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The Journal of Neurophysiology Vol. 81 No. 4 April 1999, pp. 1559-1566
Copyright ©1999 by the American Physiological Society
W. M. Keck Center for the Neurobiology of Learning and Memory; and the Department of Neurobiology and Anatomy, University of Texas Medical School, Houston, Texas 77030
Steele, Philip M. and
Michael D. Mauk.
Inhibitory control of LTP and LTD: stability of synapse strength.
Although much is known about the induction of synaptic
plasticity, the persistence of memories suggests the importance of
understanding factors that maintain synaptic strength and prevent
unwanted synaptic changes. Here we present evidence that recurrent
inhibitory connections in the CA1 region of hippocampus may contribute
to this task by modulating the relative ability to induce long-term
potentiation and depression (LTP and LTD). Bath application of the
-aminobutyric acid (GABA) type A agonist muscimol to hippocampal
slices increased the range of frequencies that produce LTD, whereas in
the presence of the GABA type A antagonist picrotoxin LTD was induced
only at very low stimulation frequencies (0.25-0.5 Hz). Because one source of GABAergic input to CA1 pyramidal cells is via recurrent inhibition, we tested the prediction that elevated postsynaptic spike
activity would increase feedback GABA inhibition and favor the
induction of LTD. By using an induction stimulation of 8 Hz, which
alone produced no net change in synaptic strength, we found that
stimulation presented during antidromic activation of pyramidal cell
spikes induced LTD. This effect was blocked by picrotoxin. The
influence of recurrent inhibition on LTP and LTD displays properties
that may decrease the potential for self-reinforcing, runaway changes
in synapse strength. A mechanism of this sort may help maintain
patterns of synaptic strengths despite the ongoing opportunities for
plasticity produced by synapse activation.
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