Circuits for Modulation of Auditory Function

Abstract

This chapter discusses anatomical, physiological, and functional aspects of circuits associated with four major neuromodulators: acetylcholine, serotonin, noradrenaline, and dopamine. These neuromodulators occur in nearly all auditory structures from the cochlea of the inner ear to the cortex of the brain. A review of the anatomy is focused on the origins of modulatory inputs to auditory structures and the patterns of termination in those areas. Sources of the modulatory inputs include widely recognized cell groups in the basal forebrain and pontomesencephalic tegmentum (for acetylcholine), raphe nuclei (for serotonin), locus coeruleus (for noradrenaline), and ventral tegmental area (for dopamine), as well as smaller cell groups in the brainstem. In addition, there are numerous examples of cells within the auditory system that release one or more of these neuromodulators. Physiology and function are discussed from several perspectives, starting with a brief overview of methods used for assessing modulatory function. Neuromodulators are directly involved in regulating auditory processing according to both internal state and stimulus salience. Many mechanisms are likely involved. Neuromodulators can reconfigure auditory circuitry through multiple receptor types and in multiple auditory regions. Furthermore, multiple neuromodulators may converge at the level of single neuron types. This makes the effects of neuromodulators complex but confers the ability to produce a range of behaviorally appropriate outputs from auditory circuitry. In addition, neuromodulators facilitate long-term plasticity. Such plasticity plays a role in many adaptive responses, including numerous changes that may play a role in the auditory dysfunction that follows hearing loss.