Endogenous Release of Neuronal Serotonin and 5‐Hydroxyindoleacetic Acid in the Caudate‐Putamen of the Rat as Revealed by Intracerebral Dialysis Coupled to High‐Performance Liquid Chromatography with Fluorimetric Detection

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Abstract
Extracellular levels of endogenous serotonin (5-HT) and its major metabolite, 5-hydroxyindoleacetic acid (5-HIAA), were measured in the caudate-putamen of anesthetized and awake rats using intracerebral microdialysis coupled to HPLC with fluorimetric detection. A dialysis probe (of the loop type) was perfused with Ringer solution at 2 μl/min, and samples collected every 30 or 60 min. Basal indole levels were followed for up to 4 days in both intact and 5,7-dihydroxytryptamine (5,7-DHT) lesioned animals. Immediately after the probe implantation, the striatal 5-HT levels were about 10 times higher than the steady-state levels that were reached after 7-8 h of perfusion. The steady-state baseline levels, which amounted to 22.5 fmol/30 min sampling time, remained stable for 4 days. In 5,7-DHT-denervated animals, the steady-state levels of 5-HT, measured during the second day after probe implantation, were below the limit of detection (M) added to the perfusion fluid produced a sharp increase in 5-HT output that was eight-fold at the 60 mM concentration and 21-fold at the 100 mM concentration. In contrast, 5-HIAA output dropped by 43 and 54%, respectively. In 5,7-DHT-lesioned animals, the KCl-evoked (100 mM) release represented less than 5% of the peak values obtained for the intact striata. Omission of Ca2+ from the perfusion fluid resulted in a 70% reduction in baseline 5-HT output, whereas the 5-HIAA levels remained unchanged. High concentrations of tetrodotoxin (TTX) added to the perfusion medium (5-50 μM) resulted in quite variable results. At a lower concentration (1 μM), however, TTX produceda 50% reduction in baseline 5-HT release, whereas the 5-HIAA output remained unchanged. The 5-HT reuptake blocker, indalpine, increased the extracellular levels of 5-HT sixfold when added to the perfusion medium (1 μM), and threefold when given intraperitoneally (5 mg/kg). By contrast, the 5-HIAA level remained unaffected during indalpine infusion. Application of TTX (1 μM) under simultaneous 5-HT uptake blockade induced a decrease in 5-HT output by 62–71%. p-Chloroamphetamine (2.5 mg/kg, i.p.) induced a 12-fold increase in 5-HT release and reduced the 5-HIAA output by about 50%. The p-chloroamphetamine-induced increase in 5-HT release was 10 times lower in the 5,7-DHT-denervated striatum. Pargyline (75 mg/kg, i.p.) increased the extracellular levels of 5-HT 11-fold within 6 h, and reduced the 5-HIAA levels by 80%. The 5-HT receptor agonist, 5-methoxy-N,N-dimethyltryptamine (1 mg/kg, i.p.), produced an immediate reduction of about 50% in 5-HT release and a small (11 %) decrease in 5-HIAA output. It is concluded (1) that intracerebral microdialysis coupled to HPLC with fluorimetric detection provides a useful method for the study of extracellular 5-HT and 5-HIAA levels; (2) that steady-state levels of 5-HT and 5-HIAA recovered in the dialysis perfusate are neuronally derived, but these steady-state levels are reached only after a minimum of 7–8 h after probe implantation; (3) that changes in striatal extracellular levels of 5-HT are closely related to changes in serotonergic synaptic activity; and (4) that extracellular levels of 5-HIAA are a poor indicator of synaptic activity, and instead primarily reflect intraneuronal metabolism.