Spectrally specific temporal analyses of spike-train responses to complex sounds: A unifying framework

Significant scientific and translational questions remain in auditory neuroscience surrounding the neural correlates of perception. Relating perceptual and neural data collected from humans can be useful; however, human-based neural data are typically limited to evoked far-field responses, which lack anatomical and physiological specificity. Laboratory-controlled preclinical animal models offer the advantage of comparing single-unit and evoked responses from the same animals. This ability provides opportunities to develop invaluable insight into proper interpretations of evoked responses, which benefits both basic-science studies of neural mechanisms and translational applications, e.g., diagnostic development. However, these comparisons have been limited by a disconnect between the types of spectrotemporal analyses used with single-unit spike trains and evoked responses, which results because these response types are fundamentally different (point-process versus continuous-valued signals) even though the responses themselves are related. Here, we describe a unifying framework to study temporal coding of complex sounds that allows spike-train and evoked-response data to be analyzed and compared using the same advanced signal-processing techniques. The framework uses a set of peristimulus-time histograms computed from single-unit spike trains in response to polarity-alternating stimuli to allow advanced spectral analyses of both slow (envelope) and rapid (temporal fine structure) response components. Demonstrated benefits include: (1) novel spectrally specific temporal-coding measures that are less confounded by distortions due to hair-cell transduction, synaptic rectification, and neural stochasticity compared to previous metrics, e.g., the correlogram peak-height, (2) spectrally specific analyses of spike-train modulation coding (magnitude and phase), which can be directly compared to modern perceptually based models of speech intelligibility (e.g., that depend on modulation filter banks), and (3) superior spectral resolution in analyzing the neural representation of nonstationary sounds, such as speech and music. This unifying framework significantly expands the potential of preclinical animal models to advance our understanding of the physiological correlates of perceptual deficits in real-world listening following sensorineural hearing loss.

The chinchilla animal model for hearing science and noise-induced hearing loss

The chinchilla animal model for noise-induced hearing loss has an extensive history spanning more than 50 years. Many behavioral, anatomical, and physiological characteristics of the chinchilla make it a valuable animal model for hearing science. These include similarities with human hearing frequency and intensity sensitivity, the ability to be trained behaviorally with acoustic stimuli relevant to human hearing, a docile nature that allows many physiological measures to be made in an awake state, physiological robustness that allows for data to be collected from all levels of the auditory system, and the ability to model various types of conductive and sensorineural hearing losses that mimic pathologies observed in humans. Given these attributes, chinchillas have been used repeatedly to study anatomical, physiological, and behavioral effects of continuous and impulse noise exposures that produce either temporary or permanent threshold shifts. Based on the mechanistic insights from noise-exposure studies, chinchillas have also been used in pre-clinical drug studies for the prevention and rescue of noise-induced hearing loss. This review paper highlights the role of the chinchilla model in hearing science, its important contributions, and its advantages and limitations.

Between-ear sound frequency disparity modulates a brain stem biomarker of binaural hearing

The auditory brain stem response (ABR) is an evoked potential that indexes a cascade of neural events elicited by sound. In the present study we evaluated the influence of sound frequency on a derived component of the ABR known as the binaural interaction component (BIC). Specifically, we evaluated the effect of acoustic interaural (between-ear) frequency mismatch on BIC amplitude. Goals were to 1) increase basic understanding of sound features that influence this long-studied auditory potential and 2) gain insight about the persistence of the BIC with interaural electrode mismatch in human users of bilateral cochlear implants, presently a limitation on the prospective utility of the BIC in audiological settings. Data were collected in an animal model that is audiometrically similar to humans, the chinchilla (Chinchilla lanigera; 6 females). Frequency disparities and amplitudes of acoustic stimuli were varied over broad ranges, and associated variation of BIC amplitude was quantified. Subsequently, responses were simulated with the use of established models of the brain stem pathway thought to underlie the BIC. Collectively, the data demonstrate that at high sound intensities (≥85 dB SPL), the acoustically elicited BIC persisted with interaurally disparate stimulation (click frequencies ≥1.5 octaves apart). However, sharper tuning emerged at moderate sound intensities (65 dB SPL), with the largest BIC occurring for stimulus frequencies within ~0.8 octaves, equivalent to ±1 mm in cochlear place. Such responses were consistent with simulated responses of the presumed brain stem generator of the BIC, the lateral superior olive. The data suggest that leveraging focused electrical stimulation strategies could improve BIC-based bilateral cochlear implant fitting outcomes.NEW & NOTEWORTHY Traditional hearing tests evaluate each ear independently. Diagnosis and treatment of binaural hearing dysfunction remains a basic challenge for hearing clinicians. We demonstrate in an animal model that the prospective utility of a noninvasive electrophysiological signature of binaural function, the binaural interaction component (BIC), depends strongly on the intensity of auditory stimulation. Data suggest that more informative BIC measurements could be obtained with clinical protocols leveraging stimuli restricted in effective bandwidth.

Biodistribution of [11C]-Metformin and mRNA Expression of Placentae Metformin Transporters in the Pregnant Chinchilla

Background

While metformin is the first-line pharmacological treatment of diabetes mellitus type 2, this drug is not considered safe to use in pregnant women because of its unknown consequences for the fetus. In this study, we aimed to investigate the biodistribution of metformin in the pregnant chinchilla, a species exhibiting placental characteristics comparable with the pregnant woman. Furthermore, we aimed to investigate the expression of metformin transporters in humans and chinchillas, respectively, in order to evaluate the pregnant chinchilla as a novel animal model for the use of metformin in pregnancy.

Methods

Three chinchillas in the last part of gestation were injected with [11C]-metformin and scanned by PET/CT for 70 minutes to visualize the distribution. To investigate the difference in expression of placenta transporters between humans and chinchillas, PCR was performed on samples from five chinchilla placentae and seven human placentae.

Results

Dynamic PET with [11C]-metformin showed that the metformin distribution in chinchillas was similar to that in nonpregnant humans, with signal from kidneys, liver, bladder, and submandibular glands. Conversely, no radioactive signal was observed from the fetuses, and no metformin was accumulated in the chinchilla fetus when measuring the SUV. PCR of placental mRNA showed that the human placentae expressed OCT3, whereas the chinchilla placentae expressed OCT1.

Conclusion

Since metformin did not pass the placenta barrier in the pregnant chinchilla, as it is known to do in humans, we do not suggest the chinchilla as a future animal model of metformin in pregnancies.

A lumped-element model of the chinchilla middle ear

An air-conduction circuit model was developed for the chinchilla middle ear and cochlea. The lumped-element model is based on the classic Zwislocki model of the same structures in human. Model parameters were fit to various measurements of chinchilla middle-ear transfer functions and impedances, using a combination of error-minimization-driven computer-automated and manual fitting methods. The measurements used to fit the model comprise a newer, more-extensive data set than previously used, and include measurements of stapes velocity and inner-ear sound pressure within the vestibule and the scala tympani near the round window. The model is in agreement with studies of the effects of middle-ear cavity holes in experiments that require access to the middle-ear air space. The structure of the model allows easy addition of other sources of auditory stimulation, e.g., the multiple sources of bone-conducted sound-the long-term goal for the model's development-and mechanical stimulation of the ossicles and round window.

Analgesic Efficacy and Safety of Hydromorphone in Chinchillas (Chinchilla lanigera)

Limited information is available regarding the efficacy of opioid analgesics in chinchillas. Here we sought to evaluate the analgesic efficacy and safety of hydromorphone in chinchillas. In a randomized, controlled, blind, complete crossover design, hydromorphone was administered at 0.5, 1, and 2 mg/kg SC to 16 chinchillas. Analgesic efficacy was determined by measuring hindlimb withdrawal latencies after a thermal noxious stimulus (Hargreaves method) at 0, 1, 2, 4, and 8 h after drug administration. Changes in daily food intake and fecal output after hydromorphone administration were recorded. At 2 mg/kg SC, but not at lower dosages, hydromorphone increased withdrawal latencies for less than 4 h. Food intake was reduced after all 3 dosages, and fecal output decreased in the 1- and 2-mg/kg groups. The decreases in these parameters were dose-dependent, with the greatest reduction measured over the first 24 h. Our current results indicate that hydromorphone at 2 mg/kg SC is an effective, short-acting analgesic drug in chinchillas that transiently reduces food intake and fecal output. Further studies are needed to evaluate the safety of hydromorphone in animals undergoing surgical procedures and general anesthesia and to determine whether lower doses provide analgesia in different nociceptive models.

Individual and sex distinctiveness in bark calls of domestic chinchillas elicited in a distress context

Animals obtain information about their social environment by means of communication signals, which provide relevant subtle cues for individual recognition. An important requisite for this process is the existence of larger between- than within-emitter signal variation. Acoustic signals are complex traits susceptible of variation in their spectral and temporal components, implying that signal distinctiveness can result from differences in single or various acoustic components. In this study, domestic chinchillas were induced to vocalize in a distress context to describe the acoustic characteristics of the bark calls, and to determine features that denote the potential value of this vocalization for individual and/or sexual recognition. The results demonstrate that the variation in spectral and temporal components of the bark calls of chinchillas elicited under a distress context is larger between than within individuals, suggesting the potential of these signals for distinctiveness between individual signalers, although the potential of this call type for sex distinctiveness is quite limited. These results combined with previous studies on auditory capabilities of chinchillas contribute to position this rodent as a valuable model species for studying auditory-vocal interactions.

Analysis of an impulse response measured at the basilar membrane of the chinchilla

In a recent paper [J. Acoust. Soc. Am. 133, 2224-2239 (2013)], Shera and Cooper report on the impulse response of the basilar membrane (BM) of a chinchilla, a waveform which shows repetitive bursts. They explain the bursts in terms of repeated coherent reflection at BM discontinuities and partial reflection at the stapes ("coherent reflection filtering"). Here the same waveform is examined in detail, highlighting features which indicate that the coherent reflection model, with calls for the same repetitive process to act on each successive burst, does not fully account for the shape of the measured impulse response.

Non-invasive endocrine monitoring of ovarian and adrenal activity in chinchilla (Chinchilla lanigera) females during pregnancy, parturition and early post-partum period

The chinchilla is a rodent that bears one of the finest and most valuable pelts in the world. The wild counterpart is, however, almost extinct because of a drastic past and ongoing population decline. The present work was developed to increase our knowledge of the reproductive physiology of pregnancy and post-partum estrus in the chinchilla, characterizing the endocrine patterns of urinary progesterone, estradiol, LH and cortisol metabolites throughout gestation and post-partum estrus and estimating the ovulation timing at post-partum estrus. Longitudinal urine samples were collected once per week throughout pregnancy and analyzed for creatinine, cortisol, LH, estrogen and progesterone metabolite concentrations. To indirectly determine the ovulation timing at post-partum estrus, a second experiment was performed using pregnant females subjected to a post-partum in vivo fertilization scheme. Urinary progestagen metabolites increased above baseline levels in early pregnancy between weeks-8 and -11 respectively to parturition, and slightly declined at parturition time. Urinary estrogens showed rising levels throughout mid- and late pregnancy (weeks-9 to -6 and a further increase at week-5 to parturition) and decreased in a stepwise manner after parturition, returning to baseline levels two weeks thereafter. Cortisol metabolite levels were relatively constant throughout pregnancy with a tendency for higher levels in the last third of gestation and after the pups' birth. Parturition was associated with dramatic reductions in urinary concentrations of sex steroids (especially progestagens). Observations in breeding farms indicated that the females that resulted in a second pregnancy after mating, did so on the second day after parturition. These data were in agreement with an LH peak detected 24h after parturition. Urinary steroid hormone patterns of estrogen and progestagen metabolites provided valuable information on endocrine events during pregnancy and after parturition in the chinchilla. Results presented in this study enhance our understanding of natural reproductive dynamics in the chinchilla and support empirical observations of breeders that post-partum ovulation occurs ∼ 48 h after parturition.

Middle-ear velocity transfer function, cochlear input immittance, and middle-ear efficiency in chinchilla

The transfer function H(V) between stapes velocity V(S) and sound pressure near the tympanic membrane P(TM) is a descriptor of sound transmission through the middle ear (ME). The ME power transmission efficiency (MEE), the ratio of sound power entering the cochlea to power entering the middle ear, was computed from H(V) measured in seven chinchilla ears and previously reported measurements of ME input admittance Y(TM) and ME pressure gain G(MEP) [Ravicz and Rosowski, J. Acoust. Soc. Am. 132, 2437-2454 (2012); J. Acoust. Soc. Am. 133, 2208-2223 (2013)] in the same ears. The ME was open, and a pressure sensor was inserted into the cochlear vestibule for most measurements. The cochlear input admittance Y(C) computed from H(V) and G(MEP) is controlled by a combination of mass and resistance and is consistent with a minimum-phase system up to 27 kHz. The real part Re{Y(C)}, which relates cochlear sound power to inner-ear sound pressure, decreased gradually with frequency up to 25 kHz and more rapidly above that. MEE was about 0.5 between 0.1 and 8 kHz, higher than previous estimates in this species, and decreased sharply at higher frequencies.

Conductive hearing loss induced by experimental middle-ear effusion in a chinchilla model reveals impaired tympanic membrane-coupled ossicular chain movement

Otitis media with effusion (OME) occurs when fluid collects in the middle-ear space behind the tympanic membrane (TM). As a result of this effusion, sounds can become attenuated by as much as 30-40 dB, causing a conductive hearing loss (CHL). However, the exact mechanical cause of the hearing loss remains unclear. Possible causes can include altered compliance of the TM, inefficient movement of the ossicular chain, decreased compliance of the oval window-stapes footplate complex, or altered input to the oval and round window due to conduction of sound energy through middle-ear fluid. Here, we studied the contribution of TM motion and umbo velocity to a CHL caused by middle-ear effusion. Using the chinchilla as an animal model, umbo velocity (V U) and cochlear microphonic (CM) responses were measured simultaneously using sinusoidal tone pip stimuli (125 Hz-12 kHz) before and after filling the middle ear with different volumes (0.5-2.0 mL) of silicone oil (viscosity, 3.5 Poise). Concurrent increases in CM thresholds and decreases in umbo velocity were noted after the middle ear was filled with 1.0 mL or more of fluid. Across animals, completely filling the middle ear with fluid caused 20-40-dB increases in CM thresholds and 15-35-dB attenuations in umbo velocity. Clinic-standard 226-Hz tympanometry was insensitive to fluid-associated changes in CM thresholds until virtually the entire middle-ear cavity had been filled (approximately >1.5 mL). The changes in umbo velocity, CM thresholds, and tympanometry due to experimentally induced OME suggest CHL arises primarily as a result of impaired TM mobility and TM-coupled umbo motion plus additional mechanisms within the middle ear.

Basilar-membrane interference patterns from multiple internal reflection of cochlear traveling waves

At low stimulus levels, basilar-membrane (BM) mechanical transfer functions in sensitive cochleae manifest a quasiperiodic rippling pattern in both amplitude and phase. Analysis of the responses of active cochlear models suggests that the rippling is a mechanical interference pattern created by multiple internal reflection within the cochlea. In models, the interference arises when reverse-traveling waves responsible for stimulus-frequency otoacoustic emissions (SFOAEs) reflect off the stapes on their way to the ear canal, launching a secondary forward-traveling wave that combines with the primary wave produced by the stimulus. Frequency-dependent phase differences between the two waves then create the rippling pattern measurable on the BM. Measurements of BM ripples and SFOAEs in individual chinchilla ears demonstrate that the ripples are strongly correlated with the acoustic interference pattern measured in ear-canal pressure, consistent with a common origin involving the generation of SFOAEs. In BM responses to clicks, the ripples appear as temporal fine structure in the response envelope (multiple lobes, waxing and waning). Analysis of the ripple spacing and response phase gradients provides a test for the role of fast- and slow-wave modes of reverse energy propagation within the cochlea. The data indicate that SFOAE delays are consistent with reverse slow-wave propagation but much too long to be explained by fast waves.

Inner-ear sound pressures near the base of the cochlea in chinchilla: further investigation

The middle-ear pressure gain GMEP, the ratio of sound pressure in the cochlear vestibule PV to sound pressure at the tympanic membrane PTM, is a descriptor of middle-ear sound transfer and the cochlear input for a given stimulus in the ear canal. GMEP and the cochlear partition differential pressure near the cochlear base ΔPCP, which determines the stimulus for cochlear partition motion and has been linked to hearing ability, were computed from simultaneous measurements of PV, PTM, and the sound pressure in scala tympani near the round window PST in chinchilla. GMEP magnitude was approximately 30 dB between 0.1 and 10 kHz and decreased sharply above 20 kHz, which is not consistent with an ideal transformer or a lossless transmission line. The GMEP phase was consistent with a roughly 50-μs delay between PV and PTM. GMEP was little affected by the inner-ear modifications necessary to measure PST. GMEP is a good predictor of ΔPCP at low and moderate frequencies where PV >> PST but overestimates ΔPCP above a few kilohertz where PV ≈ PST. The ratio of PST to PV provides insight into the distribution of sound pressure within the cochlear scalae.

Postnatal maturation of contralateral DPOAE suppression in a precocious animal model (chinchilla) of the human neonate

CONCLUSION

In the neonatal chinchilla, the degree of contralateral distortion product otoacoustic emission (DPOAE) suppression and the latency and time constants of suppression are immature for 40-60 days. This suggests that olivocochlear efferent innervation of outer hair cells is not fully mature at birth in this animal model, and this may also be the case for human neonates.

OBJECTIVES

To track postnatal changes in the dynamics of the olivocochlear efferent system in an animal model with cochlear development at birth similar to that in humans.

METHODS

Real-time measurements of contralateral DPOAE suppression were made in 79 ears of anaesthetized chinchillas, ranging in age from 1 day to 70 days. An adult control group (13 ears) was also tested. DPOAE (2f1-f2; f2 = 4.4 kHz; f2/f1 = 1.22) input/output functions were measured. Dynamics of contralateral broadband noise suppression were measured, including latency and suppression time constants.

RESULTS

DPOAE amplitude input/output functions are immature until 20-30 days postnatally. The maturation period for contralateral suppression amplitude is about 30 days. Latency of onset suppression was 40 ms at birth reducing to adult values (23 ms) at 40 days. The DPOAE suppression time constant was about 350 ms at birth and mature (230 ms) at 60 days.

Sexual hormone fluctuation in chinchillas

The data about chinchilla (Chinchilla laniger) reproduction are limited and in some cases discordant. The aim of this study was to monitor the sexual hormone fluctuation by fecal progesterone level and colpocytology analysis by vaginal smears in order to evaluate the different phases of the oestrus cycle. Twenty-four non pregnant chinchillas aged from 1 to 4 years old and subdivided in three groups were monitored. In contrast with findings reported in other study, the high values of progesterone recorded in autumn suggested the presence of a ciclicity also in this period. The data indicate that chinchilla presents a continuous cycle.

Chinchilla middle-ear admittance and sound power: high-frequency estimates and effects of inner-ear modifications

The middle-ear input admittance relates sound power into the middle ear (ME) and sound pressure at the tympanic membrane (TM). ME input admittance was measured in the chinchilla ear canal as part of a larger study of sound power transmission through the ME into the inner ear. The middle ear was open, and the inner ear was intact or modified with small sensors inserted into the vestibule near the cochlear base. A simple model of the chinchilla ear canal, based on ear canal sound pressure measurements at two points along the canal and an assumption of plane-wave propagation, enables reliable estimates of Y(TM,) the ME input admittance at the TM, from the admittance measured relatively far from the TM. Y(TM) appears valid at frequencies as high as 17 kHz, a much higher frequency than previously reported. The real part of Y(TM) decreases with frequency above 2 kHz. Effects of the inner-ear sensors (necessary for inner ear power computation) were small and generally limited to frequencies below 3 kHz. Computed power reflectance was ~0.1 below 3.5 kHz, lower than with an intact ME below 2.5 kHz, and nearly 1 above 16 kHz.

Processing pitch in a nonhuman mammal (Chinchilla laniger)

Whether the mechanisms giving rise to pitch reflect spectral or temporal processing has long been debated. Generally, sounds having strong harmonic structures in their spectra have strong periodicities in their temporal structures. We found that when a wideband harmonic tone complex is passed through a noise vocoder, the resulting sound can have a harmonic structure with a large peak-to-valley ratio, but with little or no periodicity in the temporal structure. To test the role of harmonic structure in pitch perception for a nonhuman mammal, we measured behavioral responses to noise-vocoded tone complexes in chinchillas (Chinchilla laniger) using a stimulus generalization paradigm. Chinchillas discriminated either a harmonic tone complex or an iterated rippled noise from a 1-channel vocoded version of the tone complex. When tested with vocoded versions generated with 8, 16, 32, 64, and 128 channels, responses were similar to those of the 1-channel version. Behavioral responses could not be accounted for based on harmonic peak-to-valley ratio as the acoustic cue, but could be accounted for based on temporal properties of the autocorrelation functions such as periodicity strength or the height of the first peak. The results suggest that pitch perception does not arise through spectral processing in nonhuman mammals but rather through temporal processing. The conclusion that spectral processing contributes little to pitch in nonhuman mammals may reflect broader cochlear tuning than that described in humans.

Adrenal activity and anxiety-like behavior in fur-chewing chinchillas (Chinchilla lanigera)

Due to its complexity, in combination with a lack of scientific reports, fur-chewing became one of the most challenging behavioral problems common to captive chinchillas. In the last years, the hypothesis that fur-chewing is an abnormal repetitive behavior and that stress plays a role in its development and performance has arisen. Here, we investigated whether a relationship existed between the expression and intensity of fur-chewing behavior, elevated urinary cortisol excretion and anxiety-related behaviors. Specifically, we evaluated the following parameters in behaviorally normal and fur-chewing animals of both sexes: (1) mean concentrations of urinary cortisol metabolites and (2) anxiety-like behavior in an elevated plus-maze test. Urinary cortisol metabolites were higher only in females that expressed the most severe form of the fur-chewing behavior (P≤0.05). Likewise, only fur-chewing females exhibited increased (P≤0.05) anxiety-like behaviors associated with the elevated plus-maze test. Overall, these data provided additional evidence to support the concept that fur-chewing is a manifestation of physiological stress in chinchilla, and that a female sex bias exists in the development of this abnormal behavior.

Characterizing distortion-product otoacoustic emission components across four species

Distortion-product otoacoustic emissions (DPOAEs) were measured as level/phase (L/P) maps in humans, rabbits, chinchillas, and rats with and without an interference tone (IT) placed either near the 2f(1)-f(2) DPOAE frequency place (f(dp)) or at one-third of an octave above the f(2) primary tone (1/3-oct IT). Vector differences between with and without IT conditions were computed to derive a residual composed of the DPOAE components removed by the IT. In humans, a DPOAE component could be extracted with the expected steep phase gradient indicative of reflection emissions by ITs near f(dp). In the laboratory species, ITs near f(dp) failed to produce any conclusive evidence for reflection components. For all species, 1/3-oct ITs extracted large DPOAE components presumably generated at or basal to the IT-frequency place that exhibited both distortion- and reflection-like phase properties. Together, these findings suggested that basal distortion components could assume reflection-like phase behavior when the assumptions of cochlear-scaling symmetry, the basis for shallow phase gradients for constant f(2)/f(1) ratio sweeps, are violated. The present results contradict the common belief that DPOAE components associated with steep or shallow phase slopes are unique signatures for reflection emissions arising from f(dp) or distortion emissions generated near f(2), respectively.

Concurrent development of the head and pinnae and the acoustical cues to sound location in a precocious species, the chinchilla (Chinchilla lanigera)

Sounds are filtered in a spatial- and frequency-dependent manner by the head and pinna giving rise to the acoustical cues to sound source location. These spectral and temporal transformations are dependent on the physical dimensions of the head and pinna. Therefore, the magnitudes of binaural sound location cues-the interaural time (ITD) and level (ILD) differences-are hypothesized to systematically increase while the lower frequency limit of substantial ILD production is expected to decrease due to the increase in head and pinna size during development. The frequency ranges of the monaural spectral notch cues to source elevation are also expected to decrease. This hypothesis was tested here by measuring directional transfer functions (DTFs), the directional components of head-related transfer functions, and the linear dimensions of the head and pinnae for chinchillas from birth through adulthood. Dimensions of the head and pinna increased by factors of 1.8 and 2.42, respectively, reaching adult values by ~6 weeks. From the DTFs, the ITDs, ILDs, and spectral shape cues were computed. Maximum ITDs increased by a factor of 1.75, from ~160 μs at birth (P0-1, first postnatal day) to 280 μs in adults. ILDs depended on source location and frequency exhibiting a shift in the frequency range of substantial ILD (>10 dB) from higher to lower frequencies with increasing head and pinnae size. Similar trends were observed for the spectral notch frequencies which ranged from 14.7-33.4 kHz at P0-1 to 5.3-19.1 kHz in adults. The development of the spectral notch cues, the spatial- and frequency-dependent distributions of DTF amplitude gain, acoustic directionality, maximum gain, and the acoustic axis were systematically related to the dimensions of the head and pinnae. The dimension of the head and pinnae in the chinchilla as well as the acoustical properties associated with them are mature by ~6 weeks.

Middle ear function and cochlear input impedance in chinchilla

Simultaneous measurements of middle ear-conducted sound pressure in the cochlear vestibule P(V) and stapes velocity V(S) have been performed in only a few individuals from a few mammalian species. In this paper, simultaneous measurements of P(V) and V(S) in six chinchillas are reported, enabling computation of the middle ear pressure gain G(ME) (ratio of P(V) to the sound pressure in the ear canal P(TM)), the stapes velocity transfer function SVTF (ratio of the product of V(S) and area of the stapes footplate A(FP) to P(TM)), and, for the first time, the cochlear input impedance Z(C) (ratio of P(V) to the product of V(S) and A(FP)) in individuals. mid R:G(ME)mid R: ranged from 25 to 35 dB over 125 Hz-8 kHz; the average group delay between 200 Hz and 10 kHz was about 52 mus. SVTF was comparable to that of previous studies. Z(C) was resistive from the lowest frequencies up to at least 10 kHz, with a magnitude on the order of 10(11) acoustic ohms. P(V), V(S), and the acoustic power entering the cochlea were good predictors of the shape of the audiogram at frequencies between 125 Hz and 2 kHz.

Middle-ear pressure gain and cochlear partition differential pressure in chinchilla

An important step to describe the effects of inner-ear impedance and pathologies on middle- and inner-ear mechanics is to quantify middle- and inner-ear function in the normal ear. We present middle-ear pressure gain G(MEP) and trans-cochlear-partition differential sound pressure DeltaP(CP) in chinchilla from 100 Hz to 30 kHz derived from measurements of intracochlear sound pressures in scala vestibuli P(SV) and scala tympani P(ST) and ear-canal sound pressure near the tympanic membrane P(TM). These measurements span the chinchilla's auditory range. G(MEP) had constant magnitude of about 20 dB between 300 Hz and 20 kHz and phase that implies a 40-micros delay, values with some similarities to previous measurements in chinchilla and other species. DeltaP(CP) was similar to G(MEP) below about 10 kHz and lower in magnitude at higher frequencies, decreasing to 0 dB at 20 kHz. The high-frequency rolloff correlates with the audiogram and supports the idea that middle-ear transmission limits high-frequency hearing, providing a stronger link between inner-ear macromechanics and hearing. We estimate the cochlear partition impedance Z(CP) from these and previous data. The chinchilla may be a useful animal model for exploring the effects of non-acoustic inner-ear stimulation such as "bone conduction" on cochlear mechanics.

Threshold tuning curves of chinchilla auditory-nerve fibers. I. Dependence on characteristic frequency and relation to the magnitudes of cochlear vibrations

Frequency-threshold tuning curves were recorded in thousands of auditory-nerve fibers (ANFs) in chinchilla. Synthetic tuning curves with 21 characteristic frequencies (187 Hz to 19.04 kHz, spaced every 1/3 octave) were constructed by averaging individual tuning curves within 2/3-octave frequency bands. Tuning curves undergo a gradual transition in symmetry at characteristic frequencies (CFs) of 1 kHz and an abrupt change in shape at CFs of 3-4 kHz. For CFs < or = 3 kHz, the lower limbs of tuning curves have similar slopes, about -18 dB/octave, but the upper limbs have slopes that become increasingly steep with increasing frequency and CF. For CFs >4 kHz, tuning curves normalized to the CF are nearly identical and consist of three segments. A tip segment, within 30-40 dB of CF threshold, has lower- and upper-limb slopes of -60 and +120 dB/octave, respectively, and is flanked by a low-frequency ("tail") segment, with shallow slope, and a terminal high-frequency segment with very steep slope (several hundreds of dB/octave). The tuning curves of fibers innervating basal cochlear sites closely resemble basilar-membrane tuning curves computed with low isovelocity criteria. At the apex of the chinchilla cochlea, frequency tuning is substantially sharper for ANFs than for available recordings of organ of Corti vibrations.

Pitch perception in chinchillas (Chinchilla laniger): stimulus generalization using rippled noise

Rippled noises evoke the perception of pitch in human listeners. Infinitely iterated rippled noise (IIRN) is generated when wideband noise (WBN) is delayed, attenuated, and added to the original WBN through either a positive (+) or a negative (-) feedback loop. The pitch of IIRN[+] is matched to the reciprocal of the delay, whereas the pitch of IIRN[-] for the same delay is an octave lower. Chinchillas (Chinchilla laniger) were trained to discriminate IIRN[+] with a 4-ms delay from IIRN[+] with a 2-ms delay and then tested in a stimulus generalization paradigm with IIRN[+] at delays between 2 and 4 ms. Systematic gradients in behavioral response occurred along the dimension of delay, suggesting that a perceptual dimension corresponding to pitch exists for IIRN[+]. Behavioral responses to IIRN[-] test stimuli were more variable among chinchillas, suggesting that IIRN[-] did not evoke similar pitches relative to IIRN[+]. Systematic gradients in behavioral response were observed when IIRN[-] test stimuli were presented in the context of other IIRN[-] stimuli. Thus, other perceptual cues such as timbre may dominate the pitch cues when IIRN[-] test stimuli are presented in the context of IIRN[+] stimuli.

Time-related changes in functional activity and capacitation of chinchilla (Chinchilla lanigera) spermatozoa during in vitro incubation

The application of assisted breeding programs for chinchilla, an endangered species, requires detailed knowledge about their gamete physiology. Main purposes of the present study were to examine the time-related changes during 8h in vitro incubation in parameters that reflect chinchilla sperm functional activity (including sperm motility, viability, membrane and acrosome integrity), and to determine the incubation time required for achieving in vitro sperm capacitation, evaluated through the quantification of the percentages of sperm that underwent the acrosome reaction in response to progesterone (P, 20 microM) or another acrosome reaction inducer the calcium ionophore, A23187 (20 nM). Semen was obtained by electroejaculation, subjected to swim-up and incubated for 0, 2, 4 and 8h. After these periods, sperm functional activity was assessed. In all treatments percentages of motile, viable and viable sperm with intact acrosomes decreased (p<0.001) after 8h of incubation. The percentages of swollen gametes decreased (p<0.001) after 2h of incubation. Capacitation of chinchilla sperm could be achieved within 4h, as indirectly demonstrated by the increase of acrosome reacted cells in response to P or A23187 (time x treatment interaction: p=0.02).

Gross morphological features of plexus brachialis in the chinchilla (Chinchilla lanigera)

This study documents the detailed features of the morphological structure and the innervation areas of the plexus brachialis in the chinchilla (Chinchilla lanigera). The animals (5 female and 5 male) were euthanased with ketamine hydrocloride and xylazine hydrocloride combination, 60 mg/kg and 6 mg/kg, respectively. Skin, muscles and nerves were dissected under a stereo-microscope. The brachial plexus of the chinchilla is formed by rami ventrales of C5-C8, T1 and T2, and possesses a single truncus. The subscapular nerve is formed by the rami of the spinal nerves originating from C6 (one thin ramus) and C7 (one thick and 2 thin rami). These nerves innervate the subscapular and teres minor muscles. The long thoracic nerve, before joining with the brachial plexus, obtains branches from C6 and C7 in 5 cadavers (3 male, 2 female), from C7 in 4 cadavers (2 male, 2 female) and from C6-C8 in only 1 female cadaver. These nerves disperse in variable combinations to form the extrinsic and intrinstic named, nerves of the thoracic limb. An undefined nerve branch originates from the rami ventrales of C7, C8 and T1 spinal nerves enter the coracobrachial muscle.

Structures that contribute to middle-ear admittance in chinchilla

We describe measurements of middle-ear input admittance in chinchillas (Chinchilla lanigera) before and after various manipulations that define the contributions of different middle-ear components to function. The chinchilla's middle-ear air spaces have a large effect on the low-frequency compliance of the middle ear, and removing the influences of these spaces reveals a highly admittant tympanic membrane and ossicular chain. Measurements of the admittance of the air spaces reveal that the high-degree of segmentation of these spaces has only a small effect on the admittance. Draining the cochlea further increases the middle-ear admittance at low frequencies and removes a low-frequency (less than 300 Hz) level dependence in the admittance. Spontaneous or sound-driven contractions of the middle-ear muscles in deeply anesthetized animals were associated with significant changes in middle-ear admittance.

Pitch cue learning in chinchillas: the role of spectral region in the training stimulus

Chinchillas were trained to discriminate a cosine-phase harmonic tone complex (COS) from wideband noise (WBN) and tested in a stimulus generalization paradigm with tone complexes in which phase differed between frequency regions. In this split-phase condition, responses to complexes made of random-phase low frequencies, cosine-phase high frequencies were similar to responses to the COS-training stimulus. However, responses to complexes made of cosine-phase low frequencies, random-phase high frequencies were generally lower than their responses to the COS-training stimulus. When tested with sine-phase (SIN) and random-phase (RND) tone complexes, responses were large for SIN, but were small for RND. Chinchillas were then trained to discriminate infinitely-iterated rippled noise (IIRN) from WBN and tested with noises in which the spectral ripple differed between frequency regions. In this split-spectrum condition, responses were large to noises made of rippled-spectrum low frequencies, flat-spectrum high frequencies, whereas responses were generally lower to noises made of flat-spectrum low frequencies, rippled-spectrum high frequencies. The results suggest that chinchillas listen across all frequencies, but attend to high frequencies when discriminating COS from WBN and attend to low frequencies when discriminating IIRN from WBN.

Geometry of the semicircular canals of the chinchilla (Chinchilla laniger)

The orientations of the semicircular canals determines the response of the canals to head rotations and, in turn, the brain's ability to interpret those motions. The geometry of chinchillas' semicircular canals has never been reported. Volumetric representations of three chinchilla skulls were generated using a microCT scanner. The centroids of each semicircular canal lumen were identified as they passed through the image slices and were regressed to a plane. Unit vectors normal to the plane representing canal orientations were used to calculate angles between canal pairs. Pitch and roll maneuvers required to bring any canal into the horizontal plane for physiologic investigation were calculated. The semicircular canals of the chinchilla were found to be relatively planar. The horizontal canal was found to be oriented 55.0 degrees anteriorly upward. Pairs of ipsilateral chinchilla canals were not orthogonal and contralateral synergistic pairs were not parallel. Despite this arrangement, the canal plane unit normal vectors were organized to respond with approximately equal overall sensitivity to rotations in any direction. The non-orthogonal chinchilla labyrinth may provide an opportunity to determine whether the frame of reference used by the central vestibular and oculomotor system is based on directions of afferent maximum sensitivity or prime directions.

Tone responses in core versus belt auditory cortex in the developing chinchilla

Single-unit responses to tone pip stimuli were isolated from numerous microelectrode penetrations of core primary auditory cortex (AI) and a dorsocaudal (DC) belt region in the ketamine-anesthetized chinchilla (laniger). Results are reported at postnatal day 3 (P3), P15, P30, and from adult animals. The AI core could be distinguished from the DC belt on the basis of its strict tonotopic organization, evident in all chinchillas studied (including the youngest). Averaged by age group and compared to their core counterparts, belt neurons generally had similar absolute (spike rate) thresholds and onset latencies (at a given sound pressure level), but lower maximum spike rates, broader tuning bandwidths, and more complex (multipeaked) receptive fields. Most notably, the fraction of complex belt units in the near-newborn (P3) group was high (approximately 50%), and did not systematically increase with age, while that of complex core units was approximately 10% at P3 and increased steadily to about 40% in adulthood. These results provide further evidence to support the hypothesis that, at least to some extent, core and belt auditory cortex may constitute parallel processing streams which represent different aspects of complex acoustic stimuli.

Year-round testicular volume and semen quality evaluations in captive Chinchilla lanigera

In mammals, reproductive performance is usually associated with seasons. Chinchilla lanigera, an endemic South American rodent, reproduces throughout the year in captivity but its seasonal breeding pattern is not fully understood. The present study was designed to evaluate (bi-weekly) over 1 year: (1) testicular volume variations and (2) seminal volume, sperm concentration and functional activity changes. Five animals were studied; they were individually housed indoors (22.2 +/- 1.0 degrees C) under natural photoperiod in Argentina (Córdoba, 31 degrees S-64 degrees W). Semen was obtained by electroejaculation; a total of 116 ejaculates was evaluated. Monthly values for paired testicular volume were less in the middle of the summer than in other seasons (p < 0.006), while those for seminal volume and total spermatozoa/ejaculate were not significantly different; these variables ranged between 7.2-30.9 cm(3), 10-130 microL and 0.9-432.6 x 10(6), respectively. Spermatozoa concentration was (x 10(6)/mL) 2145.9 +/- 365.3 and the pH of semen was 7.3 +/- 0.0. Spermatozoa functional activity showed no significant differences between monthly evaluations; confidence intervals were calculated for the means of: motility, 92.2-95.8%; viability, 92.2-96.1%; swollen cells (hypo-osmotic swelling test), 81.2-87.7% and viable intact acrosome, 83.5-89.0%. The present study represents the first longitudinal reproductive assessment in the chinchilla male. In conclusion, males produce spermatozoa continuously that exhibit high quality functional activity.

The effect of superior canal dehiscence on cochlear potential in response to air-conducted stimuli in chinchilla

A superior semicircular canal dehiscence (SCD) is a break or hole in the bony wall of the superior semicircular canal. Patients with SCD syndrome present with a variety of symptoms: some with vestibular symptoms, others with auditory symptoms (including low-frequency conductive hearing loss) and yet others with both. We are interested in whether or not mechanically altering the superior canal by introducing a dehiscence is sufficient to cause the low-frequency conductive hearing loss associated with SCD syndrome. We evaluated the effect of a surgically introduced dehiscence on auditory responses to air-conducted (AC) stimuli in 11 chinchilla ears. Cochlear potential (CP) was recorded at the round-window before and after a dehiscence was introduced. In each ear, a decrease in CP in response to low frequency (<2 kHz) sound stimuli was observed after the introduction of the dehiscence. The dehiscence was then patched with cyanoacrylate glue leading to a reversal of the dehiscence-induced changes in CP. The reversible decrease in auditory sensitivity observed in chinchilla is consistent with the elevated AC thresholds observed in patients with SCD. According to the 'third-window' hypothesis the SCD shunts sound-induced stapes velocity away from the cochlea, resulting in decreased auditory sensitivity to AC sounds. The data collected in this study are consistent with predictions of this hypothesis.

Prevalence of stereotypical responses to mistuned complex tones in the inferior colliculus

The human auditory system has an exceptional ability to separate competing sounds, but the neural mechanisms that underlie this ability are not understood. Responses of inferior colliculus (IC) neurons to "mistuned" complex tones were measured to investigate possible neural mechanisms for spectral segregation. A mistuned tone is a harmonic complex tone in which the frequency of one component has been changed; that component may be heard as a separate sound source, suggesting that the mistuned tone engages the same mechanisms that contribute to the segregation of natural sounds. In this study, the harmonic tone consisted of eight harmonics of 250 Hz; in the mistuned tone, the frequency of the fourth harmonic was increased by 12% (120 Hz). The mistuned tone elicited a stereotypical discharge pattern, consisting of peaks separated by about 8 ms and a response envelope modulated with a period of 100 ms, which bore little resemblance to the discharge pattern elicited by the harmonic tone or to the stimulus waveform. Similar discharge patterns were elicited from many neurons with a range of characteristic frequencies, especially from neurons that exhibited short-latency sustained responses to pure tones. In contrast, transient and long-latency neurons usually did not exhibit the stereotypical discharge pattern. The discharge pattern was generally stable when the stimulus level or component phase was varied; the major effect of these manipulations was to shift the phase of the response envelope. Simulation of IC responses with a computational model suggested that off-frequency inhibition could produce discharge patterns with these characteristics.

Electroejaculation in the Chinchilla (Chinchilla lanigera): effects of anesthesia on seminal characteristics

Repetitive electroejaculation is probably extremely stressful in conscious animals and could adversely affect fertility. The present study was designed to (a) evaluate the effects of anesthesia (40 mg ketamine/kg body weight, i.m.) on a method of electroejaculation used previously in conscious chinchillas (Chinchilla lanigera), and (b) determine the quality of the electroejaculated semen obtained under anesthesia. In Experiment 1 (8 animals), a 4 x 4 Latin square design was used to study the effects of anesthesia and ejaculatory voltage on semen collection, ejaculate volume, sperm concentration, motility, viability, response to the hypoosmotic swelling test (HOST), and acrosomal status. In Experiment 2 (12 animals), the effects of differing voltages and the number of stimuli on ejaculation by conscious or anesthetized males was determined. In both experiments, all the conscious animals ejaculated, but only 60% ejaculated under anesthesia and they required more stimuli and higher voltages to achieve ejaculation. Ejaculate volume was significantly lower in anesthetized (<5 microl) than in conscious animals (>40 microl), but sperm concentration was unaffected. None of the indices of sperm quality were affected by anesthesia. The techniques we have developed in anesthetized domestic chinchillas should be applicable to endangered chinchillas in the wild, and though the number of sperm available is reduced, there are still sufficient for assisted reproduction.

Ultrastructural and immunocytochemical studies of the viscacha (Lagostomus maximus maximus) pituitary pars tuberalis

The hypophyseal pars tuberalis (PT) has been the focus of numerous studies attempting to understand its physiological role in the reproductive regulation and modulation by the neuroendocrine system. Ultrastructural studies of the PT in a number of species have shown that it consists of a well-developed hypophyseal area with important secretory activity, demonstrated by the abundance of secretory granules in the cytoplasm and the marked blood irrigation. This article describes ultrastructural and immunocytochemical aspects of the PT in viscachas captured in their habitat. The cell types identified were PT-specific cells, agranulated cells, and Folliculostellate cells. PT-specific cells are divided into type I and II. Type I cells have cytoplasms with secretory granules of 150-500 nm diameter. The secretory granules of type II PT-specific cells are 65-200 nm in diameter. Both cellular types exhibit numerous nerve endings on the plasmatic membranes. Agranulated cells exhibit nuclei with lax chromatin, mitochondria, phagosomes, scarce Golgi complex, and rough endoplasmic reticulum. Folliculostellate cells exhibit an irregularly shaped and moderately condensed nucleus. All the described cellular types exhibit deposits of cytoplasmic glycogen. The immunocytochemical study revealed the presence of cells immunostained for LH-beta and FSH-beta in the PT caudal zone. ACTH was only detected in the zona tuberalis. No staining was observed with antiprolactin, anti-TSH-beta, and anti-GH sera. Folliculostellate cells exhibited staining with anti-S-100. The results demonstrate that the viscacha PT is a hypophyseal zone with specific cellular types, which exhibits evident secretory activity. The presence of nerve endings suggests neural control of the function of PT cells.

Response properties of single auditory nerve fibers in the mouse

The availability of transgenic and mutant lines makes the mouse a valuable model for study of the inner ear, and a powerful window into cochlear function can be obtained by recordings from single auditory nerve (AN) fibers. This study provides the first systematic description of spontaneous and sound-evoked discharge properties of AN fibers in mouse, specifically in CBA/CaJ and C57BL/6 strains, both commonly used in auditory research. Response properties of 196 AN fibers from CBA/CaJ and 58 from C57BL/6 were analyzed, including spontaneous rates (SR), tuning curves, rate versus level functions, dynamic range, response adaptation, phase-locking, and the relation between SR and these response properties. The only significant interstrain difference was the elevation of high-frequency thresholds in C57BL/6. In general, mouse AN fibers showed similar responses to other mammals: sharpness of tuning increased with characteristic frequency, which ranged from 2.5 to 70 kHz; SRs ranged from 0 to 120 sp/s, and fibers with low SR (<1 sp/s) had higher thresholds, and wider dynamic ranges than fibers with high SR. Dynamic ranges for mouse high-SR fibers were smaller (<20 dB) than those seen in other mammals. Phase-locking was seen for tone frequencies <4 kHz. Maximum synchronization indices were lower than those in cat but similar to those found in guinea pig.

Anaesthesia with Midazolam/Medetomidine/Fentanyl in Chinchillas (Chinchilla lanigera) Compared to Anaesthesia with Xylazine/Ketamine and Medetomidine/Ketamine

We studied four different drug regimes for anaesthetic management in chinchillas and evaluated and compared their cardiovascular and respiratory effects. In this randomized, cross-over experimental study, seven adult chinchillas, five females, two males [515 +/- 70 (SD) g] were randomly assigned to one of the following groups: group 1 [midazolam, medetomidine and fentanyl (MMF), flumazenil, atipamezole and naloxone (FAN); MMF-FAN] received 1.0 mg/kg midazolam, 0.05 mg/kg medetomidine and 0.02 mg/kg fentanyl i.m., and for reversal 0.1 mg/kg flumazenil, 0.5 mg/kg atipamezole and 0.05 mg/kg naloxone s.c. after 45 min; group 2 (MMF) 1.0 mg/kg midazolam, 0.05 mg/kg medetomidine and 0.02 mg/kg fentanyl i.m.; group 3 [xylazine/ketamine (X/K)] 2.0 mg/kg xylazine and 40.0 mg/kg ketamine i.m.; and group 4 [medetomidine/ketamine (M/K)] 0.06 mg/kg medetomidine and 5.0 mg/kg ketamine i.m. Reflexes were judged to determine anaesthetic stages and planes. Anaesthesia with X/K and M/K was associated with a prolonged surgical tolerance and recovery period. By reversing MMF, recovery period was significantly shortened (5 +/- 1.3 min versus 40 +/- 10.3 min in MMF without FAN, 73 +/- 15.0 min in X/K, and 31 +/- 8.5 min in M/K). Without reversal, MMF produced anaesthesia lasting 109 +/- 16.3 min. All combinations decreased respiratory and heart rate but compared with X/K and M/K, respiratory and cardiovascular complications were less in the MMF groups. Focussing on the clinical relevance of the tested combinations, completely reversible anaesthesia showed two major advantages: anaesthesia can be antagonized in case of emergency and routinely shortens recovery. In small animals particularly these advantages lead to less complications and discomfort and thus often can be lifesaving. As all analgesic components (medetomidine and fentanyl) are reversed, postoperative analgesia should be provided before reversal of anaesthesia.

Bilateral Enucleation and Captivity Influence the Reproductive Cycle of Male Viscacha (Lagostomus maximus maximus)

The viscacha (Lagostomus maximus maximus) is a seasonal rodent living in the Southern Hemisphere. The adult males exhibit an annual reproductive cycle characterized by a gonadal regression period during winter. In this study, we investigated the effects of bilateral enucleation and captivity on their annual reproductive cycle. Testicular volume relative to body weight was recorded monthly in intact and bilaterally enucleated animals placed under natural photoperiod, water, and food ad lib. and constant temperature. Testes and accessory organs were evaluated by qualitative and quantitative light microscopic studies. The intact animals showed an annual reproductive cycle with complete gonadal atrophy in the first year. In the second year, testicular regression was observed but attenuated in regard to that recorded in the first winter period, indicating that adaptive changes might be involved. Bilateral enucleation in the viscacha dampened and extended the period of its annual reproductive cycle. The results suggest that both conditions, constant captivity and enucleation, produced stimulatory effects on the reproductive system of this rodent. Furthermore, local control mechanisms might be responsible for the morphological differences observed in testes, epididymis, and seminal vesicles from both groups, which exhibited similar levels of serum testosterone. Finally, an intact retinohypothalamic-pineal axis and/or photoperiodic input would be necessary to maintain the reproductive cycle amplitude and timing in viscacha.

The nutrition of the chinchilla as a companion animal--basic data, influences and dependences

This contribution is meant to obtain basic data for feeding chinchillas (ingestion behaviour, feed and water intake) kept as companion animals. The chinchillas ingested more than 70% of their total feed intake during the dark phase (highest level of activity between 9:00 pm and 7:00 am). Daily amounts of feed intake varied between 2.5 (fresh grass) or 2.6 (hay) and 5.5 (pelleted complete diet) g of dry matter per 100 g of body weight. An offered mixed feed based on native components led to a selection of individual ingredients (high palatability: carob, beet pulp, sunflower seeds). The chinchillas' daily water intake varied between 30 (mixed feed in briquette form) and 40 ml (alfalfa cubes) and amounted on average between 1.5 and 3 ml/g of dry matter. Compared with rabbits or guinea-pigs, the chinchillas generally showed noticeable differences (rhythm of feed intake, palatability of individual ingredients, capacity for digestion, etc.) which must be considered in order to optimize the nutrition of this species.

Fiber diameter distributions in the chinchilla's ampullary nerves

A morphometric study of the chinchilla's ampullary nerves was conducted to produce an unbiased accounting of the diameter distribution of their constituent fibers. Diameter analyses were determined from 1 microm plastic-embedded nerve sections taken at a plane immediately proximal to the sensory epithelium. We found these nerves to be composed of 2094+/-573 fibers, having diameters that ranged from 0.5 to 8 microm. The distributions of diameters were positively skewed, where approximately 75% of the fibers were found to have diameters less than 3.5 microm. An analysis of the spatial distribution of diameters within the nerve section revealed that the lateralmost areas of the nerve contained larger fractions of fibers within the smallest diameter quintiles, and the central area harbored greater proportions of the larger diameter quintiles. However, significant fractions of all quintiles were found in all areas. These data were integrated with available data of Fernandez et al. (1998) to produce diameter estimates of calyx, dimorphic, and bouton morphology subpopulations. In view of a general relationship between diameter, innervation locus, and an afferent's physiologic characteristics, these data provide the basis for developing a perspective for the in situ distribution of afferent response dynamics.

Hemoglobin affinity in Andean rodents

Blood hemoglobin oxygen affinity (P50) was measured in three Andean species and in the laboratory rat (control), all raised near sea level. Chinchilla lanigera (Molina, 1792) has an altitudinal habitat range from low Andean slopes up to 3000 m., while Chinchilla brevicaudata (Waterhouse, 1848) has an altitudinal range from 3000 to 5000 m. The laboratory type guinea pig, wild type guinea pig (Cavia porcellus), (Waterhouse, 1748), and laboratory rat (Rattus norvegicus) were also raised at sea level. The Andean species had high hemoglobin oxygen affinities (low P50) compared with the rat. Chinchilla brevicaudata had a higher affinity than Chinchilla lanigera. The wild type guinea pig had a higher affinity than the laboratory type. As has been shown in other species, this is another example of an inverse correlation between the altitude level and the P50 values. This is the first hemoglobin oxygen affinity study in Chinchilla brevicaudata.

Evidence for interactions across frequency channels in the inferior colliculus of awake chinchilla

As a result of cochlear processing, information about acoustic broadband signals is distributed across many parallel frequency channels. Periodic modulations of signal envelopes - conspicuous in particular in harmonic signals - may extend across a wide frequency range and give rise to temporal response patterns in the auditory nerve, particularly useful for recombination of constituents and the separation of the signals from background noise. Herein we report evidence that across frequency processing as necessary for binding of related signal components occurs already in the auditory midbrain of mammals. Extracellular recordings were made from 231 multi and single units in the inferior colliculus of awake chinchillas. Loud pure tones evoked onset type excitation (26%) and suppression of spontaneous rate (60%) not only in the range of the units' characteristic frequency (CF), but also in a frequency range far above CF. About 80% of all units tuned to CFs below 3 kHz gave sustained responses to low level stimuli of high frequencies (>2CF) provided the tones were sinusoidally amplitude modulated (SAM) with a unit specific modulation frequency although none of the spectral components of the amplitude modulation alone was sufficient to evoke such a response, even at high intensities. Low level high carrier SAM responses and wide band onset responses as well as inhibition must have their origin in a non-linear across frequency channel interaction of neuronal information. Many aspects of these responses cannot be explained by peripheral distortion in the cochlea. We therefore propose a mechanism of integration across frequency channels that may originate within the inferior colliculus and/or the nuclei of the lateral lemniscus. This process may lead to the binding of information that shares a common periodicity and may thereby help to distinguish different acoustic objects.

Temporal and spatial coding of periodicity information in the inferior colliculus of awake chinchilla (Chinchilla laniger)

Amplitude modulation responses and onset latencies of multi-unit recordings and evoked potentials were investigated in the central nucleus of inferior colliculus (ICC) in the awake chinchilla. Nine hundred and one recording sites with best frequencies between 60 and 30 kHz showed either phasic (18%), tonic (25%), or phasic-tonic (57%) responses. Of 554 sites tested for responses to modulation frequencies 73% were responsive and 57% showed clear preference for a narrow range of modulation frequencies. Well defined bandpass characteristics were found for 32% of rate modulation transfer functions (rate-MTFs) and 36% of synchronization MTFs (sync-MTFs). The highest best modulation frequency (BMF) of a bandpass rate-MTF was 600 Hz. Neurons with phasic responses to best-frequency tones showed strong phase coupling to modulation frequencies and were dominated by bandpass rate-MTFs and sync-MTFs. Most neurons with tonic responses showed bandpass tuning only for sync-MTFs. Both BMFs and onset latencies changed systematically across frequency-band laminae of the ICC. Low BMFs and long latencies were located medially and high BMFs and short latencies laterally. Latency distributions obtained with evoked potentials to clicks showed a similar gradient to the multi-unit data. These findings are in line with previous findings in different animals including humans and support the hypothesis that temporal processing results in a topographic arrangement orthogonal to the spectral processing axis, thus forming a second neural axis of the auditory system.

Responses of inferior colliculus neurons to harmonic and mistuned complex tones

Responses of inferior colliculus neurons to simplified stimuli that may engage mechanisms that contribute to auditory scene analysis were obtained. The stimuli were harmonic complex tones, which are heard by human listeners as single sounds, and the same tones with one component 'mistuned', which are heard as two separate sounds. The temporal discharge pattern elicited by a harmonic complex tone usually resembled the same neuron's response to a pure tone. In contrast, tones with a mistuned component elicited responses with distinctive, stereotypical temporal patterns that were not obviously related to the stimulus waveform. For a particular stimulus configuration, the discharge pattern was similar across neurons with different pure-tone frequency selectivity. A computational model that compared response envelopes across multiple narrow bands successfully reproduced the stereotypical response patterns elicited by different stimulus configurations. The results suggest that mistuning created a temporally synchronous distributed representation of the mistuned component that could be identified by higher auditory centers in the presence of the ongoing response produced by the remaining components; this kind of representation might facilitate the identification of individual sound sources in complex acoustic environments.

GABAergic inputs shape responses to amplitude modulated stimuli in the inferior colliculus

The inferior colliculus (IC) is an important auditory processing center receiving inputs from lower brainstem nuclei, higher auditory and nonauditory structures, and contralateral IC. The IC, along with other auditory structures, is involved in coding information about the envelope of complex signals. Biologically relevant acoustic signals, including animal vocalizations and speech, are spectrally and temporally complex and display amplitude and frequency variations over time. Certain IC neurons respond selectively over a narrow range of modulation frequencies to sinusoidally amplitude modulated (SAM) stimuli. Responses to SAM stimuli can be measured in terms of discharge rate, with rate plotted against the modulation frequency to generate rate modulation transfer functions (rMTF). A role for the inhibitory neurotransmitter, gamma-aminobutyric acid (GABA), in shaping selective responses to SAM stimuli has been suggested. The present study examined the role of GABA in shaping responses to SAM stimuli in the IC of anesthetized chinchilla. Responses from 94 IC neurons were obtained before, during and after iontophoretic application of the GABA(A) receptor antagonist bicuculline methiodide. Complete responses to SAM stimuli were obtained from 55 extensively tested neurons, displaying band-pass (38) and low-pass rMTFs (17). For neurons showing band-pass rMTFs, GABA(A) receptor blockade selectively increased discharge rate at low modulation frequencies for 14 units, increased discharge near the best modulation frequency for 12 units. For neurons showing low-pass rMTFs, GABA(A) receptor blockade selectively increased discharge rate at low modulation frequencies for nine units. GABA(A) receptor blockade consistently reduced peak modulation gain, producing low-pass gain functions in a subset of IC neurons. In support of previous findings suggesting that selective temporal responses to SAM stimuli are coded in lower brainstem nuclei, temporal responses to SAM stimuli were relatively unaffected by GABA(A) receptor blockade. These findings support a role for GABA in shaping selective rate responses to SAM stimuli for a subset of chinchilla IC neurons.

Susceptibility to the ototoxic properties of toluene is species specific

Toluene is the most widely used industrial solvent. It has been shown to be ototoxic in mice and rats, and to increase permanent threshold shift in conjunction with exposure to noise. Chinchillas are widely used for studying noise effects on the cochlea. The present study was initiated to study toluene and noise interaction in chinchillas. Thirty-three chinchillas were exposed to a 95 dBA 500 Hz octave band noise plus 2000 ppm toluene, 8 or 12 h per day for 10 days. Auditory function was estimated using the auditory brainstem response (ABR) to tones between 500 Hz and 16 kHz. There was no effect on the ABR of toluene alone. Noise alone produced a threshold shift. There was no interaction of noise and toluene on the ear. The present study suggests that chinchillas are markedly less susceptible to the ototoxic effect of toluene than mice and rats. A working hypothesis as to the species differences was that chinchilla liver was able to detoxify the toluene. Hepatic microsomes from chinchillas, rats and humans were tested for their ability to convert toluene to the more water-soluble compound - benzyl alcohol. Chinchilla livers were found to contain more of the P450 enzymes CYP2E1 and CYP2B than rats or humans. In addition, the data show that the P450 enzymes are more active in chinchillas than in rats and humans. In conclusion, the results suggest that rats and mice are a more appropriate model for human toluene ototoxicity. However, chinchillas may provide a valuable model for investigating how ototoxic agents can be detoxified to less damaging compounds.

Diagnosis of sexual cycle by means of vaginal smear method in the chinchilla (Chinchilla lanigera)

An investigation was made as to whether the sexual cycle and pregnancy can be determined by means of vaginal smear in chinchillas. This study represents the first attempt to record changes which occur in the pattern of exfoliated cells in chinchilla's vaginal smear during anoestrus, proestrus, oestrus, metoestrus and pregnancy. Fifteen female chinchillas aged from 8 months to 3 years and bred through harem breeding method were used. The major change during proestrus was an increase in the proportion of superficial cells, with a corresponding decrease in other cells. Goblet cells were observed in the smears prepared by strong aspiration during this cycle. Neutrophils, small and large intermediates and parabasal cells were not found in the smear during oestrus and the smear consisted of superficial cells only. In the proportion of neutrophils, small and large intermediates and parabasal cells increased during metoestrus. In addition, metoestrum and foam cells were found in this cycle. In anoestrus; superficial and parabasal cells were present in small numbers. Also small and large intermediate cells as well as neutrophils were present. Traces of foam and metoestrum cells were found. During pregnancy, neutrophils generally of medium density were present, parabasal; small and large intermediate cells were present at low or medium density, and superficial cells were only present in trace amounts.

Wideband reflectance tympanometry in chinchillas and human

Wideband reflectance tympanometry was performed on twelve chinchillas ears. The complex input impedance of the middle ear, multifrequency admittance tympanograms, reflectance patterns (reflectance versus frequency), and reflectance tympanograms (reflectance versus ear-canal air pressure) were analyzed and compared to human data. The complex impedance of the chinchilla ear has a lower stiffness reactance at low frequencies, a higher mass reactance at high frequencies, and a lower resistance compared to the human. Multifrequency admittance tympanograms from chinchillas follow the same sequence of patterns as humans for low frequencies (<2 kHz). At higher frequencies tympanograms from both species are poorly organized and do not follow a consistent sequence of patterns. Reflectance patterns of chinchillas and humans are different. However, both species show high reflectance at low frequencies, regions of lower reflectance in mid-frequencies (2-6 kHz), and high reflectance at high frequencies (>8 kHz). Reflectance tympanograms for the two species show a single, centrally located minimum at low frequencies (<2 kHz) and are substantially different at higher frequencies. Results are shown for two animals that underwent eustachian tube obstruction. Reflectance patterns obtained with different ear-canal air pressures are substantially different. Reflectance results at any single ear-canal pressure (including ambient pressure) do not completely characterize the effects of middle-ear pathology.

Storage of Chinchilla lanigera semen at 4 degrees C for 24 or 72 h with two different cryoprotectants

The objectives of this study were to: (a) test the functional activity of Chinchilla lanigera spermatozoa suspended in either glycerol or ethylene glycol, cooled to 4 degrees C, and stored for 24 or 72 h and (b) investigate, after these cooling periods, the effects of incubating sperm at 37 degrees C (for 4 h) upon sperm functional activity. The ejaculate was mixed with the cryoprotectant medium (at 1 M final concentration) and cooled to 4 degrees C. After warming, sperm motility, sperm viability, hypoosmotic swelling test results, and acrosomal integrity were significantly higher for samples containing ethylene glycol than for those in glycerol, stored for 24 or 72 h, and then assayed after 0 or 4 h incubation at 37 degrees C. A significant reduction of sperm motility and viability was detected only when the glycerol cryoprotectant agent was employed, compared to the fresh samples. These results clearly indicate that under our experimental conditions, ethylene glycol is a better protectant for sperm storage than glycerol.

Mechanical bases of frequency tuning and neural excitation at the base of the cochlea: comparison of basilar-membrane vibrations and auditory-nerve-fiber responses in chinchilla

We review the mechanical origin of auditory-nerve excitation, focusing on comparisons of the magnitudes and phases of basilar-membrane (BM) vibrations and auditory-nerve fiber responses to tones at a basal site of the chinchilla cochlea with characteristic frequency approximately 9 kHz located 3.5 mm from the oval window. At this location, characteristic frequency thresholds of fibers with high spontaneous activity correspond to magnitudes of BM displacement or velocity in the order of 1 nm or 50 microm/s. Over a wide range of stimulus frequencies, neural thresholds are not determined solely by BM displacement but rather by a function of both displacement and velocity. Near-threshold, auditory-nerve responses to low-frequency tones are synchronous with peak BM velocity toward scala tympani but at 80-90 dB sound pressure level (in decibels relative to 20 microPascals) and at 100-110 dB sound pressure level responses undergo two large phase shifts approaching 180 degrees. These drastic phase changes have no counterparts in BM vibrations. Thus, although at threshold levels the encoding of BM vibrations into spike trains appears to involve only relatively minor signal transformations, the polarity of auditory-nerve responses does not conform with traditional views of how BM vibrations are transmitted to the inner hair cells. The response polarity at threshold levels, as well as the intensity-dependent phase changes, apparently reflect micromechanical interactions between the organ of Corti, the tectorial membrane and the subtectorial fluid, and/or electrical and synaptic processes at the inner hair cells.