Thesis (M.S.)--University of Michigan, 1987.

Thesis (M.S.)--University of Michigan, 1992.

Thesis (M.S.)--University of Michigan, 1993. / Includes bibliographical references.

Thesis (M.S.)--University of Michigan, 1993. / Includes bibliographical references.

Thesis (M.S.)--University of Michigan, 1992.

CoordenaÃÃo de AperfeiÃoamento de NÃvel Superior / Os cavalos-marinhos sÃo peixes Ãsseos do gÃnero Hippocampus. Algumas caracterÃsticas deste grupo os tornam bastante vulnerÃveis a sobreexploraÃÃo. O intenso comÃrcio e a degradaÃÃo de seus habitats aliados aos seus aspectos biolÃgicos, contribuem para o declÃnio de suas populaÃÃes. Diante da ameaÃa que estes animais estÃo sujeitos em todo o mundo e da sua relevÃncia como espÃcie bandeira, à de fundamental importÃncia estudos a respeito de sua ecologia. O objetivo deste trabalho foi verificar a distribuiÃÃo espacial e temporal da abundÃncia de Hippocampus reidi nos rios Pacoti e Malcozinhado, alÃm de documentar caracterÃsticas ecolÃgicas desta espÃcie na natureza. Foram realizados transectos com Ãrea de 100 m2 ao longo das margens dos dois rios. Cada rio foi dividido em duas estaÃÃes com distÃncias crescentes do mar, estas distÃncias eram aproximadamente equivalentes nos dois estuÃrios. A densidade mÃdia verificada foi de 0,008 indivÃduos/m2. O rio Pacoti apresentou uma densidade mÃdia significativamente superior à do rio Malcozinhado (p = 0,019). O Pacoti nÃo apresentou diferenÃa de densidade no gradiente de distÃncia do mar, sendo que o mesmo nÃo ocorreu com o Malcozinhado. Os dois rios apresentaram uma pequena diminuiÃÃo nas densidades em um intervalo de aproximadamente 3 anos, mas sem diferenÃa significativa (p = 0,33 e p = 0,41, para o Pacoti e Malcozinhado, respectivamente). Foi observado que nÃo hà variaÃÃo sazonal na abundÃncia de cavalos-marinhos. Os indivÃduos avistados no rio Pacoti apresentaram altura significativamente superior aos do rio Malcozinhado (p = 0,027). A razÃo sexual foi de 1:1, onde os machos apresentaram altura mÃdia superior à das fÃmeas (p = 0,013). Os indivÃduos de coloraÃÃo preta e marrom foram os mais abundantes. As raÃzes de Laguncularia racemosa foi o substrato de apoio preferido dos animais avistados. Este estudo demonstrou que diferenÃas ambientais nos estuÃrios como Ãrea do manguezal, vazÃo fluvial e vulnerabilidade a impactos ambientais podem causar diferenÃa na abundÃncia de H. reidi e que o conhecimento de aspectos ecolÃgicos destes animais na natureza à de fundamental importÃncia na elaboraÃÃo de planos de manejo para esta espÃcie. / Seahorses are bony fishes included in the Hippocampus genus. Many aspects of their biology make them extremely vulnerable to overexploitation. The intense commerce and habitat degradation allied to their biology, contribute to the decline of their populations. Considering the level of threat to these animals around the world and their relevance as a âflagship speciesâ, studies concerning their ecology are extremely important. The purpose of this study was to determine spatial and temporal distribution of the abundance of Hippocampus reidi at the Pacoti and Malcozinhado rivers, registering also other ecological aspects of the species. Transects with an area of 100 m2 were conducted along the riverbanks. Each river was divided into two stations with increasing distances from the sea. The mean density determined was of 0,008 individuals/m2. The Pacoti River showed a significantly higher mean density when compared to the Malcozinhado River (p=0,019). There was no difference between the two stations in the Pacoti River, while in the Malcozinhado an evident difference was noticed. Both rivers showed a decreasing trend in the densities during the study, but the differences were not significant (p=0,33 and p=0,41, for Pacoti and Malcozinhado, respectively). No seazonal variation in abundance was determined. The animals from the Pacoti River were significantly higher than those from the Malcozinhado (p=0,027). Sez ratio was 1:1, and males were higher on average than the females (p=0,013). Black and brown colored seahorses were more abundant than other colors. The roots of Laguncularia racemosa were preferably chosen as anchoring substrata. The present study showed that environmental differences on the estuaries, such as mangrove areas, discharge volume and vulnerability to impacts might cause differences in the abundance of H. reidi. The adequate management of this threatened species depend strongly on a good knowledge of its ecology.

The mechanisms underlying the generation and spread of seizure activity have remained elusive despite a considerable research effort over the last two decades. Most of this work has concentrated on the characteristics of neuronal excitability and burst discharge at the single cell level. These studies have provided some understanding of the possible abnormalities of neurons within an epileptic focus, but little direct insight into the factors responsible for the striking synchronization of action potentials during interictal discharge or in the spread of synchronous activity across apparently normal brain tissue. Although synaptic activation probably plays a role in the generation of seizure activity, recent evidence indicates that seizure-like discharge can occur during chemical blockade of synaptic transmission (Jefferys and Haas 1982; Taylor and Dudek 1982). This rather surprising result emphasizes the importance of considering non-synaptic mechanisms for both the synchronization and spread of abnormal neuronal activity in the central nervous system. One important non-synaptic mechanism to consider is ephaptic interactions. This term refers to the direct electrical influence of extracellular field potentials on neuronal excitability. It is possible that ephaptic interactions, generated during seizure activity, simultaneously depolarize an entire population of neurons leading to both recruitment and synchronization of action potential discharge. This thesis investigates ephaptic interactions during evoked potentials in the hippocampal formation. The hippocampus is one of the most seizure-prone regions of the brain and its anatomical structure is ideal for the generation of field effects. Evoked potentials were used as "models" of synchronous neuronal discharge since they are more reproducible, easier to control, and better understood than seizure activity. This initial investigation of ephaptic interactions lays the foundation for further studies involving the complexities of epileptic activity. The first phase of this project examined the spatial characteristics of field potentials evoked in the hippocampus and the dentate gyrus. Current source density (CSD) analysis and voltage gradient determinations obtained from these fields were used to characterize the pattern of current flow within the neuropil and to predict the polarity and relative intensity of ephaptic influences on neuronal excitability. The detailed characteristics of extracellular voltage gradients varied between CAl and the dentate gyrus, and also between anti- and orthodromic responses. In general, voltage gradients during the positive components of a somatic population spike predicted ephaptic hyperpolarization of the neuronal population, whereas gradients observed during the negative component predicted depolarization. They were often an order of magnitude greater than the smallest gradient known to influence granule cell activity. An exception to this rule was the minimal gradient observed during the negative component of the dentate response. In the second phase of the study, extracellular voltage gradients were experimentally applied to the dentate gyrus to determine the sensitivity of granule cells to ephaptic interactions. The magnitude of the applied gradients were in the range observed during the evoked potentials studied in the first phase. These experiments demonstrated a remarkable sensitivity of granule cells to the applied fields. The fields could alter the population spike from near minimal to near maximal. Surprisingly, even antidromic potentials were influenced by the gradients. On the other hand, the EPSP phase of the population spike was not influenced. These findings established that extracellular currents can influence the excitability within a neuronal population without altering synaptic drive. The final phase of the project investigated the transmembrane potential (TMP) of pyramidal and granule cells during applied fields and evoked potentials. The TMP was calculated by subtracting the extracellular from the intracellular response. This potential ultimately determines the voltage dependent behavior of a neuron and gives a direct measure of any ephaptic interactions. In order to measure the intracellular influences of applied fields, the TMP was monitored while the impaled cell was exposed to extracellular voltage gradients spanning the same range as used in phase two of the project. The TMP shifted by as much as plus or minus 5 mV, depending on the amplitude and polarity of the gradient. This large shift in TMP accounts for the observed influence of the applied field potentials, and suggests that the voltage gradients associated with evoked potentials should also have a marked effect on the TMP. A depolarizing wave of the TMP occurred during the negative component of anti- and orthodromic CA1 responses. This depolarization was capable of initiating action potentials, and decreased the latency to discharge during orthodromic responses. During epileptiform discharge, a similar depolarizing wave was associated with each negative component of the burst. These depolarizations recruit and synchronize neuronal discharge by simultaneously increasing the excitability within an entire population of cells. These data support the hypothesis that ephaptic interactions in the hippocampal formation influence the pattern of cell discharge during evoked potentials. It is postulated that similar ephaptic interactions may contribute to recruitment and synchronization during seizure activity. / Medicine, Faculty of / Cellular and Physiological Sciences, Department of / Graduate

The electrophysiological properties of somatic and dendritic membranes of CA1 pyramidal neurons were investigated using the rat in vitro hippocampal slice preparation. A comprehensive analysis of extracellular field potentials, current-source density (CSD) and intracellular activity has served to identify the site of origin of action potential (AP) discharge in CA1 pyramidal neurons. 1) Action potential discharge of CA1 pyramidal cells was evoked by suprathreshold stimulation of the alveus (antidromic) or afferent synaptic inputs in stratum oriens (SO) or stratum radiatum (SR). Laminar profiles of the "stimulus evoked" extracellular field potentials were recorded at 25µm intervals along the dendro-somatic axis of the pyramidal cell and a 1-dimensional CSD analysis applied. 2) The shortest latency population spike response and current sink was recorded in stratum pyramidale or the proximal stratum oriens, a region corresponding to somata and axon hillocks of CA1 pyramidal neurons. A biphasic positive/negative spike potential (current source/sink) was recorded in dendritic regions, with both components increasing in peak latency through the dendritic field with distance from the border of stratum pyramidale. 3) A comparative intracellular analysis of evoked activity in somatic and dendritic membranes revealed a basic similarity in the pattern of AP discharge at all levels of the dendro-somatic axis. Stimulation of the alveus, SO, or SR evoked a single spike while injection of depolarizing current evoked a repetitive train of spikes grouped for comparative purposes into three basic patterns of AP discharge. 4) Both current and stimulus evoked intracellular spikes displayed a progressive decline in amplitude and increase in halfwidth with distance from the border of stratum pyramidale. 5) The only consistent voltage threshold for intracellular spike discharge was found in the region of the cell body, with no apparent threshold for spike activation in dendritic locations. 6) Stimulus evoked intradendritic spikes were evoked beyond the peak of the population spike recorded in stratum pyramidale, and aligned with the biphasic extradendritic field potential shown through laminar profile analysis to conduct with increasing latency from the cell body layer. The evoked characteristics of action potential discharge in CA1 pyramidal cells are interpreted to indicate the initial generation of a spike in the region of the soma-axon hillock and a subsequent retrograde spike invasion of dendritic arborizations. / Medicine, Faculty of / Cellular and Physiological Sciences, Department of / Graduate

Der Hippocampus ist ein Hirnareal welches fundamental für die Generierung von bewussten Erinnerungen und für die Etablierung von flexiblen Beziehungen zwischen kontextuellen Repräsentationen ist. Hippocampale Fehlfunktion während des Alterns wurde als ein Schlüsselfaktor für den kognitiven Abbau vorgeschlagen und im Kontext einer schnell alternden Bevölkerung wird es zwingend notwenig, mögliche Mechanismen zu verstehen, die diese Einschränkungen verhindern oder rückgängig machen können. Der Hippocampus ist ein von nur zwei Arealen im Gehirn, in dem neue Neuronen ständig im Erwachsenenleben erzeugt werden. Deren Rolle beim Lernen und bei Gedächtnisfunktionen ist jedoch nicht gut verstanden. Interessanterweise wurde adulte hippocampale Neurogenese als eine zelluläre Komponente eines Gehirnreservenmechanismus vorgeschlagen, mit dem Potenzial kognitive Fähigkeiten ein Leben lang zu erhalten sowie ein mögliches Ziel für therapeutische Ansätze darzustellen. In dieser Arbeit habe ich eine spezifische, genetisch-bedingte Expandierung von hippocampalen Nervenstammzellen genutzt, um deren intrinsisches Potenzial, neugeborene Neuronen zu erzeugen, auszuschöpfen, was zu einer lebenslangen erhöhten Neurogenese geführt hat. Dies hat die hippocampale Funktion auf mehreren Ebenen gefördert, vom verbesserten flexiblen Lernen in Navigationsaufgaben in der Jugend, über Kompensation des altersbedingten kognitiven Abbaus bis hin zur Verjüngung von kontextuellem Gedächtnis beim Altern. Zusammengefasst stellt meine Arbeit ein besseres Verständnis des funktionellen Beitrags der Neurogenese zu Lernen und Gedächtnis zur Verfügung and zeigt, dass kritische Aspekte hippocampaler kognitiver Beeinträchtigung im Alter rückgängig gemacht oder ein Leben lang durch extrinsische Ausnutzung der endogenen Hirnreserven kompensiert werden können.

BACKGROUND:Aging is typically accompanied by memory decline and changes in hippocampal function. Among these changes is a decline in the activity of the dentate gyrus (DG) during behavior. Lasting memory, however, is thought to also require recapitulation of recent memory traces during subsequent rest - a phenomenon, termed memory trace reactivation, which is compromised in hippocampal CA1 with progressive age. This process has yet to be assessed in the aged DG, despite its prominent role in age-related memory impairment. Using zif268 transcription to measure granule cell recruitment, DG activity in adult and aged animals was assessed both during spatial exploration and as animals remained at rest in the home cage in order to detect potential memory-related replay.RESULTS:Consistent with the observation of memory trace reactivation in DG, the probability that an individual granule cell transcribes zif268 during rest in the animal's home cage is increased by recent experience in a novel environment. Surprisingly, a comparable increase was observed in the probability of granule cells in the aged DG expressing zif268 during rest. Moreover, no significant age-related difference was observed in the number of granule cells expressing zif268 during rest. Thus, the number and pattern of granule cell expression of zif268 during rest is preserved in aged animals, despite a significant decline in exploration-related zif268 expression.CONCLUSIONS:These data lead to the hypothesis that the input the aged DG receives from backprojections from CA3 (the region widely hypothesized to mediate reactivation) remains functionally intact despite loss of innervation from the perforant path.