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13BIOLOGICAL SCIENCES
THE INFLUENCE OF AN EXPERIMENTAL MODEL OF INFLAMMATION ON RATS COGNITIVE
FUNCTIONS AND BEHAVIOR
Tchang A.,
Student, Department of Biology, Samara National Research University, Samara, Russia
Belyakov V.,
Candidate of biological sciences, Associate professor of Department of human and animal physiology, Samara National Research University, Samara, Russia
Lemba Y. Student, Department of Biology, Samara National Research University, Samara, Russia
Abstract
The aim of the current study was to observe various behaviors of adult rats under experimental conditions of semi-chronic exposure to bacterial lipopolysaccharide. The rats' behaviors were studied using the "Open field" and "Elevated cruciform maze" setups. It was found that daily administration of Salmonella typhi lipopolysaccharide (50 ^g/kg; intraperitoneal) for 10 days led to a decrease of motor and exploratory activity with an increase of anxiety in adult rats. The results of the study suggest that bacterial lipopolysaccharide (LPS) provides the development of an inflammatory response with the corresponding production of pro-inflammatory cytokines.
It is possible that the effect of inflammation on the brain structures e. g. cerebral cortex, limbic system etc. causes an inhibition of the exploratory behavior of rats in tested experimental conditions.
Keywords: inflammation, lipopolysaccharide (LPS), cognitive functions, behavior, rats.
Introduction. Current research suggests that inflammation of the central nervous system structures is a significant trigger of the discoordination of the brain function and causes a number of neurodegenerative diseases e.g, multiple sclerosis, Parkinson's and Alzheimer's diseases) [1-4]. However, there is limited research available on the influence of inflammation on the various aspects of adaptive behavior, so this problem remains poorly understood. In preclinical studies, the various experimental approaches are used for modeling peripheral and central inflammation in laboratory animals. An introduction of pharmacological agents from the class of immunomodulators containing bacterial LPS is one of the commonly used and scientifically recognized approaches.
Purpose of the study. The aim of the current study was to investigate the influence of an experimental model of inflammation (the effect of LPS) on the behavioral responses of adult rats in various experimental conditions.
Materials and Method. The participants of the study were 16 male adult rats (235±27 g). The selection of rats occurred with compliance to all bio-ethical norms of using animals in research. The protocol of the study was approved by the ethical committee of the Samara State University. The animals were kept in a vivarium under semi-natural conditions where the free access to water and food was created for them. The rats were randomly divided into experimental (n=8) and control groups (n=8). The LPS solution, Salmonella typhi lipopolysaccharide (50 ^g/kg, i.p.; NRCEM named after N.F. Gamalei, Russia) was administered to the experimental group daily for 10 consecutive mornings. The rats of the control group were injected with
1ml of physiological saline solution. The behavior of rats in both groups was observed in "Open field" and "Elevated cruciform maze" setups (Openscience, Russia).
In the Open field setup, the following measures were coded and analyzed: horizontal locomotor activity (defined by the number of crossed sectors of the field), vertical locomotor activity (defined by the number of getting up on the hind legs), the level of anxiety (determined by the number of exits to the central sector of the field).
In the Elevated cruciform maze setup, the time (s) the rats spent in the open and clo sed sleeves of the maze was recorded, as well as the frequency of horizontal locomotor activity (determined by the number of sectors crossed by the rats in the open sleeves of the maze). The time the rats spent in the closed sleeves of the maze was coded as the high level of anxiety. The behaviour of the rats was video-recorded by a smartphone which was attached to a special tripod. The behavioral responses of the rats were assessed after one, three, five and ten days of LPS exposure.
Data were expressed as average ± SD. The collected data were statistically processed and analysed using the specialised statistical software SigmaStat 12.5. The data were considered statistically significant at p <0,05.
Results. The results of the study suggest that the effect of the bacterial toxin induces the formation of a specific pattern of behavior in rats. In the Open field under the influence of LPS, there was a decrease in the motor and exploratory activity in rats (table 1).
Table 1
Behavior of rats in the test Open field
Behavioral reactions Initial values Day 5 Day 10
Control group Experimental group Control group Experimental group Control group Experimental group
Physical activity 24,8±3,8 26,2±4,1 17,9±2,9 9,1±3,2 * 19,4±5,2 5,9±1,8 **
Outputs to the central sector 6,8±1,2 7,5±1,6 10,2±1,0 3,0±0,8 ** 11,1±2,4 2,2±0,4 **
Research activity 9,8±1,6 8,8±1,7 8,4±,1,2 5,1±0,9 * 7,8±0,9 4,7±0,3 *
Designations: * - p<0,05, ** - p<0,01 (differences bet On the 5th day of LPS administration - the number of crossed sectors decreased by 54,5% (p<0,01). Manifestations of anxious behavior also increased. The rats moved along the periphery of the test arena, the number of exits to the central sector decreased by 58 % (p<0,05). The greatest decrease in rats motor activity was observed on the 10th day. The movements through the sectors of the test arena decreased by 75% (p<0,01). The number of vertical motor reactions decreased by
Behavior of rats in the te
een groups)
14,5%. (p>0,05). LPS decreased the exploration by 50% (p<0,05).
The results obtained were confirmed in the Elevated cruciform maze test (table 2). On the 10 th day of the experiment, the time spent by rats from group experimental in the open sleeves decreased by 48,2% (p<0,05), the number of crossings through the sectors decreased by 34,7% (p<0,05).
Table 2
Elevated cruciform maze
Behavioral reactions Initial values Day 5 Day 10
Control group Experimental group Control group Experimental group Control group Experimental group
Time spent in open heats (s) 22,8±3,1 24,0±3,6 40,0±4,0 14,0±1,1 ** 37,0±4,9 13,1±0,9 **
Number of sectors crossed in open sleeves 8,7±1,9 10,2±1,5 16,7±3,6 12,9±4,2 18,4±5,2 11,6±2,3
Designations: table 1
Thus, the obtained results indicate that the administration of bacterial LPS leads to specific changes in rats behavior. Those changes are characterized by a deficit in motor and exploratory activity, a decrease in cognitive abilities associated with spatial memory and the ability to find a way out of a stressful situation quickly. Researchers refer to this behavior as «painful behavior» [5].
Discussion. The behavioral effects of bacterial LPS injections established in this study are possibly associated with the activation of immune processes and the triggering of the production of proinflammatory factors.
LPS administration is a way to model peripheral and central inflammation. Target cells for LPS are a variety of phagocytic cells expressing CD14 and Toll-4 receptors [6]. The LPS actions through Toll-4 receptors activating the production of pro-inflammatory cyto-kines (IL-ip, IL-6, TNF-a, etc.), chemokines (CCL2, CCL5, CXCL1), secondary messengers (NO and prostaglandins) and active forms of oxygen [7]. The action of proinflammatory cytokines on brain structures leads to neuroinflammation. The neuroinflammation activates microglia, which also produces proinflammatory cytokines. The influence of pro-inflammatory cyto-kines on the activity of neurons and the functioning of various neurotransmitter systems have been established. Peripheral administration of a high dose of LPS
leads to a long-term neuroinflammation and neurodegeneration of the dopaminergic et al. systems of the brain [8-11]. An increase in the metabolism of monoamine mediators: dopamine, norepinephrine and serotonin (5-HT) in various areas of the brain has been shown. This causes an imbalance between individual monoaminergic systems and dysregulation of cognitive processes, motor and autonomic functions [12].
Elevated levels of IL-ip and IL-6 in the brain are associated with depression, anxiety, insomnia and sleep disturbance. The level of proinflammatory cytokines appears to be elevated in neurodegenerative diseases [13].
Conclusion. Based on the results obtained in the study and analysis of information sources, the following conclusions were made:
1. The daily administration of bacterial LPS for 10 days causes a progressive decrease in various types of motor activity, and increased level of anxiety in the "Open Field" setting.
2. The exposure of LPS causes a reduction of time the rats spent in the open sleeves of the "Elevated Cruciform Maze", which also indicates the alarming effects of the use of an inflammation model.
3. It is hypothesized that the behavioral effects of LPS exposure are associated with the development of peripheral and central inflammation and the effect of proinflammatory cytokines on brain structures.
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