Measurement of extracellular 5-HT and 5-HIAA in hippocampus of freely moving rats using microdialysis: long-term...

Extracellular levels of 5-hydroxytryptamine (5-HT) and its metabolite 5-hydroxyindoleacetic acid (5-HIAA) were measured in the ventral hippocampus of the awake rat using microdialysis. The basal level of 5-HT in hippocampal dialysates was very close to the detection limit of our assay. However, addition of a 5-HT re-uptake blocker, citalopram, to the perfusion medium resulted in a 3-fold rise of 5-HT levels in dialysates and provided a stable baseline. Under these conditions, extracellular levels of 5-HT and 5-HIAA could be measured up to 11 days after dialysis probe implantation. 5-HT measured in dialysate was Ca(2+)-dependent both 24 h and 7 days after surgery. In comparison K(+)-induced depolarization caused a 9-fold increase in 5-HT output 24 h after probe implantation and this effect had disappeared 10 days later, although behavioral activation following K(+) application was similar 24 h and 11 days after surgery. Systematic administration of l-trytophan had no significant effect on 5-HT but increased 5-HIAA levels by 90%. These results suggest that in the presence of citalopram, 5-HT in hippocampal dialysates is derived from serotonergic neurones. The microdialysis method appears most useful for measurement of extracellular 5-HT 24-72 h after probe implantation. Finally, our data indicate that extracellular 5-HIAA mainly reflects intraneuronal metabolism of 5-HT prior to release.