Meeting Abstract
Efferent modulation of auditory encoding in the ear under natural conditions remains poorly understood. Studies in rodents report an inhibitory effect of dopamine (DA) in the cochlea, hypothesized to protect the ear from noise trauma. However, stimuli tested are louder and longer in duration than those likely to be encountered in the natural soundscape in which ears evolved. A biological function for DA in the inner ear in the context of natural behaviors remains to be demonstrated. Our work with the plainfin midshipman fish, Porichthys notatus, suggests DA modulation is important for acoustic communication in social and reproductive contexts. During the breeding season, nesting males produce a nocturnal hum-like vocalization which females use to locate males for spawning. Females undergo a hormonally regulated enhancement of peripheral auditory sensitivity that facilitates mate localization. Importantly, during the breeding season, DA innervation is reduced in the inner ear and increased in the cholinergic hindbrain nucleus projecting to the ear. A single forebrain nucleus sends DA to both the ear and the efferent hindbrain nucleus. Terminals form direct synapses in the hindbrain but not in the ear, suggesting paracrine release and the potential for DA to modulate hair cells directly. Application of DA and receptor-specific drugs during sound-evoked extracellular recordings from hair cells confirmed that DA increases auditory thresholds. The seasonal decrease of DA in the inner ear appears to serve as a release of inhibition, adaptively improving auditory sensitivity for mate localization. DA modulation of social-acoustic signals in the peripheral auditory system is a function which may be conserved in other vocal vertebrate species.
