Central Control of Postural Orientation in Flatfish

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.

Otolith regularities

The masses and the area sizes of the otoliths for the utriculus, sacculus and lagena of 15 species of the Black Sea fish are analyzed. Morphometrical otolith regularities are derived and their functional and ecomorphological explanations are suggested. The otolith regularities are summarized in four otolith rules: (1) the masses of the otoliths gradually increase with the fish growth. (2) The mass ratio of the sacculus and utriculus or the sacculus and lagena otoliths does not change with the fish growth. (3) The ratio between the otolith area s and the otolith mass m is described by the exponential equation s=alpham(2/3). (4) The ratio between the otolith and macula sizes does not change with fish growth. Mathematical modeling of the otolith displacement responses to the acoustic and the instant force stimuli is performed. Based on the modeling the functional and ecomorphological explanations of the otolith regularities are suggested: (1) the greater the otolith mass, the higher the acoustic sensitivity at low frequencies and the sharper the frequency-response curve at its maximum. (2) The separation between maxima of the frequency-response curves for the saccular and lagenar otoliths remains virtually constant with the fish growth. (3) The bottom and littoral fish have better auditory capabilities than the pelagic fish. (4) The sensitivity to vestibular stimuli for greater otoliths is higher but the response is slower. The corresponding acceleration resolution for greater otoliths is higher and the range of accelerations in which the otolith organ can operate is narrower. (5) The relative vestibular sensitivities of the utriculus, sacculus and lagena otolith organs remain constant with fish growth. (6) The otolith organs of the bottom and littoral fish are tuned to different accelerations and possess different functional properties. The otolith organs of pelagic fish are adapted to a limited range of accelerations and are less sensitive to low accelerations as compared to the bottom and littoral fish.

Directional response properties of saccular afferents of the toadfish, Opsanus tau

The displacement sensitivity, frequency response, and directional response properties of primary saccular afferents of toadfish (Opsanus tau) were studied in response to a simulation of acoustic particle motion for which displacement magnitudes and directions were manipulated in azimuth and elevation. Stimuli were 50, 100, and 200 Hz sinusoidal, translatory oscillations of the animal at various axes in the horizontal and midsagittal planes. Thresholds in these planes defined a cell's characteristic axis (the axis having the lowest threshold) in spherical coordinates. Recordings were made from afferents in rostral, middle, and caudal bundles of the saccular nerve. The most sensitive saccular afferents responded with a phase-locked response to displacements as small as 0.1 nm. This sensitivity rivals that of the mammalian cochlea and is probably common to the sacculi and other otolith organs of most fishes. Most afferents showed lower thresholds at 100 Hz than at 50 or 200 Hz. Eighty percent of afferents have three-dimensional directional properties that would be expected if they innervated a group of hair cells having the same directional orientation on the saccular epithelium. Of the afferents that are not perfectly directional, most appear to innervate just two groups of hair cells having different orientations. The directional characteristics of afferents are qualitatively correlated with anatomically defined patterns of hair cell orientation on the saccule. In general, azimuths of best sensitivity tend to lie parallel to the plane of the otolith and sensory epithelium. Elevations of best sensitivity correspond well with hair cell orientation patterns in different regions of the saccular epithelium. Directional hearing in the horizontal plane probably depends upon the processing of interaural differences in overall response magnitude. These response differences arise from the gross orientations of the sacculi and are represented, in part, as time differences among nonspontaneous afferents that show level-dependent phase angles of synchronization. Directional hearing in the vertical plane may be derived from the processing of across-afferent profiles of activity within each saccule. Fishes were probably the first vertebrates to solve problems in sound source localization, and we suggest that their solutions formed a model for those of their terrestrial inheritors.

Organization and connections of octaval and lateral line centers in the medulla of a clupeid, Dorosoma cepedianum

In the clupeid fishes, the functionally specialized utricle and cephalic lateral line respond to sound pressure by virtue of their mechanical coupling to the auditory bullae. The cytoarchitecture of, and primary inputs to, the octavolateralis area were studied in the gizzard shad, Dorosoma cepedianum, in order to determine whether first-order acoustic and lateral line areas of the medulla are likewise specialized. The octavolateralis area of Dorosoma is composed of the nuclei that have been observed in other teleosts: nucleus medialis, the descending and anterior octaval nuclei, nucleus magnocellularis, nucleus tangentialis, and a caudal granular-cell region that likely represents nucleus caudalis and the posterior octaval nucleus. The descending octaval nucleus can be divided into dorsomedial, intermediate, and ventral zones using cytoarchitectonic criteria, whereas the anterior octaval nucleus can be divided into caudal, rostral, and medial portions. Primary inputs to the octavolateralis area were determined by means of in vitro application of horseradish peroxidase to nerves from the otolithic endorgans of the inner ear and the lateral line neuromasts. These primary inputs are generally organized like those of other teleosts: the otolithic endorgans supply the posterior, descending, magnocellular, and anterior nuclei, whereas the majority of lateral line fibers project to nucleus medialis, nucleus caudalis, and to the magnocellular nucleus. However, other characteristics of these projections may be unique to clupeids. The medial subdivision of the dorsomedial zone of the descending nucleus is dominated by a bilateral projection from at least a portion of the utricle, while the lateral subdivison of the dorsomedial zone is supplied by the saccule and lagena. This pattern is not present in non-clupeid fishes; in many species, the saccule has the most dorsomedial projection zone within the descending nucleus. In Dorosoma, both lateral line nerves contribute a light, bilateral projection to the medial and lateral subdivisions of the dorsomedial zone. The apparently specialized, bilateral utricular and lateral line inputs to the dorsomedial zone of the descending nucleus may be related to the specialized sensitivity of the utricle and the cephalic lateral line to sound pressure. A prominent group of neurons, tentatively identified as a secondary octaval population, is also described. Like the secondary octaval population of otophysans, the presumed secondary octaval population of Dorosoma is composed of a dorsal, fusiform region, an intermediate spherical cell region, and a ventral fusiform cell region.

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.

Retention of generalized hair cell patterns in the inner ear of the primitive flatfish Psettodes

Flatfish are a group of uniquely asymmetrical vertebrates, lying always on one side. This postural control depends on the vestibular receptors of the inner ear. From the most primitive living flatfish, orientations of sensory hair cells in the inner ear were mapped by scanning electron microscopy. The maps of the three otolith organs, the three semicircular cristae, and the macula neglecta (newly discovered here for flatfish) show patterns that are very similar to those in many upright teleosts, particularly perches. Thus, peripheral sensory structure does not require modification for the unusual postural control of flatfish.

Organization of eighth nerve afferent projections from individual endorgans of the inner ear in the teleost, Astronotus ocellatus

Eighth nerve fibers from the saccule, utricle, lagena, and the anterior, horizontal, and posterior semicircular canals of a cichlid fish were traced to the octavolateralis region of the brainstem using HRP and degeneration methods. The anterior, magnocellular, descending, and posterior nuclei of the octavus column receive inputs from all endorgans, whereas the tangential nucleus receives projections only from the utricle and semicircular canals. The most rostral projections only from the utricle and semicircular canals. The most rostral projection from each endorgan is found in the eminentia granularis of the vestibulolateral lobe of the cerebellum. Sparse terminals are found in the medial reticular formation from the utricle adn semicircular canals, and utricular and saccular remain terminate in the vicinity of the lateral dendrite of the Mauthner cell. Utricular and semicircular canal projections consistently overlap centrally as do saccular and lagenar inputs. Afferent fibers from all endorgans end within relatively distinct regions throughout the octavus column of nuclei. Saccular and lagenar inputs lie dorsal to the semicircular canal terminations. Utricular endings are complex, however, in that they overlap dorsally with saccular and lagenar terminals and ventrally with the semicircular canal inputs. Cerebellar inputs are found only in the eminentia granularis of the vestibulolateral lobe, and the densest terminals are from the utricle and the semicircular canals; the sparsest are from the saccule. Previous studies in fish have indicated that generally the utricle and semicircular canals are concerned with he maintenance of static and dynamic equilibrium whereas the saccule and lagena are concerned with auditory reception. There is recent evidence, however, for multiple functions within individual endorgans. Our anatomical findings suggest that in Astronotus each otolithic endorgan carries more than one modality; that the semicircular canals are concerned solely with an equilibrium function; and that acoustic information is processed dorsally and vestibular information ventrally along the octavus column of nuclei. No single nucleus appears to be solely auditory in function and only the tangential nucleus, situated ventrally in the octavus column, appears to be solely vestibular.

Adaptive changes of the vestibulo-ocular reflex in flatfish are achieved by reorganization of central nervous pathways

Flatfish provide a natural model for the study of adaptive changes in the vestibulo-ocular reflex system. During metamorphosis their vestibular and oculomotor coordinate systems undergo a 90 degree relative displacement. 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 nervous connections in the flatfish with horseradish peroxidase revealed that in postmetamorphic fish secondary horizontal semicircular canal neurons contact vertical eye muscle motoneuron pools on both sides of the brain via pathways that are absent in all other vertebrates studied.

Retinotectal projection of the adult winter flounder (Pseudopleuronectes americanus)

The winter flounder shifts the orinetation of its body 90 degrees at metamorphosis so that its left side is functionally ventral and its right side functionally dorsal. Concomitantly the left eye migrates onto the right side. The net result of these complex metamorphic changes is that the dorsoventral axes of the visual fields are perpendicular to the body rather than parallel as in most other teleosts. The developing flatfish may provide a resource for studying the formation of neural connections, for the change in orientation may necessitate some shift in connections in visuomotor pathways. As a baseline for developmental studies, we have established the retinotectal projection in adult winter flounder by means of anatomical tracing techniques (autoradiography and degeneration staining) and electrophysiological mapping techniques. The histological pattern of retinal afferents to the tectum is similar to that of other teleosts; afferents are confined to the superficial white and gray zone, with a few fibers coursing in the deep white zone. Electrophysiological mapping shows that the visuotectal projection is complete over the entire extent of the tectum, symmetrical for right and left fields and patterned normally.