Electric organ corollary discharge pathways in mormyrid fish

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.

Plastic corollary discharge predicts sensory consequences of movements in a cerebellum-like circuit

The capacity to predict the sensory consequences of movements is critical for sensory, motor, and cognitive function. Though it is hypothesized that internal signals related to motor commands, known as corollary discharge, serve to generate such predictions, this process remains poorly understood at the neural circuit level. Here we demonstrate that neurons in the electrosensory lobe (ELL) of weakly electric mormyrid fish generate negative images of the sensory consequences of the fish's own movements based on ascending spinal corollary discharge signals. These results generalize previous findings describing mechanisms for generating negative images of the effects of the fish's specialized electric organ discharge (EOD) and suggest that a cerebellum-like circuit endowed with associative synaptic plasticity acting on corollary discharge can solve the complex and ubiquitous problem of predicting sensory consequences of movements.

From the intrinsic properties to the functional role of a neuron phenotype: an example from electric fish during signal trade-off

This review deals with the question: what is the relationship between the properties of a neuron and the role that the neuron plays within a given neural circuit? Answering this kind of question requires collecting evidence from multiple neuron phenotypes and comparing the role of each type in circuits that perform well-defined computational tasks. The focus here is on the spherical neurons in the electrosensory lobe of the electric fish Gymnotus omarorum. They belong to the one-spike-onset phenotype expressed at the early stages of signal processing in various sensory modalities and diverse taxa. First, we refer to the one-spike neuron intrinsic properties, their foundation on a low-threshold K(+) conductance, and the potential roles of this phenotype in different circuits within a comparative framework. Second, we present a brief description of the active electric sense of weakly electric fish and the particularities of spherical one-spike-onset neurons in the electrosensory lobe of G. omarorum. Third, we introduce one of the specific tasks in which these neurons are involved: the trade-off between self- and allo-generated signals. Fourth, we discuss recent evidence indicating a still-undescribed role for the one-spike phenotype. This role deals with the blockage of the pathway after being activated by the self-generated electric organ discharge and how this blockage favors self-generated electrosensory information in the context of allo-generated interference. Based on comparative analysis we conclude that one-spike-onset neurons may play several functional roles in animal sensory behavior. There are specific adaptations of the neuron's 'response function' to the circuit and task. Conversely, the way in which a task is accomplished depends on the intrinsic properties of the neurons involved. In short, the role of a neuron within a circuit depends on the neuron and its functional context.

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.

Multimodal integration in granule cells as a basis for associative plasticity and sensory prediction in a cerebellum-like circuit

The recoding of diverse sensory and motor signals by granule cells (GCs) is probably critical for the function of cerebellar circuits, yet the nature of these transformations and their significance for cerebellar information processing remain poorly understood. In cerebellum-like structures in fish, anti-Hebbian plasticity at parallel fiber synapses generates "negative images" that act to cancel predictable patterns of electrosensory input. Here I test the hypothesis that GCs enhance the capacity of Purkinje-like cells to generate specific negative images by selectively encoding combinations of sensory and motor signals. Using in vivo whole-cell recordings, I show (1) that a subset of GCs integrate sensory and motor signals conveyed by distinct mossy fiber classes and (2) that Purkinje-like cells exhibit plastic changes specific to the combinations of signals that individual GCs encode. Consistent with influential theories of cerebellar function, these findings suggest that selective GC output enhances the capacity of Purkinje-like cells to acquire selectivity through associative plasticity.

Receptive field properties of neurons in the electrosensory lateral line lobe of the weakly electric fish, Gnathonemus petersii

The receptive field of a sensory neuron is known as that region in sensory space where a stimulus will alter the response of the neuron. We determined the spatial dimensions and the shape of receptive fields of electrosensitive neurons in the medial zone of the electrosensory lateral line lobe of the African weakly electric fish, Gnathonemus petersii, by using single cell recordings. The medial zone receives input from sensory cells which encode the stimulus amplitude. We analysed the receptive fields of 71 neurons. The size and shape of the receptive fields were determined as a function of spike rate and first spike latency and showed differences for the two analysis methods used. Spatial diameters ranged from 2 to 36 mm (spike rate) and from 2.45 to 14.12 mm (first spike latency). Some of the receptive fields were simple consisting only of one uniform centre, whereas most receptive fields showed a complex and antagonistic centre-surround organisation. Several units had a very complex structure with multiple centres and surrounding-areas. While receptive field size did not correlate with peripheral receptor location, the complexity of the receptive fields increased from rostral to caudal along the fish's body.

New insights into corollary discharges mediated by identified neural pathways

Sensory systems respond not only to stimuli from the environment but also to cues generated by an animal's own behaviour. This leads to problems of sensory processing because self-generated information can occur at the same time as external sensory information. However, in motor regions of the CNS corollary discharges are generated during behaviour. These signals are not used to generate movements directly but, instead, interact with the processing of self-generated sensory signals. Corollary discharges transiently modulate self-generated sensory responses and can prevent self-induced desensitization or help distinguish between self-generated and externally generated sensory information. Here, we review recent work that has identified corollary discharge pathways at different levels of the CNS in vertebrates and invertebrates.

The cellular basis of a corollary discharge

How do animals discriminate self-generated from external stimuli during behavior and prevent desensitization of their sensory pathways? A fundamental concept in neuroscience states that neural signals, termed corollary discharges or efference copies, are forwarded from motor to sensory areas. Neurons mediating these signals have proved difficult to identify. We show that a single, multisegmental interneuron is responsible for the pre- and postsynaptic inhibition of auditory neurons in singing crickets (Gryllus bimaculatus). Therefore, this neuron represents a corollary discharge interneuron that provides a neuronal basis for the central control of sensory responses.

Interruption of pacemaker signals by a diencephalic nucleus in the African electric fish, Gymnarchus niloticus

The African electric fish Gymnarchus niloticus rhythmically emits electric organ discharges (EODs) for communication and navigation. The EODs are generated by the electric organ in the tail in response to the command signals from the medullary pacemaker complex, which consists of a pacemaker nucleus (PN), two lateral relay nuclei (LRN) and a medial relay nucleus (MRN). The premotor structure and its modulatory influences on the pacemaker complex have been investigated in this paper. A bilateral prepacemaker nucleus (PPn) was found in the area of the dorsal posterior nucleus (DP) of the thalamus by retrograde labeling from the PN. No retrogradely labeled neurons outside the pacemaker complex were found after tracer injection into the LRN or MRN. Accordingly, anterogradely labeled terminal fibers from PPn neurons were found only in the PN. Iontophoresis of L-glutamate into the region of the PPn induced EOD interruptions. Despite the exclusive projection of the PPn neurons to the PN, extracellular and intracellular recordings showed that PN neurons continue their firing while MRN neurons ceased their firing during EOD interruption. This mode of EOD interruption differs from those found in any other weakly electric fishes in which EOD cessation mechanisms have been known.

Neuroanatomy of the mormyrid electromotor control system

Mormyrid fish produce a diverse range of electric signals that are under the control of a central electromotor network. The anatomical organization of this network was delineated by injecting biotinylated compounds into neurophysiologically identified nuclei. Previous work using retrograde labeling with horseradish peroxidase indicated that the medullary command nucleus (CN) receives inputs from the precommand nucleus (PCN) at the mesencephalic-diencephalic border and the ventroposterior nucleus (VP) in the torus semicircularis. This study confirms these projections and identifies the dorsal posterior nucleus (DP) in the thalamus as an additional input to CN. DP and PCN form a bilateral column of cells extending ventrolaterally and caudally from the dorsal thalamus. The primary input to DP/PCN is from VP, which is identified as having two distinct subdivisions. A small group of large, multipolar cells along the ventral edge projects to DP/PCN and to CN, whereas a dorsal group of small, ovoid cells projects to DP/PCN but not to CN. VP receives input from the tectum mesencephali and the mesencephalic command-associated nucleus (MCA). As in all vertebrates, the tectum mesencephali receives input from several sources and likely provides multimodal sensory input to the electromotor system. MCA is part of the electromotor corollary discharge pathway, and its projection to VP suggests a feedback loop. These results, combined with recent physiological studies and comparisons with other taxa, suggest that modifiable feedback to DP/PCN plays a critical role in electromotor control and that the different inputs to CN may each be responsible for generating distinct electric signals.

Plasticity in an electrosensory system. III. Contrasting properties of spatially segregated dendritic inputs

Efferent neurons of the first-order electrosensory processing center of the brain, the electrosensory lateral line lobe (ELL), receive electroreceptor afferent input as well as feedback inputs descending from higher centers. These ELL efferents, pyramidal cells, adaptively filter predictable patterns of sensory input while preserving sensitivity to novel stimuli. The filter mechanism involves integration of centrally generated predictive inputs with the afferent inputs being canceled. The predictive inputs, referred to as "negative image" inputs, terminate on pyramidal cell apical dendrites and generate responses that are opposite those resulting from the predictable afference, hence integration of these signals results in attenuation of pyramidal cell responses. The system also shows a robust form of plasticity; the pyramidal cells learn, with a time course of a few minutes, to cancel new patterns of repetitive inputs. This is accomplished by adjusting the strength of excitatory and inhibitory apical dendritic inputs according to an anti-Hebbian learning rule. This study focuses on the properties of two separate pathways that convey descending information to pyramidal cell apical dendrites. One pathway terminates proximally, nearer to the pyramidal cell body, whereas the other terminates distally. Recordings of ELL evoked potentials, extracellular pyramidal cell spike responses, and intracellularly recorded synaptic potentials show that the pyramidal cells respond oppositely to moderate-frequency (> approximately 8 Hz) single pulse stimulation or repeated (1/s) tetanic activation of these two pathways. Repetitive activation of the proximally terminating pathway results in highly facilitating responses due to potentiation of pyramidal cell excitatory postsynaptic potentials (EPSPs). These same stimuli applied to the distally terminating pathway result in a reduction of pyramidal cell responses due to depression of EPSPs and potentiation of inhibitatory postsynaptic potentials (IPSPs). Anti-Hebbian plasticity was demonstrated by pairing tetanic stimulation of either pathway with changes in the postsynaptic cell's membrane potential. After stabilization of the response potentiation due to tetanic stimulation of the proximally terminating pathway, paired postsynaptic hyperpolarization resulted in further increases in spike responses and additional potentiation of pyramidal cell EPSPs. Paired postsynaptic depolarization reduced subsequent responses to the tetanus, depressed EPSP amplitudes, and, in many cases, potentiated IPSPs. The same pattern of plasticity was observed when postsynaptic hyper- or depolarization was paired with tetanic stimulation of the distally terminating pathway except that the plasticity was superimposed on the depressed pyramidal cell responses resulting from stimulating this pathway alone. Modulation of a postsynaptic form of synaptic depression is proposed to account for the anti-Hebbian plasticity associated with both pathways.

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.

Convergent designs for electrogenesis and electroreception

New- and old-world tropical electric fish lack a common electrical ancestor, suggesting that the mechanisms of signal generation and recognition evolved independently in the two groups. Recent research on convergent designs for electrogenesis and electroreception has focused on the structure of electric organs, the neural circuitry controlling the pacemaker driving the electric organ, and the neural circuitry underlying time coding of electric waveforms.