Release of Somatostatin-like Immunoreactivity from Incubated Rat Hypothalamus and Cerebral Cortex

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Abstract
Somatostatin (SRIF) is localized in the hypothalamus, extrahypothalamic brain, and throughout the gastrointestinal tract. Release of gastrointestinal SRIF-like immunoreactivity (SRIF-LI) is under nutrient regulation but the effect of nutrients on neural SRIF-LI is unknown. The present studies examined the effects of glucose uptake and metabolism and hormones influencing glucose disposition on SRIF-LI release from medial basal hypothalamus (MBH) and cerebral cortex (Cx) incubated in Krebs-Ringer bicarbonate containing bacitracin. After a preincubation to achieve stable secretion, tissues were incubated for 20 min in 14 mM glucose (basal) and then, for 20 min in fresh medium with test materials. MBH SRIF-LI release was inversely related to medium glucose concentration with release in the absence of glucose (235±42 pg/MBH per 20 min) more than five times that in the presence of 25 mM glucose (46±4 pg/20 min). In the presence of 14 mM glucose MBH SRIF-LI release was stimulated above basal by agents interfering with glucose uptake including 3-O-methyl-d-glucose (42 mM; 70±5 vs. 42±3 pg/20 min, P < 0.05), phlorizin (50 mM; 351±63 vs. 29±2 pg/20 min, P < 0.001) or cytochalasin B (20 μM; 110±7 vs. 22±2 pg/20 min, P < 0.001). Inhibition of glucose metabolism by 2-deoxy-d-glucose resulted in dose-related stimulation of MBH SRIF-LI release (maximal at 28 mM; 201±28 pg/20 min vs. 32±4 pg/20 min, P < 0.001). Viability of MBH was unimpaired by incubation in the absence of glucose or following exposure to 2-deoxy-d-glucose as determined by retention of SRIF-LI responsiveness to stimulation by potassium (60 mM) or neurotensin (5 μM). In contrast, Cx SRIF-LI release was slightly inhibited by decreases in medium glucose and unaffected by inhibition of glucose uptake or metabolism. These results provide evidence for nutrient regulation of MBH but not Cx SRIF-LI release and may explain inhibition of growth hormone seen in the rat in response to hypoglycemia. Insulin (10 nM-1 μM) stimulated MBH but not Cx SRIF-LI release while glucagon was without effect. Our previous demonstration that MBH SRIF-LI release was stimulated by somatomedin-C, but not insulin at physiologic concentrations, is consistent with an action of insulin through the somatomedin-C receptor at the doses studied. Our studies indicate a regional specificity for the control of SRIF secretion within the brain and suggests the possibility of a role for hypothalamic SRIF in metabolic regulation.