Fourth ventricular phlorizin dissociates feeding from hyperglycemia in rats

The effects of 4th ventricular injections of phlorizin and 5-thioglucose (5-TG) on feeding, plasma glucose, and plasma insulin levels were determined. Fourth ventricular applications of phlorizin (13 micrograms) and 5-TG (150 and 210 micrograms) were equally effective in stimulating feeding. Systemic injections of phlorizin (13 micrograms) did not stimulate feeding. In the absence of food, hyperglycemia was elicited by 4th ventricular injections of 5-TG. In contrast, the dose of phlorizin that stimulated feeding, did not produce hyperglycemia. Basal plasma insulin levels were not affected by either of the 4th ventricular injections. These data indicate that activation of caudal brainstem metabolic interoceptors provides an afferent limb for the production of compensatory responses and that behavioral and autonomic compensatory responses can be activated independently. The glucodynamic action of phlorizin appears selective for that mechanism mediating the behavioral compensatory response of feeding in contrast to the dual effects produced by 5-TG. These and other data suggest that different caudal brainstem mechanisms control behavioral and autonomic compensatory responses.