A new approach to detection of the bregma point on the rat skull

Stereotaxy is commonly used to implant microelectrodes or microprobes in specific structures of the brain in vivo. In this technique, the positions of the brain nuclei are determined as the distance from a defined reference point on the skull. Thus, it is crucial to correctly locate the reference point. On the rodent skull cap, the principal stereotaxic reference point is called the bregma and is defined as the midpoint of the curve of best fit along the coronal suture. Rough determination of the position of the bregma often results in error. In our experiments we developed and tested an alternative method of locating the bregma point on the skull of mature Wistar rats. In this method, a digital picture of the exposed skull cap is analyzed by a computer. The curve is mathematically fitted to the outline of the coronal suture, and the brain midline is delineated based on the temporal ridges of the skull. The crossing of these two lines is defined as the bregma. Systematic, experimental testing of this new method revealed that, in many cases, the position of the bregma point as located by two different methods (old, rough method and the new one) varied by as much as hundreds of microns. The error in stereotaxic positioning of the microprobe in the brain was significantly decreased when the bregma was determined using the new approach. These results confirm that the new method of locating the stereotaxic reference point improves the precision of in vivo electrode implantation.