A SIMPLIFIED PREPARATION FOR RELATING CELLULAR EVENTS TO BEHAVIOR - CONTRIBUTION OF LE AND UNIDENTIFIED SIPHON SENSORY NEURONS TO MEDIATIONAND HABITUATION OF THE APLYSIA GILL-WITHDRAWAL AND SIPHON-WITHDRAWAL REFLEX

We have begun to analyze several elementary forms of learning in a sim ple preparation consisting of the isolated mantle organs and abdominal ganglion of Aplysia. Previous studies suggested that plasticity at si phon sensory neuron synapses contributes to habituation and dishabitua tion of the gill- and siphon-withdrawal reflex in this preparation. We next wished to identify the sensory neurons that participate in the r eflex and examine their plasticity more directly. To investigate the c ontribution of the LE siphon mechanosensory cells, we recorded from th em and gill or siphon motor neurons during the same siphon stimulation that has been used in behavioral experiments in this preparation. Our results indicate that the LE cells make a substantial contribution to the evoked response in the motor neurons under these conditions, but they suggest that other as yet unidentified siphon sensory neurons wit h lower thresholds and shorter latencies also contribute. In addition, we find that homosynaptic depression of monosynaptic postsynaptic pot entials (PSPs) from LE sensory cells makes an important contribution t o habituation of the response in the motor neurons. To investigate pla sticity of PSPs from the unidentified sensory neurons, we recorded the PSP that was produced in a motor neuron by water-movement stimulation of the siphon, which does not cause firing of LE cells, Our results s uggest that PSPs from the unidentified sensory neurons and the LE neur ons undergo similar plasticity during habituation and dishabituation t raining. These results support the idea that plasticity at synapses of both LE and unidentified sensory neurons contributes to habituation a nd dishabituation of the reflex response in this preparation.