Physiologically Relevant Changes in Serotonin Resolved by Fast Microdialysis

Abstract

Online microdialysis is a sampling and detection method that enables continuous interrogation of extracellular molecules in freely moving subjects under behaviorally relevant conditions. A majority of recent publications using brain microdialysis in rodents report sample collection times of 20−30 min. These long sampling times are due, in part, to limitations in the detection sensitivity of high performance liquid chromatography (HPLC). By optimizing separation and detection conditions, we decreased the retention time of serotonin to 2.5 min and the detection threshold to 0.8 fmol. Sampling times were consequently reduced from 20 to 3 min per sample for online detection of serotonin (and dopamine) in brain dialysates using a commercial HPLC system. We developed a strategy to collect and to analyze dialysate samples continuously from two animals in tandem using the same instrument. Improvements in temporal resolution enabled elucidation of rapid changes in extracellular serotonin levels associated with mild stress and circadian rhythms. These dynamics would be difficult or impossible to differentiate using conventional microdialysis sampling rates.

Perinatal vs Genetic Programming of Serotonin States Associated with Anxiety

“…Somatodendritic 5-HT 1A receptors control serotonin release, and the sensitivity/function of these receptors is reciprocally regulated by extracellular serotonin levels . No-net-flux microdialysis was used to quantify extracellular serotonin in the ventral hippocampus in the two models of SERT disruption Yang et al, 2013). We found significant reductions in extracellular serotonin concentrations 10-14 months after postnatal exposure to ESC compared with saline-or FLX-exposed mice (Po0.05; Figure 5a and c; Supplementary Table S8).…”

“…Male and female mice treated postnatally with SSRIs and male SERT-deficient mice were used in experiments to determine the levels of extracellular serotonin in the ventral hippocampus. No-net-flux protocols were similar to previously published methods (Luellen et al, 2007;Mathews et al, 2004;Szapacs et al, 2004;Yang et al, 2013) with the following modifications. At 0400-0500 hours on the day following probe insertion, the flow rate of aCSF was increased to 3 ml/min at least 30 min prior to dialysate sampling.…”

“…Interpretations of behavioral similarities led to the idea that the effects of SLC6A4 gene polymorphisms might be limited to sensitive developmental periods (Homberg et al, 2010;Sibille and Lewis, 2006). Although attractive, the notion that brief early-life SERT inhibition recapitulates life-long constitutive SERT deficiency seems unlikely, because it neglects to account for the ongoing effects of genetic reductions in SERT on serotonin neurotransmission Yang et al, 2013). We alternately theorized that these two modes of SERT disruption lead to divergent neurochemical endophenotypes (hyperserotonergic vs hyposerotonergic; Figure 1) that produce opposing changes in anxiety-related behavior.…”

Serotonin States and Social Anxiety

“…7 These behaviors in rodents have strong correlations with aspects of human anxiety and depression. Genetic reductions in SERT drive neuroadaptive alterations in the serotonin system, including increased extracellular serotonin concentrations, 8 decreased serotonin 1A autoreceptor function, 9 and reduced intracellular (recycled) serotonin accompanied by compensatory increases in serotonin synthesis (Figure, C). Observations in rodent models support the notion of reduced serotonin availability, either through relative deficits in serotonin recycling (eg, via reduced SERT) or serotonin synthesis as critical mechanisms by which the buffering of emotional stress is affected in anxious and depressed states.…”

“…Mice having a primary defect in SERT expression similarly exhibit increased extracellular serotonin levels; however, in this case, increased extracellular serotonin levels were derived directly from a lack of reuptake. 8 Unfortunately, there are as yet no direct methods of assessing extracellular serotonin signaling in humans that might be useful for further investigating the hypothesis by Frick et al…”