Circadian rhythm of feeding induced changes in hypothalamic Met-enkephalin concentrations

Abstract

Accumulating evidence suggests that opioid peptides play an important role in the hunger component of the control of food intake. The enkephalins, one of the opioid peptide families, stimulate feeding when injected into specific hypothalamic areas and endogenous concentrations change with the fed/fasted condition of rats and sheep and with phase of circadian cycle. To demonstrate a possible circadian rhythm in feeding-induced changes in Met-enkephalin (MEK), 54 male rats initially weighing 255±3 g were adapted to a 12-hr fast during the light (light-fasted) or dark (dark-fasted) phase of the circadian cycle, then sacrificed before (non-fed) or after (fed) being allowed to eat a meal. In non-fed compared with fed rats, MEK concentrations were higher in the paraventricular nucleus (PVN, 170 vs. 109 pg/mg tissue, p<0.05) and ventromedial hypothalamus (VMH, 209 vs. 161 pg/mg tissue, p<0.05) in the dark (light-fasted) but not light (dark-fasted), even though rats ate a larger meal in the light (8.6 vs. 5.0 g, p<0.01). In rats fed the same amount of food in the light (dark-fasted) as ad lib fed rats in the dark (light-fasted), MEK concentrations did not differ in the PVN or VMH, suggesting that circadian rhythm is more important than meal size. Rats gavaged with an amount of milk equal in calories to dark ad lib-fed rats (light-fasted) had MEK concentrations not different from light-fasted non-fed rats (216 vs. 209 pg/mg tissue, NS) suggesting that feeding behavior, pregastric stimuli and/or form of diet is important for influencing MEK concentrations. These results demonstrate that feeding-induced MEK concentration changes in areas previously shown to be important in the control of food intake are circadian in nature and are larger in the dark when most feeding occurs in rats.