Cerebellar granule cells cultured from adolescent rats express functional NMDA receptors: an in vitro model for studying the developing cerebellum

In vitro models for neurotoxicology research

The nervous system has a highly complex organization, including many cell types with multiple functions, with an intricate anatomy and unique structural and functional characteristics; the study of its (dys)functionality following exposure to xenobiotics, neurotoxicology, constitutes an important issue in neurosciences.

Models of calcium dynamics in cerebellar granule cells

Intracellular calcium dynamics is critical for many functions of cerebellar granule cells (GrCs) including membrane excitability, synaptic plasticity, apoptosis, and regulation of gene transcription. Recent measurements of calcium responses in GrCs to depolarization and synaptic stimulation reveal spatial compartmentalization and heterogeneity within dendrites of these cells. However, the main determinants of local calcium dynamics in GrCs are still poorly understood. One reason is that there have been few published studies of calcium dynamics in intact GrCs in their native environment. In the absence of complete information, biophysically realistic models are useful for testing whether specific Ca(2+) handling mechanisms may account for existing experimental observations. Simulation results can be used to identify critical measurements that would discriminate between different models. In this review, we briefly describe experimental studies and phenomenological models of Ca(2+) signaling in GrC, and then discuss a particular biophysical model, with a special emphasis on an approach for obtaining information regarding the distribution of Ca(2+) handling systems under conditions of incomplete experimental data. Use of this approach suggests that Ca(2+) channels and fixed endogenous Ca(2+) buffers are highly heterogeneously distributed in GrCs. Research avenues for investigating calcium dynamics in GrCs by a combination of experimental and modeling studies are proposed.

An in vitro model for studying the effects of continuous ethanol exposure on N-methyl-D-aspartate receptor function

Long-term ethanol exposure has deleterious effects on both glial and neuronal function. We assessed alterations in both astrocytic and neuronal viability, and alterations in N-methyl-d-aspartate receptor (NMDAR) function, in cocultures of rat cerebellar granule cells (CGCs) and astrocytes after continuous ethanol exposure (CEE). Treatment of cells with 100 mM EtOH once every 24 h for 4 days resulted in a mean ethanol concentration of 57.3 ± 2.1 mM. Comparisons between control and post-ethanol-treated cells were made 4 days after the last ethanol treatment. CEE did not alter glial cell viability, as indicated by the absence of either changes in astrocytic morphology, actin depolymerization, or disruption of astrocytic intracellular mitochondrial distribution at any day postethanol treatment. The CGCs were healthy and viable after CEE, as indicated by phase-contrast microscopy and the trypan-blue exclusion method. Whole-cell patch-clamp experiments indicated that NMDA-induced currents (I(NMDA)) were altered by CEE treatment. Similar to previous results obtained during the withdrawal phase from chronic ethanol exposure, I(NMDA) from CEE-treated cells were significantly larger than I(NMDA) from NMDARs in control CGCs, but returned to control values by the fourth day post-CEE. However, after the last ethanol dosing and during a time when ethanol concentrations remained high, I(NMDA) were significantly smaller than control values. Identical results were observed in CGCs expressing the NR2A or NR2B subunit. In summary, both neurons and astrocytes remained healthy following exposure to CEE with no signs of neurotoxicity at the cellular level, and modulation of NMDAR function is consistent with findings from prior experiments. Thus, we conclude that the CEE paradigm in glial-neuronal cocultures readily lends itself to long-term in vitro studies of ethanol effects that include glial-neuronal interactions and the ability to study ethanol withdrawal-induced neurotoxicity.

Acute ethanol exposure prevents PMA-mediated augmentation of N-methyl-D-aspartate receptor function in primary cultured cerebellar granule cells

Many intracellular proteins and signaling cascades contribute to the ethanol sensitivity of native N-methyl-D-aspartate receptors (NMDARs). One putative protein is the serine/threonine kinase, protein kinase C (PKC). The purpose of this study was to assess if PKC modulates the ethanol sensitivity of native NMDARs expressed in primary cultured cerebellar granule cells (CGCs). With the whole-cell patch-clamp technique, we assessed if ethanol inhibition of NMDA-induced currents (I(NMDA)) (100 μM NMDA plus 10 μM glycine) were altered in CGCs in which the novel and classical PKC isoforms were activated by phorbol-12-myristate-13-acetate (PMA). Percent inhibition by 10, 50, or 100 mM ethanol of NMDA-induced steady-state current amplitudes (I(SS)) or peak current amplitudes (I(Pk)) of NMDARs expressed in CGCs in which PKC was activated by a 12.5 min, 100 nM PMA exposure at 37°C did not differ from currents obtained from receptors contained in control cells. However, PMA-mediated augmentation of I(Pk) in the absence of ethanol was abolished after brief applications of 10 or 1 mM ethanol coapplied with agonists, and this suppression of enhanced receptor function was observed for up to 8 min post-ethanol exposure. Because we had previously shown that PMA-mediated augmentation of I(NMDA) of NMDARs expressed in these cells is by activation of PKCα, we assessed the effect of ethanol (1, 10, 50, and 100 mM) on PKCα activity. Ethanol decreased PKCα activity by 18% for 1 mM ethanol and activity decreased with increasing ethanol concentrations with a 50% inhibition observed with 100 mM ethanol. The data suggest that ethanol disruption of PMA-mediated augmentation of I(NMDA) may be due to a decrease in PKCα activity by ethanol. However, given the incomplete blockade of PKCα activity and the low concentration of ethanol at which this phenomenon is observed, other ethanol-sensitive signaling cascades must also be involved.

Phorbol 12-myristate 13-acetate potentiation of N-methyl-D-aspartate-induced currents in primary cultured cerebellar granule cells is mediated by protein kinase C alpha

We have previously reported that activation of protein kinase C (PKC) by phorbol 12-myristate 13-acetate (PMA) results in potentiation of N-methyl-D-aspartate-induced currents (I(NMDA))of receptors contained in primary cultured cerebellar granule cells (CGCs). The purpose of this study was to identify which PKC isoform(s) was responsible for this effect by using the whole-cell patch-clamp technique. Experiments were conducted on CGCs that expressed both the NR2A and NR2B NMDA receptor subunits as well as the PMA-sensitive PKC isoforms alpha, betaI, betaII, delta, epsilon, gamma, and . As observed previously, N-methyl-D-aspartate-induced peak currents (I(Pk)) were enhanced by a 12.5-min, 100 nM PMA exposure at 37 degrees C under normal recording conditions. Potentiation of receptor function was not observed when extracellular Ca(2+) was removed and 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid was present inside the cell. PMA-induced potentiation of I(Pk) did not occur when PKCalpha-specific antibody was introduced into the cell via the recording electrode. However, in similar experiments with antibodies specific for PKCbetaII, delta, epsilon, gamma, and , PMA potentiation of I(Pk) was observed. Down-regulation of PMA-sensitive PKC isoforms by an overnight exposure of 100 nM PMA resulted in lack of potentiation by PMA that was rescued when catalytically active PKCalpha was introduced into the cell via the patch electrode. PMA potentiation of I(Pk) was not recovered when catalytically active PKCbetaI, PKCbetaII, or PKCgamma was introduced into the cell via the patch electrode. Collectively, our data provide strong evidence that PMA-enhanced function of native NMDA receptors expressed in primary cultured CGCs is mediated by activation of PKCalpha.