I assume, that the signal that forces a synaptic receptor/ion channel complex to change its behaviour (i.e. strengthening in the course of upregulation) must be the combination of the state of the complex ("occupied" or "open" or any other state except the ground or rest state) and an extraordinarily high membrane potential at the complex (caused by a retrograded action potential).
The not-ground state indicates that the presynaptic neuron recently has fired, the potential indicates that the postsynaptic neuron recently has fired. According to the rule "fire together, wire together", both conditions must be fulfilled for Hebb-like strengthening to occur.
I assume further that the membrane potential that triggers the upregulation of the complex must be significantly higher than the typical post-synaptic potential generated by the synapse, otherwise activation of the synapse alone would suffice for strengthening to occur, which violates Hebb's rule.
Is this way of thinking essentially correct? Or is strengthening triggered by other signals than the ones mentioned above?