Proconvulsant and anticonvulsant properties of ethanol: studies of electrographic seizures in vitro

Acute ethanol exposure has bidirectional actions on the endogenous neuromodulator adenosine in rat hippocampus

Background and Purpose

Ethanol is a widely used recreational drug with complex effects on physiological and pathological brain function. In epileptic patients, the use of ethanol can modify seizure initiation and subsequent seizure activity with reports of ethanol being both pro‐ and anticonvulsant. One proposed target of ethanol's actions is the neuromodulator adenosine, which is released during epileptic seizures to feedback and inhibit the occurrence of subsequent seizures. Here, we investigated the actions of acute ethanol exposure on adenosine signalling in rat hippocampus.

Experimental Approach

We have combined electrophysiology with direct measurements of extracellular adenosine using microelectrode biosensors in rat hippocampal slices.

Key Results

We found that ethanol has bidirectional actions on adenosine signalling: depressant concentrations of ethanol (50 mM) increased the basal extracellular concentration of adenosine under baseline conditions, leading to the inhibition of synaptic transmission, but it inhibited adenosine release during evoked seizure activity in brain slices. The reduction in activity‐dependent adenosine release was in part produced by effects on NMDA receptors, although other mechanisms also appeared to be involved. Low concentrations of ethanol (10–15 mM) enhanced pathological network activity by selectively blocking activity‐dependent adenosine release.

Conclusions and Implications

The complex dose‐dependent actions of ethanol on adenosine signalling could in part explain the mixture of pro‐convulsant and anticonvulsant actions of ethanol that have previously been reported.

Alcohol abuse promotes changes in non-synaptic epileptiform activity with concomitant expression changes in cotransporters and glial cells

Non-synaptic mechanisms are being considered the common factor of brain damage in status epilepticus and alcohol intoxication. The present work reports the influence of the chronic use of ethanol on epileptic processes sustained by non-synaptic mechanisms. Adult male Wistar rats administered with ethanol (1, 2 e 3 g/kg/d) during 28 days were compared with Control. Non-synaptic epileptiform activities (NEAs) were induced by means of the zero-calcium and high-potassium model using hippocampal slices. The observed involvement of the dentate gyrus (DG) on the neurodegeneration promoted by ethanol motivated the monitoring of the electrophysiological activity in this region. The DG regions were analyzed for the presence of NKCC1, KCC2, GFAP and CD11b immunoreactivity and cell density. The treated groups showed extracellular potential measured at the granular layer with increased DC shift and population spikes (PS), which was remarkable for the group E1. The latencies to the NEAs onset were more prominent also for the treated groups, being correlated with the neuronal loss. In line with these findings were the predispositions of the treated slices for neuronal edema after NEAs induction, suggesting that restrict inter-cell space counteracts the neuronal loss and subsists the hyper-synchronism. The significant increase of the expressions of NKCC1 and CD11b for the treated groups confirms the existence of conditions favorable to the observed edematous necrosis. The data suggest that the ethanol consumption promotes changes on the non-synaptic mechanisms modulating the NEAs. For the lower ethanol dosage the neurophysiological changes were more effective suggesting to be due to the less intense neurodegenertation.

Dose-dependent anticonvulsive effect of ethanol on lindane-induced seizures in rats

Chronic ethanol consumption is a major risk factor for epilepsy, and seizures frequently occur during the withdrawal period. The aim of our study was to investigate effects of ethanol on lindane-induced seizures in rats. Male Wistar rats were injected i.p. with one of the following 5 treatments: (i) saline, (ii) dimethylsulfoxide, (iii) lindane (8 mg/kg) (L), (iv) ethanol in doses of 0.5 g/kg (E(0.5)), 1 g/kg (E(1)), and 2 g/kg (E(2)), and (v) groups that received ethanol 30 min before lindane (LE(0.5), LE(1), and LE(2)). Behavioral changes were described by using a descriptive scale as follows: 0, no response; 1, head nodding, lower jaw twitching; 2, myoclonic body jerks, bilateral forelimb clonus; 3, generalized tonic-clonic convulsions; 4, status epilepticus. The incidence of convulsions in the LE(2) group was significantly lower than the incidence in the L (p < 0.01) and LE(0.5) groups (p < 0.05). The median grade of convulsive behavior was significantly lower in the LE(2) (p < 0.01) and LE(1) groups (p < 0.05) compared with the L group. Latencies to the first seizure response were not significantly different among groups. ED50 of ethanol was 1.40 (1.19-1.65). Our findings suggest that ethanol decreased severity and incidence of lindane-induced seizures in a dose-dependent manner.

Ethanol effects on dentate granule cell LTP

In previous studies we identified a lateral hypothalamic area (LHA) sensitive to ethanol, < 5.0 mM, when the perifornical region of the area is perfused with different concentrations of ethanol. Some of these perifornical neurons contain angiotensin (Ang) and project directly to the dentate gyrus where angiotensin is released and inhibits LTP in medial perforant path-dentate granule cell synapses. The AT1 subtype receptor is involved because pretreatment with losartan, an AT1 antagonist, prevents Ang II, diazepam, and ethanol impairment of LTP as well as their effects on behavior. There is a possibility that these effects were not specific to the LHA; but might be attributable to direct effects of ethanol on postsynaptic granule cells due to diffusion of the ethanol in the extracellular space or by the circulatory system. The purpose of the present study was to determine a dose effect of ethanol on LTP in these same synapses when the dentate gyrus was perfused with several different concentrations of ethanol under the same conditions in urethane anesthetized rats. Ethanol was administered directly into the dentate gyrus by means of a fine stainless steel cannula attached approximately 1.0 mm from the tip of the glass capillary recording electrode. Results show that the threshold for ethanol in the dentate is higher by a factor of ten, > 30 mM and < 50 mM; and that at higher doses ethanol can have a direct effect on the LHA; and possibly toxic due to increasing ethanol in the blood circulatory system.

The epileptogenic properties of the volatile anesthetics sevoflurane and isoflurane in patients with epilepsy

No study comparing epileptogenicity of sevoflurane to other volatile anesthetics has been performed. We compared the epileptogenic properties of sevoflurane to isoflurane in patients with epilepsy. In 24 mentally and/or physically disabled patients, 12 with epilepsy and 12 without epilepsy, electroencephalograms were recorded under anesthesia with 1.0 minimum alveolar anesthetic concentration (MAC), 1.5 MAC, and then 2.0 MAC sevoflurane or isoflurane under three ventilatory conditions: (A) 100% oxygen, and end-tidal CO(2) partial pressure (ETCO(2)) = 40 mm Hg, (B) 50% oxygen, 50% nitrous oxide, ETCO(2) = 40 mm Hg, and (C) 100% oxygen, ETCO(2) = 20 mm Hg. Spike activity was evaluated as a spike-and-wave index (% durations of spike and wave). The spike-and-wave index increased (P<0.05) from 1.99%+/-0.96% during 1.0 MAC sevoflurane to 6.14% +/- 4.45% during 2.0 MAC sevoflurane in (A) in the epilepsy group, while no spike activity was observed in the nonepilepsy group. Only a few spikes were observed under isoflurane anesthesia, 0.04% +/- 0.04% in (A), with no spikes in (B) and (C). Supplementation with 50% nitrous oxide or hyperventilation (P<0.05) suppressed the occurrence of spikes. Sevoflurane has a stronger epileptogenic property than isoflurane, but nitrous oxide or hyperventilation counteracts this specific epileptogenic property.

IMPLICATIONS

The stronger epileptogenicity of sevoflurane than isoflurane was confirmed in a controlled study in patients with epilepsy. Hyperventilation and supplementation of nitrous oxide under sevoflurane anesthesia suppressed epileptogenicity. A combination of sevoflurane and nitrous oxide may be a safer method for seizure-prone patients than the use of sevoflurane alone.

The effect of alcohol on convulsions and nitric oxide levels in seizure-susceptible el mice

To evaluate the influences of ethanol intake on convulsive seizures and brain nitric oxide (NO) production, EL mice, a strain highly susceptible to seizures, were given a 10% ethanol solution ad libitum. In mice consuming ethanol for 4, 8, and 12 weeks, seizures were not suppressed by zonisamide (75 mg/kg ip). Brain NO metabolite levels in mice after 12 weeks of consumption were significantly lower than those in control mice and those consuming ethanol for 4 weeks. Numbers of NADPH diaphorase-positive neurons in the hippocampal formation and parietal cortex of mice consuming for 4 and 12 weeks were significantly higher than in controls. These results suggested that increasing of numbers of NADPH diaphorase-positive neurons in the hippocampal formation and parietal cortex were assumed to develop in compensation for reduction in whole-brain NO metabolite levels of EL mice exposed to ethanol.

The contribution of electrophysiology to knowledge of the acute and chronic effects of ethanol

This review describes the effects of ethanol on the components of neuronal transmission and the relationship of such effects to the behavioural actions of ethanol. The concentrations of ethanol with acute actions on voltage-sensitive ion channels are first described, then the actions of ethanol on ligand-gated ion channels, including those controlled by cholinergic receptors, 5-hydroxytryptamine receptors, the various excitatory amino acid receptors, and gamma-aminobutyric acid receptors. Acute effects of ethanol are then described on brain areas thought to be involved in arousal and attention, the reinforcing effects of ethanol, the production of euphoria, the actions of ethanol on motor control, and the amnesic effects of ethanol; the acute effects of ethanol demonstrated by EEG studies are also discussed. Chronic effects of alcohol on neuronal transmission are described in the context of the various components of the ethanol withdrawal syndrome, withdrawal hyperexcitability, dysphoria and anhedonia, withdrawal anxiety, craving, and relapse drinking. Electrophysiological studies on the genetic influences on the effects of ethanol are discussed, particularly the acute actions of ethanol and electrophysiological differences reported in individuals predisposed to alcoholism. The conclusion notes the concentration of studies on the classical transmitters, with relative neglect of the effects of ethanol on peptides and on neuronal interactions between brain areas and integrated patterns of neuronal activity.

Bicuculline sensitive depressor response to ethanol infusion into the lateral hypothalamus

Decreased GABA function in the hypothalamus increases mean arterial pressure (MAP) and heart rate (HR). Since ethanol acts on GABA-A receptors, blocking GABA-A receptors can prevent a decrease of MAP and HR by ethanol in the lateral hypothalamus (LH). Ethanol at 5-30 mM, with or without 25 ng/microl bicuculline, was infused into the LH, and the activity of the site was validated with 100 nmoles of serotonin. Male rats were anesthetized with pentobarbital, and the femoral artery was catheterized to measure MAP and HR. Microinfusion was performed with a 28-gauge cannula placed into the LH. Serotonin increased MAP and HR within 15 sec. Ethanol decreased the MAP by -21.15 +/- 3.92 mmHg and HR by -53.61 +/- 14.95 BPM, at 15 min, which recovered by 15 min after the infusion was terminated. These maximum decreases were produced by 20 mM ethanol giving a U-shaped dose response. The aCSF vehicle had no effect. Bicuculline prevented ethanol-induced changes and had no effect when administered alone. Both serotonin and ethanol have direct effects on LH neurons with cardiovascular function. Ethanol produces this effect through GABA-A receptors.

Ethanol inhibition of synaptically evoked kainate responses in rat hippocampal CA3 pyramidal neurons

Many studies have demonstrated that intoxicating concentrations of ethanol (10-100 mM) can selectively inhibit the component of glutamatergic synaptic transmission mediated by N-methyl-D-aspartate (NMDA) receptors while having little or no effect on excitatory synaptic transmission mediated by non-NMDA receptors [i.e., alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) and/or kainate (KA) receptors]. However, until the recent development of highly selective AMPA receptor antagonists, it was not possible to assess the relative contribution of AMPA and KA receptors to non-NMDA receptor-mediated synaptic transmission or to determine whether these glutamate receptor subtypes differed in their sensitivity to ethanol. In the present experiments, we used the highly selective AMPA receptor antagonist LY 303070 to pharmacologically isolate KA receptor-mediated excitatory postsynaptic currents (EPSCs) in rat hippocampal CA3 pyramidal neurons and tested their sensitivity to ethanol. Concentrations of ethanol as low as 20 mM significantly and reversibly depressed KA EPSCs. Ethanol also inhibited KA currents evoked by direct pressure application of KA in the presence of LY 303070, suggesting that this inhibition was mediated by a postsynaptic action. In contrast, ethanol had no effect on AMPA EPSCs in these cells, even at the highest concentration tested (80 mM). Ethanol significantly inhibited NMDA EPSCs in these neurons, but these responses were less sensitive to ethanol than KA EPSCs. These results suggest that in addition to its well-described depressant effect on NMDA receptor-mediated synaptic transmission, ethanol has an even greater inhibitory effect on glutamatergic synaptic transmission mediated by KA receptors in rat hippocampal CA3 pyramidal neurons.

Potentiation of N-methyl-D-aspartate receptor-dependent afterdischarges in rat dentate gyrus following in vitro ethanol withdrawal

The contribution of NMDA receptors to neuronal hyperexcitability following in vitro ethanol withdrawal was examined using afterdischarges (ADs) in rat dentate gyrus as a model system. ADs were evoked by high frequency stimuli and blocked by N-methyl-D-aspartate (NMDA) receptor antagonists. Ethanol (75 mM) inhibited ADs, and following in vitro ethanol withdrawal, the duration of afterdischarges was significantly enhanced (37 +/- 2%, n = 21). Therefore, in vitro ethanol exposure with subsequent withdrawal is associated with an enhancement of NMDA receptor-dependent afterdischarges. These results are consistent with the involvement of NMDA receptors in ethanol withdrawal hyperexcitability.