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Instytut Biologii Doświadczalnej im. Marcelego Nenckiego PAN
Contributor:Knapska, Ewelina : Supervisor ; Konarzewski, Marek : Supervisor
Publisher:Nencki Institute of Experimental Biology PAS
Place of publishing: Date issued/created: Description:81 pages : illustrations ; 30 cm ; Affiliation of the second supervisor: Uniwersytet w Białymstoku ; Bibliography ; Summary in Polish
Degree grantor:Instytut Biologii Doświadczalnej im. Marcelego Nenckiego PAN
Type of object: Subject and Keywords:Brain plasticity ; Encephalization ; Experimental evolution ; Trade-offs
Abstract:
The enlarged brains of homeotherms bring behavioral advantages but also incur high energy expenditures. Energy fueling evolutionary increase in brain size and enhanced cognitive abilities (CA) could come from two primary sources: according to the “expensive tissue” hypothesis postulated by (Aiello and Wheeler 1995), the evolution of a larger brain was made possible by a diet-related reduction in the size of the digestive tract and by increasing of quality (energy density) of the diet. Thereby, an evolutionary increase in brain size resulted from the brain-gut trade-off. The second hypothesis, dubbed the “expensive brain” hypothesis (Isler and van Schaik 2006), predicts that the energetic costs of an evolutionary increase in brain size were covered by increased total energy intake rather than energy savings on metabolically costly organs (such as the gut) or processes (reproduction or immunocompetence). In my thesis, I asked a question: How were the energetic costs of an enlarged brain overcome in the course of evolution? To answer this question, I used the experimental evolution animal model consisting of the line types of Swiss Webster mice artificially selected for high (H-) or low (L-) Basal Metabolic Rate (BMR), maximal (VO2max) metabolic rate (a.k.a. peak, PMR), and random bread lines (RB). The metabolism rates selected in the model are proxies of the traits implicated in the evolution of homeothermy. Thus, they are a prerequisite for the encephalization and exceptional CA of mammals, including humans. The H-BMR mice had bigger guts, but not brains, than mice of other line types. Yet, they were superior to the other line types in the cognitive tasks carried out in reward and avoidance learning contexts. Conversely, when subjected to the classical paradigm of contextual fear conditioning, the L- BMR mice lost fear response much faster than the mice of other line types (that is, their memory was inferior). Furthermore, the H-BMR mice had higher neuronal plasticity (indexed as the long-term potentiation, LTP). They also had increased numbers of neurons and dendritic spines in the hippocampus compared to their counterparts. Finally, the activity of cytochrome oxidase (CCO), a proxy of the number of neuronal mitochondria, was higher in the H-BMR mice than in other line types. The results suggest that the evolutionary increase of CA in mammals was initially associated with increased BMR and brain plasticity, rather than a direct increase in brain size. Thus, an enlarged gut was not traded off for brain size. It could be that in the course of evolution, selection for increased total energy expenditures indirectly increased BMR and the metabolic rate of better connected and more plastic individual neurons, improving CA. Thus, my study does not support the existence of the brain-gut trade-offs postulated by the ET hypothesis. Conversely, my results support the link between CA fueled by high brain metabolism reflected in H-BMR as proposed by the EB concept.
Nencki Institute of Experimental Biology of the Polish Academy of Sciences
Original in:Library of the Nencki Institute of Experimental Biology PAS
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