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

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 microliters/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 (less than 10 fmol/60 min). However, during the first 6 h post-implantation, the 5-HT output was as high as in intact animals, which suggests that the high 5-HT levels recovered in association with probe implantation were blood-derived. As a consequence, all other experiments were started after a delay of at least 12 h after implantation of the dialysis probe. In awake, freely moving animals, the steady-state 5-HT levels were about 60% higher than in halothane-anesthetized animals, whereas 5-HIAA was unaffected by anesthesia. KCl (60 and 100 mM) 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 microM) resulted in quite variable results. At a lower concentration (1 microM), however, TTX produced a 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 microM), and threefold when given intraperitoneally (5 mg/kg). By contrast, the 5-HIAA level remained unaffected during indalpine infusion.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of p‐chlorophenylalanine on release of 5‐hydroxytryptamine from the rat frontal cortex in vivo

“…Increased firing seems reasonable as high cortical dialysate 5-HT and low 5-HIAA values also occur on both handling (Kalen et al, 1989) and stimulation with K+ ( Figure 2). The latter finding agrees with previous dialysis studies (Kalen et al, 1988;Auerbach et al, 1989;Carboni & DiChiara, 1989). It is relevant that transport of 5-HIAA from neurones has been suggested to be impaired during the depolarized state which occurs when 5-HT is actively released (Miyamoto et al, 1990).…”

“…Recent investigations on 5-hydroxytryptamine (5-HT) by Auerbach et al (1989), Kalen et al (1988) and Carboni & DiChiara (1989) show that various drug treatments influence dialysate 5-HT levels in the expected direction. Other studies reveal that levels rise during motor activity or handling (Kalen et al, 1989).…”

Serotonergic vulnerability and depression: assumptions, experimental evidence and implications

“…While TRP availability to the brain influences 5-HT synthesis, 350,351 extracellular 5-HT levels reflect the neuronal activity of release and reuptake of the neurotransmitter. 355 LAC ingestion not only affects 5-HT synthesis, but also its release. Long-term LAC diet was reported to elevate extracellular 5-HT and may induce beneficial effects on mood.…”

Simultaneous Increases of Extracellular Monoamines in Microdialysates from Hypothalamus of Conscious Rats by Duloxetine, a Dual Serotonin and Norepinephrine Uptake Inhibitor

“…Fluoxetine treatment consistently increased 5-HT levels in most other brain areas: Nearly fourfold in striatum (Perry and Fuller 1992), 2.5-fold in thalamus (Dailey et al 1992), and twofold in diencephalon (Rutter and Auerbach 1993). Among other selective 5-HT uptake inhibitors, indalpine increased 5-HT levels threefold in caudate-putamen (Kalen et al 1988). Sertraline (Invernizzi et al 1991), citalopram (Invernizzi et al 1992, or clomipramine (Adell and Artigas 1991) produced fi.ve-, four-, and threefold increases, respectively, in the rap he areas.…”

“…Applications of in vivo techniques (including rnicrodialysis) have demonstrated that extracellular serotonin in most rat brain areas increases several fold over basal levels upon systemic administration of serotonin uptake inhibitors, including fluoxetine (Auerbach et al 1989;Dailey et al 1992;Guan and McBride 1988;Fuller 1992, 1993a;Rutter and Auerbach 1993), indalpine (Kalen et al 1988(Kalen et al ), citaloprarn (lnvernizzi et al 1992, sertraline (Invernizzi et al 1991), and clomipramine (Adell and Artigas 1991;Carboni and Di Chiara 1989). Extracellular norepinephrine (NE) concentrations are also elevated upon local or peripheral administra-tion of the NE uptake inhibitor desipramine (Dennis et al 1987;Itoh et al 1990;Kalen et al 1988;L'Heureux et al 1986;Thomas and Holman 1991).…”