Stimulation by cochlear implant in unilaterally deaf rats reverses the decrease of inhibitory transmission in the inferior colliculus

Abstract

In the last decade, numerous studies have investigated synaptic transmission changes in various auditory nuclei after unilateral cochlear injury. However, few data are available concerning the potential effect of electrical stimulation of the deafferented auditory nerve on the inhibitory neurotransmission in these nuclei. We report here for the first time the effect of chronic electrical stimulation of the deafferented auditory nerve on alpha1 subunit of the glycinergic receptor (GlyRalpha1) and glutamic acid decarboxylase (GAD)67 expression in the central nucleus of inferior colliculus (CIC). Adult rats were unilaterally cochleectomized by intracochlear neomycin sulphate injection. Fifteen days later, the ipsilateral auditory nerve was chronically stimulated either 4, 8 or 22 h daily, for 5 days using intracochlear bipolar electrodes. GlyRalpha1 and GAD67 mRNA and protein were quantified in the CIC using in situ hybridization and immunohistofluorescence methods. Our data showed that as after surgical ablation, GlyRalpha1 and GAD67 expression were strongly decreased in the contralateral CIC after unilateral chemical cochleectomy. Most importantly, these postlesional down-modulations were significantly reversed by chronic electrical stimulation of the deafferented auditory nerve. This recovery, however, did not persist for more than 5 days after the cessation of the deafferented auditory nerve electrical stimulation. Thus, downregulations of GlyRalpha1 and GAD67 may be involved both in the increased excitability observed in the CIC after unilateral deafness and consequently in the tinnitus frequently observed in unilateral adult deaf patients. Electrical stimulation of the deafferented auditory nerve in patients may be a potential new approach for treating tinnitus with unilateral hearing loss.