Quantitative distribution of choline acetyltransferase activity in rat trapezoid body

Abstract

There is evidence for a function of acetylcholine in the cochlear nucleus, primarily in a feedback, modulatory effect on auditory processing. Using a microdissection and quantitative microassay approach, choline acetyltransferase activity was mapped in the trapezoid bodies of rats, in which the activity is relatively higher than in cats or hamsters. Maps of series of sections through the trapezoid body demonstrated generally higher choline acetyltransferase activity rostrally than caudally, particularly in its portion ventral to the medial part of the spinal trigeminal tract. In the lateral part of the trapezoid body, near the cochlear nucleus, activities tended to be higher in more superficial portions than in deeper portions. Calculation of choline acetyltransferase activity in the total trapezoid body cross-section of a rat with a comprehensive trapezoid body map gave a value 3-4 times that estimated for the centrifugal labyrinthine bundle, which is mostly composed of the olivocochlear bundle, in the same rat. Comparisons with other rats suggest that the ratio may not usually be this high, but it is still consistent with our previous results suggesting that the centrifugal cholinergic innervation of the rat cochlear nucleus reaching it via a trapezoid body route is much higher than that reaching it via branches from the olivocochlear bundle. The higher choline acetyltransferase activity rostrally than caudally in the trapezoid body is consistent with evidence that the centrifugal cholinergic innervation of the cochlear nucleus derives predominantly from locations at or rostral to its anterior part, in the superior olivary complex and pontomesencephalic tegmentum.

Keywords: Acetylcholine; Acetylcholinesterase; Auditory; Cochlear nucleus; Hearing.

Figure 1

Photomicrographs of 10 µm-thick transverse sections stained for acetylcholinesterase (AChE) activity at 4 caudal-to-rostral locations relative to the caudal limit of the right trapezoid body in rat 62582C, shown as millimeter distance at upper left of each photo. Black double-arrowhead line between upper and lower photos represents 1 mm in all 4 sections. In photos, dorsal is up and lateral to the right. AChE-stained fiber bundles that cross the trapezoid body (T) include some crossing below the spinal trigeminal tract (S), which have been reported to consist of parasympathetic salivatory system fibers (P, Shute and Lewis, 1960; Brown and Howlett, 1968, 1971). Fiber bundles within the vestibular nerve root consist of olivocochlear and centrifugal vestibular fibers that innervate the auditory and vestibular portions of the inner ear labyrinth, collectively termed the centrifugal labyrinthine bundle (L, Godfrey et al., 1984). The large fiber bundle medial to the spinal trigeminal tract is the facial nerve root (F). Since the compositions of all photos are similar, the abbreviation for a feature is given in just one, either 0.96 mm or 1.16 mm, where a feature is well represented. Other abbreviations are A, anteroventral cochlear nucleus; C, cerebellum (molecular layer, with granular layer to its right), and V, vestibular ganglion.

Figure 2

Distribution of choline acetyltransferase (ChAT) activity in a series of 28 10-µm thick transverse freeze-dried sections spaced at 60 µm intervals through the right trapezoid body of rat 62582C. Dorsal direction is up and lateral to the right in each section, and 1 mm scale is at the top. The distance rostral to the caudal limit of the trapezoid body is indicated in millimeters (mm) to the left of every other section, along with the relative percentage distance from the caudal to the rostral trapezoid body limit. The total extent of the right trapezoid body in this rat was 1.65 mm. Solid black lines indicate sample cut boundaries. Dashed black lines indicate regional boundaries, including the ventral boundary of the spinal trigeminal tract (STT) dorsal to the trapezoid body. Activities are color-coded as indicated at lower left. Gray areas correspond in location to prominent parasympathetic fiber bundles staining darkly for acetylcholinesterase (AChE) activity in adjacent sections, some of which are shown in Fig. 1 (0.96 mm and 1.00 mm locations). ChAT activities for 6 samples in those areas averaged, as mean ± SEM, 1057 ± 215 µmol/kg dry wt/min. The activity for the one sample in the gray area of the section at 0.95 mm was 1211 µmol/kg dry wt/min, and the activities for two samples in the gray area of the section at 1.01 mm were 371 and 1114 µmol/kg dry wt/min. Gray dots and lines within some samples indicate the presence of AChE-positive fiber bundles in adjacent stained sections. Zones where the auditory nerve root (ANR) is present and where the facial nerve root (FNR) and AChE-positive parasympathetic fiber bundles (PF) cross the trapezoid body are indicated on the right. Fibers of the centrifugal labyrinthine bundle (Godfrey et al, 1984) cross the lateral part of the trapezoid body between 0.84 and 1.12 mm rostral to the caudal limit of the trapezoid body -almost exactly the same sections as those in which the parasympathetic fiber bundles cross more medially. The outline of the anteroventral cochlear nucleus (A) is included in the most rostral section.

Figure 3

Plots of average choline acetyltransferase (ChAT) activity vs. percentage of the caudal-to-rostral extent of the right trapezoid body of rat 62582C (maps shown in Fig. 2) and the left trapezoid body of rat 40885A. Averages for samples located laterally, close to the cochlear nucleus, are shown by filled circles, and averages for samples located more medially, between the lateral samples and the medial limit of the spinal trigeminal tract, are shown by empty circles. An extra filled circle at the 76 % location for rat 62582C is the average of lateral samples in one section, not shown in Fig. 2, wherein there were no data for medial samples. Averages were calculated by multiplying individual sample weights in each zone by their ChAT activities, adding the resulting values, then dividing by the total weight of the samples included. Samples corresponding in location to where acetylcholinesterase (AChE)-positive fiber bundles were seen in adjacent AChE-stained sections were excluded from the averages. Some measurements were made for samples located in fiber tracts caudal (% < 0) and rostral (% > 100) to the trapezoid body, corresponding to spinocerebellar and pontocerebellar fibers, respectively.