Cellular Traces of Behavioral Classical Conditioning Can Be Recorded at Several Specific Sites in a Simple Nervous System

Abstract

We used a behavioral learning paradigm followed by electrophysiological analysis to find sites in the Lymnaea feeding network in which electrical changes could be recorded after appetitive conditioning. Specifically, we analyzed 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 conditioned animals demonstrated a network correlate of the overall feeding conditioned response, a touch-evoked CPG-driven fictive feeding rhythm. At the motoneuronal level, we found significant conditioning-induced increases in the amplitude of an early touch-evoked EPSP and spike activity, recorded from the B3 feeding motoneuron. Increases in EPSP amplitude and motoneuronal spike activity could occur independently of conditioned fictive feeding. These changes in response recorded at the level of CPG interneurons, and motoneurons were preceded by changes recorded in the CS pathway. This was demonstrated by recording a conditioning-induced increase in the number of touch-evoked spikes in the cerebrobuccal connective, which forms part of the CS pathway. The finding that electrophysiological changes after conditioning can be recorded at multiple sites in this simple system provided an important intermediate level of analysis between whole animal behavior and cellular studies on the synaptic sites of plasticity.

Multiple Types of Control by Identified Interneurons in a Sensory-Activated Rhythmic Motor Pattern

“…In suppression experiments the B3 motoneuron was used as a monitor of the effect of suppressing activity in C V1a /SO and N1M on the final motor output of the feeding system. This motoneuron type shows reliable bursting activity during fictive feeding cycles in semi-intact preparations and is therefore suitable for statistical analyses of fictive feeding activity (Staras et al, 1999a).…”

“…The preparations used in these experiments consisted of the lips and the C NS and were described in detail in previous papers (Kemenes et al, 1986Staras et al, 1998bStaras et al, , 1999a. After dissection, the preparations were transferred to a silicon elastomer (Sylgard)-lined electrophysiology chamber containing saline and pinned dorsal-side up.…”

“…The intracellular recording and stimulation techniques used in these experiments were described in detail in previous papers Staras et al, 1998bStaras et al, , 1999a. In the suppression experiments, hyperpolarization of interneurons was achieved by passing current through the recording electrode and this, together with the fact that recording from the small C V1a and N1M interneurons required the use of sharp electrodes, often resulted in bridge imbalance in the hyperpolarized C V1a and N1M traces.…”

“…Thus, the CPG itself and particularly the N1M interneuron seem to be the pivotal components in determining whether a sensory-evoked motor pattern occurs. The distributed nature of the network has been furthered by the findings that motoneurons have a feedback connection with the CPG (Staras et al, 1999a) and that the SO (Elliott and Benjamin, 1985b), and perhaps also the CV1a, have direct connections with motoneurons. Thin arrows, Weaker connections; thick arrows, stronger connections.…”

Context Learning and the Effect of Context on Memory Retrieval in Lymnaea

“…Thus although the internal state of the animal may have been altered by the lack of food, this alteration in state only had a negative impact on learning and memory when a specific training context (food odorant) was used. As in appetitive classical conditioning studies using Lymnaea Staras et al 1999), other mollusks (Susswein et al 1986;Lechner et al 2000) and vertebrates (Dickinson 1980) the internal state of the animal greatly influences the establishment of learning. For instance, it is much more difficult to associatively condition a well-fed animal to a neutral or repulsive stimulus than it is with a hungry animal (Dickinson 1980).…”