Variability of bregma in 300 gram Long-Evans and Sprague-Dawley rats

Bregma is used as an anatomical reference point for most freehand and some stereotaxic injections into the rat brain. The variability of bregma with respect to the intraaural line has been examined previously for rats of a wide range of weights. We examined this variability in 282-326 gram male rats of the Long-Evans and Sprague-Dawley strains. In both strains, mean bregma was less than that stated in a widely used stereotaxic atlas. Furthermore, the mean bregma of the Sprague-Dawley rats was significantly greater than that for the Long-Evans strain. The results suggest that bregma is inadequate as an anatomical reference for intraventricular injections in rats unless the volume injected is a substantial fraction of ventricular volume.