Editing Retrograde signaling (section)

====Background====
Long-term potentiation is the persistent increase in the strength of a [[chemical synapse]] that lasts from hours to days.<ref name="Warburton_2015">{{cite book | vauthors = Warburton EC | chapter = Long-Term Potentiation and Memory | title = Encyclopedia of Psychopharmacology | date = 2015 | pages = 928–32 | doi = 10.1007/978-3-642-27772-6_345-2 | isbn = 978-3-642-27772-6 }}</ref> It is thought to occur via two temporally separated events, with ''induction'' occurring first, followed by ''expression''.<ref name="Warburton_2015" /> Most LTP investigators agree that induction is entirely postsynaptic, whereas there is disagreement as to whether expression is principally a presynaptic or postsynaptic event.<ref name="Nicoll_1995" /> Some researchers believe that both presynaptic and postsynaptic mechanisms play a role in LTP expression.<ref name="Nicoll_1995" />

Were LTP entirely induced and expressed postsynaptically, there would be no need for the postsynaptic cell to communicate with the presynaptic cell following LTP induction. However, postsynaptic induction combined with ''presynaptic'' expression requires that, following induction, the postsynaptic cell must communicate with the presynaptic cell. Because normal [[synaptic transmission]] occurs in a presynaptic to postsynaptic direction, postsynaptic to presynaptic communication is considered a form of ''retrograde'' transmission.<ref name="Regehr_2009" />