In vivo analysis of the role of metabotropic glutamate receptors in the afferent regulation of chick cochlear nucleus neurons

Abstract

Cochlea removal results in the death of approximately 20-30% of neurons in the chick nucleus magnocellularis (NM). One early event in NM neuronal degradation is the disruption of their ribosomes. This can be visualized in the first few hours following cochlea removal using Y10B, an antibody that recognizes ribosomal RNA. Previous studies using a brain slice preparation suggest that maintenance of ribosomal integrity in NM neurons requires metabotropic glutamate receptor (mGluR) activation. Isolating the brain slice in vitro, however, may eliminate other potential sources of trophic support and only allows for evaluation of the early changes that occur in NM neurons following deafferentation. Consequently, it is not known if mGluR activation is truly required for the maintenance of NM neurons in the intact system. The current experiments evaluated the importance of mGluRs in vivo. The effects of short-term receptor blockade were assessed through Y10B labeling and the effects of long-term blockade were assessed through stereological counting of NM neurons in Nissl-stained tissue. mGluR antagonists or vehicle were administered intracerebroventricularly following unilateral cochlea removal. Vehicle-treated subjects replicated the previously reported effects of cochlea removal, showing lighter Y10B-labeling and fewer Nissl-stained NM neurons on the deafened side of the brain. Blockade of mGluRs prevented the rapid activity-dependent difference in Y10B labeling, and in some cases, had the reverse effect, yielding lighter labeling of NM neurons on the intact side of the brain. Similarly, mGluR blockade over longer survival periods resulted in a reduction in number of cells on both intact and deafferented sides of the brain, and in some cases, yielded a reverse effect of fewer neurons on the intact side versus deafened side. These data are consistent with in vitro findings and suggest that mGluR activation plays a vital role in the afferent maintenance of NM neurons.

Metabotropic glutamate receptors in auditory processing

“…This activity-dependent regulation of protein synthesis does not involve ionotropic glutamate receptors (Hyson, 1997), but rather relies on activity of mGluRs (Hyson, 1998). Specifically, group I and II mGluRs are required to maintain protein synthesis in NM neurons, because blocking either group I or II mGluRs eliminates activity-dependent regulation of ribosomes in vitro (Nicholas and Hyson, 2004) as well as in vivo (Carzoli and Hyson, 2011). These studies demonstrate the regulatory roles of mGluRs in Ca 2+ signaling and protein synthesis, suggesting potential neuroprotective effects of mGluRs under abnormal hearing conditions.…”

Activation of Metabotropic Glutamate Receptors Regulates Ribosomes of Cochlear Nucleus Neurons

“…On the other hand, blockade of mGluRs does prevent this activity-dependent regulation [21] , [23] . The importance of mGluR activation has also been confirmed in vivo where it has been shown that blockade of mGluRs reduces ribosomal antigenicity and produces NM neuronal death even with an intact cochlea [25] . The current set of experiments demonstrate that providing mGluR activation to NM neurons is sufficient, at least in the early stages that follow deafferentation, to maintain healthy ribosomes.…”

“…The Y10B antiserum was raised from a clone originally developed by J. Steitz and subsequently supplied to our laboratory by E. Rubel. This antibody is an established marker of changes in ribosomes [26] , and has been used extensively as an indicator of the early changes resulting from deafferentation (e.g., [15] , [17] , [18] , [25] , [27] ). Changes in antigenicity appear to match changes in overall protein synthesis and a deafferentation-induced reduction in Y10B labeling has been confirmed at the EM level [15] .…”

“…Through the use of the in vitro slice preparation, it has been shown that glutamate's action on ionotropic glutamate receptors (iGluRs) is responsible for the electrical activity in postsynaptic NM neurons [20] , [22] , and glutamate's action on metabotropic glutamate receptors (mGluRs) is responsible for providing trophic support to NM neurons [21] , [23] , [24] . Previous in vivo studies have also shown that mGluR activation is necessary for maintaining ribosomal antigenicity of Y10B and for the ultimate survival of NM neurons [25] . For example, application of selective group I or group II mGluR antagonists results in NM neuronal cell death even with an intact cochlea.…”

Anatomy and Physiology of Metabotropic Glutamate Receptors in Mammalian and Avian Auditory System

“…Further studies have shown that this activity-dependent regulation of protein synthesis relies on activity of mGluRs [124,125], but not involve iGluRs [126]. Specifically, group I and II mGluRs are required to maintain protein synthesis in NM neurons, because blocking either group I or II mGluRs eliminates activity-dependent regulation of ribosomes in vitro [127,128] as well as in vivo [129]. Together, these studies demonstrate the regulatory roles of mGluRs in Ca 2+ signaling and protein synthesis, suggesting potential neuroprotective effects of mGluRs under both normal and abnormal hearing conditions.…”