Evidence that mice engineered to overexpress the 5-HT transporter have a low-anxiety phenotype

Katie Jennings, John Sheward1, Tony Harmar1, Robert Deacon2 & Trevor Sharp. University Department of Pharmacology, Oxford OX1 3QT, University Department of Neuroscience, Edinburgh EH8 9JZ1, University Department of Experimental Psychology, Oxford OX1 3UD2.

Polymorphic variants of the human 5-HT transporter (5-HTT) gene which influence 5-HTT expression, have been linked to altered anxiety levels (Lesch et al. 1996; Greenberg et al. 2000; Heinz et al. 2000) . Specifically, groups of individuals with a 5-HTT genotype predicted to cause increased 5-HTT expression, exhibit reduced responses to fearful stimuli compared to other groups (Hariri et al. 2002) . However, it is not known whether this association between 5-HTT genotype and anxiety relates to variation in 5-HTT expression. Recently we characterised a novel mouse genetically engineered to overexpress the human 5-HTT (Jennings et al 2004). Here we examine the behaviour of this transgenic mouse in models of anxiety, specifically the hyponeophagia and elevated plus maze tests.

For the hyponeophagia test, food restricted mice (male, wildtype and transgenic, CBAxC57BL6J, 3-9 months, 10/group), were placed in a clear Perspex box with the floor covered with food pellets, and latency to begin eating was recorded. For the plus maze mice (8/group), either drug naïve or pre-treated with paroxetine (10 mg/kg i.p.) or saline 30 min prior to testing, were placed on the closed arm. Time spent on, and latency to emerge onto, the open arms as well the number of arm entries were recorded over 5 min. Data are expressed as mean +s.e.m. values and were analysed statistically using a Student’s t-test or ANOVA with a Bonferroni post hoc test.

In the hyponeophagia test, transgenic mice had a lower latency to eat than wildtype mice (48 +7 versus 74 +12 sec; P<0.05). In the plus maze, drug naïve transgenic mice spent more time on the open arms compared to drug naïve wildtypes (112 +15 versus 55 +18 sec; P<0.05). Likewise, saline-treated transgenic mice spent more time on the open arm compared to saline-treated wildtypes (63 +16 versus 1 +1 sec, P<0.001). The latency to emerge onto the open arm was also significantly less in the transgenic compared to wildtype mice. In comparison, both latency to emerge and time spent on the open arm, did not differ between paroxetine-treated transgenic and wildtype mice. Total arm entries were not different between genotypes.

In summary, the plus maze and hyponeophagia data show that 5-HTT overexpressing mice exhibit a low anxiety phenotype compared to wildtype mice. Furthermore, in the plus maze the low anxiety phenotype of the transgenics was reversed by a 5-HTT inhibitor, suggesting a direct link between behaviour and 5-HTT overexpression. These findings support the hypothesis that variation in 5-HTT expression leads to altered anxiety levels.

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Heinz A, et al (2000). Biol Psychiatry 47(7): 643-9.
Lesch KP, et al (1996). Science 274(5292): 1527-31.
Jennings KA et al (2003) Proc. 10 thInt. Conference in in vivo methods, p. 389-391.

Funded by a MRC studentship (KAJ) and a FP6 Integrated network ( LMSH-CT-2004-503474).