Growth hormone in the dwarf dopaminergic D2 receptor knockout mouse: somatotrope population, GH release, and responsiveness to GH-releasing factors and somatostatin

GARCIA-TORNADU ISABEL; RUBINSTEIN MARCELO; GAYLINN BRUCE D; HILL DAVID; ARANY EDITH; LOW MALCOLM; DIAZ TORGA GRACIELA; BECU-VILLALOBOS DAMASIA

JOURNAL OF ENDOCRINOLOGY

Año: 2006 vol. 190 p. 611 - 619

0022-0795

Recently the importance of dopaminergic D2 receptor subtype (D2R) in normal body growth and neonatal GH secretion has been highlighted. Disruption of D2R alters the GHRH-GH-IGF-I axis, and impairs body growth in the adult male mice. The D2R knockout dwarf mouse (KO) has not been well characterized; we therefore sought to determine somatotrope function in the adult pituitary. Using immunohistochemistry and confocal microscopy we found a significant decrease in somatotrope population in pituitaries from KO mice (P = 0.043), which was paralleled by a decreased GH output from pituitary cells cultured in vitro. In cells from adult mice the response amplitude to GHRH differed between genotypes (lower in KO), but this difference was less dramatic after taking into account the lower basal release and hormone content in the knockout cells. Furthermore, there were no significant differences in cAMP generation in response to GHRH between genotypes. By Western blot, GHRH-R in pituitary membranes from KO mice was reduced to 46% of the level found in wildtype (WT) mice (P= 0.016). Somatostatin induced a concentration dependent decrease in GH and prolactin secretion in both genotypes, and Ghrelin (1*10-7 M) released GH in cells from both genotypes (P=0.017) in a proportionate manner to basal levels. These results suggest that KO somatotropes maintain a regulated secretory function.  We finally tested the direct effect of dopamine on GH and prolactin secretion in cells from both genotypes at 20 days and 6 months of life. As expected, we found that dopamine could reduce prolactin levels at both ages in WT mice and not in KO mice, but there was no consistent effect of the neurotransmitter on GH release in either genotype at the ages studied. The present study demonstrates that in the adult D2R male knockout mouse there is a reduction in pituitary GH content and secretory activity. We point to an involvement of D2R signaling at the hypothalamic level as dopamine did not release GH acting at the pituitary level either in one month-old or adult mice. The similarity of the pituitary defect in the D2R knockout mouse to that of GHRH deficient models suggests a probable mechanism. A loss of dopamine signalling via hypothalamic D2Rs at a critical age causes the reduced release of GHRH from hypophyseotropic neurons leading to inadequate clonal expansion of the somatotrope population. Our data also reveal that somatotrope cell number is much more sensitive to changes in neonatal GHRH input than is their capacity to develop properly regulated GH secretory function.