Fig. 5. Voltage- and Ca²⁺ -sensitive outward K⁺ currents of a typical Kenyon cell and a projection neuron. (A) Currents were recorded in the standard external saline (ES std., see Materials and methods) with the Na⁺ channel blocker tetrodotoxin (TTX, 100 nmol l^-1) added. As in Fig. 3, projection neurons showed higher current amplitudes (note the different scale bars) and Kenyon cells expressed prominent transient K⁺ currents. Furthermore, the current amplitudes of Kenyon cells are increasing continuously with increasing depolarisations. By contrast, outward currents of projection neurons show an apparent non-linearity with a decrease of current amplitude increases between command potentials of 70-110 mV (see N-shaped I-V curve in Fig. 5C). The activation protocol for all experiments consisted of a hyperpolarising conditioning prepulse to -120 mV (1 s) and subsequent depolarizing voltage commands to various potentials (-100 to +120 mV, 10 mV increments, duration 100 ms; holding potential -80 mV). (B) Addition of the irreversible Ca²⁺ channel blocker 50 µmol l^-1 CdCl₂ (Cd²⁺) to the bath solution had little effect on K⁺ currents of Kenyon cells (left), but removed non-linearity of current activation in projection neurons (right). (C) Corresponding I-V curves of the two cells showing the Cd²⁺ block of the N-shaped I-V curve for the projection neuron (right), but no significant change in the I-V curve of voltage-sensitive K⁺ currents of the Kenyon cell (left). The peak currents were measured at the time points indicated by the arrows in A and B.

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