Dim Light at Night Causes a Loss in the 24-Hour Rhythm of Heart Rate in<i>db/db</i>Mice

Background: db/db mice contain a mutation in the leptin receptor gene that leads to hyperphagia, obesity, and hyperglycemia and exhibits features of type 2 diabetes (T2D). db/db mice and people with T2D can develop cardiovascular autonomic neuropathy, a condition characterized by decreased heart rate responsiveness to autonomic signaling. Recent technological advances have led to increased and widespread light exposure at night, and recent studies suggest light at night disrupts the 24-hour regulation of the heart rate by the autonomic nervous system. Hypothesis: dLAN will have a more pronounced impact on the 24-hour regulation of heart rate in db/db mice. Methods: Four to six-month-old control and db/db female mice were implanted with telemetry devices to continuously record the 24-hour rhythms in electrocardiogram (ECG) and core body temperature (Tb). Mice were housed in 12 h light: 12 h dark cycles (12LD, 200 lux: 0 lux) with ad libitum access to food. Mice were then subjected to 12 h light: 12 h dim light cycles (dLAN; 200 lux: 5 lux) for one week. Mice were studied in thermoneutral housing to limit confounds associated with cold-induced sympathetic nervous system activation. Results: In mice housed in 12LD, all control (5/5 mice) and db/db mice (4/4 mice) showed a significant 24-hour rhythm in basal heart rate. The daily rhythmic mean (mesor) was lower in control mice vs. db/db mice (364±29 beats per minute or bpm vs. 438±18 bpm, p&lt;0.01, respectively). There were no differences in the amplitude or the phase of the 24-hour rhythms in heart rate between control or db/db mice. Following exposure to dLAN, all the control mice (5/5) showed a significant 24-hour rhythm in heart rate. The daily rhythmic mean did not change, but the amplitude of the 24-hour rhythm in heart rate decreased from 40±12 bpm in 12LD to 18±5 bpm in control mice under dLAN. Exposing the db/db mice to dLAN caused a loss in 24-hour rhythm in heart rate in all the mice (0/4 mice). Conclusion: In 12LD, both control and db/db mice had 24-hour rhythms in heart rate, but db/db mice had a faster heart rate compared to the control. Exposing mice to dLAN decreased the amplitude of the 24-hour rhythm in heart rate in control mice but caused a loss in the 24-hour rhythm in heart rate in db/db mice. These data suggest that dLAN has a more pronounced effect on the 24-hour regulation of the heart rate in db/db mice. Center for Clinical and Translational Science, University of Kentucky - Barnstable Brown Diabetes Center Collaborative Award grant to Prof. Brian Delisle. This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.