TONIC EFFERENT-INDUCED COCHLEAR DAMPING IN ROOSTING AND ECHOLOCATING MOUSTACHED BATS

The activity of the medial olivocochlear (MOC) efferent system in must ached bats, Pteranotus p. parnellii, was studied by monitoring changes in the mechanical properties of the cochlea. The changing properties were expressed by the decay time (DT) of cochlear microphonic potentia ls produced by transient-induced ringing (Henson et al., 1995). Tape-r ecorded roost noise (biosonar and communication sounds) produced sudde n, marked decreases in DT when presented to the contralateral ear of a nimals adapted to the quiet. When the animals were first removed from their roosts the DT was relatively short (1.2-1.5 ms) but this gradual ly lengthened by about 0.5-1.0 ms as they rested in a quiet chamber. T he time required to reach a stable, quiet-adapted state after noise ex posure varied with SPL and exposure time; in many experiments recovery was in the range of 90-120 min. When quiet-adapted bats were isolated and allowed to fly and echolocate for 20 min, the DTs measured within a few minutes after the end of the flight were also short and only sl owly returned to longer preflight values. The administration of a sing le dose of gentamicin, which blocks MOC effects, greatly reduced the a mount of suppression (damping) observed after periods of noise and ech olocation sound exposure. We conclude that tonic MOC activity is induc ed by the natural vocalizations and roost noise and this activity prob ably regulates and protects the highly resonant cochlear partition. (C ) 1998 Elsevier Science B.V. All rights reserved.