Effects of Electrical Stimulation of the Locus Coeruleus on Rat ABR

Abstract

The present study was designed to investigate how ABRs of the rats are influenced by experimental manipulations of central noradrenergic (NA) activities. Experiments were conducted on Wistar strain male rats anesthetized with α-chloralose. ABR waveform patterns had dependencies on electrode positions. A clear pattern with a distinct separation of each peak could be recorded when an electrode was placed on the midline of the skull close to the lambda. A reference electrode was placed on the tail or pinna. Clonidine, an NA α2-agonist, depressed the amplitudes of ABR at intravenous doses of 0.05-0.1mg/kg; larger changes in peaks IIb and IV were noted. These effects were antagonized by yohimbine, an NA α2-antagonist. Electrical stimulation of the locus coeruleus also depressed peak II and beyond in a current-dependent manner. The maximum inhibitory effect was observed at the conditioning-test stimulus interval of about 50msec. Yohimbine suppressed these changes, too. These results suggested that an NA system of α2-type, originating from the locus coeruleus, was involved in the efferent modulatory function on the auditory afferent system.