Trace and contextual fear conditioning is enhanced in mice lacking the alpha4 subunit of the GABA(A) receptor

Behavioral outputs and overlapping circuits between conditional fear and active avoidance

Aversive learning can produce a wide variety of defensive behavioral responses depending on the circumstances, ranging from reactive responses like freezing to proactive avoidance responses. While most of this initial learning is behaviorally supported by an expectancy of an aversive outcome and neurally supported by activity within the basolateral amygdala, activity in other brain regions become necessary for the execution of defensive strategies that emerge in other aversive learning paradigms such as active avoidance. Here, we review the neural circuits that support both reactive and proactive defensive behaviors that are motivated by aversive learning, and identify commonalities between the neural substrates of these distinct (and often exclusive) behavioral strategies.

Fear-Conditioning to Unpredictable Threats Reveals Sex and Strain Differences in Rat Fear-Potentiated Startle (FPS)

Fear-potentiated startle (FPS) has been widely used to study fear processing in humans and rodents. Human studies showed higher startle amplitudes and exaggerated fear reactivity to unpredictable vs. predictable threats in individuals suffering from post-traumatic stress disorder (PTSD). Although human FPS studies use both sexes, a surprisingly limited number of rodent FPS studies use females. Here we investigate the effects of signal-threat contingency, signal-threat order and threat predictability on FPS in both sexes. We use a classic fear-conditioning protocol (100% contingency of cue and shock pairings, with forward conditioning such that the cue co-terminates with the shock) and compare it to modified fear-conditioning protocols (70% contingency; backward conditioning; or cue and shock un-paired). Although there are no sex differences in the startle amplitudes when corrected for body weight, females consistently demonstrate higher shock reactivity during fear-conditioning. Both sexes and strains demonstrate comparable levels of cued, non-cued, and contextual fear in the classic FPS and FPS following fear-conditioning with 70% contingency or backward order (cue co-starts with shock). However, in the classic FPS, Sprague-Dawley females show reduced proportion between cued fear and cue-elicited vigilant state than males. Lastly, a prominent sex difference is uncovered following unpredictable fear-conditioning (cue and shock un-paired), with Wistar, but not Sprague-Dawley, females showing significantly higher startle overall during the FPS recall, regardless of trial type, and higher contextual fear than males. This striking sex difference in processing unpredictable threats in rodent FPS might help to understand the mechanisms underlying higher incidence of PTSD in women.

Cyclin D1-Cdk4 regulates neuronal activity through phosphorylation of GABAA receptors

Nuclear Cyclin D1 (Ccnd1) is a main regulator of cell cycle progression and cell proliferation. Interestingly, Ccnd1 moves to the cytoplasm at the onset of differentiation in neuronal precursors. However, cytoplasmic functions and targets of Ccnd1 in post-mitotic neurons are unknown. Here we identify the α4 subunit of gamma-aminobutyric acid (GABA) type A receptors (GABAARs) as an interactor and target of Ccnd1-Cdk4. Ccnd1 binds to an intracellular loop in α4 and, together with Cdk4, phosphorylates the α4 subunit at threonine 423 and serine 431. These modifications upregulate α4 surface levels, increasing the response of α4-containing GABAARs, measured in whole-cell patch-clamp recordings. In agreement with this role of Ccnd1-Cdk4 in neuronal signalling, inhibition of Cdk4 or expression of the non-phosphorylatable α4 decreases synaptic and extra-synaptic currents in the hippocampus of newborn rats. Moreover, according to α4 functions in synaptic pruning, CCND1 knockout mice display an altered pattern of dendritic spines that is rescued by the phosphomimetic α4. Overall, our findings molecularly link Ccnd1-Cdk4 to GABAARs activity in the central nervous system and highlight a novel role for this G1 cyclin in neuronal signalling.

Fear-conditioning to unpredictable threats reveals sex differences in rat fear-potentiated startle (FPS)

Fear-potentiated startle (FPS) has been widely used to study fear processing in humans and rodents. Human studies have shown higher startle amplitudes and exaggerated fear reactivity to unpredictable vs. predictable threats in individuals suffering from post-traumatic stress disorder (PTSD). Although human FPS studies often use both sexes, a surprisingly limited number of rodent FPS studies use females. Here we investigate the effects of signal-threat contingency, signal-threat order and threat predictability on FPS in both sexes. We use a classic fear-conditioning protocol (100% contingency of cue and shock pairings, with forward conditioning such that the cue co-terminates with the shock) and compare it to modified fear-conditioning protocols (70% contingency; backward conditioning; or cue and shock unpaired). Although there are no sex differences in the startle amplitudes when corrected for body weight, females demonstrate higher shock reactivity during fear-conditioning. Both sexes demonstrate comparable levels of cued, non-cued, and contextual fear in the classic FPS but females show reduced fear discrimination vs. males. Fear-conditioning with 70% contingency or backward order (cue co-starts with shock) induces similar levels of cued, non-cued, and contextual fear in both sexes but they differ in contextual fear extinction. Lastly, a prominent sex difference is uncovered following unpredictable fear-conditioning protocol (cue and shock un-paired), with females showing significantly higher startle overall during the FPS recall, regardless of trial type, and higher contextual fear than males. This striking sex difference in processing unpredictable threats in rodent FPS might help to understand the mechanisms underlying higher incidence of PTSD in women.

Highlights

Male and female rats have comparable startle amplitudes when corrected for body weightFemale rats show higher foot-shock reactivity than males during fear-conditioningFemale rats show reduced fear discrimination vs. males in the classic FPSReversed signal-threat order increases contextual fear in both sexesExposure to unpredictable threats increases startle in general and contextual fear only in females.

Sex differences in fear responses: Neural circuits

Women have increased vulnerability to PTSD and anxiety disorders compared to men. Understanding the neurobiological underpinnings of these disorders is critical for identifying risk factors and developing appropriate sex-specific interventions. Despite the clear clinical relevance of an examination of sex differences in fear responses, the vast majority of pre-clinical research on fear learning and memory formation has exclusively used male animals. This review highlights sex differences in context and cued fear conditioning, fear extinction and fear generalization with a focus on the neural circuits underlying these behaviors in rodents. There are mixed reports of behavioral sex differences in context and cued fear conditioning paradigms, which can depend upon the behavioral indices of fear. However, there is greater evidence of differential activation of the hippocampus, amygdalar nuclei and the prefrontal cortical regions in male and female rodents during context and cued fear conditioning. The bed nucleus of the stria terminalis (BNST), a sexually dimorphic structure, is of particular interest as it differentially contributes to fear responses in males and females. In addition, while the influence of the estrous cycle on different phases of fear conditioning is delineated, the clearest modulatory effect of estrogen is on fear extinction processes. Examining the variability in neural responses and behavior in both sexes should increase our understanding of how that variability contributes to the neurobiology of affective disorders. This article is part of the Special Issue on 'Fear, anxiety and PTSD'.

Abnormal Expression of Synaptic and Extrasynaptic GABAA Receptor Subunits in the Dystrophin-Deficient mdx Mouse

Duchenne muscular dystrophy (DMD) is a neurodevelopmental disorder primarily caused by the loss of the full-length Dp427 dystrophin in both muscle and brain. The basis of the central comorbidities in DMD is unclear. Brain dystrophin plays a role in the clustering of central gamma-aminobutyric acid A receptors (GABA_ARs), and its loss in the mdx mouse alters the clustering of some synaptic subunits in central inhibitory synapses. However, the diversity of GABAergic alterations in this model is still fragmentary. In this study, the analysis of in vivo PET imaging of a benzodiazepine-binding site radioligand revealed that the global density of central GABA_ARs is unaffected in mdx compared with WT mice. In contrast, semi-quantitative immunoblots and immunofluorescence confocal imaging in tissue sections revealed complex and differential patterns of alterations of the expression levels and/or clustered distribution of a variety of synaptic and extrasynaptic GABA_AR subunits in the hippocampus, cerebellum, cortex, and spinal cord. Hence, dystrophin loss not only affects the stabilization of synaptic GABA_ARs but also influences the subunit composition of GABA_ARs subtypes at both synaptic and extrasynaptic sites. This study provides new molecular outcome measures and new routes to evaluate the impact of treatments aimed at compensating alterations of the nervous system in DMD.

Pathway-specific GABAergic inhibition contributes to the gain of resilience against anorexia-like behavior of adolescent female mice

Anorexia nervosa is one of the most debilitating mental illnesses that emerges during adolescence, especially among females. Anorexia nervosa is characterized by severe voluntary food restriction and compulsive exercising, which combine to cause extreme body weight loss. We use activity-based anorexia (ABA), an animal model, to investigate the neurobiological bases of vulnerability to anorexia nervosa. This is a Mini-Review, focused on new ideas that have emerged based on recent findings from the Aoki Lab. Our findings point to the cellular and molecular underpinnings of three ABA phenomena: (1) age-dependence of ABA vulnerability; (2) individual differences in the persistence of ABA vulnerability during adolescence; (3) GABAergic synaptic plasticity in the hippocampus and the prefrontal cortex that contributes to the suppression of the maladaptive anorexia-like behaviors. We also include new data on the contribution to ABA vulnerability by cell type-specific knockdown of a GABA receptor subunit, α4, in dorsal hippocampus. Although the GABA system recurs as a key player in the gain of ABA resilience, the data predict why targeting the GABA system, singularly, may have only limited efficacy in treating anorexia nervosa. This is because boosting the GABAergic system may suppress the maladaptive behavior of over-exercising but could also suppress food consumption. We hypothesize that a sub-anesthetic dose of ketamine may be the magic bullet, since a single injection of this drug to mid-adolescent female mice undergoing ABA induction enhances food consumption and reduces wheel running, thereby reducing body weight loss through plasticity at excitatory synaptic inputs to both excitatory and inhibitory neurons. The same treatment is not as efficacious during late adolescence but multiple dosing of ketamine can suppress ABA vulnerability partially. This caveat underscores the importance of conducting behavioral, synaptic and molecular analyses across multiple time points spanning the developmental stage of adolescence and into adulthood. Since this is a Mini-Review, we recommend additional literature for readers seeking more comprehensive reviews on these subjects.

Sex differences in contextual fear learning and generalization: a behavioral and computational analysis of hippocampal functioning

There are sex differences in anxiety disorders with regard to occurrence and severity of episodes such that females tend to experience more frequent and more severe episodes. Contextual fear learning and generalization are especially relevant to anxiety disorders, which are often defined by expressing fear and/or anxiety in safe contexts. In contextual fear conditioning, a representation of the context must first be created, and then that representation must be paired with an aversive consequence. With some variation, the experiments presented here use a 3-d procedure in which day 1 consists of pre-exposure to the to-be-shocked context, day 2 consists of a single context-shock pairing after some placement-to-shock interval (PSI), and day 3 consists of testing in either the same or a novel context. With shorter pre-exposure periods, male rats showed more contextual fear, consistent with previous literature; however, after longer pre-exposure periods, female rats showed greater contextual fear. Additionally, while pre-exposure and PSI are both periods of time prior to the shock, it was found that they were not equivalent to each other. Animals with 120 sec of pre-exposure and a 30-sec PSI show a differential level and time course of fear expression than animals who received no pre-exposure and a 150-sec PSI, and this further depended on sex of the rat. Additionally, an experiment comparing recently versus remotely acquired contextual fear was run. Males were again shown to have greater contextual fear at both time points, and this contextual fear incubated/increased over time in males but not females. To facilitate identification of what processes caused sex differences, we used BaconX, a conceptual and computational model of hippocampal contextual learning. Computational simulations using this model predicted many of our key findings. Furthermore, these simulations suggest potential mechanisms with regard to hippocampal computation; namely, an increased feature sampling rate in males, which may account for the sex differences presented here and in prior literature.

Assessing the importance of sex in a hippocampus-dependent behavioral test battery in C57BL/6NTac mice

Inclusion of male and female subjects in behavioral neuroscience research requires a concerted effort to characterize sex differences in standardized behavioral assays. Sex differences in hippocampus-dependent assays have been widely reported but are still poorly characterized. In the present study, we conducted a parametric analysis of spontaneous alternation, object recognition, and fear conditioning in a commonly used control strain, C57BL/6NTac. Our findings show largely similar performance between males and females across the majority of behavioral end points. However, we identified an important difference in nonassociative fear sensitization, whereby females showed an enhanced fear response to the 75-dB tone that is used as the conditional stimulus. In addition, we observed an impairment in object location performance in females that was ameliorated by more extensive habituation to handling. Together, these findings argue that sex differences in nonassociative fear responses to both novel auditory cues and novel objects need to be considered when designing and interpreting cognitive assays in C57BL/6 mice. Furthermore, this elevated fear sensitization could serve as a novel approach to model the increased incidence of anxiety disorders in women.

Prefrontal GABAA(δ)R Promotes Fear Extinction through Enabling the Plastic Regulation of Neuronal Intrinsic Excitability

Extinguishing the previously acquired fear is critical for the adaptation of an organism to the ever-changing environment, a process requiring the engagement of GABAA receptors (GABAARs). GABAARs consist of tens of structurally, pharmacologically, and functionally heterogeneous subtypes. However, the specific roles of these subtypes in fear extinction remain largely unexplored. Here, we observed that in the medial prefrontal cortex (mPFC), a core region for mood regulation, the extrasynaptically situated, δ-subunit-containing GABAARs [GABAA(δ)Rs], had a permissive role in tuning fear extinction in male mice, an effect sharply contrasting to the established but suppressive role by the whole GABAAR family. First, the fear extinction in individual mice was positively correlated with the level of GABAA(δ)R expression and function in their mPFC. Second, knockdown of GABAA(δ)R in mPFC, specifically in its infralimbic (IL) subregion, sufficed to impair the fear extinction in mice. Third, GABAA(δ)R-deficient mice also showed fear extinction deficits, and re-expressing GABAA(δ)Rs in the IL of these mice rescued the impaired extinction. Further mechanistic studies demonstrated that the permissive effect of GABAA(δ)R was associated with its role in enabling the extinction-evoked plastic regulation of neuronal excitability in IL projection neurons. By contrast, GABAA(δ)R had little influence on the extinction-evoked plasticity of glutamatergic transmission in these cells. Altogether, our findings revealed an unconventional and permissive role of extrasynaptic GABAA receptors in fear extinction through a route relying on nonsynaptic plasticity.SIGNIFICANCE STATEMENT The medial prefrontal cortex (mPFC) is one of the kernel brain regions engaged in fear extinction. Previous studies have repetitively shown that the GABAA receptor (GABAAR) family in this region act to suppress fear extinction. However, the roles of specific GABAAR subtypes in mPFC are largely unknown. We observed that the GABAAR-containing δ-subunit [GABAA(δ)R], a subtype of GABAARs exclusively situated in the extrasynaptic membrane and mediating the tonic neuronal inhibition, works oppositely to the whole GABAAR family and promotes (but does not suppress) fear extinction. More interestingly, in striking contrast to the synaptic GABAARs that suppress fear extinction by breaking the extinction-evoked plasticity of glutamatergic transmission, the GABAA(δ)R promotes fear extinction through enabling the plastic regulation of neuronal excitability in the infralimbic subregion of mPFC. Our findings thus reveal an unconventional role of GABAA(δ)R in promoting fear extinction through a route relying on nonsynaptic plasticity.

Neuronal Ensembles Organize Activity to Generate Contextual Memory

Contextual learning is a critical component of episodic memory and important for living in any environment. Context can be described as the attributes of a location that are not the location itself. This includes a variety of non-spatial information that can be derived from sensory systems (sounds, smells, lighting, etc.) and internal state. In this review, we first address the behavioral underpinnings of contextual memory and the development of context memory theory, with a particular focus on the contextual fear conditioning paradigm as a means of assessing contextual learning and the underlying processes contributing to it. We then present the various neural centers that play roles in contextual learning. We continue with a discussion of the current knowledge of the neural circuitry and physiological processes that underlie contextual representations in the Entorhinal cortex-Hippocampal (EC-HPC) circuit, as the most well studied contributor to contextual memory, focusing on the role of ensemble activity as a representation of context with a description of remapping, and pattern separation and completion in the processing of contextual information. We then discuss other critical regions involved in contextual memory formation and retrieval. We finally consider the engram assembly as an indicator of stored contextual memories and discuss its potential contribution to contextual memory.

Memory suppressor genes: Modulating acquisition, consolidation, and forgetting

The brain has a remarkable but underappreciated capacity to limit memory formation and expression. The term "memory suppressor gene" was coined in 1998 as an attempt to explain emerging reports that some genes appeared to limit memory. At that time, only a handful of memory suppressor genes were known, and they were understood to work by limiting cAMP-dependent consolidation. In the intervening decades, almost 100 memory suppressor genes with diverse functions have been discovered that affect not only consolidation but also acquisition and forgetting. Here we highlight the surprising extent to which biological limits are placed on memory formation through reviewing the literature on memory suppressor genes. In this review, we present memory suppressors within the framework of their actions on different memory operations: acquisition, consolidation, and forgetting. This is followed by a discussion of the reasons why there may be a biological need to limit memory formation.

Hyperbaric oxygen therapy alleviates vascular dysfunction and amyloid burden in an Alzheimer's disease mouse model and in elderly patients

Vascular dysfunction is entwined with aging and in the pathogenesis of Alzheimer's disease (AD) and contributes to reduced cerebral blood flow (CBF) and consequently, hypoxia. Hyperbaric oxygen therapy (HBOT) is in clinical use for a wide range of medical conditions. In the current study, we exposed 5XFAD mice, a well-studied AD model that presents impaired cognitive abilities, to HBOT and then investigated the therapeutical effects using two-photon live animal imaging, behavioral tasks, and biochemical and histological analysis. HBOT increased arteriolar luminal diameter and elevated CBF, thus contributing to reduced hypoxia. Furthermore, HBOT reduced amyloid burden by reducing the volume of pre-existing plaques and attenuating the formation of new ones. This was associated with changes in amyloid precursor protein processing, elevated degradation and clearance of Aß protein and improved behavior of 5XFAD mice. Hence, our findings are consistent with the effects of HBOT being mediated partially through a persistent structural change in blood vessels that reduces brain hypoxia. Motivated by these findings, we exposed elderly patients with significant memory loss at baseline to HBOT and observed an increase in CBF and improvement in cognitive performances. This study demonstrates HBOT efficacy in hypoxia-related neurological conditions, particularly in AD and aging.

GABAA receptors in GtoPdb v.2021.3

The GABA_A receptor is a ligand-gated ion channel of the Cys-loop family that includes the nicotinic acetylcholine, 5-HT₃ and strychnine-sensitive glycine receptors. GABA_A receptor-mediated inhibition within the CNS occurs by fast synaptic transmission, sustained tonic inhibition and temporally intermediate events that have been termed 'GABA_A, slow' [45]. GABA_A receptors exist as pentamers of 4TM subunits that form an intrinsic anion selective channel. Sequences of six α, three β, three γ, one δ, three ρ, one ε, one π and one θ GABA_A receptor subunits have been reported in mammals [278, 235, 236, 283]. The π-subunit is restricted to reproductive tissue. Alternatively spliced versions of many subunits exist (e.g. α4- and α6- (both not functional) α5-, β2-, β3- and γ2), along with RNA editing of the α3 subunit [71]. The three ρ-subunits, (ρ1-3) function as either homo- or hetero-oligomeric assemblies [359, 50]. Receptors formed from ρ-subunits, because of their distinctive pharmacology that includes insensitivity to bicuculline, benzodiazepines and barbiturates, have sometimes been termed GABA_C receptors [359], but they are classified as GABA _A receptors by NC-IUPHAR on the basis of structural and functional criteria [16, 235, 236]. Many GABA_A receptor subtypes contain α-, β- and γ-subunits with the likely stoichiometry 2α.2β.1γ [168, 235]. It is thought that the majority of GABA_A receptors harbour a single type of α- and β - subunit variant. The α1β2γ2 hetero-oligomer constitutes the largest population of GABA_A receptors in the CNS, followed by the α2β3γ2 and α3β3γ2 isoforms. Receptors that incorporate the α4- α5-or α 6-subunit, or the β1-, γ1-, γ3-, δ-, ε- and θ-subunits, are less numerous, but they may nonetheless serve important functions. For example, extrasynaptically located receptors that contain α6- and δ-subunits in cerebellar granule cells, or an α4- and δ-subunit in dentate gyrus granule cells and thalamic neurones, mediate a tonic current that is important for neuronal excitability in response to ambient concentrations of GABA [209, 272, 83, 19, 288]. GABA binding occurs at the β+/α- subunit interface and the homologous γ+/α- subunits interface creates the benzodiazepine site. A second site for benzodiazepine binding has recently been postulated to occur at the α+/β- interface ([254]; reviewed by [282]). The particular α-and γ-subunit isoforms exhibit marked effects on recognition and/or efficacy at the benzodiazepine site. Thus, receptors incorporating either α4- or α6-subunits are not recognised by 'classical' benzodiazepines, such as flunitrazepam (but see [356]). The trafficking, cell surface expression, internalisation and function of GABA_A receptors and their subunits are discussed in detail in several recent reviews [52, 140, 188, 316] but one point worthy of note is that receptors incorporating the γ2 subunit (except when associated with α5) cluster at the postsynaptic membrane (but may distribute dynamically between synaptic and extrasynaptic locations), whereas as those incorporating the δ subunit appear to be exclusively extrasynaptic. NC-IUPHAR [16, 235, 3, 2] class the GABA_A receptors according to their subunit structure, pharmacology and receptor function. Currently, eleven native GABA_A receptors are classed as conclusively identified (i.e., α1β2γ2, α1βγ2, α3βγ2, α4βγ2, α4β2δ, α4β3δ, α5βγ2, α6βγ2, α6β2δ, α6β3δ and ρ) with further receptor isoforms occurring with high probability, or only tentatively [235, 236]. It is beyond the scope of this Guide to discuss the pharmacology of individual GABA_A receptor isoforms in detail; such information can be gleaned in the reviews [16, 95, 168, 173, 143, 278, 216, 235, 236] and [9, 10]. Agents that discriminate between α-subunit isoforms are noted in the table and additional agents that demonstrate selectivity between receptor isoforms, for example via β-subunit selectivity, are indicated in the text below. The distinctive agonist and antagonist pharmacology of ρ receptors is summarised in the table and additional aspects are reviewed in [359, 50, 145, 223]. Several high-resolution cryo-electron microscopy structures have been described in which the full-length human α1β3γ2L GABA_A receptor in lipid nanodiscs is bound to the channel-blocker picrotoxin, the competitive antagonist bicuculline, the agonist GABA (γ-aminobutyric acid), and the classical benzodiazepines alprazolam and diazepam [198].

Entorhinal cortical Island cells regulate temporal association learning with long trace period

Temporal association learning (TAL) allows for the linkage of distinct, nonsynchronous events across a period of time. This function is driven by neural interactions in the entorhinal cortical-hippocampal network, especially the neural input from the pyramidal cells in layer III of medial entorhinal cortex (MECIII) to hippocampal CA1 is crucial for TAL. Successful TAL depends on the strength of event stimuli and the duration of the temporal gap between events. Whereas it has been demonstrated that the neural input from pyramidal cells in layer II of MEC, referred to as Island cells, to inhibitory neurons in dorsal hippocampal CA1 controls TAL when the strength of event stimuli is weak, it remains unknown whether Island cells regulate TAL with long trace periods as well. To understand the role of Island cells in regulating the duration of the learnable trace period in TAL, we used Pavlovian trace fear conditioning (TFC) with a 60-sec long trace period (long trace fear conditioning [L-TFC]) coupled with optogenetic and chemogenetic neural activity manipulations as well as cell type-specific neural ablation. We found that ablation of Island cells in MECII partially increases L-TFC performance. Chemogenetic manipulation of Island cells causes differential effectiveness in Island cell activity and leads to a circuit imbalance that disrupts L-TFC. However, optogenetic terminal inhibition of Island cell input to dorsal hippocampal CA1 during the temporal association period allows for long trace intervals to be learned in TFC. These results demonstrate that Island cells have a critical role in regulating the duration of time bridgeable between associated events in TAL.

Extended amygdala circuits are differentially activated by context fear conditioning in male and female rats

As the incidence of anxiety disorders is more prevalent in females, comparing the neural underpinnings of anxiety in males and females is imperative. The bed nucleus of the stria terminalis (BNST) contributes to long-lasting, anxiety-like states including the expression of context fear conditioning. Currently, there is conflicting evidence as to which nuclei of the BNST contribute to these behaviors. The anterolateral portion of the BNST (BNST-AL) located dorsal to the anterior commissure and lateral to the stria terminalis sends robust projections to the central nucleus of the amygdala (CE). Here we asked whether the BNST-AL is active during the expression of context fear conditioning in both male and female rats. At the cellular level, the expression of context fear produced upregulation of the immediate-early gene ARC in the BNST-AL as well as an upregulation of ARC specifically in neurons projecting to the CE, as labeled by the retrograde tracer Fluorogold infused into the CE. However, this pattern of ARC expression was observed in male rats only. Excitotoxic lesions of the BNST reduced context fear expression in both sexes, suggesting that a different set of BNST subnuclei may be recruited by the expression of fear and anxiety-like behaviors in females. Overall, our data highlight the involvement of the BNST-AL in fear expression in males, and suggest that subnuclei of the BNST may be functionally different in male and female rats.

Long-term plasticity of inhibitory synapses in the hippocampus and spatial learning depends on matrix metalloproteinase 3

Learning and memory are known to depend on synaptic plasticity. Whereas the involvement of plastic changes at excitatory synapses is well established, plasticity mechanisms at inhibitory synapses only start to be discovered. Extracellular proteolysis is known to be a key factor in glutamatergic plasticity but nothing is known about its role at GABAergic synapses. We reveal that pharmacological inhibition of MMP3 activity or genetic knockout of the Mmp3 gene abolishes induction of postsynaptic iLTP. Moreover, the application of exogenous active MMP3 mimics major iLTP manifestations: increased mIPSCs amplitude, enlargement of synaptic gephyrin clusters, and a decrease in the diffusion coefficient of synaptic GABAA receptors that favors their entrapment within the synapse. Finally, we found that MMP3 deficient mice show faster spatial learning in Morris water maze and enhanced contextual fear conditioning. We conclude that MMP3 plays a key role in iLTP mechanisms and in the behaviors that presumably in part depend on GABAergic plasticity.

Loss of α7 nicotinic acetylcholine receptors in GABAergic neurons causes sex-dependent decreases in radial glia-like cell quantity and impairments in cognitive and social behavior

The dentate gyrus (DG) is a unique brain structure in that neurons can be generated postnatally and integrated within existing circuitry throughout life. The maturation process of these newly generated neurons (granule cells) is modulated by nicotinic acetylcholine receptors (nAChRs) through a variety of mechanisms such as neural stem pool proliferation, cell survival, signal modulation, and dendritic integration. Disrupted nAChR signaling has been implicated in neuropsychiatric and neurodegenerative disorders, potentially via alterations in DG neurogenesis. GABAergic interneurons are known to express nAChRs, predominantly the α7 subtype, and have been shown to shape development, integration, and circuit reorganization of DG granule cells. Therefore, we examined histological and behavioral effects of knocking out α7 nAChRs in GABAergic neurons. Deletion of α7 nAChRs resulted in a reduction of radial glia-like cells within the subgranular zone of the DG and a concomitant trend towards decreased immature neurons, specifically in male mice, as well as sex-dependent changes in several behaviors, including social recognition and spatial learning. Overall, these findings suggest α7 nAChRs expressed in GABAergic neurons play an important role in regulating the adult neural stem cell pool and behavior in a sex-dependent manner. This provides important insight into the mechanisms by which cholinergic dysfunction contributes to the cognitive and behavioral changes associated with neurodevelopmental and neurodegenerative disorders.

Contextual processing elicits sex differences in dorsal hippocampus activation following footshock and context fear retrieval

Establishing a contextual representation of an environment places specific spatial-temporal processing demands on the mammalian hippocampus, a region showing sex-differences in processing capabilities. However, evidence for sex differences in these processing demands during contextual fear learning remains limited. Here, we examined the relationship among contextual processing, timing of footshock, and activation of the dorsal hippocampus and basolateral amygdalar nuclei (BLA) in male and female mice (C57Bl/6 J). We modified the initial exposure time to the conditioning context prior to administration, or not, of a single footshock. We then quantified Fos- ir neurons activated by acquisition or retrieval of contextual fear memories in the rostral half of the dorsal CA1 (proximal - distal regions), CA3, Dentate Gyrus and basolateral amygdalar nuclei corresponding to atlas levels of the Allen Reference Atlas. In experiment 1, we found that sex differences in context elicited freezing were evident at the longest context placement-to-shock interval and that context fear retrieval with increasing contextual exposure periods increased CA1 Fos-ir in males, but not females. In experiment 2, we observed that an aversive footshock in males potentiated CA1 activation, while it downregulated CA1 activation in females. We also found that an aversive footshock independent of sex moderated Dentate Gyrus activation that normally showed increased activation with greater context exposure periods. Lastly, we identified a heightened responsiveness in BLA neurons to both footshock and length of context exposure in females compared to males. Overall, our findings suggest that sex differences in contextual fear conditioning may arise from marked sex differences in the contextual processing demands of the dorsal hippocampus subfields largely modulated by aversive outcomes.

Transcriptomics of Gabra4 knockout mice reveals common NMDAR pathways underlying autism, memory, and epilepsy

Autism spectrum disorder (ASD) is a neuronal developmental disorder with impaired social interaction and communication, often with abnormal intelligence and comorbidity with epilepsy. Disturbances in synaptic transmission, including the GABAergic, glutamatergic, and serotonergic systems, are known to be involved in the pathogenesis of this disorder, yet we do not know if there is a common molecular mechanism. As mutations in the GABAergic receptor subunit gene GABRA4 are reported in patients with ASD, we eliminated the Gabra4 gene in mice and found that the Gabra4 knockout mice showed autistic-like behavior, enhanced spatial memory, and attenuated susceptibility to pentylenetetrazol-induced seizures, a constellation of symptoms resembling human high-functioning autism. To search for potential molecular pathways involved in these phenotypes, we performed a hippocampal transcriptome profiling, constructed a hippocampal interactome network, and revealed an upregulation of the NMDAR system at the center of the converged pathways underlying high-functioning autism-like and anti-epilepsy phenotypes.

Acute alcohol and cognition: Remembering what it causes us to forget

Addiction has been conceptualized as a specific form of memory that appropriates typically adaptive neural mechanisms of learning to produce the progressive spiral of drug-seeking and drug-taking behavior, perpetuating the path to addiction through aberrant processes of drug-related learning and memory. From that perspective, to understand the development of alcohol use disorders, it is critical to identify how a single exposure to alcohol enters into or alters the processes of learning and memory, so that involvement of and changes in neuroplasticity processes responsible for learning and memory can be identified early. This review characterizes the effects produced by acute alcohol intoxication as a function of brain region and memory neurocircuitry. In general, exposure to ethanol doses that produce intoxicating effects causes consistent impairments in learning and memory processes mediated by specific brain circuitry, whereas lower doses either have no effect or produce a facilitation of memory under certain task conditions. Therefore, acute ethanol does not produce a global impairment of learning and memory, and can actually facilitate particular types of memory, perhaps particular types of memory that facilitate the development of excessive alcohol use. In addition, the effects on cognition are dependent on brain region, task demands, dose received, pharmacokinetics, and tolerance. Additionally, we explore the underlying alterations in neurophysiology produced by acute alcohol exposure that help to explain these changes in cognition and highlight future directions for research. Through understanding the impact that acute alcohol intoxication has on cognition, the preliminary changes potentially causing a problematic addiction memory can better be identified.

A Dynamic Memory Systems Framework for Sex Differences in Fear Memory

Emerging research demonstrates that a pattern of overlapping but distinct molecular and circuit mechanisms are engaged by males and females during memory tasks. Importantly, sex differences in neural mechanisms and behavioral strategies are evident even when performance on a memory task is similar between females and males. We propose that sex differences in memory may be best understood within a dynamic memory systems framework. Specifically, sex differences in hormonal influences and neural circuit development result in biases in the circuits engaged and the information preferentially stored or retrieved in males and females. By using animal models to understand the neural networks and molecular mechanisms required for memory in both sexes, we can gain crucial insights into sex and gender biases in disorders including post-traumatic stress disorder (PTSD) in humans.

Sex differences in fear extinction

Despite the exponential increase in fear research during the last years, few studies have included female subjects in their design. The need to include females arises from the knowledge gap of mechanistic processes underlying the behavioral and neural differences observed in fear extinction. Moreover, the exact contribution of sex and hormones in relation to learning and behavior is still largely unknown. Insights from this field could be beneficial as fear-related disorders are twice as prevalent in women compared to men. Here, we review an up-to-date summary of animal and human studies in adulthood that report sex differences in fear extinction from a structural and functional approach. Furthermore, we describe how these factors could contribute to the observed sex differences in fear extinction during normal and pathological conditions.

Memory generalization after one-trial contextual fear conditioning: Effects of sex and neuropeptide S receptor deficiency

One-trial contextual fear conditioning in laboratory mice results in a fear memory which is relatively specific to the original conditioning context shortly after conditioning but becomes more unspecific after an incubation time of one month. This process is called generalization of fear memory and is used to investigate processes which might be involved in the pathogenesis of post-traumatic stress disorder. In the present study, we investigated the effects of sex and neuropeptide S receptor (NPSR) deficiency in one-trial contextual fear conditioning. In addition to contextual fear, we also measured startle reactivity, anxiety and corticosterone plasma levels of the mice. Our data show main effects of sex and NPSR-deficiency on freezing behavior, startle magnitude, and anxiety levels. However, generalization of contextual fear memory after incubation time was not affected by sex. Notably, NPSR-deficient mice had a more specific fear memory shortly after conditioning than their wildtype littermates but after incubation time, all genotypes had a generalized fear memory. The present data further show that plasma corticosterone levels are increased after incubation time. This increase was significantly more pronounced in NPSR-deficient mice. Taken together, our study confirms the suitability of one-trial contextual fear conditioning to study the effects of incubation time on fear memory generalization but also indicates the need for control groups without incubation. We further demonstrate that the increase of plasma corticosterone levels after incubation time is exaggerated in NPSR-deficient mice. The latter finding suggests an important role of the NPS system in the regulation of corticosterone release.

Sex-specific deficits in biochemical but not behavioral responses to delay fear conditioning in prenatal alcohol exposure mice

BACKGROUND

Studies in clinical populations and preclinical models have shown that prenatal alcohol exposure (PAE) is associated with impairments in the acquisition, consolidation and recall of information, with deficits in hippocampal formation-dependent learning and memory being a common finding. The glucocorticoid receptor (GR), mineralocorticoid receptor (MR), and extracellular signal-regulated kinase 2 (ERK2) are key regulators of hippocampal formation development, structure and functioning and, thus, are potential mediators of PAE's effects on this brain region. In the present studies, we employed a well-characterized mouse model of PAE to identify biochemical mechanisms that may underlie activity-dependent learning and memory deficits associated with PAE.

METHODS

Mouse dams consumed either 10% (w/v) ethanol in 0.066% (w/v) saccharin (SAC) or 0.066% (w/v) SAC alone using a limited (4-h) access, drinking-in-the-dark paradigm. Male and female offspring (∼180-days of age) were trained using a delay conditioning procedure and contextual fear responses (freezing behavior) were measured 24 h later. Hippocampal formation tissue and blood were collected from three behavioral groups of animals: 20 min following conditioning (conditioning only group), 20 min following the re-exposure to the context (conditioning plus re-exposure group), and behaviorally naïve (naïve group) mice. Plasma corticosterone levels were measured by enzyme immunoassay. Immunoblotting techniques were used to measure protein levels of the GR, MR, ERK1 and ERK2 in nuclear and membrane fractions prepared from the hippocampal formation.

RESULTS

Adult SAC control male and female mice displayed similar levels of contextual fear. However, significant sex differences were observed in freezing exhibited during the conditioning session. Compared to same-sex SAC controls, male and female PAE mice demonstrated context fear deficits While plasma corticosterone concentrations were elevated in PAE males and females relative to their respective SAC naïve controls, plasma corticosterone concentrations in the conditioning only and conditioning plus re-exposure groups were similar in SAC and PAE animals. Relative to the respective naïve group, nuclear GR protein levels were increased in SAC, but not PAE, male hippocampal formation in the conditioning only group. In contrast, no difference was observed between nuclear GR levels in the naïve and conditioning plus re-exposure groups. In females, nuclear GR levels were significantly reduced by PAE but there was no effect of behavioral group or interaction between prenatal treatment and behavioral group. In males, nuclear MR levels were significantly elevated in the SAC conditioning plus re-exposure group compared to SAC naïve mice. In PAE females, nuclear MR levels were elevated in both the conditioning only and conditioning plus re-exposure groups relative to the naïve group. Levels of activated ERK2 (phospho-ERK2 expressed relative to total ERK2) protein were elevated in SAC, but not PAE, males following context re-exposure, and a significant interaction between prenatal exposure group and behavioral group was found. No main effects or interactions of behavioral group and prenatal treatment on nuclear ERK2 were found in female mice. These findings suggest a sex difference in which molecular pathways are activated during fear conditioning in mice.

CONCLUSIONS

In PAE males, the deficits in contextual fear were associated with the loss of responsiveness of hippocampal formation nuclear GR, MR and ERK2 to signals generated by fear conditioning and context re-exposure. In contrast, the contextual fear deficit in PAE female mice does not appear to be associated with activity-dependent changes in GR and MR levels or ERK2 activation during training or memory recall, although an overall reduction in nuclear GR levels may play a role. These studies add to a growing body of literature demonstrating that, at least partially, different mechanisms underlie learning, memory formation and memory recall in males and females and that these pathways are differentially affected by PAE.

An Emerging Circuit Pharmacology of GABAA Receptors

In the past 20 years we have learned a great deal about GABA_A receptor (GABA_AR) subtypes, and which behaviors are regulated or which drug effects are mediated by each subtype. However, the question of where GABA_ARs involved in specific drug effects and behaviors are located in the brain remains largely unanswered. We review here recent studies taking a circuit pharmacology approach to investigate the functions of GABA_AR subtypes in specific brain circuits controlling fear, anxiety, learning, memory, reward, addiction, and stress-related behaviors. The findings of these studies highlight the complexity of brain inhibitory systems and the importance of taking a subtype-, circuit-, and neuronal population-specific approach to develop future therapeutic strategies using cell type-specific drug delivery.

α4-GABAA receptors of hippocampal pyramidal neurons are associated with resilience against activity-based anorexia for adolescent female mice but not for males

Activity-based anorexia (ABA) is an animal model of anorexia nervosa, a mental illness with highest mortality and with onset that is most frequently during adolescence. We questioned whether vulnerability of adolescent mice to ABA differs between sexes and whether individual differences in resilience are causally linked to α4βδ-GABAAR expression. C57BL6/J WT and α4-KO adolescent male and female mice underwent ABA induction by combining wheel access with food restriction. ABA vulnerability was measured as the extent of food restriction-evoked hyperactivity on a running wheel and body weight losses. α4βδ-GABAAR levels at plasma membranes of pyramidal cells in dorsal hippocampus were assessed by electron microscopic immunocytochemistry. Temporal patterns and extent of weight loss during ABA induction were similar between sexes. Both sexes also exhibited individual differences in ABA vulnerability. Correlation analyses revealed that, for both sexes, body weight changes precede and thus are likely to drive suppression of wheel running. However, the suppression was during the food-anticipatory hours for males, while for females, suppression was delayed by a day and during food-access hours. Correspondingly, only females adaptively increased food intake. ABA induced up-regulation of α4βδ-GABAARs at plasma membranes of dorsal hippocampal pyramidal cells of females, and especially those females exhibiting resilience. Conversely, α4-KO females exhibited greater food restriction-evoked hyperactivity than WT females. In contrast, ABA males did not up-regulate α4βδ-GABAARs, did not exhibit genotype differences in vulnerability, and exhibited no correlation between plasmalemmal α4βδ-GABAARs and ABA resilience. Thus, food restriction-evoked hyperactivity is driven by anxiety but can be suppressed through upregulation of hippocampal α4βδ-GABAARs for females but not for males. This knowledge of sex-related differences in the underlying mechanisms of resilience to ABA indicates that drugs targeting α4βδ-GABAARs may be helpful for treating stress-induced anxiety and anorexia nervosa of females but not males.

Sexual differentiation of contextual fear responses

Development and sex differentiation impart an organizational influence on the neuroanatomy and behavior of mammalian species. Prior studies suggest that brain regions associated with fear motivated defensive behavior undergo a protracted and sex-dependent development. Outside of adult animals, evidence for developmental sex differences in conditioned fear is sparse. Here, we examined in male and female Long-Evans rats how developmental age and sex affect the long-term retention and generalization of Pavlovian fear responses. Experiments 1 and 2 describe under increasing levels of aversive learning (three and five trials) the long-term retrieval of cued and context fear in preadolescent (P24 and P33), periadolescent (P37), and adult (P60 and P90) rats. Experiments 3 and 4 examined contextual processing under minimal aversive learning (1 trial) procedures in infant (P19, P21), preadolescent (P24), and adult (P60) rats. Here, we found that male and female rats display a divergent developmental trajectory in the expression of context-mediated freezing, such that context fear expression in males tends to increase toward adulthood, while females displayed an opposite pattern of decreasing context fear expression toward adulthood. Longer (14 d) retention intervals produced an overall heightened context fear expression relative to shorter (1 d) retention intervals an observation consistent with fear incubation. Male, but not Female rats showed increasing generalization of context fear across development. Collectively, these findings provide an initial demonstration that sexual differentiation of contextual fear conditioning emerges prior to puberty and follows a distinct developmental trajectory toward adulthood that strikingly parallels sex differences in the etiology and epidemiology of anxiety and trauma- and stressor-related disorders.

Sex Differences in Context Fear Generalization and Recruitment of Hippocampus and Amygdala during Retrieval

Anxiety disorders are commonly associated with increased generalization of fear from a stress- or trauma-associated environment to a neutral context or environment. Differences in context-associated memory in males and females may contribute to increased susceptibility to anxiety disorders in women. Here we examined sex differences in context fear generalization and its neural correlates. We observed stronger context fear conditioning and more generalization of fear to a similar context in females than males. In addition, context preexposure increased fear conditioning in males and decreased generalization in females. Accordingly, males showed stronger cFos activity in dorsal hippocampus during memory retrieval and context generalization, whereas females showed preferential recruitment of basal amygdala. Together, these findings are consistent with previous research showing that hippocampal activity correlates with reduced context fear generalization. Differential competition between hippocampus and amygdala-dependent processes may thus contribute to sex differences in retrieval of context fear and greater generalization of fear-associated memory.

Role of α4-containing GABAA receptors in limiting synaptic plasticity and spatial learning of female mice during the pubertal period

Expression of α4βδ GABA_A receptors (GABARs) increases at the onset of puberty on dendritic spines of CA1 hippocampal pyramidal cells. These receptors reduce activation of NMDA receptors (NMDARs), impair induction of long-term potentiation (LTP) and reduce hippocampal-dependent spatial learning. These effects are not seen in the δ-/- mouse, implicating α4βδ GABARs. Here we show that knock-out of α4 also restores synaptic plasticity and spatial learning in female mice at the onset of puberty (verified by vaginal opening). To this end, field excitatory post-synaptic potentials (fEPSPs) were recorded from the stratum radiatum of CA1 hippocampus in the slice from +/+ and α4-/- pubertal mice (PND 35-44). Induction of LTP, in response to stimulation of the Schaffer collaterals with theta burst stimulation (TBS), was unsuccessful in the +/+ hippocampus, but reinstated by α4 knock-out (~65% potentiation) but not by blockade of α5-GABARs with L-655,708 (50nM). In order to compare spatial learning in the two groups of mice, animals were trained in an active place avoidance task where the latency to first enter a shock zone is a measure of learning. α4-/- mice had significantly longer latencies by the third learning trial, suggesting better spatial learning, compared to +/+ animals, who did not reach the criterion for learning (120s latency). These findings suggest that knock-out of the GABAR α4 subunit restores synaptic plasticity and spatial learning at puberty and is consistent with the concept that the dendritic α4βδ GABARs which emerge at puberty selectively impair CNS plasticity. This article is part of a Special Issue entitled SI: Adolescent plasticity.

Tonic Inhibitory Control of Dentate Gyrus Granule Cells by α5-Containing GABAA Receptors Reduces Memory Interference

Interference between similar or overlapping memories formed at different times poses an important challenge on the hippocampal declarative memory system. Difficulties in managing interference are at the core of disabling cognitive deficits in neuropsychiatric disorders. Computational models have suggested that, in the normal brain, the sparse activation of the dentate gyrus granule cells maintained by tonic inhibitory control enables pattern separation, an orthogonalization process that allows distinct representations of memories despite interference. To test this mechanistic hypothesis, we generated mice with significantly reduced expression of the α5-containing GABAA (α5-GABAARs) receptors selectively in the granule cells of the dentate gyrus (α5DGKO mice). α5DGKO mice had reduced tonic inhibition of the granule cells without any change in fast phasic inhibition and showed increased activation in the dentate gyrus when presented with novel stimuli. α5DGKO mice showed impairments in cognitive tasks characterized by high interference, without any deficiencies in low-interference tasks, suggesting specific impairment of pattern separation. Reduction of fast phasic inhibition in the dentate gyrus through granule cell-selective knock-out of α2-GABAARs or the knock-out of the α5-GABAARs in the downstream CA3 area did not detract from pattern separation abilities, which confirms the anatomical and molecular specificity of the findings. In addition to lending empirical support to computational hypotheses, our findings have implications for the treatment of interference-related cognitive symptoms in neuropsychiatric disorders, particularly considering the availability of pharmacological agents selectively targeting α5-GABAARs.

SIGNIFICANCE STATEMENT

Interference between similar memories poses a significant limitation on the hippocampal declarative memory system, and impaired interference management is a cognitive symptom in many disorders. Thus, understanding mechanisms of successful interference management or processes that can lead to interference-related memory problems has high theoretical and translational importance. This study provides empirical evidence that tonic inhibition in the dentate gyrus (DG), which maintains sparseness of neuronal activation in the DG, is essential for management of interference. The specificity of findings to tonic, but not faster, more transient types of neuronal inhibition and to the DG, but not the neighboring brain areas, is presented through control experiments. Thus, the findings link interference management to a specific mechanism, proposed previously by computational models.

From neural to genetic substrates of panic disorder: Insights from human and mouse studies

Fear is an ancestral emotion, an intrinsic defensive response present in every organism. Although fear is an evolutionarily advantageous emotion, under certain pathologies such as panic disorder it might become exaggerated and non-adaptive. Clinical and preclinical work pinpoints that changes in cognitive processes, such as perception and interpretation of environmental stimuli that rely on brain regions responsible for high-level function, are essential for the development of fear-related disorders. This review focuses on the involvement of cognitive function to fear circuitry disorders. Moreover, we address how animal models are contributing to understand the involvement of human candidate genes to pathological fear and helping achieve progress in this field. Multidisciplinary approaches that integrate human genetic findings with state of the art genetic mouse models will allow to elucidate the mechanisms underlying pathology and to develop new strategies for therapeutic targeting.

Differential effects of dietary oils on emotional and cognitive behaviors

Several dietary oils have been used preventatively and therapeutically in the setting of neurological disease. However, the mechanisms underlying their influence on brain function and metabolism remain unknown. It was investigated whether 3 types of dietary oils affected emotional behaviors in mice. Wild-type (WT) mice and sialyltransferase ST3Gal IV-knockout (KO) mice, which exhibit increased emotional and cognitive behaviors, were fed diets containing 20% dietary oils from post-weaning to adulthood. Mice were fed pellets made from control feed AIN93G powder containing 18% fish oil, soybean oil, or a mixture of 1-palmitoyl-2-oleoyl-3-palmitoyl glycerol (POP) and 1-stearoyl-2-oleoyl-3-stearoyl glycerol (SOS), plus 2% soybean oil. Once mice reached adulthood, they were subjected to fear conditioning test to measure cognitive anxiety and forced swim test to measure depression. WT mice fed the POP-SOS diet showed a 0.6-fold decrease in percent freezing with contextual fear compared with WT mice fed the control diet. KO mice fed the fish oil diet showed a 1.4-fold increase in percent freezing with contextual fear compared with KO mice fed the control diet. These findings indicate that response to contextual fear was improved in WT mice that consumed POP-SOS but aggravated in KO mice that consumed fish oils. Furthermore, KO mice showed a 0.4-fold decrease in percent freezing in response to tone fear when they were fed POP-SOS diet compared to a control diet. Thus, POP-SOS diet reduced tone fear level of KO mice until the same level of WT mice. Finally, KO mice fed the soybean oil diet showed a 1.7-fold increase in immobility in the forced swim test compared to KO mice fed the control diet. Taken together, oil-rich diets differentially modulate anxiety and depression in normal and anxious mice. Oils rich in saturated fatty acids may alleviate anxiety more strongly than other oils.

Hippocampal metaplasticity is required for the formation of temporal associative memories

Metaplasticity regulates the threshold for modification of synaptic strength and is an important regulator of learning rules; however, it is not known whether these cellular mechanisms for homeostatic regulation of synapses contribute to particular forms of learning. Conditional ablation of mGluR5 in CA1 pyramidal neurons resulted in the inability of low-frequency trains of afferent activation to prime synapses for subsequent theta burst potentiation. Priming-induced metaplasticity requires mGluR5-mediated mobilization of endocannabinoids during the priming train to induce long-term depression of inhibition (I-LTD). Mice lacking priming-induced plasticity had no deficit in spatial reference memory tasks, but were impaired in an associative task with a temporal component. Conversely, enhancing endocannabinoid signaling facilitated temporal associative memory acquisition and, after training animals in these tasks, ex vivo I-LTD was partially occluded and theta burst LTP was enhanced. Together, these results suggest a link between metaplasticity mechanisms in the hippocampus and the formation of temporal associative memories.

Neurosteroid effects at α4βδ GABAA receptors alter spatial learning and synaptic plasticity in CA1 hippocampus across the estrous cycle of the mouse

Fluctuations in circulating levels of ovarian hormones have been shown to regulate cognition (Sherwin and Grigorova, 2011. Fertil. Steril. 96, 399-403; Shumaker et al., 2004. JAMA. 291, 2947-2958), but increases in estradiol on the day of proestrus yield diverse outcomes: In vivo induction of long-term potentiation (LTP), a model of learning, is reduced in the morning, but optimal in the afternoon (Warren et al., 1995. Brain Res. 703, 26-30). The mechanism underlying this discrepancy is not known. Here, we show that impairments in both CA1 hippocampal LTP and spatial learning observed on the morning of proestrus are due to increased dendritic expression of α4βδ GABAA receptors (GABARs) on CA1 pyramidal cells, as assessed by electron microscopic (EM) techniques, compared with estrus and diestrus. LTP induction and spatial learning were robust, however, when assessed on the morning of proestrus in α4-/- mice, implicating these receptors in mediating impaired plasticity. Although α4βδ expression remained elevated on the afternoon of proestrus, increases in 3α-OH-THP (3α-OH-5α-pregnan-20-one) decreased inhibition by reducing outward current through α4βδ GABARs (Shen et al., 2007. Nat. Neurosci. 10, 469-477), in contrast to the usual effect of this steroid to enhance inhibition. Proestrous levels of 3α-OH-THP reversed the deficits in LTP and spatial learning, an effect prevented by the inactive metabolite 3β-OH-THP (10 mg/kg, i.p.), which antagonizes actions of 3α-OH-THP. In contrast, administration of 3α-OH-THP (10 mg/kg, i.p.) on the morning of proestrus improved spatial learning scores 150-300%. These findings suggest that cyclic fluctuations in ovarian steroids can induce changes in cognition via α4βδ GABARs that are dependent upon 3α-OH-THP. This article is part of a Special Issue entitled SI: Brain and Memory.

Glycine and GABA(A) ultra-sensitive ethanol receptors as novel tools for alcohol and brain research

A critical obstacle to developing effective medications to prevent and/or treat alcohol use disorders is the lack of specific knowledge regarding the plethora of molecular targets and mechanisms underlying alcohol (ethanol) action in the brain. To identify the role of individual receptor subunits in ethanol-induced behaviors, we developed a novel class of ultra-sensitive ethanol receptors (USERs) that allow activation of a single receptor subunit population sensitized to extremely low ethanol concentrations. USERs were created by mutating as few as four residues in the extracellular loop 2 region of glycine receptors (GlyRs) or γ-aminobutyric acid type A receptors (GABA(A)Rs), which are implicated in causing many behavioral effects linked to ethanol abuse. USERs, expressed in Xenopus oocytes and tested using two-electrode voltage clamp, demonstrated an increase in ethanol sensitivity of 100-fold over wild-type receptors by significantly decreasing the threshold and increasing the magnitude of ethanol response, without altering general receptor properties including sensitivity to the neurosteroid, allopregnanolone. These profound changes in ethanol sensitivity were observed across multiple subunits of GlyRs and GABA(A)Rs. Collectively, our studies set the stage for using USER technology in genetically engineered animals as a unique tool to increase understanding of the neurobiological basis of the behavioral effects of ethanol.

Ovarian cycle-linked plasticity of δ-GABAA receptor subunits in hippocampal interneurons affects γ oscillations in vivo

GABA_A receptors containing δ subunits (δ-GABA_ARs) are GABA-gated ion channels with extra- and perisynaptic localization, strong sensitivity to neurosteroids (NS), and a high degree of plasticity. In selective brain regions they are expressed on specific principal cells and interneurons (INs), and generate a tonic conductance that controls neuronal excitability and oscillations. Plasticity of δ-GABA_ARs in principal cells has been described during states of altered NS synthesis including acute stress, puberty, ovarian cycle, pregnancy and the postpartum period, with direct consequences on neuronal excitability and network dynamics. The defining network events implicated in cognitive function, memory formation and encoding are γ oscillations (30–120 Hz), a well-timed loop of excitation and inhibition between principal cells and PV-expressing INs (PV + INs). The δ-GABA_ARs of INs can modify γ oscillations, and a lower expression of δ-GABA_ARs on INs during pregnancy alters γ frequency recorded in vitro. The ovarian cycle is another physiological event with large fluctuations in NS levels and δ-GABA_ARs. Stages of the cycle are paralleled by swings in memory performance, cognitive function, and mood in both humans and rodents. Here we show δ-GABA_ARs changes during the mouse ovarian cycle in hippocampal cell types, with enhanced expression during diestrus in principal cells and specific INs. The plasticity of δ-GABA_ARs on PV-INs decreases the magnitude of γ oscillations continuously recorded in area CA1 throughout several days in vivo during diestrus and increases it during estrus. Such recurring changes in γ magnitude were not observed in non-cycling wild-type (WT) females, cycling females lacking δ-GABA_ARs only on PV-INs (PV-Gabrd^-/-), and in male mice during a time course equivalent to the ovarian cycle. Our findings may explain the impaired memory and cognitive performance experienced by women with premenstrual syndrome (PMS) or premenstrual dysphoric disorder (PMDD).

Genes and signaling pathways involved in memory enhancement in mutant mice

Mutant mice have been used successfully as a tool for investigating the mechanisms of memory at multiple levels, from genes to behavior. In most cases, manipulating a gene expressed in the brain impairs cognitive functions such as memory and their underlying cellular mechanisms, including synaptic plasticity. However, a remarkable number of mutations have been shown to enhance memory in mice. Understanding how to improve a system provides valuable insights into how the system works under normal conditions, because this involves understanding what the crucial components are. Therefore, more can be learned about the basic mechanisms of memory by studying mutant mice with enhanced memory. This review will summarize the genes and signaling pathways that are altered in the mutants with enhanced memory, as well as their roles in synaptic plasticity. Finally, I will discuss how knowledge of memory-enhancing mechanisms could be used to develop treatments for cognitive disorders associated with impaired plasticity.

Reduced CHRNA7 expression in C3H mice is associated with increases in hippocampal parvalbumin and glutamate decarboxylase-67 (GAD67) as well as altered levels of GABA(A) receptor subunits

Decreased expression of CHRNA7, the gene encoding the α7(∗) subtype of nicotinic receptor, may contribute to the cognitive dysfunction observed in schizophrenia by disrupting the inhibitory/excitatory balance in the hippocampus. C3H mice with reduced Chrna7 expression have significant reductions in hippocampal α7(∗) receptor density, deficits in hippocampal auditory gating, increased hippocampal activity as well as significant decreases in hippocampal glutamate decarboxylase-65 (GAD65) and γ-aminobutyric acid-A (GABAA) receptor levels. The current study investigated whether altered Chrna7 expression is associated with changes in the levels of parvalbumin, GAD67 and/or GABAA receptor subunits in the hippocampus from male and female C3H Chrna7 wildtype, C3H Chrna7 heterozygous and C3H Chrna7 knockout (KO) mice using quantitative Western immunoblotting. Reduced Chrna7 expression was associated with significant increases in hippocampal parvalbumin and GAD67 and with complex alterations in GABAA receptor subunits. A decrease in α3 subunit protein was seen in both female C3H Chrna7 Het and KO mice while a decrease in α4 subunit protein was also detected in C3H Chrna7 KO mice with no sex difference. In contrast, an increase in δ subunit protein was observed in C3H Chrna7 Het mice while a decrease in this subunit was observed in C3H Chrna7 KO mice, with δ subunit protein levels being greater in males than in females. Finally, an increase in γ2 subunit protein was found in C3H Chrna7 KO mice with the levels of this subunit again being greater in males than in females. The increases in hippocampal parvalbumin and GAD67 observed in C3H Chrna7 mice are contrary to reports of reductions in these proteins in the postmortem hippocampus from schizophrenic individuals. We hypothesize that the disparate results may occur because of the influence of factors other than CHRNA7 that have been found to be abnormal in schizophrenia.

The role of the δ GABA(A) receptor in ovarian cycle-linked changes in hippocampus-dependent learning and memory

The δ subunit of the GABAAR is highly expressed in the dentate gyrus of the hippocampus where it mediates a tonic extrasynaptic inhibitory current that is sensitive to neurosteroids. In female mice, the expression level of the δ subunit within the dentate gyrus is elevated in the diestrous relative to estrous phase of the estrous cycle. Previous work in our lab found that female δ-GABAAR KO mice showed enhanced hippocampus-dependent trace but normal hippocampus-independent delay fear conditioning. Wild-type females in this study showed a wide range of freezing levels, whereas δ-GABAAR KO mice expressed only high levels of fear. We hypothesized that the variability in the wild-type mice may have been due to estrous cycle-mediated changes in the expression of the δ-GABAAR, with low levels of freezing in mice that were in the diestrous phase when dentate gyrus tonic inhibition is high. In the present study we tested this hypothesis by utilizing contextual, delay, and trace fear conditioning protocols in mice that were trained and tested in either the diestrous or estrous phases. Consistent with our hypothesis, we found a significant impairment of hippocampus-dependent learning and memory during diestrus relative to estrus in wild-type mice and this impairment was absent in δ-GABAAR mice. These findings argue that the δ-GABAAR plays an important role in estrous cycle-mediated fluctuations in hippocampus-dependent learning and memory.

Trace fear conditioning in mice

In this experiment we present a technique to measure learning and memory. In the trace fear conditioning protocol presented here there are five pairings between a neutral stimulus and an unconditioned stimulus. There is a 20 sec trace period that separates each conditioning trial. On the following day freezing is measured during presentation of the conditioned stimulus (CS) and trace period. On the third day there is an 8 min test to measure contextual memory. The representative results are from mice that were presented with the aversive unconditioned stimulus (shock) compared to mice that received the tone presentations without the unconditioned stimulus. Trace fear conditioning has been successfully used to detect subtle learning and memory deficits and enhancements in mice that are not found with other fear conditioning methods. This type of fear conditioning is believed to be dependent upon connections between the medial prefrontal cortex and the hippocampus. One current controversy is whether this method is believed to be amygdala-independent. Therefore, other fear conditioning testing is needed to examine amygdala-dependent learning and memory effects, such as through the delay fear conditioning.

Differences in memory development among C57BL/6NCrl, 129S2/SvPasCrl, and FVB/NCrl mice after delay and trace fear conditioning

Fear-conditioning testing paradigms have been used to study differences in memory formation between inbred mouse strains, including numerous mouse models of human diseases. In this study, we characterized the conditioned fear memory of 3 inbred strains: C57BL/6NCrl, 129S2/SvPasCrl, and FVB/NCrl, obtained from Charles River Laboratories. We used 2 training paradigms: delay conditioning, in which an unconditional stimulus coterminates with the presentation of a conditional stimulus, and trace conditioning, in which the conditional and unconditional stimuli are separated by a trace interval. In each paradigm, we evaluated the recent (3 d) and remote (25 d) memory of the mice by using a longitudinal design. Our results showed that both C57BL/6NCrl and 129S2/SvPasCrl mice developed strong and long-lasting context and tone memories in both paradigms, but FVB/NCrl mice showed a weaker but nevertheless consistent tone memory after delay training. Tone memory in the FVB strain was stronger in male than female mice. The remote tone memory of 129S2/SvPasCrl mice diminished after delay training but was stable and stronger than that of C57BL/6NCrl mice after trace training. In conclusion, both C57BL/6NCrl and 129S2/SvPasCrl mice showed reliable and long-lasting fear memory after delay or trace training, with 129 mice showing particularly strong tone memory after trace conditioning. The FVB/NCrl strain, especially male mice, showed reliable tone fear memory after delay training. Our findings confirm that both C57BL/6NCrl and 129S2/SvPasCrl mice develop strong context and tone memory in delay and trace fear-conditioning paradigms.

Acutely increasing δGABA(A) receptor activity impairs memory and inhibits synaptic plasticity in the hippocampus

Extrasynaptic γ-aminobutyric acid type A (GABA_A) receptors that contain the δ subunit (δGABA_A receptors) are expressed in several brain regions including the dentate gyrus (DG) and CA1 subfields of the hippocampus. Drugs that increase δGABA_A receptor activity have been proposed as treatments for a variety of disorders including insomnia, epilepsy and chronic pain. Also, long-term pretreatment with the δGABA_A receptor–preferring agonist 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP) enhances discrimination memory and increases neurogenesis in the DG. Despite the potential therapeutic benefits of such treatments, the effects of acutely increasing δGABA_A receptor activity on memory behaviors remain unknown. Here, we studied the effects of THIP (4 mg/kg, i.p.) on memory performance in wild-type (WT) and δGABA_A receptor null mutant (Gabrd^−/−) mice. Additionally, the effects of THIP on long-term potentiation (LTP), a molecular correlate of memory, were studied within the DG and CA1 subfields of the hippocampus using electrophysiological recordings of field potentials in hippocampal slices. The results showed that THIP impaired performance in the Morris water maze, contextual fear conditioning and object recognition tasks in WT mice but not Gabrd^−/− mice. Furthermore, THIP inhibited LTP in hippocampal slices from WT but not Gabrd^−/− mice, an effect that was blocked by GABA_A receptor antagonist bicuculline. Thus, acutely increasing δGABA_A receptor activity impairs memory behaviors and inhibits synaptic plasticity. These results have important implications for the development of therapies aimed at increasing δGABA_A receptor activity.

α4βδ GABAA receptors and tonic inhibitory current during adolescence: effects on mood and synaptic plasticity

The onset of puberty is associated with alterations in mood as well as changes in cognitive function, which can be more pronounced in females. Puberty onset in female mice is associated with increased expression of α4βδ γ-amino-butyric acid-A (GABAA) receptors (GABARs) in CA1 hippocampus. These receptors, which normally have low expression in this central nervous system (CNS) site, emerge along the apical dendrites as well as on the dendritic spines of pyramidal neurons, adjacent to excitatory synapses where they underlie a tonic inhibition that shunts excitatory current and impairs activation of N-methyl-D-aspartate (NMDA) receptors, the trigger for synaptic plasticity. As would be expected, α4βδ expression at puberty also prevents long-term potentiation (LTP), an in vitro model of learning which is a function of network activity, induced by theta burst stimulation of the Schaffer collaterals to the CA1 hippocampus. The expression of these receptors also impairs spatial learning in a hippocampal-dependent task. These impairments are not seen in δ knock-out (-/-) mice, implicating α4βδ GABARs. α4βδ GABARs are also a sensitive target for steroids such as THP ([allo]pregnanolone or 3α-OH-5α[β]-pregnan-20-one), which are dependent upon the polarity of GABAergic current. It is well-known that THP can increase depolarizing current gated by α4βδ GABARs, but more recent data suggest that THP can reduce hyperpolarizing current by accelerating receptor desensitization. At puberty, THP reduces the hyperpolarizing GABAergic current, which removes the shunting inhibition that impairs synaptic plasticity and learning at this time. However, THP, a stress steroid, also increases anxiety, via its action at α4βδ GABARs because it is not seen in δ(-/-) mice. These findings will be discussed as well as their relevance to changes in mood and cognition at puberty, which can be a critical period for certain types of learning and when anxiety disorders and mood swings can emerge.

γ-aminobutyric acid type A receptors that contain the δ subunit promote memory and neurogenesis in the dentate gyrus

OBJECTIVE

Extrasynaptic γ-aminobutyric acid type A receptors that contain the δ subunit (δGABAA receptors) are highly expressed in the dentate gyrus (DG) subfield of the hippocampus, where they generate a tonic conductance that regulates neuronal activity. GABAA receptor-dependent signaling regulates memory and also facilitates postnatal neurogenesis in the adult DG; however, the role of the δGABAA receptors in these processes is unclear. Accordingly, we sought to determine whether δGABAA receptors regulate memory behaviors, as well as neurogenesis in the DG.

METHODS

Memory and neurogenesis were studied in wild-type (WT) mice and transgenic mice that lacked δGABAA receptors (Gabrd(-/-)). To pharmacologically increase δGABAA receptor activity, mice were treated with the δGABAA receptor-preferring agonist 4,5,6,7-tetrahydroisoxazolo(5,4-c)pyridin-3-ol (THIP). Behavioral assays including recognition memory, contextual discrimination, and fear extinction were used. Neurogenesis was studied by measuring the proliferation, survival, migration, maturation, and dendritic complexity of adult-born neurons in the DG.

RESULTS

Gabrd(-/-) mice exhibited impaired recognition memory and contextual discrimination relative to WT mice. Fear extinction was also impaired in Gabrd(-/-) mice, although the acquisition of fear memory was enhanced. Neurogenesis was disrupted in Gabrd(-/-) mice as the migration, maturation, and dendritic development of adult-born neurons were impaired. Long-term treatment with THIP facilitated learning and neurogenesis in WT but not Gabrd(-/-) mice.

INTERPRETATION

δGABAA receptors promote the performance of certain DG-dependent memory behaviors and facilitate neurogenesis. Furthermore, δGABAA receptors can be pharmacologically targeted to enhance these processes.

Defective synaptic transmission and structure in the dentate gyrus and selective fear memory impairment in the Rsk2 mutant mouse model of Coffin-Lowry syndrome

The Coffin-Lowry syndrome (CLS) is a syndromic form of intellectual disability caused by loss-of-function of the RSK2 serine/threonine kinase encoded by the rsk2 gene. Rsk2 knockout mice, a murine model of CLS, exhibit spatial learning and memory impairments, yet the underlying neural mechanisms are unknown. In the current study, we examined the performance of Rsk2 knockout mice in cued, trace and contextual fear memory paradigms and identified selective deficits in the consolidation and reconsolidation of hippocampal-dependent fear memories as task difficulty and hippocampal demand increase. Electrophysiological, biochemical and electron microscopy analyses were carried out in the dentate gyrus of the hippocampus to explore potential alterations in neuronal functions and structure. In vivo and in vitro electrophysiology revealed impaired synaptic transmission, decreased network excitability and reduced AMPA and NMDA conductance in Rsk2 knockout mice. In the absence of RSK2, standard measures of short-term and long-term potentiation (LTP) were normal, however LTP-induced CREB phosphorylation and expression of the transcription factors EGR1/ZIF268 were reduced and that of the scaffolding protein SHANK3 was blocked, indicating impaired activity-dependent gene regulation. At the structural level, the density of perforated and non-perforated synapses and of multiple spine boutons was not altered, however, a clear enlargement of spine neck width and post-synaptic densities indicates altered synapse ultrastructure. These findings show that RSK2 loss-of-function is associated in the dentate gyrus with multi-level alterations that encompass modifications of glutamate receptor channel properties, synaptic transmission, plasticity-associated gene expression and spine morphology, providing novel insights into the mechanisms contributing to cognitive impairments in CLS.

In the rat, chronic intermittent ethanol exposure during adolescence alters the ethanol sensitivity of tonic inhibition in adulthood

BACKGROUND

Alcohol drinking by adolescents is a major public health concern. Adolescents tend to drink in a chronic, intermittent, that is, "binge," pattern, and such patterns of ethanol exposure are associated with increased risk of neurotoxicity and the development of alcohol use disorders (Crews et al., 2000; Hunt, 1993). Both adolescent humans and rats are more sensitive to acute ethanol-induced memory impairment than adults (Acheson et al., 1998; Markwiese et al., 1998). Furthermore, in rats, chronic intermittent ethanol (CIE) exposure during adolescence produces a long-lasting, perhaps permanent, maintenance of the adolescent high sensitivity to ethanol's amnestic effects (White et al., 2000a). We have previously shown that acute ethanol increases tonic inhibitory current mediated by extrasynaptic GABA(A) receptors more efficaciously in dentate granule cells (DGCs) from adolescent than adult rats (Fleming et al., 2007). In this study, we determined if CIE during adolescence produced long-lasting changes in this tonic current.

METHODS

Adolescent rats were subjected to a CIE exposure regimen and allowed to mature to full adulthood. Whole-cell voltage-clamp measurements of tonic inhibitory current and mean phasic current were made in vitro in hippocampal brain slices.

RESULTS

CIE exposure during adolescence increased the ethanol sensitivity of tonic inhibitory current mediated by extrasynaptic GABA(A) receptors and decreased the ethanol sensitivity of phasic, synaptic GABA(A) receptor-mediated current in adult DGCs.

CONCLUSIONS

CIE exposure during adolescence produces long-lasting changes in the function and ethanol sensitivity of extrasynaptic GABA(A) receptors in DGCs. These changes appear to "lock-in" and maintain the high adolescent sensitivity to ethanol in these cells. Furthermore, greater ethanol enhancement of tonic inhibition in the hippocampal formation after CIE is consistent with the greater sensitivity to ethanol-induced memory impairment after adolescent CIE. This finding represents the first demonstration of a long-term, memory-related cellular effect of CIE during adolescence, and the "lock-in" of adolescent ethanol sensitivity that these results suggest could represent a conceptual step forward in understanding the vulnerability of the adolescent brain to alcohol.