Jorge L. Arias

Spatial learning-induced increase in the argyrophilic nucleolar organizer region of dorsolateral telencephalic neurons in goldfish

Spatial learning and memory related morphological changes in the argyrophilic nucleolar organizer region (AgNOR) of telencephalic neurons in goldfish were quantitatively evaluated by means of AgNOR neurohistochemical stain. The AgNORs and nuclei of nerve cells of two different telencephalic regions of goldfish trained in a spatial task or submitted to a similar non-contingent behavioral procedure (control group) were morphometrically evaluated. Results show that the area of AgNORs in goldfish dorsolateral telencephalic neurons increased significantly in the spatial learning group but not in control group. This effect seems to be highly specific as it did not appear in the dorsolateral area of the control group neither in the dorsomedial area of both groups. As the size of AgNORs in the nerve cell nuclei reflect the level of transcriptive activity, these morphological changes could be revealing increased protein synthesis in goldfish dorsolateral telencephalic neurons related with learning and memory. These findings could contribute to determining the subregions of the teleost telencephalon implicated in spatial learning and could indicate that the AgNOR staining technique would be a useful tool in assesing learning and memory related neuronal activity.

Effects of mammillary body lesions on spatial reference and working memory tasks

This work examines the effects of electrolytic mammillary body (MB) lesions on the performance of rats in different spatial memory tasks in the Morris water maze. The first experiment assessed the effect of MB lesion on performance in a spatial reference memory task (place learning with multiple trials). The second experiment examined the effect of a lesion in this nucleus on performance in a spatial working memory task (single-trial place learning). The results show that lesion of the MB impairs the animals performance in spatial working memory tasks but does not impair acquisition in spatial reference memory tasks (place learning, transfer task, reversal task) or in a visual-cued task. However, the deficit in the spatial working memory task does not appear to vary with the delay between acquisition and retention trials (30 s and 5 min). Our results demonstrate a clear role of the mammillary bodies in the processing of spatial information in a working memory task. Lesion of the MB impairs performance in a working memory task but does not affect reference memory processes.

Selective involvement of the goldfish lateral pallium in spatial memory

The involvement of the main pallial subdivisions of the teleost telencephalic pallium in spatial cognition was evaluated in a series of three experiments. The first two compared the effects of lesions selective to the lateral (LP), medial (MP) and dorsal (DP) telencephalic pallium of goldfish, on the retention and the reversal learning of a spatial constancy task which requires the use of allocentric or relational strategies. The results showed that LP lesions produced a selective impairment on the capability of goldfish to solve the spatial task previously learned and on the reversal learning of the same procedure, whereas MP and DP lesions did not produce observable deficits. The third experiment evaluated, by means of the AgNOR stain, learning-dependent changes of the neuronal transcription activity in the pallium of goldfish trained in the spatial constancy task or in a cue version of the same procedure, which only differed on their spatial cognition demands. The results revealed that training in the spatial task produced an increment in the transcriptive activity which was selective to the neurons of the ventral lateral pallium, as indicated by increases in the size of the nucleolar organizing region (NOR), the nucleolar organelles associated with the synthesis of ribosomal proteins. In contrast, training in the cue version did not produced observable changes. These data, revealing a striking functional similarity between the lateral telencephalic pallium of the teleost fish and the amniote hippocampus, provide additional evidence regarding the homology of both structures.

Effects of chronic alcohol consumption on spatial reference and working memory tasks.

The aim of this work was to determine the spatial memory impairments induced by chronic alcohol consumption in rats. The alcoholization process began on the 21st postnatal day and alcohol concentrations were gradually increased to reach a concentration of 20% that was maintained for 4 mon. Behavioral tests were performed in the Morris Water Maze (MWM). The first study assessed the effects of chronic alcohol intake on two reference memory tasks (a place learning with multiple trials and a new place learning carried out in the same experimental context). Alcohol-treated animals presented no overall impairment in their ability to process spatial information. Deficits were restricted to reduced behavioral flexibility in spatial strategies. The second study assessed working memory in two tasks in which information about platform location was only valid for one trial. In the first working memory task, the animals had to perform one trial per day and in the second task they were submitted to four trials per day. At the end of the second experiment, all animals were trained in a visual-cued task. In the second experiment, the most important deficits in alcohol-treated animals occur in spatial working memory tasks, and this impairment was independent of the intertrial interval used. In the second spatial working memory task, performance of the alcohol-treated animals in the earlier trials affected their performance in subsequent trials, suggesting that a process of proactive interference had taken place. The visual-cued task demonstrated that these behavioral impairments were produced without visuoperceptive impairments.

Spatial memory alterations in three models of hepatic encephalopathy.

A behavioural evaluation was carried out on three chronic models of hepatic encephalopathy: two models of type B HE, portacaval shunt (PCS) and portal hypertension (PH) and one of type C HE with cirrhosis and portal hypertension from thioacetamide intoxication (TAA). The tasks selected cover a wide range of behaviours related to: locomotion (rotarod-accelerod test), anxiety (open field and elevated plus maze) and memory (Morris water maze). The results indicate that neither locomotor activity nor anxiety was affected in our models, in comparison with their respective controls. However, this is not the case for the mnesic tasks. Hence, the PCS and TAA groups displayed a severe alteration in spatial reference memory and cannot correctly perform the Morris maze task, while this alteration is less severe in the PH group. On the contrary, the PH group revealed a deficit in spatial working memory, like the TAA group, but this does not occur in subjects with PCS. These results reveal a double dissociation in spatial reference memory and spatial working memory between the PCS and PH groups, which would be of great interest to study about cerebral causes and substrates of the alterations accompanying HE.

Spatial learning of the water maze: progression of brain circuits mapped with cytochrome oxidase histochemistry.

The progression of brain circuits involved in spatial learning tasks is still a matter of debate. In addition, the participation of individual regions at different stages of spatial learning remains a controversial issue. In order to address these questions, we used quantitative cytochrome oxidase histochemistry as a metabolic brain mapping method applied to rats (Rattus norvegicus) trained in a water maze for 1, 3 or 5 days of training. Sustained changes throughout training were found in the lateral septal nucleus and anteroventral thalamic nucleus. As compared to naïve or habituation groups, rats with 1 day of training in the spatial learning task showed involvement of the lateral mammillary nucleus, basolateral amygdala and anterodorsal thalamic nucleus. By 5 days of training, there were mean changes in the hippocampal CA3 field and the prefrontal cortex. The regions involved and their pattern of network interactions changed progressively over days of training. At 1-day there was an open serial network of pairwise correlations. At 3-days there was a more closed reciprocal network of intercorrelations. At 5-days there were three separate parallel networks. In addition, brain-behavior correlations showed that CA1 and CA3 hippocampal fields together with the parietal cortex are related to the mastery of the spatial learning task. The present study extends previous findings on the progressive contribution of neural networks to spatial learning.

Sex-related differences in spatial learning during the early postnatal development of the rat

Some authors have reported that male rats younger than 21 days old are unable to perform spatial learning correctly because they have still not developed the ability to use extra-maze cues. In experiment 1, we analyzed spatial learning in 14-, 21-, 30- and 42-day-old rats using the Morris water maze (MWM). According to our results, a good performance was observed in 30-day-old male rats whereas this was not observed in female rats until they were 42 days old. In experiment 2 we studied the role of sex hormones in this kind of learning using the MWM and 30-day-old rats (castrated male rats and female rats treated with testosterone propionate (TP) after birth). The latter group, the male control group and the castrated males all solved the task correctly. The objective of experiment 3 was to determine possible differences between the sexes in the use of taxon strategies in the T water maze. To summarize, sexual dimorphism was only observed in spatial learning during development.

c-Fos expression in supramammillary and medial mammillary nuclei following spatial reference and working memory tasks.

To investigate brain substrates of spatial memory, neuronal expression of c-Fos protein was studied. Two groups of rats were trained in two spatial memory tasks in the Morris water maze, where the rats have to apply a reference memory rule or a working memory rule. In addition to the experimental groups, two control groups were used to study c-fos activation not specific to the memory processes studied. After immunohistochemical procedures, the number of c-Fos positive neuronal nuclei was quantified in the mammillary body (MB) region (medial mammillary nucleus [MMn] and supramammillary nucleus [SuM]). The results have shown that some MMn neurons expressed c-Fos nuclear immunoreactivity related to spatial working memory but not to spatial reference memory. The increased number of c-Fos immunoreactive neuronal nuclei in the SuM was related to spatial training but not to either working or reference memory demands of the tasks.

Astrocytic plasticity as a possible mediator of the cognitive improvements after environmental enrichment in aged rats

Currently, little is known about the effect of environmental enrichment (EE) on astrocytic plasticity, especially during aging. Given the newly discovered role of the astrocytes in regulating the synaptic transmission and thereby, the cognitive functions, we aimed to study the impact of EE on the performance in a spatial memory task and on the number and morphology of GFAP immunopositive cells in the dorsal hippocampus. After two months of EE (3 h/per day), the animals were tested in the Radial-Arm Water Maze (RAWM) for four days, with six daily trials. Next, we analyzed the changes in the GFAP immunopositive cells in CA1, CA3 and Dentate Gyrus (DG). Behavioral results showed that, even in advanced ages, EE improved the performance in a spatial memory task. Also, we found that aged rats submitted to EE had more GFAP immunopositive cells in the DG and more complex astrocytes, revealed by Sholl analysis, in all hippocampal subfields with respect to the other experimental conditions. Interestingly, the learning of a spatial memory task produced more morphological complexity and higher levels of GFAP immunopositive cells with regard to a standard control group, but not at the same level of the enriched groups. Thus, it is possible that the plastic changes found in the hippocampal astrocytes after EE are involved in a brain reserve to cope with age-related cognitive impairments.

Chronic portal hypertension in the rat by triple-portal stenosing ligation.

A surgical technique based on the development of a triple stenosing ligation is used to worsen the complications inherent to the prehepatic chronic portal hypertension. The results have been compared with those obtained in rats with a single-portal stenosing ligation. An increase ( p < .05) in the body, liver, spleen, and kidney weights as well as a decrease ( p < .001) in the testes weight to body weight ratio were produced in both groups of animals. In addition, the variability in the obtained weights, particularly in the liver weight, stands out. The incidence of portosystemic and portohepatic collateral circulation and of the mesenteric venous vasculopathy increases in the animals with triple-portal stenosing ligation. The new proposed technique is a valid alternative to the classic one that used single portal stenosing ligation.