Modulation of serotonin dynamics in the dorsal raphe nucleus via high frequency medial prefrontal cortex stimulation.

The subcallosal cingulate (SCC) region, or its rodent homologue the medial prefrontal cortex (mPFC), and midbrain dorsal raphe (DR) are crucial nodes of the widespread network implicated in emotional regulation. Stimulation of the SCC is being explored as a potential treatment for depression. Because modulation of the 5-HT system is the most common pharmacological means of treating depression, we sought to establish 5-HT's role in the mPFC-DR projection. Using anaesthetized mice, we recorded neuronal activity in 49 neurons of the DR before, during, and after high frequency stimulation (HFS) of the mPFC. The majority of DR cells (74%) significantly decreased firing rate during HFS (p<0.001, 65.7±9.4% of baseline, 14 mice). To see the effect of mPFC-HFS on 5-HT neurons, we used transgenic mice with expression of the channelrhodopsin fusion protein directed to the 5-HT neuronal population. Neurons were categorized as 5-HT based on their excitatory response to blue light stimulation (p<0.05, n=11). Our main finding was that identified 5-HT neurons in the DR were clearly inhibited by HFS, albeit non-selectively. Lastly, we used fast scan cyclic voltammetry (FSCV) to investigate the effects of mPFC-HFS on the release and reuptake of electrically stimulated 5-HT in the DR of C57BL/6J mice. Serotonin clearance was significantly faster following 5min HFS (2.3±1.0s, n=5, p<0.05) when compared to control levels (3.7±1.0s, n=5), indicating less release or more efficient 5-HT reuptake. Taken together, these findings imply that mPFC stimulation alters 5-HT activity dynamics in the DR. Such altered 5-HT dynamics may modulate the potential therapeutic mechanisms of SCC/mPFC stimulation.

In vivo histamine voltammetry in the mouse premammillary nucleus.

Histamine plays a major role in the mediation of allergic reactions such as peripheral inflammation. This classical monoamine is also a neurotransmitter involved in the central nervous system but its role in this context is poorly understood. Studying histamine neurotransmission is important due to its implications in many neurological disorders. The sensitivity, selectivity and high temporal resolution of fast scan cyclic voltammetry (FSCV) offer many advantages for studying electroactive neurotransmitters. Histamine has previously been studied with FSCV; however, the lack of a robust Faradaic electrochemical signal makes it difficult to selectively identify histamine in complex media, as found in vivo. In this work, we optimize an electrochemical waveform that provides a stimulation-locked and unique electrochemical signal towards histamine. We describe in vitro waveform optimization and a novel in vivo physiological model for stimulating histamine release in the mouse premammillary nucleus via stimulation of the medial forebrain bundle. We demonstrate that a robust signal can be used to effectively identify histamine and characterize its in vivo kinetics.

Voltammetric and mathematical evidence for dual transport mediation of serotonin clearance in vivo.

The neurotransmitter serotonin underlies many of the brain's functions. Understanding serotonin neurochemistry is important for improving treatments for neuropsychiatric disorders such as depression. Antidepressants commonly target serotonin clearance via serotonin transporters and have variable clinical effects. Adjunctive therapies, targeting other systems including serotonin autoreceptors, also vary clinically and carry adverse consequences. Fast scan cyclic voltammetry is particularly well suited for studying antidepressant effects on serotonin clearance and autoreceptors by providing real-time chemical information on serotonin kinetics in vivo. However, the complex nature of in vivo serotonin responses makes it difficult to interpret experimental data with established kinetic models. Here, we electrically stimulated the mouse medial forebrain bundle to provoke and detect terminal serotonin in the substantia nigra reticulata. In response to medial forebrain bundle stimulation we found three dynamically distinct serotonin signals. To interpret these signals we developed a computational model that supports two independent serotonin reuptake mechanisms (high affinity, low efficiency reuptake mechanism, and low affinity, high efficiency reuptake system) and bolsters an important inhibitory role for the serotonin autoreceptors. Our data and analysis, afforded by the powerful combination of voltammetric and theoretical methods, gives new understanding of the chemical heterogeneity of serotonin dynamics in the brain. This diverse serotonergic matrix likely contributes to clinical variability of antidepressants.