Proceedings of The Physiological Society
University of Cambridge (2008) Proc Physiol Soc 11, PC102
Serotonin-specific reuptake inhibitors (SSRI) blunt bitter taste acutely (minutes), but enhance it chronically (hours) in normal healthy humans.
S. O'Driscoll1, E. McRobie1, C. Ayres1, N. Mileusnic1, T. P. Heath1, J. K. Melichar1, L. F. Donaldson1
1. Physiology and Pharmacology, University of Bristol, Bristol, United Kingdom.
Serotonin is postulated to act as either a transmitter between taste cells and gustatory neurones, and/or as an intercellular signal, modulating taste transmission within the taste bud itself (1). Systemic SSRIs affect human taste thresholds, lowering bitter and sweet recognition thresholds while not affecting salt thresholds (2) 2 hours after administration. Bitter and salt recognition thresholds were determined in up to 26 healthy volunteers (age range 19-47;13 male,13 female) at the tip of the tongue at each of four experimental sessions. Different concentrations of bitter (quinine) and salt (NaCl) solutions were presented to each subject in a pseudorandom order, using cotton buds soaked in each solution. Each taste concentration was presented a minimum of 5 times before and after drug or placebo in each of two experiments (double-blind). Psychophysical taste functions were constructed to calculate bitter and salt taste threshold for each volunteer and intervention. Expt. 1 (n=21) Taste thresholds determined before and 15 minutes after a 5 minute application of either SSRI (Seroxat™ elixir, 2mg/ml) or placebo (proprietary cough mixture) application to the lingual epithelium. Both drugs were citrus flavoured. Expt. 2 Taste thresholds determined before, 30 minutes (n=11) and 2 hours (n=26) after systemic paroxetine (20mg) or inactive placebo. Data shown are mean ±SEM. Expt 1. Lingually applied SSRI resulted in an increase in bitter recognition threshold (threshold after - threshold before) of 54±34%, which was significantly greater than the change after placebo (-39±26%; p=0.03 paired t test). The net effect of SSRI (ΔSSRI-Δplacebo) was to increase thresholds by 27±18µM. The changes in salt threshold after lingual SSRI were not significantly different from those after placebo (-44±25% SSRI, -54±23% placebo; p=0.8 paired t test). Expt 2. Systemic SSRI tended to increase bitter thresholds at 30 minutes (ΔSSRI-Δplacebo, +17±29%). This small increase in threshold at 30 minutes was significantly different from the decrease in bitter threshold at 2 hours reported previously (-32±22%, p=0.02, Wilcoxon)(2). There was no significant effect of systemic SSRI on salt thresholds at any time. These data suggest that acute inhibition of 5-HT reuptake at the taste bud blunts bitter taste (increases thresholds) whereas chronic inhibition (2 hours) enhances bitter taste (decreases thresholds). This difference may represent a difference in the site of action of 5-HT (taste bud or CNS), or a temporal effect of acute (minutes) versus chronic (hours) reuptake inhibition.
Where applicable, experiments conform with Society ethical requirements