<i>Tph1</i> (−/−) mice have a normal response to social novelty.

<p><i>Tph1</i> (−/−) mice demonstrate similar responses to a stranger mouse (A) and to a novel mouse (B). The percentage of time that the test mouse spent exploring stranger 1 was comparable to age-matched WT animals (A) and the time that the test mouse spent exploring stranger 2 was comparable to age-matched WT mice (B).</p

<i>Tph1</i> (−/−) mice demonstrate alterations in gait dynamics.

<p>A stride is comprised of a swing duration (limb in air) and a stance duration (paw in contact with the treadmill belt). The stance is comprised of a brake duration and a propulsion duration. The paw angle is the outward angle that the paw makes relative to the long axis of the mouse during walking. Gait metrics were described previously <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0059032#pone.0059032-Kale1" target="_blank">[21]</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0059032#pone.0059032-Hampton1" target="_blank">[45]</a>.</p

Lack of peripheral serotonin does not result in attention impairment.

<p>Ability to learn the association between the stimulus light and reward in WT and Tph1 (−/−) mice showed a strong trend indicating that aged Tph1 (−/−) mice may acquire this association more slowly, however, overtime they demonstrated successful learning across daily sessions (A). Overall, aged mice of both genotypes showed a significant deficit in performance during trials of 1 (**P<0.01) and 2 seconds (*P<0.05) (B).</p