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Department of Physiology, Dokkyo University School of Medicine, Tochigi, Japan.
1. Intracellular recordings were made from zebra finch hyperstriatum ventrale pars caudale (HVc) neurones in in vitro slice preparations. 2. Small depolarizing current pulses elicited tonic firing while relatively large current pulses elicited an initial high-frequency burst followed by tonic firing. Time-dependent and fast-activated inward rectification were observed when a hyperpolarizing current pulse was applied. 3. Fast action potentials were abolished by tetrodotoxin (TTX, 0.5 micrograms/ml). When Co2+ (2.2-2.4 mM) was added to a low-Ca2+ (0-0.2 mM) solution, a plateau potential was elicited by a small depolarizing current pulse. These plateau potentials outlasted the applied current pulse and were abolished by TTX (0.5 micrograms/ml). 4. A long-lasting after-hyperpolarization (AHP), with a duration of tens of seconds, followed long trains of repetitive firing in control solution. Even in a low-Ca2+ (0-0.2 mM) solution containing Co2+ (2.2-2.4 mM) the long-lasting AHPs followed plateau potentials and were associated with an increase in input conductance. The long-lasting AHP, as well as the plateau potential, was blocked by TTX (0.5 micrograms/ml). 5. While TTX abolished fast action potentials, two types of active responses with different thresholds were elicited by depolarizing current in the presence of TTX (0.5 micrograms/ml), particularly when tetraethylammonium (5 mM) was also added to the solution. Both the low-threshold spike (LTS) and the high-threshold spike (HTS) were abolished by Co2+ (2.2-2.3 mM). 6. When Ba2+ (0.5-1 mM) was added to a solution containing TTX (0.5 micrograms/ml), an LTS elicited by a depolarizing current pulse became larger in amplitude. At membrane potentials more positive than -55 mV, an HTS, but not the LTS, was elicited. 7. These results suggest that HVc neurones have three Na(+)-dependent conductances: a fast Na+ conductance responsible for the fast spike, a persistent Na+ conductance responsible for the plateau potential, and a Na(+)-activated K+ conductance responsible for the long-lasting AHP; and that they have two Ca(2+)-dependent conductances: a low-threshold Ca2+ conductance responsible for the LTS, and a high-threshold Ca2+ conductance responsible for the HTS.
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