Abstracts

Rationale: Polymeric nanoparticles have recently been developed for quantitative, ratiometric imaging of oxygenation changes. This probe has previously been used for measuring oxygenation changes in tumor tissue. Our interest in adapting this technique for imaging the neocortex is twofold. First, imaging of oxygen sensing nanoparticles might be useful for mapping normal and epileptic neuronal activity. Second, this technique may prove to be a useful means for studying activity-evoked changes in tissue oxygenation and metabolism in neocortex. Methods: The nanoparticles were prepared as previously described (Zhang et al., Nature Materials. 8:747-751, 2009). An imaging chamber was implanted that included hand motor and sensory cortex in the field of view. A 1.0 mg/ml solution of the nanoparticles in sterile water was prepared and applied to the exposed cortex in the imaging chamber. After 30 minutes, the nanoparticle solution was washed from the cortex and replaced with a layer of physiological saline solution. The nanoparticle probe has an oxygen-insensitive fluorescence emission wavelength centered at 460nm, and an oxygen-sensitive phosphorescence emission wavelength at 540nm. Images were acquired for quantitative ratiometric measurements at both wavelengths during electrical stimulation of the cortex. Images of the intrinsic optical signal were also acquired at 535nm (blood volume) and 660nm (blood oxygenation) for comparison to images of the the cortex labeled with the oxygen sensing nanoparticles. Images were acquired while the cortex was electrically stimulated either below or above its afterdischarge threshold so that oxygenation changes could be studied in response to both normal and epileptiform activity. Results: Ratiometric maps of the luminescent nanoparticle signals showed significant increases in the oxygenation of neocortical tissue during neuronal activity that was elicited by both subthreshold electrical stimulation and afterdischarge activity. The spatial and temporal patterns of oxygenation increases were similar to the intrinsic optical signal maps acquired at 535nm, but distinct from the intrinsic optical changes acquired at 660nm. Conclusions: The oxygen nanosensors likely measure changes that occur near the cortical surface. These changes closely resembled the optical absorption changes measured at 535nm, suggesting that oxygenation increases in the upper cortical layers are the consequence of activity-evoked increases in blood perfusion. These results represent the first steps towards developing a new in vivo technique for mapping changes in tissue oxygenation in response neuronal activity. Since the oxygen sensing nanoparticles are nontoxic and biodegradable, they may potentially be useful for mapping functional and epileptic cortex during neurosurgical procedures.