Development of cochlear potentials in the neonatal gerbil.

The onset and maturation of hearing was examined in separate groups of sibling and nonsibling neonatal Mongolian gerbils (Meriones unguiculatus). Auditory nerve compound action potentials (CAP) and cochlear microphonics (CM) were measured at the round window, and the endocochlear potential (EP) was recorded at three different locations in pups aged 13 to 30 days after birth (DAB) and in 90 day-old animals. Maturational trends for the three potentials were similar to those previously reported for gerbil neonates. However, CAP thresholds continued to decrease, and CM and CAP input/output functions and EP continued to increase beyond 30 days of age, a time at which many investigators have considered hearing in the gerbil to be mature. The EP developed simultaneously throughout the cochlea and approached 80 mV by 20 DAB. CAP thresholds showed a highly correlated log-linear relationship with EP in groups of nonlittermates and in siblings studied at different ages. In contrast, maximum CAP and CM amplitudes increased with increasing EP, but did not show significant growth until the EP exceeded 70 mV.