Saccadic omnipause neurons (OPNs) are essential for the generation of saccadic eye movements. In primates OPNs are located near the midline within the nucleus raphe interpositus (rip). In the present study we used several different neuroanatomical methods to investigate the transmitters associated with OPNs in the monkey. Immunolabeling for the calcium-binding protein parvalbumin was employed to mark OPNs in the monkey and define the homologous cell group in cat and human. The use of antibodies against GABA, glycine (GLY), glutamate (GLU), serotonin (5-HT), and tyrosine hydroxylase revealed that the somata of OPNs are GLY immunoreactive, but they are devoid of GABA and 5-HT immunostaining. In situ hybridization with the GAD67 mRNA probe confirmed the negative GABA immunostaining of OPNs. 3H-GLY was injected into a projection field of OPNs, the rostral interstitial nucleus of the medial longitudinal fascicle (riMLF)--the vertical saccadic burst neuron area. This resulted in selective retrograde labeling of the OPNs in rip, while no labeling was found in the superior colliculus, which sends an excitatory projection to the riMLF. The somata and dendrites of putative burst neurons in the riMLF were contacted by numerous GLY- immunoreactive terminals. The quantitative analysis of immunoreactive terminal-like structures contacting OPNs revealed a strong input from GLY- and GABA-positive terminals on somata and dendrites, whereas GLU- positive puncta were mainly confined to the dendrites. Very few 5-HT and catecholaminergic terminals contacted OPN somata. Our findings suggest that OPNs use GLY as a neurotransmitter, and they receive numerous contacts from GABAergic, glycinergic, and glutaminergic afferents, and significantly fewer from monoaminergic inputs.

The perioculomotor region contains several functional cell groups, including parasympathetic preganglionic neurons of the ciliary ganglion, motoneurons of multiply innervated muscle fibers (MIF) of extraocular muscles, and urocortin-positive neurons. In this study, midbrain sections of monkey and human were treated with antibodies against choline acetyltransferase (ChAT), cytochrome oxidase (CytOx), nonphosphorylated neurofilaments (NP-NF), chondroitin sulfate proteoglycan (CSPG), and urocortin (UCN) to identify them by their histochemical properties. To facilitate the comparison between species, a new nomenclature was introduced (see also May et al., 2007), which designates these perioculomotor cell populations (pIII) in terms of their function and histochemical properties. The name Edinger-Westphal nucleus (EW) is kept for the cytoarchitecturally defined cell group traditionally considered as the location of preganglionic neurons of the ciliary ganglion. In monkey, the EW contains ChAT-positive presumed preganglionic neurons, and is therefore termed EW(PG), but in contrast human EW consists of noncholinergic UCN-positive neurons, and is therefore termed EW(U). In human, the presumed preganglionic neurons were found dorsal to EW(U), as an inconspicuous group of ChAT- and CytOx-positive neurons. They were interspersed with prominent CSPG-positive cells, a pattern also present in monkey. For the first time, the MIF motoneurons could be identified around the medial aspect of the human oculomotor nucleus as a group of ChAT-positive neurons that lack CSPG-positive perineuronal nets. Moreover, the Perlia nucleus was found to share the histochemical properties of oculomotor twitch motoneurons. The present results form the basis for addressing the appropriate functional cell groups in correlative clinicopathological studies.

Current investigations show that two separate sets of motoneurons control the extraocular eye muscles, and that is there is a dual final common pathway. We propose that one set of motoneurons are the major source of tension generating eye movements, whereas the other may participate in a proprioceptive system concerned more with the exact alignment and stabilization of the eyes. In this article we discuss the structures that may participate in the proprioceptive circuits; and consider several recent publications in the light of this sensory feedback hypothesis, emphasizing the relevance to eye movement disorders.

The abducens nucleus (nVI) contains several functional cell groups: motoneurons of the singly-innervated twitch muscle fibers (SIF) and those of the multiply-innervated muscle fibers (MIF) of the lateral rectus muscle (LR), internuclear neurons (INTs) projecting to the contralateral oculomotor nucleus (nIII) and paramedian tract-neurons (PMT) that receive input from premotor neurons of the oculomotor system and project to the floccular region. In monkey, these cell populations can be delineated by their chemical signature. For correlative clinico-pathological studies the identification of the homologous cell groups in the human nVI are required. In this study, we plotted the distribution of these populations in monkey nVI by combined tract-tracing and immunohistochemical staining facilitating the identification of homologous cell groups in man. Paraffin sections of two Rhesus monkeys fixed with 4% paraformaldhehyde and immunostained with antibodies directed against choline acetyltransferase (ChAT) as marker enzyme for cholinergic neurons and chondroitin sulfate proteoglycan (CSPG) to detect perineuronal nets (PNs) revealed four neuron populations in nVI with different chemical signatures: ChAT-positive and CSPG-positive SIF motoneurons, ChAT-positive, but CSPG-negative MIF motoneurons, and ChAT-negative neurons with prominent PNs that were considered as INTs. This was confirmed by combined immunofluorescence labeling of cholera toxin subunit B (CTB) or wheat germ agglutinin (WGA) and ChAT or CSPG in nVI sections from cases with tracer injections into nIII. In the rostral part of nVI and at its medial border, populations of ChAT-negative groups with weak CSPG-staining, but with strong acetylcholinesterase (AChE) activity, were identified as PMT cell groups by correlating them with the location of anterograde tracer labeling from INTs in nIII. Applying ChAT- and CSPG-immunostaining as well as AChE staining to human brainstem sections four neuron groups with the same chemical signature as those in monkey could be identified in and around the nVI in human. In conclusion, the distribution of nVI neuron populations was identified in human based on findings in monkey utilizing their markers for cholinergic neurons and their different ensheathment by PNs of the extracellular matrix.