Effect of High-Frequency Stimulation of the Perforant Path on Previously Acquired Spatial Memory in Rats: Influence of Memory Strength and Reactivation

<div><p>If memory depends on changes in synaptic strength, then manipulation of synaptic strength after learning should alter memory for what was learned. Here, we examined whether high frequency stimulation of the perforant path <i>in vivo</i> disrupts memory for a previously-learned hidden platform location in the Morris water task as well as whether this effect is modulated by memory strength or memory reactivation. We found that high frequency stimulation affected probe test performance regardless of memory strength or state of memory activation, although the precise nature of this effect differed depending on whether rats received minimal or extensive training prior to high frequency stimulation. These findings suggest that artificial manipulation of synaptic strength between the entorhinal cortex and hippocampus may destabilize memory for a previously-learned spatial location.</p></div

Training.

<p>Both rats that underwent 1 day of training (<b>AB</b>) and rats that underwent 4 days of training (<b>CD</b>) displayed a decrease in latency to reach the hidden platform across trials. £ denotes main effect of trial within 1-day training group (collapsed across reactivation/no reactivation and HFS/control groups), <i>p</i><.05. ¥ denotes main effect of trial within 4-day training group (collapsed across reactivation/no reactivation and HFS/control groups), <i>p</i><.05.</p

Retraining.

<p>Both rats that underwent 1 day of training (<b>AB</b>) and rats that underwent 4 days of training (<b>CD</b>) displayed a decrease in latency to reach the hidden platform across trials. Rats that underwent 4 days of training exhibited shorter latency than rats that underwent 1 day of training. £ denotes main effect of trial (collapsed across 1-day/4-day, reactivation/no reactivation, and HFS/control groups), <i>p</i><.05. ¥ denotes main effect of training (collapsed across reactivation/no reactivation and HFS/control groups and trial), <i>p</i><.05.</p

Experimental design and timeline.

<p>Rats were surgically (Sx) implanted with stimulating electrodes in the perforant path and recording electrodes in the dentate gyrus. After the acquisition of I/O curves, rats underwent either 1 day (upper timelines) or 4 consecutive days (lower timelines) of training in the hidden platform version of the Morris water task. Twenty-four hours after the completion of training, half of the rats received a memory reactivation treatment consisting of being briefly placed on the platform in its trained location in the pool (reactivation group), whereas the other half remained in their home cages (no reactivation group). Next, half of the rats received 10 HFS trains applied bilaterally to the perforant path (HFS group), whereas the other half received a matching number of test pulses (control group). Afterward, rats' memory for the platform location was assessed during two consecutive probe tests with the platform removed from the pool. Finally, rats underwent retraining with the platform returned to its trained location.</p

HFS.

<p>Across all experimental conditions, rats in the HFS group exhibited HFS-induced potentiation of evoked responses considering both EPSP slope (<b>ABEF</b>) and PS amplitude (<b>CDGH</b>), whereas rats in the control group exhibited no change in the size of evoked responses after a matching number of test pulses. Above graphs: representative traces of evoked responses from rats in the HFS group before (solid line) and after (dashed line) HFS. * denotes HFS-induced potentiation of evoked responses above baseline, <i>p</i><.05.</p

Probe tests.

<p>Among rats that underwent 1 day of training, rats in the HFS group exhibited a significant increase in latency (<b>AB</b>) and path length (<b>CD</b>) to the target location across consecutive probe tests, whereas rats in the control group exhibited no change across probe tests. HFS did not affect number of target crosses (<b>EF</b>), time spent in the target zone (<b>GH</b>), or time spent in the target quadrant (<b>IJ</b>). Among rats that underwent 4 days of training, HFS rats spent less time in the target zone than control rats during both probe tests (<b>QR</b>). Also, rats in the reactivation group exhibited short latency (<b>L</b>) and path length (<b>N</b>) during both probe tests, whereas rats in the no reactivation group exhibited a significant increase in latency (<b>K</b>) and path length (<b>M</b>) across consecutive probe tests. Rats in the reactivation group also spent more time in the target zone and target quadrant during both probe tests (<b>RT</b>) compared to rats in the no reactivation group (<b>QS</b>). Neither HFS nor reactivation affected number of target crosses (<b>OP</b>). * denotes main effect of probe test within HFS group (collapsed across reactivation/no reactivation groups), <i>p</i><.05.). # denotes main effect of HFS (collapsed across reactivation/no reactivation groups and probe tests), <i>p</i><.05. £ denotes main effect of probe test (collapsed across HFS/control and reactivation/no reactivation groups), <i>p</i><.05. § denotes main effect of probe test within no reactivation group (collapsed across HFS/control groups), <i>p</i><.05. ¥ denotes main effect of reactivation (collapsed across HFS/control groups and probe tests), <i>p</i><.05.</p

Evoked currents and ifenprodil sensitivity.

<p>A) Representative glutamatergic EPSCs. Representative traces from a combined AMPA/NMDA EPSC, an NMDA-mediated EPSC, and an AMPA-mediated EPSC calculated by subtracting the NMDA EPSC from the combined EPSC are shown. B) Representative evoked NMDA EPSCs. Sample traces of evoked NMDA EPSCs from both prenatal treatment conditions with amplitudes of total NMDA currents normalized between treatment conditions. C) Sensitivity of evoked NMDA EPSCs to ifenprodil. Mean percent (+SEM) reduction in evoked NMDA current following application of 3μM ifenprodil (n = 10 per prenatal treatment). Asterisk (*) indicates a significant effect (p = 0.002) of prenatal treatment condition on ifenprodil sensitivity.</p

Effect of prenatal ethanol exposure on the density of various glutamate receptor subtypes in frontal cortex.

<p>Data are the mean (SEM) specific binding, expressed as femtomoles bound / 10⁵ μm², for each radioligand for the total sample (in bold, N = 14) and separately for males (n = 7 per prenatal treatment) and females (n = 7 per prenatal treatment). R = Region, PT = Prenatal Treatment, S = Sex.</p><p>* indicates a significant effect at p < 0.05; See <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0118721#pone.0118721.g001" target="_blank">Fig. 1D</a> for additional data relevant to characterization of the PTxR interaction for specific [³H]-ifenprodil binding.</p><p>Effect of prenatal ethanol exposure on the density of various glutamate receptor subtypes in frontal cortex.</p

Effects of daily four-hour consumption of 5% ethanol on rat dams and their offspring.

<p>Data are mean (SEM) with group sample size.</p><p>Effects of daily four-hour consumption of 5% ethanol on rat dams and their offspring.</p

Characteristics of average sEPSC and mEPSC waveforms fit with a dual exponent function (A) and multiplicity ratio data (B).

<p>Data are mean (SEM) for each characteristic for the total sample (N = 10 animals per prenatal treatment) and separately for males (n = 5 animals per prenatal treatment) and females (n = 5 per prenatal treatment). There were no significant main effects of prenatal treatment or prenatal treatment X sex interactions at p < 0.05.</p><p>*indicates a significant effect at p<0.05 for <b>sex main effects</b> for either the sEPSC[s] or mEPSC[m].</p><p>** p<0.01</p><p>***p<0.001</p><p>Characteristics of average sEPSC and mEPSC waveforms fit with a dual exponent function (A) and multiplicity ratio data (B).</p