Cytology and immunocytochemistry of the nucleus of the lateral line lobe in the electric fish Gnathonemus petersii (Mormyridae): evidence suggesting that GABAergic synapses mediate an inhibitory corollary discharge

Distribution of gamma-aminobutyric acid immunoreactivity in the brain of the Siberian sturgeon (Acipenser baeri): Comparison with other fishes

Gamma-aminobutyric acid (GABA) is the main inhibitory neurotransmitter in the central nervous system (CNS) of vertebrates. Immunohistochemical techniques with specific antibodies against GABA or against its synthesizing enzyme, glutamic acid decarboxylase (GAD) allowed characterizing GABAergic neurons and fibers in the CNS. However, studies on the CNS distribution of GABAergic neurons and fibers of bony fishes are scant and were done in teleost species. With the aim of understanding the early evolution of this system in bony vertebrates, we analyzed the distribution of GABA-immunoreactive (-ir) and GAD-ir neurons and fibers in the CNS of a basal ray-finned fish, the Siberian sturgeon (Chondrostei, Acipenseriformes), using immunohistochemical techniques. Our results revealed the presence and distribution of GABA/GAD-ir cells in different regions of the CNS such as olfactory bulbs, pallium and subpallium, hypothalamus, thalamus, pretectum, optic tectum, tegmentum, cerebellum, central grey, octavolateralis area, vagal lobe, rhombencephalic reticular areas, and the spinal cord. Abundant GABAergic innervation was observed in most brain regions, and GABAergic fibers were very abundant in the hypothalamic floor along the hypothalamo-hypophyseal tract and neurohypophysis. In addition, GABA-ir cerebrospinal fluid-contacting cells were observed in the alar and basal hypothalamus, saccus vasculosus, and spinal cord central canal. The distribution of GABAergic systems in the sturgeon brain shows numerous similarities to that observed in lampreys, but also to those of teleosts and tetrapods.

Hormonal coordination of motor output and internal prediction of sensory consequences in an electric fish

Steroid hormones remodel neural networks to induce seasonal or developmental changes in behavior. Hormonal changes in behavior likely require coordinated changes in sensorimotor integration. Here, we investigate hormonal effects on a predictive motor signal, termed corollary discharge, that modulates sensory processing in weakly electric mormyrid fish. In the electrosensory pathway mediating communication behavior, inhibition activated by a corollary discharge blocks sensory responses to self-generated electric pulses, allowing the downstream circuit to selectively analyze communication signals from nearby fish. These pulses are elongated by increasing testosterone levels in males during the breeding season. We induced electric-pulse elongation using testosterone treatment and found that the timing of electroreceptor responses to self-generated pulses was delayed as electric-pulse duration increased. Simultaneous recordings from an electrosensory nucleus and electromotor neurons revealed that the timing of corollary discharge inhibition was delayed and elongated by testosterone. Furthermore, this shift in the timing of corollary discharge inhibition was precisely matched to the shift in timing of receptor responses to self-generated pulses. We then asked whether the shift in inhibition timing was caused by direct action of testosterone on the corollary discharge circuit or by plasticity acting on the circuit in response to altered sensory feedback. We surgically silenced the electric organ of fish and found similar hormonal modulation of corollary discharge timing between intact and silent fish, suggesting that sensory feedback was not required for this shift. Our findings demonstrate that testosterone directly regulates motor output and internal prediction of the resulting sensory consequences in a coordinated manner.

Chemosensation: Corollary discharge filters out self-generated chemical cues

Corollary discharge allows organisms to discriminate external sensory inputs from self-generated cues. However, the underlying synaptic and molecular mechanisms are not well understood. A new study has identified a tyraminergic corollary discharge signal that extrasynaptically modulates chemosensory neurons in Caenorhabditis elegans.

A History of Corollary Discharge: Contributions of Mormyrid Weakly Electric Fish

Corollary discharge is an important brain function that allows animals to distinguish external from self-generated signals, which is critical to sensorimotor coordination. Since discovery of the concept of corollary discharge in 1950, neuroscientists have sought to elucidate underlying neural circuits and mechanisms. Here, we review a history of neurophysiological studies on corollary discharge and highlight significant contributions from studies using African mormyrid weakly electric fish. Mormyrid fish generate brief electric pulses to communicate with other fish and to sense their surroundings. In addition, mormyrids can passively locate weak, external electric signals. These three behaviors are mediated by different corollary discharge functions including inhibition, enhancement, and predictive “negative image” generation. Owing to several experimental advantages of mormyrids, investigations of these mechanisms have led to important general principles that have proven applicable to a wide diversity of animal species.

Signal Diversification Is Associated with Corollary Discharge Evolution in Weakly Electric Fish

Communication signal diversification is a driving force in the evolution of sensory and motor systems. However, little is known about the evolution of sensorimotor integration. Mormyrid fishes generate stereotyped electric pulses (electric organ discharge [EOD]) for communication and active sensing. The EOD has diversified extensively, especially in duration, which varies across species from 0.1 to >10 ms. In the electrosensory hindbrain, a corollary discharge that signals the timing of EOD production provides brief, precisely timed inhibition that effectively blocks responses to self-generated EODs. However, corollary discharge inhibition has only been studied in a few species, all with short-duration EODs. Here, we asked how corollary discharge inhibition has coevolved with the diversification of EOD duration. We addressed this question by comparing 7 mormyrid species (both sexes) having varied EOD duration. For each individual fish, we measured EOD duration and then measured corollary discharge inhibition by recording evoked potentials from midbrain electrosensory nuclei. We found that delays in corollary discharge inhibition onset were strongly correlated with EOD duration as well as delay to the first peak of the EOD. In addition, we showed that electrosensory receptors respond to self-generated EODs with spikes occurring in a narrow time window immediately following the first peak of the EOD. Direct comparison of time courses between the EOD and corollary discharge inhibition revealed that the inhibition overlaps the first peak of the EOD. Our results suggest that internal delays have shifted the timing of corollary discharge inhibition to optimally block responses to self-generated signals.SIGNIFICANCE STATEMENT Corollary discharges are internal copies of motor commands that are essential for brain function. For example, corollary discharge allows an animal to distinguish self-generated from external stimuli. Despite widespread diversity in behavior and its motor control, we know little about the evolution of corollary discharges. Mormyrid fishes generate stereotyped electric pulses used for communication and active sensing. In the electrosensory pathway that processes communication signals, a corollary discharge inhibits sensory responses to self-generated signals. We found that fish with long-duration pulses have delayed corollary discharge inhibition, and that this time-shifted corollary discharge optimally blocks electrosensory responses to the fish's own signal. Our study provides the first evidence for evolutionary change in sensorimotor integration related to diversification of communication signals.

Sensing External and Self-Motion with Hair Cells: A Comparison of the Lateral Line and Vestibular Systems from a Developmental and Evolutionary Perspective

Detection of motion is a feature essential to any living animal. In vertebrates, mechanosensory hair cells organized into the lateral line and vestibular systems are used to detect external water or head/body motion, respectively. While the neuronal components to detect these physical attributes are similar between the two sensory systems, the organizational pattern of the receptors in the periphery and the distribution of hindbrain afferent and efferent projections are adapted to the specific functions of the respective system. Here we provide a concise review comparing the functional organization of the vestibular and lateral line systems from the development of the organs to the wiring from the periphery and the first processing stages. The goal of this review is to highlight the similarities and differences to demonstrate how evolution caused a common neuronal substrate to adapt to different functions, one for the detection of external water stimuli and the generation of sensory maps and the other for the detection of self-motion and the generation of motor commands for immediate behavioral reactions.

The cellular and circuit basis for evolutionary change in sensory perception in mormyrid fishes

Species differences in perception have been linked to divergence in gross neuroanatomical features of sensory pathways. The anatomical and physiological basis of evolutionary change in sensory processing at cellular and circuit levels, however, is poorly understood. Here, we show how specific changes to a sensory microcircuit are associated with the evolution of a novel perceptual ability. In mormyrid fishes, the ability to detect variation in electric communication signals is correlated with an enlargement of the midbrain exterolateral nucleus (EL), and a differentiation into separate anterior (ELa) and posterior (ELp) regions. We show that the same cell types and connectivity are found in both EL and ELa/ELp. The evolution of ELa/ELp, and the concomitant ability to detect signal variation, is associated with a lengthening of incoming hindbrain axons to form delay lines, allowing for fine temporal analysis of signals. The enlargement of this brain region is also likely due to an overall increase in cell numbers, which would allow for processing of a wider range of timing information.

The mammalian olivocochlear system--a legacy of non-cerebellar research in the Mugnaini lab

Although the major emphasis of Enrico Mugnaini's research has been on investigations of the cerebellum, a significant amount of work over a relatively short span of time was also done in his lab on a number of other brain systems. These centered on sensory systems. One of these extra-cerebellar systems that he embraced was the auditory system. Portions of the cochlear nucleus, the first synaptic relay station along the central auditory pathways, possess a cerebellar-like circuitry and neurochemistry, and this no doubt lured Enrico into the auditory field. As new tools became available to pursue neuroanatomical research in general, which included a novel antibody to glutamic acid decarboxylase (GAD), Enrico's lab soon branched out into investigating many other brain structures beyond the cerebellum, with an overall goal of producing a map illustrating GAD expression in the brain. In collaboration with long-term colleagues, one of these many non-cerebellar regions he took an interest in was an efferent pathway originating in the superior olive and projecting to the cochlea, the peripheral end organ for hearing. There was a need for a more complete neurochemical map of this olivocochlear efferent system, and armed with new antibodies and well-established tract tracing tools, together we set out to further explore this system. This short review describes the work done with Enrico on the olivocochlear system of rodents, and also continues the story beyond Enrico's lab to reveal how the work done in his lab fits into the larger scheme of current, ongoing research into the olivocochlear system.

Detection of transient synchrony across oscillating receptors by the central electrosensory system of mormyrid fish

Recently, we reported evidence for a novel mechanism of peripheral sensory coding based on oscillatory synchrony. Spontaneously oscillating electroreceptors in weakly electric fish (Mormyridae) respond to electrosensory stimuli with a phase reset that results in transient synchrony across the receptor population (Baker et al., 2015). Here, we asked whether the central electrosensory system actually detects the occurrence of synchronous oscillations among receptors. We found that electrosensory stimulation elicited evoked potentials in the midbrain exterolateral nucleus at a short latency following receptor synchronization. Frequency tuning in the midbrain resembled peripheral frequency tuning, which matches the intrinsic oscillation frequencies of the receptors. These frequencies are lower than those in individual conspecific signals, and instead match those found in collective signals produced by groups of conspecifics. Our results provide further support for a novel mechanism for sensory coding based on the detection of oscillatory synchrony among peripheral receptors.

DOI:http://dx.doi.org/10.7554/eLife.16851.001

Multiplexed temporal coding of electric communication signals in mormyrid fishes

The coding of stimulus information into patterns of spike times occurs widely in sensory systems. Determining how temporally coded information is decoded by central neurons is essential to understanding how brains process sensory stimuli. Mormyrid weakly electric fishes are experts at time coding, making them an exemplary organism for addressing this question. Mormyrids generate brief, stereotyped electric pulses. Pulse waveform carries information about sender identity, and it is encoded into submillisecond-to-millisecond differences in spike timing between receptors. Mormyrids vary the time between pulses to communicate behavioral state, and these intervals are encoded into the sequence of interspike intervals within receptors. Thus, the responses of peripheral electroreceptors establish a temporally multiplexed code for communication signals, one consisting of spike timing differences between receptors and a second consisting of interspike intervals within receptors. These signals are processed in a dedicated sensory pathway, and recent studies have shed light on the mechanisms by which central circuits can extract behaviorally relevant information from multiplexed temporal codes. Evolutionary change in the anatomy of this pathway is related to differences in electrosensory perception, which appears to have influenced the diversification of electric signals and species. However, it remains unknown how this evolutionary change relates to differences in sensory coding schemes, neuronal circuitry and central sensory processing. The mormyrid electric communication pathway is a powerful model for integrating mechanistic studies of temporal coding with evolutionary studies of correlated differences in brain and behavior to investigate neural mechanisms for processing temporal codes.

From sequence to spike to spark: evo-devo-neuroethology of electric communication in mormyrid fishes

Mormyrid fishes communicate using pulses of electricity, conveying information about their identity, behavioral state, and location. They have long been used as neuroethological model systems because they are uniquely suited to identifying cellular mechanisms for behavior. They are also remarkably diverse, and they have recently emerged as a model system for studying how communication systems may influence the process of speciation. These two lines of inquiry have now converged, generating insights into the neural basis of evolutionary change in behavior, as well as the influence of sensory and motor systems on behavioral diversification and speciation. Here, we review the mechanisms of electric signal generation, reception, and analysis and relate these to our current understanding of the evolution and development of electromotor and electrosensory systems. We highlight the enormous potential of mormyrids for studying evolutionary developmental mechanisms of behavioral diversification, and make the case for developing genomic and transcriptomic resources. A complete mormyrid genome sequence would enable studies that extend our understanding of mormyrid behavior to the molecular level by linking morphological and physiological mechanisms to their genetic basis. Applied in a comparative framework, genomic resources would facilitate analysis of evolutionary processes underlying mormyrid diversification, reveal the genetic basis of species differences in behavior, and illuminate the origins of a novel vertebrate sensory and motor system. Genomic approaches to studying the evo-devo-neuroethology of mormyrid communication represent a deeply integrative approach to understanding the evolution, function, development, and mechanisms of behavior.

Evolution of time-coding systems in weakly electric fishes

Weakly electric fishes emit electric organ discharges (EODs) from their tail electric organs and sense feedback signals from their EODs by electroreceptors in the skin. The electric sense is utilized for various behaviors, including electrolocation, electrocommunication, and the Jamming avoidance response (JAR). For each behavior, various types of sensory Information are embedded in the transient electrical signals produced by the fish. These temporal signals are sampled, encoded, and further processed by peripheral and central neurons specialized for time coding. There are time codes for the sex or species Identities of other fish or the resistance and capacitance of objects. In the central nervous system, specialized neural elements exist for decoding time codes for different behavioral functions. Comparative studies allow phylogenetic comparison of time-coding neural systems among weakly electric fishes.

Time-domain signal divergence and discrimination without receptor modification in sympatric morphs of electric fishes

Polymorphism in an animal communication channel provides a framework for studying proximate rules of signal design as well as ultimate mechanisms of signal diversification. Reproductively isolated mormyrid fishes from Gabon's Brienomyrus species flock emit distinctive electric organ discharges(EODs) thought to function in species and sex recognition. Species boundaries and EODs appear congruent in these fishes, with the notable exception of three morphs designated types I, II and III. Within the species flock, these morphs compose a monophyletic group that has recently been called the magnostipes complex. Co-occurring morphs of this complex express distinctive EODs, yet they appear genetically indistinguishable at several nuclear loci. In this study, we investigated EOD discrimination by these morphs using both behavioral and physiological experiments. During the breeding season, wild-caught type I and type II males showed evidence that they can discriminate their own morph's EOD waveform from that of a sympatric and genetically distinct reference species. However, we found that type I and type II males exhibited an asymmetry in unconditioned responses to paired playback of EODs recorded from type I versus type II females. Males of the type II morph responded preferentially to EODs of type II females,whereas type I males did not appear to discriminate homotypic and heterotypic EODs in our experimental paradigm. Part of this behavioral asymmetry may have resulted from a previously undetected difference in adult size, which may have enhanced apparent discrimination by the smaller morph (type II) due to a relatively higher risk of injury from the larger morph (type I). Knollenorgan receptors, which mediate electrical communication in mormyrids, showed similar spectral tuning in type I and type II. These electroreceptors coded temporal features of any single magnostipes-complex EOD with similar patterns of time-locked spikes in both morphs. By contrast, Knollenorgans exhibited distinctive responses to different EOD waveforms. These results suggest that discrete EOD variation in this rapidly diversifying complex is functional in terms of morph-specific advertisement and recognition. Time-domain signal divergence has outpaced frequency-domain divergence between sympatric morphs,requiring little to no change in receptor response properties. We discuss our findings in light of a model for EOD time-coding by the Knollenorgan pathway,as well as evolutionary hypotheses concerning sympatric signal diversification in the magnostipes complex.

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.

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.

Neural substrates for species recognition in the time-coding electrosensory pathway of mormyrid electric fish

Mormyrid electric fish have species- and sex-typical electric organ discharges (EODs). One class of tuberous electroreceptors, the knollenorgans, plays a critical role in electric communication; one function is species recognition of EOD waveforms. In this paper, we describe cell types in the knollenorgan central pathway, which appear responsible for analysis of the temporal patterns of spikes encoded by the knollenorgans in response to EOD stimuli. Secondary sensory neurons in the nucleus of the electrosensory lateral line lobe (NELL) act as relays of peripheral responses. They fire a single phase-locked spike to an outside positive-going voltage step. Axons from the NELL project to the toral nucleus exterolateralis pars anterior (ELa). Immediately after they enter the ELa, they send collaterals to terminate on one to three ELa large cells and then continue in a lengthy neuronal pathway that traverses the ELa several times. After a path length of up to 5 mm, the NELL axon terminates on as many as 70 ELa small cells. Thus the large cells appear to be excited first, followed by the small cells, with the intervening length of the axon serving as a delay line. The large cells also respond with phase-locked spikes to voltage steps. Large cell axons extend for approximately 1 mm and terminate on several small cells within the ELa. The terminals are known to be GABAergic inputs and are presumed inhibitory. We propose that small cells receive direct inhibition from large cells and delayed excitation from NELL axons. The small cells may act as anti-co-incidence detectors to analyze the temporal structure of the EOD waveform.

Projection neurons of the mormyrid electrosensory lateral line lobe: morphology, immunohistochemistry, and synaptology

This paper describes the morphological, immunohistochemical, and synaptic properties of projection neurons in the highly laminated medial and dorsolateral zones of the mormyrid electrosensory lateral line lobe (ELL). These structures are involved in active electrolocation, i.e., the detection and localization of objects in the nearby environment of the fish on the basis of changes in the reafferent electrosensory signal generated by the animal's own electric organ discharge. Electrosensory, corollary electromotor command-associated signals (corollary discharges), and a variety of other inputs are integrated within the ELL microcircuit. The organization of ELL projection neurons is analyzed at the light and electron microscopic levels based on Golgi impregnations, intracellular labeling, neuroanatomical tracer techniques, and gamma-aminobutyric acid (GABA), gamma-aminobutyric acid decarboxylase (GAD), and glutamate immunohistochemistry. Two main types of ELL projection neurons have been distinguished in mormyrids: large ganglionic (LG) and large fusiform (LF) cells. LG cells have a multipolar cell body (average diameter 13 microns) in the ganglionic layer, whereas LF cells have a fusiform cell body (on average, about 10 x 20 microns) in the granular layer. Apart from the location and shape of their soma, the morphological properties of these cell types are largely similar. They are glutamaterigic and project to the midbrain torus semicircularis, where their axon terminals make axodendritic synaptic contacts in the lateral nucleus. They have 6-12 apical dendrites in the molecular layer, with about 10,000 spines contacted by GABA-negative terminals and about 3,000 GABA-positive contacts on the smooth dendritic surface between the spines. Their somata and short, smooth basal dendrites, which arborize in the plexiform layer (LG cells) or in the granular layer (LF cells), are densely covered with GABA-positive, inhibitory terminals. Correlation with physiological data suggests that LG cells are I units, which are inhibited by stimulation of the center of their receptive fields, and LF cells are E units, excited by electric stimulation of the receptive field center. Comparison with the projection neurons of the ELL of gymnotiform fish, which constitute another group of active electrolocating teleosts, shows some striking differences, emphasizing the independent development of the ELL in both groups of teleosts.

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.

Correlating gamma-aminobutyric acidergic circuits and sensory function in the electrosensory lateral line lobe of a gymnotiform fish

Electric fish generate an electric field, which they sense with cutaneous electroreceptors. Electroreceptors project topographically onto the medullary electrosensory lateral line lobe (ELL). The ELL of gymnotiform electric fish is divided into four segments specialized to detect different aspects of the electrosensory input; it is also laminated with separate laminae devoted to electroreceptive input, interneurons, projection neurons, and feedback input. We have utilized antisera to glutamic acid decarboxylase (GAD) and gamma-aminobutyric acid (GABA) to map the distribution of GABAergic cells and fibers in the ELL of the gymnotiform fish, Apteronotus leptorhynchus. Six types of GABAergic interneurons are found in ELL: Type 2 granular cells (granular layer) project to pyramidal cells; polymorphic cells (pyramidal cell layer) project to the non-GABAergic type 1 granular cells; ovoid cells (deep neuropil layer) project bilaterally upon basilar dendrites of pyramidal cells; multipolar cells (deep neuropil layer) project bilaterally, probably to dendrites and neurons within the deep neuropil layer; and neurons of the ventral molecular layer and stellate cells (molecular layer) project to apical dendrites of pyramidal cells. GABAergic bipolar cells in the nucleus praeminentialis, a rhombencephalic structure devoted to feedback in the electrosensory system, project in relatively diffuse fashion to pyramidal cells. We hypothesize that the various GABAergic circuits of the ELL can be correlated with specific functions: type 2 granular cells with adaptation, size of receptive field center, and gain; polymorphic cells and type 1 granular cells with regulation of surround inhibition; ovoid cells with common mode rejection; and neurons of the ventral molecular layer with adaptive gain control. The feedback GABAergic input from bipolar cells of n. praeminentialis to pyramidal cells may be part of a searchlight mechanism similar to the one postulated for thalamocortical systems.

Extraordinary synapses of the unipolar brush cell: an electron microscopic study in the rat cerebellum

A neglected type of neuron, termed the unipolar brush cell, was recently characterized in the granular layer of the mammalian cerebellar cortex with several procedures, including light and electron microscopic immunocytochemistry utilizing antibodies to calretinin and neurofilament proteins. Although certain features of the unipolar brush cells were highlighted in these studies, the internal fine structure was partially obfuscated by immunoreaction product. In this study, rat cerebella were prepared for electron microscopy after perfusion fixation and Araldite embedding, and folia of the vestibulo-cerebellum, where unipolar brush cells are known to be enriched, were studied by light microscopy in semithin (0.5-1 micron) sections and by electron microscopy in ultrathin sections. Unipolar brush cells were easily identified in semithin sections immunostained with antibodies to GABA and/or glycine, and counterstained with toluidine blue. The unipolar brush cells have a pale cytoplasm and are GABA and glycine negative, while Golgi cells are darker and appear positive for GABA and, for the most part, also for glycine. Sets of identification criteria to differentiate unipolar brush cells from granule and Golgi cells in standard electron micrographs are presented. The unipolar brush cells possess many distinctive features that make them easily distinguishable from other cerebellar neurons and form unusually conspicuous and elaborate synapses with mossy rosettes. The unipolar brush cell has a deeply indented nucleus containing little condensed chromatin. The Golgi apparatus is large and the cytoplasm is rich in neurofilaments, microtubules, mitochondria, and large dense core vesicles, but contains few cisterns of granular endoplasmic reticulum. In addition, unipolar brush cells contain an unusual inclusion, which invariably lacks a limiting membrane and is made up of peculiar ringlet subunits. The cell body usually emits a thin axon and is provided with a single, large dendritic trunk that terminates with a paintbrush-like bunch of branchlets. Numerous nonsynaptic appendages emanate from the cell body, the dendritic stem, and the branchlets. The appendages contain rare organelles and lack neurofilaments. The branchlets contain numerous mitochondria, neurofilaments, large dense core vesicles, and clusters of clear, small, and round synaptic vesicles. They form extensive asymmetric synaptic junctions with one or two mossy fibers, which indicates minimal convergence of excitatory inputs. Under the postsynaptic densities, the branchlet cytoplasm displays a microfilamentous web. Besides their contact with mossy rosettes, the branchlets form symmetric and asymmetric synaptic junctions with presumed Golgi cell boutons that contain pleomorphic synaptic vesicles, indicating that the unipolar brush cells receive an inhibitory modulation. Some of these junctions are unusually extensive.(ABSTRACT TRUNCATED AT 400 WORDS)

Evidence for GABAergic interneurons in the red nucleus of the painted turtle

Immunocytochemical and electrophysiological evidence supporting the presence of GABAergic interneurons in the turtle red nucleus is presented. Injections of HRP into the spinal cord produced labeling of large neurons in the contralateral red nucleus. The peroxidase-antiperoxidase (PAP) method revealed smaller cells immunoreactive to an antibody against glutamate decarboxylase (GAD), the synthetic enzyme for the inhibitory neurotransmitter GABA, that were interspersed among larger immunonegative neurons. Similar small neurons were densely immunostained by antibodies to GABA-glutaraldehyde conjugates obtained from different sources and applied according to pre-embedding and postembedding protocols. Rubrospinal neurons retrogradely labeled with HRP measured 16 and 27 microns in mean minor and major cell body diameters, while GABA-like immunopositive neurons situated within the red nucleus measured 7 and 13 microns. There was very little overlap in soma size between the two cell populations. Therefore, we suggest that the GAD- and GABA-positive neurons may be local inhibitory interneurons. This notion is further supported by observations of pre-embedding immunostaining for GAD and postembedding immunostaining for GABA showing that the turtle red nucleus is amply innervated by immunoreactive axon terminals. These puncta are closely apposed to cell bodies and dendrites of both immunonegative large neurons and immunopositive small neurons. Moreover, immunogold staining at the electron microscopic level demonstrated that GABA-like immunoreactive axon terminals with pleomorphic synaptic vesicles formed symmetric synapses with cell bodies and dendrites of the two types of red nucleus cells. These ultrastructural features are commonly assumed to indicate inhibitory synapses. A moderately labeled bouton with round vesicles and asymmetric synapses was also observed. In addition, the two types of red nucleus neurons received asymmetric axosomatic and axodendritic synapses with GABA-negative boutons provided with round vesicles, features usually associated with excitatory functions. To obtain electrophysiological evidence for inhibition, intracellular recordings from red nucleus neurons were conducted using an in vitro brainstem-cerebellum preparation from the turtle. Small, spontaneous IPSPs were recorded from 7 out of 14 red nucleus cells studied. These morphological and physiological results provide strong support for concluding that the turtle red nucleus, like its mammalian counterpart, contains GABAergic inhibitory interneurons. While we have not identified the main source of input to these interneurons, in view of the scarce development of the reptilian cerebral cortex, this input is unlikely to come from the motor cortex as it does in mammals.(ABSTRACT TRUNCATED AT 400 WORDS)

Recent advances in the study of electroreception

Recent studies on electroreception in fish have focused on the structure and function of recurrent descending pathways, efference copy mechanisms, and multiple neuronal maps involved in the processing of sensory information. Studies on a neuronal oscillator have revealed that different neuronal inputs modulate the pattern of oscillations to produce different forms of behavioral output.

Topography and ultrastructure of commissural interneurons that may establish reciprocal inhibitory connections of the Mauthner axons in the spinal cord of the tench, Tinca tinca L

This study was made to identify the inhibitory interneurons belonging to the spinal circuitry activated by the Mauthner axons in the tench (Tinca tinca L.). The histological investigations were focused on a segmental pair of commissural interneurons that were reconstructed in toto from their distinguishing topographical and ultrastructural features. These features are: (a) the adendritic soma located 100-150 microns dorsal to the central canal; (b) the first node of Ranvier which is precommissural and connected to the ipsilateral Mauthner axon via gap junction; (c) the second node of Ranvier, from which two first-order branches arise postcommissurally each supplying roughly the rostral and caudal half of the contralateral spinal cord segment; (d) their second-order branches, which arise at intervals that correspond closely to those of the Mauthner axon collaterals; (e) the postsynaptic targets of the second-order branches, which are exclusively all the motoneurons and interneurons innervated by the contralateral Mauthner axon; (f) the axon terminals of these branches, which contain F-type vesicles, form Gray type-2 synapses, and abut either on the initial segment or on the first node of Ranvier of the target neurons. Thus, it appears that this segmental interneuron has all the appropriate features that could provide the structural basis for the reciprocal fast-acting inhibitory coupling underlying the startle reflex elicited by the Mauthner neurons in response to auditory stimuli.

Gap junction protein in weakly electric fish (Gymnotide): immunohistochemical localization with emphasis on structures of the electrosensory system

Electrotonic transmission via gap junctions appears to be essential for both the relay and integration of information in nuclear groups involved in the electrolocation and electrocommunication systems of weakly electric fish. An affinity-purified antibody against the 27 kD gap-junctional polypeptide (GJP) from rat liver was used to determine immunohistochemically the distribution of GJP-immunoreactivity (GJP-IR) in electrosensory structures and some other brain regions of the gymnotiform fish, Apteronotus leptorhynchus. At the ultrastructural level, immunolabelling with this antibody was localized, in part, to neuronal and glial gap junctions where it was assumed to recognize a junctional polypeptide. By light microscopy, the vast majority of immunoreactive elements appeared either as fine puncta or as varicosities along fibers that exhibited immunostained intervaricose segments. Diffuse immunoreactivity within cell bodies was rare, being most evident in giant relay neurons and presumptive glial cells within the pacemaker nucleus and in neurons within the posterior raphe nucleus. The distribution of punctate and fibrous GJP-IR was remarkably heterogeneous with respect to density; large areas of the forebrain and most major fiber tracts were nearly devoid of immunoreactivity, whereas concentrations of puncta delineating patches within the inferior lobe of the hypothalamus and the vagal sensory nucleus were so dense as to appear as uniform deposition of immunoperoxidase reaction product at low magnification. Some structures known to be associated with the electrosensory system, including the nucleus electrosensorius and nucleus praeeminentialis, were among the brain regions containing the highest concentrations of immunoreactivity. At the cellular level, expected patterns of GJP-IR were observed in the pacemaker nucleus, torus semicircularis, and electrosensory lateral line lobe. In each of these structures punctate immunoreactivity was seen in apposition to cell bodies or dendrites of neurons known to receive gap junction contacts. In addition, the dendrites of neurons within the prepacemaker nucleus were laden with a striking array of puncta, suggesting that interactions via gap junctions may be a significant feature of these neurons. These immunohistochemical results are consistent with previous electrophysiological and ultrastructural observations pointing to the importance of electrotonic communication in the electrosensory system of weakly electric fish, and suggest that gap junctions may also contribute to neural transmission in central nervous system related to other functions in these teleosts.(ABSTRACT TRUNCATED AT 400 WORDS)

Comparative study of glutamate decarboxylase immunoreactive boutons in the mammalian inferior olive

An antiserum raised against rat glutamate decarboxylase was used to map GABAergic boutons in the inferior olive of rabbit, cat, rhesus monkey, and human. A description of the human periolivary region is also included. The inferior olive of each species contained a dense GABAergic innervation, but immunostaining intensities varied among regions. These intensities were evaluated visually and photometrically, and the sizes and frequencies of occurrence of boutons in various olivary subnuclei were measured. The beta nucleus in all species was intensely immunostained and contained the largest boutons. The caudal subdivision of the dorsal accessory olive stained with a lower intensity than the beta nucleus, but contained similarly large GABAergic boutons. By visual analysis, the rostral subdivision and the subnucleus a of the medial accessory olive and the principal olive were stained with an intermediate intensity, and these regions contained small GABAergic boutons. Photometric analysis of focal regions of the neuropil, however, revealed species differences in teh staining intensity of the principal olive, which was lowest in rabbits and highest in primates. In all species, the lowest immunostaining intensity was observed in the subnucleus b of the medial accessory olive. Species variations in bouton sizes and regional staining intensities were observed in the dorsal cap and the dorsomedial cell column. The heterogeneous staining pattern and regional variation of bouton size argue for the existence of separate GABAergic projections to discrete regions of the inferior olive. Since glutamate decarboxylase immunostaining patterns in the olive are largely similar across species, the afferent projections producing these patterns may also be similar.

Mormyromast electroreceptor organs and their afferent fibers in mormyrid fish: I. Morphology

Mormyromast electroreceptor organs are the most numerous type of electroreceptor organs in mormyrid electric fish and provide the sensory information necessary for active electrolocation. Mormyromast organs and their primary afferent fibers have not been studied very extensively. Both morphological and physiological questions remain to be answered before the neural basis of active electrolocation in mormyrids can be understood. This paper examines four different aspects of the morphology of mormyromast organs and afferent fibers: 1) Mormyromast organs in the skin. The innervation patterns for the two types of separately innervated sensory cells in the mormyromast organ are described on the basis of silver-stained whole mounts of skin. The number of sensory cells per mormyromast organ increases linearly with fish growth for both types of sensory cells. 2) Relation between peripheral sensory cell innervated and central zone of termination for mormyromast afferent fibers. The afferent fibers arising from the two types of sensory cell in the mormyromast organ project to separate zones of the electrosensory lateral line lobe, as shown by using retrograde labeling with horseradish peroxidase. 3) Central trajectories and terminal arbors of mormyromast afferent fibers. These aspects of mormyromast fibers are described by using intracellular staining of individual fibers as well as whole nerve staining of an electrosensory nerve. 4) Fine structure of mormyromast afferent terminals in the electrosensory lateral line lobe. Afferent fibers make various synaptic contacts, including contacts of a mixed type, gap junction-chemical, onto a restricted class of granule cells. The fine structure is described based on electron microscopy of horseradish-peroxidase-labeled fibers. The results provide an anatomical base for current physiological studies on mormyromast afferent fibers.

Sound-generating (sonic) motor system in a teleost fish (Porichthys notatus): sexual polymorphisms and general synaptology of sonic motor nucleus

The sonic motor nucleus of the plainfin midshipman, Porichthys notatus, is a midline nucleus located at the junction of the caudal medulla and rostral spinal cord. Its motoneurons innervate sonic "drumming" muscles that are attached to the lateral walls of the swimbladder. There are two classes of sexually mature males referred to as Type I and Type II. The Type I males are larger and generate sounds during the breeding season. The Type II males are smaller and, like adult females, have not yet been shown to generate sounds. This study examined possible sex differences in the size of sonic motoneurons, and the type and distribution of their afferent terminal boutons. The average soma diameter of motoneurons of Type I males is about 50% larger than that of Type II males and females. There is also a small but significant difference in soma diameter between Type II males and females; they are smaller in the former class. There were no sex differences in the presence or distribution of different classes of axosomatic and axodendritic terminal boutons, which included: (1) active zones with either clear, round, or pleomorphic vesicles, (2) active zones with both clear, round vesicles, and larger dense core vesicles, (3) "mixed synapses" with gap junctions and active zones usually associated with pleomorphic vesicles. The results are discussed within the context of sexual differentiation of vertebrate motor systems and the functional organization of the sonic motor system in fishes. Sex differences in soma diameter correlate with a number of sex differences in the gross and ultrastructural features that distinguish the sonic muscles of Type I males from those of Type II males and females, which are similar to each other. The absence of qualitative sex differences in synaptic morphology suggest that the central neuronal circuitry of the sonic motor system is similar among all three adult morphs.

Serotoninergic neurons in the mormyrid brain and their projection to the preelectromotor and primary electrosensory centers: immunohistochemical study

Serotonin-containing neurons in the brain of the weak-electric fish Gnathonemus petersii (mormyridae, teleostei) were studied with the aid of immunohistochemical labeling. Study of the central serotoninergic innervation was focused on the structures subserving the command of the electric organ and the first central relay of the electrosensory system. In the midline raphe nuclei, serotoninergic neurons formed a column that stretched from the ventral caudal medulla to the dorsal midbrain, ending caudal to the cerebellar peduncle. In the dorsal tegmentum, serotoninergic neurons were found bilaterally at the anterior margin of the decussation of the lateral lemniscus. Labeled neurons were also present bilaterally immediately anterior to the cerebellar peduncle and also in the pretectal region. In the hypothalamus, many serotoninergic neurons were in contact with the ventricular wall, and a few were present in the preoptic area. This distribution of serotoninergic cell bodies showed many similarities to that in other fish and higher vertebrates but lacked the lateral spread of the serotoninergic raphe system found in the midbrain tegmentum in mammals. Labeled fibers were found in both the preelectromotor medullary relay nucleus and the electromotor command nucleus. These serotoninergic projections were traced to the posterior raphe. Serotoninergic fibers also formed a dense network in the cortex and in the nucleus of the electrosensory lobe, both of which receive primary input from electroreceptors. These results suggest that serotonin may have a role in the modulation of the intrinsic, rhythmic electromotor command and in the gating of electrosensory input.

Patterns of glutamate decarboxylase immunostaining in the feline cochlear nuclear complex studied with silver enhancement and electron microscopy

The cochlear nuclear complex of the cat was immunostained with an antiserum to glutamate decarboxylase (GAD), the biosynthetic enzyme for the inhibitory neurotransmitter GABA, and studied with different procedures, including silver intensification, topical colchicine injections, semithin sections, and immunoelectron microscopy. Immunostaining was found in all portions of the nucleus. Relatively few immunostained cell bodies were observed: most of these were in the dorsal cochlear nucleus and included stellate cells, cartwheel cells, Golgi cells, and unidentified cells in the deep layers. An accumulation of immunoreactive cells was also found within the small cell cap and along the medial border of the ventral cochlear nucleus. Immunostained cells were sparse in magnocellular portions of the ventral nucleus. Most staining within the nucleus was of nerve terminals. These included small boutons that were prominent in the neuropil of the dorsal cochlear nucleus, the granule cell domain, in a region beneath the superficial granule cell layer within the small cell cap region, and along the medial border of the ventral nucleus. Octopus cells showed small, GAD-positive terminals distributed at moderate density on both cell bodies and dendrites. Larger, more distinctive terminals were identified on the large cells in the ventral nucleus, in particular on spherical cells and globular cells. There was a striking positive correlation of the size, location, and complexity of GAD-positive terminals with the size, location, and complexity of primary fiber endings on the same cells. This correlation did not hold in the dorsal nucleus, where pyramidal cells receive many large GAD-positive somatic terminals despite the paucity of primary endings on their cell bodies. The GAD-positive terminals contained pleomorphic synaptic vesicles and formed symmetric synaptic junctions that occupied a substantial portion of the appositional surface to cell bodies, dendrites, axon hillocks, and the beginning portion of the initial axon segments. Thus, the cells provided with large terminals can be subjected to considerable inhibition that may be activated indirectly through primary fibers and interneurons or by descending inputs from the auditory brainstem.