Two distinct ways to form long-term object recognition memory during sleep and wakefulness

Abstract

Memory consolidation is promoted by sleep. However, there is also evidence for consolidation into long-term memory during wakefulness via processes that preferentially affect nonhippocampal representations. We compared, in rats, the effects of 2-h postencoding periods of sleep and wakefulness on the formation of long-term memory for objects and their associated environmental contexts. We employed a novel-object recognition (NOR) task, using object exploration and exploratory rearing as behavioral indicators of these memories. Remote recall testing (after 1 wk) confirmed significant long-term NOR memory under both conditions, with NOR memory after sleep predicted by the occurrence of EEG spindle-slow oscillation coupling. Rats in the sleep group decreased their exploratory rearing at recall testing, revealing successful recall of the environmental context. By contrast, rats that stayed awake after encoding showed equally high levels of rearing upon remote testing as during encoding, indicating that context memory was lost. Disruption of hippocampal function during the postencoding interval (by muscimol administration) suppressed long-term NOR memory together with context memory formation when animals slept, but enhanced NOR memory when they were awake during this interval. Testing remote recall in a context different from that during encoding impaired NOR memory in the sleep condition, while exploratory rearing was increased. By contrast, NOR memory in the wake rats was preserved and actually superior to that after sleep. Our findings indicate two distinct modes of long-term memory formation: Sleep consolidation is hippocampus dependent and implicates event-context binding, whereas wake consolidation is impaired by hippocampal activation and strengthens context-independent representations.

Keywords: memory consolidation; novel-object recognition memory; sleep; wake.

Fig. 1.

Remote NOR memory is present after postencoding sleep and wakefulness, but is oppositely influenced by postencoding inactivation of the hippocampus. (A) Experimental design. Rats explored two identical objects in an arena for 10 min (encoding), and then either slept or remained awake for 2 h. Retrieval of NOR memory was tested a week later. For retrieval testing, one of the two objects used in the encoding phase was replaced by a novel object. Recognition memory was assessed based on the increased exploration time the rat devoted to the novel object in comparison to that spent exploring the old object during the first 3 min of the retrieval phase (object discrimination ratio). To inactivate the hippocampus during the 2-h postencoding interval, muscimol was bilaterally infused into the dorsal hippocampus, either upon the first occurrence of continuous SWS or at a comparable time point during postencoding wakefulness. Controls included rats infused with vehicle and untreated rats. (B) Mean (±SEM) object discrimination ratios at 1 and 3 min into the remote retrieval phase, for animals of the sleep (Left) and wake (Right) groups with functioning hippocampus (empty bars), and following infusion with muscimol (red bars) during the 2-h postencoding interval n = 11, 11, 10, and 8 rats for sleep control, sleep muscimol, wake control, and wake muscimol groups, respectively. ⁺⁺⁺P < 0.001, ⁺⁺P < 0.01, ⁺P < 0.05 for one-sample t tests against chance level; **P < 0.01, *P < 0.05 for pairwise t tests (two-sided) between muscimol and control groups.

Fig. 2.

Postencoding sleep, but not wakefulness, decreases exploratory rearing at remote NOR testing. (A) Illustration of the two prominent exploratory behaviors on the NOR task. In addition to object exploration (object sniffing), rearing on the hind legs was analyzed to assess to what extent remote retrieval involved recall of environmental context, that is, distal spatial cues. (B) Percentage change in mean rearing duration (mean ± SEM) at first and third minutes of retrieval testing, compared to mean rearing duration during encoding (set to 100%). Only rats that had slept after encoding (Left) and retained hippocampus function (empty bars) showed a decrease in rearing duration at retrieval, whereas mean rearing duration in the rats that remained awake (Right) and in the rats whose hippocampi were inactivated during the postencoding interval (red bars) did not change between encoding and remote retrieval testing; n = 11, 11, 10, and 8 rats for sleep control, sleep muscimol, wake control, and wake muscimol groups, respectively. +++P < 0.001, for one-sample t tests against chance level; ***P < 0.001, **P < 0.01, for pairwise t tests (two-sided) between muscimol and control groups.

Fig. 3.

Remote NOR memory after postencoding sleep, but not after postencoding wakefulness, is context dependent. (A) Experimental procedures. During encoding, rats explored two identical objects in an arena of context A for 10 min, followed by a 2-h interval in which the rats either slept or remained awake. Retrieval of NOR memory was tested a week later in a different (but familiar) context B. Contexts A and B differed with respect to distal and proximal cues, and olfactory and auditory stimulation, and also with regard to the experimenters who performed the experiments (see Materials and Methods). (B) (Left) Mean (±SEM) object discrimination ratios at first and third minutes of the retrieval phase for animals of the sleep (gray bars) and wake (empty bars) groups, and (Right) percentage change in mean rearing duration (mean ± SEM) at first and third minutes of retrieval phase, compared to mean rearing duration during encoding (set to 100%). Note that rats that slept after encoding (gray bars) showed a decrease in NOR memory performance, but an increase in rearing duration, relative to the wake rats that remained awake during the 2-h postencoding interval (empty bars); n = 12 rats each for sleep and wake groups. ⁺⁺⁺P < 0.001, ⁺P < 0.05 for one-sample t tests against chance level; *P < 0.05 for pairwise t tests (two-sided) between groups.

Fig. 4.

Associations between behavioral performance at remote retrieval and sleep spindle–related activity during postencoding SWS. (A) Individual EEG traces from one rat (over the left prefrontal cortex) after intrahippocampal infusion of vehicle (Left) or muscimol (Right) during postencoding sleep. Original recordings are shown before (Top) and after filtering in the slow oscillatory (0.3 Hz to 4.5 Hz, Middle) and spindle (10 Hz to 16 Hz, Bottom) frequency bands. (B) Time-frequency power plots −2 s to +2 s around the negative peak (0 s) of the EEG SOs (overlaid solid line, right y axis). Data represent averages across all identified SO events and all animals during the 2-h postencoding sleep interval after vehicle (Left) and muscimol infusion (Right), n = 10 rats in each condition. Power is color coded. Note the increase in spindle-related power nesting in the upstate of the SO in both conditions (arrows). (C) NOR performance (discrimination ratios; Left), but not mean rearing duration (indicating spatial context memory; Right) at retrieval testing, is positively correlated with the number of sleep spindles and (D) the number of coupled SO–spindle events. Vehicle, gray dot plots; muscimol, red dot plots. Note that the correlation of NOR with SO–spindle event numbers is also observed after muscimol infusion. Spearman correlation coefficients and P values (uncorrected for multiple comparison) are indicated. Inserted bar graphs indicate the mean (±SEM) number of identified spindles and SO–spindle events identified during the 2-h postencoding interval.