Effect of unilateral noise exposure on the tonotopic distribution of spontaneous activity in the cochlear nucleus and inferior colliculus in the cortically intact and decorticate rat

Abstract

Effects of unilateral noise exposure on spontaneous activity (SA) in the anteroventral and dorsal cochlear nuclei (AVCN and DCN) and the central nucleus of the inferior colliculus (ICc) were studied in cortically intact and decorticate rats. SA was measured 1 week following exposure using uptake of 14C-labeled 2-deoxyglucose (2DG) in quiet. Optical density (OD) measurements were obtained in low- and high-frequency (LF and HF) areas of each nucleus. We refer to the ipsilateral AVCN and DCN (side of the noise-exposed ear) and the contralateral ICc as direct nuclei and to their opposite side counterparts as indirect nuclei. Noise exposure altered the tonotopic profile of SA in the direct pathway by causing a decrease in the ratio of HF OD to LF OD (HF/LF ratio). In intact animals, the decreased HF/LF ratio was due to decreased HF OD. In decorticate animals, it was due to decreased HF OD and increased LF OD, the latter occurring mainly in the DCN and ICc. Decorticate-intact differences may reflect corticofugal feedback inhibition. Lesion of the dorsal acoustic stria caused a substantial decrement of SA in the contralateral ICc. Furthermore, strong positive correlations between HF/LF ratios in the DCN, AVCN, and contralateral ICc suggest that the cochlear nucleus is a major contributor to SA in the ICc. Noise exposure had opposite and weaker effects on 2DG uptake in the indirect pathway that were attributed to crossed inhibition. Noise-induced changes in the tonotopic profile of SA may represent a neural correlate of tinnitus.

Hyperactivity in the Ventral Cochlear Nucleus after Cochlear Trauma

“…Salvi et al (1978) reported a decrease in spontaneous firing rates in VCN, but this study only measured activity a short time after a relatively mild sound exposure and measured mainly from low-frequency neurons (64% lower than 4 kHz). Imig and Durham (2005) reported alterations in the regional distribution of deoxyglucose uptake in cat VCN 1 week after unilateral noise exposure, but changes in absolute levels of uptake were not evident. Another indirect measure of neural activity, manganeseenhanced magnetic resonance imaging, was reported by Brozoski et al (2007), who showed increases in posteroventral cochlear nucleus in rats with behaviorally demonstrated tinnitus.…”

Spontaneous activity in the inferior colliculus of CBA/J mice after manipulations that induce tinnitus

“…By contrast, autoradiographic imaging lacks physiological detail but can provide a more complete picture of activity levels in the major auditory nuclei. When this approach was used to measure the effects of unilateral sound exposure (Imig and Durham, 2005), the tonotopic distribution of 2-deoxyglucose (2DG) labeling was strongly correlated in the DCN, AVCN, and contralateral ICC. These results provide another source of support for the physiological links between midbrain activity and ascending inputs from the cochlear nucleus.…”

The dorsal cochlear nucleus as a participant in the auditory, attentional and emotional components of tinnitus

“…This condition of hyperactivity resembles activity that is elevated during sound stimulation and therefore seems a likely candidate for a tinnitus-producing signal. Sound-induced hyperactivity has now been observed in numerous other species including rats , chinchillas (Brozoski et al, 2002), guinea pigs (Imig and Durham, 2005) and mice (Kaltenbach et al, 2001). Hyperactivity has been induced in the DCN following prolonged exposure to both moderate and intense sounds, has been observed at both the single and multiunit levels Brozoski et al, 2002), and is not the result of increased activity in the auditory nerve (Zacharek et al, 2002;Liberman and Dodds, 1984).…”