Voltammetric study of extracellular dopamine near microdialysis probes acutely implanted in the striatum of the anesthetized rat

Establishing in vivo microdialysis methods for the quantitative determination of dopamine concentrations in the extracellular space of the brain is an important yet challenging objective. The source of the challenge is the difficulty in directly measuring the microdialysis recovery of dopamine during an in vivo experiment. The recovery value is needed for quantitative microdialysis, regardless of whether conventional or no-net-flux methods are used. Numerical models of microdialysis that incorporate both diffusion and active transport processes suggest that dopamine recovery is strongly affected by processes occurring in the tissue closest to the probe. Some evidence suggests that the tissue adjacent to the probe becomes disrupted during probe implantation. Hence, the objective of the present study was to further identify whether the tissue adjacent to the probe is disrupted and, if so, whether that disruption might affect dopamine recovery. The experiments were conducted with microdialysis probes implanted acutely in the striatum of rats anesthetized with chloral hydrate. Carbon fiber voltammetric microelectrodes were used to monitor extracellular dopamine at three sites near the probes; immediately adjacent to the probe, 220-250 μm from the probe, and 1 mm from the probe. Probes were lowered slowly over a 30 min period, so that dialysate dopamine levels were stable, in the low nanomolar range, and partially TTX-sensitive by the time experiments began. Starting 2 h after probe implantation, dopamine was monitored by fast-scan cyclic voltammetry during electrical stimulation of the medial forebrain bundle and during administration of the dopamine uptake inhibitor, nomifensine. The findings of this study show that a gradient of dopamine release and uptake activity extends at least 220 μm from microdialysis probes implanted acutely in the striatum of the anesthetized rat.