Spontaneous activity in developing sensory systems has been shown to be important for the growth and survival of projection neurons as well as the refinement and stabilization of sensory maps in the brain. In the developing cochlea, bursts of action potentials occur in afferent spiral ganglion neurons prior to the onset of hearing, activity that has been traced to inner hair cells (IHCs). Although IHCs are capable of generating Ca2+ action potentials during this period, the depolarizing stimulus required to initiate these events has not been identified. Whole-cell recordings from IHCs and supporting cells located adjacent in IHCs in ex vivo cochleas from young rodents revealed the presence of spontaneous inward currents that were capable of inducing large depolarizations. This activity was coincident with changes in the optical properties of the tissue when visualized using IR/DIC imaging, indicating that these events can be monitored non- invasively. Spontaneous electrical and optical activity was blocked by P2 purinergic receptor antagonists and gap junction inhibitors, suggesting that ATP and gap junctions/hemichannels are involved in initiating these events. Remarkably, this activity is no longer observed after the onset of hearing. This discovery of spontaneous purinergic signaling in the developing organ of Corti raises many new questions about the mechanisms responsible for producing this activity, the role that this ATP-mediated signaling plays in driving afferent firing, and the cause of the disappearance of the activity after hearing onset. We hypothesize that these ATP driven depolarizations of IHCs are responsible for initiating activity in developing auditory pathways. The preservation of this activity in both acute and cultured cochleas in which appropriate cell- cell interactions are maintained provides us with an unprecedented opportunity to understand the mechanisms responsible for these robust phenomena. We propose to use IR/DIC and confocal fluorescence imaging, photolysis, and both whole cell and extracellular recording to investigate the mechanisms underlying spontaneous activity in supporting cells and hair cells in the developing organ of Corti. These studies will evaluate the specific hypothesis that spontaneous oscillations in [Ca2+]i within supporting cells triggers both inward currents and the release of ATP that depolarizes IHCs.Relevance The studies outlined in this proposal seek to understand the mechanisms responsible for initiating spontaneous activity in supporting cells, hair cells, and afferent dendrites in the developing cochlea. This activity has been shown to have a profound influence on survival of target neurons in brainstem nuclei, the physiological properties of these auditory neurons, and the pattern of synaptic connectivity in these regions. Most congenital forms of deafness result from mutations in connexin 26, a gap junction protein highly expressed by cochelar supporting cells. As our preliminary results suggest that connexin hemichannels may play a role in ATP release from supporting cells, the studies outlined here may help explain how these mutations lead to deafness. Furthermore, these studies may reveal one mechanism by which activity can be induced in afferent nerves in the absence of sound, which may have direct relevance to human conditions such as tinnitus.

Agency
National Institute of Health (NIH)
Institute
National Institute on Deafness and Other Communication Disorders (NIDCD)
Type
Research Project (R01)
Project #
5R01DC008860-03
Application #
7741241
Study Section
Special Emphasis Panel (ZRG1-IFCN-L (03))
Program Officer
Freeman, Nancy
Project Start
2007-12-01
Project End
2011-11-30
Budget Start
2009-12-01
Budget End
2011-11-30
Support Year
3
Fiscal Year
2010
Total Cost
$345,015
Indirect Cost
Name
Johns Hopkins University
Department
Neurosciences
Type
Schools of Medicine
DUNS #
001910777
City
Baltimore
State
MD
Country
United States
Zip Code
21218
Zhang-Hooks, YingXin; Agarwal, Amit; Mishina, Masayoshi et al. (2016) NMDA Receptors Enhance Spontaneous Activity and Promote Neuronal Survival in the Developing Cochlea. Neuron 89:337-50
Wang, Han Chin; Lin, Chun-Chieh; Cheung, Rocky et al. (2015) Spontaneous Activity of Cochlear Hair Cells Triggered by Fluid Secretion Mechanism in Adjacent Support Cells. Cell 163:1348-59
Tritsch, Nicolas X; Rodríguez-Contreras, Adrián; Crins, Tom T H et al. (2010) Calcium action potentials in hair cells pattern auditory neuron activity before hearing onset. Nat Neurosci 13:1050-2
Yi, Eunyoung; Roux, Isabelle; Glowatzki, Elisabeth (2010) Dendritic HCN channels shape excitatory postsynaptic potentials at the inner hair cell afferent synapse in the mammalian cochlea. J Neurophysiol 103:2532-43
Tritsch, Nicolas X; Bergles, Dwight E (2010) Developmental regulation of spontaneous activity in the Mammalian cochlea. J Neurosci 30:1539-50
Tritsch, Nicolas X; Zhang, Ying-Xin; Ellis-Davies, Graham et al. (2010) ATP-induced morphological changes in supporting cells of the developing cochlea. Purinergic Signal 6:155-66