Pharmacological magnetic resonance imaging (phMRI) is the use of functional MRI techniques to elucidate the effects that psychotropic drugs have on neural activity within the brain; it is an emerging field of research that holds great potential for the investigation of drugs that act on the central nervous system by revealing the changes in neural activity that mediate observable changes in behavior, cognition, and perception. However, the realization of this potential is hampered by several unanswered questions: Are the MRI measurements reliable surrogates of changing neural activity in the presence of pharmacological agents? Is it relevant to investigate psychiatric phenomena such as reward or anxiolysis in anesthetized, rather than conscious animals? What are the methods that yield reproducible and meaningful results from phMRI experiments, and are they consistent in the investigations of different drugs?
The research presented herein addresses many of these questions with the specific aims of
1) Developing pharmacological MRI methodologies that can be used in the conscious animal,
2) Validating these methodologies with the investigation of a non-stimulant, psychoactive compound, and
3) Applying these methodologies to the investigation of typical and atypical antipsychotic drugs, classes of compounds with unknown mechanisms of therapeutic action
Building on recent developments in the field of functional MRI research, we developed new techniques that enable the investigator to measure localized changes in metabolism commensurate with changing neural activity. We tested the hypothesis that metabolic changes are a more reliable surrogate of changes in neural activity in response to a cocaine challenge, than changes observed in the blood-oxygen-level-dependent (BOLD) signal alone. We developed a system capable of multi-modal imaging in the conscious rat, and we tested the hypothesis that the conscious brain exhibits a markedly different response to systemic morphine challenge than the anesthetized brain. We identified and elucidated several fundamental limitations of the imaging and analysis protocols used in phMRI investigations, and developed new tools that enable the investigator to avoid common pitfalls. Finally, we applied these phMRI techniques to the investigation of neuroleptic compounds by asking the question: does treatment with typical or atypical antipsychotic drugs modulate the systems in the brain which are direct or indirect (i.e. downstream) substrates for a dopaminergic agonist?
The execution of this research has generated several new tools for the neuroscience and drug discovery communities that can be used in neuropsychiatric investigations into the action of psychotropic drugs, while the results of this research provide evidence that supports several answers to the questions that currently limit the utility of phMRI investigations. Specifically, we observed that metabolic change can be measured to resolve discrepancies between anomalous BOLD signal changes and underlying changes in neural activity in the case of systemically administered cocaine. We found clear differences in the response to systemically administered morphine between conscious and anesthetized rats, and observed that only conscious animals exhibit a phMRI response that can be explained by the pharmacodynamics of morphine and corroborated by behavioral observations. We identified fundamental and drug-dependent limitations in the protocols used to perform phMRI investigations, and designed tools and alternate methods to facilitate protocol development.
By applying these techniques to the investigation of neuroleptic compounds, we have gained a new perspective of the alterations in dopaminergic signaling induced by treatment with antipsychotic medications, and have found effects in many nuclei outside of the pathways that act as direct substrates for dopamine. A clearer picture of how neuroleptics alter the intercommunication of brain nuclei would be an invaluable resource for the classification of investigational antipsychotic drugs, and would provide the basis for future studies that examine the neuroplastic changes that confer therapeutic efficacy following chronic treatment with antipsychotic medications
