Temporal patterns of gonadotropin-releasing hormone (GnRH), c-fos, and galanin gene expression in GnRH neurons relative to the luteinizing hormone surge in the rat

Abstract

Gonadotropin-releasing hormone (GnRH) neurons increase their expression of Fos and galanin coincident with the luteinizing hormone (LH) surge in the female rat. To define the temporal relationships between the expression of these genes and the GnRH gene itself and to gain insight about the possible functional interactions of these processes, we compared levels of c-fos, galanin, and GnRH mRNA in GnRH neurons and plasma levels of LH in the rat, beginning 6 hr before and continuing for 24 hr after a sex steroid-induced LH surge. LH levels were increased significantly by 1600 hr. They increased twofold further by 1800 hr and then returned to baseline by 2400 hr. Using in situ hybridization, we determined that levels of c-fos mRNA in GnRH neurons were elevated significantly at 1600 hr only, whereas levels of galanin mRNA in GnRH neurons first increased twofold by 1800 hr, increased an additional twofold by 2400 hr, and remained elevated at all time points sampled thereafter. There were no significant changes in cellular levels of GnRH mRNA over the time points sampled. These results are consistent with the hypothesis that the induction of c-fos gene expression in GnRH neurons leads to an increase in galanin gene expression, and that the sustained increase in galanin mRNA levels in GnRH neurons reflects either the need to replenish galanin stores that are depleted at the time of the LH surge or the involvement of galanin with physiological events that occur on the day of estrus.

Fig. 1.

Time course of serum LH concentrations in estrogen- and progesterone-primed ovariectomized rats in experiment 1. Values represent the mean ± SEM serum LH concentrations of serial blood samples obtained via jugular cannulae from 11 animals.

Fig. 2.

Time course of serum LH concentrations (A), relative levels of c-fos mRNA (B), relative levels of galanin mRNA (C), and relative levels of GnRH mRNA (D) in estrogen- and progesterone-primed ovariectomized rats during a 30 hr period bracketing the time of the expected LH surge (experiment 2). Values represent the mean ± SEM of four to six animals per group. Values with differentletters are significantly different from one another (p < 0.05). Values with the sameletter are not significantly different from one another (p > 0.05). The dashed lineindicates the time of the peak of the LH surge.

Fig. 3.

A, Bright-field photomicrograph of two GnRH neurons (black arrows) labeled with a digoxigenin-labeled cRNA probe for GnRH mRNA in an ovariectomized estrogen- and progesterone-primed female rat killed at 1600 hr on the day of the expected LH surge. B, Dark-field photomicrograph of the same view as in A showing the location of the same two GnRH neurons labeled with a³³P-labeled probe for c-fos mRNA (white arrows). Note the abundance of silver grains (clusters of white dots) over each cell indicating the high level of c-fos mRNA expression in these neurons. C, Bright-field photomicrograph of three GnRH neurons (black arrows) labeled with a digoxigenin-labeled cRNA probe for GnRH mRNA in the same animal shown in A and B. D, Dark-field photomicrograph of the same view as C, showing the location of the same three GnRH neurons labeled with a³⁵S-labeled probe for galanin mRNA (white arrows). Note the relative absence of silver grains indicating the low level of galanin mRNA expression in these neurons.E, Bright-field photomicrograph of two GnRH neurons (black arrows) labeled with a digoxigenin-labeled cRNA probe for GnRH mRNA in an ovariectomized estrogen- and progesterone-primed female rat killed at 2400 hr on the day of the expected LH surge. F, Dark-field photomicrograph of the same view as in E showing the location of the same two GnRH neurons labeled with a ³³P-labeled probe for c-fos mRNA (white arrows). Note the relative absence of silver grains indicating the low level of c-fos mRNA expression in these neurons.G, Bright-field photomicrograph of two GnRH neurons (black arrows) labeled with a digoxigenin-labeled cRNA probe for GnRH mRNA in the same animal shown in E andF. H, Dark-field photomicrograph of the same view as G, showing the location of the same two GnRH neurons labeled with a ³⁵S-labeled probe for galanin mRNA (white arrows). Note the abundance of silver grains (clusters of white dots) over each cell indicating the high level of galanin mRNA expression in these neurons. Magnification, 1200×.