Effect of estradiol and progesterone on daily rhythm in food intake and feeding patterns in Fischer rats

The product of meal number x meat size, over time, is food intake. Because estrogens modulate feeding activity via their action on the hypothalamus, a nd because there is a diurnal rhythm in the expression of cytoplasmic estro gen receptors and in estrogen binding activity, the present study examined the effects of ovariectomy and later hormone therapy on acute changes in bo dy weight, and on the meal number-to-meal size relationship as reflected by food intake in the dark/light feeding patterns, in adult female rats in th e intact state and after ovariectomy. Twelve female Fischer rats were rando mized into ovariectomy and sham operation groups. A rat eater meter measure d the feeding indexes for 15 days before and 25 days after ovariectomy, and later for 35 days with hormone therapy. We report: (a) mean body weight ga in was linear before and up to ovariectomy, while exponential after ovariec tomy; (b) increase in daily food consumption is mainly via an increase in f ood intake during the light phase; (c) light phase meal number remains unch anged, meal size significantly increases, with the resultant increase in ov erall food intake; (d) during the dark phase, meal size also significantly increases, but is accompanied by a proportional decrease in meal number, re sulting in unchanged dark-phase food intake; and (e) estrogen restoration w ith either estradiol valerate or estradiol-progesterone combination, revers ed the above changes. Data show that in the female Fischer 344 rat: (a) cha nges in daily rhythm in food intake are brought about by differential effec ts of the hormones on both meal size and meal number in both the total dail y levels as well as in the dark-to-light distribution; (b) estadiol appears to have a tonic inhibitory effect on the light-phase meal size and a phasi c effect on the dark phase meal size and number, but no significant effect on the light-phase meal number; and (c) in the Fischer rats, progesterone a ugments estradiol's effect on these indicies. (C) 1999 Elsevier Science Inc . All rights reserved.