Sleep lowers waking theta frequency in the rat hippocampus

Hippocampal theta oscillations coordinate computations underlying learning and memory. The frequency of theta varies with factors such as locomotion and anxiety, but the effect of an animal's sleep history on theta frequency remains unknown. Using long-duration CA1 recordings in rats, we found that awake theta frequency progressively decreases following sleep but remains elevated during sleep deprivation. These changes were not accounted for by movement but were predicted by the proportion of sleep in the preceding 30 min. In contrast, theta frequency remained relatively stable during rapid eye movement sleep. In aged rats, sleep-dependent frequency changes were less pronounced, likely due to sleep fragmentation. Systemic administration of the phosphodiesterase inhibitor rolipram during sleep deprivation reduced theta frequency, mimicking natural sleep. Collectively, these findings reveal a robust influence of sleep history on hippocampal theta frequency, with potential implications for understanding memory function, age-related sleep disruptions, and the potential for pharmacological interventions.