Dear Yan-Bo:

It seems to be expected that during LTP, excitatory pre-synaptic neurons release large amount of excitatory neurotransmitters such as glutamate.

This is an older paper -- and I'm sure there's more recent work -- but it's a starting point:

https://www.nature.com/articles/nn1220

Note that two forms of LTP are considered here: a chemically-induced LTP in culture and STDP in slices.