Olivocochlear neurons in the chinchilla: a retrograde fluorescent labelling study.

Although the chinchilla is widely used as a model for auditory research, little is known about the distribution and morphology of its olivocochlear neurons. Here, we report on the olivocochlear neurons projecting to one cochlea, as determined by single and double retrograde fluorescent tracer techniques. 10 adult chinchillas were anesthetized and given either unilateral or bilateral injections of a fluorescent tracer (either Fluoro-Gold or Fast Blue) into scala tympani or as a control, a unilateral injection into the middle ear cavity. The results indicate that there are similarities as well as significant differences between the chinchilla and other species of rodents in the distributions of their olivocochlear neurons. Based on three well-labelled cases, there was a mean total of 1168 olivocochlear neurons in the chinchilla. Of these, the majority (mean 787) were small, lateral olivocochlear neurons found almost exclusively within the ipsilateral lateral superior olivary nucleus. The next largest group consisted of a mean of 280 medial olivocochlear neurons virtually all of which were located in the dorsomedial peri-olivary nucleus. Chinchilla medial olivocochlear neurons were more predominantly crossed in their projections (4:1) than in any known species. The smallest group of olivocochlear neurons (mean 101) consisted of larger 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 olivocochlear neurons and occurred in only 1-2% of these cells. The results confirm previous findings which indicated a relative paucity of fibers belonging 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 efferent-mediated suppressive effects of contralateral acoustic stimulation in this species. Regarding the lateral olivocochlear system, the chinchilla is shown to possess both intrinsic and shell neurons, as in the rat.

Noradrenergic and serotonergic projections to the superior olive: potential for modulation of olivocochlear neurons.

The distribution and density of noradrenergic (NA) and serotonergic (5-HT) varicosities in the superior olive (SO) and periolivary region (PO) and their relationship to olivocochlear neurons was studied. Antibodies against 5-HT and the NA precursor enzyme dopamine beta-hydroxylase were utilized to examine the density of innervation of SO and PO. To determine the relationship of these varicosities to efferent neurons projecting to the cochlea, olivocochlear neurons were retrogradely labeled with biotinylated dextranamine (BDA). NA and 5-HT varicosities were found adjacent to labeled olivocochlear neuron cell bodies and dendrites. More than 50% of labeled medial olivocochlear (MOC) neurons showed likely contact with NA varicosities and more than 90% of labeled MOC neurons with 5-HT varicosities. There was no apparent difference in the number of lateral olivocochlear (LOC) neurons in close proximity to NA and 5-HT varicosities versus MOCs in close proximity to NA and 5-HT varicosities. Our results suggest that the NA and 5-HT systems are in a position to modulate auditory brainstem processing. The specific relationship of NA and 5-HT varicosities to olivocochlear neurons suggests that one possible level of modulation is prior to signal transduction.

Inferior colliculus stimulation and changes in 2f1-f2 distortion product otoacoustic emissions in the rat.

The external, central and dorsal subnuclei of the inferior colliculus (ICX, ICC, ICD respectively) have different patterns of descending projections to the periolivary region. We found that electrical stimulation of these subnuclei in anesthetized rats causes suppression of 2f1-f2 distortion product otoacoustic emissions (DPOAE). The responses in DPOAEs to stimulation of ICX and ICC are characterized by a large (7-25 dB), initial suppression which lasted 25-30 s (early phase) followed by a sustained smaller (3-15 dB) suppression (late phase). ICX stimulation produces the largest suppressions of DPOAEs in both contralateral and ipsilateral ears equally. Stimulation of the ICC causes large suppressions in the contralateral ear DPOAEs but much smaller than the suppressions of DPOAEs caused by ICX stimulation. Stimulation of the ICC causes a much smaller suppression in the ipsilateral ear DPOAEs than in the contralateral ear DPOAEs. ICD stimulation, however, produces little or no suppression of DPOAEs in either ear when compared to other subnuclei. No frequency-specific changes in DPOAEs were seen with stimulation of any of the subnuclei of the inferior colliculus (IC). Sectioning of middle ear muscles did not negate the suppressive effects of IC stimulation on DPOAEs. These findings verify that the IC has a subnucleus-specific influence over cochlear micromechanics.