Marta Méndez

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.

Associative learning deficit in two experimental models of hepatic encephalopathy

People with hepatic insufficiency can develop hepatic encephalopathy (HE), a complex neuropsychological syndrome covering a wide range of neurological and cognitive and motor alterations. The cognitive deficits include disturbances in intellectual functions such as memory and learning. In spite of its high prevalence in western societies, the causes of HE have not yet been clearly established. For this reason, experimental models of HE are used to study this condition. In this work, two experimental models were used, one Type B HE (portacaval shunt) and the other Type C HE (cirrhosis by intoxication with thioacetamide), to evaluate its effect on two tasks of associative learning: two-way active avoidance and step-through passive avoidance. The results show an impediment both in acquisition and retention of active avoidance in both models of HE. However, in passive avoidance, only the rats with portacaval shunt presented a memory deficit for the aversive event. In our opinion, these results can be explained by alterations in the neurotransmission system presented by animals with hepatic insufficiency, which are mainly caused by a rise in cerebral histamine and a dysfunction of the glutamatergic system.

The value of microsurgery in liver research

The use of an operating microscope in rat liver surgery makes it possible to obtain new experimental models and improve the already existing macrosurgical models. Thus, microsurgery could be a very valuable technique to improve experimental models of hepatic insufficiency. In the current review, we present the microsurgical techniques most frequently used in the rat, such as the portacaval shunt, the extrahepatic biliary tract resection, partial and total hepatectomies and heterotopic and orthotopic liver transplantation. Hence, reducing surgical complications allows for perfecting the resulting experimental models. Thus, liver atrophy related to portacaval shunt, prehepatic portal hypertension secondary to partial portal vein ligation, cholestasis by resection of the extrahepatic biliary tract, hepatic regeneration after partial hepatectomies, acute liver failure associated with subtotal or total hepatectomy and finally complications derived from preservation or rejection in orthotopic and heterotopic liver transplantation can be studied in more standardized experimental models. The results obtained are therefore more reliable and facilitates the flow of knowledge from the bench to the bedside. Some of these microsurgical techniques, because of their simplicity, can be performed by researchers without any prior surgical training. Other more complex microsurgical techniques require in‐depth surgical training. These techniques are ideal for achieving a complete surgical training and more select microsurgical models for hepatology research.

Sexually dimorphic c-Fos expression following spatial working memory in young and adult rats

The sex differences in the functional contribution of brain substrates were explored following acquisition of a spatial working memory task using quantification of c-Fos protein. Rats of both sexes were trained during adolescence and adulthood in Morris water maze using a hidden escape platform with different daily location. Two control groups for each sex and age were added to explore the c-Fos activation not specific to the memory process. These were a free-swimming group (yoked control) and a handled control (CO) group. Behaviorally, no age differences were found in number of days required by males to acquire the task, but females showed a delay in acquisition during adolescence (P30) that improved in adulthood (P90). Both sexes showed a learning-related increase in Fos immunoreactivity in the anterodorsal and anteroventral thalamus and medial and lateral mammillary nuclei during adolescence. Higher levels of learning-related Fos immunoreactivity were found in the infralimbic cortex, CA3 and CA1 only in females. During adulthood the common activated region was the prelimbic cortex with the addition of the infralimbic cortex in the male group and the lateral mammillary nucleus in the female group. These results indicated sex and age differences in brain functioning following working memory task. However, they could not be necessarily linked with differences in performance since similar results were found between males and females during adulthood. The activation of common and interrelated structures suggests that these structures are involved in spatial processing but it also highlights the relevance of developmental changes for understanding the memory process.

Spatial working memory learning in young male and female rats: involvement of different limbic system regions revealed by cytochrome oxidase activity.

Sex differences have been found in the spatial memory which involve several regions of the limbic system. The Morris water maze (MWM) is one of the most widely used tasks in behavioral neuroscience to explore spatial and episodic memory in rats. We evaluated the oxidative metabolic activity of the prefrontal cortex, dorsal hippocampus, anterior thalamic nuclei and mammillary region following the acquisition of a spatial working memory (WM) task in young rats (30 days) of both sexes using quantitative histochemistry of the cytochrome oxidase (COx). The rats were trained until they achieved the learning criteria in the MWM using a hidden escape platform with different daily locations, and two control groups were added to evaluate the oxidative metabolism not specific to the learning task. We found a delay in the acquisition of the WM in females. A significant decrease in COx activity was found in the prefrontal cortex in both sexes. Also, changes were found in the dentate gyrus and lateral mammillary nucleus in males, whereas females showed changes in the anterodorsal thalamus and CA3. These results suggest a sex difference in the contribution of brain limbic structures to the WM process during postnatal development.

Spatial working memory in Wistar rats: Brain sex differences in metabolic activity

Several works have shown that males and females differ in the ability to learn spatial locations in mazes. In this study, we used the Morris water maze to assess the acquisition of a spatial working memory (WM) task in adult male and female Wistar rats. The task consisted of a paired sample procedure made up of two daily identical trials, sample and retention. To study the oxidative metabolic activity of some brain limbic system regions after the WM task, we applied the cytochrome oxidase (COx) histochemistry. In addition to the experimental groups, free swimming control groups and untrained naïve groups were added to explore the COx changes not specific to the learning process. Similar spatial performances were found between sexes as only one more sample and retention trials were needed in males to reduce the escape latencies significantly. Males showed decreased COx activity as compared to control groups in the medial prefrontal cortex (prelimbic and infralimbic regions) as well as in the lateral septum and dentate gyrus. Regarding females, an increase in COx activity was found in nucleus accumbens, ventral tegmental area and supramammillary region in relation to control groups. Overall, these findings suggest that the acquisition of the spatial WM task is mediated by different subsystems in a sex-dependent manner that points to the hippocampus as the central structure in males whereas other structures would be central in females.

Effects of forced exercise on spatial memory and cytochrome c oxidase activity in aged rats

We have studied the effects of exercise in aged rats (18 months-old) on spatial learning and changes in neuronal metabolic activity associated with exercise program and the spatial learning process. The changes on neuronal oxidative metabolic activity was studied through cytochrome c oxidase histochemistry (COx) in brain regions related to spatial memory, reward, and motor activity after a forced exercise program on Rotarod. The spatial learning task was performed in the 4 arm-radial arm water maze (4-RAWM). Exercise program improved slightly the performance, with more percentage of entries into the correct arm along the days. Respect to COx activity, exercise increased the basal oxidative metabolism in frontal regions, such as motor, cingulate and retrosplenial cortex, and in central and basolateral amygdala. In the spatial memory task, the exercise group showed lower COx activity than the non-exercise group in prefrontal cortex, bed nucleus of the stria terminalis, amygdala, hippocampus, retrosplenial cortex, tegmental ventral area and supramammillary nucleus, but the neuronal activity increased in the motor cortex in exercised group. These results suggest that our exercise program produces a more accurate performance and it increased efficiency, because the exercise group had lower neuronal metabolic needs in the regions implicated in the spatial memory process. Also, the reduction of COx activity in brain regions traditionally related to stress and some behavioral parameters, such as the lower velocity or more time spent in the center of the maze, may indicate a possible reduction of anxiety in the exercise group during the spatial task.

Working memory impairment and reduced hippocampal and prefrontal cortex c-Fos expression in a rat model of cirrhosis

Hepatic encephalopathy (HE) is a frequent neurological complication observed in patients with liver malfunction. Previous studies have shown memory impairment in these patients. In order to investigate brain substrates of spatial working memory impairment in chronic HE, neuronal expression of c-Fos protein was studied in an experimental model of cirrhosis. Control and cirrhotic rats were trained on a spatial working memory task in the Morris water maze (MWM). Differences between groups were found in the working memory task. Cirrhotic rats were unable to locate the platform in the retention trial. Neuronal activation, measured by c-Fos protein, was compared between groups. No differences were found in c-Fos expression of control and cirrhotic rats that were not tested in the MWM. Working memory task produced increase in c-Fos positive cells in dorsal hippocampus, CA1 and CA3, and prefrontal cortex in control group compared to thioacetamide group or naïve, which only swam in the maze during a similar time. These findings suggest that cirrhotic rats show spatial working memory impairment that could be linked to dysfunction in neuronal activity in prefrontal cortex and hippocampus.

Unilateral hippocampal blockade reveals that one hippocampus is sufficient for learning a passive avoidance task

Understanding hippocampal participation in memory processes is one of the goals in neuroscience research. By blocking the hippocampus unilaterally in Wistar rats, we assessed the contribution of this brain structure to memory in a passive avoidance task. Subjects were distributed into four groups. Group 1 received tetrodotoxin (TTX) in the right hippocampus during acquisition and retrieval phases. Group 2 had the same procedure as group 1, except that the contralateral hippocampus was blocked during retrieval. Subjects from group 3 acquired the task with saline (both hippocampi intact) and retrieved with the right hippocampus inactivated. Finally, group 4 received TTX unilaterally 2 min after acquisition to determine the hippocampal role in consolidation. Results showed that group 2 was impaired, compared with the other groups, during retrieval. These findings reveal that the hippocampal contribution to this task differs from that in other tasks considered to be hippocampus dependent.

Similarities and differences between the brain networks underlying allocentric and egocentric spatial learning in rat revealed by cytochrome oxidase histochemistry

The involvement of different brain regions in place- and response-learning was examined using a water cross-maze. Rats were trained to find the goal from the initial arm by turning left at the choice point (egocentric strategy) or by using environmental cues (allocentric strategy). Although different strategies were required, the same maze and learning conditions were used. Using cytochrome oxidase histochemistry as a marker of cellular activity, the function of the 13 diverse cortical and subcortical regions was assessed in rats performing these two tasks. Our results show that allocentric learning depends on the recruitment of a large functional network, which includes the hippocampal CA3, dentate gyrus, medial mammillary nucleus and supramammillary nucleus. Along with the striatum, these last three structures are also related to egocentric spatial learning. The present study provides evidence for the contribution of these regions to spatial navigation and supports a possible functional interaction between the two memory systems, as their structural convergence may facilitate functional cooperation in the behaviours guided by more than one strategy. In summary, it can be argued that spatial learning is based on dynamic functional systems in which the interaction of brain regions is modulated by task requirements.