Hi Lei, for a western blot or two a small litter would be fine, you'd actually probably get enough ug of protein for a few blots from a single brain! We've just published a paper for isolation of cerebral microvessels (and culture of endothelial cells) from rat, but the techniques of isolating the capillaries would equally apply to mouse. http://www.biomedcentral.com/1471-2202/14/59
Please just drop me a line if you've any more questions Lei!