Continuous exposure of cultured rat cerebellar macroneurons to ethanol-depressed NMDA and KCl-stimulated elevations of intracellular calcium

Ethanol-Induced Alterations in Purkinje Neuron Dendrites in Adult and Aging Rats: a Review

Uncomplicated alcoholics suffer from discrete motor dysfunctions that become more pronounced with age. These deficits involve the structure and function of Purkinje neurons (PN), the sole output neurons from the cerebellar cortex. This review focuses on alterations to the PN dendritic arbor in the adult and aging Fischer 344 rat following lengthy alcohol consumption. It describes seminal studies using the Golgi-Cox method which proposed a model for ethanol-induced dendritic regression. Subsequent ultrastructural studies of PN dendrites showed dilation of the extensive smooth endoplasmic reticulum (SER) which preceded and accompanied dendritic regression. The component of the SER that was most affected by ethanol was the sarco/endoplasmic reticulum Ca(2+) ATPase pump (SERCA) responsible for resequestration of calcium into the SER. Ethanol-induced decreases in SERCA pump levels, similar to the finding of SER dilation, preceded and occurred concomitantly with dendritic regression. Discrete ethanol-induced deficits in balance also accompanied these decreases. Ethanol-induced ER stress within the SER of PN dendrites was proposed as an underlying cause of dendritic regression. It was recently shown that increased activation of caspase 12, inherent to the ER, occurred in PN of acute slices in ethanol-fed rats and was most pronounced following 40 weeks of ethanol treatment. These findings shed new light into alcohol-induced disruption in PN dendrites providing a new model for the discrete but critical changes in motor function in aging, adult alcoholics.

Transport of 5-formyltetrahydrofolate into primary cultured cerebellar granule cells

Transport of 5-formyltetrahydrofolate (5-FTHF) into primary cultured cerebellar granule cells (CGC) was studied. Uptake of 5-FTHF into CGC was saturable with K(m)=2.86 microM and V(max)=40.8 pmol/mg protein/45 min in pH 7.4 medium. Uptake of 5-FTHF in the astrocytes has a similar style in the time curve. Uptake of 5-FTHF is characterized by countertransport because adding unlabeled 5-FTHF in the medium resulted in the efflux of labeled 5-FTHF. Uptake of 5-FTHF was inhibited by the structural analogs 5-methyltetrahydrofolate, methotrexate and folic acid (K(i)=6.64, 7.69, and 19.38 microM, respectively). Uptake was significantly decreased by high concentrations of sodium azide and sodium arsenate but not by sodium cyanide. Uptake was also inhibited by p-chloromercuriphenylsulfonate and by the anions probenecid and 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid. Acute exposure of the cells to ethanol (100 mM) did not affect the uptake. It is concluded that CGC have a carrier-mediated system for the uptake of 5-FTHF and other folates.

Transport of 5-formyltetrahydrofolate into primary cultured rat astrocytes

Transport of 5-formyltetrahydrofolate (5-FTHF) into primary cultured rat astrocytes was studied. Uptake of 5-FTHF into astrocytes was linear in the first 60 min and is saturable with K(m)=3.3 microM and V(max)=27.5 pmol/mg protein/45 min in pH 7.4 medium. Uptake of 5-FTHF displayed the characteristics of countertransport. Uptake of 5-FTHF was inhibited by the structural analogs 5-methyltetrahydrofolate, methtrexate, and folic acid (K(i)=3.8, 2.7, and 18.4 microM, respectively). Uptake was significantly decreased by sodium azide but was increased by high concentration of sodium cyanide and low concentration of sodium arsenate. Uptake was also inhibited by p-chloromercuriphenylsulfonate and by the anions probenecid and 4,4(')-diisothiocyanostilbene-2,2(')-disulfonic acid. Acute exposure of the cells to ethanol (100mM) inhibited the uptake for 90 min of the experimental duration. It is concluded that astrocytes have a system for the uptake of 5-FTHF and folate analogs which is carrier mediated, this system is sensitive to energy inhibitors and alcohol exposure.

Ethanol-induced death of postnatal hippocampal neurons

Fetal alcohol exposure causes severe neuropsychiatric problems, but mechanisms of the ethanol-associated changes in central nervous system development are unclear. In vivo, ethanol's interaction with N-methyl-D-aspartate (NMDA) and gamma-aminobutyric acid type A (GABA(A)) receptors may cause increased apoptosis in the immature forebrain. We examined whether ethanol affects survival of neonatal hippocampal neurons in primary cultures. A 6-day ethanol exposure killed hippocampal neurons with an LD50 of approximately 25 mM. Elevated extracellular potassium or insulin-related growth factor 1 inhibited cell loss. Although potentiation of GABA(A) receptors or complete block of NMDA receptors also kills hippocampal neurons, pharmacological studies suggest that ethanol's interaction with GABA(A) and NMDA receptors is not sufficient to explain ethanol's effects on neuronal survival. Ca(2+) influx in response to depolarization was depressed >50% by chronic ethanol treatment. We suggest that chronic ethanol may promote neuronal loss through a mechanism affecting Ca(2+) influx in addition to effects on postsynaptic GABA and glutamate receptors.

Stimulation of cannabinoid receptor agonist 2-arachidonylglycerol by chronic ethanol and its modulation by specific neuromodulators in cerebellar granule neurons

In an earlier study, we reported that chronic ethanol (EtOH) stimulates the formation of anandamide in human SK-N-SH cells. In the present study, we investigated the effect of chronic EtOH on the formation of yet another cannabinoid receptor (CB1) agonist, 2-arachidonylglycerol (2-AG), in cerebellar granule neurons (CGNs). The formation of 2-[(3)H]AG without any stimulation was more pronounced in the older cultures than in younger cultures. Exposure of CGNs to EtOH led to a significant increase in the level of 2-[(3)H]AG (P<0.05). Incubation with the anandamidehydrolase inhibitor phenylmethylsulfonyl fluoride and EtOH did result in an additive increase in 2-[(3)H]AG, but did not with E-6-(bromomethylene)tetrahydro-3-(1-naphthelenyl)-2H-pyran-2-one. The formation of 2-[(3)H]AG was enhanced by ionomycin in both the control and EtOH-exposed CGNs, and the ionomycin-stimulated 2-[(3)H]AG synthesis was inhibited by the intracellular chelating agent 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid. Further, glutamate increased the formation of 2-[(3)H]AG only in control CGNs. MK-801 inhibited the EtOH-induced 2-[(3)H]AG synthesis, suggesting the participation of intracellular Ca(2+) in EtOH-induced 2-[(3)H]AG synthesis. The dopamine receptor (D2) agonist did not modify the 2-AG synthesis in either the control or EtOH-exposed CGNs. However, the D2 receptor antagonist inhibited the EtOH-induced formation of 2-[(3)H]AG. The EtOH-induced 2-[(3)H]AG formation was inhibited by SR141716A and pertussis toxin, suggesting the CB1 receptor- and Gi/o-protein-mediated regulation of 2-AG. The observed increase in 2-AG level in CGNs is possibly a mechanism for neuronal adaptation to the continuous presence of EtOH. These findings indicate that some of the pharmacological actions of EtOH may involve alterations in the endocannabinoid signaling system.

Chronic ethanol exposure attenuates the anti-apoptotic effect of NMDA in cerebellar granule neurons

Ethanol, added to primary cultures of cerebellar granule neurons simultaneously with NMDA, was previously shown to inhibit the anti-apoptotic effect of NMDA. The in vitro anti-apoptotic effect of NMDA is believed to mimic in vivo protection against apoptosis afforded by innervation of developing cerebellar granule neurons by glutamatergic mossy fibers. Therefore, the results suggested that the presence of ethanol in the brain at a critical period of development would promote apoptosis. In the present studies, we examined the effect of chronic ethanol exposure on the anti-apoptotic action of NMDA in cerebellar granule neurons. The neurons were treated with ethanol in vitro for 1-3 days in the absence of NMDA. Even after ethanol was removed from the culture medium, as ascertained by gas chromatography, the protective effect of added NMDA was significantly attenuated. The decreased anti-apoptotic effect of NMDA was associated with a change in the properties of the NMDA receptor, as indicated by a decrease in ligand binding, decreased expression of NMDA receptor subunit proteins, and decreased functional responses including stimulation of increases in intracellular Ca(2+) and induction of brain-derived neurotrophic factor expression. The latter effect may directly underlie the attenuated protective effect of NMDA in these neurons. The results suggest that ethanol exposure during development can have long-lasting effects on neuronal survival. The change in the NMDA receptor caused by chronic ethanol treatment may contribute to the loss of cerebellar granule neurons that is observed in animals and humans exposed to ethanol during gestation.

Effect of ethanol on muscarinic receptor-induced calcium responses in astroglia

The effects of ethanol on muscarinic receptor-mediated calcium responses were investigated in individual primary rat astrocytes and human 132 1N1 astrocytoma cells using indo-1/AM and image cytometry. After a 30-min incubation, carbachol-induced calcium responses were inhibited only at 100 or 250 mM ethanol. The effects of ethanol were more pronounced and occurred at lower concentrations with longer exposures, with significant inhibition seen at 10 mM following a 24-hr incubation. Thapsigargin- and glutamate-induced responses were unaffected by ethanol, indicating some selectivity in this inhibition. Upon removal of ethanol, inhibition of calcium responses persisted for up to 6-12 hr, with carbachol responses returning to control levels by 24 hr after washout. Ethanol exposure did not affect muscarinic-receptor binding in astrocytoma cells, but inhibited carbachol-induced IP(3) formation. Inhibition of (3)H-thymidine incorporation by ethanol also persisted upon removal of the alcohol, with a time-dependency similar to that of the calcium responses. These results indicate that ethanol inhibits muscarinic receptor-induced calcium responses in astroglia in a concentration- and duration-dependent manner. They also show that co-incubation with ethanol is not necessary for this effect, suggesting that long-term exposure to ethanol may modify, in a reversible manner, the coupling of muscarinic receptors with its effector. This effect of ethanol may play a role in ethanol's inhibition of carbachol-induced thymidine incorporation.

Chronic ethanol increases the cannabinoid receptor agonist anandamide and its precursor N-arachidonoylphosphatidylethanolamine in SK-N-SH cells

In an earlier study, we demonstrated that chronic ethanol (EtOH) exposure down-regulated the cannabinoid receptors (CB1) in mouse brain synaptic plasma membrane. In the present study, we investigated the effect of chronic EtOH on the formation of anandamide (AnNH), an endogenous cannabimimetic compound, and its precursor N-arachidonoylphosphatidylethanolamine (N-ArPE) in SK-N-SH cells that were prelabeled with [3H]arachidonic acid. The results indicate that exposure of SK-N-SH cells to EtOH (100 mM) for 72 h significantly increased levels of [3H]AnNH and [3H]N-ArPE (p < 0.05) (1.43-fold for [3H]AnNH and 1.65-fold for [3H]N-ArPE). Exposure of SK-N-SH cells to EtOH (100 mM, 24 h) inhibited initially the formation of [3H]AnNH at 24 h, followed by a progressive increase, reaching a statistical significance level at 72 h (p < 0.05). [3H]N-ArPE increased gradually to a statistically significant level after 48 and 72 h (p < 0.05). Incubation with exogenous ethanolamine (7 mM) and EtOH (100 mM, 72 h) did not result in an additive increase in the formation of [3H]AnNH. The formation of [3H]AnNH and [3H]N-ArPE by EtOH was enhanced by the Ca2+ ionophore A23187 or by the depolarizing agent veratridine and the K+ channel blocker 4-aminopyridine. Further, the EtOH-induced formation of [3H]AnNH and [3H]N-ArPE was inhibited by exogenous AnNH, whereas only [3H]AnNH formation was inhibited by the CB1 receptor antagonist SR141716A and pertussis toxin, suggesting that the CB1 receptor and G(i/o) protein mediated the regulation of AnNH levels. The observed increase in the levels of these lipids in SK-N-SH cells may be a mechanism for neuronal adaptation and may serve as a compensatory mechanism to counteract the continuous presence of EtOH. The present observation taken together with our previous results indicate the involvement of the endocannabinoid system in mediating some of the pharmacological actions of EtOH and may constitute part of a common brain pathway mediating reinforcement of drugs of abuse including EtOH.

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.

N-Methyl-D-aspartate inhibits apoptosis through activation of phosphatidylinositol 3-kinase in cerebellar granule neurons. A role for insulin receptor substrate-1 in the neurotrophic action of n-methyl-D-aspartate and its inhibition by ethanol

Primary cultured rat cerebellar granule neurons underwent apoptosis when switched from medium containing 25 mM K+ to one containing 5 mM K+. N-methyl-D-aspartate (NMDA) protected granule neurons from apoptosis in medium containing 5 mM K+. Inhibition of apoptosis by NMDA was blocked by the phosphatidylinositol 3-kinase (PI 3-kinase) inhibitor LY294002, but it was unaffected by the mitogen-activated protein kinase kinase inhibitor PD 98059. The antiapoptotic action of NMDA was associated with an increase in the tyrosine phosphorylation of insulin receptor substrate 1 (IRS-1), an increase in the binding of the regulatory subunit of PI 3-kinase to IRS-1, and a stimulation of PI 3-kinase activity. In the absence of extracellular Ca2+, NMDA was unable to prevent apoptosis or to phosphorylate IRS-1 and activate PI 3-kinase. Significant inhibition of NMDA-mediated neuronal survival by ethanol (10-15%) was observed at 1 mM, and inhibition was half-maximal at 45-50 mM. Inhibition of neuronal survival by ethanol corresponded with a marked reduction in the capacity of NMDA to increase the concentration of intracellular Ca2+, phosphorylate IRS-1, and activate PI 3-kinase. These data demonstrate that the neurotrophic action of NMDA and its inhibition by ethanol are mediated by alterations in the activity of a PI 3-kinase-dependent antiapoptotic signaling pathway.

Neonatal alcohol exposure reduces NMDA induced Ca2+ signaling in developing cerebellar granule neurons

Glutamatergic neurotransmission through NMDA receptors is critical for both neurogenesis and mature function of the central nervous system (CNS), and is thought to be one target for developmentally-induced damage by alcohol to brain function. In the current study we examined Ca2+ signaling linked to NMDA receptor activation as a potential site for alcohol's detrimental effects on the developing nervous system. We compared Ca2+ signals to NMDA in granule neurons cultured from cerebella of rat neonates exposed to alcohol (ethanol) during development with responses to NMDA recorded in separated control groups. Alcohol exposure was by the vapor chamber method on postnatal days 4-7. An intermittent exposure paradigm was used where the pups were exposed to alcohol vapor for 2. 5 h/day to produce peak BALs of approximately 320 mg%. Control pups were placed in an alcohol-free chamber for a similar time period or remained with their mother. After culture under alcohol-free conditions for up to 9 days, Ca2+ signaling in response to NMDA was measured using fura-2 Ca2+ imaging. Results show that the peak amplitude of the Ca2+ signal to NMDA was significantly smaller in cultured granule neurons obtained from alcohol-treated pups compared to granule neurons from control pups. In contrast, the Ca2+ signal to K+ depolarization was not depressed by the alcohol treatment. Resting Ca2+ levels were also altered by the alcohol treatment. These results show that intermittent alcohol exposure during development in vivo can induce long-term changes in CNS neurons that affect the Ca2+ signaling pathway linked to NMDA receptors and resting Ca2+ levels. Such changes could play an important role in the CNS dysfunction associated with alcohol exposure during CNS development.

Effects of prenatal ethanol exposure on voltage-dependent calcium entry into neonatal whole brain-dissociated neurons

The effect of prenatal ethanol exposure on voltage-dependent calcium entry into neonatal-dissociated neurons was studied. Dissociated whole brain cells were isolated from neonates of prenatally ethanol-treated (ET), pair-fed (PF) control, and ad libitum (AL) control groups and loaded with fura-2. Prenatal ethanol exposure resulted in a significant reduction of calcium entry into K(+)-depolarized cells, compared with AL and PF control treatments. Initially, in dissociated cells from AL control animals, it was found that nifedipine (1 microM), omega-agatoxin (100 nM), and omega-conotoxin (500 nM), to a much lesser extent, significantly inhibited the 45 mM KCl-stimulated calcium entry. To determine the inhibitory action of prenatal ethanol exposure on N-, P-, and L-type voltage-dependent calcium channels, treatment of neonatal-dissociated neurons with different combinations of omega-conotoxin, omega-agatoxin, and nifedipine, respectively, was compared in the prenatal ethanol and control treatment groups. The inhibition of K(+)-stimulated increase in calcium entry by prenatal ethanol exposure was significantly less in the presence or absence of single antagonist conditions (ET < AL and PF). There was no apparent interaction of ethanol exposure and antagonist condition. However, the reduced calcium entry after prenatal ethanol exposure was superseded by the stronger inhibition in dual and triple antagonist conditions. The magnitude of the calcium response inhibition by the antagonist combinations was similar among the ET, PF, and AL groups. Thus, these results suggest that prenatal ethanol exposure decreases voltage-dependent calcium entry into neonatal-dissociated neurons in a manner that does not seem to involve the selective inhibition of any individual N-, P-, or L-type calcium channel.

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.