Adult neurogenesis in the brain of chiroptera

Abstract

The current thesis, studying adult neurogenesis in the brains of Chiroptera (bats), is a collection of four related studies investigating the occurrence of neurogenesis in the two suborders of adult bats, megachiroptera (megabats) and microchiroptera (microbats), from different environments, including the wild and captive habitats. The studies were carried out in order to understand the dynamics associated with adult neurogenesis in mammals living in their natural habitat given that much of the current understanding is based on experiments done on laboratory bred or captive raised animals. The investigation of megachiropterans and microchiropterans was stimulated by the findings of a previous study which failed to show adult neurogenesis in some microchiropteran species, which is in contrast to the almost universal occurrence of the phenomenon in nearly all mammals. In addition, the use of chiropterans was appealing given their behavioural attributes, which have been previously associated with the occurrence of neurogenesis. These include such behaviours as good spatial abilities, high sociality and complex behaviours such as fusion-fission sociality. In addition, the highly debatable evolutionary history of chiropterans provided a framework in which to evaluate specific neural characters in terms of phylogenetic relationships. Using immunohistochemical methods, the presence and characteristics of proliferating and newly generated neurons in the brain of eight wild-caught adult megachiropteran species was examined. For the neurogenic patterns observed, direct homologies were evident in other mammalian species. Numerous proliferating cells and immature neurons were identified in the subventricular zone (SVZ) and the dentate gyrus. From the SVZ, these cells migrated to the olfactory bulb through a typically mammalian rostral migratory stream (RMS). Some newlygenerated cells were observed emerging from the RMS to the neocortex. Similar to primates, proliferating cells and immature neurons were identified in the SVZ of the temporal horn of the lateral ventricle of the megachiropterans and were observed to migrate to the rostral and caudal piriform cortex through a primate-like temporal migratory stream. A similar study using three microchiropteran species revealed almost similar findings. However, distinct differences to the megachiropterans were noted, especially so in the migratory pathway to the piriform cortex, where cells appeared to migrate from the RMS through an insectivore-like ventral migratory stream to populate the entire piriform cortex. In addition microchiropterans had immature axons in the anterior commissure, something which was not observed in megachiropterans but was previously reported in insectivores. Using immunohistochemical and stereological methods, the effect of animal capture and handling on the occurrence of adult neurogenesis in 10 microchiropterans species was investigated. These animals were euthanized and perfusion fixed at specific time points following capture to investigate the effect of stress as a possible explanation for the negative findings regarding adult hippocampal neurogenesis in microchiropterans reported in a previous study. This investigation revealed that when euthanized and perfused within 15 minutes of capture, abundant putative adult hippocampal neurogenesis could be detected using doublecortin immunohistochemistry, but the ability to readily observe these cells rapidly diminishes if the microchiropterans have not been euthanased within 15 minutes of capture. Also using immunohistochemical and stereological methods, proliferative and immature cells within the dentate gyrus of adult Egyptian fruit bats from three distinct environments (fifth generation captive bred, wild-caught from the primary rainforest of central Africa and wildcaught from the South African woodlands) was quantified and compared. Four previously reported methods to assess the effect of the environment on proliferative and immature cells were used. These include: (1) the comparison of raw totals of proliferative and immature cells; and these totals standardized to (2) brain mass, (3) the volume of the granular cell layer (GCL), and (4) the total number of granule cells in the dentate gyrus. For all methods, the numbers of proliferative cells did not differ statistically amongst the three groups. For the immature cells standardizations to brain mass and GCL volume revealed no difference between the three groups studied; however, the raw numbers and standardization to total granule cell numbers indicated that the two groups of wild-caught bats had significantly higher numbers of immature neurons than the captive-bred bats. In conclusion, the observation of the ventral migratory stream in the microchiropterans and insectivores, in contrast to the temporal migratory stream in megachiropterans and primates adds another neural characteristic supporting the diphyletic origin of Chiroptera, and aligns microchiropterans with insectivores and megachiropterans with primates. In microchiropterans, the presence of doublecortin, revealing adult neurogenesis, in the hippocampus is highly sensitive to capture and handling. Lastly, the interpretation of the effect of the environment on the numbers of immature neurons appears method dependent. It is possible that current methods are not sensitive enough to reveal the effect of different environments on proliferative and immature cells.