Enhancement of sensorimotor connections by conditioning-related stimulation in Aplysia depends upon postsynaptic Ca2+

Classical conditioning of Aplysia's siphon-withdrawal reflex is thought to be due to a presynaptic mechanism-activity-dependent presynaptic facilitation of sensorimotor connections. Recent experiments with sensorimotor synapses in dissociated cell culture, however, provide an alternative cellular mechanism for classical conditioning-Hebbian long-term potentiation (LTP) of sensorimotor connections. Induction of Hebbian LTP of these connections is mediated by activation of N-methyl-D-aspartate-related receptors and requires the postsynaptic elevation of intracellular Ca2+. To determine whether the enhancement of sensorimotor synapses during classical conditioning in Aplysia-like LTP of sensorimotor synapses in culture-also depends upon the elevation of postsynaptic Ca2+, we carried out experiments involving the cellular analog of classical conditioning of siphon withdrawal. We examined changes in the strength of monosynaptic siphon sensorimotor connections in the abdominal ganglion of Aplysia following paired presentations of sensory neuron activation and tail nerve shock. This training regimen resulted in significant enhancement of the monosynaptic sensorimotor excitatory postsynaptic potential, as compared with the sensorimotor excitatory postsynaptic potential in preparations that received only test stimulation. Infusing the motor neuron with 1,2-bis(2-aminophenoxy)ethane-N,N-N',N'-tetraacetic acid, a specific chelator of intracellular Ca2+, prior to paired stimulation training blocked this synaptic enhancement. Our results implicate a postsynaptic, possibly Hebbian, mechanism in classical conditioning in Aplysia.