PREMOTOR NEURONS FOR VERTICAL EYE-MOVEMENTS IN THE ROSTRAL MESENCEPHALON OF MONKEY AND HUMAN - HISTOLOGIC IDENTIFICATION BY PARVALBUMIN IMMUNOSTAINING

In the monkey, premotor neurons for vertical gaze are located in the m esencephalic reticular formation: the rostral interstitial nucleus of the medial longitudinal fascicle (riMLF) contains medium-lead burst ne urons, and the interstitial nucleus of Cajal (iC) acts as integrator f or the eye-velocity signals to eye-position signals. Both nuclei lie a djacent to each other and are similar in appearance at the transition zone in Nissl-stained sections, which makes a delineation of the funct ionally different nuclei difficult in human. For a neuropathologic ana lysis of degenerative changes in saccadic disorders of patients, the h istologic identification of the riMLF and the iC is important. The aim of this study is to identify both nuclei in human by using parvalbumi n as a histologic marker. First, in monkeys the premotor neurons in ri MLF and iC were identified by trans-synaptic labelling after injection s of tetanus toxin fragment C into vertical-pulling eye muscles. Premo tor neurons were found in the riMLF mainly ipsilateral to the correspo nding eye muscle motoneurons and on both sides within the iC, but here the labelled cell populations differed: the contralateral side contai ned more medium-sized cells compared with the mainly small-sized cell population on the ipsilateral side. Double labelling showed that almos t all premotor neurons in the iC and all premotor neurons in the riMLF were parvalbumin-immunoreactive. The immunocytochemical staining of h uman brainstem sections revealed the riMLF as a cluster of medium-size d, elongated parvalbumin-positive cells, with a similar appearance and at a similar location as that in monkey: a wing-shaped nucleus dorsom edial to the red nucleus, rostral to the traversing tractus retroflexu s, dorsally bordered by the thalamo-subthalamic paramedian artery. The adjacent iC could be distinguished easily by its more densely packed, round parvalbumin-immunoreactive neurons. The exact identification of premotor neurons of the vertical system in the normal human brain pro vides a reference basis for the neuropathologic analysis of vertical g aze disorders at a cellular level. (C) 1998 Wiley-Liss, Inc.