Eric Kandel and the Sea Slug

How a refugee from Vienna used a giant sea slug, Aplysia, to show that learning changes the strength of synapses — and that long-term memory needs new proteins and new connections.

Eric Kandel was born in Vienna in 1929. After Nazi Germany annexed Austria in 1938, his family fled to the United States. He trained in psychiatry, hoping to find the biology of memory that psychoanalysis described — and turned to laboratory science instead.

Why a sea slug?

Memory in the mammalian brain was too complex to study cell by cell. Kandel chose Aplysia californica, a large marine snail with only about 20,000 neurons, many of them big enough to see by eye and identify from animal to animal. Aplysia has a simple defensive reflex: touch its siphon and it withdraws its gill.

Learning in a circuit

The reflex can be modified. With repeated gentle touches, the slug habituates — it withdraws less. After a shock to its tail, it becomes sensitised — it withdraws more strongly. Kandel and colleagues traced the reflex circuit and showed that these simple forms of learning change the strength of specific synapses between sensory and motor neurons — confirming, at the cellular level, ideas proposed by Cajal and Hebb.

Short-term versus long-term

They then showed that short-term sensitisation involves chemical modifications of existing proteins (through the messenger cyclic AMP and the enzyme protein kinase A), while long-term memory requires switching on genes — via a protein called CREB — to make new proteins and even grow new synaptic connections. The same molecular steps were later found in mice and other animals.

Kandel shared the 2000 Nobel Prize with Arvid Carlsson and Paul Greengard. His memoir, In Search of Memory (2006), is a widely read account of the science.