The Journal of Neurophysiology Vol. 85 No. 1 January 2001, pp. 84-88
Copyright ©2001 by the American Physiological Society

Fast, But Not Slow, Effects of Olivocochlear Activation Are Resistant to Apamin

 ¹Eaton-Peabody Laboratory, Massachusetts Eye and Ear Infirmary; and  ²Department of Otology and Laryngology and  ³Program in Neuroscience, Harvard Medical School, Boston, Massachusetts 02114; and  ⁴Department of Otolaryngology, Tohoku University, School of Medicine, Sendai 980-8574, Japan

Yoshida, Naohiro, M. Charles Liberman, M. Christian Brown, and William F. Sewell. Fast, But Not Slow, Effects of Olivocochlear Activation Are Resistant to Apamin. J. Neurophysiol. 85: 84-88, 2001. Olivocochlear (OC) efferent suppression of auditory-nerve responses comprises a fast effect lasting tens of milliseconds and a slow effect building and decaying over tens of seconds. Both fast and slow effects are mediated by activation of the same alpha 9 nicotinic receptor. We have hypothesized that fast effects are generated at the OC synapse, but that slow effects reflect activation of calcium-activated potassium (K_Ca) channels by calcium release from the subsurface cisternae on the basolateral wall of the hair cells. We measured in vivo effects of apamin, a blocker of small-conductance (SK) K_Ca channels, and charybdotoxin, a blocker of large-conductance K_Ca channels, perfused through scala tympani, on fast and slow effects evoked by electrical stimulation of the OC bundle in anesthetized guinea pigs. Apamin selectively and reversibly reduced slow-effect amplitude without altering fast effects or baseline amplitude of the auditory-nerve response, but only when perfused at concentrations of 100 µM. In contrast, the effects of charybdotoxin were noted at 30 nM, but were not specific, reducing both afferent and efferent responses. The very high concentrations of apamin needed to block efferent effects contrasts with the high sensitivity of isolated hair cells to apamin's block of acetylcholine's effects. The results suggest that in vivo fast OC effects are dominated by a conductance that is not apamin sensitive.