Thickness of rat's skull is 0.6 - 0.7 mm. But it may very due to actual size of rat.

You need to do some experiments for calculated the brain area, you need a pressure injector to inject dyes in VTA area, then, the rat sacrificed and brain is sliced to sections. you can finally confirm marker site (dye) by rat brain atlas (paxinos and watson).

Hi

Have you tried to set up your method in a larger dopamine rich area eg CPu to measure dopamine? This at least will exclude problems with surgery and reveal problems (if any) in detection. 

Good luck

Zisis

You can look at the atlas book to see how deep you need to go for VTA (or any brain region of interest).  The book usually does not include the skull so it starts from dura. If you look at the left or right side of the page in the book, you will see numbers.  Those numbers are the depth in mm from dura.  But you can/should inject dyes and verify placement.   

Correction. The numbers on the right side of the page are actually depth in mm from BREGMA, not from dura.