Recovery of brainstem motor activity during cooling through temperature sensing in noradrenergic neurons

Homeostasis is a defining principle in physiology that explains how organisms maintain balance despite changes in the environment. Neurons are thought to regulate activity patterns through negative feedback homeostasis. A major hypothesis in the field suggests these regulatory responses arise by sensing variables linked to neuronal activity. For example, if activity is reduced below a set-point, cell signaling molecules that fall in proportion, such as intracellular Ca2+, induce circuit-level changes that recover activity. We discovered the brainstem respiratory network of the American bullfrog exhibits fast homeostatic regulation during activity disturbances caused by acute temperature changes: cooling from 22°C to 10°C initially silenced activity, but after ~10-15 minutes, activity spontaneously recovered. Upon rewarming, burst frequency overshot baseline and then drifted back to control levels over 30 minutes (9/9 experiments). Surprisingly, pharmacologically stopping activity over the same time course at warm temperatures did not lead to compensation, suggesting that cooling per se drove this response. We hypothesized that modulatory neurons may respond to cooling to recover activity. To address this, we first used a “thick slice” preparation that produces network activity but removes much of the modulation. All thick slices (4/4) failed to show compensation responses. To localize the cold sensor to the brainstem or midbrain/pons, we developed a “split bath” preparation in which midbrain/pontine structures were kept at control temperature (22°C) while the brainstem was cooled to 10°C. Keeping the midbrain/pons warm decreased the compensation response (n=7; p<0.0001; Holm-Sidak Multiple Comparison test). An important modulatory center in this region is the noradrenergic locus coeruleus (LC). Experiments to block different adrenergic receptors α1 (n=7) and α2 (n=6) showed similar compensation responses to control, while β-adrenoreceptor antagonists (n=7) showed reduced compensation (p=0.005; Holm-Sidak Multiple Comparison test). Whole-cell current and voltage-clamp experiments revealed that LC neurons are activated by cooling from 22°C to 10°C (n=9; p=0.0009; paired t-test) through mechanisms that involve inhibition of the Na+/K+ pump (excitatory cold current; n=14, p<0.0001; paired t-test; control vs. pump inhibitor ouabain). Opposing inhibition of the Na+ pump with 2 µM monensin reduced cold sensitivity in LC neurons (p=0.016; unpaired t-test) and also blunted the compensation response of the network (n=0.0005; Holm-Sidak Multiple Comparison test). Therefore, rather than “activity sensing” for neural homeostasis, we detail a pathway whereby temperature sensitivity of modulatory neurons combats altered activity levels caused by acute temperature changes. These findings provide evidence explaining how neural circuits regulate their activity during physiological disturbances beyond activity-dependent control mechanisms.

National Institutes of Health (R01NS114514)

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.