A role for extracellular adenosine in time-dependent reversal of long-term potentiation by low-frequency stimulation at hippocampal CA1 synapses

Abstract

The involvement of adenosine on the development of time-dependent reversal of long-term potentiation (LTP) by low-frequency stimulation (LFS) was investigated at Schaffer collateral-CA1 synapses of rat hippocampal slices. A train of LFS (2 Hz, 10 min, 1200 pulses) had no long-term effects on synaptic transmission but produced lasting depression of previously potentiated responses. This reversal of LTP (depotentiation) was observed when the stimulus was delivered </=3 min after induction of LTP. However, application at 10 min after induction had no detectable effect on potentiation. This time-dependent reversal of LTP by LFS appeared to be mediated by extracellular adenosine, because it was mimicked by bath-applied adenosine and was specifically inhibited by the selective A(1) adenosine receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (100 nM). The effect of adenosine could be mimicked by 5-HT(1A) receptor agonist buspirone, but the LFS-induced depotentiation could not be antagonized by 5-HT(1A) receptor antagonist NAN-190. The source of extracellular adenosine in response to LFS appeared to be attributable to the efflux of cAMP. In addition, this LFS-induced depotentiation was blocked by bath application of adenylyl cyclase activator forskolin or injection of a cAMP analog Sp-adenosine cAMP (10 mM) into postsynaptic neurons. Moreover, the selective protein phosphatase 1 and 2A inhibitors okadaic acid and calyculin A prevented the LFS-induced depotentiation. These results thus suggest that increasing extracellular adenosine appears to underlie the LFS-induced depotentiation via acting on the A(1) receptor subtype to interrupt the cAMP-dependent biochemical processes leading to the LTP expression.

Fig. 1.

High-frequency TS but not LFS induces long-lasting changes in synaptic transmission. A, An example of the time course of homosynaptic long-term potentiation at Schaffer collateral–CA1 synapses. The slope of fEPSP exhibited ∼95% increase after TS that slowly decayed during the first 10 min and remained stable at 45% increase afterwards. B, Summary of data from 12 experiments performed as in A.C, Example of an experiment showing that the protocol of low-frequency stimulation (2 Hz, 10 min) had no lasting effect on synaptic transmission. D, Plots the pooled data from four experiments performed as in C. The superimposed fEPSP in the inset of each graph illustrates respective recordings from example experiments taken at the time indicated bynumber. Bar denotes the period of the delivery of LFS. Calibration: 0.5 mV, 10 msec.

Fig. 2.

Time-dependent reversal of LTP by LFS.A–F, Examples in which LTP was induced at Schaffer collateral–CA1 synapses. LFS (2 Hz, 10 min) was applied at various times after LTP induction: A, 1 min after;C, 3 min after; or E, 10 min after.B, D, F, Summary of experiments similar to that shown in A,C, and E. Note that LFS erased potentiation when delivered 1 or 3 min after TS but was without effect when applied 10 min after. Calibration: 0.5 mV, 10 msec.

Fig. 3.

A₁ adenosine receptor antagonist DPCPX prevents the LFS-induced depotentiation. A, Example of an experiment showing that LFS-induced depotentiation was inhibited when A₁ adenosine receptor antagonist DPCPX (0.1 μm) was applied during TS and left until the end of LFS.B, Summary of data from seven experiments performed as in A. C, A₂ adenosine receptor antagonist DMPX (5 μm) does not affect the LFS-induced depotentiation. D, Pooled data from six experiments performed as in C. Note that only DPCPX could effectively block the LFS-induced depotentiation. Calibration: 0.5 mV, 10 msec.

Fig. 4.

Time-dependent reversal of LTP by adenosine application. A–F, Examples in which LTP was induced at Schaffer collateral–CA1 synapses. Adenosine (0.2 mm) was applied at various times after LTP induction: A, immediately; C, 3 min after; or E, 10 min after. B, D, F, Summary of experiments similar to that shown in A,C, and E. Note that adenosine erased potentiation when delivered immediately or 3 min after the TS but was without effect when applied 10 min after. Calibration: 0.5 mV, 10 msec.

Fig. 5.

Selective activation of adenylyl cyclase prevents the LFS-induced depotentiation. A, Previous activation of adenylyl cyclase by forskolin (10 μm) produced a minor increase in the basal synaptic transmission and exerted a significant inhibition on the subsequent LFS-induced depotentiation. Forskolin was applied starting 5 min before the TS and left until the end of LFS.B, Summary of seven experiments performed as inA. C, Previous application of an inactive isomer of forskolin, 1,9-dideoxy-forskolin (10 μm), did not affect the LFS-induced depotentiation. D, Summary of five experiments performed as in C. Calibration: 0.5 mV, 10 msec.

Fig. 6.

Time-dependent reversal of LTP by 5-HT_1A receptor agonist buspirone application.A–D, Examples in which LTP was induced at Schaffer collateral–CA1 synapses. Buspirone (0.2 mm) was applied at various times after LTP induction: A, 1 min; orC, 10 min after. B, D, Summary of experiments similar to that shown in A andC. Note that buspirone erased potentiation when delivered 1 min after the TS but was without effect when applied 10 min after. Calibration: 0.5 mV, 10 msec.

Fig. 7.

Activation of postsynaptic PKA prevents the LFS-induced depotentiation. A, Example of an experiment in which intracellular application of PKA activator, Sp-cAMPS (10 mm), prevented the LFS-induced depotentiation. In this experiment, extracellular fEPSP was also monitored simultaneously with intracellular EPSP. Note that LFS effectively erased potentiation of fEPSP when delivered 3 min after the TS, indicating that LFS-induced depotentiation could be induced in the population of cells that were not subjected to the Sp-cAMPS. B, Summary of six experiments performed as in A. Calibration: EPSP, 5 mV, 10 msec; fEPSP, 0.5 mV, 10 msec.

Fig. 8.

The protein phosphatase 1 and 2A inhibitors prevent the LFS-induced depotentiation. A,C, Example of an experiment showing that preincubation of slice for 2–3 hr in either 1 μm okadaic acid (PP1/2A inhibitor) or 1 μm calyculin A (PP1/2A inhibitor) effectively prevented the LFS-induced depotentiation. B,D, Summary of experiments similar to that shown inA and C. Calibration: 0.5 mV, 10 msec.