Effects of daytime, night and sleep pressure on long-term depression in the hippocampus in vivo

Although long-term depression (LTD) is generally considered as one of the underlying mechanisms of learning and memory, the induction of it in vivo seems difficult. Evidence demonstrates that the total synaptic weight is associated with circadian rhythm, with up-regulation in wakefulness and down-regulation during sleep, suggesting that the induction of LTD may also be affected by it. In this study, we found that in two well-established rat models, low-frequency stimuli (LFS) induced LTD upon daytime anesthesia, but not at night. Upon further study, we found that the induction of LTD could not be blocked at night if we deprived sleep of the rats during the daytime. These results indicate that the induction of LTD is facilitated by daytime or sleep deprivation. Since rats both in the daytime and after sleep deprivation share the same character of high sleep pressure, our results suggest that LTD is actually facilitated by high sleep pressure. Our study also provides a possible explanation why some labs can induce LTD in vivo while others cannot. Sleep pressure should be taken into account as one of the key factors on the induction of LTD in vivo. (C) 2012 Elsevier Ireland Ltd. All rights reserved.