Post-traumatic stress disorder (PTSD) is a debilitating mental illness that results from exposure to intense, life-threatening trauma. Some of the symptoms of PTSD include intrusive flashback memories, persistent anxiety, hyperarousal and cognitive impairments. The finding of reduced basal glucocorticoid levels, as well as a greater suppression of glucocorticoid levels following dexamethasone administration, has also been commonly observed in people with PTSD. Our laboratory has developed an animal model of PTSD which utilizes chronic psychosocial stress, composed of unavoidable predator exposure and daily social instability, to produce changes in rat physiology and behavior that are comparable to the symptoms observed in PTSD patients. The present set of experiments was therefore designed to 1) test the hypothesis that our animal model of PTSD would produce abnormalities in glucocorticoid levels that are comparable to those observed in people with PTSD, 2) examine the ability of antidepressant and anxiolytic agents to ameliorate the PTSD-like physiological and behavioral symptoms induced by our paradigm and 3) ascertain how long the physiological and behavioral effects of our stress regimen could be maintained. The experimental findings revealed that our animal model of PTSD produces a reduction in basal glucocorticoid levels and increased negative feedback sensitivity to the synthetic glucocorticoid, dexamethasone. In addition, chronic prophylactic administration of amitriptyline (tricyclic antidepressant) and clonidine (α2-adrenergic receptor agonist) prevented a subset of the effects of chronic stress on rat physiology and behavior, but tianeptine (antidepressant) was the only drug to block the effects of chronic stress on all physiological and behavioral measures. The final experiment indicated that only a subset of the effects of chronic stress on rat physiology and behavior could be observed 4 months following the initiation of chronic stress, suggesting that some of the effects of our animal model diminish over time. Together, these findings further validate our animal model of PTSD and may provide insight into the mechanisms underlying trauma-induced changes in brain and behavior. They also provide guidance for pharmacotherapeutic approaches in the treatment of individuals suffering from PTSD.

The causes of autism spectrum disorder (ASD) are not all known, but it is suspected that the serotonin transporter (SERT) plays an important role for some subjects with ASD. Mutations in the SLC6A4 gene, that encodes SERT, including the Ala56 mutation (Gly56Ala), have been found in some autism patients. This mutation makes the transporter more active and reduces the probability of serotonergic neurotransmission in the brain, which is linked to behavioral changes that are associated with core domain deficits of ASD 1. Depression also has been linked to decreases in the availability of serotonin (5-hydroxytryptamine; 5-HT) in the central nervous system (CNS), and is associated with reduced hippocampal neurogenesis. Selective serotonin reuptake inhibitors (SSRIs), drugs used to block SERTs, are used to treat depression and/or anxiety by inhibiting SERT to increase synaptic 5-HT levels. / Includes bibliography. / Thesis (M.S.)--Florida Atlantic University, 2019. / FAU Electronic Theses and Dissertations Collection

The ability of insulin-like growth factor-I (IGF-I) to impact the hippocampus and associated behaviors may vary depending upon estrogenic status. Previous work from our lab demonstrated that chronic antagonism of brain IGF-I receptors (IGF-IR) resulted in increased levels of hippocampal synaptic proteins in control-treated ovariectomized (OVX) rats. In contrast, antagonism of brain IGF-IR decreased levels of synaptic proteins in estradiol-treated OVX rats. The goal of the current experiment was to test the hypothesis that effects of chronic agonism of IGF-IR, via peripheral treatment with IGF-I, on hippocampal-dependent memory would also vary with estrogenic status. Furthermore, we assessed the influence of estrogenic status on the ability of IGF-I to impact levels of hippocampal synaptic proteins. OVX rats received chronic peripheral treatment with estradiol or cholesterol control via silastic implants, as well as IGF-I or vehicle via osmotic minipumps. One week after surgeries, place learning and memory on the Morris water maze was assessed via eight place-training trials on the first day and four place-training trials on the second day of testing. Place learning and memory was measured using mean swim path length. Following place training, a probe trial was conducted to assess memory for the location of the hidden platform. Memory on the probe trial was measured via percent time in the target quadrant. Animals were euthanized 24 hours following behavioral testing, and hippocampi were processed for western blotting to determine levels of hippocampal synaptic proteins PSD-95, spinophilin, and synaptophysin. Results revealed no effects of treatment on behavioral measures or on levels of hippocampal synaptic proteins. These data indicate that chronic peripheral administration of IGF-I does not affect hippocampal-dependent memory in a Morris water maze task and does not impact hippocampal synaptic protein levels in the presence or absence of peripheral estradiol. / acase@tulane.edu

Extensive research in both humans and animals has identified and isolated distinct brain regions essential for different types of memory, supporting the notion of multiple memory systems (MMS). The hippocampus and the striatum are the two systems that have been studied widely and are the focus of our studies. Research involving lesion and pharmacologic manipulations on both memory systems show strong evidence for independence. However, recent evidence suggests that both memory systems can interact as well. Though evidence point also in favor interactions, the mechanisms under which these systems interact are unknown. The experiments in this dissertation primarily focused on understanding how these two systems operate in a normally functioning brain. Two methods were used to investigate our notions: 1) Behavioral experiment measured cellular changes in the levels of phosphorylated CREB (pCREB) in the regions of interest (ROI) 2) Somatic experiments measured changes in the behavior following manipulation of CREB selectively in the ROI. Overall, these experiments demonstrate CREB as a critical neuronal marker that can be used in both interventions. Behavioral intervention experiment showed evidence as a plasticity related changes in the levels of pCREB that suggested both the hippocampus and the striatum might act in a competitive manner, bidirectionally. The somatic intervention experiments used lentiviral (LV) vectors and showed evidence that LV CREB manipulations are suitable for chronic stable expression and can be used to investigate multiple tasks following a single manipulation. LV mutant CREB in the hippocampus impaired memory across two different hippocampus-dependent tasks and demonstrated that CREB is critical for long term memory. Overexpression of wild type CREB in the striatum enhanced striatal memory, but also showed evidence for hippocampus competition and cooperation. Similarly, CREB overexpressed in the hippocampus of young and middle-aged rats demonstrate that CREB might be a rate limiting factor in young, but not in the middle-aged rats. / acase@tulane.edu

Evidence from previous studies on the multiple memory systems model suggests that specific brain regions cooperate and compete to mediate the navigational strategies used to locate a goal in a spatial environment. Specifically, the cholinergic system within these discrete brain regions plays a key role in balancing this mediation such that acetylcholine release, genomic changes, and receptor regulation at cholinergic synapses are altered following learning and subsequent memory consolidation. Based on previous findings, we proposed to test learning-induced changes in muscarinic receptor binding expression in adult male rats following training on a water maze task guided either by a cue proximal to the escape platform (stimulus-response strategy), by cues surrounding the maze (place strategy) or by alternating between the two strategies (strategy-switching). The primary findings of the current study demonstrate that adult male rats that navigated to an escape platform guided by cues surrounding a water maze (place-trained) learned the task at a significantly slower rate than males that were guided by a cue proximal to the platform (stimulus-response-trained) or males that were required to switch strategies on alternating days. Additionally, males that were required to switch strategies over alternating days expressed higher ratios of muscarinic binding in the hippocampus relative to the striatum compared to place-trained rats, stimulus-response-trained rats, and swim-only controls. These results indicate that the use of a place learning strategy slows acquisition of a water maze task while the requirement to switch strategies as the demands of the task change over days engages the cholinergic system in the hippocampus most heavily. Taken together, the results from the current study further confirm the involvement of cholinergic function in regulating the balance between multiple memory systems. / acase@tulane.edu

The goal of the current experiments was to assess the role of estrogen receptor alpha (ERë±) in the ability of a prior, short-term exposure to estradiol in mid-life to enhance cognition and affect the hippocampus in aging, ovariectomized rats, as well as to investigate a possible mechanism involving the insulin-like growth factor-1 system by which ERë± may exert these effects. In Experiment 1, rats were trained on the radial maze, ovariectomized, and implanted with estradiol or cholesterol vehicle capsules for 40 days. Rats then continuously received JB1 or aCSF vehicle i.c.v. and were tested on delay trials on the radial maze. Using Western blotting, I determined effects of treatment on protein levels of ERë±, ChAT, IGF-1R, IGF-1BP2, and phosphorylated and total p42/p44 MAPK and Akt in the hippocampus. Antagonism of IGF-1 receptors blocked the ability of prior estradiol exposure to enhance cognition and increase ERë±, phosphorylated ERK/MAPK, and ChAT in the hippocampus. The second experiment was divided into two parts. In Experiment 2A, rats were trained on the radial maze and ovariectomized. After 40 days, rats underwent stereotaxic surgery to receive lentiviral delivery of the gene encoding for ERë± or control virus to the hippocampus, and were then tested on delay trials. Using Western blotting, I determined if proteins of interest previously found to be affected by a prior exposure to estradiol in Experiment 1 would be affected by treatments for all subsequent experiments. Lentiviral delivery of ERë± to the hippocampus was sufficient to enhance cognition and increase phosphorylation of ERK/MAPK in the hippocampus. In Experiment 2B, rats were trained on the radial maze, ovariectomized, and implanted with estradiol or cholesterol vehicle capsules for 40 days. Rats then continuously received an ER antagonist or aCSF vehicle i.c.v. and were tested on delay trials. Antagonism of ERë± blocked the ability of prior estradiol exposure to enhance cognition. In the third experiment, rats were trained on the radial maze, ovariectomized, and implanted with estradiol or cholesterol vehicle capsules for 40 days. Rats then underwent stereotaxic surgery to receive i.c.v. cannula. Infusion of an ERK/MAPK inhibitor or aCSF vehicle was administered every 12 hours during delay trials. Inhibition of ERK/MAPK did not block the ability of prior estradiol exposure to enhance cognition, although it did block an increase in levels of ERë± in the hippocampus. Altogether, these results indicate that ERë± plays an important role in the ability of prior estradiol exposure to enhance cognition and affect the hippocampus in aging, ovariectomized rats, and likely interacts with the IGF-1 system and its associated ERK/MAPK signaling pathway. / acase@tulane.edu

Early sensory experiences play a critical role in the activity-dependent development of the sensorimotor system. The sources of sensory input to the neonatal nervous system involve external stimulation (exafference) and sensory feedback arising from self-generated movements (reafference). In the perinatal period, reafference from twitches of the limbs and facial muscles during active (REM) sleep is a powerful driver of neural activity across the entire neuraxis. Thus, sleep-related twitches are thought to contribute to the activity-dependent development of sensorimotor networks. In this dissertation, we first aimed to identify a motor pathway for the generation of twitching. Using newborn rats at postnatal day (P) 8, we provide evidence that the red nucleus (RN; source of the rubrospinal tract) is involved in the production of twitching. In addition, we show that reafference from twitches drives neural activity in the RN, therefore suggesting that the RN is an important site for sensorimotor integration. Also, in the RN of P8 rats, twitch-related reafference triggers theta (4–7 Hz) oscillations. By P12, theta oscillations are expressed continuously and exclusively across bouts of active sleep. Synchronized neuronal oscillations comprise a fundamental mechanism by which distant neural structures establish and express functional connectivity. Thus, we next hypothesized that sleep-dependent theta oscillatory activity enables the expression of network connectivity between the RN and associated neural networks, such as the hippocampus. Simultaneous recordings from the hippocampus and RN at P12 show that theta oscillations in both structures are synchronized, co-modulated, and mutually interactive exclusively during active sleep. Lastly, we test the hypothesis that twitches drive synchronized oscillatory activity across functionally related sensory structures at early ages when the occurrence of oscillations largely depends on sensory input. Focusing on the cortico-hippocampal network at P8, we demonstrate that, unlike periods of wake-related movements or behavioral quiescence, twitching promotes coupled oscillatory activity at Beta2 frequency (~20-30 Hz). Altogether, the findings in this dissertation suggest that one of the functions of active sleep in early infancy is to provide a context for sensorimotor processing and for synchronizing activity within and between forebrain and brainstem structures. Consequently, any condition or manipulation that restricts active sleep can deprive the infant animal of substantial sensory experience, potentially resulting in atypical developmental trajectories.

Co-speech gesture is ubiquitous in everyday conversation, facilitating comprehension, learning, and memory. Information is often provided uniquely in the gesture modality and this information is integrated with speech, affecting the listener’s comprehension and memory of a message. Despite the robust evidence that gesture supports learning, the memory mechanisms that support this learning are unclear. The current study investigates the ability of patients with hippocampal damage to integrate and retain information from co-speech gesture. Four patients with bilateral hippocampal lesions, four patients with damage to the ventral medial prefrontal cortex, and 17 healthy comparisons watched videos of a storyteller narrating four stories with gestures. Some of the gestures provided redundant information to the speech signal and some provided supplementary information that was unique. The participants retold the story immediately after, thirty-minutes after, and four weeks later. Co-speech gesture integration was measured by the proportion of words changed as a result of seeing a supplementary gesture. Memory retention for the stories was measured by the number of story features mentioned during each retelling. The patients with hippocampal amnesia were successful at integrating speech and gesture information immediately after hearing the story but did not show a benefit in memory for gestured features after delays. Though the hippocampus has previously been thought to be critical for relational memory, this finding suggests that the integration of speech and gesture may be mediated by other cognitive mechanisms.

Language production is characterized by an unlimited expressive capacity and creative flexibility that allows speakers to rapidly generate novel and complex utterances. In turn, listeners interpret language "on-line", incrementally integrating diverse representations to create meaning in real-time. A challenge for theories of language has been to understand how speakers generate, integrate, and maintain representations in service of language use and processing and how this is accomplished in the brain. Much of this work has focused prefrontal cortex mechanisms such as "working memory". The goal of this dissertation is to understand the role of the hippocampal declarative memory system (HDMS) in language use and processing, specifically in referential processing and narrative construction. To test the role of the hippocampus in referential processing, healthy comparisons, brain damaged comparisons (BDC), individuals with bilateral hippocampal damage participated in an eyetracking experiment in which individuals viewed scenes and listened to short stories. The amount of time participants spent looking at the characters after a pronoun reference was recorded. Healthy comparisons and BDC participants preferentially targeted the first mentioned character while participants with hippocampal damage did not, suggesting that the hippocampus plays a role in maintaining and integrating information, even in short discourse history. In a second experiment, participants with bilateral hippocampal damage and healthy comparisons told narratives multiple times over the course of a month. The narratives were analyzed for the number of words, the number of episodic details, the number of semantic details, the number of editorials and the consistency of details over the multiple tellings. The patients with hippocampal damage told stories that were significantly shorter, more semanticized and less consistent from telling to telling than healthy comparisons. The final goal of this study was to understand the effects of unilateral hippocampal damage on language processing. Individuals with unilateral hippocampal damage participated in all of the previous experiments. It was predicted that individuals with left hippocampal damage would perform worse than individuals with right hippocampal damage, and their performance was significantly impaired across measures. This suggests that the left hippocampus may be particularly important for processing linguistic material outside of even verbal memory.

Dans le cerveau adulte, la signalisation liée à l’adénosine facilite ou inhibe la libération vésiculaire de neurotransmetteurs suite à l’activation des récepteurs de l’adénosine de type 2A ou 1 (A2AR ou A1R), respectivement. Cependant, son rôle dans le développement est mal connu. Au cours de ma thèse, j’ai étudié le rôle de la signalisation adénosine dans la synaptogenèse GABAergiques de l’hippocampe. Nous avons mis en évidence (i) une sécrétion activité-dépendante accrue d’adénosine et d’ATP pendant la période de synaptogenèse, (ii) un pic d’expression de l’enzyme limitant la formation de l’adénosine à partir de l’ATP extracellulaire, l’ecto-5’-nucleotidase, aux synapses pendant cette période critique, et (iii) un pic d’expression péri/post- synaptique du A2AR concomitant de la période de synaptogenèse. Cette expression développementale des molécules clés de la signalisation adénosine dépendante du A2AR corrélait avec un rôle de ce récepteur dans la stabilisation des synapses GABAergiques naissantes, une régulation restreinte à la période de synaptogenèse. De plus, la suppression de A2AR par une approche shRNA dans des neurones isolés conduisait à une perte de synapses GABAergiques équivalente à celle observée après un blocage pharmacologique de l’activité du A2AR, signifiant que la stabilisation synaptique médiée par le A2AR est un processus « cellule autonome » indépendant de l’activité du réseau neuronal et qu’elle requiert l’activation du A2AR dans la cellule post-synaptique.L’ATP et l’adénosine sont secrétés par la glie et les neurones ; cependant, nous avons montré in vitro que la libération neuronale activité-dépendante suffit à stabiliser les synapses GABAergiques naissantes. En utilisant la vidéomicroscopie sur cellules vivantes, nous avons montré que la signalisation adénosine stabilise les synapses actives. Puis, nous avons caractérisé le mécanisme moléculaire sous-jacent. Nous rapportons la contribution de la cascade adénylate cyclase/adénosine monophosphate cyclique/protéine kinase A et nous avons identifié une ciblé clé, la géphrine, la molécule d’ancrage postsynaptique des récepteurs GABAA. Enfin, nous avons mis en évidence que la stabilisation de l’élément présynaptique requiert probablement le complexe trans-synaptique Slitrk3-PTPd.Puisque le GABA exerce une fonction similaire au cours du développement et que le GABA et l’adénosine sont co-libérés à certaines synapses, j’ai étudié l’interaction entre ces deux voies de signalisation. Mes résultats favorisent l’hypothèse que la signalisation GABA, en activant la calcium-calmoduline, converge vers la signalisation adénosine en potentiant les adenylates cyclases sensibles au calcium. Mon travail m’a permis de proposer que, au cours d’une période clé du développement, les A2ARs postsynaptiques agissent comme des senseurs de l’activité des terminaisons présynaptiques GABAergiques pour stabiliser les synapses actives. En absence d’activité et donc de libération d’adénosine/ATP, les synapses seraient éliminées. / In the adult brain, adenosine signaling facilitates or inhibits neurotransmitter vesicular release mainly through activation of type 2A or 1 adenosine receptors (A2AR or A1R), respectively. However, its role in development remains to be elucidated. During my PhD, I addressed the role of A2AR-mediated signalling in GABAergic synaptogenesis in the hippocampus. We found (i) a larger activity-dependent release of ATP and adenosine during the period of synaptogenesis in the hippocampus, (ii) a peak of expression of the ecto-5’-nucleotidase, the rate-limiting enzyme for the formation of adenosine from extracellular ATP in synapses during this critical period, and (iii) a peak of peri/post-synaptic expression of A2AR concomitant with the period of synaptogenesis. This developmental expression of the key molecules of the adenosine A2AR signalling pathway correlated with a role of A2AR in the stabilization of nascent GABA synapses, a regulation restricted to the period of synaptogenesis. Furthermore, suppressing A2AR with a shRNA approach in isolated neurons led to a loss of synapses equivalent to that seen upon A2AR activity blockade, reporting that the A2AR-mediated synapse stabilization is a cell autonomous process that requires A2AR activation in the postsynaptic cell. ATP/adenosine can be secreted by both glia and neurons; however, we found that activity-dependent release of neuronal adenosine is sufficient to stabilize newly formed GABA synapses in vitro. Using live cell imaging, we showed adenosine signalling stabilizes active synapses. We then characterized the molecular mechanism downstream postsynaptic A2AR. We report the contribution of adenylyl cyclase/cyclic adenosine monophosphate/protein kinase A signalling cascade and we identified a key target, the postsynaptic scaffolding molecule gephyrin. We further showed the A2AR-mediated stabilization of the presynaptic compartment most probably requires the trans-synaptic Slitrk3-PTPd complex. Since GABA exerts a similar function during development and GABA and adenosine are co-released at some synapses, I further investigated the interplay between these two pathways. My results support the hypothesis that GABA signalling converge onto the adenosine signalling pathway by potentiating calcium-sensitive adenylyl cyclases through the activation of calmodulin.Altogether these results let us propose that, during a key developmental period, postsynaptic A2ARs act as sensors of the activity of GABAergic presynaptic terminals to stabilize active nascent GABAergic synapses. In absence of activity and therefore secretion of adenosine/ATP, synapses will be eliminated.