Frequency-dependent propagation of sodium action potentials in dendrites of hippocampal CA1 pyramidal neurons

1. The propagation of antidromically activated action potentials in CA1 pyramidal neurons was examined with intrasomatic and intradendritic electrical recording and optical measurements using the fluorescent calcium indicator Calcium Green-1. 2. In somatic recordings, trains of 40 action potentials, activated at rates up to 100 Hz, showed modest amplitude reduction. Recordings in the apical dendrites, 150 microns from the soma, showed smaller initial amplitudes and much greater decrement during trains. Higher frequencies caused a greater rate of reduction with a lower final amplitude. 3. Calcium concentration changes ([Ca2+]i), measured with the fluorescent indicator Calcium Green-1 and a fast, cooled charge coupled device (CCD) camera, were detected over the entire length of the apical dendrites in response to single antidromic action potentials, although the changes in distal dendrites were smaller. These changes were rapid, decaying to half-amplitude in < 150 ms in distal dendritic locations. 4. Trains of action potentials at all frequencies up to 100 Hz caused transient [Ca2+]i, increases for each spike at 150 microns from the soma. In the last 100 microns of the distal branches, only the first few spikes caused a [Ca2+]i increase for frequencies above approximately 40 Hz. These patterns could be matched with a simple model of calcium influx and removal, where later spikes in a train brought in less calcium than earlier spikes. 5. These results show that the action-potential amplitude and the spatial extent of their propagation in the dendrites is frequency dependent.