Constant Light Desynchronizes Olfactory versus Object and Visuospatial Recognition Memory Performance

Abstract

Circadian rhythms optimize physiology and behavior to the varying demands of the 24 h day. The master circadian clock is located in the suprachiasmatic nuclei (SCN) of the hypothalamus and it regulates circadian oscillators in tissues throughout the body to prevent internal desynchrony. Here, we demonstrate for the first time that, under standard 12 h:12 h light/dark (LD) cycles, object, visuospatial, and olfactory recognition performance in C57BL/6J mice is consistently better at midday relative to midnight. However, under repeated exposure to constant light (rLL), recognition performance becomes desynchronized, with object and visuospatial performance better at subjective midday and olfactory performance better at subjective midnight. This desynchrony in behavioral performance is mirrored by changes in expression of the canonical clock genes Period1 and Period2 (Per1 and Per2), as well as the immediate-early gene Fos in the SCN, dorsal hippocampus, and olfactory bulb. Under rLL, rhythmic Per1 and Fos expression is attenuated in the SCN. In contrast, hippocampal gene expression remains rhythmic, mirroring object and visuospatial performance. Strikingly, Per1 and Fos expression in the olfactory bulb is reversed, mirroring the inverted olfactory performance. Temporal desynchrony among these regions does not result in arrhythmicity because core body temperature and exploratory activity rhythms persist under rLL. Our data provide the first demonstration that abnormal lighting conditions can give rise to temporal desynchrony between autonomous circadian oscillators in different regions, with different consequences for performance across different sensory domains. Such a dispersed network of dissociable circadian oscillators may provide greater flexibility when faced with conflicting environmental signals.SIGNIFICANCE STATEMENT A master circadian clock in the suprachiasmatic nuclei (SCN) of the hypothalamus regulates physiology and behavior across the 24 h day by synchronizing peripheral clocks throughout the brain and body. Without the SCN, these peripheral clocks rapidly become desynchronized. Here, we provide a unique demonstration that, under lighting conditions in which the central clock in the SCN is dampened, peripheral oscillators in the hippocampus and olfactory bulb become desynchronized, along with the behavioral processes mediated by these clocks. Multiple clocks that adopt different phase relationships may enable processes occurring in different brain regions to be optimized to specific phases of the 24 h day. Moreover, such a dispersed network of dissociable circadian clocks may provide greater flexibility when faced with conflicting environmental signals (e.g., seasonal changes in photoperiod).

Keywords: circadian; clock genes; hippocampus; internal desynchrony; olfactory bulb; suprachiasmatic nuclei.

Figure 1.

rLL desynchronizes object and visuospatial versus odor recognition memory performance. A, Under the standard 12 h:12 h LD condition (left), each mouse was given different types of recognition task repeatedly at midday (ZT 6) and midnight (ZT 18). Under rLL (right), the lighting condition was alternated between 2 d of LD and 2 d of LL and mice were tested at subjective midday (CT 6) and at subjective midnight (CT 18) starting from the second cycle of LL. Red and blue stars show the different time points at which recognition tasks were given. Mice in Subgroup 1 were given different recognition tasks at ZT/CT 6, 18, 18, 6, 18, 6, 6, 18, etc., across days, whereas Subgroup 2 received the reverse arrangement (i.e., ZT/CT 18, 6, 6, 18, etc.). B, Under LD, performance in the object, object-in-place, and odor recognition tasks was consistently better at ZT 6 than at ZT 18. Under rLL, object and visuospatial performance was better at CT 6 than at CT 18, similar to the results under LD (top and middle rows); asterisks (*p < 0.05) indicate main effects of time of day on recognition scores (N − F)/(N + F), where N and F represent novel versus familiar objects, object–place combinations, or odors. However, odor recognition performance was phase shifted and became better at CT 18 than at CT 6, resulting in a significant lighting condition × time of day interactive effect on odor recognition scores (bottom row); asterisks (*p < 0.05) indicate simple main effects of time of day. In the object recognition task, n = 32 under LD and n = 8 under rLL; in the object-in-place task, n = 16 under LD and n = 16 under rLL; in the odor recognition task, n = 8 under LD and n = 24 under rLL. Error bars indicate SEM.

Figure 2.

A single 48 h period of light (control LL) is not sufficient to desynchronize object and visuospatial versus odor recognition memory performance. A, Under control LL, each mouse was given a one-off 48 h period of light and received the object, object-in-place, or odor recognition task at CT 6 and 18 in a counterbalanced order. Mice in Subgroup 1 were given the recognition task first at CT 6 and subsequently at CT 18, whereas in Subgroup 2 mice were tested first at CT 18 and subsequently at CT 6. B, Performance in the object, object-in-place, and odor recognition tasks was consistently better at CT 6, as indicated by the main effect of time of day (*p < 0.05). The task × time of day interaction was not significant, indicating that control LL did not induce a significant phase shift in odor recognition performance. However, performance in the odor task was generally poorer than performance in the object task (†Bonferroni post hoc comparison, p < 0.05). In the object and odor recognition tasks, n = 16 per task; in the object-in-place task, n = 8. Error bars indicate SEM.

Figure 3.

Core body temperature rhythms are maintained after four cycles of alternation between LD and LL in five mice. A, Example of the normalized core body temperature rhythm pooled across blocks and mice. The mean ± SEM core body temperature prior to normalization was 36.29°C ± 0.14°C under LD, and it was 36.14°C ± 0.15°C under LL (pooled across blocks). Inset, Normalized time series from each 48 h period was fitted with a sinusoidal function, Asin(ωt + ϕ), to determine the amplitude (A), period length τ (2π/ω), and phase (ϕ) of each mouse's body temperature rhythm. B–D, Mean values of the three parameters across 16 d of telemetry recording. There was no significant effect on amplitude values (B). The mean τ was longer during 48 h periods of LL than during LD (main effect of lighting condition, p < 0.01; C) and there was a significant change in Δϕ across blocks (main effect of block, p < 0.0005; D). These slight changes in circadian rhythms (i.e., period lengthening and phase delay of <1.5 h) cannot account for the complete reversal of odor recognition performance under rLL. Error bars indicate SEM in B–D.

Figure 4.

rLL produces differential effects on the canonical clock genes Per1 and Per2 and the immediate-early gene Fos mRNA expression in the SCN (DAMPENED), dorsal hippocampus (NO CHANGE), and olfactory bulb (PHASE SHIFTED). A–C, Gene expression in the SCN was dampened in mice housed under rLL after 4 cycles of LD–LL alternation (n = 6 at CT 6 and n = 6 at CT 18 for each gene) relative to animals housed under LD (n = 6 at ZT 6 and n = 6 at ZT 18 for each gene). Red asterisk and daggers in A indicate the main effect of lighting condition (p < 0.05) and main effect of time of day (p < 0.005), respectively, whereas gray asterisk and dagger in C indicate the marginally significant lighting condition × time of day interaction and marginal main effect of time of day (both p = 0.057). D–F, Mean Per1, Per2, and Fos mRNA levels in the hippocampus were unaffected by rLL. Gene expression was consistently higher at ZT 6 than at ZT 18 for mice housed under LD (Per1 and Per2: n = 15 at ZT 6 and n = 14 at ZT 18; Fos: n = 6 at ZT 6 and n = 6 at ZT 18) and higher at CT 6 than at CT 18 for mice housed under rLL (Per1: n = 10 at CT 6 and n = 12 at CT 18; Per2: n = 10 at CT 6 and n = 14 at CT 18; Fos: n = 6 at CT 6 and n = 5 at CT 18). Daggers in D–F indicate main effects of time of day (all p < 0.025). G–I, Gene expression in the olfactory bulb was phased shifted in mice housed under rLL (Per1: n = 10 at CT 6 and n = 14 at CT 18; Per2: n = 12 at CT 6 and n = 14 at CT 18; Fos: n = 4 at CT 6 and n = 6 at CT 18) relative to animals housed under LD (Per1: n = 13 at ZT 6 and n = 15 at ZT 18; Per2: n = 16 at ZT 6 and n = 15 at ZT 18; Fos: n = 6 at ZT 6 and n = 6 at ZT 18). Asterisks in G and I indicate lighting condition × time of day interactions (p ≤ 0.05); double asterisk in G indicates the simple main effect of lighting condition at ZT/CT 18 (p < 0.01); dagger in I indicates the simple main effect of time of day under LD (p < 0.005). Error bars indicate SEM in all panels.

Figure 5.

Model describing the effect of rLL on the SCN and its knock-on effect on peripheral oscillators. A, Under LD, circadian oscillators in the olfactory bulb and hippocampus are tightly coupled to the central SCN pacemaker, allowing extra-SCN oscillators to maintain a constant phase relationship with the SCN, as well as with one another. B, Under rLL, the central SCN pacemaker is dampened due to abnormal light inputs and this weakens the coupling between the SCN and extra-SCN oscillators. Without a strong coupling with the SCN, the circadian oscillator in the olfactory bulb becomes autonomous, resulting in a shift in the circadian phase of odor recognition performance. This model is based on multiple sources, including Herzog and Tosini (2001), Guilding and Piggins (2007), Dibner et al. (2010), Kyriacou and Hastings (2010), and Granados-Fuentes et al. (2011). Schematics of the mouse brain are adapted from the PPT Drawing Toolkit–Neuroscience (illustrations 7111409 and 7111412; Motifolio). HPC, Hippocampus; OB, olfactory bulb; RHT, retinohypothalamic tract.