Plasticity of serotonergic innervation of the inferior colliculus in mice following acoustic trauma

Abstract

Acoustic trauma often results in permanent damage to the cochlea, triggering changes in processing within central auditory structures such as the inferior colliculus (IC). The serotonergic neuromodulatory system, present in the IC, is responsive to chronic changes in the activity of sensory systems. The current study investigated whether the density of serotonergic innervation in the IC is changed following acoustic trauma. The trauma stimulus consisted of an 8 kHz pure tone presented at a level of 113 dB SPL for six consecutive hours to anesthetized CBA/J mice. Following a minimum recovery period of three weeks, serotonergic fibers were visualized via histochemical techniques targeting the serotonin reuptake transporter (SERT) and quantified using stereologic probes. SERT-positive fiber densities were then compared between the traumatized and protected hemispheres of unilaterally traumatized subjects and those of controls. A significant effect of acoustic trauma was found between the hemispheres of unilaterally traumatized subjects such that the IC contralateral to the ear of exposure contained a lower density of SERT-positive fibers than the IC ipsilateral to acoustic trauma. No significant difference in density was found between the hemispheres of control subjects. Additional dimensions of variability in serotonergic fibers were seen among subdivisions of the IC and with age. The central IC had a slightly but significantly lowered density of serotonergic fibers than other subdivisions of the IC, and serotonergic fibers also declined with age. Overall, the results indicate that acoustic trauma is capable of producing modest but significant decreases in the density of serotonergic fibers innervating the IC.

Fig. 1

Control treatments remove SERT labeling in the IC. Sections at 20× processed in parallel from a single brain illustrating (A) normal staining of SERT fibers using primary antibody at 1:5000, (B) lack of fiber staining with omission of the primary antibody, (C) lack of fiber staining with omission of the secondary antibody, and (D) lack of fiber staining following preadsorption of the primary antibody at 1: 5000 with the control peptide. All images were normalized to the range of pixel values present.

Fig. 2

Serotonergic fiber staining in the inferior colliculus (IC). Center panel shows a transverse section of a typical mouse IC demonstrating SERT-positive fiber staining at 4× magnification. The three subdivisions of the IC (dorsal (ICd), external (ICe), and central (ICc)) are superimposed. Notice that the ICd as well as dorsomedial portions of the ICe and ICc regions contain darker staining, indicating a greater density of serotonergic fibers in these areas compared with more ventrolateral areas. Panels on the left and right show SERT-positive fiber staining in areas of the ICd and ICc subdivisions, respectively, at a magnification of 40×. A ‘space ball’ probe has been superimposed in each, appearing as a black 2D circle. Notice that at this plane of focus, SERT-positive fibers cross the perimeter of the space ball probe six times in the area of the ICd, but only three times in the area of the ICc.

Fig. 3

Mean SFLD in the dorsal cortex (ICd), external cortex (ICe), and central IC (ICc) subdivisions in both traumatized and control groups of mice. Densities in ICd and ICx were each significantly different from the central IC, but not from each other. Letters designate the outcome of a repeated measures ANOVA.

Fig. 4

Comparison of SFLD between the hemispheres within unilaterally traumatized and control subject groups. Individuals in the unilateral acoustically traumatized group had significantly lower SFLD in the right hemisphere (contralateral to acoustic trauma) compared to the left hemisphere. No significant differences were found between SFLD of the right and left hemispheres of control subjects. Circles represent individual subject values within each group. Boxes represent 25th and 75th percentiles. Whiskers represent 5th and 95th percentiles.

Fig. 5

Age-related decline in serotonergic fiber density. Age is represented in days at the time of perfusion. Circles represent untraumatized controls, and squares represent unilaterally traumatized animals. Serotonin fiber length density is averaged for all subdivisions of both colliculi.