may help to improve the development of both diagnoses and treatments. Recent advances have elucidated many of the processes and structures involved in stress response management; however, there is still much to unravel regarding this phenomenon. The main aim of the present research is to characterize the response of three brain areas deeply involved in the stress response (i.e., to an acute stressful experience). Specifically, the electrophysiological activity of the infralimbic division of the medial prefrontal cortex (IL), the basolateral nucleus of the amygdala (BLA), and the dorsal hippocampus (dHPC) was recorded after the infusion of 0.5 µl of corticosterone-releasing factor into the dorsal raphe nucleus (DRN), a procedure which has been validated as a paradigm to cause acute stress. This procedure induced a delayed reduction in slow waves in the three structures, and an increase in faster oscillations, such as those in theta, beta, and gamma bands. The mutual information at low theta frequencies between the BLA and the IL increased, and the delta and slow wave mutual information decreased. The low theta-mid gamma phase-amplitude coupling increased within BLA, as well as between BLA and IL. This electrical pattern may facilitate the activation of these structures, in response to the stressor, and memory consolidation.

Theta-Range Oscillations in Stress-Induced Mental Disorders as an Oscillotherapeutic Target

Emotional behavior and psychological disorders are expressed through coordinated interactions across multiple brain regions. Brain electrophysiological signals are composed of diverse neuronal oscillations, representing cell-level to region-level neuronal activity patterns, and serve as a biomarker of mental disorders. Here, we review recent observations from rodents demonstrating how neuronal oscillations in the hippocampus, amygdala, and prefrontal cortex are engaged in emotional behavior and altered by psychiatric changes such as anxiety and depression. In particular, we focus mainly on theta-range (4–12 Hz) oscillations, including several distinct oscillations in this frequency range. We then discuss therapeutic possibilities related to controlling such mental disease-related neuronal oscillations to ameliorate psychiatric symptoms and disorders in rodents and humans.

Juvenile exposure to acute traumatic stress leads to long-lasting alterations in grey matter myelination in adult female but not male rats

Stress early in life can have a major impact on brain development, and there is increasing evidence that childhood stress confers vulnerability for later developing psychiatric disorders. In particular, during peri-adolescence, brain regions crucial for emotional regulation, such as the prefrontal cortex (PFC), amygdala (AMY) and hippocampus (HPC), are still developing and are highly sensitive to stress. Changes in myelin levels have been implicated in mental illnesses and stress effects on myelin and oligodendrocytes (OLs) are beginning to be explored as a novel and underappreciated mechanism underlying psychopathologies. Yet there is little research on the effects of acute stress on myelin during peri-adolescence, and even less work exploring sex-differences. Here, we used a rodent model to test the hypothesis that exposure to acute traumatic stress as a juvenile would induce changes in OLs and myelin content across limbic brain regions. Male and female juvenile rats underwent 3 h of restraint stress with exposure to a predator odor on postnatal day (p) 28. Acute stress induced a physiological response, increasing corticosterone release and reducing weight gain in stress-exposed animals. Brain sections containing the PFC, AMY and HPC were taken either in adolescence (p40), or in adulthood (p95) and stained for markers of OLs and myelin. We found that acute stress induced sex-specific changes in grey matter (GM) myelination and OLs in both the short- and long-term. Exposure to a single stressor as a juvenile increased GM myelin content in the AMY and HPC in p40 males, compared to the respective control group. At p40, corticosterone release during stress exposure was also positively correlated with GM myelin content in the AMY of male rats. Single exposure to juvenile stress also led to long-term effects exclusively in female rats. Compared to controls, stress-exposed females showed reduced GM myelin content in all three brain regions. Acute stress exposure decreased PFC and HPC OL density in p40 females, perhaps contributing towards this observed long-term decrease in myelin content. Overall, our findings suggest that the juvenile brain is vulnerable to exposure to a brief severe stressor. Exposure to a single short traumatic event during peri-adolescence produces long-lasting changes in GM myelin content in the adult brain of female, but not male, rats. These findings highlight myelin plasticity as a potential contributor to sex-specific sensitivity to perturbation during a critical window of development.

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

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

Reduced Information Transmission of Medial Prefrontal Cortex to Basolateral Amygdala Inhibits Exploratory Behavior in Depressed Rats

Depression is a mental and neurological disease that reduces the desire for exploration. Dysregulation of the information transmission between medial prefrontal cortex (mPFC) and basolateral amygdala (BLA) is associated with depression. However, which direction of information transmission (mPFC-BLA or BLA-mPFC) related to the decline of exploratory interests in depression is unclear. Therefore, it is important to determine what specific changes occur in mPFC and BLA information transmission in depressed rats during exploratory behavior. In the present study, local field potentials (LFPs) were recorded via multi-electrodes implanted in the mPFC and BLA for the control and depression groups of rats when they were exploring in an open field. The theta band was determined to be the characteristic band of exploratory behavior. The direct transfer function (DTF) was used to calculate the mPFC and BLA bidirectional information flow (IF) to measure information transmission. Compared with the control group, the theta IF of mPFC-BLA in the depression group was significantly reduced, and there was no significant difference in theta IF of BLA-mPFC between the two groups. Our results indicated that the reduction of mPFC-BLA information transmission can inhibit the exploratory behavior of depressed rats.

Decoupling of Gray and White Matter Functional Networks in Medication-Naïve Patients With Major Depressive Disorder

BACKGROUND

Major depressive disorder (MDD) has been increasingly conceptualized as a disconnection syndrome. However, most studies have only focused on functional connectivity (FC) alterations in gray matter (GM), and the functional alterations in white matter (WM) remain largely unknown in MDD.

PURPOSE

To investigate WM functional alterations and the functional interaction between GM and WM networks in medication-naïve MDD.

STUDY TYPE

Prospective.

SUBJECTS

Sixty-eight patients with MDD and 66 age- and sex-matched healthy controls (HCs).

FIELD STRENGTH/SEQUENCE

Resting state-functional MRI (fMRI) using a gradient-echo imaging sequence and T₁ -weighted images were acquired at 3.0T.

ASSESSMENT

Functional GM and WM networks, based on resting-state blood oxygenation level-dependent (BOLD) signals, were identified by the K-means clustering algorithm, and FC matrices were obtained for each subject.

STATISTICAL TESTS

Two-sample t-tests, Pearson chi-square test, and Pearson correlation analysis.

RESULTS

Both the GM and WM of the visual network (GM1 and WM11) showed reduced FC with the sensorimotor network (WM5 and GM8), lateral temporal network (GM5 and WM6), cingulo-opercular network (GM9), and dorsal attention network (GM7) in MDD patients compared to controls (P < 0.05, false discovery rate [FDR]-corrected). Reduced FC between the anterior cingulum network (WM3) and the lateral temporal network (GM5 and WM6) and temporal pole network (GM13) and between GM13 and the medial temporal network (GM4) and medial prefrontal-subcortical network (GM10) were also observed in MDD patients (P < 0.05, FDR-corrected). In addition, the WM BOLD signal in the sensorimotor network was negatively correlated with illness duration (r = -0.286, P = 0.018).

DATA CONCLUSION

Disconnectivity between the GM and WM networks in the perception-motor system may be the foundation of extensively disrupted connections in MDD. Furthermore, the observed decoupling between subsystems of the default mode network may help explain previous findings of persistent negative rumination and theory of mind deficits in depression.

LEVEL OF EVIDENCE

3.

TECHNICAL EFFICACY

Stage 3.

Contribution of the prefrontal cortex and basolateral amygdala to behavioral decision-making under reward/punishment conflict

RATIONALE

Control of reward-seeking behavior under conditions of punishment is an important function for survival.

OBJECTIVES AND METHODS

We designed a task in which rats could choose to either press a lever and obtain a food pellet accompanied by a footshock or refrain from pressing the lever to avoid footshock, in response to tone presentation. In the task, footshock intensity steadily increased, and the task was terminated when the lever press probability reached < 25% (last intensity). Rats were trained until the last intensity was stable. Subsequently, we investigated the effects of the pharmacological inactivation of the ventromedial prefrontal cortex (vmPFC), lateral orbitofrontal cortex (lOFC), and basolateral amygdala (BLA) on task performance.

RESULTS

Bilateral inactivation of the vmPFC, lOFC, and BLA did not alter lever press responses at the early stage of the task. The number of lever presses increased following vmPFC and BLA inactivation but decreased following lOFC inactivation during the later stage of the task. The last intensity was elevated by vmPFC or BLA inactivation but lowered by lOFC inactivation. Disconnection of the vmPFC-BLA pathway induced behavioral alterations that were similar to vmPFC or BLA inactivation. Inactivation of any regions did not alter footshock sensitivity and anxiety levels.

CONCLUSIONS

Our results demonstrate a strong role of the vmPFC and BLA and their interactions in reward restraint to avoid punishment and a prominent role of the lOFC in reward-seeking under reward/punishment conflict situations.

A unified circuit for social behavior

Recent advances in circuit manipulation technologies have enabled the association of distinct neural circuits with complex social behaviors. The brain areas identified through historical anatomical characterizations as mediators of sexual and parental behaviors can now be functionally linked to adult social behaviors within a unified circuit. In vivo electrophysiology, optogenetics and chemogenetics have been used to follow the processing of social sensory stimuli from perception by the olfactory system to valence detection by the amygdala and mesolimbic dopamine system to integration by the cerebral and cerebellar cortices under modulation of hypothalamic neuropeptides. Further, these techniques have been able to identify the distinct functional changes induced by social as opposed to non-social stimuli. Together this evidence suggests that there is a distinct, functionally coupled circuit that is selectively activated by social stimuli. A unified social circuit provides a new framework against which synaptopathic autism related mutations can be considered and novel pharmacotherapeutic strategies can be developed.

The loop neural circuit between the medial prefrontal cortex and the amygdala in the rat brain

A major neuronal basis underlying emotion regulation is the inhibitory influence of the medial prefrontal cortex (mPFC) on amygdalar neurons. However, in spite of the importance of mPFC neuronal activities in emotion regulation, little is known about the inputs modulating activity of mPFC neurons projecting to the amygdala. To gain insight into dense reciprocal connections between mPFC and amygdala, we investigated neural circuits between these brain regions using electrophysiological techniques. We found that mPFC neurons were antidromically driven mainly by stimulation of the central nucleus of the amygdala (CeA), rather than the posterior part of the basolateral nucleus of the amygdala (pBLA), whereas pBLA, but not CeA, stimulation evoked orthodromic excitatory and inhibitory responses. mPFC neurons antidromically driven by CeA stimulation showed excitatory or inhibitory responses to pBLA stimulation. These findings indicate the existence of a functional neural loop between amygdala and mPFC, pointing to an amygdalar self-control system.

Characterization of the sensory, affective, cognitive, biochemical, and neuronal alterations in a modified chronic constriction injury model of neuropathic pain in mice

The chronic constriction injury (CCI) of the sciatic nerve is a nerve injury-based model of neuropathic pain (NP). Comorbidities of NP such as depression, anxiety, and cognitive deficits are associated with a functional reorganization of the medial prefrontal cortex (mPFC). Here, we have employed an adapted model of CCI by placing one single loose ligature around the sciatic nerve in mice for investigating the alterations in sensory, motor, affective, and cognitive behavior and in electrophysiological and biochemical properties in the prelimbic division (PrL) of the mPFC. Our adapted model of CCI induced mechanical allodynia, motor, and cognitive impairments and anxiety- and depression-like behavior. In the PrL division of mPFC was observed an increase in GABA and a decrease in d-aspartate levels. Moreover an increase in the activity of neurons responding to mechanical stimulation with an excitation, mPFC (+), and a decrease in those responding with an inhibition, mPFC (-), was found. Altogether these findings demonstrate that a single ligature around the sciatic nerve was able to induce sensory, affective, cognitive, biochemical, and functional alterations already observed in other neuropathic pain models and it may be an appropriate and easily reproducible model for studying neuropathic pain mechanisms and treatments.

Maturational Changes in Prefrontal and Amygdala Circuits in Adolescence: Implications for Understanding Fear Inhibition during a Vulnerable Period of Development

Anxiety disorders that develop in adolescence represent a significant burden and are particularly challenging to treat, due in no small part to the high occurrence of relapse in this age group following exposure therapy. This pattern of persistent fear is preserved across species; relative to those younger and older, adolescents consistently show poorer extinction, a key process underpinning exposure therapy. This suggests that the neural processes underlying fear extinction are temporarily but profoundly compromised during adolescence. The formation, retrieval, and modification of fear- and extinction-associated memories are regulated by a forebrain network consisting of the prefrontal cortex (PFC), the amygdala, and the hippocampus. These regions undergo robust maturational changes in early life, with unique alterations in structure and function occurring throughout adolescence. In this review, we focus primarily on two of these regions-the PFC and the amygdala-and discuss how changes in plasticity, synaptic transmission, inhibition/excitation, and connectivity (including modulation by hippocampal afferents to the PFC) may contribute to transient deficits in extinction retention. We end with a brief consideration of how exposure to stress during this adolescent window of vulnerability can permanently disrupt neurodevelopment, leading to lasting impairments in pathways of emotional regulation.

Acute and long-lasting effects of oxytocin in cortico-limbic circuits: consequences for fear recall and extinction

The extinction of conditioned fear responses entrains the formation of safe new memories to decrease those behavioral responses. The knowledge in neuronal mechanisms of extinction is fundamental in the treatment of anxiety and fear disorders. Interestingly, the use of pharmacological compounds that reduce anxiety and fear has been shown as a potent co-adjuvant in extinction therapy. However, the efficiency and mechanisms by which pharmacological compounds promote extinction of fear memories remains still largely unknown and would benefit from a validation based on functional neuronal circuits, and the neurotransmitters that modulate them. From this perspective, oxytocin receptor signaling, which has been shown in cortical and limbic areas to modulate numerous functions (Eliava et al. Neuron 89(6):1291-1304, 2016), among them fear and anxiety circuits, and to enhance the salience of social stimuli (Stoop Neuron 76(1):142-59, 2012), may offer an interesting perspective. Experiments in animals and humans suggest that oxytocin could be a promising pharmacological agent at adjusting memory consolidation to boost fear extinction. Additionally, it is possible that long-term changes in endogenous oxytocin signaling can also play a role in reducing expression of fear at different brain targets. In this review, we summarize the effects reported for oxytocin in cortico-limbic circuits and on fear behavior that are of relevance for the modulation and potential extinction of fear memories.

Preclinical studies of stress, extinction, and prefrontal cortex: intriguing leads and pressing questions

BACKGROUND

Stress is associated with cognitive and emotional dysfunction, and increases risk for a variety of psychological disorders, including depression and posttraumatic stress disorder. Prefrontal cortex is critical for executive function and emotion regulation, is a target for stress hormones, and is implicated in many stress-influenced psychological disorders. Extinction of conditioned fear provides an excellent model system for examining how stress-induced changes in corticolimbic structure and function are related to stress-induced changes in neural function and behavior, as the neural circuitry underlying this behavior is well characterized.

OBJECTIVES

This review examines how acute and chronic stress influences extinction and describes how stress alters the structure and function of the medial prefrontal cortex, a potential neural substrate for these effects. In addition, we identify important unanswered questions about how stress-induced change in prefrontal cortex may mediate extinction deficits and avenues for future research.

KEY FINDINGS

A substantial body of work demonstrates deficits in extinction after either acute or chronic stress. A separate and substantial literature demonstrates stress-induced neuronal remodeling in medial prefrontal cortex, along with several key neurohormonal contributors to this remodeling, and there is substantial overlap in prefrontal mechanisms underlying extinction and the mechanisms implicated in stress-induced dysfunction of-and neuronal remodeling in-medial prefrontal cortex. However, data directly examining the contribution of changes in prefrontal structure and function to stress-induced extinction deficits is currently lacking.

CONCLUSIONS

Understanding how stress influences extinction and its neural substrates as well as individual differences in this effect will elucidate potential avenues for novel interventions for stress-sensitive disorders characterized by deficits in extinction.

Inactivation of ventral hippocampus projections promotes sensitivity to changes in contingency

The loss of behavioral flexibility is common across a number of neuropsychiatric illnesses. This may be in part due to the loss of the ability to detect or use changes in action–outcome contingencies to guide behavior. There is growing evidence that the ventral hippocampus plays a critical role in the regulation of flexible behavior and reward-related decision making. Here, we investigated the role of glutamatergic projections from the ventral hippocampus in the expression of contingency-mediated reward seeking. We demonstrate that selectively silencing ventral hippocampus projections can restore the use of action–outcome contingencies to guide behavior, while sparing cue-guided behavior and extinction learning. Our findings further indicated that the ability of the ventral hippocampus to promote habitual response strategies may be in part mediated by selective projections from the ventral hippocampus to the nucleus accumbens shell. Together these results implicate glutamatergic projections from the ventral hippocampus in the regulation of behavioral flexibility and suggest that alterations in ventral hippocampus function may contribute to overreliance on habitual response strategy observed in neuropsychiatric illnesses including addiction and obsessive–compulsive disorder.

Hippocampus-driven feed-forward inhibition of the prefrontal cortex mediates relapse of extinguished fear

The medial prefrontal cortex (mPFC) has been implicated in the extinction of emotional memories, including conditioned fear. We found that ventral hippocampal (vHPC) projections to the infralimbic (IL) cortex recruited parvalbumin-expressing interneurons to counter the expression of extinguished fear and promote fear relapse. Whole-cell recordings ex vivo revealed that optogenetic activation of vHPC input to amygdala-projecting pyramidal neurons in the IL was dominated by feed-forward inhibition. Selectively silencing parvalbumin-expressing, but not somatostatin-expressing, interneurons in the IL eliminated vHPC-mediated inhibition. In behaving rats, pharmacogenetic activation of vHPC→IL projections impaired extinction recall, whereas silencing IL projectors diminished fear renewal. Intra-IL infusion of GABA receptor agonists or antagonists, respectively, reproduced these effects. Together, our findings describe a previously unknown circuit mechanism for the contextual control of fear, and indicate that vHPC-mediated inhibition of IL is an essential neural substrate for fear relapse.

A Computational Model of Major Depression: the Role of Glutamate Dysfunction on Cingulo-Frontal Network Dynamics

Major depression disease (MDD) is associated with the dysfunction of multinode brain networks. However, converging evidence implicates the reciprocal interaction between midline limbic regions (typified by the ventral anterior cingulate cortex, vACC) and the dorso-lateral prefrontal cortex (dlPFC), reflecting interactions between emotions and cognition. Furthermore, growing evidence suggests a role for abnormal glutamate metabolism in the vACC, while serotonergic treatments (selective serotonin reuptake inhibitor, SSRI) effective for many patients implicate the serotonin system. Currently, no mechanistic framework describes how network dynamics, glutamate, and serotonin interact to explain MDD symptoms and treatments. Here, we built a biophysical computational model of 2 areas (vACC and dlPFC) that can switch between emotional and cognitive processing. MDD networks were simulated by slowing glutamate decay in vACC and demonstrated sustained vACC activation. This hyperactivity was not suppressed by concurrent dlPFC activation and interfered with expected dlPFC responses to cognitive signals, mimicking cognitive dysfunction seen in MDD. Simulation of clinical treatments (SSRI or deep brain stimulation) counteracted this aberrant vACC activity. Theta and beta/gamma oscillations correlated with network function, representing markers of switch-like operation in the network. The model shows how glutamate dysregulation can cause aberrant brain dynamics, respond to treatments, and be reflected in EEG rhythms as biomarkers of MDD.

The Role of the Endocannabinoid System and Genetic Variation in Adolescent Brain Development

During adolescence, both rodent and human studies have revealed dynamic changes in the developmental trajectories of corticolimbic structures, which are known to contribute to the regulation of fear and anxiety-related behaviors. The endocannabinoid (eCB) system critically regulates stress responsivity and anxiety throughout the life span. Emerging evidence suggests that during adolescence, changes in eCB signaling contribute to the maturation of local and corticolimbic circuit populations of neurons, such as mediating the balance between excitatory and inhibitory neurotransmission within the prefrontal cortex. This function of the eCB system facilitates efficient communication within and between brain regions and serves a central role in establishing complex and adaptive cognitive and behavioral processing. Although these peri-adolescent changes in eCB signaling promote brain development and plasticity, they also render this period a particularly sensitive one for environmental perturbations to these normative fluctuations in eCB signaling, such as stress, potentially leading to altered developmental trajectories of neural circuits governing emotional behaviors. In this review, we focus on the role of eCB signaling on the regulation of stress and anxiety-related behaviors both during and after adolescence. Moreover, we discuss the functional implications of human genetic variation in the eCB system for the risk for anxiety and consequences of stress across development and into adulthood.

D1, but not D2, receptor blockade within the infralimbic and medial orbitofrontal cortex impairs cocaine seeking in a region-specific manner

Evidence suggests that the infralimbic cortex (IL), a subregion of the ventromedial prefrontal cortex (vmPFC), suppresses cocaine-seeking behavior in a self-administration paradigm, whereas the more anterior vmPFC subregion, the medial orbitofrontal cortex (mOFC), has received very little attention in this regard. Despite the established dopaminergic innervation of the vmPFC, whether dopamine receptor blockade in each subregion alters the reinstatement of cocaine seeking is unclear. To address this issue, male Sprague-Dawley rats underwent 2 weeks of cocaine self-administration, followed by extinction training and reinstatement testing. Immediately prior to each reinstatement test, rats received microinjections of the D1 receptor antagonist SCH 23390, the D2 receptor antagonist sulpiride or their respective vehicles. D1 receptor blockade in the IL reduced cued reinstatement but had no effect on cocaine prime and cue + cocaine-prime reinstatement, whereas D2 receptor blockade in the IL had no effect on reinstatement. For the mOFC, however, D1 receptor blockade reduced cocaine seeking in all reinstatement types, whereas blocking D2 receptors in the mOFC had no effect on any form of cocaine seeking. These findings suggest different roles for D1 receptors in the IL versus the mOFC in regulating cocaine-seeking behavior. Moreover, even as previous work indicates that IL inactivation does not affect reinstatement but, rather, induces cocaine seeking during extinction, the present findings suggest that dopamine receptor activation in the IL is necessary for cocaine seeking under some circumstances.

Antibiotic-induced microbiota perturbation causes gut endocannabinoidome changes, hippocampal neuroglial reorganization and depression in mice

The microbiota-gut-brain axis (MGBA) regulates the reciprocal interaction between chronic inflammatory bowel and psychiatric disorders. This interaction involves multiple pathways that are highly debated. We examined the behavioural, biochemical and electrophysiological alterations, as well as gut microbiota composition in a model of antibiotic-induced experimental dysbiosis. Inflammation of the small intestine was also assessed. Mice were exposed to a mixture of antimicrobials for 2weeks. Afterwards, they received Lactobacillus casei DG (LCDG) or a vehicle for up to 7days via oral gavage. Perturbation of microbiota was accompanied by a general inflammatory state and alteration of some endocannabinoidome members in the gut. Behavioural changes, including increased immobility in the tail suspension test and reduced social recognition were observed, and were associated with altered BDNF/TrkB signalling, TRPV1 phosphorylation and neuronal firing in the hippocampus. Moreover, morphological rearrangements of non-neuronal cells in brain areas controlling emotional behaviour were detected. Subsequent probiotic administration, compared with vehicle, counteracted most of these gut inflammatory, behavioural, biochemical and functional alterations. Interestingly, levels of Lachnospiraceae were found to significantly correlate with the behavioural changes observed in dysbiotic mice. Our findings clarify some of the biomolecular and functional modifications leading to the development of affective disorders associated with gut microbiota alterations.

Npas4 deficiency and prenatal stress interact to affect social recognition in mice

Neurodevelopmental disorders such as autism spectrum disorders and schizophrenia have an expansive array of reported genetic and environmental contributing factors. However, none of these factors alone can account for a substantial proportion of cases of either disorder. Instead, many gene-by-environment interactions are responsible for neurodevelopmental disturbances that lead to these disorders. The current experiment used heterozygous knock-out mice to examine a potential interaction between 2 factors commonly linked to neurodevelopmental disorders and cognitive deficit: imbalanced excitatory/inhibitory signaling in the cortex and prenatal stress (PNS) exposure. Both of these factors have been linked to disrupt GABAergic signaling in the prefrontal cortex (PFC), a common feature of neurodevelopmental disorders. The neuronal PAS domain protein 4 (Npas4) gene is instrumental in regulation of the excitatory/inhibitory balance in the cortex and hippocampus in response to activation. Npas4 heterozygous and wild-type male and female mice were exposed to either PNS or standard gestation, then evaluated during adulthood in social and anxiety behavioral measures. The combination of PNS and Npas4 deficiency in male mice impaired social recognition. This behavioral deficit was associated with decreased parvalbumin and cFos protein expression in the infralimbic region of the PFC following social stimulation in Npas4 heterozygous males. In contrast, females displayed fewer behavioral effects and molecular changes in PFC in response to PNS and decreased Npas4.

Role of the Astroglial Glutamate Exchanger xCT in Ventral Hippocampus in Resilience to Stress

We demonstrate that stress differentially regulates glutamate homeostasis in the dorsal and ventral hippocampus and identify a role for the astroglial xCT in ventral dentate gyrus (vDG) in stress and antidepressant responses. We provide an RNA-seq roadmap for the stress-sensitive vDG. The transcription factor REST binds to xCT promoter in co-occupancy with the epigenetic marker H3K27ac to regulate expression of xCT, which is also reduced in a genetic mouse model of inherent susceptibility to depressive-like behavior. Pharmacologically, modulating histone acetylation with acetyl-L-carnitine (LAC) or acetyl-N-cysteine (NAC) rapidly increases xCT and activates a network with mGlu2 receptors to prime an enhanced glutamate homeostasis that promotes both pro-resilient and antidepressant-like responses. Pharmacological xCT blockage counteracts NAC prophylactic effects. GFAP⁺-Cre-dependent overexpression of xCT in vDG mimics pharmacological actions in promoting resilience. This work establishes a mechanism by which vDG protection leads to stress resilience and antidepressant responses via epigenetic programming of an xCT-mGlu2 network.

Functional and neurochemical interactions within the amygdala-medial prefrontal cortex circuit and their relevance to emotional processing

The amygdala-medial prefrontal cortex (mPFC) circuit plays a key role in emotional processing. GABA-ergic inhibition within the mPFC has been suggested to play a role in the shaping of amygdala activity. However, the functional and neurochemical interactions within the amygdala-mPFC circuits and their relevance to emotional processing remain unclear. To investigate this circuit, we obtained resting-state functional magnetic resonance imaging (rs-fMRI) and proton MR spectroscopy in 21 healthy subjects to assess the potential relationship between GABA levels within mPFC and the amygdala-mPFC functional connectivity. Trait anxiety was assessed using the State-Trait Anxiety Inventory (STAI-Y2). Partial correlations were used to measure the relationships among the functional connectivity outcomes, mPFC GABA levels and STAI-Y2 scores. Age, educational level and amount of the gray and white matters within 1H-MRS volume of interest were included as nuisance variables. The rs-fMRI signals of the amygdala and the vmPFC were significantly anti-correlated. This negative functional coupling between the two regions was inversely correlated with the GABA+/tCr level within the mPFC and the STAI-Y2 scores. We suggest a close relationship between mPFC GABA levels and functional interactions within the amygdala-vmPFC circuit, providing new insights in the physiology of emotion.

Palmitoylethanolamide Reduces Neuropsychiatric Behaviors by Restoring Cortical Electrophysiological Activity in a Mouse Model of Mild Traumatic Brain Injury

Traumatic brain injury (TBI) represents a major public health problem, which is associated with neurological dysfunction. In severe or moderate cases of TBI, in addition to its high mortality rate, subjects may encounter diverse behavioral dysfunctions. Previous reports suggest that an association between TBI and chronic pain syndromes tends to be more common in patients with mild forms of brain injury. Despite causing minimal brain damage, mild TBI (mTBI) often leads to persistent psychologically debilitating symptoms, which can include anxiety, various forms of memory and learning deficits, and depression. At present, no effective treatment options are available for these symptoms, and little is known about the complex cellular activity affecting neuronal activity that occurs in response to TBI during its late phase. Here, we used a mouse model to investigate the effect of Palmitoylethanolamide (PEA) on both the sensorial and neuropsychiatric dysfunctions associated with mTBI through behavioral, electrophysiological, and biomolecular approaches. Fourteen-day mTBI mice developed anxious, aggressive, and reckless behavior, whilst depressive-like behavior and impaired social interactions were observed from the 60th day onward. Altered behavior was associated with changes in interleukin 1 beta (IL-1β) expression levels and neuronal firing activity in the medial prefrontal cortex. Compared with vehicle, PEA restored the behavioral phenotype and partially normalized the biochemical and functional changes occurring at the supraspinal level. In conclusion, our findings reveal some of the supraspinal modifications responsible for the behavioral alterations associated with mTBI and suggest PEA as a pharmacological tool to ameliorate neurological dysfunction induced by the trauma.

Interhemispheric regulation of the rat medial prefrontal cortical glutamate stress response: role of local GABA- and dopamine-sensitive mechanisms

RATIONALE

We previously reported that stressors increase medial prefrontal cortex (PFC) glutamate (GLU) levels as a result of activating callosal neurons located in the opposite hemisphere and that this PFC GLU stress response is regulated by GLU-, dopamine- (DA-), and GABA-sensitive mechanisms (Lupinsky et al. 2010).

OBJECTIVES

Here, we examine the possibility that PFC DA regulates the stress responsivity of callosal neurons indirectly by acting at D₁ and D₂ receptors located on GABA interneurons.

METHODS

Microdialysis combined with drug perfusion (reverse dialysis) or microinjections was used in adult male Long-Evans rats to characterize D₁, D₂, and GABA_B receptor-mediated regulation of the PFC GABA response to tail-pinch (TP) stress.

RESULTS

We report that TP stress reliably elicited comparable increases in extracellular GABA in the left and right PFCs. SCH23390 (D₁ antagonist; 100 μM perfusate concentration) perfused by reverse microdialysis attenuated the local GABA stress responses equally in the left and right PFCs. Intra-PFC raclopride perfusion (D₂ antagonist; 100 μM) had the opposite effect, not only potentiating the local GABA stress response but also causing a transient elevation in basal (pre-stress) GABA. Moreover, unilateral PFC raclopride microinjection (6 nmol) attenuated the GLU response to TP stress in the contralateral PFC. Finally, intra-PFC baclofen perfusion (GABA_B agonist; 100 μM) inhibited the local GLU and GABA stress responses.

CONCLUSIONS

Taken together, these findings implicate PFC GABA interneurons in processing stressful stimuli, showing that local D₁, D₂, and GABA_B receptor-mediated changes in PFC GABA transmission play a crucial role in the interhemispheric regulation of GLU stress responsivity.

Patterns of Theta Activity in Limbic Anxiety Circuit Preceding Exploratory Behavior in Approach-Avoidance Conflict

Theta oscillations within the hippocampus-amygdala-medial prefrontal cortex (HPC-AMY-mPFC) circuit have been consistently implicated in the regulation of anxiety behaviors, including risk-assessment. To study if theta activity during risk-assessment was correlated with exploratory behavior in an approach/avoidance paradigm we recorded simultaneous local field potentials from this circuit in rats exploring the elevated-plus maze (EPM). Opposing patterns of power variations in the ventral hippocampus (vHPC), basolateral amygdala (BLA), and prelimbic (PrL) mPFC, but not in the dorsal hippocampus (dHPC), during exploratory risk-assessment of the open arms preceded further exploration of the open arms or retreat back to the safer closed arms. The same patterns of theta power variations in the HPC-BLA-mPFC(PrL) circuit were also displayed by animals submitted to chronic unpredictable stress protocol known to induce an anxious state. Diverging patterns of vHPC-mPFC(PrL) theta coherence were also significantly correlated with forthcoming approach or avoidance behavior in the conflict situation in both controls and stressed animals; interestingly, vHPC-BLA, and BLA-mPFC(PrL) theta coherence correlated with future behavior only in stressed animals, underlying the pivotal role of the amygdala on the stress response.

The risk of male adult alcohol dependence: The role of the adverse childhood experiences and ecological executive function

OBJECTIVE

To explore the association between male adult alcohol dependence and their adverse childhood experiences as well as ecological executive function.

METHODS

The questionnaires of Adverse Childhood Experiences (ACEs) and Behavior Rating Inventory of Executive Function-Adult Version (BRIEF-A) were adopted for the assessments of 102 alcohol dependent patients who were diagnosed according to the criteria defined by the International Classification of diseases and related health problems (ICD-10) and 106 healthy volunteers, and the differences between patients and healthy volunteers were analyzed.

RESULTS

The percentage of adverse childhood experiences in alcohol dependent patients was significantly higher than that in healthy volunteers (χ(2)=17.28, P<0.01); and the incidences of emotional abuse, physical neglect, violence witness, and substance abuse were significantly higher in alcohol dependent patients than those in healthy volunteers (χ(2)=4.59, 4.46, 10.51, and 44.09 respectively; P<0.05). The ecological executive function analysis showed that the BRIEF total score and scores for each item were all significantly higher in alcohol dependent patients than those of healthy volunteers (P<0.01).

CONCLUSIONS

The adult alcohol dependence was associated with their adverse childhood experiences and ecological executive function. Then physical neglect and substance abuse of parents in childhood, and emotional control defect in the ecological executive function showed strong association with adult alcohol dependence.

MMPIP, an mGluR7-selective negative allosteric modulator, alleviates pain and normalizes affective and cognitive behavior in neuropathic mice

This study investigated the effects of a single administration of 6-(4-methoxyphenyl)-5-methyl-3-pyridinyl-4-isoxazolo[4,5-c]pyridin-4(5H)-one (MMPIP), a negative allosteric modulator (NAM) of metabotropic glutamate receptor 7 (mGluR7), on pain and on affective and cognitive behavior in neuropathic mice. The activity of pyramidal neurons in the prelimbic cortex (PLC), which respond to stimulation of the basolateral amygdala (BLA) with either excitation or inhibition, was also investigated. The spared nerve injury (SNI) of the sciatic nerve induced, 14 days after surgery, thermal hyperalgesia and mechanical allodynia, reduced open-arm choice in the elevated plus-maze, increased time of immobility in the tail suspension, and increased digging and burying in the marble burying test. Cognitive performance was also significantly compromised in the SNI mice. Spared nerve injury induced phenotypic changes on pyramidal neurons of the PLC; excitatory responses increased, whereas inhibitory responses decreased after BLA stimulation. mGluR7 expression, mainly associated with vesicular glutamate transporter, increased in the hippocampus and decreased in the BLA, PLC, and dorsal raphe in SNI mice. MMPIP increased thermal and mechanical thresholds and open-arm choice. It reduced the immobility in the tail suspension test and the number of marbles buried and of digging events in the marble burying test. MMPIP also improved cognitive performance and restored the balance between excitatory and inhibitory responses of PLC neurons in SNI mice. 7-hydroxy-3-(4-iodophenoxy)-4H-chromen-4-one, XAP044, another selective mGluR7 NAM, reproduced the effects of MMPIP on thermal hyperalgesia, mechanical allodynia, tail suspension, and marble burying test. Altogether, these findings show that mGluR7 NAMs reduce pain responses and affective/cognitive impairments in neuropathic pain conditions.

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

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

Antioxidative enzymes in irradiated rat brain-indicators of different regional radiosensitivity

PURPOSE

Previously, we examined manganese superoxide dismutase (MnSOD), copper-zinc superoxide dismutase (CuZnSOD), and catalase (CAT) activities in rat brain irradiated with 2 or 3 Gy of γ-rays. The results indicated that lower MnSOD activity and inducibility found in hippocampus might explain higher radiosensitivity of this brain region. Thus, in this study, we wanted to determine changes of MnSOD, CuZnSOD, and CAT activities after dose of 5 Gy and to find out if differences in MnSOD activity are caused by changes in its expression.

METHODS

Heads of 4-day-old female rats were irradiated with γ-rays, using (60)Co. Animals were sacrificed 1/24 h after exposure. Hippocampus and cortex tissues were prepared for enzyme activity measurements and Western blot analysis.

RESULTS

One hour after exposure, γ-rays significantly decreased MnSOD activity in both examined brain regions. Twenty-four hours later, MnSOD recovery showed dose and regional dependence. It was weaker at higher doses and in hippocampal region. MnSOD expression changed in the similar manner as MnSOD activity only at lower doses of γ-rays. In both examined brain regions, gamma radiation significantly decreased CuZnSOD activity and did not change activity of CAT.

CONCLUSIONS

Our results confirmed that MnSOD plays an important role in different regional radiosensitivity but also showed that depending on dose, radiation affects MnSOD level by utterly different mechanisms. Postradiation changes of CuZnSOD and CAT are not regionally specific and therefore, cannot account for the different radiosensitivity of the hippocampus and cortex.

Inactivation of the nucleus reuniens/rhomboid causes a delay-dependent impairment of spatial working memory

Inactivation of the rodent medial prefrontal cortex (mPFC) and hippocampus or disconnection of the hippocampus from the mPFC produces deficits in spatial working memory tasks. Previous studies have shown that delay length determines the extent to which mPFC and hippocampus functionally interact, with both structures being necessary for tasks with longer delays and either structure being sufficient for tasks with shorter delays. In addition, inactivation of the nucleus reuniens (Re)/rhomboid nucleus (Rh) of the thalamus, which has bidirectional connections with the mPFC and hippocampus, also produces deficits in these tasks. However, it is unknown how delay duration relates to the function of Re/Rh. If Re/Rh are critical in modulating mPFC-hippocampus interactions, inactivation of the RE/Rh should produce a delay-dependent impairment in spatial working memory performance. To investigate this question, groups of rats were trained on one of three different spatial working memory tasks: continuous alternation (CA), delayed alternation with a five-second delay (DA5), or with a thirty-second delay (DA30). The Re/Rh were inactivated with muscimol infusions prior to testing. The results demonstrate that inactivation of RE/Rh produces a deficit only on the two DA tasks, supporting the notion that the Re/Rh is a critical orchestrator of mPFC-HC interactions.

Genetic deletion of monoacylglycerol lipase leads to impaired cannabinoid receptor CB₁R signaling and anxiety-like behavior

Endocannabinoids (eCB) are key regulators of excitatory/inhibitory neurotransmission at cannabinoid-1-receptor (CB1 R)-expressing axon terminals. The most abundant eCB in the brain, that is 2-arachidonoylglycerol (2-AG), is hydrolyzed by the enzyme monoacylglycerol lipase (MAGL), whose chronic inhibition in the brain was reported to cause CB1 R desensitization. We employed the MAGL knock-out mouse (MAGL-/-), a genetic model of congenital and sustained elevation of 2-AG levels in the brain, to provide morphological and biochemical evidence for β-arrestin2-mediated CB1 R desensitization in brain regions involved in the control of emotional states, that is, the prefrontal cortex (PFC), amygdala, hippocampus and cerebellar cortex. We found a widespread CB1 R/β-arrestin2 co-expression in the mPFC, amygdala and hippocampus accompanied by impairment of extracellular signal-regulated kinase signaling and elevation of vesicular glutamate transporter (VGluT1) at CB1 R-positive excitatory terminals in the mPFC, or vesicular GABA transporter (VGAT) at CB1 R-positive inhibitory terminals in the amygdala and hippocampus. The impairment of CB1 R signaling in MAGL-/- mice was also accompanied by enhanced excitatory drive in the basolateral amygdala (BLA)-mPFC circuit, with subsequent elevation of glutamate release to the mPFC and anxiety-like and obsessive-compulsive behaviors, as assessed by the light/dark box and marble burying tests, respectively. Collectively, these data provide evidence for a β-arrestin2-mediated desensitization of CB1 R in MAGL-/- mice, with impact on the synaptic plasticity of brain circuits involved in emotional functions. In this study, the authors provide evidence that congenitally enhanced endocannabinoid levels in the neuronal circuits underlying anxiety-like behavioral states (mainly medial prefrontal cortex, amygdala and hippocampus) lead to CB1R desenistization and anxiety and depression. MAGL-/- mice, a model of congenital overactivity of the eCB system, exhibited a compensatory impairment of CB1R signaling in anxiety-associated brain areas and a subsequent change in excitatory/inhibitory tone associated with altered score in the marble burying and light/dark box test, in concomitance with anxiety and depression behavior states. These findings may have potential relevance to the understanding of the neurochemical effects of chronic CB1R overstimulation in cannabis abusers.

Astrocyte-expressed FABP7 regulates dendritic morphology and excitatory synaptic function of cortical neurons

Fatty acid binding protein 7 (FABP7) expressed by astrocytes in developing and mature brains is involved in uptake and transportation of fatty acids, signal transduction, and gene transcription. Fabp7 knockout (Fabp7 KO) mice show behavioral phenotypes reminiscent of human neuropsychiatric disorders such as schizophrenia. However, direct evidence showing how FABP7 deficiency in astrocytes leads to altered brain function is lacking. Here, we examined neuronal dendritic morphology and synaptic plasticity in medial prefrontal cortex (mPFC) of Fabp7 KO mice and in primary cortical neuronal cultures. Golgi staining of cortical pyramidal neurons in Fabp7 KO mice revealed aberrant dendritic morphology and decreased spine density compared with those in wild-type (WT) mice. Aberrant dendritic morphology was also observed in primary cortical neurons co-cultured with FABP7-deficient astrocytes and neurons cultured in Fabp7 KO astrocyte-conditioned medium. Excitatory synapse number was decreased in mPFC of Fabp7 KO mice and in neurons co-cultured with Fabp7 KO astrocytes. Accordingly, whole-cell voltage-clamp recording in brain slices from pyramidal cells in the mPFC showed that both amplitude and frequency of action potential-independent miniature excitatory postsynaptic currents (mEPSCs) were decreased in Fabp7 KO mice. Moreover, transplantation of WT astrocytes into the mPFC of Fabp7 KO mice partially attenuated behavioral impairments. Collectively, these results suggest that astrocytic FABP7 is important for dendritic arbor growth, neuronal excitatory synapse formation, and synaptic transmission, and provide new insights linking FABP7, lipid homeostasis, and neuropsychiatric disorders, leading to novel therapeutic interventions.

Estrogen receptors in the central nervous system and their implication for dopamine-dependent cognition in females

This article is part of a Special Issue "Estradiol and cognition". Over the past 30 years, research has demonstrated that estrogens not only are important for female reproduction, but also play a role in a diverse array of cognitive functions. Originally, estrogens were thought to have only one receptor, localized exclusively to the cytoplasm and nucleus of cells. However, it is now known that there are at least three estrogen receptors (ERs): ERα, ERβ and G-protein coupled ER1 (GPER1). In addition to being localized to nuclei, ERα and ERβ are localized to the cell membrane, and GPER1 is also observed at the cell membrane. The mechanism through which ERs are associated with the membrane remains unclear, but palmitoylation of receptors and associations between ERs and caveolin are implicated in membrane association. ERα and ERβ are mostly observed in the nucleus using light microscopy unless they are particularly abundant. However, electron microscopy has revealed that ERs are also found at the membrane in complimentary distributions in multiple brain regions, many of which are innervated by dopamine inputs and were previously thought to contain few ERs. In particular, membrane-associated ERs are observed in the prefrontal cortex, dorsal striatum, nucleus accumbens, and hippocampus, all of which are involved in learning and memory. These findings provide a mechanism for the rapid effects of estrogens in these regions. The effects of estrogens on dopamine-dependent cognition likely result from binding at both nuclear and membrane-associated ERs, so elucidating the localization of membrane-associated ERs helps provide a more complete understanding of the cognitive effects of these hormones.