LUTEINIZING-HORMONE-RELEASING HORMONE-RECEPTOR MESSENGER-RIBONUCLEIC-ACID EXPRESSION IN THE RAT PITUITARY DURING LACTATION AND THE ESTROUS-CYCLE

To study mechanisms underlying the modulation of luteinizing hormone-r eleasing hormone receptor (LHRH-R) during lactation and the estrous cy cle, we used a reverse transcriptase-polymerase chain reaction (RT-PCR ) procedure to generate a probe for rat LHRH-R messenger RNA (mRNA). U sing primers based on the mouse sequence, we amplified an approximatel y 300 bp fragment from rat pituitary complementary DNA. This PCR produ ct was shown to be part of LHRH-R cDNA by direct sequencing and by com paring to the rat LHRH-R cDNA reported recently. Then, this PCR fragme nt was used as a probe for northern blotting analysis. The level of LH RH-R mRNA in the pituitary was significantly decreased during lactatio n, by approximately 80%, compared to that of ovariectomized and intact (diestrous and metestrous cycling) rats while no statistical differen ce in glyceraldehyde-3-phosphate-dehydrogenase (GAPDH) mRNA level was observed between groups. During the estrous cycle, the level of LHRH-R mRNA in the pituitary was about two-fold higher on diestrous day 2 an d the morning of proestrus than that on diestrous day 1 and quickly re turned toward control level by noon of proestrus. In addition, we foun d that GAPDH mRNA levels from a so-called housekeeping gene often thou ght to be unchanged under different conditions, were significantly hig her on proestrus while levels of 18S rRNA were not significantly chang ed. The large decrease in LHRH-R mRNA during lactation could account f or the changes in LHRH binding previously reported.