Joseph A. Sisneros

Seasonal Plasticity of Auditory Saccular Sensitivity in the Vocal Plainfin Midshipman Fish, Porichthys notatus

The plainfin midshipman fish, Porichthys notatus, is a seasonally breeding species of marine teleost fish that generates acoustic signals for intraspecific social and reproductive-related communication. Female midshipman use the inner ear saccule as the main acoustic endorgan for hearing to detect and locate vocalizing males that produce multiharmonic advertisement calls during the breeding season. Previous work showed that the frequency sensitivity of midshipman auditory saccular afferents changed seasonally with female reproductive state such that summer reproductive females became better suited than winter nonreproductive females to encode the dominant higher harmonics of the male advertisement calls. The focus of this study was to test the hypothesis that seasonal reproductive-dependent changes in saccular afferent tuning is paralleled by similar changes in saccular sensitivity at the level of the hair-cell receptor. Here, I examined the evoked response properties of midshipman saccular hair cells from winter nonreproductive and summer reproductive females to determine if reproductive state affects the frequency response and threshold of the saccule to behaviorally relevant single tone stimuli. Saccular potentials were recorded from populations of hair cells in vivo while sound was presented by an underwater speaker. Results indicate that saccular hair cells from reproductive females had thresholds that were approximately 8 to 13 dB lower than nonreproductive females across a broad range of frequencies that included the dominant higher harmonic components and the fundamental frequency of the males advertisement call. These seasonal-reproductive-dependent changes in thresholds varied differentially across the three (rostral, middle, and caudal) regions of the saccule. Such reproductive-dependent changes in saccule sensitivity may represent an adaptive plasticity of the midshipman auditory sense to enhance mate detection, recognition, and localization during the breeding season.

Ontogenetic changes in the response properties of individual, primary auditory afferents in the vocal plainfin midshipman fish Porichthys notatus Girard.

SUMMARY The auditory system of adult midshipman fish Porichthys notatus Girard is an important sensory receiver system used during intraspecific social communication to encode conspecific vocalizations, but the response properties and function of this system in the pre-adult stages are unknown. Midshipman fish, like other teleosts, use the saccule as the main acoustic end organ of the inner ear. In this study, we examined the discharge properties and the frequency response dynamics of auditory saccular afferent neurons in pre-adult midshipman (∼4–12 months of age) to determine whether encoding of auditory information, inclusive of conspecific vocalizations, changes across life history stages. Extracellular single unit recordings were made from saccular afferents while sound was presented via an underwater speaker. Comparisons with adult data show that the resting discharge rate and auditory threshold sensitivity increased with age/size, while temporal encoding of frequency did not show any significant shifts. The results indicate that the saccular afferents of juveniles, like those of non-reproductive adults, are best adapted to temporally encode the low frequency components (≤100 Hz) of midshipman vocalizations. This report represents the first in vivo investigation of age-related changes in the encoding properties of individual auditory neurons for any fish species.

Auditory sensitivity of larval zebrafish (Danio rerio) measured using a behavioral prepulse inhibition assay.

SUMMARY Zebrafish (Danio rerio) have become a valuable model for investigating the molecular genetics and development of the inner ear in vertebrates. In this study, we employed a prepulse inhibition (PPI) paradigm to assess hearing in larval wild-type (AB) zebrafish during early development at 5–6 days post-fertilization (d.p.f.). We measured the PPI of the acoustic startle response in zebrafish using a 1-dimensional shaker that simulated the particle motion component of sound along the fishs dorsoventral axis. The thresholds to startle-inducing stimuli were determined in 5–6 d.p.f. zebrafish, and their hearing sensitivity was then characterized using the thresholds of prepulse tone stimuli (90–1200 Hz) that inhibited the acoustic startle response to a reliable startle stimulus (820 Hz at 20 dB re. 1 m s−2). Hearing thresholds were defined as the minimum prepulse tone level required to significantly reduce the startle response probability compared with the baseline (no-prepulse) condition. Larval zebrafish showed greatest auditory sensitivity from 90 to 310 Hz with corresponding mean thresholds of −19 to −10 dB re. 1 m s−2, respectively. Hearing thresholds of prepulse tones were considerably lower than previously predicted by startle response assays. The PPI assay was also used to investigate the relative contribution of the lateral line to the detection of acoustic stimuli. After aminoglycoside-induced neuromast hair-cell ablation, we found no difference in PPI thresholds between treated and control fish. We propose that this PPI assay can be used to screen for novel zebrafish hearing mutants and to investigate the ontogeny of hearing in zebrafish and other fishes.

Sound source localization by the plainfin midshipman fish (Porichthys notatus).

The aim of this study was to use plainfin midshipman fish (Porichthys notatus) as a general model to explore how fishes localize an underwater sound source in the relatively simple geometry of a monopole sound field. The robust phonotaxic responses displayed by gravid females toward a monopole sound projector (J-9) broadcasting a low-frequency (90 Hz) tone similar to the fundamental frequency of the males advertisement call were examined. The projectors sound field was mapped at 5 cm resolution azimuth using an eight-hydrophone array. Acoustic pressure was measured with the array and acoustic particle motion was calculated from pressure gradients between hydrophones. The response pathways of the fish were analyzed from video recordings and compared to the sound field. Gravid females at initial release were directed toward the sound source, and the majority (73%) swam to the playback projector with straight to slightly curved tracks in the direction of the source and in line with local particle motion vectors. In contrast, the initial direction of the control (sound-off) group did not differ from random. This paper reports on a comparison of fish localization behavior with directional cues available in the form of local particle motion vectors.

Saccular-Specific Hair Cell Addition Correlates with Reproductive State-Dependent Changes in the Auditory Saccular Sensitivity of a Vocal Fish

The plainfin midshipman fish, Porichthys notatus, is a seasonal breeding teleost fish for which vocal–acoustic communication is essential for its reproductive success. Female midshipman use the saccule as the primary end organ for hearing to detect and locate “singing” males that produce multiharmonic advertisement calls during the summer breeding season. Previous work has shown that female auditory sensitivity changes seasonally with reproductive state; summer reproductive females become better suited than winter nonreproductive females to detect and encode the dominant higher harmonic components in the males advertisement call, which are potentially critical for mate selection and localization. Here, we test the hypothesis that these seasonal changes in female auditory sensitivity are concurrent with seasonal increases in saccular hair cell receptors. We show that there is increased hair cell density in reproductive females and that this increase is not dependent on body size since similar changes in hair cell density were not found in the other inner ear end organs. We also observed an increase in the number of small, potentially immature saccular hair bundles in reproductive females. The seasonal increase in saccular hair cell density and smaller hair bundles in reproductive females was paralleled by a dramatic increase in the magnitude of the evoked saccular potentials and a corresponding decrease in the auditory thresholds recorded from the saccule. This demonstration of correlated seasonal plasticity of hair cell addition and auditory sensitivity may in part facilitate the adaptive auditory plasticity of this species to enhance mate detection and localization during breeding.

Reevaluating the use of aminoglycoside antibiotics in behavioral studies of the lateral line.

Van Trump et al. (2010) recently demonstrated that the aminoglycoside antibiotic gentamicin, previously thought to be selectively toxic to hair cells of the canal neuromasts (CN) of the fish lateral line system, is additionally toxic to hair cells of the superficial neuromasts (SN). The authors used the fluorescent vital dyes DASPEI (2-(4-(dimethylamino)styryl)-N-ethylpyridinium iodide) and FM1-43 ((n-(3-triethylammoniumpropyl)-4-(4-(dibutylamino)styryl) pyridinium dibromide) to visualize by fluorescence microscopy the survival of hair cells in the CN and SN of zebrafish (Danio rerio) and Mexican blind cave fish (Astyanax fasciatus) treated in a solution of 0.001% gentamicin sulfate for 24h. Extensive hair cell death was observed in populations of CN and SN of gentamicin-treated animals of both species compared to control animals. On the basis of this result, the authors concluded, (p. 49), “(1) That hair cells in the SNs of the lateral line can no longer be regarded as functionally resistant to gentamicin toxicity, (2) that this drug should therefore no longer be used as a pharmacological tool for selective blocking of CN, but not SN hair cells, and (3) that the conclusions of some previous studies need to be reevaluated.” Using an alternative approach to themselves reevaluate a 15-year-old assumption in the literature drawn from SEM imaging (e.g., Song et al., 1995), Van Trump et al. (2010) convincingly demonstrated that gentamicin in fact damages a high proportion of both SN and CN, similar in effect to another commonly used aminoglycoside, streptomycin (e.g., Kaus, 1987; Blaxter and Fuiman, 1989; Montgomery et al., 1997; Montgomery et al., 2003). Toward further caution in the interpretation of lateral line behavioral studies that have used aminoglycoside antibiotics to chemically ablate SN and/or CN without rigorous verification of treatment efficacy, we are inspired to share our own recent observations on limitations in the efficacy of both gentamicin and streptomycin in damaging the lateral line systems of three fish species.

Local acoustic particle motion guides sound-source localization behavior in the plainfin midshipman fish, Porichthys notatus

SUMMARY Sound-source localization behavior was studied in the plainfin midshipman fish (Porichthys notatus) by making use of the naturally occurring phonotaxis response of gravid females to playback of the males advertisement call. The observations took place outdoors in a circular concrete tank. A dipole sound projector was placed at the center of the tank and an 80–90 Hz tone (the approximate fundamental frequency to the males advertisement call) was broadcast to gravid females that were released from alternative sites approximately 100 cm from the source. The phonotaxic responses of females to the source were recorded, analyzed and compared with the sound field. One release site was approximately along the vibratory axis of the dipole source, and the other was approximately orthogonal to the vibratory axis. The sound field in the tank was fully characterized through measurements of the sound pressure field using hydrophones and acoustic particle motion using an accelerometer. These measurements confirmed that the sound field was a nearly ideal dipole. When released along the dipole vibratory axis, the responding female fish took essentially straight paths to the source. However, when released approximately 90 deg to the sources vibratory axis, the responding females took highly curved paths to the source that were approximately in line with the local particle motion axes. These results indicate that the acoustic cues used by fish during sound-source localization include the axes of particle motion of the local sound field.

Steroid-dependent auditory plasticity for the enhancement of acoustic communication: recent insights from a vocal teleost fish

The vocal plainfin midshipman fish (Porichthys notatus) has become an excellent model for identifying neural mechanisms of auditory perception that may be shared by all vertebrates. Recent neuroethological studies of the midshipman fish have yielded strong evidence for the steroid-dependent modulation of hearing sensitivity that leads to enhanced coupling of sender and receiver in this vocal-acoustic communication system. Previous work shows that non-reproductive females treated with either testosterone or 17beta-estradiol exhibit an increase in the degree of temporal encoding by the auditory saccular afferents to the dominant frequency content of male vocalizations produced during social-reproductive behaviors. The expanded frequency sensitivity of steroid treated females mimics the reproductive females auditory phenotype and is proposed to improve the detection and localization of calling conspecific mates during the summer breeding season. This review focuses on the novel form of steroid-dependent auditory plasticity that is found in the adult midshipman fish and its association with the reproductive biology and behavior of this species. Evidence for midshipman reproductive-state and steroid-dependent auditory plasticity is reviewed and the potential mechanisms that lead to this novel form of adaptive plasticity are discussed.

Neuroendocrine control of seasonal plasticity in the auditory and vocal systems of fish

Seasonal changes in reproductive-related vocal behavior are widespread among fishes. This review highlights recent studies of the vocal plainfin midshipman fish, Porichthys notatus, a neuroethological model system used for the past two decades to explore neural and endocrine mechanisms of vocal-acoustic social behaviors shared with tetrapods. Integrative approaches combining behavior, neurophysiology, neuropharmacology, neuroanatomy, and gene expression methodologies have taken advantage of simple, stereotyped and easily quantifiable behaviors controlled by discrete neural networks in this model system to enable discoveries such as the first demonstration of adaptive seasonal plasticity in the auditory periphery of a vertebrate as well as rapid steroid and neuropeptide effects on vocal physiology and behavior. This simple model system has now revealed cellular and molecular mechanisms underlying seasonal and steroid-driven auditory and vocal plasticity in the vertebrate brain.

Catecholaminergic connectivity to the inner ear, central auditory, and vocal motor circuitry in the plainfin midshipman fish porichthys notatus

Although the neuroanatomical distribution of catecholaminergic (CA) neurons has been well documented across all vertebrate classes, few studies have examined CA connectivity to physiologically and anatomically identified neural circuitry that controls behavior. The goal of this study was to characterize CA distribution in the brain and inner ear of the plainfin midshipman fish (Porichthys notatus) with particular emphasis on their relationship with anatomically labeled circuitry that both produces and encodes social acoustic signals in this species. Neurobiotin labeling of the main auditory end organ, the saccule, combined with tyrosine hydroxylase immunofluorescence (TH‐ir) revealed a strong CA innervation of both the peripheral and central auditory system. Diencephalic TH‐ir neurons in the periventricular posterior tuberculum, known to be dopaminergic, send ascending projections to the ventral telencephalon and prominent descending projections to vocal–acoustic integration sites, notably the hindbrain octavolateralis efferent nucleus, as well as onto the base of hair cells in the saccule via nerve VIII. Neurobiotin backfills of the vocal nerve in combination with TH‐ir revealed CA terminals on all components of the vocal pattern generator, which appears to largely originate from local TH‐ir neurons but may include input from diencephalic projections as well. This study provides strong neuroanatomical evidence that catecholamines are important modulators of both auditory and vocal circuitry and acoustic‐driven social behavior in midshipman fish. This demonstration of TH‐ir terminals in the main end organ of hearing in a nonmammalian vertebrate suggests a conserved and important anatomical and functional role for dopamine in normal audition. J. Comp. Neurol. 522:2887‐2927, 2014.