EFFECTS OF LONG-TERM RESTRICTED FEEDING ON MOTOR-ACTIVITY RHYTHM IN THE RAT

The motor activity circadian rhythm of the rat is driven by at least t wo different pacemakers, one entrained by light-dark cycles and the ot her by periodic food access. The interaction between these two pacemak ers generates the overt rhythm in many variables in the rat. In this p aper we study the effects of restricted feeding (RF) for >300 days on the motor activity of two groups of eight rats each (4 males and 4 fem ales) that were kept under different light patterns. The first group w as kept under dim red light, and the second group was kept initially f or 70 days under bright light and afterward under dim red light. Rats had access to food for only 2 h/day. Results show that strong alterati ons in the motor activity pattern, such as relative coordination, boun cing, and several cases of entrainment, are shown in the first group. When these animals return to ad libitum feeding conditions the presenc e of the food component is clear, in some cases for >20 days. In contr ast, the rats that were previously subjected to bright light did not e ntrain to RF. In this group, there was a masking effect when lights we re on, but a free-running component appeared when lights were off. The endogenous period (tau) of this component was not modified during the course of the experiment. After RF these rats showed a longer tau tha n the former group. In all the rats, the daily motor activity decrease d with time, whereas the activity corresponding to 2 h before food acc ess did not change with respect to the time RF was applied, when expre ssed as a percentage of the daily motor activity. We conclude that lon g-term RF modifies the tau of the light-entrainable pacemaker, and onl y after tau is shortened can the motor activity in the rat be entraine d.