Apical membrane currents were recorded from the taste pore of single taste buds maintained in the tongue of the rat, using a novel approach. Under a dissection microscope, the 150-mu-m opening of a saline-filled glass pipette was positioned onto single fungiform papillae, while the mucosal surface outside the pipette was kept dry. Electrical responses of receptor cells to chemical stimuli, delivered from the pipette, were recorded through the pipette while the cells remained undamaged in their natural environment. We observed monophasic transient currents of 10-msec duration and 10-100 pA amplitude, apparently driven by action potentials arising spontaneously in the receptor cells. When perfusing the pipette with a solution of increased Na but unchanged Cl concentration, a stationary inward current (from pipette to taste cell) of 50-900 pA developed and the collective spike rate of the receptor cells increased. At a mucosal Na concentration of 250 mM, the maximal collective spike rate of a bud was in the range of 6-10 sec-1. In a phasic/tonic response, the high initial rate was followed by an adaptive decrease to 0.5-2 sec-1. Buds of pure phasic response were also observed. Amiloride (30-mu-M) present in the pipette solution reversibly and completely blocked the increase in spike rate induced by mucosal Na. Amiloride also decreased reversibly the stationary current which depended on the presence of mucosal Na (inhibition constant near 1-mu-M). During washout of amiloride, spike amplitudes were first small, then increased, but always remained smaller than the amiloride-blockable stationary current of the bud. This is understandable since the stationary current of a bud arises from a multitude of taste cells, while each current spike is presumably generated by just one taste cell. We suggest that, in a Na-sensitive receptor cell, (i) the apical amiloride-blockable Na inward current serves as a generator current causing cell depolarization and firing of action potentials, and (ii) each current spike recorded from the taste pore arises mainly from a modulation of the apical Na inward current of this cell, because the action potential generated by the taste cell will transiently decrease or abolish the driving force for the apical Na inward current. The transients are indicators of receptor cell action potentials, which appear to be physiological responses of taste cells in situ.
