Long-term synaptic plasticity between pairs of individual CA3 pyramidal cells in rat hippocampal slice cultures

Abstract

1. Long-term potentiation (LTP) and depression (LTD) were investigated at synapses formed by pairs of monosynaptically connected CA3 pyramidal cells in rat hippocampal slice cultures. 2. An N-methyl-D-aspartate (NMDA) receptor-mediated component of the unitary EPSP, elicited at the resting membrane potential in response to single action potentials in an individual CA3 cell, could be isolated pharmacologically. 3. Associative LTP was induced when single presynaptic action potentials were repeatedly paired with 240 ms postsynaptic depolarizing pulses that evoked five to twelve action potentials or with single postsynaptic action potentials evoked near the peak of the unitary EPSP. LTP induction was prevented by an NMDA receptor antagonist. 4. Associative LTD was induced when single presynaptic action potentials were repeatedly elicited with a certain delay after either 240 ms postsynaptic depolarizing pulses or single postsynaptic action potentials. The time window within which presynaptic activity had to occur for LTD induction was dependent on the amount of postsynaptic depolarization. LTD was induced if single pre- and postsynaptic action potentials occurred synchronously. 5. Homosynaptic LTD was induced by 3 Hz tetanization of the presynaptic neuron for 3 min and was blocked by an NMDA receptor antagonist. 6. Depotentiation was produced with stimulation protocols that elicit either homosynaptic or associative LTD. 7. Recurrent excitatory synapses between CA3 cells display associative potentiation and depression. The sign of the change in synaptic strength is a function of the relative timing of pre- and postsynaptic action potentials.

Figure 1. The NMDA receptor-mediated component of unitary EPSPs

A single presynaptic action potential in one CA3 pyramidal cell (Pre) elicited a unitary EPSP in a second CA3 pyramidal cell (Post) at its resting membrane potential (-58 mV). Application of 40 μM AP5 decreased the duration of the unitary EPSP (middle trace). Digital subtraction of the unitary EPSP in the presence of AP5 from the control EPSP revealed the component of the EPSP blocked by AP5 (bottom trace). This NMDA receptor-mediated component had a time to peak of 27 ms and an amplitude of 0.5 mV.

Figure 2. Potentiation of EPSPs between CA3 cells

A, synchronous pairing (SP) of low frequency (at 0.3 Hz) extracellular stimuli and postsynaptic depolarization (240 ms duration, 0.5-2.5 nA) induced a long-lasting potentiation of the amplitude and initial slope of compound EPSPs evoked in CA3 pyramidal cells (n= 11 cells). A representative example is shown above. B, pooled data illustrating the potentiation of unitary CA3-CA3 EPSPs induced with the synchronous pairing of single presynaptic action potentials and postsynaptic depolarizations (240 ms duration, 30-80 repetitions) which produced 7-12 action potentials. Traces from a representative experiment are shown above the graph. C, potentiation induced by the pairing (indicated by bar in graph) of a single presynaptic action potential elicited 15 ms before a single postsynaptic action potential (repeated × 50 at 0.3 Hz). Averaged traces from the labelled time points are illustrated above the graph.

Figure 3. Associative depression of EPSPs between CA3 cells

A, asynchronous pairing (AP) of low frequency extracellular stimuli (at 0.3 Hz) and postsynaptic depolarization (240 ms, preceding the EPSP by 800 ms, repeated × 100) induced a long-lasting depression of both amplitude and slope of compound EPSPs recorded in CA3 pyramidal neurons (n= 3 cells). A representative example is shown above. B, pooled data illustrating associative LTD of 6 unitary CA3-CA3 EPSPs following the asynchronous pairing of single presynaptic action potentials and postsynaptic depolarization (240 ms duration, 7-12 action potentials, 100 repetitions) delayed by 800 ms. C, depression induced by the asynchronous pairing (indicated by bar in graph in C) of a single presynaptic action potential elicited 70 ms after a single postsynaptic action potential (repeated × 100 at 0.3 Hz). Averaged traces from the labelled time points are illustrated above the graph.

Figure 4. Changes in synaptic efficacy as a function of the time interval between pre- and postsynaptic action potentials

Potentiation was induced only when the presynaptic action potential preceded the postsynaptic action potential (-15 ms of delay). Depression was induced for synchronous (0 ms) and asynchronous pairing (15-70 ms). The change in unitary EPSP amplitude observed when the time interval was increased to 200 ms was not significant (P > 0.01, Mann-Whitney U test). The inset illustrates the timing of the NMDA receptor-mediated component of the unitary EPSP (from Fig. 1) relative to the pre- and postsynaptic action potentials when they are synchronous or when the delay is -15 ms.

Figure 5. Homosynaptic depression of EPSPs between CA3 cells

A, low frequency tetanus (3 Hz, 3 min) resulted in a persistent depression of the amplitude and the initial slope (not shown) of compound EPSPs (inset) evoked with an extracellular stimulation in the area CA3 (n= 5 cells). B, example of homosynaptic LTD of a unitary CA3-CA3 EPSP following 3 Hz tetanization of the presynaptic cell for 3 min. Averaged traces from the labelled time points are illustrated above the graph. C, pooled data illustrating homosynaptic LTD of 5 unitary CA3-CA3 EPSPs after 3 Hz tetanization for 3 min. D, no LTD of unitary CA3-CA3 EPSPs was induced when the NMDA receptor antagonist AP5 (40 μM) was present during the 3 Hz tetanus (n= 4 cell pairs).

Figure 6. Potentiation and depotentiation of a unitary CA3-CA3 EPSP induced with synchronous and asynchronous pairing

A, a 61 ± 8 % potentiation (all responses 5-10 min after the pairing) over the control amplitude was first induced by a synchronous pairing (SP) of 240 ms postsynaptic depolarizing pulses and single action potentials in the presynaptic cell (repeated × 30). After stabilization, the asynchronous pairing (AP) of 240 ms postsynaptic depolarizing pulses followed 800 ms later by single presynaptic action potentials (repeated × 100) resulted in a 58 ± 5 % decrease in amplitude relative to the potentiated level (measured 5-10 min after the AP), which persisted for 20 min. Averaged traces from the labelled time points are illustrated above the graph. B, pooled data illustrating potentiation of 3 unitary CA3-CA3 EPSPs using the SP protocol, followed by depotentiation using the AP protocol. The large standard error bars for the mean responses immediately after SP reflect the variable occurrence of the transient, short-term potentiation in some cells, such as the example in A.