Rapid activation of GABAergic interneurons and possible calcium independent GABA release in the mormyrid electrosensory lobe

Neural readout of a latency code in the active electrosensory system

The latency of spikes relative to a stimulus conveys sensory information across modalities. However, in most cases, it remains unclear whether and how such latency codes are utilized by postsynaptic neurons. In the active electrosensory system of mormyrid fish, a latency code for stimulus amplitude in electroreceptor afferent nerve fibers (EAs) is hypothesized to be read out by a central reference provided by motor corollary discharge (CD). Here, we demonstrate that CD enhances sensory responses in postsynaptic granular cells of the electrosensory lobe but is not required for reading out EA input. Instead, diverse latency and spike count tuning across the EA population give rise to graded information about stimulus amplitude that can be read out by standard integration of converging excitatory synaptic inputs. Inhibitory control over the temporal window of integration renders two granular cell subclasses differentially sensitive to information derived from relative spike latency versus spike count.

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.

Modeling latency code processing in the electric sense: from the biological template to its VLSI implementation

Understanding the coding of sensory information under the temporal constraints of natural behavior is not yet well resolved. There is a growing consensus that spike timing or latency coding can maximally exploit the timing of neural events to make fast computing elements and that such mechanisms are essential to information processing functions in the brain. The electric sense of mormyrid fish provides a convenient biological model where this coding scheme can be studied. The sensory input is a physically ordered spatial pattern of current densities, which is coded in the precise timing of primary afferent spikes. The neural circuits of the processing pathway are well known and the system exhibits the best known illustration of corollary discharge, which provides the reference to decoding the sensory afferent latency pattern. A theoretical model has been constructed from available electrophysiological and neuroanatomical data to integrate the principal traits of the neural processing structure and to study sensory interaction with motor-command-driven corollary discharge signals. This has been used to explore neural coding strategies at successive stages in the network and to examine the simulated network capacity to reproduce output neuron responses. The model shows that the network has the ability to resolve primary afferent spike timing differences in the sub-millisecond range, and that this depends on the coincidence of sensory and corollary discharge-driven gating signals. In the integrative and output stages of the network, corollary discharge sets up a proactive background filter, providing temporally structured excitation and inhibition within the network whose balance is then modulated locally by sensory input. This complements the initial gating mechanism and contributes to amplification of the input pattern of latencies, conferring network hyperacuity. These mechanisms give the system a robust capacity to extract behaviorally meaningful features of the electric image with high sensitivity over a broad working range. Since the network largely depends on spike timing, we finally discuss its suitability for implementation in robotic applications based on neuromorphic hardware.

Identifying self- and nonself-generated signals: lessons from electrosensory systems

This chapter provides a short review of the mechanisms used by electroreceptive fish to discriminate self- from nonself-generated signals. Electroreception is used by animals to detect objects of electric impedance different from the water, to detect natural electrogenic sources and to communicate signals between conspecifics. Electroreceptive animals may generate electric fields either with the purpose of electrically illuminating the neighborhood or as an epiphenomenon of other functions. In addition, the presence of the fish body as a conductive object in a scene funnels the current flow and, consequently, animal movements also generate signals by changing the body shape or the spatial relationship of the body with the surrounding objects. Therefore, mechanisms for discrimination between self and externally generated signals are very important for constructing a coherent representation of the environment. Some mechanisms facilitate and stream the flow of signals carried by the self-generated electric field. Others are designed to reject unwanted interference coming from self-generated movements or even the self-generated electric field. Finally, more complex operations involving sensory motor integration are used for discriminating between self- and conspecific- generated communication signals. Despite the evolutionary distance between animals endowed with electric sense, mechanisms for self-identification reappear with few differences between species. This suggests that many of the possible strategies are present in vertebrates may be found in these fish. Therefore, we have much to learn about self recognition from the study of electroreception.

A diversity of synaptic filters are created by temporal summation of excitation and inhibition

Temporal filtering is a fundamental operation of nervous systems. In peripheral sensory systems, the temporal pattern of spiking activity can encode various stimulus qualities, and temporal filtering allows postsynaptic neurons to detect behaviorally relevant stimulus features from these spike trains. Intrinsic excitability, short-term synaptic plasticity, and voltage-dependent dendritic conductances have all been identified as mechanisms that can establish temporal filtering behavior in single neurons. Here we show that synaptic integration of temporally summating excitation and inhibition can establish diverse temporal filters of presynaptic input. Mormyrid electric fish communicate by varying the intervals between electric organ discharges. The timing of each discharge is coded by peripheral receptors into precisely timed spikes. Within the midbrain posterior exterolateral nucleus, temporal filtering by individual neurons results in selective responses to a particular range of presynaptic interspike intervals. These neurons are diverse in their temporal filtering properties, reflecting the wide range of intervals that must be detected during natural communication behavior. By manipulating presynaptic spike timing with high temporal resolution, we demonstrate that tuning to behaviorally relevant patterns of presynaptic input is similar in vivo and in vitro. We reveal that GABAergic inhibition plays a critical role in establishing different temporal filtering properties. Further, our results demonstrate that temporal summation of excitation and inhibition establishes selective responses to high and low rates of synaptic input, respectively. Simple models of synaptic integration reveal that variation in these two competing influences provides a basic mechanism for generating diverse temporal filters of synaptic input.

Etomidate reduces initiation of backpropagating dendritic action potentials: implications for sensory processing and synaptic plasticity during anesthesia

Anesthetics may induce specific changes that alter the balance of activity within neural networks. Here we describe the effects of the GABA(A) receptor potentiating anesthetic etomidate on sensory processing, studied in a cerebellum-like structure, the electrosensory lateral line lobe (ELL) of mormyrid fish, in vitro. Previous studies have shown that the ELL integrates sensory input and removes predictable features by comparing reafferent sensory signals with a descending electromotor command-driven corollary signal that arrives in part through parallel fiber synapses with the apical dendrites of GABAergic interneurons. These synapses show spike timing-dependent depression when presynaptic activation is associated with postsynaptic backpropagating dendritic action potentials. Under etomidate, almost all neurons become tonically hyperpolarized. The threshold for action potential initiation increased for both synaptic activation and direct intracellular depolarization. Synaptically evoked inhibitory postsynaptic potentials (IPSPs) were also strongly potentiated and prolonged. Current source density analysis showed that backpropagation of action potentials through the apical dendritic arborization in the molecular layer was reduced but could be restored by increasing stimulus strength. These effects of etomidate were blocked by bicuculline or picrotoxin. It is concluded that etomidate affects both tonic and phasic inhibitory conductances at GABA(A) receptors and that increased shunting inhibition at the level of the proximal dendrites also contributes to increasing the threshold for action potential backpropagation. When stimulus strength is sufficient to evoke backpropagation, repetitive association of synaptic excitation with postsynaptic action potential initiation still results in synaptic depression, showing that etomidate does not interfere with the molecular mechanism underlying plastic modulation.

Sensory and motor effects of etomidate anesthesia

The effects of anesthesia with etomidate on the cellular mechanisms of sensory processing and sensorimotor coordination have been studied in the active electric sense of the mormyrid fish Gnathonemus petersii. Like many anesthetics, etomidate is known to potentiate GABA(A) receptors, but little is known about the effects on sensory processing at the systems level. A better understanding is necessary for experimental studies of sensory processing, in particular regarding possible effects on the dynamic structure of excitatory and inhibitory receptive fields and to improve the knowledge of the mechanisms of anesthesia in general. Etomidate slowed the electromotor discharge rhythm, probably because of feedback inhibition at the premotor level, but did not alter the structure of the electromotor command. Sensory translation through primary afferents projecting to the cerebellum-like electrosensory lobe (ELL) was not changed. However, central interneurons and projection neurons were hyperpolarized under etomidate, and their spiking activity was reduced. Although the spatial extent and the center/surround organization of sensory receptive fields were not changed, initial excitatory responses were followed by prolonged inhibition. Corollary discharge input to ELL was maintained, and the temporal sequence of excitatory and inhibitory components of this descending signal remained intact. Later inhibitory corollary discharge responses were prolonged by several hundred milliseconds. The result was that excitatory reafferent sensory input was conserved with enhanced precision of timing, whereas background activity was greatly reduced. Anti-Hebbian synaptic plasticity evoked by association of sensory and corollary discharge input was still present under anesthesia, and differences compared with the nonanesthetized condition are discussed.

Some environmental contaminants influence motor and feeding behaviors in the ornate wrasse (Thalassoma pavo) via distinct cerebral histamine receptor subtypes

Common environmental contaminants such as heavy metals and pesticides pose serious risks to behavioral and neuroendocrine functions of many aquatic organisms. In the present study, we show that the heavy metal cadmium and the pesticide endosulfan produce such effects through an interaction of specific cerebral histamine receptor subtypes in the teleost ornate wrasse (Thalassoma pavo). Treatment of this teleost with toxic cadmium levels for 1 week was sufficient to induce abnormal swimming movements, whereas reduced feeding behaviors were provoked predominantly by elevated endosulfan concentrations. In the brain, these environmental contaminants caused neuronal degeneration in cerebral targets such as the mesencephalon and hypothalamus, damage that appeared to correlate with altered binding levels of the three major histamine receptors (subtypes 1, 2, and 3). Although cadmium accounted for reduced binding activity of all three subtypes in most brain regions, it was subtype 2 that seemed to be its main target, as shown by a very great (p < 0.001) down-regulation in mesencephalic areas such as the stratum griseum central layer. Conversely, endosulfan provided very great and great (p < 0.01) up-regulating effects of subtype 3 and 1 levels, respectively, in preoptic-hypothalamic areas such as the medial part of the lateral tuberal nucleus, and in the suprachiasmatic nucleus. These results suggest that the neurotoxicant-dependent abnormal motor and feeding behaviors may well be tightly linked to binding activities of distinct histamine subtypes in localized brain regions of the Thalassoma pavo.

Dendritic spike back propagation in the electrosensory lobe of Gnathonemus petersii

Spike timing-dependent plasticity that follows anti-Hebbian rules has been demonstrated at synapses between parallel fibers and inhibitory interneurons known as medium ganglionic layer (MG) neurons in the cerebellum-like electrosensory lobe of mormyrid fish. This plasticity is expressed when presynaptic activation is associated with a characteristically broad, postsynaptic action potential, lasting 7-15 ms, occurring within a window of up to 60-80 ms following synaptic activation. Since the site of plastic change is presumably in the apical dendrites, it is important to know where, when and how this broad spike is generated and the manner in which such events propagate within the intrinsic network of the electrosensory lobe. The electrosensory lobe has a strict layered organization that makes the preparation suitable for one dimension current source density analysis. Using this technique in an 'in vitro' interface slice preparation, we found that following either parallel fiber stimulation or an orthogonal field stimulus, a sink appeared in the ganglionic layer and propagated into the molecular layer. Intracellular records from MG somata showed these stimuli evoked broad action potentials whose timing corresponds to this sink. TTX application in the deep fiber layer blocked the synaptically evoked ganglionic layer field potential and the 'N3' wave of the outer molecular layer field potential simultaneously, while the molecular layer 'N1' and 'N2' waves corresponding to synaptic activation of the apical dendrites remained intact. These results confirm the hypothesis that the broad spikes of MG cells originate in the soma and propagate through the molecular layer in the apical dendritic tree, and suggest the possibility that this backpropagation may contribute to 'boosting' of the synaptic response in distal apical dendrites in certain circumstances.

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.

Pre-receptor profile of sensory images and primary afferent neuronal representation in the mormyrid electrosensory system

Afferent responses to the fish's own electric organ discharge were explored in the electrosensory lobe of the mormyrid fish Gnathonemus petersii. In order to understand the neural encoding of natural sensory images,responses were examined while objects of different conductivities were placed at different positions along the skin of the fish, i.e. at different points within, and also outside, peripheral receptive fields. The presence of an object in the fish's self-generated electric field produces local modulation of transcutaneous current density. Measurement of the local electric organ discharge shows that object images formed at the electroreceptive sensory surface have an opposing center-surround, `Mexican hat' profile. This is a pre-receptor phenomenon intrinsic to the physical nature of the sensory stimulus that takes place prior to neural lateral inhibition and is independent of such central inhibition.

Stimulus intensity is encoded in the latency and number of action potentials in the response of primary afferent fibers. It is also reflected in changes in the amplitude and area of extracellular field potentials recorded in the deep granular layer of the electrosensory lobe. Since the object image consists of a redistribution of current density over the receptive surface,its presence is coded by change in the activity of receptors over an area much larger than the skin surface facing the object. We conclude that each receptor encodes information coming from the whole scene in a manner that may seem ambiguous when seen from a single point and that, in order to extract specific object features, the brain must process the electric image represented over the whole sensory surface.

Dye coupling without gap junctions suggests excitatory connections of gamma-aminobutyric acidergic neurons

After injections of the low-molecular-weight tracer neurobiotin into the preeminential nucleus of the brain of the mormyrid fish Gnathonemus petersii, we observed that retrogradely labeled, large fusiform projection neurons (LFd cells) of the deep granular layer of the electrosensory lobe (ELL) were surrounded by 30-50 labeled satellite granular cells. More superficially located projection cells, including large fusiform cells in the superficial granular layer (LFs) and large ganglionic (LG) cells in the ganglionic layer, were never surrounded by labeled satellites. LFd-satellite cells have a small soma (diameter 5-8 microm), a few small dendrites, and an apical axon that terminates in the plexiform and ganglionic layers of the ELL. They contact LFd projection neurons with dendrodendritic, dendrosomatic, and somatodendritic puncta adhaerentia-like appositions, designated here as "neurapses." In the electron microscope, these contacts resemble synapses without presynaptic vesicles. Because no gap junctions were found between LFd and satellite granule cells, we suggest that the neurapses allow the passage of neurobiotin, though not biocytin or biotinylated dextran amine. These contacts may provide the intermediate substrate for the postulated, but so far unknown, excitatory connection between primary afferent input and LFd projection neurons, via gamma-aminobutyric acid (GABA)-ergic granular cells. We suggest that certain puncta adhaerentia-like contacts might not be only adhesive structures and that LFd-satellite granular cells might both excite LFd projection cells via neuraptic contacts of their dendrites and cell bodies and inhibit more superficial LF and LG cells via their GABAergic axonal synapses. Our results suggest that puncta adhaerentia-like contacts could be responsible in some cases for the electrical coupling found electrophysiologically in local inhibitory circuits.

Cadmium toxicity in synaptic neurotransmission in the brain

Chronic exposure to cadmium causes central nervous system disorders, e.g. olfactory dysfunction. To clarify cadmium toxicity in synaptic neurotransmission in the brain, the movement of cadmium in the synapses was examined using in vivo microdialysis. One and 24 h after injection of (109)CdCl(2) into the amygdala of rats, (109)Cd release into the extracellular space was facilitated by stimulation with high K(+), suggesting that cadmium taken up by amygdalar neurons is released into the synaptic clefts in a calcium- and impulse-dependent manner. Moreover, to examine the action of cadmium in the synapses, the amygdala was perfused with artificial cerebrospinal fluid containing 10-30 microM CdCl(2). The release of excitatory neurotransmitters, i.e. glutamate and aspartate, into the extracellular space was decreased during perfusion with cadmium, while the release of inhibitory neurotransmitters, i.e. glycine and gamma-amino butyric acid (GABA), into the extracellular space was increased during the period. These results suggest that cadmium released from the amygdalar neuron terminals affect the degree and balance of excitation-inhibition in synaptic neurotransmission.

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.