Young dentate granule cells mediate pattern separation, whereas old granule cells facilitate pattern completion.

Adult-born granule cells (GCs), a minor population of cells in the hippocampal dentate gyrus, are highly active during the first few weeks after functional integration into the neuronal network, distinguishing them from less active, older adult-born GCs and the major population of dentate GCs generated developmentally. To ascertain whether young and old GCs perform distinct memory functions, we created a transgenic mouse in which output of old GCs was specifically inhibited while leaving a substantial portion of young GCs intact. These mice exhibited enhanced or normal pattern separation between similar contexts, which was reduced following ablation of young GCs. Furthermore, these mutant mice exhibited deficits in rapid pattern completion. Therefore, pattern separation requires adult-born young GCs but not old GCs, and older GCs contribute to the rapid recall by pattern completion. Our data suggest that as adult-born GCs age, their function switches from pattern separation to rapid pattern completion.

Linking hippocampal multiplexed tuning, Hebbian plasticity and navigation.

Three major pillars of hippocampal function are spatial navigation1, Hebbian synaptic plasticity2 and spatial selectivity3. The hippocampus is also implicated in episodic memory4, but the precise link between these four functions is missing. Here we report the multiplexed selectivity of dorsal CA1 neurons while rats performed a virtual navigation task using only distal visual cues5, similar to the standard water maze test of spatial memory1. Neural responses primarily encoded path distance from the start point and the head angle of rats, with a weak allocentric spatial component similar to that in primates but substantially weaker than in rodents in the real world. Often, the same cells multiplexed and encoded path distance, angle and allocentric position in a sequence, thus encoding a journey-specific episode. The strength of neural activity and tuning strongly correlated with performance, with a temporal relationship indicating neural responses influencing behaviour and vice versa. Consistent with computational models of associative and causal Hebbian learning6,7, neural responses showed increasing clustering8 and became better predictors of behaviourally relevant variables, with the average neurometric curves exceeding and converging to psychometric curves. Thus, hippocampal neurons multiplex and exhibit highly plastic, task- and experience-dependent tuning to path-centric and allocentric variables to form episodic sequences supporting navigation.

Cholinergic-Sensitive Theta Oscillations in Memory Encoding in Mice.

Cholinergic regulation of hippocampal theta oscillations has long been proposed to be a potential mechanism underlying hippocampus-dependent memory encoding processes. However, cholinergic transmission has been traditionally associated with type II theta under urethane anesthesia. The mechanisms and behavioral significance of cholinergic regulation of type I theta in freely exploring animals is much less clear. In this study, we examined the potential behavioral significance of cholinergic regulation of theta oscillations in the object location task in male mice that involves training and testing trials and provides an ideal behavioral task to study the underlying memory encoding and retrieval processes, respectively. Cholinergic regulation of hippocampal theta oscillations and the behavioral outcomes was examined by either intrahippocampal infusion of cholinergic receptor antagonists or knocking out cholinergic receptors in excitatory neurons or interneurons. We found that both muscarinic acetylcholine receptors (mAChRs) and α7 nicotinic AChRs (α7 nAChRs) regulated memory encoding by engaging excitatory neurons and interneurons, respectively. There is a transient upregulated theta oscillation at the beginning of individual object exploration events that only occurred in the training trials, but not in the testing trials. This transient upregulated theta is also the only theta component that significantly differed between training and testing trials and was sensitive to mAChR and α7 nAChR antagonists. Thus, our study has revealed a transient cholinergic-sensitive theta component that is specifically associated with memory encoding, but not memory retrieval, in the object location task, providing direct experimental evidence supporting a role for cholinergic-regulated theta oscillations in hippocampus-dependent memory encoding processes.

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.

Differential signalling induced by α7 nicotinic acetylcholine receptors in hippocampal dentate gyrus in vitro and in vivo.

The activation of α7 nicotinic acetylcholine receptors (nAChRs) has been shown to improve hippocampus-dependent learning and memory. α7 nAChRs are densely expressed among several different cell types in the hippocampus, with high Ca2+  permeability, although it is unclear if α7 nAChRs mobilize differential signalling mechanisms among distinct neuronal populations. To address this question, we compared α7 nAChR agonist-induced responses (i.e. calcium and cAMP changes) between granule cells and GABAergic neurons in the hippocampal dentate gyrus both in vitro and in vivo. In cultured organotypic hippocampal slices, we observed robust intracellular calcium and cAMP increases in dentate granule cells upon activation of α7 nAChRs. In contrast, GABAergic interneurons displayed little change in either calcium or cAMP concentration after α7 nAChR activation, even though they displayed much larger α7 nAChR current responses than those of dentate granule cells. We found that this was due to smaller α7 nAChR-induced Ca2+ rises in GABAergic interneurons. Thus, the regulation of the Ca2+ transients in different cell types resulted in differential subsequent intracellular signalling cascades and likely the ultimate outcome of α7 nAChR activation. Furthermore, we monitored neuronal activities of dentate granule cells and GABAergic interneurons in vivo via optic fibre photometry. We observed enhancement of neuronal activities after nicotine administration in dentate granule cells, but not in GABAergic neurons, which was absent in α7 nAChR-deficient granule cells. In summary, we reveal a mechanism for α7 nAChR-mediated increase of neuronal activity via cell type-specific intracellular signalling pathways. KEY POINTS: α7 nicotinic acetylcholine receptors (nAChRs) are widely distributed throughout the central nervous system and regulate a variety of brain functions including learning and memory. Understanding the cellular signalling mechanisms of their activations among different neuronal populations is important for delineating their actions in cognitive function, and developing effective treatment strategies for cognitive deficits. We report that α7 nAChR activation leads to Ca2+ and cAMP increases in granule cells (but not in GABAergic interneurons) in hippocampal dentate gyrus in vitro, a key region for pattern separation during learning. We also found that nicotine enhanced granule cell (but not in GABAergic interneurons) activity in an α7 nAChR-dependent manner via in vivo fibre photometry recording. Based on our findings, we propose that differential responses to α7 nAChR activation between granule cells and GABAergic interneurons is responsible for the increase of excitation by α7 nAChR agonists in hippocampal circuits synergistically.

Qualitative and Quantitative Neuropathology Approaches Using Magnetic Resonance Microscopy (Diffusion Tensor Imaging) and Stereology in a Hexachlorophene Model of Myelinopathy in Sprague-Dawley Rats.

It is well established that hexachlorophene, which is used as an antibacterial agent, causes intramyelinic edema in humans and animal models. The hexachlorophene myelinopathy model, in which male Sprague-Dawley rats received 25 to 30 mg/kg hexachlorophene by gavage for up to 5 days, provided an opportunity to compare traditional neuropathology evaluations with magnetic resonance microscopy (MRM) findings. In addition, stereology assessments of 3 neuroanatomical sites were compared to quantitative measurements of similar structures by MRM. There were positive correlations between hematoxylin and eosin and luxol fast blue stains and MRM for identifying intramyelinic edema in the cingulum of corpus callosum, optic chiasm, anterior commissure (aca), lateral olfactory tracts, pyramidal tracts (py), and white matter tracts in the cerebellum. Stereology assessments were focused on the aca, longitudinal fasciculus of the pons, and py and demonstrated differences between control and treated rats, as was observed using MRM. The added value of MRM assessments was the ability to acquire qualitative 3-dimensional (3-D) images and obtain quantitative measurements of intramyelinic edema in 26 neuroanatomical sites in the intact brain. Also, diffusion tensor imaging (fractional anisotropy [FA]) indicated that there were changes in the cytoarchitecture of the white matter as detected by decreases in the FA in the treated compared to the control rats. This study demonstrates creative strategies that are possible using qualitative and quantitative assessments of potential white matter neurotoxicants in nonclinical toxicity studies. Our results lead us to the conclusion that volumetric analysis by MRM and stereology adds significant value to the standard 2-D microscopic evaluations.

α2* Nicotinic acetylcholine receptors influence hippocampus-dependent learning and memory in adolescent mice.

The absence of α2* nicotinic acetylcholine receptors (nAChRs) in oriens lacunosum moleculare (OLM) GABAergic interneurons ablate the facilitation of nicotine-induced hippocampal CA1 long-term potentiation and impair memory. The current study delineated whether genetic mutations of α2* nAChRs (Chrna2L9'S/L9'S and Chrna2KO) influence hippocampus-dependent learning and memory and CA1 synaptic plasticity. We substituted a serine for a leucine (L9'S) in the α2 subunit (encoded by the Chrna2 gene) to make a hypersensitive nAChR. Using a dorsal hippocampus-dependent task of preexposure-dependent contextual fear conditioning, adolescent hypersensitive Chrna2L9'S/L9'S male mice exhibited impaired learning and memory. The deficit was rescued by low-dose nicotine exposure. Electrophysiological studies demonstrated that hypersensitive α2 nAChRs potentiate acetylcholine-induced ion channel flux in oocytes and acute nicotine-induced facilitation of dorsal/intermediate CA1 hippocampal long-term potentiation in Chrna2L9'S/L9'S mice. Adolescent male mice null for the α2 nAChR subunit exhibited a baseline deficit in learning that was not reversed by an acute dose of nicotine. These effects were not influenced by locomotor, sensory or anxiety-related measures. Our results demonstrated that α2* nAChRs influenced hippocampus-dependent learning and memory, as well as nicotine-facilitated CA1 hippocampal synaptic plasticity.

Impaired extinction of cued fear memory and abnormal dendritic morphology in the prelimbic and infralimbic cortices in VPAC2 receptor (VIPR2)-deficient mice.

The structurally related neuropeptides vasoactive intestinal peptide (VIP) and pituitary adenylate cyclase-activating polypeptide (PACAP) have been implicated in stress regulation and learning and memory. Several bodies of research have shown the impact of the PACAP specific receptor PAC1 on fear memory, but the roles of other PACAP receptors in regulating fear stress responses remain to be elucidated. Here we aimed to investigate the effects of genetic deletion of VIPR2 encoding the VPAC2 receptor, which binds both VIP and PACAP, on fear-related memory and on dendritic morphology in the brain regions of the fear circuitry. Male VPAC2 receptor knockout (VPAC2-KO) and littermate wild-type control mice were subjected to Pavlovian fear conditioning paradigm. VPAC2-KO mice displayed normal acquisition of fear conditioning, contextual and cued fear memory, but impaired extinction of cued fear memory. Morphological analyses revealed reductions in cell body size and total branch number and length of apical and basal dendrites of prelimbic cortex neurons in VPAC2-KO mice. In addition, Sholl analysis indicated that the amount of dendritic material distal to the soma was decreased, while proximal dendritic material was increased. In the infralimbic cortex, the amount of apical dendritic material proximal to the soma was increased in VPAC2-KO mice, while other indices of morphology did not differ. Finally, there were no differences in dendritic morphology in basolateral amygdala neurons between genotypes. These findings suggest that the VPAC2 receptor plays an important role in the fear extinction processes and the regulation of the dendritic morphology in the prelimbic and infralimbic cortices.

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.

FiPhA: an open-source platform for fiber photometry analysis.

Significance: Fiber photometry (FP) is a widely used technique in modern behavioral neuroscience, employing genetically encoded fluorescent sensors to monitor neural activity and neurotransmitter release in awake-behaving animals. However, analyzing photometry data can be both laborious and time-consuming. Aim: We propose the fiber photometry analysis (FiPhA) app, which is a general-purpose FP analysis application. The goal is to develop a pipeline suitable for a wide range of photometry approaches, including spectrally resolved, camera-based, and lock-in demodulation. Approach: FiPhA was developed using the R Shiny framework and offers interactive visualization, quality control, and batch processing functionalities in a user-friendly interface. Results: This application simplifies and streamlines the analysis process, thereby reducing labor and time requirements. It offers interactive visualizations, event-triggered average processing, powerful tools for filtering behavioral events, and quality control features. Conclusions: FiPhA is a valuable tool for behavioral neuroscientists working with discrete, event-based FP data. It addresses the challenges associated with analyzing and investigating such data, offering a robust and user-friendly solution without the complexity of having to hand-design custom analysis pipelines. This application thus helps standardize an approach to FP analysis.

Sex differences in spontaneous behavior and cognition in mice using an automated behavior monitoring system.

Isolation of sex differences as a key characteristic underlying neurobehavioral differentiation is an essential component of studies in neuroscience. The current study sought to address this concern by observing behavioral differences using an automated home cage system for neurobehavioral assessment, a method rapidly increasing in use due to advances in technology and advantages such as reduced handling stress and cross-lab variability. Sex differences in C57BL/6 mice arose for motor activity and circadian-linked behavior, with females being more active compared to males, and males having a stronger anticipatory increase in activity leading up to the onset of the light phase compared to females. These activity differences were observed not only across the lifespan, but also in different genetic background mouse strains across different testing sites showing the generalizability and robustness of these observed effects. Activity differences were also observed in performance on a spatial learning and reversal task with females making more responses and receiving a corresponding elevation in reward pellets. Notably, there were no sex differences in learning nor achieved accuracy, suggesting these observed effects were predominantly in activity. The outcomes of this study align with previous reports showcasing differences in activity between males and females. The comparison across strains and testing sites showed robust and reproducible differences in behavior between female and male mice that are relevant to consider when designing behavioral studies. Furthermore, the observed sex differences in performance on the learning and reversal procedure raise concern for interpretation of behavior differences between sexes due to the attribution of these differences to motor activity rather than cognition.

Partial or Complete Loss of Norepinephrine Differentially Alters Contextual Fear and Catecholamine Release Dynamics in Hippocampal CA1.

Background: Contextual fear learning is heavily dependent on the hippocampus. Despite evidence that catecholamines contribute to contextual encoding and memory retrieval, the precise temporal dynamics of their release in the hippocampus during behavior is unknown. In addition, new animal models are required to probe the effects of altered catecholamine synthesis on release dynamics and contextual learning. Methods: We generated 2 new mouse models of altered locus coeruleus-norepinephrine (NE) synthesis and utilized them together with GRABNE and GRABDA sensors and in vivo fiber photometry to investigate NE and dopamine (DA) release dynamics in the dorsal hippocampal CA1 during contextual fear conditioning. Results: Aversive foot shock increased both NE and DA release in the dorsal CA1, while freezing behavior associated with recall of fear memory was accompanied by decreased release. Moreover, we found that freezing at the recent time point was sensitive to both partial and complete loss of locus coeruleus-NE synthesis throughout prenatal and postnatal development, similar to previous observations of mice with global loss of NE synthesis beginning postnatally. In contrast, freezing at the remote time point was compromised only by complete loss of locus coeruleus-NE synthesis beginning prenatally. Conclusions: Overall, these findings provide novel insights into the role of NE in contextual fear and the precise temporal dynamics of both NE and DA during freezing behavior and highlight complex relationships between genotype, sex, and NE signaling.

Genetic deletion of α7 nAChRs reduces hippocampal granule and pyramidal cell number in both sexes but impairs pattern separation in males only.

Introduction: Neurogenesis within the dentate gyrus is thought to play an important role in cognitive processes such as reversal learning and pattern separation. The α7 nicotinic acetylcholine receptor (α7 nAChR) is expressed early in newly formed granule cells of the dentate gyrus, though its role in neurogenesis and related cognitive function is not fully understood. Methods: To better characterize relevant function of α7 nAChRs, we performed unbiased stereology to quantify hippocampal granule cells, pyramidal cells, and total volume and used a touchscreen operant spatial discrimination/reversal task to test pattern separation in a global α7 nAChR knockout mouse line. Results: The knockout resulted in an ≈22% reduction in granule cells and a ≈ 20% reduction in pyramidal cells in both sexes, with no change in total hippocampal volume. However, the knockout impaired performance in the touchscreen task for males only. The sex-dependent difference in behavioral, but not stereological, results suggest a divergence in the structure-function relationship in males versus females. Detailed analyses revealed males were more biased by the initial reversal contingency relative to females indicating a potential source of the sex-specific interaction with the loss of α7 nAChRs. Discussion: These findings argue that the α7 nAChR plays a critical role in hippocampal development, not just granule cell neurogenesis, and plays a sex-dependent role in cognitive function.

Entorhinal cortical delta oscillations drive memory consolidation.

Long-term memories are formed by creating stable memory representations via memory consolidation, which mainly occurs during sleep following the encoding of labile memories in the hippocampus during waking. The entorhinal cortex (EC) has intricate connections with the hippocampus, but its role in memory consolidation is largely unknown. Using cell-type- and input-specific in vivo neural activity recordings, here we show that the temporoammonic pathway neurons in the EC, which directly innervate the output area of the hippocampus, exhibit potent oscillatory activities during anesthesia and sleep. Using in vivo individual and populational neuronal activity recordings, we demonstrate that a subpopulation of the temporoammonic pathway neurons, which we termed sleep cells, generate delta oscillations via hyperpolarization-activated cyclic-nucleotide-gated channels during sleep. The blockade of these oscillations significantly impaired the consolidation of hippocampus-dependent memory. Together, our findings uncover a key driver of delta oscillations and memory consolidation that are found in the EC.

FiPhA: An Open-Source Platform for Fiber Photometry Analysis.

Significance: Fiber photometry is a widely used technique in modern behavioral neuroscience, employing genetically encoded fluorescent sensors to monitor neural activity and neurotransmitter release in awake-behaving animals, However, analyzing photometry data can be both laborious and time-consuming. Aim: We propose the FiPhA (Fiber Photometry Analysis) app, which is a general-purpose fiber photometry analysis application. The goal is to develop a pipeline suitable for a wide range of photometry approaches, including spectrally resolved, camera-based, and lock-in demodulation. Approach: FiPhA was developed using the R Shiny framework and offers interactive visualization, quality control, and batch processing functionalities in a user-friendly interface. Results: This application simplifies and streamlines the analysis process, thereby reducing labor and time requirements. It offers interactive visualizations, event-triggered average processing, powerful tools for filtering behavioral events and quality control features. Conclusions: FiPhA is a valuable tool for behavioral neuroscientists working with discrete, event-based fiber photometry data. It addresses the challenges associated with analyzing and investigating such data, offering a robust and user-friendly solution without the complexity of having to hand-design custom analysis pipelines. This application thus helps standardize an approach to fiber photometry analysis.

Mbnl2 loss alters novel context processing and impairs object recognition memory.

Patients with myotonic dystrophy type I (DM1) demonstrate visuospatial dysfunction and impaired performance in tasks requiring recognition or memory of figures and objects. In DM1, CUG expansion RNAs inactivate the muscleblind-like (MBNL) proteins. We show that constitutive Mbnl2 inactivation in Mbnl2ΔE2/ΔE2 mice selectively impairs object recognition memory in the novel object recognition test. When exploring the context of a novel arena in which the objects are later encountered, the Mbnl2ΔE2/ΔE2 dorsal hippocampus responds with a lack of enrichment for learning and memory-related pathways, mounting instead transcriptome alterations predicted to impair growth and neuron viability. In Mbnl2ΔE2/ΔE2 mice, saturation effects may prevent deployment of a functionally relevant transcriptome response during novel context exploration. Post-novel context exploration alterations in genes implicated in tauopathy and dementia are observed in the Mbnl2ΔE2/ΔE2 dorsal hippocampus. Thus, MBNL2 inactivation in patients with DM1 may alter novel context processing in the dorsal hippocampus and impair object recognition memory.

NMDA receptor hypofunction in the dentate gyrus and impaired context discrimination in adult Fmr1 knockout mice.

Fragile X syndrome (FXS) is the most common form of inherited intellectual disability in humans. This X-linked disorder is caused by the transcriptional repression of a single gene, Fmr1. The loss of Fmr1 transcription prevents the production of Fragile X mental retardation protein (FMRP) which in turn disrupts the expression of a variety of key synaptic proteins that appear to be important for intellectual ability. A clear link between synaptic dysfunction and behavioral impairment has been elusive, despite the fact that several animal models of FXS have been generated. Here we report that Fmr1 knockout mice exhibit impaired bidirectional synaptic plasticity in the dentate gyrus (DG) of the hippocampus. These deficits are associated with a novel decrease in functional NMDARs (N-methyl-D-aspartate receptors). In addition, mice lacking the Fmr1 gene show impaired performance in a context discrimination task that normally requires functional NMDARs in the DG. These data indicate that Fmr1 deletion results in significant NMDAR-dependent electrophysiological and behavioral impairments specific to the DG.

Loss of GABA co-transmission from cholinergic neurons impairs behaviors related to hippocampal, striatal, and medial prefrontal cortex functions.

Introduction: Altered signaling or function of acetylcholine (ACh) has been reported in various neurological diseases, including Alzheimer's disease, Tourette syndrome, epilepsy among others. Many neurons that release ACh also co-transmit the neurotransmitter gamma-aminobutyrate (GABA) at synapses in the hippocampus, striatum, substantia nigra, and medial prefrontal cortex (mPFC). Although ACh transmission is crucial for higher brain functions such as learning and memory, the role of co-transmitted GABA from ACh neurons in brain function remains unknown. Thus, the overarching goal of this study was to investigate how a systemic loss of GABA co-transmission from ACh neurons affected the behavioral performance of mice. Methods: To do this, we used a conditional knock-out mouse of the vesicular GABA transporter (vGAT) crossed with the ChAT-Cre driver line to selectively ablate GABA co-transmission at ACh synapses. In a comprehensive series of standardized behavioral assays, we compared Cre-negative control mice with Cre-positive vGAT knock-out mice of both sexes. Results: Loss of GABA co-transmission from ACh neurons did not disrupt the animal's sociability, motor skills or sensation. However, in the absence of GABA co-transmission, we found significant alterations in social, spatial and fear memory as well as a reduced reliance on striatum-dependent response strategies in a T-maze. In addition, male conditional knockout (CKO) mice showed increased locomotion. Discussion: Taken together, the loss of GABA co-transmission leads to deficits in higher brain functions and behaviors. Therefore, we propose that ACh/GABA co-transmission modulates neural circuitry involved in the affected behaviors.

Natural locus coeruleus dynamics during feeding.

Recent data demonstrate that noradrenergic neurons of the locus coeruleus (LC-NE) are required for fear-induced suppression of feeding, but the role of endogenous LC-NE activity in natural, homeostatic feeding remains unclear. Here, we found that LC-NE activity was suppressed during food consumption, and the magnitude of this neural response was attenuated as mice consumed more pellets throughout the session, suggesting that LC responses to food are modulated by satiety state. Visual-evoked LC-NE activity was also attenuated in sated mice, suggesting that satiety state modulates LC-NE encoding of multiple behavioral states. We also found that food intake could be attenuated by brief or longer durations of LC-NE activation. Last, we found that activation of the LC to the lateral hypothalamus pathway suppresses feeding and enhances avoidance and anxiety-like responding. Our findings suggest that LC-NE neurons modulate feeding by integrating both external cues (e.g., anxiogenic environmental cues) and internal drives (e.g., satiety).

Behavioral pharmacogenetic analysis on the role of the α4 GABA(A) receptor subunit in the ethanol-mediated impairment of hippocampus-dependent contextual learning.

BACKGROUND: A major effect of low-dose ethanol is impairment of hippocampus-dependent cognitive function. α4/δ -containing GABA(A) Rs are highly expressed within the dentate gyrus region of the hippocampus where they mediate a tonic inhibitory current that is sensitive to the enhancement by low ethanol concentrations. These receptors are also powerful modulators of learning and memory, suggesting that they could play an important role in ethanol's cognitive impairing effects. The goal of this study was to develop a high-throughput cognitive ethanol assay, amenable to use in genetically modified mice that could be used to test this hypothesis. METHODS: We developed a procedure where preexposure to a conditioning chamber is used to rescue the "immediate shock deficit." Using this task, ethanol can be specifically targeted at the hippocampus-dependent process of contextual learning without interfering with pain sensitivity or behavioral performance. RESULTS: Validation of this task in C57BL/6 mice indicated that 1.0 g/kg ethanol and 10 mg/kg allopregnanolone disrupt contextual learning. Ro15-4513 reversed the effects of ethanol but not allopregnanolone, whereas it produced an impairment when given alone. The high-throughput nature of this task allowed for its application in a large cohort of α4 GABA(A) R KO mice. Loss of the α4 GABA(A) R subunit produced an enhanced sensitivity to the cognitive impairing effects of ethanol. This is consistent with the enhanced ethanol sensitivity of synaptic GABA(A) Rs that has been previously observed in the dentate gyrus in these mice, but inconsistent with the reduced ethanol sensitivity of extrasynaptic GABA(A) Rs observed in the same cells. CONCLUSIONS: Overall, these findings are consistent with our hypothesis that ethanol acts directly at GABA(A) receptors to impair hippocampus-dependent cognitive function. Furthermore, validation of this high-throughput assay will allow for future studies to use anatomically and temporally restricted genetic manipulations to probe more deeply into the neural mechanisms of ethanol action on learning and memory circuits.