Lactation is associated with a reduction in pituitary GnRH receptor content (GnRH-R) when compared with diestrus of the estrous cycle. These studies examined whether the changes in GnRH-R during lactation involved alterations in GnRH-R gene expression. GnRH-R messenger RNA (mRNA) levels were assessed by in situ hybridization, using a35S-labeled antisense riboprobe coding for the transmembrane region of the mouse GnRH-R gene. The area occupied by grains in 20- fim sections of the pituitary was assessed with an image analysis program. The specificity of the probe for GnRH-R mRNA was demonstrated by the presence of only background grains over the pituitary when a sense riboprobe to GnRH-R mRNA was used. Also, there was an absence of clusters of grains over the posterior pituitary, an area devoid of gonadotropes. In lactating rats suckling eight pups on day 10 postpartum, GnRH-R mRNA levels in the anterior pituitary were suppressed by 60% when compared with the levels observed during diestrus. Removal of the eight-pup suckling stimulus for 24 h reestablished the diestrous pattern of GnRH-R mRNA expression. These results demonstrate that the suckling stimulus results in a significant suppression of GnRH-R mRNA levels that is reversed by removal of the stimulus. These changes in GnRH-R mRNA most likely contribute to the changes in GnRH-R content observed in response to the suckling stimulus.
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