CHOLINE-ACETYLTRANSFERASE IMMUNOREACTIVE NEURONS INNERVATING LABYRINTHINE AND LATERAL LINE SENSE-ORGANS IN AMPHIBIANS

The goal of the present study was to investigate aspects of the centra l organization of the neurons belonging to the octavolateralis efferen t system of amphibians. The perikarya of three genera, Pleurodeles, Xe nopus, and Discoglossus, were located in the brainstem by applying ret rograde tracers to the appropriate cranial nerves and choline acetyltr ansferase immunohistochemistry was used to identify cholinergic neuron s. The efferent neurons supplying lateral line (Pleurodeles, Xenopus) and labyrinthine (Pleurodeles, Xenopus, and Discoglossus) end organs w ere found to intermingle in a single octavolateralis efferent nucleus. The neurons lie bilateral to the labelled nerves in Pleurodeles and i psilateral in Xenopus and Discoglossus. Separate labelling of the ante rior and posterior octavus rami provided no evidence for distinct grou pings of efferent neurons that could be associated with auditory and v estibular end organs. In all three species many if not all octavolater al efferent neurons displayed immunoreactivity for choline acetyltrans ferase. They could be distinguished from the cholinergic facial motone urons, with which they sometimes intermingle, on the basis of either t heir distinctive size and shape (Pleurodeles, Xenopus) or their locati on (Discoglossus). Double labelling in Xenopus confirmed the cholinerg ic nature of the efferent neurons.