Abstracts Biological Sciences

The role of calcium spikes in neocortical pyramidal cell dendrites: implications for the transduction of dendritic current into spike output

by John Christopher Oakley

The possible role of long-lasting, Ca2+-dependent, dendritic, action potentials (plateaus) in the transduction of synaptic current into spike output was investigated by whole-cell recording from the soma of layer 5 pyramidal neurons visualized in a slice preparation of rat neocortex by the use of IR-DIC microscopy. Plateaus, evoked by focal glutamate iontophoresis on visualized dendrites, resulted in the initial amplification and subsequent limitation of the ligand-gated dendritic current that arrived at the soma and thus in a nonlinear relation between firing rate (F) and dendritic iontophoretic current (I). Plateaus and these nonlinear F-I relations were evoked in apical, oblique and basal dendrites. In contrast, plateaus could not be evoked by iontophoresis at the soma or the proximal 100 mum of the apical dendrite and 50 mum of the basal dendrites. Firing rate was a linear function of applied current in these regions. Plateau amplitude recorded in the soma decreased as the site of iontophoresis was moved farther from the soma. Plateaus evoked large local [Ca2+]i transients but did not propagate actively to the soma or into the dendritic tuft based on measurements of plateau-evoked [Ca2+]i transients using the Ca2+ sensitive dye Fluo-4. Currents arriving at the soma summed if plateaus were evoked on separate dendrites or if subthreshold responses were evoked at 2 sites on the same dendrite. If plateaus were evoked at 2 sites on the same dendrite, only the proximal plateau was seen at the soma. Just-subthreshold depolarizations at 2 sites on the same dendrite evoked a plateau at the proximal site. I conclude that the most important consequence of the plateau is to prevent additional current from ligand-gated channels at and distal to the plateau-generating region from having a direct influence on the firing rate once a plateau is initiated. Plateau interaction experiments suggest that the cell would continue to sum whole-cell currents during increasing, uniformly distributed, excitatory synaptic input, but that the total current reaching the soma would be limited by dendritic plateaus and thus the firing rate of the cell would be restricted to a nonsaturating dynamic range.