Correlating the alpha rhythm to BOLD using simultaneous EEG/fMRI: Inter-subject variability

Simultaneous recording of electroencephalogram/functional magnetic resonance images (EEG/fMRI) was applied to identify blood oxygenation level-dependent (BOLD) changes associated with spontaneous variations of the alpha rhythm, which is considered the hallmark of the brain resting state. The analysis was focused on inter-subject variability associated with the resting state. Data from 7 normal subjects are presented. Confirming earlier findings, three subjects showed a negative correlation between the BOLD signal and the average power time series within the alpha band (8--12 Hz) in extensive areas of the occipital, parietal and frontal lobes. In small thalamic areas, the BOLD signal was positively correlated with the alpha power. For subjects 3 and 4, who displayed two different states during the data acquisition time, it was shown that the corresponding correlation patterns were different, thus demonstrating the state dependency of the results. In subject 5, the changes in BOLD were observed mainly in the frontal and temporal lobes. Subject 6 only showed positive correlations, thus contradicting the negative BOLD alpha power cortical correlations that were found in most subjects. Results suggest that the resting state varies over subjects and, sometimes, even within one subject. As the resting state plays an important role in many fMRI experiments, the inter-subject variability of this state should be addressed when comparing fMRI results from different subjects.

A whole head MEG study of the amplitude-modulation-following response: phase coherence, group delay and dipole source analysis

OBJECTIVE

The amplitude-modulation-following response (AMFR) is the frequency component detectable in the electroencephalogram (EEG) or magnetoencephalography (MEG) corresponding to the modulation frequency of an amplitude modulated tone used as a continuous acoustic stimulus. Various properties of the AMFR depend on modulation frequency, suggesting that different generators along the auditory pathway are involved. The present study addresses these issues on the basis of a whole head MEG experiment.

METHODS

AM tones with modulators in the 40 Hz and 80 Hz range were presented unilaterally to 10 normal hearing subjects. Biomagnetic responses were recorded with a 151 channel MEG system. The data analysis concentrated on the phase coherence of the responses, group delays and the estimated location of underlying equivalent dipole sources.

RESULTS

MEG AMFR is more reliably detected in the 40 Hz than in the 80 Hz range. Both response amplitude and phase coherence indicate clear bilateral activation over the parietal/temporal region. Dipole source analysis confirms that sources are located in or near the auditory cortex. Group delays at 80 Hz are shorter than at 40 Hz.

CONCLUSIONS

In both modulation frequency ranges MEG responses are dominated by activity in the auditory cortex, in apparent contrast with EEG data in the literature, pointing to dominant contributions of thalamic sources to the 80 Hz AMFR.

Decreasing fibrogenesis: an immunohistochemical study of paired liver biopsies following lamivudine therapy for chronic hepatitis B

BACKGROUND

Activation of hepatic stellate cells is the earliest step in fibrogenesis. Alpha-smooth muscle actin (alpha-SMA), expressed by activated hepatic stellate cells, and C-terminal procollagen alpha1(III) propeptide (PIIICP) are early markers of fibrogenesis and should precede fibrosis.

AIM

Determine if suppression of hepatitis B virus replication with lamivudine would decrease fibrogenesis as measured by immunohistochemical markers.

METHODS

Paired liver biopsies from patients with hepatitis B before and after therapy with lamivudine (n=47) or placebo (n=33) were studied. alpha-SMA and PIIICP were detected in paraffin-embedded tissue by immunohistochemistry and quantified in a blinded manner by video imaging analysis.

RESULTS

Liver biopsies from patients treated with lamivudine showed a significant decrease in alpha-SMA expression (1.06+/-0.23 vs. 0.58+/-0.11, pre vs. post, P<0.05). Placebo recipients had increased levels of alpha-SMA (0.82+/-0.14 vs. 1.32+/-0.21, P<0.05). PIIICP was similarly decreased after lamivudine. Among subjects whose Histologic Activity Index fibrosis score was unchanged or worsened, the mean change in alpha-SMA expression was significantly decreased in the lamivudine group compared with placebo.

CONCLUSIONS

Lamivudine decreased markers of hepatic stellate cell activation and collagen synthesis. Immunohistochemical techniques are sensitive for assessing fibrogenesis and will be useful in trials of antiviral and antifibrotic agents.

Amplitude of distortion product otoacoustic emissions in the guinea pig in f(1)- and f(2)-sweep paradigms

The amplitude versus frequency relations of distortion product otoacoustic emissions (DPOAEs) were studied in the guinea pig, using both the f(1)- and the f(2)-sweep paradigms to vary the primary frequency separation. The amplitude of the DPOAEs 2f(1)-f(2), 3f(1)-2f(2), 4f(1)-3f(2), and 2f(2)-f(1), plotted as a function of DP frequency, exhibited a bandpass structure. The separation of the primaries for which the DPOAE level is maximum is referred to as the optimum ratio f(2)/f(1). For the lower sideband DPOAEs (f(dp)<f(1), f(2)), the optimum ratio varies non-monotonically with the primary frequency region. At an f(2) around 4.4 kHz, the optimum ratio for 2f(1)-f(2) reaches a maximum of about 1.46 while elsewhere it is in the more commonly found 1.2-1.3 range. The width of the amplitude profiles was studied by determining their Q(10 dB). The f(2)-sweep yielded significantly larger Q(10 dB) than f(1)-sweep, for the lower sideband DPOAEs. The amplitude versus frequency functions of the lower sideband DPOAEs approximately line up. Upon closer inspection, however, with f(1)-sweep the 2f(1)-f(2) DPOAE has its maximum at a slightly smaller DP frequency than the higher order DPOAEs. With f(2)-sweep, on the contrary, the 2f(1)-f(2) tends to peak at a higher DP frequency than the other lower sideband distortion products. When the amplitude is considered as a function of the ratio between f(dp) and f(2), the difference between f(1)- and f(2)-sweep with respect to the width and the alignment of the amplitude functions disappears. The amplitude profiles of the lower sideband DPOAEs are a function of the DPOAE frequency f(dp) relative to f(2).

Repeated inhalation exposure to octamethylcyclotetrasiloxane produces hepatomegaly, transient hepatic hyperplasia, and sustained hypertrophy in female Fischer 344 rats in a manner similar to phenobarbital

Octamethylcyclotetrasiloxane (D4) has been described as a phenobarbital-like inducer of hepatic enzymes. Phenobarbital (PB) and phenobarbital-like chemicals induce transient hepatic and thyroid hyperplasia and sustained hypertrophy in rats and mice. The extent to which these processes are involved with D4-induced hepatomegaly is not known. The present study has evaluated the effects of repeated inhalation exposure to D4 vapors on hepatic and thyroid cell proliferation and hypertrophy with respect to time and exposure concentration. Female Fischer 344 rats were exposed via whole body inhalation to 0 ppm D4, 700 ppm D4 vapors (6 h/day; 5 days/week), or 0.05% PB in drinking water over a 4-week period. Incorporation of 5'-bromo-2-deoxyuridine (BrdU) and the abundance of proliferating cell nuclear antigen were used as indicators of cell proliferation. Designated animals from each treatment group were euthanized on study days 6, 13, and 27. The effect of D4 exposure concentration on hepatic cell proliferation was evaluated at 0, 7, 30, 70, 150, 300, or 700 ppm. Liver-to-body weight ratios in animals exposed to 700 ppm D4 were increased 18, 20, and 22% over controls while PB-treated animals showed increases of 33, 27, and 27% over controls on days 6, 13, and 27 respectively. Hepatic incorporation of BrdU following exposure to D4 was highest on day 6 (labeling index = 15-22%) and was at or below control values by day 27. This pattern of transient hyperplasia was observed in all hepatic lobes examined and was similar to the pattern observed following treatment with PB.

DPOAE group delays versus electrophysiological measures of cochlear delay in normal human ears

Group delays of 2 f1-f2 distortion product otoacoustic emissions (DPOAEs) were determined using both f1- and f2-sweep paradigms in 24 normal-hearing subjects. These DPOAE group delays were studied in comparison with cochlear delays estimated from derived band VIIIth nerve compound action potentials (CAPs) and auditory brainstem responses (ABRs) in the same subjects. The center frequencies of the derived bands in the electrophysiological experiment were matched with the f2-frequencies in the DPOAE recording to ensure that DPOAEs and derived CAPs and ABRs were generated at the same places along the cochlear partition, thus allowing for a direct comparison. The degree to which DPOAE group delays are larger in the f2- than in the f1-sweep paradigm is consistent with a theoretical analysis of the so-called wave-fixed model. Both DPOAE group delays are highly correlated with CAP- and ABR-derived measures of cochlear delay. The principal result of this study is that "roundtrip" DPOAE group delay in the f1-sweep paradigm is exactly twice as large as the neural estimate of the "forward" cochlear delay. The interpretation of this notion in the context of cochlear wave propagation properties and DPOAE-generating mechanisms is discussed.

Gene delivery of Cu/Zn-superoxide dismutase improves graft function after transplantation of fatty livers in the rat

Oxygen-derived free radicals play a central role in reperfusion injury after organ transplantation, and fatty livers are particularly susceptible. Endogenous radical scavengers such as superoxide dismutase (SOD) degrade these radicals; however, SOD is destroyed rapidly when given exogenously. Therefore, an adenoviral vector encoding the Cu/Zn-SOD gene (Ad.SOD1) was used here to test the hypothesis that organ injury would be reduced and survival increased in a rat model of transplantation of fatty livers. Donors received chow diet (untreated), high-fat diet, or ethanol-containing high-fat diet. Some of the ethanol-fed donors were infected either with the gene lacZ encoding bacterial beta-galactosidase (Ad.lacZ), or Ad.SOD1. After liver transplantation, SOD activity and protein expression in liver, survival, histopathology, release of transaminases, free radical adducts in bile, and activation of NF-kappaB, IkappaB kinase (IKK), Jun-N-terminal kinase (JNK), and TNFalpha were evaluated. Ad.SOD1 treatment increased survival dramatically, blunted transaminase release, and reduced necrosis and apoptosis significantly. Free radical adducts were increased two-fold in the ethanol group compared with untreated controls. Ad. SOD1 blunted this increase and reduced the activation of NF-kappaB. However, release of TNFalpha was not affected. Ad.SOD1 also blunted JNK activity after transplantation. This study shows that gene therapy with Ad.SOD1 protects marginal livers from failure after transplantation because of decreased oxygen radical production. Genetic modification of fatty livers using viral vectors represents a new approach to protect marginal grafts against primary nonfunction.

Expression of small heat shock protein alphaB-crystallin is induced after hepatic stellate cell activation

Using the differential PCR display method to select cDNA fragments that are differentially expressed after hepatic stellate cell (HSC) activation, we have isolated from activated HSCs a cDNA that corresponds to rat alphaB-crystallin. Northern blots confirmed expression of alphaB-crystallin in culture-activated HSCs but not in quiescent HSCs. Western blot analysis and immunocytochemical staining confirmed expression of alphaB-crystallin protein in activated but not quiescent HSCs. alphaB-crystallin is induced as early as 6 h after plating HSCs on plastic and continues to be expressed for 14 days in culture. Expression of alphaB-crystallin was also induced in vivo in activated HSCs from experimental cholestatic liver fibrosis. Confocal microscopy demonstrated a cytoplasmic distribution of alphaB-crystallin in a cytoskeletal pattern. Heat shock treatment resulted in an immediate perinuclear redistribution that in time returned to a normal cytoskeletal distribution. The expression pattern of alphaB-crystallin was similar to that of HSP25, another small heat shock protein, but differed from the classic heat shock protein HSP70. Therefore, alphaB-crystallin represents an early marker for HSC activation.

Improving the accuracy of the boundary element method by the use of second-order interpolation functions

The boundary element method (BEM) is a widely used method to solve biomedical electromagnetic volume conduction problems. The commonly used formulation of this method uses constant interpolation functions for the potential and flat triangular surface elements. Linear interpolation for the potential on a flat triangular mesh turned out to yield a better accuracy. In this paper, we introduce quadratic interpolation functions for the potential and quadratically curved surface elements, resulting from second-order spatial interpolation. Theoretically, this results in an accuracy that is inversely proportional to the third power of element size. The method is tested on a four concentric sphere geometry, representative for electroencephalogram modeling, and compared to previous solutions of this problem in literature. In addition, a cylindrical test configuration is used. We conclude that the use of quadratic interpolation functions for the potential and of quadratically curved surface elements in BEM results in a significant increase in accuracy and in some cases even a reduction of the computation time with the same number of nodes involved in the calculations.

Oxidants from nicotinamide adenine dinucleotide phosphate oxidase are involved in triggering cell proliferation in the liver due to peroxisome proliferators

It was shown that 4-chloro-6-(2,3-xylidino)-2-pyrimidinylthio acetic acid (Wy-14,643), a potent peroxisome proliferator, caused rapid oxidant-dependent activation of nuclear factor kappaB (NF-kappaB) in Kupffer cells in vivo and activated superoxide production by isolated Kupffer cells. Here, we tested the hypothesis that NADPH oxidase (NADPH OX) is the source of oxidants increased by Wy-14,643. Indeed, both activation of NF-kappaB and increases in cell proliferation due to a single dose of Wy-14,643 (100 mg/kg) were prevented completely when rats were pretreated with diphenyleneiodonium (1 mg/kg), an inhibitor of NADPH OX. p47phox is a critical subunit of NADPH OX; therefore, p47phox knockout mice were used to specifically address the hypothesis of NADPH OX involvement. In livers of wild-type mice, Wy-14,643 activated NF-kappaB, followed by an increase in mRNA for tumor necrosis factor a. Importantly, these changes did not occur in p47phox knockouts. Moreover, when Kupffer cells were treated with Wy-14,643 in vitro, superoxide production was increased in cells from wild-type but not p47phox-null mice. Finally, when mice were fed a Wy-14,643-containing (0.1%) diet for 7 days, the increase in liver weight and cell proliferation caused by Wy-14,643 in wild-type mice was blocked in p47phox-null mice. Combined, these results are consistent with the hypothesis that Wy-14,643 activates NADPH OX, which leads to NF-kappaB-mediated production of mitogens that causes hepatocellular proliferation characteristic of this class of nongenotoxic carcinogens.

Nuclear factor kappaB in proliferation, activation, and apoptosis in rat hepatic stellate cells

BACKGROUND/AIMS

Activation of the transcription factor NFkappaB has been demonstrated in activated hepatic stellate cells (HSCs). We investigated the role of NFkappaB in proliferation, in activation, and in TNFalpha-induced apoptosis of HSCs.

METHODS

NFkappaB activation was inhibited using an adenovirus expressing an IkappaB dominant negative protein (Ad5IkappaB) in both quiescent and activated HSCs. Quiescent HSCs were infected with Ad5IkappaB or an adenovirus expressing beta-galactosidase (Ad5LacZ). The cells were cultured for 7 days. HSCs activation was determined by cell morphology, smooth muscle alpha-actin (alpha-sma) expression, and steady-state mRNA levels of alpha1(I) collagen as assessed by Western blot and RNase protection assay, respectively. Proliferation was determined in culture-activated HSCs by 3H-thymidine incorporation and direct cell counting. Apoptosis was analyzed by infecting quiescent or activated HSCs with Ad5IkappaB or Ad5LacZ, and then treating with TNFalpha. Apoptosis was demonstrated by determining cell number, assessing nuclear morphology, TUNEL assay and caspase 3 activity.

RESULTS

After 7 days in culture no differences were noted between the Ad5IkappaB- and the Ad5LacZ-infected cells in the morphology, alpha-sma expression or in alpha1(I) collagen mRNA levels. Ad5IkappaB infection did not modify proliferation in activated HSCs. TNFalpha induced apoptosis only in Ad5IkappaB-infected activated, but not quiescent HSCs. Apoptosis was initially demonstrated 12 h after exposure to TNFalpha. Twenty-four h after the TNFalpha treatment, 60% of the activated HSCs were apoptotic.

CONCLUSION

NFkappaB activity is not required for proliferation or activation of HSCs; however, NFkappaB protects activated HSCs against TNFalpha-induced apoptosis.

Group delays of distortion product otoacoustic emissions: relating delays measured with f1- and f2-sweep paradigms

A theoretical analysis is presented of group delays of distortion product otoacoustic emissions (DPOAEs) measured with the phase-gradient method. The aim of the analysis is to clarify the differences in group delays D1 and D2, obtained using the f1- and the f2-sweep paradigms, respectively, and the dependence of group delays on the order of the DPOAE. Two models are considered, the place-fixed and the wave-fixed models. While in the former model the generation place is assumed to be invariant with both f1- and f2-sweeps, in the latter model the shift of generation place is fully accounted for. By making a simple local approximation of the cochlear scale invariance, a mathematical conversion from phase-place to phase-frequency gradients is incorporated in the wave-fixed model. Under the assumption that the DPOAE (as recorded at the best f2/f1 ratio) is dominated by the contribution from the generation site and not by, e.g., reflection components, the analysis leads to simple expressions for the ratio and difference between D1 and D2. Validation of the models against experimental data indicates that lower sideband DPOAEs (2f1-f2, 3f1-2f2, 4f1-3f2) are most consistent with the wave-fixed model. Upper sideband components (2f2-f1), in contrast, are not properly described by either the place-fixed or the wave-fixed model, independent whether DPOAE generation is assumed to originate at the f2 or at the more basally located f(dp) characteristic place.

Long-term audiometric follow-up of click-evoked auditory brainstem response in hearing-impaired infants

Conventional pure-tone thresholds were collected as determined at ages between 4 and 8 years from a group of 163 infants, tested by auditory brainstem response (ABR) in the age range between 1 and 3 years old for objective hearing assessment. The subjects suffered from a variety of degrees and types of sensorineural hearing impairment. The prognostic value of the ABR peak V thresholds in response to 0.1 ms clicks with respect to the behavioural thresholds at octave frequencies from 125 to 8,000 Hz obtained later is evaluated. Correlation between ABR and behavioural thresholds is largest in the 1,000- to 8,000-Hz frequency range. Predicted pure-tone audiograms (mean and SD) were determined for each 10-dB class of ABR thresholds. SDs are in the order of 15 to 18 dB in the 500- to 4,000-Hz range and slightly higher at adjacent frequencies (i.e., somewhat larger than in comparable adult studies). Mean pure-tone thresholds in the 1,000- to 8,000-Hz frequency range are up to 20 dB worse than ABR thresholds, which is opposite to findings in normally-hearing subjects. Thus, with an increasing degree of sensorineural hearing impairment, pure-tone thresholds increase at a significantly higher rate than ABR thresholds. The observation is explained in terms of reduced temporal integration in cochlear hearing loss. ABR thresholds worse than 80 dB nHL are demonstrated to have very limited predictive value with respect to the amount of residual hearing, not only in the low- but also in the high-frequency range. The presence of otitis media during ABR testing is shown to make estimation errors increase to more than 25 dB (SD).

Delivery of Cu/Zn-superoxide dismutase genes with a viral vector minimizes liver injury and improves survival after liver transplantation in the rat

BACKGROUND

Oxygen-derived free radicals play a central role in pathomechanisms of reperfusion injury after organ transplantation. Endogenous radical scavenger systems such as superoxide dismutase (SOD) degrade toxic radicals; however, SOD is degraded rapidly when given exogenously. Therefore, the hypothesis that treatment of the donor liver with an adenoviral vector encoding the Cu/Zn-SOD gene (Ad-SOD1) would lead to permanent gene expression and therefore protect the organ against injury and increase survival in a rat model of liver transplantation was tested.

METHODS

Some donors were infected with Ad-SOD1, whereas untreated grafts and livers infected with the indicator gene lacZ encoding bacterial beta-galactosidase (Ad-lacZ) served as controls. After orthotopic liver transplantation, survival, serum transaminases, and histopathology were evaluated.

RESULTS

Approximately 80% of hepatocytes expressed beta-galactosidase 72 hr after injection of Ad-lacZ. Moreover, SOD1 gene expression and activity were increased 3- and 10-fold in the Ad-SOD1 group, respectively. After transplantation, 20-25% of rats treated with Ad-lacZ survived. In contrast, all SOD1-treated animals survived. Transaminases measured 8 hr after transplantation in Ad-SOD1 rats were only 40% of those in controls, which increased 40-fold above normal values. Approximately 20% of hepatocytes in untreated and Ad-lacZ-infected organs were necrotic 8 hr after reperfusion, whereas necrosis was nearly undetectable in grafts from rats treated with Ad-SOD1.

CONCLUSIONS

This study provides clear evidence for the first time that gene therapy with Ad-SOD1 increases survival and decreases hepatic injury after liver transplantation. Genetic modification of the liver represents a future approach to protect organs against injury where oxygen-derived free radicals are involved.

Inhibition of chronic rejection of aortic allografts by dietary glycine

BACKGROUND

Chronic rejection is influenced by a variety of risk factors, including histoincompatibility and ischemia. Glycine, a cytoprotective agent, has been shown to protect against ischemia-reperfusion injury in the liver, inactivate hepatic resident macrophages, minimize cyclosporin A-induced nephrotoxicity, and exhibit immunosuppressive properties in vitro. The aim of this study was to investigate whether dietary glycine could reduce development of chronic rejection.

METHODS

Lewis recipients of Fisher-344 abdominal aortic allografts received diets that contained either 5% glycine plus 15% casein or 20% casein as control for 10 weeks. Vascular lesions of aortic isografts and allografts were evaluated quantitatively with image analysis and cell counting.

RESULTS

No significant vascular changes were observed in isografts (mean medial areas of 3.3 +/- 0.3x0(5) microm2). However, dramatic intimal thickening (neointimal area 2.1+/-0.3) and medial thinning (1.5+/-0.3) were observed in allografts from rats fed control diet. In contrast, glycine significantly reduced the neointima by 45% (1.2+/-0.3) and protected the media (3.5+/-0.2). This led to intima to media area ratios almost twice as large in the control group as in glycine-fed rats (2.2+/-0.4 vs. 1.1+/-0.3, P<0.05). Moreover, infiltrating leukocytes, especially macrophages, were reduced significantly in the adventitia by glycine. In addition, glycine inhibited proliferation and migration of rat aortic smooth muscle cells in culture by 45 and 60%, respectively.

CONCLUSION

These results indicate that dietary glycine minimizes histopathological changes of chronic rejection by reducing the immune response and, in part, by minimizing proliferation and migration of smooth muscle cells.

Gentle organ manipulation during harvest as a key determinant of survival of fatty livers after transplantation in the rat

Both in situ organ manipulation during harvest and steatosis reduce survival after liver transplantation via mechanisms involving Kupffer cells; thus, their effect on survival was compared here. Moderate steatosis was induced by a single dose of ethanol to Lewis rats, while long-term administration of ethanol yielded severe steatosis in donor animals. After minimal dissection during the first 12 min, livers were either manipulated gently or left alone for 13 min subsequently. Orthotopic liver transplantation was performed after 1 h of cold storage in UW solution. Ethanol increased hepatic lipid content to a level of moderate or severe steatosis that reduced survival after transplantation from 100% to approximately 70% (P < 0.05). However, gentle manipulation decreased survival to approximately 30% (P < 0.05) in livers from normal, saline-treated rats and in livers from rats fed a high-fat control diet. Moreover, after short- or long-term ethanol administration, manipulation of fatty livers decreased survival from 70% to approximately 13% (P < 0.05). Further, manipulation elevated serum transaminases, total bilirubin, and necrosis significantly about 2- to 20-fold in fatty grafts after transplantation. At the end of harvest, trypan blue distribution time and hypoxia assessed from 2-nitroimidazole binding were elevated significantly about two- to fourfold by manipulation of fatty grafts. Gadolinium chloride, a Kupffer cell toxicant, blocked the detrimental effects of manipulation. These data demonstrate for the first time that, while steatosis is detrimental for survival, organ manipulation plays a much greater role than fat in mechanisms of primary nonfunction.

Corn oil rapidly activates nuclear factor-kappaB in hepatic Kupffer cells by oxidant-dependent mechanisms

N-6 polyunsaturated fatty acids (N-6 PUFAs), major constituents of corn oil and natural ligands for peroxisome proliferator-activated receptors, increase the rate of growth of established tumors. It has been proposed that chemical peroxisome proliferators increase hepatocyte proliferation by mechanisms involving activation of nuclear factor-kappaB (NF-kappaB) and production of low levels of tumor necrosis factor alpha (TNFalpha) by Kupffer cells; however, how N-6 PUFAs are involved in increased cell proliferation in liver is not well understood. Here, the hypothesis that N-6 PUFAs increase production of mitogens by activation of Kupffer cell NF-kappaB was tested. A single dose of corn oil (2 ml/kg, i.g.), but not olive oil or medium-chain triglycerides (saturated fat), caused an approximately 3-fold increase in hepatocyte proliferation. Similarly, when activity of NF-kappaB in whole rat liver or isolated hepatocytes and Kupffer cells was measured at various time intervals for up to 36 h, only corn oil activated NF-kappaB. Corn oil increased NF-kappaB activity approximately 3-fold 1-2 h after treatment exclusively in the Kupffer cell fraction. In contrast, increases were small and only occurred after approximately 8 h in hepatocytes. The activation of NF-kappaB at 2 h and increases in cell proliferation at 24 h due to corn oil were prevented almost completely when rats were pretreated for 4 days with either dietary glycine (5% w/w), an agent that inactivates Kupffer cells, or the NADPH oxidase inhibitor, diphenyleneiodonium (s.c., 1 mg/kg/day). Furthermore, arachidonic acid (100 microM) activated superoxide production approximately 4-fold when added to isolated Kupffer cells in vitro. This phenomenon was not observed with oleic or linoleic acids. Interestingly, a single dose of corn oil increased TNFalpha mRNA nearly 2-fold 8 h after treatment. It is concluded that corn oil rapidly activates NF-kappaB in Kupffer cells via oxidant-dependent mechanisms. This triggers production of low levels of TNFalpha which is mitogenic in liver and promotes growth of hepatocytes.

Group delays of distortion product otoacoustic emissions in the guinea pig

This paper presents a comprehensive set of experimental data on group delays of distortion product otoacoustic emissions (DPOAEs) in the guinea pig. Group delays of the DPOAEs with frequencies 2f1-f2, 3f1-2f2, 4f1-3f2, and 2f2-f1 were measured with the phase gradient method. Both the f1- and the f2-sweep paradigm were used. Differences between the two sweep paradigms were investigated for the four DPOAEs, as well as the group delay differences between the DPOAEs. Analysis revealed larger group delays with the f2-sweep paradigm, but only for the lower sideband DPOAEs (with fdp < f1,f2). For the lower sideband cubic distortion product 2f1-f2, the f2-sweep delays were a factor of 1.17-1.54 larger than the f1-sweep delays, depending on frequency. The upper sideband DPOAE 2f2-f1 showed no significant difference between f1- and f2-sweep group delays, except for the highest and lowest f2 frequencies. Comparing the group delays of the DPOAEs for each sweep paradigm separately, equal group delays were found for all four DPOAEs measured with the f1-sweep. With the f2-sweep paradigm on the other hand, the group delays of the three lower sideband DPOAEs occurred to be larger than the group delays of the upper sideband DPOAE 2f2-f1. A tentative interpretation of the data in the context of proposed explanatory hypotheses on DPOAE group delays is given.

The prognostic value of electrocochleography in severely hearing-impaired infants

This paper presents a longitudinal evaluation of electrocochleographic assessment in severely hearing-impaired infants. Electrophysiological data were obtained by transtympanic electrocochleography to tone-burst stimuli at octave frequencies of 500 to 8000 Hz at the age of 0-6 years in a group of 126 subjects. The results are compared with auditory thresholds determined at school age in the same children by means of pure-tone audiometry. Cochlear microphonics could be recorded in virtually all ears, although the majority of subjects had hearing losses of 90 dB and more. Compound action potentials (CAPs) showed waveforms varying from normal to a wide range of abnormalities. Audiometric thresholds correlated generally well with the compound action potential (CAP) thresholds obtained in infancy. The error in the predicted audiometric thresholds is between 15 and 20 dB, as compared with 11 dB reported for more moderate hearing losses. It is shown that, in spite of the high stimulus levels used, substantial frequency-specific threshold information is retained. Occasional large discrepancies in thresholds were often associated with markedly abnormal response waveforms. Among the many cases in which no ABR could be elicited, 68 per cent produced detectable electrocochleographic responses in the 1000-4000 Hz range. It is concluded that electrocochleography is a valuable method for the assessment of residual hearing in infants suspected of having a severe hearing impairment.

Kupffer cell oxidant production is central to the mechanism of peroxisome proliferators

Increased cell proliferation most likely plays a key role in peroxisome proliferator-induced liver cancer. Recently, Kupffer cells were shown to be responsible for Wy-14,643-induced cell proliferation. However, the mechanism by which peroxisome proliferators activate Kupffer cells is unknown. Since gut-derived endotoxin is a known activator of Kupffer cells, the hypothesis that it is involved was evaluated. Increased cell proliferation and peroxisome induction were unaffected by gut sterilization. Moreover, endotoxin was not detectable in portal blood following treatment with Wy-14,643. Therefore, it is concluded that gut-derived endotoxin is not responsible for Kupffer cell activation. To test the hypothesis that Kupffer cells are activated by Wy-14,643 directly, Kupffer cell superoxide production was measured following treatment in vitro. Wy-14,643 increased superoxide production in a dose-dependent manner (0.1 and 50 microM) with half-maximal stimulation at 2.5 microM. Diethylhexylphthalate (DEHP) and ethylhexanol did not increase superoxide production even at doses 50 times higher than Wy-14,643; however, monoethylhexylphthalate (MEHP) activated superoxide production as effectively as Wy-14,643 with half-maximal stimulation at 5 microM. Treatment with Wy-14,643 for 21 days caused a 2-fold increase in Kupffer cell superoxide production while DEHP did not. Pretreatment of Kupffer cells with staurosporine (0.01-10 pM) completely blocked generation of superoxide demonstrating that protein kinase C is required. Moreover, Wy-14,643 increased Kupffer cell protein kinase C activity 3-fold. Pretreatment of Kupffer cells with the amino acid glycine (0.01-3 mM), which blunts calcium signaling, inhibited Wy-14,643-stimulated superoxide production and increased protein kinase C activity completely. These data are consistent with the hypothesis that potent peroxisome proliferators (Wy-14,643 and MEHP) directly activate Kupffer cell production of oxidants via mechanisms involving protein kinase C. Further, peroxisome proliferator treatments that sustain elevated rates of cell proliferation (e.g. Wy-14,643) activate Kupffer cell superoxide production following long-term dietary treatment supporting the hypothesis that Kupffer cell-derived oxidants are involved in peroxisome proliferator-induced neoplasia.

CDK inhibitors p18(INK4c) and p27(Kip1) mediate two separate pathways to collaboratively suppress pituitary tumorigenesis

INK4 and CIP/KIP are two distinct families of cyclin-dependent kinase (CDK) inhibitors implicated in mediating a wide range of cell growth control signals. We have created p18(INK4c)-deficient mice. These mice develop gigantism and widespread organomegaly. The pituitary gland, spleen, and thymus are disproportionately enlarged and hyperplastic. T and B lymphocytes develop normally in p18-deficient mice, but both exhibit increased cellularity and a higher proliferative rate upon mitogenic stimulation. Loss of p18, like that of p27, but not other CDK inhibitor genes, leads to a gradual progression from intermediate lobe pituitary hyperplasia in young mice to an adenoma by 10 months of age with a nearly complete penetrance. Mice lacking both p18 and p27, like mice chimeric for Rb deficiency, invariably died from pituitary adenomas by 3 months. Hence, p18 and p27 mediate two separate pathways to collaboratively suppress pituitary tumorigenesis, likely by controlling the function of Rb.

Gentle in situ liver manipulation during organ harvest decreases survival after rat liver transplantation: role of Kupffer cells

BACKGROUND

The etiology of primary graft nonfunction and dysfunction is unknown but most likely involves Kupffer cell-dependent reperfusion injury. However, the donor operation and surgical technique may also have an effect on the outcome after transplantation. Because liver manipulation during harvest cannot be prevented completely with standard procedures, its effect on survival was assessed here.

METHODS

Donor livers were harvested from female Sprague-Dawley rats (200-230 g). Briefly, after minimal dissection during the first 12 min, livers were either manipulated gently or left alone for 13 subsequent minutes. At 25 min, all livers were perfused with cold University of Wisconsin solution via the portal vein, and transplantation was performed after cold storage (1 hr). In some rats, Kupffer cells were destroyed with gadolinium chloride or inactivated with dietary glycine before harvest. Survival, proteolytic activity in the rinse effluent, serum transaminases, trypan blue distribution to index microcirculation, and histology were compared.

RESULTS

In the nonmanipulated group, survival was 100% after transplantation; however, gentle manipulation decreased survival by 70%. Further, manipulation elevated transaminases fivefold and caused about 200% necrosis. At harvest, proteolytic activity and the time for trypan blue to distribute homogeneously were elevated three- to eightfold by manipulation. Gadolinium chloride and glycine prevented the effects of manipulation on all parameters studied.

CONCLUSION

These data indicate for the first time that brief, gentle manipulation of the donor liver has a marked detrimental effect on survival by priming or activating Kupffer cells. This may represent an important early event in pathogenesis, because Kupffer cells play an important role in primary graft nonfunction.

[The cochlea: modern physiologic insights into sensorineural hearing loss]

The cochlea is organized tonotopically: each frequency between 20 Hz and 20 kHz has its own location on the basilar membrane. In addition the cochlea has a large dynamic range (sound intensity). Active movements of the outer hair cells of the sensory organ of Corti play a crucial part in determining hearing acuity and frequency selectivity. Long-known properties such as combination tones and more recent observations like spontaneous and evoked otoacoustic emissions (the cochlea as sound source) fit in the same concept of a cochlea acting as a non-linear fine-tuned amplifier. Most sensorineural losses in people with diminished hearing are caused by injury to the hair cells, e.g. by medicines. Damage of outer hair cells impairs the cochlear amplifier, leading to loss of sensitivity to soft sounds and loss of frequency selectivity. Damage to the inner hair cells has much the same effect as conductive hearing loss. Tinnitus is not caused by otoacoustic emissions, but probably by denervation hypersensitivity following loss of cochlear input.

NFkappaB prevents apoptosis and liver dysfunction during liver regeneration

Although NFkappaB binding activity is induced during liver regeneration after partial hepatectomy, the physiological consequence of this induction is unknown. We have assessed the role of NFkappaB during liver regeneration by delivering to the liver a superrepressor of NFkappaB activity using an adenoviral vector expressing a mutated form of IkappaBalpha. This adenovirus (Ad5IkappaB) was almost exclusively expressed in the liver and inhibited NFkappaB DNA binding activity and transcriptional activity in cultured cells as well as in the liver in vivo. After partial hepatectomy, infection with Ad5IkappaB, but not a control adenovirus (Ad5LacZ), resulted in the induction of massive apoptosis and hepatocytes as demonstrated by histological staining and TUNEL analysis. In addition, infection with Ad5IkappaB but not Ad5LacZ decreased the mitotic index after partial hepatectomy. These two phenomena, increased apoptosis and failure to progress through the cell cycle, were associated with liver dysfunction in animals infected with the Ad5IkappaB but not Ad5LacZ, as demonstrated by elevated serum bilirubin and ammonia levels. Thus, the induction of NFkappaB during liver regeneration after partial hepatectomy appears to be a required event to prevent apoptosis and to allow for normal cell cycle progression.

Transmitter release in inner hair cell synapses: a model analysis of spontaneous and driven rate properties of cochlear nerve fibres

The inner hair cell (IHC) synapse is one of the stages of cochlear processing that determine the relation between sound pressure level and spike rate in auditory nerve fibres. Transmitter released in the non-stimulated condition is held responsible for the wide range of spontaneous spike rates (SR) observed in these fibres. Properties of stimulated spike activity in auditory nerve fibres, including rate threshold and operating range of a fibre, are known to systematically vary with SR. This paper presents a model analysis of the relation between IHC transmembrane potential and transmitter release rate as becoming manifest in these spontaneous and driven rate properties. A previously developed computational model is used to identify those transfer properties of its synapse section which lead to reproduction of the variation of rate thresholds, shapes of rate-intensity functions and maximal driven rate with SR known from the literature. First a simple additive release model, in which driven transmitter release depends linearly on IHC potential, is elaborated. Its results lead to the hypothesis that the true release function is non-linear and variable across synapses generating different SR. An exponential release function is then introduced, with parameters varying across SR in a physiologically dictated way. This approach leads to adequate reproduction of the variation in rate thresholds and rate-intensity functions with SR. Finally, the model is applied in an inverse way to directly estimate the release function from given rate-intensity functions. The conclusion of both forward and inverse model analyses is that transmitter release is a non-linear function of IHC potential which, by the systematic variation of its parameters across SR, effectively leads to the physiological variation in dynamic range across fibres of different SR. Possible relations of these results with ultrastructural morphology and basic physiology of IHC synapses are discussed.

Dietary glycine prevents increases in hepatocyte proliferation caused by the peroxisome proliferator WY-14,643

Peroxisome proliferators are a group of nongenotoxic carcinogens which include a number of hypolipidemic drugs, solvents, and industrial plasticizers. Although the mechanism by which they cause cancer remains unknown, one likely possibility is that they act as tumor promoters by increasing cell proliferation. Hepatic Kupffer cells represent a rich source of mitogenic cytokines (e.g., tumor necrosis factor alpha, TNF alpha) and are stimulated by peroxisome proliferators. Since glycine prevents activation of Kupffer cells, these experiments were designed to test the hypothesis that a diet containing glycine could block the mitogenic effect of the peroxisome proliferator [[4-chloro-6-(2,3-xylidino)pyrimidinyl]thio]acetic acid (WY-14,643). The effects of a glycine-enriched diet on WY-14,643-induced increases in cell proliferation after a single dose or after feeding WY-14,643 in the diet for 3 weeks were assessed. As expected, 24 h after a single dose of WY-14,643, rates of cell proliferation increased from basal values of 0.7 +/- 0.3% to 5.1 +/- 0.5%. Glycine largely prevented the increase caused by WY-14,643 with proliferation only reaching 1.9 +/- 0.4% (p < 0.05). Acyl CoA oxidase increased from 1.4 +/- 0.1 to 3.5 +/- 0.6 nmol of H2O2 min-1 (mg of protein)-1 (p < 0.05) indicating that peroxisome-specific enzyme activity was induced about 2-fold in livers of WY-14,643-treated rats after 24 h. Unlike cell proliferation, however, acyl CoA oxidase was not affected by dietary glycine, consistent with the hypothesis that cell proliferation and peroxisome proliferation occur via different mechanisms. After 3 weeks, dietary glycine reduced basal rates of cell proliferation by about 50% and completely prevented the sustained 5-fold increase in cell proliferation caused by feeding WY-14,643. Moreover, the 3-fold increase in TNF alpha mRNA caused by WY-14,643 was blocked completely by the glycine-enriched diet. Similarly, immunohistochemical staining for TNF alpha was increased 6-fold by WY-14,643, an increase which was prevented by dietary glycine. However, the 6-fold increase in acyl CoA oxidase activity was unaffected by glycine under similar conditions demonstrating that a diet containing 5% glycine prevents increased hepatocyte proliferation caused by a potent peroxisome proliferator without affecting induction of peroxisomes. These data demonstrate that a glycine-enriched diet prevents stimulated cell proliferation most likely by inhibiting TNF alpha production and raise the possibility that dietary glycine will be effective in preventing cancer caused by nongenotoxic carcinogens such as WY-14,643.

Kupffer cells are causally responsible for the mitogenic effect of peroxisome proliferators

WY-14,643 [4-chloro-6-(2,3-xylidino)pyrimidinylthio-acetic acid] is a well-known non-genotoxic carcinogen and peroxisome proliferator that causes liver cancer in rodents by unknown mechanisms. Its ability to sustain elevated rates of hepatocyte DNA synthesis is most likely pivotal in the ultimate development of tumors. The source of this mitogenic stimulus following treatment of rats with WY-14,643 has been hypothesized to be Kupffer cells, the resident hepatic macrophages, since they are activated by peroxisome proliferators in vivo. Therefore, these studies were designed to determine if Kupffer cells are causally responsible for WY-14 643-induced increases in hepatocyte DNA synthesis in vivo. WY-14,643 (100 mg/kg) increased DNA synthesis 8-fold 24 h after treatment; however, inactivation of Kupffer cells with methyl palmitate, a nonhydrolyzable fatty acid ester and known Kupffer cell inhibitor, completely prevented the mitogenic effect of WY-14,643. On the other hand, the ability of WY-14,643 to induce peroxisomes was not affected by methyl palmitate. These data demonstrate that induction of peroxisomes is not dependent on factors from Kupffer cells and support the idea that stimulation of DNA synthesis and induction of peroxisomes occur via distinct mechanisms. Additionally, WY-14,643 increased liver mRNA transcripts of the hepatocyte mitogen tumor necrosis factor alpha (TNF alpha) more than twofold. This increase was also prevented by inactivating Kupffer cells with methyl palmitate. Therefore, it is concluded that Kupffer cells are causally responsible for WY-14,643-induced increases in hepatocyte DNA synthesis most likely by increasing production of TNF alpha, a hepatic mitogen.

Antibodies to tumor necrosis factor alpha prevent increases in cell replication in liver due to the potent peroxisome proliferator, WY-14,643

Several structurally dissimilar hypolipidemic drugs, plasticizers and halogenated hydrocarbons induce peroxisomes in hepatocytes, and cause hepatocellular adenoma and carcinoma in rats and mice. The mechanism by which these agents act is unknown, although recent studies have suggested a link between increased cell proliferation and hepatic cancer caused by peroxisome proliferators. Here, we demonstrate that neutralizing antibodies to tumor necrosis factor alpha (TNF alpha) block increases in protein kinase C and cell proliferation due to [4-chloro-6-(2,3-xylidino)-2-pyrimidinylthio]acetic acid (WY-14,643), a hypolipidemic drug and potent peroxisome proliferator that causes tumors. WY-14,643 moderately elevated the level of TNF alpha mRNA in the liver. TNF alpha was detected immunohistochemically exclusively in Kupffer cells. These results demonstrate that WY-14,643 acts as an indirect mitogen on hepatocytes via TNF alpha. We propose that the Kupffer cell, a major source of TNF alpha in the liver, is involved in the mechanism of the mitogenic effect of WY-14,643.

Auditory-nerve fiber responses to clicks in guinea pigs with a damaged cochlea

This paper describes auditory-nerve single-fiber responses to clicks in noise-damaged cochleas. Poststimulus time histograms (PSTHs) were recorded for various click intensities and for the two click polarities. The PSTHs found in fibers with elevated thresholds are discussed in relation to the frequency threshold curves (FTCs) measured in these fibers. Five types of abnormal FTCs are distinguished. Type I is elevated as a whole, type II has an elevated (and often broadened) tip and a tail at normal level, type III has low thresholds in the tail (often hypersensitive), type IV represents a flat tuning, and type V has no tip but shows a clear appearance of the tail (often hypersensitive). The click PSTHs of abnormal fibers were compared to normal PSTHs at equal sound-pressure levels, and various abnormal trends were found corresponding to the type of FTC. PSTHs for type I have longer dominant-peak latencies and smaller amplitudes; PSTHs for type II were normal well above the fiber's threshold; PSTHs for type III revealed remarkable patterns with multiple peaks, part of them with a latency strongly varying with polarity; PSTHs for type IV showed narrow peaks and steep amplitude/intensity curves; PSTHs for type V showed a multiple peaked pattern and large amplitudes and steep amplitude/intensity curves to rarefaction polarity. The various features in the click responses were in most cases consistent with the type of FTC. The results can be used to explain deviations in whole-nerve recordings in abnormal cochleas.

The effects of exposure route on DNA adduct formation and cellular proliferation by 1,2,3-trichloropropane

1,2,3-Trichloropropane (TCP) induces high incidences of tumors at multiple sites in mice and rats when administered chronically by gavage. The animal tumor data are being used to predict human risk from potential exposure to TCP in drinking water. Risk assessment may be affected by differences in the route of exposure. Gavage administration, which results in high bolus concentrations compared to drinking water exposure, may quantitatively affect toxicokinetics, cytotoxicity, and genotoxicity. We have examined the effects of TCP exposure by the two routes on the formation of DNA adducts and the induction of cellular proliferation. Male B6C3F1 mice were administered [14C]TCP for 1 week by gavage or in drinking water at the low dose (6 mg/kg) used in the NTP carcinogenesis bioassay. Two target organs (forestomach and liver) and two nontarget organs (glandular stomach and kidney) were examined for DNA adduct formation. Adducts were hydrolyzed from DNA, isolated by HPLC, and quantitated by measuring HPLC fractions for radioactivity. In the forestomach, liver, and kidney, gavage administration of TCP resulted in 1.4-to 2.4-fold greater yields of the major DNA adduct, previously identified as S-[1-(hydroxymethyl)-2-(N7-guanyl)ethyl]glutathione. Significant differences in cell proliferation, as determined by incorporation of bromodeoxyuridine into DNA, were also observed for the two routes. Gavage administration of TCP for 2 weeks resulted in up to a threefold greater cell proliferation rate relative to administration in drinking water. Our findings of exposure-related differences in TCP-induced DNA adduct formation and cell proliferation suggest that a risk assessment based on the existing gavage study may overestimate human risk.

Response thresholds in electrocochleography and their relation to the pure tone audiogram

OBJECTIVE

The relations between response thresholds obtained by 1) transtympanic (TT) and 2) extratympanic (ET) electrocochleography, and by 3) pure tone audiometry, were investigated in a population of patients with cochlear hearing loss of various types and degrees, with the prediction of audiometric from electrocochleographic thresholds as the primary focus.

DESIGN

Analyses were made of the relation between TT response thresholds for tone bursts with octave frequencies from 500 to 8000 Hz and audiometric thresholds in 148 ears. Similar analyses of ET thresholds were given for a subset of 30 ears in which TT and ET physiological responses were simultaneously recorded.

RESULTS

TT electrocochleographic thresholds are highly correlated with audiometric thresholds. Linear regression analysis shows that audiometric thresholds can be predicted from physiological thresholds with an error in the estimate of 11 dB. ET electrocochleography permits similar predictions but with a larger uncertainty of 16 dB. Both methods demonstrate a clear frequency specificity. As a remarkable secondary observation, it appears that electrocochleographic thresholds increase slightly less with increasing cochlear dysfunction than do pure tone thresholds. The hypothesis is elaborated that this is due to the different stimulus durations on which the two threshold measurements are based in interaction with the difference in temporal integration between normal and pathological ears.

CONCLUSION

Electrocochleography is a powerful method for the objective and frequency-specific determination of cochlear response thresholds. TT recording is the preferred method, but ET recording is suitable with a larger uncertainty in the estimate. Electrocochleographic thresholds directly measure the cochlear sensitivity as such, whereas pure tone thresholds measure the combination of loss of cochlear sensitivity and of reduced temporal integration.

Input/output curves to tone bursts and clicks in extratympanic and transtympanic electrocochleography

OBJECTIVE

The aim of this study is to explore the applicability of two specific methods in extratympanic (ET) electrocochleography that have rarely been used there, but are more commonly applied in transtympanic (TT) approaches. These two methods are the use of tone burst stimulation in addition to clicks, and the analysis of amplitude input/output curves as a measure of cochlear recruitment due to a pathological spread of excitation.

DESIGN

Simultaneous extratympanic and transtympanic recordings were made in 30 patients with various types and degrees of cochlear hearing loss. ET recordings were also made in a group of normal subjects. Stimuli were tone bursts at octave frequencies from 500 to 8000 Hz and clicks, at intensity levels of about 90 dB down to response threshold in 10 dB steps. First, a general comparison is made of ET versus TT response properties, then the initial slopes of I/O curves in both sets of data are evaluated.

RESULTS

ET responses are reduced in amplitude with respect to TT responses by a factor of 0.43 on average. Within each subject this factor is independent of stimulus type or level, but it has a substantial intersubject variation. ET and TT latencies are identical. From the normal data a criterion for abnormality of the slope of the I/O curve is derived in a similar way as available for TT data. Using this criterion ET I/O curves appear to be significantly steeper than normal in recruiting ears, albeit with a lesser sensitivity than for the associated TT data.

CONCLUSIONS

Extratympanically recorded responses to tone burst stimuli provide basically the same information as transtympanic recordings. However, for a reliable interpretation of responses to low-intensity stimuli a larger amount of signal averaging is required. The analysis of I/O curves is a useful extension of current ET methods as a contribution to assessing a pathological spread of excitation in the cochlea.

Conduction velocity distributions compared to fiber size distributions in normal human sural nerve

Compound action potentials (CAPs) were recorded from the sural nerve of healthy volunteers. A mathematical technique (inverse modeling) was used to compute conduction velocity (CV) histograms from the data. Results were compared to the morphology of age-matched normal sural nerve biopsies. Coefficients of variation (CoVs) revealed the statistical relationship between morphological data (diameter histograms) and electrophysiological data (CV histograms and conventional CAP parameters). No differences were found for the thick fiber group when comparing the CoVs of the diameter histogram parameters with the corresponding CV histogram parameters. Apparently, the same inherent biological interindividual variability is encountered. The CoVs of the CVs of the CAP's main phases are in good agreement with the CoVs of the estimated mean velocity of the thick fiber group. Inverse modeling increases the reliability of the estimation of the number of active fibers as compared to direct CAP amplitude interpretation.

Potential distributions and neural excitation patterns in a rotationally symmetric model of the electrically stimulated cochlea

In spite of many satisfactory results, the clinical outcome of cochlear implantation is poorly predictable and further insight into the fundamentals of electrical nerve stimulation in this complex geometry is necessary. For this purpose we developed a rotationally symmetric volume conductor model of the implanted cochlea, using the Boundary Element Method (BEM). This configuration mimics the cochlear anatomy more closely than previous, unrolled models. The calculated potential distribution in the cochlea due to stimulating electrodes is combined with a multiple non-linear node model of auditory nerve fibres, which we recently developed. The combined model is used to compute excitation profiles of the auditory nerve for a variety of stimulus levels and electrode positions. The model predicts that the excitation threshold, the spatial selectivity and the dynamic range depend on the exact position of the electrode in the scala tympani. These results are in good agreement with recently published electrical ABR data. It is shown that the use of actively modelled nerve fibres is essential to obtain correct predictions for the biphasic stimuli typically used in cochlear implants and that unrolling the cochlear duct as done in previous models leads to erroneous predictions regarding modiolar stimulation.

A dual filter model describing single-fiber responses to clicks in the normal and noise-damaged cochlea

This paper presents a composite model of the normal and noise-damaged guinea pig cochlea. The model incorporates a phenomenologically defined cochlear filter, and physiological descriptions of inner hair cell transduction, synaptic adaptation, and spike generation. The latter three model sections were taken from recent literature. The paper first deals with validation and evaluation of the model and adaptation of the relevant parameters to the guinea pig. Then the model is applied to explore to what extent changes in the cochlear filter can be held responsible for abnormal responses to clicks that were recorded in single auditory nerve fibers in noise-damaged animals. Focus is on those fibers in which the tip-to-tail sensitivity ratio of the frequency threshold curve (FTC) has decreased and/or in which the FTC tail has become hypersensitive. Inspired by this type of W-shaped FTC the mechanical response of the basilar membrane is phenomenologically modeled by two parallel filters, one responsible for the tip of the FTC, the other for its tail. Model simulations show that most abnormal temporal response properties can be explained by pathological alterations in the mechanical response. Residual discrepancies between model and experiment are identified which presumably point to pathological changes in other stages of cochlear processing.

Recovery characteristics of auditory nerve fibres in the normal and noise-damaged guinea pig cochlea

Spontaneous activity was analysed in auditory-nerve fibres innervating normal and noise-damaged cochleas. Spike occurrences were conceived as point processes. Joint interval distributions and serial correlation coefficients reveal a weak history effect for succeeding intervals. The point process is regarded as a renewal and the recovery function, being proportional to the hazard function, is determined from the interval probability density function. In 29 out of 60 fibres the latter shows peculiarities which result in a deviation from a monotonically increasing recovery function. For three fibers of low characteristic frequency the interval probability function shows an oscillatory pattern and for 26 fibres this function exhibits an early, sharp peak around 1.1 ms irrespective of characteristic frequency, spontaneous rate, or cochlear damage. The recovery function is not different between fibres with normal and those with abnormally high thresholds and exhibits an exponential recovery with one time constant of average value 1.6 ms. Bursting activity is found in only one fibre from the abnormally high threshold group.

Characterization of p53 mutations in methylene chloride-induced lung tumors from B6C3F1 mice

Mutations of the p53 tumor suppressor gene are the most common defined genetic alterations seen in a wide variety of human cancers. In contrast, little is known about the importance of the p53 gene in chemically induced tumors of rodents, which are widely used as models for the evaluation of human health risks. In this study we examined 54 methylene chloride-induced and seven spontaneously arising lung tumors from female B6C3F1 mice for losses of heterozygosity (LOH) at markers near the p53 gene on chromosome 11. LOH was detected in seven methylene chloride-induced lung carcinomas by Southern analysis of a restriction fragment length polymorphism and PCR analysis of five simple sequence length polymorphisms. In each case allele loss was observed at all six markers; thus, these chromosomal alterations were likely to have resulted from mitotic nondisjunction. In contrast, LOH was not detected in 20 liver tumors from methylene chloride-treated mice at the Acrb locus, which is tightly linked to the p53 gene on chromosome 11. In addition single strand conformation polymorphism analysis was performed to screen for mutations in the most conserved regions of the p53 gene (exons 5 to 8). Consequently, potential mutations identified by direct sequencing, were only detected in four of the seven tumor samples with LOH, but not in any of the remaining lung tumors. Overexpression of the p53 protein by immunohistochemical staining was detected only in the four tumors that contained p53 point mutations and in a focal area of another tumor. Finally, using a simple sequence length polymorphism within the retinoblastoma tumor suppressor gene, LOH on mouse chromosome 14 was also detected in three lung carcinomas and one liver tumor. Inactivation of p53 and possibly the retinoblastoma tumor suppressor gene appear to be infrequent events in lung and liver tumors from methylene chloride treated mice.

Round-window recorded potential of single-fibre discharge (unit response) in normal and noise-damaged cochleas

Unit responses (URs) of eighth-nerve fibres have been determined at the round window by spike-triggered averaging in both normal and pathological guinea pig cochleas. The pathology was mainly noise-induced damage. The URs have been analysed with respect to their dependence on the fibre's threshold, characteristic frequency (CF) and spontaneous rate (SR). The results from normal cochleas confirmed earlier data (Prijs, 1986): the UR has a diphasic waveform and the amplitude of its negative first peak is about 0.1 microV. From the six parameters (amplitude, latency, and width of the two peaks) by which the UR was described only the amplitude of the positive peak showed a significant variation with CF: a small decrease with increasing CF (CF-range 0.1 to 20 kHz). This finding may possibly be caused by oscillations in the spike-triggered average for low CFs. URs for most low- and medium-SR fibres were found to be large (greater than 0.3 microV). However, this result is interpreted as an artefact caused by synchrony of fibre spontaneous activity. In damaged cochleas only slight changes of the UR were found: the waveform duration became significantly shorter and on some occasions the positive peak increased in amplitude, but latency and amplitude of the negative component of the UR remained unchanged.

Single-fibre and whole-nerve responses to clicks as a function of sound intensity in the guinea pig

This paper describes a study of the intensity dependence of click-evoked responses of auditory-nerve fibres in relation to the simultaneously recorded compound action potential (CAP). Condensation and rarefaction clicks were presented to normal hearing guinea pigs over an intensity range of 60 dB. The recorded poststimulus time histograms (PSTHs) were characterized by the latency (tp), amplitude (Ap) and synchronization (Sp) of their dominant peak, parameters that are particularly important for the understanding of the CAP. For all fibres tp decreased monotonically with increasing intensity, in a continuous way for fibres with high characteristic frequency (CF greater than 3 kHz), and in discrete steps of one CF-cycle for low-CF (CF less than or equal to 3 kHz) fibres. An additional analysis of PSTH envelopes revealed that average latency shifts with intensity are similar for all CFs above 2 kHz. For all fibres Ap increased monotonically with intensity; the increase was stronger and maximum values were larger for low-CF than for high-CF fibres. A schematic model PSTH was then formulated on the basis of the experimental data. A sum of these model PSTHs from a hypothesized fibre population was convolved with an elemental unit response (Versnel et al., 1992) in order to simulate the compound action potential. Synthesized CAPs agreed with experimental CAPs in their main aspects.

Dependence of auditory brainstem response on click polarity and high-frequency sensorineural hearing loss

An investigation is presented on the effect of click polarity upon the auditory brainstem response (ABR) in cases of simulated high-frequency hearing loss. A high-frequency hearing loss was stimulated in 12 normal subjects by applying a high-pass noise masker, the noise cutoff frequency being varied in half-octave steps through the range of audiometric frequencies. With this paradigm, the effects of click polarity on the ABR are similar to those reported in patients with high-frequency hearing loss, including changes in waveform morphology and substantial peak latency shifts. To evaluate the role of individual cochlear frequency channels in a more direct way, half-octave narrow-band responses were determined by pair-wise subtraction of the high-pass-noise-masked responses. As far as ABR peak latencies are concerned, stimulus polarity effects tend to increase towards the lower frequency bands. Thus, the selective elimination of the contribution to the ABR of cochlear high-frequency channels can be held responsible for the increased dependence of ABR on click polarity observed in high-frequency hearing loss. However, statistical evaluation of peak I, III and V latencies, both in high-pass-noise-masked and in narrow-band ABRs, reveals that the latency effects are not systematic between subjects. Mechanisms producing the observed rarefaction-condensation differences, possible explanations for the large intersubject variability and the consequences for clinical interpretation of ABR are discussed.

A longitudinal electrocochleographic study of a case of long-standing bilateral Lermoyez's syndrome

An 18-year follow-up of a case of bilateral Lermoyez's syndrome is presented. The left ear having reached a stabilized hearing loss about 9 years after the onset of the disease, the right ear, apart from some isolated early periods of hearing loss, started to show the full extent of clinical symptoms after about 16 years. Electrocochleographic observations are presented. Studies were performed twice in the right ear during a period of strongly fluctuating hearing thresholds, once in an impaired and once in a relatively good condition. Electrocochleography of the stabilized left ear was performed as well. The data are compared with electrocochleographical observations in the left ear in its early fluctuating stage. Variation of cochlear physiological data during the fluctuating stage of the disease shows remarkable correspondence between the two ears. The stabilized Lermoyez ear is shown to have developed considerable hair cell loss, but may still have preserved its endolymphatic hydrops. These findings in Lermoyez's syndrome fit well into the observations reported in patients with Menière's disease.

Single-fibre responses to clicks in relationship to the compound action potential in the guinea pig

Poststimulus time histograms (PSTHs) to clicks of standard level were measured in eighth-nerve fibres of normal-hearing guinea pigs. In the context of studying the fibres' contribution to the compound action potential (CAP), the PSTHs are described with the parameters latency (tp), amplitude (Ap) and synchronization (Sp) of the dominant PSTH peak. These parameters are considered in relation to characteristic frequency (CF) and spontaneous rate (SR). An adequate description for tp is one in which tp is constant for non-phase-locking fibres (CF above 3 kHz) and it is an exponential function of CF for phase-following fibres. The low-SR fibres (SR below 5 spikes/s) had smaller amplitudes and longer latencies than the other ones. The variations of Ap with CF can be explained by the varying synchronization of the response.

Latencies of eighth nerve fibre responses with respect to their relative contribution to the compound action potential in the guinea pig

We present a study of the latencies of click-evoked post-stimulus time histograms (PSTHs) in the guinea pig in the context of the auditory nerve fibre's contribution to the compound action potential (CAP) recorded at the round window. The latencies of the dominant PSTH peak were studied as a function of relevant physiological fibre properties, in particular the characteristic frequency (CF) and the spontaneous discharge rate (SR). We found that high-SR fibres have shorter latencies than low-SR fibres. These findings are discussed in the context of correlation between synaptic morphology and SR as described in the literature. The PSTH latency as a function of CF is described separately for low- and high-CF fibres for each of the two SR subgroups. Finally, we discuss to what extent the various subgroups of fibres contribute to the N1 peak of the CAP, the most commonly studied component.

Lermoyez's syndrome. A follow-up study in 12 patients

Clinical studies were performed in 12 patients with the Lermoyez syndrome during a close follow-up of 2 to 31 years. The nature and temporal sequences of the symptoms of these patients are described. The incidence of Lermoyez's syndrome, as compared with Meniere's disease, is almost 18%. This is much higher than usually is assumed. In 6 out of 12 patients typical Meniere attacks were found besides their Lermoyez attacks. Almost every one of the 12 patients showed, besides the Lermoyez attacks, also hearing fluctuations without vertigo, and vertigo without hearing fluctuations. The type of vertigo was usually rotating, often with unsteadiness and in 3 patients dropping attacks were sometimes observed. The typical Lermoyez attack lasted several hours. The preceding hearing loss lasted for days to months. The hearing recovered after the attacks and remained stable for days to months. An improved hearing was noticed within some hours after the vertiginous attack in 9 out of 12 cases. Three patients noticed the hearing improvement already during the attack. In 8 out of 12 cases the disease became bilateral.

Sensory potentials and sural nerve biopsy: a model evaluation

Compound action potentials (CAPs) were recorded from the sural nerve prior to nerve biopsy in patients with various kinds of polyneuropathy. A previously developed volume conductor model for the CAP is applied to analyze the recorded CAPs in close relation with the morphological findings in the biopsy. First, the fiber diameter histograms obtained from the biopsy are used to simulate CAPs, by assuming a linear relation between fiber diameter and propagation velocity. It is concluded that the simulated CAPs deviate systematically from the recorded CAPs. Next, the assumption of a linear diameter-velocity relation is left, and the assumed fiber velocity distribution is adapted to obtain optimal model reconstructions of the recorded CAPs. It is concluded that the model is capable of reconstructing the recorded CAPs, including slow components and small polyphasic potentials in the case of severe fiber loss. It is demonstrated how the diameter histogram and the optimal velocity distribution can be combined to an empirical estimate of the diameter-velocity relation.