K. Svoboda, W. Denk, D. Kleinfeld, D. W. Tank.
Biological Computation Research Department,
Bell Laboratories, Lucent Technologies
Over the last few years a rich collection of active conductances have been discovered in the dendrites of mammalian pyramidal neurons, but their role in neural information processing remains obscure. These conductances can, at least under some experimental conditions, support sodium and calcium action potentials and may subserve a variety of dendritic computations. But even such seemingly straightforward issues as the presence, site of initiation, and direction of propagation of dendritic action potentials remain controversial, primarily because they depend on resting membrane potential, ionic composition, degree of channel inactivation and on the intensity and pattern of synaptic stimulation. This makes it difficult to extrapolate from in vitro experiments to the situation in the intact brain. We used two-photon laser scanning microscopy to image dendritic calcium dynamics of layer 2/3 neurons in the rat primary vibrissa (Sm1) cortex in vivo. Simultaneous recordings of intracellular voltage and dendritic calcium dynamics during whisker stimulation or current injection showed increases in calcium only in coincidence with sodium action potentials. The amplitude of these calcium transients at a given location was approximately proportional to the number of sodium action potentials in a short burst. This suggests that under our conditions calcium action potentials are not generated and any significant calcium increase depends on somatically triggered sodium action potentials. The amplitude for a given number of action potentials was greatest in the proximal apical dendrite and declined steeply with increasing distance from the soma, with little calcium in the tuft branches of layer 1. We hypothesize that inhibitory activity in vivo prevents active propagation of sodium action potentials into the dendrites of pyramidal neurons. We are currently testing this hypothesis directly by recording intracellularly from dendrites in vivo.