The pharmacology of peripheral auditory processes; cochlear pharmacology

GluA2-Containing AMPA Receptors Distinguish Ribbon-Associated from Ribbonless Afferent Contacts on Rat Cochlear Hair Cells

Mechanosensory hair cells release glutamate at ribbon synapses to excite postsynaptic afferent neurons, via AMPA-type ionotropic glutamate receptors (AMPARs). However, type II afferent neurons contacting outer hair cells in the mammalian cochlea were thought to differ in this respect, failing to show GluA immunolabeling and with many “ribbonless” afferent contacts. Here it is shown that antibodies to the AMPAR subunit GluA2 labeled afferent contacts below inner and outer hair cells in the rat cochlea, and that synaptic currents in type II afferents had AMPAR-specific pharmacology. Only half the postsynaptic densities of type II afferents that labeled for PSD-95, Shank, or Homer were associated with GluA2 immunopuncta or presynaptic ribbons, the “empty slots” corresponding to ribbonless contacts described previously. These results extend the universality of AMPAergic transmission by hair cells, and support the existence of silent afferent contacts.

Sensory-evoked potentials in rats chronically exposed to trichloroethylene: predominant auditory dysfunction

Sensory-evoked potentials (EPs) were studied in male Long-Evans and Fischer-344 rats in order to characterize the electrophysiological consequences of chronic inhalation exposure to trichloroethylene (TCE). Groups of ten Long-Evans rats were exposed to air or 1600 ppm or 3200 ppm TCE for twelve weeks and evaluated periodically with a multisensory test battery. Brainstem auditory-evoked response (BAER) amplitudes were depressed by TCE, whereas somatosensory and visual potentials remained normal. The effects on BAERs, which varied with tone intensity and frequency, suggested that TCE causes a predominantly high-frequency hearing loss. Comparable effects were obtained in both strains of rats and were like those previously observed following exposure to toluene.

Hearing loss in rats caused by inhalation of mixed xylenes and styrene

We have reported that inhalation exposure of rats to toluene causes permanent hearing loss, e.g. Pryor et al. Neurobehav. Toxicol. Teratol. 5, 53-62 (1983). Therefore, it was of considerable interest to examine the ototoxic potential of two structurally related solvents--mixed xylenes and styrene--compared with that of toluene. Male, weanling Fischer-344 rats were exposed to clean air or solvents in four identical 62.5 l Plexiglas chambers. Exposures to 800, 1000, and 1200 ppm were daily for 14 hours/day and lasted 6 weeks for mixed xylenes, 3 weeks for styrene. An additional experiment with xylenes examined the effect of exposure for only 4 (1700 ppm) or 8 (1450 ppm) hours or for 8 (1450 ppm) hours on three consecutive days to compare the results with those obtained with toluene under comparable exposure schedules. Both xylenes and styrene caused marked hearing loss as assessed by behavioral (conditioned avoidance) and electrophysiologic (brainstem auditory-evoked response) methods. Moreover, both solvents appeared to be more potent ototoxicants than toluene, as indicated by effective concentration, effective durations of exposure, and the range of frequencies affected.

Neurochemistry of the auditory system

Two areas of auditory biochemistry are reviewed: the identification and characterization of neurotransmitters in the auditory system and the biochemical approach to the study of genetic hearing disorders. Studies to identify neurotransmitters at major auditory synapses are outlined. Evidence supporting glutamate or aspartate as the neurotransmitter for the auditory nerve is presented. The application of biochemistry to the study of genetic hearing disorders is discussed.

Effect of nicotine on cochlear function and noise-induced hair cell loss

Nicotine administered in high doses intravenously (1 mg/kg) and intracochlearly (10 mM) exhibited very little effect on guinea pig cochlear potentials. When administered chronically (1-20 mg/kg, i.p. for 20 days), nicotine demonstrated no effect on cochlear potentials or the number of observed hair cell scars monitored 21 days after the treatment was terminated. Neither chronic nicotine treatment (1-20 mg/kg, i.p. for 20 days) nor acute reserpine pretreatment (5 mg/kg, i.p. 24 hours before) altered the hair cell damaging effects of an intense pure tone (4 kHz, 126 dB SPL, 30 min). No nicotine ototoxicity was detected by the methods used in this study.