Heterosynaptic plasticity (or also heterotropic modulation) is a change in synaptic strength that results from the activity of other neurons. Again, the plasticity can alter the number of vesicles or their replenishment rate or the relationship between calcium and vesicle release.  Additionally, it could directly affect calcium influx.  Heterosynaptic plasticity can also be postsynaptic in nature, affecting receptor sensitivity.
One example is again neurons of the sympathetic nervous system, which release noradrenaline, which, in addition, generates an inhibitory effect on presynaptic terminals of neurons of the parasympathetic nervous system.