Dendritic spine plasticity and chronic alcoholism in rats

Ethanol-Induced Alterations in Purkinje Neuron Dendrites in Adult and Aging Rats: a Review

Uncomplicated alcoholics suffer from discrete motor dysfunctions that become more pronounced with age. These deficits involve the structure and function of Purkinje neurons (PN), the sole output neurons from the cerebellar cortex. This review focuses on alterations to the PN dendritic arbor in the adult and aging Fischer 344 rat following lengthy alcohol consumption. It describes seminal studies using the Golgi-Cox method which proposed a model for ethanol-induced dendritic regression. Subsequent ultrastructural studies of PN dendrites showed dilation of the extensive smooth endoplasmic reticulum (SER) which preceded and accompanied dendritic regression. The component of the SER that was most affected by ethanol was the sarco/endoplasmic reticulum Ca(2+) ATPase pump (SERCA) responsible for resequestration of calcium into the SER. Ethanol-induced decreases in SERCA pump levels, similar to the finding of SER dilation, preceded and occurred concomitantly with dendritic regression. Discrete ethanol-induced deficits in balance also accompanied these decreases. Ethanol-induced ER stress within the SER of PN dendrites was proposed as an underlying cause of dendritic regression. It was recently shown that increased activation of caspase 12, inherent to the ER, occurred in PN of acute slices in ethanol-fed rats and was most pronounced following 40 weeks of ethanol treatment. These findings shed new light into alcohol-induced disruption in PN dendrites providing a new model for the discrete but critical changes in motor function in aging, adult alcoholics.

Eyeblink Classical Conditioning in Alcoholism and Fetal Alcohol Spectrum Disorders

Alcoholism is a debilitating disorder that can take a significant toll on health and professional and personal relationships. Excessive alcohol consumption can have a serious impact on both drinkers and developing fetuses, leading to long-term learning impairments. Decades of research in laboratory animals and humans have demonstrated the value of eyeblink classical conditioning (EBC) as a well-characterized model system to study the neural mechanisms underlying associative learning. Behavioral EBC studies in adults with alcohol use disorders and in children with fetal alcohol spectrum disorders report a clear learning deficit in these two patient populations, suggesting alcohol-related damage to the cerebellum and associated structures. Insight into the neural mechanisms underlying these learning impairments has largely stemmed from laboratory animal studies. In this mini-review, we present and discuss exemplary animal findings and data from patient and neuroimaging studies. An improved understanding of the neural mechanisms underlying learning deficits in EBC related to alcoholism and prenatal alcohol exposure has the potential to advance the diagnoses, treatment, and prevention of these and other pediatric and adult disorders.

Abnormal Immunoreactivity to Serotonin in Cerebellar Purkinje Cells after Neonatal Cocaine Exposure

Neonatal cocaine is known to affect the developing serotonergic system in many brain structures, including the cerebellum. Changes in the cerebellar Purkinje cells after drug exposure are well documented and result in impairment of movement and other cerebellar disorders such as ataxia. These cells have a major postnatal developmental pattern; therefore, neonatal exposure to cocaine is likely to affect them. In this work, male and female Wistar rats were injected with 15 mg of cocaine hydrochloride/kg body weight/day, subcutaneously, in two daily doses, from postnatal day 1 (PND1) to PND29. Controls were given 0.9% of saline. On PND14, PND21, and PND30, rats were transcardially perfused, and brains removed and cryoprotected. Coronal sections from the cerebellum were processed for immunocytochemistry of cells containing serotonin (5-hydroxytryptamine, or 5-HT). At the same postnatal age, rats from at least three different litters were sacrificed by decapitation, and brains were dissected for determination of 5-HT in the cerebellum by high-performance liquid chromatography with electrochemical detection. Upon the expected distribution of immunoreactivity to 5-HT, an abnormal immunoreactivity to 5-HT was observed in the Purkinje cells of six cocaine-exposed animals, but not in control animals. Also, levels of cerebellar 5-HT in cocaine-exposed rats were significantly increased on PND21. These results, together with previously reported observations of altered patterns of motor behavior, indicate that neonatal cocaine exposure affects the serotonergic cerebellar system, altering the standard development of Purkinje cells and possibly compromising the motor function.

Reduction of Purkinje cell volume in cerebellum of alcoholics

It is generally believed that chronic alcohol consumption results in cerebellar atrophy and Purkinje cell loss, especially in the anterior vermal region. A post-mortem stereological design was applied to cerebella from 10 chronic male alcoholics (mean age 45.5 years) with a minimum of 10 years of severe addiction and 10 male controls (mean age 42.5 years). All alcoholics had pathoanatomical evidence of alcohol abuse but no clinical signs of Wernicke's encephalopathy. Cerebellum was divided into five different areas: the anterior and posterior lobe, the anterior and posterior vermis, and the flocculonodular lobe. The total cortex and white matter volume, the cerebellar surface area, the total Purkinje and granule cell number and density, and the mean volume of Purkinje cells and their cell nuclei were measured in all five regions using stereological methods. The volume of the granular layer was increased by 13% with an increase in layer thickness by 17% possibly due to oedema. Globally, the mean volume of the Purkinje cell perikaryon was decreased by 24% with a decrease in the volume of Purkinje cell nuclei by 16%. The increase of the granular layer and the decrease of Purkinje cell size resulted in a 21% global reduction of Purkinje cell density without a concomitant loss of neurons. No significant regional or global cortical and white matter atrophy was found in cerebella from alcoholics compared to controls.

EAAT4 mRNA expression is preserved in the cerebellum of prion protein-deficient mice

To study the mechanism underlying the selective degeneration of Purkinje cells in the cerebellum of the Nagasaki (Ngsk) prion protein-deficient (PrP(-/-)) mice, the mRNA levels of glutamate transporter EAAT4, the marker highly specific for Purkinje cell synapses, were analyzed by semi-quantitative reverse transcription-polymerase chain reaction. EAAT4 mRNA was expressed in the cerebellum of PrP(-/-) mice presenting with cerebellar ataxia, at the levels identical to those in the cerebellum of non-ataxic PrP(+/-) mice. Furthermore, EAAT4 mRNA was identified in the cerebrum of both PrP(-/-) and PrP(+/-) mice, although its levels were much lower than those in the cerebellum. These results indicate that Purkinje cell degeneration found in the cerebellum of PrP(-/-) mice is not primarily caused by glutamate neurotoxicity, although it remains to be investigated whether preserved expression of EAAT4 might represent a compensatory mechanism for protecting against Purkinje cell degeneration in the PrP(-/-) mice cerebellum.

Expression of hNP22 Is Altered in the Frontal Cortex and Hippocampus of the Alcoholic Human Brain

BACKGROUND

Human neuronal protein (hNP22) is a gene with elevated messenger RNA expression in the prefrontal cortex of the human alcoholic brain. hNP22 has high homology with a rat protein (rNP22). These proteins also share homology with a number of cytoskeleton-interacting proteins.

METHODS

A rabbit polyclonal antibody to an 18-amino acid epitope was produced for use in Western and immunohistochemical analysis. Samples from the human frontal and motor cortices were used for Western blots (n = 10), whereas a different group of frontal cortex and hippocampal samples were obtained for immunohistochemistry (n = 12).

RESULTS

The hNP22 antibody detected a single protein in both rat and human brain. Western blots revealed a significant increase in hNP22 protein levels in the frontal cortex but not the motor cortex of alcoholic cases. Immunohistochemical studies confirmed the increased hNP22 protein expression in all cortical layers. This is consistent with results previously obtained using Northern analysis. Immunohistochemical analysis also revealed a significant increase of hNP22 immunoreactivity in the CA3 and CA4 but not other regions of the hippocampus.

CONCLUSIONS

It is possible that this protein may play a role in the morphological or plastic changes observed after chronic alcohol exposure and withdrawal, either as a cytoskeleton-interacting protein or as a signaling molecule.

Dendritic spine pathology: cause or consequence of neurological disorders?

Altered dendritic spines are characteristic of traumatized or diseased brain. Two general categories of spine pathology can be distinguished: pathologies of distribution and pathologies of ultrastructure. Pathologies of spine distribution affect many spines along the dendrites of a neuron and include altered spine numbers, distorted spine shapes, and abnormal loci of spine origin on the neuron. Pathologies of spine ultrastructure involve distortion of subcellular organelles within dendritic spines. Spine distributions are altered on mature neurons following traumatic lesions, and in progressive neurodegeneration involving substantial neuronal loss such as in Alzheimer's disease and in Creutzfeldt-Jakob disease. Similarly, spine distributions are altered in the developing brain following malnutrition, alcohol or toxin exposure, infection, and in a large number of genetic disorders that result in mental retardation, such as Down's and fragile-X syndromes. An important question is whether altered dendritic spines are the intrinsic cause of the accompanying neurological disturbances. The data suggest that many categories of spine pathology may result not from intrinsic pathologies of the spiny neurons, but from a compensatory response of these neurons to the loss of excitatory input to dendritic spines. More detailed studies are needed to determine the cause of spine pathology in most disorders and relationship between spine pathology and cognitive deficits.

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.

The number of granule cells and spine density on Purkinje cells in aged, ethanol-fed rats

The purpose of this study was to determine whether chronic intake of ethanol by aged F344 rats was associated with a reduction in parallel fiber input to cerebellar Purkinje neurons (PN). Previous results from this laboratory provided direct evidence that synaptic density in PN dendritic arbors was significantly decreased and indirect evidence that terminal dendritic segments of PN were deleted during chronic ethanol treatment. From these results, it was hypothesized that an ethanol-related deletion of PN terminal dendritic segments might result from 1) a reduction in parallel fiber input to PN from cerebellar granule neurons or 2) a reduction in dendritic spines, the postsynaptic sites for parallel fiber input to PN dendrites. Measurements of the total number of cerebellar granule neurons (GN) and the volume of the GN layer, and measurements of the density of spines on PN terminal dendritic segments were made in separate groups of aged, ethanol-treated and control rats. There were no significant ethanol-related changes in these parameters after 40-48 weeks of ethanol treatment.

Effects of alcohol on the synthesis and expression of hypothalamic peptides

Studies aimed at analyzing the deleterious effects of excess alcohol in the brain have revealed structural alterations that are often associated with functional and behavioral disturbances. Among the neuronal damage related to prolonged alcohol exposure, alterations in the synthesizing capabilities and levels of expression of neuroactive peptides have been increasingly reported. Actually, such changes frequently represent the sole repercussion of acute and short-term exposure to ethanol. This review gathers the existing data on the effects of ethanol exposure on the synthesis and expression of hypothalamic peptides. Amid those that can act both as neurotransmitters and neurohormones, we allude to vasopressin, corticotropin-releasing hormone, thyrotropin-releasing hormone and pro-opiomelanocortin and related peptides produced by paraventricular, supraoptic and arcuate neurons. With respect to peptides that act exclusively as neurotransmitters, we address the effects of alcohol on vasoactive intestinal polypeptide, gastrin-releasing peptide, somatostatin and vasopressin synthesized by suprachiasmatic neurons. Hypothalamic neurons that produce peptides that act as neurotransmitters are supposed to be modulated primarily by influences exerted by neuronal afferents, whereas those producing peptides that additionally act as neurohormones are also regulated by peripheral stimuli (e.g., plasma levels of circulating hormones, osmotic challenges). These peculiar features endue the hypothalamus with characteristics that are particularly propitious to enlighten the still cryptic mechanisms underlying the ethanol effects on protein synthesis.

3-D confocal laser scanning microscopy used in morphometric analysis of rat Purkinje cell dendritic spines after chronic ethanol consumption

A confocal laser scanning microscope (with a 543 nm laser) was used for imaging rat Purkinje cell dendritic spines at high 3-D resolution. In a nutritionally controlled study of the rat, 5 months of ethanol consumption was demonstrated to alter the spines of Purkinje cell dendrites in rat cerebellum. Intact spines showed significant elongation after ethanol exposure, whereas this neuromorphological alteration could not be detected in controls. Spine elongation could be regarded as compensative growth of spines in search of new synaptic contacts due to alcohol induced cell loss.

Ultrastructure of rat Purkinje neurons after chronic ethanol consumption and prolonged abstinence

In the present study of nutrition control of Wistar white rats, the ultrastructure of cerebellar Purkinje cells was studied after chronic ethanol exposure and a subsequent period of prolonged abstinence: a qualitative investigation of the perikarya of Purkinje cells was performed in age-matched controls (group A) and rats alcohol-fed for 5 months and withdrawn from this diet for 3 months (group B). The results showed massive accumulation of small dense bodies as well as obvious deposition of lipofuscin in the Purkinje cells of group B. Furthermore, ring-shaped Golgi apparatus units, lamellar bodies and degenerative foci dispersed throughout the cytoplasm of the alcohol-treated animals referred to degeneration processes and neuronal cell death, the morphological characteristics and the aetiology of which are discussed.

Can megadoses of thiamine prevent ethanol-induced damages of rat hippocampal CA1 pyramidal neurones?

The specific aim of this study was to evaluate whether high doses of thiamine can compensate or prevent alcohol-induced damages of rat hippocampus CA1 pyramids. Twenty weeks of ethanol consumption together with a dose of thiamine in the range of 1.19 mg/100 mg food induced significant enlargement (parameters measured were length of the whole spine and diameter of the end-bulb) of dendritic spines. Hypertrophy can be interpreted as a compensation process due to alcohol-induced cell death because viable spines are in search of new synaptic contacts. In contrast, dendritic spines of the alcohol group fed at the same time with a high dose of thiamine (119 mg/ 100 g food = megavitamintherapy) showed normal data concerning these parameters. From these results it may be concluded that a megavitamin therapy supports a neuron's carbohydrate metabolism and therefore could be able to prevent or reduce alcohol-induced damages of hippocampal CA1 pyramidal cells in rat central nervous system.

The dendritic trees of neurons from the hippocampal formation of protein-deprived adult rats. A quantitative Golgi study

We have recently shown that lengthy periods of low-protein feeding of the adult rat lead to deficits in the number of hippocampal granule and pyramidal cells, and in the number of mossy fiber synapses. These findings prompted us to analyze the dendrites of these neurons to evaluate whether, under the same experimental conditions, degenerative and/or plastic changes also take place at the dendritic level. The hippocampal formations from five 8-month-old rats fed a low-protein diet (casein 8%) for 6 months from the age of 2 months and from five age-matched controls were Golgi-impregnated and the morphology of the dendritic trees quantitatively studied. We found that in malnourished animals there was a reduction in the number of dendritic branches in the dentate granule cells and in the apical dendritic arborizations of CA3 pyramidal neurons. In addition, in the dentate granule cells the spine density was markedly increased and the terminal dendritic segments were elongated in malnourished animals. No alterations were found in the apical dendrites of CA1 pyramidal cells. The results obtained show that long periods of malnutrition induce marked, although not uniform, changes in the dendritic domain of the hippocampal neurons, which reflect the presence of both degenerating and regrowing mechanisms. These alterations are likely to affect the connectivity pattern of the hippocampal formation and, hence, the activity of the neuronal circuitries in which this region of the brain is involved.

Synapse-to-neuron ratio in the lateral geniculate nucleus of rats exposed chronically to ethanol

Effects of chronic ethanol exposure on synapse-to-neuron ratio in the rat lateral geniculate nucleus were investigated. Male Sprague-Dawley rats were exposed to ethanol, using the Lieber-DeCarli liquid diets, for 4 months starting at the age of 5 weeks. Brains were perfusion-fixed, and the region containing the dorsal lateral geniculate nucleus was cut into slabs (500 microns thick) that were epoxy resin-embedded. From each rat, three slabs containing the structure were serially sectioned for electron microscopy. Using the double disector method, the study shows an unaltered synapse-to-neuron ratio in ethanol-treated rats when compared with controls. The findings are in agreement with previous studies on the visual system using the same exposure model. In contrast, a previous study has shown that the synapse-to-neuron ratio in locus ceruleus of ethanol-treated rats is reduced by 50%. Other studies have shown that, whereas the glutamatergic NMDA receptor is very sensitive to ethanol, the kainate/AMPA type of receptor is very much less so. Thus, the difference in ethanol-induced synapse elimination between the two regions may reflect this different sensitivity of the glutamatergic receptors, which are of the kainate/AMPA type in the lateral geniculate nucleus and of the NMDA type in the locus ceruleus.

Reorganization of mossy fiber synapses in male and female hypothyroid rats: a stereological study

Thyroid hormone deficiency has been reported to interfere with synaptogenesis, particularly in those regions of the brain where the neurons display a late and protracted histogenesis, although the extent of the synaptic alterations remains unknown. To provide detailed quantitative data on the effects of hypothyroidism upon synapses, a link of the hippocampal circuitry was selected: the contact between mossy fibers and dendritic excrescences of CA3 pyramidal cells (MF-CA3 synapses). Groups of six male and six female rats aged 30 and 180 days were analyzed separately after being treated as follows: (1) hypothyroid from day 0 until day 30 (30 day old hypothyroid group); (2) hypothyroid from day 0 until day 180 (180 day old hypothyroid group); (3) hypothyroid until day 30 and thenceforth maintained euthyroid (recovery group); and (4) and (5) 30 and 180 day old control groups, respectively. Timm staining, Golgi impregnation, and electron microscopy were employed to estimate the volume of the mossy fiber system, the number and size of mossy fiber boutons, and the number and related features of MF-CA3 synapses. The volume of the mossy fiber system and the number of synaptic boutons were reduced in all experimental groups. The total number of synapses was decreased in 30 day old hypothyroid rats, but did not differ among 180 day old animals. Postsynaptic densities were shorter in hypothyroid and recovery groups than in controls, although the reduction was not as marked in recovery rats as it was in hypothyroid animals. Structural alterations were noted in the pre- and postsynaptic compartments of MF-CA3 synapses of both 180 day old hypothyroid and recovery rats. These changes can be regarded as mechanisms of reorganization as they underlie the compensation for the hypothyroid-induced numerical reduction of synapses observed in 30 day old animals and enable a complete catch-up of their total number. However, synaptic reorganization was not fully achieved, as revealed by the reduction in the size of the synaptic sites in hypothyroid and recovery animals. Finally, we demonstrate that hypothyroidism did not interfere with the sex-related differences of MF-CA3 synapses described in normal rats.

Effects of chronic ethanol administration on acetylcholinesterase activity in the somatosensory cortex and basal forebrain of the rat

A chronic diet of ethanol has detrimental effects on the cholinergic system in adult humans and rats. This study examined the effects of chronic exposure to dietary ethanol on the anatomical organization of true acetylcholinesterase (AChE) active elements in rat cerebral cortex. We focused on the somatosensory cortex because of its highly organized chemical and cellular structure. Following 42 days of exposure to an ethanol diet (6.7% v/v), there were marked changes in the cortical plexus of AChE-positive fibers. The AChE-positive plexus in ethanol-treated rats was reduced in all cortical layers, in comparison to age-matched pair-fed control and chow-fed rats. The most marked reduction was evident in layers II/III, IV, and VIa. Moreover, the density of AChE-positive cell bodies was significantly reduced in the cortices of ethanol-fed rats, particularly in the deep laminae. These alterations in the chemoarchitecture of somatosensory cortex occurred in the absence of changes in the cytoarchitectonic organization of neocortex. There was no detectable ethanol-induced change in the density of Cresyl violet-stained neurons either in the horizontal limb of the diagonal band of Broca or in the nucleus basalis. The density of AChE-positive neurons in the nucleus basalis, however, was significantly lower in ethanol-fed rats than in controls. Thus, it appears that a mere 6 weeks of ethanol exposure is sufficient to alter the cholinergic innervation of the cerebral cortex. These cortical alterations occur despite the lack of an ethanol-induced death of neurons in the basal forebrain. Such changes may contribute to the memory loss associated with alcohol dementia.

Structural changes in the hippocampal formation after long-term alcohol consumption and withdrawal in the rat

The effects of long-term alcohol consumption and withdrawal upon the structure of the rat hippocampal formation were studied by applying morphometric methods to material processed for light and electron microscopy. The somatostatinergic neurons of the hilus were also studied. Groups of 6 rats were treated as follows: (a) given alcohol for 6, 12 and 18 months; (b) paired controls; and (c) rats switched to a normal diet in the 6 months after 6 and 12 months of alcohol intake. A progressive loss of hippocampal neurons after chronic alcohol consumption was found. The loss was aggravated during withdrawal from alcohol, with the exception of the hilar cells. The dendrites of granule cells from the alcohol-treated rats displayed signs of regrowing, but they did not do so in rats withdrawn from alcohol. The synapses between mossy fibre terminals and CA3 dendrites appear to be rather resistant to alcohol insult, and evidence of morphological plasticity was found in withdrawn rats. If an homology can be established between humans and rodents then the changes observed in alcohol-fed rats can be regarded as underpinning some of the functional and behavioural alterations depicted under these circumstances. The peculiar changes found in some nerve cell populations after withdrawal of alcohol could be related to the deficient or incomplete functional recovery often seen after abstinence from alcohol.

Dendritic hypertrophy in Purkinje neurons of old Fischer 344 rats after long-term ethanol treatment

Metric parameters of Purkinje cell dendritic networks in 24- to 26-month-old Fischer 344 rats were determined after 48 weeks of chronic ethanol intake. Measurements included the total number and length of all segments/network, the total number and length of segments within topologically defined segment categories, and the mean length of segments in each category. A main effect of ethanol was expressed as a significant increase in cumulative length within one category of terminal segments. This increase was the result of a significant increase in mean length/segment in that category. Metric changes in other segment categories were not significant, showing that changes in the networks during ethanol treatment were not distributed randomly. Recovery after ethanol treatment was associated with further nonrandom remodeling of these networks. Significant differences in lengths of terminal segments were no longer present, but internal segments in networks of both pair-fed and ethanol-fed rats were significantly longer. Only one category of internal segments showed this change during recovery. The data showed that long term ethanol treatment in old rats, at a time when effects of aging processes were prominent in Purkinje cell networks, was associated with remodeling of those networks through dendritic extension. This effect was interpreted as compensatory growth in surviving networks following ethanol-related neuronal loss and/or damage to the surrounding neuropil. Recovery from dietary treatment produced further internal remodeling of those networks that was not related specifically to ethanol. It could be shown, however, that the restructuring processes that resulted in longer internal segments after dietary recovery were different in the pair-fed and the ethanolfed rats.

Effect of perinatal administration of ethanol on the CA1 pyramidal cell of the hippocampus and Purkinje cell of the cerebellum: an ultrastructural survey

Previous studies of Golgi-impregnated material demonstrated the vulnerability of the dendritic arbors of the CA1 pyramidal cells of the hippocampus and the Purkinje and granule cells of the cerebellum to perinatal exposure to ethanol. The purpose of this investigation was to determine whether the alterations seen in the Golgi material could be corroborated at the ultrastructural level, and to qualitatively survey the neuropil surrounding the affected neurons. Pregnant mice were maintained on either a chocolate-flavoured liquid diet containing 25% of its caloric value as ethanol (5.4% v/v) or a chow and water diet. The treatment period began on gestational day 12 and continued until postnatal day 7, at which time the ethanol-exposed animals were returned to a chow and water diet. Pups were killed on postnatal day 14, and the tissue was processed for ultrastructural and Golgi analysis. The results confirm our Golgi findings that ethanol severely compromises the dendritic arbor, and also indicate that ethanol exposure produces changes in the organization of the neurophil, as well as alterations in the organization of the perikaryal organelles, and glial swelling.

Contributions of dendritic spines and perforated synapses to synaptic plasticity

The dynamic nature of synaptic connections has presented morphologists with considerable problems which, from a structural perspective, have frustrated the development of ideas on synaptic plasticity. Gradually, however, progress has been made on concepts such as the structural remodelling and turnover of synapses. This has been considerably helped by the recent elaboration of unbiased stereological procedures. The major emphasis of this review is on naturally occurring synaptic plasticity, which is regarded as an ongoing process in the postdevelopmental CNS. The focus of attention are PSs, with their characteristically discontinuous synaptic active zone, since there is mounting evidence that this synaptic type is indicative of synaptic remodelling and turnover in the mature CNS. Since the majority of CNS synapses can only be considered in terms of their relationship to dendritic spines, the contribution of these spines to synaptic plasticity is discussed initially. Changes in the configuration of these spines appears to be crucial for the plasticity, and these can be viewed in terms of the significance of the cytoskeleton, of various dendritic organelles, and also of the biophysical properties of spines. Of the synaptic characteristics that may play a role in synaptic plasticity, the PSD, synaptic curvature, the spinule, coated vesicles, polyribosomes, and the spine apparatus have all been implicated. Each of these is assessed. Special emphasis is placed on PSs because of their ever-increasing significance in discussions of synaptic plasticity. The possibility of their being artefacts is dismissed on a number of grounds, including consideration of the results of serial section studies. Various roles, other than one in synaptic plasticity have been put forward in discussing PSs. Although relevant to synaptic plasticity, these include a role in increasing synaptic efficacy, as a more permanent type of synaptic connection, or as a route for the intercellular exchange of metabolites or membrane components. The consideration of many estimates of synaptic density, and of PS frequency, have proved misleading, since studies have reported diverse and sometimes low figures. A recent reassessment of PS frequency, using unbiased stereological procedures, has provided evidence that in some brain regions PSs may account for up to 40% of all synapses. All ideas that have been put forward to date regarding the role of PSs are examined, with particular attention being devoted to the major models of Nieto-Sampedro and co-workers.(ABSTRACT TRUNCATED AT 400 WORDS)

Length changes in dendritic networks of cerebellar Purkinje cells of old rats after chronic ethanol treatment

Dendritic networks of cerebellar Purkinje neurons from aging ethanol-treated Fisher 344 rats were analyzed for metric changes in terminal and internal segments of the networks. Mean lengths of three categories of dendritic segments were determined. No significant metric changes in segment lengths were detectable immediately at the termination of 24 weeks of ethanol treatment, but significant changes were apparent after 8 weeks of recovery from ethanol treatment. Return to a diet of standard laboratory chow was associated with a period of dendritic extension in networks from pair-fed control rats but not in those from the ethanol-treated rats. The resulting significant differences in mean segment length were restricted to the paired terminal segments at the peripheral tips of the bifurcating networks. Unpaired terminal segments and internal segments of the networks showed no significant changes in length during the recovery period.

In situ localization of myosin and actin in dendritic spines with the immunogold technique

The in situ detection of macromolecules by means of immunoelectron microscopy provides information about their ultrastructural localization in cellular compartments. With this technique, we have demonstrated that the contractile proteins actin and myosin are both localized in dendritic spines at densities exceeding those of other neuronal compartments. Myosin was associated with actin filaments, with spine plasma membrane, and with membranes of the spine apparatus. Given the dynamic properties of actin and myosin, these data suggest that these proteins may be involved in the mechanism of synaptic plasticity in general and in morphometric change resulting from intense synaptic activation in particular.

Granule cell loss and dendritic regrowth in the hippocampal dentate gyrus of the rat after chronic alcohol consumption

The effects of chronic alcohol consumption (CAC) on the relative number of dentate gyrus granule cells and their dendritic trees, were studied in animals fed alcohol for 6, 12 and 18 months and in their respective controls. The granule cell density was estimated with the unbiased disector method. Following 6 months of alcohol consumption, the thickness of the dentate gyrus granular layer and the relative number of dentate granule cells were significantly decreased when compared with controls. The granule cell dendritic arborizations showed an increase of their dendritic extent in alcohol-treated rats. No significant differences were found in the density of dendritic spines between alcohol-fed and control animals. These results indicate the existence of hippocampal granule cell dendritic regrowth in alcohol-fed rats, probably occurring as a compensatory response to the granule cell deficit which follows the alcohol-induced granule cell degeneration. These degenerative and regenerative changes might have functional implications for the organization of the synaptic hippocampal circuitry after long periods of alcohol consumption.

Time course and reversibility of ethanol's suppressive effects on axon sprouting in the dentate gyrus of the adult rat

Ethanol was administered chronically to adult rats in a liquid diet for 14 days preceding and for 5, 7, 8, 9, or 10 days following the unilateral destruction of the entorhinal cortex. Control groups received a diet of lab chow and water and were sacrificed at comparable survival times. An additional experimental group was given ethanol until 9 days after the lesion, then switched to lab chow and water and sacrificed 1 day later. Coronal sections through the dorsal hippocampal formation were stained and analyzed histochemically for the localization of acetylcholinesterase (AChE). Quantitative measurements of the histochemical patterns in the molecular layer of the dentate gyrus were obtained. Ethanol exposure inhibited the withdrawal of the acetylcholinesterase-stained septohippocampal fibers and limited the typical lesion-induced expansion of the pale-staining commissural/associational zone in the molecular layer of the denervated dentate gyrus. However, abstinence from ethanol for just 24 h released the inhibitory effect on the acetylcholinesterase-staining fibers, resulting in a significant expansion of the commissural/associational zone.

Morphometric parameters of Purkinje dendritic networks after ethanol treatment during aging

Spine densities on terminal branches of Purkinje cell dendrites of Fischer 344 rats were significantly altered by ethanol treatment and aging processes. An effect of the control liquid diet on the lengths of terminal branches and an interaction effect of this diet with age on the numbers of terminal branches/cell also occurred, but there were no detectable interaction effects of ethanol with age on dendritic parameters. Changes in spine density on terminal branches, which accounted for 63-67% of the total dendritic length/cell, represented a major quantitative modulation of synaptic input to these neurons during age and following ethanol treatment.

Synapses of the cerebellar cortex molecular layer after chronic alcohol consumption

The cerebellar molecular layer of chronic alcohol treated rats showed degenerated parallel fiber boutons and vacated Purkinje cell spines after 6 months of alcohol feeding; degenerated Purkinje cell dendrites were concomitantly observed. The number of synapses between parallel fibers and Purkinje cell spines decreased after 6 months whereas their mean synaptic diameter increased throughout the experiment. Conversely, synapses between parallel fibers and dendrites of interneurons increased in 18-month alcohol-fed group. Quantifications were done using a discrete unfolding procedure. It was also observed that some parallel fiber terminals simultaneously established synapses with multiple Purkinje cell spines and with dendrites of interneurons. These results were obtained from groups of 6 rats alcohol-fed for 1, 3, 6, 12 and 18 months and compared with age-matched pair-fed controls. Previous reports of alcohol-induced degenerative changes in the adult rat central nervous system were, thus, fully corroborated. Furthermore, there was evidence of remodeling processes pointing to compensatory plastic mechanisms in the cerebellar circuitry albeit not sufficient to overcome its alcohol-induced deterioration.

Ultrastructural pathology of dendritic spines in epitumorous human cerebral cortex

Ultrastructural changes in the human epitumorous cerebral cortex were examined. A swelling of nerve cell perikarya, dendrites and axon terminals, and an occasional hyperplasia and disarray of microtubules and neurofilaments were observed. Accumulations of lysosomes, tubuloreticular structures and intracytoplasmic or intramitochondrial crystalloid inclusions were also found. Some myelinated axons were degenerated. Astrocytes and their processes were focally swollen. A mild swelling was found also in microgliocytes. Oligodendrocytes occasionally contained accumulations of dense bodies. Special attention was paid to dendritic spines. The spine surface morphology changed distinctly on swollen dendritic segments. The necks of most spines were short and wide, and numerous sessile forms were present in this location. The spine apparatus was often hypertrophied and disorganized. Smoothing out of spines on swollen dendrites is described and a possible functional significance of the observed changes in the symptomatology of brain tumors is hypothesized.

Effects of chronic ethanol treatment on neocortex

Neocortical inhibition and neuronal morphology were studied in rats following chronic ethanol treatment (CET). In terminal acute experiments, spontaneous neuronal discharges in pair-fed and naive rats were inhibited by epicortical stimulation, a procedure known to produce postsynaptic inhibition. Few units in CET rats were inhibited by such stimulation. Cortical recurrent inhibition, indicated by a surface-negative potential in response to antidromic stimulation of the cerebral peduncle, was little affected by a challenge dose of ethanol, compared with the response in pair-fed animals. Recurrent inhibition involves inhibitory interneurons. CET apparently made inhibitory interneurons and inhibitory postsynaptic receptors less responsive to ethanol. Apical dendritic spines on some portions of pyramidal neurons increased in number with CET. This could reflect a compensatory growth in neurons not damaged by CET. The overall observations are consistent with ethanol affecting one or more specific systems of cortical motor control as opposed to its presumed general disinhibitory effect.

Actin filament organization within dendrites and dendritic spines during development

The myosin S-1 subfragment was used to label actin filaments in the developing rat brain. The results show actin filaments present throughout the dendritic region with highest concentrations within growth cones and regions of spine development. Between 6 and 25 days postnatal, spines became more complex and actin filaments within them increased in number and formed a complex network. The observed organization of actin supports the hypothesis that actin has a role in the protrusion of spines from the dendrite during development.

Long term alcohol consumption induces microtubular changes in the adult rat cerebellar cortex

The effects of prolonged alcohol consumption on the microtubules of Purkinje cell dendrites and granule cell axons were studied in adult rats fed alcohol for 1, 3, 6, 12 and 18 months and compared with respective age-matched controls. A significant consequential decrease in the number of dendritic microtubules in alcohol-fed rats was found when compared with the respective controls. Conversely, an increase in the number of these organelles was found in both ascending and parallel portion of the axons in the experimental animals. The possibility of a relationship between microtubular changes and previously reported cerebellar cortex alcohol-induced structural alterations is advanced.

Lipofuscin granules in cerebellar interneurons after long-term alcohol consumption in the adult rat

Lipofuscin deposition in nerve cells is one of the most reliable and consistent neurocytological features correlated with ageing. Purkinje cells of long-term alcohol-fed rats show large agglomerates of lipofuscin granules after six months of alcohol experiment, whereas in normal biological ageing, this happens only after 25 months of age. Cerebellar interneurons have specific patterns of lipofuscin accumulation during ageing concerning both its morphological type and chronology of deposition. We studied the effects of chronic alcohol treatment on cerebellar interneurons taking particular account of lipofuscin pigment accumulation. Control and alcohol-fed groups for 1, 3, 6, 12 and 18 months were used. A precocious and progressive accumulation of lipofuscin granules occurred in granule, Golgi and basket cells. Stellate cells remained pigment-free. The lipofuscin deposited in the granule and Golgi cells was of the granular type, whereas that of basket and stellate cells was lamellar (fingerprint-like pattern). These results parallel those observed during normal ageing, and reinforce the hypothesis of the existence of a close relationship between chronic alcohol consumption and precocious nerve cell ageing.