Promotion of intragranular co-aggregation with LH by enhancement of secretogranin II storage resulted in increased intracellular granule storage in gonadotrophs of GnRH-deprived male mice

Intracellular associations indicate that granins may play a role in the regulatory mechanisms involved in differential secretion of gonadotrophins. The effect of GnRH on mRNA expression, storage and secretory patterns of granins and gonadotrophins was investigated in male mice. GnRH antiserum (G/A) was injected into mice in the treatment group (n = 15) at 12 h intervals for 2 days and a subset (n = 9) was killed. Buserelin (G/A + B) was administered to the remaining mice (n = 6), which were killed 2 h later; control mice (n = 6) were killed at the onset of the study. LHb mRNA content was lower in G/A and G/A + B mice compared with controls, whereas plasma LH concentrations were higher in G/A + B mice. FSHbeta mRNA content did not change, whereas plasma FSH concentrations were lower in G/A mice compared with controls, and higher in G/A + B mice compared with both G/A and control mice. Secretogranin II (SgII) and CgA mRNA contents were not different between experimental groups. There were more granules per gonadotroph in G/A mice, and considerably fewer after Buserelin treatment. Immunogold labelling of gonadotrophs revealed the presence of LH(+ve)/SgII(+ve) and LH(+ve)/SgII(-ve) granules, and negligible numbers of LH(-ve)/SgII(+ve) granules. Both the numbers of LH(+ve)/SgII(+ve) granules and overall granule antigenicity for SgII were higher in G/A mice compared with controls and G/A + B mice. In contrast, there were fewer LH(+ve)/SgII(-ve) granules per gonadotroph in G/A mice compared with controls. In conclusion, absence of GnRH input to the pituitary gland resulted in preferential storage of SgII and subsequently increased intragranular co-aggregation with LH. Administration of Buserelin to G/A mice resulted in the apparent release of LH(+ve)/SgII(+ve) granules that was reflected by an increase in plasma LH concentrations, indicating that these granules were in the regulated secretory pathway. In contrast, secretion of LH(+ve)/SgII(-ve) granules did not appear to be influenced by the actions of Buserelin and, therefore, may have been destined for constitutive release, possibly to maintain basal plasma LH concentrations.

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