Driving and regulating temporal association learning coordinated by entorhinal-hippocampal network

Dynamics of hippocampal reactivation for temporal association memory in mice

Reactivation refers to the re-emergence of activity in neuronal ensembles that were active during information encoding. Hippocampal CA1 neuronal ensembles generate firing activities that encode the temporal association among time-separated events. However, whether and how temporal association memory-related CA1 neuronal ensembles reactivate during sleep and their role in temporal association memory consolidation remain unclear. We utilized multiple unit recordings to monitor CA1 neuronal activity in mice learning a trace eyeblink conditioning (tEBC) task, in which presentation of the conditioned stimulus (CS, a light flash) was paired with presentation of the unconditioned stimulus (US, corneal puff) by a time-separated interval. We found that the CS-US paired training mice exhibited few conditioned eyeblink responses (CRs) at the initial-learning stage (ILS) and an asymptotic level of CRs at the well-learning stage (WLS). More than one third of CA1 pyramidal cells (PYR) in the CS-US paired training mice manifested a CS-evoked firing activity that was sustained from the CS to time-separated interval. The CS-evoked PYR firing activity was required for the tEBC acquisition and was greater when the CRs occurred. Intriguingly, the CS-evoked firing PYR ensembles reactivated, which coincided with increased hippocampal ripples during post-training sleep. The reactivation of CS-evoked firing PYR ensembles diminished across learning stages, with greater strength in the ILS. Disrupting the ripple-associated PYR activity impaired both the reactivation of CS-evoked firing PYR ensembles and tEBC consolidation. Our findings highlight the features of hippocampal CA1 neuronal ensemble reactivation during sleep, which support the consolidation of temporal association memory.

Differential roles of medial/lateral entorhinal cortex in spatial/object memory and contribution to hippocampal functional neuronal organization

Episodic memory is subserved by interactions between entorhinal cortex (EC) and hippocampus. Within EC, a functional dissociation has been proposed for medial (MEC) and lateral (LEC) subregions, whereby, MEC processes spatial information while LEC processes information about objects and their location in space. Most of these studies, however, used classical methods which lack both spatial and temporal specificity, thus, the precise role of MEC/LEC in memory could use further clarification. First, we show a possible functional dissociation of MEC/LEC for place/object fear memory, by optogenetic suppression of these areas during memory acquisition. The main output of EC is to the hippocampus. MEC projects mainly towards proximal/superficial CA1 and deep CA3 while LEC towards distal/deep CA1 and superficial CA3. Dentate gyrus (DG), terminations of MEC/LEC are dissociated septotemporally. A functional dissociation has also been proposed for subregions of the hippocampus. Previous studies reported that proximal/distal CA1 process spatial/nonspatial information, respectively. For the second part of the study, we used the immediate-early gene Zif-268 to map neuronal activity in CA1. We first show enhanced Zif-268 expression and cluster-type organization in the proximal CA1 by place exposure and enhanced Zif-268 expression/cluster organization in distal CA1 following object exposure. Second, direct optogenetic stimulation of MEC/LEC, produced a similar enhancement/cluster-type organization in the same areas. Enhanced Zif-268 expression was also observed in CA3 and DG. These results substantiate previous findings and are proof positive that the hippocampus is organized in clusters to encode information generally ascribed to this structure.

Disentangling in-vivo microstructural changes of white and gray matter in mild cognitive impairment and Alzheimer's disease: a systematic review and meta-analysis

The microstructural characteristics of white and gray matter in mild cognitive impairment (MCI) and the early-stage of Alzheimer's disease (AD) remain unclear. This study aimed to systematically identify the microstructural damages of MCI/AD in studies using neurite orientation dispersion and density imaging (NODDI), and explore their correlations with cognitive performance. Multiple databases were searched for eligible studies. The 10 eligible NODDI studies were finally included. Patients with MCI/AD showed overall significant reductions in neurite density index (NDI) of specific white matter structures in bilateral hemispheres (left hemisphere: -0.40 [-0.53, -0.27], P < 0.001; right: -0.33 [-0.47, -0.19], P < 0.001), involving the bilateral superior longitudinal fasciculus (SLF), uncinate fasciculus (UF), the left posterior thalamic radiation (PTR), and the left cingulum. White matter regions exhibited significant increased orientation dispersion index (ODI) (left: 0.25 [0.02, 0.48], P < 0.05; right: 0.27 [0.07, 0.46], P < 0.05), including the left cingulum, the right UF, and the bilateral parahippocampal cingulum (PHC), and PTR. Additionally, the ODI of gray matter showed significant reduction in bilateral hippocampi (left: -0.97 [-1.42, -0.51], P < 0.001; right: -0.90 [-1.35, -0.45], P < 0.001). The cognitive performance in MCI/AD was significantly associated with NDI (r = 0.50, P < 0.001). Our findings highlight the microstructural changes in MCI/AD were characterized by decreased fiber orientation dispersion in the hippocampus, and decreased neurite density and increased fiber orientation dispersion in specific white matter tracts, including the cingulum, UF, and PTR. Moreover, the decreased NDI may indicate the declined cognitive level of MCI/AD patients.

Learning-prolonged maintenance of stimulus information in CA1 and subiculum during trace fear conditioning

Temporal associative learning binds discontiguous conditional stimuli (CSs) and unconditional stimuli (USs), possibly by maintaining CS information in the hippocampus after its offset. Yet, how learning regulates such maintenance of CS information in hippocampal circuits remains largely unclear. Using the auditory trace fear conditioning (TFC) paradigm, we identify a projection from the CA1 to the subiculum critical for TFC. Deep-brain calcium imaging shows that the peak of trace activity in the CA1 and subiculum is extended toward the US and that the CS representation during the trace period is enhanced during learning. Interestingly, such plasticity is consolidated only in the CA1, not the subiculum, after training. Moreover, CA1 neurons, but not subiculum neurons, increasingly become active during CS-and-trace and shock periods, respectively, and correlate with CS-evoked fear retrieval afterward. These results indicate that learning dynamically enhances stimulus information maintenance in the CA1-subiculum circuit during learning while storing CS and US memories primarily in the CA1 area.

Dissecting cell-type-specific pathways in medial entorhinal cortical-hippocampal network for episodic memory

Episodic memory, which refers to our ability to encode and recall past events, is essential to our daily lives. Previous research has established that both the entorhinal cortex (EC) and hippocampus (HPC) play a crucial role in the formation and retrieval of episodic memories. However, to understand neural circuit mechanisms behind these processes, it has become necessary to monitor and manipulate the neural activity in a cell-type-specific manner with high temporal precision during memory formation, consolidation, and retrieval in the EC-HPC networks. Recent studies using cell-type-specific labeling, monitoring, and manipulation have demonstrated that medial EC (MEC) contains multiple excitatory neurons that have differential molecular markers, physiological properties, and anatomical features. In this review, we will comprehensively examine the complementary roles of superficial layers of neurons (II and III) and the roles of deeper layers (V and VI) in episodic memory formation and recall based on these recent findings.

Neuroanatomical and psychological considerations in temporal lobe epilepsy

Temporal lobe epilepsy (TLE) is the most common form of focal epilepsy and is associated with a variety of structural and psychological alterations. Recently, there has been renewed interest in using brain tissue resected during epilepsy surgery, in particular 'non-epileptic' brain samples with normal histology that can be found alongside epileptic tissue in the same epileptic patients - with the aim being to study the normal human brain organization using a variety of methods. An important limitation is that different medical characteristics of the patients may modify the brain tissue. Thus, to better determine how 'normal' the resected tissue is, it is fundamental to know certain clinical, anatomical and psychological characteristics of the patients. Unfortunately, this information is frequently not fully available for the patient from which the resected tissue has been obtained - or is not fully appreciated by the neuroscientists analyzing the brain samples, who are not necessarily experts in epilepsy. In order to present the full picture of TLE in a way that would be accessible to multiple communities (e.g., basic researchers in neuroscience, neurologists, neurosurgeons and psychologists), we have reviewed 34 TLE patients, who were selected due to the availability of detailed clinical, anatomical, and psychological information for each of the patients. Our aim was to convey the full complexity of the disorder, its putative anatomical substrates, and the wide range of individual variability, with a view toward: (1) emphasizing the importance of considering critical patient information when using brain samples for basic research and (2) gaining a better understanding of normal and abnormal brain functioning. In agreement with a large number of previous reports, this study (1) reinforces the notion of substantial individual variability among epileptic patients, and (2) highlights the common but overlooked psychopathological alterations that occur even in patients who become "seizure-free" after surgery. The first point is based on pre- and post-surgical comparisons of patients with hippocampal sclerosis and patients with normal-looking hippocampus in neuropsychological evaluations. The second emerges from our extensive battery of personality and projective tests, in a two-way comparison of these two types of patients with regard to pre- and post-surgical performance.

Sustained Activity of Hippocampal Parvalbumin-Expressing Interneurons Supports Trace Eyeblink Conditioning in Mice

Although recent studies have revealed an involvement of hippocampal interneurons in learning the association among time-separated events, its underlying cellular mechanisms remained not fully clarified. Here, we combined multichannel recording and optogenetics to elucidate how the hippocampal parvalbumin-expressing interneurons (PV-INs) support associative learning. To address this issue, we trained the mice (both sexes) to learn hippocampus-dependent trace eyeblink conditioning (tEBC) in which they associated a light flash conditioned stimulus (CS) with a corneal air puff unconditioned stimuli (US) separated by a 250 ms time interval. We found that the hippocampal PV-INs exhibited learning-associated sustained activity at the early stage of tEBC acquisition. Moreover, the PV-IN sustained activity was positively correlated with the occurrence of conditioned eyeblink responses at the early learning stage. Suppression of the PV-IN sustained activity impaired the acquisition of tEBC, whereas the PV-IN activity suppression had no effect on the acquisition of delay eyeblink conditioning, a hippocampus-independent learning task. Learning-associated augmentation in the excitatory pyramidal cell-to-PVIN drive may contribute to the formation of PV-IN sustained activity. Suppression of the PV-IN sustained activity disrupted hippocampal gamma but not theta band oscillation during the CS-US interval period. Gamma frequency (40 Hz) activation of the PV-INs during the CS-US interval period facilitated the acquisition of tEBC. Our current findings highlight the involvement of hippocampal PV-INs in tEBC acquisition and reveal insights into the PV-IN activity kinetics which are of key importance for the hippocampal involvement in associative learning.SIGNIFICANCE STATEMENT The cellular mechanisms underlying associative learning have not been fully clarified. Previous studies focused on the involvement of hippocampal pyramidal cells in associative learning, whereas the activity and function of hippocampal interneurons were largely neglected. We herein demonstrated the hippocampal PV-INs exhibited learning-associated sustained activity, which was required for the acquisition of tEBC. Furthermore, we showed evidence that the PV-IN sustained activity might have arisen from the learning-associated augmentation in excitatory pyramidal cell-to-PVIN drive and contributed to learning-associated augmentation in gamma band oscillation during tEBC acquisition. Our findings provide more mechanistic understanding of the cellular mechanisms underlying the hippocampal involvement in associative learning.

Wolframin is a novel regulator of tau pathology and neurodegeneration

Selective neuronal vulnerability to protein aggregation is found in many neurodegenerative diseases including Alzheimer's disease (AD). Understanding the molecular origins of this selective vulnerability is, therefore, of fundamental importance. Tau protein aggregates have been found in Wolframin (WFS1)-expressing excitatory neurons in the entorhinal cortex, one of the earliest affected regions in AD. The role of WFS1 in Tauopathies and its levels in tau pathology-associated neurodegeneration, however, is largely unknown. Here we report that WFS1 deficiency is associated with increased tau pathology and neurodegeneration, whereas overexpression of WFS1 reduces those changes. We also find that WFS1 interacts with tau protein and controls the susceptibility to tau pathology. Furthermore, chronic ER stress and autophagy-lysosome pathway (ALP)-associated genes are enriched in WFS1-high excitatory neurons in human AD at early Braak stages. The protein levels of ER stress and autophagy-lysosome pathway (ALP)-associated proteins are changed in tau transgenic mice with WFS1 deficiency, while overexpression of WFS1 reverses those changes. This work demonstrates a possible role for WFS1 in the regulation of tau pathology and neurodegeneration via chronic ER stress and the downstream ALP. Our findings provide insights into mechanisms that underpin selective neuronal vulnerability, and for developing new therapeutics to protect vulnerable neurons in AD.

Developmental Hypothyroidism Influences the Development of the Entorhinal-Dentate Gyrus Pathway of Rat Offspring

BACKGROUND

Developmental hypothyroidism impairs learning and memory in offspring, which depend on extensive neuronal circuits in the entorhinal cortex, together with the hippocampus and neocortex. The entorhinal-dentate gyrus pathway is the main entrance of memory circuits. We investigated whether developmental hypothyroidism impaired the morphological development of the entorhinal-dentate gyrus pathway.

METHODS

We examined the structure and function of the entorhinal-dentate gyrus pathway in response to developmental hypothyroidism induced using 2-mercapto-1-methylimidazole.

RESULTS

1,1´-Dioctadecyl-3,3,3´,3´-tetramethylindocarbocyanine perchlorate tract tracing indicated that entorhinal axons showed delayed growth in reaching the outer molecular layer of the dentate gyrus at postnatal days 2 and 4 in hypothyroid conditions. The proportion of fibers in the outer molecular layer was significantly smaller in the hypothyroid group than in the euthyroid group at postnatal day 4. At postnatal day 10, the pathway showed a layer-specific distribution in the outer molecular layer, similar to the euthyroid group. However, the projected area of entorhinal axons was smaller in the hypothyroid group than in the euthyroid group. An electrophysiological examination showed that hypothyroidism impaired the long-term potentiation of the perforant and the cornu ammonis 3-cornu ammonis 1 pathways. Many repulsive axon guidance molecules were involved in the formation of the entorhinaldentate gyrus pathway. The hypothyroid group had higher levels of erythropoietin-producing hepatocyte ligand A3 and semaphorin 3A than the euthyroid group.

CONCLUSION

We demonstrated that developmental hypothyroidism might influence the development of the entorhinal-dentate gyrus pathway, contributing to impaired long-term potentiation. These findings improve our understanding of neural mechanisms for memory function.

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.

Ventral hippocampal shock encoding modulates the expression of trace cued fear

The hippocampus is crucial for associative fear learning when the anticipation of threat requires temporal or contextual binding of predictive stimuli as in trace and contextual fear conditioning. Compared with the dorsal hippocampus, far less is known about the contribution of the ventral hippocampus to fear learning. The ventral hippocampus, which is highly interconnected with defensive and emotional networks, has a prominent role in both innate and learned affective behaviors including anxiety, fear, and reward. Lesions or temporary inactivation of the ventral hippocampus impair both cued and contextual fear learning, but whether the ventral hippocampal role in learning is driven by affective processing, associative encoding, or both is not clear. Here, we used trace fear conditioning in mixed sex cohorts to assess the contribution of shock-encoding to the acquisition of cued and contextual fear memories. Trace conditioning requires subjects to associate an auditory conditional stimulus (CS) with a shock unconditional stimulus (UCS) that are separated in time by a 20-s trace interval. We first recorded neuronal activity in the ventral hippocampus during trace fear conditioning and found that ventral CA1 predominantly encoded the shock reinforcer. Potentiated firing to the CS was evident at testing, but no encoding of the trace interval was observed. We then tested the necessity of shock encoding for conditional fear acquisition by optogenetically silencing ventral hippocampal activity during the UCS on each trial of training. Contrary to our predictions, preventing hippocampal shock-evoked firing did not impair associative fear. Instead, it led to a more prolonged expression of CS freezing across test trials, an effect observed in males, but not females. Contextual fear learning was largely intact, although a subset of animals in each sex were differentially affected by shock-silencing. Taken together, the results show that shock encoding in the ventral hippocampus modulates the expression of learned fear in a sex-specific manner.

Pten haploinsufficiency causes desynchronized growth of brain areas involved in sensory processing

How changes in brain scaling relate to altered behavior is an important question in neurodevelopmental disorder research. Mice with germline Pten haploinsufficiency (Pten ^+/-) closely mirror the abnormal brain scaling and behavioral deficits seen in humans with macrocephaly/autism syndrome, which is caused by PTEN mutations. We explored whether deviation from normal patterns of growth can predict behavioral abnormalities. Brain regions associated with sensory processing (e.g., pons and inferior colliculus) had the biggest deviations from expected volume. While Pten ^+/- mice showed little or no abnormal behavior on most assays, both sexes showed sensory deficits, including impaired sensorimotor gating and hyporeactivity to high-intensity stimuli. Developmental analysis of this phenotype showed sexual dimorphism for hyporeactivity. Mapping behavioral phenotypes of Pten ^+/- mice onto relevant brain regions suggested abnormal behavior is likely when associated with relatively enlarged brain regions, while unchanged or relatively decreased brain regions have little predictive value.

Differential Effects of Lateral and Medial Entorhinal Cortex Lesions on Trace, Delay and Contextual Fear Memories

The entorhinal cortex (EC), with connections to the hippocampus, amygdala, and neocortex, is a critical, yet still underexplored, contributor to fear memory. Previous research suggests possible heterogeneity of function among its lateral (LEC) and medial (MEC) subregions. However, it is not well established what unique roles these subregions serve as the literature has shown mixed results depending on target of manipulation and type of conditioning used. Few studies have manipulated both the LEC and MEC within the same experiment. The present experiment systematically manipulated LEC and MEC function to examine their potential roles in fear memory expression. Long-Evans rats were trained using either trace or delay fear conditioning. The following day, rats received an N-methyl-D-aspartate (NMDA)-induced lesion to the LEC or MEC or received a sham surgery. Following recovery, rats were given an 8-min context test in the original context. The next day, rats were tested for tone freezing in a novel context with three discrete tone presentations. Further, rats were tested for hyperactivity in an open field under both dark and bright light gradient conditions. Results: Following either LEC or MEC lesion, freezing to context was significantly reduced in both trace and delay conditioned rats. LEC-lesioned rats consistently showed significantly less freezing following tone-offset (trace interval, or equivalent, and intertrial interval) in both trace and delay fear conditioned rats. Conclusions: These data suggest that the LEC may play a role in the expression of a conjunctive representation between the tone and context that mediates the maintenance of post-tone freezing.

Complementary roles of differential medial entorhinal cortex inputs to the hippocampus for the formation and integration of temporal and contextual memory (Systems Neuroscience)

In humans and rodents, the entorhinal cortical (EC)-hippocampal (HPC) circuit is crucial for the formation and recall of memory, preserving both spatial information and temporal information about the occurrence of past events. Both modeling and experimental studies have revealed circuits within this network that play crucial roles in encoding space and context. However, our understanding about the time-related aspects of memory is just beginning to be understood. In this review, we first describe updates regarding recent anatomical discoveries for the EC-HPC network, as several important neural circuits critical for memory formation have been discovered by newly developed neural tracing technologies. Second, we examine the complementary roles of multiple medial entorhinal cortical inputs, including newly discovered circuits, into the hippocampus for the temporal and spatial aspects of memory. Finally, we will discuss how temporal and contextual memory information is integrated in HPC cornu ammonis 1 cells. We provide new insights into the neural circuit mechanisms for anatomical and functional segregation and integration of the temporal and spatial aspects of memory encoding in the EC-HPC networks.

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.

Cell-Type-Specific Optogenetic Techniques Reveal Neural Circuits Crucial for Episodic Memories

The formation and maintenance of episodic memories are important for our daily life. Accumulating evidence from extensive studies with pharmacological, electrophysiological, and molecular biological approaches has shown that both entorhinal cortex (EC) and hippocampus (HPC) are crucial for the formation and recall of episodic memory. However, to further understand the neural mechanisms of episodic memory processes in the EC-HPC network, cell-type-specific manipulation of neural activity with high temporal resolution during memory process has become necessary. Recently, the technological innovation of optogenetics combined with pharmacological, molecular biological, and electrophysiological approaches has significantly advanced our understanding of the circuit mechanisms for learning and memory. Optogenetic techniques with transgenic mice and/or viral vectors enable us to manipulate the neural activity of specific cell populations as well as specific neural projections with millisecond-scale temporal control during animal behavior. Integrating optogenetics with drug-regulatable activity-dependent gene expression systems has identified memory engram cells, which are a subpopulation of cells that encode a specific episode. Finally, millisecond pulse stimulation of neural activity by optogenetics has further achieved (a) identification of synaptic connectivity between targeted pairs of neural populations, (b) cell-type-specific single-unit electrophysiological recordings, and (c) artificial induction and modification of synaptic plasticity in targeted synapses. In this chapter, we summarize technological and conceptual advancements in the field of neurobiology of learning and memory as revealed by optogenetic approaches in the rodent EC-HPC network for episodic memories.

Involvement of microglia in disturbed fear memory regulation: Possible microglial contribution to the pathophysiology of posttraumatic stress disorder

Microglia, immune cells in the brain, play a crucial role in brain inflammation and synaptic plasticity by releasing inflammatory mediators and neurotrophic factors as well as, phagocytosing synaptic elements. Recent studies have shown peripheral inflammation, immune alteration in the brain are associated with post-traumatic stress disorder (PTSD) in humans. Several preclinical studies using Pavlovian fear conditioning have suggested that microglia are involved in fear memory dysregulation and altered fear neuronal networks. Microglial priming resulting from previous stressful experiences may also have an effect. This review will introduce the current knowledge of microglial contribution to disturbed fear memory regulation, a fundamental feature of PTSD.

Neurotoxic effects of lactational exposure to perfluorooctane sulfonate on learning and memory in adult male mouse

The present study aims to examine the effect of early lactational perfluorooctane sulfonate (PFOS) exposures on learning and memory in male mice and reveal the underlying mechanisms involved. PFOS solution was orally administered to dams from the postpartum days 1-14, so that pups would be exposed through breast milk. At 8-10 weeks of age, we performed object location test (OLT), object recognition test (ORT), and pairwise visual discrimination (VD) task. We also performed in vivo microdialysis, and mRNA and protein analysis of genes responsible for hippocampal development and function. In both OLT and ORT, the performance of mice in the PFOS-exposed group was significantly lower than those in the control group. In the VD task, the PFOS-exposed group learned significantly slower than the control group. Concentrations of glutamate and gamma-aminobutyric acid in the dorsal hippocampus were significantly higher in the PFOS-exposed group than in the control group. No notable differences were shown in our mRNA and protein studies. The present study showed that lactational PFOS exposure has a profound, long-lasting neurotoxic effect in the hippocampus and consequently leads to learning and memory deficits.

Novel nose poke-based temporal discrimination tasks with concurrent in vivo calcium imaging in freely moving mice

The hippocampus has been known to process temporal information as part of memory formation. While time cells have been observed in the hippocampus and medial entorhinal cortex, a number of the behavioral tasks used present potential confounds that may cause some contamination of time cell observations due to animal movement. Here, we report the development of a novel nose poke-based temporal discrimination task designed to be used with in vivo calcium imaging for the analysis of hippocampal time cells in freely moving mice. First, we developed a ten second held nose poke paradigm for use in mice to deliver a purer time metric for the analysis of time cell activity in hippocampus CA1. Second, we developed a temporal discrimination task that involves the association of held nose poke durations of differing lengths with differential spatial cues presented in arms on a linear I-maze. Four of five mice achieved successful temporal discrimination within three weeks. Calcium imaging has been successfully performed in each of these tasks, with time cell activity being detected in the 10s nose poke task, and calcium waves being observed in discrete components of the temporal discrimination task. The newly developed behavior tasks in mice serve as novel tools to accelerate the study of time cell activity and examine the integration of time and space in episodic memory formation.

The evolutionary old forebrain as site of action to develop new psychotropic drugs

BACKGROUND

Previously, the authors have developed a model of how reward-seeking and distress- avoiding behaviour is regulated by the human brain. The forebrain's evolution in vertebrates was taken as a starting point.

AIMS

The authors want to inspire colleagues to study in particular the pharmacological effects on the described ancient forebrain structures in order to modify specific symptoms of mental disorders.

METHODS

Compilation of data and ideas of previous articles, with examples to illustrate.

RESULTS

A primary (lamprey-like), secondary (frog-like) and tertiary (mammal-like) forebrain can be distinguished, organized according to a Russian doll model. The first constituent is primarily involved in producing the emotional response, while the last is principally concerned with constructing conscious cognitive behaviour (including verbal and written communication). Mental disorders comprise (partly related and partly unrelated) biological and rational phenomena. The secondary system regulates the intensity of reward-seeking and distress-avoiding behaviour. An essential component of the primary forebrain evaluates the results of behavioural actions: the lateral habenula-projecting pallidum. These neurons regulate the activity of ascending dopaminergic pathways. The authors suggest that these habenula-projecting pallidum neurons are targeted by subanaesthetic dosages of ketamine. The medial habenula is enriched with nicotinergic acetylcholine receptors and regulates the activity of ascending adrenergic and serotonergic neurons. This may link varenicline-induced hostility to selective serotonin reuptake inhibitor-induced aggression.

CONCLUSIONS

Studying the effects of new compounds on the primary and secondary brains in lampreys and frogs may yield interesting new treatments of mental disorders.