A ROLE FOR POSTSYNAPTIC NEURONS IN DETERMINING PRESYNAPTIC RELEASE PROPERTIES IN THE CRICKET CNS - EVIDENCE FOR RETROGRADE CONTROL OF FACILITATION

Intracellular recordings in the cricket cercal system show that the sy naptic terminals of a single sensory neuron can facilitate at one targ et, the medial giant interneuron (MGI), and simultaneously depress at another target, interneuron 10-3. A quantal analysis of transmission a t these synapses demonstrates that facilitation and depression are pro perties of the presynaptic cell. For facilitating synapses contacting MGI, the mean quantal content (m), determined from the probability of the failures, increases for the second EPSP, while the quantal size (q ) remains constant. Similarly, an analysis of depression for those syn apses contacting 10-3 supports a presynaptic mechanism for depression. Since facilitation and depression are presynaptic and their expressio n at the synapses of a single, identified sensory neuron are correlate d with the target interneuron, we conclude that these properties are r egulated locally, at the synapse, possibly by an interaction with the postsynaptic cell.