Toll-like Receptor 8 (TLR8) Is Expressed in Mouse Hippocampal Neurons and Modulates Neuronal Excitability

Toll-like receptors (TLRs) are essential in the innate immune response. Furthermore, neuronal TLRs have been involved in regulating neuronal dendritic outgrowth and excitability. However, a deeper understanding of neuronal-TLR function is essential. We investigated the expression and role of TLR8 using mouse postnatal hippocampal neuronal cultures. We assessed TLR8 (and TLR7) expression by immunocytochemistry and TLR8 functionality via electrophysiological recordings in a high-density multi-electrode array system and calcium flux imaging. Pathway activation following fast TLR8 stimulation was performed via Western blot. TLR8 and TLR7 are expressed in postnatal hippocampal neurons at DIV14 and in neurosphere-derived neurons. TLR8 expression was detected in the cell body and dendrites of GABAergic, parvalbumin-expressing interneurons. Stimulation with the TLR8 agonist TL8-506 led to increased neuronal activity in the short term and enhanced neuronal synchronization in short- and long-term recordings. The calcium flux induced by TLR8 was reduced by blocking NMDA receptors with D-AP5. TLR8 stimulation did not lead to IkB-α degradation. Our findings demonstrate that TLR8 and TLR7 are expressed in mouse postnatal hippocampal neurons. TLR8 activation leads to increased neuronal excitability that was decreased by NMDA antagonism. However, acute TLR8 stimulation failed to robustly activate the canonical TLR signaling pathway. We propose that TLR8 activation might lead to the enhancement of excitatory NMDA receptors.