The ghrelin-mimetic drug growth-hormone releasing peptide 6 (GHRP-6) has been shown to inhibit light-induced cFos expression in the SCN and attenuate a light induced phase shift (Yi et al., 2006; Yi et al., 2008), suggesting that ghrelin can act as a non-photic stimulus to alter the timing of light-signaled behaviour. Therefore, it is not surprising that the absence of ghrelin could alter the timing of activity, especially in LL, where photic Zeitgebers are also absent. In this situation, the absence of ghrelin activity at the GHRS receptor did not have a significant effect on comsummatory behaviour, as the two groups ate the same amount of food and there were no differences

in body weight. One question that must be addressed is the surprising lack of food anticipatory activity in WT mice housed in LL. Indeed, food anticipatory activity has been previously demonstrated in rats housed Bleomycin in LL (Bolles & Stokes, 1965; Edmonds & Adler, 1977a,b; Lamont et al., 2005). In Lamont et al. (2005), no attempt

was made to quantify the amount of anticipatory activity, but certainly overall activity levels were very low after an extended period in LL, as can be seen in the actograms presented in that article. Species differences may p38 MAPK inhibitor account for the lack of food anticipatory activity observed in the present study in WT mice. In one study using spiny mice, Acomys cahirinus, wheel-running activity was reduced dramatically in LL compared to LD and only two of the 11 mice studied actually showed entrainment to a restricted feeding schedule under LL, although all 11 had shown significant food anticipatory activity on an LD schedule prior to exposure to

LL (Chabot et al., 2012). In the current experiment, pentoxifylline 30 days in LL reduced daily activity levels in WT mice to fewer than 200 wheel revolutions per day, as compared to 600 in KO mice. With such a low level of activity in WT mice, it may simply be difficult to detect food anticipatory activity in these animals. Sampling of brain and peripheral tissues for clock gene protein and RNA at different time points during the temporal feeding period would have demonstrated whether central and peripheral circadian oscillators were entrained to the time of food availability, although the large number of animals required for this type of study was prohibitive. Alternately, a circadian-controlled measurement that is suppressed by light to a lesser degree, such as body temperature, may have been useful in detecting food anticipation in these mice. Regrettably, these data were not collected. Together, these data provide further support for the hypothesis that ghrelin plays a role in the food-entrainable clock, but also suggest that there may be an interaction between the effect of light and ghrelin that extends beyond a simple deficit in the ability of GHSR-KO animals to entrain to scheduled feeding.