The topography of the mesencephalic and pontine projections from the cingulate cortex of the rat.

A projection from the rat midline cortex to the midbrain and pons has been recognized for several years. The present study is a detailed analysis of this projection using the autoradiographic technique. Small injections of [3H]amino acids were placed within individual segments of the cingulate cortex in 68 rats. The resulting material reaffirmed the existence of the cingulo-brainstem projections and demonstrated that a precise topographical relationship exists between the cingulate cortex cells of origin and their termination fields within the brainstem. The most ventral and anterior segment of the cortex (IRaa) projects to the ventral periaqueductal gray, to the dorso-medial ventral pontine nuclei and to the lateral tegmental region. Conversely, the dorso-anterior cortex (IRca) projects to the superior colliculus, the dorso-lateral periaqueductal gray, and the medio-ventral ventral pontine nuclei. The intermediate anterior cortex projects to both dorsal and ventral periaqueductal gray, lightly to the superior colliculus, and to the medio-intermediate ventral pontine nuclei. The posterior half of the infraradiata (IR) cortex projects to the dorso-lateral periaqueductal gray, to the superior colliculus, and to the region of the ventral pontine nuclei slightly lateral to the terminal zone occupied by the anterior IR cortex. Increasingly dorsal segments of the IR beta cortex project to more increasingly ventral areas of the ventral nuclei. The posterior portion of the midline cortex (retrosplenial cortex, R) does not project to the dorsal midbrain, but it does topographically project to the ventral pontine nuclei, lateral to the terminal zone of the IR axons. Increasingly, posterior regions of the R cortex project to more lateral regions of the ventral nuclei, and increasingly, dorsal cells of the R cortex project to more dorsal regions of the ventral nuclei. These data demonstrate a very precise topography of brainstem projections which may underlie the visceral and somatic motor functions of the cingulate cortex, as well as the ability of the cingulate cortex to modulate sensory information and emotional behavior.