Olivocochlear neurons in the chinchilla: a retrograde fluorescent labelling study

Although the chinchilla is widely used as a model for auditory research, li ttle is known about the distribution and morphology of its olivocochlear ne urons. Here, we report on the olivocochlear neurons projecting to one cochl ea, as determined by single and double retrograde fluorescent tracer techni ques. 10 adult chinchillas were anesthetized and given either unilateral or bilateral injections of a fluorescent tracer (either Fluoro-Gold or Fast B lue) into scala tympani or as a control, a unilateral injection into the mi ddle ear cavity. The results indicate that there are similarities as well a s significant differences between the chinchilla and other species of roden ts in the distributions of their olivocochlear neurons. Based on three well -labelled cases, there was a mean total of 1168 olivocochlear neurons in th e chinchilla. Of these, the majority (mean 787) were small, lateral olivoco chlear neurons found almost exclusively within the ipsilateral lateral supe rior olivary nucleus. The next largest group consisted of a mean of 280 med ial olivocochlear neurons virtually all of which were located in the dorsom edial peri-olivary nucleus. Chinchilla medial olivocochlear neurons were mo re predominantly crossed in their projections (4:1) than in any known speci es. The smallest group of olivocochlear neurons (mean 101) consisted of lar ger lateral olivocochlear neurons (shell neurons) which were located on the margins of the superior olivary nucleus and which projected mainly (2.2:1) ipsilaterally. Double retrograde labelling was observed only in medial oli vocochlear neurons and occurred in only 1-2% of these cells. The results co nfirm previous findings which indicated a relative paucity of fibers belong ing to the uncrossed as compared to the crossed olivocochlear bundle. This, together with the strong apical bias of the uncrossed projection reported previously, offers possible explanations for the apparent absence of effere nt-mediated suppressive effects of contralateral acoustic stimulation in th is species. Regarding the lateral olivocochlear system, the chinchilla is s hown to possess both intrinsic and shell neurons, as in the rat. (C) 1999 E lsevier Science B.V. All rights reserved.