Ventromedial mesencephalic tegmental (VMT) projections to ten functionally different cortical areas in the cat: Topography and quantitative analysis

Abstract

The cortical projections of the ventromedial mesencephalic tegmentum (VMT) were studied in the cat by the retrograde transport of wheat germ agglutinin-conjugated HRP (WGA-HRP). The following cortical regions were injected: prefrontal, cingulate, motor, somatic sensory, auditory, primary visual, secondary visual, parasplenial, insular, and entorhinal. Labelled cells in the five component nuclei of the VMT (rostral linear, parabrachial pigmented, central linear, interfascicular, and paranigral nuclei) were counted and the numbers correlated with the total cell populations of the nuclei. In all experiments large numbers of cells were labelled in the VMT, particularly in the rostral linear and parabrachial pigmented nuclei. Following entorhinal cortical injections, significant labelling occurred in the interfascicular and central linear nucleus, although some cells of these nuclei were labelled in every experiment. The cells of the paranigral nucleus remained unlabelled in all experiments. Several hundred VMT cells were labelled in most experiments, over 1000 in the case of the parasplenial and over 2000 in the case of prefrontal injections. The most strongly labelled nucleus was the rostral linear, in which up to 36% of the total cell population could be labelled in a single experiment. These results indicate that the VMT of the cat is a major source of brainstem projections to the cerebral cortex and that all cortical areas are reached by this system. Although the projections to the prefrontal and limbic cortical areas are particularly prominent, there appears to be a consistent and significant projection to all areas. Further, the projection of the VMT to the cortex is topographically organized, indicating that the functional roles of the component nuclei of the VMT may be substantially different.