Postsynaptic membrane shifts during frequency potentiation of the hippocampal EPSP.

1. In some classes of central neurons, repetitive synaptic stimulation induces substantial changes in the postsynaptic membrane, in conjunction with robust frequency potentiation of the excitatory postsynaptic potential (EPSP). However, the nature and time course of these postsynaptic membrane shifts, or their possible contributions to EPSP frequency potentiation (e.g., by altering driving force or current pathways), have not been examined extensively. We therefore studied the simultaneous patterns of change in composite EPSP amplitude, postsynaptic input resistance (Rin), and postsynaptic membrane potential during a 4-min train of 10-Hz monosynaptic stimulation in CA1 neurons of hippocampal slices. Slices were maintained in media containing either control (4 mM) or high (6.5 mM) concentrations of K+. 2. Potentiation of the EPSP, hyperpolarization of the membrane, and a decline of Rin, all developed rapidly during 10-Hz synaptic stimulation; these responses reached maximal levels by 5-15 s of the stimulation train. In most cells, a membrane depolarization phase occurred between 15 and 45 s of stimulation, followed by rehyperpolarization by 1 min of stimulation. During the depolarization phase, both EPSP potentiation and the decline in Rin remained near maximal. No significant differences were seen as a function of K+ concentrations. 3. These results show that hyperpolarization is not invariably associated temporally with EPSP frequency potentiation. Moreover, if driving force and membrane conductance changes are assumed to be approximately similar in large dendrites and soma, then the increase in driving force due to membrane hyperpolarization was not sufficient to account for the three- and fourfold increases in EPSP amplitude seen during frequency potentiation. Further, based on similar assumptions and on dendritic models of EPSP attenuation, the decline in Rin should reduce EPSP amplitude at the dendritic synaptic site and, to a proportionately greater extent, at the soma. 4. Studies in which the membrane was hyperpolarized with injected current to approximately the IPSP reversal potential, or in which bicuculline methiodide was applied to the slices, indicated that depression of the IPSP by repetitive stimulation did not account for frequency potentiation of EPSP amplitude. 5. These data are therefore consistent with the conclusion that the frequency potentiation of composite EPSPs in central neurons depends on presynaptic mechanisms, rather than on generalized postsynaptic changes. However, our findings do not rule out localized postsynaptic changes in receptors or spines as possible contributing factors.(ABSTRACT TRUNCATED AT 400 WORDS)