Disparate effects of long-term potentiation on evoked potentials and single CA1 neurons in the hippocampus of anesthetized rats

To examine the effects of long-term potentiation (LTP) on individual neuron s in the intact brain, anesthetized rats were implanted with a recording st ereotrode in the right CA1 layer of the hippocampus and a stimulating elect rode in the right and left CA3 layers. The evoked and spontaneous firing of single CA1 neurons was characterized before and after LTP of the contralat eral (commissural) Schaffer collaterals and again after LTP of the ipsilate ral (associational) Schaffer collaterals. Individual CA1 neurons displayed either increases or decreases in evoked and spontaneous firing after LTP. A s many as five discriminated cells were recorded simultaneously, and they t ypically responded discordantly, so that after LTP, firing in some neurons increased while in others it decreased. The response of individual neurons to in vivo LTP may be modulated by heterogeneous synaptic changes on indivi dual and local groups of cells, and by changes in feed-forward excitation a nd inhibition provided by local hippocampal circuitry. (C) 2000 Wiley-Liss, Inc.