Ethanol inhibits the sensory responses of cerebellar granule cells in anesthetized cats

The Cerebellar GABAAR System as a Potential Target for Treating Alcohol Use Disorder

In the brain, fast inhibitory neurotransmission is mediated primarily by the ionotropic subtype of the gamma-aminobutyric acid (GABA) receptor subtype A (GABA_AR). It is well established that the brain's GABA_AR system mediates many aspects of neurobehavioral responses to alcohol (ethanol; EtOH). Accordingly, in both preclinical studies and some clinical scenarios, pharmacologically targeting the GABA_AR system can alter neurobehavioral responses to acute and chronic EtOH consumption. However, many of the well-established interactions of EtOH and the GABA_AR system have been identified at concentrations of EtOH ([EtOH]) that would only occur during abusive consumption of EtOH (≥40 mM), and there are still inadequate treatment options for prevention of or recovery from alcohol use disorder (AUD, including abuse and dependence). Accordingly, there is a general acknowledgement that more research is needed to identify and characterize: (1) neurobehavioral targets of lower [EtOH] and (2) associated brain structures that would involve such targets in a manner that may influence the development and maintenance of AUDs.Nearly 15 years ago it was discovered that the GABA_AR system of the cerebellum is highly sensitive to EtOH, responding to concentrations as low as 10 mM (as would occur in the blood of a typical adult human after consuming 1-2 standard units of EtOH). This high sensitivity to EtOH, which likely mediates the well-known motor impairing effects of EtOH, combined with recent advances in our understanding of the role of the cerebellum in non-motor, cognitive/emotive/reward processes has renewed interest in this system in the specific context of AUD. In this chapter we will describe recent advances in our understanding of cerebellar processing, actions of EtOH on the cerebellar GABA_AR system, and the potential relationship of such actions to the development of AUD. We will finish with speculation about how cerebellar specific GABA_AR ligands might be effective pharmacological agents for treating aspects of AUD.

Mini-Review: Effects of Ethanol on GABAA Receptor-Mediated Neurotransmission in the Cerebellar Cortex--Recent Advances

Studies from several laboratories have shown that ethanol impairs cerebellar function, in part, by altering GABAergic transmission. Here, we discuss recent advances in our understanding of the acute effects of ethanol on GABA(A) receptor-mediated neurotransmission at cerebellar cortical circuits, mainly focusing on electrophysiological studies with slices from laboratory animals. These studies have shown that acute ethanol exposure increases GABA release at molecular layer interneuron-to-Purkinje cell synapses and also at reciprocal synapses between molecular layer interneurons. In granule cells, studies with rat cerebellar slices have consistently shown that acute ethanol exposure both potentiates tonic currents mediated by extrasynaptic GABA(A) receptors and also increases the frequency of spontaneous inhibitory postsynaptic currents mediated by synaptic GABA(A) receptors. These effects have been also documented in some granule cells from mice and nonhuman primates. Currently, there are two distinct models on how ethanol produces these effects. In one model, ethanol primarily acts by directly potentiating extrasynaptic GABA(A) receptors, including a population that excites granule cell axons and stimulates glutamate release onto Golgi cells. In the other model, ethanol acts indirectly by increasing spontaneous Golgi cell firing via inhibition of the Na(+)/K(+) ATPase, a quinidine-sensitive K(+) channel, and neuronal nitric oxide synthase. It was also demonstrated that a direct inhibitory effect of ethanol on tonic currents can be unmasked under conditions of low protein kinase C activity. In the last section, we briefly discuss studies on the chronic effect of ethanol on cerebellar GABA(A) receptor-mediated transmission and highlight potential areas where future research is needed.

Ethanol-Induced Cerebellar Ataxia: Cellular and Molecular Mechanisms

The cerebellum is an important target of ethanol toxicity given that cerebellar ataxia is the most consistent physical manifestation of acute ethanol consumption. Despite the significance of the cerebellum in ethanol-induced cerebellar ataxia (EICA), the cellular and molecular mechanisms underlying EICA are incompletely understood. However, two important findings have shed greater light on this phenomenon. First, ethanol-induced blockade of cerebellar adenosine uptake in rodent models points to a role for adenosinergic A1 modulation of EICA. Second, the consistent observation that intracerebellar administration of nicotine in mice leads to antagonism of EICA provides evidence for a critical role of cerebellar nitric oxide (NO) in EICA reversal. Based on these two important findings, this review discusses the potential molecular events at two key synaptic sites (mossy fiber-granule cell-Golgi cell (MGG synaptic site) and granule cell parallel fiber-Purkinje cell (GPP synaptic site) that lead to EICA. Specifically, ethanol-induced neuronal NOS inhibition at the MGG synaptic site acts as a critical trigger for Golgi cell activation which leads to granule cell deafferentation. Concurrently, ethanol-induced inhibition of adenosine uptake at the GPP synaptic site produces adenosine accumulation which decreases glutamate release and leads to the profound activation of Purkinje cells (PCs). These molecular events at the MGG and GPP synaptic sites are mutually reinforcing and lead to cerebellar dysfunction, decreased excitatory output of deep cerebellar nuclei, and EICA. The critical importance of PCs as the sole output of the cerebellar cortex suggests normalization of PC function could have important therapeutic implications.

Neuronal oscillations in Golgi cells and Purkinje cells are accompanied by decreases in Shannon information entropy

Neuronal oscillations have been shown to contribute to the function of the cerebral cortex by coordinating the neuronal activities of distant cortical regions via a temporal synchronization of neuronal discharge patterns. This can occur regardless whether these regions are linked by cortico-cortical pathways or not. Less is known concerning the role of neuronal oscillations in the cerebellum. Golgi cells and Purkinje cells are both principal cell types in the cerebellum. Purkinje cells are the sole output cells of the cerebellar cortex while Golgi cells contribute to information processing at the input stage of the cerebellar cortex. Both cell types have large cell bodies, as well as dendritic structures, that can generate large currents. The discharge patterns of both these cell types also exhibit oscillations. In view of the massive afferent information conveyed by the mossy fiber-granule cell system to different and distant areas of the cerebellar cortex, it is relevant to inquire the role of cerebellar neuronal oscillations in information processing. In this study, we compared the discharge patterns of Golgi cells and Purkinje cells in conscious rats and in rats anesthetized with urethane. We assessed neuronal oscillations by analyzing the regularity in the timing of individual spikes within a spike train by using autocorrelograms and fast-Fourier transform. We measured the differences in neuronal oscillations and the amount of information content in a spike train (defined by Shannon entropy processed per unit time) in rats under anesthesia and in conscious, awake rats. Our findings indicated that anesthesia caused more prominent neuronal oscillations in both Golgi cells and Purkinje cells accompanied by decreases in Shannon information entropy in their spike trains.

Identification of gene expression profile in the rat brain resulting from acute alcohol intoxication

This study aimed to identify gene expression profile in the rat brain resulting from acute alcohol intoxication (AAI). Eighteen SD rats were divided into the alcohol-treated group (n = 9) and saline control group (n = 9). Periorbital blood samples were taken to determine their blood alcohol content by gas chromatography. Tissue sections were analyzed by H and E staining and biochemical assays. Real-time reverse transcription PCR was used to validate microarray data. Statistical analysis was carried out using SPSS18.0 software (Version 18.0, SPSS Inc., Chicago, IL, USA). H and E staining demonstrated that alcohol-treated rats showed no obvious pathological changes in nerve cells compared with those in the control group. Biochemical tests revealed that alcohol-treated rats had lower superoxide dismutase activity than those in the control group (167.3 ± 10.3 U/mg vs. 189.2 ± 5.9 U/mg, P < 0.05). Furthermore, the malondialdehyde levels in alcohol-treated rats were higher than those in the control group (3.48 ± 0.24 mmol/mg vs. 2.51 ± 0.23 mmol/mg, P < 0.05). Microarray data presented 366 up-regulated genes and 300 down-regulated genes in the AAI rat brain. Gene ontology analysis identified 31 genes up-regulated and 39 down-regulated among all differentially expressed genes. Twenty-four pathways showed significant differences, including 12 pathways involved with up-regulated genes and 12 pathways involved with down-regulated genes. Selected genes showed significantly different expression in both alcohol-treated and control groups (P < 0.05). Gene expression analysis enabled clustering of alcohol intoxication-related genes by function. These genes expression may be potential targets for treatment or drug screening for acute alcohol intoxication.

Three types of ependymal cells with intracellular calcium oscillation are characterized by distinct cilia beating properties

Ependymal cells are multiciliated epithelial cells that line the ventricles in the adult brain. Abnormal function or structure of ependymal cilia has been associated with various neurological deficits. For the first time, we report three distinct ependymal cell types, I, II, and III, based on their unique ciliary beating frequency and beating angle. These ependymal cells have specific localizations within the third ventricle of the mouse brain. Furthermore, neither ependymal cell types nor their localizations are altered by aging. Our high-speed fluorescence imaging analysis reveals that these ependymal cells have an intracellular pacing calcium oscillation property. Our study further shows that alcohol can significantly repress the amplitude of calcium oscillation and the frequency of ciliary beating, resulting in an overall decrease in volume replacement by the cilia. Furthermore, the pharmacological agent cilostazol could differentially increase cilia beating frequency in type II, but not in type I or type III, ependymal cells. In summary, we provide the first evidence of three distinct types of ependymal cells with calcium oscillation properties.

Acute ethanol exposure inhibits silencing of cerebellar Golgi cell firing induced by granule cell axon input

Golgi cells (GoCs) are specialized interneurons that provide inhibitory input to granule cells in the cerebellar cortex. GoCs are pacemaker neurons that spontaneously fire action potentials, triggering spontaneous inhibitory postsynaptic currents in granule cells and also contributing to the generation tonic GABA_A receptor-mediated currents in granule cells. In turn, granule cell axons provide feedback glutamatergic input to GoCs. It has been shown that high frequency stimulation of granule cell axons induces a transient pause in GoC firing in a type 2-metabotropic glutamate receptor (mGluR2)-dependent manner. Here, we investigated the effect ethanol on the pause of GoC firing induced by high frequency stimulation of granule cell axons. GoC electrophysiological recordings were performed in parasagittal cerebellar vermis slices from postnatal day 23 to 26 rats. Loose-patch cell-attached recordings revealed that ethanol (40 mM) reversibly decreases the pause duration. An antagonist of mGluR2 reduced the pause duration but did not affect the effect of ethanol. Whole-cell voltage-clamp recordings showed that currents evoked by an mGluR2 agonist were not significantly affected by ethanol. Perforated-patch experiments in which hyperpolarizing and depolarizing currents were injected into GoCs demonstrated that there is an inverse relationship between spontaneous firing and pause duration. Slight inhibition of the Na⁺/K⁺ pump mimicked the effect of ethanol on pause duration. In conclusion, ethanol reduces the granule cell axon-mediated feedback mechanism by reducing the input responsiveness of GoCs. This would result in a transient increase of GABA_A receptor-mediated inhibition of granule cells, limiting information flow at the input stage of the cerebellar cortex.

Ethanol attenuates sensory stimulus-evoked responses in cerebellar granule cells via activation of GABA(A) receptors in vivo in mice

Acute alcohol intoxication affects cerebellar motor regulation possibly by altering the transfer and integration of external information in cerebellar cortical neurons, resulting in a dysfunction of cerebellar motor regulation or a cerebellar atexia. However, the synaptic mechanisms of ethanol induced impairments of sensory information processing in cerebellar cortical neurons are not fully understand. In the present study, we used electrophysiological and pharmacological methods to study the effects of ethanol on the sensory stimulation-evoked responses in cerebellar granule cells (GCs) in vivo in urethane anesthetized mice. Air-puff stimulation of the ipsilateral whisker-pad evoked stimulus-on (P1) and stimulus-off responses (P2) in GCs of cerebellar Crus II. Cerebellar surface perfusion of ethanol did not alter the onset latency of the sensory stimulation-evoked responses, but reversible reduced the amplitude of P1 and P2. The ethanol-induced reduction of the GCs sensory responses was concentration-dependent. In the presence of ethanol, the mean half-width, area under curve, rise Tau and decay Tau of P1 were significantly decreased. Blockade of gamma-aminobutyric acid type A (GABA(A)) receptors activity induced an increase in amplitude of P1, and abolished the ethanol induced inhibition of the GCs sensory responses. These results indicate that ethanol inhibits the tactile evoked responses in cerebellar GCs through enhancement of GABA(A) receptors activity.

Na+/K+-ATPase inhibition partially mimics the ethanol-induced increase of the Golgi cell-dependent component of the tonic GABAergic current in rat cerebellar granule cells

Cerebellar granule cells (CGNs) are one of many neurons that express phasic and tonic GABAergic conductances. Although it is well established that Golgi cells (GoCs) mediate phasic GABAergic currents in CGNs, their role in mediating tonic currents in CGNs (CGN-I(tonic)) is controversial. Earlier studies suggested that GoCs mediate a component of CGN-I(tonic) that is present only in preparations from immature rodents. However, more recent studies have detected a GoC-dependent component of CGN-I(tonic) in preparations of mature rodents. In addition, acute exposure to ethanol was shown to potentiate the GoC component of CGN-I(tonic) and to induce a parallel increase in spontaneous inhibitory postsynaptic current frequency at CGNs. Here, we tested the hypothesis that these effects of ethanol on GABAergic transmission in CGNs are mediated by inhibition of the Na(+)/K(+)-ATPase. We used whole-cell patch-clamp electrophysiology techniques in cerebellar slices of male rats (postnatal day 23-30). Under these conditions, we reliably detected a GoC-dependent component of CGN-I(tonic) that could be blocked with tetrodotoxin. Further analysis revealed a positive correlation between basal sIPSC frequency and the magnitude of the GoC-dependent component of CGN-I(tonic). Inhibition of the Na(+)/K(+)-ATPase with a submaximal concentration of ouabain partially mimicked the ethanol-induced potentiation of both phasic and tonic GABAergic currents in CGNs. Modeling studies suggest that selective inhibition of the Na(+)/K(+)-ATPase in GoCs can, in part, explain these effects of ethanol. These findings establish a novel mechanism of action of ethanol on GABAergic transmission in the central nervous system.

Commentary: How ethanol short-circuits the cerebellum-actions on Golgi cells in freely-moving animals

This commentary discusses the important contributions of the article published in this journal by Huang and colleagues, titled, "Acute ethanol exposure increases firing and induces oscillations in cerebellar Golgi cells of freely moving rats." In this manuscript, Huang and colleagues present a number of interesting and important findings. While it has been shown previously that ethanol (EtOH) causes an increase in the firing of cerebellar Golgi cells in brain slice preparations and anesthetized animals, here the authors provide the first evidence that this action of EtOH occurs in vivo in freely moving, unanesthetized animals. These results also enhance our understanding of cerebellar functioning by describing the mechanism by which EtOH essentially de-afferentates (blocks specific inputs to) the cerebellum from the normal processing of sensory signals due to EtOH-induced Golgi neuron excitation, resulting in inhibition of granule cells. Furthermore, the authors characterize the novel observation of EtOH-induced neuronal oscillations, which was not previously observed in other preparations.

Acute ethanol exposure increases firing and induces oscillations in cerebellar Golgi cells of freely moving rats

BACKGROUND

Alcohol is a widely abused substance and is responsible for significant morbidity and mortality worldwide. The precise mechanisms underlying ethanol (EtOH)'s actions in the central nervous system (CNS) remain elusive. In vitro studies suggest that GABAergic interneurons are important targets of EtOH action in the CNS. Although EtOH generally acts to inhibit CNS neurons, it appears to cause an increase in GABAergic interneuron excitability. However, it has yet to be demonstrated that EtOH produces this effect in the brain of behaving animals. Here, we demonstrate for the first time that acute EtOH exposure excites a subtype of GABAergic interneuron (cerebellar Golgi cell [GoC]) in a freely moving animal.

METHODS

Electrophysiological recordings were made from microwire arrays implanted in the anterior cerebellum of freely moving rats.

RESULTS

Cerebellar GoCs display a slow, irregular, spontaneous action potential firing pattern under control conditions. EtOH caused dramatic and consistent increases in the rate and regularity of GoC discharges, including a redistribution of the power in the GoC spike train, such that power became concentrated in the 26.7 ± 7.3 Hz region.

CONCLUSIONS

Taken together with our previous findings, these data suggest that a major mechanism of EtOH actions on cerebellar function is an EtOH-induced de-afferentation at the input stage of the cerebellar cortex in the form of granule cell inhibition, and that this inhibition is caused by an increase in GoC firing. It is likely that GoCs may play a significant role both in the gating of information transmission to granule cells and in the modulation of the overall excitability of the cerebellum by tonically controlling granule cell activity.

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.

Ethanol decreases Purkinje neuron excitability by increasing GABA release in rat cerebellar slices

Cerebellar Purkinje neurons (PNs) receive inhibitory GABAergic input from stellate and basket cells, which are located in the outer and inner portions of the molecular layer, respectively. Ethanol (EtOH) was recently shown to increase GABAergic transmission at PNs via a mechanism that involves enhanced calcium release from presynaptic internal stores (J Pharmacol Exp Ther 323:356-364, 2007). Here, we further characterized the effect of EtOH on GABA release and assessed its impact on PN excitability. Using whole-cell patch-clamp electrophysiological techniques in cerebellar vermis parasagittal slices, we found that EtOH acutely increases the frequency but not the amplitude or half-width of miniature and spontaneous inhibitory postsynaptic currents (IPSCs). EtOH significantly increased the amplitude and decreased the paired pulse ratio of IPSCs evoked by stimulation in the outer but not inner molecular layer. In current clamp, EtOH decreased both the amplitude of excitatory postsynaptic potentials evoked in PNs by granule cell axon stimulation and the number of action potentials triggered by these events; these effects depended on GABA(A) receptor activation because they were not observed in presence of bicuculline. Loose-patch cell-attached PN recordings revealed that neither the spontaneous action potential firing frequency nor the coefficient of variation of the interspike interval was altered by acute EtOH exposure. These findings suggest that EtOH differentially affects GABAergic transmission at stellate cell- and basket cell-to-PN synapses and that it modulates PN firing triggered by granule cell axonal input. These effects could be in part responsible for the cerebellar impairments associated with acute EtOH intoxication.

Modulation of GABAA receptors in cerebellar granule neurons by ethanol: a review of genetic and electrophysiological studies

Cerebellar granule neurons (CGNs) receive inhibitory input from Golgi cells in the form of phasic and tonic currents that are mediated by postsynaptic and extrasynaptic gamma-aminobutyric acid type A (GABAA) receptors, respectively. Extrasynaptic receptors are thought to contain alpha6betaxdelta subunits. Here, we review studies on ethanol (EtOH) modulation of these receptors, which have yielded contradictory results. Although studies with recombinant receptors expressed in Xenopus oocytes indicate that alpha6beta3delta receptors are potently enhanced by acute exposure to low (>or=3 mM) EtOH concentrations, this effect was not observed when these receptors were expressed in Chinese hamster ovary cells. Slice recordings of CGNs have consistently shown that EtOH increases the frequency of phasic spontaneous inhibitory postsynaptic currents (sIPSCs), as well as the tonic current amplitude and noise. However, there is a lack of consensus as to whether EtOH directly acts on extrasynaptic receptors or modulates them indirectly; that is, via an increase in spillover of synaptically released GABA. It was recently demonstrated that an R to Q mutation of amino acid 100 of the alpha6 subunit increases the effect of EtOH on both sIPSCs and tonic current. These electrophysiological findings have not been reproducible in our hands. Moreover, it was shown the alpha6-R100Q mutation enhances sensitivity to the motor-impairing effects of EtOH in outbred Sprague-Dawley rats, but this was not observed in a line of rats selectively bred for high sensitivity to EtOH-induced motor alterations (Alcohol Non-Tolerant rats). We conclude that currently there is insufficient evidence conclusively supporting a direct potentiation of extrasynaptic GABAA receptors following acute EtOH exposure in CGNs.