Characterization of the extracellular serotonin release in the substantia nigra of the freely moving rat using microdialysis

The characteristics of the serotonin release were investigated in the substantia nigra (SN) of the freely moving rat using microdialysis. We also examined whether the delay between surgery and microdialysis experiments might influence these characteristics by implanting rats with a guide cannula 1 or 2 days prior to microdialysis experiments. In the first group, the tissue was not punctured until the microdialysis probe was inserted the evening before the experiment. In the second group, the nigral tissue was punctured with an extended obturator which was then replaced by a microdialysis probe the evening before the experiment. After administration of 60 mM K⁺ a more pronounced increase in serotonin was observed in the first group (260%) compared to the second group (159%). Calcium-free and tetrodotoxin (TTX, a sodium channel blocker) (1 μM) perfusion reduced extracellular serotonin to respectively 77% and 80% in the first group and 70% and 64% in the second group. These results suggest that vesicular release of nigral serotonin only occurs partially in this region and that minimizing the damage caused by implantation of the probe results only in 10% more vesicular release of serotonin. However, blockade of the serotonin reuptake carrier caused more TTX sensitivity of the serotonin release. Also, stimulation of the dorsal raphe by locally perfusing 60 mM K⁺ decreased serotonin in the SN, confirming the anatomical and functional link between both areas.