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WORKSHOP ON:
CENTRAL PROBLEMS IN SINGLE CELL COMPUTATION
16-18 September 2002
By invitation only
Venue
B10 Seminar Room, Alexandra House, 17 Queen Square, London, WC1N 3AR
| Active properties of layer 2/3
pyramidal cell dendrites in vitro and in vivo |
| Fritjof Helmchen, Department
of Cell Physiology, Max Planck Institut e for Medical Reserach, Germany |
| Pyramidal cells in layer 2/3 of neocortex are
morphologically heterogeneous with somata located 150-550 µm from the pia, an apical
trunk of variable length, and apical tufts extending in layer 1. Integration of synaptic
inputs arriving at the apical tuft critically depends on whether voltage-dependent ion
channels are present in the dendrites. We therefore investigated active properties of
layer 2/3 apical dendrites both in brain slices (in vitro) and in urethane-anesthetized
rats (in vivo). Experiments were performed under as similar as possible conditions in
somatosensory cortex of 28-day old rats using whole-cell patch-clamp recordings and
calcium imaging. Both in vitro and in vivo, action potentials (AP) backpropagated in a
decremental fashion along the apical trunk (measured using dendritic recordings).
Backpropagation was actively supported by sodium channels because application of TTX
caused a much stronger attenuation. Both in vitro and in vivo single action potentials
evoked large calcium transients in the apical trunk but generally failed to evoke calcium
influx in layer 1. In contrast, distal calcium influx could be induced by multiple APs
delivered at high frequencies (>100 Hz). In addition, all-or-none regenerative
dendritic potentials causing large distal calcium influx could be evoked by direct
dendritic current injection. When paired with a somatic AP these distal dendritic events
could cause a second rebound AP. We conclude that layer 2/3 pyramidal cell dendrites
contain sodium and calcium channels, which are activated similarly in brain slices and
during anesthesia. These active properties may allow layer 2/3 pyramidal neurons to
associate basal and distal synaptic inputs. |
| Joint work with: Jack Waters, Matthew
Larkum, Bert Sakmann |
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