Electrophysiological correlates of attention in the locus coeruleus-prelimbic cortex circuit during the rodent continuous performance test.

Sustained attention, the ability to focus on an activity or stimulus over time, is significantly impaired in many psychiatric disorders, and there remains a major unmet need in treating impaired attention. Continuous performance tests (CPTs) were developed to measure sustained attention in humans, non-human primates, rats, and mice, and similar neural circuits are engaged across species during CPT performance, supporting their use in translational studies to identify novel therapeutics. Here, we identified electrophysiological correlates of attentional performance in a touchscreen-based rodent CPT (rCPT) in the locus coeruleus (LC) and prelimbic cortex (PrL), two inter-connected regions that are implicated in attentional processes. We used viral labeling and molecular techniques to demonstrate that neural activity is recruited in LC-PrL projections during the rCPT, and that this recruitment increases with cognitive demand. We implanted male mice with depth electrodes within the LC and PrL for local field potential (LFP) recordings during rCPT training, and identified an increase in PrL delta and theta power, and an increase in LC delta power during correct responses in the rCPT. We also found that the LC leads the PrL in theta frequencies during correct responses while the PrL leads the LC in gamma frequencies during incorrect responses. These findings may represent translational biomarkers that can be used to screen novel therapeutics for drug discovery in attention.

Degrading stimuli by reducing image resolution impairs performance in a rodent continuous performance test.

Attention is a cognitive domain often disrupted in neuropsychiatric disorders and continuous performance tests (CPTs) are common clinical assays of attention. In CPTs, participants produce a behavioral response to target stimuli and refrain from responding to non-target stimuli. Performance in CPTs is measured as the ability to discriminate between targets and non-targets. Rodent versions of CPTs (rCPTs) have been validated with both anatomical and pharmacological studies, providing a translational platform for understanding attention function. In humans, stimulus degradation, the inclusion of visual noise in the image to reduce resolution, in CPTs impairs performance. Reduced image contrast, changes in the relative luminescence of elements in the image, has been used in rCPTs to test similar constructs, but, to our knowledge, reduced image resolution has not been tested in an rCPT. In this study, we tested multiple levels of stimulus degradation in a touchscreen version of the rCPT in mice. We found that stimulus degradation significantly decreased performance in males and females. Specifically, we found decreased stimulus discrimination and increases in hit reaction time and reaction time variability. These findings are in line with the effects of stimulus degradation in human studies. These data extend the utility and translational value of the family of rCPTs by demonstrating that stimulus degradation in the form of reduced image resolution produces qualitatively similar behavioral responses in mice as those in previous human studies.

Electrophysiological correlates of attention in the locus coeruleus - anterior cingulate cortex circuit during the rodent continuous performance test.

Sustained attention, the ability to focus on an activity or stimulus over time, is significantly impaired in many psychiatric disorders, and there remains a major unmet need in treating impaired attention. Continuous performance tests (CPTs) were developed to measure sustained attention in humans, non-human primates, rats, and mice, and similar neural circuits are engaged across species during CPT performance, supporting their use in translational studies to identify novel therapeutics. Here, we identified electrophysiological correlates of attentional performance in a touchscreen-based rodent CPT (rCPT) in the locus coeruleus (LC) and anterior cingulate cortex (ACC), two inter-connected regions that are implicated in attentional processes. We used viral labeling and molecular techniques to demonstrate that neural activity is recruited in LC-ACC projections during the rCPT, and that this recruitment increases with cognitive demand. We implanted male mice with depth electrodes within the LC and ACC for local field potential (LFP) recordings during rCPT training, and identified an increase in ACC delta and theta power, and an increase in LC delta power during correct responses in the rCPT. We also found that the LC leads the ACC in theta frequencies during correct responses while the ACC leads the LC in gamma frequencies during incorrect responses. These findings may represent translational biomarkers that can be used to screen novel therapeutics for drug discovery in attention.

The basolateral amygdala to lateral septum circuit is critical for regulating social novelty in mice.

The lateral septum (LS) is a basal forebrain GABAergic region that is implicated in social novelty. However, the neural circuits and cell signaling pathways that converge on the LS to mediate social behaviors aren't well understood. Multiple lines of evidence suggest that signaling of brain-derived neurotrophic factor (BDNF) through its receptor TrkB plays important roles in social behavior. BDNF is not locally produced in LS, but we demonstrate that nearly all LS GABAergic neurons express TrkB. Local TrkB knock-down in LS neurons decreased social novelty recognition and reduced recruitment of neural activity in LS neurons in response to social novelty. Since BDNF is not synthesized in LS, we investigated which inputs to LS could serve as potential BDNF sources for controlling social novelty recognition. We demonstrate that selectively ablating inputs to LS from the basolateral amygdala (BLA), but not from ventral CA1 (vCA1), impairs social novelty recognition. Moreover, depleting BDNF selectively in BLA-LS projection neurons phenocopied the decrease in social novelty recognition caused by either local LS TrkB knockdown or ablation of BLA-LS inputs. These data support the hypothesis that BLA-LS projection neurons serve as a critical source of BDNF for activating TrkB signaling in LS neurons to control social novelty recognition.

The ventral midline thalamus coordinates prefrontal-hippocampal neural synchrony during vicarious trial and error.

When faced with difficult choices, the possible outcomes are considered through a process known as deliberation. In rats, deliberation is thought to be reflected by pause-and-reorienting behaviors, better known as vicarious trial and errors (VTEs). While VTEs are thought to require medial prefrontal cortex (mPFC) and dorsal hippocampal (dHPC) interactions, no empirical evidence has yet demonstrated such a dual requirement. The nucleus reuniens (Re) of the ventral midline thalamus is anatomically connected with both the mPFC and dHPC, is required for HPC-dependent spatial memory tasks, and is critical for mPFC-dHPC neural synchronization. Currently, it is unclear if, or how, the Re is involved in deliberation. Therefore, by examining the role of the Re on VTE behaviors, we can better understand the anatomical and physiological mechanisms supporting deliberation. Here, we examined the impact of Re suppression on VTE behaviors and mPFC-dHPC theta synchrony during asymptotic performance of a HPC-dependent delayed alternation (DA) task. Pharmacological suppression of the Re increased VTE behaviors that occurred with repetitive choice errors. These errors were best characterized as perseverative behaviors, in which some rats repeatedly selected a goal arm that previously yielded no reward. We then examined the impact of Re suppression on mPFC-dHPC theta synchrony during VTEs. We found that during VTEs, Re inactivation was associated with a reduction in mPFC-dHPC theta coherence and mPFC-to-dHPC theta directionality. Our findings suggest that the Re contributes to deliberation by coordinating mPFC-dHPC neural interactions.

Molecularly Defined Hippocampal Inputs Regulate Population Dynamics in the Prelimbic Cortex to Suppress Context Fear Memory Retrieval.

BACKGROUND: Context fear memory dysregulation is a hallmark symptom of several neuropsychiatric disorders, including generalized anxiety disorder and posttraumatic stress disorder. The hippocampus (HC) and prelimbic (PrL) subregion of the medial prefrontal cortex have been linked with context fear memory retrieval in rodents, but the mechanisms by which HC-PrL circuitry regulates this process remain poorly understood. METHODS: Spatial and genetic targeting of HC-PrL circuitry was used for RNA sequencing (n = 31), chemogenetic stimulation (n = 44), in vivo calcium imaging (n = 20), ex vivo electrophysiology (n = 8), and molecular regulation of plasticity cascades during fear behavior (context fear retrieval) (n = 16). RESULTS: We showed that ventral HC (vHC) neurons with projections to the PrL cortex (vHC-PrL projectors) are a transcriptomically distinct subpopulation compared with adjacent nonprojecting neurons, and we showed complementary enrichment for diverse neuronal processes and central nervous system-related clinical gene sets. We further showed that stimulation of this population of vHC-PrL projectors suppresses context fear memory retrieval and impairs the ability of PrL neurons to dynamically distinguish between distinct phases of fear learning. Using transgenic and circuit-specific molecular targeting approaches, we demonstrated that unique patterns of activity-dependent gene transcription associated with brain-derived neurotrophic factor signaling within vHC-PrL projectors causally regulated activity in excitatory and inhibitory PrL neurons during context fear memory retrieval. CONCLUSIONS: Together, our data show that activity-dependent brain-derived neurotrophic factor release from molecularly distinct vHC-PrL projection neurons modulates postsynaptic signaling in both inhibitory and excitatory PrL neurons, modifying activity in discrete populations of PrL neurons to suppress freezing during context fear memory retrieval.

TrkB Signaling Influences Gene Expression in Cortistatin-Expressing Interneurons.

Brain-derived neurotrophic factor (BDNF) signals through its cognate receptor tropomyosin receptor kinase B (TrkB) to promote the function of several classes of inhibitory interneurons. We previously reported that loss of BDNF-TrkB signaling in cortistatin (Cort)-expressing interneurons leads to behavioral hyperactivity and spontaneous seizures in mice. We performed bulk RNA sequencing (RNA-seq) from the cortex of mice with disruption of BDNF-TrkB signaling in cortistatin interneurons, and identified differential expression of genes important for excitatory neuron function. Using translating ribosome affinity purification and RNA-seq, we define a molecular profile for Cort-expressing inhibitory neurons and subsequently compare the translatome of normal and TrkB-depleted Cort neurons, revealing alterations in calcium signaling and axon development. Several of the genes enriched in Cort neurons and differentially expressed in TrkB-depleted neurons are also implicated in autism and epilepsy. Our findings highlight TrkB-dependent molecular pathways as critical for the maturation of inhibitory interneurons and support the hypothesis that loss of BDNF signaling in Cort interneurons leads to altered excitatory/inhibitory balance.

Manipulation of a genetically and spatially defined sub-population of BDNF-expressing neurons potentiates learned fear and decreases hippocampal-prefrontal synchrony in mice.

Brain-derived neurotrophic factor (BDNF) signaling regulates synaptic plasticity in the hippocampus (HC) and prefrontal cortex (PFC), and has been extensively linked with fear memory expression in rodents. Notably, disrupting BDNF production from promoter IV-derived transcripts enhances fear expression in mice, and decreases fear-associated HC-PFC synchrony, suggesting that Bdnf transcription from promoter IV plays a key role in HC-PFC function during fear memory retrieval. To better understand how promoter IV-derived BDNF controls HC-PFC connectivity and fear expression, we generated a viral construct that selectively targets cells expressing promoter IV-derived Bdnf transcripts ("p4-cells") for tamoxifen-inducible Cre-mediated recombination (AAV8-p4Bdnf-ERT2CreERT2-PEST). Using this construct, we found that ventral hippocampal (vHC) p4-cells are recruited during fear expression, and that activation of these cells causes exaggerated fear expression that co-occurs with disrupted vHC-PFC synchrony in mice. Our data highlight how this novel construct can be used to interrogate genetically defined cell types that selectively contribute to BDNF-dependent behaviors.

Cortistatin-expressing interneurons require TrkB signaling to suppress neural hyper-excitability.

Signaling of brain-derived neurotrophic factor (BDNF) via tropomyosin receptor kinase B (TrkB) plays a critical role in the maturation of cortical inhibition and controls expression of inhibitory interneuron markers, including the neuropeptide cortistatin (CST). CST is expressed exclusively in a subset of cortical and hippocampal GABAergic interneurons, where it has anticonvulsant effects and controls sleep slow-wave activity (SWA). We hypothesized that CST-expressing interneurons play a critical role in regulating excitatory/inhibitory balance, and that BDNF, signaling through TrkB receptors on CST-expressing interneurons, is required for this function. Ablation of CST-expressing cells caused generalized seizures and premature death during early postnatal development, demonstrating a critical role for these cells in providing inhibition. Mice in which TrkB was selectively deleted from CST-expressing interneurons were hyperactive, slept less and developed spontaneous seizures. Frequencies of spontaneous excitatory post-synaptic currents (sEPSCs) on CST-expressing interneurons were attenuated in these mice. These data suggest that BDNF, signaling through TrkB receptors on CST-expressing cells, promotes excitatory drive onto these cells. Loss of excitatory drive onto CST-expressing cells that lack TrkB receptors may contribute to observed hyperexcitability and epileptogenesis.

Recognition Without Words: Using Taste to Explore Survival Processing.

Many educational demonstrations of memory and recall employ word lists and number strings; items that lend themselves to semantic organization and "chunking." By applying taste recall to the adaptive memory paradigm, which evaluates memory from a survival-based evolutionary perspective, we have developed a simple, inexpensive exercise that defies mnemonic strategies. Most adaptive memory studies have evaluated recall of words encountered while imagining survival and non-survival scenarios. Here, we've left the lexical domain and hypothesized that taste memory, as measured by recognition, would be best when acquisition occurs under imagined threat of personal harm, namely poisoning. We tested participants individually while they evaluated eight teas in one of three conditions: in one, they evaluated the toxicity of the tea (survival condition), in a second, they considered the marketability of the tea and, in the third, they evaluated the bitterness of the tea. After a filler task, a surprise recognition task required the participants to taste and identify the eight original teas from a group of 16 that included eight novel teas. The survival condition led to better recognition than the bitterness condition but, surprisingly, it did not yield better recognition than the marketing condition. A second experiment employed a streamlined design more appropriate for classroom settings and failed to support the hypothesis that planning enhanced recognition in survival scenarios. This simple technique has, at least, revealed a robust levels-of-processing effect for taste recognition and invites students to consider the adaptive advantages of all forms of memory.

Ventral Midline Thalamus Is Critical for Hippocampal-Prefrontal Synchrony and Spatial Working Memory.

UNLABELLED: Maintaining behaviorally relevant information in spatial working memory (SWM) requires functional synchrony between the dorsal hippocampus and medial prefrontal cortex (mPFC). However, the mechanism that regulates synchrony between these structures remains unknown. Here, we used a unique dual-task approach to compare hippocampal-prefrontal synchrony while rats switched between an SWM-dependent task and an SWM-independent task within a single behavioral session. We show that task-specific representations in mPFC neuronal populations are accompanied by SWM-specific oscillatory synchrony and directionality between the dorsal hippocampus and mPFC. We then demonstrate that transient inactivation of the reuniens and rhomboid (Re/Rh) nuclei of the ventral midline thalamus abolished only the SWM-specific activity patterns that were seen during dual-task sessions within the hippocampal-prefrontal circuit. These findings demonstrate that Re/Rh facilitate bidirectional communication between the dorsal hippocampus and mPFC during SWM, providing evidence for a causal role of Re/Rh in regulating hippocampal-prefrontal synchrony and SWM-directed behavior. SIGNIFICANCE STATEMENT: Hippocampal-prefrontal synchrony has long been thought to be critical for spatial working memory (SWM) and the ventral midline thalamic reuniens and rhomboid nuclei (Re/Rh) have long been considered a potential site for synchronizing the hippocampus and medial prefrontal cortex. However, the hypothesis that Re/Rh are critical for hippocampal-prefrontal synchrony and SWM has not been tested. We first used a dual-task approach to identify SWM-specific patterns of hippocampal-prefrontal synchrony. We then demonstrated that Re/Rh inactivation concurrently disrupted SWM-specific behavior and the SWM-specific patterns of hippocampal-prefrontal synchrony seen during dual-task performance. These results provide the first direct evidence that Re/Rh contribute to SWM by modulating hippocampal-prefrontal synchrony.

Transient inactivation of the thalamic nucleus reuniens and rhomboid nucleus produces deficits of a working-memory dependent tactile-visual conditional discrimination task.

Working memory depends on communication between the hippocampus and the prefrontal cortex (PFC); however, the neural circuitry that mediates interactions between these brain areas has not been well characterized. Two candidate structures are the thalamic reuniens (RE) and rhomboid (Rh) nuclei, which are reciprocally connected with both the hippocampus and PFC. These known anatomical connections suggest that RE/Rh may be involved in mediating hippocampal-prefrontal communication, and therefore may be critical for working memory processing. To test the hypothesis that RE/Rh are necessary for working memory, we trained separate groups of rats to perform 1 of 2 tasks in a T-maze. The first task was a working memory-dependent conditional discrimination (CDWM) task, and the second task was a nonworking memory-dependent conditional discrimination (CD) task. These tasks took place in the same maze, featured the same number of trials, and utilized the same cue (a tactile-visual maze insert). After rats had learned either task, RE/Rh were transiently inactivated with the GABAA receptor agonist muscimol, and performance was assessed. RE/Rh inactivation caused performance deficits on the CDWM task, but not the CD task. This result suggests that RE/Rh are a necessary component of working memory task performance, which is also thought to depend on the hippocampal-prefrontal circuit. RE/Rh inactivation did not cause a performance deficit on the CD task, suggesting that RE/Rh have dissociable contributions to working memory-dependent and nonworking memory-dependent tasks, independently of the known contributions of these 2 thalamic nuclei to the sensorimotor and attention-related aspects of other memory tasks.

Hippocampal signatures of episodic memory: evidence from single-unit recording studies.

What hippocampal neural firing patterns signal memory and, more importantly, how is this memory code used by associated structures to translate a memory into a decision or action? Candidate hippocampal activity patterns will be discussed including (1) trajectory-specific firing of place cells with place fields on an overlapping segment of two (or more) distinct trajectories (2) prospective firing of hippocampal neurons that signal an upcoming event or action, and (3) place cell remapping to changes in environment and task. To date, there has not been compelling evidence for any of these activity patterns being the neural substrate of episodic memory. New findings suggest that learning and memory processes are emergent properties of interregional interactions and not localized within any one discrete brain region. Therefore, the next step in understanding how remapping and trajectory coding participate in memory coding may be to investigate how these activity patterns relate to activity in anatomically connected structures such as the prefrontal cortex.

Dissociable roles of the dorsal striatum and dorsal hippocampus in conditional discrimination and spatial alternation T-maze tasks.

The roles of the dorsal hippocampus (DH) and dorsal striatum (DS) in the learning and retention of conditional discrimination (CD) rules is a subject of debate. Although previous studies have examined the relationship between the DH and DS and the performance of CD tasks in operant chambers, the relative contributions of these two brain regions to the retention of CD rules requiring an association between a cue and a spatial location have not been characterized. We designed an experiment to assess the roles of the DH and DS in the retention of a visuospatial CD task by transiently inactivating either structure with muscimol in separate groups of rats and measuring performance on a previously learned CD task. The performance of two other groups of rats on a previously learned delayed spatial alternation (DA) task was also measured following inactivation of either DS or DH, which allowed us to control for any possibly confounding effects of spatial cues present in the testing room, length of the intertrial interval period on the performance of the CD task, and muscimol on sensorimotor or motivational processing. Muscimol inactivation of dorsal striatum, but not dorsal hippocampus, impaired CD performance, while inactivation of dorsal hippocampus, but not dorsal striatum impaired DA performance. These results demonstrate a double dissociation between the roles of the DH and DS in these two tasks, and provide a systematic characterization of the relationship between these two brain areas and CD performance.

The role of the medial prefrontal cortex in the acquisition, retention, and reversal of a tactile visuospatial conditional discrimination task.

The medial prefrontal cortex (mPFC) is responsible for executive functions such as abstract rule coding, strategy switching, and behavioral flexibility; however, there is some debate regarding the extent to which mPFC is involved in reversal learning, especially in complex multisensory tasks such as conditional discrimination. Therefore, we investigated the effects of mPFC inactivation on the acquisition, retention, and reversal of a visuospatial conditional discrimination (CD) task. In experiment 1, muscimol was infused through bilateral cannulae on days 1, 2, and 3 to test the effects of mPFC inactivation on task acquisition and days 19, 20, and 21 to test the effects on retention of the task. For experiment 2, rats were trained on the CD task for 21 days with no infusions given, after which the reward contingency was reversed, with infusions given during the first six days of reversal. The results of experiment 1 showed that the muscimol and saline groups did not differ on acquisition or retention. However, experiment 2 showed that the muscimol group displayed significantly more performance errors than the control group during reversal. Compared to the control group, the muscimol group also showed a decreased tendency to use a side-bias strategy during the intermediate stages of reversal. The failure of the muscimol group to exhibit a side bias suggests that the mPFC is necessary for sampling strategies necessary for the reversal of a visuospatial CD task.

Dynamic coding of dorsal hippocampal neurons between tasks that differ in structure and memory demand.

Hippocampal place fields show remapping between environments that contain sufficiently different contextual features, a phenomenon that may reflect a mechanism for episodic memory formation. Previous studies have shown that place fields remap to changes in the configuration of visual landmarks in an environment. Other experiments have demonstrated that remapping can occur with experience, even when the visual features of an environment remain stable. A special case of remapping may be trajectory coding, the tendency for hippocampal neurons to exhibit different firing rates depending upon recently visited or upcoming spatial locations. To further delineate the conditions under which different task features elicit remapping, we recorded from place cells in dorsal CA1 of hippocampus while rats switched between tasks that differed in memory demand and task structure; continuous spatial alternation (CA), delayed spatial alternation (DA), and tactile-visual conditional discrimination (CD). Individual hippocampal neurons and populations of simultaneously recorded neurons showed coherent remapping between CA and CD. However, task remapping was rarely seen between DA and CD. Analysis of individual units revealed that even though the population retained a coherent representation of task structure across the DA and CD tasks, the majority of individual neurons consistently remapped at some point during recording sessions. In contrast with previous studies, trajectory coding on the stem of the T-maze was virtually absent during all of the tasks, suggesting that experience with multiple tasks in the same environment reduces the likelihood that hippocampal neurons will represent distinct trajectories. Trajectory coding was, however, observed during the delay period of DA. Whether place fields change in response to task or trial type or remain stable within the same environment may depend on which aspects of the context are most salient or relevant to behavior.