Developmental changes in metabotropic glutamate receptor-mediated calcium homeostasis

Neurons of the chick cochlear nucleus, nucleus magnocellularis (NM), require eighth nerve activation of metabotropic glutamate receptors (mGluRs) for maintenance of intracellular calcium homeostasis. Interrupting this activation results in an increase in intracellular calcium concentration ([Ca(2+)](i)) followed by cell atrophy, degeneration, and death of many neurons. Although these phenomena are well characterized in late embryonic and posthatch chicks, little is known about the role of mGluRs and calcium homeostasis during the development of synaptic activity in NM. Using Fura-2 imaging, fluorescent immunohistochemistry, and Western immunoblotting, we investigated (1) the expression and function of group I mGluRs and their role in calcium regulation during development of NM, and (2) the expression of two other key molecules involved in regulating neuronal [Ca(2+)](i) : inositol trisphosphate receptors (IP(3)Rs) and sarcoplasmic/endoplasmic reticulum calcium ATPases (SERCAs). Confocal imaging of Fluo-3-labeled NM was used to investigate the kinetics of global NM neuron calcium signals. Measurements were made at four ages that extend from before synaptic function begins in NM, through functional onset, to mature patterns of spontaneous activity, namely, embryonic days (E) 10, 13, 15, and 18. mGluR5, mGluR1, and SERCA expression peaked at E13 and then decreased with age. IP(3)R expression increased to peak at E18. [Ca(2+)](i) response to mGluR activation increased with age. The rise time of [Ca(2+)](i) signals in NM neurons did not change with development, but E13 neurons were slower to reestablish baseline [Ca(2+)](i). These results suggest that the mGluR-mediated calcium homeostasis of NM neurons develops in parallel with synaptic activity and appears to be refined with increasing synaptic activity.