Histochemical characterization of inputs to motoneurons of extraocular muscles subserving different functions

Eye movements are important to aid vision, and they serve two main
functions: to stabilize a moving visual target on the retina and to
stabilize gaze during own body movements. Six types of eye
movements have been evolved fulfilling this function: saccades,
smooth pursuit, vestibulo-ocular reflex, optokinetic response,
convergence and gaze holding. In all vertebrates the eyes are moved
by six pairs of extraocular muscles that enable horizontal,
vertical and rotatory eye movements. The motoneurons of these
muscles are located in the oculomotor (nIII), trochlear (nIV) and
abducens (nVI) nucleus in the brainstem. Motoneurons of the lateral
rectus muscle (LR) in nVI and of the medial rectus muscle (MR) in
nIII provide horizontal eye movements, those of inferior oblique
(IO) and superior rectus muscle (SR) in nIII upward eye movements.
Motoneurons of the superior oblique (SO) and the inferior rectus
muscle (IR) in nIII convey downward eye movements. Recently, it was
shown that each extraocular muscle is controlled by two
motoneuronal groups: 1. Motoneurons of singly innervated muscle
fibers (SIF) that lie within the boundaries of motonuclei providing
a fast muscle contraction (twitch) and 2. motoneurons of multiply
innervated muscle fibers (MIF) in the periphery of motonuclei
providing a tonic muscle contraction (non-twitch). Tract-tracing
studies indicate that both motoneuronal groups receive premotor
inputs from different brainstem areas. A current hypothesis
suggests that pathways controlling twitch motoneurons serve to
generate eye movements, whereas the non-twitch system is involved
in gaze holding. Lesions of inputs to the twitch motoneuron system
may lead to supranuclear gaze palsies, whereas impairment of the
non-twitch motoneuron system may result in gaze holding deficits,
like nystagmus, or strabismus. Up to date only limited data are
available about the histochemical characteristics including
transmitters to the SIF- (twitch) and MIF (non-twitch) motoneurons.
The present study was undertaken to investigate the histochemical
profile of inputs to motoneuronal groups of individual eye muscles
mediating horizontal and vertical eye movements including the
inputs to MIF- and SIF motoneurons. The MIF motoneurons of the IR
and MR are located in the periphery dorsolateral to nIII, close to
the Edinger-Westphal nucleus (EW), which is known to contain
preganglionic cholinergic neurons. Other scientists have found that
the EW is composed of urocortin-positive neurons involved in food
intake or stress. In order to delineate these different cell
populations within the supraoculomotor area dorsal to nIII, a
comparative study in different mammals was conducted to locate the
cholinergic preganglionic neurons and urocortin-positive neurons.
Only then, it became obvious that the cytoarchitecturally defined
EW labels different cell populations in different species. In rat,
ferret and human the cytoarchitecturally defined EW is composed of
urocortin-positive neurons. Only in monkey the EW contains
cholinergic preganglionic neurons, which lie close to the
MIF-motoneurons of MR and IR in the C-group. In monkey, I performed
a systematic study on the histochemical profile and transmitter
inputs to the different motoneuron subgroups, including MIF- and
SIF motoneurons. Brainstem sections containing prelabelled
motoneurons were immunostained for the calcium-binding protein
calretinin (CR), gamma-aminobutyric acid (GABA) or glutamate
decarboxylase (GAD), glycine transporter 2, glycine receptor 1, and
the vesicular glutamate transporters (vGlut) 1 and 2. The study on
the histochemical profile of the motoneuron inputs revealed three
main results: 1.The inhibitory control of SIF motoneurons for
horizontal and vertical eye movements differs. Unlike previous
studies in the primate a considerable GABAergic input was found to
all SIF motoneuronal groups, but a glycinergic input was confined
to motoneurons of the MR mediating horizontal eye movements. 2. The
excitatory inputs to motoneurons for upgaze and downgaze differ in
their histochemistry. A striking finding was that CR-positive nerve
endings were confined to the motoneurons of muscles involved in
upgaze, e.g. SR, IO and the levator palpebrae, which elevates the
upper eyelid and acts in synchrony with the SR. Since
double-immunoflourescence labelling with anti-GAD did not reveal
any colocalization of GAD and CR, the CR-input to upgaze
motoneurons is considered as excitatory. 3. The histochemistry of
MIF- and SIF motoneurons differs only for vGlut1. Whereas SIF- and
MIF motoneurons of individual eye muscles do not differ in their
GABAergic, glycinergic and vGlut2 input, vGlut1 containing
terminals were covering the supraoculomotor area and targeting only
MR MIF motoneurons. It is reasonable to assume that the vGlut1
input affects the near response system in the supraoculomotor area,
which houses the preganglionic neurons in the EW mediating
pupillary constriction and accommodation and the MR MIF
motoneurones involved in vergence. The histochemical data in monkey
enabled the localization of the corresponding motoneuronal
subgroups of individual eye muscles in human with the development
of an updated nIII map. Taken together the present work provides
new data on the histochemical properties of premotor inputs to
motoneuronal groups of the twitch- and non-twitch eye muscle
systems in primates. Especially the selective association of CR in
premotor upgaze pathways may open the possibility for a targeted
research of this system in human post-mortem studies of clinical
cases with impairment of upward eye movements, such as progressive
supranuclear palsy (PSP) or Niemann-Pick disease (NPC).