Target neuron specification of short-term synaptic facilitation and depression in the cricket CNS

We investigated the role of retrograde signals in the regulation of short-term synaptic depression and facilitation by characterizing the form of plasticity expressed at novel synapses on four giant interneurons in the cricket cercal sensory system. We induced the formation of novel synapses by transplanting a mesothoracic leg and its associated sensory neurons to the cricket terminal abdominal segment. Axons of ectopic leg sensory neurons regenerated and innervated the host terminal abdominal ganglion forming monosynaptic connections with the medial giant interneuron (MGI), lateral giant interneuron (LGI), and interneurons 7-1a and 9-2a. The plasticity expressed by these synapses was characterized by stimulating a sensory neuron with pairs of stimuli at various frequencies or with trains of 10 stimuli delivered at 100 Hz and measuring the change in excitatory postsynaptic potential amplitude recorded in the postsynaptic neuron. Novel synapses of a leg tactile hair on 7-1a depressed, as did control synapses of cercal sensory neurons on this interneuron. Novel synapses of leg campaniform sensilla (CS) sensory neurons on MGI, like MGI's control synapses, always facilitated. The form of plasticity expressed by novel synapses is thus consistent with that observed at control synapses. Leg CS synapses with 9-2a also facilitated; however, the plasticity expressed by these sensory neurons is dependent on the identity of the postsynaptic cell since the synapses these same sensory neurons formed with LGI always depressed. We conclude that the form of plasticity expressed at these synaptic connections is determined retrogradely by the postsynaptic cell.