Neurons die faster in the spring (yellow) unless they are treated with BDNF (blue).

Nottebohm/NAS

Not all memories live forever. Some memories may, at least in birds, die with the death of the neurons that encode them. Now, Benjamin Alvarez-Borda, Bhagwattie Haripal, and Fernando Nottebohm (Rockefeller University, New York, NY) suggest that the life expectancy of certain canary neurons, and perhaps of their associated memories, depends on one factor.

Canaries start singing and learning songs in the fall as the mating season approaches. Neurons entering the high vocal center (HVC) during this time stick around so that song memories persist. But by spring most of the birds have found a mate and ignore singing in favor of other pursuits. Neurons born in the spring come and go more rapidly.

The Rockefeller team turned the short-lived spring neurons into long-lived neurons by a short burst of brain-derived neurotrophic factor (BDNF) given 14–20 days after the neurons were born, which is when differentiation is occurring. Adding BDNF one week earlier or later did not enhance survival.

BDNF may increase survival time by strengthening neural connections. But why is this regulation needed? The memory capacity of adult bird brains may be limited by the number of available nerve cells. If cell number can't be increased, survival time may need to be limited to make space for new memories. BDNF is the first handle on how canary brains assign lifetimes to neurons and perhaps memories. Similar processes may even apply to our own memory-processing hippocampus. ▪

Reference:

Alvarez-Borda, B., et al. 2004. Proc. Natl. Acad. Sci. USA. 10.1073/pnas.0308118101.