Medial olivocochlear reflex interneurons are located in the posteroventral cochlear nucleus: a kainic acid lesion study in guinea pigs

Abstract

The medial olivocochlear (MOC) reflex arc is probably a three-neuron pathway consisting of type I spiral ganglion neurons, reflex interneurons in the cochlear nucleus, and MOC neurons that project to the outer hair cells of the cochlea. We investigated the identity of MOC reflex interneurons in the cochlear nucleus by assaying their regional distribution using focal injections of kainic acid. Our reflex metric was the amount of change in the distortion product otoacoustic emission (at 2f(1)-f(2)) just after onset of the primary tones. This metric for MOC reflex strength has been shown to depend on an intact reflex pathway. Lesions involving the posteroventral cochlear nucleus (PVCN), but not the other subdivisions, produced long-term decreases in MOC reflex strength. The degree of cell loss within the dorsal part of the PVCN was a predictor of whether the lesion affected MOC reflex strength. We suggest that multipolar cells within the PVCN have the distribution and response characteristics appropriate to be the MOC reflex interneurons.

Fig. 1

Schematic of medial olivocochlear (MOC) reflex pathways that can affect cochlear responses from the ipsilateral (right) ear. The Ipsi reflex (black lines, arrows, and cells) starts with sound-evoked activity in the auditory nerve, continues with projections from the cochlear nucleus (CN) to “Ipsi” MOC neurons, which then project back across the midline to the ipsilateral cochlea. The Contra reflex (gray line, arrows and cells) begins with the contralateral auditory nerve, and continues as CN projections across the midline to “Contra” MOC neurons. In addition to these dominant inputs from the CN on the opposite side of the brain, MOC neurons receive smaller inputs assumed to be from the CN on the same side of the brain (small arrows), which are postulated to explain the phenomenon of binaural facilitation. KA, kainic acid; DPOAE, distortion product otoacoustic emissions; LSO, lateral superior olive; MSO, medial superior olive. See text for further details.

Fig. 2

A,B: Illustration of the distortion product otoacoustic emissions (DPOAEs) -based metric used to assess medial olivocochlear (MOC) reflex strength and the changes seen after kainic acid (KA) injection in two cases. Left: The amplitude of the DPOAE vs. post-onset time for the primary tones. The onset timing and duration for ipsilateral primaries and contralateral noise are schematized below A; they apply to B as well. In both cases, several response traces are superimposed (see key in A). DPOAE amplitude is expressed both in normalized terms (left vertical axis) with respect to the steady-state value achieved before contralateral-noise onset, and in absolute dB sound pressure level (SPL, right vertical axis) for the DPOAE before the injection. The magnitude of Ipsi and Contra preinjection reflex metrics (i.e., the changes in the preinjection DPOAE) are indicated by the double-headed arrows in A. The traces shown in each case are for the primary level combinations resulting in the largest DPOAE changes (A: f₂ = 10 kHz; before and short-term, L₁ = 77 dB, L₂ = 65 dB; long-term, L₁ = 78 dB, L₂ = 67 dB; B: f₂=12 kHz; before, L₁ = 80 dB, L₂ = 73 dB, short-term, L₁ = 80 dB, L₂ = 72 dB; long-term, L₁ = 82 dB, L₂ = 74 dB). Right: Bar graphs show percentage changes in MOC reflex strength for the traces shown in A and B. Stars indicate significant changes, i.e., changes larger than the test–retest variability as described in the text.

Fig. 3

Summary of effects of kainic acid (KA) lesions. A,D,G,J: The four test conditions are schematized (also see Fig. 1 and text for explanation). B,C,E,F,H,I,K,L: Each set of bar graphs from middle (B,E,H,K) and right (C,F,I,l) columns illustrates data for the test condition schematized at the left. Bar graphs show average percentage changes in reflex strength after KA injection in the short-term (< 2 hours; middle column) and in the long-term (> 6 hours; right column). In each panel, each bar shows the result from one animal, averaged from two to four traces such as those of Figure 2. Individual experiment numbers are listed along the X-axis in each panel. The experiment numbers are arranged, in all panels, according to lesion locus, as described in the key above C. Filled and unfilled bars indicate whether the lesion involved or spared the posteroventral cochlear nucleus (PVCN), respectively (see key in B). Gray bar indicates surgical cut of the dorsal and intermediate acoustic striae. AVCN, anteroventral cochlear nucleus; DCN, dorsal cochlear nucleus.

Fig. 4

A,B: Photomicrographs of rostral posteroventral cochlear nucleus (PVCN) from the lesioned side (A) and control side (B) of Experiment 75. In A, the area of nearly complete cell loss is indicated by a dashed line. The lesion section is from slide 82: schematics of the full lesion in this case are shown in Figure 6A. NR, nerve root of auditory nerve; lam, lamina of granule cells; sgl, superficial granular layer. Scale bar = 0.5 mm in B (applies to A,B).

Fig. 5

Data from four reflex-sparing lesions. Left: Bars show the long-term changes in Ipsi reflex strength on the injected side. Right: Camera-lucida drawings of lesion-containing sections indicate the regions of almost-complete cell loss from the kainic acid (KA) injection. A–D: Histology shown for every fourth (B,D), fifth (C), or sixth (A) section, with more caudal sections toward the right. Within-lesion subdivision boundaries not obvious because of missing neurons are drawn with dashed lines using analogous sections from the opposite side in the same animal. cap, small cell cap; AVCN, anteroventral cochlear nucleus; DCN, dorsal cochlear nucleus; PVCN, posteroventral cochlear nucleus; NR, nerve root of auditory nerve; lam, lamina of granule cells; sgl, superficial granular layer.

Fig. 6

A–D: Data from four reflex-interrupting lesions, all of which involved the PVCN. There was minor involvement of the DCN in one case (C) and more substantial involvement of the AVCN in another case (D). Format and abbreviations are as for Figure 5. oca, octopus cell area.

Fig. 7

Data from four PVCN lesions that spared the reflex. Format and abbreviations are as for Figure 5.

Fig. 8

A–G: The six reflex-interrupting cases plotted on an atlas constructed from a template cochlear nucleus. Column G shows overlap of all the cases with areas of cell loss common to all six cases indicated in black. The first four cases are also illustrated in Figure 6. For abbreviations, see Figure 5.

Fig. 9

A–F: Six reflex-sparing cases plotted on the same atlas as Figure 8. These six cases were chosen because they were the largest PVCN lesions that spared the reflex. There was no area of cell loss common to all of these cases. Black areas show the areas of overlap of the six reflex-interrupting cases from Figure 8. The first four cases are illustrated in Figure 7 For abbreviations, see Figure 5.

Fig. 10

Neuron counts for posteroventral cochlear nucleus (PVCN) lesions that either interrupted (dashed lines) or spared (thin solid lines) the medial olivocochlear reflex. Included are the six cases shown in Figure 8 and the six cases shown in Figure 9. A,B: Remaining neurons were counted in the dorsal half (A) or the ventral half (B) of the PVCN. Arrow points to the region with the largest difference in neuron counts between reflex interrupting and reflex sparing cases. Average counts from the opposite (noninjected) sides are shown as a thick solid line. Number of neurons is plotted as a function of section number, beginning with the most rostral section of PVCN (section 1) and proceeding caudally. The reflex-interrupting case with almost complete cell loss is Exp. 31. DCN, dorsal cochlear nucleus.

Fig. 11

Comparison of the effects of kainic acid (KA) on medial olivocochlear (MOC) reflex strength and auditory brainstem response (ABR). A: Data from a large lesion in posterior anteroventral cochlear nucleus (AVCN)/anterior posteroventral cochlear nucleus (PVCN) that caused large long-term decreases in ABR peaks P2–P5 but that did not cause any long-term decrease in MOC reflex strength. Data were taken 10.5–12 hours after injection. B: Scatter plot of the change in ABR peak P2 and the change in MOC Ipsi reflex strength. Stars indicate data from the reflex-interrupting cases; dots indicate reflex-sparing cases.

Fig. 12

Summary wiring diagram of the Ipsi medial olivocochlear (MOC) reflex. The reflex begins with hair cells of the cochlea and continues with the three neurons of the pathway: (1) auditory-nerve fibers from spiral ganglion cells, (2) multipolar cells of the posteroventral cochlear nucleus (PVCN), and (3) Ipsi MOC neurons, which terminate on outer hair cells (OHCs). IHC, inner hair cell.