AJP - Endocrinology and Metabolism, Vol 244, Issue 1 31-E36, Copyright © 1983 by American Physiological Society
Acute inhibition of PRL and TSH secretion after intraventricular injection of PRL in ovariectomized rats
H. K. Mangat and S. M. McCann
Prolactin (PRL) is under short-loop inhibitory control via the
hypothalamus. However, earlier studies evaluated the effects on PRL
secretion of PRL levels elevated for periods of days. In this study we
evaluated the acute effects of intraventricular and systemic injection of
PRL on the release of a variety of pituitary hormones. Ovariectomized (OVX)
rats, bearing implanted third ventricular and jugular cannulas were used.
Blood was withdrawn in unanesthetized, freely moving animals before and
after intraventricular injection of 0.9% NaCl or 1 or 3 micrograms of
bovine (b) or ovine (o) PRL. Prolactin was also administered intravenously
in doses of 3 or 6 micrograms. No effect on plasma levels of any of the
pituitary hormones occurred after intraventricular or systemic injection of
saline. Intraventricular injection of both doses of bPRL or oPRL
significantly lowered plasma PRL within 15-30 min. In animals with elevated
initial PRL values because of stress or estradiol (E) priming, greater
lowering of PRL occurred. Inconsistent reductions in plasma PRL occurred
after intravenous injection of oPRL but not bPRL, which elevated PRL values
via cross-reaction in the immunoassay. In contrast, only small and
inconsistent declines in luteinizing hormone (LH) were seen after
intraventricular injection of PRL in either OVX or OVX E-primed rats.
Plasma follicle-stimulating hormone (FSH) and growth hormone (GH) were not
affected by PRL in any of the experiments; however, a significant lowering
of thyrotropin (TSH) occurred in OVX or OVX E-primed rats within 30 min
after intravenous injection of 3 micrograms of oPRL, but no change occurred
after intravenous PRL. The data indicate that PRL can acutely inhibit PRL
and TSH release via a hypothalamic action, whereas release of LH is only
slightly inhibited and that of FSH and GH is unaltered.
