NEURAL ANALOG OF LONG-TERM SENSITIZATION TRAINING PRODUCES LONG-TERM (24 AND 48 H) FACILITATION OF THE SENSORY-TO-MOTOR NEURON CONNECTION IN APLYSIA

1. An in vitro analogue of long-term sensitization training was used to gain insights into the mechanisms and time course of the memory for long-term sensitization in Aplysia. The analogue, consisting of four blocks of shocks, was delivered to peripheral nerves of the isolated pleural-pedal ganglia, which contain the sensory neurons and motor neurons that mediate the tail withdrawal reflex. 2. Long-term facilitation of the connections between the sensory neurons and motor neurons was produced by the conjoint stimulation of two peripheral nerves, P8 and P9. Long-term facilitation, however, was not observed after conjoint stimulation of three nerves, P7, P8, and P9. 3. The preparation was viable and stable (no changes in the amplitudes of excitatory postsynaptic potentials (EPSPs) and membrane properties in controls) for at least 48 h. Moreover, the long-term facilitation persisted for at least 48 h. 4. We observed no significant long-term changes in the resting membrane potentials of the sensory and motor neurons or in the input resistance of the motor neurons 24 and 48 h after the conjoint stimulation of nerves P8 and P9. Thus changes in these biophysical properties do not appear to contribute to the expression of long-term facilitation. 5. The finding that conjoint stimulation of three nerves, P7, P8, and P9, produced no long-term facilitation raised the possibility that stimulation of nerve P7 alone might produce long-term inhibition that opposes the facilitatory effects induced by conjoint stimulation of nerves P8 and P9. Stimulation of nerve P7 alone, however, had no long-term inhibitory effect on the EPSPs. Stimulation of nerve P7 may activate pathways that block the long-term facilitation produced by conjoint stimulation of nerves P8 and P9. 6. This study represents the first demonstration that long-term synaptic facilitation can persist for 48 h, which is consistent with results indicating that the memory for long-term sensitization persists for >1 day in intact animals. Moreover, the results suggest that long-term synaptic facilitation is a mechanism underlying the memory for long-term sensitization that persists for a period of time >24 h. Finally, the results provide a starting point for an analysis of the biophysical and molecular mechanisms for very long-term (e.g., 48 h) alterations in synaptic efficacy as well as a basis for comparing mechanisms underlying different stages of long-term memory.