Endocrinology, Vol 113, 1452-1458, Copyright © 1983 by Endocrine Society
Failure of prolactin short loop feedback mechanism to operate in old as compared to young female rats
DK Sarkar, N Miki and J Meites
The short loop feedback effect of PRL was studied in young (4-5 months of
age) and old (24-26 months of age) ovariectomized rats after a single iv
injection of bovine PRL (bPRL, 500 micrograms/100 g BW) or BSA (500
micrograms/100 g BW). Blood samples were collected via intraatrial cannula
every 20 min for assay of PRL. Plasma PRL levels in both young and old
ovariectomized rats were pulsatile in nature, and showed approximately one
PRL pulse per hour. The magnitude of the PRL peaks and concentrations of
plasma PRL, but not the number of PRL peaks, were significantly greater in
the old than in the young rats. The effect of bPRL on in situ PRL release
was studied after verifying that bPRL does not cross-react with rat PRL
RIA, but does significantly increase the release of [3H] dopamine from the
median eminence in vitro. This latter effect was dose dependent. In young
rats, a single injection of bPRL minimally reduced the concentration of
plasma PRL between 100 min and 5 h, but by 22-25 h it decreased plasma PRL
to approximately one third of preinjection levels. The magnitude of the PRL
pulses, but not the pulse frequency was significantly reduced after
administration of bPRL treatment to young rats. Treatment with BSA did not
alter the concentration of plasma PRL or the magnitude and frequency of the
PRL pulses in young rats. In old rats, plasma PRL concentrations and the
frequency and magnitude of the PRL pulses were not significantly decreased
after injection of either bPRL or BSA. Thus, the feedback inhibition of PRL
on PRL release may not be operative in old rats. The loss of the short loop
feedback inhibition of PRL is believed to be due to the reduction in
hypothalamic dopaminergic activity previously reported by our and other
laboratories in old rats.
