The use of anesthesia during evoked potential audiometry in goldfish (Carassius auratus)

Boat noise interferes with Lusitanian toadfish acoustic communication

Anthropogenic noise is considered a major underwater pollutant as increasing ocean background noise due to human activities is impacting aquatic organisms. One of the most prevalent anthropogenic sounds is boat noise. Although motorboat traffic has increased in the past few decades, its impact on the communication of fish is still poorly known. The highly vocal Lusitanian toadfish (Halobatrachus didactylus) is an excellent model to test the impact of this anthropogenic stressor as it relies on acoustic communication to attract mates. Here, we performed two experiments to test the impact of boat noise on the acoustic communication of the Lusitanian toadfish. Using the auditory evoked potential (AEP) technique, we first compared the maximum distance a fish can perceive a boatwhistle (BW), the mate attraction acoustic signal, before and after embedding it in boat noise. Noises from a small motorboat and from a ferryboat reduced the active space from a control value of 6.4-10.4 m to 2.0-2.5 m and 6.3-6.7 m, respectively. In the second experiment we monitored the acoustic behaviour of breeding males exposed to boat noise playbacks and we observed an increase in the inter-onset interval of BWs and a disruption of the usual vocal interactions between singing males. These results demonstrate that boat noise can severely reduce the acoustic active space and affect the chorusing behaviour in this species, which may have consequences in breeding success for individuals and could thus affect fitness.

Comparative analysis and gut bacterial community assemblages of grass carp and crucian carp in new lineages from the Dongting Lake area

Gut microbiota are known to play an important role in health and nutrition of the host and have been attracting an increasing attention. Farming of new lineages of grass carp and crucian carp has been developed rapidly as these species were found to outperform indigenous ones in terms of growth rate and susceptibility to diseases. Despite this rapid development, no studies have addressed the characteristics of their gut microbiota as a potential factor responsible for the improved characteristics. To reveal whether microbiomes of the new lineages are different from indigenous ones, and therefore could be responsible for improved growth features, intestinal microbiota from the new lineages were subjected to high-throughput sequencing. While the phyla Firmicutes, Fusobacteria and Proteobacteria were representing the core bacterial communities that comprised more than 75% in all fish intestinal samples, significant differences were found in the microbial community composition of the new linages versus indigenous fish populations, suggesting the possibility that results in the advantages of enhanced disease resistance and rapid growth for the new fish lineages. Bacterial composition was similar between herbivorous and omnivorous fish. The relative abundance of Bacteroidetes and Actinobacteria was significantly higher in omnivores compared to that of herbivores, whereas Cetobacterium_sp. was abundant in herbivores. We also found that the gut microbiota of freshwater fish in the Dongting lake area was distinct from those of other areas. Network graphs showed the reduced overall connectivity of gut bacteria in indigenous fish, whereas the bacteria of the new fish lineage groups showed hubs with more node degree. A phylogenetic investigation of communities by reconstruction of unobserved states inferred function profile showed several metabolic processes were more active in the new lineages compared to indigenous fish. Our findings suggest that differences in gut bacterial community composition may be an important factor contributing to the rapid growth and high disease resistance of the new fish lineages.

Differential processing in modality-specific Mauthner cell dendrites

KEY POINTS

The present study examines dendritic integrative processes that occur in many central neurons but have been challenging to study in vivo in the vertebrate brain. The Mauthner cell of goldfish receives auditory and visual information via two separate dendrites, providing a privileged scenario for in vivo examination of dendritic integration. The results show differential attenuation properties in the Mauthner cell dendrites arising at least partly from differences in cable properties and the nonlinear behaviour of the respective dendritic membranes. In addition to distinct modality-dependent membrane specialization in neighbouring dendrites of the Mauthner cell, we report cross-modal dendritic interactions via backpropagating postsynaptic potentials. Broadly, the results of the present study provide an exceptional example for the processing power of single neurons.

ABSTRACT

Animals process multimodal information for adaptive behavioural decisions. In fish, evasion of a diving bird that breaks the water surface depends on integrating visual and auditory stimuli with very different characteristics. How do neurons process such differential sensory inputs at the dendritic level? For that, we studied the Mauthner cells (M-cells) in the goldfish startle circuit, which receive visual and auditory inputs via two separate dendrites, both accessible for in vivo recordings. We investigated whether electrophysiological membrane properties and dendrite morphology, studied in vivo, play a role in selective sensory processing in the M-cell. The results obtained show that anatomical and electrophysiological differences between the dendrites combine to produce stronger attenuation of visually evoked postsynaptic potentials (PSPs) than to auditory evoked PSPs. Interestingly, our recordings showed also cross-modal dendritic interaction because auditory evoked PSPs invade the ventral dendrite (VD), as well as the opposite where visual PSPs invade the lateral dendrite (LD). However, these interactions were asymmetrical, with auditory PSPs being more prominent in the VD than visual PSPs in the LD. Modelling experiments imply that this asymmetry is caused by active conductances expressed in the proximal segments of the VD. The results obtained in the present study suggest modality-dependent membrane specialization in M-cell dendrites suited for processing stimuli of different time domains and, more broadly, provide a compelling example of information processing in single neurons.

Cellular Mechanisms of Cortisol-Induced Changes in Mauthner-Cell Excitability in the Startle Circuit of Goldfish

Predator pressure and olfactory cues (alarm substance) have been shown to modulate Mauthner cell (M-cell) initiated startle escape responses (C-starts) in teleost fish. The regulation of such adaptive responses to potential threats is thought to involve the release of steroid hormones such as cortisol. However, the mechanism by which cortisol may regulate M-cell excitability is not known. Here, we used intrasomatic, in vivo recordings to elucidate the acute effects of cortisol on M-cell membrane properties and sound evoked post-synaptic potentials (PSPs). Cortisol tonically decreased threshold current in the M-cell within 10 min before trending towards baseline excitability over an hour later, which may indicate the involvement of non-genomic mechanisms. Consistently, current ramp injection experiments showed that cortisol increased M-cell input resistance in the depolarizing membrane, i.e., by a voltage-dependent postsynaptic mechanism. Cortisol also increases the magnitude of sound-evoked M-cell PSPs by reducing the efficacy of local feedforward inhibition (FFI). Interestingly, another pre-synaptic inhibitory network mediating prepulse inhibition (PPI) remained unaffected. Together, our results suggest that cortisol rapidly increases M-cell excitability via a post-synaptic effector mechanism, likely a chloride conductance, which, in combination with its dampening effect on FFI, will modulate information processing to reach threshold. Given the central role of the M-cell in initiating startle, these results are consistent with a role of cortisol in mediating the expression of a vital behavior.

Effect of the Level of Anesthesia on the Auditory Brainstem Response in the Emei Music Frog (Babina daunchina)

Anesthesia is known to affect the auditory brainstem response (ABR) in mice, rats, birds and lizards. The present study investigated how the level of anesthesia affects ABR recordings in an amphibian species, Babina daunchina. To do this, we compared ABRs evoked by tone pip stimuli recorded from 35 frogs when Tricaine methane sulphonate (MS-222) anesthetic immersion times varied from 0, 5 and 10 minutes after anesthesia induction at sound frequencies between 0.5 and 6 kHz. ABR thresholds increased significantly with immersion time across the 0.5 kHz to 2.5 kHz frequency range, which is the most sensitive frequency range for hearing and the main frequency range of male calls. There were no significant differences for anesthetic levels across the 3 kHz to 6 kHz range. ABR latency was significantly longer in the 10 min group than in the 0 and 5 min groups at frequencies of 0.5, 1.0, 1.5, 2.5 kHz, while ABR latency did not differ across the 3 kHz to 4 kHz range and at 2.0 kHz. Taken together, these results show that the level of anesthesia affects the amplitude, threshold and latency of ABRs in frogs.

Glycine and GABAA receptors mediate tonic and phasic inhibitory processes that contribute to prepulse inhibition in the goldfish startle network

Prepulse inhibition (PPI) is understood as a sensorimotor gating process that attenuates sensory flow to the startle pathway during early stages (20–1000 ms) of information processing. Here, we applied in vivo electrophysiology and pharmacology to determine if PPI is mediated by glycine receptors (GlyRs) and/or GABA_A receptors (GABA_ARs) in the goldfish auditory startle circuit. Specifically, we used selective antagonists to dissect the contributions of target receptors on sound-evoked postsynaptic potentials (PSPs) recorded in the neurons that initiate startle, the Mauthner-cells (M-cell). We found that strychnine, a GlyR antagonist, disrupted a fast-activated (5 ms) and rapidly (<50 ms) decaying (feed-forward) inhibitory process that contributes to PPI at 20 ms prepulse/pulse inter-stimulus intervals (ISI). Additionally we observed increases of the evoked postsynaptic potential (PSP) peak amplitude (+87.43 ± 21.53%, N = 9) and duration (+204 ± 48.91%, N = 9). In contrast, treatment with bicuculline, a GABA_AR antagonist, caused a general reduction in PPI across all tested interstimulus intervals (ISIs) (20–500 ms). Bicuculline also increased PSP peak amplitude (+133.8 ± 10.3%, N = 5) and PSP duration (+284.95 ± 65.64%, N = 5). Treatment with either antagonist also tonically increased post-synaptic excitability in the M-cells, reflected by an increase in the magnitude of antidromically-evoked action potentials (APs) by 15.07 ± 3.21%, N = 7 and 16.23 ± 7.08%, N = 5 for strychnine and bicuculline, respectively. These results suggest that GABA_ARs and GlyRs are functionally segregated to short- and longer-lasting sound-evoked (phasic) inhibitory processes that contribute to PPI, with the mediation of tonic inhibition by both receptor systems being critical for gain control within the M-cell startle circuit.

The 5-HT5A receptor regulates excitability in the auditory startle circuit: functional implications for sensorimotor gating

Here we applied behavioral testing, pharmacology, and in vivo electrophysiology to determine the function of the serotonin 5-HT5A receptor in goldfish startle plasticity and sensorimotor gating. In an initial series of behavioral experiments, we characterized the effects of a selective 5-HT5A antagonist, SB-699551 (3-cyclopentyl-N-[2-(dimethylamino)ethyl]-N-[(4'-{[(2-phenylethyl)amino]methyl}-4-biphenylyl)methyl]propanamide dihydrochloride), on prepulse inhibition of the acoustic startle response. Those experiments showed a dose-dependent decline in startle rates in prepulse conditions. Subsequent behavioral experiments showed that SB-699551 also reduced baseline startle rates (i.e., without prepulse). To determine the cellular mechanisms underlying these behaviors, we tested the effects of two distinct selective 5-HT5A antagonists, SB-699551 and A-843277 (N-(2,6-dimethoxybenzyl)-N'[4-(4-fluorophenyl)thiazol-2-yl]guanidine), on the intrinsic membrane properties and synaptic sound response of the Mauthner cell (M-cell), the decision-making neuron of the startle circuit. Auditory-evoked postsynaptic potentials recorded in the M-cell were similarly attenuated after treatment with either 5-HT5A antagonist (SB-699551, 26.41 ± 3.98% reduction; A-843277, 17.52 ± 6.24% reduction). This attenuation was produced by a tonic (intrinsic) reduction in M-cell input resistance, likely mediated by a Cl(-) conductance, that added to the extrinsic inhibition produced by an auditory prepulse. Interestingly, the effector mechanisms underlying neural prepulse inhibition itself were unaffected by antagonist treatment. In summary, these results provide an in vivo electrophysiological characterization of the 5-HT5A receptor and its behavioral relevance and provide a new perspective on the interaction of intrinsic and extrinsic modulatory mechanisms in startle plasticity and sensorimotor gating.

Auditory evoked potential audiometry in fish

A recent survey lists more than 100 papers utilizing the auditory evoked potential (AEP) recording technique for studying hearing in fishes. More than 95 % of these AEP-studies were published after Kenyon et al. introduced a non-invasive electrophysiological approach in 1998 allowing rapid evaluation of hearing and repeated testing of animals. First, our review compares AEP hearing thresholds to behaviorally gained thresholds. Second, baseline hearing abilities are described and compared in 111 fish species out of 51 families. Following this, studies investigating the functional significance of various accessory hearing structures (Weberian ossicles, swim bladder, otic bladders) by eliminating these morphological structures in various ways are dealt with. Furthermore, studies on the ontogenetic development of hearing are summarized. The AEP-technique was frequently used to study the effects of high sound/noise levels on hearing in particular by measuring the temporary threshold shifts after exposure to various noise types (white noise, pure tones and anthropogenic noises). In addition, the hearing thresholds were determined in the presence of noise (white, ambient, ship noise) in several studies, a phenomenon termed masking. Various ecological (e.g., temperature, cave dwelling), genetic (e.g., albinism), methodical (e.g., ototoxic drugs, threshold criteria, speaker choice) and behavioral (e.g., dominance, reproductive status) factors potentially influencing hearing were investigated. Finally, the technique was successfully utilized to study acoustic communication by comparing hearing curves with sound spectra either under quiet conditions or in the presence of noise, by analyzing the temporal resolution ability of the auditory system and the detection of temporal, spectral and amplitude characteristics of conspecific vocalizations.

Dopaminergic-induced changes in Mauthner cell excitability disrupt prepulse inhibition in the startle circuit of goldfish

Prepulse inhibition (PPI) is a widespread sensorimotor gating phenomenon characterized by a decrease in startle magnitude if a nonstartling stimulus is presented 20-1,000 ms before a startling stimulus. Dopaminergic agonists disrupt behavioral PPI in various animal models. This provides an important neuropharmacological link to schizophrenia patients that typically show PPI deficits at distinct (60 ms) prepulse-pulse intervals. Here, we study time-dependent effects of dopaminergic modulation in the goldfish Mauthner cell (M-cell) startle network, which shows PPI-like behavioral and physiological startle attenuations. The unique experimental accessibility of the M-cell system allows investigating the underlying cellular mechanism with physiological stimuli in vivo. Our results show that the dopaminergic agonist apomorphine (2 mg/kg body wt) reduced synaptic M-cell PPI by 23.6% (n = 18; P = 0.009) for prepulse-pulse intervals of 50 ms, whereas other intervals showed no reduction. Consistently, application of the dopamine antagonist haloperidol (0.4 mg/kg body wt) restored PPI to control level. Current ramp injections while recording M-cell membrane potential revealed that apomorphine acts through a postsynaptic, time-dependent mechanism by deinactivating a M-cell membrane nonlinearity, effectively increasing input resistance close to threshold. This increase is most pronounced for prepulse-pulse intervals of 50 ms (47.9%, n = 8; P < 0.05) providing a time-dependent, cellular mechanism for dopaminergic disruption of PPI. These results provide, for the first time, direct evidence of dopaminergic modulation of PPI in the elementary startle circuit of vertebrates and reemphasize the potential of characterizing temporal aspects of PPI at the physiological level to understand its underlying mechanisms.

Sound pressure and particle acceleration audiograms in three marine fish species from the Adriatic Sea

Fishes show great variability in hearing sensitivity, bandwidth, and the appropriate stimulus component for the inner ear (particle motion or pressure). Here, hearing sensitivities in three vocal marine species belonging to different families were described in terms of sound pressure and particle acceleration. In particular, hearing sensitivity to tone bursts of varying frequencies were measured in the red-mouthed goby Gobius cruentatus, the Mediterranean damselfish Chromis chromis, and the brown meagre Sciaena umbra using the non-invasive auditory evoked potential-recording technique. Hearing thresholds were measured in terms of sound pressure level and particle acceleration level in the three Cartesian directions using a newly developed miniature pressure-acceleration sensor. The brown meagre showed the broadest hearing range (up to 3000 Hz) and the best hearing sensitivity, both in terms of sound pressure and particle acceleration. The red-mouthed goby and the damselfish were less sensitive, with upper frequency limits of 700 and 600 Hz, respectively. The low auditory thresholds and the large hearing bandwidth of S. umbra indicate that sound pressure may play a role in S. umbra's hearing, even though pronounced connections between the swim bladder and the inner ears are lacking.

Objective threshold estimation and measurement of the residual background noise in auditory evoked potentials of goldfish

A survey of papers using auditory evoked potentials (AEPs) published over the last 10 years (Table I) demonstrates that most AEP studies in animals have used subjective methods for auditory threshold determination. Subjective methods greatly reduce the value of statistical hypothesis testing and jeopardize tests of hypothetical experimental group differences in hearing sensitivity. Correspondingly, many attempts have been made to develop objective threshold determination methods, but these have not been used widely. Further, they seldom include an appreciation of the effects of residual noise in the AEP. In this study, AEPs evoked by tonal and noise stimuli in goldfish (Carassius auratus) were recorded and the residual background noise was measured and analyzed in detail. High variability was found in residual noise, but can be effectively controlled with a simple modification of averaging routines. Considerable interobserver disagreements were found using subjective threshold estimation. An objective method of threshold determination was developed based on comparison between AEP amplitude and controlled residual noise, using a signal detection theory approach to set specific threshold criteria. The usefulness of AEP in hypothesis testing for auditory function requires more control over residual background noise amplitudes and the use of objective threshold determination techniques.