Topographically organized projections from the nucleus subceruleus to the hypoglossal nucleus in the rat: A light and electron microscopic study with complementary axonal transport techniques

Abstract Projections from the nucleus subceruleus (nSC) to the hypoglossal nucleus (XII) were investigated with complementary retrograde and anterograde axonal transport techniques at the light and electron microscopic level in the rat. Injections of WGA‐HRP into XII resulted in labeling of neurons in and around the nSC. Labeled nSC neurons were few in number (<4 per 40–60 μm sections) and variable in size and shape. Most labeled nSC neurons were medium‐sized (X̄ = 16.89 μm), fusiform, triangular, or oval, with 3–4 dendrites typically oriented dorsomedially and ventrolaterally. These neurons were found throughout the rostrocaudal extent of the nSC but were most numerous medial, dorsomedial, and ventromedial to the motor trigeminal nucleus. Others were observed rostral to the motor trigeminal nucleus and ventral to the parabrachial nuclear complex. Confirmation of retrograde results was obtained following injections of tritiated amino acids or WGA‐HRP into the nSC. This resulted in labeling throughout the rostrocaudal extent of XII mainly ipsilaterally. Labeled fibers descended the brainstem in the dorsolateral and, to a lesser extent, in the ventromedial component of Probst's tract. Fibers entered XII mainly rostrally along the lateral border of the nucleus. All regions of XII were recipients of nSC afferents, but the caudoventromedial quadrant contained the greatest density of terminal labeling. Electron microscopic evaluation confirmed that nSC afferents synapsed on motoneurons in XII. Axon terminals containing WGA‐HRP reaction product were found contacting dendrites and somata, but primarily the former (81.3% versus 10.6%). Axodendritic terminals synapsed mainly on medium‐to‐small sized dendrites (<3 μm in diameter). The majority of labeled axodendritic terminals (90.1%) contained small, round, and clear synaptic vesicles (S‐type: 20–50 nm) and were associated with an asymmetric (60.6%), symmetric (11.4%), or no (18%) postsynaptic specialization. By contrast, most axosomatic terminals contained flattened vesicles (F‐type) and formed a symmetric or no postsynaptic specialization (75%). Large dense core vesicles (55–90 nm) were observed within a small proportion of all labeled axon terminals (1.3%). The results from this study demonstrate that the nSC projects to XII, preferentially targets a specific subgrouping of protrusor motoneurons, and synapses on both somata and dendrites, although mainly on the latter. The implications of these data are discussed relative to tongue control.