Plasticity between excitatory and inhibitory neurons?
Various forms of plasticity has been reported for inhibitory-excitatory synapses as well. See below figure from [Woodlin et al. 2003].



Holmgren CD, Zilberter Y (2001) Coincident spiking activity induces long-term changes in inhibition of neocortical pyramidal cells. J Neurosci 21: 8270–7
Woodin MA, Ganguly K, Poo MM (2003) Coincident pre- and postsynaptic activity modifies GABAergic synapses by postsynaptic changes in Cl- transporter activity. Neuron 39: 807–20