Presynaptic induction and expression of homosynaptic depression at Aplysia sensorimotor neuron synapses

The cellular mechanisms underlying the induction and expression of homosynaptic depression at the glutamatergic synapse between Aplysia sensory and motor neurons were studied in dissociated cell culture. Intracellular microelectrodes were used to stimulate action potentials in the presynaptic sensory neuron and record the depolarizing EPSP from the motor neuron. Homosynaptic depression (HSD) was induced by repeatedly stimulating the sensory neuron at rates as low as one action potential per minute. Activation of postsynaptic Glu receptors was neither sufficient nor necessary to induce HSD. Thus, repeated applications of exogenous Glu did not depress the synaptically evoked EPSP. Moreover, normal HSD was observed when the sensory neuron was stimulated during a period when the Glu receptors were blocked with the antagonist DNQX. The induction of HSD is thus likely to occur within the presynaptic terminal. We explored the role of presynaptic calcium in the induction of HSD by injecting the sensory neuron with EGTA, a relatively slow calcium chelator that does not alter rapid release but effectively buffers the slow residual calcium transient thought to be important for plasticity. EGTA had little effect on HSD, indicating that residual Cai is not involved. HSD does not appear to involve a decrease in presynaptic calcium influx, because there was no change in the presynaptic calcium transient, measured by calcium indicator dyes, during HSD. We conclude that HSD is induced and expressed in the presynaptic terminal, possibly by a mechanism directly coupled to the release process.