In a quest to unravel brain mysteries, cellular neuroscientists at UCLA are studying learning and memory in the marine snail Aplysia, and are providing new insights into the mechanisms that underlie long-term memory.
Aplysia has almost 20,000 neurons in its central nervous system, while humans have approximately 1 trillion.
This research by David Glanzman, UCLA professor of physiological science and neurobiology, has the potential to help with human brain disorders. His research may lead to such applications as developing interventions for people with memory-related disorders and reducing age-related memory loss.
"The more we know about how long-term memory is induced in the brain and how our memories are maintained in the brain, the more we are going to be able to treat long-term memory loss," said Glanzman
He added: "All the things that we find in the snail, we eventually find in the mammalian brain. Knowledge about learning and memory in Aplysia will inform us about the kinds of changes that take place in our brains when we learn."
He also said that during long-term memory, in the snail and in our brains, synaptic connections become stronger.
While it was believed that during learning in snails, the neurotransmitter serotonin binds to receptors on the presynaptic axon and, through a complicated process, causes the growth of new presynaptic axons, in the new study researchers have said that "the process is not actually initiated in the presynaptic axon, but that this presynaptic change is actually initiated in the postsynaptic neuron."
"Surprisingly, we're seeing that there is a specific presynaptic protein whose synthesis is actually regulated by postsynaptic calciumWhat we think happens is when serotonin binds to receptors on the postsynaptic neuron, it causes an elevation of calcium within the postsynaptic neuron, and somehow this elevation of postsynaptic calcium causes synthesis of presynaptic proteins. In other words, the information is going backwards. We don't know yet how that happens," said Glanzman.
He speculated that this complex process may be the brain's way of preventing mistaken learning. For memories that last for weeks or longer, the presynaptic and postsynaptic cell have to talk to each other, and "we're beginning to understand the chemical signals of the conversation," he said.
"As far as I can tell, the main reason why snails don't learn Shakespeare and do algebra is they just don't have the computational power, because they have only 20,000 neurons. However, in terms of learning, all the cellular and molecular processes seem to be very, very similar. The fundamental mechanisms of learning and memory are identical, as far as we can tell," said Glanzman.
He pointed out that synaptic change requires an interaction between the two sides of the synapse.
"The synthesis of presynaptic proteins depends on postsynaptic calcium. Now the question for us to understand is what are the signals that are activated by postsynaptic calcium that travel across the synapse or somehow affect the presynaptic cell to trigger the synthesis of these proteins," he said.
The researchers think that they are the first scientists ever to see the synthesis of a specific presynaptic protein that is mediated by postsynaptic calcium during a learning-related synaptic change.
The study appeared recently in the early online edition of the journal Current Biology.