BIOPHYSICAL PROPERTIES AND DISTRIBUTION OF LARGE-CONDUCTANCE CALCIUM-DEPENDENT POTASSIUM CHANNELS IN NEOCORTICAL LAYER 5 PYRAMIDAL NEURONS

N. Benhassine.
Institute of Physiology, University of Bern.

Layer 5 pyramidal cells in the somatosensory cortex are characterized by two action potential initiation zones, one in the soma and one in the apical dendrite. Somatic action potentials can backpropagate into the dendrite and activate the dendritic calcium action potential initiation zone. Because calcium-activated high-conductance potassium channels (BK channels) on the dendrite could critically modulate the interaction between both zones, I studied the properties and distribution of BK channels in layer 5 pyramidal neurons. In an acute slice preparation of rat somatosensory cortex, patches were excised from this cell type. BK channels were characterized by their particularly high conductance of 138.5 pS and by their calcium dependence. In addition, BK channels were blocked by 1mM TEA and 3uM of the specific blocker paxilline. Their opening probability increased due to depolarization and increasing calcium concentration on the inner side of the membrane. The opening probabilities were 28, 27, 87, and 78 % at -100, -50, +50, and +100 mV, respectively. The mean channel open and closed time constants were also voltage-dependent. They reveal two open states at depolarized potentials but only one at hyperpolarized potentials. The open state time constants were: t(o1), 0.3, 0.6, 1.3, and 2.8 ms at -100, -50, +50, and +100 mV, respectively; t(o2), 16.0 and 28.8 ms at +50 and +100 mV, respectively. The closed state time constant was 4.6, 4.6, 0.8, and 1.6 ms at -100, -50, +50, and +100 mV, respectively. In order to clarify a possible role of BK channels in somato-dendritic interaction, I studied their distribution. Recordings were performed at the soma and along the dendrite up to 850 um from the soma. BK channels were found either isolated or in clusters containing up to four channels with a mean density of 22.5 channels per um2 (n=81). Surprisingly, BK channel distribution was found to be homogeneous along the dendrite (Kendall´s statistical test, tau=0.08). In addition an immunofluoresecence study confirmed their presence on soma and dendrite. This study presents the first evidence for the presence of BK channels on dendrites. Finally, a preliminary functional study showed that BK channels are critically influencing the action potential frequency. Hence, BK channel activation might modulate the shape of the action potential, its backpropagation into the dendrite, and thereby the electrotonic distance between the two action potential initiation zones.
Supported by the SNF (Grant 3100-066651.01) and the Bonizzi-Theler Foundation