LTP forms 1, 2 and 3: different mechanisms for the 'long' in long-term potentiation

Long-term potentiation (LTP) of synaptic transmission is a primary experimental model of memory formation in neuronal circuits. Because of the intellectual appeal and scientific fecundity of the field, it is perhaps unsurprising that the literature on LTP contains many complex and often contradictory findings. Recognition that LTP is not a unitary phenomenon and mechanisms can differ between brain regions has resolved some controversy. However, further categorization can be made of mechanistically discrete forms of LTP at the same set of synapses. LTP1, LTP2 and LTP3 have previously been defined according to differences in the longevity and general molecular mechanisms of LTP. This review aims to reinvigorate and extend this concept as a useful framework to disentangle the mechanisms of LTP.