We used a trephine drill to eliminate a circular part of the skull centered posterio-lateral to the viral injection site. location in which events occurred (O’Keefe and Dostrovsky, 1971; O’Keefe, 1978). Whereas ample knowledge exists regarding the encoding of location, relatively little is known Irbesartan (Avapro) about the neural mechanisms that enable the encoding of the time in which events occur. Recent work has revealed that in familiar environments hippocampal place cell activity is dynamic over timescales that range from minutes to weeks (Howard and Kahana, 2002; Mankin et al., 2015; Mankin et al., 2012; Manns et al., 2007; Ziv et al., 2013). For timescales that are greater than one day, these dynamics primarily result from ongoing changes in the subsets of place cells that are active during repeated visits to the same fixed environment (Ziv et al., 2013). Such dynamics may contribute information about the temporal relationship between events by providing a unique code that functions as a timestamp. If such timestamps exist, they would likely aid long-term memory by reducing interference between traces of events that occur at different times at the same Irbesartan (Avapro) place, or that are similar in that they share contextual components such as sensory experience and behavior. Moreover, to support the formation of a mental timeline of experienced events in long-term memory, and the capacity to mentally time-travel during memory recall (Kragel et al., 2015; Nyberg et al., 2010), timestamps should change gradually and continuously with time. Such gradual changes in the ensembles of place cells active during similar events on different days have been recently reported, but the extent that these dynamics actually carry temporal information remains unclear (Mankin et al., 2012; Ziv et al., 2013). We consider two alternative hypotheses regarding the possible contribution of the observed dynamics to Irbesartan (Avapro) coding of time. According to one hypothesis, the dynamics in the ensemble activity over days is unique to the environment in which it is observed, and independent from the dynamics in other, dissimilar environments. In this case, the dynamics may contribute ordinal information about different events that occur within a given environment, but will not contribute to associations in memory between events that happen close in time if these events occurred in different or dissimilar environments. An alternative hypothesis asserts that certain aspects of the days-scale dynamics in the ensemble activity are common to different environments. Such environment-nonspecific dynamics could support a linkage in long-term memory between dissimilar events that occur at temporal proximity. If this is the case, we would expect the hippocampal representations of events that occur in different spatial environments but in temporal proximity (e.g. the same day) to share common time-varying components. To test these alternative hypotheses we investigated hippocampal neuronal representations of different spatial contexts over multiple days and weeks. We combined head-mounted miniaturized fluorescence microscopes (Ghosh et al., 2011; Ziv et al., 2013), chronic microendoscopy (Barretto et al., 2011), and viral-vector based expression of a genetically encoded Ca2+ indicator (Chen et al., 2013), to longitudinally image the Ca2+ dynamics of large populations ( 1,000 per mouse) of hippocampal CA1 pyramidal cells in freely behaving mice that repeatedly explored two familiar environments (Figure 1A). To avoid circadian effects we alternated the two environments between AM and PM sessions, 4C5 hr apart. Each session consisted of five 3-min trials. To maximize the perceived differences between the environments, we constructed linear tracks (environments A and B) that differed in shape, floor texture, surrounding proximal and distal visual cues, odor, and flavor of the Rabbit polyclonal to KBTBD8 water reward (see Materials and methods). To uncover the time-dependent coding dynamics, while minimizing changes in place codes induced by learning, we familiarized the mice with the two environments before starting the experiment (Figure 1A). During pre-training, the mice ran on each linear track for 15 min per day for 8C11 days, until they performed at least 60 laps in each environment for two consecutive days. Time-lapse imaging began two days after the last pre-training session, and was performed every other day for two weeks (days 1C15, Figure 1B). To confirm that changes in the representations of the two environments are also evident within a single session, we performed on days 16C17.