REGULATION OF GROWTH HORMONE-RELEASING HORMONE AND SOMATOSTATIN FROM PERIFUSED, BOVINE HYPOTHALAMIC SLICES .3. 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 somatostat in (SRIF) modulate each other's release, and whether SRIF mediates D-1 -agonist-induced suppression of GHRH in cattle. Up to three sagittal d ices (600 mu m) of bovine hypothalamus, immediately parallel to the mi dline, were cut in an oxygenated balanced salt solution at 4 degrees C , placed in 5 cc syringe barrels, and perifused at 37 degrees C with o xygenated minimum essential medium-alpha 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 curv es (AUG), adjusted by covariance for pretreatment values, were calcula ted from samples collected during the treatment/post-treatment period. Perifusion of SRIF at 10(-6) M and 10(-4) M decreased AUC for GHRH fr om 86.3 (control) co 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%, respectivel y. Activation of D-1 receptors with 10(-6) M SKF 38393 increased AUC f or SRIF from 12.5 ng . ml(-1) min (control) to 484.9 ng . ml(-1) min a nd 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 s uppression of GKRH. We concluded that GHRH and SRIF interact within th e bovine hypothalamus/pituitary stalk to modulate the release of the o ther. Moreover, SRIF mediates the inhibitory effects of activation of D-1 receptors on release of GHRH in cattle. (C) Elsevier Science Inc. 1997.