Study of the olivocochlear neurons using two different tracers, fast blue and cholera toxin, in hypothyroid rats

Congenital hypothyroidism results in deafness that is caused by changes in the auditory receptor, including scanty development of the outer hair cells and a lack of synaptogenesis between these cells and the efferent system. although the afferent population is present. The normal efferent innervatio n of the cochlea originates in the superior olivary complex, arising from e fferent neurons belonging to the lateral or to the medial olivocochlear sys tem. In the rat, the former is constituted by neurons located in the latera l superior olivary nucleus, that project to the inner hair cells, while the later originates in the ventral nuclei of the trapezoid body and project t o the outer hair cells. The aim of this work is to study the localization, number and morphology of the olivochochlear neurons in congenital hypothyro id animals by means of the injections of the retrograde tracers, either fas t blue or cholera toxin, in the cochlea. The mean total number of labeled o livocochlear neurons after injection of fast blue in hypothyroid animals wa s 1,016, and in control ones was 1,027. Using cholera toxin, the mean total number of labeled olivocochlear neurons was slightly lower: 863 in hypothy roid animals versus 910 in control ones. Although both tracers showed no si gnificant differences between groups, when the somatic area of the labeled olivocochlear neurons is considered, the size of all of the three different population of cells (lateral olivocochlear neurons, medial olivocochlear n eurons and shell neurons) was significantly lower in the hypothyroid rats. This is the first study of the olivocochlear neurons in hypothyroid animals . The conclusion from this work is that in hypothyroid rats the labeled oli vocochlear neurons are significantly smaller but that there is not any modi fication in the localization and number of the labeled olivocochlear neuron s, suggesting that thyroid hormones are necessary for the neuronal growth. However, most of the medial olivocochlear neurons do not make contact with their target, so their maintenance suggests that the axons are in contact w ith other structures of the cochlea.