Sound and head movements are sensed by the inner ear, which conveys the signals to the brain, where they are processed and relevant information is sorted out. The brain also signals back to the ear and can influence its activity. For example, if a person is exposed to continuous loud noise, then the auditory system adapts. Generally, the same signaling molecules that mediate communication among neurons in the brain are also used for signaling to the ear. One such molecule is dopamine, which is better known for its role in the reward system in the brain and in Parkinson’s disease. The senses of hearing and balance are modulated by signaling including the release of dopamine (DA), however, communication and feedback from the brain to the ear is in most cases not very well understood, partially because of the complexity of the inner ear.
In a recent paper entitled “Dopamine modulates the activity of sensory hair cells,” published as a featured article in the Journal of Neuroscience, researchers in the Nicolson lab, led by Cecilia Toro, Ph.D., demonstrated that dopamine receptors are present in sensory hair cells at synapses involved in signaling the brain. Release of dopamine activates these receptors and increases neurotransmission of signals related to hearing and balance.
In most vertebrates, the inner ear expresses multiple types of receptors that signal in different ways when dopamine binds to them. To get around this issue, the research team took advantage of the relatively simple system of the lateral line organ in fish. This mechanosensory organ detects water movements in aquatic animals, and the mechanosensory cells of the lateral line organ are analogous to those employed by the ear to sense sounds and head movements. It turns out that only one type of dopamine receptor is present in the lateral line organ.
The authors discovered that dopamine enhances the activity of the mechanosensory cells. Imaging experiments suggest that enhancement of activity appears to be through ion channels that let more calcium into the cells. Moreover, sensory cells of the zebrafish and mammalian inner ear also express the same type of dopamine receptor, suggesting a similar mechanism is working in inner ear sensory cells.
Why would enhancement of sensitivity to sound or head movements by dopamine be a desired outcome? The answer to this question is not known, but the global nature of the dopamine signaling discovered by Toro and colleagues suggest that it might have to do with ensuring the function or maintenance of the system (‘homeostasis’) or that sensitivity may be affected by circadian rhythms. Perhaps having increased hearing sensitivity during the day would be an advantage while less sensitivity at night might just lead to a better night’s sleep. These await further investigation.
Members of the research team: (at the time, postdoctoral fellows) Cecilia Toro, Ph.D. (now at Linfield College), Josef G. Trapani, Ph.D. (now at Amherst College), and Lavinia Sheets, Ph.D. (now at Harvard Medical School); Itallia Pacentine (graduate student); Reo Maeda, Ph.D. (postdoctoral fellow); (at the time, graduate student) Weike Mo, Ph.D., (now CEO and founder of UniteGen); and Teresa Nicolson, Ph.D.