Citrate ions enhance behavioral and cellular responses to taste stimuli

Behavioral and electrophysiological experiments have been performed on male Sprague-Dawley rats to evaluate the effectiveness of citrate ions as taste enhancers. In two- and four-day two bottle preference tests (vs. water), citrate (1-25 mM) significantly enhanced preference for sweet compounds and the amino acid glycine over control (non-citrate containing) solutions. Under conditions in which animals were "forced" to choose between test solutions with or without citrate, saccharin, sucrose, glycine and NaCl solutions containing citrate were significantly preferred over controls. In addition to its effects on sweet, glycine and NaCl taste, short term preference tests following water deprivation revealed that citrate modulated acid taste preference, as well. Bitter taste preference remained insensitive to citrate in all behavioral assays. As a preliminary attempt to elucidate the target of citrate's actions, electrophysiological responses of isolated fungiform taste receptor cells (TRCs) to saccharin and glycine were recorded using patch clamp techniques. Saccharin (20 mM) and glycine (50 mM) elicited action potentials from TRCs in current clamp mode. Addition of citrate caused a significant increase in number of action potentials generated per 30 s stimulation. Citrate caused a slight but significant depolarization from resting potentials in most TRCs, independent of tastant effects. Taken together, these results suggest that citrate acts at the receptor cell level to enhance only those responses to tastants which depolarize TRCs (i.e. sweet, salt, glycine, acid) while leaving unaffected those which do not depolarize TRCs (bitter).