Transmitter neurochemistry of the efferent neuron system innervating the labyrinth

It is likely that several mechanisms contribute to the efferent control of cochlear and vestibular function. Different effects are probably mediated by different neuronal transmitters. In spite of a number of transmitter candidates, it is still widely assumed that the entire efferent system can be globally characterized as cholinergic. We attempted to label retrogradely identified efferent neurons in the brainstem with a monoclonal antibody against choline acetyltransferase (ChAT), the acetylcholine (ACh) synthesizing enzyme. Only a portion of the vestibular efferents could thus be shown to be cholinergic in the rat. Medial cochlear efferents, terminating under outer hair cells, may also be cholinergic since they stain intensely for acetylcholine esterase (AChE) after pre-treatment with the AChE inhibitor diisopropylfluorophosphate (DFP). The lateral cochlear efferents terminating under inner hair cells, as well as more than half of the vestibular efferent neuron population, reacted negatively with either method designed to identify cholinergic neurons. Half of the lateral olivo-cochlear neuron population filled retrogradely with tritiated gamma-amino butyric acid [( 3H]-GABA). These cells were similar in size and distribution to neurons staining for the GABA synthesizing enzyme glutamic acid decarboxylase (GAD). Retrograde transport of [3H]-aspartate from the inner ear to the brainstem was seen in half of the lateral olivocochlear population, as well as in part of the efferent vestibular population in group E and in the caudal pontine reticular nucleus (CPR). Since various peptides have also been located in efferent neurons, this system is chemically diversified. Several distinct mechanisms of efferent control with presumably differing functions must, therefore, exist.