BASAL AND HYPERINSULINEMIA-INDUCED IMMUNOREACTIVE HYPOTHALAMIC INSULIN CHANGES IN LEAN AND GENETICALLY-OBESE ZUCKER RATS REVEALED BY MICRODIALYSIS

Lean and genetically obese Zucker rats were implanted with permanent intravenous catheters and a guide canula was aimed at the region of the ventromedial (VMH) and paraventricular (PVN) nuclei to measure immunoreactive insulin collected by means of microdialysis. Preliminary experiments assessed the validity of a novel assay of insulin in microdialysates by a sensitized radioimmunoassay technique. This method was then used to measure basal levels of insulin and those induced by i.v. infusion of 0.5 U of insulin over 30 min in both lean and obese rats. Basal hypothalamic immunoreactive insulin levels were lower in the obese rats than in the lean Zucker rats. When insulin was infused i.v. for 30 min, hypothalamic immunoreactive insulin showed an increase in the 30-60 min sample, which was twice as great in the obese rats. Two facts suggest that the insulin found in the microdialysates was of cerebral, not vascular origin: the short latency in the response and the finding that the response was greater in obese rats.