Regulation of growth hormone-releasing hormone and somatostatin from perifused,bovine hypothalamic slices. III. Reciprocal feedback between growth hormone-releasing hormone and somatostatin

An in vitro perifusion system for bovine hypothalamic tissue was used to determine if growth hormone-releasing hormone (GHRH) and somatostatin (SRIF) modulate each other's release, and whether SRIF mediates D₁-agonist-induced suppression of GHRH in cattle. Up to three sagittal slices (600 μm) of bovine hypothalamus, immediately parallel to the midline, were cut in an oxygenated balanced salt solution at 4° C, placed in 5 cc syringe barrels, and perifused at 37° C with oxygenated minimum essential medium-α at a flow rate of 0.15 ml/min. Three experiments were conducted, and medium effluent was collected every 20 min before (two samples), during (one or three samples), and after (six samples) treatment. Areas under GHRH and SRIF response curves (AUC), adjusted by covariance for pretreatment values, were calculated from samples collected during the treatment/post-treatment period. Perifusion of SRIF at 10^?6 M and 10^?4 M decreased AUC for GHRH from 86.3 (control) to 65.4 and 59.5 ± 6.3 ng · ml^?1 min, but 10^?8 M SRIF was ineffective. Relative to controls, 10^?8, 10^?6, and 10^?4 M GHRH increased release of SRIF 190, 675, and 1,135%, respectively. Activation of D₁ receptors with 10^?6 M SKF 38393 increased AUC for SRIF from 12.5 ng · ml^?1 min (control) to 484.9 ng · ml^?1 min and decreased AUC for GHRH from 36.4 ng · ml^?1 min (control) to 18.2 ng · ml^?1 min. Blockade of SRIF action with a SRIF antagonist, cyclo-7-aminoheptanoyl-phe-d-trp-lys-thr(bzl)], increased release of GHRH 1.9-fold. In addition, the SRIF antagonist blocked SKF 38393-induced suppression of GHRH. We concluded that GHRH and SRIF interact within the bovine hypothalamus/pituitary stalk to modulate the release of the other. Moreover, SRIF mediates the inhibitory effects of activation of D₁ receptors on release of GHRH in cattle.