Pain afferents release as a transmitter. descending analgesic (pain relief) neurons that synapse with the of the pain afferent may interrupt the transmission of pain information by affecting voltage gated channels

The answer to this question would be: substance P; axon terminal

The transmitter that used by pain afferent would be substance P. This transmission can be disrupted by the descending analgesic drug. The blockade will prevent the transmission of pain from reaching the brain.
Since this drug works in CNS system it can't be used for local anesthesia. Drug that directly blocks Na+ channel can be used instead.