Hippocampal ensemble discharge during the stable and rotating conditions.

<p>(A) A photograph of a rat on the arena (A1) and a schematic drawing of the apparatus during the rotating condition (A2). A rat was placed on a circular arena that was surrounded by a black curtain with a white cue card. The arena rotated slowly in the rotating condition and was otherwise stable. The rat was reinforced to avoid two unmarked shock zones. A room frame shock zone (red) was defined relative to room landmarks and did not rotate. An arena frame shock zone (blue) was defined relative to arena landmarks and rotated together with the arena as indicated by the blue arrow. (B) Schematics of the experimental protocol. Each hippocampal ensemble was recorded during one session of rotating condition, flanked by two sessions of the stable condition. (C) Raster plots of activity of an ensemble of 15 cells during the stable and rotating conditions in the same environment. For each 10-s interval, the ensemble activity was characterized by a spike-count vector (red rectangles). The similarity of ensemble activity during any two intervals was assessed by computing the Pearson correlation between the corresponding ensemble vectors. (D) The correlation matrix shows that the correlation of ensemble activity for each pair of 10-s intervals recorded during the stable condition tends to be high. Similarly, the intervals recorded during the rotating condition tend to have highly correlated activity. Intervals during rotation were often dissimilar to the intervals during the stable condition, as is indicated by blue pixels. (E) Average of the correlation between ensemble activity during different intervals in the stable and rotating conditions (data from all recordings). Correlations are high when activity in the same conditions is compared and lower when activity is compared between the stationary and rotating conditions.</p

Influence of external cues on cross-episode retrieval.

<p>(A) Examples comparing the ability to decode the rat's position during the stable and rotating conditions. The upper 10 plots display the 10 intervals with the best decoding during the stable condition. The middle 10 plots display the 10 cross-episode intervals with the best decoding. The bottom 10 plots display the 10 intra-episode intervals with the best decoding. In all cases the position was decoded using the spatial activity during the stable session as a template. The observed position of the rat is shown in black, the decoded position in red. The number next to each plot indicates the decoding error in cm. (B) The proportion of 1-s cross-episode intervals was computed for different angular displacements of the arena from its orientation in the stable condition. The probability of cross-episode retrieval was highest when the arena displacement was close to 0°—similar to the arena orientation in the stable condition. (C) The average arena displacement vectors are shown for each session during the cross-episode intervals and the intra-episode intervals. The distribution was not random in 11 out of 12 ensemble recording sessions (Raleigh's test, <i>p</i>s<0.005); cross-episode retrieval was most likely when the arena displacement within the room was between 270° and 360°. (D) The position of the rat during cross-episode intervals and intra-episode intervals is shown in the spatial frame of the room (D1) and the spatial frame of the arena (D2). Red and blue dots mark the beginnings and ends of the intervals. The data from a single session are shown.</p

Organization of hippocampal ensemble activity according to two distinct categories of spatial information.

<p>(A) Cross-correlation plots indicating probability of the code for spatial frame-specific position (red) and the code for task variant (blue) to switch to representing the other value. The code for spatial frame: Time 0 marks that activity was preferentially signaling position in either the room or the rotating arena. The <i>y</i>-axis shows the probability of preferentially signaling position in the other frame within the time interval indicated on the <i>x</i>-axis. The probability of observing activity that represents position in the other frame remains low for approximately 7 s when it reaches a plateau. The code for task variant: Time 0 is the time that cross-episode or intra-episode activity was preferentially expressed, and the <i>y</i>-axis shows the probability of the other activity pattern being expressed. The probability of observing the other pattern remains low for about twice as long as is the case for the spatial frame code, before reaching a plateau. Thus the code for task variant predicts activity further into the future than the code for spatial frame. (B1) The firing rate similarity between room frame intervals and arena frame intervals was high. The similarity was indistinguishable from the similarity of firing rates during two stable sessions in the same environment, and it was greater than is expected by chance. (B2) The firing rate similarity between cross-episode intervals and intra-episode intervals was low. The similarity was not higher than would be expected by chance.</p

The probability of cross-episode retrieval is influenced by ongoing hippocampal ensemble activity.

<p>(A) The temporal dynamics of the code for task variant and code for spatial frame during a single rotating session flanked by stable sessions. (Upper plot) The code for task variant. A time series measuring the similarity (r transformed to z scores) of ongoing ensemble activity to activity during the stable and rotating conditions. Cross-episode retrieval is apparent as deflections during rotation that are similar to the values during the stable condition. (Lower plot) The code for spatial frame. A time series measuring the preference in ongoing ensemble activity for representing information about the current position in the room or arena. Each few seconds, positional information toggles between preferentially representing positions in the room (red) or arena (blue) spatial frame. (B) Averaged normalized spatial frame preference during rotating condition intervals of cross-episode retrieval and intra-episode activity. Time 0 corresponds to the moment when cross-episode activity was detected (orange) and the time when intra-episode activity was detected (purple). The analysis shows that cross-episode retrieval is preferentially associated with information about position in the room, whereas the intra-episode activity during rotation is preferentially associated with information about position on the arena.</p

Experiments 5 and 6—Stress is necessary for the forced swim to alter consolidated memories.

<p>(A) Experiment 5—Propranolol caused amnesia of inhibitory avoidance memory only if it was administered after the forced swim. Rats were trained in the inhibitory avoidance paradigm on Day 1. On Day 2, they were either forced to swim (Sw) or just handled (NoSw), and immediately afterwards injected with 10 mg/ml/kg Propranolol or saline (NoSw-Sal <i>n</i> = 11, NoSw-Pro <i>n</i> = 10, Sw-Sal <i>n</i> = 10, Sw-Pro <i>n</i> = 11). Rats in the Sw-delPro group (<i>n</i> = 12) were injected with Propranolol 5 h after the swim. The average ± step-through latencies on Day 1 were: NoSw-Sal = 8.5±1.7; NoSw-Pro = 6.5±0.79; Sw-Sal = 7.4±1.5; Sw-Pro = 5.5±0.91; Sw-delPro = 7.9±0.98). The step-through latencies recorded on Day 3 are plotted. The groups did not differ on Day 1, but they differed on Day 3 (F_4,49 = 3.0; <i>p</i> = 0.03). Amnesia, manifested as reduced step-through latencies, was observed only in the Sw-Pro group (all post hoc <i>p</i>s <0.05) (* <i>p</i><0.05). The data indicate the swim activated the consolidated memory. Whether or not inhibitory avoidance was enhanced by the swim could not be determined in this experiment because performance was already maximal after the single conditioning trial. The data cannot be explained by previous work showing that exposure to a novel alerting stimulus can enhance retrieval of conditioned inhibitory avoidance because in that case, beta-endorphin activation triggered beta-noradrenergic and cholinergic processes that acted at the time of retrieval only if the retrieval test was given within less than 6 h <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.1000570#pbio.1000570-Izquierdo2" target="_blank">[63]</a>,<a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.1000570#pbio.1000570-Netto1" target="_blank">[64]</a>. (B) Experiment 6—Dexamethasone blocks the swim-induced enhancement of memory. Rats were trained in the intensive left/right discrimination on Day 1. The next day, Dexamethasone (Dex; 0.2 mg/kg i.p.) or saline (Sal) was administered 2 h prior to the forced swim (Sw) or no swim (NoSw) shallow-water control treatments. Retention of the Day 1 left/right discrimination memory was tested on Day 3 by the reversal test. Enhanced memory was observed in the saline-treated animals that were forced to swim (Sal-Sw), but the effect was blocked by the action of Dexamethasone in the (Dex-Sw) group. (* <i>p</i><0.05 compared to the saline-treated no swim (Sal-NoSw) control group).</p

Experiments 8 and 9—Acquisition and retrieval of left-right discrimination does not depend on hippocampus but the swim-induced enhancement of memory does.

<p>(A) Experiment 8a—Bilateral TTX inactivation of dorsal hippocampus in the D1-TTX (black, <i>n</i> = 9) group did not influence left/right discrimination learning in the Y-maze task compared with saline controls (D1-Sal, white, <i>n</i> = 11; <i>p</i>>0.05). (B) Experiment 8b—Another two groups of animals were trained on Day 1 with the intensive training protocol. One hour before the Day 3 reversal test, TTX (D3-TTX, black, <i>n</i> = 7) or saline (D3-Sal, white, <i>n</i> = 11) was infused into both dorsal hippocampi. The TTX injection did not impair retrieval. In fact, there was an opposite tendency for enhanced retrieval in the hippocampus-inactivated group <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.1000570#pbio.1000570-McDonald1" target="_blank">[65]</a>, but the trend did not reach significance (<i>p</i>>0.05). Thus hippocampus was not necessary for learning or expressing left/right discrimination memory. (C) Experiment 9—Hippocampus was necessary for the swim-induced enhancement of memory. Left/right discrimination was conditioned on Day 1 using the intensive training protocol. On Day 2, rats received bilateral intrahippocampal injections of saline (Sw-Sal, white, <i>n</i> = 11) or TTX (Sw-TTX, black, <i>n</i> = 8), and 1 h later they were forced to swim. Memory was tested by reversal training on Day 3. The numbers of to-criterion errors are reported. The TTX injection attenuated the swim-induced memory enhancement (t₁₇ = 3.47; <i>p</i> = 0.003) (* <i>p</i><0.01). The placement of 20 bilateral injections are depicted on schematic coronal sections <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.1000570#pbio.1000570-Paxinos1" target="_blank">[66]</a>. The number indicates the section's location posterior to bregma.</p

Experiment 10—The swim-induced inter-hemispheric transfer of lateralized memory required hippocampal function.

<p>Left/right discrimination was conditioned on Day 1 using the intensive training protocol with one hemicortex inactivated by cortical spreading depression (CSD). On Day 2 rats received bilateral hippocampal injections of saline (Lat-Sw-Sal, white, <i>n</i> = 11) or lidocaine (Lat-Sw-Lid, black, <i>n</i> = 21), and then they were forced to swim. Memory was assessed by reversal training 2 h after the swim with the originally trained hemicortex inactivated by CSD. The number of to-criterion errors is reported. The groups did not differ on Day 1 but they differed on Day 2 (t₃₀ = 2.27; <i>p</i> = 0.03). The Day 1 memory was lateralized because rats in the Lat-Sw-Lid group performed as if naïve on Day 2. The swim induced IHT of the lateralized memory because rats in the Lat-Sw-Sal group made more reversal errors on Day 2 (* <i>p</i><0.05).</p