Excitatory modulation in the cochlear nucleus through group I metabotropic glutamate receptor activation

Activation of group I metabotropic glutamate receptors (mGluRs) has been suggested to modulate development of auditory neurons. However, the acute effects of mGluR activation on physiological response properties are unclear. To address this, we studied the effects of mGluRs in bushy cells (BCs) of the mammalian anteroventral cochlear nucleus (AVCN). Activation of mGluRs with dihydroxyphenyl-glycine (DHPG) caused depolarization of BCs in mice as old as P42, but did not affect neurotransmitter release by presynaptic auditory nerve (AN) fibers. Application of mGluR antagonists indicated that mGluRs are tonically active, and are highly sensitive to small elevations in ambient glutamate by the glutamate reuptake blocker threo-β-benzyloxyaspartic acid (TBOA). mGluR-mediated depolarization enhanced the firing probability in response to AN stimulation, and reduced the latency and jitter. Furthermore, excitation through postsynaptic mGluRs can significantly counterbalance the inhibitory effects of presynaptic GABA_B receptors. Thus, interaction between these two modulatory pathways may provide additional flexibility for fine-tuning the BC relay.