京都大学 / 新制・課程博士 / 博士(医学) / 甲第23076号 / 医博第4703号 / 新制||医||1049(附属図書館) / 京都大学大学院医学研究科医学専攻 / (主査)教授 花川 隆, 教授 古川 壽亮, 教授 髙橋 良輔 / 学位規則第4条第1項該当 / Doctor of Medical Science / Kyoto University / DFAM

The work presented in this thesis investigates whether primitive pallial brain circuits such as those found in teleost fish may also encode complex information such as spatial memory despite its circuitry being “simpler” than those found in species with much larger brains such as primates and rodents. Previous behavioral studies have already shown that most teleost fish are capable of spatially orienting themselves and remembering past food locations. Behavioral studies combined with selective brain lesions and related anatomical studies have identified a hippocampal-like region in the fish’s pallium; however, it is unknown whether the neurons located in this structure can also perform cortical-like computations as those found in the mammalian hippocampus. Consequently, this thesis will first present an anatomical characterization of the intrinsic circuitry of this hippocampal-like structure, followed by an in vitro electrophysiological characterization of its constituent neurons. Surprisingly, we have found that this hippocampal-like structure possesses many features reminiscent of the mammalian cortex, including recurrent local connectivity as well as a laminar/columnar-like organization. Furthermore, we have also identified many biophysical properties which would describe these hippocampal-like neurons as sparse coders, including a prominent after-hyperpolarizing potential and an adapting spike threshold with slow recovery. Since this particular dynamic spike threshold mechanism has not been thoroughly characterized in the mammalian hippocampus, we have further investigated the dynamic threshold in the major rodent hippocampal cell types. We have found that only a subset of excitatory neurons displayed this dynamic spike threshold on the time scale that was observed in teleost pallial cells, which allowed us to discuss its potential role in encoding spatial information in both species. Nevertheless, the fact that this teleost hippocampal homologue possesses characteristics that are both akin to the cortex and hippocampus suggest that it may perform computations that, in a mammalian brain, would require both structures and makes this ancestral structure a very interesting candidate to study the mechanism(s) underlying spatial memory.

Dysfunction in the Sigma-1 receptor (Sig-1R) is implicated in many neurodegenerative diseases such as Alzheimer’s Disease (AD). Recently, agonists of the Sig-1R have been found to be neuroprotective in AD and provide significant improvements in symptoms. The hallmarks of AD are aggregation of amyloid-β (Aβ) plaques and development of neurofibrillary tau tangles in the brain, which are thought to be correlated with progressive neuronal cell death in AD. Aβ leads to increased endoplasmic reticulum (ER) stress, decreased autophagy, and increased apoptosis, all of which may be contributing to the neuronal cell death that is seen in AD. The Sig-1R is known to reduce ER stress, increase autophagy, and decrease apoptosis. However, as of yet there is little research on the ability of the Sig-1R to specifically reduce Aβ toxicity through these pathways. Therefore, through the use of in vitro and ex vivo models, this study examined the pathways through which activation of the Sig-1R may exert its protective effects against Aβ toxicity. Here, it is shown that activation of the Sig-1R reduces neuronal cell apoptosis in vitro, and reduces tissue death in the CA3 region of the hippocampus ex vivo. Furthermore, this reduction in cell and tissue death may be a result of reduction of ER stress and a return towards baseline levels of autophagy. Together, this research provides insight as to how the Sig-1R may be an important therapeutic target in AD through protection against apoptosis and tissue death.

The hippocampus is known to play a critical role in episodic memory function. Understanding the relation between electrophysiological activity in a rat hippocampus and rat behavior may be helpful in studying pathological diseases that corrupt electrical signaling in the hippocampus, such as Parkinson’s and Alzheimer’s. Additionally, having a method to interpret rat behaviors from neural activity may help in understanding the dynamics of rat neural activity that are associated with certain identified behaviors. In this thesis, neural networks are used as a black-box model to map electrophysiological data, representative of an ensemble of neurons in the hippocampus, to a T-maze, wheel running or open exploration behavior. The velocity and spatial coordinates of the identified behavior are then predicted using the same neurological input data that was used for behavior identification. Results show that a nonlinear autoregressive process with exogenous inputs (NARX) neural network can partially identify between different behaviors and can generally determine the velocity and spatial position attributes of the identified behavior inside and outside of the trained interval

The hippocampus is a brain structure known for its role in declarative memory- our ability to consciously recall facts and events. The hippocampus is a highly heterogeneous brain structure, and the small subregion CA2 has been shown to be necessary for the formation of social memories, the ability of an animal to recognize previously encountered conspecifics. Changes in excitatory/inhibitory balance have been observed in CA2 in humans with schizophrenia and in mouse models of schizophrenia, suggesting that these alterations may lead to some of the social dysfunction seen in schizophrenia. Although the hippocampal CA2 region has been implicated in social memory and neuropsychiatric disorders, little is known about how CA2 neural activity may encode social interactions and how this coding may be altered in disease. To see if and how CA2 codes for social interactions, I recorded extracellularly from CA2 pyramidal neurons as mice engage in a three-chamber social interaction task where the mice interact with the following task dimensions: space, novel objects, familiar social stimuli, novel social stimuli, and the passage of time. I found that whereas CA2 activity fails to provide a stable representation of space, unlike most other dorsal hippocampal subregions, it does code for contextual changes and for novel social stimuli. In Df(16)A+/- mice, which model the 22q11.2 microdeletion, a major schizophrenia risk factor, CA2 activity fails to encode context or social novelty, consistent with the deficit in social memory seen in these mice. In contrast, CA2 activity shows a surprising increase in spatial coding in Df(16)A+/- mice. These mice were previously shown to have a loss of inhibitory neurons within CA2, and a hyperpolarization of the CA2 pyramidal neuron resting potential. This hyperpolarization is likely due to upregulation of the outward rectifying TREK-1 K+ channel. I found that administration of a TREK-1 K+ channel antagonist rescued social memory and restored the normal CA2 coding properties in the mutants. These results demonstrate a crucial role for CA2 in the encoding of social stimuli and the expression of social memory, and suggest that dysfunction in CA2 may underlie deficits in social function seen in some forms of neuropsychiatric disease.

The goal of this study is to find an optimal dose of cytostatic Temozolomide (TMZ) for Long-Evans strain of rats. This dose should reduce neurogeneis while having a minimal pernicious health side-effects. Temozolomide is newly used to suppress neurogenesis but similarly to any other cytostatic has an effect on all dividing cells in an organism. This can affect health of an animal. Contrary to the mice, there was no systematic attempt to establish optimal dose. In our experiment rats were divided into four groups - one control group and three treatment groups which received different doses of temozolomide (10, 25, 40 mg/kg of TMZ). To detect level of neurogenesis cells were labeled by bromodeoxyuridine. During the experiment blood element counts were assessed, sensorimotoric tests were conducted, and weight increment was monitored. The results indicate that dose of 10mg/kg is adequate as it reduces neurogenesis by 64% compared to the control group and does not significantly differ from higher doses. In this group weight increment is comparable with the control group, while in the higher doses of TMZ weight increment is significantly lower. Effect of myelosupression is same for all treatment groups.

Traumatic brain injury (TBI) is becoming increasingly recognized as a global health issue. Each year over 160,000 Canadians experience some form of TBI, which can be caused by sport-related injuries, motor vehicle accidents, or assault. Adolescents are especially susceptible to repeat head injury and represent an at-risk population for sustaining sports-related concussions. The hippocampus, known for its role in learning and memory, is vulnerable to this injury. Although most TBI studies exclude females, there are important sex differences in outcomes and recovery following brain injury. A greater understanding of how sex differences contribute to the heterogeneity of this disease is critical for clinical care and potential treatments. Currently, few preclinical studies have assessed sex differences in adolescents following repeated mild traumatic brain injury (rmTBI). This study uses an awake closed head injury (ACHI) paradigm in male and female adolescent rats to investigate acute injury-induced changes to the hippocampus after rmTBI. A neurological assessment protocol (NAP) administered immediately after each impact showed that the ACHI acutely alters state of consciousness, and results in deficits after each impact. Following 8 ACHIs spaced 2 hours apart, adolescent rats were injected with the thymidine analogue BrdU and perfused 2 hours later on either post injury day (PID) 1 or 3. BrdU was used to identify cells undergoing DNA synthesis, and Ki-67 - expressed during all active phases of the cell cycle - was used as an endogenous marker for proliferation. Results indicate a robust and diffuse increase in cellular proliferation in male rmTBI animals that was not present to the same extent in female rmTBI animals. Triple labeling experiments revealed a higher proportion of microglia/macrophages in the subgranular zone of rmTBI animals, indicating an immediate inflammatory response in both sexes. This study shows sex differences in the pathophysiology of rmTBI in adolescent rats. Further investigation will reveal the detrimental versus neuroprotective contributions of this effect on learning and memory. / Graduate

Accumulating evidence suggests dysfunction of the glutamate signaling system in major depressive disorder. Previously, we observed elevated levels of NR2C subunit of N-methyl-D-aspartate receptor (NMDAR) and lower levels of neuronal nitric oxide synthase (nNOS), intracellular mediator of NMDAR activation, in the noradrenergic locus coeruleus in depressed subjects (Karolewicz et al. 2004, 2005). The aim of the present study was to investigate potential abnormalities in the NMDAR signaling molecules in the amygdala and hippocampus from the same depressed subjects. Tissue samples containing the lateral nucleus of amygdala, and hippocampal dentate gyrus were obtained from 10 -11 matched pairs of depressed subjects and healthy controls. Subjects were matched for age, sex, brain pH, and postmortem interval. Changes in concentration of NMDAR subunits NR1 and NR2 as well as intracellular associated proteins, neuronal nitric oxide synthase (nNOS) and postsynaptic density protein (PSD-95), that might occur in depression were assessed by immunoblotting. NR1 subunit immunoreactivity appeared as a prominent band at 120 kDa; NR2A as a band at 180 kDa; PSD-95 and nNOS immunoreactivities appeared as bands at 95 kDa and 155 kDa, respectively. The overall amount of NR1 and nNOS was normal in subjects diagnosed with depression as compared to controls. Amounts of PSD-95 were significantly higher in the amygdala (+115%, p<0.05) and hippocampus (+34%, p<0.05), respectively. The level of NR2A subunit was elevated in the amygdala (+51%) in depressed subjects as compared to controls. Our data indicate that glutamatergic signaling is abnormal in depression. Higher levels of NMDA receptor subunits and its associated protein may represent an adaptive response to decreased synaptic release of glutamate. This hypothesis is in agreement with postmortem and neuroimaging findings of altered glutamatergic transmission in depression. Further studies of the glutamatergic signaling system may lead to the development of novel therapeutics for the treatment of depressive disorder.

No description available.

Aktuell studie syftar till att undersöka relationen mellan stress och episodiskt minne, processhastighet, hippocampusvolym och den vita substansens mikrostruktur. Därtill undersöktes huruvida hippocampusvolym och den vita substansens mikrostruktur medierar sambandet mellan stress och kognitiv funktion. Data inhämtades från Betula-projektet från två separata mättillfällen med fem års mellanrum. Data analyserade för 153 deltagare, vilka grupperades baserat på nivå av självskattad stress. Mellangruppsanalys visade att den stressade gruppen hade signifikant mindre hippocampusvolym än den ostressade gruppen, men att grupperna inte skilde sig avseende förändring av hippocampusvolym över fem år. Inomgruppsanalys visade att det inom den ostressade gruppen fanns en signifikant negativ korrelation mellan stressnivå och processhastighet, och att det inom den stressade gruppen förekom en signifikant positiv korrelation mellan stressnivå och integritet av vitsubstansbanor. Mediationsanalyser visade inte några signifikanta resultat. Diskussion förs kring samband mellan stress och hippocampus samt begränsningar i aktuell studie. Därtill diskuteras huruvida ett potentiellt dos-responssamband kan föreligga mellan stressnivå och utfall, baserat på resultat från inomgruppsanalys. Framtida forskning bör undersöka om stress är relaterat till specifika områden av hippocampus samt specifika vitsubstansbanor. Därtill bör framtida forskning överväga att undersöka kliniskt stressade populationer.