THE INTRINSIC ELECTROPHYSIOLOGICAL CHARACTERISTICS OF FLY LOBULA PLATE TANGENTIAL CELLS .2. ACTIVE MEMBRANE-PROPERTIES

The voltage-gated currents in the fly lobula plate tangential cells we re examined using the switched electrode voltage clamp technique. In C H cells, two currents were identified (Figs. 1, 2): a slow calcium inw ard current and a delayed rectifying, noninactivating potassium outwar d current. HS and VS cells appear to possess similar currents to CH ce lls, but in addition, exhibit a fast-activating sodium inward current and a sodium-activated potassium outward current (Figs. 3, 4). While t he delayed rectifying potassium current in all three cell classes is r esponsible for the observed outward rectification described previously (Borst and Haag, 1996), the sodium inward current produces the fast a nd irregular spikelike depolarizations found in HS and VS cells but no t in CH cells: When the sodium current is blocked by either TTX or int racellular QX314, no more action potentials can be elicited in HS cell s under current-clamp conditions (Fig. 5). As is demonstrated in HS ce lls, space clamp conditions are sufficient to suppress synaptically in duced action potentials (Fig. 6). The currents described above were in corporated with the appropriate characteristics into compartmental mod els of the cells (Figs. 7, s). The anatomical and electrically passive membrane parameters of these cells were determined in a preceding pap er (Borst and Haag, 1996). After fitting the current parameters to the voltage-clamp data (Fig. 9), the model cells qualitatively mimicked t he fly tangential cells under current clamp conditions in response to current injection (Fig. 10). The simulations demonstrated that the ele ctrical compactness seen in the HS and VS cells, either in passive mod els or in active models during continuous hyperpolarization, decreased significantly in the active models during continuous depolarization ( Fig. 11). Active HS models reproduce the frequency-dependent amplifica tion of current injected into their axon (Fig. 12).