The ability to recognize dangerous situations and environments is crucial for survival, but overestimating risk can lead to pathological avoidance of normal activities, potentially leading to anxiety disorders. Many studies over the past several decades have begun to identify the brain regions underlying threat detection and anxiety behavior. In particular, the ventral hippocampus has emerged as a critical structure for emotional behaviors, including innate anxiety. Recent work from our lab and others has shown that ventral CA1 pyramidal neurons encode information about anxiety, and these CA1 neurons preferentially target downstream structures such as hypothalamus and medial prefrontal cortex. However, the neural representation of anxiogenic environments in the initial stage of the trisynaptic circuit— the dentate gyrus— is unknown. Here, I use Dock10-Cre and Drd2-Cre mouse lines to gain optical access to granule cells and mossy cells, respectively, in the ventral dentate gyrus. Calcium activity was recorded during free exploration of the elevated plus maze (EPM) and open field test (OFT). Single cell activity and population coding were analyzed for mossy cells, granule cells, and CA1 pyramidal neurons. I found that anxiety-related activity was present in granule cells and vCA1, however mossy cells encoded novelty and spatial position. Furthermore, chemogenetic inhibition of mossy cells did not disrupt behavior in the EPM or OFT, but did disrupt acquisition of a contextual fear memory. These findings support the notion that different features of an anxiogenic environment are encoded by different cell types, and that anxiety information is present at the earliest stage of the trisynaptic circuit.

Neural circuits within the hippocampus, a mammalian brain structure critical for both the encoding and consolidation of episodic memories, are composed of intimately connected excitatory pyramidal cells and inhibitory interneurons. While decades of research have focused on how the in vivo physiological properties of pyramidal cells may support these cognitive processes, and the anatomical and physiological properties of interneurons have been extensively studied in vitro, relatively little is known about how the in vivo activity patterns of interneurons support memory encoding and consolidation. Here, I have utilized Acousto-Optic Deflection (AOD)-based two-photon calcium imaging and post-hoc immunohistochemistry to perform large-scale recordings of molecularly-defined interneuron subtypes, within both CA1 and CA3, during various behavioral tasks and states. I conclude that the subtype-specific dynamics of inhibitory circuits within the hippocampus are critical in supporting its role in memory encoding and consolidation.

The behavioral deficits produced by trimethyltin (TMT) are usually attributed to the hippocampal damage caused by this toxicant. The purpose of this experiment was to determine the effects of TMT administration on acquisition and reversal of a discrete trial light-dark discrimination. Acquisition of this task is impaired by hippocampal lesions but the effects of TMT on it are not known. Forty-five days after some of the rats were given one of three doses of TMT, adult, male Long-Evans rats were given 100 trials per day for 20 days to acquire a discrete trial lever press discrimination with lit cue lights located above the correct lever. At the end of this time the contingencies were reversed and the rats were given 30 more days of training. No significant group differences occurred during the first 20 days. A significant group effect was found for the 30 days of reversal training. The rats given the highest dose of TMT (6 mg/kg) obtained significantly more reinforcements during reversal training than the other groups. Because surgical hippocampal lesions generally impair both acquisition and reversal of visual discriminations, these data were unexpected and suggest that other factors than hippocampal damage enter into the behavioral effects of TMT.

The environmental neurotoxin trimethyltin (TMT) destroys parts of the hippocampal formation as well as the entorhinal cortex but leaves the septal cholinergic projection to the hippocampus and dentate gyrus intact. In this study we measured choline acetyltransferase (ChAT) activity in micropunch samples of the dentate gyrus, the CA1 region of Ammon's horn, and the caudate-putamen as a measure of density of cholinergic innervation in control rats and rats exposed to 7 mg/kg TMT by means of gastric intubation. Three months after the rats were exposed to a single dose of TMT both the dentate gyrus and CA1 demonstrated significantly higher ChAT activity in TMT-exposed rats than in control rats. No differences were found between groups for the caudate-putamen samples. These results support the hypothesis that exposure to TMT causes reactive synaptogenesis in the cholinergic septohippocampal system.

Trimethyltin (TMT) produces prominent neuron death in the hippocampus. The time-course of TMT-induced damage was studied using reduced-silver procedures for impregnation of degenerating axons and their terminals, and a modified Timm's stain procedure for visualization of hippocampal transitional metals. Standard cell body stains were also used. Fifty-four, adult, Long-Evans rats were gavaged with 6.0 mg TMT/kg b.wt. and 10 rats were gavaged with distilled water as controls. Five TMT-gavaged rats and one saline-gavaged rat were sacrificed on either postgavage day 1, 3, 6, 9, 14, 19, 30, 45, 70 or 99. Histological examination revealed a band of degenerating terminals in the stratum lucidum, below the hippocampal subfields CA3a,b pyramidal cells, by postgavage day 3. This preceded dentate gyrus granule cell loss supplying the mossy fiber input to the stratum lucidum by several days. Hippocampal pyramidal cell necrosis continued through the examination period while dentate granule cell loss subsided between postgavage days 9 and 14. Fiber and terminal degeneration was more extensive in the dorsal hippocampus than in the ventral hippocampus, although Timm's-stained sections revealed "bleaching" of stainable metal in the mossy fiber pathway of the ventral hippocampus. These data suggest that loss of ventral dentate granule cells might reduce TMT-induced necrosis of pyramidal cells in the ventral (temporal) part of the Ammon's horn, possibly by preventing the spread of seizure activity in this region of the hippocampus. Additionally, although previous studies have reported the toxic effects of TMT to last approximately 60 days, the results of the present study indicate that TMT-induced degeneration continues for more than 3 months. Reduced-silver stains, such as the Fink-Heimer procedure, appear to be more sensitive indicators of enduring neuropathology than more traditional cell stains.

Adult male Long-Evans rats were given 6 mg/kg trimethyltin (TMT). Rats were killed 1, 3, 7, 14, 21, 35 or 60 d later. An untreated control group was included. Brain sections were processed using film autoradiography to visualize in the hippocampus either total muscarinic receptor binding ([3H]quinuclidiny] benzilate: [3H]QNB), or M1 receptors ([3H]pirenzepine; [3H]PZ), or M2 receptors ([3H]oxotremorine-M; [3H]OXO-M). A reduction in [3H]QNB binding was found in CA1 and CA3c 7 d after TMT, but not in CA3a,b, or the dentate gyrus. [3H]PZ binding was decreased throughout Ammon's horn by 14 d after treatment. [3H]OXO-M binding decreased 1 d after exposure in CA1 and in all subfields of Ammon's horn by d 3. Neither [3H]PZ or [3H]OXO-M binding decreased in the dentate gyrus of TMT-treated rat at any time point. The temporal patterns of receptor loss may be explicable by reference to timing of fiber and cell body degeneration reported in previous studies and the regional differences may account for discrepancies between reports of either substantial decreases or no loss in hippocampal muscarinic receptors after TMT exposure.

Vascular risk factors affect cerebral blood flow (CBF) and cerebral vascular reactivity, contributing to cognitive decline. Hippocampus is vulnerable to both Alzheimer's disease (AD) pathology and ischemia; nonetheless, the information about the impact of vascular risk on hippocampal perfusion is minimal. Cognitively, healthy elderly (NL18, 69.96.7 years) and subjects with mild cognitive impairment (MCI15, 74.98.1 years) were evaluated for the Framingham cardiovascular risk profile (FCRP). All underwent structural imaging and resting CBF assessment with arterial spin labeling (ASL) at 3T magnetic resonance imaging (MRI). In 24 subjects (NL17, MCI7), CBF was measured after a carbon dioxide rebreathing challenge. Across all subjects, FCRP negatively correlated with hippocampal (0.41, P0.049) and global cortical (0.46, P0.02) vasoreactivity to hypercapnia (VRh). The FCRP-VRh relationships were most pronounced in the MCI group: hippocampus (0.77, P=0.04); global cortex (0.83, P=0.02). The FCRP did not correlate with either volume or resting CBF. The hippocampal VR h was lower in MCI than in NL subjects (Z2.0, P=0.047). This difference persisted after age and FCRP correction (F 3,20 4.6, P0.05). An elevated risk for vascular pathology is associated with a reduced response to hypercapnia in both hippocampal and cortical tissue. The VR h is more sensitive to vascular burden than either resting CBF or brain volume.

Agmatine, an endogenous amine derived from decarboxylation of l-arginine catalyzed by arginine decarboxylase, has been proposed as a neurotransmitter or neuromodulator in the brain. In the present study, we examined whether agmatine has neuroprotective effects against repeated immobilization-induced morphological changes in brain tissues and possible effects of immobilization stress on endogenous agmatine levels and arginine decarboxylase expression in rat brains. Sprague-Dawley rats were subjected to 2 h immobilization stress daily for 7 days. This paradigm significantly increased plasma corticosterone levels, and the glutamate efflux in the hippocampus as measured by in vivo microdialysis. Immunohistochemical staining with β-tubulin III showed that repeated immobilization caused marked morphological alterations in the hippocampus and medial prefrontal cortex that were prevented by simultaneous treatment with agmatine (50 mg/kg/day), i.p.). Likewise, endogenous agmatine levels measured by high-performance liquid chromatography in the prefrontal cortex, hippocampus, striatum and hypothalamus were significantly increased by immobilization, as compared to controls. The increased endogenous agmatine levels, ranging from 92 to 265% of controls, were accompanied by a significant increase of arginine decarboxylase protein levels in the same regions. These results demonstrate that the administration of exogenous agmatine protects the hippocampus and medial prefrontal cortex against neuronal insults caused by repeated immobilization. The parallel increase in endogenous brain agmatine and arginine decarboxylase protein levels triggered by repeated immobilization indicates that the endogenous agmatine system may play an important role in adaptation to stress as a potential neuronal self-protection mechanism.

The hippocampus, a brain region that is part of the limbic system in the medial temporal lobe, is critical to episodic memory, or the memory of autobiographical events. The hippocampus plays an important role in the consolidation of information from short-term memory into more permanent long-term memory and spatial memory which enables navigation. Hippocampal damage in humans has been linked to memory loss, such as in Alzheimer’s disease and other dementias, as well as in amnesia such as in the case of patient H.M. The role of the hippocampus has been well characterized in humans but is less understood in rodents due to contradictory findings. While rodents have served well as model organisms in developing our understanding of the cognitive map that is critical for spatial navigation, there has been substantial contention over the degree to which the rodent hippocampus supports non-spatial memory, specifically the memory for items or objects previously encountered. The overall objective of this research is to gain a better understanding of how neuronal circuits involving the hippocampus and perirhinal cortex function to support object memory in the brain. Chemogenetic technologies such as DREADDs (designer receptor exclusively activated by designer drugs) have proven to be effective tools in remote manipulation of neuronal activity. First, a series of behavioral tasks was used to validate the effects of DREADD inactivation in the CA1 region of dorsal hippocampus in C57BL/6J male mice. DREADD inhibition resulted in significant impairment in the spontaneous object recognition (SOR) task and of spatial memory in the Morris water maze. In conjunction, mice were implanted with bilateral perirhinal cortex guide cannulae to allow for temporary muscimol inactivation during distinct time points in the SOR task to further investigate the nature of its relationship with the hippocampus. The results reveal an unexpected role for the perirhinal cortex in the retrieval of strong object memory. Finally, Arc mRNA expression was quantified in CA1 of dorsal hippocampus and perirhinal cortex following both weak and strong object memory formation. The results indicate that the perirhinal cortex and hippocampus have distinct, yet complementary roles in object recognition memory and that distinction is gated by memory strength. Understanding the neural mechanisms supporting the weak-strong object memory distinction in mice is an important step not only in validating mice as a suitable model system to study episodic memory in humans, but also in developing treatments and understanding the underlying causes of diseases affecting long-term memory such as Alzheimer’s disease. / Includes bibliography. / Dissertation (Ph.D.)--Florida Atlantic University, 2020. / FAU Electronic Theses and Dissertations Collection

The hippocampus supports the initial formation and recall of episodic memories, as well as the consolidation of short-term into long-term memories. The ability of hippocampal neurons to rapidly change their connection strengths during learning and maintain these changes over long time-scales may provide a mechanism supporting memory. However, little evidence currently exists concerning the long-term stability of information contained in hippocampal neuronal activity, likely due to limitations in recording extracellular activity in vivo from the same neurons across days. In this thesis I employ calcium imaging in freely moving mice to longitudinally track the activity of large ensembles of hippocampal neurons. Using this technology, I explore the proposal that long-term stability of hippocampal information provides a substrate for episodic memory in three different ways. First, I tested the hypothesis that hippocampal activity should remain stable across days in the absence of learning. I found that place cells – hippocampal neurons containing information about a mouse’s position – maintain a coherent map relative to each other across long time-scales but exhibit instability in how they anchor to the external world. Furthermore, I found that coherent maps were frequently used to represent a different environment and incorporated learning via changes in a subset of neurons. Next, I examined how learning a spatial alternation task impacts neuron stability. I found that splitter neurons whose activity patterns reflected an animal’s future or past trajectory emerged relatively slowly when compared to place cells. However, splitter neurons remained more consistently active and relayed more consistent spatial information across days than did place cells, suggesting that the utility of information provided by a neuron influences its long term stability. Last, I investigated how protein synthesis, known to be necessary for long-term maintenance of changes in hippocampal neuron connection strengths and for proper memory consolidation, influences their activity patterns across days. I found that along with blocking memory consolidation, inhibiting protein synthesis induced a profound, long-lasting decrease in neuronal activity up to two days later. These results combined demonstrate the importance of rapid, lasting changes in the hippocampal neuronal code to supporting long-term memory.