Frontal cortical projections to the periaqueductal gray in the rat: a retrograde and orthograde horseradish peroxidase study.

The retrograde and orthograde capabilities of the horseradish peroxidase (HRP) technique were used to study the projection from the frontal cortex to the periaqueductal gray (PAG) in the rat. Small pyramidal neurons in layer V of the frontal (MD-dependent) cortex were the primary source of these projections and the system terminated primarily in the lateral PAG. Since the projection is homologous to that in the monkey, the rat may serve as a model for the study of the cortical influence on PAG-mediated behaviours such as analgesia, aversion and reward.