The pharmacology of neurotransmission in semicircular canals

Current evidence has established acetylcholine as the efferent neurotransmitter in the inner ear vestibular system and an excitatory amino acid, possibly glutamate, as the afferent neurotransmitter. The efferent neurotransmitter exerts a biphasic effect on vestibular sensory cell activity and the actions of the afferent transmitter are modulated by adenosine through negative feedback control of release. The leading candidate for the role of afferent neurotransmitter in labyrinthine systems is glutamate. In the semicircular canal glutamate, in moderate doses, causes an increase in evoked and spontaneous activity. This effect is blocked by glutamate blockers such as d-alpha amino adipate. However, there is some evidence to suggest that glutamate acts on the sensory cells themselves rather than postsynaptically on the afferent neuronal dendrites. Other neurochemicals such as GABA have also been proposed to the afferent neurotransmitter but, the evidence for these is not nearly as strong as that supporting glutamate´s candidacy.<>