POLARITY OF LONG-TERM SYNAPTIC GAIN CHANGE IS RELATED TO POSTSYNAPTICSPIKE FIRING AT A CEREBELLAR INHIBITORY SYNAPSE

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