Positron emission tomography (PET) permits the study of neuroreceptor systems non-invasively in the living brain, and thus offers a means to define the neurophysiological basis of unconsciousness produced by general anesthetics. This application for a isoflurane of the functional state of the GABAa-receptor (GABAA-R), as well as postsynaptic processes directly linked to the receptor, such as neuronal metabolism, in the intact, living brain. Abundant data obtained from in vitro and small animal models support that the GABAA-R is an important target for general anesthetics, which potential the actions of GABA at the receptor complex. The relevance of this potential of receptor function, however, has not yet been explored; this is due to inherent limitations of previous experiments employing destructive techniques. In sharp contrast, using PET methodology and the benzodiazepine ligand 11C-flumazenil (11C- FMZ), we recently demonstrated in fully intact brain, that isoflurane dose-dependently, and specifically, enhances GABAA-R ligand binding, indicating that modulation of GABAA-R conformational state occurs. This provides the first in vivo support in humans for the well known GABAA-R hypothesis of general anesthesia. To determine whether 11C- FMZ binding is a valid reflection of GABAA-R functional state in the living brain, we propose to test the specific hypothesis in non-human primates that the GABAA-R agonist muscimol enhances 11C-FMZ binding in a dose-dependent manner, as has been shown in in vitro studies. If however, binding of 11C-FMZ, a benzodiazepine antagonist, proves to be insensitive to increasing doses of muscimol, the binding of the benzodiazepine agonist midazolam will be used as a probe for GABAA-R function by measuring the displacement of 11C-FMZ by midazolam. To determine whether muscimol-related increase in GABAA- R function translates into enhanced inhibitory transmission, we propose to measure the effect of increasing doses of muscimol on regional neuronal metabolism (rCMR/glu) by 18F-deoxyglucose PET. Subsequently, analogous muscimol 11C-FMZ (midazolam) binding, and muscimol- rCMR/glu effect curves indicating an increase in agonist affinity for the receptor, as well as a decrease in regional neuronal metabolism due to enhanced GABAA-R function, respectively. The proposed experiments are expected to yield insights into the relationship between isoflurane's effect on the GABAA-receptor alone, as well as the translation of this effect into enhanced inhibitory transmission in various brain regions. This project, combined with laboratory experience and coursework, designed as a vehicle for development of the candidate's potential as an investigator of anesthetic mechanisms in the intact, living brain.
