Activity-dependent regulation of calcium and ribosomes in the chick cochlear nucleus

- [Ca2+]i and ribosomes (Y10B-ir) show rapid changes in NM following deafferentation.
- There is a dissociation between how [Ca2+]i and ribosomes are regulated.
- Selective mGluR receptor blockade during stimulation dissociates the two events.
- Blockade of group I mGluRs affects changes in both [Ca2+]i and ribosomes.
- Blockade of group II mGluRs affects only ribosomal changes.

Cochlea removal results in the death of 20-30% of neurons in the chick cochlear nucleus, nucleus magnocellularis (NM). Two potentially cytotoxic events, a dramatic rise in intracellular calcium concentration ([Ca2+]i) and a decline in the integrity of ribosomes are observed within 1 h of deafferentation. Glutamatergic input from the auditory nerve has been shown to preserve NM neuron health by activating metabotropic glutamate receptors (mGluRs), maintaining both normal [Ca2+]i and ribosomal integrity. One interpretation of these results is that a common mGluR-activated signaling cascade is required for the maintenance of both [Ca2+]i and ribosomal integrity. This could happen if both responses are influenced directly by a common messenger, or if the loss of mGluR activation causes changes in one component that secondarily causes changes in the other. The present studies tested this common-mediator hypothesis in slice preparations by examining activity-dependent regulation of [Ca2+]i and ribosomes in the same tissue after selectively blocking group I mGluRs (1-Aminoindan-1,5-dicarboxylic acid (AIDA)) or group II mGluRs (LY 341495) during unilateral auditory nerve stimulation. Changes in [Ca2+]i of NM neurons were measured using fura-2 ratiometric calcium imaging and the tissue was subsequently processed for Y10B immunoreactivity (Y10B-ir), an antibody that recognizes a ribosomal epitope. The group I mGluR antagonist blocked the activity-dependent regulation of both [Ca2+]i and Y10B-ir, but the group II antagonist blocked only the activity-dependent regulation of Y10B-ir. That is, even when group II receptors were blocked, stimulation continued to maintain low [Ca2+]i, but it did not maintain Y10B-ir. These results suggest a dissociation in how calcium and ribosomes are regulated in NM neurons and that ribosomes can be regulated through a mechanism that is independent of calcium regulation.