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

GAD65 deficient mice are susceptible to ethanol-induced impairment of motor coordination and facilitation of cerebellar neuronal firing

γ-aminobutyric acid (GABA) is a major inhibitory neurotransmitter and its concentrations in the brain could be associated with EtOH-induced impairment of motor coordination. GABA is synthesized by two isoforms of glutamate decarboxylase (GAD): GAD65 and GAD67. Mice deficient in GAD65 (GAD65-KO) can grow up to adulthood, and show that GABA concentration in their adult brains was 50-75% that of wild-type C57BL/6 mice (WT). Although a previous study showed that there was no difference in recovery from the motor-incoordination effect of acute intraperitoneally administered injections of 2.0 g/kg EtOH between WT and GAD65-KO, the sensitivity of GAD65-KO to acute EtOH-induced ataxia has not been fully understood. Here, we sought to determine whether motor coordination and spontaneous firing of cerebellar Purkinje cells (PCs) in GAD65-KO are more sensitive to the effect of EtOH than in WT. Motor performance in WT and GAD65-KO was examined by rotarod and open-field tests following acute administration of EtOH at lower-doses, 0.8, 1.2 and 1.6 g/kg. In a rotarod test, there was no significant difference between WT and GAD65-KO in terms of baseline motor coordination. However, only the KO mice showed a significant decrease in rotarod performance of 1.2 g/kg EtOH. In the open-field test, GAD65-KO showed a significant increase in locomotor activity after 1.2 and 1.6 g/kg EtOH injections, but not WT. In in vitro studies of cerebellar slices, the firing rate of PCs was increased by 50 mM EtOH in GAD65-KO compared with WT, whereas no difference was observed in the effect of EtOH at more than 100 mM between the genotypes. Taken together, GAD65-KO are more susceptible to the effect of acute EtOH exposure on motor coordination and PC firing than WT. This different sensitivity could be attributed to the basal low GABA concentration in the brain of GAD65-KO.

Effects of Acute Alcohol Exposure on Layer 5 Pyramidal Neurons of Juvenile Mice

Early-onset drinking during childhood or preadolescence is a serious social problem. Yet, most of the basic neurobiological research on the acute effects of ethanol has been carried out on adult or early postnatal animals. We studied the effect of alcohol exposure on the basic electrophysiological properties and cell viability of layer 5 pyramidal neurons from the somatosensory cortex of juvenile (P21-P23) C57BL/6N mice. After bath application of 50 mM ethanol to acute slices of the somatosensory cortex, no adverse effects were detected on cells survival, whereas the input resistance and firing rate of layer 5 neurons were significantly reduced. While the effect on the input resistance was reversible, the depressing effect on cell firing remained stable after 6 min of alcohol exposure. Ethanol application did not result in any significant change of mIPSC frequency, amplitude, and rise time. A slight increase of mIPSC decay time was observed after 6 min of ethanol exposure. The molecular mechanisms leading to these alterations and their significance for the physiology of the cerebral cortex are briefly discussed.

Sensitivity of GABAergic Tonic Currents to Acute Ethanol in Cerebellar Granule Neurons is Not Age- or δ Subunit-Dependent in Developing Rats

BACKGROUND

The age of first exposure to ethanol (EtOH), as well as reduced sensitivity to its motor-impairing effects, are associated with a future predisposition to abuse EtOH. In adolescence, acute EtOH potentiates GABA transmission, including tonic inhibition mediated by δ-containing extrasynaptic GABAA receptors (GABAA Rs) in cerebellar granule neurons (CGNs), an effect that likely contributes to EtOH-induced motor impairment. Prenatal EtOH exposure is strikingly prevalent and is associated with increased EtOH abuse later in life; however, the acute effects of EtOH on GABA transmission in developing CGNs are unknown.

METHODS

Using whole-cell patch-clamp electrophysiological techniques in acute brain slices, we examined the acute effects of EtOH on GABA transmission and functionally assessed the role of δ-containing GABAA Rs in CGNs of preweanling (postnatal day [P] 12 to 14) and postweanling (P28 to 30) male Sprague-Dawley rats.

RESULTS

The magnitude of basal tonic currents were similar at both ages. However, 4,5,6,7-Tetrahydroisoxazolo[5,4-c]pyridin-3-ol hydrochloride, an agonist with preferential affinity for δ-containing GABAA Rs, significantly potentiated tonic currents to a larger magnitude in CGNs from postweanlings compared to preweanlings. Conversely, acute application of EtOH (80 mM) significantly increased tonic currents and the frequency of spontaneous inhibitory postsynaptic currents to a similar extent in CGNs from pre- and postweanlings.

CONCLUSIONS

These findings highlight the sensitivity of the developing cerebellum to EtOH. Furthermore, this study demonstrates age-dependent functional changes in a well-characterized circuitry that may contribute to the short- and long-term effects of prenatal exposure to EtOH.

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.

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.