Projections of medial terminal accessory optic nucleus, ventral tegmental nuclei, and substantia nigra of rabbit and rat as studied by retrograde axonal transport of horseradish peroxidase

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Abstract
Projections of the medial terminal nucleus (MTN) of the accessory optic system, the ventral tegmental area of Tsai, and the substantia nigra of the rabbit and the rat have been studied by the method of retrograde axonal transport of horseradish peroxidase. The data show that MTN projections are remarkably similar in the rabbit and rat. The MTN projects heavily to the ipsilateral nucleus of the optic tract and dorsal terminal nucleus of the accessory optic system and to a portion of the contralateral ventral tegmental area of Tsai that we have termed the visual tegmental relay zone (VTRZ). Further, the MTN sends projections to the ipsilateral mesencephalic (deep mesencephalic nucleus, pars medialis) and pontine (nucleus reticularis pontis oralis) reticular formations; the contralateral dorsolateral division of the basal pontine complex; the superior and lateral vestibular nuclei (contralateral in rat; bilateral in rabbit); and the ipsi‐ and contralateral interstitial nucleus of Cajal, nucleus of Darkschewitsch, and supraoculomotor‐periaqueductal gray. The findings also indicate that the MTN has a small bilateral, but mainly ipsilateral, projection to the dorsal cap, its ventrolateral outgrowth, and the B division of the inferior olivary complex. This study further reveals that ventral tegmental nuclei (n. parabrachialis pigmentosus and n. paranigralis) and subdivisions of the substantia nigra (pars compacta and pars reticulata) project to many brain stem targets of the MTN. Thus, the VTRZ projections are similar to those of the MTN in both distribution and density except that the VTRZ projection to the inferior olive is substantially stronger. The nucleus parabrachialis pigmentosus sends a small contralateral projection to the VTRZ and a moderate‐sized bilateral projection to the supraoculomotor‐periaqueductal gray. The nucleus paranigralis sends a moderate number of axons to the ipsilateral deep mesencephalic nucleus, pars medialis, and the nucleus reticularis pontis oralis and provides a strong bilateral projection to the supraoculomotor‐periaqueductal gray. The pars compacta of the substantia nigra provides a sparse input to the ipsilateral deep mesencephalic nucleus, pars medialis, and nucleus reticularis pontis oralis, and to the contralateral VTRZ and sends a moderate number of axons, bilaterally, to the supraoculomotor‐periaqueductal gray. The pars reticulata of the substantia nigra sends an ipsilateral projection of moderate size to the intermediate and deep layers of the superior colliculus, sparse ipsilateral projections to the deep mesencephalic nucleus, pars medialis, and nucleus reticularis pontis oralis, and a sparse bilateral projection to the supraoculomotor‐periaqueductal gray. In addition, the precise origins of nigrostriatal and ventral tegmental‐striatal projections have been deter‐mined after injections in the anteromedial caudate‐putamen. These cases result in the labeling of large numbers of densely packed neurons in the ipsilateral pars compacta of the substantia nigra and nucleus paranigralis, with moderate numbers of labeled neurons in nucleus parabrachialis pig‐ mentosus. Further, we observe a sparse projection from the contralateral nucleus paranigralis and parabrachialis pigmentosus to the anteromedial caudate‐putamen. The findings presented indicate that (1) the visual areas of the ventral midbrain tegmentum (MTN and VTRZ) project to primary optic, preoculomotor, and precerebellar nuclei involved in eye movement control; and (2) the adjacent ventral tegmentum and substantia nigra project to many of the same nuclei as the MTN and VTRZ.