Abstract:
Galanin is colocalized with GnRH, and its expression in these neurons is
enhanced at proestrus, a time of activation of GnRH neurons. We tested the
hypothesis that the expression of both the GnRH and galanin mRNAs in GnRH
neurons decrease during lactation in the rat, a reproductive state
characterized by reduced gonadotropin secretion. For double label in situ
hybridization, GnRH mRNA was detected with an antisense cRNA probe labeled
with the hapten digoxigenin, whereas galanin mRNA was detected with a cRNA
probe labeled with 35S. The number of silver grains deposited over a
digoxigenin-labeled cell body provided an index of galanin mRNA levels in
GnRH cells. We observed a 60% reduction in signal (grains per cell) for
galanin mRNA in GnRH neurons of lactating animals compared with those of
diestrus animals (P < 0.004), with no difference in the number of GnRH
neurons between groups. To compare cellular GnRH mRNA content between
groups, we used single label in situ hybridization and image analysis.
Signal levels (grains per cell) for GnRH mRNA were not different between
diestrus and lactating animals in either an initial (diestrus, 121.4 +/-
5.9; lactation, 117.3 +/- 8.0; P > 0.7) or in a subsequent trial
(diestrus, 184.0 +/- 10.4; lactation, 197.5 +/- 13.0; P > 0.7). To confirm
and extend these findings, we used a RNAse protection assay to measure and
compare the content of GnRH mRNA in hypothalamic fragments between
diestrus and lactating animals. The concentration of GnRH mRNA (picograms
of mRNA per 25 micrograms total RNA) was not different between the two
groups (diestrus, 1.21 +/- 0.25; lactation, 1.25 +/- 0.13; P > 0.7). A
determination of the total GnRH peptide content by RIA in a separate set
of hypothalamic dissections revealed no difference between groups in the
level of GnRH content (nanograms) per hypothalamus (diestrus, 6.0 +/- 0.6;
lactation, 5.7 +/- 0.4; P > 0.4). We conclude that galanin mRNA expression
in GnRH neurons of the rat is diminished during lactation, whereas GnRH
expression continues unabated. This decrease in galanin gene expression
associated with lactation may lead to decreased synthesis and secretion of
galanin, which, in turn, could diminish the pulsatile secretion of GnRH or
reduce its activity at the pituitary.
