Nonretinal projections to the medial terminal accessory optic nucleus in rabbit and rat: a retrograde and anterograde transport study.

The distribution and density of the nonretinal projections to the rabbit medial terminal accessory optic nucleus (MTN) have been studied after injections of horseradish peroxidase (HRP) into the MTN in seven rabbits, and confirmation for the presence of certain of these projections has been made in the rabbit or rat by utilizing anterograde transport of tritiated leucine or leucine/proline after appropriate injections into cerebral cortical areas and brainstem nuclei. In seven cases studied by the retrograde axonal transport method, HRP-labeled neurons have been identified: (A) In four visual or preoculomotor nuclei in which available autoradiographic brain series have confirmed the presence of projections to the MTN: (1) The nucleus of the optic tract/dorsal terminal accessory optic nucleus, (2) the interstitial nucleus of the superior fasciculus (posterior fibers), (3) the periaqueductal gray (including its supraoculomotor portion), and (4) the medial division of the deep mesencephalic nucleus. (B) Within the ventral lateral geniculate nucleus, from which a projection to the MTN has been confirmed autoradiographically in the rat by other workers. (C) In brainstem nuclei and cerebral cortical areas in which available autoradiographic brain series fail to confirm the presence of afferents to the MTN: (1) The nucleus reticularis pontis, pars oralis and pars caudalis, (2) the intermediate interstitial nucleus of the medial longitudinal fasciculus, (3) the nucleus raphe pontis, and (4) five cerebral cortical areas (the area retrosplenialis granularis dorsalis, the striate area, the parietal area 3, the subicular cortex, and the regio praecentralis granularis). Finally, we report retrograde labeling which, on the basis of published connectional data, we believe to result from the spread to and uptake from axons en passant. The false-positive labeling in this category is likely to result from spread of HRP into ventral tegmental nuclei or tracts adjacent to the MTN. Thus, as a result, in the medulla and pons, labeled neurons are found in the medial, lateral, and superior vestibular nuclei, the medullary reticular formation including the nucleus reticularis gigantocellularis, the lateral reticular nucleus, the nucleus raphe magnus, the spinal nucleus of V, the nucleus gracilis/nucleus cuneatus, the dorsal and ventral divisions of the parabrachial nucleus, the central pontine gray, the nucleus K of Meessen and Olszewski, and the dorsal nucleus of the lateral lemniscus. In the midbrain, labeled neurons are present in the interstitial nucleus of Cajal and the strata griseum intermedium and profundus of the superior colliculus. In the diencephalon labeled neurons are found in the posterior and medial pretectal nuclei, the zona incerta, and the ventral lateral geniculate nucleus. Labeled somata also are observed in the caudate-putamen, fundus striati, lateral preoptic area, and lateral hypothalamic nucleus of the telencephalon The present study represents the first comprehensive analysis of the total nonretinal projections to the MTN in any vertebrate species. It provides data whichimpinge in two ways upon other findings that show that the terminal accessory optic nuclei, including the MTN, contain substantial populations of GABAergic neurons and axon terminals: First, in relation to the present demonstration of the nonretinal afferents of the MTN, which may contribute to the GABAergic innervation of MTN neurons, and, second, related to the fact that these nonretinal afferents may activate GABAergic neurons of the MTN (Penny et al.: J. Comp. Neurol. 228:38–56, '84; Giolli et al.: Exp. Brain Res. 61:194–203, '85) triggering a fine tuning of compensatory eye movements to retinal slip.