Noradrenergic Modulation in the Mouse Auditory Midbrain

Abstract

The orchestration of diffuse actions is essential for the efficient functioning and behavior of animals. Information processing across the central nervous system is regulated and coordinated by small groups of neurons in the brain that express neurochemicals known as neuromodulators. The modulator norepinephrine is important for regulating arousal and attention. Norepinephrine modulates many sensory systems, including the central auditory system. Here we examined the modulatory effects of norepinephrine in the principal midbrain auditory nucleus, the inferior colliculus (IC). Using calcium imaging, we found that norepinephrine modulates the physiology of cells in the IC. We observed increases in intracellular calcium in cells across the inferior colliculus. This effect is mediated by all three adrenergic receptor subtypes. Noradrenergic effects were not observed in excitatory neurons in the IC. We also examined cellular expression of adrenergic receptor-encoding mRNA using fluorescent in situ hybridization. We found expression of α1, α2A, and β2 receptor-encoding mRNA throughout the IC. We observed similar levels of expression of α1 and α2A receptor-encoding mRNA across the IC, while β2 receptor-encoding mRNA was expressed in a higher proportion of cells in the outer subregions of the IC. Co-expression of adrenergic receptor-encoding mRNA was greater than expected. Together, this research provides the first evidence for noradrenergic modulation of the inferior colliculus and furthers our understanding of the expression of adrenergic receptors in the auditory midbrain.