Dopaminergic olivocochlear neurons originate in the high frequency region of the lateral superior olive of guinea pigs.

Dopaminergic neurons are known to exist within the lateral superior olive (LSO). The LSO is the nucleus of origin of the lateral olivocochlear neurons, which project to the cochlea and synapse onto the primary afferents contacting the inner hair cells. We investigated whether the dopaminergic neurons in the LSO are part of the lateral olivocochlear neuron population. We combined intracochlear injections of a fluorescent retrograde tracer with immunofluorescent staining of tyrosine hydroxylase (TH). TH was used as a marker for dopaminergic neurons. After the injection with retrograde tracer most of the TH-labelled neurons in the LSO also contained the tracer, which directly demonstrates for the first time that the TH-labelled, dopaminergic neurons in the LSO are lateral olivocochlear neurons. TH-labelled neurons were not equally distributed over the LSO as is observed for the lateral olivocochlear neurons in general. TH-labelled neurons were almost exclusively seen in the medial, high frequency, limb of the LSO. Since the projection of the lateral olivocochlear neurons to the cochlea is known to be tonotopic, we investigated the TH-labelling in the cochlea as well. We found that the staining pattern of TH in the cochlea is in broad agreement with the distribution of TH-labelling in the LSO. Cochlear sections showed dense labelling in the basal and second, high frequency, turns and decreasing intensity of staining in the third turn, while the extreme apical, low frequency, turn was almost devoid of any positive TH-labelling. These observations imply that the dopaminergic neurons of the lateral olivocochlear system may play a role in the selective suppression of the high frequency fibers of the auditory system.