GnRH receptor site increase on the surface of cultured gonadotropes of senescent C57BL/6NNia mice.

Anterior pituitary cells were cultured for 2 days from 6-, 14-, and 23-25-month-old C57BL/6NNia mice. The cells were then stimulated with one of three biotinylated GnRHs [biotinyl-Lys6]-[D-Lys6]GnRH, [biotinyl-Ser4]-[D-Lys6]GnRH, [biotinyl-Ser4]-[D-Trp6, des-Gly10]GnRH) at 4 degrees C for 1 h. Some of these cells were processed unfixed, attaching avidin-fluorescein and examined with fluorescent microscopy. Other cells were fixed and the biotinylated GnRH coupled to avidin-gold (20 nm particles), which was subsequently silver-enhanced (70 nm particles) and examined with light and scanning electron microscopy (SEM). To enhance the location of receptor sites for quantitation, a back-scattering electron detector was employed with the SEM. Gonadotropes from all three age groups ranged in size from 3 to 13 microns in diameter. The largest gonadotropes in each age group displayed the highest number of GnRH receptors, but the concentration/surface area (microns2) was less than for smaller cells. Smaller gonadotropes (3-6 microns diameter) from 14- and 23-25-month-old mice contained more GnRH receptor sites/microns2 (p < .001) than those from 6-month-old mice; the percentage of these gonadotropes was markedly higher for 14-(65%) and 25-(52%) versus 6-month-old (5%) mice. The number of receptor sites in larger gonadotropes (6.1-13 microns diameter) did not decline with the increasing age of the mouse. Decreased secretory activity in gonadotropes of 25-month-old mice may result from age-related changes in pituitary peptide processing or release, but it is not due to a lack of GnRH receptors or the inability of GnRH to bind to its receptor site.