Microglial modulators reduce respiratory rhythm long-term facilitation in vitro

Inflammation inhibits the expression of some, but not all forms of respiratory motor plasticity. For example, systemic application of lipopolysaccharide (LPS) inhibits the phrenic long-term facilitation induced by moderate-intermittent hypoxia in vivo. There are multiple pro-inflammatory processes triggered by the systemic application of LPS, including neuroinflammation in the CNS. Considering that microglia can be activated by the systemic application of LPS, it is likely that this cell type influences the response of the respiratory circuits to intermittent hypoxia (IH). Thus, we aimed to test whether modulators of microglial function would affect the response to IH of the preBötzinger complex (preBötC) isolated in a brainstem slice preparation. This experimental approach avoids the systemic influences of these microglial modulators and limits their effects on cells, mostly microglia, included in the slice. First, we found that IH (3 × 5-min episodes of bubbling with 95% N₂ and 5% CO₂, mixed with 5-min normoxic intervals by bubbling with 95% O₂ and 5% CO₂) induces a long-lasting increase in the respiratory rhythm frequency recorded directly from the preBötC, called in vitro long-term facilitation (LTF), which occurs simultaneously with a long-lasting decrease in burst amplitude. Moreover, we found that bath applications of "microglial activators" (LPS and fractalkine), "microglial inhibitors" (minocycline and fucoidan) and a microgliotoxin (liposomal clodronate) partially reduce in vitro LTF. These findings reveal a complex scenario in which both the activation and the inhibition of microglia halts IH-induced preBötC plasticity and suggest that experimental or pathological conditions that affect this cell type, almost in any way, could affect breathing and its plastic responses.