ABSTRACT
Ovariectomized (OVX) rats suckling 8 pups have a complete suppression of pulsatile LH secretion and a decrease in pituitary GnRH receptor (GnRH-R) content. Removing the suckling stimulus for 24 h results in a sharp increase in GnRH-R and a restoration of pulsatile LH secretion. These findings suggest that the suckling stimulus induces a suppression of GnRH secretion, and removal of the suckling stimulus permits the restoration of GnRH secretion. Indeed, if GnRH antiserum is injected at the time of pup removal, the restoration of pituitary GnRH-R and LH secretion is prevented. The present studies were designed to test our hypothesis that the deficits in pituitary gonadotroph function observed during lactation are due to suckling-induced suppression of GnRH. Exogenous GnRH was administered in a pulsatile regimen to OVX lactating rats on days 10 and 11 postpartum, and the effects on pituitary GnRH-R levels, pituitary sensitivity to GnRH, and pulsatile LH secretion were assessed. GnRH doses of 0, 0.5, 2.0 or 5.0 ng/pulse were administered every 50 min for 24 h beginning on day 10. Administration of 0.5 ng GnRH/pulse for 24 h increased GnRH-R from 35 +/- 3 to 63 +/- 8 fmol/pituitary. There was a clear GnRH dose-related upregulation of GnRH-R to approach nonsuckling levels (140-160 fmol/pituitary) with the 5 ng GnRH dose. At the beginning of GnRH administration, the pituitary was very unresponsive to GnRH. Consistent LH pulses were only observed with 5 ng GnRH/pulse.(ABSTRACT TRUNCATED AT 250 WORDS)