Branch-specific heterosynaptic facilitation in Aplysia siphon sensory cells

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Abstract
Aplysia siphon sensory cells exhibit heterosynaptic facilitation of transmitter release during both sensitization and classical conditioning of the siphon withdrawal response. Whether facilitation must invariably enhance transmission at all terminals of a neuron or whether facilitation can instead occur at one set of terminals without also occurring at other terminals of the same cell was studied. To examine this question, effects of local application of serotonin [5-hydroxytryptamine] and of connective stimulation on transmission at central and peripheral branches of single sensory cells were compared. Heterosynaptic facilitation can be branch-specific and can occur at either central or peripheral synapses independently. Siphon sensory cells exhibit homosynaptic post-tetanic potentiation, allowing us to compare effects of hetero- and homosynaptic facilitation, in the same cells. By contrast to heterosynaptic facilitation, homosynaptic facilitation occurs concomitantly at both central and peripheral synapses of siphon sensory cells. While both heterosynaptic and homosynaptic facilitation involve increases in transmitter release from sensory neuron terminals, heterosynaptic facilitation provides a greater specificity and flexibility in the modification of synaptic connections.