IPSPs modulate spike backpropagation and associated [Ca 2+ ] i changes in the dendrites of hippocampal CA1 pyramidal neurons. Tsubokawa, Hiroshi and William N. Ross. Department of Physiology, New York Medical College, Valhalla, NY 10595, Tel: 914-993-4092, Fax: 914-993-4018, email: ross@nymc.edu.
APStracts 3:014N, 1996.
SUMMARY AND CONCLUSIONS
1. We studied the effects of synaptic inhibition on backpropagating Na + spikes in the apical dendrites of CA1 pyramidal neurons in transverse slices from the rat hippocampus. Action potentials were evoked synaptically by stimulation in the stratum radiatum or antidromically by stimulation in the alveus. 2. IPSPs, evoked by stimulation in the stratum lacunosum moleculare, reduced the amplitude of single spikes in the distal dendrites, but did not change the amplitudes in the somatic or proximal regions. Inhibition also reduced the spike-associated [Ca 2+ ] i changes in the distal dendrites but had little effect on the changes in the proximal part of the cell. Both of these results are consistent with inhibition converting actively backpropagating spikes into passively spreading potentials at some point in the arbor. 3. In most cells the spike amplitude reduction in the distal dendrites was blocked by bicuculline methiodide (10 [mu]M) and inhibition was most effective when evoked in a time window less than 10 ms preceding the action potential. This suggests that the amplitude reduction was due to a conductance shunt activated by GABA A receptors. Synaptically evoked GABA B responses were detected but usually did not block spike propagation. 4. Direct hyperpolarization in the distal dendrites was also effective in blocking antidromically evoked spike backpropagation but probably does not contribute when the action potentials are evoked synaptically. 5. This effect of inhibition is different from its usual function in synaptic integration since spike generation and propagation down the axon are not significantly affected. This kind of inhibition might be important in regulating transient [Ca 2+ ] i changes in the dendrites including individual dendritic branches.

Received  1996; accepted in final form  1996.
APS Manuscript Number J.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 25 July 1996