Polarity of long-term synaptic gain change is related to postsynaptic spike firing at a cerebellar inhibitory synapse

Long-term potentiation and depression (LTP and LTD) in excitatory synapses can coexist, the former being triggered by stimuli that produce strong postsynaptic excitation and the latter by stimuli that produce weaker postsynaptic excitation. It has not been determined whether these properties also apply to LTP and LTD in the inhibitory synapses between Purkinje neurons and the neurons of the deep cerebellar nuclei (DCN), a site that has been implicated in certain types of motor learning. DCN cells exhibit a prominent rebound depolarization (RD) and associated spike burst upon release from hyperpolarization. In these cells, LTP can be elicited by short, high-frequency trains of inhibitory postsynaptic potentials (IPSPs), which reliably evoke an RD. LTD is induced if the same protocol is applied with conditions where the amount of postsynaptic excitation is reduced. The polarity of the change in synaptic strength is correlated with the amount of RD-evoked spike firing during the induction protocol. Thus, some important computational principles that govern the induction of use-dependent change in excitatory synaptic efficacy also apply to inhibitory synapses.