Hypothesis: The brain serotonin (5-HT) system is known to play a key role in regulating critical processes such as circadian clock timing, motivation, reward response and sleep. 5-HT knockout (SKO) mouse models have previously been used to understand 5-HT actions, but such global 5-HT depletion has profound non-targeted neurophysiological and developmental consequences. Therefore, conditional SKOs are needed. Here, we performed preliminary behavioral phenotyping in a conditional 5-HT knock down model where 5-HT synthesis blockade was targeted specifically to the midbrain raphe nuclear complex (site of hypothalamic serotonergic innervation) in adult mice.

Methods: Male Tph2-floxed mice were injected with AAV-Cre (experimental/ Tph2-/-) or blank AAV (control/ Tph2+/+) into the dorsal and median raphe nucleus (DRN and MRN). Animals were given 1wk to recover prior to experimentation. Behavior was monitored for a 2 wk post-injection period, during which circadian parameters (period [tau] nocturnal activity length [alpha] and daily activity bouts) were measured using overhead infrared motion detectors interfaced with a computerized data acquisition system.

Results and Conclusions: There was no difference in tau between Tph2-/- and Tph2+/+ (p < 0.05). Notably, there was a large difference in alpha, with Tph2-/- mice having longer alpha (p).

Keywords: serotonin, thp2, 5-ht, circadian, biological rhythms, raphe nucleus, dorsal raphe nucleus, median raphe nucleus, AAV-Cre, knockout mouse