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Abstract
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 agglutinin conjugated with horseradish peroxidase with choline acetyltransferase
immunohistochemistry. All major sensory (medial geniculate, lateral geniculate, ventrobasal),
motor (ventroanterior, ventrolateral, ventromedial), associational (mediodorsal, pulvinar,
lateral posterior) and limbic (anteromedial, anteroventral) 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-pontine
junction where the cholinergic peribrachial 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.(ABSTRACT
TRUNCATED AT 400 WORDS)