Circadian Time-Place Learning in Mice Depends on Cry Genes

Abstract

Endogenous biological clocks allow organisms to anticipate daily environmental cycles. The ability to achieve time-place associations is key to the survival and reproductive success of animals. The ability to link the location of a stimulus (usually food) with time of day has been coined time-place learning, but its circadian nature was only shown in honeybees and birds. So far, an unambiguous circadian time-place-learning paradigm for mammals is lacking. We studied whether expression of the clock gene Cryptochrome (Cry), crucial for circadian timing, is a prerequisite for time-place learning. Time-place learning in mice was achieved by developing a novel paradigm in which food reward at specific times of day was counterbalanced by the penalty of receiving a mild footshock. Mice lacking the core clock genes Cry1 and Cry2 (Cry double knockout mice; Cry1(-/-)Cry2(-/-)) learned to avoid unpleasant sensory experiences (mild footshock) and could locate a food reward in a spatial learning task (place preference). These mice failed, however, to learn time-place associations. This specific learning and memory deficit shows that a Cry-gene dependent circadian timing system underlies the utilization of time of day information. These results reveal a new functional role of the mammalian circadian timing system.

Time–place learning over a lifetime: absence of memory loss in trained old mice

“…In a pilot study, we found a 26% increase in CRY2 (the protein product of the core molecular clock gene Cry2) positive cells in the hippocampal dentate gyrus of TPL-trained mice compared with homecage control mice (data not included). In line with this, we previously showed that young-adult Cry1/Cry2 double knockout mice were unable to acquire TPL (Van der Zee et al 2008). Therefore, we suspect that experience-related cues entrain local hippocampal timekeeping mechanisms that are functional to cTPL ).…”

“…Whether an animal is capable of mastering a TPL task may depend on the species or used strain, but is also greatly determined by the experimental setup (see Mulder et al 2013b). Previously, we designed a now well-established TPL paradigm for mice based on a balanced approach between reward and punishment ( Van der Zee et al 2008). Mice are food deprived to 85% of ad libitum body weight.…”

“…This paradigm emulates the natural situation in which hungry animals seek food while different feeding locations can be predictably safe or unsafe to visit depending on the TOD. We have shown repeatedly that young wild-type mice readily acquire this paradigm and will structurally use a circadian strategy (Van der Zee et al 2008;Mulder et al 2013aMulder et al , 2014.…”

Functional motifs in the (6-4) photolyase crystal structure make a comparative framework for DNA repair photolyases and clock cryptochromes

“…In vertebrates having both (6-4) PHR and clock CRY proteins, (6-4) PHR does not disturb the circadian clock, and conversely clockwork CRYs do not appear to directly contribute to DNA repair (22,23). Clockwork CRYs from different species have also been implicated in diverse processes, including nonvisual photoreception, sun compass orientation, and time-place learning (24)(25)(26)(27)(28)(29)(30)(31)). Yet, delays in mechanistic characterization of clock CRYs, caused in part by technical difficulties in protein expression and thus structure determination, have hampered biological understanding.…”

The effects of response cost and species-typical behaviors on a daily time–place learning task

“…While daily TPL has been well documented in a variety of species, including birds (garden warblers, Biebach, Gordijn, & Krebs, 1989;pigeons, Saksida & Wilkie, 1994), fish (inangas, Reebs, 1999;golden shiner, Reebs, 1996), honeybees (Wahl, 1932, as cited in Reebs, 1993), ants (Schatz, Beugnon, & Lachaud, 1994), and mice (Van der Zee et al, 2008), research with rats has surprisingly resulted in inconsistent results (see Thorpe &Wilkie, 2006, for a review). The ease with which researchers demonstrate daily TPL in rats seems to be dependent, in part, on the type of task.…”