Diurnal variation in the effect of chronic mild stress on sucrose intake and preference.

Diurnal variation (depression worse in the morning) is one of the diagnostic criteria for the melancholic subtype of major depression. This study examined diurnal variation in the effects of chronic mild stress (CMS), an animal model of depression, by testing Wistar rats at different phases of the light-dark cycle. CMS decreased sucrose intake and sucrose preference in animals tested at the start of the dark phase (the most active period in this nocturnal species), but not in animals tested during the light phase. CMS also decreased body weight in both groups; however, the effects of CMS on sucrose intake in the dark phase were not secondary to body weight changes. On the contrary, loss of body weight led to underestimates in the magnitude of the effects of CMS on sucrose intake. The results support the validity of the CMS procedure as a model of melancholia. The discussion addresses criticisms of this position that have been raised in two recent publications.