Disruption of lateral efferent pathways: functional changes in auditory evoked responses

Abstract

The functional consequences of selectively lesioning the lateral olivocochlear efferent system in guinea pigs were studied. The lateral superior olive (LSO) contains the cell bodies of lateral olivocochlear neurons. Melittin, a cytotoxic chemical, was injected into the brain stem using stereotaxic coordinates and near-field evoked potentials to target the LSO. Brain stem histology revealed discrete damage to the LSO following the injections. Functional consequences of this damage were reflected in depressed amplitude of the compound action potential of the eighth nerve (CAP) following the lesion. Threshold sensitivity and N1 latencies were relatively unchanged. Onset adaptation of the cubic distortion product otoacoustic emission (DPOAE) was evident, suggesting a reasonably intact medial efferent system. The present results provide the first report of functional changes induced by isolated manipulation of the lateral efferent pathway. They also confirm the suggestion that changes in single-unit auditory nerve activity after cutting the olivocochlear bundle are probably a consequence of disrupting the more lateral of the two olivocochlear efferent pathways.

Figure 1

Representative brain stem section from an acute test animal scored as an LSO hit. Left: Intact (control) side of brain stem. Both the U-shaped lateral superior olive (LSO) and the more medially located medial superior olive (MSO) are evident (see labeled arrows). Right: The experimental (cytotoxin-injected) side of the same brain stem as depicted in left panel. The lesion is centered over the LSO, with only the most ventromedial aspects of LSO appearing undamaged. The damage to the neural tissue does not extend to the MSO. Scale bar = 400 µm.

Figure 2

Area of intact (undamaged) lateral superior olive (LSO) was measured following injection of a cytotoxin into the vicinity of LSO (e.g., “Lesioned”). Control comparisons were provided from measurements of LSO area in contralateral, uninjected LSO (e.g., “Unlesioned”). Section 1 was the most caudal section in which LSO appeared. Sections were sequentially numbered until LSO was no longer evident. Asterisks denote statistically reliable differences between lesioned and unlesioned sections (p's < 0.05).

Figure 3

Cochlear tissues were immunolabeled with antisynaptophysin to indicate the density of efferent terminals. Tissues from the cochlea contralateral to the injection site (i.e., the control cochlea receiving intact lateral efferent projections from the unlesioned side of the brain stem) are depicted in the left panels. Tissues ipsilateral to the injection site (i.e., tissues receiving projections originating in the region of the lesion) are depicted in the right panels. All images were focused at the level of the efferent vesicles in the vicinity of the inner hair cell and auditory nerve fibers. A. Animal GP2-1, basal turn, 63X. B. Animal CGL067, basal turn, 63X. C. Animal CGL067, third turn, 40X.D. Animal CGL067, apex, 40X. Arrows indicate specific regions of interest. a. Medial efferent vesicles under outer hair cells. b. Inner spiral bundle. c. Lateral efferent vesicles below the inner hair cells. d. Tunnel crossing fibers. All scale bars = 50 µm.

Figure 4

Surface area of synaptophysin immunolabeling in the vicinity of the lateral efferent neurons was measured for cochlear tissues ipsilateral (lesioned) and contralateral (control) to the brain stem injection site. Quantification was conducted on tissues from animal CGL067 (see Fig. 3); this animal was euthanized 1 week after we injected melittin into the brain stem. Labeled area was measured across a 325 µm region representative of labeling in each cochlear turn. Each region selected for analysis was broken into four analysis windows measuring approximately 81 µm in length (total analysis window for each segment was 974 µm²). Average labeling within each of the four 81 µm segments is depicted. We note that the analysis region did not include inner spiral bundle or tunnel-crossing fiber labeling.

Figure 5

The sound-evoked whole-nerve compound action potential (CAP) response of the auditory nerve is shown for a subset of the levels tested (20, 40, 60, 80- dB SPL). Waveforms are in response to a 14-kHz acoustic signal (5 ms, see horizontal scale bar) and are from a control animal (top) and an animal in which the lateral superior olive (LSO) was lesioned (bottom). Data from the LSO-lesioned animal were obtained on the seventh day postlesion. Vertical scale bar = 170 µV.

Figure 6

Threshold, defined as the lowest signal level (dB SPL) that elicited a compound action potential (CAP) response, was determined for all animals. Top: Mean threshold (± SE) for acute electrophysiology animals. All lesioned animal thresholds were determined 1 week postlesion. Asterisks denote statistically reliable differences between lesioned and unlesioned control animals (p's < 0.05). Middle: Thresholds for chronic animal HIT1 were determined before and 1 week after lesioning the lateral superior olive (LSO). Bottom: Thresholds for chronic animals HIT2 and MISS1 were determined at 10 kHz before and after the lesion surgery. Lesion-induced change from prelesion threshold is depicted. Brain stem sections from animal HIT2 exhibited targeted disruption of LSO; brain stem sections from animal MISS1 showed no cytotoxin-induced trauma. Postlesion assessment was conducted at time points extending to 17 weeks.

Figure 7

CAP amplitude, defined as the amplitude of the N1–P1 response component, was determined for all frequency/level combinations for acute electrophysiology test animals. The 20-kHz 100-dB SPL data were discarded due to acoustic signal distortion at this frequency/level combination. Mean CAP amplitude (± SE), averaged within levels and across frequencies, is depicted. All lesioned animals were assessed 1 week postlesion. Top: CAP amplitude for sound intensity levels measured in dB SPL. Asterisks denote statistically reliable pairwise differences between lesioned and unlesioned control animals (p's < 0.05). Diamonds denote differences that approached statistical reliability (p's < 0.10). Bottom: CAP amplitude for sound intensity levels normalized to account for differences in threshold sensitivity. Intensity is expressed as dB sensation level (dB SL, or, equivalently, dB above threshold). Because the overall statistical reliability of the lesion-induced change in CAP amplitude was greater than 0.10, pairwise comparisons were not conducted.

Figure 8

CAP amplitude, defined as the amplitude of the N1–P1 response component, was determined for all frequency/level combinations for animal HIT1 prior to and 1 week after melittin was injected into the lateral superior olive (LSO). The 20- kHz 100-dB SPL data were discarded due to acoustic signal distortion at this frequency/level combination. CAP amplitude, averaged within levels and across frequencies, is depicted. Top: CAP amplitude for sound intensity levels measured in dB SPL. Bottom: CAP amplitude for sound intensity levels normalized to account for differences in threshold sensitivity. Intensity is expressed as dB sensation level (dB SL, or, equivalently, dB above threshold).

Figure 9

Distortion product otoacoustic emissions (DPOAEs) show a rapid adaptation shortly after signal onset when the medial efferent pathway is intact. DPOAE adaptation is depicted for an unlesioned control animal (top, CGL073) and HIT2 (middle); HIT2 underwent a targeted disruption of the lateral superior olive (LSO). Variation in amplitude of the adaptation component is consistent with unlesioned animals tested in this laboratory as well as other laboratories. The ratio of levels of Fl and F2 were systematically varied across a wide range for both animals. The amplitude of the adaptation across level combinations is depicted in the bottom panel for animal HIT2.

Figure 10

Top. Area of intact (undamaged) lateral superior olive (LSO) was measured following injection of melittin into the vicinity of LSO (e.g., “Lesioned”) in the animal identified as HIT1. Control comparisons were provided from measurements of LSO area in contralateral, uninjected LSO (e.g., “Unlesioned”). Section 1 was the most caudal section in which LSO appeared. Sections were sequentially numbered until LSO was no longer evident. Bottom, left. Amplitude of the compound action potential (CAP) response of the auditory nerve is depicted across stimulus levels (dB SPL) at three frequencies (6, 12, and 18 kHz). CAP was assessed prelesion using a chronically implanted electrode, as well as 1 week postlesion. Right. Percent change in CAP amplitude induced by the LSO lesion, at frequencies corresponding to those depicted in the left panels. Negative numbers represent a depression in CAP amplitude. Percent change in CAP amplitude at other test frequencies is summarized for a subset of stimulus intensities in Tables 1 and 2.