BDNF: GOOD? BAD? UGLY?

I have bee researching many papers on the effect of exercise on neurochemistry and how it may relate to autism spectrum disorder. There have been a number of recent studies that identified an increase in brain derived neurotrophic factor (BDNF), particularly in the hippocampus, the centre of learning and memory in the brain.  BDNF is a compound that promotes neuronal growth and protection and synaptic connections which all relate to the idea that the brain has the ability to grow and change (brain plasticity).

This has huge ramifications for elderly people with Alzheimer's (as some investigators suggest).  

What about autism?

Here is where I hit a wall and need to find more research in the area. In fact I found a couple of articles that suggested some people with ASD had up three times as much BDNF in their blood serum. I could not find out if this was because it was not being utilized or if it had a negative effect on the brain.

There are also connections between exercise, BDNF, and serotonin, and how exercise can alleviate symptoms of depression

I need more on the direct positive relationships between EXERCISE and AUTISM!
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Modie, Jonathan. (2003). "'Good' Chemical, Neurons in Brain Elevated Among Exercise Addicts." OHSU, online.

"Exercise can help brain healing process." (2004). Medical Research News, online.

Russo-Neustadt, A.A., R.C. Beard, Y.M. Huang, and C.W. Cotman. (2000). "Physical Activity and Antidepressant Treatment Potentiate the Expression of Specific Brain-Derived Neurotrophic Factor Transcripts in the Rat Hippocampus." Neuroscience, 101, 305-312