Ethanol inhibits muscarinic receptor-stimulated phosphoinositide metabolism and calcium mobilization in rat primary cortical cultures

Fluorescent assessment of intracellular calcium in individual cells

Calcium is an important intracellular ion involved in numerous cell processes. There are multiple factors that contribute to the release of Ca²⁺. Some factors induce release as part of intracellular signaling cascades, while others result in unwanted changes to both basal and inducible Ca²⁺ levels. The accurate measurement of intracellular Ca²⁺ is, therefore, an important tool in neurotoxicology for assessing compounds/substances that disrupt Ca²⁺ homeostasis. Fluorescent, Ca²⁺-sensitive probes Indo-1 and Fluo-3 allow for the quantification of intracellular Ca²⁺ in individual cells to determine what effects neurotoxins have on both basal and stimulus-dependent Ca²⁺ concentrations.

Ethanol inhibits muscarinic receptor-induced axonal growth in rat hippocampal neurons

BACKGROUND

In utero alcohol exposure can lead to fetal alcohol spectrum (FAS) disorders characterized by cognitive and behavioral deficits. In vivo and in vitro studies have shown that ethanol alters neuronal development. One mechanism through which ethanol has been shown to exert its effects is the perturbation of activated signaling cascades. The cholinergic agonist carbachol has been shown to induce axonal outgrowth through intracellular calcium mobilization, protein kinase C (PKC) activation, and ERK1/2 phosphorylation. This study investigated the effect of ethanol on the differentiation of rat hippocampal pyramidal neurons induced by carbachol as a possible mechanism involved in the developmental neurotoxicity of ethanol.

METHODS

Prenatal rat hippocampal pyramidal neurons were treated with ethanol (50 to 75 mM) in the presence or absence of carbachol for 24 hours. Neurite outgrowth was assessed spectrophotometrically; axonal length was measured in neurons fixed and immunolabeled with the neuron-specific betaIII tubulin antibody; cytotoxicity was analyzed using the thiazolyl blue tetrazolium bromide assay. The effect of ethanol on carbachol-stimulated intracellular calcium mobilization was assessed utilizing the fluorescent calcium probe, Fluo-3AM. The PepTag(R) assay for nonradioactive detection of PKC from Promega was used to measure PKC activity, and ERK1/2 activation was determined by densitometric analysis of Western blots probed for phospo-ERK1/2.

RESULTS

Ethanol treatment (50 to 75 mM) caused an inhibition of carbachol-induced axonal growth, without affecting neuronal viability. Neuron treatment for 15 minutes with ethanol did not inhibit the carbachol-stimulated rise in intracellular calcium, while inhibiting PKC activity at the highest tested concentration and ERK1/2 phosphorylation at both the concentrations used in this study. On the other hand, neuron treatment for 24 hours with ethanol significantly inhibited carbachol-induced increase in intracellular calcium.

CONCLUSIONS

Ethanol inhibited carbachol-induced neurite outgrowth by inhibiting PKC and ERK1/2 activation. These effects may be, in part, responsible for some of the cognitive deficits associated with in utero alcohol exposure.

Phosphatidic acid modulates G protein regulation of phospholipase C-beta1 activity in membranes

Regulation of G protein stimulated phospholipase C-beta1 (PLC-beta1) activity by phosphatidic acid (PA) was determined in membranes. In cerebral cortical membranes, PLC-beta1 is under dual regulation by G protein stimulatory and inhibitory mechanisms. PA stimulated basal activity and was synergistic with G protein activation in increasing PLC-beta1 activity. Lysophosphatidic acid (LPA) also stimulated PLC-beta1 activity, but was less effective then PA. PA stimulation of PLC-beta1 activity was relatively independent of acyl chain length. PA decreased the Ca2+ dependence for G protein stimulation of PLC-beta1 activity. PA modulated the dual G protein regulation of PLC-beta1 activity, increasing stimulatory regulation and reducing inhibitory G protein regulation. The sensitivity to guanosine 5'-[gamma-thio]trisphosphate (GTP-gamma-S) and carbachol stimulation of PLC-beta1 activity was increased by PA. These results demonstrate that PA regulates both basal activity and G protein stimulation of PLC-beta1 activity. The data indicates that PA regulates the PLC-beta1 signaling pathway and thus may have an important role in the modulation of cell activation.

Amyloid beta peptide 1-40 and the function of rat hippocampal hemicholinium-3 sensitive choline carriers: effects of a proteolytic degradation in vitro

Effects of amyloid beta peptide 1-40 (Abeta) and of plant cysteine proteases bromelain and papain on the high-affinity uptake of choline (HACU) and the specific binding of [3H]hemicholinium-3 ([3H]HC-3) have been investigated on hippocampal synaptosomes from young adult male Wistar rats under basal and stimulated conditions (55 mM KCl). Depolarization increased significantly the HACU levels (the changes were predominantly in Vmax) and mildly the [3H]HC-3 binding (the changes especially in K(D)). Nonaggregated Abeta at low nM concentrations suppressed the depolarization effects but was ineffective under basal conditions during a short-term incubation. Higher microM concentrations decreased the HACU and binding under basal conditions in a time-dependent manner. The binding changes were firstly associated with alterations in K(D) and secondarily were accompanied also by a drop in Bmax. The results suggest that Abeta directly influences high-affinity carriers, inhibits their transport activity and enhances their sensitivity to proteoLytic cleavage. Stimulation increases the sensitivity of carriers to the interaction with Abeta.

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.

Agonist-stimulated Ca2+ response in neutrophils from patients with primary alcoholism and alcoholized healthy subjects

The sensitivity of the inositol phosphate (IP)/Ca2+-second messenger generating system was assessed in neutrophils from healthy volunteers before and after ingestion of approximately 1%o ethanol for 2 h. In addition, isolated neutrophils from healthy subjects were incubated with ethanol in vitro. Furthermore, the sensitivity of the IP/Ca2+ system was evaluated in neutrophils from alcoholic patients in the state of active drinking, and after 2-3 weeks and 6 months of abstinence. EC50 values of the concentration-response curves obtained by agonist stimulation with formyl-methionyl-leucylphenylalanine (fMLP) of the intracellular Ca2+ accumulation were determined as an indicator of the sensitivity of the system. Ingestion of ethanol by healthy volunteers (both in the ex vivo and in vitro experiments) induced a rightward shift of the concentration-response curve (higher EC50 values) in neutrophils, indicating a reduced sensitivity to agonist stimulation evoked by ethanol. The sensitivity of the Ca2+ response in neutrophils from alcoholic patients decreased intraindividually after a period of 2-3 weeks of abstinence (higher EC50 values) and was at this time also significantly lower compared to a group of matched healthy controls In contrast, the maximal Ca2+ release induced by a saturating concentration of fMLP was increased after 2-3 weeks of abstinence, both intraindividually and in comparison to healthy controls. These alterations of the EC50 values and the maximal Ca2+ response were normalized after 6 months of abstinence. It is concluded that ethanol attenuates the sensitivity of the IP/Ca2+ system in neutrophils in healthy subjects. In neutrophils from alcoholic subjects complex alterations appear to persist up to several weeks, which are only normalized after a prolonged period of abstinence.

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.

Neuronal signaling systems and ethanol dependence

In recent years there have been remarkable developments toward the understanding of the molecular and/or cellular changes in the neuronal second-messenger pathways during ethanol dependence. In general, it is believed that the cyclic adenosine 3',5'-monophosphate (cAMP) and the phosphoinositide (PI) signal-transduction pathways may be the intracellular targets that mediate the action of ethanol and ultimately contribute to the molecular events involved in the development of ethanol tolerance and dependence. Several laboratories have demonstrated that acute ethanol exposure increases, whereas protracted ethanol exposure decreases, agonist-stimulated adenylate cyclase activity in a variety of cell systems, including the rodent brain. Recent studies indicate that various postreceptor events of the cAMP signal transduction cascade (i.e., Gs protein, protein kinase A [PKA], and cAMP-responsive element binding protein [CREB]) in the rodent brain are also modulated by chronic ethanol exposure. The PI signal-transduction cascade represents another important second-messenger system that is modulated by both acute and chronic ethanol exposure in a variety of cell systems. It has been shown that protracted ethanol exposure significantly decreases phospholipase C (PLC) activity in the cerebral cortex of mice and rats. The decreased PLC activity during chronic ethanol exposure may be caused by a decrease in the protein levels of the PLC-beta 1 isozyme but not of PLC-delta 1 or PLC-gamma 1 isozymes in the rat cerebral cortex. Protein kinase C (PKC), which is a key step in the PI-signaling cascade, has been shown to be altered in a variety of cell systems by acute or chronic ethanol exposure. It appears from the literature that PKC plays an important role in the modulation of the function of various neurotransmitter receptors (e.g., gamma-aminobutyrate type A [GABAA], N-methyl-D-aspartate [NMDA], serotonin2A [5-HT2A], and 5-HT2C, and muscarinic [m1] receptors) resulting from ethanol exposure. The findings described in this review article indicate that neuronal-signaling proteins represent a molecular locus for the action of ethanol and are possibly involved in the neuro-adaptational mechanisms to protracted ethanol exposure. These findings support the notion that alterations in the cAMP and the PI-signaling cascades during chronic ethanol exposure could be the critical molecular events associated with the development of ethanol dependence.

Ethanol selectively interferes with the trophic action of NMDA and carbachol on cultured cerebellar granule neurons undergoing apoptosis

Exposure of mature rat cerebellar granule neurons to non-depolarizing conditions (5 mM K+) for 24 h resulted in the onset of apoptosis. NMDA, forskolin, carbachol and GABA attenuated low K+-induced toxicity, although to a different extent, with NMDA and GABA being the most effective agents. When cells were co-exposed for 24 h to ethanol, the survival promoting action of NMDA and carbachol, but not that of forskolin and GABA, was attenuated. By contrast, a 24 h cell pre-treatment with ethanol, followed by its removal prior to K+ deprivation, was ineffective towards the neurotrophic action of NMDA and carbachol. The concomitant presence of alcohol and neurotrophic factors was not required for the pro-apoptotic effect of ethanol to be manifest after a long-term alcohol exposure: inhibition of NMDA- and carbachol-mediated neurotrophism was still observed when cells were pre-exposed for 72 h to alcohol in depolarizing conditions, prior to the challenge with 5 mM K+-containing medium and the test compounds in the absence of ethanol. The present study shows that ethanol promotes apoptotic cell death of cultured cerebellar neurons by selectively inhibiting the neurotrophic effect of NMDA and carbachol, and suggests that alcohol may cause permanent changes in the control mechanisms of apoptosis: this finding may have significant implications for the in vivo toxicity of prenatal ethanol exposure on the developing cerebellum.

Carbachol-stimulated Ca2+ increase in single neuroblastoma SH-SY5Y cells: effects of ethanol

The effect of ethanol on the characteristics of carbachol-stimulated release of Ca2+ from intracellular Ca2+ stores was studied in single SH-SY5Y cells. Stimulation with carbachol, in the absence of extracellular Ca2+, elicited a rapid Ca2+ increase in SH-SY5Y cells peaking within seconds after addition of maximal agonist concentration. The Ca2+ response pattern in single cells resembled the population response, and there was no evidence of oscillatory changes in cytosolic [Ca2+] ([Ca2+]i). However, cell-to-cell variability could be detected in the magnitude and the latency time of the response, and in the rate of [Ca2+]i increase. In a carbachol dose-response analysis, the EC50 for the number of responsive cells and for the peak [Ca2+]i response was lower than that for carbachol-induced inositol 1,4,5-trisphosphate formation by a factor of 5 to 50. Ethanol (100 mM) caused a significant suppression of the number of responsive cells, but only when cells were stimulated with nonsaturating carbachol concentrations (1 and 10 microM). The suppression by ethanol was evident primarily in those cells that gave a Ca2+ response after several seconds of stimulation, whereas cells that responded within the initial seconds of receptor stimulation remained relatively unaffected. In responding cells stimulated with 10 microM carbachol, ethanol exposure also suppressed the maximal Ca2+ increase primarily in those cells that responded late. We suggest that ethanol suppression of muscarinic receptor-mediated signal transduction through the phospholipase C pathway may depend on the potentiation of feedback inhibition that requires receptor stimulation.

Xestospongins: potent membrane permeable blockers of the inositol 1,4,5-trisphosphate receptor

Xestospongins (Xe's) A, C, D, araguspongine B, and demethylxestospongin B, a group of macrocyclic bis-1-oxaquinolizidines isolated from the Australian sponge, Xestospongia species, are shown to be potent blockers of IP3-mediated Ca2+ release from endoplasmic reticulum vesicles of rabbit cerebellum. XeC blocks IP3-induced Ca2+ release (IC50 = 358 nM) without interacting with the IP3-binding site, suggesting a mechanism that is independent of the IP3 effector site. Analysis of Pheochromocytoma cells and primary astrocytes loaded with Ca2+-sensitive dye reveals that XeC selectively blocks bradykinin- and carbamylcholine-induced Ca2+ efflux from endoplasmic reticulum stores. Xe's represent a new class of potent, membrane permeable IP3 receptor blockers exhibiting a high selectivity over ryanodine receptors. Xe's are a valuable tool for investigating the structure and function of IP3 receptors and Ca2+ signaling in neuronal and nonneuronal cells.