Site of action of putative lipostatic factor: hypothalamic metabolism of parabiotic rats

Studies were conducted to determine whether metabolic adaptation occurred in the hypothalamus of overfed parabiotic rats and their partners to distinguish between the adaptations caused by increased caloric intake and those caused by the production of a "lipostatic factor." The induction of overfed obesity in one parabiotic partner was employed to test the hypothesis that a putative lipostatic factor produced in the obese parabiotic elicited the hypophagic-lipid-mobilizing effect observed in the lean parabiotic via alterations in hypothalamic fatty acid and glucose metabolism. Fatty acid oxidation in the ventrolateral hypothalamus (VLH) of overfed parabiotic rats and their partners was lower than in ad libitum parabiotic rats. Net flux of glucose through the VLH gamma-aminobutyric acid (GABA) shunt was elevated in overfed parabiotic rats compared with the net flux observed in their partners and ad libitum parabiotic rats, the levels being similar in these last two groups. Net flux of glucose through the ventromedial hypothalamic (VMH) pentose shunt in overfed parabiotic rats and their partners was elevated relative to ad libitum parabiotic rats. The putative lipostatic factor may act to regulate energy balance through modification of VLH fatty acid oxidation and/or glucose flux via the VMH pentose shunt.