Microglia-triggered hypoexcitability plasticity of pyramidal neurons in the rat medial prefrontal cortex

Lipopolysaccharide (LPS), an outer component of Gram-negative bacteria, induces a strong response of innate immunity via microglia, which triggers a modulation of the intrinsic excitability of neurons. However, it is unclear whether the modulation of neurophysiological properties is similar among neurons. Here, we found the hypoexcitability of layer 5 (L5) pyramidal neurons after exposure to LPS in the medial prefrontal cortex (mPFC) of juvenile rats. We recorded the firing frequency of L5 pyramidal neurons long-lastingly under in vitro whole-cell patch-clamp, and we found a reduction of the firing frequency after applying LPS. A decrease in the intrinsic excitability against LPS-exposure was also found in L2/3 pyramidal neurons but not in fast-spiking interneurons. The decrease in the excitability by immune-activation was underlain by increased activity of small-conductance Ca²⁺-activated K⁺ channels (SK channels) in the pyramidal neurons and tumor necrosis factor (TNF)-α released from microglia. We revealed that the reduction of the firing frequency of L5 pyramidal neurons was dependent on intraneuronal Ca²⁺ and PP2B. These results suggest the hypoexcitability of pyramidal neurons caused by the upregulation of SK channels via Ca²⁺-dependent phosphatase during acute inflammation in the mPFC. Such a mechanism is in contrast to that of cerebellar Purkinje cells, in which immune activation induces hyperexcitability via downregulation of SK channels. Further, a decrease in the frequency of spontaneous inhibitory synaptic transmission reflected network hypoactivity. Therefore, our results suggest that the directionality of the intrinsic plasticity by microglia is not consistent, depending on the brain region and the cell type.