I want to inject a virus expressing Cre-recombinase in the mouse nucleus accumbens, but I need to do it in a way so that I don't hit the ventricle that is right above it. The most obvious solution would be to inject at an angle, and thus bypass the ventricle altogether, however, the arm of our stereotaxic apparatus can't bend at an angle. Is it feasible and reliable to perform these injections at an angle by tilting the head of the mice while keeping the stereotax arm at a perpendicular angle, instead of tilting the arm itself (as I usually see in the literature)? If so, what is the best way to calculate the tilt angle need to access a given deep target region?
It is always possible, but you will run into all kind of problems, the least one being reproducibility and confidence that you tilted the head the right way.
Can you revise your coordinates so you inject in the N Acc forward ? In the rat you can inject at bregma +2mm and almost by pass the ventricle ?
That's really not an option... I need to get as much coverage across the rostral-caudal axis as possible, and with the virus I'm using I need to do 2-3 microinjections of 1uL to get close to that. We can even get some good expression in the more rostral NAcc, but in the more caudal regions we always end up hitting the ventricle and losing viral load due to dilution. THis is why why we want to try angled injections...
I agree with Stephane that it may not be feasible, albeit possible. I should note though that one of my labmates has been performing the upward head-tilt approach for viral injection into mouse accumbens shell (mostly anterior). His suggestion for increasing injection placement consistency between mice was to confirm flat skull positioning before tilting. One could do this by measuring dorsal-ventral coordinates at bregma, then dorsal-ventral coordinates 1-2 mm in front of bregma; the DV coordinates should start off the same before tilting. This would also be your baseline to tilt from. However, I'm not sure beyond that how much one would have to tilt the head to hit anterior accumbens and tilt again to hit posterior accumbens while avoiding the ventricles. This would vary based on mouse skull size/length.
The need to tilt could be circumvented by revising the injection method in a way that avoids spillage along the injection tract. Perhaps you should perform microinjections via glass micropipette, and eject virus from the micropipette either via pressurized system or via iontophoresis. Since micropipette tips are so small (10-25 um), injected fluids have less tendency to leak out into surrounding structures such as the ventricles.
Dear Kevin, thanks for the input! Maybe the upward head-tilt approach isn't the best option for me (considering that I need to target the posterior accumbens), but using micropipettes seems like a really good idea. :-)
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