Transcriptional response of human microglial cells to interferon-gamma.

Microglia, the resident macrophages in the central nervous system (CNS), play a pivotal role in innate and adaptive immune responses in the brain. The immune functions of microglia are regulated by cytokines, including interferon (IFN)-gamma, which is a major mediator of macrophage activation. We describe the transcriptional profile of human fetal microglial cells at 1, 6, and 24 h after IFN-gamma treatment. The results show a change in the expression of 405 genes including transcriptionally induced chemokines, IFN-gamma signaling factors, and major histocompatibility complex genes. Our results demonstrate that activation of microglia by IFN-gamma induces proinflammatory T-lymphocyte-related chemokine genes as well as genes involved in antigen presentation. As a result, signals for T-cell infiltration and antigen presentation are produced to allow for microglia-T-cell interactions that likely contribute to defense against invading pathogens. In sum, our results provide a foundation for the molecular mechanisms of the microglial response to IFN-gamma-a key to understanding cell-mediated immunity of the CNS.

Pasteurella multocida gene expression in response to iron limitation.

Pasteurella multocida is the causative agent of a wide range of diseases in avian and mammalian hosts. Gene expression in response to low iron conditions was analyzed in P. multocida using whole-genome microarrays. The analysis shows that the expression of genes involved in energy metabolism and electron transport generally decreased 2.1- to 6-fold while that of genes used for iron binding and transport increased 2.1- to 7.7-fold in P. multocida during the first 2 h of growth under iron-limiting conditions compared with controls. Notably, 27% of the genes with significantly altered expression had no known function, illustrating the limitations of using publicly available databases to identify genes involved in microbial metabolism and pathogenesis. Taken together, the results of our investigations demonstrate the utility of whole-genome microarray analyses for the identification of genes with altered expression profiles during varying growth conditions and provide a framework for the detailed analysis of the molecular mechanisms of iron acquisition and metabolism in P. multocida and other gram-negative bacteria.

Relationship between the auditory brainstem response and auditory nerve thresholds in cats with hearing loss.

This study explored the relationship between the auditory brainstem response (ABR) and auditory nerve sensitivity in cats with normal hearing and with noise-induced permanent threshold shifts. A statistically significant linear correlation was found between each cat's ABR thresholds and the most sensitive single neuron thresholds at the same frequency. ABR thresholds were approximately 25 dB higher than the thresholds of the most sensitive neural responses in cats with normal hearing. The two measures produced equivalent thresholds at impaired frequencies in subjects with sensorineural hearing loss. Two factors may have contributed to this convergence of ABR and neural thresholds. First, our results suggest that the elevation of the most sensitive neural responses led to a compressed threshold distribution. Consequently, only a narrow range of sound levels separated stimulus conditions that activated relatively few fibers from those that were sufficient to evoke a robust population response. In addition, the threshold responses of impaired auditory nerve fibers may have been augmented by activity in the more sensitive 'off-frequency' regions that surrounded a discrete cochlear lesion. Across varying degrees of hearing loss, the ABR maintained a systematic relationship to auditory nerve fiber thresholds, and therefore has the potential to be used as a functional assay of cochlear pathology.

Complete genomic sequence of Pasteurella multocida, Pm70.

We present here the complete genome sequence of a common avian clone of Pasteurella multocida, Pm70. The genome of Pm70 is a single circular chromosome 2,257,487 base pairs in length and contains 2,014 predicted coding regions, 6 ribosomal RNA operons, and 57 tRNAs. Genome-scale evolutionary analyses based on pairwise comparisons of 1,197 orthologous sequences between P. multocida, Haemophilus influenzae, and Escherichia coli suggest that P. multocida and H. influenzae diverged approximately 270 million years ago and the gamma subdivision of the proteobacteria radiated about 680 million years ago. Two previously undescribed open reading frames, accounting for approximately 1% of the genome, encode large proteins with homology to the virulence-associated filamentous hemagglutinin of Bordetella pertussis. Consistent with the critical role of iron in the survival of many microbial pathogens, in silico and whole-genome microarray analyses identified more than 50 Pm70 genes with a potential role in iron acquisition and metabolism. Overall, the complete genomic sequence and preliminary functional analyses provide a foundation for future research into the mechanisms of pathogenesis and host specificity of this important multispecies pathogen.

Role of the dorsal cochlear nucleus in the sound localization behavior of cats.

The role of the dorsal cochlear nucleus (DCN) in directional hearing was evaluated by measuring sound localization behaviors before and after cats received lesions of the dorsal and intermediate acoustic striae (DAS/IAS). These lesions are presumed to disrupt spectral processing in the DCN without affecting binaural time and level difference cues that exit the cochlear nucleus via the ventral acoustic stria. Prior to DAS/IAS lesions, cats made accurate head orientation responses toward sound sources in the frontal sound field. After a unilateral DAS/IAS lesion, subjects showed increased errors in the azimuth and elevation of their responses; in addition, the final orientation of head movements tended to be more variable. Largest deficits in response elevation were observed in the hemifield that was ipsilateral to the lesion. When a second lesion was placed in the opposite DAS/IAS, increased orientation errors were observed throughout the frontal field. Nonetheless, bilaterally lesioned cats showed normal discrimination of changes in sound source location when tested with a spatial acuity task. These findings support previous interpretations that the DCN contributes to sound orientation behavior, and further suggest that the identification of absolute sound source locations and the discrimination between spatial locations involve independent auditory processing mechanisms.

Behavioral assessments of auditory sensitivity in transgenic mice.

This report summarizes positive reinforcement conditioning procedures for assessing sensory function in transgenic mice. To illustrate these behavioral methods auditory sensitivity was measured in mice lacking alpha9 acetylcholine receptor subunits (alpha9 knock-out mice). These receptors are known to play an important role in the efferent pathways that modify cochlear responses to sound stimuli. The strategies of parameter manipulation that led these subjects through their preliminary training stages to stable threshold performances are described in detail. Techniques for estimating and interpreting sensory thresholds are discussed from the perspective of signal detection analyses. This study found no significant differences between alpha9 knock-out mice and control subjects when hearing thresholds were measured under quiet conditions, as predicted by previous behavioral and electrophysiological evidence.

Rate representation of tones in noise in the inferior colliculus of decerebrate cats.

Neurons in the central nucleus of the inferior colliculus (ICC) of decerebrate cats show three major response patterns when tones of different frequencies and sound-pressure levels (SPLs) are presented to the contralateral ear. The frequency response maps of type I units are uniquely defined by a narrow excitatory area at best frequency (BF: a unit's most sensitive frequency) and surrounding inhibition at higher and lower frequencies. As a result of this receptive field organization, type I units exhibit strong excitatory responses to BF tones but respond only weakly to broadband noise (BBN). These response characteristics predict that type I units are well suited to encode narrowband signals in the presence of background noise. To test this hypothesis, the dynamic range properties of ICC unit types were measured under quiet conditions and in multiple levels of continuous noise. As observed in previous studies of the auditory nerve and cochlear nucleus, type I units showed upward threshold shifts and discharge rate compression in background noise that partially degraded the dynamic range properties of neural representations at high noise levels. Although the other two unit types in the ICC showed similar trends in threshold shift and noise compression, their ability to encode auditory signals was compromised more severely in increasing noise levels. When binaural masking effects were simulated, only type I units showed an enhanced representation of spatially separated signals and maskers that was consistent with human perceptual performance in independent psychoacoustic observations. These results support the interpretation that type I units play an important role in the auditory processing of narrowband signals in background noise and suggest a physiological basis for spatial factors that govern signal detection under free-field listening conditions.

Single-unit responses in the inferior colliculus of decerebrate cats. I. Classification based on frequency response maps.

This study proposes a classification system for neurons in the central nucleus of the inferior colliculus (ICC) that is based on excitation and inhibition patterns of single-unit responses in decerebrate cats. The decerebrate preparation allowed extensive characterization of physiological response types without the confounding effects of anesthesia. The tone-driven discharge rates of individual units were measured across a range of frequencies and levels to map excitatory and inhibitory response areas for contralateral monaural stimulation. The resulting frequency response maps can be grouped into the following three populations: type V maps exhibit a wide V-shaped excitatory area and no inhibition; type I maps show a more restricted I-shaped region of excitation that is flanked by inhibition at lower and higher frequencies; and type O maps display an O-shaped island of excitation at low stimulus levels that is bounded by inhibition at higher levels. Units that produce a type V map typically have a low best frequency (BF: the most sensitive frequency), a low rate of spontaneous activity, and monotonic rate-level functions for both BF tones and broadband noise. Type I and type O units have BFs that span the cat's range of audible frequencies and high rates of spontaneous activity. Like type V units, type I units are excited by BF tones and noise at all levels, but their rate-level functions may become nonmonotonic at high levels. Type O units are inhibited by BF tones and noise at high levels. The existence of distinct response types is consistent with a conceptual model in which the unit types receive dominant inputs from different sources and shows that these functionally segregated pathways are specialized to play complementary roles in the processing of auditory information.

Single-unit responses in the inferior colliculus of decerebrate cats. II. Sensitivity to interaural level differences.

Single units in the central nucleus of the inferior colliculus (ICC) of unanesthetized decerebrate cats can be grouped into three distinct types (V, I, and O) according to the patterns of excitation and inhibition revealed in contralateral frequency response maps. This study extends the description of these response types by assessing their ipsilateral and binaural response map properties. Here the nature of ipsilateral inputs is evaluated directly using frequency response maps and compared with results obtained from methods that rely on sensitivity to interaural level differences (ILDs). In general, there is a one-to-one correspondence between observed ipsilateral input characteristics and those inferred from ILD manipulations. Type V units receive ipsilateral excitation and show binaural facilitation (EE properties); type I and type O units receive ipsilateral inhibition and show binaural excitatory/inhibitory (EI) interactions. Analyses of binaural frequency response maps show that these ILD effects extend over the entire receptive field of ICC units. Thus the range of frequencies that elicits excitation from type V units is expanded with increasing levels of ipsilateral stimulation, whereas the excitatory bandwidth of type I and O units decreases under the same binaural conditions. For the majority of ICC units, application of bicuculline, an antagonist for GABAA-mediated inhibition, does not alter the basic effects of binaural stimulation; rather, it primarily increases spontaneous and maximum discharge rates. These results support our previous interpretations of the putative dominant inputs to ICC response types and have important implications for midbrain processing of competing free-field sounds that reach the listener with different directional signatures.

Vowel representations in the ventral cochlear nucleus of the cat: effects of level, background noise, and behavioral state.

Single-unit responses were studied in the ventral cochlear nucleus (VCN) of cats as formant and trough features of the vowel /epsilon/ were shifted in the frequency domain to each unit's best frequency (BF; the frequency of greatest sensitivity). Discharge rates sampled with this spectrum manipulation procedure (SMP) were used to estimate vowel representations provided by populations of VCN neurons. In traditional population measures, a good representation of a vowel's formant structure is based on relatively high discharge rates among units with BFs near high-energy formant features and low rates for units with BFs near low-energy spectral troughs. At most vowel levels and in the presence of background noise, chopper units exhibited formant-to-trough rate differences that were larger than VCN primary-like units and auditory-nerve fibers. By contrast, vowel encoding by primary-like units resembled auditory nerve representations for most stimulus conditions. As is seen in the auditory nerve, primary-like units with low spontaneous rates (SR <18 spikes/s) produced better representations than high SR primary-like units at all but the lowest vowel levels. Awake cats exhibited the same general response properties as anesthetized cats but larger between-subject differences in vowel driven rates. The vowel encoding properties of VCN chopper units support previous interpretations that patterns of auditory nerve convergence on cochlear nucleus neurons compensate for limitations in the dynamic range of peripheral neurons.

Effects of bilateral olivocochlear lesions on vowel formant discrimination in cats.

Operant conditioning procedures were used to measure the effects of bilateral olivocochlear lesions on the cat's discrimination thresholds for changes in the second formant frequency (deltaF2) of the vowel /epsilon/. Three cats were tested with the formant discrimination task under quiet conditions and in the presence of continuous broadband noise at signal-to-noise ratios (S/Ns) of 23, 13, and 3 dB. In quiet, vowel levels of 50 and 70 dB produced average deltaF2s of 42 and 47 Hz, respectively, and these thresholds did not change significantly in low levels of background noise (S/Ns = 23 and 13 dB). Average deltaF2s increased to 94 and 97 Hz for vowel levels of 50 and 70 dB in the loudest level of background noise (S/N = 3 dB). Average deltaF2 thresholds in quiet and in lower noise levels were only slightly affected when the olivocochlear bundle was lesioned by making bilateral cuts into the floor of the IVth ventricle. In contrast, post-lesion deltaF2 thresholds in the highest noise level were significantly larger than pre-lesion values; the most severely affected subject showed post-lesion discrimination thresholds well over 200 Hz for both 50 and 70 dB vowels. These results suggest that olivocochlear feedback may enhance speech processing in high levels of ambient noise.

Spectral cues for sound localization in cats: a model for discharge rate representations in the auditory nerve.

Neural representations of pinna-based spectral cues for sound localization were modeled by simulating auditory nerve discharge rates to noise bursts that had been shaped by filtering properties of the cat's head-related transfer functions (HRTFs) at 179 locations in the frontal field. The auditory nerve model transformed spectral differences between HRTFs into simulated neural rate differences. Linear equations for this transformation were developed from actual auditory nerve responses to a limited subset of HRTF-filtered noise bursts [Rice et al., J. Acoust. Soc. Am. 97, 1764-1776 (1995)]. Signal detection methods were used to investigate simulated neural responses to pairwise changes between HRTFs. The quality of neural representation for these changes, in terms of d' values, declined when the reference HRTF was moved from a central location (0 degree AZ, 0 degree EL) to a large positive azimuth in the horizontal plane (75 degrees AZ, 0 degree EL) or a high elevation in the median plane (0 degree AZ, 75 degrees EL). Most simulated responses exhibited large d' values for comparisons of contralateral versus ipsilateral azimuths, or eccentric versus frontal elevations. This rate information resulted from directionally dependent changes in the overall gain of HRTFs. In addition, fibers with best frequency (BF: the frequency of greatest sensitivity for individual fibers) between 5 and 18 kHz showed large d' values for HRTF contrasts in the immediate frontal field because of the effects of spectral notches (i.e., sharp drops in gain over a narrow frequency range). Spectral notches also played a prominent role in simulations that required identification of HRTF location in the absence of a fixed reference stimulus. These modeling results correspond well with previously described patterns in the cat's localization behaviors.

Spectral cues for sound localization in cats: effects of frequency domain on minimum audible angles in the median and horizontal planes.

Rice et al. [Hear. Res. 58, 132-152 (1992)] classified directional properties of the cat's head-related transfer function (HRTF) into three frequency domains. Low frequencies (< 5 kHz) display a broad azimuth-sensitive spectral peak that establishes interaural level differences, mid frequencies (5-18 kHz) are marked with a single deep spectral notch that changes in frequency as a function of both azimuth and elevation, and high frequencies (18-50 kHz) exhibit a complex pattern of peaks and notches that shows extensive but less systematic changes with sound location. Spectral cues conveyed by the mid frequencies of broadband sounds are important in tasks that require cats to identify the actual location of acoustic stimuli [Huang and May, J. Acoust. Soc. Am. (in press)]. The present study investigates how directional cues conveyed by the mid- and high-frequency spectrum of the HRTF influence the cat's ability to discriminate between sound locations. Thresholds for spatial acuity were measured as minimum audible angles (MAAs) [Mills, J. Acoust. Soc. Am. 30, 237-246 (1958)] at positive azimuths in the interaural horizontal plane and at positive and negative elevations in the median vertical plane. The frequency domain of the noise burst had little effect on MAAs in the horizontal plane, but removal of high-frequency spectral information significantly increased thresholds at positive and negative elevations in the median plane. These results suggest that cats are sensitive to directional properties of the HRTF at frequencies above 18 kHz and may use this information to detect small changes in sound source elevation.

Vowel discrimination in cats: thresholds for the detection of second formant changes in the vowel /epsilon/.

The ability of cats to discriminate changes in the second formant of the vowel /epsilon/ was examined across a range of stimulus levels. Cats were trained to press and hold down a lever to produce a pulsed train of the standard vowel /epsilon/, and to release the lever only when a variant of [epsilon] occurred. Six synthetic variants of /epsilon/ had the same first and third formants (F1 and F3), but with the second formant (F2) located between 1700 and 2000 Hz. All stimuli were tested at levels of 10, 30, 50, and 70 dB SPL. Average difference thresholds for changes in F2 (delta F2) of the vowel /epsilon/ ranged from 87 to 36 Hz across levels of 10 to 70 dB SPL, and were only slightly above those of humans. Further, the delta F2 values were lower than pure-tone delta F values in the same frequency range, whereas humans exhibit higher delta F2 values than pure-tone delta F values in the same frequency range. Changes in the second formant in a negative direction (downward F2 shifts) were also found to be more difficult to detect than upward F2 shifts. These results suggest that, compared to pure tones, cats are better able to discriminate small changes in more complex, vowel-like stimuli.

Sound orientation behavior in cats. I. Localization of broadband noise.

Behavioral experiments measured the accuracy of the cat's voluntary head orientation responses to bursts of broadband noise at 104 locations in the frontal sound field. Cats were presented sound stimuli at randomly selected locations and received a food reward for releasing a lever when a light-emitting diode (LED) flashed at the same location. Head movements to the perceived location of the acoustic stimulus, and therefore expected location of the LED cue, were tracked by an electromagnetic sensor. Orientation responses to single sound bursts were directed to within 5 degrees of the azimuth and elevation of sounds within 15 degrees of the immediate frontal field and did not change for burst durations of 40, 100, and 200 ms. An increasing underestimation of target location was observed as the sound stimulus moved to more lateral azimuths and higher elevations. The "undershoot" phenomenon was reduced by allowing subjects to track paired stimulus bursts that repeated from the same location. These effects of sound location on the accuracy of orientation responses are predicted by the availability of mid-frequency spectral cues for sound localization.

Sound orientation behavior in cats. II. Mid-frequency spectral cues for sound localization.

The cat's head-related transfer function creates a directionally dependent mid-frequency notch in the amplitude spectrum of a broadband sound as the stimulus propagates to the tympanic membrane [Rice et al., Hear. Res. 58, 132-152 (1992)]. Our previous behavioral studies [May and Huang, J. Acoust. Soc. Am. 100, 1059-1069 (1996)] have indicated that the cat's sound-evoked orientation responses are well directed to the azimuth and elevation of broadband noise bursts in the frontal sound field, where pinna-based spectral notches are prominent and change systematically with sound direction. In the present study, the importance of mid-frequency directional cues in the cat's sound localization behavior was further evaluated by manipulating the frequency and bandwidth of orientation stimuli. The accurate pattern of orientation behavior seen previously with bursts of broadband noise was relatively unaffected when stimulus bandwidth was decreased to mid-frequency bandpass noise of 5-18 kHz. In contrast, poorly directed head orientation responses were observed in tests with high-pass noise (> 18 kHz) and mid-frequency pure tones. When tested with narrow bands of mid-frequency noise, cats oriented toward the spatial location where HRTF-filtering properties most closely matched the stimulus spectrum. These results suggest that important sound localization cues are derived from mid-frequency spectral features of the cat's HRTF.

Vowel discrimination in cats: acquisition, effects of stimulus level, and performance in noise.

The ability of cats to discriminate accurately among different synthetic, steady-state vowels was examined across a range of stimulus levels and in background noise. Cats were trained to press and hold down a lever to produce a pulsed train of a standard vowel stimulus, and to release the lever only when a different vowel sound occurred. Five synthetic vowels were tested (/e/, /ae/, /a/, /o/, and /u/) at levels of 30, 50, 70, and 90 dB SPL. In separate experiments, each of these vowels served in turn as the standard vowel. All cats discriminated among the vowels accurately, and in general performed at least as well at high stimulus levels as at low levels. Where differences in vowel discriminability occurred, they were correlated with the relative changes in first and second formant peaks. Cats appear to predominantly utilize upward frequency changes in either the first or second formants of the vowels to make the discriminations; downward formant changes produced considerably lower discrimination performances. In background noise, high vowel discriminability was still maintained at an average signal/noise ratio of -12.3 dB. Thus cats can discriminate among vowels at high signal levels and in background noise, despite the fact that the neural representations of vowels based on rate responses in the auditory nerve can be severely degraded under these conditions.

Frequency organization of the dorsal cochlear nucleus in cats.

Sensory epithelia are often spatially reiterated throughout their representation in the central nervous system. Differential expression of this representation can reveal specializations of the organism's behavioral repertoire. For example, the nature of the central representation of sound frequency in the auditory system has provided important clues in understanding ecological pressures for acoustic processing. In this context, we used electrophysiological techniques to map the frequency organization of the dorsal cochlear nucleus in nine cats. Frequency responses were sampled in increments of 100-200 microns along electrode tracks that entered the dorsomedial border of the nucleus and exited at the ventrolateral border. Electrode tracks were oriented parallel to the long (or strial) axis of the nucleus so that each penetration sampled neural responses for most of the cat's audible frequencies and remained in or near the pyramidal cell layer for several millimeters. Nearly identical distance versus frequency relationships were obtained for different rostral-caudal locations within the same cat as well as for different cats. Frequency responses systematically decreased from above 50 kHz at the most dorsomedial locations in the nucleus to below 1 kHz in the most ventrolateral regions. The rate of frequency change was roughly three times greater in high frequency regions than in low frequency regions. In addition, the highest pyramidal cell density and longest rostral-caudal axis was observed for the middle third of the dorsal-ventral axis of the nucleus. As a result, roughly half of all pyramidal cells responded to frequencies between 8-30 kHz. The representation of neural tissue for these frequencies may be related to the importance of spectral cues in sound locations.

Dynamic range of neural rate responses in the ventral cochlear nucleus of awake cats.

1. Response thresholds and dynamic range properties of neurons in the ventral cochlear nucleus (VCN) of awake cats were measured by fitting a computational model to rate-level functions for best frequency (BF) tone bursts and for bursts of broad-band noise. Dynamic range measurements were performed in quiet and in the presence of continuous background noise. 2. The sample of neurons obtained in the VCN of awake cats exhibited a variety of peristimulus histograms (PSTHs) and thresholds. All PSTH response types previously described in the VCN of anesthetized cats were found in awake cats. The lowest thresholds for neural responses were observed at sound pressure levels that were equivalent to behavioral thresholds of absolute auditory sensitivity. 3. When responses to BF tones or bursts of broad-band noise were recorded in quiet backgrounds, the dynamic range properties of most units in the VCN of awake cats were not significantly different from dynamic range properties of auditory nerve fibers (ANFs) in anesthetized cats or VCN units in decerebrate cats. All auditory units showed a larger dynamic range for noise bursts than for tone bursts, but VCN units with primary-like and onset PSTHs showed larger dynamic ranges for responses to noise bursts than that of ANFs and VCN chopper units. 4. When tests were performed in the presence of continuous noise, rate-level functions for BF tone bursts shifted to higher tone levels and showed a more compressed range of driven rates in comparison with data obtained in quiet. Compression of the rate-level function in noise resulted from an increase in driven rate at low tone levels and a decrease in rate at high tone levels. These changes in the rate-level function suggest that noise may reduce the range of BF tone levels that are potentially encoded by a unit's rate responses. By exhibiting larger shifts and less compression in background noise, VCN units in awake cats better preserved the dynamic range of their rate responses to BF tones than ANFs in anesthetized cats or VCN units in decerebrate cats. 5. Rate-level functions were obtained from a small sample of VCN units not only with the cat performing the behavioral task but also with the cat awake and sitting quietly in the testing apparatus. No differences in noise-induced shift or compression were noted between the two testing conditions.(ABSTRACT TRUNCATED AT 400 WORDS)