Neither endocochlear potential nor Tegmentum vasculosum are a¡ected in hearing impaired Belgian Waterslager canaries O. Gleich a; *, R.J. Dooling b , B.M. Ryals c a ENT-Department, University of Regensburg, Franz-Josef-StrauM-Allee 11, 93042 Regensburg, Germany b Department of Psychology, University of Maryland, College Park, MD 20742, USA c Department of Communication Sciences and Disorders, James Madison University, Harrisonburg, VA 22807, USA Received 20 July 1999; received in revised form 13 December 1999; accepted 13 December 1999 Abstract We previously showed that the Belgian Waterslager canary strain is affected by a hereditary hearing loss that is associated with a reduced number of hair cells and hair cell pathologies in the basilar papilla. Since hair cell pathologies were also present in the sacculus, Weisleder et al. (1994) suggested that these birds are afflicted by Scheibe's like dysplasia, a cochleo-saccular defect. In mammals, cochleo-saccular defects are characterized primarily by the lack of an endocochlear potential and abnormalities in the Stria vascularis which only secondarily lead to hair cell loss (Steel and Bock, 1983; Steel, 1994; 1995). Here we report the endocochlear potential of six ears from three non-Belgian Waterslager canaries and three ears of two Belgian Waterslager canaries to decide if Waterslager canaries are affected by a cochleo-saccular or by a neuroepithelial defect. The mean endocochlear potential was 17.6 þ 2.5 mV in the non-Waterslager canaries and 20.3 þ 0.6 mV in Waterslager canaries. In addition, and consistent with the presence of a normal endocochlear potential, light microscopy of the tegmentum vasculosum provided no evidence for pathology. These data show that Belgian Waterslager canaries are affected by a neuroepithelial rather than a cochleo-saccular inner ear defect. ß 2000 Elsevier Science B.V. All rights reserved. Key words: Inner ear; Bird; Cochlear pathology; Hereditary hearing Loss; Endocochlear potential 1. Introduction A signi¢cant threshold elevation, especially at higher frequencies, has been demonstrated in a special strain of canary, the Belgian Waterslagers (BWS; Okanoya and Dooling, 1985, 1987; Okanoya et al., 1990). Physiolog- ical measurements revealed that threshold elevations are present in the periphery within the cochlea (Gleich et al., 1994b, 1995). Speci¢cally the observation of ele- vated thresholds of the cochlear microphonic potential in BWS demonstrates de¢cits at the level of or periph- eral to the transduction by hair cells, although middle ear transmission was una¡ected (Gleich et al., 1995). In birds and mammals the £uid in Scala media con- tains a high concentration of potassium and a low con- centration of sodium ions although the DC potential in Scala media di¡ers between both groups (see discussion in Runhaar et al., 1991). In mammals the normal endo- cochlear potential (EP) is around 80 mV (e.g. v. Bekesy, 1952; Runhaar and Manley, 1987; McGuirt et al., 1995). Lowering the EP acutely by loop diuretics leads to increased auditory thresholds (e.g. Sewell, 1984; Whitworth et al., 1993) and a chronic decline of the EP in aging gerbils is associated with increased auditory thresholds (Schulte and Schmiedt, 1992; Hellstrom and Schmiedt, 1996). Mills et al. (1996) showed that increas- ing the EP to 90 mV by current injection into Scala media of aged gerbils with a pathologically low EP can temporarily improve compound action potential thresholds. In addition, genetic defects that a¡ect the Stria vascularis and consequently the EP are associated with elevated thresholds (Steel et al., 1987). In birds the EP is much lower than in mammals, having values up to approximately 20 mV (Necker, 0378-5955 / 00 / $ ^ see front matter ß 2000 Elsevier Science B.V. All rights reserved. PII:S0378-5955(00)00006-X * Corresponding author. Tel.: +49 (941) 944 9426; Fax: +49 (941) 944 9402; E-mail: otto.gleich@klinik.uni-regensburg.de Hearing Research 142 (2000) 56^62 www.elsevier.com/locate/heares