Mice underexpressing 5 HT2C receptors in brain are hyperphagic, hyperactive and healthy

Atsuko Kimura1, Gavin MacColl1, Karen French1, David Melton2, Karen Chapman1 and Megan Holmes1. 1Endocrinology Unit, University of Edinburgh, EH16 4TJ. 2Molecular Medicine Centre, University of Edinburgh, EH4 2XU.

5-HT2C receptors (2CR) have been implicated in serotonergic control of appetite and mood. Antagonists of 2CR antagonists may be useful for the treatment of eating disorders such as anorexia as well as depression. Supporting this hypothesis, 2CR-null mice are hyperphagic, susceptible to late-onset obesity and hyperactive, but a concern emerged that they are also prone to premature sudden death from seizures (Tecott et al., 1995; Nonogaki et al., 2003), with only 50% of null mice surviving to 6 months of age. We have now produced a gene targeted mouse with tetracycline regulatable expression of the endogenous 2CR gene. Here we report our initial characterisation of mice with reduced, or “knockdown” 2CR expression. All studies were carried out on male mice, carrying the targeted allele of the 2CR gene, which is on the X chromosome. The level of 2CR mRNA expression was studied by in situ hybridisation. The distribution of 2CR mRNA was similar in 2CR knockdown (targeted) and littermate control mice. However, the level of expression was greatly reduced (>90%) in targeted mice, but was still detectable above background. Despite severely reduced 2CR levels, the targeted animals were viable and healthy; expected numbers (according to Mendelian segregation) of targeted animals were born from normal sized litters. Over at least 6 months, only 7% unexpected deaths were observed in the targeted mice, similar to the number observed in wildtype mice (7%). These results demonstrate that very low 2CR function is not detrimental to survival. Growth curves for targeted mice show they have significantly higher body weight compared with littermate controls (Two-way ANOVA, interaction of genotype and age, p<0.0001, n=13-20). Furthermore, targeted mice show 28% increased food intake per day (average of 4 days, Student’s t-test, p<0.02, n=5). Following overnight fast, a 2CR agonist, m-chlorophenylpiperazine (3mg.kg -1, i.p.) inhibited food consumption over 1hr refeeding by 56±11.9% (n=5) in targeted mice compared with 83±16.1% in control mice (n=5), demonstrating the low levels of 2CR present in these mice are functional. In order to determine the effects of 2CR reduction upon physical activity, mice were given free access to running wheels. Mice with reduced 2CR expression were hyperactive; wheel revolutions during the dark phase for targeted mice were significantly higher compared with controls (for targeted 30,340±3,643 revolutions, control 15,572±3,673 revolutions, Student’s t-test, p<0.009, n=6), indicating that targeted mice are hyperactive.

In conclusion, we have created mice underexpressing 5-HT2C receptors in brain. These mice show increased weight, elevated food intake and hyperactivity, but there is no adverse effect on their survival.

Nonogaki et al., 2003 Diabetes. 52, 315-320;
Tecott et al., 1995 Nature. 374, 542-546.

This work was supported by the Wellcome Trust (MCH, KEC).