Olivocochlear neurons in the squirrel monkey brainstem

Centrifugal projections from the brain to the cochlea have been well described in rodents and cats. In order to gain a better understanding of the general mammalian features of this efferent projection system—the olivocochlear (OC) system—we have begun to extend its description to other mammalian orders, particularly primates. This report describes the origin, cellular morphology, and cholinergic nature of OC neurons in squirrel monkey. Olivocochlear neurons were identified after cochlear injection and subsequent retrograde transport of one of the tracers, horseradish peroxidase, True Blue, or Diamidino Yellow. One series of sections was processed to demonstrate the tracer and an adjacent series was processed to demonstrate acetylcholinesterase (AChE). In some cases, a series of sections was immunohistochemically processed to identify the presence of choline acetyltransferase (CAT), the synthesizing enzyme for acetylcholine. Approximately 1,700–1,800 OC neurons were contained in five distinct regions surrounding the major nuclei of the superior olivary complex (SOC), namely: (1) dorsal to medial superior olive (MSO); (2) between MSO and lateral superior olive (LSO); (3) lateral to LSO; (4) medial to SOC; and (5) in the ventral nucleus of the trapezoid body (VTB). These neurons were larger in the regions dorsal to MSO, lateral to LSO, and within VTB; they tended to be smaller in the regions between MSO and LSO and medial to SOC. Neuronal shapes varied among regions and included oval, elongate, round, and multipolar cells. In further support of their cholinergic nature as implied by AChE reactivity, OC neurons also stained positively for the cholinergic marker, CAT.