Impact of Microdialysis Probes on Vasculature and Dopamine in the Rat Striatum: a Combined Fluorescence and Voltammetric Study and The Design and Optimization of a Glutamate Sensor to be used under Hypoxic Conditions

Measuring extracellular dopamine in the brain of living animals by means of microdialysis and/or voltammetry is a route towards understanding both normal brain function and pathology. Previous reports, however, suggest that the tissue response to implantation of devices may affect the outcome of the measurements. To address the source of the tissue response and its impact on striatal dopamine systems microdialysis probes were placed in the striatum of anesthetized rats. Images obtained by dual-label fluorescence microscopy show signs of ischemia, or reduced blood flow, and opening of the blood-brain barrier near the probe tracks. Opening of the blood-brain barrier was further examined by determining dialysate concentrations of carbi-DOPA, a drug that normally does not penetrate the brain. Although carbi-DOPA was recovered in brain dialysate, it did not alter dialysate dopamine levels or evoked dopamine release as measured by voltammetry near the probes. Microdialysis probes also significantly diminished the effect of intrastriatal infusion of kynurenate on extracellular dopamine levels as measured by voltammetry near the probes. Glutamate is a very important neurotransmitter in the central nervous system. It is implicated in diseases such as schizophrenia, epilepsy, and stroke. Obtaining accurate information on glutamate concentrations is important in order to understand brain functioning. Glutamate is also of primary focus in terms of understanding the effects of hypoxia on the brain. iii It is believed that the large release of glutamate in the absence of oxygen is what is causing the toxic effects of hypoxia. To date, sensors used to monitor glutamate required oxygen to be present. Here, the design and optimization of a glutamate sensor to be used under hypoxic conditions is presented. Figure 1: Confocal microscopy of dual-labeled blood vessels in a horizontal section of the rat striatum contralateral to probe implantation (probe-free tissue). These images represent a Z-stack of optical sections taken at 1.5 mm intervals in the z-direction across a 30-μm thick brain slice. Figure 2: Fluorescence microscopy of dual-labeled blood vessels in a 30-µm thick horizontal section of the rat striatum after a 4-hr probe implantation. The white circle represents the probe track. Figure 4: Fluorescence microscopy as in Figures 2 and 3 except that the microdialysis probe was extracted from the tissue prior to transcardial perfusion. Note the presence of nanobeads (green) outside blood vessels (white arrow). The white circle indicates the probe implantation track. Figure 5: Systemic administration of carbi-DOPA (150 mg/kg i.p.) causes a significant …