Effect of diazoxide on brain capillary insulin receptor binding and food intake in hyperphagic obese Zucker rats

Insulin is believed to act as a central adiposity signal by binding to hypothalamic and other brain insulin receptors. Entry of circulating insulin into the brain is accomplished by a saturable receptor-mediated transendothelial transport system and is believed to be impaired in hyperinsulinemic, insulin-resistant, and hyperphagic obese Zucker rats. Theoretically, if hyperinsulinemia is decreased simultaneously while brain capillary insulin binding is increased, uptake of insulin into the brain can be enhanced leading to reduced food intake. To test this hypothesis, we administered diazoxide (DZ, 150 mg/kg/day), an inhibitor of glucose-mediated insulin secretion, or vehicle (control) to 7-week-old female obese and lean Zucker rats for 4 weeks (n = 24-28/subgroup-strain). Animals were assigned to either fasted (FD) or free-fed (FF) protocol for determination of plasma and cerebrospinal fluid (CSF) insulin and brain capillary insulin binding at the end of 4 weeks. DZ obese consumed fewer calories (P < 0.01) and gained less weight than control obese (P < 0.01), whereas DZ lean had similar amounts of caloric intake and weight gain compared with lean controls. DZ obese had lower fasting and random plasma glucose than control obese (P < 0.05). FD and FF DZ-treated obese and lean rats had lower plasma insulin than their respective obese (P < 0.01) and lean (P < 0.01) controls. FD and FF DZ-treated obese rats demonstrated higher CSF insulin (P < 0.05) and CSM plasma insulin ratio (P < 0.01) than their controls, while only FF DZ lean animals showed higher CSF/plasma insulin ratio (P < 0.01) than their controls (P < 0.05). This was associated with enhanced brain capillary insulin binding in FD and FF DZ-treated obese (P < 0.01) and lean (P < 0.05) animals compared with their respective controls. It was concluded that DZ treatment in obese Zucker rats caused a decrease in insulin secretion and partially reversed impaired insulin binding to brain capillaries, leading to enhanced brain insulin uptake, and resulted in reduced food intake and weight gain observed in these animals.