Glucagon-like peptide-1 stimulates luteinizing hormone-releasing hormone secretion in a rodent hypothalamic neuronal cell line

To examine the influence of the putative satiety factor (GLP-1) on the hypothalamo-pituitary-gonadal axis, we used GT(1)-7 cells as a model of neuronal luteinizing hormone-releasing hormone (LHRH) release. GLP-1 caused a concentration-dependent increase in LHRH release from GT(1)-7 cells. Specific, saturable GLP-1 binding sites were demonstrated on these cells. The binding of [I-125]GLP-1 was time-dependent and consistent with a single binding site (K-d = 0.07 +/- 0.016 nM; binding capacity = 160 +/- 11 fmol/mg protein). The specific GLP-1 receptor agonists, exendin-3 and exendin-B also showed high affinity (K-i = 0.3 +/- 0.05 and 0.32 +/- 0.06 nM, respectively) as did the antagonist exendin-(9-39) (K-i = 0.98 +/- 0.24 nM). At concentrations that increased LHRH release, GLP-1 (0.5-10 nM) also caused an increase in intracellular cAMP in GT(1)-7 cells (10 nM GLP-1: 7.66 +/- 0.4 vs. control: 0.23 +/- 0.02 nmol/mg protein; P < 0.001). Intracerebroventricular injection of GLP-1 at a single concentration (10 mu g) produced a prompt increase in the plasma luteinizing hormone concentration in male rats (GLP-1: 1.09 +/- 0.11 vs. saline: 0.69 +/- 0.06 ng/ml; P < 0.005). GLP-1 levels in the hypothalami of 48-h-fasted male rats showed a decrease, indicating a possible association of the satiety factor with the low luteinizing hormone levels in animals with a negative energy balance.