Morphological identification of nitric oxide sources and targets in the cat oculomotor system

Species Differences in the Organization of the Ventral Cochlear Nucleus

The mammalian cochlear nuclei (CN) consist of two major subdivisions, the dorsal (DCN) and ventral (VCN) nuclei. We previously reported differences in the structural and neurochemical organization of the human DCN from that in several other species. Here we extend this analysis to the VCN, considering both the organization of subdivisions and the types and distributions of neurons. Classically, the VCN in mammals is composed of two subdivisions, the anteroventral (VCA) and posteroventral cochlear nuclei (VCP). Anatomical and electrophysiological data in several species have defined distinct neuronal types with different distributions in the VCA and VCP. We asked if VCN subdivisions and anatomically defined neuronal types might be distinguished by patterns of protein expression in humans. We also asked if the neurochemical characteristics of the VCN are the same in humans as in other mammalian species, analyzing data from chimpanzees, macaque monkeys, cats, rats and chinchillas. We examined Nissl- and immunostained sections, using antibodies that had labeled neurons in other brainstem nuclei in humans. Nissl-stained sections supported the presence of both VCP and VCA in humans and chimpanzees. However, patterns of protein expression did not differentiate classes of neurons in humans; neurons of different soma shapes and dendritic configurations all expressed the same proteins. The patterns of immunostaining in macaque monkey, cat, rat, and chinchilla were different from those in humans and chimpanzees and from each other. The results may correlate with species differences in auditory function and plasticity. Anat Rec, 301:862-886, 2018. © 2017 Wiley Periodicals, Inc.

A perioculomotor nitridergic population in the macaque and cat

PURPOSE

We determined the distribution of cells containing synthetic enzymes for the unconventional neurotransmitter, nitric oxide, with respect to the known populations within the oculomotor complex.

METHODS

The oculomotor complex was investigated in monkeys and cats by use of histochemistry to demonstrate nicotinamide adenine dinucleotide phosphate diaphorase positive (NADPHd(+)) cells and antibodies to localize neuronal nitric oxide synthase positive (NOS(+)) cells. In some cases, wheat germ agglutinin conjugated horseradish peroxidase (WGA-HRP) was injected into extraocular muscles to allow comparison of retrogradely labeled and NADPHd(+) cell distributions.

RESULTS

The distribution of the NADPHd(+) and NOS(+) neurons did not coincide with that of preganglionic and extraocular motoneurons in the oculomotor complex. However, labeled perioculomotor neurons were observed. Specifically, in monkeys, they lay in an arc that extended from between the oculomotor nuclei into the supraoculomotor area (SOA). Comparison of WGA-HRP-labeled medial and superior rectus motoneurons with NADPHd staining confirmed that the distributions overlapped, but showed that the C- and S-group cells were not NADPHd(+). This suggested that NADPHd(+) cells are part of the centrally projecting Edinger-Westphal population (EWcp). Examination of the NADPHd(+) cell distribution in the cat showed that these cells were indeed found primarily within its well-defined EWcp.

CONCLUSIONS

Based on their similar distributions, it appears that the peptidergic EWcp neurons, which project widely in the brain, also may be nitridergic. While the preganglionic and C- and S-group motoneuron populations do not use this nonsynaptic neurotransmitter, nitric oxide produced by surrounding NADPHd(+) cells may modulate the activity of these motoneurons.

The anatomy and physiology of the ocular motor system

Accurate diagnosis of abnormal eye movements depends upon knowledge of the purpose, properties, and neural substrate of distinct functional classes of eye movement. Here, we summarize current concepts of the anatomy of eye movement control. Our approach is bottom-up, starting with the extraocular muscles and their innervation by the cranial nerves. Second, we summarize the neural circuits in the pons underlying horizontal gaze control, and the midbrain connections that coordinate vertical and torsional movements. Third, the role of the cerebellum in governing and optimizing eye movements is presented. Fourth, each area of cerebral cortex contributing to eye movements is discussed. Last, descending projections from cerebral cortex, including basal ganglionic circuits that govern different components of gaze, and the superior colliculus, are summarized. At each stage of this review, the anatomical scheme is used to predict the effects of lesions on the control of eye movements, providing clinical-anatomical correlation.

NO orchestrates the loss of synaptic boutons from adult "sick" motoneurons: modeling a molecular mechanism

Synapse elimination is the main factor responsible for the cognitive decline accompanying many of the neuropathological conditions affecting humans. Synaptic stripping of motoneurons is also a common hallmark of several motor pathologies. Therefore, knowledge of the molecular basis underlying this plastic process is of central interest for the development of new therapeutic tools. Recent advances from our group highlight the role of nitric oxide (NO) as a key molecule triggering synapse loss in two models of motor pathologies. De novo expression of the neuronal isoform of NO synthase (nNOS) in motoneurons commonly occurs in response to the physical injury of a motor nerve and in the course of amyotrophic lateral sclerosis. In both conditions, this event precedes synaptic withdrawal from motoneurons. Strikingly, nNOS-synthesized NO is "necessary" and "sufficient" to induce synaptic detachment from motoneurons. The mechanism involves a paracrine/retrograde action of NO on pre-synaptic structures, initiating a downstream signaling cascade that includes sequential activation of (1) soluble guanylyl cyclase, (2) cyclic guanosine monophosphate-dependent protein kinase, and (3) RhoA/Rho kinase (ROCK) signaling. Finally, ROCK activation promotes phosphorylation of regulatory myosin light chain, which leads to myosin activation and actomyosin contraction. This latter event presumably contributes to the contractile force to produce ending axon retraction. Several findings support that this mechanism may operate in the most prevalent neurodegenerative diseases.

Expression of calcium-binding proteins and nNOS in the human vestibular and precerebellar brainstem

Information about the position and movement of the head in space is coded by vestibular receptors and relayed to four nuclei that comprise the vestibular nuclear complex (VNC). Many additional brainstem nuclei are involved in the processing of vestibular information, receiving signals either directly from the eighth nerve or indirectly via projections from the VNC. In cats, squirrel monkeys, and macaque monkeys, we found neurochemically defined subdivisions within the medial vestibular nucleus (MVe) and within the functionally related nucleus prepositus hypoglossi (PrH). In humans, different studies disagree about the borders, sizes, and possible subdivisions of the vestibular brainstem. In an attempt to clarify this organization, we have begun an analysis of the neurochemical characteristics of the human using brains from the Witelson Normal Brain Collection and standard techniques for antigen retrieval and immunohistochemistry. Using antibodies to calbindin, calretinin, parvalbumin, and nitric oxide synthase, we find neurochemically defined subdivisions within the MVe similar to the subdivisions described in cats and monkeys. The neurochemical organization of PrH is different. We also find unique neurochemical profiles for several structures that suggest reclassification of nuclei. These data suggest both quantitative and qualitative differences among cats, monkeys, and humans in the organization of the vestibular brainstem. These results have important implications for the analysis of changes in that organization subsequent to aging, disease, or loss of input.

The nitric oxide/cyclic guanosine monophosphate pathway modulates the inspiratory-related activity of hypoglossal motoneurons in the adult rat

Motoneurons integrate interneuronal activity into commands for skeletal muscle contraction and relaxation to perform motor actions. Hypoglossal motoneurons (HMNs) are involved in essential motor functions such as breathing, mastication, swallowing and phonation. We have investigated the role of the gaseous molecule nitric oxide (NO) in the regulation of the inspiratory-related activity of HMNs in order to further understand how neural activity is transformed into motor activity. In adult rats, we observed nitrergic fibers and bouton-like structures in close proximity to motoneurons, which normally lack the molecular machinery to synthesize NO. In addition, immunohistochemistry studies demonstrated that perfusion of animals with a NO donor resulted in an increase in the levels of cyclic guanosine monophosphate (cGMP) in motoneurons, which express the soluble guanylyl cyclase (sGC) in the hypoglossal nucleus. Modulators of the NO/cGMP pathway were micro-iontophoretically applied while performing single-unit extracellular recordings in the adult decerebrated rat. Application of a NO synthase inhibitor or a sGC inhibitor induced a statistically significant reduction in the inspiratory-related activity of HMNs. However, excitatory effects were observed by ejection of a NO donor or a cell-permeable analogue of cGMP. In slice preparations, application to the bath of a NO donor evoked membrane depolarization and a decrease in rheobase, which were prevented by co-addition to the bath of a sGC inhibitor. These effects were not prevented by reduction of the spontaneous synaptic activity. We conclude that NO from afferent fibers anterogradely modulates the inspiratory-related activity of HMNs by a cGMP-dependent mechanism in physiological conditions.

Neurochemical organization of the nucleus paramedianus dorsalis in the human

We have characterized the neurochemical organization of a small brainstem nucleus in the human brain, the nucleus paramedianus dorsalis (PMD). PMD is located adjacent and medial to the nucleus prepositus hypoglossi (PH) in the dorsal medulla and is distinguished by the pattern of immunoreactivity of cells and fibers to several markers including calcium-binding proteins, a synthetic enzyme for nitric oxide (neuronal nitric oxide synthase, nNOS) and a nonphosphorylated neurofilament protein (antibody SMI-32). In transverse sections, PMD is oval with its long axis aligned with the dorsal border of the brainstem. We identified PMD in eight human brainstems, but found some variability both in its cross-sectional area and in its A-P extent among cases. It includes calretinin immunoreactive large cells with oval or polygonal cell bodies. Cells in PMD are not immunoreactive for either calbindin or parvalbumin, but a few fibers immunoreactive to each protein are found within its central region. Cells in PMD are also immunoreactive to nNOS, and immunoreactivity to a neurofilament protein shows many labeled cells and fibers. No similar region is identified in atlases of the cat, mouse, rat or monkey brain, nor does immunoreactivity to any of the markers that delineate it in the human reveal a comparable region in those species. The territory that PMD occupies is included in PH in other species. Since anatomical and physiological data in animals suggest that PH may have multiple subregions, we suggest that the PMD in human may be a further differentiation of PH and may have functions related to the vestibular control of eye movements.

Anatomical and pharmacological relationship between acetylcholine and nitric oxide in the prepositus hypoglossi nucleus of the cat: Functional implications for eye-movement control

The prepositus hypoglossi (PH) nucleus has been proposed as a pivotal structure for horizontal eye-position generation in the oculomotor system. Recent studies have revealed that acetylcholine (ACh) in the PH nucleus could mediate the persistent activity necessary for this process, although the origin of this ACh remains unknown. It is also known that nitric oxide (NO) in the PH nucleus plays an important role in the control of velocity balance, being involved in a negative feedback control of tonic signals arriving at the PH nucleus. As it could be expected that neurons taking part in eye-position generation must control their tonic background inputs, the existence of a relationship between nitrergic and cholinergic neurons is hypothesized. In the present study we analyzed the distribution, size, and morphology of choline acetyltransferase-positive neurons, and their relationship with neuronal nitric oxide synthase in the PH nucleus of the cat. As presumed, some 96% of cholinergic neurons were also nitrergic in the PH nucleus, suggesting that NO is regulating the level of ACh released by cholinergic PH neurons. Furthermore, we studied the alterations induced by muscarinic-receptor agonists and antagonists on spontaneous and vestibularly induced eye movements in the alert cat and compared them with those induced in previous studies by modification of NO levels in the same animal preparation. The results suggest that ACh is necessary for the generation of saccadic and vestibular eye-position signals, whereas the NO is stabilizing the eye-position generator by controlling background activity reaching cholinergic neurons in the PH nucleus.

Neuronal nitric oxide synthase immunopositive neurons in cat vestibular complex: a light and electron microscopic study

Nitric oxide is a unique neurotransmitter, which participates in many physiological and pathological processes in the organism. Nevertheless, there are little data about the neuronal nitric oxide synthase immunoreactivity (nNOS-ir) in the vestibular complex of a cat. In this respect, the aims of this study were to: (1) demonstrate nNOS-ir in the neurons and fibers, from all major and accessory vestibular nuclei; (2) describe their light microscopic morphology and distribution; (3) investigate and analyze the ultrastructure of the NOS I-immunopositive neurons, fibers, and synaptic boutons. For demonstration of the nNOS-ir, the peroxidase-antiperoxidase-diaminobenzidin method was applied. Immunopositive for nNOS neurons and fibers were present in all major and accessory vestibular nuclei. On the light microscope level, the immunopositive neurons were different in shape and size. According to the latter, they were divided into four groups--small (with diameter less than 15 microm), medium-sized (with diameter from 15 to 30 microm), large type I (with diameter from 30 to 40 microm), and large type II (with diameter greater than 40 microm). On the electron microscope level, the immunoproduct was observed in neurons, dendrites, and terminal boutons. According to the ultrastructural features, the neurons were divided into three groups--small (with diameter less than 15 microm), medium-sized (with diameter from 15 to 30 microm), and large (with diameter greater than 30 microm). At least two types of nNOS-ir synaptic boutons were easily distinguished. As a conclusion, we hope that this study will contribute to a better understanding of the functioning of the vestibular complex in cat and that some of the data presented could be extrapolated to other mammals, including human.

Neurochemically defined cell columns in the nucleus prepositus hypoglossi of the cat and monkey

Many studies have shown that the nucleus prepositus hypoglossi (PH) participates with the vestibular nuclear complex, the cerebellum and the oculomotor nuclei in the control of eye movements. We have looked at the neurochemical organization of PH in the cat and monkey using a recently developed antibody, 8B3, that recognizes a chondroitin sulfate proteoglycan. In the cat, immunoreactivity to 8B3 labels a set of cells in PH. On frontal sections, these cells form a cluster that is seen over the entire anterior-posterior (A-P) extent of PH, but the number of cells in the cluster changes with A-P level. Earlier studies have identified an A-P cell column in PH of the cat whose neurons synthesize nitric oxide. We have used both single- and double-label protocols to investigate the relation between the two cell groups. Single-label studies show spatial overlap but that the cells immunoreactive to nitric oxide synthase (nNOS) are more numerous than cells immunoreactive to 8B3. Double-label studies show that all cells immunoreactive to 8B3 were also immunoreactive to nNOS, but, as suggested by the single-label data, there are many nNOS-immunoreactive cells not immunoreactive to 8B3. Populations of 8B3 and nNOS-immunoreactive cells are also found in PH of squirrel and macaque monkeys. The results suggest that nNOS-immunoreactive cells in PH may consist of two functionally different populations.

Expression of cyclic nucleotide-gated channels in the rat medial vestibular nucleus

The role of cyclic nucleotide-gated (CNG) channels in sensory signal transduction in retinal and olfactory cells is widely recognized, but there is increasing evidence that they also play more general functions in the central nervous system as downstream effectors of cyclic nucleotides. Here, we demonstrate the expression of the alpha-subunit of rod- and olfactory-type CNG channels (CNG1 and CNG2, respectively) in the rat medial vestibular nucleus (MVN). Nested polymerase chain reaction revealed CNG channel mRNA in the MVN, and CNG1 and CNG2 proteins were also detected by Western blotting and immunohistochemistry. Finally, electrophysiological evidence is provided suggesting that CNG channels play a functional role in the MVN.

The extraocular motor nuclei: organization and functional neuroanatomy

The organization of the motoneuron subgroups in the brainstem controlling each extraocular eye muscle is highly stable through the vertebrate species. The subgroups are topographically organized in the oculomotor nucleus (III) and are usually considered to form the final common pathway for eye muscle control. Eye muscles contain a unique type of slow non-twitch, fatigue-resistant muscle fiber, the multiply innervated muscle fibers (MIFs). The recent identification the MIF motoneurons shows that they too have topographic organization, but very different from the classical singly innervated muscle fiber (SIF) motoneurons. The MIF motoneurons lie around the periphery of the oculomotor nucleus (III), trochlear nucleus (IV), and abducens nucleus (VI), slightly separated from the SIF subgroups. The location of four different types of neurons in VI are described and illustrated: (1) SIF motoneurons, (2) MIF motoneurons, (3) internuclear neurons, and (4) the paramedian tract neurons which project to the flocculus. Afferents to the motoneurons arise from the vestibular nuclei, the oculomotor and abducens internuclear neurons, the mesencephalic and pontine burst neurons, the interstitial nucleus of Cajal, nucleus prepositus hypoglossi, the supraoculomotor area and the central mesencephalic reticular formation and the pretectum. The MIF and SIF motoneurons have different histochemical properties and different afferent inputs. The hypothesis that SIFs participate in moving the eye and MIFs determine the alignment seems possible but is not compatible with the concept of a final common pathway.

Nucleus prepositus

The cytoarchitecture and the histochemistry of nucleus prepositus hypoglossi and its afferent and efferent connections to oculomotor structures are described. The functional significance of the afferent connections of the nucleus is discussed in terms of current knowledge of the firing behavior of prepositus neurons in alert animals. The efferent connections of the nucleus and the results of lesion experiments suggest that it plays a role in a variety of functions related to the control of gaze.

A cholinergic mechanism underlies persistent neural activity necessary for eye fixation

It is generally accepted that the prepositus hypoglossi (PH) nucleus is the site where horizontal eye-velocity signals are integrated into eye-position ones. However, how does this neural structure produce the sustained activity necessary for eye fixation? The generation of the neural activity responsible for eye-position signals has been studied here using both in vivo and in vitro preparations. Rat sagittal brainstem slices including the PH nucleus and the paramedian pontine reticular formation (PPRF) rostral to the abducens nucleus were used for recording intracellularly the synaptic activation of PH neurons from the PPRF. Single electrical pulses applied to the PPRF showed a monosynaptic projection on PH neurons. This synapse was found to be glutamatergic in nature, acting on alpha-amino-3-hydroxy-5-methylisoxazole propionate (AMPA)/kainate receptors. Train stimulation (100 ms, 50-200 Hz) of the PPRF evoked a depolarization of PH neurons, exceeding (by hundreds of ms) the duration of the stimulus. Both duration and amplitude of this long-lasting depolarization were linearly related to train frequency. The train-evoked sustained depolarization was demonstrated to be the result of the additional activation of cholinergic fibers projecting onto PH neurons, because it was prevented by slice superfusion with atropine sulfate and pirenzepine (two cholinergic antagonists), and mimicked by carbachol and McN-A-343 (two cholinergic agonists). These results were confirmed in alert behaving cats. Microinjections of atropine and pirenzepine evoked an ipsilateral gaze-holding deficit consisting of an exponential-like, centripetal eye movement following saccades directed toward the injected site. These findings suggest that the sustained activity present in PH neurons carrying eye-position signals is the result of the combined action of PPRF neurons and the facilitative role of cholinergic terminals, both impinging on PH neurons. The present results are discussed in relation to other proposals regarding integrative properties of PH neurons and/or related neural circuits.

Light microscopical study of nitric oxide synthase I-positive neurons, including fibres in the vestibular nuclear complex of the cat

Nitric oxide is a gaseous neurotransmitter that is synthesized by the enzyme nitric oxide synthase I (NOS I). At present, little is known of NOS I-positive neurons in the vestibular nuclear complex of the cat (VNCc). The aim of the present study was to examine the morphology, distribution patterns and interconnections of NOS I-positive neurons, including fibres in the VNCc. Five adult cats were used as experimental animals. All cats were anaesthetized and perfused transcardially. Brains were removed, postfixed, cut on a freezing microtome and stained in three different ways. Every third section was treated with the Nissl method, other sections were stained either histochemically for NADPH diaphorase or immunohistochemically for NOS I. The atlas of Berman (1928) was used for orientation in the morphometric study. NOS I-positive neurons and fibres were found in all parts of VNCc: medial vestibular nucleus (MVN); lateral vestibular nucleus (LVN); superior vestibular nucleus (SVN); inferior vestibular nucleus (IVN); X, Y, Z groups and Cajal's nucleus. The NOS I-positive neurons were classified according to their size (small, medium-sized, large neurons type I and type II) and their shape (oval, fusiform, triangular, pear-shaped, multipolar and irregular). In every nucleus, a specific neuronal population was observed. In SVN, a large number of interconnections between NOS I-positive neurons were identified. In MVN, chain-like rolls of small neurons were found. Tiny interconnections between MVN and mesencephalic reticular formation were present. Our data provide information on the morphology, distribution patterns and interconnections of NOS I-positive neurons in the VNCc and can be extrapolated to other mammals.

Immunoreactivity for calcium-binding proteins defines subregions of the vestibular nuclear complex of the cat

The vestibular nuclear complex (VNC) is classically divided into four nuclei on the basis of cytoarchitectonics. However, anatomical data on the distribution of afferents to the VNC and the distribution of cells of origin of different efferent pathways suggest a more complex internal organization. Immunoreactivity for calcium-binding proteins has proven useful in many areas of the brain for revealing structure not visible with cell, fiber or Golgi stains. We have looked at the VNC of the cat using immunoreactivity for the calcium-binding proteins calbindin, calretinin and parvalbumin. Immunoreactivity for calretinin revealed a small, intensely stained region of cell bodies and processes just beneath the fourth ventricle in the medial vestibular nucleus. A presumably homologous region has been described in rodents. The calretinin-immunoreactive cells in this region were also immunoreactive for choline acetyltransferase. Evidence from other studies suggests that the calretinin region contributes to pathways involved in eye movement modulation but not generation. There were focal dense regions of fibers immunoreactive to calbindin in the medial and inferior nuclei, with an especially dense region of label at the border of the medial nucleus and the nucleus prepositus hypoglossi. There is anatomical evidence that suggests that the likely source of these calbindin-immunoreactive fibers is the flocculus of the cerebellum. The distribution of calbindin-immunoreactive fibers in the lateral and superior nuclei was much more uniform. Immunoreactivity to parvalbumin was widespread in fibers distributed throughout the VNC. The results suggest that neurochemical techniques may help to reveal the internal complexity in VNC organization.

Nitric oxide increases the spontaneous firing rate of rat medial vestibular nucleus neurons in vitro via a cyclic GMP-mediated PKG-independent mechanism

The effects of nitric oxide (NO) on the discharge rate of medial vestibular nucleus neurons (MVNn) were investigated in rat brainstem slices. The NO-donor sodium nitroprusside (SNP, 200 microM) caused a marked enhancement (+36.7%) of MVNn spontaneous firing rate, which was prevented by the NO-scavenger, carboxy-PTIO (300 microM). The SNP effects were not modified (+37.4%) by synaptic uncoupling, suggesting that NO influences intrinsic membrane properties of MVNn rather than the synaptic input they receive. The excitatory action of SNP was virtually abolished by slice pretreatment with the soluble guanylyl cyclase inhibitor, ODQ (10 microM), and it was mimicked (+33.1%) by the cGMP analogue 8-Br-cGMP (400 microM). Protein kinase G (PKG) and cAMP/protein kinase A (PKA) were both excluded as downstream effectors of the NO/cGMP-induced excitation. However, the cyclic nucleotide-gated (CNG) channel blockers, L-cis-diltiazem (LCD, 100 microM) and Sp-8-Br-PET-cGMPS (100 microM), significantly reduced the firing rate increase produced by 8-Br-cGMP. Moreover, LCD alone decreased spontaneous MVNn firing (-19.7%), suggesting that putative CNG channels may contribute to the tonic control of resting MVNn discharge. 8-Br-cAMP (1 mM) also elicited excitatory effects in MVNn (+40.8%), which occluded those induced by 8-Br-cGMP, indicating that the two nucleotides share a common target. Finally, nested-polymerase chain reaction assay revealed the expression of CNG channel alpha subunit transcript in MVNn. Our data provide the first demonstration that NO/cGMP signalling modulates MVNn spontaneous firing through a mechanism that is independent of PKG or PKA and probably involves activation of CNG channels.

Nitric oxide facilitates GABAergic neurotransmission in the cat oculomotor system: a physiological mechanism in eye movement control

Nitric oxide (NO) synthesis by prepositus hypoglossi (PH) neurons is necessary for the normal performance of horizontal eye movements. We have previously shown that unilateral injections of NO synthase (NOS) inhibitors into the PH nucleus of alert cats produce velocity imbalance without alteration of the eye position control, both during spontaneous eye movements and the vestibulo-ocular reflex (VOR). This NO effect is exerted on the dorsal PH neuropil, whose fibres increase their cGMP content when stimulated by NO. In an attempt to determine whether NO acts by modulation of a specific neurotransmission system, we have now compared the oculomotor effects of NOS inhibition with those produced by local blockade of glutamatergic, GABAergic or glycinergic receptors in the PH nucleus of alert cats. Both glutamatergic antagonists used, 2-amino-5-phosphonovaleric acid (APV) and 2,3-dihydro-6-nitro-7-sulphamoyl-benzo quinoxaline (NBQX), induced a nystagmus contralateral to that observed upon NOS inhibition, and caused exponential eye position drift. In contrast, bicuculline and strychnine induced eye velocity alterations similar to those produced by NOS inhibitors, suggesting that NO oculomotor effects were due to facilitation of some inhibitory input to the PH nucleus. To investigate the anatomical location of the putative NO target neurons, the retrograde tracer Fast Blue was injected in one PH nucleus, and the brainstem sections containing Fast Blue-positive neurons were stained with double immunohistochemistry for NO-sensitive cGMP and glutamic acid decarboxylase. GABAergic neurons projecting to the PH nucleus and containing NO-sensitive cGMP were found almost exclusively in the ipsilateral medial vestibular nucleus and marginal zone. The results suggest that the nitrergic PH neurons control their own firing rate by a NO-mediated facilitation of GABAergic afferents from the ipsilateral medial vestibular nucleus. This self-control mechanism could play an important role in the maintenance of the vestibular balance necessary to generate a stable and adequate eye position signal.

The neuroanatomical basis of oculomotor disorders: the dual motor control of extraocular muscles and its possible role in proprioception

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.