How many RNA I need for detecting plasma miR expression? - (Aug/08/2011 )

Hi, i am working on miR expression in plasma. I used miRNesay to extract miRNA, but i only got 30-100ng RNA per 200ul plasma(mice). I am planning to use microarray to test miR expression. But the core facility asked me to provide 5ug RNA at least for each chip. That is too much. Because I usually get 500-600ul plasma from each mouse, and the max amount of RNA is 300ng/mouse. So if I want to get 5ug, I must sac 20 mice for one chip.
Have anyone done the similar research? How many RNA usually you put on one chip?
The concentration of RNA from plasma is around 5-10ng/ul, could I condense RNA? How?
Thanks.

Use a kit to amplify your RNA. The MessageAmp II kit from Ambion/Applied Biosystems is commonly used for this:

http://www.ambion.com/catalog/CatNum.php?1751