In vivo electrochemical evidence for simultaneous 5‐HT and histamine release in the rat substantia nigra pars reticulata following medial forebrain bundle stimulation

Abstract

Exploring the mechanisms of serotonin (5-hydoxytryptophan (5-HT)) in the brain requires an in vivo method that combines fast temporal resolution with chemical selectivity. Fast-scan cyclic voltammetry (FSCV) is a technique with sufficient temporal and chemical resolution for probing dynamic 5-HT neurotransmission events; however, traditionally it has not been possible to probe in vivo 5-HT mechanisms. Recently, we optimized FSCV for measuring 5-HT release and uptake in vivo in the substantia nigra pars reticulata (SNR) with electrical stimulation of the dorsal raphe nucleus (DRN) in the rat brain. Here, we address technical challenges associated with rat DRN surgery by electrically stimulating 5-HT projections in the medial forebrain bundle (MFB), a more accessible anatomical location. MFB stimulation elicits 5-HT in the SNR; furthermore, we find simultaneous release of an additional species. We use electrochemical and pharmacological methods and describe physiological, anatomical and independent chemical analyses to identify this species as histamine. We also show pharmacologically that increasing the lifetime of extracellular histamine significantly decreases 5-HT release, most likely due to increased activation of histamine H-3 receptors that inhibit 5-HT release. Despite this, under physiological conditions, we find by kinetic comparisons of DRN and MFB stimulations that the simultaneous release of histamine does not interfere with the quantitative 5-HT concentration profile. We therefore present a novel and robust electrical stimulation of the MFB that is technically less challenging than DRN stimulation to study 5-HT and histamine release in the SNR.

Brain dopamine and serotonin differ in regulation and its consequences

“…Distinct storage and releasable pools for both 5-HT and dopamine have been described (31,32), but the large difference in their relative size had not previously been appreciated. Moreover, we previously noted that evoked release of 5-HT in the SNr is much lower in vivo compared with its electrically evoked release from SNr slice preparations (14). This suggests that in vivo 5-HT release is subject to tighter control mechanisms than both 5-HT in slices and dopamine in vivo.…”

“…Varying the dorsoventral location of the stimulating electrode revealed that 5-HT release can be evoked over greater region than dopamine, suggesting that serotonergic fibers have a broader topographic distribution. However, this is not sufficient to explain the 300-fold greater release of dopamine because even direct stimulation of serotonergic cell bodies in the dorsal raphe evokes comparably low 5-HT release (14).…”

“…1073/pnas.1201547109/-/DCSupplemental. uptake in the NAc (k = V max /K m ) is ∼14 s −1 (16), and in the SNr, it is ∼4 s −1 for 5-HT uptake (14). We first examined dopamine and 5-HT release as a function of the stimulating electrode's dorsoventral position.…”

Medial Forebrain Bundle Deep Brain Stimulation has Symptom-specific Anti-depressant Effects in Rats and as Opposed to Ventromedial Prefrontal Cortex Stimulation Interacts With the Reward System

“…As dopamine levels in the NAcc are considered important for reward-processing and motivation, and since FSL rats exhibit an abnormal serotonergic system [35,82], it was proposed that impairment in the serotonindopamine interactions could be related to the behavioral abnormalities of FSL rats [33,81]. As the rewarding effect of MFB stimulation has repeatedly been associated with activation of the serotonergic system [63,[91][92][93] this leads us to speculate that the effect of MFB-DBS in combination with haloperidol might be due to serotonergic-dopaminergic interaction. This could also explain why this effect is only seen in control but not in FSL rats.…”

A Comparison of the Subsecond Dynamics of Neurotransmission of Dopamine and Serotonin

“…14,21,22 In addition to fouling problems, the pattern of brain 5-HT innervation has also contributed a challenge to in vivo FSCV measurement. While FSCV allows identification of individual monoamines, it does not allow the separate quantification of 5-HT and DA when they are simultaneously present at the electrode.…”