A well defined spinocerebellar system in the weakly electric teleost fish Gnathonemus petersii. A tracing and immuno-histochemical study.

Long ascending fiber systems were investigated in the spinal cord of a teleost fish, Gnathonemus petersii. Concomitant results of Fink-Heimer degeneration tracing as well as CaBP28K immunohistochemical labelling demonstrate the existence of a well defined direct pathway from the very lowest spinal level to the caudal lobe of the cerebellum. HRP retrograde labelling shows that this pathway originates in a cellular column located in the most ventral part of the lateral column next to the lateral extremity of the ventral horn. From each spinal segment, the large axons of these cells gather and form a strip shaped tract at the periphery of the lateral column immediately dorsal to the cell column from which they originate. The spinal course of these fibers is ipsilateral; they give off a large number of collaterals to the lateral reticular nucleus. Bypassing the trigeminal motor nucleus, the lateral column tract courses dorsally to the paratrigeminal command associated nucleus between the lateral lemniscus and the nucleus preeminentialis and with a ventro-dorsally oriented large loop, turns in the caudal direction and penetrates into the cerebellar caudal lobe. Running caudally in the dorsal granular layer of the caudal lobe, it shifts more and more medially and crosses the midline whilst decussating with the contralateral tract on the dorsal margin of the molecular layer of the caudal lobe. Finally, the tract splits off and terminates throughout the granular layer of the caudal lobe. The main characteristics of this pathway are similar to those of the ventral spinocerebellar tract of higher vertebrates; it conveys information from all spinal levels directly to the contralateral cerebellum. However, it does not seem to receive direct synaptic input from the periphery, since projection of the dorsal root fibers appears to be limited to the dorsal ipsilateral half of the spinal cord. The appearance of such a pathway in a teleost fish is probably related to the existence of a well developed proprioceptive system in this species.     

The development of lateral line placodes: Taking a broader view

The lateral line system of anamniote vertebrates enables the detection of local water movement and weak bioelectric fields. Ancestrally, it comprises neuromasts – small sense organs containing mechanosensory hair cells – distributed in characteristic lines over the head and trunk, flanked on the head by fields of electroreceptive ampullary organs, innervated by afferent neurons projecting respectively to the medial and dorsal octavolateral nuclei in the hindbrain. Given the independent loss of the electrosensory system in multiple lineages, the development and evolution of the mechanosensory and electrosensory components of the lateral line must be dissociable. Nevertheless, the entire system arises from a series of cranial lateral line placodes, which exhibit two modes of sensory organ formation: elongation to form sensory ridges that fragment (with neuromasts differentiating in the center of the ridge, and ampullary organs on the flanks), or migration as collectives of cells, depositing sense organs in their wake. Intensive study of the migrating posterior lateral line placode in zebrafish has yielded a wealth of information concerning the molecular control of migration and neuromast formation in this migrating placode, in this cypriniform teleost species. However, our mechanistic understanding of neuromast and ampullary organ formation by elongating lateral line placodes, and even of other zebrafish lateral line placodes, is sparse or non-existent. Here, we attempt to highlight the diversity of lateral line development and the limits of the current research focus on the zebrafish posterior lateral line placode. We hope this will stimulate a broader approach to this fascinating sensory system.     

Afferent and efferent components of the facial nerve in a frog,Rana pipiens

Summary The seventh cranial nerve in Rana pipiens is a slender nerve with limited peripheral distribution. We investigated the afferent and efferent components of this nerve by labeling its major branch, the hyomandibular, with horseradish peroxidase. The efferent portion of the seventh nerve originates from a small cell group in the upper medulla which contains two subdivisions. Afferent fibers carried in nerve VII travel in the solitary tract and the dorsolateral funiculus. The solitary component consists of a small number of ascending fibers that reach the level of the trigeminal nucleus and a large descending component that terminates slightly caudal to the obex in the commissural nuclei of the solitary complex. Afferent fibers also descend in the dorsolateral funiculus; many of these fibers cross dorsal to the central canal in the lower medulla. Most of the fibers in the dorsolateral funiculus terminate in the ipsilateral and contralateral dorsal horns and in nuclei of the dorsal column. A few ipsilateral fibers reach lower thoracic levels of the spinal cord.     

The cerebellar dorsal granular ridge in an elasmobranch has proprioceptive and electroreceptive representations and projects homotopically to the medullary electrosensory nucleus

1. Response properties of neurons in the dorsal granular ridge (DGR) of the little skate, Raja erinacea, were studied in decerebrate, curarized fish. Sensory responses included proprioceptive (426 of 952; 45%) and electroreceptive units (526 of 952; 55%). Electroreceptive units responded to weak electric fields with a higher threshold than lower-order units and had large ipsilateral receptive fields, whose exact boundaries were often unclear but contained smaller, identifiable best areas. Proprioceptive units responded to displacement of the ipsilateral fin and were either position-or movement-sensitive.
2. Both proprioceptive and electroreceptive units showed a progression of receptive fields from anterior to posterior body in the rostral to caudal direction along the length of DGR. Sensory maps in DGR projected homotopically to the electrosensory somatotopy in the dorsal nucleus. Peak evoked potentials and units responding to local DGR stimulation occurred only in areas of the dorsal nucleus with receptive fields located within the composite receptive field at the DGR stimulation site.
3. Single shocks to DGR produced a short spike train followed by a prolonged suppression period in the medullary dorsal nucleus. These results have implications for the role of the parallel fiber system in medullary electrosensory processing.

     

Behavioral evidence of a latency code for stimulus intensity in mormyrid electric fish

Mormryid electric fish (Gnathonemus petersii) respond to novel stimuli with an increase in the rate of the electric organ discharge (EOD). These novelty responses were used to measure the fish's ability to detect small changes in the amplitude and latency of an electrosensory stimulus. Responses were evoked in curarized fish in which the EOD was blocked but in which the EOD motor command continued to be emitted. An artificial EOD was provided to the fish at latencies of 2.4 to 14.4 ms following the EOD motor command.Novelty responses were evoked in response to transient changes in artificial EOD amplitude as small as 1% of baseline amplitude, and in latency as small as 0.1 ms. Changes in latency were effective only at baseline delays of less than 12.4 ms.The sensitivity to small changes in latency supports the hypothesis that latency is used as a code for stimulus intensity in the active electrolocation system of mormyrid fish. The results also indicate that a corollary discharge signal associated with the EOD motor command is used to measure latency.Abbreviations EOD electric organ discharge - ELL electrosensory lateral line lobe - epsp excitatory post synaptic potential     

Neural simulations of adaptive reafference suppression in the elasmobranch electrosensory system

The electrosensory system of elasmobranchs is extremely sensitive to weak electric fields, with behavioral thresholds having been reported at voltage gradients as low as 5 nV/cm. To achieve this amazing sensitivity, the electrosensory system must extract weak extrinsic signals from a relatively large reafferent background signal associated with the animal's own movements. Ventilatory movements, in particular, strongly modulate the firing rates of primary electrosensory afferent nerve fibers, but this modulation is greatly suppressed in the medullary electrosensory processing nucleus, the dorsal octavolateral nucleus. Experimental evidence suggests that the neural basis of reafference suppression involves a common-mode rejection mechanism supplemented by an adaptive filter that fine tunes the cancellation. We present a neural model and computer simulation results that support the hypothesis that the adaptive component may involve an anti-Hebbian form of synaptic plasticity at molecular layer synapses onto ascending efferent neurons, the principal output neurons of the nucleus. Parallel fibers in the molecular layer carry a wealth of proprioceptive, efference copy, and sensory signals related to the animal's own movements. The proposed adaptive mechanism acts by canceling out components of the electrosensory input signal that are consistently correlated with these internal reference signals.Abbreviations AEN ascending efferent neuron - AFF primary afferent nerve fiber - DGR dorsal granular ridge - DON dorsal octavolateral nucleus - ELL electrosensory lateral line lobe - GABA -aminobutyric acid - IN inhibitory interneuron - ISI interspike interval - ST stellate cell     

A reinvestigation of the Gn-RH (gonadotrophin-releasing hormone) systems in the goldfish brain using antibodies to salmon Gn-RH

Summary The organization of Gn-RH systems in the brain of teleosts has been investigated previously by immunohistochemistry using antibodies against the mammalian decapeptide which differs from the teleostean factor. Here, we report the distribution of immunoreactive Gn-RH in the brain of goldfish using antibodies against synthetic teleost peptide.Immunoreactive structures are found along a column extending from the rostral olfactory bulbs to the pituitary stalk. Cell bodies are observed within the olfactory nerves and bulbs, along the ventromedial telencephalon, the ventrolateral preoptic area and the latero-basal hypothalamus. Large perikarya are detected in the dorsal midbrain tegmentum, immediately caudal to the posterior commissure. A prominent pathway was traced from the cells located in the olfactory nerves through the medial olfactory tract and along all the perikarya described above to the pituitary stalk. In the pituitary, projections are restricted to the proximal pars distalis. A second immunoreactive pathway ascends more dorsally in the telencephalon and arches to the periventricular regions of the diencephalon. Part of this pathway forms a periventricular network in the dorsal and posterior hypothalamus, whereas other projections continue caudally to the medulla oblongata and the spinal cord. Lesions of the ventral preoptic area demonstrate that most of the fibers detected in the pituitary originate from the preoptic region.     

Electrosensory systems in the mormyrid fish, Gnathonemus petersii : special emphasis on the fast conducting pathway

1. Rhombencephalic and mesencephalic structures involved in electroreception were investigated by electrophysiological methods in the weakly electric fish Gnathonemus petersii. 2. The existence of a synchronous response to electric field stimulation of the fish in the mesencephalic exterolateral nucleus (n.ext.-lat.mes) with 2.5-3 ms latency was confirmed. The lateral line lobe nucleus (nLLL) is identified as the rhombencephalic relay for the mesencephalic responses because of the short latency synchronous response in the nLLL obtained by threshold stimulation of the posterior lateral line nerve. Responses in both the nLLL and the n.ext.-lat.mes. appear and their amplitudes increase simultaneously with increasing stimulus intensity. 3. Comparison of latencies supports a three-neuron pathway hypothesis which also agrees well with the various functional properties described. 4. The nLLL-n.ext.-lat.mes. pathway is blocked sharply for a period of 1 ms occurring 3 ms after the electric organ discharge (EOD). This inhibitory period is phase-related to the Mesencephalic Command Associated Signal (MCAS) of Aljure (1946) ; The phase relation is such that no response is observed to the fish's own EOD. 5. Long-lasting responses of 10-12 ms duration to higher stimulation intensities were obtained in the ganglionic layer of the lateral line lobe (LLL). Intensities evoking maximal responses in the nLLL and n.ext.lat.mes. are still threshold stimulation for lateral line lobe responses. 6. Long-lasting responses (of the same order as in the LLL) to the fish' own EOD were observed in the mesencephalic lateral nucleus. Responses to artificial electric pulses were obtained only if delivered in a certain phase realtion to the MCAS. The MCAS displays a facilitating effect on the slow conducting electrosensory system. 7. Results indicate the existence in mormyrids of a double, fast and slow conducting, electrosensory system similar to that of gymnotid fish. The mormyrids can control both of these electrosensory systems by means of the MCAS, the effect of which is opposite for the same time period on the two systems.     

Commissural neurons of the electrosensory lateral line lobe of Apteronotus leptorhynchus: morphological and physiological characteristics

Extracellular injections of horseradish peroxidase were used to label commissural cells connecting the electrosensory lateral line lobes of the weakly electric fish Apteronotus leptorhynchus. Multiple commissural pathways exist; a caudal commissure is made up of ovoid cell axons, and polymorphic cells' axons project via a rostral commissure. Intracellular recording and labeling showed that ovoid cells discharge spontaneously at high rates, fire at preferred phases to the electric organ discharge, and respond to increased receptor afferent input with short latency partially adapting excitation. Ovoid cell axons ramify extensively in the rostro-caudal direction but are otherwise restricted to a single ELL subdivision. Polymorphic cells are also spontaneously active, but their firing is unrelated to the electric organ discharge waveform. They respond to increased receptor afferent activity with reduced firing frequency and response latency is long. Electrical stimulation of the commissural axons alters the behavior of pyramidal cells in the contralateral ELL. Basilar pyramidal cells are hyperpolarized and nonbasilar pyramidal cells are depolarized by this type of stimulation. The physiological results indicate that the ovoid cells participate in common mode rejection mechanisms and also suggest that the ELLs may function in a differential mode in which spatially restricted electrosensory stimuli can evoke heightened responses.Abbreviations ccELL caudal commissure of the ELL - CE contralaterally excited - DML dorsal molecular layer - ELL electrosensory lateral line lobe - EOD electric organ discharge - HRP horseradish peroxidase - IE ipsilaterally excited - MTI mouth-tail inverted - MTN mouth-tail normal - rcELL rostral commissure of the ELL - TRI transverse inverted - TRN transverse normal     

Electrophysiological analysis of the organization of somatosensory thalamic inputs in the frog

Potentials produced in the frog thalamus by electrical stimulation of the peripheral nerves were investigated by sink and current source-density analysis. Sinks, which are viewed as potential generation sites, were located in three regions: the cell-free zone of the ventral thalamus adjoining the ventrolateral nucleus, the ventromedial and ventrolateral nuclei, and the caudal section of the dorsal thalamus. Evoked activity was recorded in individual neurons in the area of the second and third of these sinks. The first sink failed to form after section of the dorsal tracks of the spinal cord, while the remaining two only appeared after a considerably extended latency. It is suggested that nuclei of the ventral and caudal sections of the dorsal thalamus receive somatic impulses through the systems connected with the dorsal as well as the ventrolateral columns of the spinal cord. The direct projections of the primordial nuclei of dorsal columns may be involved in afferentation the ventral thalamus.I. M. Sechenov Institute of Evolutionary Physiology and Biochemistry, Academy of Sciences of the USSR, Leningrad. Translated from Neirofiziologiya, Vol. 18, No. 5, pp. 687–695, September–October, 1986.     

Distribution of Kv1-like potassium channels in the electromotor and electrosensory systems of the weakly electric fish Apteronotus leptorhynchus

The electromotor and electrosensory systems of the weakly electric fish Apteronotus leptorhynchus are model systems for studying mechanisms of high-frequency motor pattern generation and sensory processing. Voltage-dependent ionic currents, including low-threshold potassium currents, influence excitability of neurons in these circuits and thereby regulate motor output and sensory filtering. Although Kv1-like potassium channels are likely to carry low-threshold potassium currents in electromotor and electrosensory neurons, the distribution of Kv1 alpha subunits in A. leptorhynchus is unknown. In this study, we used immunohistochemistry with six different antibodies raised against specific mammalian Kv1 alpha subunits (Kv1.1-Kv1.6) to characterize the distribution of Kv1-like channels in electromotor and electrosensory structures. Each Kv1 antibody labeled a distinct subset of neurons, fibers, and/or dendrites in electromotor and electrosensory nuclei. Kv1-like immunoreactivity in the electrosensory lateral line lobe (ELL) and pacemaker nucleus are particularly relevant in light of previous studies suggesting that potassium currents carried by Kv1 channels regulate neuronal excitability in these regions. Immunoreactivity of pyramidal cells in the ELL with several Kv1 antibodies is consistent with Kv1 channels carrying low-threshold outward currents that regulate spike waveform in these cells (Fernandez et al., J Neurosci 2005;25:363-371). Similarly, Kv1-like immunoreactivity in the pacemaker nucleus is consistent with a role of Kv1 channels in spontaneous high-frequency firing in pacemaker neurons. Robust Kv1-like immunoreactivity in several other structures, including the dorsal torus semicircularis, tuberous electroreceptors, and the electric organ, indicates that Kv1 channels are broadly expressed and are likely to contribute significantly to generating the electric organ discharge and processing electrosensory inputs.     

Caudal fin in the white shark, Carcharodon carcharias (Lamnidae): a dynamic propeller for fast, efficient swimming

The caudal peduncle and caudal fin of Carcharodon carcharias together form a dynamic locomotory structure. The caudal peduncle is a highly modified, dorsoventrally compressed and rigid structure that facilitates the oscillations of the caudal fin. Its stiffness appears to be principally achieved by a thick layer of adipose tissue ranging from 28-37% of its cross-sectional area, reinforced by cross-woven collagen fibers. Numerous overlying layers of collagen fibers of the stratum compactum, oriented in steep left- and right-handed helices (approximately 65 degrees to the shark's long axis), prevent bowstringing of the perimysial fibers, which lie just below the dermal layer. Perimysial fibers, muscles, and the notochord are restricted to the dorsal lobe of the caudal fin and comprise the bulk of its mass. Adipose tissue reinforces the leading edge of the dorsal lobe of the caudal fin and contributes to maintaining the ideal cross-sectional geometry required of an advanced hydrofoil. Most of the mass of the ventral lobe consists of the ceratotrichia or fin rays separated by thin partitions of connective tissue. Dermal fibers of the stratum compactum of the dorsal lobe occur in numerous distinct layers. The layers are more complex than in other sharks and appear to reflect a hierarchical development in C. carcharias. The fiber layer comprises a number of thick fiber bundles along the height of the layer and the layers get thicker deeper into the stratum compactum. Each of these layers alternates with a layer a single fiber-bundle deep, a formation thought to give stability to the stratum compactum and to enable freer movements of the fiber system. In tangential sections of the stratum compactum the fiber bundles in the dorsal lobe can be seen oriented with respect to the long axis of the shark at approximately 55-60 degrees in left- and right-handed helices. Because of the backward sweep of the dorsal lobe (approximately 55 degrees to the shark's long axis) the right-handed fibers also parallel the lobe's long axis. In the dorsal lobe, ceratotrichia are present only along the leading edge (embedded within connective tissue), apparently as reinforcement. Stratum compactum fiber bundles of the ventral lobe, viewed in transverse section, lack the well-ordered distinctive layers of the dorsal lobe, but rather occur as irregularly arranged masses of tightly compacted fiber bundles of various sizes. In tangential sections the fiber bundles are oriented at angles of approximately 60 degrees, generally in one direction, i.e., lacking the left- and right-handed helical pattern. Tensile load tests on the caudal fin indicate high passive resistance to bending by the skin. The shear modulus G showed that the skin's contribution to stiffness (average values from three specimens at radians 0.52 and 1.05) is 33.5% for the dorsal lobe and 41.8% for the ventral. The load tests also indicate greater bending stiffness of the ventral lobe compared to the dorsal. Overall, the anatomy and mechanics of the dorsal lobe of C. carcharias facilitate greater control of movement compared to the ventral lobe. The helical fiber architecture near the surface of the caudal fin is analogous to strengthening of a thin cylinder in engineering. High fiber angles along the span of the dorsal lobe are considered ideal for resisting the bending stresses that the lobe is subjected to during the locomotory beat cycle. They are also ideal for storing strain energy during bending of the lobe and consequently may be of value in facilitating the recovery stroke. The complex fiber architecture of the caudal fin and caudal peduncle of C. carcharias provides considerable potential for an elastic mechanism in the animal's swimming motions and consequently for energy conservation.     

Medullary electrosensory processing in the little skate

1. Previous studies have demonstrated that the resting activity of electrosensory ALLN fibers is modulated by the animal's own respiratory activity and that all fibers innervating a single ampullary cluster are modulated with the same amplitude and phase relationship to ventilation. We demonstrate that ALLN fibers in the skate are modulated in this common-mode manner bilaterally, regardless of receptor group, orientation, or position of the receptor pore on the body surface (Fig. 2). 2. Ascending efferent neurons (AENs), which project to the electrosensory midbrain from the DON, are modulated through a much smaller portion of their dynamic range. AENs give larger responses to an extrinsic local electric field than to the respiratory driving, indicating that a mechanism exists for suppressing ventilatory electrosensory reafference. 3. In paralyzed animals no modulation of resting activity or of responses of extrinsic electric fields could be observed with respect to the animal's respiratory motor commands in the absence of electrosensory reafference. 4. Cells of the dorsal granular ridge (DGR) project to medullary AENs via the DON molecular layer. A majority of proprioceptive DGR neurons are modulated by ventilatory activity, however, in a given fish the modulation is not in the same phase relationship to ventilation among DGR units. 5. The modulation of AENs during respiration was increased following transection of the contralateral ALLN (Fig. 9). Resting activity and responses to excitatory stimuli were inhibited by simultaneous stimulation of the transected contralateral ALLN indicating that a common-mode rejection mechanism is mediated via the commissural interconnections of the DONs.     

Receptive field properties of neurons in the electrosensory lateral line lobe of the weakly electric fish, <Emphasis Type="Italic">Gnathonemus petersii</Emphasis>

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.     

The area octavo-lateralis in Xenopus laevis

Summary Central projections of afferents from the lateral line nerves and from the individual branches of the VIIIth cranial nerve in Xenopus laevis and Xenopus mülleri were studied by the application of HRP to the cut end of the nerves.Upon entering the rhombencephalon, the lateral line afferents form a longitudinal fascicle of ascending and descending branches in the ventro-lateral part of the lateral line neuropile. The fascicle exhibits a topographic organization, that is not reflected in the terminal field of the side branches. The terminal field can be subdivided into a rostral, a medial and a caudal part, each of which shows specific branching and terminal pattern of the lateral line afferents. These different patterns within the terminal field are interpreted as the reflection of functional subdivisions of the lateral line area. The study did not reveal a simple topographic relationship between peripheral neuromasts and their central projections.Two nuclei of the alar plate with significant lateral line input were delineated: the lateral line nucleus (LLN) and the medial part of the anterior nucleus (AN). An additional cell group, the intermediate nucleus (IN), is a zone of lateral line and eighth nerve overlap, although such zones also exist within the ventral part of the LLN and the dorsal part of the caudal nucleus (CN). Six nuclei which receive significant VIIIth nerve input are recognized: the cerebellar nucleus (CbN), the lateral part of the anterior nucleus, the dorsal medullary nucleus (DMN), the lateral octavus nucleus (LON), the medial vestibular nucleus (MVN) and the caudal nucleus (CN).All inner ear organs have more than one projection field. All organs project to the dorsal part of the LON and the lateral part of the AN. Lagena, amphibian papilla and basilar papilla project to separate regions of the dorsal medullary nucleus (DMN). There is evidence for a topographic relation between the hair cells of the amphibian papilla (AP) and the central projections of AP fibers. The sacculus projects extensively to a region between the DMN and the LON. Fibers from the sacculus and the lagena project directly to the superior olive. Fibers from the utriculus and the three crista organs terminate predominantly in the medial vestibular nucleus (MVN) and in the adjacent parts of the reticular formation, and their terminal structures appear to be organotopically organised. Octavus fiber projections to the cerebellum and to the spinal cord are also described.     

Excitation of cat spinal cord dorsal column fibers during stimulation of different fiber groups of cutaneous and somatic nerves

The effects of electrically stimulating different groups of nerve fibers supplying the skin and muscle on evoked potentials in cat spinal cord dorsal columns were studied. Significant differences in the configuration of dorsal column potentials recorded in response to stimulation of these nerves were found. It was shown that cutaneous nerve unmyelinated fibres were connected to unmyelinated dorsal column fibers. In addition, excitation of cutaneous C-fibers lead to activation of dorsal column fibers with the maximum conduction velocity. The somatic nerve was only connected to myelinated dorsal column fibers, and excitation of its non-myelinated fibers did not cause other types of dorsal column fibers to be activated. It is suggested that the acceleration of cutaneous signal transmission in the dorsal column system may be brought about by the necessity for rapid warning of potentially harmful stimuli.Medical Institute, Russian Federation Ministry of Public Health, Nizhny Novgorod. Translated from Neirofiziologiya, Vol. 24, No. 5, pp. 625–635, September–October, 1992.     

Comparison of calbindin D 28K and cytochrome c oxidase in electrosensory nuclei of high- and low-frequency weakly electric fish (Gymnotiformes)

Summary Weakly electric fish (Gymnotiformes) emit quasi-sinusoidal electric organ discharges within speciesspecific frequency ranges. The electrosensory system is organized into 2 parallel pathways which convey either the amplitude or the timing of each electric organ discharge cycle. Two putative metabolic activity markers, calbindin D 28K and cytochrome c oxidase, and their relationship with the electrosensory nuclei of high- and low-frequency species were studied. Calbindin is found in the somata of the spherical neurons in the first-order electrosensory recipient nucleus, the electrosensory lateral-line lobe, and in layer VI of the midbrain's torus semicircularis, in Eigenmannia virescens, an intermediate-frequency species, and Apteronotus leptorhynchus, a high-frequency species. Calbindin immunoreactivity was completely absent in these nuclei in Sternopygus macrurus, a closely related, low-frequency species. Cytochrome c oxidase levels were inversely related to calbindin immunoreactivity since relatively high levels were observed in the electrosensory lateral-line lobe and torus semicircularis of S. macrurus but were absent in these nuclei in A. leptorhynchus. Our studies indicate that calbindin immunoreactivity is present in tonic, repetitively firing neurons with high frequencies.     

Pyramidal-cell plasticity in weakly electric fish: a mechanism for attenuating responses to reafferent electrosensory inputs

Recordings within the posterior eminentia granularis of the weakly electric fish, Apteronotus leptorhynchus, revealed multiple types of proprioceptive units responsive to changes in the position of the animal's trunk and tail. Intracellular labelling showed that the proprioceptor recordings were made from axons that ramify extensively within the EGp. The location of the somata giving rise to these axons is presently unknown. Electroreceptor afferent responses to electric organ discharge amplitude modulations caused by movement of the animal's tail were compared to responses caused by electronically generated AMs of similar amplitude and time course. These did not differ. Electrosensory lateral line lobe pyramidal cells responded significantly less to electric organ discharge amplitude modulations caused by changing the animal's posture as compared to electronically produced AMs, suggesting that central mechanisms attenuate pyramidal cell responses to reafferent electrosensory inputs. Experiments in which the pattern of reafferent input associated with changes in posture was altered revealed that the pyramidal cells learn, over a time course of several minutes, to reject new patterns of input. Both proprioceptive input and descending electrosensory input to the posterior eminentia granularis are involved in generating the observed plastic changes in pyramidal cell responsiveness.Abbreviations AM amplitude modulation - EGp posterior eminentia granularis - ELL electrosensory lateral line lobe - EOD electric organ discharge - HRP horseradish peroxidase - LTD long-term depression - LTP long-term potentiation     

The role of dorsal columns pathway in visceral pain

Traditionally, the dorsal column-medial lemniscus system has been viewed as a pathway not involved in pain perception. However, recent clinical and experimental studies have provided compelling evidence that implicates an important role of the dorsal column pathway in relaying visceral nociceptive information. Several clinical studies have shown that a small lesion that interrupts fibers of the dorsal columns (DC) that ascend close to the midline of the spinal cord significantly relieves pain and decreases analgesic requirements in patients suffering from cancer originating in visceral organs. Behavioral, electrophysiological and immunohistochemical methods used under experimental situations in animals showed that DC lesion lead to decreased activation of thalamic and gracile neurons by visceral stimuli, suppressed inhibition of exploratory activity induced by visceral noxious stimulation and prevented potentiation of visceromotor reflex evoked by colorectal distention under inflammatory conditions. Whereas the surgical lesion of the DC tract has proven to be clinically successful, a pharmacological approach would be a better strategy to block this pathway and thus to improve visceral pain conditions under less dramatic circumstances than cancer pain. Our finding that PSDC neurons start to express receptors for substance P after colon inflammation suggests new targets for the development of pharmacological strategies for the control of visceral pain.     

Design principles of sensory processing in cerebellum-like structures

Cerebellum-like structures are compared for two sensory systems: electrosensory and auditory. The electrosensory lateral line lobe of mormyrid electric fish is reviewed and the neural representation of electrosensory objects in this structure is modeled and discussed. The dorsal cochlear nucleus in the auditory brainstem of mammals is reviewed and new data are presented that characterize the responses of neurons in this structure in the mouse. Similarities between the electrosensory and auditory cerebellum-like structures are shown, in particular adaptive processes that may reduce responses to predictable stimuli. We suggest that the differences in the types of sensory objects may drive the differences in the anatomical and physiological characteristics of these two cerebellum-like structures.