Time dependent effects of restraint stress on food intake in rats

We investigated whether 3 hours restraint stress (RS) applied at different times of the light cycle produced differential effects on food intake and body weight. Male Sprague Dawley rats were divided into 4 weight matched groups: Exp. 1-control and RS with stress from 4:00 to 7:00 pm and Exp. 2-control and RS with stress from 8:00 to 11:00 am (lights off 7:00 pm to 7:00 am). Food and water intake, and body weight were measured during three periods: 7 days baseline, day of RS, and 11 days of recovery. On the day of stress food and water intake were measured at intervals. RS significantly inhibited 24 hour food, but not water, intake. In Exp. 1 food intake was suppressed during the first 2 hours of the dark cycle. In Exp. 2 intake was suppressed for 4 hours immediately after RS and during the first 2 hours of the dark cycle. There was no effect of RS on intake during recovery. There was no significant effect of RS on body weight in Exp. 1. In Exp.2 RS rats weighed less than controls throughout recovery. After recovery treatment groups were switched (i.e. control were exposed to RS). Measurements made within the first hour of the dark cycle revealed no significant differences in hypothalamic NPY mRNA expression. NPY protein in the paraventricular nucleus of hypothalamus was significantly higher in RS rats from Exp. 2, compared with their controls or RS rats from Exp. 1. There were no obvious Rs-induced changes in hypothalamic monoamine concentration. RS has greater effect on metabolism and energy balance when it is applied in the morning.