Many experiences in our daily lives are temporally structured, enabling prediction of events that will occur in the future. We can anticipate upcoming subway stops during our daily commute, or plan multiple steps ahead when cooking a meal we have made many times. Although sequences of events in daily life can be multiple steps long, like the stations along a subway line, it is unknown how extended temporal structure enables predictions over multiple timescales. The three studies reported in this dissertation investigate how extended temporal structure is flexibly represented in memory and in the brain to guide multistep prediction. Chapter 1 demonstrates that memory for temporal structure is enhanced with memory consolidation, enabling more efficient judgements about predictable future events over time. Chapter 2 shows that temporal structure is represented bidirectionally and hierarchically across the hippocampus and across visual regions during multistep anticipation. Finally, Chapter 3 addresses how internal models are updated when regularities in our environment change: the hippocampus rapidly reconfigures memories of temporal structure in response to learning new information, which supports the planning of novel trajectories. Together, the studies presented in this dissertation shed light on how we represent internal models of the world that span multiple timescales to guide adaptive behavior.

The entorhinal cortex (EC) is central to the brain’s navigation system. Its subregions are conventionally thought to compute dichotomous representations for spatial processing: medial entorhinal cortex (MEC) provides a global spatial map, while lateral entorhinal cortex (LEC) encodes specific sensory details of experience. While local recordings of EC circuits have amassed a vast catalogue of specialized cell types that could support navigational computations in the brain, we have little direct evidence for how these signals are actually transmitted outside of the EC to its primary downstream reader, the hippocampus, which itself is critical for the formation of spatial and episodic memories. Here we exploit in vivo sub-cellular imaging to directly record from EC axon terminals as they locally innervate hippocampal area CA1, while mice performed navigation and spatial learning tasks in virtual reality. We find both distinct and overlapping representations of task, location, and context in both MEC and LEC axons. While MEC transmitted a highly location- and context-specific code, LEC inputs were strongly biased by ongoing navigational goals and reward. Surprisingly, the position of the animal could be accurately decoded from either entorhinal subregion. Our results challenge prevailing dogma on the routing of spatial and non-spatial information from the cortex to the hippocampus, indicating that cortical interactions upstream of the hippocampus are critical for combining these processing streams to support navigation and memory.

The dorsal CA2 subregion (dCA2) of the hippocampus exerts a critical role in social novelty recognition (SNR) memory and in the promotion of social aggression. Whether the SNR memory and social aggression functions of dCA2 are related or represent independent processes is unknown. Here I investigated the hypothesis that an animal is more likely to attack a novel compared to familiar animal and that dCA2 promotes social aggression through its ability to distinguish between novel and familiar animals. To test this hypothesis, I conducted a multi-day resident intruder (R-I) test to assess aggression towards familiarized and novel conspecifics. I found that residents were indeed more likely to attack a novel intruder, and that silencing of dCA2 caused a more profound suppression of aggression towards a novel than a familiarized intruder. To explore whether and how dCA2 pyramidal neurons encode aggression, I recorded calcium signals from resident dCA2 pyramidal neurons using microendoscopy during the R-I test. I found that a fraction of dCA2 neurons were selectively activated or inhibited during exploration, dominance, and attack behaviors and that the responses varied with conspecific novelty. Based on dCA2 population activity, a set of binary linear classifiers could accurately predict whether an animal was engaged in each of these forms of social behavior. Notably, the accuracy of decoding aggression was greater for novel compared to familiar intruders. Moreover, calcium signals were more highly correlated during R-I tests with the same familiarized intruder on successive days compared to R-I tests with a familiar and novel intruder on successive days. Similarly, I found significant cross-day decoding results during attack-related behaviors towards familiar-familiar but not for familiar-novel intruder pairs. Together, these findings demonstrate that dCA2 integrates social experience to guide future behavior and provides insight into how SNR memory adaptively influences aggressive behavior. Encounters with novel intruders generally promote aggression while familiarization leads to its stabilization. Moreover, my results are consistent with the hypothesis that dCA2 promotes aggression by computing social novelty.

Complex social connections are essential for health and survival, and memory-impacting disorders like schizophrenia and Alzheimer’s disease can be debilitating for the relationships between patients and loved ones. To form and sustain relationships requires the ability to, first, identify strangers versus familiar individuals (identification) and, second, revise one’s representations of them based on past experience (learning). This ability is called social memory. A range of evidence confirms that the CA2 subregion of the hippocampus is crucial for social memory, and CA2-specific abnormalities are linked to social memory deficits in disease mouse models. However, the specific social cues that CA2 processes to inform social memory—as well as how CA2 adapts its responses to representations of other individuals through learning and experience—remains unclear.Since mice rely most heavily on olfaction to investigate conspecifics, odor sensory cues likely inform the basis of social identification processes in the murine brain. Furthermore, the hippocampus receives information from the olfactory bulb through the entorhinal cortex, suggesting that CA2 may be capable of processing odor sensory information for memory storage. It is already known the neighboring hippocampal subregion CA1 processes nonsocial odor cues and encodes the relationship between nonsocial odors and positive valence through learned experience. Therefore, since CA2 is necessary for social recognition overall, and since it is possible CA2 receives odor information through the same circuits as CA1, I hypothesized that CA2 processes social odor cues for social identification and combines this information with contextual information to develop and maintain social memory. In my thesis, I used two-photon calcium imaging to confirm that CA2 indeed encodes and distinguishes social odors belonging to unique individuals, as well as nonsocial odors. I also found that CA2 neurons adapt their responses to odor stimuli when a reward contingency is introduced—pairing some odors and not others with an artificial reward. Intensive decoding analyses further revealed that CA2 is capable of forming a generalized or abstract representation of social versus nonsocial and rewarded versus unrewarded social odor stimuli. Finally, with archaerhodopsin-mediated CA2 silencing, I confirmed that CA2 is necessary for social—but not nonsocial—odor-reward associative learning, further promoting the specificity of this brain region in the encoding of socially-relevant episodic memory. A link exists between CA2-specific dysfunction (namely, poor CA2 neuronal excitability) and social recognition deficits in the Df(16)A+/- microdeletion mouse model of the human 22q11.2 Deletion Syndrome—in which nearly a third of patients develop schizophrenia. I next hypothesized that CA2 in this model has a deficit in processing social sensory cues and forming the appropriate association between those cues and learned valence. Indeed, I discovered behavioral deficits in both social and nonsocial odor-reward associative learning in the Df(16)A+/- model. I further showed that CA2 is important in this impairment because selective expression of a dominant negative TREK-1 potassium channel subunit, which has been shown to improve CA2 function in these mice, rescued the deficits in social and nonsocial odor-reward learning. With two-photon imaging, I found that CA2 neurons in Df(16)A+/- mice were able to discriminate between social and nonsocial odors with an accuracy that was similar to that seen in wild-type mice, which was surprising given the CA2-dependent deficit in odor-reward learning in the Df(16)A+/- mice. However, the Df(16)A+/- mice did show a reduced fraction of neurons that were selectively activated by the rewarded odor compared to the wild-type mice. Perhaps the most salient finding is that CA2 representations in Df(16)A+/- mice showed a reduced generalized or abstract coding of odor-reward across the social and nonsocial odor categories. This suggests that the Df(16)A+/- mice failed to generalize the task variable of reward, but rather learned separate rules for social and nonsocial odor-reward association. This is reminiscent of a reduction in abstract thought in individuals with schizophrenia. Overall, my thesis provides evidence for the first time that CA2 encodes social odors and odor-reward learned experiences, that these identification and learning-related adaptation mechanisms are impaired in a disease model harboring social memory deficits, and that specific manipulations to restore CA2 function can rescue abnormal learning in this model. These results reinforce the notion that CA2 may provide a novel target for therapeutic intervention in restoring cognitive function associated with neuropsychiatric disease.

Developments in materials science and engineering have significantly enhanced the recording and stimulation capabilities of electrophysiological neural interface devices. Enhanced biocompatibility has increased the viability and longevity of such systems, with particularly interesting advances resulting from the utilization of organic and mixed-conductive conjugated polymers. These materials tend to improve biocompatibility and signal quality by overcoming material limitations of conventional metallic electrodes. Simultaneously, advanced microfabrication methods have increased the spatiotemporal resolution and signal quality of recorded data without added invasiveness. The research work presented in this dissertation touches upon three aspects of relevance for improved neural interface use: i) improving the accuracy of histological verification procedures in research using naturally abundant organic materials; ii) introducing unconventional electrodes for neural recordings and localized drug delivery by utilizing conductive organic polymers and clinical supply items; iii) applying a high-density electrocorticography (ECoG) array to study differential neural oscillation patterns underlying memory processing in vivo. First, we showcase a chitosan (CS) based, solution-processable film for localizing neural implants by leveraging CSs intrinsic fluorescence, without impeding data quality or cell viability. Second, we develop a mixed-conductive suture by using standard silk sutures and the mixed-conductive polymer PEDOT:PSS. The resulting device (E-Suture) is shown to safely interface with live tissue and possess high-fidelity recording and stimulation capabilities as well as applicability for localized drug delivery thanks to the mixed-conductivity of PEDOT:PSS. Finally, we leverage high spatiotemporal resolution ECoG arrays to show that distinctive oscillatory memory biomarkers in the neocortex and hippocampus show significant but differential temporal coupling patterns in response to consolidation of new information and reconsolidation of that information at a later time in rats. This dissertation demonstrates the utility of different organic materials for the enhancement of neural interface functions at multiple phases of the device life cycle as well as a concrete demonstration of improved electrophysiological recording devices in answering key questions of foundational neuroscience.

Memories are not static records of our experiences. They are dynamic and undergo continuous change. Yet how memories transform awaits further characterization. This dissertation examines different forms of memory transformation through the lens of reactivation. Across three studies, I leverage the close link between memory transformation and reactivation to explore the transformation of repeated and once-encoded memories. In chapter one, I focus on the process of ongoing memory consolidation, examining the offline reactivation dynamics of thrice- and once-encoded memories. I show that repeated learning leads to enhanced cortical reactivation and hippocampal-cortical coordinated reactivation during post-encoding rest periods, potentially accelerating systems-level consolidation. Chapter two examines how successful retrieval of repeated memories is supported by reactivation in functionally distinct brain regions. This study demonstrates that the ventral temporal cortex preferentially reactivates the first occurrence of repeated events during active retrieval, but reactivation of the last occurrence in the left ventrolateral prefrontal cortex predicts memory accessibility. Lastly, chapter three addresses the mechanistic role of memory reactivation in modulating the updating of repeated (strong) and once-encoded (weak) memories. I show that strongly encoded memories are more resistant to updating with new, conflicting information than weak memories. However, updating of a strong memory can be predicted by the degree of its reactivation at the time of new encoding. Further, weak memories, which are more flexible in incorporating new information compared to strong memories, exhibit overall heightened reactivation upon memory violation. Together, these findings highlight that reactivation both reflects and modulates memory transformation, and that memory transformation and reactivation are adaptive to the repetitive yet ever-changing nature of our everyday experiences.

Traumatic brain injury (TBI) is a major cause of hospitalization and death. To mitigate these human costs, the search for effective drugs to treat TBI continues. Even mild injuries can lead to long-term deficits in memory and cognition. Predicting which patients will have long lasting memory issues following mild TBI is challenging. Our group has previously shown that in vitro models of TBI result in cell death, decreased long-term potentiation (LTP), and glial activation. In this thesis, we used chemical and electrical treatments to modulate the outcome following injury to inform future therapies. In the first aim of this thesis, we evaluated the efficacy of a novel neurosteroid, NTS-105, to reduce post-traumatic pathobiology in an in vitro model of moderate TBI that utilizes an organotypic hippocampal slice culture. Treatment with NTS-105 starting an hour after injury decreased post-traumatic cell death in a dose-dependent manner, with 10 nM NTS-105 being most effective. Post-traumatic administration of 10 nM NTS-105 also prevented deficits in LTP without adversely affecting neuronal activity in naïve cultures. Our results suggest that the pleiotropic pharmacology (affinity for the androgen, mineralocorticoid, and progesterone receptors) of NTS-105 may be a promising strategy for the effective treatment of TBI. In the second aim, we evaluated the mechanisms of NTS-105 in an in vitro model of mild blast TBI, a model in which NTS-105 is known to preserve LTP. Treatment with NTS-105 starting an hour after injury reduced a marker of microglial activation and increased expression of the GluR1 subunit of the AMPAR, which is a postsynaptic protein associated with LTP. NTS-105 is known to inhibit activation of the androgen receptor and the mineralocorticoid receptor, partially activate the progesterone B receptor and not activate the glucocorticoid receptor. NTS-105 treatment did not alter the expression of any of the oxosteroid receptors (progesterone, androgen, mineralocorticoid, and glucocorticoid). In order to demonstrate the benefits of mineralocorticoid antagonism following TBI, we administered eplerenone immediately after injury. Eplerenone treatment preserved LTP, but did increase spike magnitude at high concentrations. In the third aim, organotypic hippocampal slice cultures were biaxially stretched to model a mild TBI and serial electrophysiological recordings were collected. In this in vitro model, stretchable microelectrode arrays were embedded within the culture substrate to both deform the adhered culture and record neural signals, which are indicators of neuronal health and network connectivity. Multiple spontaneous and evoked recordings were obtained while maintaining sterility to study and modulate the electrophysiological response to injury. Bursting activity increased 2 hours after injury but returned to baseline by 24 hours. However, 24 hours after injury, both LTP and long-term depression (LTD) were impaired. In another experiment, LTP was induced multiple times, both 24 hours before and 24 hours after injury, to study how the state of the pre-injury network affected electrophysiological outcome after injury. We provide preliminary evidence that induction of LTP before injury to increase synaptic strength was detrimental to neuronal plasticity (LTP) after injury. This thesis has expanded upon the understanding of TBI injury mechanisms and hormone receptor modulators following TBI. Future studies will continue to examine NTS-105 and study the benefits of androgen receptor antagonism. Future studies will also continue to use the stretchable microelectrode arrays and our induction paradigm to test if induction of LTD, a weakening of synaptic strength, could increase resiliency to injury.

The hippocampal formation has been implicated in spatial formation for many decades. The hippocampus proper has received the most attention but other regions of the hippocampal formation contribute largely to spatial cognition. This thesis concentrated on one such region, the postsubiculum. The postsubiculum is considered important because it contains head direction cells and because it thought to be a major input to the hippocampus, via the entorhinal cortex. This thesis aims to test the functional role of the rat postsubiculum under two types of situation: one where the rat must rely on idiothetic cues for navigation, and another where the rat has visual cues present and can rely on these for orientation. The thesis also investigates hippocampal place cells and their stability over time after short exposures to novel environments. Chapter 3 of this thesis aimed to test whether the postsubiculum is necessary for path integration during a homing task. Rats were trained on a homing task on a circular platform maze. Once the task was acquired, rats were given lesions of the postsubiculum or sham lesions and then re-tested on the path integration task. The homing performance of rats with lesions of the postsubiculum was as good as that of the sham rats. A series of manipulations suggests that the rats were homing by path integration, confirmed by probe tests. The rats were then tested on a forced-choice delayed alternation T-maze task that revealed a significant impairment in alternation with delays of 5, 30, and 60 seconds. This suggests that the postsubiculum is not necessary for path integration in a homing task but is necessary for avoiding previously visited locations as is necessary in an alternation task. The experiments in Chapters 4 and 5 of this thesis aimed to investigate the effects of postsubiculum pharmacological inactivation on hippocampal CA1 place cells when rats were introduced to a novel environment with visual cues. A necessary first step was to assess place cells without any manipulation of the postsubiculum (Chapter 4) and then use information gained from this in the design of experiments in Chapter 5. Rats chronically implanted with recording electrodes in the CA1 region of the hippocampus were exposed to novel cue-rich environments whilst place fields were recorded. Following delays of 3, 6, or 24 hours, the same cells were recorded again in the same environment but with the cues rotated by 90°. Pixel-by-pixel correlations of the place fields show that stability of the place fields was significantly lower at 24 hours than at 3 hours. Stability after 6 hours was not significantly different from 3 hours. In the third set of experiments, rats were implanted with drug infusion cannulae in the postsubiculum and recording electrodes in CA1. Following infusions of either the AMPA receptor antagonist CXQX, the NMDA receptor antagonist D-AP5 or a control infusion of ACSF, place field stability was assessed as rats were exposed to a cylindrical environment with a single polarising cue card for 3 x 10 minute sessions and then again 6 hours later. There were no differences in place field correlations between the 3 drug conditions, although there was evidence of larger changes in spatial information content between cells in the CNQX and AP5 drug condition, but not the ACSF condition. The results suggest that, under the present testing conditions, place fields stability did not depend upon AMPA receptor-mediated transmission nor did it depend on NMDA receptor-mediated synaptic plasticity.

Experimentally naive rats show variance in their locomotor reactivity to novelty, some displaying higher (HR) while others displaying lower (LR) reactivity, associated with vulnerability to stress. LRHR phenotype is proposed as an antecedent to the development of stress hyper responsiveness. Results presented here show emergence of antidepressive-like behavior following peripubertal-juvenile exposure to chronic variable physical (CVP) and chronic variable social stress (CVS) in HR rats, and depressive-like behavior following CVP in the LRs. The antidepressive-like behavior in HR rats was accompanied by increased levels of acetylated Histone3 (acH3) and acetylated Histone4 (acH4) at the hippocampal brain-derived neurotrophic factor (BDNF) P2 and P4 promoters respectively. This effect may mediate increased mossy fibre (MF) terminal field size, particularly the suprapyramidal mossy fibre projection volume (SP-MF), in the HR animals following both stress regimens. These findings show that chronic variable stress during adolescence induces individual differences in molecular, neuromorphological and behavioral parameters between LRs and HRs, which provides further evidence that individual differences in stress responsiveness is an important factor in resistance or vulnerability to stress-induced depression and/or anxiety. / by Ozge Oztan. / Thesis (Ph.D.)--Florida Atlantic University, 2013. / Includes bibliography. / Mode of access: World Wide Web. / System requirements: Adobe Reader.

Transcription and translation of proteins are required for the consolidation of episodic memory. Arc, an effector immediate early gene, has been linked to synaptic plasticity following learning and memory. It is well established that the rodent hippocampus is essential for processing spatial memory, but its role in processing object memory is a point of contention. Using immunohistochemical techniques, hippocampal sections were stained for arc proteins in the CA1 region of the dorsal hippocampus in mice following two variations of the novel object recognition (NOR) task. Results suggest mice that acquired strong object memory showed significant hippocampal activation. In mice that acquired weak object memory, hippocampal activation was not significantly different from controls. Arc expression was also examined in other hippocampal sub-regions, as well as in the perirhinal cortex. These results suggest that the mice must acquire a threshold amount of object information before the hippocampal CA1 region is engaged. / Includes bibliography. / Thesis (M.A.)--Florida Atlantic University, 2015 / FAU Electronic Theses and Dissertations Collection