SWEET TASTE TRANSDUCTION IN HAMSTER - SWEETENERS AND CYCLIC-NUCLEOTIDES DEPOLARIZE TASTE CELLS BY REDUCING A K+ CURRENT

1. The gigaseal voltage-clamp technique was used to record responses o f hamster taste receptor cells to synthetic sweeteners and cyclic nucl eotides. Voltage-dependent currents and steady-state currents were mon itored during bath exchanges of saccharin, two high-potency sweeteners , 8-chlorophenylthio-adenosine 3',5'-cyclic monophosphate (8cpt-cAMP), and dibutyryl-guanosine 3',5'-cyclic monophosphate (db-cGMP). 2. Of t he 237 fungiform taste cells studied, only one in eight was sweet resp onsive. Outward currents, both voltage-dependent and resting, were red uced by all of the sweeteners tested in sweet-responsive taste cells, whereas these currents were unaffected by sweeteners in sweet-unrespon sive taste cells. 3. In every sweet-responsive cell tested, 8cpt-cAMP and db-cGMP mimicked the response to the sweeteners, but neither nucle otide elicited responses in sweet-unresponsive cells. Thus there was a one-to-one correlation between sweet responsivity and cyclic nucleoti de responsivity. 4. Sweet responses showed cross adaptation with cycli c nucleotide responses. This indicates that the same ion channel is mo dulated by sweeteners and cyclic nucleotides. 5. The sweetener- and cy clic nucleotide-blocked current had an apparent reversal potential of -50 mV, which was dose to the potassium reversal potential in these ex periments. In addition, there was no effect of sweeteners and cyclic n ucleotides in the presence of the K+ channel blocker tetraethylammoniu m bromide (TEA). These data suggest that block of a resting, TEA-sensi tive K+ current is the final common step Lading to taste cell depolari zation during sweet transduction. 6. These data, together with data fr om a previous study (Cummings et al. 1993), suggest that both syntheti c sweeteners and sucrose utilize second-messenger pathways that block a resting K+ conductance to depolarize the taste cell membrane.