Small doses of the opiate antagonist naloxone selectively abolished overeating in genetically obese mice (ob/ob) and rats (fa/fa). Elevated concentrations of the naturally occurring opiate beta-endorphin were found in the pituitaries of both obese species and in the blood plasma of the obese rats. Brain levels of beta-endorphin and Leu-enkephalin were unchanged. These data suggest that excess pituitary beta-endorphin may play a role in the development of the overeating and obesity syndrome.

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Kinetics of the catecholainine uptake process in brain were altered by fighting. Significant increases in the apparent Michaelis constant (Km) for the uptake of norepinephrine into cerebral cortical homnogenates and significant increases in the inhibition constant (Ki) for d-amphetamnine inhibition of this uptake occurred in group-caged mice living under chronic attack from aggressive cage mates. Also, significant increases in the apparent Km and maximum velocity (Vmax) for norepinephrine uptake were observed 18 to 20 hours after the last of a series of short intense daily fights between male mice previously made aggressive by long-term individual caging. These results suggest that the natural stress of fighting leads to (i) lowered affinity for reuptake of norepinephrine into nerve endings of the cerebral cortex, (ii) an increase in the number of uptake sites, and (iii) lowered affinity for d-amphetamine.