Chronic inflammation has been reported to be a significant factor in the induction and progression of a number of chronic neurological disorders including Alzheimer's disease and Down syndrome. It is believed that inflammation may promote synaptic dysfunction, an effect that is mediated in part by pro-inflammatory cytokines such as interleukin-1β (IL-1β). However, the role of IL-1β and other cytokines in synaptic transmission is still poorly understood. In this study, we have investigated how synaptic transmission and neuronal excitability in hippocampal pyramidal neurons are affected by chronic inflammation induced by exposing organotypic slices to the bacterial cell-wall product lipopolysaccharide (LPS). We report that CA1 pyramidal neurons recorded in whole cell from slices previously exposed to LPS for 7 days had resting membrane potential and action potential properties similar to those of the controls. However, they had significantly lower membrane resistance and a more elevated action potential threshold, and displayed a slower frequency of action potential discharge. Moreover, the amplitude of pharmacologically isolated postsynaptic γ-aminobutyric acid (GABA)ergic potentials, but not excitatory glutamatergic postsynaptic potentials, was significantly larger following chronic LPS exposure. Interestingly, co-incubation of the IL-1 receptor antagonist (IL-1Ra) concurrently with LPS prevented the increase in GABAergic transmission, but not the reduction in intrinsic neuronal excitability. Finally, we confirmed that LPS dramatically increased IL-1β, and IL-1β-dependent IL-6 levels in the culture medium for 2 days before returning to baseline. We conclude that CA1 pyramidal neurons in slices chronically exposed to LPS show a persistent decrease in excitability due to a combined decrease in intrinsic membrane excitability and an enhancement in synaptic GABAergic input, the latter being dependent on IL-1β. Therefore, chronic inflammation in hippocampus produces IL-1β-dependent and -independent effects in neuronal and synaptic function that could contribute significantly to cognitive disturbances. © 2005 Wiley-Liss, Inc.