Neither the SCN nor the adrenals are required for circadian time-place learning in mice.
Bottom Line: During Time-Place Learning (TPL), animals link biological significant events (e.g. encountering predators, food, mates) with the location and time of occurrence in the environment.Abrupt FEO phase-shifts (induced by advancing and delaying feeding time) affected TPL performance in specific test sessions while a LEO phase-shift (induced by a light pulse) more severely affected TPL performance in all three daily test sessions.We conclude that, although cTPL is sensitive to timing manipulations with light as well as food, neither the SCN nor the adrenals are required for cTPL in mice.
Affiliation: Department of Molecular Neurobiology and.
During Time-Place Learning (TPL), animals link biological significant events (e.g. encountering predators, food, mates) with the location and time of occurrence in the environment. This allows animals to anticipate which locations to visit or avoid based on previous experience and knowledge of the current time of day. The TPL task applied in this study consists of three daily sessions in a three-arm maze, with a food reward at the end of each arm. During each session, mice should avoid one specific arm to avoid a foot-shock. We previously demonstrated that, rather than using external cue-based strategies, mice use an internal clock (circadian strategy) for TPL, referred to as circadian TPL (cTPL). It is unknown in which brain region(s) or peripheral organ(s) the consulted clock underlying cTPL resides. Three candidates were examined in this study: (a) the suprachiasmatic nucleus (SCN), a light entrainable oscillator (LEO) and considered the master circadian clock in the brain, (b) the food entrainable oscillator (FEO), entrained by restricted food availability, and (c) the adrenal glands, harboring an important peripheral oscillator. cTPL performance should be affected if the underlying oscillator system is abruptly phase-shifted. Therefore, we first investigated cTPL sensitivity to abrupt light and food shifts. Next we investigated cTPL in SCN-lesioned- and adrenalectomized mice. Abrupt FEO phase-shifts (induced by advancing and delaying feeding time) affected TPL performance in specific test sessions while a LEO phase-shift (induced by a light pulse) more severely affected TPL performance in all three daily test sessions. SCN-lesioned mice showed no TPL deficiencies compared to SHAM-lesioned mice. Moreover, both SHAM- and SCN-lesioned mice showed unaffected cTPL performance when re-tested after bilateral adrenalectomy. We conclude that, although cTPL is sensitive to timing manipulations with light as well as food, neither the SCN nor the adrenals are required for cTPL in mice.
Mentions: A detailed overview of the experiments and experimental groups is provided in Table 1. CORT measurements at TPL training times were performed on animals from experiments 1 and 4. Experiment 1 included intact mice, which had successfully mastered cTPL (TPL, n = 9), and homecage control mice (HCC, n = 8). Next to investigating differences between TPL-trained and HCC mice, the measurements from experiment 1 serve as a positive control for the measurements of the adrenalectomized mice from experiment 4 (Figure 7). The light pulse and food shift manipulations, to investigated LEO/FEO involvement in cTPL, were performed in experiment 2, including mice which had successfully mastered cTPL (n = 7) and two HCC mice, which were not food-deprived in contrast to all other HCC groups (so that we could clearly distinguish the effect size of the light pulse in these mice). The SCN lesion experiment (experiment 3) was performed in two separate batches. Mice of the first batch, three months old at reception, were habituated to the climate room and housing conditions for 1 month before receiving bilateral SCN lesions (n = 14) or SHAM lesions (n = 4). After recovery for at least 10 days, mice were phenotyped for arrhythmic running-wheel behavior in constant darkness (DD) over a 2 week period. Based on a visual and statistical rhythmicity assessment, five completely arrhythmic SCN-lesioned (SCNx) and all four SHAM-lesioned (SHAM) mice were selected for TPL testing (the maximal number of mice supported by the protocol). One week later mice were put back on LD and the spontaneous alternation (SA) test was performed. Ad libitum body weights were determined after the SA test and on the two following days, after which food deprivation (timed feeding) was initiated on the SCNx, SHAM and HCC mice (mice received minimally 1.5 g food per day). TPL testing was started the next day. Animals were tested daily during 38 days, starting with 10 days of habituation steps in LD, followed by 20 days of testing in LD, 3 days of testing in DD and 5 days of testing in LD. Session skips were performed on following days, with the number between brackets indicating which session was skipped: 14(1); 16(2); 21(1); 22(1,2,3); 35(1). Animals were sacrificed the day after their last TPL test day, at the time of their first daily test session (deviation ± 5 minutes).