Voltammetric evidence for discrete serotonin circuits, linked to specific reuptake domains, in the mouse medial prefrontal cortex

Abstract

The medial prefrontal cortex (mPFC) is an important brain region, that controls a variety of behavioral and functional outputs. As an important step in characterizing mPFC functionality, in this paper we focus on chemically defining serotonin transmission in this area. We apply cutting-edge analytical methods, fast-scan cyclic voltammetry (FSCV) and fast-scan controlled adsorption cyclic voltammetry (FSCAV), pioneered in our laboratory, for the first real-time in vivo analysis of serotonin in the mPFC. In prior in vivo work in the substantia nigra, pars reticulata, we found that our sub-second measurements of a single evoked serotonin release were subject to two clearance mechanisms. These mechanisms were readily modeled via Uptake 1, mediated by the serotonin transporters (SERTs), and Uptake 2, mediated by monoamine transporters (dopamine transporters (DATs), norepinephrine transporters (NETs), and organic cation transporters (OCTs)). Here in the mPFC, for the first time to our knowledge, we observe two release events in response to a single stimulation of the medial forebrain bundle (MFB). Of particular note is that each response is tied to a discrete reuptake profile comprising both Uptake 1 and 2. We hypothesize that two distinct populations of serotonin axons traverse the MFB and terminate in different domains with specific reuptake profiles. We test and confirm this hypothesis using a multifaceted pharmacological, histological and mathematical approach. We thus present evidence for a highly elaborate biochemical organization that regulates serotonin chemistry in the mPFC. This knowledge provides a solid foundation on which to base future studies of the involvement of the mPFC in brain function and behavior.

Fast serotonin voltammetry as a versatile tool for mapping dynamic tissue architecture: I. Responses at carbon fibers describe local tissue physiology

“…In contrast to the CA2 and SNr where a single serotonin release and uptake event is evoked upon stimulation, in the mPFC we often also observe a dual peak, the origins of which we've recently described (West et al ). Two representative examples of mPFC signals are illustrated in Fig.…”

“…Here, we analyze the experimental curves with previously established models and allow the model to generate an independent hypothesis to explain the experimental data. The model utilizes the Uptake 1, Uptake 2, autoreceptor, and two domain theories to determine the best parameters for fitting the experimental data presented here (Wood et al ; West et al ). Figure shows the mean experimental curves for the CA2, mPFC, and the SNr (solid are experimental and dotted are models).…”

“…For details and discussion, see Wood et al (). To model the double peak in the mPFC we use the extended model presented in West et al () that allows diffusion of serotonin from regions with only Uptake 1 to regions that have both Uptake 1 and Uptake 2.…”

“…It is very important to address the chemical functionality associated with such biophysical changes. We have performed pilot studies in these and other brain areas (Srejic et al ; Abdalla et al ; West et al ), here we present a formal characterization of the voltammetry stimulation and measurement circuits for serotonin in the CA2 region of the hippocampus and the mPFC, compared with the SNr.…”

“…Comparison of the evoked and ambient serotonin chemistry in these three regions reveals significant differences which we postulate to arise from differences in tissue physiology local to the CFM, specifically in the characteristics of serotonin reuptake via different monoamine transporters. The chemical signals are modeled mathematically according to our previously established models of serotonin transmission which accounts for Uptake 1 [Serotonin transporter (SERT) mediated] and Uptake 2 [Dopamine transporter (DAT), norepinephrine transporter (NET), and organic cation transporter (OCT) mediated] (Wood et al ) and single versus dual‐evoked events (West et al ). Our modeling also, independently, hypothesizes differences in how serotonin is reuptaken via Uptake 1 and 2, while bringing forth additional hypotheses about the strength of input to the three locales and autoreceptor contribution.…”