Hello,
Apologies if this question was addressed somewhere on this site, but I could not find an exact answer after some digging. I was discussing this idea with someone - per the question - about how one adjusts the anterior-posterior (AP; rostral-caudal) coordinate of a brain target in stereotaxic surgery when a rat is older/larger than what a brain atlas is based off of. Specifically, I know that the Paxinos & Watson rat brain atlases are based on a ~270 g Wistar rat, IIRC. But often, researchers end up performing surgeries on rats above this weight - sometimes significantly above. I expect this will add more error to targeting a brain area in the surgery, as the skull seems to get larger in size with age up to a certain point/weight (but, let me know if that is incorrect!). I don't ever recall seeing/hearing a consistent way to address that. This issue applies mainly to adult male rats, as female rats (at least the strains I've used in the past) cap out at 350 g or less, whereas male rats can keep growing much more.
Some factors I've read about / considered that I'm not sure how to factor in:
1. Weight is not always tied to the AP growth of the skull - some rat strains and diet conditions affect body mass without affecting skull growth.
2. I am unsure if the AP skull growth is equal across its whole extent. In other words, if there is some formula people use, like +0.5mm anterior for every +50 or +100 grams body weight, would that be inconsistent across the AP extent of the skull? Would there be more of an effect on targets & coordinates closer to bregma, and less of an effect for those closer to the interaural line (and by proxy lambda)?
Let me know your thoughts - at least I and one other person are wondering about this one.
Hi Kevin R. Urstadt
I conducted stereotaxic surgery on rats weighing approximately 320-530 grams. What I did was to measure the distance between bregma and lambda, which was approximately 9 mm. This distance remained constant across the surgeries I performed.
Thanks and regards
The weight of the rat is irrelevant. The rat brain stops developing at 3 month of age and the scull solidifies. After that, the coordinates remain the same regardless the weight. Its the same for all species. A 80kg human and a 300Kg human have the same brain coordinates.
Thanks for these answers. I understand that the brain size should remain the same in adulthood, and I appreciate that the bregma-lambda distance does not change. As long as these translate also to no age-related shift in where the brain is within the skull (which, it shouldn't), then this is far less of a concern than what I was originally taught.
I would also add that piloting/optimization across weights (if you're consistently dealing with different age groups, for example) would be helpful to ensure that you are most consistently hitting your target.
Indeed, it's always a good idea to confirm these things through initial surgeries. I think that a lot of folks go into doing these surgeries expecting them to be consistent across these variables, but sometimes inconsistencies arise.