IPSPS MODULATE SPIKE BACKPROPAGATION AND ASSOCIATED [CA2+](I) CHANGESIN THE DENDRITES OF HIPPOCAMPAL CA1 PYRAMIDAL NEURONS

1. We studied the effects of synaptic Inhibition on backpropagating Na + spikes in the apical dendrites of CA1 pyramidal neurons in transvers e slices from the rat hippocampus. Action potentials were evoked synap tically by stimulation in the stratum radiatum or antidromically by st imulation in the alveus. 2. Inhibitory postsynaptic potentials, evoked by stimulation in the stratum lacunosum moleculare, reduced the ampli tude of single spikes in the distal dendrites but did not change the a mplitudes in the somatic or proximal regions. Inhibition also reduced the spike-associated [Ca2+](i) changes in the distal dendrites but had little effect on the changes in the proximal part of the cell. Both o f these results are consistent with inhibition converting actively bac kpropagating spikes into passively spreading potentials at some point in the arbor. 3. In most cells, the spike amplitude reduction in the d istal dendrites was blocked by bicuculline methiodide (10 mu M) and in hibition was most effective when evoked in a time window <10 ms preced ing the action potential. This suggests that the amplitude reduction w as due to a conductance shunt activated by gamma-aminobuturic acid-A ( GABA(A)) receptors. Synaptically evoked GABA(B) responses were detecte d but usually did not block spike propagation. 4. Direct hyperpolariza tion in the distal dendrites was also effective in blocking antidromic alIy evoked spike backpropagation but probably does not contribute whe n the action potentials are evoked synaptically. 5. This effect of inh ibition is different from its usual function in synaptic integration b ecause spike generation and propagation down the axon are not signific antly affected, This kind of inhibition might be important in regulati ng transient [Ca2+](i) changes in the dendrites including individual d endritic branches.