Medullary and spinal efferents of the pedunculopontine tegmental nucleus and adjacent mesopontine tegmentum in the rat

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Abstract
The medullary and spinal efferents of the pedunculopontine tegmental nucleus and adjacent mesopontine tegmentum were investigated by employing (1) the anterograde autoradiographic methodology and (2) the retrograde transport of HRP and/or WGA-HRP in combination with choline acetyltransferase immunohistochemistry. The anterograde experiments identified five descending pathways from the mesopontine tegmentum: (1) Probst's tract, which descends in the dorsolateral reticular formation in close relation to the nucleus of the solitary tract; (2) a ventrolateral branch of Probst's tract that extends ventrolaterally alongside the spinal trigeminal nucleus; (3) a ventromedial branch of Probst's tract that extends ventromedially through the gigantocellular field of the medulla; (4) the medial reticulospinal tract, which descends in parallel with the medial longitudinal fasciculus and turns ventrolaterally along the dorsal surface of the inferior olive to enter the ventrolateral funiculus of the spinal cord; and (5) a crossed ventromedial pathway, which descends in a ventral paramedian position through the magnocellular field of the medulla. The origins of these pathways reflected a rough lateral-to-medial topography of mesopontine tegmental cell groups. The parabrachial nucleus, situated furthest laterally, for example, projected primarily through Probst's tract and its ventrolateral branch. The pedunculopontine tegmental nucleus, midbrain extrapyramidal area, and the subceruleal region, situated more medially, projected descending axons largely through the ventromedial branch of Probst's tract. The pontine tegmental field, situated furthest medially and ventromedially, was the largest contributor to the medial reticulospinal tract. The retrograde transport experiments confirmed these general organizational features. The combination of retrograde transport with choline acetyltransferase immunohistochemistry established that the cholinergic pedunculopontine tegmental nucleus contributes a large portion to the mesopontine tegmental innervation of the medullary reticular formation. A much smaller number of cholinergic pedunculopontine neurons project as far as the spinal cord. Spinal projections from the mesopontine tegmentum originate largely from non-cholinergic neurons of the midbrain extrapyramidal area, subceruleal region, Kölliker-Fuse division of the parabrachial nucleus, and pontine tegmental field.