The rat cochlea in the absence of circulating adrenal hormones: an electrophysiological and morphological study

Circulating adrenal hormones affect strial function. Removal of endogenous levels of adrenal steroids by bilateral adrenalectomy (ADX) in rats causes a decrease of Na+/K+-ATPase activity in the cochlear lateral wall [Rarey et al., 1989. Arch. Otolaryngol. Head Neck Surg. 115, 817-821] and a decrease of the volume of the marginal cells in the stria vascularis [Lohuis et al. , 1990. Acta Otolaryngol. (Stockh.) 110, 348-356]. To study further the eff ect of absence of circulating adrenocorticosteroids on cochlear function, 1 8 male Long Evans rats underwent either an ADX or a SHAM operation. Electro cochleography was performed 1 week after surgery for tone bursts in a frequ ency range of 1-16 kHz. Thereafter, the cochleas were harvested and examine d histologically. No significant changes in the amplitude growth curves of the summating potential (SP), the compound action potential (CAP) and the c ochlear microphonics (CM) were detected after ADX. However, visually, there appeared to be a decrease of endolymphatic volume (tentatively called imdr ops). Reissner's membrane (RM) extended less into scala vestibuli in ADX an imals than in SHAM-operated animals. The ratio between the length of RM and the straight distance between the medial and lateral attachment points of RM were used as an objective measure to quantify this effect in each sub-ap ical half turn of the cochlea. The decrease in length of RM was statistical ly significant. Thus, circulating adrenal hormones appear to be necessary f or normal cochlear fluid homeostasis. Absence of one or more of these hormo nes leads to shrinkage of the scala media (imdrops). However, the absence o f adrenal hormones does not affect the gross cochlear potentials. Apparentl y, the cochlea is capable of compensating for the absence of circulating ad renal hormones to sustain the conditions necessary for proper cochlear tran sduction. (C) 2000 Elsevier Science B.V. All rights reserved.