morphology and soma-dendritic distribution of synaptic endings from the rostral interstitial nucleus...

THE JOURNAL OF COMPARATIVE NEUROLOGY 366~149-162 ( 1996)

Morphology and Soma-Dendritic Distribution of Synaptic Endings From the Rostra1 Interstitial Nucleus of the Medial

Longitudinal Fasciculus (riMLF) on Motoneurons in the Oculomotor and

Trochlear Nuclei in the Cat

SHWU-FEN WANG AND ROBERT F. SPENCER Departments o f h a t o m y (S.-F.W., R.F.S.) and Otolaryngoloa-Head and Neck Surgery (R.F.S.),

Medical College of Virginia, Virginia Commonwealth University, Richmond, Virginia 23298

ABSTRACT The morphology and soma-dendritic distribution of anterograde biocytin-labelled rostra1

interstitial nucleus of the medial longitudinal fasciculus (riMLF) synaptic endings in the oculomotor and trochlear nuclei have been examined by electron microscopy by using both preembedding immunoperoxidase and postembedding immunogold methods. The results indicate that three morphological types of riMLF synaptic endings are distinguishable on the basis of synaptic vesicle morphology (spheroidal, pleiomorphic, or ellipsoidal) and postsynaptic membrane specializations (asymmetrical or symmetrical j. All three morphological types of riMLF synaptic endings establish synaptic connections predominantly with dendrites. Synaptic endings that contain ellipsoidal synaptic vesicles have a more proximal soma-dendritic distribution than those that contain either spheroidal or pleiomorphic synaptic vesicles. Furthermore, all three morphological types of synaptic endings are encountered in the same motoneuron subdivisions of the oculomotor and trochlear nuclei in the same experiments. The findings suggest that subregions of the riMLF contain coexistent populations of excitatory and inhibitory premotor neurons that are related to opposite directions of vertical saccadic eye movements but that project to the same motoneuron subgroups on the ipsilateral side. Both the morphology and the mode, pattern, and soma-dendritic distribution of saccade-related riMLF synaptic endings that establish synaptic connections with vertical motoneurons differ from those of excitatory and inhibitory second-order vertical vestibular synaptic endings. These differences in the synaptic organization of riMLF and second-order vestibular inputs to oculomotor and trochlear motoneurons may be related to differences in the information transferred by each source, the riMLF input conveying eye-velocity signals, and the vestibular input conveying eye-position signals.

Indexing terms: mesencephalic reticular formation, excitatory synapses, inhibitory synapses, biocytin,

, IUSC; WiIey-Iiss. Inc.

electron microscopy

Motoneurons in the extraocular motor nuclei are the final common pathway upon which afferents converge from brainstem premotor areas that are related intimately to the control of different types of eye movements (e.g., vestibuloocular reflex, optokinetic nystagmus, smooth pursuit, saccadic, and vergence). For vertical eye movements, the major inputs to motoneurons in the oculomotor (inferior rectus, superior rectus, and inferior oblique) and trochlear (superior oblique) nuclei are derived from the vestibular nuclei for vertical vestibuloocular eye movements (for review,

see Spencer et al., 1992), the interstitial nucleus of Cajal (INC; Carpenter et al., 1970; Graybiel and Hartwieg, 1974; Buttner-Ennever and Biittner, 1978; Steiger and Buttner-

Accepted October 10, 1996. Dr. Shwu-Fen Wang is currently at the School of Physical Therapy.

College of Medicine, National Taiwan University, Taipei, Taiwan R.O.C. Address reprint requests to Dr. Robert F. Spencer, Department of

Anatomy, Medical College of Virginia, Virginia Commonwealth University, 1101 East Marshall Street. Richmond, VA 23298-0709.

( 1996 WILEY-LISS, INC.

150 S.-F. WANG AND R.F. SPENCER

Ennever, 1979; Labandeira-Garcia et al., 1989; Carpenter et al., 1992; Li et al., 1993; Robinson et al., 1994), and the rostra1 interstitial nucleus of the medial longitudinal fasciculus (riMLF; Graybiel and Hartwieg, 1974; Graybiel, 1977; Biittner-Ennever and Biittner, 1978; Steiger and Biittner- Ennever, 1979; Nakao and Shiraishi, 1983, 1985; Isa et al., 1988; Labandeira-Garcia et al., 1989; Moschovakis et al., 1990, 1991a,b; Nakao et al., 1990; Carpenter et al., 1992; Isa and Itouji, 1992; Li et al., 1993; Moschovakis and Highstein, 1994; Robinson et al., 1994; Shiraishi et al., 1994). The INC and riMLF in the mesencephalic reticular formation are related specifically to the control of vertical saccadic eye movements (Biittner-Ennever et al., 1982; Fukushima, 1987, 1991; Buttner-Ennever and Biittner, 1988; Hepp et al., 1989; Fukushima and Fukushima, 1992; Isa and Naito, 1994; Moschovakis and Highstein, 1994).

Reciprocal excitatory and inhibitory synaptic connections of second-order vestibular neurons with motoneurons in the oculomotor and trochlear nuclei provide the physiological basis for the vertical vestibuloocular reflex (Highstein and Ito, 1971; Precht and Baker, 1972; Berthoz et al., 1973; Highstein, 1973; Uchino et al., 1978; Iwamoto et al., 1990). The excitatory second-order vestibular input terminates predominantly on the distal dendrites of oculomotor and trochlear motoneurons (Dememes and Raymond, 1980; Spencer and Baker, 1983). By contrast, second-order inhibitory vestibular neurons establish synaptic connections predominantly with the somata and proximal dendrites of motoneurons in the oculomotor and trochlear nuclei (Bak et al., 1976; Dememes and Raymond, 1980; Spencer and Baker, 1983). Although both excitatory and inhibitory synaptic effects have been observed in oculomotor and trochlear motoneurons following electrical stimulation of the region of the riMLF (Schwindt et al., 1974; Nakao and Shiraishi, 1983, 1985), neither the morphology nor the mode (i.e., single vs. multiple synaptic contact zones), pattern (i.e,, single vs. multiple postsynaptic targets), or soma-dendritic distribution of these inputs have been examined. Thus, it is unknown how the saccade-related inputs interact with the vestibular inputs and to what extent these inputs correspond to the known types of synaptic endings in the oculomotor (Tredici et al., 1976; Waxman and Pappas, 1979) and trochlear (Bak and Choi, 1974) nuclei.

In the present study, the morphology and synaptic organization of riMLF synaptic endings in the oculomotor and trochlear nuclei labelled by anterograde transport of biocytin have been examined by electron microscopy by using preembedding peroxidase and postembedding colloidal-gold localization procedures. Quantitative analyses have been used to determine the soma-dendritic distribution of presumed excitatory and inhibitory riMLF synaptic endings on oculomotor and trochlear motoneurons.

MATERIALS AND METHODS The data obtained in this study were derived from

experiments performed in a previous study (Wang and Spencer, 1995) in which biocytin was injected stereotax- cally into different regions of the riMLF to anterogradely label synaptic endings in the oculomotor and trochlear nuclei. Two groups of sections from those experiments, each representing 150 IJ-m intervals through the oculomotor and trochlear nuclei, were processed for the ultrastructural localization of biocytin by using preembedding or postembedding procedures, as described below.

Preembedding localization of biocytin One group of sections was incubated for 2 hours in avidin

D-horseradish peroxidase (HRP; Vector; 1500) in 0.1 M sodium phosphate buffer containing 0.1% Triton X-100. For the histochemical demonstration of HRP, sections were treated in 0.05% 3,3’-diaminobenzidine tetrahydrochloride (DAB; Aldrich) and 0.01% hydrogen peroxide with 0.005% cobalt acetate and 0.005% nickel chloride in 0.1 M sodium phosphate buffer, pH 7.2, for 5-10 minutes. After washing through several changes of buffer, sections were processed for electron microscopy. Sections were postfixed in 1.0% osmium tetroxide in 0.1 M phosphate buffer with 7% dextrose for 1 hour at 4°C and were stained en bloc with 2% uranyl acetate in 0.05 M maleate buffer for 1 hour at 4°C. Then, sections were dehydrated in graded methanols and propylene oxide, infiltrated with plastic resin (Fullam), and flat embedded between two glass microscope slides coated with a water-soluble release agent (Electron Microscopy Sciences). After curing, the embedded sections were at- tached to prelabelled BEEM capsules, removed from the slides, and trimmed to include regions of the oculomotor and trochlear nuclei that were determined in the previous light microscopic study to contain anterogradely labelled terminals. Semithin (0.2 IJ-m) sections were cut with glass knives on an LKB Ultratome IV ultramicrotome and were stained with 0.1% toluidine blue in 1.0% sodium borate for light microscopy as a reference for the electron microscopic analysis. Ultrathin (60-80 nm) sections were cut with a diamond knife on the ultramicrotome, collected on Formvar- coated, single-slot copper grids, and stained with 1.0% uranyl acetate in distilled water and 0.1% lead citrate in 0.1 N sodium hydroxide. Sections were examined and photographed using a Zeiss EM-10CA electron microscope.

Postembedding localization of biocytin Another group of untreated sections was processed for

electron microscopy, as described above. Serial ultrathin (60-90 nm) sections were cut on the ultramicrotome and collected on Formvar-coated, single-slot, gold grids. The sections were pretreated for 6-7 minutes in 1.0% sodium metaperiodate (Fluka) in distilled water, washed through three changes of distilled water, and then immersed in two changes of 0.05 M Tris-buffered saline (TBS) containing 0.1% bovine serum albumin (BSA) for 15 minutes each. The sections were placed on a drop of 15 nm gold-tagged streptavidin (Jansen) diluted 1:20 with 0.05 M TBS containing 0.1% BSA, pH 8.2, in a ceramic dish for 2 hours with constant agitation a t room temperature. The grids were washed in two changes of 0.05 M TBS followed by three changes of distilled water. All solutions used in the process- ing were filtered through a 0.2 IJ-m Acrodisc (Gelman) syringe filter before use. Sections were counterstained with uranyl acetate and lead citrate and then were examined and photographed with the electron microscope.

Quantitative analysis The cross-sectional areas of biocytin-labelled synaptic

endings localized by both the preembedding and postembedding procedures and the diameters of the postsynaptic profiles were measured from electron micrographs at a magnification of ~ 2 4 , 0 0 0 using a BIOQUANT (R and M Biometrics) digital image analysis system operating on a Hewlett-Packard Vectra microcomputer with standard mor- phometric, spreadsheet, and statistical software. Colloidal-

riMLF SYNAPTIC CONNECTIONS WITH OCULOMOTOR AND TROCHLEAR MOTONEUKONS 151

gold particles overlying synaptic endings labelled by the postembedding method were counted, and their density was calculated as the number of particles per ym2. Diam- eter measurements of the proximal dendrites of choline acetyltransferase (ChAT)-immunoreactive trochlear motoneurons were made directly from the light microscope at a magnification of x 1,350 by using images captured from a DageiMTI Newvicon NC-70 video camera with a Truevi- sion Targa M8 frame-grabber board interfaced to the microcomputer and image-analysis software.

RESULTS Morphology of biocytin-labelled riMLF

synaptic endings With the preembedding method, biocytin-labelled riMLF

synaptic endings were recognized easily by dense peroxidase reaction product that permitted their unequivocal identification compared to unlabelled synaptic endings. Labelled synaptic endings varied considerably in profile in apparent direct relation to their soma-dendritic distribution. Most of the labelled synaptic endings associated with somata (Fig. 1A-C) and proximal dendrites (Fig. 1D) were dome shaped and tended to form shallow depressions in the surface membrane of the postsynaptic profile. At these proximal sites, labelled synaptic endings occurred either singly (Fig. 1A) or infrequently in a series reminiscent of en passant boutons that arise from a single axonal arboriza- tion (Fig. 1B). Labelled synaptic endings also were observed occasionally in association with somatic spine-like appendages (Fig. lC) , particularly in the superior rectus subdivision of the oculomotor nucleus and in the trochlear nucleus. In all cases, riMLF synaptic endings on the motoneuron somata and proximal dendrites, which were identified on the basis of elaborate arrays of granular endoplasmic reticulum, were associated with only one postsynaptic profile.

The overwhelming majority of riMLF labelled synaptic endings were encountered in the neuropil of the oculomotor and trochlear nuclei. Synaptic endings almost invariably were dome shaped when they were associated with only a single postsynaptic dendritic profile (Fig. 2A,C,E). Most of the labelled synaptic endings, however, were associated with two or more small- and/or medium-caliber dendrites (Fig. 2A-D) and, consequently, had irregular profiles. Irre- spective of their soma-dendritic location, riMLF synaptic endings typically contained numerous small mitochondria clustered near the center of each terminal. Labelled myelinated axons also were observed in the neuropil, usually in close proximity to the labelled synaptic endings (Figs. lD, 2E).

The unobstructed view of the ultrastructural features of the synaptic endings labelled by the postembedding colloidal- gold procedure permitted the classification of riMLF synaptic endings into three populations on the basis of synaptic vesicle morphology. All three populations of anterograde biocytin-labelled riMLF synaptic endings coexisted within the same motoneuron subdivisions that were targeted by injections in different regions of the riMLF. One population of riMLF synaptic endings contained large spheroidal synaptic vesicles and formed asymmetrical synaptic contacts (Fig. 3A-D). Synaptic contact zones were characterized by a prominent postsynaptic densification with occasional sub- junctional dense bodies (Fig. 3A, inset). In most instances, only a single synaptic contact was observed with each

postsynaptic dendritic profile (Fig. SA,B,D). Occasionally, multiple synaptic contact zones were associated with axosomatic synaptic endings of this variety (Fig. 3C).

A second population of riMLF synaptic endings contained predominantly flattened or ellipsoidal synaptic vesicles and established symmetrical synaptic contacts (Fig. 4A-D). Synaptic contact zones were characterized by a modest or inconspicuous postsynaptic densification (Fig. 4D, inset). Synaptic endings of this variety most often established synaptic contacts with dendrites (Fig. 4B-E) and, to a lesser extent, with somata (Fig. 4A) and spine-like profiles (Fig. 4A,B).

The third population of riMLF synaptic endings contained pleiomorphic synaptic vesicles and established synaptic contacts with a modest postsynaptic thickening along the postsynaptic membrane (Fig. 5A, inset). Whereas most of the synaptic vesicles were spheroidal, these were smaller than the uniformly spheroidal synaptic vesicles in the synaptic endings that established obvious asymmetrical contacts. Synaptic endings in this category established synaptic contacts predominantly with one (Fig. 5B,C) or more (Fig. 5A,D) dendrites.

Size and labelling density of biocytin-labelled riMLF synaptic endings

The riMLF synaptic endings in the oculomotor and trochlear nuclei, as a population, were relatively heteroge- neous in size. The cross-sectional areas of 168 riMLF synaptic endings that were labelled by using the preembedding procedure ranged from 0.30 to 6.11 Fm2 (Fig. 61, with a mean of 1.67 i- 1.19 Fm'. Without regard to differences in synaptic vesicle morphology, as a group, the cross-sectional areas of 384 riMLF synaptic endings that were labelled by using the postembedding procedure ranged from 0.03 to 9.46 Fm2 (Fig. 6), with a mean of 2.47 2 1.46 Fm2. Although the synaptic endings that were labelled by the preembedding procedure were significantly smaller than those labelled with the postembedding procedure ( P < 0.01; t test), the difference between the two samples was attributable most likely to sampling bias toward larger terminals. Smaller terminals labelled with the small colloidal-gold particles were more difficult to find in the electron microscope.

No significant differences were observed in the gold- particle density associated with the three categories of riMLF synaptic endings labelled with the postembedding procedure. The particle density of synaptic endings that contained spheroidal synaptic vesicles ranged from 4.44 to 8.86 particles/pm2, with a mean of 6.04 * 1.02 particles/ km2. The density of gold particles overlying synaptic endings that contained flattened or ellipsoidal synaptic vesicles ranged from 4.86 to 8.04 particlesipm2, with a mean of 5.96 ? 1.32 partic1esiIJ-m'. Synaptic endings that contained pleiomorphic synaptic vesicles exhibited a gold-particle density that ranged from 4.23 to 8.33 particles/pm2, with a mean of 6.46 * 1.22 particles/km2. For comparison, back- ground levels measured from unlabelled synaptic endings in neighboring regions of the oculomotor nucleus (e.g., medial rectus subdivision) ranged from 0.22 to 1.21 particles/pm2, with a mean of 0.41 * 0.31 particles/Fm2. The quantitative data, therefore, indicate that biocytin labelled the different populations of riMLF synaptic endings in an approximately equivalent manner without regard to their presumed physiological action or neurotransmitter content.

Fig. 1. A-D: Electron micrographs of biocytin-labelled (preembedding) synaptic endings from the rostra1 interstitial nucleus of the medial longitudinal fasciculus (riMLF) associated with somata (A-C) and a proximal dendrite (d; D ) in the oculomotor and trochlear nuclei. Although relatively few in number, labelled axosomatic synaptic end-

ings from the riMLF are observed either in isolation (A) or as a series (B). Less frequently, synaptic connections are established with somatic spine-like (s) appendages (C). Note the labelled myelinated axons in D. Asterisks indicate unlabelled synaptic endings. Scale bars = 1 km.

riMLF SYNAPTIC CONNECTIONS WITH OCULOMOTOR AND TROCHLEAR MOTONEURONS

Fig. 2. A-E: Electron micrographs of biocytin-labelled (preembedding) synaptic endings from the riMLF contacting medium-and small-diameter dendrites (d) in the oculomotor and trochlear nuclei. Individual synaptic endings in A-D are associated with two or more postsynaptic profiles. Note the labelled myelinated axon ( a ) in E. Scale bars = 1 ym.

153


Fig. 3 . A-D: Electron micrographs of presumed excitatory biocytin- labelled riMLF synaptic endings containing spheroidal synaptic vesicles and forming asymmetric synaptic contact profiles (large arrows) in the oculomotor and trochlear nuclei. Biocytin is localized by the postembedding procedure using 15 nm colloidal-gold particles (small arrows).

Labelled synaptic endings often contact more than one medium- diameter and/or small-diameter dendrite (A,B,D). Labelled axosomatic synaptic endings with similar synaptic vesicle morphology (C) were encountered only rarely. d, Dendrite; psd, postsynaptic densification. Scale bars = 0.5 pm in A-D; 0.25 pm in inset.

Soma-dendritic distribution of biocvtin-labelled riMLF svnaDtic endinm

For comparison, the diameters of 252 proximal dendrites oftrochlear motoneurons that were labelled by the immuno-

Y I Y

Most biocytin-labelled riMLF synaptic endings were encountered in the neuropil, and comparatively fewer labelled synaptic endings were associated with the somata of oculomotor and trochlear motoneurons. A quantitative analysis of the postsynaptic diameters of profiles that were in synaptic contact with labelled synaptic endings was under- taken to determine the soma-dendritic distribution of riMLF synaptic endings. With the preembedding procedure, the diameters of postsynaptic profiles ranged from 0.17 to 11.79 pm (Fig. 7), with a mean of 3.22 2 2.63 pm. With the postembedding procedure, postsynaptic profiles ranged from 0.20 to 14.51 pm in diameter (Fig. 71, with a mean of 2.54 2 2.36 pm.

histochemical localization of ChAT (McHaffie et al., 1991) were measured by light microscopy. The diameters of proximal dendrites ranged from 2.42 to 9.59 pm (Fig. 7), with a mean of 5.36 2 1.48 pm. The 95% confidence interval (CI) ranged from 2.06 to 8.66 pm. Using the CI of ChAT-immunoreactive proximal dendrites as a reference, postsynaptic profiles were characterized as being somata ( > 8.66 pm), proximal dendrites (2.06-8.66 pm), and distal dendrites ( < 2.06 pm), including spines. These results indicated that the majority (92.86%) of the synaptic endings labelled with the preembedding procedure contacted dendrites, although very few (7.14%) contacted somata (Figs. 7, 8) . Axodendritic synaptic endings established synaptic con-

Fig. 4. A-E: Electron micrographs of presumed inhibitory biocytin- labelled riMLF synaptic endings containing ellipsoidal synaptic vesicles and forming symmetrical synaptic contact profiles (large arrows) in the cat oculomotor and trochlear nucleus. Biocytin is localized by the postembedding procedure using 15 nm colloidal-gold particles (small

arrows). Labelled synaptic endings are associated occasionally with somata or spine-like appendages (A), but, more frequently, they contact medium- and small-diameter dendrites id; B-E). d, Dendrite; s, spine- like appendage; psd, postsynaptic densification. Asterisks indicate unlabelled synaptic endings. Scale bars = 0.5 pm, 0.25 pm in inset.


Fig. 5. A-D: Electron micrographs of biocytin-labelled riMLF synaptic endings containing pleiomorphic synaptic vesicles and forming synaptic contacts (large arrows) in the oculomotor and trochlear nuclei. Biocytin is localized by the postembedding procedure using 15 nm colloidal-gold particles (small arrows). Labelled synaptic endings in A, B, and D contact more than one medium- and small-diameter dendrite.

tacts with a greater weighting toward proximal (52.98%) than distal (39.88%) dendrites. However, the three populations of synaptic endings labelled with the postembedding procedure overall made synaptic contacts with a greater weighting toward distal (62.83%) than proximal (32.74%) dendrites (Figs. 7, 8).

Because the diameters measured from single ultrathin sections were dependent on the plane of the section through the process, postsynaptic profiles were characterized subjec- tively on the basis of their ultrastructural features. For example, a distinction was made between proximal and distal dendrites by the content of granular endoplasmic reticulum, Golgi apparatus, and polyribosomes. Proximal dendrites were distinguished from somata by their content of parallel arrays of microtubules. Somatic or dendritic

The ultrastructural features (e.g., size and morphology of synaptic vesicles, postsynaptic densification) vary but are distinct from those that characterize typical excitatory and inhibitory synaptic endings (see Figs. 2 , 4 ) . d, Dendrite; psd, postsynaptic densification. Scale bars = 0.5 pm, 0.25 pm in inset.

spines were characterized by their sac-like protrusions from a soma or a dendrite, respectively (Figs. lC, 4A,B). Classify- ing the postsynaptic profiles by their ultrastructural features, the soma-dendritic distributions were determined in the trochlear nucleus for the three categories of riMLF synaptic endings characterized on the basis of synaptic vesicle morphology (Fig. 9). Using these criteria, the majority (53.10%) of the labelled riMLF synaptic endings contacted distal dendrites. A smaller proportion of the synaptic endings contacted proximal dendrites (27.43% ), somata (7.96% ), and spine-like appendages (11.50%). The majority of labelled synaptic endings that contained spheroidal (61.11%,) and pleiomorphic (72.41%) synaptic vesicles contacted distal dendrites. Very few of these two types of synaptic endings contacted proximal dendrites ( 16.67%

riMLF SYNAPTIC CONNECTIONS WITH OCULOMOTOR AND TROCHLEAR MOTONEURONS 157

0 0 0 0 0 0 0 0 0 0 0 0 0 0 g ~ ~ y ~ x ~ , , , " " " ~ ~ & ~ A

Cross-Sectional Area (pm2)

Fig. 6. Distribution of cross-sectional areas of biocytin-labelled riMLF synaptic endings in the trochlear nucleus localized by pre- and postembedding procedures. The mean size of 168 riMLF synaptic endings labelled with the preembedding procedure is 1.67 t 1.19 pm',

and the median is 1.28 pm2. The mean size of 384 riMLF synaptic endings labelled with the postembedding procedure is 2.47 ? 1.46 pm2, and the median is 2.19 km2.

Post-embedding

ChAT

9 ~ 9 ~ 9 ~ 9 ~ 9 " 9 ~ 9 ~ 9 ~ 9 ~ 9 ~ 9

Postsynaptic Diameter (vm)

o o - - ~ ~ m o ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ o A 7

Fig. 7. Diameter distribution of postsynaptic profiles contacted by that are smaller in diameter than the proximal dendrites of the riMLF synaptic endings labelled with the pre- and postembedding motoneurons. Synaptic endings labelled by the preembedding proce- procedures in the trochlear nucleus compared to the diameter distribu- dure have a wider postsynaptic distribution than those labelled by the tion of proximal dendrites of choline acetyltransferase (ChATI- postembedding procedure. Very few of the labelled synaptic endings immunoreactive trochlear motoneurons. Note that the overall distribu- make synaptic contact on the soma. tion ofriMLF synaptic endings is weighted toward postsynaptic profiles

spheroidal, 17.24% pleiomorphic). The synaptic endings that contained ellipsoidal synaptic vesicles were weighted more toward proximal dendrites (41.67%) than toward distal dendrites (35.42% ). By contrast, significantly fewer

synaptic endings of each type were located on the somata (1 1.11% spheroidal, 3.45% pleiomorphic, 8.33% ellipsoidal) of trochlearmotoneuronsoron somaticordendriticspines (11.11% spheroidal, 6.90% pleiomorphic, 14.58% ellipsoidal).


asymmetrical, with a prominent postsynaptic densification. A second population of riMLF synaptic endings contains predominantly ellipsoidal synaptic vesicles and exhibits a symmetrical pre-/postsynaptic membrane profile. The third category of riMLF synaptic endings has ultrastructural features that are intermediate between those of the spheroidal and those of the ellipsoidal types. These synaptic endings contain pleiomorphic synaptic vesicles and are associated with a modest postsynaptic specialization. All three morphological types of riMLF synaptic endings establish synaptic connections predominantly with dendrites. Synaptic endings containing ellipsoidal synaptic vesicles have a more proximal soma-dendritic distribution than those that contain either spheroidal or pleiomorphic synaptic vesicles. Furthermore, all three morphological types of synaptic endings were encountered in the same motoneuron subdivisions of the oculomotor and trochlear nuclei in the same experiments.

To characterize the soma-dendritic distribution of riMLF synaptic endings, two methods have been used to classify proximal and distal dendrites. One method is based on the diameter of the postsynaptic dendrites with reference to the CI of the diameter distributions of proximal dendrites of ChAT-immunoreactive trochlear motoneurons. The other method is based on the ultrastructural features of the postsynaptic dendrites, including the content of granular endoplasmic reticulum, Golgi apparatus, and ribosomes. For the sample population in this study, both methods produced very similar results. In general, therefore, at least in the present situation, the diameters of the postsynaptic dendrites can serve as a useful indicator of the proximodistal location of synaptic endings on the soma-dendritic surfaces.

Two of the morphological types of riMLF synaptic endings in the trochlear nucleus are similar in ultrastructural features to unlabelled synaptic endings that were identified previously (Bak and Choi, 1974; Tredici et al., 1976; Waxman and Pappas, 1979). The riMLF synaptic endings that contain ellipsoidal synaptic vesicles appear to correspond to types I and I1 (Bak and Choi, 1974), which are characteristic of inhibitory synapses (Uchizono, 1965; Lar- ramendi et al., 1967). By contrast, riMLF synaptic endings that contain spheroidal synaptic vesicles are similar to type I11 synapses in the trochlear nucleus (Bak and Choi, 1974), which are considered to be excitatory in nature (Uchizono, 1965; Larramendi et al., 1967). The third population of riMLF synaptic endings contains pleiomorphic synaptic vesicles and does not make typical symmetrical or asymmetrical synaptic contacts. The nature of this population of synaptic ending is unknown.

Synaptic endings of types 1-111 have been identified previously as originating from the vestibular nuclei (Bak et al., 1976; Demcmes and Raymond, 1980). However, the ipsilateral second-order, inhibitory, vestibular input to oculomotor and trochlear motoneurons is distributed predominantly on the somata and the proximal dendrites (Bak et al., 1976; Demcmes and Raymond, 1980; Spencer and Baker, 1983), whereas presumed inhibitory synaptic endings from the ipsilateral riMLF have a more widespread distribution on proximal and distal dendrites. Further- more, the mode (i.e., single vs. multiple synaptic contact zones) and pattern (i.e., single vs. multiple postsynaptic targets) of synaptic termination of riMLF inhibitory synaptic endings differ from those previously demonstrated for the second-order inhibitory vestibular synapses. Each inhibi-

95 % Confidence In te rva I Distribution

A <2.06 2.06-8.66 ~ 3 . 6 6

Postsynaptic Diameter (pm)

~ p ~ . ~ ~ - .

I Pre-embedding i i ~

~ Post-embedding lp-pppp- 1

Ultrastructural Distribution

1 6o ~~ p ~ - ~ - ~ ~- ~~ ~p~

B Distal Proximal Soma Spine

Soma-Dendritic Distribution

Fig. 8. Comparison of the soma-dendritic distributions of riMLF synaptic endings labelled by the preembedding and postembedding methods with reference to the 95% confidence interval of postsynaptic diameters of ChAT-immunoreactive proximal dendrites of trochlear motoneurons ( A ) and the identification of postsynaptic profiles based on ultrastructural criteria (B) .

DISCUSSION The findings in this study indicate that three morphologi-

cal types of anterograde, biocytin-labelled, riMLF synaptic endings, which are distinguishable on the basis of synaptic vesicle morphology and synaptic specializations, establish synaptic connections with motoneurons in the cat oculomotor and trochlear nuclei. One type of riMLF synaptic ending contains a uniform population of large, spheroidal, synaptic vesicles, and the synaptic contact zones are distinctly

riMLF SYNAPTIC CONNECTIONS WITH OCULOMOTOR AND TROCHLEAR MOTONEURONS 159

Distal 1

Proximal

1 0 Pleiomorphic I I 1 ‘1 Spheroidal ,

I r’ . .- -

Soma Spine

Soma-Dendritic Distribution

Fig. 9. Soma-dendritic distribution of different categories of riMLF synaptic endings characterized by synaptic vesicle morphology Le., ellipsoidal, pleiomorphic, and spheroidal) in the trochlear nucleus labelled with the postembedding procedure. The postsynaptic profiles are classified into distal and proximal dendrites, soma, and spine according to their ultrastructural features.

tory riMLF synaptic ending exhibits only a single synaptic contact zone with each postsynaptic profile, but synaptic connections usually are established simultaneously with multiple postsynaptic dendrites. By contrast, each inhibitory vestibular synaptic ending exhibits multiple, spatially separated synaptic contact zones with only one postsynaptic profile (Spencer and Baker, 1983). Consistent with the notion of multiple types of inhibitory synaptic endings, recent findings have demonstrated at least two types of synaptic endings in the cat oculomotor and trochlear nuclei that are immunoreactive toward glutamate decarboxylase, the synthesizing enzyme of the putative inhibitory neurotransmitter GABA, and that differ on the basis of the mode, pattern, and soma-dendritic distribution of their synaptic connections (Spencer et al., 1992). Furthermore, these findings are compatible with the notion that synaptic endings with distinctly different ultrastructural and/or synaptic features originate from different sources (Rose and Neuber-Hess, 1991).

The type I11 contralateral, excitatory, second-order, vestibular synaptic endings on oculomotor (DemGmes and Raymond, 1980; Spencer and Baker, 1983) and trochlear (Bak et al., 1976; Spencer and Baker, 1983) motoneurons are distributed predominantly on dendrites, although the overall soma-dendritic weighting ke . , proximal vs. distal dendrites) of this input has not been determined. Although the excitatory riMLF synaptic endings also are distributed on the dendrites of oculomotor and trochlear motoneurons, this input terminates preferentially on small- and medium- caliber distal dendrites. Based on evidence for the spatial segregation of inputs on the soma-dendritic surface of spinal motoneurons (Rose and Neuber-Hess, 1991), the proximodistal distribution of vestibular and riMLF inputs to oculomotor and trochlear motoneurons might be ex- pected to differ as well. Such a segregation of inputs on different portions of the soma-dendritic extent of a neuron,

however, does not preclude the possibility of interactions between the inputs that might influence specific individual or combined synaptic effects (White et al., 1990; Tomasulo et al., 1993).

The soma-dendritic distribution of riMLF synaptic endings is likely to have a significant role in influencing the postsynaptic physiological responses of oculomotor and trochlear motoneurons. Given the relatively constant elec- trotonic location of the riMLF excitatory input onto oculomotor and trochlear motoneurons, the shape of the EPSPs produced by activation of the vertical saccadic premotor neurons should be similar for all motoneurons that receive excitatory synaptic inputs from the riMLF (Liischer and Clamann, 1992). The time course of the excitatory postsynaptic potentials (EPSPs) also should be similar for all motoneurons, because the spatial and temporal dispersion of the input appears to be minimal (Walmsley and Stuklis, 1989). Other factors, such as the size-related rheobase and input resistance of the postsynaptic motoneurons, might influence the modulation of EPSP amplitude by high- frequency stimulation (Koerber and Mendell, 1991) and, presumably, play a role in the recruitment of the motoneurons during eye movements.

Given the divergence of individual riMLF synaptic inputs to oculomotor and trochlear motoneurons, as indicated by single synaptic endings that establish simultaneous synaptic connections with multiple postsynaptic profiles, the presynaptic control of motoneuron recruitment during vertical saccadic eye movements is likely to be less impor- tant than that proposed for second-order excitatory vestibular inputs that function in the vertical vestibuloocular reflex (Spencer and Baker, 1983). One intriguing possibility is that the differences in the synaptic organization of riMLF and second-order vestibular inputs to oculomotor and trochlear motoneurons may be related to differences in the information transferred by each source, the riMLF input


conveying eye-velocity signals (Buttner et al., 1977; King and Fuchs, 1979; Vilis et al., 1989; Moschovakis et al., 1991a,b) and the vestibular input conveying eye-position signals (McCrea et al., 1980; 1987a,b; Ohgaki et al., 1988; Iwamoto et al., 1990; Scudder and Fuchs, 1992). Further- more, these morphological and physiological differences between the riMLF and vestibular inputs to vertical motoneurons translate to distinctive clinical deficits in vertical gaze following lesions in the midbrain a t the thalamomesen- cephalic junction (Christoff, 1974; Cogan, 1974; Halmagyi et al., 1978; Jacobs et al., 1978; Kompf et al., 1979; Trojanowski and LaFontaine, 1981; Buttner-Ennever et al., 1982; Pierrot-Deseilligny et al., 1982; Moffie et al., 1983; Bogousslavsky and Regli, 1984; Ranalli et al., 1988; Deleu et al., 1989; Bogousslavsky et al., 1990; Thomke and Hopf, 1992; Green et al., 1993) vs. lesions in the medulla (Meien- berget al., 1978).

The synaptic organization of riMLF terminals in the oculomotor and trochlear nuclei that are related to vertical saccadic eye movements also differs substantially from the organization of pontomedullary reticular inputs to abducens neurons that are related to horizontal saccadic eye movements. In the present study, riMLF synaptic endings establish synaptic contacts predominantly with dendrites (particularly distal dendrites). By contrast, synaptic endings from excitatory and inhibitory burst neurons in the pontomedullary reticular formation terminate on the somata and proximal dendrites of abducens neurons (Destombes and Rouviere, 1981). The reason for this difference in the soma-dendritic distribution of reticular inputs to vertical vs. horizontal motoneurons is unclear, unless the electrolytic lesions that were employed in the latter study disrupted fibers of passage (particularly those from the vestibular nuclei). A more intriguing possibility is that these differences also may be related to different neurotransmitters that are utilized by premotor neurons involved in the control of horizontal vs. vertical eye movements (e.g., glycine vs. GABA; Spencer et al., 1989, 1992; Spencer and Baker, 1992; Spencer and Wang, 1996).

More significantly, the present study has demonstrated that both excitatory and inhibitory inputs from the riMLF establish synaptic connections with the same motoneuron subgroups ipsilaterally. This arrangement, as such, con- trasts with the reciprocal organization of excitatory and inhibitory connections that characterize the horizontal eye-movement systems (Escudero and Delgado-Garcia, 1988) and the vertical vestibuloocular reflex (Highstein and Ito, 1971; Precht and Baker, 1972; Berthoz et al., 1973; Highstein, 1973; Uchino et al., 1978; Iwamoto et al., 1990). The results from a previous study indicate a topographical organization of premotor neurons in the riMLF that project to vertical-upward or vertical-downward motoneurons in the oculomotor and trochlear nuclei (Wang and Spencer, 1996). For example, rostral regions of the riMLF target predominantly inferior rectus and superior oblique motoneurons. The present study has documented that this projection to the trochlear nucleus has both excitatory and inhibitory components. Consistent with this finding, electrical stimulation of the riMLF region elicits both monosynaptic EPSPs and inhibitory postsynaptic potentials (IPSPs) in oculomotor and trochlear motoneurons (Schwindt et al., 1974; Nakao and Shiraishi, 1983, 1985). Given the fact that physiological studies have demonstrated that neurons dis- charging in relation to vertical-upward or vertical-downward saccadic eye movements are intermingled within the

riMLF (Buttner et al., 1977; King and Fuchs, 1979; Vilis et al., 1989; Moschovakis et al., 1991a,b; Crawford and Vilis, 1992), it is logical to assume that a single region of the riMLF contains excitatory neurons that are related to one vertical axis of movement and contains inhibitory neurons that are activated during movements in the opposite direction. This arrangement would require that supranuclear inputs to the riMLF must distinguish between the two populations of neurons that are related to opposite directions of vertical saccadic eye movements.

ACKNOWLEDGMENTS This study was supported by U S . Public Health Service

MERIT Award EY02191 from the National Eye Institute, National Institutes of Health. The excellent technical assis- tance of Lynn Davis and Nancy Smith is greatly appreci- ated.

LITERATURE CITED Bak, I.J., and W.B. Choi (1974) Electron microscopic investigation of

synaptic organization of the trochlear nucleus in cat. I. Normal ultrastructure. Cell Tissue Kes. 150:409-423.

Bak, I.J., R. Baker, W.B. Choi, and W. Precht (1976) Electron microscopic investigation of the vestibular projection to the cat trochlear nuclei. Neuroscience 1.477-482.

Berthoz, A., R. Baker, and W. Precht (1973) Labyrinthine control of inferior oblique motoneurons. Exp. Brain Res. 18:225-241

Bogousslavsky, J., and F. Regli (1984) Upgaze palsy and monocular paresis of downward gaze from ipsilateral thalamo-mesencephalic infarction: A vertical “one-and-a-half’ syndrome. J . Neurol. 231:4345.

Bogousslavsky, J . , J . Miklossy, F. Regli, and R. Janzer (1990) Vertical gaze palsy and selective unilateral infarction of the rostral interstitial nucleus of the medial IonL~tudinal fasciculus (riMLF). J . Neurol. Neurasurg. Psychiatr. 53r67-71.

Buttner, U., J.A. Biittner-Ennever, and V. Henn (1977) Vertical eye movement related unit activity in the rostral mesencephalic reticular formation of the alert monkey. Brain Res. 130:239-252.

Buttner-Ennever, J.A., and U. Buttner (1978) A cell group associated with vertical eye movements in the rostral mesencephalic reticular formation of the monkey. Brain Res. 151:3147.

Biittner-Ennever, J.A., and U. Biittner (1988) The reticular formation. In J.A. Biittner-Ennever (ed): Neuroanatomy of the Oculomotor System. Amsterdam: Elsevier Science Publishers BV, pp. 119-176.

Biittner-Ennever, J.A., U. Buttner, B. Cohen, and G. Baumgartner (1982) Vertical gaze paralysis and the rostral interstitial nucleus of the medial longitudinal fasciculus. Brain 105:125-149.

Carpenter, M.B., J.W. Harbison, and P. Peter (1970) Accessory oculomotor nuclei in the monkey: Projections and effects nfdiscrete lesions. J. Comp. Neurol. 140:131-154.

Carpenter, M.B., A.B. Periera, and N. Guha (1992) Immunocytochemistry of oculomotor afferents in the squirrel monkey (Saimiri sciureus). J . Hirnforsch. 33151-167.

Christoff, N.A. (1974) A clinicopathologic study of vertical eye movements. Arch. Neurol. 31:1-8.

Cogan, D.C. (1974) Paralysis of down-gaze. Arch. Ophthalmol. 91:192-199. Crawford, J.D., and T. Vilis (1992) Symmetry of oculomotor burst neuron

coordinates about Listing’s plane. J. Neurophysiol. 68:432-448. Deleu, D., T. Buisseret, and G. Ebinger (1989) Vertical one-and-a-half

syndrome: Supranuclear downgaze paralysis with monocular elevation palsy. Arch. Neurol. 46:1361-1363.

Dememes, D., and J. Raymond (1980) Identification des terminaisons vestibulaires dans les noyaux oculomoteurs communs chez le chat par radioautographie en microscopie electronique. Brain Kes. 196:381-345.

Destombes, J . , and A. Kouviere (1981) Ultrastructural study of vestibular and reticular projections to the abducens nucleus. Exp. Brain Res. 43253-260.

Escudero, M., and J . M . Delgado-Garcia (1988) Behavior of reticular, vestibular and prepositus neurons terminating in the abducens nucleus of the alert cat. Exp. Brain Kes. 71:218-222.

riMLF SYNAPTIC CONNECTIONS WITH OCULOMOTOR AND TROCHLEAK MOTONEURONS 161

Fukushima, K. (1987) The interstitial nucleus of Cajal and its role in the control of movements of head and eyes. Progr. Neurobiol. 29:107-192.

Fukushima, K. (1991) The interstitial nucleus of Cajal in the midbrain reticular formation and vertical eye movement. Neurosci. Res. 10:159- 187.

Fukushima, K., and J . Fukushima (1992) The interstitial nucleus of Cajal is involved in generating downward fast eye movement in alert cats. Neurosci. Res. 15:299-303.

Graybiel, A.M. 11977) Organization of oculomotor pathways in the cat and rhesus monkey. In R. Baker and A. Berthoz ieds): Control of Gaze by Brainstem Neurons. Amsterdam: ElsevieriNorth-Holland Biomedical Press, pp. 79-88.

Grayhiel, A.M., and E.A. Hartwieg (1974) Some afferent connections of the oculomotor complex in the cat: An experimental study with tracer techniques. Brain Res. #1:543-551.

Green. J .P. , N.J. Newman, and J.S. Winterkorn (1993) Paralysis of downgaze in two patients with clinical-radiologic correlation. Arch. Ophthal- mol. 111:219-222.

Halmagyi, G.M., A.W. Evans, and J.M. Hallinan 11978) Failure of downward gaze: The site and nature of the lesion. Arch. Neurol. 35.22-26.

Hepp, K., V. Henn, T. Vilis, and B. Cohen (1989) Brainstem regons related to saccade generation. In R.H. Wurtz and M.E. Goldberg ieds): The Neurobiology of Saccadic Eye Movements. Amsterdam: Elsevier Science Publishers BV, pp. 105-212.

Highstein, S.M. ( 1973) The organization of the vestibulo-oculomotor and trochlear reflex pathways in the rabbit. Exp. Brain Res. 17:285-300.

Highstein, S.M., and M. Ito (1971) Differential localization within the vestibular nuclear complex of the inhibitory and excitatory cells innervating IIIrd nucleus oculomotor neurons in rabbit. Brain Res. 29358-362.

Isa. T., and T. Itouji (1992) Axonal trajectories of single Forel’s field H neurones in the mesencephalon, pons and medulla oblongata in the cat. Exp. Brain Kes. 89:484495.

Isa. T. , and K. Naito (19941 Activity of neurons in Forel’s field H during orienting head movements in alert head-free cats. Exp. Brain Res. 100:187-199.

Isa, T. , T. Itouji, S. Nakao, and S. Sasaki (1988) Subtypes of neurones in Forel’s field H as defined by their axonal projection. Neurosci. Lett. 90:95-99.

Iwamoto, Y., T. Kitami, and K. Yoshida (1990) Vertical eye movement- related secondary vestibular neurons ascending in medial longitudinal fasciculus in cat. 11. Direct connections with extraocular motoneurons. J . Neurophysiol. 63:918-935.

Jacobs, L., P.L. Anderson, and M.B. Bender (1978) The lesion producing paralysis of downward but not upward gaze. Arch. Neurol. 28.319-323.

King, W.M., and A.F. Fuchs 11979) Reticular control of vertical saccadic eye movements by mesencephalic burst neurons. J. Neurophysiol. 42:861- 876.

Koerber. H.R., and L.M. Mendell (1991) Modulation of synaptic transmis- sion at Ia-afferent fiber connections on motoneurons during high- frequency stimulation: Role of postsynaptic target. J. Neurophysiol. 65590-597.

Kompf, D., T. Pasik, P. Pasik, and M.B. Bender (1979) Downward gaze in monkeys. Stimulation and lesion studies. Brain 102:527-538.

Labandeira-Garcia, J.L., M.J. Guerra-Seijas, and J.A. Labandeira-Garcia ( 1989, Oculnmotor nucleus afferents from the interstitial nucleus of Cajil and the region surrounding the fasciculus retroflexus in the rabbit. Neurosci. Lett. 101:ll-16.

Larramendi, L.M.H., L. Fickenscher, and N. Lemkey-Johnston 11967) Synaptic vesicles of inhibitory and excitatory terminals in the cerebel- lum. Science 156:967-969.

Li, W.-B.. Y. Shiraishi, and S. Nakao (1993) Topographical organization of cat mesodiencephalic areas for monosynaptic activation of vertical oculomotoneurons. Exp. Brain Res. W43-52.

Luscher. H-R. , and H.P. Clamann (1992) Relation between structure and function in information transfer in spinal monosynaptic reflex. Physiol. Rev. 7271-99.

McCrea, K.A., K. Yoshida, A. Berthoz, and R. Baker (1980) Eye movement related activity and morphology of second order vestibular neurons terminating in the cat abducens nucleus. Exp. Brain Res. 40t468-473.

McCrea, R.A., A. Strassman. and S.M. Highstein l1987aJ Anatomical and physiological characteristics of vestibular neurons mediating the vertical vestibulo-ocular reflexes of the squirrel monkey. J. Comp. Neurol. 264.57 1-594.

McCrea. R.A., A. Strassman, E. May, and S.M. Highstein l1987b) Anatomi- cal and physiologcal characteristics of vestibular neurons mediating the

horizontal vestibulo-ocular reflex of the squirrel monkey. J . Comp. Neurol. 264:547-570.

McHaffie, J .G., M. Beninato, B.E. Stein, and K.F. Spencer (1991) Postnatal development of acetylcholinesterase in, and cholinergic projections to, the cat superior colliculus. J . Comp. Neurol. %13:113-131.

Meienberg, O., J. Rover, and G. Kommerell (1978) Prenuclear paresis of homolateral inferior rectus and contralateral superior oblique eye muscles. Arch. Neurol. 35231-233.

Moffie, D., B.W.O. de Visser, and S.Z. Stefanko (1983) Parinaud’s syndrome. J . Neurol. Sci. 581175-183.

Moschovakis, A.K., and S.M. Highstein (19941 The anatomy and physiology of primate neurons that control rapid eye movements. Annu. Rev. Neurosci. 17:465-488.

Moschovakis, A.K., C.A. Scudder, and S.M. Highstein (1990) A structural basis for Hering’s law: Projections to extraocular motoneurons. Science 248: 1 1 18- 1 1 19.

Moschovakis, A.K., C.A. Scudder, and S.M. Highstein (1991al Structure of the primate oculomotor burst generator. I. Medium-lead burst neurons with downward on-direction. J . Neurophysiol. 65.203-217.

Moschovakis, A.K., C.A. Scudder, S.M. Highstein, and J.D. Warren (1991b) Structure of the primate oculomotor burst generator. 11. Medium-lead burst neurons with upward on-direction. J . Neurophysiol. 65:218-229.

Nakao, S., and Y. Shiraishi (1983) Excitatory and inhibitory synaptic inputs from the medial mesodiencephalic junction to vertical eye movement- related motoneurons in the cat oculomotor nucleus. Neurnsci. Lett. 42: 12.5-130.

Nakao, S., and Y. Shiraishi 11985) Direct excitatory and inhibitory synaptic inputs from the medial mesodiencephalic junction to motoneurons innervating extraocular oblique muscles in the cat. Exp. Brain Res. 61:62-72.

Nakao, S., Y. Shiraishi, W.-B. Li, and T. Oikawa 11990) Mono- and disynaptic excitatory inputs from the superior colliculus to vertical saccade-related neurons in the cat Forel’s field H. Exp. Brain Res. 82:222-226.

Ohgaki, T., I S . Curthoys, and C.H. Markham 11988) Morphology of physiologically identified second-order vestibular neurons in cat, with intracellularly injected HRP. J. Comp. Neurol. 276:387-411.

Pierrot-Deseilligny, C., F. Chain, F. Gray, M. Serdaru, R. Escourolle, and F. Lhermitte ( 1982) Parinaud’s syndrome. Electro-oculographic and anatomical analyses of six vascular cases with deductions about vertical gaze organization in the premotor structures. Brain 105:667-696.

Precht, W., and K. Baker (1972) Synaptic organization of the vestibulo- trochlear pathway. Exp. Brain Res. 14:158-184.

Ranalli, P.J., J.A. Sharpe, and W.A. Fletcher (1988) Palsy of upward and downward saccadic, pursuit, and vestibular movements with a unilateral midbrain lesion: Pathophysiologic correlations. Neurology 3 8 114-122.

Robinson, F.R., J.O. Phillips, and A.F. Fuchs (1994) Coordination of gaze shifts in primates: Brainstem inputs to neck and extraocular motoneuron pools. J . Comp. Neurol. 346:43-62.

Rose, P.K., and M. Neuher-Hess 11991) Morphology and frequency of axon terminals on the somata, proximal dendrites, and distal dendrites of dorsal neck motoneurons in the cat. J . Comp. Neurol. 307:259-280.

Schwindt, P.C., W. Precht, and A. Richter ( 1974) Monosynaptic excitatory and inhibitory pathways from medial midbrain nuclei to trochlear motoneurones. Exp. Brain Kes. 20:223-238.

Scudder, C.A., and A.F. Fuchs 11992) Physiological and behavioral identification ofvestibular nucleus neurons mediating the horizontal vestibuloocular reflex in trained rhesus monkeys. J. Neurophysiol. 682444264.

Shiraishi, Y., S. Matsuo, and S. Nakao (1994) Medium-lead burst neurons related to vertical saccades in cat Forel’s field H: Input from vestibular nucleus. Neurosci. Lett. 17224-26.

Spencer, R.F., and K. Baker (1983) Morphology and synaptic connections of physiologcally-identified second-order vestibular axonal arborizations related to cat oculomotor and trochlear motoneurones. Soc. Neurosci. Abstr. 9:1088.

Spencer, K.F., and Baker, R. (1992) GABA and glycine as inhibitory neurotransmitters in the vestibuloocular reflex. Ann. N.Y. Acad. Sci. 656:602-611.

Spencer, R.F., and Wang, S:F. ( 1996) Immunohistochemical localization of neurotransmitters utilized by neurons in the rostra1 interstitial nucleus of the medial longitudinal fasciculus (riMLF) that project to the oculomotor and trochlear nuclei in the cat. J . Comp. Neurol. ( this issue).

Spencer, R.F., R.J. Wenthold, and R. Baker i 1989) Evidence for glycine as an inhibitory neurotransmitter of vestibular, reticular, and prepositus hypoglossi neurons that project to the cat abducens nucleus. J . Neurosci. 9:2718-2736.


Spencer. K.F.. S:F. Wang, and K. Baker (1992) The pathways and functions of GABA in the oculomotor system. Progr. Brain Kes. 90.307-331.

Steiger, H.-J., and J.A. Buttner-Ennever (1979) Oculomotor nucleus afferents in the monkey demonstrated with horseradish peroxidase. Brain Kes. 160:1-15.

Thiimke. F., and H.C. Hopf i 1992) Acquired monocular elevation paresis. Brain 115:1901-1910.

Tomasulo. R.A.. J.J. Ramirez, and 0. Steward (1993) Synaptic inhibition regulates associative interactions between afferents during the induction of long-term potentiation and depression. Proc. Natl. Acad. Sci. USA 90:11578-11582.

Tredici. G.. G. Pizzini, and S. Milanesi (1976) The ultrastructure of the nucleus of the oculomotor nerve (somatic efferent portion) of the cat. Anat. Embryol. 149.323-346.

Trojanowski, J.Q., and M.H. LaFontaine (1981) Neuroanatomical correlates of selective downgaze paralysis. J . Neurol. Sci. 52191-101.

Uchino. Y., N. Hirai. and S. Watanabe (1978) Vestibulo-ocular reflex from the posterior canal nerve to extraocular motoneurons in the cat. Exp. Brain Kes. 32r377-388.

Uchizono, K. (1965) Characteristics of excitatory and inhibitory synapses in the central nervous system. Nature (London) 207:642-643.

Vilis, T., K. Hepp, U. Schwarz, and V. Henn (1989) On the generation of vertical and torsional rapid eye movements in the monkey. Exp. Brain Kes. 77:l-11.

Walmsley, B., and K. Stuklis (1989) Effects of spatial and temporal dispersion of synaptic input on the time course of synaptic potentials. J. Neurophysiol. 61:681-687.

Wang, S.-F., and K.F. Spencer (1995) Spatial organization of premotor neurons related to vertical upward and downward saccadic eye movements in the rostra1 interstitial nucleus of the medial longitudinal fasciculus (riMLF) in the cat. J . Camp. Neurol. ( this issue).

Waxman, S.G., and G.D. Pappas (1979) Ultrastructure of synapses and cellular relationships in the oculomotor nucleus of the rhesus monkey. Cell Tiss. Res. 204:161-169.

White, G., W.B. Levy, and 0. Steward (1990) Spatial overlap between populations of synapses determines the extent of their associative interaction during the induction of long-term potentiation and depression. J. Neurophysiol. 6'4.1 186-1198.

morphology and soma-dendritic distribution of synaptic endings from the rostral interstitial nucleus...

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