Neurotrophins: which way did they go?

The classic view of the neurotrophin as a messenger produced by the postsynaptic cell that signals survival to the presynaptic cell has become expanded. Segal discusses the evidence that the neurotrophin brain-derived nerve growth factor (BDNF) can be also released from the presynaptic cell onto the dendritic spine of a postsynaptic cell to regulate the neuronal circuit in two directions: forward (from presynaptic cell to postsynaptic dendrite) and the classic backward direction (from postsynaptic cell to the presynaptic axon). These results remind us that in the brain, neurons exist in a complex circuit and one neuron's presynaptic cell is another neuron's postsynaptic cell. The evidence that a single growth factor may have different actions based on the localization of the receipt of the signal adds more complexity to the brain's neuronal circuitry.