Presynaptic Inhibition is a mechanism by which the amount of neurotransmitter released by an individual synapse can be reduced, resulting of less excitation of the post-synaptic neurone.
When this occurs the 'inhibition' is actually due to less excitatory input.
What's more the 'inhibition' is targeted at specific types of input to a neurone, in contrast with the IPSP, which acts post-synapticially, and inhibits all activity in the neurone.
The anatomical basis of the process is the axo-axonic synapse, a diagram of which is shown opposite. The neurone on the right has its axon terminating on the synaptic bouton of the bottom neurone. These axo-axonic contacts are found in many parts of the CNS, including the dorsal horn of the spinal cord.
The physiological significance of the axo-axonic synapse is the existence of primary afferent depolarisation, and is of importance because of their involvement in the modulation of nociceptive messages.