The contribution of facilitation of monosynaptic PSPs to dishabituation and sensitization of the Aplysia siphon withdrawal reflex

To examine the relationship between synaptic plasticity and learning and me mory as directly as possible, we have developed a new simplified preparatio n for studying the siphon-withdrawal reflex of Aplysia in which it is relat ively easy to record synaptic connections between individual identified neu rons during simple forms of learning. We estimated that monosynaptic EPSPs from LE siphon sensory neurons to LFS siphon motor neurons mediate approxim ately one-third of the reflex response measured in this preparation, which corresponds to siphon flaring in the intact animal. To investigate cellular mechanisms contributing to dishabituation and sensitization, we recorded e voked firing of LFS neurons, the siphon withdrawal produced by stimulation of an LFS neuron, the complex PSP in an LFS neuron, and the monosynaptic PS P from an "on-field" or "off-field" LE neuron to an LFS neuron during behav ioral training. Unlike the simplified gill-withdrawal preparation (Cohen et al., 1997; Frost et al., 1997), in the siphon-withdrawal preparation we fo und no qualitative differences between the major cellular mechanisms contri buting to dishabituation and sensitization, suggesting that dissociations t hat have been observed previously may be attributable to transient inhibiti on that does not occur for this component of the reflex. Furthermore, in th e siphon-withdrawal preparation, all of the various cellular measures, incl uding monosynaptic PSPs from either on-field or off-field LE neurons, chang ed approximately in parallel with changes in the behavior. These results pr ovide the most direct evidence so far available that both dishabituation an d sensitization involve multiple mechanisms, including heterosynaptic facil itation of sensory neuron-motor neuron PSPs.