Voltammetric Detection of 5-Hydroxytryptamine Release in the Rat Brain

Abstract

Abstract5-HT is an important molecule in the brain that is implicated in mood and emotional processes. In vivo, its dynamic release and uptake kinetics are poorly understood due to a lack of analytical techniques for its rapid measurement. Whereas fast-scan cyclic voltammetry with carbon fiber microelectrodes is used frequently to monitor sub-second dopamine release in freely-moving and anesthetized rats, the electrooxidation of 5-HT forms products that quickly polymerize and irreversibly coat the carbon electrode surface. Previously described modifications of the electrochemical waveform allow stable and sensitive 5-HT measurements in mammalian tissue slice preparations and in the brain of fruit fly larvae. For in vivo applications in mammals, however, the problem of electrode deterioration persists. We identify the root of this problem to be fouling by extracellular metabolites such as 5-HIAA, which is present in 200-1000 times the concentration of 5-HT and displays similar electrochemical properties, including filming of the electrode surface. To impede access of the 5-HIAA to the electrode surface, a thin layer of Nafion®, a cation exchange polymer, has been electrodeposited onto cylindrical carbon-fiber microelectrodes. The presence of the Nafion® film was confirmed with environmental scanning electron microscopy and was demonstrated by the diminution of the voltammetric signals for 5-HIAA as well as other common anionic species. The modified microelectrodes also display increased sensitivity to 5-HT, yielding a characteristic cyclic voltammogram that is easily distinguishable from other common electroactive brain species. The thickness of the Nafion® coating and a diffusion coefficient (D) in the film for 5-HT were evaluated by measuring permeation through Nafion®. In vivo, we used physiological, anatomical and pharmacological evidence to validate the signal as 5-HT. Using Nafion®-modified microelectrodes, we present the first endogenous recording of 5-HT in the mammalian brain.

Fast-Scan Cyclic Voltammetry Analysis of Dynamic Serotonin Reponses to Acute Escitalopram

“…11,19 Specifically, serotonin was found to increase by 380% 11 and 480%. 19 Current and previous findings in the intact brain of rats and mice are not in agreement with the data from tissue slice preparations.…”

“…These cyclic voltammograms identify serotonin based on the positions of the oxidation and reduction peaks. 11 We chose to study ESCIT because it is one of the most clinically useful SSRIs. 3,20 However, other SSRIs such as fluoxetine and sertraline exhibit similar modes of action (selective SERT inhibition), and we have observed similar increases in serotonin amplitude in vivo with these two agents (unpublished observations).…”

“…36 A Nafion modified carbon fiber microelectrode was lowered into the SNr (AP: −1.58, ML: +1.40, DV: −4.2 to −4.8). The procedure for nafion electrodeposition is described in ref 11. This electrode was adjusted in the dorsal/ventral plane until a desired serotonin signal was observed.…”

Influence of different biological environments on the stability of serotonin detection on carbon-based electrodes

“…At this scan rate, no reversal peak was observed, suggestive of an irreversible oxidation mechanism occurring at both electrodes. However other researchers have shown that at higher scan rates (>100 V s -1 ), the cathodic peak (Epc) can be observed [11,15]. 5-HT undergoes an electrochemical-chemical mechanism.…”

Serotonin and Dopamine: Unifying Affective, Activational, and Decision Functions

“…For instance, in order to fully understand these neuromodulatory effects, it will be particularly important to consider their timescale (tonic or phasic). Specifically, it will be important to obtain better insights in the degree to which commonly used 5-HT manipulations affect phasic versus tonic transmission, thus highlighting the necessity of combining temporally precise methods in freely behaving animals, such as neurophysiological recording of single 5-HT and DA neurons, electrochemical voltammetric approaches (Hashemi et al, 2009), and/or optogenetics with procedures used to study the effects of 5-HT, for example, 5,7-DHT lesions, ATD, SSRI administration, and the 5-HT transporter gene-linked polymorphism (5HTTLPR).…”