Pharmacologic Microglial Depletion Disrupts Breathing and Body Temperature Regulation During Torpor in 13-lined Ground Squirrels.

Mammalian hibernation initiates dramatic changes in physiology that include large reductions in ventilation (V̇ ), heart rate, oxygen consumption rate (V̇o ), and body temperature (T ) upon entrance into torpor, effects that reverse upon arousal. Several physiologic processes must maintain function throughout a wide range of temperatures. Recent evidence indicates that neuronal synapses loosen and retract in regions of the brain that are silenced during a torpor bout, although little is understood yet about how these connections are regulated during hibernation. Microglia, brain resident immune cells, are potential facilitators of these essential neural homeostatic activities due to their critical roles in neuronal support and synaptic strengthening and pruning. Thus, we hypothesized that microglia are required for maintaining normal ventilatory neural control during hibernation. To test this, we treated 13-lined ground squirrels with either vehicle or colony-stimulating factor receptor-1 (CSF1R) antagonist, PLX3397 (80mg/kg, po), a receptor whose activation is necessary for microglial survival in the adult nervous system, and measured V̇ , V̇o , and T throughout a torpor bout. During entrance, PLX-treated squirrels took ~1.5 hours longer to reach their minimum torpor temperature (T ; 12°C) compared to control animals (T ; 9.3°C) (ambient temperature; 5-6°C). Similarly, arousal time in microglia-depleted animals was as much as 1 hour longer relative to controls, despite starting at a higher minimum T . In both treatment groups, V̇o during entrance fell and spiked periodically. However, in PLX-treated animals, these spikes were much greater and more frequent. Minimum V̇o in torpor was ~3 times greater in PLX-treated animals compared to control animals. Upon arousal from torpor, V̇o remained elevated, longer, in PLX-treated squirrels. Total V̇ tended to be elevated in PLX-treated animals through entrance, steady state torpor, and arousal, but it was most striking during steady state torpor, where V̇ in PLX-treated animals was 15-fold higher than control animals. The net result in torpor was an elevated air convection requirement, thus microglia-depleted animals hyperventilated throughout the torpor bout. Together, these results suggest that microglia play an important regulatory role in normal ventilatory and temperature control throughout torpor and influence the rate at which animals enter into and arouse from torpor. Mechanisms whereby microglia contribute to hibernation neurophysiology are currently under investigation.