Most trace amine-associated receptors (TAARs), including TAAR2 and TAAR5, are olfactory receptors found in vertebrates, which carry a chemosensory function distinct from odorant receptors and are thought to participate in recognition of socially relevant cues [1]. Several components of mouse urine, an important source of social cues in rodents, have been found to act as ligands to some TAARs, including TAAR5 [2]. TAAR5 ligand trimethylamine is thought to play a role in sexual recognition since it acts as a species-specific olfactory attractant, while its content in mouse urine is sex-dependent [3]. In contrast to TAAR5, other olfactory TAARs, including TAAR2, are poorly studied. To evaluate the possible roles of TAAR2 and TAAR5 in sexual recognition, we used sexual incentive motivation test [4] to assess sexual behavior of TAAR2-knockout (TAAR2-KO) and TAAR5-KO mice.

Methods: Experiments were conducted on four groups of 3-6 months old sexually experienced male mice: 8 TAAR5-KO, 8 TAAR5-WT; 8 TAAR2-KO, 7 TAAR2-WT. Sexual incentive motivation test was used to evaluate sexual behavior. A female mouse in estrous was used as sexual incentive, while a male mouse was used as social (non sexual) incentive. To induce estrous, adult female mice were given 10 mkg estrogen benzoate and 500 mkg progesterone intraperitoneally 48 and 2 hours before the experiment, respectively. The setup consisted of 4 experimental chambers (15x30 cm), each accompanied by two adjacent incentive cages separated by permeable walls. 20 min before the experiment, incentives were placed in cages. A male mouse was then placed in the experimental chamber for 20 min, while behavior was recorded and then processed using Noldus EthoVision XT. To assess sexual recognition, “female” and “male” zones were differentiated for the analysis, each adjacent to the wall separating an incentive cage. Total time was divided into 5 min increments, then percentages of time when both the nose-point and the center-point of the animal were in the “female” and “male” zones were calculated for all increments. Two-way ANOVA was used to compare all data.