Differentiation among cells is critical to human development, not just the brain. We all start from a single cell containing the genetic material that determines our individual characteristics. As cells divide, and divide some more, they start to differentiate -- i.e., exhibit different characteristics, play specific roles or functions, and so forth. Scientists have linked growth factor chemicals with the regulation of cell growth and differentiation. In the case of nerve growth factors like neurotrophins, they activate the process by attaching to neuron cell receptors -- molecules that act like tiny antennas, sitting on the surfaces of cells like neurons. A good analogy is the lock and key. The receptors are the lock, and can only be opened by certain protein "keys" (known as ligands), and once opened, the receptors send a series of internal signals ricocheting through the nervous system.
Neurotrophins are keys that fit the locks of specific neuronal receptors in the brain. There are four related types of neurotrophins: BDNF, nerve growth factor (NGF), neurotrophin-3 (NT-3), and neurotrophin 4 (NT-4). Each of these correspond to a specific family of three receptors (A, B and C), known as Tyrokinase (Trk) receptors. NGF binds to TrkA; NT-3 binds to YrkC; and BDNF and NT-4 bind to Trk-B. There is a second class of receptor called p75 to which all the neurotrophins can bind, just with lower affinities. And p75 seems to play a critical role in triggering programmed cell death.
Once those "locks" have been opened, they trigger nerve growth, survival, and differentiation via a complex signaling pathway (so complex that the underlying mechanism is still not completely understood by neuroscientists). As such, they seem to play a vital role in behavior, learning and memory. BDNF acts specifically on certain neurons in the central nervous system and the peripheral nervous system (i.e., the lower spinal cord) to support the survival of existing neurons and encourage the growth and differentiation of new neurons and synapses. It isn't just found in the brain: BDNF is also expressed in the retina, motor neurons, the kidneys, and the prostate. Stress, or exposure to the stress hormone corticosterone, has been shown to decrease the expression of BDNF in laboratory rats, and if the exposure persists, and BDNF levels continue to drop, the entire hippocampus can atrophy. Similar atrophy has been shown in people suffering from clinical depression -- hence, the suspicion that there may be a critical link between that condition and low levels of BDNF.
Complete article can be read at:
http://twistedphysics.typepad.com/cocktail_party_physics/2008/03/survival-of-the.html
