Neuronal mechanisms underlying appetitive learning in the pond snail Lymnaea stagnalis.
1. Lymnaea was the subject of an established behavioural conditioning paradigm where
pairings of a neutral lip tactile stimulus (CS) and a sucrose food stimulus (US) results in a
conditioned feeding response to the CS alone. The current objective was to dissect
trained animals and examine electrophysiological changes in the feeding circuitry which
may underlie this learning.
2. Naive subjects were used to confirm that US and CS responses in vivo persisted in vitro
since this is a pre-requisite for survival of a learned memory trace. This required the
development of a novel semi-intact preparation facilitating CS presentation and
simultaneous access to the CNS.
3. The nature and function of the CS response was investigated using naive animals.
Intracellular recordings revealed that the tactile CS evokes specific, consistent synaptic
responses in identified feeding neurons. Extracellular recording techniques and
anatomical investigations showed that these responses occurred through a direct pathway
linking the lips to the feeding circuitry. A buccal neuron was characterized which showed
lip tactile responses and supplied synaptic inputs to feeding neurons indicating that it was
a second-order mechanosensory neuron involved in the CS pathway.
4. Animals trained using the behavioural conditioning paradigm were tested for
conditioned responses and subsequently dissected~ Intracellular recording from specific
identified feeding motoneurons revealed that CS presentation resulted in significant
activation of the feeding network compared to control subjects. This activation was
combined both with an increase in the amplitude of a specific synaptic input and an
elevation in the extracellular spike activity recorded from a feeding-related connective. A
neuronal mechanism to account for these findings is presented.
5. The role of motoneurons in the feeding circuit was reassessed. It is demonstrated,
contrary to the current model, that muscular motoneurons have an important contribution
during feeding rhythms through previously unreported electrotonic CPG connections.