FOOD EFFICIENCY IN RATS FOLLOWING BRAIN-LESIONS WHICH AFFECT TARGET BODY-SIZE - IMPLICATIONS ON THE SET-POINT FOR TARGET SIZE

Target size, i.e. body size appropriate for age, may be reset by bilat eral lesions of several brain areas. The mechanism for control of targ et body size is unknown, but some of the loci have marked effects on g ustatory behavior and/or energy metabolism. We have tested the possibi lity that a disturbance in energy metabolism may be a common factor in resetting target size. Food efficiency for body weight gain and for m etabolic size (the 0.75 power of body weight) was determined in rats t hat were experimentally stunted by neonatal head-irradiation or by bil ateral electrolytic lesions produced soon after weaning in the dorsome dial hypothalamic nuclei (DMH) or the substantia nigra (SN). The irrad iations were carried out in males and females; the surgical lesions we re produced only in males. Observations were carried out from weaning through early adulthood. Subgroups of irradiated rats and controls wer e fasted for 48 hours at 40 days of age. Irradiated rats had reduced f ood efficiency for weight gain and for metabolic size, more marked in males than in females. DMH or SN lesions did not change food efficienc y for weight gain. Food efficiency for metabolic size increased after DMH lesions and declined after SN lesions. During refeeding after a fa st, irradiated rats showed a normal transient increase in food efficie ncy for weight gain, but not for-metabolic size. The differences in fo od efficiency following different lesions tend to exclude altered ener gy metabolism as a common factor in the reset of target body size. Alt hough, the mechanism for resetting target size in these models remains unknown. the fact that lesions of several brain areas produce a reset strongly suggests that target size depends on a neural network.