We need more information: 1. How big of a volume? Bigger than 2uL is not specific enough for a determination. 2. What is the rate of your infusion? Are you infusing your injection slow enough, i.e. over how may minutes are you putting your tracer into the Intracerebral space (allowing for absorption)? 3. What is the absorption/ clearance rate of your tracer. The problem here is volume and infusion time are correlated, too much volume too fast will result in damage. However, there is also a saturation point in which the body must clear the tracer before more can be accepted. If the tracer is absorbed and cleared rapidly, no problem; but if it sits in the space for long periods, you have to wait longer. Changes to internal fluids of the brain can increase intracranial pressure (ICP), if you are not carefully monitoring. The result of too high ICP is damage and/or death.

we were considering 2 bilateral injections of 2ul of CTB tracer. The rate of injection is 0.1ul/2min. After each injection we wait 15min with the pipette still in place and than move to inject the next.

I haven't found information on the absorption rate of CTB

Short answer: Large volumes can indeed cause damage.

Infusion rate, absorption rate and total volume play an important role.

Are you saying that you are injecting 2ul into each side? Two times? So, 8ul total?

Or a single bilateral injection of 2ul per side?

Or a single bilateral injection 2ul total volume?

Or two bilateral injections of 2ul total volume?

Please clarify.

Injecting more than 2 micro lit is always risky because fluid pressure will act on controlling organ fast making risk of hemorrhage. Careful, select only pharmacological dose