Anatomical organization of the brainstem octavolateralis area of the oyster toadfish, Opsanus tau

Serotonin-immunoreactivity in the brain of the cichlid fish Oreochromis mossambicus

Serotonin (5-HT) is an evolutionarily conserved monoaminergic neurotransmitter found in the central nervous system and peripheral nervous system across invertebrates and vertebrates. Although the distribution of 5-HT-immunoreactive (5-HT-ir) neurons is investigated in various fish species, the organization of these neurons in cichlid fishes is poorly understood. These fish are known for their adaptability to diverse environments, food habits, and complex mating and breeding behaviors, including parental care. In this paper, we describe the organization of 5-HT-ir neurons in the brain of the cichlid fish Oreochromis mossambicus. Aggregations of 5-HT-ir neurons were spotted in the granule cell layer of the olfactory bulb and near the ventricular border in the preoptic area and magnocellular subdivisions of the nucleus preopticus. Although the presence of 5-HT-ir cells and fibers in the hypothalamic and thalamic regions, cerebellum, and raphe nuclei was comparable to that of other teleosts, the current study reveals the occurrence of 5-HT-ir cells and fibers for the first time in some areas, such as the nucleus posterior tuberis, nucleus oculomotorius, and nucleus paracommissuralis in the tilapia. While the presence of 5-HT-ir cells and fibers in gustatory centers suggests a role for serotonin in the processing of gustatory signals, distinctive pattern of 5-HT immunoreactivity was seen in the telencephalon, pretectal areas, mesencephalic, and rhombencephalic regions, suggesting a cichlid fish specific organization of the serotonergic system. In conclusion, the 5-HT system in the tilapia brain may serve several neuroendocrine and neuromodulatory roles, including regulation of reproduction and sensorimotor processes.

Reproductive state modulates utricular auditory sensitivity in a vocal fish

The plainfin midshipman, Porichthys notatus, is a seasonally breeding vocal fish that relies on acoustic communication to mediate nocturnal reproductive behaviors. Reproductive females use their auditory senses to detect and localize "singing" males that produce multiharmonic advertisement (mate) calls during the breeding season. Previous work showed that the midshipman saccule, which is considered the primary end organ used for hearing in midshipman and most other fishes, exhibits reproductive state and hormone-dependent changes that enhance saccular auditory sensitivity. In contrast, the utricle was previously posited to serve primarily a vestibular function, but recent evidence in midshipman and related toadfish suggests that it may also serve an auditory function and aid in the detection of behaviorally relevant acoustic stimuli. Here, we characterized the auditory-evoked potentials recorded from utricular hair cells in reproductive and nonreproductive female midshipman in response to underwater sound to test the hypothesis that variation in reproductive state affects utricular auditory sensitivity. We show that utricular hair cells in reproductive females exhibit up to a sixfold increase in the utricular potential magnitude and have thresholds based on measures of particle acceleration (re: 1 ms-2) that are 7-10 dB lower than nonreproductive females across a broad range of frequencies, which include the dominant harmonics of male advertisement calls. This enhanced auditory sensitivity of the utricle likely plays an essential role in facilitating midshipman social and reproductive acoustic communication.NEW & NOTEWORTHY In many animals, vocal-acoustic communication is fundamental for facilitating social behaviors. For the vocal plainfin midshipman fish, the detection and localization of social acoustic signals are critical to the species' reproductive success. Here, we show that the utricle, an inner ear end organ often thought to primarily serve a vestibular function, serves an auditory function that is seasonally plastic and modulated by the animal's reproductive state effectively enhancing auditory sensitivity to courting male advertisement calls.

Response of toadfish (Opsanus tau) utricular afferents to multimodal inputs

The inner ear of teleost fishes is composed of three paired multimodal otolithic end organs (saccule, utricle, and lagena), which encode auditory and vestibular inputs via the deflection of hair cells contained within the sensory epithelia of each organ. However, it remains unclear how the multimodal otolithic end organs of the teleost inner ear simultaneously integrate vestibular and auditory inputs. Therefore, microwire electrodes were chronically implanted using a 3D printed micromanipulator into the utricular nerve of oyster toadfish (Opsanus tau) to determine how utricular afferents respond to conspecific mate vocalizations termed boatwhistles (180 Hz fundamental frequency) during movement. Utricular afferents were recorded while fish were passively moved using a sled system along an underwater track at variable speeds (velocity: 4.0 - 12.5 cm/s; acceleration: 0.2 - 2.6 cm/s²) and while fish freely swam (velocity: 3.5 - 18.6 cm/s; acceleration: 0.8 - 29.8 cm/s²). Afferent fiber activities (spikes/s) increased in response to the onset of passive and active movements; however, afferent fibers differentially adapted to sustained movements. Additionally, utricular afferent fibers remained sensitive to playbacks of conspecific male boatwhistle vocalizations during both passive and active movements. Here, we demonstrate in alert toadfish that utricular afferents exhibit enhanced activity levels (spikes/s) in response to behaviorally-relevant acoustic stimuli during swimming.

Serotonin (5-hydroxytryptamine)-immunoreactive neurons in the brain of the viviparous fish Gambusia affinis

The monoaminergic neurotransmitter serotonin (5-HT) acts as a neuromodulator and is associated with a wide range of functions in fish. In this investigation, 5-HT immunoreactivity was studied in the central nervous system (CNS) of the viviparous mosquitofish Gambusia affinis. 5-HT-immunoreactive (5-HT-ir) cells/fibres were observed throughout the subdivisions of ventral and dorsal telencephalon including the olfactory bulb. Several intensely stained 5-HT-ir cells and/or fibres were detected in different areas of the hypothalamus as well as the proximal pars distalis of the pituitary gland. 5-HT-ir cells were restricted to the dorsal and ventral part of the pretectal diencephalic cluster, but only fibres were detected in the anterior, ventromedial and posterior subdivisions of the thalamic nucleus and in the preglomerular complex. In the mesencephalon, 5-HT-ir perikarya, and fibres were seen in the optic tectum, midbrain tegmentum and torus semicircularis. A cluster of prominently labelled 5-HT-ir neurons was observed in the superior raphe nucleus, whereas numerous 5-HT-ir fibres were distributed throughout the rhombencephalic divisions. In addition, a bundle of rostrocaudally running 5-HT-ir fibres was noticed in the spinal cord. This is the first detailed neuroanatomical study in a viviparous teleost, reporting a widespread distribution of 5-HT-ir somata and fibres in the CNS. The results of this study provide new insights into the evolutionarily well conserved nature of the monoaminergic system in the CNS of vertebrates and suggest a role for 5-HT in regulation of several physiological, behavioural and neuroendocrine functions in viviparous teleosts.

Evolution of Sound Source Localization Circuits in the Nonmammalian Vertebrate Brainstem

The earliest vertebrate ears likely subserved a gravistatic function for orientation in the aquatic environment. However, in addition to detecting acceleration created by the animal's own movements, the otolithic end organs that detect linear acceleration would have responded to particle movement created by external sources. The potential to identify and localize these external sources may have been a major selection force in the evolution of the early vertebrate ear and in the processing of sound in the central nervous system. The intrinsic physiological polarization of sensory hair cells on the otolith organs confers sensitivity to the direction of stimulation, including the direction of particle motion at auditory frequencies. In extant fishes, afferents from otolithic end organs encode the axis of particle motion, which is conveyed to the dorsal regions of first-order octaval nuclei. This directional information is further enhanced by bilateral computations in the medulla and the auditory midbrain. We propose that similar direction-sensitive neurons were present in the early aquatic tetrapods and that selection for sound localization in air acted upon preexisting brain stem circuits like those in fishes. With movement onto land, the early tetrapods may have retained some sensitivity to particle motion, transduced by bone conduction, and later acquired new auditory papillae and tympanic hearing. Tympanic hearing arose in parallel within each of the major tetrapod lineages and would have led to increased sensitivity to a broader frequency range and to modification of the preexisting circuitry for sound source localization.

What the Toadfish Ear Tells the Toadfish Brain About Sound

Of the three, paired otolithic endorgans in the ear of teleost fishes, the saccule is the one most often demonstrated to have a major role in encoding frequencies of biologically relevant sounds. The toadfish saccule also encodes sound level and sound source direction in the phase-locked activity conveyed via auditory afferents to nuclei of the ipsilateral octaval column in the medulla. Although paired auditory receptors are present in teleost fishes, binaural processes were believed to be unimportant due to the speed of sound in water and the acoustic transparency of the tissues in water. In contrast, there are behavioral and anatomical data that support binaural processing in fishes. Studies in the toadfish combined anatomical tract-tracing and physiological recordings from identified sites along the ascending auditory pathway to document response characteristics at each level. Binaural computations in the medulla and midbrain sharpen the directional information provided by the saccule. Furthermore, physiological studies in the central nervous system indicated that encoding frequency, sound level, temporal pattern, and sound source direction are important components of what the toadfish ear tells the toadfish brain about sound.

Catecholaminergic connectivity to the inner ear, central auditory, and vocal motor circuitry in the plainfin midshipman fish porichthys notatus

Although the neuroanatomical distribution of catecholaminergic (CA) neurons has been well documented across all vertebrate classes, few studies have examined CA connectivity to physiologically and anatomically identified neural circuitry that controls behavior. The goal of this study was to characterize CA distribution in the brain and inner ear of the plainfin midshipman fish (Porichthys notatus) with particular emphasis on their relationship with anatomically labeled circuitry that both produces and encodes social acoustic signals in this species. Neurobiotin labeling of the main auditory end organ, the saccule, combined with tyrosine hydroxylase immunofluorescence (TH-ir) revealed a strong CA innervation of both the peripheral and central auditory system. Diencephalic TH-ir neurons in the periventricular posterior tuberculum, known to be dopaminergic, send ascending projections to the ventral telencephalon and prominent descending projections to vocal-acoustic integration sites, notably the hindbrain octavolateralis efferent nucleus, as well as onto the base of hair cells in the saccule via nerve VIII. Neurobiotin backfills of the vocal nerve in combination with TH-ir revealed CA terminals on all components of the vocal pattern generator, which appears to largely originate from local TH-ir neurons but may include input from diencephalic projections as well. This study provides strong neuroanatomical evidence that catecholamines are important modulators of both auditory and vocal circuitry and acoustic-driven social behavior in midshipman fish. This demonstration of TH-ir terminals in the main end organ of hearing in a nonmammalian vertebrate suggests a conserved and important anatomical and functional role for dopamine in normal audition.

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.

Vocal-motor and auditory connectivity of the midbrain periaqueductal gray in a teleost fish

The midbrain periaqueductal gray (PAG) plays a central role in the descending control of vocalization across vertebrates. The PAG has also been implicated in auditory-vocal integration, although its precise role in such integration remains largely unexplored. Courtship and territorial interactions in plainfin midshipman fish depend on vocal communication, and the PAG is a central component of the midshipman vocal-motor system. We made focal neurobiotin injections into the midshipman PAG to both map its auditory-vocal circuitry and allow evolutionary comparisons with tetrapod vertebrates. These injections revealed an extensive bidirectional pattern of connectivity between the PAG and known sites in both the descending vocal-motor and the ascending auditory systems, including portions of the telencephalon, dorsal thalamus, hypothalamus, posterior tuberculum, midbrain, and hindbrain. Injections in the medial PAG produced dense label within hindbrain auditory nuclei, whereas those confined to the lateral PAG preferentially labeled hypothalamic and midbrain auditory areas. Thus, the teleost PAG may have functional subdivisions playing different roles in vocal-auditory integration. Together the results confirm several pathways previously identified by injections into known auditory or vocal areas and provide strong support for the hypothesis that the teleost PAG is centrally involved in auditory-vocal integration.

Exposure to advertisement calls of reproductive competitors activates vocal-acoustic and catecholaminergic neurons in the plainfin midshipman fish, Porichthys notatus

While the neural circuitry and physiology of the auditory system is well studied among vertebrates, far less is known about how the auditory system interacts with other neural substrates to mediate behavioral responses to social acoustic signals. One species that has been the subject of intensive neuroethological investigation with regard to the production and perception of social acoustic signals is the plainfin midshipman fish, Porichthys notatus, in part because acoustic communication is essential to their reproductive behavior. Nesting male midshipman vocally court females by producing a long duration advertisement call. Females localize males by their advertisement call, spawn and deposit all their eggs in their mate’s nest. As multiple courting males establish nests in close proximity to one another, the perception of another male’s call may modulate individual calling behavior in competition for females. We tested the hypothesis that nesting males exposed to advertisement calls of other males would show elevated neural activity in auditory and vocal-acoustic brain centers as well as differential activation of catecholaminergic neurons compared to males exposed only to ambient noise. Experimental brains were then double labeled by immunofluorescence (-ir) for tyrosine hydroxylase (TH), an enzyme necessary for catecholamine synthesis, and cFos, an immediate-early gene product used as a marker for neural activation. Males exposed to other advertisement calls showed a significantly greater percentage of TH-ir cells colocalized with cFos-ir in the noradrenergic locus coeruleus and the dopaminergic periventricular posterior tuberculum, as well as increased numbers of cFos-ir neurons in several levels of the auditory and vocal-acoustic pathway. Increased activation of catecholaminergic neurons may serve to coordinate appropriate behavioral responses to male competitors. Additionally, these results implicate a role for specific catecholaminergic neuronal groups in auditory-driven social behavior in fishes, consistent with a conserved function in social acoustic behavior across vertebrates.

Does the magnocellular octaval nucleus process auditory information in the toadfish, Opsanus tau?

Previous work on auditory processing in Opsanus tau has focused on the descending octaval nucleus; however, the magnocellular octaval nucleus receives similar inputs from the otolithic endorgans. The purpose of this study was to assess whether cells in any of the three subdivisions of the magnocellular nucleus respond to auditory frequencies and encode sound source direction. Extracellular recording sites were chosen based on anatomical landmarks, and neurobiotin injections confirmed the location of auditory sites in subdivisions of the magnocellular nucleus. In general, the auditory cells in M2 and M3 responded best to frequencies at or below 100 Hz. Most auditory cells responded well to directional stimuli presented along axes in the horizontal plane. Cells in M3 (not M2) also responded to lateral line stimulation, consistent with otolithic endorgan and lateral line inputs to M3. The convergence of auditory and lateral line inputs in M3, the lack of Mauthner cells in this species, and previous evidence that the magnocellular nucleus does not contribute to ascending auditory pathways suggest to us that the large cells of M3 may play a role in rapid behavioral responses to particle motion stimuli in oyster toadfish.

The tangential nucleus controls a gravito-inertial vestibulo-ocular reflex

BACKGROUND

Although adult vertebrates sense changes in head position by using two classes of accelerometer, at larval stages zebrafish lack functional semicircular canals and rely exclusively on their otolithic organs to transduce vestibular information.

RESULTS

Despite this limitation, we find that larval zebrafish perform an effective vestibulo-ocular reflex (VOR) that serves to stabilize gaze in response to pitch and roll tilts. By using single-cell electroporations and targeted laser ablations, we identified a specific class of central vestibular neurons, located in the tangential nucleus, that are essential for the utricle-dependent VOR. Tangential nucleus neurons project contralaterally to extraocular motoneurons and in addition to multiple sites within the reticulospinal complex.

CONCLUSIONS

We propose that tangential neurons function as a broadband inertial accelerometer, processing utricular acceleration signals to control the activity of extraocular and postural neurons, thus completing a fundamental three-neuron circuit responsible for gaze stabilization.

Gamma-aminobutyric acid is a neurotransmitter in the auditory pathway of oyster toadfish, Opsanus tau

Binaural computations involving the convergence of excitatory and inhibitory inputs have been proposed to explain directional sharpening and frequency tuning documented in the brainstem of a teleost fish, the oyster toadfish (Opsanus tau). To assess the presence of inhibitory neurons in the ascending auditory circuit, we used a monoclonal antibody to GABA to evaluate immunoreactivity at three levels of the circuit: the first order descending octaval nucleus (DON), the secondary octaval population (dorsal division), and the midbrain torus semicircularis. We observed a subset of immunoreactive (IR) cells and puncta distributed throughout the neuropil at all three locations. To assess whether contralateral inhibition is present, fluorescent dextran crystals were inserted into dorsal DON to fill contralateral, commissural inputs retrogradely prior to GABA immunohistochemistry. GABA-IR somata and puncta co-occurred with retrogradely filled, GABA-negative auditory projection cells. GABA-IR projection cells were more common in the dorsolateral DON than in the dorsomedial DON, but GABA-IR puncta were common in both dorsolateral and dorsomedial divisions. Our findings demonstrate that GABA is present in the ascending auditory circuit in the brainstem of the toadfish, indicating that GABA-mediated inhibition participates in shaping auditory response characteristics in a teleost fish as in other vertebrates.

Physiological evidence for binaural directional computations in the brainstem of the oyster toadfish, Opsanus tau (L.)

Comparisons of left and right auditory input are required for sound source localization in most terrestrial vertebrates. Previous physiological and neuroanatomical studies have indicated that binaural convergence is present in the ascending auditory system of the toadfish. In this study, we introduce a new technique, otolith tipping, to reversibly alter directional auditory input to the central nervous system of a fish. The normal directional response pattern (DRP) was recorded extracellularly for auditory cells in the first-order descending octaval nucleus (DON) or the midbrain torus semicircularis (TS) using particle motion stimuli in the horizontal and mid-sagittal planes. The same stimuli were used during tipping of the saccular otolith to evaluate changes in the DRPs. Post-tipping DRPs were generated and compared with the pre-tipping DRPs to ensure that the data had been collected consistently from the same unit. In the DON, ipsilateral or contralateral tipping most often eliminated spike activity, but changes in spike rate (+/-) and DRP shape were also documented. In the TS, tipping most often caused a change in spike rate (+/-) and altered the shape or best axis of the DRP. The data indicate that there are complex interactions of excitatory and inhibitory inputs in the DON and TS resulting from the convergence of binaural inputs. As in anurans, but unlike other terrestrial vertebrates, binaural processing associated with encoding the direction of a sound source begins in the first-order auditory nucleus of this teleost.

Directional and frequency response characteristics in the descending octaval nucleus of the toadfish (Opsanus tau)

This study is a continuation of a long-term investigation of the auditory circuit in the oyster toadfish, Opsanus tau. Input from the auditory periphery projects to the ipsilateral descending octaval nucleus (DON). Ipsilateral and contralateral DONs project to the auditory midbrain, where a previous study indicated that both frequency tuning and directional sharpening are present. To better understand the transformation of auditory information along the auditory pathway, we have examined over 400 units in the DON to characterize frequency and directional information encoded in the dorsolateral division of the nucleus. Background activity was primarily low (<10 spikes/s) or absent. The maximum coefficient of synchronization was equivalent to the periphery (R = 0.9) and substantially better than in the midbrain. The majority of DON units (79%) responded best to stimulus frequencies of 84-141 Hz and were broadly tuned. DON cells retain or enhance the directional character of their peripheral input (s); however, characteristic axes were distributed in all quadrants around the fish, providing further evidence that binaural computations may first occur in the DON of this species.

Morphology and physiology of the cerebellar vestibulolateral lobe pathways linked to oculomotor function in the goldfish

Intracellular recording and single-cell labeling were combined to investigate the oculomotor circuitry of the goldfish cerebellar vestibulolateral lobe, consisting of the eminentia granularis (Egr) and caudal lobe. Purkinje cells exhibiting highly conserved vertebrate electrophysiological and morphological properties provide the direct output from the caudal lobe to the vestibular nuclei. Biocytin labeling of the Egr distinguished numerous hindbrain precerebellar sources that could be divided into either putative mechano- or vestibulosensitive nuclei based on cellular location and axon trajectories. Precerebellar neurons in a hindbrain nucleus, called Area II, were electrophysiologically characterized after antidromic activation from the Egr (>50% bilateral) and their morphology analyzed after intracellular biocytin labeling (n = 28). Bipolar spindle-shaped somas ranged widely in size with comparably scaled dendritic arbors exhibiting largely closed field configuration. Area II neurons (85%) projected to the ipsilateral Egr with most (93%) sending a collateral through the cerebellar commissure to the contralateral Egr; however, 15% projected to the contralateral Egr by crossing in the ventral hindbrain. Axon terminals in the vestibular nucleus were the only collaterals within the hindbrain. Every Area II neuron received a disynaptic EPSP after contralateral horizontal canal nerve stimulation and a disynaptic IPSP, preceded by a small EPSP (>50%), after ipsilateral activation. Vestibular synaptic potentials were of varying shape/amplitude, unrelated to neuron location in the nucleus, and thus likely a correlate of somadendritic size. The exceptional separation of eye position and eye velocity signals into two separate hindbrain nuclei represents an ideal model for understanding the precerebellar projection to the vestibulocerebellum.

Auditory physiology and anatomy of octavolateral efferent neurons in a teleost fish

Vertebrate hair cell systems receive innervation from efferent neurons in the brain. Here we report the responses of octavolateral efferent neurons that innervate the inner ear and lateral lines in a teleost fish, Dormitator latifrons, to directional linear accelerations, and compare them with the afferent responses from the saccule, the main auditory organ in the inner ear of this species. Efferent neurons responded to acoustic stimuli, but had significantly different response properties than saccular afferents. The efferents produced uniform, omnidirectional responses with no phase-locking. Evoked spike rates increased monotonically with stimulus intensity. Efferents were more broadly tuned and responsive to lower frequencies than saccular afferents, and efferent modulation of the otolithic organs and lateral lines is likely more pronounced at lower frequencies. The efferents had wide dynamic ranges, shallow rate-level function slopes, and low maximum discharge rates. These findings support the role of the efferent innervation of the otolithic organs as part of a general arousal system that modulates overall sensitivity of the peripheral octavolateral organs. In addition, efferent feedback may help unmask biologically relevant directional stimuli, such as those emitted by a predator, prey, or conspecific, by reducing sensitivity of the auditory system to omnidirectional ambient noise.

Sharpening of directional responses along the auditory pathway of the oyster toadfish, Opsanus tau

Our previous studies have shown that the peripheral auditory system of the toadfish encodes the direction of a sound source. Here, we compare directional responses of peripheral saccular afferents, cells in the descending octaval nucleus (DON) of the medulla, and the torus semicircularis (TS) of the midbrain. Recording locations in the brain were labeled with neurobiotin to confirm the site. To compare directional responses among cells, we calculated an index [sharpening ratio (SR)] that weights the relative strength of responses to the best direction for that cell and to the adjacent stimulus angles tested. Unsharpened saccular afferents tend to have a cosinusoidal directional response pattern (DRP) with an expected SR of 0.87. In DON, more than 60% of the cells exhibited directional sharpening (defined as SR <0.8). In TS, more than 80% of the cells exhibited directional sharpening. We conclude that directional auditory sharpening first occurs in DON and some additional sharpening occurs in the ascending pathway to the midbrain, particularly in azimuth. The sharpening of directional selectivity is likely to be an important component of the neural computations underlying directional hearing.

Projections to Bimodal Sites in the Torus semicircularis of the Toadfish, Opsanus tau

Bimodal cells in the torus semicircularis of the toadfish respond to both directional acoustic stimuli and hydrodynamic stimuli. Our previous physiological work indicated that bimodal cells may be distributed throughout the torus semicircularis. In this study, neurobiotin was used to compare the distribution of auditory-only and bimodal sites and to assess the inputs to those sites. A brief neurobiotin injection with short survival time was used to document the recording location. In other fish, a longer injection and survival time was used at an auditory-only or a bimodal site to fill the axons of the medullary inputs. Auditory-only sites were located in the most dorsal and medial sites in nucleus centralis. Bimodal sites were identified within both nucleus centralis and nucleus ventrolateralis. The greatest number of retrogradely filled cell bodies was found in the descending octaval nucleus following injection at auditory-only recording sites in nucleus centralis. In contrast, retrogradely filled cell bodies were found in both the descending octaval nucleus and the lateral line nucleus medialis following injection at bimodal sites in nucleus centralis or nucleus ventrolateralis. Retrogradely filled cell bodies were located in the dorsal and ventral divisions of the secondary octaval population from injections at either bimodal or auditory-only sites. The secondary octaval population has been implicated in auditory processing based on previous studies of both auditory specialist and auditory generalist fishes; however, this study is the first to reveal the potential role of the secondary octaval population in directional hearing in a fish. Relatively large numbers of retrogradely filled cells around the lateral lemniscus at consistent locations in the medulla indicate that a perilemniscal cell group also might be a component of the directional hearing circuit.

Octavolateral projections and organization in the medulla of a teleost fish, the sleeper goby (Dormitator latifrons)

This study is the first to employ simultaneous labeling with different colored fluorescent dyes and confocal microscopy to investigate the central projections of the octavolateral nerves in any fish. Three-dimensional reconstructions of the hindbrain octavolateral nuclei were made and overlap of octavolateral projections was assessed in a teleost, the sleeper goby (Dormitator latifrons). The octavolateral nerves, which innervate the otolithic organs, semicircular canals, and lateral lines, project to seven hindbrain nuclei in diverse, complex patterns. The medulla is generally organized with auditory regions dorsal to vestibular regions. The intermediate subdivision of the descending octaval nucleus (DON) receives interdigitating projections from the otolithic organs, and the dorsomedial DON likely integrates multiple auditory inputs. Afferents from the three otolithic organs (the utricle, saccule, and lagena) project to the intermediate DON in approximately equal proportion, supporting physiological evidence that suggests auditory roles for all three otolithic organs in the sleeper goby. The anterior octaval nucleus receives partially segregated inputs from the octavolateral organs. The dorsal division of the magnocellular octaval nucleus (MgON) receives highly overlapping otolithic organ and semicircular canal input, and we propose that this region is a major octaval integration center. Regions in the ventral medulla (the tangential octaval nucleus, ventral DON, and ventral MgON) receive mainly utricular and semicircular canal inputs, suggesting vestibular roles. Each semicircular canal nerve projects to distinct regions of the hindbrain, with little overlap in most octaval nuclei. Efferent neurons receive bilateral input and project unilaterally to the octavolateral organs.

Directional selectivity and frequency tuning of midbrain cells in the oyster toadfish, Opsanus tau

Single-unit recordings were made from areas in the midbrain (torus semicircularis) of the oyster toadfish. We evaluated frequency tuning and directional responses using whole-body oscillation to simulate auditory stimulation by particle motion along axes in the horizontal and mid-sagittal planes. We also tested for bimodality in responses to auditory and hydrodynamic stimuli. One recording location in each animal was marked by a neurobiotin injection to confirm the recording site. Recordings were made in nucleus centralis, nucleus ventrolateralis, and the deep cell layer. Most units were frequency-selective with best frequencies between 50 and 141 Hz. Suppression of activity was apparent in 10% of the cells. Bimodality was common, including inhibition and suppression of background activity by auditory or hydrodynamic stimulation. The majority of the cells were directionally selective with directional response patterns that were sharpened compared with those of primary saccular afferents. The best directional axes were arrayed widely in spherical space, covering most azimuths and elevations. This representation is adequate for the computation of the motional axis of an auditory stimulus for sound source localization.

Neural mechanisms and behaviors for acoustic communication in teleost fish

Sound communication is not unique to humans but rather is a trait shared with most non-mammalian vertebrates. A practical way to address questions of vocal signal encoding has been to identify mechanisms in non-mammalian model systems that use acoustic communication signals in their social behavior. Teleost fishes, the largest group of living vertebrates, include both vocal and non-vocal species that exploit a wide range of acoustic niches. Here, we focus on those vocal species where combined behavioral and neurobiological studies have recently begun to elucidate a suite of adaptations for both the production and the perception of acoustic signals essential to their reproductive success and survival. Studies of these model systems show that teleost fish have the vocal-acoustic behaviors and neural systems both necessary and sufficient to solve acoustic problems common to all vertebrates. In particular, behavioral studies demonstrate that temporal features within a call, including pulse duration, rate and number, can all be important to a call's communicative value. Neurobiological studies have begun to show how these features are produced by a vocal motor system extending from forebrain to hindbrain levels and are encoded by peripheral and central auditory neurons. The abundance and variety of vocal fish present unique opportunities for parallel investigations of neural encoding, perception, and communication across a diversity of natural, acoustic habitats. As such, investigations in teleosts contribute to our delineating the evolution of the vocal and auditory systems of both non-mammalian and mammalian species, including humans.

Patterns of canal and otolith afferent input convergence in frog second-order vestibular neurons

Second-order vestibular neurons (2 degrees VN) were identified in the isolated frog brain by the presence of monosynaptic excitatory postsynaptic potentials (EPSPs) after separate electrical stimulation of individual vestibular nerve branches. Combinations of one macular and the three semicircular canal nerve branches or combinations of two macular nerve branches were stimulated separately in different sets of experiments. Monosynaptic EPSPs evoked from the utricle or from the lagena converged with monosynaptic EPSPs from one of the three semicircular canal organs in ~30% of 2 degrees VN. Utricular afferent signals converged predominantly with horizontal canal afferent signals (74%), and lagenar afferent signals converged with anterior vertical (63%) or posterior vertical (37%) but not with horizontal canal afferent signals. This convergence pattern correlates with the coactivation of particular combinations of canal and otolith organs during natural head movements. A convergence of afferent saccular and canal signals was restricted to very few 2 degrees VN (3%). In contrast to the considerable number of 2 degrees VN that received an afferent input from the utricle or the lagena as well as from one of the three canal nerves (~30%), smaller numbers of 2 degrees VN (14% of each type of 2 degrees otolith or 2 degrees canal neuron) received an afferent input from only one particular otolith organ or from only one particular semicircular canal organ. Even fewer 2 degrees VN received an afferent input from more than one semicircular canal or from more than one otolith nerve (~7% each). Among 2 degrees VN with afferent inputs from more than one otolith nerve, an afferent saccular nerve input was particularly rare (4-5%). The restricted convergence of afferent saccular inputs with other afferent otolith or canal inputs as well as the termination pattern of saccular afferent fibers are compatible with a substrate vibration sensitivity of this otolith organ in frog. The ascending and/or descending projections of identified 2 degrees VN were determined by the presence of antidromic spikes. 2 degrees VN mediating afferent utricular and/or semicircular canal nerve signals had ascending and/or descending axons. 2 degrees VN mediating afferent lagenar or saccular nerve signals had descending but no ascending axons. The latter result is consistent with the absence of short-latency macular signals on extraocular motoneurons during vertical linear acceleration. Comparison of data from frog and cat demonstrated the presence of a similar organization pattern of maculo- and canal-ocular reflexes in both species.

Microgravity (STS-90 Neurolab-Mission) influences synapse formation in a vestibular nucleus of fish brain

Synapse counting was undertaken by conventional electron microscopy in primary vestibular integration centers (i.e., Nucleus descendens, Nd, and Nucleus magnocellularis, Nm, of the brainstem Area octavolateralis) and in the diencephalic visual Nucleus corticalis (Nc) of spaceflown neonate swordtail fish Xiphophorus helleri as well as in 1 g control siblings. Spaceflight (16 days microgravity, STS-90 Neurolab-Mission) yielded an increase in synaptic contacts only within the vestibular Nd indicating that lack of input resulted in compensation processes. No effect of microgravity, however, was observed in the visual Nc and in the vestibular Nm which is situated in the close vicinity of the Nd. In contrast to the latter, the Nm does not receive exclusively vestibular input, but inputs from the lateral line as well, possibly providing sufficient input at microgravity.

Effects of hypergravity on the development of cell number and asymmetry in fish brain nuclei

Larval cichlid fish (Oreochromis mossambicus) siblings were subjected to 3 g hypergravity (hg) and total darkness for 21 days during development and subsequently processed for conventional histology. Further siblings reared at 1 g and alternating light/dark (12h:12h) conditions served as controls. Cell number counts of the visual Nucleus isthmi (Ni) versus the vestibular Nucleus magnocellularis (Nm) revealed that in experimental animals total cell number was decreased in the Ni, possibly due to retarded growth as a result of the lack of visual input whereas no effect was observed in the Nm. Calculating the percentual asymmetry in cell number (i.e., right vs. the left side of the brain), no effects of hg/darkness were seen in the Ni, whereas asymmetry was slightly increased in the Nm. Since the asymmetry of inner ear otoliths is decreased under hg, this finding may indicate efferent vestibular action of the CNS on the level of the Nm by means of a feedback mechanism.

Location of dye-coupled second order and of efferent vestibular neurons labeled from individual semicircular canal or otolith organs in the frog

Vestibular nerve branches innervating the sensory epithelia of the three semicircular canals or of the three otolith organs of frogs were selectively labeled in-vitro with biocytin. Labeled afferent fibers from the semicircular canals, utricle, and lagena were encountered in each of the four vestibular nuclei and their projections overlapped considerably. Saccular afferent fibers projected to the dorsal (acoustic) nuclei and smaller projections to the vestibular nuclei were regionally restricted. Per semicircular canal or otolith organ about equal numbers (11-14) of medium sized vestibular neurons (between 7.5 and 17 microm in diameter) were dye-coupled to afferent fibers. Most of these dye-coupled vestibular neurons were located in the lateral and descending vestibular nuclei between the VIIIth and IXth nerves. The superior vestibular nucleus was relatively free of dye-coupled vestibular neurons. The location of this subpopulation of central vestibular neurons supports the notion that these neurons are part of a particular vestibulospinal pathway. In addition, from each of the canal and/or otolith organs about 3-4 efferent vestibular neurons were labeled retrogradely. These neurons (between 15 and 26 microm in diameter) were located ventral to the vestibular nuclear complex. The branching of efferent vestibular neurons was shown by the presence of neurons that were double labeled by two different fluorescent dyes applied in the same experiment to the anterior and posterior ramus of the same VIIIth nerve, respectively. The branching of these efferent neuron axons explained the presence of collaterals and terminals in the sensory epithelia of a number of untreated ipsilateral endorgans.

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.

Weightlessness during spaceflight results in enhanced synapse formation in a fish brain vestibular nucleus

Synapse counts were undertaken by conventional electron microscopy in primary vestibular integration centers, (i.e. nucleus descendens and nucleus magnocellularis of the brainstem area octavolateralis) and in the diencephalic visual nucleus corticalis of spaceflown neonate swordtail fish Xiphophorus helleri as well as in 1 g control siblings. Spaceflight (16 days microgravity, (microg), STS-90 Neurolab Mission) yielded an increase in synaptic contacts within the vestibular nucleus descendens indicating that lack of input resulted in compensation processes. No effect of microg, however, was observed in the visual nucleus corticalis and in the vestibular nucleus magnocellularis which is situated in the close vicinity of the nucleus descendens. In contrast to the latter, the nucleus magnocellularis does not receive exclusively vestibular input, but inputs from the lateral line as well, possibly providing sufficient input at microgravity.

Relationship between vestibular primary afferents and vestibulospinal neurons in lampreys

The distribution of vestibular primary afferents as well as their relationship with vestibulospinal and other brainstem neurons were studied in lampreys using anatomical tracers. Afferents from the anterior (aVIIIn) and the posterior (pVIIIn) branches of the vestibular nerve were located mainly in the ventral nucleus of the octavolateral area. The relationship between afferents and vestibulospinal neurons was studied by applying one fluorescent tracer to the whole vestibular nerve or one of its branches and applying another tracer to the spinal cord. Some afferents showed large, bulb-like enlargements (bulbs) and about 20 of these were found in the anterior and the intermediate octavomotor nucleus, whereas about 40 were found in the posterior octavomotor nucleus. Some of the bulbs made apparent contact with vestibulospinal neurons in the intermediate octavomotor nucleus and originated mostly from the aVIIIn, whereas bulbs in the posterior octavomotor nucleus originated from the pVIIIn. Applications of biocytin to hemisegments of rostral spinal cord labeled vestibulospinal neurons located in the ipsilateral intermediate octavomotor nucleus and the contralateral posterior octavomotor nucleus. In addition, vestibular primary afferents with bulbs in apparent contact with vestibulospinal neurons were transneuronally labeled by biocytin. They were observed in the ipsilateral aVIIIn and the contralateral pVIIIn and could be followed in the labyrinths, where they innervated the vertical and horizontal arms of the semicircular canal crests. Taken together, these results indicate that vestibular primary afferents from the aVIIIn innervate predominantly vestibulospinal neurons of the intermediate octavomotor nucleus, whereas afferents from the pVIIIn innervate vestibulospinal neurons in the posterior octavomotor nucleus. This anatomical organization suggests that afferents carrying bulbs convey dynamic information to vestibulospinal neurons, which, in turn, project to the spinal cord networks.

Spatial distribution of semicircular canal nerve evoked monosynaptic response components in frog vestibular nuclei

Most second-order vestibular neurons receive a canal-specific monosynaptic excitation, although the central projections of semicircular canal afferents overlap extensively. This remarkable canal specificity prompted us to study the spatial organization of evoked field potentials following selective stimulation of individual canal nerves. Electrically evoked responses in the vestibular nuclei were mapped systematically in vitro. Constructed activation maps were superimposed on a cytoarchitectonically defined anatomical map. The spatial activation maps for pre- and postsynaptic response components evoked by stimulation of a given canal nerve were similar. Activation maps for monosynaptic inputs from different canals tended to show a differential distribution of their peak amplitudes, although the overlap was considerable. Anterior vertical canal signals peaked in the superior vestibular nucleus, posterior vertical canal signals peaked in the descending and in the dorsal part of the lateral vestibular nucleus, whereas horizontal canal signals peaked in the descending and in the ventral part of the lateral vestibular nucleus. A similar, differential but overlapping, spatial organization of the canal inputs was described also for other vertebrates, suggesting a crude but rather conservative topographical organization of semicircular canal nerve projections within the vestibular nuclei. Differences in the precision of topological representations between vestibular and other sensory modalities are discussed.

Midbrain acoustic circuitry in a vocalizing fish

The mapping of auditory circuitry and its interface with vocal motor systems is essential to the investigation of the neural processing of acoustic signals and its relationship to sound production. Here we delineate the circuitry of a midbrain auditory center in a vocal fish, the plainfin midshipman. Biotin injections into physiologically identified auditory sites in nucleus centralis (NC) in the torus semicircularis show a medial column of retrogradely filled neurons in the medulla mainly in a dorsomedial division of a descending octaval nucleus (DO), dorsal and ventral divisions of a secondary octaval nucleus (SO), and the reticular formation (RF) near the lateral lemniscus. Biotin-filled neurons are also located at midbrain-pretectal levels in a medial pretoral nucleus. Terminal fields are identified in the medulla (ventral SO, RF), isthmus (nucleus praeeminentialis), midbrain (nucleus of the lateral lemniscus, medial pretoral nucleus, contralateral NC, tectum), diencephalon (lateral preglomerular, central posterior, and anterior tuber nuclei), and telencephalon (area ventralis). The medial column of toral afferent neurons is adjacent to and overlapping the positions of DO and SO neurons shown previously to be linked to the vocal pacemaker circuitry of the medulla. Midshipman are considered "hearing generalists" because they lack the peripheral adaptations of "specialists" that enhance the detection of the pressure component of acoustic signals. Whereas the results indicate a general pattern of acoustic circuitry similar to that of specialists, they also show central adaptations, namely, a vocal-acoustic interface in DO and SO related to this species' vocal abilities.

Characteristics of regenerating horizontal semicircular canal afferent and efferent fibers in the toadfish, Opsanus tau

The horizontal semicircular canal nerve of the toadfish, Opsanus tau, was transected and allowed to regenerate. The time course, morphometrics, and projection patterns of regenerating afferent and efferent vestibular fibers were determined. Nerve transections were performed both pre- and postganglionically, and regeneration was assessed in afferent and efferent fibers by bulk labeling the peripheral axons of the horizontal semicircular canal nerve with biocytin after nerve regrowth. Afferent fibers regrew through the transection site within 14 days and projected to all vestibular nuclei within 3 weeks. Bouton and branch number, axon length, surface area, volume, fiber diameter, and internodal distance were quantified for afferent fibers from eight sites within the vestibular nuclei, and axon number and soma size was quantified for the efferent fibers. Extensive regeneration was seen within 5 weeks of transection in all nuclei, and most morphometric parameters approached or exceeded control levels within 10 weeks. Regeneration appeared to recapitulate morphogenesis with an initial overproduction of boutons and branch points followed by elimination of presumably superfluous structures. Internodal distance remained significantly shorter in regenerating afferent axons than in control fish throughout the 15-week observation period. Efferent fibers also were observed to regenerate. Efferent axon number, diameter, and soma size were indistinguishable from those in controls from 3 weeks posttransection through week 15. Electrophysiological recordings from the horizontal canal nerve during mechanical stimuli of the canal confirmed that the regenerated axons transmitted normal signals. The return of normal equilibrium and behavior coincided with the projection of afferent fibers into the central vestibular nuclei, indicating that functional connections had been reestablished.

Neurobiology of fish under altered gravity conditions

In vertebrates (including man), an altered gravitational environment such as weightlessness can induce malfunction of the inner ear, based on an irregular dislocation of the otoliths from the corresponding sensory epithelia. This dislocation leads to an illusionary tilt, since the otolithic inputs are not in register with other sensory organs. This results in an intersensory conflict. Vertebrates in orbit therefore face severe orientation problems. In humans, the intersensory conflict may additionally lead to a malaise, commonly referred to as space motion sickness (SMS). During the first days in weightlessness, the orientation problems (and SMS) disappear, since the brain develops a new compensatory interpretation of the available sensory data. The present review reports the neurobiological responses-particularly in fish-observed at altered gravitational states, concerning behaviour and neuroplastic reactivities. Recent investigations employing microgravity (spaceflight, parabolic aircraft flights, clinostat) and hyper-gravity (laboratory centrifuges as ground based research tools) yielded clues and insights into the understanding of the respective basic phenomena. The possible sources of human space sickness (a kinetosis) and of the space adaptation syndrome (when a sensory reinterpretation of gravitational and visual cues takes place) are particularly highlighted with regard to the functional significance of bilaterally asymmetric otoliths (weight, size).

Anatomical evidence for binaural processing in the descending octaval nucleus of the toadfish (Opsanus tau)

The connections of a potential auditory circuit were determined in the medulla of the toadfish (Opsanus tau). Fluorescent dextran amines placed in the medial torus semicircularis (mTS) retrogradely filled cells primarily in the dorsal region of the descending octaval nuclei (DON) with contralateral predominance. Fluorescent dextran amines placed in the DON revealed commissural fibers that cross the midline with the internal arcuate tract. The interconnections are consistent with a dorsal-ventral organization of the DON: reciprocal innervation is present for the left and right dorsal zones of the DON and for the left and right ventral zones of the DON. Based on projections to the medial (auditory) TS and the reciprocal connections, the dorsal region of the DON appears to be the major auditory processing site in the medulla and also may be a site for directional, binaural comparisons. The ventral region of the DON may be a site for bilateral vestibular processing. Double-labelling experiments revealed that some of the descending octaval cells projecting to the contralateral DON also project to the mTS. Based on the auditory pathway indicated by this study, future neurophysiological investigations of sensitivity to directional sound stimuli should begin in the dorsal DON of the toadfish.

Projections of primary afferents from regions of the saccule in toadfish (Opsanus tau)

The hair cell orientation patterns present on the saccules of fishes may be important for encoding the direction of a sound source. This study was conducted to determine whether primary afferent projections to the medulla are organized by the best directions for the hair cells they innervate. The toadfish saccule has hair cells oriented primarily in the vertical plane: both the rostral and the caudal saccule have hair cell orientations sweeping from 0 degrees to 45 degrees, and the middle saccule has hair cells oriented at 90 degrees. Fluorescent dextran amines were used singly and in combination to label regions of the saccular nerve innervating rostral, middle, and caudal saccule. The projections of those afferents were examined in detail in the anterior and descending octaval nuclei, which are auditory nuclei in this species. There was no evidence of topographic projections based on location along the length of the saccule or based on hair cell orientation. There was some evidence that parallel inputs are present from each region of the saccule examined, which may be based on the 180 degrees opposition of hair cells found throughout the saccule; however, afferents from the rostral, middle, and caudal saccule appear to have overlapping projections to the anterior and descending octaval nuclei. These data suggest that in toadfish, calculations of the direction of the sound source may begin in either of these primary auditory nuclei by comparing afferent input from along the saccule.

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.

Electrophysiology and pharmacology of outward potassium currents in semicircular canal hair cells of toadfish, Opsanus tau

Outward currents from hair cells from the horizontal semicircular canal (HSCC) of the toadfish were investigated using whole cell patch clamp methods. Two classes of hair cells are found. One class (approx. 10% of cells) showed only a non-inactivating current (IKCa) which was blocked by 2 mM TEA. A second class had both inactivating and non-inactivating currents. The former (IA) was blocked by 4-AP (1 mM) and the latter (IKCa) by TEA (2-20 mM) . While the majority of the cells expressed both these outward currents, due to IA inactivation both currents are functionally present in the same cell only between -60 and -40 mV. At more depolarized membrane potentials, IA was inactivated, suggesting that a single hair cell may have two distinct signalling modes, one dominated by IA at more hyperpolarized membrane potentials and the other by IKCa at depolarized values where ICa is beginning to grow, increasing both amplitude and activation rate of IKCa. The switch between modes will be determined by the amplitude and frequency characteristics of the stimulus and possibly also by actions of efferent transmitters. In current clamp mode, 10% of the HSCC hair cells showed high Q and high frequency resonance, from 44 to 360 Hz at 12 degrees C. These cells expressed only one outward calcium dependent, non-inactivating, TEA sensitive current, characteristic of IKCa. A suggested role for high frequency resonance is as positive feedback to produce a high frequency updating of the stereociliary compliance to most faithfully transduce angular acceleration.

Differential central projections of vestibular afferents in pigeons

The question of whether a differential distribution of vestibular afferent information to central nuclear neurons is present in pigeons was studied using neural tracer compounds. Discrete tracing of afferent fibers innervating the individual semicircular canal and otolith organs was produced by sectioning individual branches of the vestibular nerve that innervate the different receptor organs and applying crystals of horseradish peroxidase, or a horseradish peroxidase/cholera toxin mixture, or a biocytin compound for neuronal uptake and transport. Afferent fibers and their terminal distributions within the brainstem and cerebellum were visualized subsequently. Discrete areas in the pigeon central nervous system that receive primary vestibular input include the superior, dorsal lateral, ventral lateral, medial, descending, and tangential vestibular nuclei; the A and B groups; the intermediate, medial, and lateral cerebellar nuclei; and the nodulus, the uvula, and the paraflocculus. Generally, the vertical canal afferents projected heavily to medial regions in the superior and descending vestibular nuclei as well as the A group. Vertical canal projections to the medial and lateral vestibular nuclei were observed but were less prominent. Horizontal canal projections to the superior and descending vestibular nuclei were much more centrally located than those of the vertical canals. A more substantial projection to the medial and lateral vestibular nuclei was seen with horizontal canal afferents compared to vertical canal fibers. Afferents innervating the utricle and saccule terminated generally in the lateral regions of all vestibular nuclei in areas that were separate from the projections of the semicircular canals. In addition, utricular fibers projected to regions in the vestibular nuclei that overlapped with the horizontal semicircular canal terminal fields, whereas saccular afferents projected to regions that received vertical canal fiber terminations. Lagenar afferents projected throughout the cochlear nuclei, to the dorsolateral regions of the cerebellar nuclei, and to lateral regions of the superior, lateral, medial, and descending vestibular nuclei.

Fine structure of the medullary lateral line area of Chelon labrosus (order perciformes), a nonelectroreceptive teleost

The ultrastructure and synaptic organization of the nucleus medialis and cerebellar crest of the teleost Chelon labrosus have been investigated. The nucleus medialis receives projections from the anterior and posterior lateral line nerves. This nucleus consists of oval neurons and large crest cells ("Purkinje-like" cells) whose apical dendrites branch in the overlying molecular layer, the cerebellar crest. In the dorsal region of the nucleus medialis, the perikarya and smooth primary dendrites of the crest cells are interspersed among myelinated fibers and nerve boutons. The ventral layer of the nucleus medialis contains crest cell perikarya and dendrites as well as oval neurons. The cerebellar crest lacks neuronal bodies, but the apical dendrites of crest cells receive synapses from unmyelinated and myelinated fibers. In the cerebellar crest, two types of terminals are presynaptic to the crest cell dendrites: boutons with spherical vesicles that form asymmetric synapses with dendritic spines and boutons containing pleomorphic vesicles that form symmetric synapses directly on the dendritic shaft. Most axon terminals found on the somata and primary dendrites of crest cells in the nucleus medialis have pleomorphic vesicles and form symmetric contacts, though asymmetric synapses with spherical vesicles and mixed synapses can be observed; these mixed synapses exhibit gap junctions and contain spherical vesicles. Unlike crest cells, the oval neuron perikarya receive three types of contacts (symmetric, asymmetric, and mixed). The origins and functions of these different bouton types in the nucleus medialis are discussed.

Eye position and eye velocity integrators reside in separate brainstem nuclei

Two types of central nervous system integrators are critical for oculomotor performance. The first integrates velocity commands to create position signals that hold fixation of the eye. The second stores relative velocity of the head and visual surround to stabilize gaze both during and after the occurrence of continuous self and world motion. We have used recordings from single neurons to establish that the "position" and "velocity" integrators for horizontal eye movement occupy adjacent, but nonoverlapping, locations in the goldfish medulla. Lidocaine inactivation of each integrator results in the eye movement deficits expected if horizontal eye position and velocity signals are processed separately. These observations also indicate that each brainstem compartment generates and stores these signals. Consequently, each integrator exhibits functional autonomy. Therefore, we propose that the intrinsic electrophysiological properties of the constituent neurons in each brainstem subnucleus may be sufficient for producing integrator rhythmicity.