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Distance-Dependent Modifiable Threshold for Action Potential Back-Propagation in Hippocampal Dendrites

Division of Neuroscience, Baylor College of Medicine, Houston, Texas 77030

Submitted 24 March 2003; accepted in final form 6 May 2003

In hippocampal CA1 pyramidal neurons, action potentials generated in the axon back-propagate in a decremental fashion into the dendritic tree where they affect synaptic integration and synaptic plasticity. The amplitude of back-propagating action potentials (b-APs) is controlled by various biological factors, including membrane potential (V_m). We report that, at any dendritic location (x), the transition from weak (small-amplitude b-APs) to strong (large-amplitude b-APs) back-propagation occurs when V_m crosses a threshold potential, _x. When V_m > _x, back-propagation is strong (mostly active). Conversely, when V_m < _x, back-propagation is weak (mostly passive). _x varies linearly with the distance (x) from the soma. Close to the soma, _x << resting membrane potential (RMP) and a strong hyperpolarization of the membrane is necessary to switch back-propagation from strong to weak. In the distal dendrites, _x >> RMP and a strong depolarization is necessary to switch back-propagation from weak to strong. At 260 µm from the soma, ₂₆₀ RMP, suggesting that in this dendritic region back-propagation starts to switch from strong to weak. _x depends on the availability or state of Na⁺ and K⁺ channels. Partial blockade or phosphorylation of K⁺ channels decreases _x and thereby increases the portion of the dendritic tree experiencing strong back-propagation. Partial blockade or inactivation of Na⁺ channels has the opposite effect. We conclude that _x is a parameter that captures the onset of the transition from weak to strong back-propagation. Its modification may alter dendritic function under physiological and pathological conditions by changing how far large action potentials back-propagate in the dendritic tree.

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