The projections of brainstem core neurons to relay and associational thalamic nuclei were studied in the cat and macaque monkey by combining the retrograde transport of wheat germ agglulinin conjugated with horseradish peroxidase with choline acetyltransferase immunohistochemistry. All major sensory (medial geniculate. lateral geniculate, ventrobasal). motor (venlroanterior. ventrolateral. ventromedial), associational (mediodorsal, pulvinar, lateral posterior) and limbic (anteromedial.,interoventral) thalamic nuclei of the cat were found to receive projections from cholinergic neurons located in the peribrachial area of the pedunculopontine nucleus and in the laterodorsal tegmental nucleus as well as from non-cholinergic neurons in the rostral (perirubral) part of the central tegmental mesencephalic field. Specific relay nuclei receive less than 10%) of their brainstem afferents from non-cholinergic neurons located at rostral midbrain levels and receive 85 96% of their brainstem innervation from a region at midbrain pontinc junction where the cholinergic perihrachial area and laterodorsal tegmental nucleus are maximally developed. of the total number of horseradish peroxidase-positive brainstem neurons seen after injections in various specific relay nuclei, the double-labeled (horseradish peroxidase + choline acetyltransferase) neurons represent approximately 70 85%. Three to eight times more numerous horseradish peroxidase-labeled brainstem cells were found after injections in associational (mediodorsal and pulvinar lateral posterior complex) and diffusely cortically projecting (ventromedial) thalamic nuclei of cat than after injections in specific relay nuclei. The striking retrograde cell labeling observed after injections in nuclei with associative functions and widespread cortical projections was due to massive afferentation from non-cholinergic parts of the midbrain and pontine reticular formation, on both ipsi- and contralateral sides. After wheat germ agglutinin horseradish peroxidase injections in the associative pulvinar lateral posterior complex and mediodorsal nucleus of Macaca sylvana.. 45 50% of horseradish peroxidase-positive brainstem peribrachial neurons were also choline acetyltransferase-positive. While cells in the medial part of the cholinergic peribrachial area were found to project especially towards the pulvinar lateral posterior nuclear complex in monkey, the retrograde cell labeling seen after the mediodorsal injection was mostly confined to the lateral part of both dorsal and ventral aspects of the peribrachial area. These findings support the role played by cholinergic and non-cholinergic hrainstem thalamic projections in various tonic and phasic events of behavioral slates of vigilance.
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