5-HT2C KO Mouse Strain

Overview

5-HT_2C KO mice are used to study the in-vivo behaviors and cognitive functions associated with the serotonin 5-HT_2C subtype receptor. Since serotonin is a key neuromodulatory transmitter implicated in many behaviors and complex cognitive functions, studying the specific receptors that it interacts with is a major interest for neuroscientists.

History

To study the function of serotonin (5-HT) receptor subtypes, researchers typically relied on administering antagonists to lab animals. To study the 5-HT_2C receptor, researchers relied on the antagonists SB242084 and SDZ SER 082.

With time, N=numerous pharmacological studies have concluded that the 5-HT_2C receptor is involved in major cognitive functions and behaviors like anxiety, fear memory, and locomotion.^[1] This led to the increased demand and interest in creating 5-HT_2C KO mice, in order to be able to study the specific receptor in-vivo.

5-HT_2C knockout (KO) mice are genetically mutant mice that lack the 5-HT_2C subtype receptor which is a G-protein-coupled receptor located throughout the central nervous system.^[2]

Engineering 5-HT_2C KO Mice

This mouse strain can be acquired by ordering breeding pairs, enabling labs to have their own in-house colony of 5-HT_2C KO mice for prolonged experimentation. Through breeding at least 5 generations,  5-HT_2C KO can be developed through targeted disruption of the X-chromosome linked 5-HT_2C gene from a 129 embryonic stem (ES) cell line.^[3]

Since the 5-HT_2C gene is X-linked, breeding ultimately leads to litters where 50% of males are hemizygous and the other 50% are wild-type. This permits researchers to use littermates that are genetically and environmentally similar, except for the gene in question. This is convenient as it does not require parallel maintenance of multiple litters, ultimately reducing experimental variability.^[3]

Physical Characteristics

5-HT_2C KO mice are overweight, however, they generally appear healthy and normal.

They can breed normally and do not exhibit any visible abnormalities.^[3]

Behavioral Characteristics & Handling

The following behavioral characteristics characterize 5-HT_2C KO mice:

• Increased feeding behaviors: 5-HT_2C KO mice consume more food than wild-type littermates, indicating that they have increased appetite levels. Increased feeding is likely to contribute to their overweight status.
• Increased chewing: This strain shows higher levels of chewing non-nutritious material, such as clay and plastic screens than normal wild-type mice.
• Anxiolytic phenotype: 5-HT_2C KO mice spend large amounts of time in the open-arms of the Zero Maze, suggesting an anxiolytic phenotype to some extent. They also spend more time investigating a novel object than wild-type mice, showing that their anxiety levels are lower.
• Hesitant: In the Elevated Plus Maze, this strain shows hesitating behavior by staying in the maze’s closed-off central area. Since 5-HT_2C KO mice have also been deemed anxiolytic in certain circumstances, their hesitant behaviors demonstrate the behavioral complexity that lies beneath the modulatory function of 5-HT_2C
• Slow habituation: This mouse strain is also slow to habituate to a novel environment. As mice habituate, their locomotor activity decreases. However, this mouse strain takes longer to habituate, as indicated by sustained locomotor activity levels, when compared with wild-type mice.^[3]

Handling

Before experimentation begins, it is recommended to acclimate mice to where they will be housed. Just as with any mouse strain, it is important to have in place and follow a protocol for the light-dark cycle. Lights on usually occur at either 7 a.m. or 8 a.m.

These mice can be group-housed, typically 3-5 mice per cage. Researchers also provide ad libitum food and water access.

Health Characteristics

5-HT_2C KO mice have many abnormal health characteristics and are prone to many health issues, such as seizures, being overweight, and spontaneous death.

Given 5-HT_2C KO mice’s high instance of spontaneous death attributed to severe seizures, researchers have concluded that the 5-HT_2C receptor may be implicated in the neuronal excitability.^[2]

Despite being mutants, 5-HT_2C KO mice do not have any abnormalities of their central nervous system and their expression of other subtypes of serotonin receptors is normal.^[3][4]

Major Experimental Uses

Serotonin, as a monoaminergic neurotransmitter, is a key modulator of motor, sensory, and behavioral processes. Using 5-HT_2C KO mice is a way to establish the role that a specific serotonin receptor subtype can have on behavior.

By using mice that lack this receptor, it helps researchers unmask whether drug of interest interacts with other serotonin receptors as well.^[5]

The following areas of research use 5-HT_2C KO mice experimentally:

• Stereotypy: This strain is commonly used in behavioral research focusing on stereotypy and impulsive behaviors
• Compulsive behavior: In addition to stereotypy, 5-HT_2C KO mice also demonstrate compulsive and impulsive behaviors, due to their inappropriate levels of chewing non-nutritious items. Ultimately, this makes this strain a plausible choice amongst researchers using animal models to study obsessive-compulsive disorder (OCD).^[6]
• Anxiety: 5-HT_2C KO mice have been used to establish that 5-HT_2C receptors are implicated in anxiety regulation. 5-HT_2C KO mice, as mentioned previously, show a reduction in anxiety-like responses in the Zero Maze. This trend is also observed in their performance in the Open Field Test, Novel Object Recognition, and the Mirrored Chamber.
• Neuroplasticity: Experiments focusing on neuroplasticity have shown that hippocampal dentate gyrus plasticity is modulated by intact 5-HT_2C This has significant implications for memory processes linked to the hippocampus.^[3]
• Spatial learning and memory: 5-HT_2C receptors are crucial for spatial learning and memory as indicated by 5-HT_2C KO mice’s poor performance in the Morris Water Maze.
• Fear memory: Currently, there is a lot of variability in the literature regarding 5-HT_2C KO mice’s fear memory. Thus, this strain is being used across various protocols in order to come to a conclusion regarding fear memory.^[1]
• Feeding behaviors: Due to their abnormal feeding behaviors, 5-HT_2C KO mice are being explored for their use in experiments focusing on feeding behaviors in disease models such as obesity.^[6]

Conclusion

5-HT_2C KO mice are mutant mice engineered through generational breeding which also provides wild-type controls.

Researchers rely on this mutant strain in order to explore and identify the roles that the 5-HT_2C receptor has on behavior and cognition.

Since serotonin is a neuromodulatory transmitter affecting many aspects of behavior and cognition, the 5-HT_2C KO strain has many experimental uses, including pharmaceutical testing.

References

1. Nebuka, M., Ohmura, Y., Izawa, S., Bouchekioua, Y., Nishitani, N., Yoshida, T., & Yoshioka, M. (2020). Behavioral characteristics of 5-HT2C receptor knockout mice: Locomotor activity, anxiety-, and fear memory-related behaviors. Behavioural Brain Research, 379, 112394.
2. Tecott, L. H., Sun, L. M., Akana, S. F., Strack, A. M., Lowenstein, D. H., Dallman, M. F., & Julius, D. (1995). Eating disorder and epilepsy in mice lacking 5-HT2c serotonin receptors. Nature, 374(6522), 542-546.
3. Tecott, L. H., Logue, S. F., Wehner, J. M., & Kauer, J. A. (1998). Perturbed dentate gyrus function in serotonin 5-HT2C receptor mutant mice. Proceedings of the National Academy of Sciences, 95(25), 15026-15031.
4. López‐Giménez, J. F., Tecott, L. H., Palacios, J. M., Mengod, G., & Vilaró, M. T. (2002). Serotonin 2C receptor knockout mice: Autoradiographic analysis of multiple serotonin receptors. Journal of neuroscience research, 67(1), 69-85.
5. Fletcher, P. J., Tampakeras, M., Sinyard, J., Slassi, A., Isaac, M., & Higgins, G. A. (2009). Characterizing the effects of 5-HT2C receptor ligands on motor activity and feeding behaviour in 5-HT2C receptor knockout mice. Neuropharmacology, 57(3), 259-267.
6. Chou-Green, J. M., Holscher, T. D., Dallman, M. F., & Akana, S. F. (2003). Compulsive behavior in the 5-HT2C receptor knockout mouse. Physiology & behavior, 78(4-5), 641-649.