Inhibitory neurotransmission in animal models of tinnitus: maladaptive plasticity

Abstract

Tinnitus is a phantom auditory sensation experienced by up to 14% of the United States population with a smaller percentage experiencing decreased quality of life. A compelling hypothesis is that tinnitus results from a maladaptive plastic net down-regulation of inhibitory amino acid neurotransmission in the central auditory pathway. This loss of inhibition may be a compensatory response to loss of afferent input such as that caused by acoustic insult and/or age-related hearing loss, the most common causes of tinnitus in people. Compensatory plastic changes may result in pathologic neural activity that underpins tinnitus. The neural correlates include increased spontaneous spiking, increased bursting and decreased variance of inter-spike intervals. This review will examine evidence for chronic plastic neuropathic changes in the central auditory system of animals with psychophysically-defined tinnitus. Neurochemical studies will focus on plastic tinnitus-related changes of inhibitory glycinergic neurotransmission in the adult dorsal cochlear nucleus (DCN). Electrophysiological studies will focus on functional changes in the DCN and inferior colliculus (IC). Tinnitus was associated with increased spontaneous activity and altered response properties of fusiform cells, the major output neurons of DCN. Coincident with these physiologic alterations were changes in glycine receptor (GlyR) subunit composition, its anchoring/trafficking protein, gephyrin and the number and affinity of membrane GlyRs revealed by receptor binding. In the IC, the primary afferent target of DCN fusiform cells, multi-dimensional alterations in unit-spontaneous activity (rate, burst rate, bursting pattern) were found in animals with behavioral evidence of chronic tinnitus more than 9 months following the acoustic/cochlear insult. In contrast, immediately following an intense sound exposure, acute alterations in IC spontaneous activity resembled chronic tinnitus-related changes but were not identical. This suggests that long-term neuroplastic changes responsible for chronic tinnitus are likely to be responsible for its persistence. A clear understanding of tinnitus-related plasticity in the central auditory system and its associated neurochemistry may help define unique targets for therapeutic drug development.