Differential expression of gonadotropin and prolactin antigens by GHRH target cells from male and female rats

There is a 2- to 3-fold increase in luteinizing hormone-beta (LH beta) or f ollicle-stimulating hormone-beta (FSH beta) antigen-bearing gonadotropes du ring diestrus in preparation for the peak LH or FSH secretory activity. Thi s coincides with an increase in cells bearing LH beta or FSH beta mRNA. Sim ilarly, there is a 3- to 4-fold increase in the percentage of cells that bi nd GnRH. In 1994, we reported that this augmentation in gonadotropes may co me partially from subsets of somatotropes that transitionally express LH be ta or FSH beta mRNA and GnRH-binding sites. The next phase of the study foc used on questions relating to the somatotropes themselves. Do these putativ e somato-gonadotropes retain a somatotrope phenotype? As a part of ongoing studies that address this question, a biotinylated analog of GHRH was produ ced, separated by HPLC and characterized for its ability to elicit the rele ase of GH as well as bind to pituitary target cells. The biotinylated analo g (Bio-GHRH) was detected cytochemically by the avidin-peroxidase complex t echnique. It could be displaced by competition with 100-1000 nM GHRH but no t corticotropin-releasing hormone or GnRH. In cells from male rats exposed to 1 nM Bio-GHRH, 28 +/- 6% (mean +/- S.D) of pituitary cells exhibited lab el for Bio-GHRH (compared with 0.8 +/- 0.6% in the controls). There were no differences in percentages of GHRH target cells in populations from proest rous (28 +/- 5%) and estrous (25 +/- 5%) rats. Maximal percentages of label ed cells were seen following addition of 1 nM analog for 10 min. In dual-la beled fields, GHRH target cells contained all major pituitary hormones, but their expression of ACTH and TRH was very low (less than 3% of the pituita ry cell population) and the expression of prolactin (PRL) and gonadotropins varied with the sex and stage of the animal. In all experimental groups, 7 8-80% of Bio-GHRH-reactive cells contained GH (80-91% of GH cells). In male rats, 33 +/- 6% of GHRH target cells contained PRL (37 +/- 9% of PRL cells ) and less than 20% of these GHRH-receptive cells contained gonadotropins ( 23 +/- 1% of LH and 31 +/- 9% of FSH cells). In contrast, expression of PRL and gonadotropins was found in over half of the GHRH target cells front pr oestrous female rats (55 +/- 10% contained PRL; 56 +/- 8% contained FSH bet a; and 66 +/- 1% contained I;HF). This reflected GHRH binding by 71 +/- 2% PRL cells, 85 +/- 5% of LH cells and 83 +/- 9% of FSH cells. In estrous fem ale rats, the hormonal storage patterns in GHRH target cells were similar t o those in the male rat. Because the overall percentages of cells with :Bio -GHRH or GH label do not vary among the three groups, the differences seen in the proestrous group reflect internal changes within a single group of s omatotropes that retain their GHRH receptor phenotype. Hence, these data co rrelate with earlier findings that showed that somatotropes may be converte d to transitional gonadotropes just before proestrus secretory activity. Th e LH and FSH antigen content of the GHRH target cells from proestrous rats demonstrates that the LH beta and FSH beta mRNAs are indeed translated. Fur thermore, the :increased expression of PRL antigens by these cells signifie s that these convertible somatotropes may also be somatomammotropes.