Central distribution of octavolateral afferents and efferents in a teleost (Mormyridae)

Anatomical adventures in the fish auditory medullaa)

This paper provides an overview of my work on the central auditory system of fish. It focuses on my comparative analyses of a nucleus that receives input from the inner ear, the descending nucleus, and more specifically on that part of the descending nucleus supplied by the otolith end organs, the dorsal descending nucleus. I begin by summarizing my initial work on the bowfin, Amia calva, and go on to explain the importance of taking a comparative approach to understanding ancestral and specialized anatomical and putative functional characteristics of the dorsal descending nucleus in modern bony fishes, the teleosts.

Fish hearing revealed: Do we understand hearing in critical fishes and marine tetrapods

Hearing evolved in lampreys with a frequency range of 50-200 Hz. This hearing range is comparable to that of elasmobranchs, most non-teleosts, and lungfish. Elasmobranchs most likely use the saccule and the papilla neglecta (PN) for hearing. In non-teleosts and teleosts, lungfish, and certain tetrapods the saccule is the likely sensor for sound reception while the lagena and the PN are important for gravistatic sensing. Coelacanth and most tetrapods have a basilar papilla (BP) for hearing. In coelacanth and tetrapods, the hair cells of the BP are in contact with a basilar and a tectorial membrane. These membranes transmit mechanical vibrations. A cochlear aqueduct (CA) provides a connection between the cerebrospinal fluid that has a sodium rich space in coelacanth and tetrapods while the potassium rich endolymph is known in vertebrates. A unique feature is known in basic sarcopterygians, the intracranial joint, that never developed in actinopterygians and has been lost in lungfish and tetrapods. The BP in coelacanths is thought to generate pressure with the intracranial joint that will be transmitted to the CA. Lungs or a swim bladder are not forming in Chondrichthyes, structures that have a major impact on hearing in teleosts and tetrapods.

The continued importance of comparative auditory research to modern scientific discovery

A rich history of comparative research in the auditory field has afforded a synthetic view of sound information processing by ears and brains. Some organisms have proven to be powerful models for human hearing due to fundamental similarities (e.g., well-matched hearing ranges), while others feature intriguing differences (e.g., atympanic ears) that invite further study. Work across diverse "non-traditional" organisms, from small mammals to avians to amphibians and beyond, continues to propel auditory science forward, netting a variety of biomedical and technological advances along the way. In this brief review, limited primarily to tetrapod vertebrates, we discuss the continued importance of comparative studies in hearing research from the periphery to central nervous system with a focus on outstanding questions such as mechanisms for sound capture, peripheral and central processing of directional/spatial information, and non-canonical auditory processing, including efferent and hormonal effects.

Neuronal and Astroglial Localization of Glucocorticoid Receptor GRα in Adult Zebrafish Brain (Danio rerio)

Glucocorticoid receptor α (GRα), a ligand-regulated transcription factor, mainly activated by cortisol in humans and fish, mediates neural allostatic and homeostatic functions induced by different types of acute and chronic stress, and systemic inflammation. Zebrafish GRα is suggested to have multiple transcriptional effects essential for normal development and survival, similarly to mammals. While sequence alignments of human, monkey, rat, and mouse GRs have shown many GRα isoforms, we questioned the protein expression profile of GRα in the adult zebrafish (Danio rerio) brain using an alternative model for stress-related neuropsychiatric research, by means of Western blot, immunohistochemistry and double immunofluorescence. Our results identified four main GRα-like immunoreactive bands (95 kDa, 60 kDa, 45 kDa and 35 kDa), with the 95 kDa protein showing highest expression in forebrain compared to midbrain and hindbrain. GRα showed a wide distribution throughout the antero-posterior zebrafish brain axis, with the most prominent labeling within the telencephalon, preoptic, hypothalamus, midbrain, brain stem, central grey, locus coeruleus and cerebellum. Double immunofluorescence revealed that GRα is coexpressed in TH+, β₂-AR+ and vGLUT+ neurons, suggesting the potential of GRα influences on adrenergic and glutamatergic transmission. Moreover, GRα was co-localized in midline astroglial cells (GFAP+) within the telencephalon, hypothalamus and hindbrain. Interestingly, GRα expression was evident in the brain regions involved in adaptive stress responses, social behavior, and sensory and motor integration, supporting the evolutionarily conserved features of glucocorticoid receptors in the zebrafish brain.

Development and migration of the zebrafish rhombencephalic octavolateral efferent neurons

In vertebrate animals, motor and sensory efferent neurons carry information from the central nervous system (CNS) to peripheral targets. These two types of efferent systems sometimes bear a close resemblance, sharing common segmental organization, axon pathways, and chemical messengers. Here, we focus on the development of the octavolateral efferent neurons (OENs) and their interactions with the closely-related facial branchiomotor neurons (FBMNs) in zebrafish. Using live-imaging approaches, we investigate the birth, migration, and projection patterns of OENs. We find that OENs are born in two distinct groups: a group of rostral efferent neurons (RENs) that arises in the fourth segment, or rhombomere (r4), of the hindbrain and a group of caudal efferent neurons (CENs) that arises in r5. Both RENs and CENs then migrate posteriorly through the hindbrain between 18 and 48 hrs postfertilization, alongside the r4-derived FBMNs. Like the FBMNs, migration of the r4-derived RENs depends on function of the segmental identity gene hoxb1a; unlike the FBMNs, however, both OEN populations move independently of prickle1b. Further, we investigate whether the previously described "pioneer" neuron that leads FBMN migration through the hindbrain is an r4-derived FBMN/REN or an r5-derived CEN. Our experiments verify that the pioneer is an r4-derived neuron and reaffirm its role in leading FBMN migration across the r4/5 border. In contrast, the r5-derived CENs migrate independently of the pioneer. Together, these results indicate that the mechanisms OENs use to navigate the hindbrain differ significantly from those employed by FBMNs.

Tracing of Afferent Connections in the Zebrafish Cerebellum Using Recombinant Rabies Virus

The cerebellum is involved in some forms of motor coordination and learning, and in cognitive and emotional functions. To elucidate the functions of the cerebellum, it is important to unravel the detailed connections of the cerebellar neurons. Although the cerebellar neural circuit structure is generally conserved among vertebrates, it is not clear whether the cerebellum receives and processes the same or similar information in different vertebrate species. Here, we performed monosynaptic retrograde tracing with recombinant rabies viruses (RV) to identify the afferent connections of the zebrafish cerebellar neurons. We used a G-deleted RV that expressed GFP. The virus was also pseudotyped with EnvA, an envelope protein of avian sarcoma and leucosis virus (ALSV-A). For the specific infection of cerebellar neurons, we expressed the RV glycoprotein (G) gene and the envelope protein TVA, which is the receptor for EnvA, in Purkinje cells (PCs) or granule cells (GCs), using the promoter for aldolase Ca (aldoca) or cerebellin 12 (cbln12), respectively. When the virus infected PCs in the aldoca line, GFP was detected in the PCs’ presynaptic neurons, including GCs and neurons in the inferior olivary nuclei (IOs), which send climbing fibers (CFs). These observations validated the RV tracing method in zebrafish. When the virus infected GCs in the cbln12 line, GFP was again detected in their presynaptic neurons, including neurons in the pretectal nuclei, the nucleus lateralis valvulae (NLV), the central gray (CG), the medial octavolateralis nucleus (MON), and the descending octaval nucleus (DON). GFP was not observed in these neurons when the virus infected PCs in the aldoca line. These precerebellar neurons generally agree with those reported for other teleost species and are at least partly conserved with those in mammals. Our results demonstrate that the RV system can be used for connectome analyses in zebrafish, and provide fundamental information about the cerebellar neural circuits, which will be valuable for elucidating the functions of cerebellar neural circuits in zebrafish.

The Mormyrid Optic Tectum Is a Topographic Interface for Active Electrolocation and Visual Sensing

The African weakly electric fish Gnathonemus petersii is capable of cross-modal object recognition using its electric sense or vision. Thus, object features stored in the brain are accessible by multiple senses, either through connections between unisensory brain regions or because of multimodal representations in multisensory areas. Primary electrosensory information is processed in the medullary electrosensory lateral line lobe, which projects topographically to the lateral nucleus of the torus semicircularis (NL). Visual information reaches the optic tectum (TeO), which projects to various other brain regions. We investigated the neuroanatomical connections of these two major midbrain visual and electrosensory brain areas, focusing on the topographical relationship of interconnections between the two structures. Thus, the neural tracer DiI was injected systematically into different tectal quadrants, as well as into the NL. Tectal tracer injections revealed topographically organized retrograde and anterograde label in the NL. Rostral and caudal tectal regions were interconnected with rostral and caudal areas of the NL, respectively. However, dorsal and ventral tectal regions were represented in a roughly inverted fashion in NL, as dorsal tectal injections labeled ventral areas in NL and vice versa. In addition, tracer injections into TeO or NL revealed extensive inputs to both structures from ipsilateral (NL also contralateral) efferent basal cells in the valvula cerebelli; the NL furthermore projected back to the valvula. Additional tectal and NL connections were largely confirmatory to earlier studies. For example, the TeO received ipsilateral inputs from the central zone of the dorsal telencephalon, torus longitudinalis, nucleus isthmi, various tegmental, thalamic and pretectal nuclei, as well as other nuclei of the torus semicircularis. Also, the TeO projected to the dorsal preglomerular and dorsal posterior thalamic nuclei as well as to nuclei in the torus semicircularis and nucleus isthmi. Beyond the clear topographical relationship of NL and TeO interconnections established here, the known neurosensory upstream circuitry was used to suggest a model of how a defined spot in the peripheral sensory world comes to be represented in a common associated neural locus both in the NL and the TeO, thereby providing the neural substrate for cross-modal object recognition.

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.

Estimation of distance and electric impedance of capacitive objects in the weakly electric fish Gnathonemus petersii

During active electrolocation, the weakly electric fish Gnathonemus petersii judges the distance and impedance of nearby objects. Capacitive objects, which modulate local amplitude and waveform of the fish's electric probing signals, cast amplitude and waveform images onto the fish's electroreceptive skin. For an unambiguous estimation of the impedance and distance of an object, the animal has to deal with multiple dependencies of object and image parameters. Based on experimentally recorded amplitude and waveform images, we investigated possible strategies of the fish to unequivocally determine both the distance and the impedance of capacitive objects. We show that the relative slope in amplitude images, but not in waveform images, is independent of object impedance and is a measure of object distance. Distance-invariant impedance estimators were obtained by two different analytical strategies. The peak modulations of both image types were 'calibrated' with the relative slope of the amplitude image. Impedance estimators were obtained whenever these pairs of image features (peak and relative slope) were related dynamically over two consecutive distances. A static impedance estimator termed 'electric colour' is postulated to arise from the relationship of an amplitude and waveform image. Our results confirm that electric colour is indeed unaffected by object distance. For electric colour estimation we suggest a minimalistic approach of just relating the peak modulations of both image types to the basal amplitude and waveform condition. Our results are discussed with regard to the anatomical and physiological organization of the fish's electrosensory neuronal pathways and behavioural strategies of electrolocating fish.

A quest for excitation: Theoretical arguments and immunohistochemical evidence of excitatory granular cells in the ELL of Gnathonemus petersii

The Electrosensory Lateral Line lobe (ELL) is the first central target where the electrosensory information encoded in the spatiotemporal pattern electroreceptor afferent discharges is processed. These afferents encode the minute amplitude changes of the basal electric field through both a change in latency and discharge rate. In the ELL the time and rate-coded input pattern of the sensory periphery goes through the granular cell layer before reaching the main efferent cells of the network: large fusiform (LF) and large ganglion (LG) cells. The evidence until now shows that granular cells are inhibitory. Given that large fusiform cells are excited by the sensory input, it remains a mystery how the afferent input produce excitation through a layer composed by only inhibitory cells. We addressed this problem by modeling how the known circuitry of the ELL could produce excitation in LF cells with only inhibitory granular cells. Alternatively we show that a network composed of a mix of excitatory and inhibitory granular cell not only performs better, as expected, carrying excitation to LF cells but it does so robustly and at higher sensitivity by enhancing the contrast of the electric image between the periphery and the ELLs output. We then show with refined histological methods that a subpopulation of the granular cells indeed are excitatory, providing the necessary input for this contrast enhancing mechanism.

More a finger than a nose: the trigeminal motor and sensory innervation of the Schnauzenorgan in the elephant-nose fish Gnathonemus petersii

The weakly electric fish Gnathonemus petersii uses its electric sense to actively probe the environment. Its highly mobile chin appendage, the Schnauzenorgan, is rich in electroreceptors. Physical measurements have demonstrated the importance of the position of the Schnauzenorgan in funneling the fish's self-generated electric field. The present study focuses on the trigeminal motor pathway that controls Schnauzenorgan movement and on its trigeminal sensory innervation and central representation. The nerves entering the Schnauzenorgan are very large and contain both motor and sensory trigeminal components as well as an electrosensory pathway. With the use of neurotracer techniques, labeled Schnauzenorgan motoneurons were found throughout the ventral main body of the trigeminal motor nucleus but not among the population of larger motoneurons in its rostrodorsal region. The Schnauzenorgan receives no motor or sensory innervation from the facial nerve. There are many anastomoses between the peripheral electrosensory and trigeminal nerves, but these senses remain separate in the sensory ganglia and in their first central relays. Schnauzenorgan trigeminal primary afferent projections extend throughout the descending trigeminal sensory nuclei, and a few fibers enter the facial lobe. Although no labeled neurons could be identified in the brain as the trigeminal mesencephalic root, some Schnauzenorgan trigeminal afferents terminated in the trigeminal motor nucleus, suggesting a monosynaptic, possibly proprioceptive, pathway. In this first step toward understanding multimodal central representation of the Schnauzenorgan, no direct interconnections were found between the trigeminal sensory and electromotor command system, or the electrosensory and trigeminal motor command. The pathways linking perception to action remain to be studied.

Otolith end organ projections to auditory neurons in the descending octaval nucleus of the goldfish, Carassius auratus: a confocal analysis

The distribution of axons from the saccule, lagena, and utricle to descending octaval nucleus neurons that project to the auditory midbrain in the goldfish is reported. We have divided these auditory projection neurons, located in the dorsal portion of the descending octaval nucleus (dDO), into two groups, medial and lateral, each of which contains several neuronal populations based on morphology and location. At most levels of the dDO, there are three medial and three lateral populations; the rostral dDO contains an additional lateral population. The saccule provides input to each of the seven medial and lateral populations but appears to be the exclusive/nearly exclusive source of primary input to the most dorsal cell group of the medial population. Along with the saccule, the lagena and utricle each supply the remaining six medial and lateral populations. Neurons in each of these populations receive input from more than one end organ. One medial and one lateral population include neurons that receive remarkably large contacts from utricular afferents. Overall, the results reveal a more substantial input from the lagena and utricle to the main first-order auditory nucleus in the goldfish than was previously recognized, suggest this nucleus is composed of functionally distinct populations, and relate to functional and evolutionary issues about hearing in early vertebrates.

Functional changes in the snail statocyst system elicited by microgravity

BACKGROUND

The mollusk statocyst is a mechanosensing organ detecting the animal's orientation with respect to gravity. This system has clear similarities to its vertebrate counterparts: a weight-lending mass, an epithelial layer containing small supporting cells and the large sensory hair cells, and an output eliciting compensatory body reflexes to perturbations.

METHODOLOGY/PRINCIPAL FINDINGS

In terrestrial gastropod snail we studied the impact of 16- (Foton M-2) and 12-day (Foton M-3) exposure to microgravity in unmanned orbital missions on: (i) the whole animal behavior (Helix lucorum L.), (ii) the statoreceptor responses to tilt in an isolated neural preparation (Helix lucorum L.), and (iii) the differential expression of the Helix pedal peptide (HPep) and the tetrapeptide FMRFamide genes in neural structures (Helix aspersa L.). Experiments were performed 13-42 hours after return to Earth. Latency of body re-orientation to sudden 90° head-down pitch was significantly reduced in postflight snails indicating an enhanced negative gravitaxis response. Statoreceptor responses to tilt in postflight snails were independent of motion direction, in contrast to a directional preference observed in control animals. Positive relation between tilt velocity and firing rate was observed in both control and postflight snails, but the response magnitude was significantly larger in postflight snails indicating an enhanced sensitivity to acceleration. A significant increase in mRNA expression of the gene encoding HPep, a peptide linked to ciliary beating, in statoreceptors was observed in postflight snails; no differential expression of the gene encoding FMRFamide, a possible neurotransmission modulator, was observed.

CONCLUSIONS/SIGNIFICANCE

Upregulation of statocyst function in snails following microgravity exposure parallels that observed in vertebrates suggesting fundamental principles underlie gravi-sensing and the organism's ability to adapt to gravity changes. This simple animal model offers the possibility to describe general subcellular mechanisms of nervous system's response to conditions on Earth and in space.

Electrophysiological characteristics of cells in the anterior caudal lobe of the mormyrid cerebellum

We have examined the basic electrophysiology and pharmacology of cells in the anterior caudal lobe (CLa) of the mormyrid cerebellum. Intracellular recordings were performed in an in vitro slice preparation using the whole-cell patch recording method. The responses of cells to parallel fiber (PF) and climbing fiber (CF) stimulation and to somatic current injection were recorded, and then characterized by bath application of receptor and ion channel blockers. Using biocytin or neurobiotin, these cells were also morphologically identified after recording to ensure their classification. Efferent cells and two subtypes of Purkinje cells were identified on the basis of their physiology and morphology. While the majority of Purkinje cells fire a single type of spike that is mediated by Na(+), some fire a large broad spike mediated by Ca(2+) and a narrow spike mediated by Na(+) at resting potential levels. By patching one recording electrode to the soma and another to one of the proximal dendrites of the same cell simultaneously, it was found that the Na(+) spike has an axonal origin and the Ca(2+) spike is generated in the soma-dendritic region of Purkinje cells. Efferent cells fire a single type of Na(+) spike only. Despite variations in their physiology and morphology, all cell types responded to PF stimulation with graded excitatory postsynaptic potentials (EPSPs) mediated by AMPA receptors. However, none of the efferent cells and only some of the Purkinje cells responded to CF activation with a large, AMPA receptor-mediated all-or-none EPSPs. We conclude that the functional circuitry of the CLa resembles that of other regions of the mormyrid cerebellum and is largely similar to that of the mammalian cerebellum.

Origin and early development of the posterior lateral line system of zebrafish

The lateral line system of teleosts has recently become a model system to study patterning and morphogenesis. However, its embryonic origins are still not well understood. In zebrafish, the posterior lateral line (PLL) system is formed in two waves, one that generates the embryonic line of seven to eight neuromasts and 20 afferent neurons and a second one that generates three additional lines during larval development. The embryonic line originates from a postotic placode that produces both a migrating sensory primordium and afferent neurons. Nothing is known about the origin and innervation of the larval lines. Here we show that a "secondary" placode can be detected at 24 h postfertilization (hpf), shortly after the primary placode has given rise to the embryonic primordium and ganglion. The secondary placode generates two additional sensory primordia, primD and primII, as well as afferent neurons. The primary and secondary placodes require retinoic acid signaling at the same stage of late gastrulation, suggesting that they share a common origin. Neither primary nor secondary neurons show intrinsic specificity for neuromasts derived from their own placode, but the sequence of neuromast deposition ensures that neuromasts are primarily innervated by neurons derived from the cognate placode. The delayed formation of secondary afferent neurons accounts for the capability of the fish to form a new PLL ganglion after ablation of the embryonic ganglion at 24 hpf.

The neuronal organization of a unique cerebellar specialization: the valvula cerebelli of a mormyrid fish

The distal valvula cerebelli is the most prominent part of the mormyrid cerebellum. It is organized in ridges of ganglionic and molecular layers, oriented perpendicular to the granular layer. We have combined intracellular recording and labeling techniques to reveal the cellular morphology of the valvula ridges in slice preparations. We have also locally ejected tracer in slices and in intact animals to examine its input fibers. The palisade dendrites and fine axon arbors of Purkinje cells are oriented in the horizontal plane of the ridge. The dendrites of basal efferent cells and large central cells are confined to the molecular layer but are not planar. Basal efferent cell axons are thick and join the basal bundle leaving the cerebellum. Large central cell axons are also thick, and they traverse long distances in the transverse plane, with local collaterals in the ganglionic layer. Vertical cells and small central cells also have thick axons with local collaterals. The dendrites of Golgi cells are confined to the molecular layer, but their axon arbors are either confined to the granular layer or proliferate in both the granular and ganglionic layers. Dendrites of deep stellate cells are distributed in the molecular layer, with fine axon arbors in the ganglionic layer. Granule cell axons enter the molecular layer as parallel fibers without bifurcating. Climbing fibers run in the horizontal plane and terminate exclusively in the ganglionic layer. Our results confirm and extend previous studies and suggest a new concept of the circuitry of the mormyrid valvula cerebelli.

Local circuitry in the anterior caudal lobe of the mormyrid cerebellum: a study of intracellular recording and labeling

The caudal lobe of the mormyrid cerebellum includes the anterior portion, which is associated with the lateral line and eighth nerve senses, and the posterior portion, which is associated with the electrosense. This study examines the physiology and morphology of cells in the anterior portion in slice preparations. Two subtypes of Purkinje cells, efferent cells and stellate cells, are described. Multipolar Purkinje cells are located in the central region of the lobe, with large, multipolar, spiny dendrites and locally ending axons. Small Purkinje cells are located along its anterior border with the eminentia granularis anterior (EGa), with spiny dendrites in the molecular region. Axons of some small Purkinje cells end locally, whereas axons of other such cells are cut at the surface of the slices, suggesting that they project outside the lobe. Efferent cells are also distributed along the border with EGa. These cells have thin, smooth dendrites in the molecular region, and their axons are cut at the sliced surface. Stellate cells have thin, smooth dendrites and locally terminating axons. Physiologically, all types of cells respond to parallel fiber activation, but only multipolar Purkinje cells showed characteristic all-or-none climbing fiber responses. Although the majority of Purkinje cells fire a single type of spikes at resting level, a subset of small Purkinje cells fire small, narrow and large, broad spikes. Thus, the anterior caudal lobe of the mormyrid cerebellum is different from the mammalian cerebellum in having different subtypes of Purkinje cells and local termination of many Purkinje cell axons.

Otolith endorgan input to the Mauthner neuron in the goldfish

The Mauthner (M-) cell of the goldfish, Carassius auratus, triggers the rapid escape response of the fish in response to various stimuli, including visual and auditory. The large size and accessibility of the M-cell make it an ideal model system for the study of synaptic transmission, membrane properties, and sensory-motor gating. Although physiological recordings have suggested that afferents from all three of the inner ear endorgans (the saccule, lagena, and utricle) synapse directly on the ipsilateral M-cell, the specific contacts and anatomical distributions of these inputs along the M-cell lateral dendrite remain unknown. We traced specific branches of the auditory (VIIIth) nerve from the three otolith organs of the fish inner ear to the M-cell. The goldfish sacculus gives rise to the vast majority of inputs that contact a large portion of the M-cell lateral dendrite, and these inputs vary greatly in size. In contrast to the ubiquitous distribution of saccular inputs, those from the lagena are segregated to distal regions of the M-cell and synapse on the distal dorsal branch of the lateral dendrite. Similarly, inputs from the utricle are also segregated to distal regions, synapsing on the ventral branch of the lateral dendrite. These results demonstrate that nerves from all three endorgans contact the M-cell, with input-specific segregation of synapses along the M-cell lateral dendrite.

Anatomy of the posterior caudal lobe of the cerebellum and the eminentia granularis posterior in a mormyrid fish

The cerebellum of mormyrid fish is of interest for its large size and unusual histology. The mormyrid cerebellum, as in all ray-finned fishes, has three subdivisions--valvula, corpus, and caudal lobe. The structures of the mormyrid valvula and corpus have been examined previously, but the structure of the mormyrid caudal lobe has not been studied. The mormyrid caudal lobe includes a posterior caudal lobe associated with the electrosense and an anterior caudal lobe associated with lateral line and eighth nerve senses. In this article we describe cellular elements of the posterior caudal lobe and of the eminentia granularis posterior (EGp) in the mormyrid fish Gnathonemus petersii. The EGp gives rise to the parallel fibers of the posterior caudal lobe. We used intracellular injection of biocytin, extracellular injection of biotinylated dextran amine, and immunohistochemistry with antibodies to gamma-aminobutyric acid, inositol triphosphate receptor I, calretinin, and Zebrin II. The histological structure of the posterior caudal lobe is markedly irregular in comparison to that of the corpus and the valvula, and a tight modular organization of cerebellar elements is less apparent here. Most Purkinje cell bodies are in the middle of the molecular region. Their dendrites are only roughly oriented in the sagittal plane, extend both ventrally and dorsally, and branch irregularly. Climbing fibers terminate only on smooth dendrites near the soma. Most Purkinje cell axons terminate locally on eurydendroid cells that project outside the cortex. The results provide an additional variant to the already large set of different cerebellar and cerebellum-like structures.

Deficits in acquisition of spatial learning after dorsomedial telencephalon lesions in goldfish

Acquisition of spatial learning is an important function of mammalian hippocampus. In order to identify the brain areas in teleost fish that are homologous to mammalian hippocampus, the present study examined the effects of lesions in the dorsal area of the caudal telencephalon of goldfish (Carassius auratus) on the acquisition of spatial learning. An open-field maze that was similar to the dry version of the Morris water maze was used. The task consisted of habituation and postoperative training to reach the position of the bait. Extramaze cues were visible in the habituation sessions in experiment 1, while they were blocked and not visible in the habituation sessions in experiment 2. Only in experiment 2, there was a significant deficit in the performance in the training sessions in the goldfish with damage to the dorsomedial area of the caudal telencephalon (DM). These data showed that blocking of the extramaze cues in the habituation sessions caused deficits in postoperative acquisition of spatial learning in the training sessions in the goldfish with DM lesions. Latent learning in the habituation sessions, however, eliminated the effects of the DM lesions on spatial learning. The present study suggests that the DM plays a critical role in acquisition of spatial learning.

Afferent and efferent connections of the cerebellum of a salmonid, the rainbow trout (Oncorhynchus mykiss): A tract-tracing study

The connections of the cerebellum of the rainbow trout were studied by experimental methods. The pretectal paracommissural nucleus has reciprocal connections with the cerebellum. Three additional pretectal nuclei project to both the corpus and valvula cerebelli, and seem to receive cerebellar afferents. A large number of cells of the lateral nucleus of the valvula project to wide regions of the cerebellum, including the valvula, the corpus, the granular eminences, and the caudal lobe, whereas the contralateral inferior olive and scattered reticular cells project only to the corpus and valvula cerebelli. Afferents to the corpus were also observed from the ventral tegmental nucleus, the "paraisthmic nucleus," the perilemniscal nucleus, the central gray, and the octavolateral area. Valvular afferents were also observed from the torus semicircularis and the midbrain tegmental areas. In most cases of cerebellar application, labeled fibers were seen in the thalamus, the pretectum, the torus longitudinalis and torus semicircularis, the nucleus of the medial longitudinal fascicle, and midbrain and rhombencephalic reticular areas. From the corpus cerebelli some fibers also project to the posterior tubercle and the hypothalamus. Moreover, the granular eminences project to the cerebellar crest. DiI application to most of the areas showing labeled fibers after cerebellar tracer application led to the labeling of characteristic eurydendroid cells, mainly in the valvula cerebelli and the caudal lobe. A few putative eurydendroid cells were labeled from the octavolateralis regions. These results in a teleost with a generalized brain indicate several differences with respect to the cerebellar connections reported in other teleost fishes that have specialized brains.

Role of SDF1 chemokine in the development of lateral line efferent and facial motor neurons

Most sensory systems are innervated by efferent neurons as well as by afferent neurons. The efferent innervation modulates the sensitivity of the receptor cells or of the sensory terminals. In the posterior lateral line system of the zebrafish, two efferent nuclei have been described in the hindbrain. Here we examine the development of the efferent neurons. We show that their axons are guided toward the target organ along the lateral line nerve while their cell bodies migrate posteriorward across rhombomeres to achieve their final position in rhombomeres 6/7. This migration depends on the SDF1 chemokine. We show that the migration of motor neurons of the facial nucleus from rhombomere 4 to 6 is also affected in sdf1a morphants (embryos injected with morpholine-conjugated antisense oligonucleotides). We propose that SDF1/CXCR4-mediated cell migration is preferentially associated with movement along the anteroposterior axis of the animal.

Immunocytochemical identification of cell types in the mormyrid electrosensory lobe

The electrosensory lobes (ELLs) of mormyrid and gymnotid fish are useful sites for studying plasticity and descending control of sensory processing. This study used immunocytochemistry to examine the functional circuitry of the mormyrid ELL. We used antibodies against the following proteins and amino acids: the neurotransmitters glutamate and gamma-aminobutyric acid (GABA); the GABA-synthesizing enzyme glutamic acid decarboxylase (GAD); GABA transporter 1; the anchoring protein for GABA and glycine receptors, gephyrin; the calcium binding proteins calbindin and calretinin; the NR1 subunit of the N-methyl-D-aspartate glutamate receptor; the metabotropic glutamate receptors mGluR1alpha, mGluR2/3, and mGluR5; and the intracellular signaling molecules calcineurin, calcium calmodulin kinase IIalpha (CAMKIIalpha) and the receptor for inositol triphosphate (IP3R1alpha). Selective staining allowed for identification of new cell types including a deep granular layer cell that relays sensory information from primary afferent fibers to higher order cells of ELLS. Selective staining also allowed for estimates of relative numbers of different cell types. Dendritic staining of Purkinje-like medium ganglion cells with antibodies against metabotropic glutamate receptors and calcineurin suggests hypotheses concerning mechanisms of the previously demonstrated synaptic plasticity in these cells. Finally, several cell types including the above-mentioned granular cells, thick-smooth dendrite cells, and large multipolar cells of the intermediate layer were present in the two zones of ELL that receive input from mormyromast electroreceptors but were absent in the zone of ELL that receives input from ampullary electroreceptors, indicating markedly different processing for these two types of input. J. Comp. Neurol. 483:124-142, 2005. (c) 2005 Wiley-Liss, Inc.

Neuronal encoding of ultrasonic sound by a fish

Many species of odontocete cetaceans (toothed whales) use high-frequency clicks (60-170 kHz) to identify objects in their environment, including potential prey. Behavioral studies have shown that American shad, Alosa sapidissima, can detect ultrasonic signals similar to those of odontocetes that are potentially their predators. American shad also show strong escape behavior in response to ultrasonic pulses between 70 and 110 kHz and can determine the location of the sound source at least in the horizontal plane. The present study examines physiological aspects of ultrasound detection by American shad and provides the first insights into the neural encoding of ultrasound signals in any nonmammalian vertebrate. The recordings were obtained by penetration through the cerebellar surface. All but two units responded exclusively to ultrasound. Ultrasound-sensitive units did not phase-couple to any stimulus frequency. Some units resembled the response of constant latency neurons found in the ventral nucleus of the lateral lemniscus of bats. We suggest that ultrasonic and sonic signals are processed along different pathways in Alosa. The ultrasonic pathway in Alosa appears to be a feature detector that is likely to be adapted (e.g., frequency, intensity) to odontocete echolocation signals.

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.

Reflections of efferent activity in rotational responses of chinchilla vestibular afferents

To study presumed efferent-mediated responses, we determined if afferents responded to head rotations that stimulated semicircular canals other than the organ being innervated. To minimize stimulation of an afferent's own canal, its plane was placed nearly orthogonal to the rotation plane. Otolith units were tested in a horizontal head position with the ear placed near the rotation axis to minimize linear forces. Under these circumstances, angular-velocity trapezoids (2-s ramps, 2-s plateau) evoked excitatory responses for both rotation directions. These type III responses were considerably larger in decerebrate than in anesthetized preparations. In addition to their being exclusively excitatory, the responses resembled those obtained with electrical stimulation of efferent pathways in including per-stimulus and more prolonged post-stimulus components and in being larger in irregularly discharging than in regularly discharging units. Responses, which were not seen for rotations <80 degrees/s, grew as velocity increased between 80 and 500 degrees/s but were seldom larger than 20 spikes/s. Complete section of the VIIIth nerve abolished type III responses, leaving conventional afferent responses intact. To study the separate contributions of canals on the two sides, responses were compared when the labyrinths were intact and when the ipsilateral or contralateral horizontal canal was mechanically inactivated. Both sides contributed to the efferent-mediated responses. That afferents could be influenced from the contralateral labyrinth was confirmed with the use of unilateral galvanic currents. Following inactivation, excitatory responses were produced by rotations exciting or inhibiting the intact horizontal canal with the responses resulting from excitatory rotations being much larger. Such a response asymmetry is consistent with a semicircular-canal origin for the type III responses. A similar asymmetry was seen in the post-stimulus responses to contralateral cathodal (excitatory) and anodal (inhibitory) galvanic currents. We conclude that the efferent system receives a sufficiently powerful vestibular input from both the ipsilateral and contralateral labyrinths to affect afferent discharge.

Early efferent innervation of the zebrafish lateral line

We examined the efferent innervation of the lateral line in zebrafish larvae. Three efferent nuclei were previously reported for the posterior line, two in the hindbrain and one in the ventral hypothalamus. Here we show that the same three nuclei innervate the anterior line as well. The rhombencephalic neurons innervate either the anterior or the posterior line. The diencephalic neurons seem to innervate both lines as well as the ear. The diencephalic efferents are labeled by anti-tyrosine hydroxylase antibodies and probably use dopamine as a transmitter. They are among the very first catecholaminergic neurons to differentiate in the brain and extend branches into the lateral line system almost as soon as the latter forms. We discuss possible functions of the rhombencephalic and diencephalic efferents.

Comparison and evolution of the lagena in various animal species

The structure of the vestibular organs of the teleost fish (bluegill), newts (Japanese fire-belly newt), frogs (black-spotted pond frog), snapping turtles and birds (chicks) was morphologically compared, with particular attention to the lagena macula, and the differences between animal species with relation to evolution were considered. Teleost fish had no striola on the lagena macula. The striola of newts were short and restricted to the central area of the macula, but those of frogs, snapping turtles and chicks extended from the anterior to posterior edges of the macula. This indicates that the frog is more highly evolved than the newt. The length of the kinocilium of sensory hairs was equal to that of the longest stereocilium in teleost fish and newts, but the kinocilia of frogs, snapping turtles and chicks were longer than the longest stereocilium. This indicates that the function of the lagena of teleost fish and newts is for hearing whilst in the other animals they are for posture. The diameter of the sensory hair bundles is small in teleost fish and frogs, but large in newts and snapping turtles. This indicates that the sensitivity of the sensory cells of the lagena towards outer force is low in teleost fish and frogs, high in newts and snapping turtles and intermediate in birds. The lagena of snapping turtles protrudes from the basilar papilla into the vestibule but the lagena of chicks lies on the tip of the long projecting basilar papilla. From observation of the locations of lagenae it is natural to speculate that there must have been some species of animal now extinct that had the evolving location of the lagena prior to that of chicks. In future it will be very interesting and useful to identify this extinct animal using DNA techniques.

Myelinated dendrites in the mormyrid electrosensory lobe

This is the third paper in a series on the morphology, immunohistochemistry, and synaptology of the mormyrid electrosensory lateral line lobe (ELL). The ELL is a highly laminated, cerebellum-like structure in the rhombencephalon that subserves an active electric sense: Objects in the nearby environment are detected on the basis of changes in the reafferent electrosensory signals that are generated by the animal's own electric organ discharge. This paper concentrates on the intermediate (cell and fiber) layer of the medial zone of the ELL and pays particular attention to the large multipolar neurons of this layer (LMI cells). LMI cells are gamma-aminobutyric acid (GABA)ergic and have one axon and three to seven proximal dendrites that all become myelinated after their last proximal branching point. The axon projects to the contralateral homotopic region and has ipsilateral collaterals. Both ipsilaterally and contralaterally, it terminates in the deep and superficial granular layers. The myelinated dendrites end in the deep granular layer, where they most likely do not make postsynaptic specializations, but do make presynaptic specializations, similar to those of the LMI axons. Because it is not possible to distinguish between axonal and dendritic LMI terminals in the granular layer, the authors refer to both as LMI terminals. These are densely filled with small, flattened vesicles and form large appositions with ELL granular cell somata and dendrites with symmetric synaptic membrane specializations. LMI cells do not receive direct electrosensory input on their somata, but electrophysiological recordings suggest that they nevertheless respond strongly to electrosensory signals (Bell [1990] J. Neurophysiol. 63:303-318). Consequently, the authors speculate that the myelinated dendrites of LMI cells are excited ephaptically (i.e., by electric field effects) by granular cells, which, in turn, are excited via mixed synapses by mormyromast primary afferents. The authors suggest that this ephaptic activation of the GABAergic presynaptic terminals of the myelinated dendrites may trigger immediate synaptic release of GABA and, thus, may provide a very fast local feedback inhibition of the excited granular cells in the center of the electrosensory receptive field. Subsequent propagation of the dendritic excitation down the myelinated dendrites to the somata and axon hillocks of LMI cells probably generates somatic action potentials, resulting in the spread of inhibition through axonal terminals to a wide region around the receptive field center and in the contralateral ELL. Similar presynaptic myelinated dendrites that subserve feedback inhibition, until now, have not been described elsewhere in the brain of vertebrates.

Efferent innervation in the goldfish saccule examined by acetylcholinesterase histochemistry

In contrast to the abundance of information available regarding the anatomy and physiology of afferents within the goldfish saccule, the efferent system of this auditory endorgan has been scarcely studied morphologically. In this study, acetylcholinesterase histochemistry with diaminobenzidine enhancement was used to describe the morphology of efferents. Under light microscopy, labeled fibers appeared in the distal portion of the saccular nerve, penetrated the basement membrane and formed a horizontal mesh-like plexus near the base of hair cells. Many vertical branchlets with terminal swellings protruded upward toward hair cells from the plexus. Under electron microscopy, dense extracellular labeling was present around efferent terminals, which often formed clusters on hair cells. Labeling was also present around unmyelinated fibers of passage within the sensory epithelium and the distal saccular nerve. These fibers contained coarse microtubules and small vesicles, and often ran in a bundle with other similar fibers. Based on their position within the epithelium, histochemistry and ultrastructural characteristics, these fibers were concluded to be efferents. These fibers became myelinated and unlabeled in the proximal saccular nerve. These results suggest that acetylcholinesterase can be a marker of entire distal unmyelinated portions of efferent fibers and demonstrated abundant efferent innervation in the goldfish saccule.

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.

Structural organization of the mormyrid electrosensory lateral line lobe

The electrosensory lateral line lobe (ELL) of mormyrid teleosts is the first central stage in electrosensory input processing. It is a well-developed structure with six main layers, located in the roof of the rhombencephalon. Its main layers are, from superficial to deep, the molecular, ganglionic, plexiform, granular, intermediate and deep fiber layers. An important input arises from electroreceptors, but corollary electromotor command signals and proprioceptive, mechanosensory lateral line and descending electrosensory feedback inputs reach the ELL as well. The ELL input is processed by at least 14 cell types, which frequently show plastic responses to different inputs. The large ganglionic and large fusiform cells are the ELL projection cells. They are glutamatergic and project to the isthmic preeminential nucleus and the midbrain lateral toral nucleus. Interneurons are located in all ELL layers and are mostly GABAergic. The most remarkable interneurons are large multipolar cells in the intermediate layer, which have myelinated dendrites making presynaptic terminals contacting granular cells. With respect to the synaptic organization and microcircuitry of the ELL, a number of qualitative and quantitative aspects have been elucidated using electron microscopical and intracellular labeling techniques. However, the pathways by which primary afferent input influences the ELL projection cells are still undetermined: primary afferents do not seem to contact large fusiform or large ganglionic cells directly, but seem to terminate exclusively on granular cells, the axonal properties of which are not known. Consequently, more information of the structural organization of the ELL is still necessary for a detailed understanding of the neural basis of the plastic electrosensory input processing in mormyrids.

Central mechanisms of temporal analysis in the knollenorgan pathway of mormyrid electric fish

Mormyrid electric fish communicate using pulse-type electric organ discharges (EODs). The fine temporal structure of the waveforms of EODs varies widely throughout the 200 or more species of mormyrids. These signals carry information about the species, the sex and even the individual identity of the signaller. Behavioral experiments have shown that some species of fish are capable of using this information. Of the four known types of electroreceptors in mormyrid fish, the knollenorgan electroreceptor is the one most likely to be involved in the detection of conspecific EOD signals. Here, we review some recent advances in understanding how the central knollenorgan pathway might be analyzing the temporal structure of the EOD waveform. Fine temporal analysis appears to take place in the nucleus exterolateralis pars anterior (ELa), where tightly phase-locked inputs from the hindbrain drive a direct, excitatory input through a long axonal delay line and also drive an indirect, inhibitory input with negligible delay through the ELa large cell. These two inputs converge on ELa small cells, where they are hypothesized to interact in a ‘delay-line/blanking’ model. This initial temporal analysis is further refined in the nucleus exterolateralis pars posterior, where units tuned to ranges of pulse durations have been identified physiologically.

Anatomical connections of auditory and lateral line areas of the dorsal telencephalon (Dm) in the osteoglossomorph teleost, Gnathonemus petersii

Local field potentials evoked either by auditory or by mechanosensory (water displacement) lateral line stimuli were recorded in sensory subregions of the telencephalic nucleus dorsalis pars medialis (Dm) in the weakly electric fish Gnathonemus petersii. The neural tracer Neurobiotin was injected into these two physiologically defined subregions. A reciprocal connection between the two subregions of Dm, as well as cell bodies and terminals in other telencephalic regions, whose distribution was somewhat different for the two injection types, were found. The course of labeled fibers outside the telencephalon was similar after injections in both Dm regions. Fibers were seen running through the lateral forebrain bundle (lfb) to the ventral surface area of the brain within the diencephalic preglomerular region (PGv). Within a narrow streak along the ventral side of the brain densely arranged cell bodies were labeled. The locations of labeled cells within PGv were indistinguishable after tracer was injected into either acoustical or lateral line areas of Dm. Only after injection into the mechanosensory Dm region labeled cell bodies were found in the anterior preglomerular nucleus (PGa), in addition. When crystals of the fluorescent tracer DiI were inserted in the ventral part of PGv, a path of labeled fibers similar to that after telencephalic injections was found. Labeled terminals, but no cell bodies, were located both in the acoustical and in the mechanosensory regions of Dm as well as in several other telencephalic areas. Even though sensory regions in Dm that process acoustical and mechanical stimuli are segregated and unimodal, they both receive input from neurons of PGv. The specificity of the mechanosensory region of Dm might originate from the additional input from PGa and from other endbrain areas.

Parallel projection of amplitude and phase information from the hindbrain to the midbrain of the African electric fish Gymnarchus niloticus

Two distinct sensory cues in electrosensory signals, amplitude modulation and differential phase modulation, are essential for an African wave-type electric fish, Gymnarchus, to perform the jamming avoidance responses. Individual neurons in the first brain station for central processing, the electrosensory lateral line lobe (ELL), were investigated by the in vivo whole-cell recording and labeling technique for their physiological responses, location, morphology, and projection areas. Neurons in the dorsal zone of the ELL responded selectively to amplitude modulation. Neurons in the outer cell layer of the medial zone were categorized physiologically into two groups: amplitude-sensitive and differential phase-sensitive. All but one neuron in the inner cell layer of the medial zone responded exclusively to differential phase modulation. All neurons recorded and labeled in the ELL had pyramidal morphology with large and extensive apical dendrites and less extensive basal dendrites. They were found to project to two midbrain nuclei: the nucleus praeeminentialis and the torus semicircularis. Amplitude-sensitive neurons in the dorsal zone projected exclusively to the lateral posterior subdivision, the torus semicircularis. Neurons in the medial zone projected to the medial dorsal and lateral anterior subdivisions of the torus semicircularis. Although some neurons in the ELL responded to both amplitude and differential phase modulation, they did not differentiate between temporal patterns of the two cues that encode necessary information for the jamming avoidance response. Overlapping projection of amplitude and differential phase-sensitive neurons to the torus semicircularis suggests integration of the two sensory cues in this nucleus.

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.

Interneurons of the ganglionic layer in the mormyrid electrosensory lateral line lobe: morphology, immunohistochemistry, and synaptology

This is the second paper in a series that describes the morphology, immunohistochemistry, and synaptology of the mormyrid electrosensory lateral line lobe (ELL). The ELL is a highly laminated cerebellum-like structure in the rhombencephalon that subserves an active electric sense: Objects in the nearby environment of the fish are detected on the basis of changes in the reafferent electrosensory signals that are generated by the animal's own electric organ discharge. The present paper describes interneurons in the superficial (molecular, ganglionic, and plexiform) layers of the ELL cortex that were analyzed in the light and electron microscopes after Golgi impregnation, intracellular labeling, neuroanatomical tracing, and gamma-aminobutyric acid (GABA) immunohistochemistry. The most numerous interneurons in the ganglionic layer are GABAergic medium-sized ganglionic (MG) cells and small ganglionic (SG) cells. MG cells have 10-20 spiny apical dendrites in the molecular layer, a cell body of 10-12 microns diameter in the ganglionic layer, a single basal dendrite that gives rise to fine, beaded, axon-like branches in either the plexiform layer (MG1 subtype) or the deeper granular layer (MG2 subtype), and an axon that terminates in the plexiform layer. Their apical dendritic tree has 12,000-22,000 spines that are contacted by GABA-negative terminals, and it receives, 1,250-2,500 GABA-positive contacts on the smooth dendritic surface between the spines. The average ratio of GABA-negative to GABA-positive contacts on the interneuron apical dendrites (14:1) is significantly higher than that for the efferent projection cells that have been described previously (Grant et al. [1996] J. Comp. Neurol., this issue). The somata and basal dendrites of MG cells receive a low to moderate density of GABAergic synaptic input, and their axons make GABAergic synaptic contacts with the somata and cell bodies of MG as well as with large ganglionic (LG) cells. SG cells probably represent immature, growing MG cells. Other interneurons in the superficial ELL layers include GABAergic stellate cells in the molecular layer, two types of non-GABAergic cells with smooth dendrites in the deep molecular layer that are named thick-smooth dendrite cells and deep molecular layer cells, and horizontal cells that are encountered particularly in the plexiform layer. Comparison with the ELL of waveform gymnotiform fish, which is another group of active electrolocating teleosts that has been investigated thoroughly, shows striking differences. In these fish, no GABAergic interneurons are found in the ganglionic (pyramidal) layer of the ELL, and GABA-negative interneurons with smooth dendrites in the molecular layer also seem to be lacking. At present, the phylogenetic origin of the described superficial interneurons in the mormyrid ELL is uncertain.

Efferent neurons of the lateral line system and their innervation of lateral line branches in a euteleost and an osteoglossomorph

The efferent neurons of the lateral line system of the euteleost Aplocheilus lineatus and the osteoglossomorph Pantodon buchholzi, both surface feeding fish, were examined by neuronal tract tracing. Besides horse-radish peroxidase, fluorescent dextrans were used as tracers to allow simultaneus visualization of projections from different lateral line branches. Labeled efferent neurons were found in nuclei situated in the medulla ventral of ventricle IV. This position resembles the octa-volateralis efferent nucleus of previous studies. The number of labeled cells in the efferent nucleus is low in both species. Most neurons were found ipsilaterally to the application site, some along the midline and only very few contralaterally. The size of efferent cells differs distinctly between Aplocheilus, possessing small cell-bodies (length 16.5 microm), and Pantodon, which has very large efferent cells (length 47.0 micron). Efferent axon bundles course rostrally in both species, leaving the brain at the level of the anterior lateral line nerve. Only Aplocheilus has in addition lateral axon bundles leaving the brain at the level of the posterior lateral line nerve. After application of one fluorescent tracer to the lateral ramus and a different fluorescent tracer to the superficial ophtalmic ramus in a given animal, double-labeling of efferent cells hardly ever occurs. If the neuromasts I and IV of the dorsal skull of Pantodon are applied with one fluorescent tracer each, approximately 10% of centrally labeled cells are double-labeled. Considering the results of double-labeling, the concept of a differential innervation of lateral line branches is supported and discussed.

Cytoarchitecture of the medial octavolateralis nucleus in the goldfish, Carassius auratus

The medial octavolateralis nucleus (MON) is the principal first-order medullary lateral line sensory nucleus found in the majority of anamniotic vertebrates. Although its presence has been confirmed in numerous taxa, the cytoarchitecture of this region has not been extensively studied in any species. The purpose of this study was to examine in detail the cytoarchitecture of the MON in the goldfish using Golgi staining and HRP histochemical techniques. The results of this study demonstrated the presence of a number of cell types with distinct cellular morphologies, several of which strongly resemble those described in octavolateralis nuclei dedicated to audition and electroreception. The most prominent of these MON neurons included crest cells of two varieties, either possessing or lacking basilar dendrites. Additionally, we described stellate and cristal interneurons and granule-like cells in the molecular layer, and lateral interneurons and granule-like neurons in deeper MON layers. These morphological similarities together with similarities in functional organization, and the probable close phyletic relationships of this "family" of hair cell sensory systems, argue for parallels in mechanisms of sensory processing and analysis in strongly divergent sensory modalities.

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.

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.