A good raw estimate (sufficient for most applications) is 10% of fresh brain weight (with meninges discarded); will differ between various brain regions.

For total protein extraction if you use 100 ul of lysis buffer for each 5 mg of brain tissue, you must obtain aprox 2-3 ug/ul

i isolate the brain and remove the cerebellum part ...my tissue is 100- 150 mg , and i usually have 5mg/ml total protein . is it low?

can that be the reason that i have faint bands in western blot even with house keeping ptn ? (although i calculate amount to load that have 40 ug ptn )

It’s a good concentration. Check your transfer conditions (for wet 100V, 1h or 30V, 8h. Membrane pore depends of protein weight), maybe you’re losing proteins. Or maybe the faint bands are because the blocker. Try with non-fat milk 5% in TBS. If faint bands persist, try with BSA 5% in TBS.