Characterization of the in vivo release of dopamine as recorded by different types of intracerebral microdialysis probes

Abstract

Microdialysis of dopamine was performed in the striatum with four different microdialysis cannulas: a trans-cerebral probe, a U-shaped probe, a I-shaped probe and a commercially available I-shaped probe (Carnegie). The commercial cannula was studied with as well as without a guide cannula. The effect of infusion of tetrodotoxin (TTX) or calcium free-Ringer solution on the dialysate content of dopamine released was determined 2 h after implantation (day 1) as well as 24 h after implantation (day 2). Two hours after implantation, all cannulas displayed a certain amount of TTX-independent overflow of dopamine. The best results were obtained with the I-shaped cannula: already 2 h after implantation of the probe, about 85% of the release of dopamine was TTX-dependent. In case of the trans-cerebral cannula and the Carnegie cannula (implanted without guide in anesthetized rats), 70-75% of the output of dopamine was TTX-dependent. When the Carnegie cannula was implanted with a guide cannula, only 45% of the dopamine output was TTX-dependent. The responses to calcium-free perfusion were much more variable. Dopamine output in I-shaped and the U-shaped probes was virtually insensitive to calcium-free perfusion in acute experiments. Twenty-four hours after implantation of the probes, the calcium- and TTX-dependency was much more pronounced. All types of cannulas studied now sampled dopamine that was completely TTX-dependent. Calcium-free perfusion caused a reproducible disappearance of dopamine from the dialysates to 30% of controls, in all cannulas studied. When the removable Carnegie cannula was re-implanted 24 h after its first implantation, 91% of the dopamine overflow was TTX-sensitive. These results confirm earlier conclusions that microdialysis experiments should be carried out at least 24 h after implantation of the probe.