The pond snails, Lymnaea stagnalis, change their ability of conditioned taste aversion (CTA) during their development, for example, stage 29 embryos can acquire the CTA, whereas immature snails come to use a long-term memory to maintain the conditioned response. We thus examined the relationships between the learning ability and the development of key neurons (cerebral giant cells: CGCs) for this CTA. The immunoreactivity of serotonin, which is a main neurotransmitter employed in the feeding circuitry, was first observed in the CGCs at the stage 29. After hatching, the CGCs developed their neuropile faster than other cells in the buccal and cerebral ganglia, resulting in their early innervation at the immature stage. The present results, therefore, indicate that the development of key neurons for learning stimulates the developmental changes in learning ability.
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