Effects of in vitro ethanol and chronic ethanol consumption on the release of excitatory amino acids in the rat hippocampus

The Effects of Alcohol Intoxication and Withdrawal on Hypothalamic Neurohormones and Extrahypothalamic Neurotransmitters

The aim of the present study was to determine the effects of alcohol intoxication and withdrawal on hypothalamic neurohormones such as corticotropin-releasing factor (CRF) and arginine vasopressin (AVP), and extrahypothalamic neurotransmitters such as striatal dopamine (DA), amygdalar gamma aminobutyric acid (GABA), and hippocampal glutamate (GLU). In addition, the participation of the two CRF receptors, CRF1 and CRF2, was investigated. For this purpose, male Wistar rats were exposed to repeated intraperitoneal (ip) administration of alcohol every 12 h, for 4 days and then for 1 day of alcohol abstinence. On the fifth or sixth day, intracerebroventricular (icv) administration of selective CRF1 antagonist antalarmin or selective CRF2 antagonist astressin₂B was performed. After 30 min, the expression and concentration of hypothalamic CRF and AVP, the concentration of plasma adrenocorticotropic hormone (ACTH) and corticosterone (CORT), and the release of striatal DA, amygdalar GABA, and hippocampal GLU were measured. Our results indicate that the neuroendocrine changes induced by alcohol intoxication and withdrawal are mediated by CRF1, not CRF2, except for the changes in hypothalamic AVP, which are not mediated by CRF receptors.

Imipramine attenuates anxiety- and depressive-like effects of acute and prolonged ethanol-abstinence in male rats by modulating SERT and GR expression in the dorsal hippocampus

AIMS

Considering that serotoninergic agents attenuate symptoms of anxiety and are used to treat depression, we investigated whether subchronic treatment with imipramine, a serotonin/noradrenaline reuptake inhibitor, would prevent the anxiogenic-like behaviour induced by acute and/or chronic ethanol withdrawal. We also investigated whether those changes were related to the disfunctioning of hypothalamic-pituitary-adrenal (HPA) axis and serotonergic neurotransmission.

MAIN METHODS

264 Male Wistar rats were treated with ethanol 6% (vol./vol.) for 21 days. Acute ethanol withdrawal was induced by abrupt discontinuation of treatment and sustained for 48 h. Protracted abstinence was sustained for an additional period of 21 days. Behavioural tests included the Elevated Plus Maze (EPM) or Light/Dark Box (LDB) after acute abstinence, and the Forced Swim Test (FST) after protracted abstinence. Imipramine (15 mg/kg, i.p.) was administered 24, 19 and 1 h before EPM or LDB tests.

KEY FINDINGS

Acute abstinence decreased exploration of the open arms of the EPM, without changing exploration of LDB. Additionally, chronic abstinent rats displayed more time immobile in the FST, when compared to control animals. These effects were attenuated by imipramine treatment, without changing basal response. Imipramine prevented protracted abstinence -induced decrease in glucocorticoid receptor (GR) and serotonin transporter (SERT) expression in the dorsal hippocampus.

SIGNIFICANCE

Our findings indicate that chronic ethanol withdrawal affects the hippocampal serotonergic system by decreasing serotonin transporter expression. It also disturbs the HPA axis functioning through an imbalance on GR and mineralocorticoid (MR) expression.

Nicotine-ethanol interactions in flash-evoked potentials and behavior of Long-Evans rats

Although nicotine and ethanol are often used together, little is known about their combined effects on visual system electrophysiology. This experiment examined the separate and combined effects of nicotine and ethanol on flash-evoked potentials (FEPs) recorded from both the visual cortex (VC) and superior colliculus (SC) of chronically implanted male Long-Evans rats. There were four treatment conditions administered on separate days: either saline or ethanol (2.0 g/kg, i.p.) was given 10 min before either saline or nicotine (1.0 mg/kg, s.c.). FEPs were recorded at 5, 20, and 40 min following the second injection. In the VC, ethanol significantly decreased the amplitude of most components, but increased P46. Peaks P22 and N53 were unchanged. Nicotine enhanced most component amplitudes, but decreased N29 and P234, while P22 and N139 were unchanged. In the SC, ethanol depressed the amplitude of all components studied. In contrast, nicotine significantly depressed only P27 and N48. Latencies of most components in both structures were increased by ethanol, nicotine, and the combination treatment, although a nicotine-induced enhancement of the effects of ethanol on latencies was not typically observed. Each drug treatment also produced significant hypothermia, with the combination treatment resulting in the greatest hypothermia. Ethanol, either alone or in combination with nicotine, significantly reduced body movements during the FEP recording sessions. In subsequent open-field observations, ethanol, but not nicotine, significantly increased the number of squares crossed, while the combination treatment produced the greatest increase in movement. Nicotine significantly increased rearing behavior, but both ethanol and the combination treatment eliminated rearings. Overall, data suggesting that nicotine can counteract some of the effects of ethanol was demonstrated in varying degrees in the amplitude of VC components N39, P46, N53, N65, and P88, the latency of VC component N53, the amplitude of SC component N59, and the latency of SC components N48 and N54. In contrast, a nicotine-induced enhancement of the effects of ethanol was found for only the latency of VC components N39, P88, and P234, body temperature, and open-field ambulation.

Alcohol and withdrawal: from animal research to clinical issues

The withdrawal syndrome in alcohol-dependent patients appears to be a major stressful event whose intensity increases with repetition of detoxifications according to a kindling process. Disturbances in the balance between excitatory and inhibitory neural processes are reflected in a perturbed physical state while disturbances in the balance between positive and negative reinforcements are reflected in a perturbed mood state. Our purpose is to link the different behavioral outcomes occurring during withdrawal with specific biological brain mechanisms from the animal to the human being. Better understanding of the various biological mechanisms underlying withdrawal from alcohol will be the key to design and to apply appropriate pharmaceutical management, together with appropriate therapy aimed at inducing protracted abstinence.

Importance of magnesium ions in development of tolerance to ethanol: studies on cultured cerebral vascular smooth muscle cells, type-2 astrocytes and intact rat brain

This study was designed to examine the roles of intracellular free magnesium ion concentration ([Mg(2+)](i)) in ethanol-induced intoxication and development of tolerance in cultured canine cerebral vascular smooth muscle cells and astrocytes as well as intact rat brain. The basal, resting level of [Mg(2+)](i) in cerebrovascular cells was 732.5 +/- 82.4 microM. Exposure of cultured canine cerebral vascular smooth muscle cells to ethanol (10 and 25 mM) for 24 h reduced the concentrations of [Mg(2+)](i) to 521.1 +/- 59.6 microM, and 308.2 +/- 37.8 microM, respectively. However, exposure of these cultured vascular cells to the same concentrations of ethanol, after initial pretreatment with ethanol for 24 h, failed to interfere with the levels of [Mg(2+)](i). Measurement of [Mg(2+)](i) at 48 h and 72 h indicated that the decreased levels of [Mg(2+)](i) induced by ethanol at 24 h treatment returned toward baseline. Similar experiments were performed in cultured type-2 astrocytes isolated from neonatal rat brain. The basal level of [Mg(2+)](i) in type-2 astrocytes was about 125 microM. Incubation of these cells with 10 mM ethanol for 10 min resulted in a 27% reduction in the level of [Mg(2+)](i), whereas incubation with 25 mM ethanol resulted in almost a 50% reduction in [Mg(2+)](i). The decreased levels of [Mg(2+)](i) lasted around 30 min, until the measurement finished. Continuous incubation of these cultured astrocytes, with ethanol (either 10 mM or 25 mM), for more than 24 h, indicated that the concentrations of [Mg(2+)](i) in type-2 astrocytes were equivalent to those at basal, resting levels. In vivo 31P-NMR spectroscopy, performed on intact rat brains, indicated that an initial administration of 4 mg/kg ethanol ( approximately 20-25 mM blood alcohol level) resulted (after 20-40 min of exposure) in severe deficits in whole brain [Mg(2+)](i) (550 +/- 33 microM to 358 +/- 24 microM). Repeated injections of ethanol (4 mg/kg) over the next 24-72 h resulted in progressively diminishing effects on brain [Mg(2+)](i). These experimental data indicate that chronic ethanol treatment can induce a tolerance to depletion of [Mg(2+)](i) in cerebrovascular smooth muscle cells, type-2 astrocytes as well as intact rat brain. The results suggest that [Mg(2+)](i) might play a major role in alcohol-induced tolerance in the brain.

Effects of ethanol on flash-evoked potentials of rats: lack of antagonism by naltrexone

The present study examined the effects of ethanol and naltrexone hydrochloride (a nonselective opiate receptor antagonist) on flash-evoked potentials recorded from both the visual cortex (VC) and the superior colliculus (SC) of chronically implanted hooded rats. There were four treatment conditions administered on separate days: Either saline or naltrexone (10 mg/kg; volume of 1.0 ml/kg) was given 10 min before either saline or ethanol (2.0 g/kg; 20% ethanol solution in a volume of 1.26 ml/100 g). Evoked potentials were recorded 15 min after the intraperitoneal injections were completed. Animals were tested at 23.1 degrees C room temperature. In the VC, ethanol significantly decreased the amplitude of components N1, P3, and N3, whereas it increased the amplitude of P2. Components P1 and N2 were unaffected by ethanol treatment. The SC components P3 and N4 were reduced in amplitude by ethanol, but component P1 was not altered. Latencies of all components in both structures were increased by ethanol. Naltrexone alone did not significantly affect the potentials, nor did naltrexone pretreatment significantly alter the effects of ethanol on the potentials. Naltrexone produced a modest hypothermia of about 0.25 degrees C, whereas ethanol resulted in hypothermia of about 1.0 degrees C. Ethanol, either alone or in combination with naltrexone, significantly reduced body movement during the evoked-potential recording sessions. The results indicate that endogenous opioid systems do not play a major role in the acute effects of ethanol on flash-evoked potentials recorded from primary areas of the visual system.

The anxiolytic effect of acute ethanol or diazepam exposure is unaltered in mu-opioid receptor knockout mice

Previous researchers demonstrate an opioidergic involvement in the anxiolytic and rewarding actions of ethanol and diazepam. Therefore, to further characterize the role of the opioid system in the anxiolytic action of ethanol and diazepam, normal (C57BL/6J), hybrid (B6129F1) and mu-opioid receptor knockout mice were given i.p. ethanol (0, 1.0 or 1.6 g/kg) or diazepam (1.5 mg/kg). The anxiolytic properties of these agents were then tested in the elevated plus-maze. Additional ethanol-treated mu-opioid receptor knockout mice (1 g/kg) were pretreated with the kappa-opioid receptor antagonist nor-BNI (0 or 3 mg/kg) to assess the involvement of kappa-opioid activity in ethanol's anxiolytic actions. The anxiolytic action of ethanol and diazepam in the mu-opioid receptor knockout mouse did not differ from the effects obtained in normal mice and pretreatment with nor-BNI did not significantly attenuate ethanol's actions in mu-opioid receptor knockout mice. Thus, the anxiolytic actions of ethanol and diazepam appear to be independent of opioid system activity in the mu-opioid receptor knockout mouse.

Aspirin attenuates the anxiolytic actions of ethanol

The present study examined the effect of aspirin on the anxiolytic action of ethanol. Previous research has shown that ethanol reliably produces an anxiolytic effect on rodent's plus-maze performance while aspirin has been demonstrated to attenuate several of ethanol's behavioral actions. Female Sprague-Dawley rats were given s.c. aspirin doses of 0 or 150 mg/kg, followed 30 min later by s.c. ethanol doses of 0, 1.0 or 1.6 g/kg. After 5 min, animals were tested in the elevated plus-maze. Although aspirin did not have a significant effect on anxiety-related behavior, it did attenuate the anxiolytic action of ethanol at the dose of 1.0 g/kg, but not at the 1.6 g/kg dose. Thus, aspirin by itself does not appear to possess anxiolytic actions, but does modify the anxiolytic actions of 1.0 g/kg, but not 1.6 g/kg ethanol.

Anxiety in mice following acute aspartame and ethanol exposure

The purpose of the present study was to look at the effect of aspartame on the anxiolytic actions of ethanol. Previous research has shown that ethanol reliably produces an anxiolytic effect on rodent's plus-maze performance. There have been anecdotal reports that aspartame increases anxiety. CD-1 male mice were given i.p. aspartame doses of vehicle, 1000, or 2000 mg/kg, followed 30 min later by i.p. ethanol doses of 1.6 g/kg or vehicle. Animals were then placed in an open field, then tested in the plus-maze. Results determined that the aspartame condition had no significant effect on anxiety-related behavior, nor did it alter the anxiolytic actions of ethanol. Thus, acute high dose exposure to aspartame does not appear to affect anxiety-related behaviors.

Chronic intermittent ethanol exposure alters CA1 synaptic transmission in rat hippocampal slices

We investigated the neuroadaptive changes in synaptic transmission in the CA1 region of the hippocampus as a result of chronic intermittent ethanol exposure. Male Wistar rats were exposed daily (14 h) to ethanol vapors (blood alcohol levels = 150-200 mg%) for 12-14 days, and synaptic field potentials elicited by Schaffer collateral stimulation were compared in hippocampal slices from control and chronic ethanol-treated rats. Excitatory postsynaptic responses of slices were recorded under three conditions: (i) normal physiological saline; (ii) continued presence of 33 mM (150 mg%) ethanol (chronic ethanol-treated rats only); (iii) acute exposure to 33 mM ethanol. When recorded in ethanol-free physiological saline, the mean amplitude of the dendritic synaptic potential and the somatic population spike were significantly smaller in slices from chronic ethanol-treated rats compared to slices from control rats. Under conditions of continuous ethanol exposure, somatic and dendritic synaptic responses of slices taken from chronic ethanol-treated rats were further depressed, suggesting that neural pathways in area CA1 remained sensitive to ethanol. Acute application of ethanol led to a more pronounced reduction of the mean somatic population spike amplitude in slices from chronic ethanol-treated rats than in slices from control rats. However, dendritic synaptic responses were unaffected by acute ethanol in slices from both control and chronic ethanol-treated rats. In addition, we examined the involvement of presynaptic mechanisms in the effects of chronic intermittent ethanol using paired-pulse protocols. When recorded in the continued presence of ethanol, slices from chronic ethanol-treated rats exhibited a significant reduction in paired-pulse facilitation of the dendritic synaptic response compared to slices from control rats, indicating a presynaptic component to the neuroadaptive effects of chronic intermittent ethanol exposure. Conversely, acute ethanol exposure resulted in an enhancement of paired-pulse facilitation of the dendritic synaptic response, an effect that was similar in slices from both control and chronic ethanol-treated rats. Paired-pulse facilitation of the somatic population spike amplitude was not altered by chronic ethanol treatment. However, acute ethanol exposure significantly enhanced paired-pulse facilitation of the somatic population spike in slices from chronic ethanol-treated rats. This effect of acute ethanol was not observed in slices from control rats. Paired-pulse inhibition was not significantly altered in slices from chronic ethanol-treated rats, suggesting that GABAergic inhibitory mechanisms were not involved in the neuroadaptive effects of chronic intermittent ethanol exposure. We suggest that chronic intermittent ethanol exposure can induce multiple neuroadaptive changes in synaptic transmission of CA1 pyramidal neurons that are detectable at both the pre- and postsynaptic levels. Alterations in paired-pulse facilitation indicate presynaptic changes involving the release of the excitatory neurotransmitter glutamate, whereas changes in dendritic synaptic responses suggest postsynaptic changes in the responsiveness of neurons to synaptic input. Moreover, differential effects of chronic ethanol treatment on synaptic responses recorded in the dendrites versus the somatic region implicate additional effects of ethanol on somatically located mechanisms of CA1 pyramidal neurons. Furthermore, we suggest that complete tolerance to ethanol does not occur in the CA1 region of the hippocampus following chronic intermittent ethanol exposure.

Ethanol and neurotransmitter interactions--from molecular to integrative effects

There is extensive evidence that ethanol interacts with a variety of neurotransmitters. Considerable research indicates that the major actions of ethanol involve enhancement of the effects of gamma-aminobutyric acid (GABA) at GABAA receptors and blockade of the NMDA subtype of excitatory amino acid (EAA) receptor. Ethanol increases GABAA receptor-mediated inhibition, but this does not occur in all brain regions, all cell types in the same region, nor at all GABAA receptor sites on the same neuron, nor across species in the same brain region. The molecular basis for the selectivity of the action of ethanol on GaBAA receptors has been proposed to involve a combination of benzodiazepine subtype, beta 2 subunit, and a splice variant of the gamma 2 subunit, but substantial controversy on this issue currently remains. Chronic ethanol administration results in tolerance, dependence, and an ethanol withdrawal (ETX) syndrome, which are mediated, in part, by desensitization and/or down-regulation of GABAA receptors. This decrease in ethanol action may involve changes in subunit expression in selected brain areas, but these data are complex and somewhat contradictory at present. The sensitivity of NMDA receptors to ethanol block is proposed to involve the NMDAR2B subunit in certain brain regions, but this subunit does not appear to be the sole determinant of this interaction. Tolerance to ethanol results in enhanced EAA neurotransmission and NMDA receptor upregulation, which appears to involve selective increases in NMDAR2B subunit levels and other molecular changes in specific brain loci. During ETX a variety of symptoms are seen, including susceptibility to seizures. In rodents these seizures are readily triggered by sound (audiogenic seizures). The neuronal network required for these seizures is contained primarily in certain brain stem structures. Specific nuclei appear to play a hierarchical role in generating each stereotypical behavioral phases of the convulsion. Thus, the inferior colliculus acts to initiate these seizures, and a decrease in effectiveness of GABA-mediated inhibition in these neurons is a major initiation mechanism. The deep layers of superior colliculus are implicated in generation of the wild running behavior. The pontine reticular formation, substantia nigra and periaqueductal gray are implicated in generation of the tonic-clonic seizure behavior. The mechanisms involved in the recruitment of neurons within each network nucleus into the seizure circuit have been proposed to require activation of a critical mass of neurons. Achievement of critical mass may involve excess EAA-mediated synaptic neurotransmission due, in part, to upregulation as well as other phenomena, including volume (non-synaptic diffusion) neurotransmission. Effects of ETX on receptors observed in vitro may undergo amplification in vivo to allow the excess EAA action to be magnified sufficiently to produce synchronization of neuronal firing, allowing participation of the nucleus in seizure generation. GABA-mediated inhibition, which normally acts to limit excitation, is diminished in effectiveness during ETX, and further intensifies this excitation.

Adenosinergic modulation of ethanol-induced motor incoordination in the rat motor cortex

1. On going work in our laboratory has shown that adenosine modulates ethanol-induced motor incoordination (EIMI) when given systemically as well as directly into the cerebral ventricles, cerebellum and corpus striatum of the rat and/or mouse. 2. The objective of this study was to determine what effect adenosine agonists and antagonists would have within the rat motor cortex on EIMI. 3. The participation of the motor cortex in EIMI was suggested when microinfusion of the anti-ethanol compound, Ro15-4513, an inverse agonist of the benzodiazepine binding site, directly into the motor cortex significantly attenuated EIMI. Further, the adenosine agonists N6-cyclohexyladenosine (CHA) and 2-p-(2-carboxyethyl)-phenethylamino-5'-N-carboxaminoadenosine++ + hydrochloride (CGS-21680) significantly accentuated EIMI in a dose-related manner. The adenosine A1 receptor-selective agonist, CHA, appeared most potent in this modulatory effect when compared to the A2-selective agonist, CGS-21680. 4. The extent of diffusion of the adenosine drugs within the cortical tissue after their microinfusion was also checked by measuring the dispersion of microinfused [3H]CHA. The [3H]CHA dispersion study indirectly confirmed that the results of the present investigation were based on the effect of adenosine drugs within the motor cortex only. 5. Accentuation by the A1- and A2-selective adenosine agonists was significantly attenuated by the A1-selective antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) but not by the A2 receptor-selective antagonist 8-(3-chlorostyryl)caffeine (CSC) further suggesting modulation mainly by the A1-subtype. 6. Pretreatment of the motor cortex with pertussis toxin (PT) significantly reduced the capacity of both A1- and A2-selective adenosine agonists to accentuate EIMI suggesting the involvement of a PT-sensitive Gi/Go protein. 7. These data support earlier work which showed that adenosine modulates EIMI within the central nervous system (CNS), most likely via the A1 receptor, and moreover, extend that work by including the motor cortex as a brain area participating in the adenosinergic modulation of ethanol-induced motor impairment.

Discriminative stimulus effects of glutamate release inhibitors in rats trained to discriminate ethanol

In a drug discrimination paradigm with rats trained to discriminate ethanol (1 g/kg IP) from saline we studied two substances, lamotrigine and riluzole, which are regarded as glutamate release inhibitors concerning their ability to substitute for ethanol. Both substances have been shown to act primarily on voltage-gated sodium channels; however, Lamotrigine dose dependently generalized to the ethanol cue, whereas riluzole did not. These results reflect the different high-dose effects of both sustances at voltage-gated calcium channels, where lamotrigine has inhibitory effects, but not riluzole, and provide further evidence for a role of voltage-gated calcium channels in the mediation of the effects of ethanol.