Nikolai V. Lukoyanov

Reorganization of the morphology of hippocampal neurites and synapses after stress-induced damage correlates with behavioral improvement.

We recently demonstrated that stress-induced cognitive deficits in rats do not correlate with hippocampal neuronal loss. Working on the premise that subtle structural changes may however be involved, we here evaluated the effects of chronic stress on hippocampal dendrite morphology, the volume of the mossy fiber system, and number and morphology of synapses between mossy fibers and CA3 dendritic excrescences. To better understand the mechanisms by which stress exerts its structural effects, we also studied these parameters in rats given exogenous corticosterone. Further, to search for signs of structural reorganization following the termination of the stress and corticosterone treatments, we analysed groups of rats returned to treatment-free conditions. All animals were assessed for spatial learning and memory performance in the Morris water maze. Consistent with previous findings, dendritic atrophy was observed in the CA3 hippocampal region of chronically stressed and corticosterone-treated rats; in addition, we observed atrophy in granule and CA1 pyramidal cells following these treatments. Additionally, profound changes in the morphology of the mossy fiber terminals and significant loss of synapses were detected in both conditions. These alterations were partially reversible following rehabilitation from stress or corticosterone treatments. The fine structural changes, which resulted from prolonged hypercortisolism, were accompanied by impairments in spatial learning and memory; the latter were undetectable following rehabilitation. We conclude that there is an intimate relationship between corticosteroid levels, hippocampal neuritic structure and hippocampal-dependent learning and memory.

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 Spatial reference and working memory of alcohol-consuming and withdrawn rats were evaluated by comparison of their performance with age-matched controls in the Morris water maze.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memory task we did not observe significant cognitive deficits in rats continuously exposed to ethanol, whereas withdrawn animals showed an obvious impairment of their overall performance.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.

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.

Effects of age and sex on the water maze performance and hippocampal cholinergic fibers in rats

We have examined if age-related deterioration of spatial memory and cholinergic innervation of the dentate gyrus is gender-specific. Aging progressively affected the performance of male and female rats in place discrimination version of the water maze task. On repeated acquisition task, only old males, but not old females, were significantly impaired relative to young and adult animals of both sexes. In parallel, we found that the age-associated reduction of the density of cholinergic fibers in the dentate gyrus was significantly more profound in old males than in age-matched females. These results suggest that, although male and female rats have an identical pattern of reference memory decline, impairment of the working memory and deterioration of the hippocampal cholinergic system are slower to develop in females than in males.

Chronic green tea consumption prevents age-related changes in rat hippocampal formation

The hippocampal formation undergoes considerable structural and functional modifications during aging and oxidative stress emerges as a key player in the process. In the present study, we investigated whether prolonged consumption of green tea (GT), which contains large amounts of polyphenols, could interfere with age-related changes in this brain region using biochemical, morphological and behavioral approaches. Ten male Wistar rats aged 19 months were fed with GT since 12 months of age and results compared to those obtained from controls aged 19 months (C-19M). At 12 months of age, another group of rats was evaluated to provide baseline data. Oxidative stress markers (protein carbonyls and malondialdehyde) were quantified in hippocampal homogenates and stereological methods were applied to estimate the deposition of lipofuscin in hippocampal CA3 pyramidal neurons. Morris water maze was used to assess spatial learning and memory. Aging increased oxidative markers and lipofuscin accumulation and was associated with impaired memory acquisition. However, GT treatment protected proteins and lipids against oxidation and prevented the increase of lipofuscin deposition compared to age-matched controls. Furthermore, the spatial learning abilities of GT-treated rats were significantly improved when compared to those from C-19M group. Taken together, these findings confirm the neuroprotective ability of GT in the hippocampal formation probably due to the reduction of oxidative stress-related damage observed during aging.

Memantine, but not dizocilpine, ameliorates cognitive deficits in adult rats withdrawn from chronic ingestion of alcohol.

Adult rats were given a 20% ethanol solution as their only source of fluid for 6 months and then withdrawn from alcohol. During the first 4 weeks of the withdrawal period, animals were intraperitoneally injected with either memantine (20 mg/kg bolus followed by 1 mg/kg every 12 h) or dizocilpine (MK-801; 0.1 mg/kg every 12 h), both of which are antagonists of N-methyl-D-aspartate receptors. Ten weeks after initiation of the withdrawal procedure, cognitive status of animals was assessed using the Morris water maze. Withdrawal from alcohol produced robust deficits in the performance of rats on the acquisition task and on the probe trial. Treatment with memantine resulted in a complete reversal of these behavioral impairments. In contrast, treatment with MK-801 was found to be ineffective in preventing cognitive alterations associated with chronic alcohol consumption and withdrawal.

Retrosplenial cortex lesions impair acquisition of active avoidance while sparing fear-based emotional memory.

There is strong evidence that the rat retrosplenial cortex (RC) is implicated in spatial navigation and in learning of both aversive and reward-based discrimination tasks. However, its involvement in other functions subserved by the limbic system to which it belongs has not yet been documented. We compared the performance of rats with bilateral excitotoxic damage to RC with that of control rats in a battery of conventional tests, including an open field, plus maze, fear conditioning, step-through passive avoidance, and two-way active avoidance techniques. In the open field, RC-lesioned rats showed somewhat decreased locomotion in the inner zone and increased defecation, which is suggestive of an anxiogenic effect. However, no differences between groups were detected in the plus-maze and inhibitory avoidance tests. Freezing scores recorded during fear conditioning, as well as during the context and tone tests, which were performed, respectively, 24 and 48 h after conditioning, did not differ between the groups. In contrast, acquisition of the active avoidance response was significantly impaired in rats with damaged RC, regarding both response latency and correctness. These data suggest that although the rat RC may play a role in the regulation of emotional responsiveness to new situations, it does not appear to contribute to emotional memory. They are also consistent with the idea that RC is a part of the limbic system that is involved in the compilation of motor programs for complex stereotyped movements such as approach and avoidance.

Restricted feeding facilitates time-place learning in adult rats.

Many species can acquire time-of-day discrimination when tested in food reinforced place learning tasks. It is believed that this type of learning is dependent upon the ability of animals to consult their internal circadian pacemakers entrained by various environmental zeitgebers, such as light-dark cycles and scheduled restricted feeding. In the present study, we examined, (1) whether rats can acquire time-of-day discrimination in an aversively motivated water maze task wherein an escape platform is located in one position in the morning and in another position in the afternoon; (2) whether time-of-day cues provided by the light- and feeding-entrainable pacemakers may have divergent impacts upon the ability of rats to learn this task. Two groups of rats, both maintained on 12-h light:12-h dark cycle, were used; in one group, animals had free access to food, whereas in the other, they were subjected to a restricted feeding protocol (60% of food consumed by rats fed ad libitum, once daily). Despite the heightened difficulty of the task, food-restricted rats were apparently able to acquire associations between two different platform positions and two different times of day, as indicated by the fact that the percentage of discrimination errors in this group declined progressively, as a function of training, and stabilized at the level of 22+/-9%. In contrast, rats that were fed ad libitum, even after extensive training, failed to perform the task above level of chance. These data indicate that time-place learning is a universal, reward-nonspecific, cognitive phenomenon. They furthermore suggest that the ability of animals to integrate spatial and temporal information can be dependent on the access to timing stimuli provided by the feeding-entrainable circadian system.

Impaired water maze navigation of Wistar rats with retrosplenial cortex lesions: effect of nonspatial pretraining.

Damage to the retrosplenial cortex (RC) impairs the performance of rodents on spatial learning and memory tasks, but the extent of these deficits was previously reported to be influenced by the lesion type, rat strain, and behavioral task used. The present study addressed the issue of whether or not cytotoxic damage to RC impairs place navigation of Wistar rats in the Morris water maze and, if so, whether this is merely attributable to spatial learning deficits or to impaired learning of general (nonspatial) behavioral strategies required to correctly perform this task or both. Behaviorally naive rats with bilateral lesions to RC were significantly impaired relative to sham-lesioned rats both during the period of initial learning of the task and during the later phases of training. In addition, these animals showed enhanced thigmotaxis, indicating that the lesion was associated with considerable abnormalities in nonspatial learning. In contrast, RC-lesioned animals that have been previously familiarized with general task rules in a series of shaping trials did not show more thigmotaxis than did their respective controls. Furthermore, although these rats were still impaired in the middle of the training process, their performance during the period of initial learning as well as by the end of training was found to now be normal. Our results confirm those of earlier studies indicating that RC is important for spatial navigation. The findings herein reported are also consistent with the notion that, in addition to spatial information processing, RC is involved in cognitive processes underlying the ability of subjects to properly respond to general task demands.

Effects of repeated electroconvulsive shock seizures and pilocarpine-induced status epilepticus on emotional behavior in the rat

Affective symptoms are frequently observed in patients with epilepsy. Although the etiology of these behavioral complications remains unknown, it is possible that brain damage associated with frequent or prolonged seizures may contribute to their development. To address this issue, we examined the behavioral sequelae of repeated brief seizures evoked by electroconvulsive shock (ECS) and compared them with those resulting from prolonged status epilepticus (SE) induced with pilocarpine. Using the open-field and elevated plus-maze tests, we detected the presence of behavioral alterations indicative of elevated levels of anxiety in rats that were administered a course of ECS seizures. Fear conditioning was also enhanced in these animals. However, the rats that had experienced SE exhibited less anxiety-like behavior than controls and were severely impaired in fear conditioning. These results support the notion that brain lesions caused by either brief repeated seizures or SE is sufficient to induce some affective disturbances.