Neurochemical evidence for afferent GABAergic and efferent cholinergic neurotransmission in the frog vestibule.

Glutamate decarboxylase and choline acetyltransferase activities with magnitudes similar to those of their homologous enzymes in frog nervous tissue were found in homogenates of the frog labyrinth. Transection of the vestibular nerve resulted in a gradual diminution of choline acetyltransferase activity until it reached an 88% decrease 6 weeks after surgery. In contrast, glutamate decarboxylase activity did not suffer any alteration at any time after nerve excision. The presence of their enzymes of synthesis is evidence of the neurotransmitter participation of GABA and acetylcholine in the frog vestibule; the observed decrease of choline acetyltransferase following vestibule nerve excision supports the efferent synaptic bouton localization of choline acetyltransferase. The suggestion that glutamate decarboxylase is located in a cell type (or compartment) that may well be the hair cell is supported by the fact that this enzyme does not suffer any modification after surgery. These results are in accordance with an efferent cholinergic neurotransmission and a putative afferent role of GABA in the frog vestibule.