The vestibuloocular reflex of the adult flatfish. II. Vestibulooculomotor connectivity

GABA-immunoreactive neurons in the Central Nervous System of the viviparous teleost Poecilia sphenops

Gamma-aminobutyric acid (GABA) functions as the primary inhibitory neurotransmitter within the central nervous system (CNS) of vertebrates. In this study, we examined the distribution pattern of GABA-immunoreactive (GABA-ir) cells and fibres in the CNS of the viviparous teleost Poecilia sphenops using immunofluorescence method. GABA immunoreactivity was seen in the glomerular, mitral, and granular layers of the olfactory bulbs, as well as in most parts of the dorsal and ventral telencephalon. The preoptic area consisted of a small cluster of GABA-ir cells, whereas extensively labelled GABA-ir neurons were observed in the hypothalamic areas, including the paraventricular organ, tuberal hypothalamus, nucleus recessus lateralis, nucleus recessus posterioris, and inferior lobes. In the thalamus, GABA-positive neurons were only found in the ventral thalamic and central posterior thalamic nuclei, whereas the dorsal part of the nucleus pretectalis periventricularis consisted of a few GABA-ir cells. GABA-immunoreactivity was extensively seen in the alar and basal subdivisions of the midbrain, whereas in the rhombencephalon, GABA-ir cells and fibres were found in the cerebellum, motor nucleus of glossopharyngeal and vagal nerves, nucleus commissuralis of Cajal, and reticular formation. In the spinal cord, GABA-ir cells and fibres were observed in the dorsal horn, ventral horn, and around the central canal. Overall, the extensive distribution of GABA-ir cells and fibres throughout the CNS suggests several roles for GABA, including the neuroendocrine, viscerosensory, and somatosensory functions, for the first time in a viviparous teleost.

Vestibular blueprint in early vertebrates

Central vestibular neurons form identifiable subgroups within the boundaries of classically outlined octavolateral nuclei in primitive vertebrates that are distinct from those processing lateral line, electrosensory, and auditory signals. Each vestibular subgroup exhibits a particular morpho-physiological property that receives origin-specific sensory inputs from semicircular canal and otolith organs. Behaviorally characterized phenotypes send discrete axonal projections to extraocular, spinal, and cerebellar targets including other ipsi- and contralateral vestibular nuclei. The anatomical locations of vestibuloocular and vestibulospinal neurons correlate with genetically defined hindbrain compartments that are well conserved throughout vertebrate evolution though some variability exists in fossil and extant vertebrate species. The different vestibular subgroups exhibit a robust sensorimotor signal processing complemented with a high degree of vestibular and visual adaptive plasticity.

Thyroid hormone mediates otolith growth and development during flatfish metamorphosis

Flatfish begin life as bilaterally symmetrical larvae that swim up-right, then abruptly metamorphose into asymmetrically shaped juveniles with lateralized swimming postures. Flatfish metamorphosis is mediated entirely by thyroid hormone (TH). Changes in flatfish swim posture are thought to be regulated via vestibular remodeling, although the influence of TH on teleost inner ear development remains unclear. This study addresses the role of TH on the development of the three otolith end-organs (sacculus, utricle, and lagena) during southern flounder (Paralichthys lethostigma) metamorphosis. Compared with pre-metamorphosis, growth rates of the sacculus and utricle otoliths increase dramatically during metamorphosis in a manner that is uncoupled from general somatic growth. Treatment of P. lethostigma larvae with methimazol (a pharmacological inhibitor of endogenous TH production) inhibits growth of the sacculus and utricle, whereas treatment with TH dramatically accelerates their growth. In contrast with the sacculus and utricle otoliths that begin to form and mineralize during embryogenesis, a non-mineralized lagena otolith is first visible 10-12 days after hatching. The lagena grows during pre- and pro-metamorphosis, then abruptly mineralizes during metamorphic climax. Mineralization of the lagena, but not growth, can be induced with TH treatment, whereas treatment with methimazol completely inhibits lagena mineralization without inhibiting its growth. These findings suggest that during southern flounder metamorphosis TH exerts differential effects on growth and development among the three types of otolith.

Twisted story of eye migration in flatfish

Early molecular markers for flatfish metamorphosis and eye migration must be linked to the ethmoid region, the earliest part of the flatfish cranium to change, as well as chondral and dermal ossification processes. Serial sections, morphological landmarks, and stereology were used to determine where and when the remodeling of tissues and asymmetry occurs in the head region of metamorphosing Atlantic halibut, Hippoglossus hippoglossus. Not all parts of the head remodel or migrate, and those that do may be asynchronous. Normal metamorphosis limits the torsion of the Atlantic halibut head to the anterior part of the neurocranium and excludes the tip of the snout and the general jaw area. The first cranial structure displaying eye migration-related asymmetric development is the paraethmoid part of the ethmoid cartilage. In early eye migration the medial frontal process moves apace with the eyes, whereas near completion the migrating eye moves significantly closer to the frontal process. Structures of the jaw remain mostly symmetrical, with the exception of the adductor mandibulae muscle and the bone maxillare, which are larger on the abocular than on the ocular side, the muscle occupying the space vacated by the migration of the eye. Thus, normal eye migration involves a series of temperospatially linked events. In juveniles lacking eye migration (arrested metamorphosis), the dermal bone, the prefrontal, does not develop. The two abnormal paraethmoids develop symmetrically as two plate-like structures curving anteriorly, whereas normal elongate fused paraethmoids curve at their posterior. The abocular side retrorbital vesicles are largest in volume only after the completion of normal eye migration. Factors involved in completion of normal metamorphosis and eye migration in flatfish affect chondral and dermal ossification signals in the ethmoid group, as well as remodeling of the mineralized frontal, a series of linked events not involving the entire neurocranium.

The anatomy of the vestibular nuclei

The vestibular portion of the eighth cranial nerve informs the brain about the linear and angular movements of the head in space and the position of the head with respect to gravity. The termination sites of these eighth nerve afferents define the territory of the vestibular nuclei in the brainstem. (There is also a subset of afferents that project directly to the cerebellum.) This chapter reviews the anatomical organization of the vestibular nuclei, and the anatomy of the pathways from the nuclei to various target areas in the brain. The cytoarchitectonics of the vestibular brainstem are discussed, since these features have been used to distinguish the individual nuclei. The neurochemical phenotype of vestibular neurons and pathways are also summarized because the chemical anatomy of the system contributes to its signal-processing capabilities. Similarly, the morphologic features of short-axon local circuit neurons and long-axon cells with extrinsic projections are described in detail, since these structural attributes of the neurons are critical to their functional potential. Finally, the composition and hodology of the afferent and efferent pathways of the vestibular nuclei are discussed. In sum, this chapter reviews the morphology, chemoanatomy, connectivity, and synaptology of the vestibular nuclei.

Intrinsic membrane properties of vertebrate vestibular neurons: function, development and plasticity

Central vestibular neurons play an important role in the processing of body motion-related multisensory signals and their transformation into motor commands for gaze and posture control. Over recent years, medial vestibular nucleus (MVN) neurons and to a lesser extent other vestibular neurons have been extensively studied in vivo and in vitro, in a range of species. These studies have begun to reveal how their intrinsic electrophysiological properties may relate to their response patterns, discharge dynamics and computational capabilities. In vitro studies indicate that MVN neurons are of two major subtypes (A and B), which differ in their spike shape and after-hyperpolarizations. This reflects differences in particular K(+) conductances present in the two subtypes, which also affect their response dynamics with type A cells having relatively low-frequency dynamics (resembling "tonic" MVN cells in vivo) and type B cells having relatively high-frequency dynamics (resembling "kinetic" cells in vivo). The presence of more than one functional subtype of vestibular neuron seems to be a ubiquitous feature since vestibular neurons in the chick and frog also subdivide into populations with different, analogous electrophysiological properties. The ratio of type A to type B neurons appears to be plastic, and may be determined by the signal processing requirements of the vestibular system, which are species-variant. The membrane properties and discharge pattern of type A and type B MVN neurons develop largely post-natally, through the expression of the underlying ion channel conductances. The membrane properties of MVN neurons show rapid and long-lasting plastic changes after deafferentation (unilateral labyrinthectomy), which may serve to maintain their level of activity and excitability after the loss of afferent inputs.

Vestibuloocular reflex of the adult flatfish. III. A species-specific reciprocal pattern of excitation and inhibition

In juvenile flatfish the vestibuloocular reflex (VOR) circuitry that underlies compensatory eye movements adapts to a 90 degrees relative displacement of vestibular and oculomotor reference frames during metamorphosis. VOR pathways are rearranged to allow horizontal canal-activated second-order vestibular neurons in adult flatfish to control extraocular motoneurons innervating vertical eye muscles. This study describes the anatomy and physiology of identified flatfish-specific excitatory and inhibitory vestibular pathways. In antidromically identified oculomotor and trochlear motoneurons, excitatory postsynaptic potentials (EPSPs) were elicited after electrical stimulation of the horizontal canal nerve expected to provide excitatory input. Electrotonic depolarizations (0.8-0.9 ms) preceded small amplitude (<0.5 mV) chemical EPSPs at 1.2-1.6 ms with much larger EPSPs (>1 mV) recorded around 2.5 ms. Stimulation of the opposite horizontal canal nerve produced inhibitory postsynaptic potentials (IPSPs) at a disynaptic latency of 1.6-1.8 ms that were depolarizing at membrane resting potentials around -60 mV. Injection of chloride ions increased IPSP amplitude, and current-clamp analysis showed the IPSP equilibrium potential to be near the membrane resting potential. Repeated electrical stimulation of either the excitatory or inhibitory horizontal canal vestibular nerve greatly increased the amplitude of the respective synaptic responses. These observations suggest that the large terminal arborizations of each VOR neuron imposes an electrotonic load requiring multiple action potentials to maximize synaptic efficacy. GABA antibodies labeled axons in the medial longitudinal fasciculus (MLF) some of which were hypothesized to originate from horizontal canal-activated inhibitory vestibular neurons. GABAergic terminal arborizations were distributed largely on the somata and proximal dendrites of oculomotor and trochlear motoneurons. These findings suggest that the species-specific horizontal canal inhibitory pathway exhibits similar electrophysiological and synaptic transmitter profiles as the anterior and posterior canal inhibitory projections to oculomotor and trochlear motoneurons. Electron microscopy showed axosomatic and axodendritic synaptic endings containing spheroidal synaptic vesicles to establish chemical excitatory synaptic contacts characterized by asymmetrical pre/postsynaptic membrane specializations as well as gap junctional contacts consistent with electrotonic coupling. Another type of axosomatic synaptic ending contained pleiomorphic synaptic vesicles forming chemical, presumed inhibitory, synaptic contacts on motoneurons that never included gap junctions. Altogether these data provide electrophysiological, immunohistochemical, and ultrastructural evidence for reciprocal excitatory/inhibitory organization of the novel vestibulooculomotor projections in adult flatfish. The appearance of unique second-order vestibular neurons linking the horizontal canal to vertical oculomotor neurons suggests that reciprocal excitation and inhibition are a fundamental, developmentally linked trait of compensatory eye movement circuits in vertebrates.

Secondary vestibulo-oculomotor projections in larval sea lamprey: anterior octavomotor nucleus

The ventral octavolateral area of lampreys contains three nuclei: the anterior, intermediate and posterior octavomotor nuclei, formed of large neurons that are contacted by thick primary vestibular fibres. We used horseradish peroxidase (HRP) or fluorescein-dextran-amine (FDA) labelling to study the projections of the anterior octavomotor nucleus (AON) in the larval sea lamprey, Petromyzon marinus. The tracers were injected either in the AON, the oculomotor nucleus or the rostralmost spinal cord. HRP injection in the AON labelled thick axons that coursed to the basal mesencephalic tegmentum, where most decussate and project to the oculomotor nucleus and the third Müller cell. Electron microscopy confirmed that AON axons contact with the contralateral third Müller cell and with oculomotor neurons. Some AON axons run in the mesencephalic tegmentum and the ventral diencephalon. An AON axon was observed to run close to the axon of the contralateral third Müller cell, establishing what appeared to be en passant contacts. HRP injection in the AON also revealed commissural fibres projecting to the contralateral octavolateral area. HRP or FDA injections in the oculomotor nucleus labelled both large and small neurons of the AON, mostly contralateral to the injection site, as well as of cells in the intermediate octavomotor nucleus, mainly ipsilateral. HRP injection in the AON or in the rostral spinal cord did not reveal any projections from the AON to the spinal cord. Our results indicate that the pattern of octavo-oculomotor connections in the lamprey is different from that observed in other vertebrates.

Pre- and postmetamorphic organization of the vestibular nuclear complex in the turbot examined by retrograde tracer substances

During metamorphosis of flatfish larvae, eye migration leads to a 90 degrees misalignment of the visual and vestibular frames of reference. In order to maintain vestibular eye stabilization, the vestibulo-ocular (V-O) pathways have to be radically reorganized. Here, we have examined the vestibular projections in turbot larvae and juveniles by means of conventional neurohistological techniques using horseradish peroxidase and fluorescent dextranamines as tracers. We have found that the vestibular projections to the rostral eye motor nuclei consist of five densely clustered groups of neurons projecting to the rostral eye motor nuclei, some through the ipsilateral, others through the contralateral medial longitudinal fascicle (MLF). In addition, there are three groups of vestibulo-spinal neurons. The most prominent of these gives rise to the ipsilateral vestibulo-spinal tract. The other two project contralaterally, one descending in the MLF, the other more laterally in the anterior funiculus of the spinal cord. These subnuclei of the vestibular complex are easily identifiable in larvae before metamorphosis, as well as in juvenile turbots. The number of projection neurons in each of the subnuclei is approximately doubled over the period of metamorphosis. Applying different tracers to rostrally and caudally projecting pathways, we found no double-labeled neurons, indicating that the V-O and vestibulo-spinal groups are distinct entities. However, by applying the two tracers ipsi- and contralaterally in the terminal fields in the rostral eye motor nuclei after metamorphosis, we found many double-labeled neurons in all the V-O subgroups. In contrast, we found only a small fraction of double-labeled vestibular neurons when the same strategy was applied to larval preparations. We conclude that 1) the basic organization of the vestibular nuclei of the turbot is similar to that of other teleosts, in larvae as well as juveniles; 2) there is a substantial increase in projection neurons over the period of metamorphosis in all the subgroups of the vestibular nuclear complex; and 3) many more of the V-O neurons project bilaterally to the rostral eye motor nuclei in juvenile than in larval turbots.

Afferents to the oculomotor nucleus in the goldfish (Carassius auratus) as revealed by retrograde labeling with horseradish peroxidase

The goal of this work was to compare the distribution and morphology of neurons projecting to the oculomotor nucleus in goldfish with those previously described in other vertebrate groups. Afferent neurons were revealed by retrograde labeling with horseradish peroxidase. The tracer was electrophoretically injected into the oculomotor nucleus. The location of the injection site was determined by the antidromic field potential elicited in the oculomotor nucleus by electrical stimulation of the oculomotor nerve. Labeled axons whose trajectories could be reconstructed were restricted to the medial longitudinal fasciculus. In order of quantitative importance, the afferent areas to the oculomotor nucleus were: (1) the ipsilateral anterior nucleus and the contralateral tangential and descending nuclei of the octaval column. Furthermore, a few labeled cells were found dorsomedially to the caudal pole of the unlabeled anterior octaval nucleus; (2) the contralateral abducens nucleus. The labeled internuclear neurons were arranged in two groups within and 500 microns behind the caudal subdivision of the abducens nucleus; (3) a few labeled cells were observed in the rhombencephalic reticular formation near the abducens nucleus, most of which were contralateral to the injection site. Specifically, stained cells were found in the caudal pole of the superior reticular nucleus, throughout the medial reticular nucleus and in the rostral area of the inferior reticular nucleus; (4) eurydendroid cells of the cerebellum, located close to the contralateral eminentia granularis pars lateralis, were also labeled; and (5) a small and primarily ipsilateral group of labeled cells was located at the mesencephalic nucleus of the medial longitudinal fasciculus. The similarity in the structures projecting to the oculomotor nucleus in goldfish to those in other vertebrates suggests that the neural network involved in the oculomotor system is quite conservative throughout phylogeny. Nevertheless, in goldfish these projections appeared with some specific peculiarities, such as the cerebellar and mesencephalic afferents to the oculomotor nucleus.

Role of the visual input in recovery of function following unilateral vestibular lesion in the goldfish. I. Short-term behavioural changes

Vestibular compensation, i.e. the partial or complete recovery of function following lesion of a labyrinth, has been suggested to be based on a multisensory substitution process involving vestibular, visual and somatosensory information. Teleost fish would seem ideal subjects for testing this proposal, because equilibrium orientation in the intact animal is largely under the bimodal control of the vestibular and visual systems. The role of the visual input in the compensation of behavioural deficits elicited by hemilabyrinthectomy (HL) was studied in goldfish maintained over the first two postoperative hours under different lighting conditions. HL caused severe postural and locomotory symptoms, which were the same or similar under all lighting conditions. The rate at which the various deficits disappeared, however, was dependent not only on the presence of light but also on its direction of incidence. Animals maintained under overhead illumination reached the criterion of successful compensation (completion of 5-min continuous and unimpaired swimming) within 10 min, whilst under unilateral illumination (90 degrees to the vertical) the time to criterion was significantly increased; animals exposed to illumination from below or infra-red illumination showed little or no signs of compensation up to 2 h after HL. It is concluded (1) that the immediately postoperative stage of vestibular compensation in the goldfish represents an integral part of the recovery process, within which all observable deficits can be compensated with remarkable rapidity; and (2) that the visual input is both necessary and sufficient for the compensation of the immediately postoperative deficits. We term this early stage of recovery the 'acute' phase and consider it to be based on a visual substitution process, whereby the missing labyrinthine input to the (partially) deafferented vestibular neurons is functionally replaced by visual afferents.

Neuronal adaptation accompanying metamorphosis in the flatfish

Flatfish provide a natural paradigm to investigate adaptive changes in the central nervous system of vertebrates. During their metamorphosis, the animals undergo a 90 degrees tilt to one side or the other to become the bottom-adapted adult flatfish. The eye on the down side is pushed over to the up side. Thus, vestibular and oculomotor coordinate systems rotate 90 degrees relative to each other. As a result, during swimming movements different types of compensatory eye movements are produced before and after metamorphosis by the same vestibular stimulation. Intracellular staining of central neurons with horseradish peroxidase revealed that in postmetamorphic flatfish second-order horizontal canal neurons contact vertical eye muscle motoneuron pools on both sides of the brain via pathways that are absent in all other vertebrates studied. These unique connections provide the necessary and sufficient connectivity to adapt the flatfish's eye movement system to the animals' postmetamorphic existence. Although the adult fish has a bilaterally asymmetric appearance, the central nervous connectivity reestablishes symmetry in the vestibulo-oculomotor system.

Vestibulo-ocular projections in the 11-day chicken embryo: pathway specificity

The organization of vestibulo-ocular projections and their spatial relationship to vestibulo-spinal projections were ascertained in the 11-day chicken embryo through retrograde tracing experiments. An in vitro preparation of the brainstem facilitated precisely localized application of tracers. Unilateral labelling of the ascending medial longitudinal fascicle (MLF), just caudal to the trochlear nucleus, labelled coherent groups of vestibular neurons that had an asymmetrical distribution on the two sides of the brainstem. Axons originating from either side followed one of four trajectories, and the groups of neurons could therefore be identified on the basis of their position and their projection pathway. Labelling each side of the MLF with a different tracer showed that although neurons projecting in the ipsi- and contralateral MLF were intermingled in some areas, individual neurons projected on one side only. Labelling the ascending MLF and the high cervical spinal cord with different tracers showed that vestibulo-ocular neurons had a wider rostro-caudal distribution than vestibulospinal neurons. Some vestibulospinal and vestibulo-ocular groups were spatially segregated, others were intermingled. Very few (in most preparations no) neurons were found to project to both targets. Together with our previous study on vestibulospinal projections in the 11-day chicken embryo (Glover and Pétursdóttir, J. Comp. Neurol. 270:25-38, '88) the results show that vestibular neurons in different regions project in characteristic subsets of the available pathways.

Properties and localization of the anterior semicircular canal-activated vestibulocollic neurons in the cat

Unit activities of secondary vestibular neurons that selectively responded to stimulation of the anterior semicircular canal nerve (ACN) were recorded extracellularly in the anesthetized cat. Axonal pathways and projections in the spinal cord of the ACN-activated neurons were examined by recording their antidromic responses to stimulation of the lateral and medial vestibulospinal tracts (LVST and MVST), and the bilateral neck extensor motoneuron pools in the C1 segment (C1 dorsal rami [DR] motoneuron pools). In order to determine whether the neurons had ascending axon collaterals to the extraocular motoneurons, the contralateral (c-) inferior oblique (IO) motoneuron pool was also stimulated. Twenty-seven neurons sent their axons to the ipsilateral (i-) C1DR motoneuron pool via the LVST without any projection to the extraocular motoneuron pool. All the cells except one were located in the ventral part of the lateral vestibular nucleus. This pathway produced monosynaptic EPSPs with short time-to-peak and short half-width in C1DR motoneurons (16/16 motoneurons). Eight neurons sent axons to the i-C1DR motoneuron pool via the MVST without any to the extraocular motoneuron pool. Cell somata were located in the descending nucleus or in the ventral part of the lateral nucleus. These neurons did not produce postsynaptic potentials (PSPs) in any C1DR motoneurons. All thirty-five neurons sending axons to the c-C1DR motoneuron pool have ascending axon collaterals to the c-IO motoneuron pool.

Descending projection neurons to the spinal cord of the goldfish, Carassius auratus

The sources of descending spinal tracts in the goldfish, Carassius auratus, were visualized by retrograde transport of horseradish peroxidase (HRP) administered to the hemisected spinal cord. In the diencephalon, HRP-positive neurons were identified in the nucleus preopticus magnocellularis pars magnocellularis and ventromedial nucleus of the thalamus of the ipsilateral side. In the mesencephalic tegmentum, a few somata of the contralateral nucleus ruber and several ipsilateral neurons of the nucleus of the median longitudinal fasciculus were labeled. The reticular formation of the rhombencephalon was the major source of descending afferents to the spinal cord. A larger number of neurons were retrogradely labeled in the ipsilateral superior, middle, and inferior nuclei than in the contralateral nuclei. A few raphe neurons and the contralateral Mauthner neuron were also HRP-positive. The octaval area showed retrogradely labeled neurons in the anterior, magnocellular, descending, and posterior octaval nuclei of the ipsilateral side. A large number of neurons in the facial lobe and a few somata located adjacent to the descending trigeminal tract were labeled on the ipsilateral side. The pattern of descending spinal projections in goldfish is comparable to that of tetrapods and suggests that the spinal tracts have originated quite early in the course of vertebrate evolution.