Abstract
The first experiments in this thesis examined the influence of chronic methamphetamine and methamphetamine-paired contexts on S-R habits in undertrained animals. We found that chronic methamphetamine exposure prior to instrumental learning caused a rapid transition to the dominance of S-R habits over goal-directed actions. This was not due to chronic methamphetamine entirely abolishing the capability of goal-directed behaviour. When instrumental learning occurred prior to training but after test goal-directed behaviour was observed. We also found evidence of a return to goal-directed behaviour following a period of abstinence prior to training. Lastly, we found distinct differences in behavioural control when animals were tested in a methamphetamine- or saline-paired context. A second series of experiments examined the influence of acute and chronic methamphetamine on animals’ ability to use contextual information to resolve conflict using a contemporary animal model of human executive function. Here we found a difference between the effects of acute and chronic methamphetamine administration on executive function. Acutely, methamphetamine had no impact on animals’ ability to use contextual information to resolve conflict. However, chronic methamphetamine led to significantly impaired performance in this task. A third set of experiments investigated whether an identical chronic methamphetamine regimen caused drug-dependent neural plasticity in brain regions known to be involved in S-R habits and executive function, and to ascertain whether different periods of abstinence had any effect on methamphetamine-dependent neural plasticity. We found evidence of persistent changes in spine density of pyramidal cells in the prelimbic and infralimbic cortices. This implies that perhaps neurochemical adaptations occur over time in order to restore balance to the system, despite enduring changes to structure of the systems. Such restoration of balance may allow for a return to normal function.