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Previous Article | Next Article
The Journal of Neuroscience, May 1, 2002, 22(9):3404-3413
Selective Coupling of T-Type Calcium Channels to SK Potassium Channels Prevents Intrinsic Bursting in Dopaminergic Midbrain Neurons
Jakob Wolfart and Jochen Roeper Medical Research Council, Anatomical Neuropharmacology Unit, Department of Pharmacology, Oxford University, Oxford OX1 3TH, United Kingdom
Dopaminergic midbrain (DA) neurons display two principal activity patterns in vivo, single-spike and burst firing, the latter coding for reward-related events. We have shown recently that the small-conductance calcium-activated potassium channel SK3 controls pacemaker frequency and precision in DA neurons of the substantia nigra (SN), and previous studies have implicated SK channels in the transition to burst firing. To identify the upstream calcium sources for SK channel activation in DA SN neurons, we studied the sensitivity of SK channel-mediated afterhyperpolarization (AHP) currents to inhibitors of different types of voltage-gated calcium channels in perforated patch-clamp recordings. Cobalt-sensitive AHP currents were not affected by L-type and P/Q-type calcium channel inhibitors and were reduced slightly (26%) by the N-type channel inhibitor
-conotoxin-GVIA. In contrast, AHP currents were blocked substantially (85-94%) by micromolar concentrations of nickel (IC50, 33.75 µM) and mibefradil (IC50, 4.83 µM), indistinguishable from the nickel and mibefradil sensitivities of T-type calcium currents (IC50 values, 33.86 and 4.59 µM, respectively). These results indicate that SK channels are activated selectively via T-type calcium channels in DA SN neurons. Consequently, SK currents displayed use-dependent inactivation with similar time constants when compared with those of T-type calcium currents and generated a transient rebound inhibition. Both SK and T-type channels were essential for the stability of spontaneous pacemaker activity, and, in some DA SN neurons, T-type channel inhibition was sufficient to induce intrinsic burst firing. The functional coupling of SK to T-type channels has important implications for the temporal integration of synaptic input and might help to understand how DA neurons switch between pacemaker and burst-firing modes in vivo.
Key words: dopamine; substantia nigra (A9); electrophysiology; apamin; nifedipine; agatoxin-TK; FTX-3.3; amphotericin; cyclopiazonic acid (CPA)
Copyright © 2002 Society for Neuroscience 0270-6474/02/2293404-10$05.00/0
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