Dianne Durham

Conductive Hearing Loss Results in a Decrease in Central Auditory System Activity in the Young Gerbil

Objectives/Hypothesis: The impact of childhood conductive HL (CHL) on development of auditory function has long been debated. The present study was conducted to define and compare the consequences of CHL and cochlear ablation (CA) in young and adult animals, using 2‐deoxyglucose (2‐DG) uptake as a measure of metabolic activity. It was hypothesized that, for both ages, CHL would result in a decrease in activity in the major ascending central auditory system pathway of the manipulated ear, but that this decrease would be significantly less than that observed with CA.

Round window administration of gentamicin: a new method for the study of ototoxicity of cochlear hair cells

Damage to inner ear sensory hair cells after systemic administration of ototoxic drugs has been documented in humans and animals. Birds have the ability to regenerate new hair cells to replace those damaged by drugs or noise. Unfortunately, the systemic administration of gentamicin damages both ears in a variable fashion with potentially confounding systemic drug effects. We developed a method of direct application of gentamicin to one cochlea of hatchling chickens, allowing the other ear to serve as a within-animal control. We tested variables including the vehicle for application, location of application, dosage, and duration of gentamicin exposure. After 5 or 28 days survival, the percent length damage to the cochlea and regeneration of hair cells was evaluated using scanning electron microscopy. Controls consisted of the opposite unexposed cochlea and additional animals which received saline instead of gentamicin. Excellent damage was achieved using gentamicin-soaked Gelfoam pledgets applied to the round window membrane. The percent length damage could be varied from 15 to 100% by changing the dosage of gentamicin, with exposures as short as 30 min. No damage was observed in control animals. Regeneration of hair cells was observed in both the base and apex by 28 days survival.

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.

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. J. Comp. Neurol. 490:391–413, 2005.

Effects of conductive hearing loss on gerbil central auditory system activity in silence

Animal models of conductive hearing loss (CHL) show altered structure and function in the central auditory system (CAS), particularly following unilateral deprivation. Assessment of neuronal activity as measured by 2-deoxyglucose (2-DG) uptake following CHL has been reported by two groups of investigators, with different findings. Woolf and colleagues [Brain Res. 274 (1983) 119] found that 2-DG uptake increased in the cochlear nucleus ipsilateral to the CHL, while Tucci et al. [Laryngoscope 109 (1999) 1359] found a decrease in 2-DG uptake in the ipsilateral cochlear nucleus. One significant difference between the protocols in the two studies was that, in the first study, animals were maintained in silence following 2-DG injection, whereas in the Tucci et al. study, animals were exposed to sound. The current study was designed to replicate the protocol used by Woolf et al. Young adult gerbils underwent unilateral malleus removal with bilateral canal ligation (n=6) or a sham procedure (n=7) 48 h prior to 2-DG administration and sacrifice. Optical density measurements were made from CAS nuclei. 2-DG uptake decreased in the ipsilateral cochlear nucleus and contralateral inferior colliculus, and in nuclei of the superior olivary complex bilaterally, supporting the finding that CHL is associated with a decrease in CAS neuronal activity.

Conductive Hearing Loss Results in Changes in Cytochrome Oxidase Activity in Gerbil Central Auditory System

Conductive hearing loss (CHL) restricts auditory input to an intact peripheral auditory system. Effects of deprivation on the central auditory system (CAS) have been debated, although a number of studies support the hypothesis that CHL can cause modification of CAS structure and function. The present study was designed to test the hypothesis that unilateral CHL results in a decrease in cytochrome oxidase (CO) activity in CAS nuclei that receive major afferent input from the affected ear. Gerbils at postnatal day 12 (P21) or 6–8 weeks underwent left unilateral CHL (malleus removal), cochlear ablation, or a sham surgical procedure. After a survival time of 48 hours or 3 weeks, animals were sacrificed and tissue was processed for cytochrome oxidase histochemistry. Optical density (OD) measurements were made from individual neurons in the anteroventral cochlear nucleus (AVCN) and from medial and lateral dendritic fields in the medial superior olivary nucleus (MSO), the lateral superior olivary nucleus, and the inferior colliculus. The width of the CO-stained neuropil in MSO was also measured as an estimate of dendritic length. OD measures were corrected to neutral areas of the brain. Cochlear ablation caused significant decreases in CO activity in left lower brainstem nuclei, particularly in adult animals. Following CHL, a significant decrease in CO activity was observed in the ipsilateral AVCN and a significant increase was observed in the contralateral AVCN. Cochlear ablation resulted in decreased width of MSO neuropil containing dendrites that receive primary input from the ablated ear. CHL resulted in a significant increase in the width of MSO neuropil on both sides of the brain in the P21 animals that survived 3 weeks but not in P21 animals that survived only 48 hours or in the adult animals. Unilateral CHL is associated with changes in CO activity in the AVCN and may affect MSO dendritic length in younger animals.

Breed differences in deafferentation-induced neuronal cell death and shrinkage in chick cochlear nucleus

Removal of functional presynaptic input can result in a variety of changes in postsynaptic neurons in the central nervous system, including altered metabolism, changes in neuronal cell size, and even death of the postsynaptic cell. Age-dependent neuronal cell death and shrinkage has been documented in second order auditory neurons in the chick brainstem (nucleus magnocellularis, NM) following cochlea removal (Born and Rubel, 1985. J. Comp. Neurol. 231, 435-445). Here we examined whether the extent of neuronal cell death and shrinkage is also breed-dependent. We performed unilateral cochlea removal on both hatchling and adult birds of either a broiler breed (Arbor Acres Cross) or egg layer breed (Hy-Line, H and N) and killed birds one week later. Changes in neuronal cell number and cross sectional area were determined from Nissl-stained sections. We observed 25% neuronal cell loss and a 15-20% decrease in neuronal cross sectional area after cochlea removal in either broiler or egg layer hatchling birds. In adult birds, however, neuronal cell loss is breed-dependent. Adult egg layer birds lose an average of 37% of NM neurons after cochlea removal, while adult broiler birds show no cell loss. In both breeds of adult birds, cochlea removal results in a 20% decrease in neuronal cross sectional area. These results suggest that analysis of differences between breeds as well as ages of birds will prove fruitful in determining how afferent input controls neuronal survival and metabolism.

Central nervous system plasticity during hair cell loss and regeneration

Following cochlear ablation, auditory neurons in the central nervous system (CNS) undergo alterations in morphology and function, including neuronal cell death. The trigger for these CNS changes is the abrupt cessation of afferent input via eighth nerve fiber activity. Gentamicin can cause ototoxic damage to cochlear hair cells responsible for high frequency hearing, which seems likely to cause a frequency-specific loss of input into the CNS. In birds, these hair cells can regenerate, presumably restoring input into the CNS. This review summarizes current knowledge of how CNS auditory neurons respond to this transient, frequency-specific loss of cochlear function. A single systemic injection of a high dose of gentamicin results in the complete loss of high frequency hair cells by 5 days, followed by the regeneration of new hair cells. Both hair cell-specific functional measures and estimates of CNS afferent activity suggest that newly regenerated hair cells restore afferent input to brainstem auditory neurons. Frequency-specific neuronal cell death and shrinkage occur following gentamicin damage to hair cells, with an unexpected recovery of neuronal cell number at longer survival times. A newly-developed method for topical, unilateral gentamicin application will allow future studies to compare neuronal changes within a given animal.

Consequences of Unilateral Hearing Loss: Cortical Adjustment to Unilateral Deprivation

The effect of unilateral hearing loss on 2-deoxyglucose (2-DG) uptake in the central auditory system was studied in postnatal day 21 gerbils. Three weeks following a unilateral conductive hearing loss (CHL) or cochlear ablation (CA), animals were injected with 2-DG and exposed to an alternating auditory stimulus (1 and 2kHz tones). Uptake of 2-DG was measured in the inferior colliculus (IC), medial geniculate (MG), and auditory cortex (fields AI and AAF) of both sides of the brain in experimental animals and in anesthesia-only sham animals (SH). Significant differences in uptake, compared to SH, were found in the IC contralateral to the manipulated ear (CHL or CA) and in AAF contralateral to the CHL ear. We hypothesize that these findings may result from loss of functional inhibition in the IC contralateral to CA, but not CHL. Altered states of inhibition at the IC may affect activity in pathways ascending to auditory cortex, and ultimately activity in auditory cortex itself. Altered levels of activity in auditory cortex may explain some auditory processing deficits experienced by individuals with CHL.

Evidence for loss and recovery of chick brainstem auditory neurons during gentamicin-induced cochlear damage and regeneration.

It is well documented that damage to the chick cochlea caused by acoustic overstimulation or ototoxic drugs is reversible. Second-order auditory neurons in nucleus magnocellularis (NM) are sensitive to changes in input from the cochlea. However, few experiments studying changes in NM during cochlear hair cell loss and regeneration have been reported. Chicks were given a single systemic dose of gentamicin, which results in maximal hair cell loss in the base of the cochlea after 5 days. Many new hair cells are present by 9 days. These new hair cells are mature but not completely recovered in organization by 70 days. We counted neurons in Nissl-stained sections of the brainstem within specific tonotopic regions of NM, comparing absolute cell number between gentamicin- and saline-treated animals at both short and long survival times. Our data suggest that neuronal number in rostral NM parallels hair cell number in the base of the cochlea. That is, after a single dose of gentamicin, we see a loss of both cochlear hair cells and NM neurons early, followed by a recovery of both cochlear hair cells and NM neurons later. These results suggest that neurons, like cochlear hair cells, can recover following gentamicin-induced damage.

Efficacy and Toxicity of Peritumoral Delivery of Nanoconjugated Cisplatin in an In Vivo Murine Model of Head and Neck Squamous Cell Carcinoma

IMPORTANCE Treatment of locally advanced head and neck squamous cell carcinoma (HNSCC) uses a multidisciplinary approach often limited by the toxicity and drug resistance of platinum agents. OBJECTIVES To test whether a nanocarrier-conjugated cisplatin boosting locoregional drug delivery improves tumor efficacy while decreasing systemic toxicity over systemic cisplatin in a murine model of locally advanced HNSCC. DESIGN A randomized, controlled, in vivo study compared standard cisplatin with nanocarrier (hyaluronan [HA])-conjugated cisplatin (HA-cisplatin) each at 50% of the maximum tolerated doses in a murine model of locally advanced HNSCC (10 mice/arm, each injected with 1 × 106 MDA-1986 HNSCC cells, with phosphate-buffered saline and HA-only control arms). Mice were treated for 3 weeks and observed for 3 additional weeks. SETTING Academic medical center. PARTICIPANTS Forty female Nu/Nu mice. Randomization and treatment arms were initiated once tumor volumes reached 30 mm3. INTERVENTION Injection with MDA-1986 HNSCC cells followed by 3 weeks of treatment with cisplatin, HA-cisplatin, phosphate-buffered saline, or HA only. MAIN OUTCOMES AND MEASURES Animal weights and tumor volumes were measured 3 times each week (modified RECIST [Response Evaluation Criteria in Solid Tumors]). At necropsy, animal kidneys were examined for nephrotoxic effects and cochleae were examined for ototoxic effects. RESULTS The mice treated with HA-cisplatin showed superior tumor efficacy (1 with complete clinical response, 3 with partial response, 1 with stable disease, and 5 with progressive disease) compared with standard cisplatin (no animals with complete clinical response, 1 with partial response, 1 with stable disease, and 8 with progressive disease), which was statistically significant (P = .003). All control animals developed progressive disease. Weight loss and body score were surrogate measures of treatment toxicity. The HA-cisplatin group had the least weight loss (mean [SD], 10.8% [4.7%]) compared with the cisplatin group (13.6% [5.6%]; P = .25). Body score dropped to 2 or less in all cisplatin-treated mice but not in any HA-cisplatin-treated mice, which also lacked any histologic signs of nephrotoxic or ototoxic effects. CONCLUSIONS AND RELEVANCE Nanoconjugated HA-cisplatin significantly improves tumor efficacy with lower toxicity compared with standard cisplatin in locally advanced HNSCC in vivo, justifying additional translational studies.