Spatial representation of frequency in the rat dorsal nucleus of the lateral lemniscus as revealed by acoustically induced c- fos mRNA expression

Abstract

The conventional view, based largely on studies in cats, holds that the dorsal nucleus of the lateral lemniscus (DNLL) is tonotopically organized with a dorsal (low-frequency) to ventral (high-frequency) representation. Based on the topography of projections between the DNLL and inferior colliculus, it has been proposed that the rat DNLL has a concentric, inside-to-outside, tonotopic organization with high frequencies represented along the rind and low frequencies represented in the core. We used acoustic stimulation and c- fos mRNA expression to examine this issue. Results suggest that the rat DNLL does have a crude tonotopic organization and that this tonotopy has a concentric component. Following high-frequency stimulation, labeled neurons were found most frequently along the margins of DNLL, although they also tended to be more concentrated ventrally. Many fewer neurons labeled following middle-frequency stimulation, and these tended to be more uniformly distributed throughout the nucleus. Still fewer neurons labeled after low-frequency stimulation and these tended to be scattered mostly in the dorsal half of the nucleus. We conclude that: (i) many more neurons in the rat DNLL are responsive to high-frequency than to low-frequency acoustic stimulation; and (ii) that the frequency representation of the rat DNLL has both concentric and dorsal-to-ventral components.