[Neuroscience] Long-term potentiation and depression

[blueocean12 from tom.com]

Hi,
I am confused about the LTP, LTD things.  Are LTP and LTD  only
expressed at excitatory synapse? Why people often include bicuculline
in their eternal solution during recording? How long the potentiation
should last we can call it long-term potentiation contrast to short-
term plasticity. Thanks!