Calcium signals elicited by quisqualate in cultured Purkinje neurons show developmental changes in sensitivity to acute alcohol

Homer proteins control neuronal differentiation through IP(3) receptor signaling

Neurons expand, sustain or prune their dendritic trees during ontogenesis [Cline, H.T. (2001). Dendritic arbor development and synaptogenesis. Curr. Opin. Neurobiol. 11, 118-126; Wong, W.T. and Wong, R.O.L. (2000) Rapid dendritic movements during synapse formation and rearrangement. Curr. Opin. Neurobiol. 10, 118-124] which critically depends on neuronal activity [Wong, W.T., Faulkner-Jones, B.E., Sanes, J.R. and Wong, R.O.L. (2000) Rapid dendritic remodeling in the developing retina: dependence on neurotransmission and reciprocal regulation by Rac and Rho. J. Neurosci. 20, 5024-5036; Li, Z., Van Aelst, L. and Cline, H.T. (2000) Rho GTPases regulate distinct aspects of dendritic arbor growth in Xenopus central neurons in vivo. Nat. Neurosci. 3, 217-225; Wong, W.T. and Wong, R.O.L. (2001) Changing specificity of neurotransmitter regulation of rapid dendritic remodeling during synaptogenesis. Nat. Neurosci. 4, 351-352.] and sub-cellular Ca(2+) signals [Lohmann, C., Myhr, K.L. and Wong, R.O. (2002) Transmitter-evoked local calcium release stabilizes developing dendrites, Nature 418, 177-181.]. The role of synaptic clustering proteins connecting both processes is unclear. Here, we show that expression levels of Vesl-1/Homer 1 isoforms critically control properties of Ca(2+) release from intracellular stores and dendritic morphology of CNS neurons. Vesl-1L/Homer 1c, an isoform with a functional WH1 and coiled-coil domain, but not isoforms missing these features were capable of potentiating intracellular calcium signaling activity indicating that such regulatory interactions function as a general paradigm in cellular differentiation and are subject to changes in expression levels of Vesl/Homer isoforms.

Chronic interleukin-6 exposure alters metabotropic glutamate receptor-activated calcium signalling in cerebellar Purkinje neurons

Chronic central nervous system expression of the cytokine interleukin-6 (IL-6) is thought to contribute to the histopathological, pathophysiological, and cognitive deficits associated with various neurological disorders. However, the effects of chronic IL-6 expression on neuronal function are largely unknown. Previous studies have shown that chronic IL-6 exposure alters intrinsic electrophysiological properties and intracellular Ca2+ signalling evoked by ionotropic glutamate receptor activation in cerebellar Purkinje neurons. In the current study, using primary cultures of rat cerebellum, we investigated the effects of chronic IL-6 exposure on metabotropic glutamate receptor (mGluR)-activated Ca2+ signalling and release from intracellular Ca2+ stores. Chronic exposure (6-10 days) of Purkinje neurons to 500 units/mL IL-6 resulted in elevated resting Ca2+ levels and increased intracellular Ca2+ signals evoked by the group I mGluR agonist (S)-3,5-dihydroxyphenylglycine (DHPG) compared to untreated control neurons. Chronic IL-6 treatment also augmented Ca2+ signals evoked by brief 100 mm K+ depolarization, although to a lesser degree than responses evoked by DHPG. Depleting intracellular Ca2+ stores with sarcoplasmic-endoplasmic reticulum ATPase inhibitors (thapsigargin or cyclopiazonic acid) or blocking ryanodine receptor-dependent release from intracellular stores (using ryanodine) resulted in a greater reduction of DHPG- and K+-evoked Ca2+ signals in chronic IL-6-treated neurons than in control neurons. The present data show that chronic exposure to elevated levels of IL-6, such as occurs in various neurological diseases, alters Ca2+ signalling involving release from intracellular stores. The results support the hypothesis that chronic IL-6 exposure disrupts neuronal function and thereby may contribute to the pathophysiology associated with many neurological diseases.

Chronic Ethanol-Induced Subtype- and Subregion-Specific Decrease in the mRNA Expression of Metabotropic Glutamate Receptors in Rat Hippocampus

BACKGROUND

Chronic ethanol consumption is known to induce adaptive changes in the hippocampal glutamatergic transmission and alter NMDA receptor binding and subunit expression. Metabotropic glutamate (mGlu) receptors have been shown to function as modulators of neuronal excitability and can fine tune glutamatergic transmission. This study was aimed to determine whether chronic ethanol treatment could change the messenger RNA (mRNA) expression of mGlu receptors in the hippocampus.

METHODS

Male Sprague Dawley rats were fed a Lieber-DeCarli liquid diet with 5% (w/v) ethanol or isocaloric amount of maltose for 2 months. Quantitative in situ hybridization was carried out using coronal brain sections through the hippocampus.

RESULTS

The results revealed decreases in mRNA expression of several mGlu receptors in different subregions of the hippocampus. In the dentate gyrus, mGlu3 and mGlu5 receptor mRNA levels were significantly lower in the ethanol-treated rats than in the control rats. In the CA3 region, the mRNA expression of mGlu1, mGlu5, and mGlu7 receptors showed substantial decreases after ethanol exposure. The mGlu7 receptor mRNA levels were also declined in the CA1 region and the polymorph layer of the dentate gyrus. No changes were found in mRNA expression of mGlu2, mGlu4, and mGlu8 receptors.

CONCLUSIONS

Considering the involvement of hippocampal mGlu receptors in learning and memory processes as well as in neurotoxicity and seizure production, the reduced expression of these receptors might contribute to ethanol withdrawal-induced seizures and also may play a role in cognitive deficits and brain damage caused by long-term ethanol consumption.

Effects of acute ethanol on the Ca2+ response to AMPA in cultured rat cortical GABAergic nonpyramidal neurons

AIMS AND METHODS

Immunocytochemical studies revealed that the vast majority of neurons in our primary cultures of rat cortical cells are GABA-positive and represent nonpyramidal interneuron-like cells. The influence of ethanol on (S)-alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA)-induced Ca2+ influx was investigated in multipolar, medium-sized neurons by using single-cell fura-2 microfluorimetry.

RESULTS

In a first series of experiments, the results showed a small but significant decrease of 17-22% by ethanol (100 mm) of the intracellular Ca2+ signals induced by slowly superfused AMPA (10, 30, 100 microm). This finding is comparable with the inhibitory activity of ethanol on N-methyl-D-aspartic acid (NMDA)-induced Ca2+ signals in these cells. Further studies with a fast pressure-application of AMPA (30 microm) showed a similar degree of inhibition by ethanol (100 mm). Superfusion with tetrodotoxin/bicuculline, to rule out possible effects of spontaneously released GABA and synaptic spike activity, did not significantly influence the AMPA-induced Ca2+ response nor the inhibitory effect of ethanol.

CONCLUSIONS

The present findings indicate that ethanol at high concentrations inhibits Ca2+ signaling via both AMPA and NMDA glutamate receptors in cortical interneuron-like cells. These effects may contribute to the central depressant action of this drug.

Chronic interleukin-6 exposure alters electrophysiological properties and calcium signaling in developing cerebellar purkinje neurons in culture

The cytokine interleukin-6 (IL-6) is chronically expressed at elevated levels within the CNS in many neurological disorders and may contribute to the histopathological, pathophysiological, and cognitive deficits associated with such disorders. However, the effects of chronic IL-6 exposure on neuronal function in the CNS are largely unknown. Therefore using intracellular recording and calcium imaging techniques, we investigated the effects of chronic IL-6 exposure on the physiological properties of cerebellar Purkinje neurons in primary culture. Two weeks of exposure to 1,000 units/ml (U/ml) IL-6 resulted in altered electrophysiological properties of Purkinje neurons, including a significant reduction in action potential generation, an increase in input resistance, and an enhanced electrical response to the ionotropic glutamate receptor agonist, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) compared with untreated neurons. Lower concentrations of IL-6 (100 and 500 U/ml) had no effects on these electrophysiological parameters. However, neurons exposed to 500 U/ml chronic IL-6 resulted in significantly elevated resting levels of intracellular calcium as well as an increase in the intracellular calcium signal of Purkinje neurons in response to AMPA, effects not observed in neurons exposed to 1,000 U/ml chronic IL-6. Morphometric analysis revealed a lack of gross structural changes following chronic IL-6 treatment, such as in the number, size, and extent of dendritic arborization of Purkinje neurons in culture. Using immunohistochemistry, we found that cultured Purkinje neurons express both the IL-6 receptor and its intracellular signaling subunit, gp130, indicating that IL-6 may act directly on Purkinje neurons to alter their physiological properties. The present data show that chronic exposure to elevated levels of IL-6, such as occurs in various neurological diseases, produces alterations in several important physiological properties of Purkinje neurons and that these changes occur in the absence of neuronal toxicity, damage, or death. The results support the hypothesis that chronic IL-6 exposure can disrupt normal CNS function and thereby contribute to the pathophysiology associated with many neurological diseases.

Short-term ethanol exposure alters calbindin D28k and glial fibrillary acidic protein immunoreactivity in hippocampus of mice

The effects of a short-term ethanol treatment on hippocampus have been studied in mice exhibiting intoxication signs. The alterations of neurons and astrocytes as well as quantitative changes of calbindin D28k-immunoreactivity and glial fibrillary acidic protein-immunoreactivity (GFAP-IR) in selected regions of the dorsal hippocampus were examined using anti-calbindin and anti-GFAP monoclonal anti-body (mAb), respectively. The administration of 6% (v/v) ethanol during first week led to the neuronal death and decrease of the total number of calbindin-IR neurons in the examined brain regions. Moreover, the calbindin positive neurons were shown to have diminished processes following short-term ethanol exposure. These neuronal changes were associated with the increase of the GFAP-IR astrocytes. Hypertrophy of cell bodies and cytoplasmic processes of reactive astrocytes were also seen. In addition, dense, thick and highly-stained GFAP-IR cells with long processes in granular cell layer appeared entering into molecular layer of dentate gyrus. In agreement with the discrepancy percentage of neuronal cell loss and increase of reactive astrocytes detected by calbindin and GFAP-IR using image quantitative analysis, the regional differences in the vulnerability to the neurotoxic effects following short-term ethanol exposure were found: CA3>CA2>CA1>DG. These findings also illustrate the importance of correlation between calbindin and GFAP-IR when determining the morphological alteration of neuron and astroglial following short-term ethanol treatment. The disruption of calbindin and GFAP could affect neuronal-astroglial interaction, resulting in disturbance of behaviors dependent on hippocampus.

Effect of short-term ethanol exposure on the suprachiasmatic nucleus of hypothalamus: immunohystochemical study in mice

Morphological changes of the suprachiasmatic nucleus (SCN) of the hypothalamus were investigated in mice exhibiting intoxication signs of stages 2 or 3 after a short application term of 6% ethanol. Alterations in glial cells and neurons were examined using anti-glial fibrillary acidic protein (GFAP) and anti-calbindin D28k monoclonal antibody, respectively. The results revealed that short-term ethanol exposure led to strong expression of GFAP-immunoreactivity (GFAP-IR) in the dorsomedial part of the SCN. Furthermore, GFAP-IR astrocytes showed an increase in number and hypertrophy with longer processes. However, calbindin D28k-IR neurons were apparently little changed in the SCN. It is concluded that neuroadaptive response of astrocytes could occur before the neurotoxic effects emerge on neurons on the SCN.

Chronic ethanol exposure enhances AMPA-elicited Ca2+ signals in the somatic and dendritic regions of cerebellar Purkinje neurons

Intracellular Ca2+ signals produced by the glutamate receptor agonist alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA; 5 microM) were measured in the somatic and dendritic regions of cerebellar Purkinje neurons in mature cerebellar control cultures (> or = 20 days in vitro) and cultures chronically treated with 32 mM ethanol (146 mg%; 8-11 days). Recordings were made in physiological saline without ethanol. The mean peak amplitude of the Ca2+ signal elicited by AMPA (applied by brief 1-s microperfusion) in the somatic region was enhanced 38% in chronic ethanol-treated Purkinje neurons compared with control neurons. In contrast, Ca2+ signals evoked by AMPA in the dendritic region were similar in magnitude between control and chronic ethanol-treated Purkinje neurons. When tetrodotoxin (TTX; 500 nM) was included in the bath saline to block spike activity and synaptically-generated events, the mean peak amplitude of the Ca2+ signal elicited by AMPA was enhanced 60% in both the somatic and dendritic regions of chronic ethanol-treated Purkinje neurons compared with control neurons. Thus, TTX-sensitive mechanisms (i.e., spike or synaptic activity) appear to play a role in normalizing neuronal functions involved in Ca2+ signaling in the chronic ethanol-treated neurons. In parallel current clamp experiments, the resting membrane potential of chronic ethanol-treated neurons was slightly depolarized compared with control neurons. However, no differences were found between control and chronic ethanol-treated Purkinje neurons in input resistance or the peak amplitude or duration of the depolarizations or hyperpolarizations elicited by AMPA. AMPA receptors mediate fast excitatory neurotransmission in the majority of neurons in the central nervous system (CNS) and Ca2+ signals in response to AMPA receptor activation contribute to synaptic function. Thus, our results suggest that modulation of Ca2+ signals to AMPA receptor activation (or other cellular inputs) may provide an important mechanism contributing to the actions of prolonged ethanol exposure in the CNS.

Chronic ethanol treatment alters AMPA-induced calcium signals in developing Purkinje neurons

Cerebellar Purkinje neurons developing in culture were treated chronically with 30 mM (140 mg%; 3-11 days in vitro) ethanol to study the actions of prolonged ethanol exposure on responses to exogenous application of AMPA, a selective agonist at the AMPA subtype of ionotropic glutamate receptors. There was no consistent difference between control and chronic ethanol-treated neurons in resting membrane potential, input resistance, or the amplitude or duration of the membrane responses to AMPA (1 or 5 microM applied by brief microperfusion) as measured using the nystatin patch method of whole cell recording. In additional studies, the Ca2+ signal to AMPA was examined using the Ca2+ sensitive dye fura-2. The mean peak Ca2+ signal elicited by 5 microM AMPA was enhanced in the dendritic region (but not the somatic region) of chronic ethanol-treated Purkinje neurons compared to control neurons. In contrast, there was no difference between control and chronic ethanol-treated neurons in the peak amplitude of the Ca2+ signal to 1 microM AMPA, whereas the recovery of the Ca2+ signals was more rapid in both somatic and dendritic regions of ethanol-treated neurons. Resting Ca2+ levels in the somatic and dendritic regions were similar between control and ethanol-treated neurons. These data show that the membrane and Ca2+ responses to AMPA in Purkinje neurons are differentially affected by prolonged ethanol exposure during development. Moreover, chronic ethanol exposure produces a selective enhancement of AMPA-evoked dendritic Ca2+ signals under conditions reflecting intense activation (i.e., 5 microM AMPA), whereas both somatic and dendritic Ca2+ signals are attenuated with smaller levels of activation (i.e., 1 microM AMPA). Because Ca2+ is an important regulator of numerous intracellular functions, chronic ethanol exposure during development could produce widespread changes in the development and function of the cerebellum.

Effects of ethanol on calcium homeostasis in the nervous system: implications for astrocytes

Ethanol is a major health concern, with neurotoxicity occurring after both in utero exposure and adult alcohol abuse. Despite a large amount of research, the mechanism(s) underlying the neurotoxicity of ethanol remain unknown. One of the cellular aspects that has been investigated in relationship to the neuroteratogenicity and neurotoxicity of ethanol is the maintenance of calcium homeostasis. Studies in neuronal cells and other cells have shown that ethanol can alter intracellular calcium levels and affect voltage and receptor-operated calcium channels, as well as G protein-mediated calcium responses. Despite increasing evidence of the important roles of glial cells in the nervous systems, few studies exist on the potential effects of ethanol on calcium homeostasis in these cells. This brief review discusses a number of reported effects of alcohol on calcium responses that may be relevant to astrocytes' functions.

Ethanol and neurotransmitter interactions--from molecular to integrative effects

There is extensive evidence that ethanol interacts with a variety of neurotransmitters. Considerable research indicates that the major actions of ethanol involve enhancement of the effects of gamma-aminobutyric acid (GABA) at GABAA receptors and blockade of the NMDA subtype of excitatory amino acid (EAA) receptor. Ethanol increases GABAA receptor-mediated inhibition, but this does not occur in all brain regions, all cell types in the same region, nor at all GABAA receptor sites on the same neuron, nor across species in the same brain region. The molecular basis for the selectivity of the action of ethanol on GaBAA receptors has been proposed to involve a combination of benzodiazepine subtype, beta 2 subunit, and a splice variant of the gamma 2 subunit, but substantial controversy on this issue currently remains. Chronic ethanol administration results in tolerance, dependence, and an ethanol withdrawal (ETX) syndrome, which are mediated, in part, by desensitization and/or down-regulation of GABAA receptors. This decrease in ethanol action may involve changes in subunit expression in selected brain areas, but these data are complex and somewhat contradictory at present. The sensitivity of NMDA receptors to ethanol block is proposed to involve the NMDAR2B subunit in certain brain regions, but this subunit does not appear to be the sole determinant of this interaction. Tolerance to ethanol results in enhanced EAA neurotransmission and NMDA receptor upregulation, which appears to involve selective increases in NMDAR2B subunit levels and other molecular changes in specific brain loci. During ETX a variety of symptoms are seen, including susceptibility to seizures. In rodents these seizures are readily triggered by sound (audiogenic seizures). The neuronal network required for these seizures is contained primarily in certain brain stem structures. Specific nuclei appear to play a hierarchical role in generating each stereotypical behavioral phases of the convulsion. Thus, the inferior colliculus acts to initiate these seizures, and a decrease in effectiveness of GABA-mediated inhibition in these neurons is a major initiation mechanism. The deep layers of superior colliculus are implicated in generation of the wild running behavior. The pontine reticular formation, substantia nigra and periaqueductal gray are implicated in generation of the tonic-clonic seizure behavior. The mechanisms involved in the recruitment of neurons within each network nucleus into the seizure circuit have been proposed to require activation of a critical mass of neurons. Achievement of critical mass may involve excess EAA-mediated synaptic neurotransmission due, in part, to upregulation as well as other phenomena, including volume (non-synaptic diffusion) neurotransmission. Effects of ETX on receptors observed in vitro may undergo amplification in vivo to allow the excess EAA action to be magnified sufficiently to produce synchronization of neuronal firing, allowing participation of the nucleus in seizure generation. GABA-mediated inhibition, which normally acts to limit excitation, is diminished in effectiveness during ETX, and further intensifies this excitation.

Alcohol-induced Purkinje cell loss depends on developmental timing of alcohol exposure and correlates with motor performance

Several reports indicate that neonatal ethanol exposure induces cerebellar Purkinje and granule cell loss if exposure occurs before postnatal day (PD) 7, and that cerebellar damage may underlie ethanol-induced motor deficits. The present study used an unbiased stereological method, the optical fractionator, to count total cerebellar Purkinje cell number in groups of Sprague-Dawley rats given binge-like ethanol exposure at different neonatal ages. Correlations between Purkinje cell number (of 55-day-old rats) and parallel bar motor performance (previously tested on PD 30-32) were also evaluated. One group was given binge-like exposure to 6.6 g/kg per day of ethanol via artificial rearing on PD 4 and 5 (PD 4/5); a second group on PD 8 and 9 (PD 8/9); and a third group on both PD 4 and 5 and 8 and 9 (Comb). Gastrostomy (CG) and suckle (SC) control groups were also included. Purkinje cells were significantly reduced in all three ethanol-treated groups compared to controls, but the severity of loss was significantly greater in the PD 4/5 and Comb groups (reduced by 42% and 45%, respectively, relative to GC) compared to the PD 8/9 group (reduced by 15%). Across treatment groups, the total cerebellar Purkinje cell number was significantly correlated with successful parallel bar traversal (r = +0.74), supporting the contention that ethanol-induced motor deficits may be associated with cerebellar cell loss. These data confirm the presence of windows of vulnerability of Purkinje cells to neurotoxic effects of binge ethanol treatment, and demonstrate that both the behavioral and neuroanatomical consequences of binge exposure depend on the developmental timing of the exposure.

Cerebellar degeneration associated with human immunodeficiency virus infection

Cerebellar disorders associated with HIV infection are typically the result of discrete cerebellar lesions resulting from opportunistic infections such as toxoplasmosis and progressive multifocal leukoencephalopathy or primary CNS lymphoma. Clinical symptoms and pathologic abnormalities related to the cerebellum may also be observed with HIV dementia. A primary cerebellar degeneration with HIV has not previously been reported. Ten patients were identified over an 8-year period at five medical centers. All patients had clinical, laboratory, and radiologic evaluations, and three had neuropathologic examinations. Patients presented with progressively unsteady gait, slurred speech, and limb clumsiness. Examination revealed gait ataxia, impaired limb coordination, dysarthria, and abnormal eye movements. Cognition, strength, and sensory function remained normal. CD4 lymphocyte counts varied between 10 and 437 cells/mm3. Neuroimaging studies showed prominent cerebellar atrophy. Neuropathology showed focal degeneration of the cerebellar granular cell layer and unusual focal axonal swellings in the brainstem and spinal cord. Cultures, histopathology, and immunochemical studies showed no conclusive evidence of infection. We report a syndrome of unexplained degeneration of the cerebellum occurring in association with HIV infection.

Developmental changes in the inhibitory actions of ethanol on glutamate-induced translocation of protein kinase C in cerebellar granule neurons

The effects of increasing concentrations of ethanol (25-200 mM) on the enhancement of [3H]phorbol-12,13-dibutyrate ([3H]PDBu) binding produced by different glutamate receptor agonists, indicative of a translocation of the intracellular enzyme protein kinase C (PKC), were studied in rat cerebellar granule cells at 2, 4, 8, and 12 days in vitro (DIV). Glutamate-produced stimulation of [3H]PDBu binding was inhibited by 50 mM ethanol at 2 DIV, whereas higher ethanol concentrations (> 100 mM) were needed to reduce the increase of [3H]PDBu binding in cells grown for 4, 8, and 12 DIV. Ethanol significantly inhibited NMDA-stimulated [3H]PDBu binding in a concentration-dependent fashion in cells maintained in culture for 4 and 8 days, respectively, with a slightly less pronounced inhibition by ethanol (50 mM) seen in cells kept for 2 and 12 DIV. Application of higher ethanol concentrations (> 100 mM), inhibited the NMDA-induced stimulation in all cell preparations. Following kainic acid-induced enhancement of [3H]PDBu binding, ethanol (100 mM) reduced the binding only in cells maintained for 2 DIV. Even higher ethanol concentrations (200 mM) inhibited the effects of kainic acid only in cells maintained for 2 and 4 DIV, respectively. Our data suggest that various subclasses of glutamate receptors display a developmentally determined differential sensitivity to ethanol at least in cerebellar granule cells in vitro.

Acute ethanol alters calcium signals elicited by glutamate receptor agonists and K+ depolarization in cultured cerebellar Purkinje neurons

The effect of acute ethanol on Ca2+ signals evoked by ionotropic (iGluR) and metabotropic (mGluR) glutamate receptor (GluR) activation and K+ depolarization was examined in cultured rat cerebellar Purkinje neurons to assess the ethanol sensitivity of these Ca2+ signaling pathways. Mature Purkinje neurons approximately 3 weeks in vitro were studied. iGluRs were activated by (RS)-alpha-amino-3-hydroxyl-5 methyl-4-isoxazolepropionic acid (AMPA; 1 and 5 microM) and domoate (5 microM). mGluRs were activated by (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD; 300 microM) and (R,S)-3,5-dihydroxyphenylglycine (DHPG; 200 microM). These agents and K+ (150 mM) were applied from micropipettes by brief (1 s) microperfusion pulses. Ca2+ levels were monitored at 2-3 s intervals during pre- and post-stimulus periods using microscopic digital imaging and the Ca2+ sensitive dye fura-2. iGluR and mGluR agonists and K+ produced abrupt increases in intracellular Ca2+ that slowly recovered to baseline resting levels. Acute exposure to ethanol at 33 mM (150 mg%) and 66 mM (300 mg%) significantly reduced the amplitude of the Ca2+ signals to iGluR agonists and K+ with little or no effect on Ca2+ signals to mGluR agonists. In contrast, acute ethanol at 10 mM (45 mg%) had no effect on the Ca2+ signals to the iGluR agonist AMPA but significantly enhanced the Ca2+ signals to the mGluR agonist DHPG. These results show that ethanol modulates Ca2+ signaling linked to GluR activation in a receptor subtype specific manner, and suggest that Ca2+ signaling pathways linked to GluR activation and membrane depolarization may be important mechanisms by which ethanol alters the transduction of excitatory synaptic signals at glutamatergic synapses and thereby affects intercellular and intracellular communication in the CNS.

Metabotropic glutamate receptor agonists alter neuronal excitability and Ca2+ levels via the phospholipase C transduction pathway in cultured Purkinje neurons

Selective agonists for metabotropic glutamate receptor (mGluR) subtypes were tested on mature, cultured rat cerebellar Purkinje neurons (> or = 21 days in vitro) to identify functionally relevant mGluRs expressed by these neurons and to investigate the transduction pathways associated with mGluR-mediated changes in membrane excitability. Current-clamp recordings (nystatin/perforated-patch method) were used to measure the membrane response of Purkinje neurons to brief microperfusion pulses (1.5 s) of the group I (mGluR1/mGluR5) agonists (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (300 microM), quisqualate (5 microM), and (R,S)-3,5-dihydroxyphenylglycine (50-500 microM). All group I mGluR agonists elicited biphasic membrane responses and burst activity in the Purkinje neurons. In addition, the group I mGluR agonists produced alterations in the active membrane properties of the Purkinje neurons and depressed the OFF response after hyperpolarizing current injection. In parallel microscopic Ca2+ imaging experiments, application of the group I mGluR agonists to fura-2-loaded cells elicited increases in intracellular Ca2+ in both the somatic and dendritic regions. The group II (mGluR2/mGluR3) agonist (2S,3S,4S)-alpha-(carboxycyclopropyl)-glycine (10 microM) and the group III (mGluR4/mGluR6/mGluR7/mGluR8) agonists L(+)-2-amino-4-phosphonobutyric acid (1 mM) and O-phospho-L-serine (200 microM) had no effect on the membrane potential or intracellular Ca2+ levels of the Purkinje neurons. The cultured Purkinje neurons, but not granule neurons or interneurons, showed immunostaining for mGluR1alpha in both the somatic and dendritic regions. All effects of the group I mGluR agonists were blocked by (+)-alpha-methyl-4-carboxyphenylglycine (1 mM), an mGluR antagonist. Furthermore, the phospholipase C inhibitor 1-[6-((17beta-3-methoxyestra-1,3,5(10)-trien-17-yl)amino)hexyl]-1H -pyrrole-2,5-dione (2 microM) blocked the group I mGluR agonist-mediated electrophysiological response and greatly attenuated the Ca2+ signal elicited by group I mGluR agonists, particularly in the dendrites. The inactive analogue 1-[6-((17beta-3-methoxyestra-1,3,5(10)-trien-17-yl)amino)hexyl]-2, 5-pyrrolidine-dione (2 microM) was relatively ineffective against the electrophysiological response and Ca2+ signal. These results indicate that functional group I mGluRs (but not group II or III mGluRs) can be activated on mature Purkinje neurons in culture and result in changes in neuronal excitability and intracellular Ca2+ mediated through phospholipase C. These data obtained from a defined neuronal type, the Purkinje neuron, confirm biochemical and molecular studies on the transduction mechanisms of group I mGluRs and show that this transduction pathway is linked to neuronal excitability and intracellular Ca2+ release in the Purkinje neurons.

Chronic alcohol reduces calcium signaling elicited by glutamate receptor stimulation in developing cerebellar neurons

The effect of chronic alcohol (33 mM ethanol) on Ca2+ signals elicited by glutamate receptor agonists (quisqualate and NMDA) was examined in developing cerebellar Purkinje and granule neurons in culture. The neurons were exposed to alcohol during the second week in culture, the main period of morphological and physiological development. The Ca2+ signals were measured with fura-2 based microscopic video imaging. Chronic exposure to alcohol during development significantly reduced the peak amplitude of the Ca2+ signals to quisqualate (1 microM; Quis) in both the somatic and dendritic regions of the Purkinje neurons. The dendritic region was affected to a greater extent than the somatic region. Granule neurons also showed a reduced somatic Ca2+ signal to Quis (dendrites not measured) in the alcohol-treated cultures, indicating that the effect was not limited to Purkinje neurons. In addition to the effects on in the response to Quis, the peak amplitude of the Ca2+ signals to NMDA (100 microM) was reduced by chronic alcohol exposure during development in both the cultured Purkinje and granule neurons. Resting Ca2+ levels were not consistently affected by alcohol treatment in either neuronal type. These results indicate that Ca2+ signaling linked to glutamate receptor activation is an important target of alcohol in the developing nervous system and could be a contributing factor in the altered CNS function and development observed in animal models of fetal alcohol syndrome.

Effects of ethanol on ion channels

Ion channels play critical roles in nervous system function, from initiating rapid synaptic activity to propagation of action potentials. Studies have indicated that many of the effects of ethanol on the nervous system are likely caused by the actions of ethanol on ion channels. Ion channels are multimeric structures that gate ions through subtle changes in tertiary structure. Ethanol readily enters molecular sites within multimeric ion channels, modifying intermolecular forces and bonds that are important for the open-close-inactivation kinetic properties of channels. The diversity of channel composition caused by the multimeric structure results in subtypes of channels that have a spectrum of sensitivity to ethanol that translates into brain regional differences in ethanol sensitivity, in part caused by differences in ion channel subunit composition. Ethanol has been shown to affect both receptor-activated ion channels and voltage-gated ion channels. The acute intoxicating and incoordinating effects of ethanol are probably related to inhibition of subtypes of NMDA-glutamate receptor ion channels and potentiation of certain subtypes of GABAA receptor ion channels. Effects on these channels, as well as glycine, nicotinic cholinergic, serotonergic, and other ion channels, likely contribute to the euphoric, sedative, and other acute actions of ethanol. Changes in ion channel subunit composition, density, and properties probably also contribute to ethanol tolerance, dependence, withdrawal hyperexcitability, and neurotoxicity. A substantial number of studies have implicated glutamate NMDA receptor, GABAA, and L-type voltage-gated calcium channels in the adaptive changes in the brain during chronic ethanol exposure. The diversity of ion channels subunits, their prominent role in brain function, and ethanol action are likely to make them important contributors to alcoholism and alcohol abuse.