Comparison of early feeding pattern dynamics in female and male rats after reversible ventromedial nucleus of hypothalamus block

Background: In the hypothalamus, a number of interconnected foci, including the ventromedial nucleus of hypothalamus (VMN), interact to control food intake (FI). FI is a function of meal number (MN) and meal size (MZ). Because gender differences exist in feeding patterns, we aimed at investigating the role of the VMN in determining the relationship of MZ and MN in female and male rats. Methods: FI and feeding patterns of 10 female and 12 male Fischer-344 rats were studied after VMN block, achieved via stereotaxically located intra-VMN microinjection of the neuronal blocker, colchicine (0.32 μg dissolved in 50 nL of isotonic injectate and instilled on each side into the VMN). Results: After colchicine injection in normal female rats, an immediate and significant increase in FI occurs as a result of the following: 1) increased MZ in dark and light phases and 2) increased light phase MN with consequent loss of the normal diurnal cycle in FI. Recovery of feeding cycle and normal vaginal smear pattern occurred by study's end. In normal male rats, VMN block resulted in the following: 1) an increase in FI resulting from increased MN occurring predominantly during the light phase, thereby 2) disrupting the usual light/dark feeding cycle. Conclusions: Sexual differences in regulation of FI occur after temporary reversible VMN block: in female rats, MZ is more sensitive to experimental modulation, whereas in male rats it is MN.