Glutamate Regulates IP₃-Type and CICR Stores in the Avian Cochlear Nucleus

Abstract

Neurons of the avian cochlear nucleus, nucleus magnocellularis (NM), are activated by glutamate released from auditory nerve terminals. If this stimulation is removed, the intracellular calcium ion concentration ([Ca2+]i) of NM neurons rises and rapid atrophic changes ensue. We have been investigating mechanisms that regulate [Ca2+]i in these neurons based on the hypothesis that loss of Ca2+ homeostasis causes the cascade of cellular changes that results in neuronal atrophy and death. In the present study, video-enhanced fluorometry was used to monitor changes in [Ca2+]i stimulated by agents that mobilize Ca2+ from intracellular stores and to study the modulation of these responses by glutamate. Homobromoibotenic acid (HBI) was used to stimulate inositol trisphosphate (IP3)-sensitive stores, and caffeine was used to mobilize Ca2+ from Ca2+-induced Ca2+ release (CICR) stores. We provide data indicating that Ca2+ responses attributable to IP3- and CICR-sensitive stores are inhibited by glutamate, acting via a metabotropic glutamate receptor (mGluR). We also show that activation of C-kinase by a phorbol ester will reduce HBI-stimulated calcium responses. Although the protein kinase A accumulator, Sp-cAMPs, did not have an effect on HBI-induced responses. CICR-stimulated responses were not consistently attenuated by either the phorbol ester or the Sp-cAMPs. We have previously shown that glutamate attenuates voltage-dependent changes in [Ca2+]i. Coupled with the present findings, this suggests that in these neurons mGluRs serve to limit fluctuations in intracellular Ca2+ rather than increase [Ca2+]i. This system may play a role in protecting highly active neurons from calcium toxicity resulting in apoptosis.

Synaptic activity-induced Ca2+ signaling in avian cochlear nucleus magnocellularis neurons

“…2F-H, t-test, P < 0.001). These results essentially agree with previous studies (Lachica et al, 1995, 1998; Kato and Rubel, 1999; Zirpel et al, 2000; Diaz et al, 2009) (with minor discrepancies, see Discussion) and indicate that under these conditions, Ca 2+ influx from extracellular space is the major pathway for cytoplasmic Ca 2+ responses in NM neurons.…”

“…The amplitudes of the glutamate- and GABA-induced [Ca 2+ ] i increases in normal ACSF were 0.069 ± 0.006 (n = 115 cells, N = 19 slices) and 0.017 ± 0.001 (n = 34 cells, N = 8 slices), respectively (t-test, P < 0.001). The amplitude of [Ca 2+ ] i increases in Ca 2+ –free ACSF was smaller than that in normal ACSF (0.030 ± 0.005, n = 20 cells, N = 4 slices, t-test, P < 0.01, compared to normal ACSF; and 0.001 ± 0.001, n = 14 cells, N = 4 slices, t-test, P < 0.001, compared to normal ACSF, for glutamate and GABA, respectively), indicating that pathways other than ion channels on the cellular membrane may be involved, e.g., Ca 2+ release from internal stores through mGluR-activated signaling pathways (Zirpel et al, 1995; Lachica et al, 1998; Kato and Rubel, 1999), or Ca 2+ signaling induced by other neurotransmitters such as ATP (Milenkovic et al, 2009). Indeed, in Ca 2+ -free ACSF, tACPD (100 μM), a broad-spectrum agonist for mGluRs, induced [Ca 2+ ] i increases (n = 28 cells, N = 7 slices), whereas baclofen (100 μM), a specific agonist for GABA B Rs, did not (n = 25 cells, N = 7 slices) (Fig.…”

“…To date, studies of intracellular Ca 2+ signaling in NM neurons have focused on the [Ca 2+ ] i increases in response to exogenous agonists for glutamate or GABA receptors (Lachica et al, 1995, 1998; Kato and Rubel, 1999; Zirpel et al, 2000; Diaz et al, 2009). This approach has the advantage of using drugs with known concentrations and known specificity.…”

Metabotropic glutamate receptors in auditory processing

“…In the following years, mGluR-mediated regulation of Ca 2+ signaling in NM neurons was extensively studied, primarily by the same research group (Lachica et al, 1995; 1998; Kato et al, 1996; Zirpel and Rubel 1996; Zirpel et al, 1995, 1998; Kato and Rubel, 1999; Zirpel and Parks, 2001). The key conclusion from these studies is that mGluRs, especially group I mGluRs, play critical roles in regulating Ca 2+ signaling and maintaining Ca 2+ homeostasis and cell survival in NM neurons.…”

Role of intracellular calcium stores in hair-cell ribbon synapse

“…Neuronal RyRs can be activated following Ca 2+ influx through voltage-dependent Ca 2+ channels or ionotropic glutamate receptors (Berridge, 1998). Despite the lack of a unified mechanistic model, presynaptic CICR is present in several neuronal types such as motor neurons (Erulkar and Rahamimoff, 1978; Soga-Sakakibara et al, 2010), sympathetic neurons (Hua et al, 1993), cerebellar basket cells (Galante and Marty, 2003), striatal neurons (Plotkin et al, 2013), thalamocortical neurons (Cheong et al, 2011), Purkinje cells (Llano et al, 2000), sensory neurons (Shmigol et al, 1995), and cochlear nucleus neurons (Kato and Rubel, 1999). The presence of action potential-evoked as well as spontaneous CICR has been demonstrated by pharmacological effects of ryanodine, caffeine and other drugs on presynaptic Ca 2+ levels and postsynaptic currents.…”