A postsynaptic potential is considered inhibitory when the resulting change in membrane voltage makes it more difficult for the cell to fire an action potential, lowering the firing rate of the neuron. They are the opposite of excitatory postsynaptic potentials (EPSPs), which result from the flow of ions like sodium into the cell.
In addition IPSPs may be produced by closure of sodium or calcium channels. The permeability to Cl- of the GABA receptoror and glycine receptors produces IPSPs.
Sometimes increasing the amount of intracellular Chloride in relation to the extracellular chloride , will cause the IPSP to switch and become excitatory.
Dudel J., Voltage dependence of amplitude and time course of inhibitory synaptic current in crayfish muscle. Pflugers Arch. 1977 Oct 19;371(1-2):167-74.
Akasu T, Koketsu K., Electrogenesis of the slow inhibitory postsynaptic potential in bullfrog sympathetic ganglia. Jpn J Physiol. 1983;33(2):279-300.