GABAergic neurons in the rostral mesencephalon of the macaque monkey that control vertical eye movements.

The mesencephalic reticular formation is important for the generation of vertical eye movements, but up until now the location of inhibitory premotor neurons is not known in primates. With tract-tracer methods combined with immunocytochemistry or in situ hybridization, we investigated the location of GABAergic premotor neurons in the rostral interstitial nucleus of the medial longitudinal fascicle (riMLF) and interstitial nucleus of Cajal (iC) in macaque monkeys. In the present work, only the premotor pathways of the downward pulling eye muscles, superior oblique (SO) and inferior rectus (IR), were studied. We found that very few, small GABAergic neurons are present in the riMLF, and none of them was found to project to the oculomotor nuclei, suggesting the presence of exclusively excitatory projections from the riMLF to the oculomotor neurons. However, in the iC, medium-sized and large GABAergic neurons were identified projecting contralaterally to the SO and IR motoneurons, and presumably the iC of the other side. These commissural GABAergic projections are well suited to inhibit the SO and IR motoneurons and possibly premotor down-burst-tonic neurons during upward eye movements.