Endocrine Abstracts

Prolactin releasing peptide (PrRP) is reported to decrease plasma growth hormone (GH) levels in the rat but increase GH release from human pituitary cells. A barbiturate anaesthetised model was used in the rat studies (where basal plasma growth hormone is elevated) and in humans the GH stimulatory effects were of a small magnitude and seen only in foetal or tumourous pituitary cells. Our studies have examined the role of PrRP in freely moving male rats without the influence of anaesthesia.

We have used ICV injection to investigate the effect of PrRP on GH levels in vivo. To examine possible mechanisms by which PrRP can alter GH levels we have used primary culture of pituitary cells and measured neuropeptide release from hypothalamic explants in vitro.

PrRP between 10nmol/l and 1mmol/l did not significantly alter growth hormone secretion from dispersed pituitary cells from male rats following 1, 2 or 4 hours incubation (PrRP 1mmol/l 106.5±25.6% of control at 1 hour; PrRP 1mmol/l 83.7±13.3% of control at 2 hours and PrRP 1mmol/l 79.2±5.4% of control at 4 hours, all p n.s.). PrRP also did effect GHRH stimulated growth hormone secretion from dispersed pituitary cells from male rats following 2 or 4 hours incubation.

Intracerebroventricular (ICV) injection of PrRP 5nmol had no effect on plasma GH levels at 10 20 or 60 minutes post injection (PrRP 5nmol 17.1±6.8ng/ml vs. saline 18.9±3.3ng/ml at 10 minutes; PrRP 5nmol 9.2±2.4ng/ml vs. saline 8.3±3.0 ng/ml at 20 minutes and PrRP 5nmol 5.9±1.3ng/ml vs. 11.2+4.3ng/ml at 60 minutes, all p n.s.)

PrRP had no effect on the release of somatostatin from hypothalamic explants in culture (PrRP 100nmol/l 89.2±15.9pmol/explant vs. basal release 90.6±9.8pmol/explant, p n.s.).

In conclusion our studies suggest that PrRP does not have a role in the control of growth hormone secretion in freely moving male rats.