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The serotonin 1A (5-HT1A) -receptor is involved in a wide range of physiological functions such as in the regulation of body temperature. Studies have shown that in mice, the hypothermic response induced by the full agonist at the 5-HT1A-receptor 8-OH-DPAT is mediated by presynaptic 5-HT1A-receptors. In contrast, investigations in humans and rats detected an involvement of postsynaptic 5-HT1A-receptors. In our study we used a transgenic mouse line with a permanent overexpression of the 5-HT1A-receptor in the projection areas of serotonergic neurons. We investigated the basal body temperature and the hypothermic effect of different dosages of 8-OH-DPAT (0.1mg/kg – 4 mg/kg ip.) in male transgenic mice in comparison to NMRI wild-type males using radio telemetry, a method that allows non-invasive recordings of body temperature. The basal body temperature of transgenic mice was lower than in NMRI wild-type mice (transgenic mice: 36.0 °C; NMRI wild-type mice: 37.4 °C). In both genotypes, hypothermia by systemic administration of 8-OH-DPAT was induced in a dose dependent manner. However, the temperature decrease was more pronounced in transgenic mice with -2.8 °C compared to -1.5 °C in NMRI wild-types. Dose response curves of 8-OH-DPAT revealed an ED50 = 0.4 mg/kg in transgenic and ED50 = 0.57 mg/kg in NMRI wild-type mice. Our results suggest that the postsynaptic 5-HT1A-receptor is involved in the regulation of body temperature in mice.