Q. Since neurotransmitters are not consumed in the synaptic process 

What are the mechanisms to reduce their concentrations in the synaptic cleft after they have been used?

Since the binding of neurotransmitters to the postsynaptic receptors is reversible after these neurochemicals perform their role they must be eliminated from the synaptic cleft. Neurotransmitters can then bind to specific proteins that carry them back to the axon they came from in a process called as neurotransmitter re-uptake. They can also be destroyed by specific enzymes like acetylcholinesterase an enzyme that destroys acetylcholine. Or they can simply diffuse out of the synaptic cleft.