Simultaneous functional diffuse optical tomography and EEG in freely moving rats.

Abstract

Diffuse Optical Tomography (DOT) is a non-invasive technology that uses near-infrared light to measure oxyhemoglobin (oxyHb) and deoxyhemoglobin (deoxyHb) levels and spatial distributions in the entire brain of anesthetized and freely moving rats. To spatially calibrate DOT imaging, local anesthetic was stereotactically injected into known brain regions. Rats (n=3) were anesthetized with urethane and 20% procaine was injected into the left hippocampus or left striatum (1uL over 2min). Shortly after procaine injection near the hippocampus, oxyHb decreased and deoxyHb increased in the left side of the brain in a posterior position (presumably the hippocampus). Likewise, following injection aimed at the striatum the same pattern of oxyHb decrease and deoxyHb increase was observed in an anterior position (presumably the striatum). OxyHb returned to baseline after ~60 min, in good agreement with the expected duration of procaine action. Results from anesthetized rats demonstrate the utility and reliability of DOT imaging and allow for the investigation of natural brain hemodynamics in awake, freely moving rats. To show that DOT can detect and localize oxyHb and deoxyHb in the brains of freely moving rats, DOT and hippocampal EEG recordings were made as rats (n=6) foraged for food pellets and slept in a bucket (n=3). When the EEG switched from large irregular activity (LIA, seen when the rat is not walking or is in slow-wave sleep) to theta (5-12 Hz rhythm seen when the rat walks or enters REM sleep) whole brain oxyHb decreased for ~1 sec and then increased over ~4 sec in a way that resembled the Blood Oxygenation Level Dependent (BOLD)-response observed with fMRI. EEG shifts from theta to LIA, however, were accompanied by decreases in oxyHb levels. Since oxyHb dynamics after transitions into theta mimic BOLD responses, and are oppositely directed to the oxyHb changes seen in LIA, it is suggested that the theta is a higher energy state than LIA. When the oxyHb shifts that accompanied LIA to theta switches were analyzed for their spatial distribution, major increases were found in regions that co-localize with the hippocampal procaine injections in anesthetized rats, implying that DOT can correctly detect sources of signals in the brain of freely moving rats. These results demonstrate that DOT imaging is a reliable tool for measuring hemodynamic changes linked to physiological changes in the brain. These studies serve as the basis for future experiments utilizing DOT that have the potential to provide useful information on hemodynamic changes related to learning and memory, as well as acute and chronic pathologies that affect blood flow, such as traumatic brain injury, cancer, and others.