Long days enhance recognition memory and increase insulin-like growth factor 2 in the hippocampus

Abstract

Light improves cognitive function in humans; however, the neurobiological mechanisms underlying positive effects of light remain unclear. One obstacle is that most rodent models have employed lighting conditions that cause cognitive deficits rather than improvements. Here we have developed a mouse model where light improves cognitive function, which provides insight into mechanisms underlying positive effects of light. To increase light exposure without eliminating daily rhythms, we exposed mice to either a standard photoperiod or a long day photoperiod. Long days enhanced long-term recognition memory, and this effect was abolished by loss of the photopigment melanopsin. Further, long days markedly altered hippocampal clock function and elevated transcription of Insulin-like Growth Factor2 (Igf2). Up-regulation of Igf2 occurred in tandem with suppression of its transcriptional repressor Wilm’s tumor1. Consistent with molecular de-repression of Igf2, IGF2 expression was increased in the hippocampus before and after memory training. Lastly, long days occluded IGF2-induced improvements in recognition memory. Collectively, these results suggest that light changes hippocampal clock function to alter memory, highlighting novel mechanisms that may contribute to the positive effects of light. Furthermore, this study provides insight into how the circadian clock can regulate hippocampus-dependent learning by controlling molecular processes required for memory consolidation.

Jet Lag Recovery and Memory Functions Are Correlated with Direct Light Effects on Locomotion

“…To restrict our analyses to the ZT14-15 time point during jet lag, we analyzed short-term memory with the object placement and novel object recognition tests. In a study that was not as ZT restricted as our study, testing memory at 2 h or 24 h after the initial pretest demonstrated that a 20:4 LD cycle positively affected memory performance when compared with 12:12 LD (Dellapolla et al, 2017). Although we did not observe a difference in memory in our pre–jet lag analyses, this study supports our findings during jet lag that the least severe disruption to memory was observed in 16:8 LD compared with 8:16 LD.…”

Effect of photoperiod and light intensity on learning ability and memory formation of the pond snail Lymnaea stagnalis

“…Importantly, we can conclude that the pond snail's ability to learn, consolidate, and maintain memory may be impaired in habitats where they are exposed to a bright artificial nightlight. This finding is consistent with previous reports suggesting that photoperiod influences circadian activity, learning, and memory in mammals (Dellapolla et al 2017;LeGates et al 2012;Loh et al 2010;Ma et al 2007;MacDonald et al 2007;Smarr et al 2014). Moreover, a recent study using Drosophila melanogaster also has emphasized that disrupting its light-dark cycle effect LTM (Inami et al 2020).…”

Photoperiod-Induced Neuroplasticity in the Circadian System

“…In contrast to other studies in which light produces cognitive deficits in nocturnal rodents, Dellapolla et al [ 116 ] found that long photoperiod enhances long-term recognition memory in mice, measured by spatial object recognition and novel object recognition tasks. Interestingly, whereas clock genes in the hippocampus were rhythmic in a neutral 12 hr photoperiod, rhythms of Per1 , Per2 , Cry1 , and Cry2 were reduced in long days.…”