I am trying to study the effect of a drug on dentate oscillations in urethane anesthetized rats, where I have a guide cannula + a tungsten wire lowered to the dentate gyrus. The tungsten wire extends ~1mm below the guide cannula. After recording a stable baseline, I lower a hamilton syringe attached to a syringe pump into the guide tube so that it extends ~0.5mm below the guide cannula. Regardless of how carefully this is done, I always end up with a spreading depression which recovers ~95% eventually . Is this common? how do i prevent this from happening?
Any suggestions will be appreciated.
Hi Deepak,
Are you saying the drug infusion affects the subsequent oscillation recordings? If this is the issue, you could try doing the infusion without any extension past the end of the guide cannula. You could test and see what this looks like outside the animal, in terms of what the injection volume does. I would guess the infusion would still work without the 0.5mm extension, just make sure you leave the injector in there 2-3 minutes after completing the infusion. The other possible issue is that the infusion is too rapid, and you could try doing it more slowly, but it sounds like you have tried this.
Hello Benjamin,
Thanks for your response. I infuse drug (1ul) at 0.2ul/min. I was worried if i dont have the 0.5mm extension, the drug might stay in the cannula and not infuse into the tissue completely. I do however let the needle in sit for 2-3 minutes after infusion. But, due to the spreading depression, I need to give it a ~10 min to recover, so technically I am recording a baseline before lowering needle and one after its in place. I can also see that the oscillations that recover are not a 100%. I was wondering if the process of lowering the needle below the guide cannula is causing some form of mechanical stress to the neurons resulting in the spreading depression.
Hi Deepak,
If you have some extra animals, you could test to see if the infusion works with extension with some dye. Colin Saldaha has done some interesting work on the effects of canula insertion in neural tissue, but that is mostly chronic effects. It is certainly possible you are seeing acute effects that are affecting the oscillation recordings. Makes you wonder about optogenetic work.
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