COMPARISON OF LONG-TERM POTENTIATION IN THE PROXIMAL VERSUS DISTAL STRATUM-RADIATUM OF HIPPOCAMPAL FIELD CA1

Recent studies indicate that long-term potentiation is accompanied by changes in the waveform of field excitatory postsynaptic potentials in the CA1 field of disinhibited hippocampal slices, suggesting that long-term potentiation alters the kinetics of the glutamate receptor channels that mediate excitatory synaptic responses. The present study examined the effects of stimulating and recording location within stratum radiatum on the magnitude of long-term potentiation and the associated waveform changes. Stimulation of stratum radiatum proximal to the cell body layer evoked field excitatory postsynaptic potentials in proximal stratum radiatum that had decay phases well-fit by single exponentials; long-term potentiation reduced the decay time constant of these potentials, as reported previously. Stimulation of distal stratum radiatum evoked field excitatory postsynaptic potentials in distal stratum radiatum that were contaminated by positive after-potentials; meaningful estimates of decay time constants for these responses could not be obtained. Long-term potentiation of distal responses tended to be smaller than that obtained proximally. Comparisons were also made between responses recorded distally and proximally for either distal or proximal stimulation. For both stimulation loci, the distally-recorded responses had positive after-potentials and the proximally-recorded responses did not. The decay time constants for proximally-recorded responses to distal stimulation decreased significantly after long-term potentiation. Proximal and distal stimulation both produced greater long-term potentiation recorded proximally than distally and the difference was significantly greater for proximal stimulation. When long-term potentiation was induced by stimulation of proximal and distal sites simultaneously, the difference between proximally- and distally-recorded long-term potentiation was significantly reduced. Paired-pulse facilitation was greater when recorded proximally but the variation in facilitation with stimulation and recording position was different from that obtained for long-term potentiation. Paired-pulse Facilitation of response amplitude was slightly reduced after long-term potentiation, but the change did not depend on the stimulation-recording arrangement; facilitation of response slope was not reliably affected by long-term potentiation. These results indicate that the shape of the dendritic field excitatory postsynaptic potential is influenced by the relative position of recording electrodes in stratum radiatum; when the position is such that after-potentials are minimized, long-term potentiation produces a decrease in the decay time constant of the synaptic field potential. Comparisons of the magnitude of long-term potentiation at proximal and distal recording sites after high-frequency stimulation of proximal and distal sites suggest that distal stimulation is sufficient to potentiate synapses located both proximal and distal to the cell body layer, but that proximal stimulation does not potentiate distal synapses. A proximal-distal gradient in the ease for triggering long-term potentiation may be a general feature of dendritic architectures.