Evaluation of prefrontal ischemic lesions on higher order cognitive behaviours in the rat

Stroke is the most common cause of disability worldwide, and it is expected that the global number of stroke survivors will increase significantly over the coming decades. Many survivors of stroke have both short and long-term deficits in function, the most common of which are motor deficits. However, an often forgotten aspect of stroke is its negative and often long-lasting effects on cognitive function. At the moment, there are some effective intervention strategies for treating motor dysfunction post-stroke, but we currently have very limited strategies for treating cognitive dysfunction. Furthermore, there are very few stroke models specifically focusing on higher-order cognitive dysfunctions, also known as executive functions, which have been shown to be affected in a substantial portion of stroke survivors. This makes it difficult to effectively study the effects of potential therapies to alleviate post-stroke cognitive dysfunction. It is also known that the seat of higher-order cognitive function is the prefrontal cortex (PFC), and that damage to this region or by damaging connections to the area can result in executive dysfunction. Therefore, the goal of this thesis is to develop new animal models of post-stroke cognitive dysfunction, targeting specific regions of the rodent PFC. The overarching hypothesis of the project was that injections of the vasoconstrictor, endothelin-1, into particular regions of the prefrontal cortex will cause ischemic lesions in this area, as well as measureable cognitive deficits in higher-order cognitive functions. The overall objectives of the thesis were: 1. To refine and evaluate the effects of endothelin-1 injections into the prefrontal cortex on brain lesion size, and initial behavioural effects in the rat. 2. To measure the effects of refined, focal ischemic lesions within the medial prefrontal cortex on aspects of set-shifting and temporal order memory, both of which are higher-order cognitive functions, in the rat. 3. To further evaluate the model using endothelin-1 to create focal lesions in the orbital prefrontal cortex versus lesions in the medial prefrontal cortex to assess its effects on decision making in the rat. The initial modelling of post-stroke cognitive dysfunction was attempted using the compound endothelin-1 (ET-1), an endogenous polypeptide capable of constricting blood vessels in the brain. Two separate medial prefrontal cortical (mPFC) ET-1 injection protocols (four injections versus two injections into the mPFC) in the rat were examined in an attempt to understand the size and location of the resulting brain lesion. It was found that the two injection protocol, as compared to the four injection protocol, resulted in a more targeted lesion in the mPFC and resulted in fewer animal mortalities. As well, it appeared that mPFC lesions caused anxiogenic behaviours in these animals as assessed through the elevated plus maze, indicating that these lesions were having some effects on post-stroke cognitive behaviour. Further experiments with the two-injection endothelin-1 model previously established assessed for deficits in specific higher-order cognitive domains, specifically set-shifting and temporal order memory. This was done using a modified set-shifting maze and a temporal object recognition task. It was found that lesions to the medial prefrontal cortex caused specific context-dependent set-shifting dysfunctions in stroke animals. However, attempts to determine the lesions effects on temporal order memory did not function as expected, indicating that the methods used to assess this behaviour need to be modified in the future. Finally, two separate models of prefrontal stroke targeting the medial and orbital prefrontal cortices were both examined for their effects on decision making and inhibitory control as assessed through a delay discounting maze, wherein animals would have the choice between an immediately available, but small, food reward or a larger food reward, to which access was delayed. As well, during this testing the animals ultrasonic vocalizations, which can indicate affective or aversive emotional states, were also recorded while the animals were anticipating a food reward. What was found was that lesions to the orbital, but not the medial, prefrontal cortex caused rats to choose the lower food reward as compared to control animals. Interestingly, only the medial prefrontal lesioned animals changed the relative frequency and duration of their ultrasonic vocalization calls post-stroke, indicating that changes in animal vocalizations are not necessarily correlated to changes in behaviour. Overall, the work presented in this thesis suggests that specific higher-order cognitive dysfunctions can be accomplished using endothelin-1, thereby providing a potentially new model for the study of post-stroke cognitive dysfunction