Understanding changes in uptake and release of serotonin from gastrointestinal tissue using a novel electroanalytical approach

Abstract

Serotonin (5-HT) is well known to be a key neurotransmitter within the gastrointestinal (GI) tract, where it is responsible for influencing motility. Obtaining dynamic information about the neurotransmission process (specifically the release and reuptake of 5-HT) requires the development of new approaches to measure the extracellular 5-HT concentration profile. In this work constant-potential amperometry has been utilised at +650 mV vs. Ag|AgCl to measure in vitro the overflow of 5-HT. Steady-state levels of 5-HT have been observed, due to continuous mechanical stimulation of the tissue from the experimental protocol. Measurements are conducted at varying tissue–electrode distances in the range of 5 to 1100 µm. The difference in the current from the bulk media and that from each tissue–electrode distance is obtained, and the natural log of this current is plotted versus the tissue–electrode distance. The linear fit to the log of the current is derived, and its intercept, I₀, with the vertical axis and its slope are calculated. The reciprocal of the slope, indicated as slope⁻¹, is used as a marker of reuptake. The ratio between intercept, I₀, and the reciprocal of the slope, I₀/slope⁻¹, is a measure of the flux at the tissue surface and it can be used as a marker for the 5-HT release rate. Current measurements for ileum and colon tissue indicated a significantly higher reuptake rate in the colon, showed by a lower slope⁻¹. In addition, the ratio, I₀/slope⁻¹, indicated that the colon has a higher 5-HT flux compared to the ileum. Following the application of the serotonin selective reuptake inhibitor (SSRI), fluoxetine, both tissues showed a higher value of slope⁻¹, as the reuptake process is blocked preventing clearance of 5-HT. No differences were observed in the ratio, I₀/slope⁻¹, in the ileum, but a decrease was observed in the colon. These results indicate that ileum and colon are characterised by different reuptake and release processes. The new approach we propose provides pivotal information on the variations in the signalling mechanism, where steady state levels are observed and can be a vital tool to study differences between normal and diseased tissue and also the efficacy of pharmacological agents.