Spontaneous neuronal activity that occurs in the auditory pathway prior to hearing onset is characterized by highly stereotyped, periodic bursts of action potentials that originate in the cochlea and propagate through auditory centers of the brain. Neuronal activity in the developing inferior colliculus (IC) occurs in discrete bands, corresponding to tonotopic frequency maps that are apparent after hearing onset. In preliminary studies, I found that astrocytes in the IC show periodic, tonotopically organized elevations of intracellular calcium before hearing onset that are strikingly similar to the activity patterns exhibited by IC neurons at this age. This study aims to define the molecular mechanisms underlying this signaling in astrocytes and assess whether this dynamic interaction between astrocytes and neurons promotes neuronal circuit maturation. The studies will test the hypothesis that periodic elevation of intracellular calcium in astrocytes is dependent on activation of astrocyte metabotropic glutamate receptors (mGluR5) by glutamate released from ascending projections into the developing IC. In the first aim I will image neuronal and astrocyte calcium activity simultaneously in transgenic mice that express genetically encoded calcium indicators to define the spatial and temporal interactions between neurons and astrocytes in the developing IC. I will selectively manipulate neuronal activity and measure the effect on astrocytic calcium signaling. In the second aim I will determine whether the correlated activity of astrocytes is mediated by mGluR5 receptors. First I will define the mRNA expression levels and spatial localization of mGluR5 using qRT-PCR and in situ hybridization during different developmental time points. Next, I will use pharmacological manipulations and astrocyte specific conditional knockout mice to inhibit mGluR5 receptors in vivo while imaging calcium activity in astrocytes. In the third aim, I will investigate whether manipulation of this early pattern of astrocyte activity affects the firing behavior of IC neurons prior to hearing onset and the tonotopic representation of sound in hearing animals. I will use transgenic mouse lines that impair astrocyte calcium activity and measure the consequences on the spatial pattern of neuronal activity in the IC, induced spontaneously or in response to stereotyped sound stimuli. To define specifically the role of astrocytes in the IC, I will manipulate astrocyte calcium locally using viruses and measure the effect on the functional refinement of neuronal circuits. This analysis of reciprocal interactions between neurons and astrocytes in the developing auditory system will provide new insight into the mechanisms that induce neuronal circuit refinement to enable hearing. These studies may reveal new pathways to promote circuit maturation and identify new targets for treatment of developmental disorders by expanding our understanding of how alterations in astrocytes during this critical period of development impact the ability of auditory circuits to process sensory information.
Neurons in the developing auditory system fire bursts of action potentials prior to the onset of hearing. This proposal will determine if this highly stereotyped, tonotopically-organized neuronal activity induces the maturation of neuronal circuits by stimulating metabotropic glutamate receptor signaling in astrocytes.
