Question based on osmosis

Recent experiments in a lab have focused on trying to understand cellular mechanisms that underlie epilepsy. Many of the studies involve recording electrical activity in neurons from brains of adult fruit flies (Drosophila). Brains in all animals are made up of a collection of highly specialized cells surrounded by fluid. In humans and other mammals the fluid surrounding the brain is called cerebrospinal fluid (CSF). This contains lots of molecules including sodium, K+, glucose and proteins with a total solute concentration of ~300 milliosmols (mOsm). In fruit flies the brains are surrounded by fluid called hemolymph with a total solute concentration of ~250 mOsm.
To record from neurons we dissect the brain out of the fruit fly head. The brain tissue is placed immediately into a solution. When we make this solution what should the total solute concentration be and why?