Light and time-of-day differentially impact synaptic excitability and plasticity across the mouse hippocampus-prefrontal cortex network

Background Light, acting via the circadian system or other brain networks, can exert both immediate and longer-term impacts on memory performance. Despite the clear behavioural evidence for such effects, information about how light and time-of-day directly shape the activity of integrated brain memory circuits remains scarce. To this end, we employed large-scale in vivo electrophysiological recordings in anaesthetised mice, to assess synaptic excitability and plasticity across the hippocampus, medial prefrontal cortex (mPFC) and nucleus reuniens (NRe) circuit that underpins key aspects of learning and memory. Results Notably, we found an enhancement of CA1 synaptic excitability during the day vs. night but antiphase nocturnal increases in short-term and long-term synaptic plasticity, specifically localised to the CA1 molecular layer. Moreover, we found a time-of-day-dependent impact of acute light exposure on mPFC responses to NRe (but not hippocampal) input, with light driving increased synaptic facilitation during the day and suppressing it at night. Conclusion In sum, our data provide new insight into the neural mechanisms that underpin daily variations in memory performance and their modulation by environmental light.