It is known that a subset of taste cells generate action potentials in response to taste stimuli. However, responsiveness of these cells to particular tastants remains unknown. In the present study, by using a newly developed extracellular recording technique, we recorded action potentials from the basolateral membrane of single receptor cells in response to taste stimuli applied apically to taste buds isolated from mouse fungiform papillae. By this method, we examined taste-cell responses to stimuli representing the four basic taste qualities (NaCl, Na saccharin, HCl, and quinine-HCl). Of 72 cells responding to taste stimuli, 48 (67%) responded to one, 22 (30%) to two, and 2 (3%) to three of four taste stimuli. The entropy value presenting the breadth of responsiveness was 0.158 +/- 0.234 (mean +/- SD), which was close to that for the nerve fibers (0.183 +/- 0.262). In addition, the proportion of taste cells predominantly sensitive to each of the four taste stimuli, and the grouping of taste cells based on hierarchical cluster analysis, were comparable with those of chorda tympani (CT) fibers. The occurrence of each class of taste cells with different taste responsiveness to the four taste stimuli was not significantly different from that of CT fibers except for classes with broad taste responsiveness. These results suggest that information derived from taste cells generating action potentials may provide the major component of taste information that is transmitted to gustatory nerve fibers.

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Electrical properties of isolated frog primary afferent neurons were examined by suction pipette technique, which combines internal perfusion with current or voltage clamp using a switching circuit with a single electrode. When K+ in the external and internal solutions was totally replaced with Cs+, extremely prolonged Ca spikes, lasting for 5 to 10 sec, and Na spikes, having a short plateau phase of 10 to 15 msec, were observed in Na+-free and Ca2+-free solutions, respectively. Under voltage clamp, Ca2+ current (ICa) appeared at around -30 mV and maximum peak current was elicited at about 0 mV. With increasing test pulses to the positive side, ICa became smaller and flattened but did not reverse. Increases of [Ca]o induced a hyperbolic increase of ICa and also shifted its I-V curve along the voltage axis to the more positive direction. Internal perfusion of F- blocked ICa time-dependently. The Ca channel was permeable to foreign divalent cations in the sequence of ICa greater than IBa greater than ISr much greater than IMn greater than IZn. Organic Ca-blockers equally depressed the divalent cation currents dose- and time-dependently without shifting the I-V relationships, while inorganic blockers suppressed these currents dose-dependently and the inhibition appeared much stronger in the order of IBa = ISr greater than ICa greater than IMn = IZn.

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The electrophysiological properties of mossy cells were examined in developing mouse hippocampal slices using whole-cell patch-clamp techniques, with particular reference to the dorsoventral difference. Dorsal mossy cells exhibited a higher spontaneous excitatory postsynaptic potential (EPSP) frequency and larger maximal EPSP amplitude than ventral mossy cells. On the other hand, the blockade of synaptic inputs with glutamatergic and GABAergic antagonists disclosed a remarkable dorsoventral difference in the intrinsic activity: none (0/27) of the dorsal mossy cells showed intrinsic bursting, whereas the majority (35/47) of the ventral mossy cells exhibited intrinsic rhythmic bursting. To characterize the ionic currents underlying the rhythmic bursting of mossy cells, we used somatic voltage-clamp recordings in the subthreshold voltage range. Ventral bursting cells possessed both hyperpolarization-activated current (Ih) and persistent sodium current (INaP), whereas dorsal and ventral nonbursting cells possessed Ih but no INaP. Blockade of Ih with cesium did not affect the intrinsic bursting of ventral mossy cells. In contrast, the blockade of INaP with tetrodotoxin or phenytoin established a stable subthreshold membrane potential in ventral bursting cells. The current-voltage curve of ventral bursting cells showed a region of tetrodotoxin-sensitive negative slope conductance between -55 mV and a spike threshold ( approximately -45 mV). On the other hand, no subthreshold calcium conductances played a significant role in the intrinsic bursting of ventral mossy cells. These observations demonstrate the heterogeneous electrophysiological properties of hilar mossy cells, and suggest that the subthreshold INaP plays a major role in the intrinsic rhythmic bursting of ventral mossy cells.