Turn-specific and pigment-dependent differences in the stria vascularis of normal and gentamicin-treated albino and pigmented guinea pigs

Assessment of nutrient supplement to reduce gentamicin-induced ototoxicity

Gentamicin is an aminoglycoside antibiotic used to treat gram-negative bacterial infections. Treatment with this antibiotic carries the potential for adverse side effects, including ototoxicity and nephrotoxicity. Ototoxic effects are at least in part a consequence of oxidative stress, and various antioxidants have been used to attenuate gentamicin-induced hair cell death and hearing loss. Here, a combination of nutrients previously shown to reduce oxidative stress in the hair cells and attenuate hearing loss after other insults was evaluated for potential protection against gentamicin-induced ototoxicity. Guinea pigs were maintained on a nutritionally complete standard laboratory animal diet or a diet supplemented with β-carotene, vitamins C and E, and magnesium. Three diets with iterative increases in nutrient levels were screened; the final diet selected for study use was one that produced statistically reliable increases in plasma levels of vitamins C and E and magnesium. In two separate studies, significant decreases in gentamicin-induced hearing loss at frequencies including 12 kHz and below were observed, with less benefit at the higher frequencies. Consistent with the functional protection, robust protection of both the inner and outer hair cell populations was observed, with protection largely in the upper half of the cochlea. Protection was independently assessed in two different laboratories, using two different strains of guinea pigs. Additional in vitro tests did not reveal any decrease in antimicrobial activity with nutrient additives. Currently, there are no FDA-approved treatments for the prevention of gentamicin-induced ototoxicity. The current data provide a rationale for continued investigations regarding translation to human patients.

Trafficking of systemic fluorescent gentamicin into the cochlea and hair cells

Aminoglycosides enter inner ear hair cells across their apical membranes via endocytosis, or through the mechanoelectrical transduction channels in vitro, suggesting that these drugs enter cochlear hair cells from endolymph to exert their cytotoxic effect. We used zebrafish to determine if fluorescently tagged gentamicin (GTTR) also enters hair cells via apically located calcium-sensitive cation channels and the cytotoxicity of GTTR to hair cells. We then examined the serum kinetics of GTTR following systemic injection in mice and which murine cochlear sites preferentially loaded with systemically administered GTTR over time by confocal microscopy. GTTR is taken up by, and is toxic to, wild-type zebrafish neuromast hair cells. Neuromast hair cell uptake of GTTR is attenuated by high concentrations of extracellular calcium or unconjugated gentamicin and is blocked in mariner mutant zebrafish, suggestive of entry via the apical mechanotransduction channel. In murine cochleae, GTTR is preferentially taken up by the stria vascularis compared to the spiral ligament, peaking 3 h after intra-peritoneal injection, following GTTR kinetics in serum. Strial marginal cells display greater intensity of GTTR fluorescence compared to intermediate and basal cells. Immunofluorescent detection of gentamicin in the cochlea also revealed widespread cellular labeling throughout the cochlea, with preferential labeling of marginal cells. Only GTTR fluorescence displayed increasing cytoplasmic intensity with increasing concentration, unlike the cytoplasmic intensity of fluorescence from immunolabeled gentamicin. These data suggest that systemically administered aminoglycosides are trafficked from strial capillaries into marginal cells and clear into endolymph. If so, this will facilitate electrophoretically driven aminoglycoside entry into hair cells from endolymph. Trans-strial trafficking of aminoglycosides from strial capillaries to marginal cells will be dependent on as-yet-unidentified mechanisms that convey these drugs across the intra-strial electrical barrier and into marginal cells.

Absence of strial melanin coincides with age-associated marginal cell loss and endocochlear potential decline

Cochlear stria vascularis contains melanin-producing intermediate cells that play a critical role in the production of the endocochlear potential (EP) and in maintaining the high levels of K(+) that normally exist in scala media. The melanin produced by intermediate cells can be exported to the intrastrial space, where it may be taken up by strial marginal cells and basal cells. Because melanin can act as an antioxidant and metal chelator, evidence for its role in protecting the stria and organ of Corti against noise, ototoxins, and aging has long been sought. While some evidence supports a protective role of melanin against noise and ototoxins, no evidence yet presented has demonstrated a clear role for melanin in maintaining the EP during aging. We tested this by comparing basal turn EPs and a host of cochlear cellular metrics in aging C57BL/6 (B6) mice and C57BL/6-Tyr(c-2J) mice. The latter mice carry a naturally occurring inactivating mutation of the tyrosinase locus, and produce no strial melanin. Because these two strains are coisogenic, and because pigmented B6 mice show essentially no age-related EP decline, they provide an ideal test of importance of melanin in the aging stria. Pigmented and albino B6 mice showed identical rates of hearing loss and sensory cell loss. However, after two years of age, basal turn EPs significantly diverged, with 42% of albinos showing EPs below 100 mV versus only 18% of pigmented mice. The clearest anatomical correlate of this EP difference was significantly reduced strial thickness in the albinos that was highly correlated with loss of marginal cells. Combined with findings in human temporal bones, plus recent work in BALB/c mice and gerbils, the present findings point to a common etiology in strial presbycusis whereby EP reduction is principally linked to marginal cell loss or dysfunction. For any individual, genetic background, environmental influences, and stochastic events may work together to determine whether marginal cell density or function falls below some critical level, and thus whether EP decline occurs.

Cellular correlates of age-related endocochlear potential reduction in a mouse model

Age-related degeneration of cochlear stria vascularis and resulting reduction in the endocochlear potential (EP) are the hallmark features of strial presbycusis, one of the major forms of presbycusis, or age-related hearing loss (ARHL) (Schuknecht, H.F., 1964. Further observations on the pathology of presbycusis. Archives of Otolaryngology 80, 369-382; Schuknecht, H.F., 1993. Pathology of the Ear. Lea and Febiger, Philadelphia; Schuknecht, H.F., Gacek, M.R., 1993. Cochlear pathology in presbycusis. Annals of Otology, Rhinology and Laryngology 102, 1-16). It is unclear whether there are multiple forms of strial ARHL having different sequences of degenerative events and different risk factors. Human temporal bone studies suggest that the initial pathology usually affects strial marginal cells, then spreads to other strial cell types. While inheritance studies support a moderate genetic influence, no contributing genes have been identified. Establishment of mouse models of strial ARHL may promote the identification of underlying genes and gene/environment interactions. We have found that BALB/cJ mice show significant EP reduction by 19 months of age. The reduction only occurs in a subset of animals. To identify key anatomical correlates of the EP reduction, we compared several cochlear lateral wall metrics in BALBs with those in C57BL/6J (B6) mice, which show little EP reduction for ages up to 26 months. Among the measures obtained, marginal cell density and spiral ligament thickness were the best predictors of both the EP decline in BALBs, and EP stability in B6. Our results indicate that the sequence of strial degeneration in BALBs is like that suggested for humans. Additional strain comparisons we have performed suggest that genes governing strial melanin production do not play a role.

A novel RFP-RET transgenic mouse model with abundant eumelanin in the cochlea

We report on the cochlea of a novel metallothionein-I (MT)/RFP-RET transgenic mouse model with severe systemic melanosis. Electron microscopy revealed that these transgenic mice possess abundant quantities of melanin in the intermediate cells of the stria vascularis. High performance liquid chromatography analysis indicated that cochleae of these transgenic mice contained about twice as much eumelanin as cochleae of control C57BL/6 mice and that the amount of pheomelanin was approximately equal in these two strains. Auditory brainstem responses at 2, 4, 8, and 16 kHz were not significantly different between transgenic and control mice. This is the first report on a mouse model of overproduction of cochlear eumelanin, and our results suggest that this transgenic mouse is an excellent model for investigating the effects of overexpression of cochlear eumelanin. In addition, we provide evidence that eumelanin overproduction in the cochlea does not affect normal hearing.

Sound needs sound melanocytes to be heard

Intermediate cells in the stria vascularis of the mammalian cochlea are melanocytes, which contain melanin pigments and are capable of synthesizing melanin. These melanocytes are required for normal development of the cochlea, as evidenced by studies of mutant mice with congenital melanocyte anomalies. Melanocytes are also needed for developed cochleae to function normally, as evidenced by studies of mutant mice with late-onset melanocyte anomaly and humans with acquired melanocyte anomaly. Melanin, per se, does not seem to be essential for normal hearing function, but it may protect against traumata to the cochlea, e.g., noise and ototoxic aminoglycosides. Recent electrophysiological studies have revealed that strial intermediate cells are provided with specific ionic channels, such as inwardly rectifying K+ channels (Kir4.1) and voltage-dependent outwardly rectifying K+ channels. These channels may play central roles in strial function and thus in normal hearing.

Ultrastructural localization of megalin in the rat cochlear duct

Megalin is an endocytic receptor predominantly expressed in the kidney proximal tubule cells. In the present study, localization of megalin was examined using a post-embedding immunogold method in the rat cochlear duct. Marginal cells of the stria vascularis were labeled on the apical surface, but not on the basolateral surface. This localization pattern resembles kidney proximal tubule cells. Immunoreactivity was also detected on various other cells, including epithelial cells of the spiral prominence and epithelial cells of Reissner's membrane. In contrast, virtually no gold particles were seen on intermediate cells and basal cells of the stria vascularis, mesothelial cells of Reissner's membrane or fibrocytes in the lateral wall. Also unlabeled were cells in the tympanic wall of the cochlear duct, including sensory cells and supporting cells of the organ of Corti. The present findings show the involvement of megalin in endocytosis of marginal cells and are suggestive of different uptake mechanisms for aminoglycosides in the kidney proximal tubule cells and in the cochlear sensory cells.

Postnatal vascular development in the lateral wall of the cochlear duct of gerbils: quantitative analysis by electron microscopy and confocal laser microscopy

The development of the capillary network in the stria vascularis and in the underlying spiral ligament of gerbils was systematically and quantitatively investigated by conventional electron microscopy and confocal laser microscopy in association with vascular labeling with fluorescent gelatin. The developmental changes of capillaries in the lateral wall were observed as the following series of events. (i) At 0 days after birth (DAB) capillaries already existed in the spiral ligament as a network. (ii) At 3-9 DAB the capillary network developed into two layers starting from the scala vestibuli side to the scala tympani side; one layer was located in the stria and the other in the spiral ligament. (iii) At 9 DAB capillaries in the stria became separated from the spiral ligament, and the capillary network consisting of a two-layered structure was complete. (iv) Total capillary length and capillary density in the lateral wall increased until 9 DAB and leveled off thereafter, but changes in the relative position of capillaries in the stria toward the luminal surface of marginal cells continued until 31 DAB. On the basis of the above observations, we propose two possible mechanisms underlying the vascular development in the lateral wall: (i) the formation of new vasculature (angiogenesis), and (ii) changes in the position of cellular components relative to capillaries in association with the differentiation and maturation of marginal cells and intermediate cells.

Quantitative differences in endolymphatic calcium and endocochlear potential between pigmented and albino guinea pigs

A number of previous studies have suggested that melanin may play a role in Ca2+ homeostasis of endolymph. In the present study, endolymph Ca2+ levels and endocochlear potential (EP) were measured in all four cochlear turns of pigmented or albino guinea pigs. Auditory sensitivity was also evaluated using cochlear action potential (AP) thresholds. In pigmented animals we found that endolymph Ca2+ tended to increase from base to apex of the cochlea, while EP systematically decreased towards the apex. In contrast, no significant Ca2+ gradient was found in albinos and the EP decline was far less. As a result, the apical turn of albino animals had significantly lower Ca2+ and significantly higher EP than in pigmented animals. AP thresholds pooled across all test frequencies were significantly lower in albino animals although no differences at individual frequencies reached significance. Even after correction for EP differences, the endolymph Ca2+ levels in albino animals were significantly lower than in pigmented ones. These results confirm that there are significant physiologic differences between pigmented and albino animals, which are a likely consequence of the absence of melanin in the albino cochlea. They are consistent with the involvement of melanin in the active transport of Ca2+ into endolymph.

Differential effects of gentamicin on the distribution of cochlear function in albino and pigmented guinea pigs

It has been suggested that the high affinity of melanin pigment for aminoglycoside antibiotics may cause these drugs to bind preferentially to the pigmented inner ear, producing greater ototoxicity than in the amelanotic albino cochlea. However, evidence of greater ototoxicity in albinos has led to the hypothesis that melanin inhibits the toxicity of these drugs in the pigmented inner ear. On the other hand, ototoxicity in the pigmented animals may simply be delayed relative to the albinos, only to become equal or even more severe with time. The present study was conducted to determine whether a relatively low dose of gentamicin (68.5 mg/kg) would produce differential ototoxicity between albino and pigmented guinea pigs which would persist long after drug exposure had stopped. Nine pigmented and eight albino guinea pigs were given gentamicin sulfate for 14 consecutive days, and were then allowed a two-month recovery period before cochlear analysis; 11 pairs of saline-injected or untreated albino and pigmented guinea pigs served as controls. The results showed that the gentamicin-treated albinos had significantly elevated thresholds for the compound action potential from the auditory nerve (CAP), and significantly lower endocochlear potentials (EP) and cochlear microphonic (CM) input-output voltage functions when compared to their respective controls, or to either group of pigmented guinea pigs. The CAP in drug-treated pigmented animals did not differ significantly from controls, and the differences in EP and CM were marginally significant. The results indicate that the pigmented cochlea is less susceptible to gentamicin than the albino cochlea, and support the hypothesis that melanin may inhibit aminoglycoside ototoxicity in the pigmented inner ear.

Distribution and features of melanocytes during inner ear development in pigmented and albino rats

In this developmental study, the distribution and features of melanocytes in the inner ear of pigmented and albino rats was investigated with the use of an antibody, which specifically reacts with a melanocyte differentiation antigen present in the membranes of (pre)melanosomes. Melanocyte precursors could be traced from 13 days post conception onwards and the course was followed to their targets in the inner ear. Melanocytes which settle in the modiolus appeared to reach their target along another pathway than strial and vestibular melanocytes. No difference was observed in the melanocyte distribution between pigmented and albino rats. The integration of melanocytes into the stria vascularis was associated with an increased rate of melanosome production in both strains, but in the albinos far fewer melanosomes were produced. After the stria had reached maturity, melanosome production was arrested and melanosomes were subject to lysosomal digestion. In the stria of the pigmented rats, cells with aggregations of disintegrating melanosomes appeared and persisted into adulthood. In the adult, the majority of the intermediate cells contained only a few scattered melanosomes, while melanosomes could only rarely be detected in the albinos. These observations indicate that there is a close relationship between melanosome production and the process of interdigitation of melanocytes with the marginal cells. It seems unlikely that melanosomes or melanin make any important contribution to the function of the adult stria vascularis. Outside the stria, the features of melanocytes in both strains were similar to skin melanocytes.

Dysgenesis of melanocytes and cochlear dysfunction in mutant microphthalmia (mi) mice

In order to evaluate the cytological homology of intermediate cells and melanocytes, and to investigate the function of melanocytes in the inner ear, hearing acuity and cochlear pathology were studied in three strains of mice, namely, wild type mice (+/+), albino mice without melanin (c2J/c2J), and microphthalmia mice with no melanocytes (mibw/mibw). Our histochemical data indicated that intermediate cells showed cytological characteristics almost identical to those of melanocytes and that disorders of melanin and/or melanocytes were reflected in the stria vascularis of each mouse. While c2J/c2J presented the same normal hearing acuity and normal structure of the stria vascularis as +/+, the hearing acuity of mibw/mibw mutants was severely impaired. Their stria vascularis was abnormally thin, lacking intermediate cells. According to these results, lack of melanin has little influence on hearing acuity; however, the absence of intermediate cells or melanocytes causes severe hearing loss, presumably due to a strial dysfunction.

Ongoing proliferation of melanocytes in the stria vascularis of adult guinea pigs

The intermediate cells of the stria vascularis are melanocytes derived from the neural crest. These internalized pigment cells have been thought to be a static population, distinct from those found in the skin. However, this investigation demonstrates that the melanocytes of the adult stria vascularis undergo continuous replication. Cell proliferation was studied using [3H]-thymidine autoradiography and bromodeoxyuridine (BrdU) immunohistochemistry. Single or multiple injections of [3H]-thymidine within a six hour period labeled a mean of 9 intermediate cells. In pigmented guinea pigs, single daily injections of [3H]-thymidine for 2 or 4 days labeled a mean of 24 and 69 intermediate cells, respectively; Pigmented guinea pigs given BrdU once daily for 2 or 4 days labeled a mean of 38 and 75 intermediate cells. By contrast, albino littermates also given BrdU averaged only 23 and 42 labeled intermediate cells in the same 2 and 4 day experiments. The mean number of proliferating cells/mm of stria per 24 h was 1.54 in the pigmented animals and 0.88 in the albinos. Both the total number and density of labeled intermediate cells were significantly smaller in the albino than the pigmented guinea pigs. These results demonstrate that the melanocytes in the stria vascularis undergo continuous baseline mitosis, and at a rate comparable to the melanocytes of the skin. This surprising similarity promotes the speculation that the proliferative rate of the strial melanocytes may be influenced by some of the same factors known to affect replication and pigment production in the skin.

Albino and pigmented gerbil auditory function: influence of genotype and gentamicin

Auditory function of albino and pigmented gerbils was examined before and after treatment with the ototoxic aminoglycoside antibiotic gentamicin. Auditory brainstem responses (ABRs) and cochlear nerve compound action potentials (CAPs) were measured in response to pure tones having frequencies between 2 and 32 kHz. Age-matched albinos had significantly lower CAP, but not ABR, thresholds than pigmented gerbils. Gentamicin treatment elevated CAP and ABR thresholds in both genotypes, but pigmented gerbils were less severely affected. Compared to the ABR, the CAP is a more sensitive measure of ototoxicity and pigmentation differences. CAP tuning curves (TCs) were another sensitive measure of genotypic differences in susceptibility to ototoxicity. TC tip thresholds from pigmented animals given gentamicin were not as elevated as the TC tip thresholds of albinos.

Pigment-dependent differences in the stria vascularis of albino and pigmented guinea pigs and rats

Functional models of the stria vascularis (SV) have ascribed roles for the marginal and basal cells, but not for the intermediate cells, which remain poorly understood. Intermediate cells have been identified as melanocytes, which produce melanin in most pigmented animals including humans. The relationship of melanin to intermediate cell function may be addressed through comparisons with the albino inner ear. Albinos have a normal distribution of melanocytes that are unable to synthesize melanin pigment. In the present study, the SV was compared between albino and pigmented littermates in both the guinea pig and the rat. Photomicrographic montages of the SV were analyzed from each of 7 cochlear regions in the guinea pig and 5 regions in the rat. Stereological procedures were used to determine the volume density (Vv) for each of the three main cell types in the stria, the surface density (Sv) of the marginal cells, and to derive estimates of absolute cell volume and surface area. In the guinea pig, comparisons between pigment groups showed that marginal cell Vv was larger across cochlear turns in the albinos, while intermediate cell Vv was smaller. Intermediate cell cytoplasmic and total cell volumes were smaller in the albino guinea pigs; however, marginal cell Sv and absolute area were larger. In the rat, intermediate cell Vv was alos smaller across cochlear turns in the albinos. Similarly, intermediate cell cytoplasmic and total cell volumes were smaller in the albinos, while marginal cell total surface area per radial cross-section of the SV was larger. These results demonstrate that amelanotic melanocytes occupy significantly less volume than do pigmented melanocytes, and suggest that melanin may influence the structure and function of the SV.

Turn-specific differences in the endocochlear potential between albino and pigmented guinea pigs

Recent findings indicate that structural differences exist in the stria vascularis (SV) between albino and pigmented guinea pigs. In the higher cochlear turns, volume density for marginal cells in the albino SV is abnormally large, while that for intermediate cells (melanocytes) is abnormally small. These anatomical variations suggest that functional differences between albino and pigmented inner ears also may be found. To examine this possibility, four strains of guinea pigs were studied, consisting of Hartley albino (N = 9) and NIH outbred pigmented (N = 15) guinea pigs, as well as albino (N = 11) and pigmented (N = 15) guinea pig siblings born to mixed litters. Tracheotomy and carotid artery cannulation were performed. Animals were mechanically ventilated, with periodic samples drawn for arterial blood gas analysis. Blood pressure, heart rate and rectal temperature were monitored. Compound action potentials were measured first to assess cochlear viability. Positive endocochlear potentials (+EP) then were recorded, beginning with the fourth turn, followed by the first, second and third turns. Results showed that the +EP in albinos remained relatively constant across cochlear turns, but decreased significantly from base to apex in the pigmented inner ears. Across all animals, mean +EPs (mV +/- S.E.M.) for turns 1-4 in albinos were: 72.5 (2.5), 68.7 (2.3), 59.2 (2.7), 68.1 (3.3); pigmented values were: 72.9 (2.9), 66.9 (2.6), 53.8 (3.0), 57.0 (2.7). One-way ANOVAs did not show a significant difference in albino +EPs between any of the cochlear turns, but did indicate a highly significant difference between turns in the pigmented inner ears (P < 0.000004). Post hoc comparisons demonstrated +EPs in turns 3 and 4 were smaller than in turn 1. Since turn 3 was recorded last in these experiments, and was reduced in value relative to turn 4 in both groups, it is likely that cochlear deterioration contributed to this result more than any other factor. These results, combined with previous anatomical data, indicate that a diminution of melanocyte cell volume in the albino SV is accompanied by an increase in marginal cell volume density and larger +EPs in the higher cochlear turns, at least at resting levels.