STAT3 activation and c-FOS expression in the brain following peripheral administration of bacterial DNA

To study the role of bacterial DNA in the brain function, we investigated signal transducer and activator of transcription 3 (STAT3) activation and c-FOS expression in the brain by immunohistochemistry in response to peripheral administration of CpG-DNA. CpG-DNA induced phospho-STAT3-immunoreactive cells and c-FOS-positive cells in several brain regions in a different manner. Phospho-STAT3-immunoreactive cells were observed in the circumventricular organs where the blood-brain barrier is weak. On the other hand, CpG-DNA increased c-FOS-positive cells in the paraventricular nucleus of the hypothalamus, and the nucleus of the tractus solitarius (NTS) and the area postrema. Unilateral cervical vagotomy did not modify CpG-DNA-induced c-FOS expression in the NTS, indicating that CpG-DNA-induced activation of the NTS is independent of the afferent vagus nerve input originating from the subdiaphragmatic organs. On the other hand, Toll-like receptor 9 mRNA was expressed in the nodose ganglion. Therefore, it is possible that CpG-DNA activates afferent vagus nerve through the nodose ganglion which subsequently activates the NTS. Present observations represented that peripheral CpG-DNA induced immune event in the brain, and that not only c-FOS but also phosphorylation of STAT3 can be a useful indicator for evaluation of neuro-immune interaction.