Complex dynamics of glutamate-induced calcium responses in astrocytes from the nucleus of the solitary tract of mice

Astrocytes play critical roles in the physiological responses of the central nervous system (CNS). Located near pre- and postsynaptic sites, they detect released neurotransmitters and gliotransmitters that modulate neuronal function. Astrocytic responses to neurotransmitter excitation often involve increases in cytoplasmic calcium, triggering the release of gliotransmitters that further influence neuronal activity. The nucleus of the solitary tract (NTS), a brainstem region integrating diverse physiological functions such as cardiovascular, respiratory, digestive, and metabolic reflexes, is modulated by astrocytic activity. To better understand the dynamics and diversity of calcium responses in NTS astrocytes to the primary excitatory neurotransmitter glutamate, we investigated individual subpostremal NTS astrocytes in brainstem slices from mice using the calcium fluorescent dye Fluo-4. We observed that only a subset of astrocytes exhibited an increase in cytoplasmic calcium in response to glutamate, while a smaller fraction showed a decrease in cytoplasmic calcium. Interestingly, in the presence of tetrodotoxin, which inhibits action potentials, the proportion of astrocytes with increased calcium levels was halved, and most astrocytes instead exhibited decreased calcium levels. Further analysis revealed that response peaks were correlated with total calcium levels after glutamate application, whereas response latencies and widths of positive calcium signals were not correlated with peak values. Negative peaks have distinct kinetics from positive peaks, corroborating that they represent different processes. These findings demonstrate that NTS astrocytes constitute a heterogeneous population with diverse responses to extracellular glutamate, highlighting their complexity in modulating brainstem functions.