High-Speed Chronoamperometry to Study Kinetics and Mechanisms for Serotonin Clearance In Vivo

Excerpt

Serotonin (5-HT) regulates many complex behaviors and physiological functions. These include mood, sleep, feeding and thermoregulation, to name but a few. As a result, dysregulation of 5-HT neurotransmission can have severe consequences. For example, reduced serotonin neurotransmission is thought to underlie such affective disorders as depression and anxiety (Owens and Nemeroff 1994; Nemeroff and Owens 2004) and predispose to addictive disorders such as alcoholism (McBride et al. 1993; Virkkunen and Linnoila 1997). In addition, the hyperserotonergic state resulting from high doses of substituted amphetamines such as 3,4- methylenedioxymethamphetamine (MDMA, “Ecstasy”) contributes to the “serotonin syndrome,” symptoms of which include agitation, high body temperature, tachycardia, convulsions, coma, and in some cases death (Hegadoren et al. 1999; Green et al. 2003). Given the clear importance of maintaining homeostatic 5-HT neurotransmission, the regulation of extracellular concentration of 5-HT has been the focus of intense study for the past several decades.

The principle mechanism for terminating the neurochemical actions of 5-HT released from nerve terminals is its high affinity uptake from extracellular fluid by the serotonin transporter (5-HTT; SERT). Consequently, since this transporter was first identified in the brain (Shaskan and Snyder 1970; Kuhar et al. 1972), it has been studied extensively (Fuller and Wong 1990; Blakely et al. 1991; Rudnick and Wall 1993; Bengel et al. 1998; Blakely et al. 1998; Kilic et al. 2003). Until relatively recently, active uptake of 5-HT by its transporter has been studied using in vitro techniques such as [³H]5-HT uptake into brain synaptosomes (Shaskan and Snyder 1970; Kokoshka et al. 1998) or into cells transfected with the 5-HTT (Wall et al. 1995; Ramamoorthy and Blakely 1999; Zhu et al. 2005). However, modern voltammetric technologies that stemmed from the pioneering work of Ralph Adams in the 1970s (Kissinger et al. 1973) have allowed 5-HTT function to be measured in vivo. Herein we describe applications of high-speed chronoamperometry, an amperometric technique, to study 5-HTT function in vivo as well as to identify alternative mechanisms for 5-HT clearance from extracellular fluid.