Pro-inflammatory cytokines, such as Tumor Necrosis Factor-alpha (TNFalpha), Interlukin-1 (IL-1) and Interlukin-6 (IL-6), were first discovered in the context of cellular activation and cell-to-cell communication in the immune system. Early studies on the role of cytokines in the brain suggested that their expression and activity are induced in response to an infection, head trauma, stroke, or neurodegenerative diseases. A growing body of data suggests that hyperactivation of the immune system has been implicated in the pathophysiology of major depressive disorder. Indeed, pro-inflammatory cytokines are expressed in the brain where they have been shown to regulate synaptic plasticity. TNFalpha, which is secreted from glial cells, regulates amino-3-hydroxyl-5-methyl-4-isoxazole-propionate (AMPA) glutamate receptor trafficking and neuroplasticity. However, the effect of TNF-alpha on synaptogenesis and mood associated behaviors remains unclear. In this research, we found that treatment with TNF-alpha at low dose (60 nM) for 1 day significantly enhanced the number of synapses and co-localization efficiency of GluR1 and PSD95 in the hippocampal neuron cultures. This effect sustained up to 3 days. However, high dose (600 nM) of TNF-alpha significantly down-regulated the number of synapses and co-localization efficiency of GluR1 and PSD95. These data were confirmed by electrophysiological studies. In addition, TNF-alpha infusion at low dosage to the third ventricle of mouse brain could enhance anxiolytic/risk taking behaviours in open field and elevated plus maze tests. However, high dosage of TNF-alpha reduced anxiolytic/risk taking behaviours in elevated plus maze. In conclusion, TNF-alpha induced AMPA receptor containing synaptogenesis was in an inverted U-shape in cultured hippocampal neurons and animal behaviour models. Our findings provide novel evidence that TNF-alpha regulates synaptogenesis, which might play a crucial role during traumatic brain damage, inflammation and other psychiatric conditions.
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NIH ExPorter