Ethanol reduces the duration of single evoked spikes by a selective inhibition of voltage-gated calcium currents in acutely dissociated supraoptic neurons of the rat

Forced Abstinence from Volitional Ethanol Intake Drives a Vulnerable Period of Hyperexcitability in BNST-Projecting Insular Cortex Neurons

The insular cortex (IC) integrates sensory and interoceptive cues to inform downstream circuitry executing adaptive behavioral responses. The IC communicates with areas involved canonically in stress and motivation. IC projections govern stress and ethanol recruitment of bed nucleus of the stria terminalis (BNST) activity necessary for the emergence of negative affective behaviors during alcohol abstinence. Here, we assess the impact of the chronic drinking forced abstinence (CDFA) volitional home cage ethanol intake paradigm on synaptic and excitable properties of IC neurons that project to the BNST (IC→BNST). Using whole-cell patch-clamp electrophysiology, we investigated IC→BNST circuitry 24 h or 2 weeks following forced abstinence (FA) in female C57BL6/J mice. We find that IC→BNST cells are transiently more excitable following acute ethanol withdrawal. In contrast, in vivo ethanol exposure via intraperitoneal injection, ex vivo via ethanol wash, and acute FA from a natural reward (sucrose) all failed to alter excitability. In situ hybridization studies revealed that at 24 h post FA BK channel mRNA expression is reduced in IC. Further, pharmacological inhibition of BK channels mimicked the 24 h FA phenotype, while BK activation was able to decrease AP firing in control and 24 h FA subjects. All together these data suggest a novel mechanism of homeostatic plasticity that occurs in the IC→BNST circuitry following chronic drinking.

Voltage-Sensitive Calcium Channels in the Brain: Relevance to Alcohol Intoxication and Withdrawal

Voltage-sensitive Ca²⁺ (Ca_V) channels are the primary route of depolarization-induced Ca²⁺ entry in neurons and other excitable cells, leading to an increase in intracellular Ca²⁺ concentration ([Ca²⁺]_i). The resulting increase in [Ca²⁺]_i activates a wide range of Ca²⁺-dependent processes in neurons, including neurotransmitter release, gene transcription, activation of Ca²⁺-dependent enzymes, and activation of certain K⁺ channels and chloride channels. In addition to their key roles under physiological conditions, Ca_V channels are also an important target of alcohol, and alcohol-induced changes in Ca²⁺ signaling can disturb neuronal homeostasis, Ca²⁺-mediated gene transcription, and the function of neuronal circuits, leading to various neurological and/or neuropsychiatric symptoms and disorders, including alcohol withdrawal induced-seizures and alcoholism.

Thirst sensation and oral dryness following alcohol intake

Substantial acute and chronic intakes of alcohol or ethanol (EtOH) severely influence oral sensations, such as thirst and oral dryness (dry mouth, xerostomia). Thirst sensation and oral dryness are primarily caused by the activation of neurons in brain regions, including the circumventricular organs and hypothalamus, which are referred to as the dipsogenic center, and by a decrease in salivary secretion, respectively. The sensation of thirst experienced after heavy-alcohol drinking is widely regarded as a consequence of EtOH-induced diuresis; however, EtOH in high doses induces anti-diuresis. Recently, it has been proposed that the ethanol metabolite acetaldehyde induces thirst via two distinct processes in the central nervous system from EtOH-induced diuresis, based on the results of animal experiments. The present review describes new insights regarding the induction mechanism of thirst sensation and oral dryness after drinking alcohol.

Voltage-Sensitive Potassium Channels of the BK Type and Their Coding Genes Are Alcohol Targets in Neurons

Among all members of the voltage-gated, TM6 ion channel superfamily, the proteins that constitute calcium- and voltage-gated potassium channels of large conductance (BK) and their coding genes are unique for their involvement in ethanol-induced disruption of normal physiology and behavior. Moreover, in vitro studies document that BK activity is modified by ethanol with an EC₅₀~23 mM, which is near blood alcohol levels considered legal intoxication in most states of the USA (0.08 g/dL = 17.4 mM). Following a succinct introduction to our current understanding of BK structure and function in central neurons, with a focus on neural circuits that contribute to the neurobiology of alcohol use disorders (AUD), we review the modifications in organ physiology by alcohol exposure via BK and the different molecular elements that determine the ethanol response of BK in alcohol-naïve systems, including the role of an ethanol-recognizing site in the BK-forming slo1 protein, modulation of accessory BK subunits, and their coding genes. The participation of these and additional elements in determining the response of a system or an organism to protracted ethanol exposure is consequently analyzed, with insights obtained from invertebrate and vertebrate models. Particular emphasis is put on the role of BK and coding genes in different forms of tolerance to alcohol exposure. We finally discuss genetic results on BK obtained in invertebrate organisms and rodents in light of possible extrapolation to human AUD.

Ethanol affects network activity in cultured rat hippocampus: mediation by potassium channels

The effects of ethanol on neuronal network activity were studied in dissociated cultures of rat hippocampus. Exposure to low (0.25–0.5%) ethanol concentrations caused an increase in synchronized network spikes, and a decrease in the duration of individual spikes. Ethanol also caused an increase in rate of miniature spontaneous excitatory postsynaptic currents. Higher concentrations of ethanol eliminated network spikes. These effects were reversible upon wash. The effects of the high, but not the low ethanol were blocked by the GABA antagonist bicuculline. The enhancing action of low ethanol was blocked by apamin, an SK potassium channel antagonist, and mimicked by 1-EBIO, an SK channel opener. It is proposed that in cultured hippocampal networks low concentration of ethanol is associated with SK channel activity, rather than the GABAergic receptor.

Nonsocial functions of hypothalamic oxytocin

Oxytocin (OXT) is a hypothalamic neuropeptide composed of nine amino acids. The functions of OXT cover a variety of social and nonsocial activity/behaviors. Therapeutic effects of OXT on aberrant social behaviors are attracting more attention, such as social memory, attachment, sexual behavior, maternal behavior, aggression, pair bonding, and trust. The nonsocial behaviors/functions of brain OXT have also received renewed attention, which covers brain development, reproduction, sex, endocrine, immune regulation, learning and memory, pain perception, energy balance, and almost all the functions of peripheral organ systems. Coordinating with brain OXT, locally produced OXT also involves the central and peripheral actions of OXT. Disorders in OXT secretion and functions can cause a series of aberrant social behaviors, such as depression, autism, and schizophrenia as well as disturbance of nonsocial behaviors/functions, such as anorexia, obesity, lactation failure, osteoporosis, diabetes, and carcinogenesis. As more and more OXT functions are identified, it is essential to provide a general view of OXT functions in order to explore the therapeutic potentials of OXT. In this review, we will focus on roles of hypothalamic OXT on central and peripheral nonsocial functions.

Alcohol excites cerebellar Golgi cells by inhibiting the Na+/K+ ATPase

Alcohol-induced alterations of cerebellar function cause motor coordination impairments that are responsible for millions of injuries and deaths worldwide. Cognitive deficits associated with alcoholism are also a consequence of cerebellar dysfunction. The mechanisms responsible for these effects of ethanol are poorly understood. Recent studies have identified neurons in the input layer of the cerebellar cortex as important ethanol targets. In this layer, granule cells (GrCs) receive the majority of sensory inputs to the cerebellum through the mossy fibers. Information flow at these neurons is gated by a specialized pacemaker interneuron known as the Golgi cell, which provides divergent GABAergic input to thousands of GrCs. In vivo electrophysiological experiments have previously shown that acute ethanol exposure abolishes GrC responsiveness to sensory inputs carried by mossy fibers. Slice electrophysiological studies suggest that ethanol causes this effect by potentiating GABAergic transmission at Golgi cell-to-GrC synapses through an increase in Golgi cell excitability. Using patch-clamp electrophysiological techniques in cerebellar slices and computer modeling, we show here that ethanol excites Golgi cells by inhibiting the Na(+)/K(+) ATPase. Voltage-clamp recordings of Na(+)/K(+) ATPase currents indicated that ethanol partially inhibits this pump and this effect could be mimicked by low concentrations of ouabain. Partial inhibition of Na(+)/K(+) ATPase function in a computer model of the Golgi cell reproduced these experimental findings. These results establish a novel mechanism of action of ethanol on neuronal excitability, which likely has a role in ethanol-induced cerebellar dysfunction and may also contribute to neuronal functional alterations in other brain regions.

Alcohol Pathophysiology

There is no specialized alcohol addiction area in the brain; rather, alcohol acts on a wide range of excitatory and inhibitory nervous networks to modulate neurotransmitters actions by binding with and altering the function of specific proteins. With no hemato-encephalic barrier for alcohol, its actions are strongly related to the amount of intake. Heavy alcohol intake is associated with both structural and functional changes in the central nervous system with long-term neuronal adaptive changes contributing to the phenomena of tolerance and withdrawal. The effects of alcohol on the function of neuronal networks are heterogeneous. Because ethanol affects neural activity in some brain sites but is without effect in others, its actions are analyzed in terms of integrated connectivities in the functional circuitry of neuronal networks, which are of particular interest because of the cognitive interactions discussed in the manuscripts contributing to this review. Recent molecular data are reviewed as a support for the other contributions dealing with cognitive disturbances related to alcohol acute and addicted consumption.

BK channel and alcohol, a complicated affair

Alcohol is a fast acting molecule that alters behavior within a few minutes of absorption. Its rapid behavioral impact suggests early action on ion channels. Of all voltage-gated potassium ion channels, BK channels, a subcategory of potassium channels characterized by their large unitary conductance, and by their capacity of being activated synergistically by membrane potential and intracellular free calcium, are unique due to their high sensitivity to alcohol. In this review, we discuss BK channels structure and function, and how they help us understand the various ways BK channel mediates alcohol's effects on neuronal function and on behavior in the striatum.

Compartmentalized beta subunit distribution determines characteristics and ethanol sensitivity of somatic, dendritic, and terminal large-conductance calcium-activated potassium channels in the rat central nervous system

Neurons are highly differentiated and polarized cells, whose various functions depend upon the compartmentalization of ion channels. The rat hypothalamic-neurohypophysial system (HNS), in which cell bodies and dendrites reside in the hypothalamus, physically separated from their nerve terminals in the neurohypophysis, provides a particularly powerful preparation in which to study the distribution and regional properties of ion channel proteins. Using electrophysiological and immunohistochemical techniques, we characterized the large-conductance calcium-activated potassium (BK) channel in each of the three primary compartments (soma, dendrite, and terminal) of HNS neurons. We found that dendritic BK channels, in common with somatic channels but in contrast to nerve terminal channels, are insensitive to iberiotoxin. Furthermore, analysis of dendritic BK channel gating kinetics indicates that they, like somatic channels, have fast activation kinetics, in contrast to the slow gating of terminal channels. Dendritic and somatic channels are also more sensitive to calcium and have a greater conductance than terminal channels. Finally, although terminal BK channels are highly potentiated by ethanol, somatic and dendritic channels are insensitive to the drug. The biophysical and pharmacological properties of somatic and dendritic versus nerve terminal channels are consistent with the characteristics of exogenously expressed alphabeta1 versus alphabeta4 channels, respectively. Therefore, one possible explanation for our findings is a selective distribution of auxiliary beta1 subunits to the somatic and dendritic compartments and beta4 to the terminal compartment. This hypothesis is supported immunohistochemically by the appearance of distinct punctate beta1 or beta4 channel clusters in the membrane of somatic and dendritic or nerve terminal compartments, respectively.

Ethanol Interactions With Calcium-Dependent Potassium Channels

In most neurons and other excitable cells, calcium-activated potassium channels of small (SK) and large conductance (BK; MaxiK) control excitability and neurotransmitter release. The spontaneous activity of dopamine neurons of the ventral tegmental area is increased by ethanol. This ethanol excitation is potentiated by selective blockade of SK, indicating that SK channels modulate ethanol stimulation of neurons that are critical in reward and reinforcement. On the other hand, ethanol directly modulates BK channel activity in a variety of systems, including rat neurohypophysial nerve endings, primary sensory dorsal root ganglia, nucleus accumbens neurons, Caenorhabditis elegans type-IV dopaminergic CEP neurons, and nonneuronal preparations, such as rat pituitary cells, cerebrovascular myocytes and human umbilical vein endothelial cells. Ethanol action on BK channels can modify neuropeptide and growth hormone release, nociception, cerebrovascular tone, and endothelial proliferation. Ethanol modulates BK channels even when the drug is evaluated using recombinant BK channel-forming alpha (slo) subunits or channel reconstitution in artificial, binary lipid bilayers, indicating that the slo subunit and its immediate lipid microenvironment are the essential targets of ethanol. Consistent with this, single amino acid slo channel mutants display altered ethanol sensitivity. Furthermore, C. elegans slo1 null mutants are resistant to ethanol-induced motor incoordination. On the other hand, Drosophila melanogaster slo null mutants fail to acquire acute tolerance to ethanol sedation. Ethanol action on slo channels, however, may be tuned by a variety of factors, including posttranslational modification of slo subunits, coexpression of channel accessory subunits, and the lipid microenvironment, resulting in increase, refractoriness, or even decrease in channel activity. In brief, both SK and BK channels are important targets of ethanol throughout the body, and interference with ethanol effects on these channels could form the basis for novel pharmacotherapies to ameliorate the actions or consequences of alcohol abuse.

Somatic localization of a specific large-conductance calcium-activated potassium channel subtype controls compartmentalized ethanol sensitivity in the nucleus accumbens

Alcohol is an addictive drug that targets a variety of ion channels and receptors. To address whether the effects of alcohol are compartment specific (soma vs dendrite), we examined the effects of ethanol (EtOH) on large-conductance calcium-activated potassium channels (BK) in cell bodies and dendrites of freshly isolated neurons from the rat nucleus accumbens (NAcc), a region known to be critical for the development of addiction. Compartment-specific drug action was indeed observed. Clinically relevant concentrations of EtOH increased somatic but not dendritic BK channel open probability. Electrophysiological single-channel recordings and pharmacological analysis of the BK channel in excised patches from each region indicated a number of differences, suggestive of a compartment-specific expression of the beta4 subunit of the BK channel, that might explain the differential alcohol sensitivity. These parameters included activation kinetics, calcium dependency, and toxin blockade. Reverse transcription-PCR showed that both BK channel beta1 and beta4 subunit mRNAs are found in the NAcc, although the signal for beta1 is significantly weaker. Immunohistochemistry revealed that beta1 subunits were found in both soma and dendrites, whereas beta4 appeared restricted to the soma. These findings suggest that the beta4 subunit may confer EtOH sensitivity to somatic BK channels, whereas the absence of beta4 in the dendrite results in insensitivity to the drug. Consistent with this idea, acute EtOH potentiated alphabeta4 BK currents in transfected human embryonic kidney cells, whereas it failed to alter alphabeta1 BK channel-mediated currents. Finally, an EtOH concentration (50 mm) that increased BK channel open probability strongly decreased the duration of somatic-generated action potential in NAcc neurons.

Ethanol targets: a BK channel cocktail in C. elegans

A genetic screen for resistance to ethanol intoxication in Caenorhabditis elegans isolated mutants of the gene slo-1. slo-1 encodes the pore-forming subunit of a large-conductance Ca(2+)-activated K(+) channel previously shown to limit excitatory neurotransmitter release in C. elegans. Electrophysiological data recorded in vivo are consistent with a model in which ethanol potentiation of SLO-1 produces intoxication in C. elegans by reducing excitatory neurotransmitter release.

Ethanol withdrawal seizure susceptibility is associated with upregulation of L- and P-type Ca2+ channel currents in rat inferior colliculus neurons

Calcium currents in the inferior colliculus (IC) are thought to play an important role in ethanol withdrawal hyperexcitability. Here, we report on the modulation of Ca(2+) channel currents in acutely dissociated IC neurons of rats, exhibiting higher incidence of audiogenic seizures when subjected to ethanol withdrawal. Whole cell Ca(2+) channel currents were activated by depolarizing pulses from a holding potential of -90 mV, in 10 mV increments, using barium (Ba(2+)) as the charge carrier. The high threshold voltage-activated (HVA) Ca(2+) channel current density increased significantly in IC neurons following ethanol withdrawal. The gating parameters of HVA Ca(2+) channel currents were only slightly altered, while the fraction of current that did not fully inactivate at positive potentials increased significantly following ethanol withdrawal. Pharmacological dissection of HVA Ca(2+) channel currents suggested that the enhanced current, associated with increased incidence of audiogenic seizures following ethanol withdrawal, was carried by L- and P-type Ca(2+) channels. The upregulation of L- and P-type currents may be responsible for IC neuronal hyperexcitability associated with increased susceptibility to ethanol withdrawal seizures.

Molecular modelling of specific and non-specific anaesthetic interactions

There has been rapid progress in molecular modelling in recent years. The convergence of improved software for molecular mechanics and dynamics, techniques for chimeric substitution and site-directed mutations, and the first x-ray structures of transmembrane ion channels have made it possible to build and test models of anaesthetic binding sites. These models have served as guides for site-directed mutagenesis and as starting points for understanding the molecular dynamics of anaesthetic-site interactions. Ligand-gated ion channels are targets for inhaled anaesthetics and alcohols in the central nervous system. The inhibitory strychnine-sensitive glycine and gamma-aminobutyric acid type A receptors are positively modulated by anaesthetics and alcohols; site-directed mutagenesis techniques have identified amino acid residues important for the action of volatile anaesthetics and alcohols in these receptors. Key questions are whether these amino acid mutations form part of alcohol- or anaesthetic-binding sites or if they alter protein stability in a way that allows anaesthetic molecules to act remotely by non-specific mechanisms. It is likely that molecular modelling will play a major role in answering these questions.

Integrated channel plasticity contributes to alcohol tolerance in neurohypophysial terminals

Short-term ethanol challenge results in the reduction of peptide hormone release from the rat neurohypophysis. However, rats that have been maintained on an ethanol-containing diet for 3 to 4 weeks exhibit tolerance to this effect. Mechanistic underpinnings of this tolerance were probed by examining four ion channel conductances critical for neurohormone release. The voltage-gated L-type calcium channel and the functionally linked calcium-activated BK channel represent a functional dyad. Although these channels show opposite drug responses in the naive terminal (i.e., the L-type Ca2+ channel is inhibited whereas the BK channel is potentiated), the effect of long-term alcohol exposure is to decrease sensitivity to the short-term administration of drug in both instances. In addition to the shift in sensitivity, current density increased for the L-type Ca2+ current and decreased for the BK current, consistent with a compensatory change. Sensitivity to alcohol was also altered for two other channel types studied. Inhibition of the voltage-gated transient Ca2+ current was lessened after long-term treatment. I(A,) which is not sensitive to the drug at clinically relevant concentrations in terminals from the naive rat, acquires sensitivity after long-term exposure, representing a potentially novel type of tolerance. However, neither the transient Ca2+ current nor I(A) shows a change in current density, demonstrating the selectivity of this aspect of tolerance. Overall, these results demonstrate that channel plasticity can explain at least a portion of the behavioral tolerance resulting from changes in sensitivity of peptide hormone release. Furthermore, they suggest that an understanding of tolerance requires the examination of dynamically coupled channel populations.

Verapamil prevents withdrawal excitation of oxytocin neurones in morphine-dependent rats

We investigated whether the full expression of morphine withdrawal excitation by supraoptic nucleus (SON) oxytocin neurones is a property of the neurones themselves or a partial function of their afferent inputs, by interrupting synaptic input activity via central administration of the L-type Ca(2+) channel blocker verapamil. In morphine-dependent rats, withdrawal-induced release of oxytocin from the posterior pituitary was suppressed by prior administration of intracerebroventricular (i.c.v.) verapamil (160 microg), as was release of oxytocin within the SON measured by microdialysis. During morphine withdrawal the increased electrical activity of SON neurones was also reduced both by i.c.v. verapamil and microdialysis application of verapamil or nifedipine into the SON. Oxytocin secretion evoked by electrical stimulation of the pituitary stalk was unaffected by i.c.v. verapamil suggesting a central site of action. To determine whether the inhibitory actions of verapamil were specific to morphine withdrawal, we also investigated the effects of verapamil on other oxytocin-secreting stimuli. I.C.V. verapamil given to morphine-naïve rats abolished pituitary oxytocin release in response to activation of brainstem or rostral excitatory inputs by cholecystokinin (20 microg kg(-1), i.v.) and 1.5 M saline (4 ml kg(-1), i.p.) respectively, whilst in lactating rats, i.c.v. verapamil reduced suckling-induced release of oxytocin within the SON. These results suggest that verapamil has a central site of action on stimulated oxytocin release (including an action within the SON) and that both pre and post-synaptic L-type Ca(2+) channels are required for the full expression of morphine withdrawal in SON oxytocin neurones.

Rat supraoptic magnocellular neurones show distinct large conductance, Ca2+-activated K+ channel subtypes in cell bodies versus nerve endings

1. Large conductance, Ca2+-activated K+ (BK) channels were identified in freshly dissociated rat supraoptic neurones using patch clamp techniques. 2. The single channel conductance of cell body BK channels, recorded from inside-out patches in symmetric 145 mM K+, was 246.1 pS, compared with 213 pS in nerve ending BK channels (P<0.01). 3. At low open probability (Po), the reciprocal of the slope in the ln(NPo)-voltage relationship (N, number of available channels in the patch) for cell body and nerve ending channels were similar: 11 vs. 14 mV per e-fold change in NPo, respectively. 4. At 40 mV, the [Ca2+]i producing half-maximal activation was 273 nM, as opposed to >1.53 microM for the neurohypophysial channel, indicating the higher Ca2+ sensitivity of the cell body isochannel. 5. Cell body BK channels showed fast kinetics (open time constant, 8.5 ms; fast closed time constant, 1.6 and slow closed time constant, 12.7 ms), identifying them as 'type I' isochannels, as opposed to the slow gating (type II) of neurohypophysial BK channels. 6. Cell body BK activity was reduced by 10 nM charybdotoxin (NPo, 37% of control), or 10 nM iberiotoxin (NPo, 5% of control), whereas neurohypophysial BK channels are insensitive to charybdotoxin at concentrations as high as 360 nM. 7. Whilst blockade of nerve ending BK channels markedly slowed the repolarization of evoked single spikes, blockade of cell body channels was without effect on repolarization of evoked single spikes. 8. Ethanol reversibly increased neurohypophysial BK channel activity (EC50, 22 mM; maximal effect, 100 mM). In contrast, ethanol (up to 100 mM) failed to increase cell body BK channel activity. 9. In conclusion, we have characterized BK channels in supraoptic neuronal cell bodies, and demonstrated that they display different electrophysiological and pharmacological properties from their counterparts in the nerve endings.