Just over 100 years ago Santiago Ramón Y Cajal proposed that long-term memories are expressed in the brain, in part, by changes in synaptic connectivity. Since then many, if not most, neuroscientists have accepted that memories are maintained by persistent molecular and cellular alterations in synaptic structures themselves.
However, a team from the University of California Los Angeles recently challenged that theory with a series of experiments that involved manipulating the defensive syphon or gill withdrawal reflex of a marine mollusc, Aplysia californica. Their work suggests that long-term memory is not stored in the synapses themselves but somewhere else, possibly in the nucleus of the neurons. If so, this could be a major breakthrough for early stage Alzheimer’s sufferers since, even though the disease destroys synapses in the brain, the memory could still be there as long as the neurons are still alive. Then, if the synaptic connections can be repaired, some of those memories lost in the early stages of Alzheimer’s could be restored.
The research reminded me that A californica has proved to be a highly valuable laboratory animal in the field of neurobiology and more specifically for studying the mechanisms of movement, learning and memory. The main interest to neuroscientists is that its neurones are among the largest in the animal kingdom and, as there are relatively few of them (about 20,000), it is feasible to identify the specific nerve cells responsible for specific mechanisms. Additionally, the cellular and molecular processes in A californica and humans seem to be very similar.
One of many species of Aplysia found in the world’s oceans, A californica is also known as the Californian brown sea hare. The name derives from the large anterior tentacles or rhinophores which, allied to its rounded body shape, gives the creature the appearance of a crouching hare or rabbit. They are herbivores feeding on kelp, eelgrass and algae and can grow up to 40cm long and weigh up to 2.3kg.
Sea hares are hermaphrodites but cannot self-fertilise. In the wild, they congregate in lines like daisy chains. Individual sea hares can act as either a male, a female, or simultaneously as both. The one at the front of the line is female only, the following ones are male to the animal in front of it and female to the one behind and the one bringing up the end of the line is male only. The eggs are laid in long gelatinous strings which may hold millions of embryos and usually hatch after about 12 days releasing planktonic larvae. Sea hares usually die after laying their eggs.
The animals used in research programmes nowadays are supplied by a carefully controlled breeding establishment in the University of Miami’s Rosenstiel School of Marine & Atmospheric Science.
