Exposure to interferon-gamma during synaptogenesis increases inhibitory activity after a latent period in cultured rat hippocampal neurons.

Certain disorders of the nervous system may have their origin in disturbances in the development of synaptic connections and network structure that may not become overt until later in life. As inflammatory cytokines can influence synaptic activity in neuronal cultures, we analysed whether cytokine exposure during synaptogenesis can lead to imbalances in a neuronal network. Short-term application of interferon-gamma (IFN-gamma), but not tumour necrosis factor-alpha, during peak synaptogenesis (but not before or after) in Sprague-Dawley rat hippocampal cultures, caused both a decrease in the frequency of spontaneous excitatory postsynaptic currents (EPSCs) and an increase in the frequency of spontaneous inhibitory postsynaptic currents (IPSCs). These effects were only detected in recordings made weeks later. This was not due to a depression of glutamatergic synapses or to a change in the relative number of neurons containing glutamic acid decarboxylase (GAD). There was an increase in the average amplitude of miniature IPSCs, and in GAD-expressing neurons the amplitude of miniature EPSCs were larger as well as the responses to glutamate. This indicates that IFN-gamma-treatment induced increased inhibition via postsynaptic changes. These effects of IFN-gamma treatment were not observed when neuronal nitric oxide synthase was inhibited. Our study therefore shows that exposure to IFN-gamma during a restricted period of development, which coincides with the peak of excitatory synaptogenesis, can cause progressive changes in synaptic activity in the network. Thus, cytokine exposure at a critical period of development may constitute a 'hit-and-run' mechanism for certain nervous system disorders that become manifest after a latency period.