Cellular traces of behavioral classical conditioning can be recorded at several specific sites in a simple nervous system

We used a behavioral learning paradigm followed by electrophysiological ana lysis to find sites in the Lymnaea feeding network in which electrical chan ges could be recorded after appetitive conditioning. Specifically we analyz ed conditioning-induced changes in cellular responses in the mechanosensory conditioned stimulus (CS) pathway, in the central pattern generator (CPG) network, and in feeding motoneurons. During training, experimental animals received 15 pairings of lip touch (the CS) with sucrose (the unconditioned stimulus, US). Control animals received 15 random CS and US presentations. Electrophysiological tests on semi-intact preparations made from conditione d animals demonstrated a network correlate of the overall feeding condition ed response, a touch-evoked CPG-driven fictive feeding rhythm. At the moton euronal level, we found significant conditioning-induced increases in the a mplitude of an early touch-evoked EPSP and spike activity, recorded from th e B3 feeding motoneuron. Increases in EPSP amplitude and motoneuronal spike activity could occur independently of conditioned fictive feeding. These c hanges in response recorded at the level of CPG interneurons, and motoneuro ns were preceded by changes recorded in the CS pathway. This was demonstrat ed by recording a conditioning-induced increase in the number of touch-evok ed spikes in the cerebrobuccal connective, which forms part of the CS pathw ay. The finding that electrophysiological changes after conditioning can be recorded at multiple sites in this simple system provided an important int ermediate level of analysis between whole animal behavior and cellular stud ies on the synaptic sites of plasticity.