Food Anticipatory Activity on Circadian Time Scales Is Not Dependent on Central Serotonin: Evidence From Tryptophan Hydroxylase-2 and Serotonin Transporter Knockout Mice

Abstract

A number of studies implicate biogenic amines in regulating circadian rhythms. In particular, dopamine and serotonin influence the entrainment of circadian rhythms to daily food availability. To study circadian entrainment to feeding, food availability is typically restricted to a short period within the light cycle daily. This results in a notable increase in pre-meal activity, termed "food anticipatory activity" (FAA), which typically develops within about 1 week of scheduled feeding. Several studies have implicated serotonin as a negative regulator of FAA: (1) aged rats treated with serotonin 5-HT2 and 3 receptor antagonists showed enhanced FAA, (2) mice lacking for the 2C serotonin receptor demonstrate enhanced FAA, and (3) pharmacologically increased serotonin levels suppressed FAA while decreased serotonin levels enhanced FAA in mice. We sought to confirm and extend these findings using genetic models with impairments in central serotonin production or re-uptake, but were surprised to find that both serotonin transporter (Slc6a4) and tryptophan hydroxylase-2 knockout mice demonstrated a normal behavioral response to timed, calorie restricted feeding. Our data suggest that FAA is largely independent of central serotonin and/or serotonin reuptake and that serotonin may not be a robust negative regulator of FAA.

Keywords: Slc6a4; circadian rhythm; food anticipatory activity; food entrainment; tryptophan hydroxylase-2.

FIGURE 1

Food anticipatory behavior in serotonin transporter knockout mice. (A) Mean ± SEM seconds of high activity behaviors (hanging, jumping, rearing, and walking) on the first day (“day 0”) of scheduled CR feeding. The yellow area indicated lights on and the gray area indicates lights off. (A’) Normalized mean ± SEM high activity of data shown in (A). (B) Mean ± SEM seconds of high activity behaviors after 7 days of timed CR feeding. (B’) Normalized mean ± SEM high activity of data shown in (B). (C) Mean ± SEM seconds of high activity behaviors after 14 days of timed CR feeding. (C’) Normalized mean ± SEM high activity of data shown in (C). (D) Mean ± SEM seconds of high activity behaviors after 21 days of timed CR feeding. (D’) Normalized mean ± SEM high activity of data shown in (D). (E) Mean ± SEM seconds of high activity data in the 3 h preceding scheduled feeding across the experiment (sum of ZT 5, 6, and 7). (E’) Mean ± SEM normalized high activity data shown in (E) in the 3 h preceding scheduled feeding across the experiment. n = 5–9 controls and n = 6–9 KO per time point.

FIGURE 2

Food anticipatory behavior in Tryptophan hydroxylase 2 knockout mice. (A) Mean ± SEM seconds of high activity behaviors (hanging, jumping, rearing, and walking) on the first day (“day 0”) of scheduled CR feeding. (A’) Normalized mean ± SEM high activity of data shown in (A). (B) Mean ± SEM seconds of high activity behaviors after 7 days of timed CR feeding. (B’) Normalized mean ± SEM high activity of data shown in (B). (C) Mean ± SEM seconds of high activity behaviors after 14 days of timed CR feeding. (C’) Normalized mean ± SEM high activity of data shown in (C). (D) Mean ± SEM seconds of high activity behaviors after 21 days of timed CR feeding. (D’) Normalized mean ± SEM high activity of data shown in (D). (E) Mean ± SEM seconds of high activity data in the 3 h preceding scheduled feeding across the experiment. (E’) Mean ± SEM normalized high activity data in the 3 h preceding scheduled feeding across the experiment. n = 12–15 WT and n = 10–16 KO per time point.