Optical mapping of neural responses and their gamma-aminobutyric acid-ergic inhibitory effects in the auditory brainstem of early postnatal mice.

gamma-Aminobutyric acid (GABA)ergic neurons play important tropic and modulatory roles in the auditory pathway, especially in the early stage between postnatal Days 0 and 5. The effects of GABA and GABAa receptor antagonist were observed in this experimental study. Numerous histological and electrophysiological studies have been performed on the contribution of GABA to the auditory pathway; however, the spatio-temporal patterns of excitatory propagation and the relationships between GABA receptor and excitatory propagation have yet to be reported. Using an optical recording technique and a voltage-sensitive dye, the spatio-temporal patterns of excitatory propagation were observed in the auditory brainstem slices of early postnatal mice. A bath containing 50 microM GABA was applied, which largely inhibited the excitatory activities along the vestibulocochlear pathway. Bicuculline methiodide (BMI), a competitive antagonist against GABAa receptor, partially reversed the effects of GABA on the optical signals. Bath application of BMI alone helped to facilitate the depolarization course and its effect was apparent as an enlargement of the depolarized region from the cochlear nucleus and vestibular nucleus to some adjacent brainstem nuclei, as well as enhancing the amplitude of changes in the optical signals. The experimental results seem to suggest that GABAa receptors are widely distributed in an early postnatal auditory brainstem. GABA exhibited a greater modulating effect in the adjacent brainstem nuclei, which are involved in complex information processes, than that observed in the modulating primary auditory pathway. In the present experiment, significant GABAergic contributions to the optical recordings in the auditory brainstem were observed.

Optical imaging of glycinergic inhibition in the vestibular and cochlear nuclei.

Using multiple-site optical recording techniques, the spatiotemporal activity was observed in both the cochlear and vestibular nucleus in newborn mice. The optical responses were obviously enhanced by bath-applied strychnine. A quantitative analysis showed higher enhancements to occur in the cochlear nucleus than in the vestibular nucleus. Optical imaging enables us to visualize the spatiotemporal extent of the inhibitory receptive field after the application of strychnine.