Methylphenidate is a psychostimulant used to treat attention deficit hyperactivity disorder. Neurogenesis occurs throughout adulthood within the dentate gyrus of the hippocampus and can be altered by psychoactive medications; however, the impact of methylphenidate on neurogenesis is not fully understood. We investigated the effects of chronic low (1 mg/kg) and high (10 mg/kg) intraperitoneal doses of methylphenidate on neurogenesis in mouse hippocampus following 28 days and 56 days of treatment. Interestingly, methylphenidate, at both doses, increased neurogenesis. However, if methylphenidate treatment was not continued, the newly generated cells did not survive after 28 days. If treatment was continued, the newly generated neurons survived only in the mice receiving low-dose methylphenidate. To investigate the mechanism for this effect, we examined levels of proteins linked to cell proliferation in the hippocampus, including brain-derived neurotrophic factor (BDNF), glial cell line-derived neurotrophic factor (GDNF), vascular endothelial growth factor (VEGF), tropomyosin receptor kinase B (TrkB), and beta-catenin. BDNF or GDNF levels were not significantly different between groups. However, hippocampal VEGF, TrkB, and beta-catenin were significantly increased in mice receiving low-dose methylphenidate for 28 days compared to controls. Interestingly, high-dose methylphenidate significantly decreased beta-catenin after 28 days and decreased VEGF, beta-catenin, and TrkB after 56 days compared to controls. Thus, low-dose methylphenidate appears to increase cell proliferation and cell survival in the hippocampus, and these effects may be mediated by increase in VEGF, TrkB, and beta-catenin. While high dose methylphenidate may initially increase neuronal proliferation, newly generated neurons are unable to survive long-term, possibly due to decrease in VEGF, TrkB and beta-catenin.

Alzheimer’s disease (AD) is widely accepted as being linked with abnormal atrophy of the hippocampus. In the nonhuman-focused literature, the hippocampus has been identified as one of the prominent regions of interest with mechanisms of adult neurogenesis from aerobic exercise. Several human studies over the past decade have shown the effect of exercise that improves cardiorespiratory fitness on the size and function of the hippocampus in participants. However, the size of hippocampal subfields, especially the dentate gyrus (DG), has not been examined in humans even though various animal studies have identified the DG subfield as the primary region of adult neurogenesis induced by aerobic exercise. The point of this investigation, therefore, was to investigate the effect of an exercise intervention on the size of the DG subfield and the related subfield of cornu ammonis (CA) 3. The hypothesis was that an endurance training intervention, designed to improve cardiorespiratory fitness, would increase the volume of the DG and CA3 subfields of the hippocampus more than a resistance training intervention, designed to increase strength, flexibility, and balance, and that improvement in cardiorespiratory fitness would positively correlate with the change in volumes of these subfields. For this investigation, 32 participants (young adults from age 20 to 33 with sedentary lifestyles) were selected from a data set collected for an ongoing study by the Brain Plasticity and Neuroimaging (BPN) Laboratory at Boston University School of Medicine (Boston, MA, USA). The fitness data and T1-weighted and T2-weighted structural magnetic resonance imaging (MRI) data were used in the analysis. FreeSurfer v6.0 software was used to extract volumetric data of the hippocampal subfields using a hippocampal subfield segmentation algorithm. Analysis of variance (ANOVA) with repeated measures and linear regression were used to analyze the statistical significance of the results. The change in volumes for the whole hippocampus, DG, and CA3 did not show any statistically significant differences after endurance training compared with after resistance training. The effect of exercise on the volume of the CA3 subfield appeared to be asymmetrical from left to right, with heavier impact on the left CA3 than on the right CA3. There was no statistically significant correlation between the change in cardiorespiratory fitness and the change in volume of any of the regions analyzed. However, the left whole hippocampus showed a slight trend (p = 0.078; R = 0.317) of weak positive correlation between its volume change and the cardiorespiratory fitness change of the participants. This result was consistent with the previous human literature. Although statistically not significant, most data showed that the endurance training group saw more preservation or increase in volume. This result is encouraging and should be explored further to validate the efficacy of cardiorespiratory exercise as a possible prevention mechanism against AD for young adults later in life.

Thesis (M.Sc.)--Memorial University of Newfoundland, 2002. / Bibliography: leaves 78-89.

Diss. (sammanfattning) Stockholm : Karolinska institutet, 2007. / Härtill 6 uppsatser.

Thesis (M.A.)--University of North Carolina Wilmington, 2008. / Title from PDF title page (October 20, 2008) Includes bibliographical references (59-71)

Microglia are essential to the brain’s innate immune response and play a vital role in neuropathology related to tauopathies. Understanding how microglia change in response to differential expression of RNA binding protein T-cell intracellular antigen 1 (TIA1) will lend insight to microglial function in tauopathy. In preliminary studies our laboratory has shown that decreasing the expression of TIA1 has an inverse and dose dependent effect on activated microglial density in the dentate gyrus of the P301S mouse (PhD Candidate Chelsey LeBlang, 2018). Here, we utilized serial sectioning electron microscopy to define whether this relationship between TIA1 level and microglia remains consistent in the hilus and granule cell layer (GCL) of the dentate gyrus. Our analyses of microglial volume and microglial interactions within the neuropil have yielded four conclusions. First, the hilus, but not the GCL, exhibited a significant decrease in microglial volume per volume of tissue with the knock out and heterozygous expression of TIA1. Second, the number of appositions on microglia steadily increased on AT8+ and AT8- presynaptic and postsynaptic appositions in the hilus with decreasing TIA1 expression. Third, with the exception of one AT8- somatic apposition, the surface area of microglia apposing AT8+ somata is greater than any other structure and exhibits a dose dependent decrease with decreasing TIA1 expression. Fourth, in the GCL there is a larger fractional surface area of AT8- presynaptic and AT8+ postsynaptic structures when compared to their respective synaptic counterparts. Though not entirely consistent with previous data, this study has important implications for microglial function in tauopathy and related diseases.

The capacity for memory is one of the most profound features of the mammalian brain, and the proper encoding and retrieval of information are the processes that form the basis of learning. The goal of this thesis is to further our understanding of the network-level mechanisms supporting learning and memory in the mammalian brain. The hippocampus has been long recognized to play a central role in learning and memory. Although being one of the most extensively studied structures in the brain, the precise circuit mechanisms underlying its function remain elusive. Principal cells in the hippocampus form complex representations of an animal's environment, but in stark contrast to the interneuron population -- and despite the apparent need for functional segregation -- these cells are largely considered a homogeneous population of coding units. Much work, however, has indicated that principal cells throughout the hippocampus, from the input node of the dentate gyrus to the output node of area CA1, differ developmentally, genetically, anatomically, and functionally. By employing in vivo two-photon calcium imaging in awake, behaving mice, we attempted to characterize the role of dened subpopulations of neurons in memory-related behaviors. In the first part of this thesis, we focus on the dentate gyrus input node of the hippocampus. Chapter 2 compares the functional properties of adult-born and mature granule cells. Chapter 3 expands on this work by comparing granule cells with mossy cells, another glutamatergic but relatively understudied cell type. The second part of this thesis focuses on the hippocampal output node, area CA1. In chapter 4, we characterize an inhibitory microcircuit that differentially targets the sublayers of area CA1. And in chapter 5, we directly compare the contributions of these sublayers to episodic and semantic memory.

Fear and anxiety disorders are potentially crippling conditions that often stem from past experience of trauma and chronic stress. One clear feature of these disorders is the failure to use proximate spatial and contextual information presented in the environment to regulate reflexive physiological threat responses. The central nervous system networks that govern spatial navigation and contextual learning and memory are a series of complex circuitries in which the hippocampus is integrally involved. Deficits in hippocampal function have been linked to severe anterograde and mild retrograde amnesia of semantic and episodic memory, and specific deficits in contextual processing. These deficits manifest as failure to distinguish between the details of contexts that help predict for danger or safety and can thus lead to the overexpression of threat responses that compose the behavioral symptoms of fear disorders. The dentate gyrus (DG) is a subdivision of the hippocampus that serves as the first filter of excitatory flow through the hippocampus. The DG is hypothesized to function in “pattern separation” or the dissociation of similar inputs into dissimilar outputs. Failure in this domain leads to generalization between contexts, a common feature of pathology. Pituitary adenylate cyclase activating polypeptide (PACAP) and the PAC1 receptor are associated with multiple behavioral disorders such as post-traumatic stress disorder, schizophrenia, and bipolar disorder. Mutations in the PAC1 receptor gene are associated with hypervigilance, and modified amygdalar and hippocampal activity. These results are mirrored by rodent studies where central PACAP infusion causes anxiety-like behavior, pain hypersensitivity, anorexia, and reinstatement of drug-seeking. PAC1 receptor transcript is found in high abundance in granule cells of the dentate gyrus and potentiation of DG synapses is impaired in PAC1 knockout mice. PACAP is known to have effects of long-duration, such as those in injury repair, growth, and development, but it also can affect ion channel physiology to control neuronal excitability through several parallel intracellular signaling cascades including those dependent on adenylyl cyclase, phospholipase C, and extracellular signal regulated kinase. Accumulated evidence suggests that recruitment of extracellular signal regulated kinase can be through either adenylyl cyclase-, phospholipase C-, or a receptor endocytosis-dependent mechanism. The experiments described in this dissertation address the role of PACAP in the DG in regulating expression of fear behavior, the effects of PACAP on the excitability of DG granule cells, and the signaling pathways and ion channels responsible for these effects. We found that PACAP infused into the DG amplifies expression of fear to a context but does not affect fear acquisition. Electrophysiology studies demonstrate that treating DG neurons with PACAP increases their excitability, and that parallel signaling mechanisms recruit extracellular signal regulated kinase to drive this excitability. Furthermore, these effects on excitability are attenuated by blocking a persistent inward sodium current. This work represents novel regulation of the DG and its impacts on behavior and identifies a current that likely participates in modulating granule cell excitability in multiple domains. In aggregate, this research traces the path from ligand, to receptor and intracellular signaling, to neurophysiology in order to propose a comprehensive description of behavioral regulation by these processes.

New neurons continue to be added to the dentate gyrus (DG) throughout adulthood and enhancing neurogenesis in this region holds therapeutic potential. However, the molecular mechanisms underlying DG neurogenesis remain elusive. Since developmental and adult neurogenesis often share the same signaling pathways, understanding how the DG develops is crucial to understanding adult neurogenesis. This study aims to determine the role of the retinoblastoma (Rb) protein in DG development and to determine if modulation of this pathway holds potential for enhancing neurogenesis in an adult system. A FoxG1 driven Cre is used to delete Rb in the developing forebrain and the resulting effects are analyzed in in vitro and in vivo mouse models. We show that Rb deletion enhances DG neurogenesis by specifically increasing proliferation of immature neurons. Overall this study suggests that Rb pathway modulation could hold potential for enhancing neurogenesis in the adult.

Stem/progenitor cells are present in the adult brain; they undergo constantproliferation and differentiate into mature neurons in certain brain areas, a phenomenoncalled neurogenesis. This study investigated the effects of GDNF, a potent trophic factorof dopaminergic neurons, on neurogenesis in the brain. Nestin and 5-Bromo-2'-deoxyuridine (BrdU) were used as stem/progenitor cells markers.First, we observed extensive bilateral increases of stem/progenitor cells in thedentate gyrus and substantia nigra after continuous infusion of GDNF into the normal ratbrain. However, none of the BrdU+ cells showed neuronal features in the substantia nigraas characterized by immunocytochemical procedures. Next, we identified themorphology of BrdU+ cells after infusing the marker into the brain. While the proceduresincreased the BrdU labeling, neurogenesis was not observed in the basal ganglia. Underelectron microscope, the BrdU+ cells either were undifferentiated or showedcharacteristics of astrocytes. This observation is consistent with suggestions thatastrocytes serve as multipotent progenitors. Later, we repeated GDNF intrastriatalinfusion one month after a severe 6-hydroxydopamine (6-OHDA) lesion. The number ofBrdU+ cells was significantly higher in the GDNF recipients in the ipsilateral substantianigra and both sides of the dentate gyrus. However, no neurogenesis was observed. Inaddition, motor functions were not improved by GDNF treatment. Thus, we measured theeffects of GDNF administration directly into the substantia nigra six hours before apartial 6-OHDA lesion. HPLC measurements of dopamine and its metabolites showed asignificant increase of tissue level in the substantia nigra and striatum, respectively.Despite this, no newly generated dopaminergic neurons was detected in the basal ganglia.Taken together, our studies investigated the effects of GDNF on adultstem/progenitor cells in normal and lesioned rat brain. For the first time, we demonstratedthat GDNF promoted their proliferation in the dentate gyrus, suggesting it has a role inneurogenesis and the function of learning and memory. In each scenario, GDNFpromoted stem/progenitor cell proliferation, but failed to induce neurogenesis in thesubstantia nigra. We believed that the local microenvironment in the substantia nigra mayprevent the stem/progenitor cells to mature into functional neurons.