Effects of GM1 ganglioside upon neuronal degeneration during withdrawal from alcohol

Nerve growth factor prevents cell death and induces hypertrophy of basal forebrain cholinergic neurons in rats withdrawn from prolonged ethanol intake

We have previously reported that the hippocampal cholinergic fiber network is severely damaged in animals withdrawn from ethanol, and that a remarkable recovery in fiber density occurs following hippocampal grafting, a finding that we suggested to be underpinned by the graft production of neurotrophic factors, which are known to be decreased after ethanol exposure. It is widely accepted that nerve growth factor (NGF) signals the neurons of the brain cholinergic system, including those of the medial septum/vertical limb of the diagonal band of Broca (MS/VDB) nuclei, from which the septohippocampal projection arises. Because neurons in these nuclei are vulnerable to ethanol consumption and withdrawal we thought of interest to investigate, in withdrawn rats previously submitted to a prolonged period of ethanol intake, the effects of intraventricular delivery of NGF upon the MS/VDB cholinergic neurons. Stereological methods were applied to estimate neuron numbers and neuronal volumes in choline acetyltransferase (ChAT)-immunostained and Nissl-stained material. We have found that in ethanol-fed rats there was a significant reduction in the total number of Nissl-stained and cholinergic neurons in the MS/VDB, and that the suppression of ethanol intake further decreased neuron numbers. In addition, the somatic size of ChAT-IR neurons was reduced by ethanol intake, and withdrawal further aggravated neuronal atrophy. NGF treatment prevented the withdrawal-associated loss, and induced hypertrophy, of cholinergic neurons. These findings show that exogenous NGF protects the phenotype and prevents the withdrawal-induced degeneration of cholinergic neurons in the MS/VDB. These effects might be due to the trophic action of NGF upon the basal forebrain cholinergic neurons, including the hippocampal fiber network that conveys this neurotrophin retrogradely to the MS/VDB, and/or upon their targets, that is, the hippocampal formation neurons.

Synaptic reorganization in the hippocampal formation of alcohol-fed rats may compensate for functional deficits related to neuronal loss

We have examined the behavioral and neuroanatomical effects of long-term alcohol intake in rats ingesting a 20% solution of ethanol for 30 weeks. Previous studies have shown that this treatment provokes neuronal degeneration in the hippocampal formation, which occurs in parallel with remodeling processes. Spatial reference and working memory of alcohol-fed rats were evaluated during last 4 weeks of treatment by comparison of their performance with age-matched controls on the Morris water maze. Alcohol consumption did not affect the performance of rats in the reference memory task as indicated by the measures derived from the acquisition trials and from the probe-trial, which were highly similar for alcohol-fed and control animals. Also, performance in the working memory task was not significantly altered in alcohol-treated animals. No treatment-related changes in swim speed or impairments of sensorimotor abilities, tested in the visible platform task, were detected. Stereological methods were applied to evaluate the damage inflicted by alcohol intake in the structure of the hippocampal formation. In the alcohol-treated animals, there was a noticeable cell loss in the granular layer of the dentate gyrus (10%), and in CA3 (18%) and CA1 (19%) hippocampal subdivisions. In spite of the neuronal loss, the total number of synapses between mossy fibers and CA3 pyramids was unaffected by alcohol treatment suggesting that new synaptic contacts were formed between the surviving neurons. We show that, regardless the marked hippocampal cell loss in rats exposed to chronic alcohol intake, the reorganization that takes place at the synaptic level may alleviate the expected functional deficits.

GM1 and piracetam do not revert the alcohol-induced depletion of cholinergic fibers in the hippocampal formation of the rat

Chronic alcohol consumption causes a depletion of the cholinergic fiber network in the rat hippocampal formation, which is not ameliorated by alcohol withdrawal. Following withdrawal from alcohol, there is a further loss of intrinsic hippocampal cholinergic neurons. In this study, we investigated whether treatment with putative neuroprotective agents during the entire withdrawal period would have beneficial effects upon the hippocampal cholinergic innervation. Adult male rats were alcohol-fed for 6 months and subsequently withdrawn from alcohol for 6 months. Some animals were treated with either ganglioside GM1 (35 mg/kg body weight s.c.), vehicle (saline s.c.), or piracetam (800 mg/kg body weight p.o.) for the entire withdrawal period. Choline acetyltransferase (ChAT) immunoreactive (IR) fibers and neurons were analyzed quantitatively in all four animal groups. There were no significant differences in the density of the ChAT-IR hippocampal fiber network when the pure withdrawal and withdrawal + vehicle groups were compared to the withdrawal + GM1 or withdrawal + piracetam groups. In contrast, the number of ChAT-IR interneurons in the hippocampal formation was higher in the withdrawal + GM1 or withdrawal + piracetam groups than in the pure withdrawal and withdrawal + vehicle groups. These results indicate that, in the doses used, neither neuroprotective agent had an effect upon the extrinsic cholinergic innervation, but they had a beneficial effect upon the hippocampal intrinsic cholinergic system.

Behavioral and neuroanatomical consequences of chronic ethanol intake and withdrawal

We have examined if long-term (13 months) alcohol consumption and the same treatment followed by a 6-week withdrawal period cause different neuropathological changes in rats. Spatial reference and working memory of alcohol-consuming and withdrawn rats were evaluated by comparison of their performance with age-matched controls in the Morris water maze. In the reference memory task we did not observe significant cognitive deficits in rats continuously exposed to ethanol, whereas withdrawn animals showed an obvious impairment of their overall performance. The reference memory deficit in withdrawn rats was evident in the spatial probe trial; these animals required significantly longer swimming distances to approach the former position of the platform when compared with controls and alcohol-consuming animals. In contrast, working memory was not significantly altered in either experimental group. Stereological methods were applied to compare the neurodegenerative changes produced by alcohol intake and withdrawal in the hippocampal formation. In the alcohol-consuming animals there was a significant cell loss in CA1 (18%) and CA3 (19%) hippocampal regions. Moreover, in withdrawn rats there was a further decay in the total number of pyramidal neurons, which amounted to 15% relative to nonwithdrawn animals. In the granular layer of the dentate gyrus there was a trend in the same direction, but it did not reach significance. Thus, our findings indicate that withdrawn rats are cognitively impaired relative to animals submitted to continuous alcohol consumption and to age-matched controls, which fits the morphological data showing that withdrawal aggravates ethanol-induced degenerative processes in the hippocampal formation.

Piracetam promotes mossy fiber synaptic reorganization in rats withdrawn from alcohol

Prolonged alcohol intake affects the morphology of the hippocampal formation of the rat and the resulting alterations do not reverse after withdrawal. Actually, an increase of the degenerative activity might occur in this condition. This unexpected observation prompted us to test the efficacy of neuronoprotective drugs during withdrawal. Because in a previous study we found that piracetam, a cyclic derivative of GABA, once added during withdrawal impedes hippocampal cell loss, we decided to evaluate the effect of this compound at the synaptic level. Using unbiased stereological techniques, we estimated the total number of contacts between mossy fibers and CA3 pyramids, as well as the volume and the surface area of the respective pre- and postsynaptic compartments. We found that in piracetam-treated withdrawn rats the number of synapses was higher than that observed in nonpiracetam-treated and alcohol-fed animals. The mechanisms leading to the synaptic reorganization took place at the mossy fiber level. The postsynaptic compartment does not seem to participate in the reorganization. It is suggested that the role of piracetam in this process might depend on the protective effect that this compound has upon glutamatergic receptors.

Effect of an NMDA receptor antagonist and a ganglioside GM1 derivative upon ethanol-induced modification of parameters of oxidative stress in several brain regions

Dietary administration of ethanol to rats for 2 weeks was able to depress levels of glutathione (GSH) and Cu/Zn superoxide dismutase (SOD) in several brain regions. This was indicative of the generation of excess levels of reactive oxygen in treated animals. The potentially protective effect of both an NMDA receptor blocker (MK-801) and an internally esterified derivative of ganglioside GM1 (AGF2) upon ethanol-induced changes in these indices of oxidative stress, was studied. Both of these agents are reported to have neuroprotective properties, but neither was able to prevent ethanol-induced reduction of GSH and SOD levels in any brain area studied. In fact, both agents depressed SOD and GSH levels in midbrain independently of ethanol. MK-801 had a pronounced pro-oxidant potential, and when administered in combination with ethanol. GSH and SOD were reduced in midbrain and striatum to levels below those obtained with either agent alone. The pro-oxidant properties of ethanol may thus act independently of its actions upon the NMDA receptor. The protective properties of NMDA receptor inhibitors or gangliosides cannot be attributed to any antioxidant effect.

GM1 ganglioside reduces ethanol intoxication and the development of ethanol dependence

The monosialoganglioside, GM1, protects the nervous system against a variety of insults. In this study, we evaluated the protective properties of GM1 on ethanol intoxication and development of dependence. GM1 (20-40 mg/kg, IP) reduced the extent and duration of ataxia produced by ethanol (2 g/kg, IP, 15-95 min), and delayed the onset of loss and reduced the duration of the righting reflex (LORR) produced by ethanol (4.2 g/kg, IP). GM1 did not alter ethanol-induced hypothermia or the rate of ethanol clearance. Rather, GM1 increased the waking blood ethanol concentration. In animals fed a complete liquid diet containing 4.5% ethanol, concurrent administration of GM1 (40 mg/kg/day) blocked the tremors, hypolocomotion, and anxiety-like behavior associated with ethanol withdrawal. These findings demonstrate that GM1 reduces both ethanol's acute intoxication and the signs and symptoms of ethanol withdrawal by a mechanism not related to ethanol pharmacokinetics.

Prevention of ethanol-induced sympathetic overactivity and degeneration by dexmedetomidine

The effects of dexmedetomidine, a selective alpha 2-adrenoceptor agonist, on rat sympathetic neurons were studied during a 12-day, heavy ethanol exposure. Adult male Wistar rats were given ethanol or isocaloric sucrose three times a day by intragastric intubation. Both acute (a single dose of 300 micrograms/kg p.o.) and chronic (100 micrograms/kg x 2 P.O. throughout the experiment) effects of dexmedetomidine were tested. The superior cervical ganglia (SCG) of the ethanol-exposed, non-dexmedetomidine-treated rats showed an abnormally high overall level of tyrosine hydroxylase immunoreactivity (TH-IR) and catecholamine histofluorescence. However, a subpopulation of neurons had apparently lost their catecholamine synthetic activity, as they exhibited no TH-IR or catecholamine fluorescence. The ethanol-exposed ganglia also showed structural alterations (e.g., decreased neuronal size and increased occurrence of vacuolated neurons). In the ethanol-exposed, chronically dexmedetomidine-treated group, by contrast, the SCG exhibited TH-IR and catecholamine fluorescence intensities comparable to those seen in the control ganglia. All the structural parameters studied, as well, were at the control level in the chronically dexmedetomidine-treated group. The single dose of dexmedetomidine offered only marginal protection against the ethanol-induced alterations. These results suggest that chronic dexmedetomidine treatment may prevent ethanol-induced overactivity and degeneration of catecholaminergic neurons.

Attenuation of glutamate-induced neurotoxicity in chronically ethanol-exposed cerebellar granule cells by NMDA receptor antagonists and ganglioside GM1

Ethanol, acutely, is a potent inhibitor of the function of the N-methyl-D-aspartate (NMDA) subtype of glutamate receptor. After chronic exposure of animals to ethanol, however, the NMDA receptor in brain is upregulated. This upregulation is associated with the occurrence of ethanol withdrawal seizures. When cultured cerebellar granule neurons are exposed chronically to ethanol, the resulting upregulation of NMDA receptor function renders the cells more susceptible to glutamate-induced neurotoxicity. The present studies show that chronic ethanol exposure produces an increase in NMDA receptor number in the cells, measured by ligand binding to intact cells. Glutamate-induced excitotoxicity, both in control and ethanol-exposed cells, is blocked by the same NMDA receptor antagonists previously shown to block ethanol withdrawal seizures in animals. In addition, glutamate neurotoxicity is blocked by acute (2-hr) pretreatment of cells with ganglioside GM1 or by chronic (3 days) treatment with the ganglioside. Acute ganglioside treatment does not interfere with the initial rise in intracellular calcium caused by glutamate, whereas this response is downregulated after chronic ganglioside treatment. These results suggest that therapeutic agents can be developed to block both ethanol withdrawal signs and the neuronal damage that accompanies ethanol withdrawal. Furthermore, chronic ganglioside treatment during ethanol exposure has the potential to prevent changes in the NMDA receptor that lead to withdrawal seizures and enhanced susceptibility to excitotoxicity.

Piracetam impedes hippocampal neuronal loss during withdrawal after chronic alcohol intake

In previous studies we have demonstrated that prolonged ethanol consumption induced hippocampal neuronal loss. In addition, we have shown that withdrawal after chronic alcohol intake augmented such degenerative activity leading to increased neuronal death in all subregions of the hippocampal formation but in the CA3 field. In an attempt to reverse this situation, we tested, during the withdrawal period, the effects of piracetam (2-oxo-1-pyrrolidine acetamide), a cyclic derivative of gamma-aminobutyric acid, as there is previous evidence that it might act as a neuronoprotective agent. The total number of dentate granule, hilar, and CA3 and CA1 pyramidal cells of the hippocampal formation were estimated using unbiased stereological methods. We found out that in animals treated with piracetam the numbers of dentate granule, hilar, and CA1 pyramidal cells were significantly higher than in pure withdrawn animals, and did not differ from those of alcohol-treated rats that did not undergo withdrawal. These data suggest that piracetam treatment impedes, during withdrawal, the pursuing of neuronal degeneration.

Glutamate receptors in alcohol withdrawal-induced neurotoxicity

Chronic ethanol ingestion results in an "up-regulation" of the N-methyl-D-aspartate (NMDA) subtype of glutamate receptor in mouse brain. This increase in receptors is associated with ethanol withdrawal seizures, which can be attenuated by NMDA receptor antagonists. Chronic exposure to ethanol (3 days) of rat cerebellar granule cells in primary culture also produces an increase in NMDA receptor number and function, which leads to enhanced susceptibility to glutamate-induced neurotoxicity. Antagonists acting at various sites on the NMDA receptor can block glutamate excitotoxicity in both control and ethanol-exposed cells. These results suggest the possibility of developing agents that will ameliorate ethanol withdrawal seizures as well as withdrawal-induced neuronal damage. In addition, acute (2 hr) or chronic (3 day) exposure of cerebellar granule cells to ganglioside GM1 protects control and ethanol-treated cells against glutamate neurotoxicity. However, while the acute GM1 treatment does not interfere with the initial response to glutamate (increase in intracellular Ca2+), this response is "down-regulated" after chronic ganglioside treatment. These findings suggest that the mechanism by which acute and chronic ganglioside treatments protect against glutamate neurotoxicity may differ. Furthermore, chronic ganglioside treatment during ethanol exposure has the potential to prevent the ethanol-induced up-regulation of NMDA receptors that underlies withdrawal seizures and increased susceptibility to excitotoxicity.

Ethanol neurotoxicity in vitro: effects of GM1 ganglioside and protein synthesis inhibition

Cultures of septal and hippocampal neurons from fetal rat and dorsal root ganglion neurons from embryonic chick were pretreated with GM1 ganglioside or cycloheximide and then supplemented with toxic concentrations of ethanol. GM1 provided significant protection against ethanol neurotoxicity in each population. The inhibition of protein synthesis by cycloheximide, however, which protects against cell death resulting from withdrawal of neurotrophic factor support, did not ameliorate ethanol-induced neuronal loss.