Effects of insulin-induced hypoglycemia on somatostatin level and binding in rat cerebral cortex and hippocampus

Response of rat cerebral somatostatinergic system to a high ammonia diet

It has been reported that ingestion of an ammonium-containing diet produces hyperammonemia without encephalopathy, thus permitting the study of the specific effects of ammonia toxicity. The present study investigated the rat cerebral somatostatinergic system using this experimental model of hyperammonemia. Wistar rats were fed a high ammonia diet prepared by mixing a standard diet with ammonium acetate (20% w/w); in addition, 5 mM of ammonium acetate was added to their water supply. Control rats were fed with a standard diet. The animals were sacrificed at 3, 7 and 15 days of ammonia ingestion. Ammonia levels in blood had increased approximately 3-fold at 7 days of ammonia ingestion. These changes were associated with a significant decrease in the specific binding of somatostatin (SS) to putative receptors sites in the frontoparietal cortex and hippocampus at 7 and 15 days after starting the high ammonia diet. Scatchard analysis shows that the decrease in SS binding resulted from a decrease in the number of available SS receptors rather than a change in receptor affinity. No changes in the somatostatin-like immunoreactivity content (SSLI) were detected in either brain area at the three study times. These results suggest that hyperammonemia alone can affect the rat brain somatostatinergic system. However, the animal model of hyperammonemia used here is insufficient to produce encephalopathy despite the significant increase in serum ammonia.

Effect of streptozotocin-induced diabetes on somatostatin receptors in the anterior pituitary, hypothalamus and cerebral cortex of the male rat

In order to better understand the mechanisms underlying the reduction in GH secretion in diabetic rats, we have characterized and measured SRIH receptors in the hypothalamus and anterior pituitary gland 5 and 9 days after induction of diabetes in the rat. Experimental diabetes was induced by an intraperitoneal injection of streptozotocin (STZ) at a dose of 65 mg/kg. Basal plasma GH was significantly reduced in diabetic rats. Chronic insulin replacement therapy partly restored plasma GH and blood glucose levels in these animals. A significant reduction in SRIH receptor concentrations was demonstrated in the hypothalamus and anterior pituitary gland, 5- and 9- days after STZ injection. These changes were not significantly corrected by insulin replacement. Cerebral cortex SRIH receptor concentrations were unaffected by experimental diabetes. We conclude that hypothalamic and pituitary SRIH receptor levels are lowered in diabetic rats. These changes may contribute to aberrant GH secretion in diabetes and they indicate that pituitary sensitivity to exogenous somatostatin should be tested in diabetic patients.