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Directional selectivity and frequency tuning of midbrain cells in the oyster toadfish, Opsanus tau.

Single-unit recordings were made from areas in the midbrain (torus semicircularis) of the oyster toadfish. We evaluated frequency tuning and directional responses using whole-body oscillation to simulate auditory stimulation by particle motion along axes in the horizontal and mid-sagittal planes. We also tested for bimodality in responses to auditory and hydrodynamic stimuli. One recording location in each animal was marked by a neurobiotin injection to confirm the recording site. Recordings were made in nucleus centralis, nucleus ventrolateralis, and the deep cell layer. Most units were frequency-selective with best frequencies between 50 and 141 Hz. Suppression of activity was apparent in 10% of the cells. Bimodality was common, including inhibition and suppression of background activity by auditory or hydrodynamic stimulation. The majority of the cells were directionally selective with directional response patterns that were sharpened compared with those of primary saccular afferents. The best directional axes were arrayed widely in spherical space, covering most azimuths and elevations. This representation is adequate for the computation of the motional axis of an auditory stimulus for sound source localization.

Acoustic Stimulation↗

Sharpening of directional responses along the auditory pathway of the oyster toadfish, Opsanus tau.

Our previous studies have shown that the peripheral auditory system of the toadfish encodes the direction of a sound source. Here, we compare directional responses of peripheral saccular afferents, cells in the descending octaval nucleus (DON) of the medulla, and the torus semicircularis (TS) of the midbrain. Recording locations in the brain were labeled with neurobiotin to confirm the site. To compare directional responses among cells, we calculated an index [sharpening ratio (SR)] that weights the relative strength of responses to the best direction for that cell and to the adjacent stimulus angles tested. Unsharpened saccular afferents tend to have a cosinusoidal directional response pattern (DRP) with an expected SR of 0.87. In DON, more than 60% of the cells exhibited directional sharpening (defined as SR <0.8). In TS, more than 80% of the cells exhibited directional sharpening. We conclude that directional auditory sharpening first occurs in DON and some additional sharpening occurs in the ascending pathway to the midbrain, particularly in azimuth. The sharpening of directional selectivity is likely to be an important component of the neural computations underlying directional hearing.

Animals↗

Saccular potentials of the vocal plainfin midshipman fish, Porichthys notatus.

The plainfin midshipman fish, Porichthys notatus, is a vocal species of teleost fish that generates acoustic signals for intraspecific communication during social and reproductive behaviors. All adult morphs (females and males) produce single short duration grunts important for agonistic encounters, but only nesting males produce trains of grunts and growls in agonistic contexts and long duration multiharmonic advertisement calls to attract gravid females for spawning. The midshipman fish uses the saccule as the main acoustic endorgan for hearing to detect and locate vocalizing conspecifics. Here, I examined the response properties of evoked potentials from the midshipman saccule to determine the frequency response and auditory threshold sensitivity of saccular hair cells to behaviorally-relevant single tone stimuli. Saccular potentials were recorded from the rostral, medial and caudal regions of the saccule while sound was presented by an underwater speaker. Saccular potentials of the midshipman, like other teleosts, were evoked greatest at a frequency that was twice the stimulus frequency. Results indicate that midshipman saccular hair cells of non-reproductive adults had a peak frequency sensitivity that ranged from 75 (lowest frequency tested) to 145 Hz and were best suited to detect the low frequency components (<or=105 Hz) of midshipman vocalizations.

Acoustic Stimulation↗

Peripheral encoding of behaviorally relevant acoustic signals in a vocal fish: harmonic and beat stimuli.

Nesting male midshipman fish, Porichthys notatus, emit simple, long-duration sounds known as hums, which are attractive to gravid females. While hums share the multi-harmonic structure typical of many vertebrate communication sounds, their lack of amplitude modulation gives individual hums unusually simple temporal envelopes. However, hums often overlap, producing beats in the summed acoustic waveform. This study presents responses of individual saccular afferent fibers to two-tone harmonic and beat stimuli presented via an underwater loudspeaker. Spike activity was quantified as vector strength of synchronization and average spike rate. Responses to harmonic stimuli depended on harmonic phase; these effects apparently resulted primarily from variation in waveform fine temporal structure rather than auditory non-linearities. At most phases, addition of the harmonic enhanced afferent synchronization compared to the fundamental alone. Two-tone beat stimuli evoked stronger synchronization to the component frequencies than to the beat modulation rate. Vector strength tended to be higher to the lower frequency component, and this pattern appeared to derive from afferent tuning. Midshipman saccular afferents encoded both the temporal envelope and waveform fine structure of these naturalistic signals, information that may be important in conspecific communication.

Acoustics↗

Movement and sound generation by the toadfish swimbladder.

Although sound-producing (sonic) muscles attached to fish swimbladders are the fastest known vertebrate muscles, the functional requirement for such extreme speed has never been addressed. We measured movement of the swimbladder caused by sonic muscle stimulation in the oyster toadfish Opsanus tau and related it to major features of the sound waveform. The movement pattern is complex and produces sound inefficiently because the sides and bottom of the bladder move in opposite in and out directions, and both movement and sound decay rapidly. Sound amplitude is related to speed of swimbladder movement, and slow movements do not produce perceptible sound. Peak sound amplitude overlaps fundamental frequencies of the male's mating call because of muscle mechanics and not the natural frequency of the bladder. These findings suggest that rapid muscle speed evolved to generate sound from an inefficient highly damped system.

Action Potentials↗

Coding of concurrent vocal signals by the auditory midbrain: effects of duration.

Neural selectivity to signal duration within the auditory midbrain has been observed in several species and is thought to play a role in signal recognition. Here we examine the effects of signal duration on the coding of individual and concurrent vocal signals in a teleost fish with exceptionally long duration vocalizations, the plainfin midshipman, Porichthys notatus. Nesting males produce long-duration, multi-harmonic signals known as hums to attract females to their nests; overlapping hums produce acoustic beats at the difference frequency of their spectral components. Our data show that all midbrain neurons have sustained responses to long-duration hum-like tones and beats. Overall spike counts increase linearly with signal duration, although spike rates decrease dramatically. Neurons show varying degrees of spike rate decline and hence, differential changes in spike rate across the neuron population may code signal duration. Spike synchronization to beat difference frequency progressively increases throughout long-duration beats such that significant difference frequency coding is maintained in most neurons. The significance level of difference frequency synchronization coding increases by an order of magnitude when integrated over the entirety of long-duration signals. Thus, spike synchronization remains a reliable difference frequency code and improves with integration over longer time spans.

Animals↗

Branchial and renal excretion of urea and urea analogues in the plainfin midshipman, Porichthys notatus.

This study investigated whether urea transport mechanisms were present in the gills of the ammoniotelic plainfin midshipman (Porichthys notatus), similar to those recently documented in its ureotelic relative (family Batrachoididae), the gulf toadfish (Opsanus beta). Midshipmen were fitted with internal urinary and caudal artery catheters for repetitive sampling of urine and blood in experiments and radiolabeled urea analogues ([(14)C]-thiourea and [(14)C]-acetamide) were used to evaluate the handling of these substances. Isosmotically balanced infusions of urea were used to raise plasma and urine urea concentrations to levels surpassing physiological levels by 8.5-fold and 6.4-fold, respectively. Despite these high urea levels, there was no observable transport maximum in either renal or branchial urea excretion rate, a result mirrored by the total uptake of fish exposed to a range of environmental urea concentrations. Permeability to urea appeared to be symmetrical in the two directions. At comparable plasma concentrations the branchial clearance rate of acetamide was 74% that of urea while branchial clearance rate of thiourea was 55% that of urea. For influx, the comparable values were 60% and 36%, indicating the same pattern. In contrast, the secretion clearance rate of acetamide by the kidney was 56% that of urea while the rate of thiourea secretion clearance was 137% greater than that of urea, with both urea and thiourea being more concentrated in the urine than in the plasma. In addition, the secretion clearance rates of thiourea and urea were significantly greater than those of water and Cl(-), whereas acetamide, water and Cl(-) were found equally in the plasma and urine, appearing to passively equilibrate between the two fluids. Based on our findings, there appear to be two distinct transport mechanisms involved in urea excretion in the plainfin midshipmen, one in the gill (a facilitated diffusion type transporter) and one in the kidney (an active transport mechanism), each of which does not saturate even at plasma urea concentrations that greatly exceed physiological levels. These transporters appear to be similar to those in the midshipman's ureotelic relative, the gulf toadfish.

Animals↗

Glucocorticoid receptors are involved in the regulation of pulsatile urea excretion in toadfish.

The objectives of this study were to characterize the pattern of pulsatile urea excretion in the gulf toadfish in the wake of exogenous cortisol loading and to determine the receptors involved in the regulation of this mechanism. Toadfish were fitted with indwelling arterial catheters and were infused with isosmotic NaCl for 48 h after which fish were treated with cortisol alone, cortisol + peanut oil, cortisol + RU486 (a glucocorticoid receptor antagonist) or cortisol + spironolactone (a mineralocorticoid receptor antagonist). Upon cortisol loading, fish treated with cortisol alone, cortisol + oil or cortisol + spironolactone experienced a two- to threefold reduction in pulsatile urea excretion. This reduction was due to a decrease in urea pulse size with no effect on pulse frequency compared to values measured during the control NaCl infusion period. In addition, these fish showed an increase in plasma urea concentrations upon treatment. These apparent effects of cortisol treatment were abolished in fish treated with cortisol + RU486. In contrast, these fish showed an increase in pulsatile urea excretion mediated by a twofold increase in pulse size with no change in frequency. Likewise, fish treated with cortisol + RU486 showed a significant decrease in plasma urea concentrations over the course of the experiment. The findings of this study indicate that high levels of cortisol reduce pulsatile urea excretion by decreasing pulse size. In addition, it appears that glucocorticoid receptors and not mineralocorticoid receptors are involved in the regulation of the toadfish pulsatile urea excretion mechanism.

Analysis of Variance↗

Myosin flares and actin leptomeres as myofibril assembly/disassembly intermediates in sonic muscle fibers.

The sonic muscle of type 1 male midshipman fish produces loud and enduring mating calls. Each sonic muscle fiber contains a tubular contractile apparatus with radially arranged myofibrillar plates encased in a desmin-rich cytoskeleton that is anchored to broad Z bands (approximately 1.2 micro m wide). Immunomicroscopy has revealed patches of myosin-rich "flares" emanating from the contractile tubes into the peripheral sarcoplasm along the length of the fibers. These flares contain swirls of thick filaments devoid of associated thin filaments. In other regions of the sarcoplasm at the inner surface of the sarcolemma and near Z bands, abundant ladder-like leptomeres occur with rungs every 160 nm. Leptomeres consist of dense arrays of filaments (approximately 4 nm) with a structure that resembles myofibrillar Z band structure. We propose that flares and leptomeres are distinct filamentous arrays representing site-specific processing of myofibrillar components during the assembly and disassembly of the sarcomere. Recent reports that myosin assembles into filamentous aggregates before incorporating into the A band in the skeletal muscles of vertebrates and Caenorhabditis elegans suggest that sonic fibers utilize a similar pathway. Thus, sonic muscle fibers, with their tubular design and abundant sarcoplasmic space, may provide an attractive muscle model to identify myofibrillar intermediates by structural and molecular techniques.

Actins↗

Relationship between inner-ear fluid pressure and semicircular canal afferent nerve discharge.

The present study was designed to determine (1) the transcupular fluid pressure (deltaP) generated across the semicircular canal cupula in response to sinusoidal head rotation, (2) the translabyrinthine dilational pressure (P0) generated across the membranous labyrinth in response to an increase in endolymph fluid volume (hydrops), (3) afferent nerve discharge patterns generated by these distinct pressure stimuli and, (4) threshold values of deltaP and P0 required to elicit afferent neural responses. The experimental model was the oyster toadfish, Opsanus tau. Micromechanical indentation of the horizontal canal (HC) duct and utricular vestibule was used to simulate sinusoidal head rotation and fluid volume injection. Single-unit neural spike trains and endolymph pressure within the ampulla, on both sides of the cupula, were recorded simultaneously. deltaP averaged 0.013 Pa per 1 degrees/s of sinusoidal angular head velocity and P0 averaged 0.2 Pa per 1 nL of endolymph volume injection. The most responsive afferents had a threshold sensitivity to deltaP of 10(-3) Pa and to P0 of 5 x 10(-2) Pa based on a discharge modulation criterion of 1 impulse/s per cycle for 2 Hz pressure stimuli. Neural sensitivity to AP was expected on the basis of transverse cupular and hair bundle deflections. Analysis of mechanics of the end organ, neuronal projections into the crista, and individual neural firing patterns indicates that P0 sensitivity resulted from pressure-induced distension of the ampulla that led to a nonuniform cupular deformation pattern and hair bundle deflections. This explanation is consistent with predictions of a finite element model of the end organ. Results have implications regarding the role of deltaP in angular motion transduction and the role of P0 under transient hydropic conditions.

Action Potentials↗

Assessing genetic diversity of Brazilian reef fishes by chromosomal and DNA markers.

Little is known on genetics of Brazilian coral reef fish and most of this information is limited to chromosome characterization of major representative species. The diploid chromosome number in marine fish varies from 2n= 22-26 to 2n = 240-260. Despite of this apparent diversity, most studied marine species have a diploid complement with 48 acrocentric chromosomes. This latter trend is mostly observed among Perciformes, an important major taxon of coral reef fishes. Studies in the families Pomacentridae, Pomacanthidae and Chaetodontidae, for example, have shown a common karyotype pattern entirely formed by 48 uniarmed chromosomes. However, rare numerical and structural chromosome polymorphisms and cryptic chromosome rearrangements involving heterochromatin segments and/or nucleolar organizing sites have been reported among such fishes. Although new chromosome forms can contribute to the establishment of genetically isolated populations, their role in reef fish speciation at marine realm still is an open question. More recently, genomic DNA analyses using RAPD and microsatellites, and sequencing and RFLP of mitochondrial DNA have increasingly been used in Atlantic reef fish species. Genetic homogeneity over wide geographical ranges has been reported for different fish groups, in contrast to several cases of population substructuring related to environmental constraints or evolutionary history. Amazonas outflow and upwelling on the Southeastern coast of Brazil are believed to be strong barriers to dispersal of some reef species. Moreover, it is suggested that the pattern of speciation and population structure at South Atlantic is quite distinctive from Pacific Ocean, even when comparing closely related taxa. Further genetic studies are strongly encouraged in Brazilian reef fishes in order to provide a reliable scenario of the genetic structure in this important and diverse fish group.

Animals↗

Chromosomal mapping of the major and minor ribosomal genes, (GATA)n and (TTAGGG)n by one-color and double-color FISH in the toadfish Halobatrachus didactylus (Teleostei: Batrachoididae).

The karyotype of Halobatrachus didactylus presents 46 chromosomes, composed of eight metacentric, 18 submetacentric, four subtelocentric, and 16 acrocentric chromosomes. The results of FISH showed that the major ribosomal genes were located in the terminal position of the short arm of a large submetacentric chromosome. They also showed a high variation in the hybridization signals. The products of amplification of 5S rDNA produced bands of about 420 pb. The PCR labeled products showed hybridization signals in the subcentromeric position of the long arm of a submetacentric chromosome of medium size. Double-color FISH indicated that the two ribosomal families are not co-located since they hybridized in different chromosomal pairs. Telomeres of all the chromosomes hybridized with the (TTAGGG)n probe. The GATA probe displayed a strong signal in the long arm of a submetacentric chromosome of medium size, in the subcentromeric position. The double-color FISH showed that the microsatellite GATA and the 5S rDNA gene are located in different chromosomal pairs. The majority presence of GATA probes in one pair of chromosomes is unusual and considering its distribution through different taxa it could be due to evolutionary mechanisms of heterochromatine accumulation, leading to the formation of differentiated sex chromosomes.

Animals↗

Bioavailability of silver and its relationship to ionoregulation and silver speciation across a range of salinities in the gulf toadfish (Opsanus beta).

Silver is taken up as a Na(+) analog (Ag(+)) by freshwater organisms, but little is known about its bioavailability in relation to salinity. Adult Opsanus beta were acclimated to 2.5, 5, 10, 20, 40, 60, 80, and 100% seawater (Cl(-)=545 mM) and exposed for 24 h to 2.18 microg L(-1) silver as (110m)Ag-labelled AgNO(3), a concentration close to the U.S. EPA marine criterion and less than 0.1% of the acute 96-h LC50 in seawater. Plasma osmolality, Na(+), and Cl(-) remained approximately constant from 100% down to 20-40% seawater, thereafter declining to 89% (osmolality) and 82% (Na(+), Cl(-)) of seawater values at the lowest salinity (2.5% seawater), while plasma Mg(2+) was invariant. Ionic measurements in intestinal fluids and urine supported the view that above the isosmotic point (about 32% seawater), toadfish drink the medium, absorb Na(+), Cl(-), and water across the gastrointestinal tract, actively excrete Na(+) and Cl(-) across the gills, and secrete Mg(2+) into the urine. Below this point, toadfish appear to stop drinking, actively take up Na(+) and Cl(-) at the gills, and retain ions at the kidney. Silver accumulation varied greatly with salinity, by nine-fold (whole body), 26-fold (gill tissue), and 18-fold (liver), with the maxima occurring in 2.5% seawater, the minima in 40% seawater (close to the isosmotic point), and slightly greater values at higher salinities. Highest silver concentrations occurred in liver, second highest in gills, intermediate concentrations in kidney, spleen, and gastrointestinal tissues, and lowest in swim bladder and white muscle, though patterns changed with salinity. There were substantial biliary but minimal urinary levels of silver. The salinity-dependent pattern of silver accumulation best correlated with the abundance of the neutral complex AgCl(0), though the presence of small amounts of Ag(+) at the lowest salinities may also have been important. In contrast, silver accumulation in the esophagus-stomach was greatest in 100% seawater and least at the isosmotic salinity (five-fold variation), a pattern probably explained by drinking and silver uptake into the blood through the gills. Models of silver bioavailability across salinity must consider the presence of silver-binding ligands on both gills and gastrointestinal tract, changing silver speciation, and the changing ionoregulatory physiology of the organism.

Animals↗

Ammonia affects brain nitrogen metabolism but not hydration status in the Gulf toadfish (Opsanus beta).

Laboratory rodents made hyperammonemic by infusing ammonia into the blood show symptoms of brain cell swelling and increased intracranial pressure. These symptoms could be caused in part by an increase in brain glutamine formed when brain glutamine synthetase (GS) naturally detoxifies ammonia to glutamine. Previous studies on the Gulf toadfish (Opsanus beta) demonstrated that it is resistant to high ammonia exposure (HAE) (96 h LC(50)=10mM) despite an increase in brain glutamine. This study attempts to resolve whether the resistance of O. beta is mediated by special handling of brain water in the face of changing glutamine concentrations. Methionine sulfoximine (MSO), an inhibitor of GS, was used to pharmacologically manipulate glutamine concentrations, and magnetic resonance imaging (MRI) was used to assess the status of brain water. Ammonia or MSO treatment did not substantially affect blood acid-base parameters. Exposure to 3.5mM ammonium chloride in seawater for 16 or 40 h resulted in a parallel increase in brain ammonia (3-fold) and glutamine (2-fold) and a decrease in brain glutamate (1.3-fold). Pre-treatment with MSO prevented ammonia-induced changes in glutamine and glutamate concentrations. HAE also induced an increase in plasma osmolality (by 7%) which was probably due to a disturbance of osmoregulatory processes but which did not result in broader whole body dehydration as indicated by muscle water analysis. The increase in brain glutamine was not associated with any changes in brain water in toadfish exposed to 3.5 mM ammonia for up to 40 h or even at 10, 20 and 30 mM ammonia consecutively and for one hour in each concentration. The lack of brain water accumulation implies that ammonia toxicity in toadfish appears to be via pathways other than cerebral swelling. Furthermore, toadfish pre-treated with MSO did not survive a normally sub-lethal exposure to 3.5 mM ammonia for 40 h. The enhancement of ammonia toxicity by MSO suggests that GS function is critical to ammonia tolerance in this species.

Ammonia↗

The effects of acute and chronic ammonia exposure during early life stages of the gulf toadfish, Opsanus beta.

The gulf toadfish (Opsanus beta) is unusual among teleosts in that it is facultatively ureotelic and adults and juveniles have a particularly high tolerance to environmental ammonia. Male toadfish brood their offspring in confined nests. It has been hypothesized that the potential accumulation of ammonia in nests from the male and the offspring, coupled with suspected low ammonia tolerance in offspring would provide the selective pressure necessary for excretion of the less toxic urea by adult toadfish. This study examines this so-called 'nest-fouling' hypothesis through acute and chronic ammonia toxicity testing on early life stages of O. beta. In addition, nitrogen elimination was examined among embryos, yolk-sac larvae and juveniles where we found an ontogenic shift from ammonotely to ureotely with advancing life history stages. The acute ammonia 96 h LC50 values for embryos and larvae were 63.6 and 5.45 mmol-Nl(-1) total ammonia (TAmm), respectively. Thus, these early life stages are more tolerant to ammonia than either juveniles or adults and LC50 values are at least 2 orders of magnitude greater than concentrations naturally occurring at nest sites. Furthermore, 40 days exposures at mean and maximum NH3 concentrations normally found within nests revealed no observable detrimental effects. In fact, growth in terms of wet or dry weight was greatest at the maximum NH3 concentration. We therefore conclude that the nest-fouling hypothesis is not a viable explanation for ureotely in the gulf toadfish.

Age Factors↗

Influence of salinity and organic matter on silver accumulation in Gulf toadfish (Opsanus beta).

To help extend the freshwater based biotic ligand model for silver (Ag) into brackish and saltwater conditions, 50g Gulf toadfish (Opsanus beta) were acclimated to 2.5%, 5%, 10%, 20%, 40%, 80%, or 100% salt water and exposed for 6d to 1.0microM AgNO(3), with or without 10mg C/L organic matter. Suwannee River natural organic matter collected by reverse osmosis was used. Silver accumulation in toadfish gills and plasma decreased as salinity increased, indicating low bioavailability of AgCl complexes. Complexation of Ag by organic matter, normally important in freshwater conditions, was less important as salinity increased. Although relatively little intestinal Ag uptake was observed, both liver and bile accumulated Ag from water imbibed past the isosmotic salinity point ( approximately 1/3 salt water). Toadfish also produced intestinal carbonate pellets, minerals which did not influence Ag accumulation. Our results further stress the importance of Ag speciation, physiological mechanisms, and intestinal Ag uptake when modelling Ag uptake and toxicity beyond freshwater conditions.

Animals↗

Hydromineral balance in the marine gulf toadfish (Opsanus beta) exposed to waterborne or infused nickel.

The effects of acute Ni exposure on the marine gulf toadfish (Opsanus beta) were investigated via separate exposures to waterborne nickel (Ni) and arterially infused Ni. Of the plasma electrolytes measured after 72 h of waterborne exposure (215.3 and 606.1 microM Ni in SW (salinity of 34)), only plasma [Ca2+] was significantly impacted (approximately 55% decrease at both exposure concentrations). At both exposure concentrations, plasma [Ni] was regulated for 24h, after which a linear accumulation over time occurred. Accumulation of Ni in the plasma, and in tissues in direct contact with seawater (gill, stomach, and intestine), was roughly proportional to the Ni concentration of the exposure water. Hydromineral balance in the intestinal fluid (IF) was markedly impacted, with Na(+), Cl(-), SO(4)(2-), K+, and Mg2+ concentrations elevated after 72 h of exposure to waterborne Ni. Following arterial Ni infusion (0.40 micromolNikg(-1)h(-1)), perturbation of hydromineral balance of the intestinal fluid was specific only to Na+ (significantly elevated by Ni infusion) and Mg2+ (significantly decreased by Ni infusion). Nitrogen excretion was not significantly impacted by Ni infusion. In all tissues save the kidney, Ni accumulation via infusion was only a fraction of that observed during waterborne exposures. Remarkably, the kidney Ni burden following infusion was almost identical to that resulting from both waterborne exposures, suggesting homeostatic control. Ni excretion, dominated at 24 h by extrarenal routes, was primarily a function of renal excretion by 72 h of infusion. The sum excretion from infused toadfish was relatively efficient, accounting for over 40% of the infused dose by 72 h. Mechanistic knowledge of the mechanisms of toxicity of waterborne Ni in marine systems is a critical component to the development of physiologically based modeling approaches to accurately predict Ni toxicity in marine and estuarine ecosystems.

Animals↗