Role of the GTP-binding protein G(o) in the suppressant effect of ethanol on voltage-activated calcium channels of murine sensory neurons

Ethanol alters opioid regulation of Ca(2+) influx through L-type Ca(2+) channels in PC12 cells

BACKGROUND

Studies at the behavioral and synaptic level show that effects of ethanol on the central nervous system can involve the opioid signaling system. These interactions may alter the function of a common downstream target. In this study, we examined Ca(2+) channel function as a potential downstream target of interactions between ethanol and μ or κ opioid receptor signaling.

METHODS

The studies were carried out in a model system, undifferentiated PC12 cells transfected with μ or κ opioid receptors. The PC12 cells express L-type Ca(2+) channels, which were activated by K(+) depolarization. Ca(2+) imaging was used to measure relative Ca(2+) flux during K(+) depolarization and the modulation of Ca(2+) flux by opioids and ethanol.

RESULTS

Ethanol, μ receptor activation, and κ receptor activation all reduced the amplitude of the Ca(2+) signal produced by K(+) depolarization. Pretreatment with ethanol or combined treatment with ethanol and μ or κ receptor agonists caused a reduction in the amplitude of the Ca(2+) signal that was comparable to or smaller than that observed for the individual drugs alone, indicating an interaction by the drugs at a downstream target (or targets) that limited the modulation of Ca(2+) flux through L-type Ca(2+) channels.

CONCLUSIONS

These studies provide evidence for a cellular mechanism that could play an important role in ethanol regulation of synaptic transmission and behavior through interactions with the opioid signaling.

Ethanol alters calcium signaling in axonal growth cones

Calcium (Ca2+) channels are sensitive to ethanol and Ca2+ signaling is a critical regulator of axonal growth and guidance. Effects of acute and chronic exposure to ethanol (22, 43, or 87 mM) on voltage-gated Ca2+ channels (VGCCs) in whole cells, and KCl-induced Ca2+ transients in axonal growth cones, were examined using dissociated hippocampal cultures. Whole-cell patch-clamp analysis in neurons with newly-formed axons (Stage 3) revealed that rapidly inactivating, low-voltage activated (LVA) and non-inactivating, high-voltage activated (HVA) currents were both inhibited in a dose-dependent manner by acute ethanol, with relatively greater inhibition of HVA currents. When assessed by Fluo-4-AM imaging, baseline fluorescence and Ca2+ response to ethanol in Stage 3 neurons was similar compared to neurons without axons, but peak Ca2+ transient amplitudes in response to bath-applied KCl were greater in Stage 3 neurons and were decreased by acute ethanol. The amplitude of Ca2+ transients elicited specifically in axonal growth cones by focal application of KCl was also inhibited by acute exposure to moderate-to-high concentrations of ethanol (43 or 87 mM), whereas a lower concentration (22 mM) had no effect. When 43 or 87 mM ethanol was present continuously in the medium, KCl-evoked Ca2+ transient amplitudes were also reduced in growth cones. In contrast, Ca2+ transients were increased by continuous exposure to 22 mM ethanol. Visualization using a fluorescent dihydropyridine analog revealed that neurons continuously exposed to ethanol expressed increased amounts of L-type Ca2+ channels, with greater increases in axonal growth cones than cell bodies. Thus, acute ethanol reduces Ca2+ current and KCl-induced Ca2+ responses in whole cells and axonal growth cones, respectively, and chronic exposure is also generally inhibitory despite apparent up-regulation of L-type channel expression. These results are consistent with a role for altered growth cone Ca2+ signaling in abnormal neuromorphogenesis associated with fetal alcohol spectrum disorders.

The role of G proteins in the activity and ethanol modulation of glycine-induced currents in rat neurons freshly isolated from the ventral tegmental area

In freshly isolated neurons of the ventral tegmental area of young rats, we first examined the role of G proteins in the functional modulation of the glycine receptor (GlyR). GTP-gamma-S [guanosine-5'-0-(2-thiotriphosphate)] (2 mM) or GDP-beta-S [guanosine 5'-0-(2-thiodiphosphate)] (2 mM) was added to the pipette solution of whole-cell recordings to regulate G protein activities. GTP-gamma-S enhanced the amplitude of glycine-induced current (I(Gly)), suggesting modulation of GlyRs via a G protein-coupled pathway. GDP-beta-S suppressed I(Gly), suggesting that basal G protein activity positively modulates the GlyRs. We next examined effects of G proteins in ethanol potentiation of GlyR function. Activation of G proteins with 2 mM GTP-gamma-S attenuated, but did not eliminate, ethanol-induced potentiation of I(Gly). These results suggest that GTP-gamma-S and ethanol share the same pathway of activating GlyRs. When G proteins are maximally activated by GTP-gamma-S, the action of ethanol was partially occluded. When 2 mM GDP-beta-S was added in pipette solution, ethanol-induced potentiation of I(Gly) was significantly attenuated, suggesting that GDP-beta-S partially blocked the action of ethanol. However, the inability of GTP-gamma-S (or GDP-beta-S) to eliminate completely the potentiating effect of ethanol indicates that some other factors, in addition to G proteins, may also contribute to the action of ethanol on GlyRs.

Regulation of neuronal voltage-gated calcium channels by ethanol

Voltage-gated calcium channels are key regulators of neuronal excitability. Several studies indicate that intoxicating concentrations of ethanol inhibit L-type, N-type and possibly T-type channels. The effects of ethanol on other channel subtypes are not yet clear. Chronic exposure to ethanol is associated with increases in functional L-type channels and this may contribute to signs of ethanol withdrawal. Preclinical studies in animals suggest that L-type calcium channel antagonists decrease ethanol consumption and signs of alcohol withdrawal. Although L-type channel antagonists do not appear to alter the performance impairing or psychological effects of acute ethanol administration, clinical trials will be needed to determine if L-type channel antagonists reduce ethanol consumption in humans.

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.

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.

Ethanol suppresses neuronal Ca2+ currents by effects on intracellular signal transduction

The mechanism of action of ethanol on voltage-activated Ca2+ currents in neurons of the mollusk, Helix pomatia, was studied focusing on intracellular signaling. Ethanol suppressed inward Ca2+ currents in a time- and voltage-dependent manner. Buffering of intracellular Ca2+ with bis(o-aminophenoxy)ethane-N,N,N',N-tetraacetic acid (BAPTA) abolished the ethanol effects on Ca2+ currents. Intracellular GTP-gamma-S injection decreased Ca2+ currents whereas GDP-beta-S injection was ineffective. Ethanol had no further blocking effect on Ca2+ currents in GTP-gamma-S injected cells. In the presence of dopamine, which is known to suppress Ca2+ currents by G0-protein activation, ethanol application was ineffective. The protein kinase C (PKC) blockers, staurosporine and chelerythrine, prevented the ethanol effects on Ca2+ currents. The PKC activators, 1,2-oleoylacetylglycerol (OAG) and beta-phorbol-12,13-dibutyrate (PdBu), both, after maximum stimulation, also occluded the effect of ethanol on Ca2+ currents, whereas in the presence of 4-alpha-phorbol-12,13-didecanoate (4-alpha-PDD), an ineffective phorbol ester, ethanol suppressed Ca2+ currents. Ethanol increased the threshold of Ca2+-dependent action potentials and decreased their duration. Our results indicate that the suppression of voltage-activated Ca2+ currents by ethanol and its effects on action potentials involve activation of a G-protein/protein kinase transduction pathway.

Ethanol actions on the mechanisms of Ca2+ mobilization in rat hippocampal cells are mediated by protein kinase C

The effects of ethanol on intracellular free Ca(2+) concentration, [Ca](i), were studied in cultured rat hippocampal neurons using fluo-3 and confocal microscopy. Ethanol application transiently elevAted [Ca](i) due to Ca(2+)-induced Ca(2+) release from internal stores since the effect was observed also in solutions containing zero Ca(2+) or 0.3 mM La(3+) and restoration of external Ca(2+) content led to secondary response in presence of ethanol. The sites of highest [Ca]i increases correlated well with those obtained after Ca(2+) release from caffeine-and IP3-sensitive internal stores. After single ethanol exposure the caffeine-evoked [Ca](i) transients were potentiated whereas Ca(2+) release induced by IP(3)-mobilizing agonists was suppressed. Similar effects were observed by activation of protein kinase C (PKC) by phorbol esters which also occluded ethanol actions. Ethanol increased fluorescence of Rim-1, a PKC indicator dye. The data obtained are consistent with ethanol activation of PKC whereby Ca(2+) release via ryanodine receptors is potentiated and IP(3) receptors are down-modulated. Since the effects of both ethanol and phorbol esters were mimicked by cytochalasins B and D, PKC-induced cytoskeleton phosphorylation and its subsequent rearrangements can be responsible for observed effects.