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Background Alcoholics have been reported to have reduced levels of magnesium in both their extracellular and intracellular compartments. Calcium-dependent potassium channels (BK) are known to be one of ethanol (EtOH)'s better known molecular targets. Methods Using outside-out patches from hippocampal neuronal cultures, we examined the consequences of altered intracellular Mg2+ on the effects that EtOH has on BK channels. Results We find that the effect of EtOH is bimodally influenced by the Mg2+ concentration on the cytoplasmic side. More specifically, when internal Mg2+ concentrations are ≤200 μM, EtOH decreases BK activity, whereas it increases activity when Mg2+ is at 1 mM. Similar results are obtained when using patches from HEK cells expressing only the α-subunit of BK. When patches are made with the actin destabilizer cytochalasin D present on the cytoplasmic side, the potentiation caused by EtOH becomes independent of the Mg2+ concentration. Furthermore, in the presence of the actin stabilizer phalloidin, EtOH causes inhibition even at Mg2+ concentrations of 1 mM. Conclusions Internal Mg2+ can modulate the EtOH effects on BK channels only when there is an intact, internal actin interaction with the channel, as is found at synapses. We propose that the EtOH-induced decrease in cytoplasmic Mg2+ observed in frequent/chronic drinkers would decrease EtOH's actions on synaptic (e.g., actin-bound) BK channels, producing a form of molecular tolerance. [ABSTRACT FROM AUTHOR]

Release of neurotransmitter is activated by the influx of calcium. Inhibition of Ca2+ channels results in less calcium influx into the terminals and presumably a reduction in transmitter release. In the neurohypophysis (NH), Ca2+ channel kinetics, and the associated Ca2+ influx, is primarily controlled by membrane voltage and can be modulated, in a voltage-dependent manner, by G-protein subunits interacting with voltage-gated calcium channels (VGCCs). In this series of experiments we test whether the κ- and µ-opioid inhibition of Ca2+ currents in NH terminals is voltage-dependent. Voltage-dependent relief of G-protein inhibition of VGCC can be achieved with either a depolarizing square pre-pulse or by action potential waveforms. Both protocols were tested in the presence and absence of opioid agonists targeting the κ- and µ-receptors in neurohypophysial terminals. The κ-opioid VGCC inhibition is relieved by such pre-pulses, suggesting that this receptor is involved in a voltage-dependent membrane delimited pathway. In contrast, µ-opioid inhibition of VGCC is not relieved by such pre-pulses, indicating a voltage-independent diffusible second-messenger signaling pathway. Furthermore, relief of κ-opioid inhibition during a physiologic action potential (AP) burst stimulation indicates the possibility of activity-dependent modulation in vivo. Differences in the facilitation of Ca2+ channels due to specific G-protein modulation during a burst of APs may contribute to the fine-tuning of Ca2+-dependent neuropeptide release in other CNS terminals, as well. J. Cell. Physiol. 225: 223–232, 2010. © 2010 Wiley-Liss, Inc. [ABSTRACT FROM AUTHOR]

Peptidergic nerve terminals of the neurohypophysis (NH) secrete both oxytocin and vasopressin upon stimulation with peptide-specific bursts of action potentials from magnocellular neurons. These bursts vary in both frequency and action potential duration and also induce in situ ionic changes both inside and outside the terminals in the NH. These temporary effects include the increase of external potassium and decrease of external calcium, as well as the increase in internal sodium and chloride concentrations. In order to determine any mechanism of action that these ionic changes might have on secretion, stimulus-induced capacitance recordings were performed on isolated terminals of the NH using action potential burst patterns of varying frequency and action potential width. The results indicate that in NH terminals: (1) increased internal chloride concentration improves the efficiency of action potential-induced capacitance changes, (2) increasing external potassium increases stimulus-induced capacitance changes, (3) decreasing external calcium decreases the capacitance induced by low frequency broadened action potentials, while no capacitance change is observed with high frequency un-broadened action potentials, and (4) increasing internal sodium increases the capacitance change induced by low frequency bursts of broadened action potentials, more than for high frequency bursts of narrow action potentials. These results are consistent with previous models of stimulus-induced secretion, where optimal secretory efficacy is determined by particular characteristics of action potentials within a burst. Our results suggest that positive effects of increased internal sodium and external potassium during a burst may serve as a compensatory mechanism for secretion, counterbalancing the negative effects of reduced external calcium. In this view, high frequency un-broadened action potentials (initial burst phase) would condition the terminals by increasing internal sodium for optimal secretion by the physiological later phase of broadened action potentials. Thus, ionic changes occurring during a burst may help to make such stimulation more efficient at inducing secretion. Furthermore, these effects are thought to occur within the initial few seconds of incoming burst activity at both oxytocin and vasopressin types of NH nerve terminals. [ABSTRACT FROM AUTHOR]

Exogenous ATP induces inward currents and causes the release of arginine-vasopressin (AVP) from isolated neurohypophysial terminals (NHT); both effects are inhibited by the P2X2 and P2X3 antagonists, suramin and PPADS. Here we examined the role of endogenous ATP in the neurohypophysis. Stimulation of NHT caused the release of both AVP and ATP. ATP induced a potentiation in the stimulated release of AVP, but not of oxytocin (OT), which was blocked by the presence of suramin. In loose-patch clamp recordings, from intact neurohypophyses, suramin or PPADS produces an inhibition of action potential currents in a static bath, that can be mimicked by a hyperpolarization of the resting membrane potential (RMP). Correspondingly, in a static versus perfused bath there is a depolarization of the RMP of NHT, which was reduced by either suramin or PPADS. We measured an accumulation of ATP (3.7 ± 0.7 µM) released from NHT in a static bath. Applications of either suramin or PPADS to a static bath decreased burst-stimulated capacitance increases in NHT. Finally, only vasopressin release from electrically stimulated intact neurohypophyses was reduced in the presence of Suramin or PPADS. These data suggest that there was sufficient accumulation of ATP released from the neurohypophysis during stimulations to depolarize its nerve terminals. This would occur via the opening of P2X2 and P2X3 receptors, inducing an influx of Ca2+. The subsequent elevation in [Ca2+]i would further increase the stimulated release of only vasopressin from NHT terminals. Such purinergic feedback mechanisms could be physiologically important at most CNS synapses. J. Cell. Physiol. 217: 155–161, 2008. © 2008 Wiley-Liss, Inc. [ABSTRACT FROM AUTHOR]

Bursts of action potentials (APs) are crucial for the release of neurotransmitters from dense core granules. This has been most definitively shown for neuropeptide release in the hypothalamic neurohypophysial system (HNS). Why such bursts are necessary, however, is not well understood. Thus far, biophysical characterization of channels involved in depolarization-secretion coupling cannot completely explain this phenomenon at HNS terminals, so purinergic feedback mechanisms have been proposed. We have previously shown that ATP, acting via P2X receptors, potentiates release from HNS terminals, but that its metabolite adenosine, via A1 receptors acting on transient Ca2+ currents, inhibit neuropeptide secretion. We now show that endogenous adenosine levels are sufficient to cause tonic inhibition of transient Ca2+ currents and of stimulated exocytosis in HNS terminals. Initial non-detectable adenosine levels in the static bath increased to 2.9 µM after 40 min. These terminals exhibit an inhibition (39%) of their transient inward Ca2+ current in a static bath when compared to a constant perfusion stream. CPT, an A1 adenosine receptor antagonist, greatly reduced this tonic inhibition. An ecto-ATPase antagonist, ARL-67156, similarly reduced tonic inhibition, but CPT had no further effect, suggesting that endogenous adenosine is due to breakdown of released ATP. Finally, stimulated capacitance changes were greatly enhanced (600%) by adding CPT to the static bath. Thus, endogenous adenosine functions at terminals in a negative-feedback mechanism and, therefore, could help terminate peptide release by bursts of APs initiated in HNS cell bodies. This could be a general mechanism for controlling transmitter release in these and other CNS terminals. J. Cell. Physiol. 210: 309–314, 2007. © 2006 Wiley-Liss, Inc. [ABSTRACT FROM AUTHOR]

Background The large conductance Ca2+- and voltage-activated K+ channel ( BK) is an important player in molecular and behavioral alcohol tolerance. Trafficking and surface expression of ion channels contribute to the development of addictive behaviors. We have previously reported that internalization of the BK channel is a component of molecular tolerance to ethanol (EtOH). Methods Using primary cultures of hippocampal neurons, we combine total internal reflection fluorescence microscopy, electrophysiology, and biochemical techniques to explore how exposure to Et OH affects the expression and subcellular localization of endogenous BK channels over time. Results Exposure to Et OH changed the expression of endogenous BK channels in a time-dependent manner at the perimembrane area (plasma membrane and/or the area adjacent to it), while total protein levels of BK remain unchanged. These results suggest a redistribution of the channel within the neurons rather than changes in synthesis or degradation rates. Our results showed a temporally nonlinear effect of Et OH on perimembrane expression of BK. First, there was an increase in BK perimembrane expression after 10 minutes of Et OH exposure that remained evident after 3 hours, although not correlated to increases in functional channel expression. In contrast, after 6 hours of Et OH exposure, we observed a significant decrease in both BK perimembrane expression and functional channel expression. Furthermore, after 24 hours of Et OH exposure, perimembrane levels of BK had returned to baseline. Conclusions We report a complex time-dependent pattern in the effect of Et OH on BK channel trafficking, including successive increases and decreases in perimembrane expression and a reduction in active BK channels after 3 and 6 hours of Et OH exposure. Possible mechanisms underlying this multiphasic trafficking are discussed. As molecular tolerance necessarily underlies behavioral tolerance, the time-dependent alterations we see at the level of the channel may be relevant to the influence of drinking patterns on the development of behavioral tolerance. [ABSTRACT FROM AUTHOR]

It has been suggested that drug tolerance represents a form of learning and memory, but this has not been experimentally established at the molecular level. We show that a component of alcohol molecular tolerance (channel internalization) from rat hippocampal neurons requires protein synthesis, in common with other forms of learning and memory. We identify β-catenin as a primary necessary protein. Alcohol increases β-catenin, and blocking accumulation of β-catenin blocks alcohol-induced internalization in these neurons. In transfected HEK293 cells, suppression of Wnt/β-catenin signaling blocks ethanol-induced internalization. Conversely, activation of Wnt/β-catenin reduces BK current density. A point mutation in a putative glycogen synthase kinase phosophorylation site within the S10 region of BK blocks internalization, suggesting that Wnt/β-catenin directly regulates alcohol-induced BK internalization via glycogen synthase kinase phosphorylation. These findings establish de novo protein synthesis and Wnt/β-catenin signaling as critical in mediating a persistent form of BK molecular alcohol tolerance establishing a commonality with other forms of long-term plasticity. [ABSTRACT FROM AUTHOR]