pituitrin and exendin-(9-39)

To date, glucagon-like peptide-1 (7-36) amide (tGLP-1) has been found to enhance the vasopressin and oxytocin secretion in vivo but not in vitro (i.e., when the isolated neurointermediate lobe of the pituitary was used for experiments). The goal of this study was to investigate whether tGLP-1 can influence the function of the hypothalamo-neurohypophysial complex in vitro. Also, the effect of a tGLP-1 agonist, exendin-4, and antagonist, exendin-(9-39), on the release of vasopressin/oxytocin from the isolated rat hypothalamo-neurohypophysial complex was tested. tGLP-1 enhanced the basal but not the potassium-stimulated release of vasopressin and oxytocin from the hypothalamo-neurohypophysial complex. On the other hand, tGLP-1 failed to affect the release of both hormones from the isolated neurointermediate lobe. The tGLP-1 agonist increased the secretion of oxytocin and vasopressin from the hypothalamo-neurohypophysial system whilst the tGLP-1 antagonist completely abolished the stimulatory effect of tGLP-1 on the secretion of both hormones. It is concluded that tGLP-1 affects the function of vasopressin- and oxytocinergic neurones through specific hypothalamic receptors.

Topics: Animals; Cyclic AMP; Glucagon; Glucagon-Like Peptide 1; Glucagon-Like Peptides; Hypothalamo-Hypophyseal System; Hypothalamus, Middle; In Vitro Techniques; Male; Oxytocin; Peptide Fragments; Peptides; Rats; Rats, Wistar; Vasopressins

Within the central nervous system, glucagon-like peptide-1-(7-36) amide (GLP-1) acts as a transmitter, inhibiting feeding and drinking behavior. Hypothalamic neuroendocrine neurons are centrally involved in the regulatory mechanisms controlling these behaviors, and high densities of GLP-1 binding sites are present in the rat hypothalamus. In the present study we have, over a period of 4 h, followed the effect of centrally injected GLP-1 on plasma levels of the neurohypophysial hormones vasopressin and oxytocin. Plasma levels of corticosterone and glucose were also followed across time after central administration of GLP-1. In conscious, freely moving, and unstressed rats, central injection of GLP-1 significantly elevated plasma levels of vasopressin 15 and 30 min after administration (basal, 0.8 +/- 0.2 pg/ml; 15 min, 7.5 +/- 2.0 pg/ml; 30 min, 5.6 +/- 1.1 pg/ml; mean +/- SEM) and elevated corticosterone 15 min after administration (52 +/- 13 vs. 447 +/- 108 ng/ml, basal vs. 15 min; mean +/- SEM). In contrast, plasma oxytocin levels were unaffected by intracerebroventricular (icv) injections of GLP-1 over a period of 4 h after the injection. The animals given a central injection of GLP-1 developed transient hypoglycemia 20 min after the injection, which was fully restored to normal levels at 30 min. Furthermore, we used c-fos immunocytochemistry as an index of stimulated neuronal activity. The distribution and quantity of GLP-1-induced c-fos immunoreactivity were evaluated in a number of hypothalamic neuroendocrine areas, including the magnocellular neurons of the paraventricular (PVN) and supraoptic (SON) nuclei and the parvicellular neurons of the medial parvicellular subregion of the PVN. The number of c-fos-expressing nuclei in those areas was assessed 30, 60, and 90 min after icv administration of GLP-1. Intracerebroventricular injection of GLP-1 induced c-fos expression in the medial parvicellular subregion of the PVN as well as in magnocellular neurons of the PVN and SON. A slight induction of c-fos expression was seen in the arcuate nucleus and the nucleus of the solitary tract, including the area postrema. In contrast, the subfornical organ, which is a rostrally situated circumventricular organ, was free of c-fos-positive cells after central administration of GLP-1. When the GLP-1 antagonist exendin-(9-39) was given before the GLP-1, c-fos expression in these neuroendocrine areas was almost completely abolished, suggesting that the effect of GLP-

Topics: Animals; Arcuate Nucleus of Hypothalamus; Blood Glucose; Corticosterone; Corticotropin-Releasing Hormone; Gene Expression Regulation; Glucagon; Glucagon-Like Peptide 1; Glucagon-Like Peptides; Hypothalamo-Hypophyseal System; Hypothalamus; Immunohistochemistry; Injections, Intraventricular; Male; Neurons; Oxytocin; Paraventricular Hypothalamic Nucleus; Peptide Fragments; Phenotype; Pituitary-Adrenal System; Proto-Oncogene Proteins c-fos; Rats; Rats, Wistar; Supraoptic Nucleus; Time Factors; Vasopressins