The output information of a neuron is encoded in temporal patterns of action potentials and amounts and temporal patterns of neurotransmitter release at synapses. When a neuron fires action potentials frequently, the rate of neurotransmitter release at the synapse may exceed the rate at which it can diffuse away into the interstitial fluid.
Therefore, neurotransmitter may need to be actively removed to reset the synapse so that it can communicate more information. This may occur in several ways. Synapses may contain enzymes that inactivate the neurotransmitter.
Presynaptic membranes may contain reuptake pumps that actively transport the neurotransmitter back into the presynaptic membrane for recycling.
Astrocyte end feet covering synapses may also contain uptake pumps to actively remove neurotransmitter.
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Introduction to Neurology 