Setsuko Takeda

Endolymphatic hydrops induced by chronic administration of vasopressin

Recently, many lines of evidence have supported the possibilities that vasopressin (VP) is closely linked to the formation of endolymphatic hydrops in Menieres disease. In the present study, it was examined whether or not the chronic administration of VP might induce endolymphatic hydrops. For this purpose, histological studies and VP radioimmunoassay were independently performed in 20 and 40 guinea pigs, respectively. The degree of hydrops was quantitatively assessed by the increase ratio (IR) of the scala media area in the mid-modiolar sections of the cochlea. The IR was defined by the following equation: 100x(A-B)/B (A: the cross-sectional area of the bulging scala media; B: the no-bulging scala media, enclosed by an idealized straight Reissners membrane). VP was administered at the rates of 200 microU/kg/min, 400 microU/kg/min and 1000 microU/kg/min for 1 week via the osmotic mini-pump. The IR of the total of the apical, second, third and basal turns (means+/-S.D.s) were 4.4+/-0.7, 10.4+/-1.8, 17.4+/-7.9 (n=10 ears, each) in respective doses of VP. Comparing with that of the control animals (5.2+/-1.7, n=10 ears), the area increased significantly in the VP dosage of 400 and 1000 microU/kg/min (Bonferronis method, P<0.05). Plasma VP concentrations produced by the VP administration in these dosages were 2.2+/-0.4, 3.5+/-0.8 and 14.0+/-3.9 (n=10, each) pg/ml. Although 3.5 pg/ml is the upper limit of plasma VP concentration in normal human subjects, 14.0 pg/ml was almost the same concentration as those observed in the acute phase of Menieres disease (Takeda et al., 1995). Therefore, the formation of endolymphatic hydrops in cases of Menieres disease might be caused by high concentrations of plasma VP.

The effects of V2 antagonist (OPC-31260) on endolymphatic hydrops.

In the present study, two experiments were performed to investigate the influence of OPC-31260 on experimentally induced endolymphatic hydrops in guinea pigs and the regulation of aquaporin-2 (AQP2) mRNA expression in the rat inner ear. In morphological studies, the increases in the ratios of the length of Reissners membrane (IR-L) and the cross-sectional area of the scala media (IR-S) were quantitatively assessed among normal guinea pigs (normal ears) and three groups with hydropic ears: hydropic ears with no infusion (non-infusion hydropic ears), hydropic ears with an infusion of physiological saline into the scala tympani (saline-infused hydropic ears) and hydropic ears with infusion of 0.3% OPC-31260 into the scala tympani (OPC-infused hydropic ears). IR-Ls in the experimental groups were markedly larger than in the normal ear group, but there was no significant difference among the groups of non-infusion hydropic ears, saline-infused hydropic ears and OPC-infused hydropic ears. The IR-Ss of non-infusion hydropic ears and saline-infused hydropic ears (48.8-49.3%) were statistically different from that of normal ears (6.5%) (Dunnet multiple comparison test, P<0.01). However, IR-S of the OPC-infused hydropic ears (-14.8%) was significantly smaller than those of non-infusion hydropic ears and saline-infused hydropic ears (one-way ANOVA, P<0.01). In the quantitative polymerase chain reaction study, a comparison of the ratio of AQP2 and beta-actin mRNA (MAQP2/Mbeta-actin) was made between water-injected and OPC-31260-injected rats. An intravenous injection of OPC-31260 resulted in a significant decrease in MAQP2/Mbeta-actin both in the cochlea and in the endolymphatic sac (t-test, P<0.001). These results indicate that water homeostasis in the inner ear is regulated via the vasopressin-AQP2 system, and that the vasopressin type-2 antagonist OPC-31260 is a promising drug in the treatment of Menieres disease.

Hormonal Aspects of Ménière’s Disease on the Basis of Clinical and Experimental Studies

Conclusions: Endolymph homeostasis is thought to be mediated by the vasopressin-aquaporin-2 (VP-AQP2) system in the inner ear. Endolymphatic hydrops, the morphological characteristics of Ménière’s disease (MD), seems to reflect the malregulation of the VP-AQP2 system in inner ear fluid. The elevation of plasma vasopressin (p-VP) level, which is often observed in MD and its related diseases, might be one of the causative factors underlying these diseases. Purpose of Review: Review of the role of the VP-AQP2 system in the inner ear fluid homeostasis and in the formation and development of endolymphatic hydrops. Recent Clinical and Experimental Findings: A clinical survey has revealed that the p-VP level is often elevated in MD and its related diseases and that the increase in the p-VP level was closely linked to vertigo attacks in MD. Experimental studies have revealed that proteins and mRNAs of aquaporin-2 and vasopressin type 2 receptor were expressed in the stria vascularis of the cochlea and the epithelium of the endolymphatic sac, and that the volume of the endolymphatic compartment was mediated by the activity of the VP-AQP2 system in the inner ear.

Expression of aquaporins, vasopressin type 2 receptor, and Na+-K+-Cl– cotransporters in the rat endolymphatic sac

Conclusion. Since many water channels and pumps, such as aquaporin (AQP) 1–4, AQP6–9, vasopressin type 2 receptor (V2-R), Na+-K+-Cl– cotransporter (NKCC) 1, and NKCC2 were expressed in the rat endolymphatic sac, it should play an important role in the physiological function of acid-based metabolism and water balance in endolymphatic fluid homeostasis. Objective: To evaluate the expression and immunolocalization of AQP1–9, V2-R, NKCC1, and NKCC2 in the rat endolymphatic sac. Materials and methods. Wistar rats were used. Expression of AQP1–9, V2-R, NKCC1, and NKCC2 mRNA in the rat endolymphatic sac was investigated using the reverse transcription-polymerase chain reaction (RT-PCR) method, and detailed immunolocalization of AQP1–9, V2-R, NKCC1, and NKCC2 was investigated using immunohistochemical methods, including immunofluorescence microscopy. Results. mRNAs and proteins of AQP1–4, AQP6–9, V2-R, NKCC1, and NKCC2 were expressed, but AQP5 was not expressed in the rat endolymphatic sac.

Effects of lithium on endolymph homeostasis and experimentally induced endolymphatic hydrops

There is evidence to suggest that water homeostasis in the inner ear is regulated via the vasopressin (VP)-aquaporin 2 (AQP2) system in the same fashion as in the kidney. The VP-AQP2 system in the kidney is well known to be inhibited by lithium, resulting in polyuria due to a decrease in reabsorption of water in the collecting duct of the kidney. Therefore, lithium is also likely to inhibit the VP-AQP2 system in the inner ear, and consequently exert some influence on inner ear fluid homeostasis. In this study, we investigated the effects of lithium on AQP2 expression in the rat inner ear, and on the cochlear fluid volume in hydropic ears of guinea pigs. A quantitative PCR study revealed that lithium reduced AQP2 mRNA expression in the cochlea and endolymphatic sac. Lithium application also decreased the immunoreactivity of AQP2 in the cochlea and endolymphatic sac. In a morphological study, lithium intake significantly reduced endolymphatic hydrops dose-dependently. These results indicate that lithium acts on the VP-AQP2 system in the inner ear, consequently producing a dehydratic effect on the endolymphatic compartment.

Decompression effects of erythritol on endolymphatic hydrops.

OBJECTIVE The effect of the uptake of erythritol with and without the addition of pectin on fecal condition, p-OSM and p-AVP levels, and the endolymphatic volume was investigated to consider the possibility that erythritol is applicable as a therapeutic agent for Menieres disease. MATERIALS AND METHODS Two experiments were performed using 100 female Hartley guinea pigs. Experiment 1 was designed to morphologically investigate the influence of the uptake of erythritol with or without the addition of pectin on the endolymphatic volume. Experiment 2 was designed to investigate changes in p-OSM and p-AVP levels after the uptake of erythritol with or without the addition of pectin. RESULTS (1) Endolymphatic hydrops significantly decreased after the uptake of a mixture of erythritol and pectin, but did not decrease after the uptake of pectin or erythritol (p<0.001). (2) The fecal condition was muddy in all animals with the uptake of erythritol alone, but muddy or very soft feces were not observed in animals with a mixture of pectin and erythritol. p-AVP and p-OSM levels were significantly elevated in animals with the uptake of erythritol alone or a mixture of erythritol and pectin. Notably, the increase in p-AVP and p-OSM levels was significantly more evident in animals with the uptake of erythritol alone (one-way ANOVA, p<0.001). CONCLUSIONS The addition of pectin almost completely suppressed erythritol-induced diarrhea. Consequently, the secondary elevation of p-AVP and p-OSM due to diarrhea was also reduced. The uptake of a mixture of erythritol and pectin markedly decompressed endolymphatic hydrops, although the uptake of erythritol alone did not. The difference of the decompression effect between animal groups with the uptake of erythritol alone and a mixture of erythritol and pectin seemed to be attributable to the difference of p-AVP levels due to diarrheal state.

A comparison of dehydration effects of V2-antagonist (OPC-31260) on the inner ear between systemic and round window applications

V2-antagonist (OPC-31260 (OPC)) application to the scala tympani reduced endolymphatic hydrops. In the present study, we investigated whether systemic administration or local infusion via the round window (RW application) of OPC would be more suitable for clinical use. In Experiment 1, the increase ratios of the cross-sectional area of the scala media of experimentally induced endolymphatic hydrops were quantitatively assessed among four groups of non-OPC application, RW application of xanthan gum, systemic application of OPC and RW application of OPC. In Experiment 2, the effects of systemic and RW applications of OPC on plasma vasopressin (p-VP) concentrations and plasma osmolality (p-OSM) were investigated. In Experiment 3, endocochlear DC potential (EP) was measured in guinea pigs with the RW application of OPC. Electron microscopic observations of the stria vascularis and the hair cells were also made. Both systemic and RW applications of OPC significantly reduced endolymphatic hydrops. However, systemic application resulted in the distension of the Reissners membrane in the non-operated ear, which seemed to be caused by elevated p-VP levels resulting from the systemic application of OPC. In contrast, RW application of OPC produced no apparent toxic effects in the inner ear, as indicated electrophysiological or morphological changes. Thus, drug delivery via the round window is more useful for the clinical application of OPC for medical decompression.

The Rebound Phenomenon of Glycerol-Induced Changes in the Endolymphatic Space

Volumetric changes of the scala media were histologically investigated in normal guinea pigs to see whether glycerol-induced volumetric change of the scala media followed a biphasic course similar to the auditory threshold in the glycerol test. Glycerol was administered orally in a 12 ml/kg dose. The volume of the scala media was assessed by examining the cross-sectional area of the scala media in the mid-modiolar sections of the cochlea. Histological study revealed that the time-course of the change in the volume of the scala media after glycerol intake showed biphasic changes. Specifically, the early phase of the glycerol effect is a decrease in endolymph volume. The volume of the scala media significantly decreased by 11.4+/-2.9% 2 h after glycerol intake. Thereafter, the volume began to increase, and reached its peak 6-12 h after intake. In addition, the volume of the scala media significantly increased by 17.6+/-1.1% after 6 h. The present study indicated that the secondary increase in the volume of the scala media following glycerol intake played an important role in the rebound phenomenon in the glycerol test, although the mechanism underlying the hearing loss with the endolymphatic hydrops remains to be elucidated.

Time course of dehydrating effects of isosorbide on experimentally induced endolymphatic hydrops in guinea pigs.

Osmotic diuretics are therapeutic agents used to reduce endolymphatic hydrops. However, glycerol-induced change in endolymph volume is followed by a rebound phenomenon. In this study, we investigated the rebound phenomenon occurring with isosorbide, an osmotic diuretic used as a therapeutic agent for Ménière’s disease in Japan. Forty guinea pigs underwent surgical obliteration of the endolymphatic sac. Thirty received isosorbide orally 1 month after surgery. These animals were sacrificed 3, 6, or 12 h after isosorbide intake. The remaining 10 animals served as controls. Quantitative assessment of changes in the endolymphatic space was performed light-microscopically. Isosorbide reduced cochlear endolymph volume, with a peak reduction 6 h after intake. Thereafter, no prominent rebound phenomenon was noted. Clinically, since isosorbide is orally administered every 8 h, rebound phenomenon need not be considered in the treatment with isosorbide.

An Animal Model of Ischemic Facial Palsy

We investigated facial palsy which was induced by the interruption of the petrosal artery in guinea pigs. Forty animals were observed for 2 months regarding their behavioral facial nerve function and assessed by the blink reflex. Morphological changes in the intratemporal portion were observed with transmission electron microscopy in 20 animals with an interrupted petrosal artery. Facial palsy developed in 85.0% within 3 days after the interruption. The degree of palsy varied from mild to severe. Remission of palsy required 2–3 months in severe cases, 3 weeks or less in mild/moderate cases. Histological studies revealed a striking difference in the degree of degenerative changes between severe cases and mild/moderate cases. Animals with severe palsy showed extensive axonal atrophy and myelin disruption from the early stage. Meanwhile, degenerative changes were slight in cases with mild/moderate palsy. Regenerating unmyelinated fibers appeared 1 week after the interruption, but diminished in number 4 weeks later. Thereafter, new myelin was reformed on fibers. In cases of severe nerve damage, however, this regeneration process did not always seem to work well. A decrease in number and an irregular shape of the fibers were noted in animals with incomplete recovery. This animal model may be helpful for understanding the pathophysiology of ischemic facial palsy.